Volume 62, Issue 1, January 2018 Published by Johnson Matthey © Copyright 2017 Johnson Matthey

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Johnson Matthey’s international journal of research exploring science and technology in industrial applications

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Johnson Matthey’s international journal of research exploring science and technology in industrial applications

Contents Volume 62, Issue 1, January 2018

2 Editorial: Field to Fork: Challenges in Ensuring a Sustainable Food Supply By Sara Coles 4 The Role of Noble Metal Catalysts in Conversion of Biomass and Bio-derived Intermediates to Fuels and Chemicals By Banothile C. E. Makhubela and James Darkwa 32 Ammonia and the Fertiliser Industry: The Development of Ammonia at Billingham By John Brightling 48 A Re-assessment of the Thermodynamic Properties of Palladium By John Arblaster 60 A Facile Green Tea Assisted Synthesis of Palladium Nanoparticles Using Recovered Palladium from Spent Palladium Impregnated Carbon By Ansari Palliyarayil, Kizhakoottu Kunjunny Jayakumar, Sanchita Sil and Nallaperumal Shunmuga Kumar 74 “Food Packaging” A book review by Chun-tian Zhao 81 “Air Pollution Control Technology Handbook”, Second Edition A book review by Martyn Twigg 86 Johnson Matthey Highlights 89 Coordination Compounds of Hexamethylenetetramine with Metal Salts: A Review By Jia Kaihua and Ba Shuhong 107 Recent Advances in Controlled and Modified Atmosphere of Fresh Produce By Natalia Falagán and Leon A. Terry 118 Challenges and Opportunities in Fast Pyrolysis of Biomass: Part I By Tony Bridgwater https://doi.org/10.1595/205651317X696667 Johnson Matthey Technol. Rev., 2018, 62, (1), 2–3

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Editorial Field to Fork: Challenges in Ensuring a Sustainable Food Supply

The United Nations Sustainable Development and ways are being sought to ensure both Goals for 2030 include targets around reducing effectiveness and, increasingly, to reduce the hunger for people worldwide (1). There are a social and environmental impacts of chemicals in number of challenges which industry is working agriculture (4, 5). There is no doubt that this will hard to address. In farming, the challenges be an important area of research for some time include crop growth and nutrition and making the to come. most of available land for food production. Once the food is harvested, there are further challenges Land Use for the food and beverage industry in ensuring that enough of the produce reaches the people It is now recognised that food security can who need it in a fit state for human consumption. be impacted by other pressures on land use (6). When biofuels were introduced the first Crop Nutrition and Protection generation were based on food crops such as sugars or oils. Now second- and third-generation One of the key crop nutrients is nitrogen. Although biofuels, made from ‘waste’ or other forms of the earth’s atmosphere is around 78% nitrogen, non-food competing biomass, are becoming most of this is locked in a form that plants cannot available (7, 8). The challenges in processing matter use. To make it accessible, it must be ‘fixed’ or such as cellulose and lignin are many: catalytic reduced into a more active chemical form. Some and pyrolytic approaches have been investigated natural processes can fix nitrogen, including to create efficient ways to prepare fuels that can microorganisms living in the roots of certain meet future energy needs as part of an energy mix vegetables as well as events such as lightning without detrimental impact on the food supply. strikes. However much of the nitrogen that is used today to produce food for humans comes from the Preventing Food Waste industrial fixation of nitrogen into ammonia, which has grown today into a huge area of industry The story of food does not stop at harvest. There carried out in many countries worldwide. are other areas where industry plays a role in There are challenges in this industry – not least getting food onto people’s plates. Studies suggest of which is ensuring the cost effectiveness and that up to one third of food produced worldwide energy efficiency of the process. By far the most is lost or wasted (9). There is a need for widely used process is the Haber-Bosch process low-cost, sustainable solutions to the problem which produces up to half the nitrogen entering of food spoilage and waste in increasingly global the human food chain. Alternatives are being food supply chains. looked at that are not yet ready for commercial The platinum group metals (pgms) have proved scale roll out but they are gaining interest in the unlikely allies in the fight against food waste as a academic community (2). number of past articles in this journal have shown Another area important to ensuring secure (10, 11, 12). There are commercial solutions food supply is the protection of crop plants from on the market which involve both pgm and pests and diseases. The pesticides industry non-pgm materials. For example, the use of nickel was worth around US$58 billion in 2016 (3) catalysts to improve oxidative stability of edible

oils, extending the shelf life, as well as adsorbents, 4. “Novel Methods for Accelerating Next-Generation packaging materials and other technological Agrochemical Development”, C&EN Whitepapers, solutions to control the environment around American Chemical Society, Washington, DC, fresh food and prevent spoilage. This is a vibrant USA, 3rd January, 2017 area and more research will no doubt reveal new 5. Climate Smart Agriculture, Yara International, approaches. Oslo, Norway, November, 2015 In this journal look out for articles detailing the 6. J. Valentine, J. Clifton-Brown, A. Hastings, P. history of ammonia production, the development Robson, G. Allison and P. Smith, Glob. Change. of new processes for biofuels that do not compete Biol. Bioenergy, 2012, 4, (1), 1 with food and work on new packaging technologies that can help ensure food reaches people, reducing 7. A. Demirbas, ‘Fuels from Biomass’ in “Biorefineries: For Biomass Upgrading Facilities”, Springer-Verlag waste in the worldwide food chain. Ltd, London, UK, 2010 8. A. D. Heavers, M. J. Watson, A. Steele and J. MS SARA COLES Simpson, Platinum Metals Rev., 2013, 57, (4), Editor, Johnson Matthey Technology Review 322 9. J. Gustavson, C. Cederberg, U. Sonesson, R. van References Otterdijk and A. Meybeck, “Global Food Losses and Food Waste: Extent, Causes and Prevention”, 1. United Nations, Sustainable Development Food and Agriculture Organization of the United Knowledge Platform, Sustainable Development Nations, Rome, Italy, 2011 Goals: https://sustainabledevelopment.un.org/ 10. A. W. J. Smith, S. Poulston, L. Rowsell, L. A. Terry sdgs (Accessed on 23rd August 2017) and J. A. Anderson, Platinum Metals Rev., 2009, 2. J. Nørskov and J. Chen, ‘Sustainable Ammonia 53, (3), 112 Synthesis’, DOE Roundtable Report, US Department of Energy, Virginia, USA, 18th February, 2016 11. P. Whitehead, Platinum Metals Rev., 1989, 33, (4), 193 3. ‘Global Markets for Agrochemicals’, BCC Research, Massachusetts, USA, March, 2017 12. Platinum Metals Rev., 1990, 34, (1), 24

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The Role of Noble Metal Catalysts in Conversion of Biomass and Bio-derived Intermediates to Fuels and Chemicals A review of promising approaches on plant based biomass as a renewable alternative feedstock

Banothile C. E. Makhubela* levulinate and valeric biofuels suitable as fuels Department of Chemistry, University of and fuel additives as well as renewable alkanes

Johannesburg, Auckland Park Kingsway in the C5–C15 range for gasoline and diesel fuel Campus, Auckland Park 2006, South Africa applications.

# James Darkwa 1. Introduction Department of Chemistry, University of Johannesburg, Auckland Park Kingsway Over the past century, the world has grown Campus, Auckland Park 2006, South Africa extremely reliant on finite fossil derived sources of carbon. Fossil derived sources such as crude Email: *[email protected], oil, natural gas and coal supply more than #[email protected] three quarters of the world’s energy (Figure 1) and 90% of chemicals and materials (1, 2). The consumption of energy and chemicals is In the face of growing oil demand the use of increasing at ~7% per annum and this has been renewable feedstocks for the potential to supply mostly driven by the growth experienced by the transportation fuels, electricity, chemicals and emerging economies. More specifically, global materials is increasingly attractive. This review oil demand is expected to grow until 2040, and covers novel technologies and pathways to this is largely as a result of lack of alternatives to produce liquid fuels and chemical intermediates in an efficient and cost-effective way. Several 2.4% 1.4% commercial and pilot scale projects by companies 4.8% Hydro Othera including Anellotech, USA; Johnson Matthey, Nuclear 10.3% UK; GFBiochemicals, Italy; Quaker Oats, USA; Biofuels Changchun Dacheng Group, China; Avantium, The and waste 31.3% Oil Netherlands; BASF, Germany and Rennovia, USA 21.2% Natural are highlighted. The review focuses on the use 28.6% gas of non-food competing biomass, namely cellulose Coal and hemicellulose biomass, and the use of precious metals to effect the key reaction steps: hydrolysis, dehydration, hydrodeoxygenation, hydrogenation Fig. 1. Total primary world energy supply in 2014. and oxidation. The value added products achieved aOther includes geothermal, wind, solar, heat include fine chemicals and functional materials. and tidal. (Based on IEA data from the Key World Among these are dimethylfuran, methylfuran, Energy Statistics © OECD/IEA 2016, www.iea.org/statistics. Licence: www.iea.org/t&c) 5-(ethoxymethyl)furfural, γ-valerolactone, ethyl

4 © 2018 Johnson Matthey https://doi.org/10.1595/205651317X696261 Johnson Matthey Technol. Rev., 2018, 62, (1) crude oil for the production of transportation fuels development of technologies and novel pathways and chemicals (1–3). to produce renewable liquid fuels and platform This growth in demand, combined with diminishing chemicals in an efficient and cost-effective way. fossil derived fuel reserves, environmental Plant biomass is available on a recurring basis degradation (arising from emission of harmful and has great potential as a renewable alternative greenhouse gases associated with processing of feedstock for the production of fuels and chemicals fossil derived feedstocks), unreliable supply and to effectively reduce reliance on fossil derived price fluctuations have been the main motivations resources. Plant biomass is inexpensive and is for the exploration of renewable alternatives to widely available throughout many geographical fossil derived sources of carbon for the production locations of the world, making it more accessible of energy and chemicals. than fossil deposits (4–6). One primary source of The global energy consumption renewables share chemicals is lignocellulose which is abundant in (including biofuels and waste, geothermal, hydro, plants. solar, wind and tidal) was 15.5% in 2014 and is expected to experience the fastest growth (led by 2. Structure of Lignocellulose solar and wind) between now and 2040 (3). It is important to point out that among the renewable The primary constituents of plant biomass are energy sources, biomass has the potential to lignin, carbohydrate polymers ((hemi)cellulose), supply renewable transportation fuels, electricity, oils, proteins and other chemicals such as dyes and organic chemicals and materials demand. For vitamins. Lignocellulose is highly functionalised and renewable liquid fuels and chemicals to make a is the main fraction (~90%) of plant biomass. It is meaningful contribution to this projected rapid composed of the non-edible biopolymers: cellulose, growth, substantial investment is needed into the hemicellulose and lignin (Figure 2) (5, 6). In the

Hemicellulose 15–30%

Lignin 15–30%

Biomass

Fig. 2. The main components of lignocellulose

Cellulose 35–60%

5 © 2018 Johnson Matthey https://doi.org/10.1595/205651317X696261 Johnson Matthey Technol. Rev., 2018, 62, (1) present article, we review the use of carbohydrates hemicellulose, contain far too much functionality (sugars and starchy biomass) and carbohydrate to use directly as fuels or bulk chemicals, and polymers (cellulose and hemicellulose biomass) in therefore require selective defunctionalisation biorefinery. Recent reviews on the valorisation of strategies (5, 12, 13). lignin are available (7–9). The main sources of cellulosic feedstock include 3.2 Biomass Conversion Technologies agricultural, forestry, industrial and municipal wastes and residues (10). Unlike carbohydrates The three common conversion technologies for (sugar and starchy biomass) feedstock, cellulosic carbohydrates and cellulose are shown in Figure 4 feedstock is non-edible and does not compete for (2, 14–16). Although production of first generation arable land and therefore avoids conflict with food biofuels (bio-ethanol and bio-butanol) is well supply (11). established, this process relies on starch and sugar feeds which compete with the food chain (16). 3. Conversion of Biomass and Hydrolysis of cellulosic feedstock to fermentable sugars has been achieved and a number of Biomass-derived Intermediates commercial scale cellulosic ethanol plants have 3.1 Biomass Conversion versus Crude been and are being developed in several countries Oil Conversion: Diverging Approaches (for example the USA, Italy, Brazil and Canada) as alternative routes to bio-alcohols as fuels in order Crude oil feedstock has low functionality which to avoid food sources as feedstock (17–20). Slow makes it directly suitable for use as a fuel after reaction rate, high cost and sensitivity of enzymes prior processing (for example cracking and and energy intensive subsequent distillation and isomerisation). Functional groups, such as C=O drying steps remain challenges to achieving cost- and OH, are added to crude oil derived feedstock effectiveness in these processes. A cost-effective to produce bulk and specialty chemicals. Here, process that transforms biomass (such as wood, special care is taken to ensure selective addition of sugarcane bagasse or corn stover) to aromatic the functional group without over functionalisation compounds has been developed by Anellotech, of the substrates (Figure 3). Contrastingly, USA. This process uses zeolite-based catalysts, co- biomass derived feedstocks, such as cellulose and developed with Johnson Matthey, to produce gases

High functionality

Fine chemicals Renewable cleavage Selective C–C and C–O Reduction Dehydration Depolymerisation Hydroxydeoxygenation Defunctionalisation

Bulk chemicals Functionalisation Polymerisation/ oligomerisation Oxidation Carbonylation bond formation C–C Fuels

Low functionality

Non-renewable

Fig. 3. Conversion of biomass derived and fossil derived feedstocks to chemicals and fuels: contrasting approaches (From (13) published by The Royal Society of Chemistry)

Feedstock: Forest waste (wood, logging residues), bagasse (sugarcane or sorghum stocks), agricultural residue (corn stover, crop residues), energy crops (grasses), aquatic plants (seaweed, water hyacinth), solid municipal waste (SMW)

Cellulosic biomass Hemicellulose/cellulose

Pretreatment >> Hydrolysis Thermochemical (enzymatic/acid and thermocatalytic catalysed) conversion

Pyrolysis or Sugars torrefaction Aqueous phase Gasification reforming Bio-oils (tars, acids, (APR) chars, alcohols, Biological Catalytic conversion aldehydes, esters, Syngas conversion (Dehydration, ketones, aromatics) (CO + H2) (enzymatic rehydration, fermentation) hydrogenation, hydrolysis, aldol condensation, Aromatisation Fischer- hydrogenolysis, oxidation, Refining Tropsch Water-gas etc.) synthesis shift

Bio-alcohols Biofuels (fuel blends, alkanes, etc.) Biofuels Biofuels Bio-hydrogen Bio-chemicals (ethanol and Bio-chemicals/bio-materials (liquid (alkanes) (H2) (aromatics) butanol) (solvents, biodegradable polymers, etc.) fuels) Bio-BTXa

Fig. 4. Current strategies for production of biofuels, biochemicals and biomaterials from biomass derived carbohydrates. aBTX = benzene, toluene, xylenes (2, 14–16)

which are then converted to benzene, xylene and (HDO), hydrogenation and oxidation leading to toluene (bio-BTX) (21). Thermochemical processes value added products from cellulosic biomass. such as gasification, pyrolysis, torrefaction and The active catalysts’ performance and function, liquefaction require intense heating at elevated mechanistic pathways of reactions, opportunities temperatures, therefore raising energy efficiency for designing better catalytic systems, current concerns. In addition, the selectivity in bio- industrial processes and barriers to upscaling to oils produced from pyrolysis is extremely poor, demonstration or commercial scale processing are therefore inevitably requiring expensive additional also discussed. We write this review cognisant of upgrading and separation steps (22–23). This the fact that many reviews have been published on has led to a number of investigations into novel the conversion of cellulosic and cellulose derived pathways to reduce costs (24, 25). intermediates using solid and soluble mineral, organic and Lewis acids in aqueous, aqueous- 4. Recent Advances in Catalytic organic biphasic media and ionic liquids (IL) (26, 27). There has also been a large body of work Approaches for Cellulose Conversion published on noble metal catalysts or noble metal In this review, we discuss recent advances in co-catalyst systems promoting key biorefinery catalytic conversion of cellulosic biomass and reaction steps and processes, thus demonstrating cellulose derived intermediates to fuels, fuel that these metal catalysts are crucial to the additives and chemicals. We focus specifically on advancement of renewable liquid fuels, chemicals highlighting the roles and contributions of noble and materials. Yet few reports exist that critically metal catalyst systems in the key reaction steps: assess and summarise the contribution of noble hydrolysis, dehydration, hydrodeoxygenation metal catalytic systems. Noble metals included in

this review are: platinum, palladium, ruthenium, SO3H groups, sulfonated resins, heteropoly acids rhodium, iridium, osmium, silver and gold. We (HPAs) and zeolites have been investigated. The have also taken the latitude to include copper use of Lewis acids and acidic IL are also known and rhenium in a few examples. These metals are (41–45). While SO3H group bearing carbonaceous known to be resistant to oxidation and corrosion catalysts give good performance, leaching of and have long been utilised, in catalytic amounts, the SO3H groups from the support is a problem. in chemical transformations. Several metal chlorides (for example chromium(II)

chloride (CrCl2), copper(II) chloride (CuCl2), 5. Hydrolysis of Cellulose and dysprosium(III) chloride (DyCl3), ytterbium(III) chloride (YbCl ), indium(III) chloride (InCl ), Hemicellulose 3 3 gallium trichloride (GaCl3), lanthanum(III) chloride

Hydrolysis of cellulosic biomass (cellulose and/or (LaCl3), aluminum chloride (AlCl3), hafnium(IV) hemicellulose) involves cleavage of β-1,4‑glycosidic chloride (HfCl4), tin(IV) chloride (SnCl4), bonds of the biopolymer and is the first step in germanium(IV) chloride (GeCl4), chromium(III) unlocking the potential of cellulosic feedstock chloride (CrCl3), zirconium(IV) chloride (ZrCl4),

(Figure 4). As mentioned previously, hydrolysis titanium(IV) chloride (TiCl4) and niobium(V) by enzymes is established (28). Several kinds of chloride (NbCl5)) have been employed as soluble mineral acids, such as hydrochloric acid (HCl), Lewis acids for cellulosic biomass hydrolysis. In sulfuric acid (H2SO4), hydrofluoric acid (HF), most cases the Lewis acid catalyst plays a dual nitric acid (HNO3), phosphoric acid (H3PO4), boric role of hydrolysis followed by dehydration to acid (H3BO3) and organic acids (for example 5-hydroxymethylfurfural (HMF) (46). formic acid, oxalic acid, boronic acid, fumaric and The use of acidic IL is a unique strategy which acetic acid) have been employed in hydrolysis of can tackle both cellulose solubility issues and bring cellulose to sugars (4, 14, 29). The first report about efficient hydrolysis. In this catalyst system, on cellulose hydrolysis was with H2SO4 (0.5 wt%) the IL acts as the solvent and acid catalyst. In in 1923. This catalyst promoted the hydrolysis of the presence of minimal amounts of water to cellulose in wood waste and gave 50% yield of aid hydrolysis, several IL have been reported to sugars (glucose and oligosaccharides) at 170°C efficiently dissolve and depolymerise cellulose (47– in aqueous media (30–32). H2SO4 has been found 49). While reports on the hydrolysis of cellulose to show better catalytic activity than most mineral to glucose by Lewis acid catalysts abound in the acids due to its higher acidity (pKa<1) (33). The literature, reports on the use of noble metal catalyst insolubility of cellulose in aqueous media has been systems solely for hydrolysis are rare. a challenge and this has led to the exploration An example of a noble metal catalyst for the of alternative solvents such as biphasic and ILs hydrolysis of cellulose is supported Ru catalysts. (34–38), which turn out to dissolve cellulosic The effect of the support has been investigated biomass better and enhance the yields of sugars. by screening Ru supported on activated carbon,

The use of microwave irradiation has accelerated C60, mesoporous carbon and carbon black, with reaction rates and lowered reaction temperatures mesoporous carbon giving the best results. The (70–100°C) all the while also affording better yields Ru catalyst on mesoporous carbon support gave of sugars (39–41). However, problems associated a combined glucose and oligosaccharide yield with the use of mineral acids, which include reactor of 41%. Interestingly, the mesoporous carbon corrosion, separation of product-catalyst solution, alone catalysed the conversion of cellulose to recyclability and the use of large amounts of acid oligosaccharides, while the Ru on the mesoporous that result in waste effluent, have rendered mineral carbon converted oligosaccharides to glucose. The acid catalytic systems unappealing for industrial active species in the conversion of oligosaccharides scale reactions. to glucose was identified as 2RuO •H2O (49, 50). Various other acid catalysts have been developed This indicates the effect of the support on the for hydrolysis of cellulosic biomass (cellulose and hydrolysis of cellulose. hemicellulose). These include solid acid catalysts, Recently, methyltrioxorhenium (MTO) was which have advantages over homogenous acid used as a catalyst for cellulose hydrolysis, catalysts, such as ease of separation of the product- at 150°C under microwave irradiation in catalyst mixture, less waste and thus lower impact 1-allyl‑3‑methylimidazolium chloride, to give a on the environment. Solid acid catalysts, generally glucose yield of ≈25%. The authors proposed that in the form of metal oxides, carbon-supported the mechanism of the hydrolysis involves

β-1,4‑glycosidic bonds by the electron deficient the CuCl2/PdCl2 pair gave the best yield (73%) rhenium centre, eventually leading to cleavage of of monosaccharides, oligosaccharides and the β-1,4-glycosidic bond (Scheme I) (51). polysaccharides in a reaction where the yield of The reaction was carried out in the IL glucose was 42%. Other products from the reaction 1-allyl‑3-methylimidazolium chloride, [AMIM] were levulinic acid (LA), formic acid, sorbitol and

Cl (52). A tetramethylammonium perrhenate HMF. It must be noted that neither CuCl2 nor + – [(CH3)4N] ReO4 in [AMIM]Cl system has also PdCl2 on their own were active in hydrolysing been reported for cellulose hydrolysis. This cellulose, thus pointing to a vital synergistic effect catalyst system produced a mixture of glucose, between the CuCl2/PdCl2 system (53). In all the oligosaccharides and polysaccharides. Notably, the reports of noble metal-catalysed hydrolysis of alkyl chain length of the quaternary ammonium cellulose discussed, no isolation and subsequent perrhenate impacted negatively on the glucose characterisation of the products was reported. yield; thus as the chain length increased the Once sugars are formed they can be further glucose yield decreased. It was suggested that the transformed into other chemicals which are mechanism of this reaction involves the perrhenate feedstocks for fuels and chemicals. Scheme II – (ReO4 ) forming hydrogen bonding with the shows how sugars formed from hydrolysis of hydroxyl groups in cellulose; thus weakening the cellulose can be transformed to these products. β-1,4-glycosidic bonds, which in turn enables water molecules to access the β-1,4-glycosidic bond more 6. Conversion of Polysaccharides easily, leading to hydrolysis (52). and Monosaccharides to Su et al. have investigated CuCl coupled with 2 5-Hydroxymethylfurfural and either chromium(III) chloride (CrCl ), CrCl , 3 2 Furfural palladium(II) chloride (PdCl2) or iron(III) chloride (FeCl3) in the hydrolysis of cellulose. They found 6.1 Polysaccharides that paired metal chlorides were particularly and Monosaccharides to active and showed better reaction rates for 5-Hydroxymethylfurfural β-1,4-glycosidic bond cleavage in comparison to mineral acid catalysed hydrolysis. Paired catalytic The dehydration of hexoses affords HMF, some systems exhibited improved activity compared furfural, LA, formic acid and humins. On the other

Scheme I. Proposed mechanism of cellulose hydrolysis by MTO Hydrolysis (Reprinted from (51). Copyright (2016), with permission from Elsevier)

Cellulose

Glucose

HMF Fructose

Cellulosic biomass Hydrodeoxygenation Hydrogenation (HDO) Cellulose 2,5-Furandimethanol Dimethylfuran Dimethyltetrahydrofuran Hemicellulose (DHMF) (DMF) (DMTHF)

Brønsted acid, Hydrolysis Lewis acid or Lewis acid/noble Selective Direct hydrodeoxygenation hydrogenation (HDO) metal catalysts Hydrogenation Rehydration g-Valerolactone (GVL) Acid H+, H O Fructose dehydration 2 Levulinic acid (LA) Formic acid } 5-Hydroxymethylfurfural (HMF) D

Glucose Isomerisation

Acid Hydrodeoxygenation dehydration (HDO) Xylose Hydrodeoxygenation (HDO) Hydrogenation Methyltetrahydrofuran Methylfuran (MTHF) Furfural (MF)

Scheme II. Reaction pathways to chemicals starting with cellulosic biomass

hand, dehydration of pentoses produces furfural at C2 to form a carbenium cation, followed by (Scheme II) (2, 4). consecutive β-dehydrations in the ring to afford HMF and furfural are important platform chemicals HMF (61). that can be upgraded to valuable commodity The acyclic pathway (Scheme IV) proceeds chemicals, fuels and fuel additives. HMF is formed via formation of 1,2-enediol, which is the as an intermediate during dry heating and roasting rate determining step. This is followed by two of foods containing high levels of carbohydrates β-dehydrations and finally ring closure to yield (54). It has been found in foods and beverages such HMF (61). as dried fruits, baked foods, coffee, honey, citrus In both pathways, the selective conversion of juice, grape juice, prune juice, wine and brandy hexoses to HMF is influenced by the isomerisation amongst others. Daily HMF intake is estimated at of glucose to fructose. Glucose isomerisation 30 to 150 mg per person and is toxic to humans is catalysed by Lewis acids and Brønsted if ingested at high concentrations (55–57). HMF acids (63). Brønsted acids tend to degenerate can be accessed by three dehydration steps of monosaccharides to various byproducts (64–72) hexose sugars. The mechanistic pathway for acid and afford low (<10%) yields of fructose. The use catalysed dehydration of hexoses by expulsion of of organic bases such as piperazine, triethylamine, three water molecules is said to proceed via acyclic pyrrolidine, ethylenediamine, piperidine and intermediates or cyclic intermediates (58–62). The morpholine has been reported. Fructose yields of cyclic mechanism (Scheme III) begins from the up to 32% and 63% selectivity were achieved (66). cyclic ketofuranose which undergoes dehydration These are the first organic base catalysed glucose

Fructose HMF

Scheme III. Cyclic pathway for the dehydration of fructose to HMF (Reprinted from (61). Copyright (1990), with permission from Elsevier)

D-Glucose 1,2-Enediol D-Fructose

3,4-Deoxyglucosene 3-Deoxyglucose-2-ene 3-Deoxyglucosone

Scheme IV. Acyclic pathway for the dehydration of hexoses to HMF (Reprinted from (61). Copyright (1990), with permission from Elsevier)

HMF

to fructose isomerisation systems that gave yields to transform cellulose into HMF (in 59% yield) in a and selectivities for fructose that are comparable single step. This catalytic system achieved this by to widely used Lewis acids (68–72). prior hydrolysis of cellulose, followed by dehydration The conversion of hexoses and cellulosic biomass of glucose units into HMF. The authors suggest to HMF has been dominated by Lewis acid and that CrCl2 activates CuCl2 to hydrolyse cellulose mineral acid catalysts and has recently been into glucose. Separate experiments with a CuCl2 reviewed (70). Some noble metal-containing catalyst only and with a paired CuCl2/CrCl2 catalyst catalytic systems have demonstrated good activity (where the relative proportion of CuCl2 in the pair for this transformation (Table I) (71–79). was higher than CrCl2), show that the paired metal In 2007, Zhang and coworkers reported that chlorides yield more than three times the amount fructose dehydration proceeded in the presence of product (i.e. glucose from cellulose) than the of catalytic amounts of MCl2 and M′Cl3 (where CuCl2 catalyst on its own. On the other hand, the

M = Cr, Fe, Cu, Pd, Pt and M′ = Rh, Rh, Ru, Mo). CuCl2/CrCl2 pair with a relatively higher proportion

The catalytic systems were selective to HMF and of CrCl2 resulted in a high HMF yield, consistent negligible amounts (0.08%) of LA were produced. with CrCl2 being a highly selective catalyst for the

Notably, only CrCl2 gave high yields (70%) of HMF conversion of glucose to HMF. The catalytic system from glucose, whereas other Lewis acid catalysts could be reused up to three times with consistent afforded yields less than 10% (71). The role of Cr2+ is activity and selectivity (71). to convert the α-glucose to β-glucose and isomerise In another report a solid Ag-containing HPA the β-glucose to fructose, which improves the yield (Ag3PW12O40 HPA) was used as a solid catalyst to of HMF. A mixture of CrCl2 and CuCl2 in 1-ethyl- dehydrate fructose and glucose to HMF. HMF yields 3-methylimidazolium chloride ([EMIM]Cl) was used of 78% and 76% were obtained from fructose

Table I Selected Noble Metal Catalysed Conversion of Monosaccharide, Polysaccharide and Lignocellulosic Biomass to HMFa (71–77)

Catalyst Carbohydrate Temp., °C Time, h HMF Yield, % Ref. (catalyst loading)

Fructose MCl2 or M’Cl3 (6 mol%) 80 3 63–83 (71) M = Cr, Fe, Cu, Pd, Pt M′ = Rh, Ru, Mo

Fructose Ag3PW12O40 (3.3 wt%) 120 1 78 (73)

Glucose Ag3PW12O40 (3.3 wt%) 130 4 76 (73)

Cellulose CrCl2/CuCl2 (6 mol%) 120 8 59 (72)

Cellulose CuCl2 (20 mol%) 160 3.5 70 (75)

Cellulose CrCl2/RhCl3 (10 mol%) 120 2 60 (74)

b reed CrCl2/RhCl3 (10 mol%) 120 2 41 (74)

Fructose CuCl2 (18 mol%) 80 0.10 80 (76)

Fructose IrCl3 (7 mol%) 120 0.5 89 (77)

Fructose AuCl3.HCl (7 mol%) 120 3 44 (77) aThese yields are HPLC yields and were not isolated yields b26% furfural was also obtained and glucose respectively. The Ag3PW12O40 cluster Good catalytic performance has been recorded for exhibited higher catalytic activity and selectivity the dehydration of fructose to HMF with iridium(III) as compared to when Brønsted acids (HCl and chloride (IrCl3) and AuCl3•HCl catalysts in HCl and phosphotungstic acid (H3PW12O40)) and Lewis acid 1-butyl-3-methylimidazolium chloride ([BMIM Cl).

(silver nitrate (AgNO3)) were used separately in A HMF yield of 89% was obtained with IrCl3 catalyst glucose dehydration to HMF. Therefore, the higher in 30 min at 120°C while AuCl3•HCl gave 44% HMF catalytic activity of Ag3PW12O40 can be attributed yield under the same conditions (77). to the synergistic effect of Lewis acid and Brønsted acid sites in the Ag3PW12O40 cluster. Infrared 6.2 Polysaccharides and (IR) experiments revealed that the substrate Monosaccharides to Furfural accumulated on the Ag3PW12O40 catalysts and could be the reason for the high activity displayed by The synthesis of furfural has been achieved by this catalyst. Fourier transform (FT)-IR spectra of using Brønsted and Lewis acid catalysts (80–82).

Ag3PW12O40 and its adsorbed fructose and glucose In a report by Zhang et al. CrCl3, CrCl3/lithium confirmed the absorption of substrate on part of chloride (LiCl), FeCl3•6H2O, CuCl2•H2O, copper(I) the catalyst. Notably, the Ag3PW12O40 cluster as chloride (CuCl), LiCl and AlCl3 were screened for a catalyst could be recycled and reused up to six catalytic activity and selectivity in xylose and times (73). xylan conversion to furfural, under microwave

In a further report, CrCl2 and RuCl3 in a 4:1 irradiation. AlCl3 gave the best performance for molar ratio catalysed the conversion of cellulose the production of furfural from xylose (82.2%) and to HMF in [EMIM]Cl IL in a 60% yield. Gram xylan (84.8%) (80). scale production of HMF was achieved with this Furfuryl alcohol (FA) has also been obtained

CrCl2/RuCl3 system. Furthermore, lignocellulosic directly from xylose using a combination of Pt/SiO2 biomass from reed was also converted to HMF and ZrO2-SO4 catalysts. The SiO2 was reported (41% yield) and furfural (26%) using the same to promote xylose to xylulose isomerisation (with catalytic system and conditions (74). CuCl2 in xylose conversion of 65%), while the ZrO2-SO4 1-ethyl-3-methylimidazolium acetate ([EMIM [Ac]) catalysed the dehydration of xylulose to furfural. has been used to produce HMF in ≈70% Finally, furfural was selectively transformed to FA yield (75). by Pt/SiO2 (81).

6.3 5-Hydroxymethylfurfural and valuable chemicals and fuel additives (89–90). Furfural as Sources of Biofuels These can be accessed via specific reaction pathways including hydrogenation, rehydration, Sections 6.1 and 6.2 provide avenues to HMF and etherification, esterification, aldol condensation, furfural, versatile renewable platform chemicals hydrogenolysis and HDO (89–92). This review that can provide access to a variety of biofuels and highlights recent literature reports of dimethylfuran chemicals (Schemes V and VI). Since the 1920s, (DMF), 2-methylfuran (2-MF), 5-(ethoxymethyl) the Quaker Oats Process has been producing furfural (EMF), 2,5-dimethyl-tetrahydrofuran furfural and FA from agricultural residues (83). (DMTHF), 2-methyl tetrahydrofuran (MTHF), FA, AVA Biochem, Switzerland, produces 300 tonnes of cyclopentanone (CPO) and pentanediols produced 90% purity aqueous HMF and 20 tonnes of highly by noble metal containing catalytic systems pure crystalline HMF per annum, (84) and there are (88–124). examples of patented pilot scale processes that have produced HMF using organic catalysts, mineral acid 7.1 Polysaccharides catalysts and without a catalyst (85–88). Although and Monosaccharides to Lewis and Brønsted acid catalysts have dominated 2,5-Dimethylfuran, 2-Methylfuran, efforts to produce these platform chemicals, noble metal containing catalytic systems (including 2,5-Dimethyltetrahydrofuran, monometallic and bimetallic species) have shown 2-Methyltetrahydrofuran and remarkable efficacy in converting HMF and furfural 5-Ethoxymethylfurfural to the chemicals illustrated in Schemes V (89) and VI (90). In Section 7 we will discuss some of the In comparison to the leading biofuel, bioethanol, noble metal promoted catalytic transformations DMF has a higher energy density (30 MJ l–1), illustrated in Schemes V and VI. a higher octane number of 119 and a lower volatility (boiling point = 92–94°C) than bioethanol 7. Conversion of Furfural and (Table II). Furthermore, it is immiscible with water therefore, in this regard, is similar to gasoline. 5-Hydroxymethylfurfural to Value These properties make DMF a better biomass Added Chemicals and Fuels derived liquid fuel. Policy limitations and/or cost- As shown in Schemes V and VI, HMF and furfural effective production of DMF at an industrial scale are precursors for the synthesis of a variety of are likely reasons why DMF has not yet been taken

GVL FDCA Formic acid

DFF LA

HMF EL DMTHF DMF Key HMF: 5-Hydroxymethylfurfural EL: Ethyl levulinate EMF: 5-Ethoxymethylfurfural EMF LA: Levulinic acid DHMF C9–C15 alkanes GVL: g-Valerolactone DMF: 2,5-Dimethylfuran DMTHF: 2,5-Dimethyltetrahydrofuran DHMF: 2,5-Dihydroxymethylfuran DHMTHF DHMTHF: 2,5-Dihydroxymethyltetra- hydrofuran FDCA: 2,5-Furandicarboxylic acid DFF: 2,5-Diformylfuran

Scheme V. Various derivatives of 5-HMF (Reprinted with permission from (89). Copyright (2014) American Chemical Society)

CPO MTHF THF

MF Furan

THFA FA Furanone GBL Furfural

1,5-PDO

Key LA GVL: g-Valerolactone 1,4-PDO: 1,4-Pentanediol GVL 1,2-PDO: 1,2-Pentanediol 1,2-PDO 1,5-PDO: 1,5-Pentanediol FA: Furfuryl alcohol LA: Levulinic acid THFA: 2-Hydroxymethyltetra- hydrofuran 1,4-PDO MF: 2-Methylfuran MTHF: 2-Methyltetrahydrofuran CPO: Cyclopentanone THF: Tetrahydrofuran GBL: g-Butyrolactone

Scheme VI. Various derivatives of furfural (Reprinted (adapted) with permission from (90). Copyright (2016) American Chemical Society)

Table II Comparison of Dimethylfuran and 2-Methylfuran to Ethanol and Gasoline (92)a

Property DMF MF Ethanol Gasoline

Molecular formula C6H8O C5H6O C2H6O C6–C9 Molecular weight, g mol–1 96.13 82.04 46.07 100–0105

Boiling point, °C 92–94 63–66 78 95–99

Energy density, MJ l–1 30 28.5 21 31

Water solubility immiscible immiscible miscible immiscible

Research octane number (RON) 119 103 110 97

Density @ 20°C, kg m–3 890 913 791 745

Flash point, °C –1 –11 14 0.71–0.77 aAdapted with permission from (92). (Copyright (2015) American Chemical Society) up as a fuel additive of choice, given its many to promote the conversion of HMF to DMF, where technical advantages over bioethanol. DMF can be the HMF was initially prepared by mineral acid obtained from the HDO of HMF in the presence of a catalysed dehydration of corn stover (93). catalyst. Metal catalysts have been widely used to An even better catalyst system was reported by promote this reaction (88). For example, Dumesic Wang et al. with a selective bimetallic catalyst et al. used a CuRu/C catalyst system that produced (PtCo nanoparticles supported on hollow carbon 71% DMF in 10 h when 1-butanol was used as the nanospheres, PtCo@HCS) for the conversion o solvent of the reaction at 220°C and 6.8 bar H2 of HMF to DMF at 180 C with a 98% yield in (91). This catalyst system could also be utilised 1-butanol. The PtCo@HCS could be recycled and

reused up to three times, although by the third Similarly, in the presence of a RhCl3/HX catalytic cycle, a drop in DMF yield (from 98 to 72%) system (X = I and Cl), a mixture of furan ring occurred (94). The role of the catalyst support in saturated and ring-opening products were also this reaction appears to be critical. For example, obtained from xylose, glucose, fructose, sucrose, when Pt nanoparticles supported on activated inulin, cellulose and corn stover. The reaction carbon (Pt/AC) and graphitic carbon (Pt/GC) were is conducted in aqueous media and the main employed in this reaction, only 9% and 56% DMF products obtained were DMTHF (up to 85% from yields respectively were achieved. However, PtCo fructose and 80% from cellulose) and MTHF (up nanoparticles on activated carbon (PtCo/AC) and to 80% from xylose and 56% from corn stover) graphitic carbon (PtCo/GC) gave conversions of (99, 100). These are gas chromatography (GC) 100% HMF and 98% DMF, effectively proving that yields and have not been isolated. DMHTHF has the two metals work cooperatively to transform been hydrogenated to 1,6-hexandiol, which was HMF into DMF. Selective HDO of HMF over converted into caprolactone and caprolactam (101).

Ru/Co3O4 and PdAu/C has also been reported. Generally, molecular hydrogen has been the These reactions were conducted in tetrahydrofuran source of hydrogen in HDO reactions; but the

(THF) using H2 at 130°C for 24 h and at 60°C use of hydrogen transfer reagents is safer than for 6 h respectively; giving excellent DMF yields using molecular hydrogen in HDO reactions. Here, of 100% and 93.4% respectively (95, 96). It is formic acid as a hydrogen source in the conversion interesting to note that Ru and Pd alone without of HMF, fructose and lignocellulose (with Ru/C either Co or Au are less effective in catalysing HMF catalyst) to chemicals provide useful examples to DMF. X-Ray photoelectron spectroscopy (XPS) of non-molecular hydrogen in HDO reactions. For spectra of PdAu/C catalyst reveal that Pd atoms in example, when HMF was used as a substrate, 37% the PdAu/C may have electronic poor states (after DMF, 43% 5-formyloxymethyl furfural (FMF) and charge transfer from Pd to Au atoms occurs) which 3% LA were obtained. Other examples include strongly contributed to the enhancement of HDO the initial direct conversion of lignocellulose and activity thus improving DMF yield. carbohydrate substrates in a Brønsted acid IL to

Notably, Ru/C and Pd/C with and without ZnCl2 catalyse the conversion of these substrates to HMF, in converting HMF to DMF were reported by Zu and then subsequent use of formic acid as hydrogen et al. (96); Ru/C/ZnCl2 produced 41% DMF and source and Ru/C as catalyst to convert HMF to FMF, 52% 2,5-dihydroxymethyl furan (DHMF) and Ru/C which was further converted to DMF. Interestingly, alone gave a minimal amount of DMF from HMF. the feedstock in this reaction appears to be crucial The higher activity displayed by the bimetallic in determining the yield of DMF. The yields of DMF catalyst (Ru/C/ZnCl2) as compared to Ru/C from cellulose is 16% whilst the yield is 10% from suggests enhanced HDO in the presence of Zn2+, sugars compared to when pure HMF was used provided by the synergy between the two metals. as the substrate (37%) (102). Isopropyl alcohol

It also explains why Pd/C/ZnCl2) gave better can also be used as a hydrogen source in place yield than Ru/C/ZnCl2) in this study (97, 98). In of formic acid in the Ru/C catalysed conversion the presence of a Brønsted acid Amberlyst®-15, of HMF to DMF. The yield of DMF obtained was instead of ZnCl2, a mixture of products was 81% and the reaction was said to proceed by the obtained from HMF, namely DMTHF (13%), DMF formation of 2,5-bis(hydroxymethyl)furfural as an (6%), tetrahydrofurfuryl alcohol (THFA) (3%), intermediate (103). 2,5-bis(hydroxymethyl)tetrahydrofuran (DHM MF has a high density (913 kg cm–3), low flash THF) (23%), 2-methyltetrahydrofuran-5-aldehyde point (–11°C) and low solubility in water, unlike (4.5%) and methyltetrahydrofuryl alcohol (6%) ethanol. The energy density of MF is higher than and 2-hexanone (1.6%) (Schemes V and VI). This that of ethanol and similar to DMF (Table II) (91). suggests that the acidity of ZnCl2 is not the only MF is a water soluble liquid, it is flammable and has reason for the enhanced activity of the catalyst, a chocolate odour. It is also used as feedstock, as a resulting in high DMF yield. Since the authors solvent and has been widely used in pesticides and detect leached zinc in the solution, it is likely that the perfume industry (104, 105). MF production the role of Zn2+ which leaches into solution during proceeds by dehydration of xylose followed by HDO the reaction is as described by Abu-Omar and co- of furfural. The best catalyst for this reaction is a workers, where Zn2+ ions bind to the substrate and polymer supported Pd(II) complex which exhibited activates its cleavage upon encountering Ru–H or 100% yield conversion of furfural into MF with H2 Pd–H sites on the catalyst surface (98). pressure <1 bar at 180°C (106).

EMF is another compound that can be accessed at 230°C has been reported by Sitthisa et al. to by etherification of HMF (Scheme V). It is a produce FA in a yield of 65% (109). Similarly, potential biofuel and has an energy density of furfural transformation to FA has been performed 30.3 MJ l–1, which is higher than that of ethanol in the vapour phase using silica supported Pt and comparable to gasoline. It has, thus, been nanoparticles. With small Pt nanoparticles on SiO2, proposed as an additive and/or substitute for decarboxylation of furfural to furan was observed diesel as a fuel (89–91). Lin and coworkers have while larger sized Pt nanoparticles produced furan + used AgxH3‑xPW catalysts, prepared through a H and FA (Scheme V) (110). exchange for Ag+, for esterification of HMF to EMF. HDO of furfural can also produce 1,2-pentanediol

In using this series of catalysts, AgH2PW displayed (1,2-PDO), 1,4-pentanediol (1,4-PDO) and higher catalytic performance than Ag2HPW and 1,5-pentanediol (1,5-PDO) which can be used as

Ag3PW in the etherification of HMF; producing a monomers for the synthesis of polyesters (Scheme 69.5% yield of EMF when fructose was the starting VI) (111). In an investigation of the effect of material. The catalyst could be recycled and adding noble metals (including Ru, Rh, Pt, Pd) to showed no marked decrease in activity (107). Ir-ReOx/SiO2 in the one-pot conversion of furfural

In addition to EMF, 2,5-bisalkoxymethylfurans to 1,5-PDO, Pd 0.66 wt%-Ir-ReOx/SiO2 was found have also been identified as potential renewable to be superior. The Pd 0.66 wt%-Ir-ReOx/SiO2 liquid fuels. Bell and coworkers have reported gave 71.4% 1,5-PDO while 28.6% of the products a one-pot process involving the sequential consisted of 1,4-PDO, 1,2-PDO, 1-pentanol, dehydration and reductive etherification of fructose 2-pentanol, THFA and 2-MTHF (112). to 2,5-bisalkoxymethylfurans. The dehydration Catalyst characterisation revealed that and etherification of fructose was performed using Pd–Ir–ReOx/SiO2 catalyst consisted of Pd metal ® Amberlyst -15 and Pt/Sn/alumina at 110°C to form particles with ReOx species (Pd–ReOx) and Ir metal

5-(alkoxymethyl)furfurals and 2-(2,2-alkoxyethyl)- particles with ReOx species (Ir–ReOx) (Figure 5).

5-(alkoxymethyl)furans. At 60°C and under The Pd–ReOx species catalyse hydrogenation of

H2 pressure, 5-bisalkoxymethylfurans and furfural into THFA and the Ir–ReOx species catalyse

2,5-bisalkoxymethyl furans could still be obtained in the HDO of THFA to 1,5-PDO. The ReOx species good yields (108). modified Pd sites (i.e. decreased Pd metal average particle size) and increased the dispersion of Pd 7.2 Polysaccharides and metal. This effect of ReOx is said to be linked to the high performance of Pd–Ir–ReO /SiO in the Monosaccharides to Furfuryl Alcohol x 2 hydrogenation of furfural into THFA. Supported Ir and Pentanediols was found to be in the metallic state and the Re

FA is prepared industrially by the catalytic species form low valent oxide clusters (ReOx) that hydrogenation of furfural using Ni and Cu/CrO partially cover the Ir metal surface, therefore the catalysts. If the catalyst used is not selective, Ir–ReOx species can catalyse the hydrogenolysis of hydrogenation of furfural yields not only the THFA to 1,5-PDO (112). desired products but also a variety of byproducts More recently, Huber et al. reported a new including furan, THF, THFA and ring-opening process for the production of 1,5-PDO (84% yield) products, such as pentanol and pentanediols from furfural. This route to 1,5-PDO proceeds via

(Scheme VI) (109–114). Pd/SiO2 as catalyst for hydrogenation → dehydration/hydration → ring- the hydrogenation of furfural using 1 bar of H2 opening tautomerisation and → hydrogenation

O OH Fig. 5. Model structure of O O Pd–Ir–ReO /SiO and the HO OH x 2 role of each species in H2 – + the conversion of furfural H H H H to 1,5-PDO (Reproduced from (112) with Ir Pd ReOx permission of The Royal ReOx Society of Chemistry) SiO2

reactions, using Ni, γ-Al2O3 and Ru catalysts carried out in ethanol, then esterification of LA with respectively. While this process has more reaction ethanol leads to ethyl levulinate (EL) (Scheme V). steps than conventional hydrogenation of furfural EL is an acceptable diluent for biodiesel fuels with to THFA followed by HDO of THFA to 1,5-PDO, high saturated fatty acid content (127). techno-economic assessments demonstrate that GFBiochemicals’ Caserta plant in Milan, Italy, the process is economically viable (113). currently produces LA and its coproduct FA from Hronec and co-workers described a novel, non-edible biomass feedstock (wood, grass, highly selective transformation of furfural to CPO wheat straw and cellulose) via an acid catalysed (Scheme V). Selective rearrangement of furfural process (ATLAS TechnologyTM). This company also to CPO by 5%Pt/C is achieved in water, at reaction has plans to expand its production portfolio to temperatures of 140°C to 190°C and hydrogen LA derivatives, such as LA-esters and LA-ketals, pressures of 30 to 80 bar. Notably, when 5% Ru/C GVL, MTHF and methylbutanediol (MeBDO) (128). and 5% Pd/C were used under the same conditions, Hydrogenation of LA in the presence of a metal a mixture of typical furfural hydrogenation products catalyst with an external H2 source or formic acid (FA, THFA, MF and MTHF) were obtained (114). as the source of hydrogen gives GVL. GVL is a

Homogeneous catalysts such as [Pd(OAc)2]/ promising bio-derived molecule and it is attractive DtBPF (DtBPF = 1,1′-bis(di-tertbutylphosphino) for its physical and chemical properties. It has low ferrocene), [Rh(CO)2(acac)]/H2WO4, {[Cp*Ru toxicity, an acceptable and definitive smell which + – (CO)2]2(μ-H)} (OTf )/HOTf (Cp* = C5Me5 and makes detection of leaks and spills easy, it is safe – OTf = CF3SO3 ) and cis-[Ru(6,6′-Cl2bpy)2(OH2)2] to store and move globally in large quantities, has

(OTf)2 (6,6′-Cl2bpy = 6,6′-dichloro-2,2′-bipyridine) high flash (96°C) and boiling (207°C) points as catalyse HDO of C–O bonds (115–118). For well as a low melting point (–31°C). It is also used + – example, {[Cp*Ru(CO)2]2(μ-H)} (OTf ) is an as an additive in the food industry and as a green efficient catalyst for the dehydration of 1,2-PDO solvent (129–133). Bond et al. have proposed a followed by hydrogenation to form n-propanol pathway to renewable butene, using GVL, which under acidic conditions. This catalyst produced 54% can be oligomerised to long chain alkenes for n-propanol after 30 h at 92% 1,2-PDO conversion liquid fuels and commodity chemicals production. (119, 120). These homogeneously catalysed Using formic acid as the source of hydrogen in the reactions are typically carried out at temperatures hydrogenation of LA is more economical because ranging from 80°C to 120°C, meaning the active dehydration of hexoses produces formic acid in species is likely a molecular catalyst or a ‘cocktail’ equimolar amounts to LA. In the presence of a of catalysts, comprising molecular catalyst and catalyst, formic acid decomposes to carbon dioxide nanoparticles (121). and H2 and hence can be used as an internal source Direct conversion of furfural to γ-valerolactone of hydrogen (Scheme VII) (131). (GVL) by Brønsted and Lewis acid zeolite catalysts Quaker Oats, USA, produces GVL on a commercial has been reported. Up to 80% GVL yield was scale from LA using chromium(III) oxide (Cr2O3) achieved through Meerwein-Ponndorf-Verley (MPV) and copper(II) oxide (CuO) at 200°C (134). Metal reduction of furfural to FA, followed by hydrolysis catalysts, such as Pd, Ir, Ni, Cu, Pt, Re, Rh, Au of FA to LA and then selective MPV hydrogenation and Ru have been used in the presence of H2 to of LA to GVL (122, 123). produce GVL from LA (135–138). Manzer studied this reaction using Ir, Rh, Ru, Pd, Rh, Pt and Re 7.3 Polysaccharides and supported on carbon, and found that 5% Ir, Pd and Rh were active in the conversion of LA to Monosaccharides to Levulinic Acid, GVL but their selectivity to GVL was rather low. Ni Ethyl Levulinate and γ-Valerolactone had the lowest activity and selectivity under the Acid catalysed dehydration of hexoses leads same conditions and Ru had the best activity and to the formation of HMF, which can undergo selectivity (Figure 6) (135). rehydration in acidic medium to give LA and formic While heterogeneous catalysts have dominated LA acid (Scheme V). LA is a particularly attractive to GVL reactions, (135–141) homogenous catalysts platform molecule, because it can be converted into have also been studied. Particularly, water soluble valuable chemicals and advanced biofuels (124). homogeneous catalysts have been attractive because For example, valeric biofuels which are compatible GVL does not form an azeotropic mixture with water, with current transportation fuels are accessible therefore the catalyst can be recovered by distillation from LA (125, 126). If dehydration of hexoses is and reused (129–131, 142). Hydrogenation of LA

Scheme VII. Production of GVL using formic acid as a source of hydrogen; the use of H2 external molecular Catalyst hydrogen is also known (Reprinted in part HMF LA Formic acid GVL with permission from (131), Copyright (2014) American Chemical or external Society) molecular H2

Ir trihydride catalysts bearing a PNP pincer

LA conversion,% ligand showed extremely high activity in LA to GVL conversion giving up to 98% GVL yield. 100 GVL selectivity, % Recovery of the catalyst was demonstrated. This 80 was done by removal of the product by vacuum and the remaining catalyst was redissolved in LA 60 and exhibited similar activity to before distillation 40 (149). Conversion of LA to GVL was studied 20 using p-cymene Ru(II) N-heterocyclic carbene (NHC) complexes as (pre)catalyst in water. The 0 Ir Rh Pd Ru Pt Re Ni complex (Table III) was reduced in situ to form highly catalytically active Ru nanoparticles which Fig. 6. Manzer’s results from catalytic deactivated in subsequent catalytic runs due to hydrogenation of LA at 150°C, 2 h reaction time, aggregation (150). 800 psi H2 pressure, 5% metal loading on carbon (Reprinted from (135). Copyright (2004), with Horváth and coworkers showed that LA and a permission from Elsevier) small excess of formic acid can be converted to GVL in the presence of Shvo’s catalysts. At 100°C, the catalyst rearranged to form a Ru hydrido to produce GVL has been examined in aqueous catalytically active species (Table III) and yields solution using various water soluble phosphine higher than 99% after 8 h were recorded. The ligands in combination with the metal precursors formation of 1,4-PDO and MTHF, the typical side

Ru(acac)3 and RuCl3•xH2O (Table III) (143–147). products of the reduction of GVL with molecular GVL was obtained in 97% yield within 5 h, using hydrogen, was not observed (131). a Ru(acac)3/TPPTS (TPPTS = p(C6H4-m-SO3Na)3) Solvent free conversion of LA to GVL has been catalytic system (144, 147). achieved with pyrazolyl-phosphite and pyrazolyl- Formic acid was used as a hydrogen phosphinite Ru(II) complexes as (pre)catalysts, 6 source in [(η ‑C6Me6)Ru(bpy)(H2O)]SO4 catalysed using both molecular hydrogen and formic acid transformation of LA, in water, to give in GVL as hydrogen sources. With H2 (15 bar) 100% GVL and 1,4-PDO. In this same study, [Ru(acac)3]/ yield obtained at 110°C, while with formic acid

PBu3/NH4PF6 afforded 100% GVL from LA (142). at 100°C, 100% GVL was obtained. In order to Mika et al. demonstrated that LA could be understand whether formic acid acts as a hydrogen reduced effectively to GVL in the presence of transfer reagent or a source of H2 through prior an in situ catalyst generated from Ru(acac)3 decomposition, in situ NMR studies, where the and different sulfonated phosphines with the reaction was monitored in a J Young nuclear general formula RnP(C6H4-m-SO3Na)3–n (n = 1, 2; magnetic resonance (NMR) tube containing the R = Me, nPr, iPr, nBu, Cp) without the need for a base. (pre)catalyst, KOH and formic acid at 120°C, n The BuP(C6H4-m-SO3Na)2 showed the highest were carried out. Within 30 min of the reaction, activity at 99% GVL. The catalyst was successfully hydrogen gas was observed by proton (1H)- recycled for six consecutive runs without loss of NMR spectroscopy with a H2 signal at 4.56 ppm. activity (146). The active species was found to be a cocktail of

Table III Selected Homogeneous Noble Metal Homogenous Catalysts That Have Been Used in the Hydrogenation of LA to GVL (131, 142–151) Catalyst T, °C Reductant Time, GVL, TOF, h–1, Ref. –1 –1 h % molGVLmol Ruh a [Ru(acac)3]/PBu3/NH4PF6 160 H2 100 bar 18 100 (143, 144) a [Ru(acac)3]/P(n-Oct)3 160 H2 100 bar 18 99 (143) c [Cp*Ir(4,4-di-OH-bpy)(H2O)] 120 H2 10 bar 1 98 12,200 (142) SO 4 120 H2 10 bar 4 87 4326 (148) 6 b [η -(C6Me6)Ru(bpy)(H2O)]SO4 70 HCO2Na 18 25 (143) d [Ru(acac)3]/TPPTS 140 H2 70 bar 12 95 (146)

H2 50 bar 5 97 (147) e [Ru(acac)3]/R2P(C6H4-m- 140 H2 100 bar 1.8 92 6370 (TON) (145) SO3Na)2

RuCl3.xH2O/PPh3 200 HCO2H/ 6 93 (147) 200 pyridine 6 95 (147) HCO2H/NEt3 t f P Bu2 100 H2 50 bar 24 98 71,000 (TON) (149) H N Ir H H t P Bu2

130 H2 12 bar 2 99 493 (150)

Br N Ru Br N

g [Ru(acac)3]/Triphos 160 H2 100 bar 18 5 (144)

Triphos = PPh 2 PPh PPh2 2 Ph 100 8 99.9 177 (131) Ar OH Ar Ph OC Ru OC H Ar = p-MeOPh

h + BArF– 110 H2 20 bar 12 100 202 (151) Cl Me Ru

N PPh2 N O Me

i 120 HCO2H/ 16 100 200 (151) KOH Ru Me Cl Cl N P OEt N O OEt Me aNo base added b1,4-PDO and MTHF were also produced c 5 Cp* = η -C5Me5 d TPPTS = p(C6H4-m-SO3Na)3 eR = Me, iPr, cyclopentyl, nBu, nPr fPotassium hydroxide (KOH) 1.2 equiv. g95% ,1,4-PDO was produced hKOH 0.05 equiv. iKOH 1 equiv.

19 © 2018 Johnson Matthey https://doi.org/10.1595/205651317X696261 Johnson Matthey Technol. Rev., 2018, 62, (1) nanoparticles and molecular catalysts as evidenced and cellobiose to sorbitol. Sorbitol yield was 91.5% by mercury poisoning tests. The catalyst could be from cellobiose and 58.9% from cellulose (156). recycled and reused up to three times (151). Ru and Os complexes have been used, as homogeneous catalysts, in conversion of fructose 8. Conversion of Polysaccharides, (to glycerol, propylene glycol (PG), ethylene glycol (EG) and sugar alcohols) (157, 158), mannose Oligosaccharides and and inulin (to sugar alcohols) (159) and glucose Monosaccharides to Sugar Alcohols (to sorbitol) (160). Recently, Wang and coworkers and Hydrocarbons reported the first example of a highly selective

8.1 Polysaccharides and homogenous Ru(II)/H2SO4 catalytic system which Monosaccharides to Sugar Alcohols promoted the conversion of cellobiose and cellulose and Diols (at 100°C and 50 bar H2 over 16 h) to sorbitol, 1,4-sorbitan, glucose and a negligible amount Hydrogenation of cellulose, oligosaccharides, of HMF (0.1%). Starting from cellobiose, 91.4% disaccharides and monosaccharides results in sorbitol, 3.1% 1,4-sorbitan and 5.3% glucose the formation of sugar alcohols or polyols such as were afforded at 100% conversion, whilst from xylitol, sorbitol and mannitol which are important cellulose 21.9% sorbitol, 7.6% 1,4-sorbitan and chemicals widely used as monomers in the plastics 8.4% glucose were produced at 40% conversion. industry. Sugar alcohols also have applications in Sorbitol can be further converted to sorbitan and the food industry in the substitution of sugars isosorbide; thus suggesting that the sorbitan in such as glucose, sucrose and fructose. Xylitol is the above study may have been produced from most frequently used in place of sugar, as it has further reaction of the sorbitol. The authors similar sweetness and lower energy content than further proposed a mechanism for the reaction sucrose. From sugar alcohols it is also possible (Scheme VIII) which involved in situ generation to produce renewable hydrogen and hydrocarbons of the catalytically active Rh–H species 1 which which will be discussed in Section 8.2 of this inserted into the C=O bond of glucose (to form 2) review (152–161). of the initially hydrolysed cellulose. This is then The catalytic conversion of cellulose to sugar followed by a metal-ligand assisted proton transfer alcohols is a two-step process, which includes to the glucose O1 and C1 3 coupled with sorbitol hydrolysis of cellulose to monosaccharides, followed expulsion to regenerate the (pre)catalyst (161). by the hydrogenation of the sugars to sugar EG, 1,2-PG, 1,3-PG and glycerol are the simplest alcohols. Sels and coworkers used water soluble sugar alcohols and are used as emulsifiers, HPAs and a Ru/C catalyst to transform cellulose dehumidifying agents, anti-freeze agents, to sugar alcohols. At 190°C and (95 bar H2), the lubricants, solvents, polymer monomers and as conversion of cellulose was 100% with 85% yield pharmaceutical intermediates. Diols are currently (152). Palkovits et al. have also demonstrated that produced from fossil derived feedstock (petroleum) the transformation of cellulose to sugar alcohols by hydration of propylene and ethylene oxide was possible using a combination of HPAs and (162). In 2008, Chen and coworkers reported Ru/C. Sugar alcohols were obtained in a yield of the direct conversion of cellulose to EG in a yield 81% (153). Liu et al. have also demonstrated of 61% using a nickel-tungsten carbide catalyst cellulose to sugar alcohols conversion with Ru/C (163). Since then, studies aimed at understanding at 245°C and 60 bar H2. They obtained 40% the reaction mechanism of cellulose to EG and sugar alcohols of which 30% was sorbitol and 1,2-PG have been conducted (164–167). The 10% mannitol (154). It is striking to see that in major mechanistic pathway involves hydrolysis the presence of HPAs, which are acidic, catalytic of cellulose (to glucose and oligosaccharides) → conversion of cellulose to sugar alcohols on Ru/C retro-aldol condensation (of glucose to form glycol proceeds to give higher yields at comparably lower aldehyde and erythrose) → hydrogenation of glycol temperatures than when no HPAs are added. aldehyde to EG. If glucose isomerises to fructose, Hydrolytic hydrogenation of cellulose was shown then 1,2-PG is formed (Scheme IX) (165, 166). by Fukuoka et al. The reaction gave 58% sorbitol Catalysts such as Ru/AC/H2WO4 have been yield at 190°C in water over a Ru/AC catalyst employed in cellulose conversion to EG (168). Liu (155). Tsubaki et al. have presented a catalytic and coworkers reported efficient conversion of system that consists of Pt/RGO (RGO = reduced cellulose into EG, 1,2-PG and sorbitol, as dominant graphene oxide) for the hydrogenation of cellulose products on Ru/C in the presence of tungsten

Scheme VIII. Proposed mechanism for the Cellulose catalytic hydrogenation Sorbitol of cellulose with Ru(II) molecular catalyst (Reprinted with (Pre)Catalyst permission from (161). Copyright (2016) American Chemical Society)

Glucose

trioxide (WO3). It was found that WO3 crystallites 1,2-PG, EG and 2,3-BD are obtained, in which the catalysed hydrolysis of cellulose as well as selective yield of 2,3-BD is 5% (167). C–C bond cleavage, while Ru/C was responsible for hydrogenation (169). 8.2 Renewable Hydrogen and EG and 1,2-PG production from sugar alcohols has Hydrocarbons from Polysaccharides, also been reported. The most studied are sorbitol Disaccharides and Monosaccharides and xylitol while Ni- and Ru-based catalysts are most used. For example, Ru nanofibres, Ru/C, Cellulose, oligosaccharide, disaccharide or

Ni-Re and Ni/Al2O3 in the presence of bases such monosaccharide and sugar alcohols have been as calcium hydroxide (Ca(OH)2), calcium oxide transformed to hydrogen and hydrocarbons (CaO), KOH and sodium methoxide (NaOMe) (alkanes). This work was pioneered by Dumesic catalyse sugar alcohols to EG and 1,2-PG (26, 174–177). Aqueous phase dehydration/ (170–173). hydrogenation (APD/H) produces alkanes and The Changchun Dacheng Group, China, has aqueous phase reforming (APR) leads to hydrogen developed a process that produces 10,000 tonnes and gaseous alkanes (primarily methane) (26, 174– of 2,3-butandiol (2,3-BD) annually. This process 177). Cortright and coworkers have demonstrated integrates bio and catalytic conversion by firstly that hydrogen can be produced from sugars and enzymatically (by glucoamylase) hydrolysing sugar alcohols in an aqueous phase reforming starch to glucose. Then a catalyst is employed in process using a Pt-based catalyst. Glucose was conversion of the glucose to sorbitol, which is then converted to hydrogen and gaseous alkanes, with catalytically cracked into a mixture of C2 to C4 diols a hydrogen yield of 50% (174). Later, they used and polyols. After purification steps, 97% purity Pt/SiO2–Al2O3, Pd/SiO2–Al2O3 and Pt/Al2O3 to

Cellulose

Hydrogenation

Erythrose Erythritol

Hydrolysis RAC

Fructose Retro-aldol condensation (RAC) } Hydrogenation D Glucose Glycoaldehyde Ethylene glycol (EG)

Oligo- Isomerisation saccharides

Xylose RAC

Isomerisation Hydrogenation Hydrogenation Dihydroacetone Acetol 1,2-Propylene glycol (1,2-PG) Hydrogenation

Dehydration Isomerisation Hydrogenation Glyceraldehyde Hydration Lactic acid

Hydrogenation Dehydrogenation Retro-Claisen reaction

Glycerol Formic acid Glycolaldehyde

Scheme IX. Reaction pathways involved in cellulose transformation to EG and 1,2-PG (Reprinted (adapted) with permission from (165, 166). Copyright (2016, 1995) American Chemical Society)

convert sorbitol to hexane. Sorbitol is transformed (pre)catalysts. Hydrogen was produced using ppm on these bifunctional catalysts by a pathway that amounts of (pre)catalyst at a turnover frequency –1 involves the formation of hydrogen and CO2 on (TOF) of up to 10,269 h (from cellobiose in 1 h) the appropriate metal catalyst (Pt or Pd) and the and a turnover number (TON) of up to 6125 (from dehydration of sorbitol on a solid acid catalyst (SiO2 cellulose in 1 h) (176). or Al2O3). This is followed by hydrogenation of the Dumesic proposed that C7 to C15 alkanes dehydrated intermediates (for example isosorbide (which have a molecular weight appropriate for or enolic species) by hydrogen produced in the transportation fuel components) can be produced water gas shift reaction on Pt or Pd, which leads from carbohydrates by acid catalysed dehydration, to the overall conversion of sorbitol to alkanes plus followed by aldol condensation (over solid base

CO2 and water (175). catalysts) to form large organic compounds. Beller has recently described a procedure These molecules may be converted into alkanes for hydrogen generation from biomass (such by HDO over bifunctional catalysts (Mg-Al oxide and as glucose, fructose, cellobiose, cellulose and Pd/Al2O3) (177). The total HDO of cellulosic biomass ligncellulose) using Ir and Ru pincer-complexes as including biomass derived substrates such as

22 © 2018 Johnson Matthey https://doi.org/10.1595/205651317X696261 Johnson Matthey Technol. Rev., 2018, 62, (1) glucose, xylose, sugar alcohols, LA, HMF and temperatures (185–187). This topic has been furfural to produce C5 to C15 alkanes has recently reviewed (188). been reviewed (178). Another approach reported by Jin et al. is the conversion of glucose or cellulose to HMF and lactic 9. Conversion of Cellulose, acid, then oxidation of the lactic acid and HMF to give acetic acid (189). Glycolic acid production Disaccharides and Monosaccharides from cellulose has been less studied, nonetheless to Organic acids Keggin-type Mo polyoxometalates (POMs) have 9.1 Polysaccharides, Disaccharides been successfully employed in the conversion of and Monosaccharides to Levulinic cellulose to glycolic acid (190). Acid, Formic Acid, Acetic Acid and Apart from being a coproduct of HMF hydration, Glycolic Acid formic acid can be produced from cellulose via oxidation. Formic acid is a starting material for the Cellulose and glucose derived carbohydrates can production of formate esters, which can be utilised be transformed into a range of organic acids in the production of a large variety of useful organic (Scheme X) (89–91, 179). derivatives such as aldehydes, ketones, carboxylic The formation of LA from cellulose in the presence acids and amides. It has been used in the rubber, of mineral acids or Lewis acids has been extensively agricultural and pharmaceutical industries. It has studied. This multistep process involves hydrolysis applications as a mordant, auxiliary agent in the of cellulose to glucose ↔ fructose → dehydration dyeing industry, a disinfectant and its formate salt of fructose to HMF → and rehydration of HMF to is a useful de-icing agent (191). Notably, formic LA and formic acid (Section 7.3) (180–182). Acetic acid has energy content at least five times higher acid can be obtained by hydrothermal oxidation of than that of commercially available lithium ion glucose or cellulose in the presence of a base, such batteries and therefore represents a convenient as NaOH and Ca(OH)2 (183, 184). hydrogen carrier in fuel cells making it a highly Efficient, VO2+, Pb2+ and Ni2+ catalysed conversion exploitable chemical on the hydrogen energy of glucose, cellulose and lignocellulose into lactic storage front (192). acid, which is a high value chemical used for the Currently formic acid is produced from fossil production of fine chemicals and biodegradable derived feedstock. The production of formic acid plastics, has been achieved. Lactic acid yields from renewable resources such as cellulose is ranging from 25% to 60% were obtained at high desirable for the advancement of green chemistry.

Scheme X. Reaction Cellulose pathways for the conversion of biomass derived glucose into organic acids (89–91, 179)

4% H Hydrolysis FDCA Levulinic acid (LA) 2SO 190–200ºC 4

Anaerobic fermentation Formic acid Glucose Or metal catalysed Lactic acid

Gluconic acid

Glucaric acid Acetic acid Glycolic acid

Formic acid has been produced from glucose and catalyst and in the aqueous phase (using O2 cellulose using alkali metal bases (193), vanadium oxidant) at 40–60°C, 100% conversion of glucose substituted Keggin-type POM (H5PV2Mo10O40) and 100% selectivity of gluconic acid was recorded

(194) and ferric sulfate (Fe2(SO4)3) (194). in 4 h (212). Comotti et al. used mono and bimetallic catalysts 9.2 Polysaccharides, Disaccharides (Rh, Pd, Pt and Au) supported on carbon to effect the transformation of glucose to gluconic acid in and Monosaccharides to water. The combination of Au-Pd resulted in 100% 2,5-Furandicarboxylic Acid, Gluconic selectivity (213). The direct conversion of cellulose Acid and Glucaric Acid to gluconic acid is more necessary, however few The oxidation of HMF can generate reports of the route are known. The conversion of 2,5-furandicarboxylic acid (FDCA) (Scheme X), cellobiose and cellulose to gluconic acid over POM- which is an important platform molecule, which supported Au nanoparticles (Au/Cs2.2H0.8PW12O40) has been identified as a potential alternative was reported. Cs2.2H0.8PW12O40 without Au to petroleum-based terephthalic acid for the exhibited a low activity for the conversion (25%) production of resins and polymers (195, 196). of cellobiose and the main product was glucose. FDCA is produced by a two-step process from Upon loading of Au particles, the conversion of cellulose or sugars (197). HMF is first prepared cellobiose increased to 96% and gluconic acid from C6 sugars or cellulose (59–62), followed by was the major product. This meant that the acidic

HMF oxidation to FDCA with oxidants (198), metal sites (Cs2.2H0.8PW12O40) catalysed the hydrolysis of catalysts (199–204) or enzymes (205). cellobiose and cellulose, and Au nanoparticles are Recently, Avantium in The Netherlands and BASF, responsible for the oxidation of glucose to gluconic Germany, have announced that they are jointly acid (214). Cellobiose conversion to gluconic acid preparing for the construction of the world’s first has also been demonstrated using sulfonated commercial plant for FDCA production from sugars. carbon supported Pt (Pt/C–SO3H) where 47% yield The Avantium process transforms fructose to HMF was obtained (215). methyl ether, which is then oxidised (by [Co(OAc)] There are some studies on the oxidation of or [Mn(OAc)] and NaBr) to FDCA. The FDCA glucose to glucaric acid. A Pt/SiO2 (4 wt%) catalyst (replacement for petroleum-based terephthalic afforded 66% yield of glucaric acid at 90°C under acid) will be combined with EG to produce 5 bar of O2, while at 119°C under 280 bar of O2 polyethylenefuranoate (PEF), a next generation over a Pt–Au/TiO2 (4Pt–4Au wt%) catalyst a 71% polyester. PEF is 100% bio-based and has a high yield of glucaric acid was recorded (216, 217). market potential as a future packaging and film Rennovia, USA, has developed a two-step catalytic material (206). process for producing adipic acid from glucose. The Gluconic and glucaric acid are important process starts by a selective catalytic oxidation of intermediates and are used in the pharmaceutical, glucose to glucaric acid, followed by a selective detergent and food industries (207). Currently, catalytic HDO of glucaric acid to adipic acid gluconic and glucaric acid are mainly produced (Scheme XI) (216–218). The Rennovia renewable by the enzymatic oxidation of glucose. Research adipic acid process offers potential for significant efforts have been devoted to developing catalytic commercial and environmental advantages oxidation of glucose to gluconic acid (208–211). For compared to the current petrochemical process for instance Au/TiO2 is an effective glucose oxidation the production of adipic acid (218).

O2 (air) H2

Catalyst Catalyst

Glucose Glucaric acid Adipic acid

Scheme XI. Rennovia’s two-step process for production of bio-adipic acid from glucose (216–218) (Reprinted with permission from (218))

10. Conclusion and Outlook site. Accessing these compounds by HDO and hydrogenation requires hydrogen and hence, Efficient and environmentally friendly conversion where will the hydrogen come from? Renewable of cellulosic biomass and cellulose derived hydrogen can be obtained from biomass by APR intermediates to renewable fuels, fuel additives or sourced from water by its electrolysis with and chemicals is of great interest to meet future electricity derived from wind or solar energy. liquid fuels and chemicals demand. (d) The conversion of cellulose and sugars to (a) Much effort has been focused on hydrolysis of organic acids and sugar alcohols has been cellulose and hemicellulose to sugars by mineral studied extensively, and further HDO of sugar acids and Lewis acids in biphasic liquids and IL. alcohols can lead to renewable alkanes in

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The Authors

Banothile Makhubela is currently a Senior Lecturer in the Department of Chemistry at the University of Johannesburg, South Africa. She qualified with a BSc in Chemistry and Mathematics from the University of Zululand, South Africa, in 2005 and obtained a BSc (Hons) in Chemistry in 2006 from the University of Cape Town (UCT), South Africa. In 2008, she completed a MSc degree under the supervision the late Professor John R. Moss and Professor Gregory S. Smith, and later obtained a PhD from UCT (supervised by Professor Smith and Dr Anwar Jardine). Picture credit: Chemical Services (CAS) Abstract

James Darkwa is a visiting Professor of Chemistry at the University of Johannesburg, South Africa, and currently a Lead Researcher at the Botswana Institute for Technology Research and Innovation (BITRI), Botswana. He worked in academia for nearly 30 years before joining BITRI in January 2017. He holds a BSc (Hons) in Chemistry from Kwame Nkrumah University of Science and Technology, Ghana, and a PhD in Chemistry from the University of New Brunswick, Canada.

www.technology.matthey.com

Ammonia and the Fertiliser Industry: The Development of Ammonia at Billingham A history of technological innovation from the early 20th century to the present day

By John Brightling for their production, ammonia is the most complex Johnson Matthey, PO Box 1, Belasis Avenue, requiring the highest number of catalytic steps. Billingham, Cleveland TS23 1LB, UK Ammonia is one of the most important chemicals produced globally with approximately 85% being Email: [email protected] used as fertiliser for food production (3). The other 15% of ammonia production is used in diverse industrial applications including explosives It is over 100 years since the Haber-Bosch process and polymers production, as a refrigeration fluid began in 1913 with the world’s first ammonia and a reducing agent in nitrogen oxides (NOx) synthesis plant. It led to the first synthetic fixed emissions control. Ammonia synthesis from nitrogen, of which today over 85% is used to atmospheric nitrogen was made possible in the make fertiliser responsible for feeding around 50% first part of the 20th century by the development of the world’s human population. With a growing of the Haber-Bosch process. It remains the only population and rising living standards worldwide, chemical breakthrough recognised by two Nobel the need to obtain reliable, economic supplies prizes for chemistry, awarded to Fritz Haber of this vital plant nutrient for crop growth is as in 1918 (4) and to Carl Bosch in 1931 (5). The important as ever. This article details the historic development of ammonia synthesis directly background to the discovery and development of a addressed “The Wheat Problem” as foretold by Sir process “of greater fundamental importance to the William Crookes in 1898 (6) whereby a shortage modern world than the airplane, nuclear energy, of available reserves (of wheat) would only allow spaceflight or television” (1, 2). It covers the the world’s population to continue to expand to role of the Billingham, UK, site in developing the about two billion which would be reached around process up to the present day. The technology was 1930. Thus, in the early 20th century, the need to pioneered in Germany and developed commercially increase food production led to the development by BASF. In 1998 ICI’s catalyst business, now of the fertiliser industry. Johnson Matthey, acquired BASF’s catalytic Today, the global value of ammonia production expertise in this application and now Johnson is estimated to be over US$100 billion, with Matthey is a world-leading supplier of catalyst and the largest individual plants being capable of technology for ammonia production globally. producing 3300 metric tonnes per day (mtpd) or 3640 short tonnes per day (stpd) (7). To achieve 1. Introduction this scale many improvements have been made over the last 100 years in both process and Ammonia is the second most produced industrial catalyst technology. chemical worldwide. Of the four chemicals, After describing historical aspects of the original ammonia, methanol, hydrogen and carbon ammonia technology development by Haber, monoxide that rely on similar syngas processes Bosch et al. in Germany, and the background to

32 © 2018 Johnson Matthey https://doi.org/10.1595/205651318X696341 Johnson Matthey Technol. Rev., 2018, 62, (1) the requirement for efficiency improvements, this 450 paper uses perspectives from Billingham, UK, to Birkeland-Eyde electric arc method 400 describe some of the technological contributions 350 that came from there in the development of 3

NH 300 ammonia production. –1 Cyanamid method 250 200 2. The Growing Need for Nitrogen 150 Haber-Bosch synthesis In just over 100 years the ammonia production 100 Steam reforming natural gas Energy, GJ mt Energy, industry has grown massively and continues to do 50 } so to feed the ever expanding world population. 0 1910 2010 The development of the remarkable iron catalyst by Alwin Mittasch (8) and the technology for the Year synthesis of ammonia from nitrogen and hydrogen Fig. 1. Historical efficiencies of ammonia process by Fritz Haber and Carl Bosch led to BASF starting technologies to operate the world’s first ammonia synthesis plant in 1913. Researchers estimate that about half consumption can only be reduced marginally, if at of today’s food supply is dependent on the nitrogen all, for the most efficient modern plants. originating from ammonia-based fertilisers (9). Worldwide ammonia production is largely based on Between now and 2050, while the world population modifications of the Haber-Bosch process in which will grow by 30%, the demand for agricultural NH is synthesised from a 3:1 volume mixture of goods will rise by 70% and demand for meat by 3 H2:N2 at elevated temperature and pressure in the 200% (10). This is linked with fundamental shifts presence of an iron catalyst. All the nitrogen used in the demand curve for food, especially caused is obtained from the air and the hydrogen may be by population growth, rising affluence leading to obtained by one of the following processes: changes in diet in many countries and in some • Steam reforming of natural gas or other light regions increasing use of food crops to produce hydrocarbons (natural gas liquids, liquefied fuel. The environmental, human health and climatic petroleum gas or naphtha) aspects of ammonia and fertilisers in the growth • Partial oxidation of heavy fuel oil or coal. scenarios have been reviewed elsewhere (11, 12). In ammonia production technology the type of Ammonia production technology has and feedstock plays a significant role in the amount continues to advance under the competitive of energy that is consumed and carbon dioxide challenges in the industry that demands an ever (CO2) produced. About 70% of global ammonia more energy efficient process, with lower emissions production is based on steam reforming concepts that can operate with high reliability for extended using natural gas, with the use of steam reforming periods between shutdowns. There have been of natural gas considered the best available dramatic increases in environmental performance technology from the point of view of energy use and energy efficiency over the last 100 years, but and CO2 emissions, Table I (14). The use of coal with modern steam reforming processes energy and fuel oil are predominately restricted to China, utilisation is nearing the theoretical minimum (13) which exhibits a strong divergence in the ammonia (Figure 1) and looking forward, specific energy feedstock versus the rest of the world. China

Table I Comparative Energy and CO2 Emissions of Different Ammonia Processes and Feedstocks

Energy, CO2 emissions, Energy source Process –1 –1 GJ t NH3 tonnes t NH3 Natural gas Steam reforming 28 1.6

Naphtha Steam reforming 35 2.5

Heavy fuel oil Partial oxidation 38 3.0

Coal Partial oxidation 42 3.8

33 © 2018 Johnson Matthey https://doi.org/10.1595/205651318X696341 Johnson Matthey Technol. Rev., 2018, 62, (1) accounts for 95% of global coal-based ammonia and Carl Bosch on the technology (5) the ammonia capacity with around 80% of the plants in China synthesis process came to Billingham, UK, in the being coal-based. early 20th century. An ammonia factory being The production of ammonia is a very energy located at Billingham, UK, grew out of the needs of demanding process, the energy use of the World War I when the British government needed to steam reforming process is about 28–35 GJ per develop technology to produce synthetic ammonia –1 tonne ammonia (GJ t NH3). Figure 2 shows the for producing explosives. Billingham was chosen theoretical, practical and operating level energy partly for its proximity to a then-new North Tees efficiencies for ammonia plants based on steam electricity generating station nearby; although reforming. Energy efficiencies vary widely for later developments to the process required less ammonia plants currently in operation due to age, electric power than had been assumed. It is worth feedstock, energy costs and utility constraints. noting that even before the plant was begun the Most plants operate well above the practical possibility for post-war use for fertiliser production minimum energy consumption with the best was recognised. This was recorded in a report by performers (top quartile) ranged between 28 and the Chemical Society in 1916: –1 –1 33 GJ t NH3 and an average efficiency of 37 GJ t NH3. It has been estimated that if all plants worldwide “With some foresight a plant erected were to achieve the efficiency of the best plants, primarily for a military purpose might be energy consumption could fall by 20–25% (15). easily adapted in peace time to agricultural A feature of the industry is that most plants are objects” (16). being continually reviewed for improvements and revamp ideas can be subsequently implemented However by the time the plant (known as the that improve efficiency. Government Nitrate Factory) was completed, World War I was over. The site was put up for sale in 1919 3. Technology Development at (Figure 3), and was purchased by Brunner Mond & Co Ltd (16) who converted it to make ammonia- Billingham, UK based fertilisers. The company was set up as a Following pioneering work by Fritz Haber on the subsidiary called Synthetic Ammonia and Nitrates process (4), Alwin Mittasch on the catalyst (8), Ltd. This became part of ICI in December 1926,

Fig. 2. Energy requirements for Energy loss to Operating ammonia plants Most plants inefficient equipment, level operate in this poor design, limited energy region heat recovery and other factors

28.0 GJ mt–1 NH3 Energy loss to process Practical irreversibility, non- minimum standard conditions energy and byproducts

18.0 GJ mt–1 NH3 Theoretical minimum energy Energy based on ideal chemical reactions, 100% yield, standard state and irreversibility

Fig. 4. Original flowsheet for ammonia production at Billingham

Fig. 3. Advertisement for the Government sale of Billingham Nitrate Plant, November 1919

when ICI was formed from the merger of Brunner Mond, Nobel Explosives, the United Alkali Company and the British Dyestuffs Corporation.

3.1 The Coke Oven Process of Syngas Production

The ammonia plants built at Billingham in the 1920s and 1930s employed the classic Haber-Bosch process based on coke, the same as the production technology used in Oppau, Germany. The first Billingham plant was a 24 mtpd (26 stpd) unit that made its first ammonia in December 1924. The original process is shown in Figure 4. The first stages of gas production were at atmospheric pressure. Alternate streams of steam Fig. 5. Bank of parallel high pressure ammonia and then air were fed into gas generators containing converters, Billingham hot coke to make ‘water-gas’ (hydrogen-rich) and producer gas (nitrogen-rich). These streams were purified using iron oxides to remove hydrogen Using this technology the rise in output from the sulfide and a shift converter to convert most of the site is shown in Figure 6. carbon monoxide to CO2 and H2. The ‘catalysed gas’ As well as scale improvement there were was compressed in reciprocating compressors. CO2 improvements in effectiveness. In 1929, A. H. was removed by counter-current scrubbing with Cowap, Chief Engineer, noted: “a striking feature circulating water and the scrubbed gas was further is an ever increasing rapidity of work. The first compressed, washed with copper liquor to remove large unit No. 3 Unit cost £5¼ million pounds residual CO and CO2 and then fed as make-up and was completed in 27 months (of which 7 gas to the synthesis loop which contained a large months was a labour stoppage for a coal strike). number of parallel converter vessels (Figure 5). No. 4 and No. 5 units cost £11 million pounds and

800 Year Plant developments 1924 No. 2 unit 600 1926 No. 2 unit extension 400 1928 No. 3 unit

200 1929 No. 4 and No. 5 units

Ammonia, mtpd 1932 Extension 0 1941 Process improvement 1920 1930 1940 1950 1960 1951 Converter internals Year redesign

Fig. 6. Ammonia output at Billingham from 1924 to 1951 and the plant developments that coincide with these have been completed in 2 months” (16). Despite Plants, was held in 1955. This meeting became an improvements, by the late 1950s increasing costs important event organised annually to improve of coal and the intrinsic inefficiency of syngas the safety performance of the ammonia industry. generation from coke had made this process It continues accomplishing these objectives by uncompetitive. sharing information on incidents, safety practices, plant performance and technology improvements, 3.2 Partial Oxidation and Plant Safety with the 62nd meeting of AIChE Safety in Ammonia Plants and Related Facilities Symposium being held The first step to improve process efficiency from in 2017 (18). coke-oven syngas production was utilisation of higher pressure oil gasification units, a Texaco 3.3 Steam Reforming of Light gasification unit at Billingham for heavy fuel oil Naphthas was later converted for naphtha feed. Syngas was produced at 30 bar (440 psi) pressure by reaction of Steam reforming of hydrocarbons provides the the hydrocarbon with steam and a limited supply of most economic source of hydrogen gas for ammonia oxygen at 1500°C (2732°F). The partial oxidation synthesis. The general steam reforming reaction is process reduced both the capital and operating shown in Equation (i): costs of low pressure gas generation, eliminated CnH2n+2 + nH2O → nCO + (2n+1)H2 (i) the need for low pressure compression and offered greater feedstock flexibility. The principle The reaction was known to proceed at 700–800°C disadvantage of the process was its requirement (1292–1470°F) over a promoted and supported for an air separation plant to supply oxygen. In nickel catalyst. ICI was amongst pioneers in these early days the challenges for safe operations methane steam reforming and commercial units and engineering of these air separation units (ASU) had been built at Billingham in 1936 to reform were significant. propane/butane byproducts of hydrogenation of In 1959 at Billingham’s partial oxidation coal as part of synthetic hydrocarbons production plant a serious explosion occurred during the (Oil Works). This reforming process was operating commissioning of the ASU which led to three at atmospheric pressure. fatalities (17). This incident resulted in a long In the 1950s natural gas was not available in delay in the partial oxidation plant achieving the UK (discovery and exploitation of North Sea beneficial operation by which time steam reforming gas was still some 15–20 years distant), however technology development had advanced sufficiently increasing quantities of light distillate hydrocarbons to make it a more competitive route for syngas (naphtha) were available at falling prices. Sulfur production. free naphthas had been successfully reformed by Within the industry frequent explosions in oxygen the catalyst research group at Billingham in 1938 plants encouraged engineers to meet and share at atmospheric pressure. What was needed was the information. The very first symposium to discuss development of the process to operate at higher safety in air and ammonia plants, called Safe Design pressures to avoid compression costs. The world’s and Operation of Low Temperature Air Separation first pressurised steam naphtha reforming process

Air Steam Product gas to final purification

Steam Steam

Liquid naphtha

Vapouriser Reforming Boiler Boiler CO2 furnace removal

Hydro- Secondary Shift desulfuriser reformer reactor

Fig. 7. Two stage (primary-secondary) reforming (21)

was designed at Billingham and was brought into (200 psi), the reformed gas, containing 10–12% commercial operation at Heysham, UK, in 1962 (19). CH4, was collected in headers near the ground and The main problems were adequate desulfurisation passed to the air injection burner in the secondary of the feed, and suppression of carbon deposition reformer. After secondary reforming were waste on the reforming catalyst without the use of heat recovery, two stage CO shift, further heat excessive steam ratios. Desulfurisation of the feed recovery, cooling and CO2 removal. The process was addressed by development of feed purification was rapidly adopted and by the mid-1960s over technology involving hydrogenation catalysts 100 steam reforming process licences had been (nickel-molybdenum, cobalt-molybdenum) along sold from Billingham to the following reputed with zinc oxide absorbents capable of reducing engineering contractor licensors: Power Gas sulfur to very low levels. The problem of carbon Corporation (later Davy Power Gas, now Johnson formation was solved by the development of new Matthey), Foster Wheeler (now AMEC), Selas, M. types of alkalised catalysts (20). W. Kellogg (now KBR), Friedrich Uhde GmbH (now Due to equilibrium considerations, to achieve a thyssenkrupp Industrial Solutions GmbH) and low methane slip a temperature of around 1000°C Humphreys & Glasgow (now Jacobs). (1830°F) is required, however the metallurgical limit for a ten-year life of the available tube materials was a design exit temperature of 800°C (1470°F). To overcome this constraint, the new steam reforming process adopted two reforming stages as shown in Figure 7 (21). Now familiar to us as primary and secondary reformers, these unit operations are still present in nearly all ammonia plants.

3.4 Steam Reforming Modernisation

Having developed a viable steam reforming process, the syngas units at Billingham were modernised with four pressured naphtha steam reforming units built in 1962–1963 (Figure 8). Each unit included a primary (tubular) reformer Fig 8. First pressured naphtha steam reforming with 4″ (100 mm) internal diameter tubes and a units for ammonia reaction length of 20 ft (6 m). Operating at 14 bar

The new steam reforming front end occupied Early design memos for the Billingham plants in an area of 14,160 m2 (3.5 acres) – a little less the 1920s had discussed the relative economics than 10% of the area occupied by the coke based of reciprocating and centrifugal compression. processes that it replaced. Using space freed up by They showed that the relative efficiency of rotary the reformers, improvements to the gas purification compressors for the later compression stages would and compression were introduced. The existing be low except at high throughputs (Figure 9). This

CO2 removal process employed water washing low efficiency was one of the reasons for excluding at 55 bar (798 psi) and consumed significant centrifugal compressors for all but the low energy leading to high capital and operating costs. pressure stages and this reasoning still prevailed

Chemical absorbents with higher capacity for CO2 at the beginning of the 1960s; however it was now removal had become available, such as the Benfield challenged. A second plant for ICI Severnside, with process (potassium carbonate) and Vetrocoke a capacity of 545 mtpd (600 stpd), led to what (arsenious oxide), and these were adopted on was described as “possibly the most important different Billingham plants in the early 1960s. event in the history of the development” of the

These processes achieved CO2 slips of <0.1% dry. single stream ammonia plant (23). Figure 10 (24) showing the increase in output of ammonia 3.5 Development of Single Stream converters from 1930 to the mid-1960s illustrates the revolution in scale taking place. By then the Plants normal ammonia unit size was already 600 mtpd As can be seen with the four reformers above, in (662 stpd) capacity and plants with lower capacity contrast to a modern single stream plant, in the than this were being regarded as small. early 1960s many units of process equipment In late 1962, a meeting was held in which Ron (such as compressors and synthesis reactors) still Smith, Vice President of Operations at M. W. Kellogg, had to be used in parallel. A ‘single stream’ concept opined that the capacity of ammonia plants was emerged for new ammonia plants and Billingham bound to increase and queried why the synthesis engineers designed and engineered a 360 mtpd loop pressure could not be reduced from 325 bar (397 stpd) ‘single stream’ plant. Commissioned at (4700 psi) to 150 bar (2200 psi), thus removing Severnside, UK, in 1963, it was, at that time, the the need for reciprocating compressors. Although largest single stream ammonia plant in the world. not successful for that plant bid soon afterwards The plant used the steam naphtha reforming discussions began on how a large plant could be process, a hot potassium carbonate based CO2 built – and the plant design was established which removal system, a copper liquor CO removal quickly became a new technology era that came system and had two high temperature shift (HTS) beds, however parallel reciprocating compressors driven by electric motors were used for synthesis gas compression. 80 Although more active copper-based catalysts were Reciprocating compressors known to be able to accomplish the shift reaction at ~250°C (482°F), they were very sensitive to 70 poisoning by sulfur and could not be used with syngas made from coke. The virtually sulfur free syngas obtained from steam reforming allowed Rotary compressors 60 these copper-based shift catalysts to be used, achieving an equilibrium CO conversion of ~0.2%

dry. ICI developed its own robust and reactive Cu % Stage efficiency, 50 catalyst for this application (22). The combined low level of residual carbon oxides from CO2 removal and CO shift was now low enough that they could be made inert by methanation before the synthesis 40 0 200 400 600 800 1000 loop. As a result of all of the improvements Ammonia output, mtpd considered so far – steam reforming, shift, CO2 removal and methanation – by the mid-1960s it Fig. 9. Mid-1960s final stage compressor was possible to carry out all operations in single efficiencies (%)vs . ammonia output (mtpd) (24) stream reactors.

1000

800

600

400

Ammonia output, mtpd 200

0 1930 1940 1950 1960 1970 Fig. 11. Three M. W. Kellogg reformers Year

Fig. 10. The increase in output (mtpd) of ammonia In keeping with its status as an operator, designer, converters from 1930 to the 1960s (24) technology licensor and catalyst manufacturer, ICI continued to develop its own technology. Two to dominate the industry with a capacity of ‘1000 Billingham designed ammonia plants, constructed stpd’ (900 mtpd). in Kanpur, India, in 1969, featured the first In January 1964, M. W. Kellogg was awarded a application of ICI’s single nozzle secondary burner contract for two ‘1000 stpd’ (900 mtpd) plants to (Figure 12); the forerunner of a design used in be built at Billingham. The design incorporated a many ICI (and subsequently Johnson Matthey) number of important features: designed plants using autothermal reforming • The steam naphtha reforming process at 31 bar technology. (450 psi) pressure • A loop pressure of 131 bar (1900 psi) allowing the use of centrifugal compressors • Improved plant efficiency by recovering heat to generate 103.5 bar (1500 psi) high pressure steam superheated to 450°C (850°F) for use on steam turbine drives. The steam was generated at a higher pressure than that required by the process, so energy was recovered by expanding the steam through turbines to the pressure level required by the process. This greatly enhanced process efficiency. Within a few weeks a third plant was announced, they were the largest plants built at that time.

3.6 M. W. Kellogg Ammonia Units Fig. 12. Single nozzle secondary burner As the M. W. Kellogg plants incorporated the steam naphtha reforming process, Billingham engineers 3.7 Use of Natural Gas Feedstock worked closely with their counterparts from M. W. Kellogg in the design of the reformers, shown in In the 1970s Billingham’s ammonia plants Figure 11. As by the mid-1960s exploration was changed from naphtha feeds to run on the newly ongoing for North Sea gas this was considered commercialised natural gas from the North Sea, and a feature of the reforming process was that however the favourable gas contract was on an the plants could be readily converted to lighter interruptible supply basis, meaning that with short hydrocarbons. notice the feedstock could be cut when demand

39 © 2018 Johnson Matthey https://doi.org/10.1595/205651318X696341 Johnson Matthey Technol. Rev., 2018, 62, (1) for natural gas was high. If natural gas supply was for 1000 mtpd (1100 stpd), this was commissioned interrupted the plants were configured to switch in 1978 and was able to achieve a throughput of feedstock on-line to liquefied petroleum gas (LPG) about 1125 mtpd, (1240 stpd) without significant propane feedstock (which was stored locally in modification. underground salt caverns), bringing a demand for catalysts that could cope with feedstock 3.8 The Ammonia V Process flexibility. This brought new requirements for a catalyst with lower potash and higher activity in Ammonia technology continued to develop and order to optimise the reformer for this feedstock. Billingham-based engineers were tasked with the By the end of the decade there were two light design of a fifth ammonia plant for Billingham potash catalysts: 25-3 (1.6% K2O) for natural (Ammonia V or ‘AMV’). Economic considerations gas feeds and 46-9 (2.2% K2O) for LPG feeds. By meant that capital cost had to be reduced whilst the end of the 1970s, ICI Katalco had a product improving plant efficiency. Although market range very similar to the present: 57-series non conditions in the early 1980s meant that the potash, 25-series light potash, and 46-series plant was never built at Billingham, the designs naphtha catalyst. By this point the catalyst beds for Ammonia V evolved into the AMV process. were operating at temperatures up to 1000°C The first AMV design was commissioned at (1832°F) and 35.6 bar (516 psi), primarily due to Courtright, Canada, in August 1985 (Figure 14), improvements in metallurgy. producing 1120 mptd (1234 stpd) at a total energy In the 1970s, it was recognised that appropriately requirement of 29 GJ per metric tonne (lower formulated low-temperature shift (LTS) catalysts heating value, LHV). Ammonia production was could be self-guarding not only in regard to sulfur, achieved 43 hours after feed gas introduction, but also towards chloride. It was also recognised believed to be a record at that time (25). The AMV that the benefits in terms of shift activity and process also featured a low pressure synthesis bed life accruing from the use of fresh LTS loop operating at about 85 bar (1230 psi) featuring catalyst outweighed the cost savings realised by a new cobalt-promoted high-activity ammonia reusing discharged LTS catalyst. All LTS catalysts synthesis catalyst (KATALCOTM 74-1) which had subsequently developed by ICI and Johnson been developed specifically for the project. Matthey were therefore optimised to maximise As a highly efficient process operating with a their self-guarding capability. low steam ratio, the plant was one of the first to These catalyst systems were utilised in the three suffer from byproduct formation and pressure drop M. W. Kellogg ammonia plants and also in the ICI increase due to HTS over reduction. Copper was designed Ammonia IV plant (Figure 13). Designed added to the HTS catalyst formulation to create an

Fig. 13. Ammonia IV (Ammonia 4) plant at Fig. 14. AMV process plant in Courtright, Canada Billingham

40 © 2018 Johnson Matthey https://doi.org/10.1595/205651318X696341 Johnson Matthey Technol. Rev., 2018, 62, (1) over reduction resistant formulation which was first second plant at Severnside made ammonia only 19 installed in 1987. hours after natural gas was first introduced.

3.9 The Leading Concept Ammonia 3.10 Catalyst Developments Process Catalyst developments continued into the 1990s. By the mid-1980s, the two ammonia plants at Figure 16 illustrates the dramatic improvement in Severnside were becoming uncompetitive and a the activity of one particular catalyst which resulted decision had to be made: improve their efficiency, from a combination of on-going development and replace them or close the site. Improving the the incorporation of learning from the development efficiency was thought unfeasible and it was of the technology for LTS catalysts. decided to develop a new process to replace them. A step change occurred in 1997 due to the This led to the leading concept ammonia (LCA) acquisition of the BASF syngas catalyst business by process technology being developed at Billingham ICI’s catalyst business (since acquired by Johnson (Figure 15). Matthey). The acquisition of the BASF activities The LCA process used a combination of allowed the knowledge of two historic companies new equipment, new catalysts and improved to be combined and in this case the best of both construction and procurement techniques. The companies created a new improved LTS catalyst. range of developments included: Methanol is an unwanted byproduct that may be • KATALCO 61-2 (the first low-temperature formed in LTS reactors and is the main volatile hydro-desulfurisation (HDS) catalyst) organic compound (VOC) emitted from ammonia • PURASPECTM 2020 (the first low-temperature production plants. It is formed as a byproduct in sulfur removal absorbent) both high-temperature and low-temperature shift.

• KATALCO 83-1 (the first application of a process Through the 1990s byproduct methanol was an gas heated reformer (GHR), isothermal shift increasing concern for plants as environmental catalyst specifically developed to resist the high emissions came under closer attention. More operating temperature) selective catalysts became available that made

• KATALCO 11-4 (a low-temperature methanation less methanol. BASF previously had low methanol catalyst) LTS catalysts, K3-110 and K3-111, which suffered

• KATALCO 74-1 (a catalyst which could be used from issues relating to physical strength and in an ammonia synthesis loop at 80 bar (1160 poisons resistance. ICI had LTS products with psi) pressure, even lower than in the AMV good strength characteristics, but could not mimic process). the BASF low methanol recipe due to patent The unique process together with extensive protection. The combination of the two businesses automation start-up sequences meant the plants meant that a low methanol, high-strength product were amongst the most automated ever. The could be developed. The results were KATALCO

83-3K, launched in 1997, and KATALCO 83-3X,

1.4

1.2

1.0

0.8 83-3 0.6 83-2 0.4

Relative shift activity Relative 0.2 53-1 52-2 0 1983 1986 1989 1992 1995 1998 Year

Fig. 16. Relative LTS activity of successive generations of the KATALCO catalysts. The Fig. 15. LCA plant in Severnside, UK numbers within the bars refer to the catalyst series

41 © 2018 Johnson Matthey https://doi.org/10.1595/205651318X696341 Johnson Matthey Technol. Rev., 2018, 62, (1) which was launched in 1998 with 90% reduction purchased in 2014. CATACEL SSRTM is a stackable in methanol byproduct formation compared to structural catalytic reactor for the production previous generations of catalyst (Figure 17). of hydrogen from natural gas. It is made from a high-temperature stainless steel foil coated with a reforming catalyst. This structure allows higher 120 heat transfer and can provide significant capacity Traditional 100 increase to reformers or lower pressure drop 83-3 compared to standard pelleted catalysts. 80 83-3S Further developments have been made by shaping the pellets in some of the other reactors 60 which follow the steam reformer in the production of syngas at the front-end of the plant, notably 40 83-3K the HTS and methanator. For example, KATALCO Relative methanol rate Relative 20 83-3X 71-5F (Figure 18) is a shaped 5-lobe pellet HTS catalyst which exhibits lower pressure 0 drop, increased strength and increased voidage. pre-1993 1993 1996 1997 1998 Similarly, for the methanation reactor, KATALCO Year 11-6MC (Figure 19) uses a 4-hole clover leaf shape Fig. 17. Relative LTS selectivity of the KATALCO to provide lower pressure drop with increased bed catalysts, measured by methanol production rate. voidage. The benefit of pressure drop reduction in The numbers above the bars refer to the catalyst the front end varies from plant to plant depending series on the individual process constraints. Generally

3.11 Developments in Catalyst Shape

The effect of shape on reforming catalysts has been recognised for a long time (21). For steam reforming catalysts, the reaction occurs in a very thin layer at the surface of the pellet. Therefore developments focused on techniques to develop the shape, maximising the external surface area of the catalyst pellets whilst at the same time considering the resistance to flow caused by the way the catalyst packs in the tube. The shape of the steam Fig. 18. Shaped HTS catalyst KATALCO 71-5F reforming catalysts evolved from the original cubes (circa 1930s) to Raschig rings (circa 1940s) to ICI Katalco 4-hole (circa 1980s) and finally the current KATALCO QUADRALOBETM shape. At each iteration, for similar sized pellets the activity increased and the pressure drop decreased (20). Increasing the catalyst activity also allowed the reforming reaction to progress at a lower temperature, which meant the tubes were also at a lower temperature as shown by the measured tube wall temperature (TWT). The lower the peak maximum TWT, the longer the tube metallurgy lasts before failure, with a difference of as little as 20°C (68°F) doubling the tube life. Since the 1990s design tools such as finite element analysis have been used to assist with the design and optimisation of catalyst shape. The latest Fig. 19. Shaped methanation catalyst KATALCO development for the steam reforming process is 11-6MC the CATACELTM technology, which Johnson Matthey

42 © 2018 Johnson Matthey https://doi.org/10.1595/205651318X696341 Johnson Matthey Technol. Rev., 2018, 62, (1) pressure drop is welcome and a small increase in Since then tkIS’s Uhde dual-pressure process has energy efficiency can be gained if it is reduced. been implemented in other similar scale plants recently commissioned in regions of the world with 3.12 The Dual-Pressure Process an abundance of low cost natural gas feedstock (Table II). In 1998, ICI and Uhde (now thyssenkrupp Industrial Solutions (tkIS)) formed an alliance in the field of 4. Ammonia Production Today ammonia technology resulting in a variety of new developments, the most public of which was the Figure 22 shows the current plant capacity and dual-pressure ammonia process (28). The resulting year of construction for all operating ammonia 3300 mtpd (3640 stpd) plant was a step change plants. There is a clear progression of increasing in the scale of plant design available and offered plant scale with time. Market needs for individual a reduction of specific production costs through plants will differ, leading to a range in plant economies of scale. These are still being built today capacities. There are however preferred plant sizes as the world’s largest ammonia plants. which have become ‘standard’ in the industry for The key innovation in the Uhde dual-pressure which references and documented plant designs ammonia process was an additional medium- exist. These can be clearly seen in Figure 22 at pressure once-through ammonia synthesis step capacities of 600 mtpd, 1000 mtpd, 1360 mtpd, operating at around 110 bar (1595 psi), connected 1500 mtpd, 2000–2200 mtpd and most recently in series with the conventional high-pressure 3300 mtpd (3640 stpd). It is notable that, as ammonia synthesis loop at around 200 bar (2900 well as being the largest production units in the psi), Figure 20. The first plant based on this world, the emission limits for the new US fertiliser process was the SAFCO IV ammonia plant in Al projects (ammonia and downstream plants) are Jubail, Saudi Arabia, started up in 2006. With a amongst the lowest in the world, with the design capacity of 3300 mtpd (3640 stpd) it was by far levels for emissions of NOx, N2O, CO and volatile the largest ammonia plant worldwide, Figure 21. organic compounds (VOC) being significantly

Off-gas Second ammonia converter

HP- HP- ~210 bar Purge gas recovery steam steam CW First ammonia converter ~110 bar NH3 chillers NH3

LP HP Once-through Ammonia casing casing ammonia converter from HP loop

Note: molecular CW sieves not shown

NH3 NH3 chillers chiller HP- steam

H2O

CW Make up gas NH3 NH3 chiller Ammonia from once-through conversion from front-end

Fig. 20. Schematic of the Uhde process dual-pressure ammonia synthesis section

below current recognised best available techniques (BAT) values (26). In just over 100 years, the nitrogen fertiliser industry based on ammonia production has grown massively (Figure 23). Drivers behind this growth have been, and remain, increasing global population (Figure 24) (9) coupled with increased plant size to achieve better economies of scale. Although the picture is more complex than this (for example, one could ask which came first: fertiliser or population growth?), together this has created demand for increased capacity and increased reliability from that capacity.

5. Conclusion

Over the last century, scientists and engineers have made a significant contribution to the nitrogen industry. Some of these have been based Fig. 21. SAFCO IV Uhde dual-pressure process (Image courtesy of tkIS) at Billingham, UK, whose heritage now resides with Johnson Matthey and the challenge is to continue

Table II 3300 mtpd tkIS Uhde Ammonia Plants with Johnson Matthey Catalyst

Plant Location Capacity, mtpd Start-up year

Saudi Arabian Fertiliser Al Jubail, Saudi Arabia 3300 2006 Company, SAFCO 4

Saudi Arabian Mining Raz Az Zwor, Saudi Arabia 3300 2011 Company, Ma’aden

CF Industries, Donaldsonville, Donaldsonville, LA, USA 3300 2016 Ammonia 6

Saudi Arabian Mining Raz Az Zwor, Saudi Arabia 3300 2016 Company, Ma’aden 2

3500 Fig. 22. Trends of plant 3000 capacity vs. year of construction 3000 2500 2500 Plant capacity, stpd 2000 2000

1500 1500

Plant capacity, mtpd Plant capacity, 1000 1000

500 500

0 0 1957 1967 1977 1987 1997 2007 2017 Year of construction

170 160 150 140 130 120 110 100 90 80 70 60 50 40 Ammonia production, millions of mtpd 30 20 10 0 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 2010 Year Fig. 23. Global Haber-Bosch ammonia production from mid-20th century to the present. Over 99% of fixed nitrogen production today is by the Haber-Bosch process (2) (Copyright The Fertilizer Institute, used with permission)

7 World population 50 6 World population (no Haber-Bosch nitrogen) Average fertiliser input, kg

40 Meat production, kg person 5 % world population fed by Haber-Bosch nitrogen World population, % /

4 Average fertiliser input 30

Meat production N 20 ha –1 –1 World population, billions World yr 2 yr –1 –1 / 10 1

0 0 1900 1950 2000 Year Fig. 24. Demographic drivers for Haber-Bosch nitrogen and its use in fertiliser: “...the lives of around half of humanity are made possible by Haber-Bosch nitrogen” (2, 9) (Copyright The Fertilizer Institute, used with permission)

this legacy and make an equally significant London, UK, 1899, 207 pp contribution to the future of this vital industry. 7. P. Heffer and M. Prud’homme, ‘Global Nitrogen The fundamental ammonia synthesis process and Fertilizer Demand and Supply: Trend, Current catalysts developed by Haber-Bosch and Mittasch Level and Outlook’, 7th International Nitrogen can still be clearly recognised in even the most Initiative Conference, Melbourne, Australia, 4th– modern ammonia plants. However, the process 8th December, 2016 efficiencies and environmental performances have 8. A. Mittasch and W. Frankenburg, Adv. Catal., been dramatically improved over the last 100 1950, 2, 81 years, most particularly in the preparation of the 9. J. W. Erisman, M. A. Sutton, J. Galloway, Z. synthesis gas, benefiting both ammonia production Klimont and W. Winiwarter, Nat. Geosci., 2008, 1, and other syngas-based processes. Because energy (10), 636 utilisation within modern processes is near the 10. N. Alexandratos, ‘World Food and Agriculture to theoretical minimum, specific energy consumption 2030/50: Highlights and Views from Mid-2009’, can be reduced only marginally, if at all. There Expert Meeting on How to Feed the World in 2050, are many future challenges for ammonia and the Rome, Italy, 12th–13th October, 2009 fertiliser industry, which fall outside the scope of 11. W. Winiwarter, J. W. Erisman, J. N. Galloway, Z. this historical overview. Klimont and M. A. Sutton, Climatic Change, 2013, For now, the ammonia industry will be with us 120, (4), 889 more or less in its present form for decades to come 12. H. J. M. Van Grinsven, J. H. J. Spiertz, H. J. (27). The present production capacity for synthetic Westhoek, A. F. Bouwman and J. W. Erisman, J. ammonia of over 175 million metric tonnes per year Agr. Sci., 2014, 152, (S1), 9 will continue to grow at 1–2% every year to satisfy 13. ‘Energy Efficiency and CO Emissions in Ammonia the increasing demands for food and ammonia- 2 Production’, Feeding the Earth, International based intermediates from an increasing number of Fertiliser Industry Association, Paris, France, people enjoying increasing welfare. December, 2009 14. M. Appl, “Ammonia, Methanol, Hydrogen, Carbon Trademarks Monoxide: Modern Production Technologies”, CRU Publishing Ltd, London, UK, 1997, 144 pp KATALCO, PURASPEC, QUADRALOBE, CATACEL and 15. “Tracking Industrial Energy Efficiency and SSR are trademarks of Johnson Matthey. CO2 Emissions”, International Energy Agency/ Organisation for Economic Co-operation and References Development, Paris, France, 2007, 324 pp 16. V. E. Parke, “Billingham – The First Ten Years”, 1. V. Smil, “Enriching the Earth: Fritz Haber, Carl Imperial Chemical Industries, Billingham, UK, Bosch, and the Transformation of World Food 1957, 110 pp Production”, Massachusetts Institute of Technology, Cambridge, Massachusetts, 2001, 358 pp 17. W. D. Matthews and G. G. Owen, ‘Safety Aspects of Reconstructed ICI Tonnage Oxygen Plant’, in 2. H. Vroomen, ‘The History of Ammonia to 2012’, “Ammonia Plant Safety (and Related Facilities)”, The Fertilizer Institute, Washington, DC, USA, 19th November, 2013 Vol. 5, American Institute of Chemical Engineers, New York, USA, 1963 3. P. Heffer and M. Prud’homme, ‘Fertiliser Outlook 2013–2017’, 81st IFA Annual Conference, Chicago, 18. 62nd Annual Safety in Ammonia Plants and Related USA, 20th–22nd May, 2013 Facilities Symposium, New York, USA, 10th–14th September, 2017 4. ‘The Nobel Prize in Chemistry 1918 – Fritz Haber’, “Nobel Prizes and Laureates”, The Nobel 19. United Nations Interregional Seminar on the Foundation, Stockholm, Sweden, 1918 Production of Fertilisers, Kiev, Ukrainian Soviet Socialist Republic, 24th August–11th September, 5. ‘The Nobel Prize in Chemistry 1931 – Carl 1965, “Fertilizer Production, Technology and Bosch and Friedrich Bergius’, “Nobel Prizes and Use: Papers Presented at the United Nations Laureates”, The Nobel Foundation, Stockholm, Interregional Seminar on the Production of Sweden, 1931 Fertilisers”, United Nations, New York, USA, 1968 6. W. Crookes, “The Wheat Problem: Based on 20. C. Murkin and J. Brightling, Johnson Matthey Remarks Made in the Presidential Address to the Technol. Rev., 2016, 60, (4), 263 British Association at Bristol in 1898: Revised with an Answer to Various Critics”, John Murray, 21. “Materials Technology in Steam Reforming

Processes”, Proceedings of the Materials Haber-Bosch to Current Times’, Proceeding Technology Symposium, Billingham, UK, 21st– 747, International Fertiliser Society, Colchester, 22nd October, 1964, ed. C. Edeleanu, Pergamon UK, 2014 Press Ltd, Oxford, UK, 1966 22. P. Davies, A. J. Hall and D. A. Dowden, ICI Ltd, Further Reading ‘Catalysts of High Activity at Low Temperature’, British Patent Appl., 1968/1,131,631 T. Hager, “The Alchemy of Air”, Harmony Books, New 23. R. H. Multhaup and G. P. Eschenbrenner, York, USA, 2008 “Technology’s Harvest: Feeding a Growing World J. Korkhaus and M. Bachtler, ‘The Ammonia Process Population”, Gulf Publishing Co, Houston, Texas, – A Challenge for Materials, Fabrication and USA, 1996 Design of the Components’, 58th Annual Safety in 24. P. W. Reynolds, ‘Manufacture of Ammonia’, Ammonia Plants and Related Facilities Symposium, Proceeding 89, International Fertiliser Society, Frankfurt, Germany, 25th–29th August, 2013, Vol. Colchester, UK, 1965, 27 pp 54, American Institute of Chemical Engineers, 25. W. K. Taylor and A. Pinto, Proc. Safety Prog., New York, USA, 2013, pp. 167–180 1987, 6, (2), 106 M. Cousins and J. Brightling, ‘Make More From 26. K. Ruthardt, ‘Environmental Constraints on New Less’, Nitrogen + Syngas, 2017, 345, (January- Plant Construction in the USA’, Proceedings 743, International Fertiliser Society, Colchester, UK, February), 52 2014, 24 pp J. Larsen, D. Lippmann and C. W. Hooper, ‘A New Process 27. J. G. Reuvers, J. R. Brightling and D. T. Sheldon, for Large-Capacity Ammonia Plants’, Nitrogen & ‘Ammonia Technology Development from Methanol, 2001, 253, (September-October), 41

The Author

John Brightling is Ammonia Commercial Manager at Johnson Matthey Process Technologies. He obtained his BSc (Hons) in mechanical engineering from the University of Leicester, UK, and has worked in the chemical industry for 30 years. Initially working at ICI with a variety of roles covering plant design, operation and maintenance; for the past 18 years he has worked in the catalyst business with responsibilities for sales, marketing, technical service and product development for the ammonia market. He is a member of the American Institute of Chemical Engineers (AIChE) and has served as Chair of the Ammonia Safety Committee.

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A Re-assessment of the Thermodynamic Properties of Palladium Improved values for the enthalpy of fusion

John W. Arblaster et al. (2), Clusius and Schachinger (3) and Mitacek Droitwich, Worcestershire, UK and Aston (4) and continue to be considered satisfactory. However only limited information Email: [email protected] was given in the original review which above 50 K consisted of specific heat values at 10 K intervals from 50 to 100 K and at 20 K intervals from 100 The thermodynamic properties were reviewed to 280 K and with the value at 298.15 K also by the author in 1995. A new assessment of the included. This is now considered to be far too enthalpy of fusion has led to a revision of the little information and instead a comprehensive thermodynamic properties of the liquid phase and review of the low-temperature thermodynamic although the enthalpy of sublimation at 298.15 K properties is given in Table I. is retained as 377 ± 4 kJ mol–1 the normal boiling In the high-temperature solid region closely point is revised to 3272 K at one atmosphere agreeing specific heat values derived from pressure. the enthalpy measurements of Cordfunke and Konings (5) (528 to 848 K) and the direct specific Introduction heat measurements of Miiller and Cezairliyan (6) (1400 to 1800 K) were selected since they were The thermodynamic properties of palladium were in excellent agreement with an extrapolation of reviewed by the author in 1995 (1). At that time the selected low-temperature values and allowed the value for the enthalpy of fusion was considered a single smooth specific heat curve to represent to be tentative since it differed significantly both the low- and high-temperature regions. More from the only known experimental value. Newly recent high-temperature specific heat values by considered enthalpy of fusion values now indicate Milošević and Babić (7) (248 to 1773 K) differ that the original selected value was almost significantly from the selected values showing a certainly incorrect and a more likely value has trend from 1.3% lower at 323 K to 3.9% lower been suggested which leads to a major alteration at 723 K to 2.3% lower at 1223 K to 5.9% lower to the selected values for the liquid phase. at 1773 K. However, at the low-temperature Additional measurements of the vapour pressure change over point at 270 K the measurements do not alter the selected enthalpy of sublimation of Milošević and Babić show a sharp change in value of 377 kJ mol–1 but the accuracy can now the slope of the specific heat curve compared to be refined to give a 95% confidence value of the selected low-temperature values, indicating ±4 kJ mol–1. that these high-temperature measurements are incompatible with those obtained at low Solid temperatures. On these grounds the original high-temperature values were retained and can Selected values in the low-temperature region be represented by Equation (i) to cover the range as given in the previous review (1) were based from 298.15 K to the accepted melting point at mainly on the specific heat values of Boerstoel 1828.0 K:

Table I Low Temperature Thermodynamic Data C° , H° – H° , S° , –G° – H° , –(G° – H° )/T, T, K p T 0 K T T 0 K T 0 K J mol–1 K–1 J mol–1 J mol–1 K–1 J mol–1 J mol–1 K–1 5 0.0594 0.133 0.0512 0.123 0.0246 10 0.196 0.722 0.128 0.554 0.0554 15 0.490 2.355 0.256 1.485 0.0990 20 0.999 5.978 0.461 3.241 0.162 25 1.768 12.78 0.761 6.253 0.250 30 2.794 24.13 1.172 11.04 0.368 35 3.998 40.97 1.689 18.15 0.519 40 5.388 64.40 2.313 28.11 0.703 45 6.809 94.90 3.030 41.43 0.921 50 8.190 132.4 3.819 58.53 1.171 60 10.724 227.3 5.541 105.2 1.753 70 12.921 345.8 7.364 169.7 2.424 80 14.803 484.6 9.215 252.6 3.157 90 16.411 640.9 11.054 353.9 3.933 100 17.784 812.1 12.856 473.5 4.735 110 18.949 995.9 14.607 610.9 5.554 120 19.926 1190 16.299 765.5 6.379 130 20.746 1394 17.927 936.7 7.205 140 21.443 1605 19.491 1124 8.027 150 22.043 1822 20.991 1326 8.842 160 22.561 2046 22.431 1543 9.646 170 23.012 2273 23.813 1775 10.439 180 23.406 2506 25.139 2020 11.219 190 23.752 2741 26.414 2277 11.986 200 24.059 2980 27.641 2548 12.738 210 24.334 3222 28.821 2830 13.476 220 24.582 3467 29.959 3124 14.200 230 24.806 3714 31.057 3429 14.909 240 25.007 3963 32.117 3745 15.604 250 25.189 4214 33.141 4071 16.285 260 25.352 4467 34.133 4408 16.952 270 25.499 4721 35.092 4754 17.607 280 25.629 4977 36.022 5109 18.248 290 25.751 5234 36.923 5474 18.876 298.15 25.845 5444 37.638 5778 19.379

Notes to Table I

C°P is specific heat

H°T – H0 K is enthalpy

S°T is entropy

–G°T – H°0 K and –(G°T – H°0 K)/T are free energy functions

–1 –1 –3 C°p (J mol K ) = 24.0658 + 9.54408 × 10 T (300 to 1825 K) trend from 2.6% lower at 300 K to – 5.31329 × 10–6 T 2 + 2.02516 × 10–9 T 3 5.3% lower at 800 K to an average of 1.5% higher – 57,835.6/ T 2 (i) above 1600 K (Figure 1). Enthalpy measurements of Holzmann (9) (575 to 1177 K) trend from 0.4% Equivalent enthalpy and entropy equations are higher to 1.7% higher whilst those of Jaeger and given in Table II and derived thermodynamic Veenstra (10) (573 to 1772 K) scatter from 0.7% values are given in Table III. lower to 1.4% lower with an average of 1.1% lower In comparison with the selected equations the (Figure 2). More recent enthalpy measurements specific heat values of Vollmer and Kohlhaas (8) of Cagran and Pottlacher (11) (1550 to 1828 K)

Table II Thermodynamic Equations Above 298.15 K

Solid: 298.15 to 1828.0 K

–1 –1 –3 –6 2 –9 3 2 C°p (J mol K ) = 24.0658 + 9.55408 × 10 T – 5.31329 × 10 T + 2.02516 × 10 T – 57,835.6/T

–1 –3 2 –6 3 –10 4 H°T – H°298.15 K (J mol ) = 24.0658 T + 4.77704 × 10 T – 1.77109667 × 10 T + 5.06290 × 10 T + 57,835.6/T – 7750.91

–1 –1 –3 –6 2 –10 3 S°T (J mol K ) = 24.0658 ln (T) + 9.55408 × 10 T – 2.656645 × 10 T + 6.75053333 × 10 T + 28,917.8/ T 2 – 102.4348

Liquid: 1828.0 to 3300 K

–1 –1 C°p (J mol K ) = 41.2000

–1 H°T – H298.15 K (J mol ) = 41.2000 – 10,903.0

–1 –1 S°T (J mol K ) = 41.2000 ln (T) – 208.9240

Notes to Table II

C°P is specific heat

H°T – H298.15 K is enthalpy

S°T is entropy

Table III High Temperature Thermodynamic Data

C° , H° – H° , S° , –(G° – H° )/T, T, K p T 298.15 K T T 298.15 K J mol–1 K–1 J mol–1 J mol–1 K–1 J mol–1 K–1

298.15 25.845 0 37.638 37.638

300 25.866 48 37.798 37.639

400 26.805 2684 45.375 38.665

500 27.536 5402 51.438 40.633

600 28.162 8188 56.515 42.868

700 28.727 11,033 60.899 45.138

800 29.255 13,932 64.770 47.355

900 29.766 16,883 68.245 49.486

1000 30.274 19,885 71.407 51.522

1100 30.794 22,938 74.317 53.464

1200 31.339 26,045 77.019 55.316

1300 31.922 29,207 79.551 57.084

1400 32.555 32,431 81.939 58.774

(Continued)

C° , H° – H° , S° , –(G° – H° )/T, T, K p T 298.15 K T T 298.15 K J mol–1 K–1 J mol–1 J mol–1 K–1 J mol–1 K–1

1500 33.251 35,720 84.209 60.395

1600 34.023 39,083 86.379 61.952

1700 34.882 42,528 88.467 63.450

1800 35.841 46,063 90.487 64.896

1828 (s) 36.129 47,071 91.043 65.293

1828 (l) 41.200 64,411 100.528 65.293

1900 41.200 67,377 102.120 66.658

2000 41.200 71,497 104.232 68.485

2100 41.200 75,617 106.243 70.235

2200 41.200 79,737 108.160 71.916

2300 41.200 83,857 109.991 73.532

2400 41.200 87,977 111.745 75.088

2500 41.200 92,097 113.427 76.588

2600 41.200 96,217 115.043 78.036

2700 41.200 100,337 116.597 79.436

2800 41.200 104,457 118.096 80.790

2900 41.200 108,577 119.542 82.101

3000 41.200 112,697 120.938 83.373

3100 41.200 116,817 122.289 84.606

3200 41.200 120,937 123.597 85.805

3300 41.200 125,057 124.865 86.969

Notes to Table III

C°P is specific heat

H°T – H298.15 K is enthalpy

S°T is entropy

–(G°T – H°295.15 K)/T is the free energy function

2 Fig. 1. Percentage deviations of specific heat in the high- 0 temperature region 300 500 700 900 1100 1300 1500 1700 1900 Temperature, K

–2 Deviation, %

–4

–6

Ref. (7) Ref. (8)

51 © 2018 Johnson Matthey http://dx.doi.org/10.1595/205651318X696648 Johnson Matthey Technol. Rev., 2018, 62, (1) determined using the rapid pulse heating technique for atomic weight, to the melting point of 1828.0 K trend from 1.4% to 3.7% lower in the solid range and to the International Temperature Scale of (Figure 3). 1990 (ITS-90), the liquid enthalpy at the melting –1 point is H°T – H°298.15 K = 63,151 J mol which in Liquid combination with the enthalpy of the solid at the melting point leads to an enthalpy of fusion of Drop calorimetry enthalpy measurements by 16.08 ± 0.74 kJ mol–1. This is notably lower than Treverton and Margrave (12) were initially actual experimental values of 17.64 kJ mol–1 determined over the range 1846 to 2334 K on determined by Nedumov (13) using differential the International Practical Temperature Scale thermal analysis and rapid pulse heating values of 1948 (IPTS-48) but a short note added in the of 17.4 ± 2.0 kJ mol–1 determined by Seydel et al. proof indicated further corrections for radiation (14, 15) and 16.98 kJ mol–1 determined by Cagran losses and a conversion to the temperature scale and Pottlacher (11). The reason why the indirect IPTS-68. However only the amended liquid specific determination of the enthalpy of fusion was low is not heat value and the liquid enthalpy at the melting known but may be due to either an overestimation point (given as 1827 K) were given. After correction of the enthalpy values for the solid or, more likely,

2 Fig. 2. Percentage deviations of enthalpy in the high- 1 temperature region

0 500 700 900 1100 1300 1500 1700 1900

Deviation, % Temperature, K

–1

–2

Ref. (9) Ref. (10)

Temperature, K 1500 1700 1900 2100 2300 2500 2700 2900 Fig. 3. Percentage 0 deviations of enthalpy in the high- –1 temperature region

–2

–3

–4 Deviation, %

–5

–6

Ref. (11) Melting point

52 © 2018 Johnson Matthey http://dx.doi.org/10.1595/205651318X696648 Johnson Matthey Technol. Rev., 2018, 62, (1) to the fact that even the revised values of Treverton are due to a constant systematic error. Over the and Margrave had still not accounted for all of the range 1828 to 3300 K the actual enthalpy can be systematic errors that may have been present in their expressed as (Equation (ii)): measurements. Not including the measurement –1 H°T – H°298.15 K (J mol ) = 41.2000 T – 10,903.0 of Seydel et al. because of the large uncertainty, (ii) the two other enthalpy values were equivalent to entropy of fusion values of 9.65 J mol–1 K–1 and Equivalent specific heat and entropy equations 9.29 J mol–1 K–1 respectively. Kats and Chekhovskoi are also given in Table II whilst derived (16) noted that for a particular structure type the thermodynamic properties are given in Table III. entropies of fusion (ΔSM) could be linearly related The more recent liquid enthalpy measurements of to the melting points (Tm) by means of the equation Cagran and Pottlacher (11) trend from 3.3% lower

ΔSM = a Tm + b where a and b are constants. For at 1828 K to 5.8% lower at 2900 K. the face-centred cubic platinum group metals rhodium, iridium and platinum selected entropy Gas of fusion values of 12.21 ± 0.38 J mol–1 K–1 at a melting point of 2236 K for rhodium (17), Selected values are based on the 143 energy 15.20 ± 0.41 J mol–1 K–1 at a melting point of 2719 levels selected by Engleman et al. (23). The K for iridium (18) and 10.83 ± 0.46 J mol–1 K–1 thermodynamic properties were calculated using at a melting point of 2041.3 K for platinum (19) the method of Kolsky et al. (24) and the 2014 when fitted to the above equation extrapolated Fundamental Constants selected by Mohr et al. to 9.52 J mol–1 K–1 for palladium, in excellent (25, 26). Derived thermodynamic values based agreement with the above experimental entropy on a one bar standard state pressure are given in values. The two experimental entropy values Table IV. for palladium were therefore combined with the values for rhodium, iridium and platinum and were –3 Enthalpy of Sublimation at 298.15 K fitted to the equation:M ΔS = 6.4574 × 10 Tm – 2.3211 with the derived value for palladium as Because of a general lack of detail as to what 9.483 ± 0.40 J mol–1 K–1 where the error is assigned temperature scales were used and problems to match those obtained for the other elements. associated with the exact measurement of The derived enthalpy of fusion value is rounded to temperature, no attempt was made to correct 17.34 ± 0.73 kJ mol–1 and leads to an enthalpy for vapour pressure measurements to ITS-90 from the liquid at the melting point of 64,411 J mol–1, what would have been contemporary scales. suggesting that even with corrections applied the For results given in the form of the Clausius- enthalpy measurements of Treverton and Margrave Clapeyron equation, log(p) = A + B/T (where p is were 2.0% too low. This confirms the suggestion pressure and T is temperature), the enthalpy of that other liquid enthalpy measurements obtained sublimation was calculated at the two temperature by Treverton and Margrave also appear to be extremes and averaged. For the measurements of systematically low with, for example, the values Walker et al. (27), Lindscheid and Lange (28) and obtained for vanadium (20) being on average 2.4% Chegodaev et al. (29) no temperature ranges were lower than the preferred values of Berezin et al. given and therefore these measurements were (21) and Lin and Frohberg (22). not included. From Table V, in view of possible The liquid specific heat determined by Treverton systematic errors in the earlier measurements, and Margrave (12) on IPTS-68 corrected to the only the twelve determinations from Taberko et al. value 41.20 ± 1.38 J mol–1 K–1 on ITS-90 which (42, 43) to Ferguson et al. (51) were considered. is notably higher than the value for the liquid The unweighted average value of 377 kJ mol–1 of 37.3 J mol–1 K–1 determined by Cagran and is assigned an accuracy of ±4 kJ mol–1 which is Pottlacher (11) (1828 to 2900 K) using rapid pulse equivalent to a 95% confidence level (two standard heating. However, it is noted that Cagran and deviations). Pottlacher obtained a specific heat value for the solid which is some 10% to 16% lower than the Vapour Pressure Equations selected values. Therefore, the value of Treverton and Margrave was selected on the assumption that The vapour pressure equations as given in Table VI the apparently lower enthalpy values obtained were evaluated for the solid from free energy

Table IV Thermodynamic Properties of the Gaseous Phase C° , H° – H° , S° , –(G° – H° )/T, T , K p T 298.15 K T T 298.15 K J mol–1 K–1 J mol–1 J mol–1 K–1 J mol–1 K–1 298.15 20.786 0 167.066 167.066 300 20.786 38 167.194 167.066 400 20.786 2117 173.174 167.881 500 20.786 4196 177.812 169.421 600 20.788 6274 181.602 171.145 700 20.802 8354 184.807 172.874 800 20.854 10,436 187.588 174.543 900 20.991 12,527 190.051 176.132 1000 21.273 14,639 192.276 177.637 1100 21.763 16,789 194.324 179.062 1200 22.508 19,000 196.248 180.414 1300 23.537 21,300 198.088 181.704 1400 24.850 23,717 199.878 182.938 1500 26.424 26,279 201.646 184.126 1600 28.213 29,009 203.407 185.276 1700 30.152 31,926 205.175 186.395 1800 32.167 35,042 206.955 187.488 1828 32.734 35,951 207.456 187.790 1900 34.179 38,360 208.749 188.559 2000 36.115 41,875 210.551 189.614 2100 37.908 45,578 212.358 190.654 2200 39.505 49,450 214.159 191.681 2300 40.869 53,471 215.946 192.698 2400 41.977 57,615 217.709 193.703 2500 42.823 61,858 219.441 194.698 2600 43.411 66,171 221.133 195.682 2700 43.757 70,532 222.779 196.656 2800 43.883 74,916 224.373 197.617 2900 43.815 79,302 225.912 198.567 3000 43.582 83,673 227.394 199.503 3100 43.213 88,014 228.817 200.426 3200 42.734 92,312 230.182 201.334 3300 42.172 96,558 231.489 202.229

Notes to Table IV

C°P is specific heat

H°T – H298.15 K is enthalpy

S°T is entropy

–(G°T – H°295.15 K)/T is the free energy function

–1 H°298.15 K – H°0 K = 6197.4 J mol

Table V Enthalpies of Sublimation at 298.15 K

ΔH° ΔH° Temperature 298.15 K 298.15 K Authors Ref. Method (II), (III), Notes range, K kJ mol–1 kJ mol–1

Babeliowsky (30) MS 1250–1730 385 ± 4 – (a)

Trulson and Schissel (31) MS 1370–1785 382 ± 5 –

Haefling and Daane (32) KE 1388–1675 332 ± 7 352.9 ± 0.5

Alcock and Hooper (33) Trans 1673–1773 459 376.1 ± 2.4

Zanitsanov (34) KE 1537–1841 368 ± 22 375.9 ± 1.2 (b)

Dreger and Margrave (35) L 1220–1640 362 ± 11 380.6 ± 1.0

Hampson and Walker (36) L 1294–1488 365 ± 4 373.4 ± 0.2

Norman et al. (37) KEMS 1485–1710 381 380.5 ± 0.1 (c)

Strassmair and Stark (38) L 1361–1603 372 ± 14 373.3 ± 0.6

Myles (39) TE 1515–1605 372 372.0 ± 0.1 (c)

Darby and Myles (40) TE 1517–1608 372 371.8 ± 0.1 (c)

Novosolov et al. (41) TE 1730–1938 363 370.0 ± 0.4 (c)

(42, Taberko et al. Evap 1828–2023 381 ±10 376.9 ± 0.3 43)

Zaitsev et al. (44) KE 1267–1598 377 ± 1 377.3 ± 0.1

1511–1678 (s) 389 ± 4 376.6 ± 0.2 Bodrov et al. (45) AA 1842–2046 (l) 394 ±12 373.1 ± 0.5

Naito et al. (46) KEMS 1567–1758 386 375.5 ± 0.5 (c)

Chandrasekharaiah (47) KEMS 1439–1724 373 ± 5 377.9 ± 0.2 (d) et al.

Stlen et al. (48) KEMS 1523–1743 389 375.5 ± 0.5 (c)

1627–1818 (s) 378 ± 7 377.7 ± 0.2 (d) Bharadwaj et al. (49) KEMS 1833–2041 (l) 380 ± 8 376.7 ± 0.2 (d)

Kulkarni et al. (50) KEMS 1237–1826 375 ± 2 381.7 ± 0.3 (e)

1473–1825 (s) 373 ± 7 377.8 ± 0.4 Ferguson et al. (51) KE 1840–1973 (l) 385 ± 7 377.4 ± 0.2

Selected 377 ± 4

Notes to Table V

ΔH°298.15 K (II) and ΔH°298.15 K (III) are the Second Law and Third Law enthalpies of sublimation at 298.15 K (a) Value given only at 298.15 K (b) Two runs combined since individual Second Law values were 433 kJ mol–1 and 312 kJ mol–1 respectively (c) Values given only in terms of the Clausius-Clapeyron equation (d) Average of two runs (e) Average of five runs Methods AA: atomic absorption Evap: evaporation KE: Knudsen effusion KEMS: Knudsen effusion mass spectrometry L: Langmuir free evaporation MS: mass spectrometry TE: torsion effusion Trans: transpiration

Table VI Vapour Pressure Equations

Temperature Phase A B C D E range, K

–1.26392 × 2.03201 × Solid 900–1828 14.71536 0.220029 –45,349.16 10–3 10–7

3.94374 × –2.33142 Liquid 1828–3300 92.57304 –10.78530 –51,305.62 10–3 × 10–7

Table VII Free Energy Equations Above 298.15 K

Solid: 298.15 to 1828.0 K

–1 –3 2 –7 3 G°T – H°298.15 K (J mol ) = 126.5006 T – 4.77704 × 10 T + 8.85548333 × 10 T – 1.68763333 × 10–10 T 4 + 28,917.8/ T – 24.0658 T ln (T) – 7750.91

Liquid: 1828.0 to 3300 K

–1 G°T – H°298.15 K (J mol ) = 250.1240 T – 41.2000 T ln (T) – 10,903.0

Note to Table VII

G°T – H°298.15 K is the free energy function

Table VIII Transition Values Involved with the Free Energy Equations

–1 –1 –1 Transition Temperature, K ΔHM , J mol ΔSM , J mol K Fusion 1828.0 17,340.00 9.4858

Table IX Vapour Pressure

–1 –1 T, K p, bar ΔG°T , J mol ΔH°T , J mol p, bar T, K 298.15 5.16 × 10–60 338,411 377,000 10–15 911

300 1.32 × 10–59 338,172 376,991 10–14 956

400 3.31 × 10–43 325,314 376,433 10–13 1005

500 2.20 × 10–33 312,606 375,794 1012 1060

600 7.59 × 10–27 300,034 375,087 10–11 1122

700 3.47 × 10–22 287,585 374,321 10–10 1191

1269 800 1.07 × 10–18 275,250 373,504 10–9

900 5.43 × 10–16 263,019 372,644 10–8 1359

1000 7.86 × 10–14 250,886 371,754 10–7 1462

1100 4.56 × 10–12 238,843 370,851 10–6 1582

1725 1200 1.33 × 10–10 226,882 369,956 10–5

1899 1300 2.30 × 10–9 214,994 369,093 10–4

1400 2.63 × 10–8 203,171 368,286 10–3 2121

1500 2.16 × 10–7 191,403 367,558 10–2 2403

2770 1600 1.36 × 10–6 179,680 366,926 10–1

(Continued)

–1 –1 T, K p, bar ΔG°T , J mol ΔH°T , J mol p, bar T, K 1700 6.89 × 10–6 167,994 366,399 1 3268.52

1800 2.91 × 10–5 156,336 365,979 NBP 3271.88

1828 (s) 4.23 × 10–5 153,076 365,880

1828 (l) 4.23 × 10–5 153,076 348,540

1900 1.01 × 10–4 145,388 347,983

2000 3.03 × 10–4 134,742 347,378

2100 8.17 × 10–4 124,121 346,961

2200 2.02 × 10–3 113,516 346,713

2300 4.60 × 10–3 102,919 346,614

2400 9.79 × 10–3 92,323 346,638

2500 1.96 × 10–2 81,725 346,761

2600 3.73 × 10–2 71,120 346,954

2700 6.75 × 10–2 60,506 347,195

2800 0.117 49,883 347,459

2900 0.196 39,251 347,745

3000 0.318 28,609 347,976

3100 0.498 17,960 348,197

3200 0.760 7304 348,375

3268.52 1.000 0 348,467

3300 1.130 –3356 348,501

Notes to Table IX

ΔG°T is the free energy of formation at one bar standard state pressure and temperature T and ΔH°T is the enthalpy of sublimation at temperature T

–1 Enthalpy of sublimation at 0 K: ΔH°0 = 376.247 ± 4.000 kJ mol NBP is the normal boiling point at one atmosphere pressure (1.01325 bar)

functions for the solid and the gas at 50 K intervals References from 900 K to 1800 K and the melting point and 1. J. W. Arblaster, Calphad, 1995, 19, (3), 327 for the liquid at the melting point and at 50 K intervals from 1850 to 3300 K and were fitted to 2. B. M. Boerstoel, J. J. Zwart and J. Hansen, Physica, 1971, 54, (3), 442 the following equation (Equation (iii)): 3. K. Clusius and L. Schachinger, Z. Naturforsch. A, 2 ln (p, bar) = A + B ln (T) + C/T + D T + E T 1947, 2a, (2), 90 (iii) 4. P. Mitacek and J. G. Aston, J. Am. Chem. Soc., 1963, 85, (2), 137 Acknowledgement 5. E. H. P. Cordfunke and R. J. M. Konings, Thermochim. Acta, 1989, 139, 99 The author is indebted to Venkatarama Venugopal, 6. A. P. Miiller and A. Cezairliyan, Int. J. Thermophys., Bhabha Atomic Research Centre, India, for 1980, 1, (2), 217 supplying the vapour pressure data corresponding 7. N. Milošević and M. Babić, Int. J. Mater. Res., to the measurements of Kulkarni et al. (50). 2013, 104, (5), 462

8. O. Vollmer and R. Kohlhaas, Z. Naturforsch. A, 31. O. C. Trulson and P. O. Schissel, J. Less Common 1969, 24, (10), 1669 Metals, 1965, 8, (4), 262 9. H. Holzmann, ‘Sieberts Festschrift zum 50 Jahr 32. J. F. Haefling and A. H. Daane, Trans. Met. Soc. Bestehen der Platinschmelze’, 1931, p. 149 AIME, 1958, 212, (1), 115 10. F. M. Jaeger and W. A. Veenstra, Proc. R. Acad. 33. C. B. Alcock and G. W. Hooper, Proc. R. Soc. A, Amsterdam, 1934, 37, (5), 280 1960, 254, (1279), 551 11. C. Cagran and G. Pottlacher, Platinum Metals Rev., 34. P. D. Zavitsanos, J. Phys. Chem., 1964, 68, (10), 2006, 50, (3), 144 2899 12. J. A. Treverton and J. L. Margrave, J. Phys. Chem., 35. L. H. Dreger and J. L. Margrave, J. Phys. Chem., 1971, 75, (24), 3737 1960, 64, (9), 1323 13. N. A. Nedumov, “Differential Thermal Analysis: 36. R. F. Hampson and R. F. Walker, J. Res. Nat. Bur. Fundamental Aspects”, ed. R. C. Mackenzie, Vol. Stand.: A. Phys. Chem., 1962, 66A, (2), 177 1, Academic Press, London, UK, 1970, p. 161 37. J. H. Norman, H. G. Staley and W. E. Bell, J. Phys. 14. U. Seydel and U. Fischer, J. Phys. F: Met. Phys., Chem., 1965, 69, (4), 1373 1978, 8, (7), 1397 38. H. Strassmair and D. Stark, Z. Angew. Phys., 15. U. Seydel, H. Bauhof, W. Fucke and H. Wadle, High 1967, 23, (1), 40 Temp.-High Press., 1979, 11, (6), 635 39. K. M. Myles, Trans. Met. Soc. AIME, 1968, 242, 16. S. A. Kats and V. Y. Chekhovskoi, High Temp.-High (8), 1523 Press., 1979, 11, (6), 629 40. J. B. Darby and K. M. Myles, Met. Trans., 1972, 17. J. W. Arblaster, Calphad, 1995, 19, (3), 357 3, (3), 653 18. J. W. Arblaster, Calphad, 1995, 19, (3), 365 41. B. M. Novoselov, E. L. Dubinin and A. I. Timofeev, 19. J. W. Arblaster, Platinum Metals Rev. 2005, 49, Izv. Vyssh. Uchebn. Zaved. Tsvetn. Metall., 1978, (3), 141 (6), 41 20. J. A. Treverton and J. L. Margrave, J. Chem. 42. A. V. Taberko, S. E. Vaisburd and L. Sh. Thermodyn., 1971, 3, (4), 473 Tsemekhman, Zh. Fiz. Khim., 1977, 51, (1), 273; 21. B. Ya. Berezin, V. Ya. Chekhovskoy and A. E. Transl. Russ. J. Phys. Chem., 1977, 51, (1), 164 Sheindlin, High Temp. Sci., 1972, 4, (6), 478 43. S. E. Vaisburd, L. Sh. Tsemekhman, A. V. Taberko 22. R. Lin and M. G. Frohberg, Z. Metallkd., 1991, 82, and Yu. A. Karasev, in “Protsessy Tsvetn. Metall. (1), 48 Nizk. Davleniyakh”, ed. A. I. Manokhin, Izd. Nauka, Moscow, Russia, 1983, p. 120 23. R. Engleman, U. Litzén, H. Lundberg and J.-F. Wyart, Phys. Scr., 1998, 57, (3), 345 44. A. I. Zaitsev, Yu. A. Priselkov and A. N. Nesmeyanov, Teplofiz. Vys. Temp., 1982, 20, (3), 589 24. H. G. Kolsky, R. M. Gilmer and P. W. Gilles, “The Thermodynamic Properties of 54 Elements 45. N. V. Bodrov, G. I. Nikolaev and A. M. Nemets, Zh. Considered as Ideal Monatomic Gases”, Los Prikl. Spektrosk., 1985, 43, (4), 535; Transl. J. Alamos Scientific Laboratory Report No. LA–2110, Appl. Spectrosc., 1985, 43, (4), 1063 New Mexico, USA, 20th December, 1956 46. K. Naito, T. Tsuji, T. Matsui and A. Date, J. Nucl. 25. P. J. Mohr, D. B. Newell and B. N. Taylor, Rev. Mod. Mater., 1988, 154, (1), 3 Phys., 2016, 88, (3), 035009 47. M. S. Chandrasekharaiah, M. J. Stickney and K. 26. P. J. Mohr, D. B. Newell and B. N. Taylor, J. Phys. A. Gingerich, J. Less Common Metals, 1988, 142, Chem. Ref. Data, 2016, 45, (4), 043102 329 27. R. F. Walker, J. Efimenko and N. L. Lofgren,Planet. 48. S. Stlen, T. Matsui and K. Naito, J. Nucl. Mater., Space Sci., 1961, 3, 24 1990, 173, (1), 48 28. H. Lindscheid and K. W. Lange, Z. Metallkd., 1975, 49. S. R. Bharadwaj, A. S. Kerkar, S. N. Tripathi and 66, (9), 546 R. Kameswaran, J. Chem. Thermodyn., 1990, 22, 29. A. I. Chegodaev, E. L. Dubinin, A. I. Timofeev, (5), 453 N. A. Vatolin and V. I. Kapitanov, Zh. Fiz. Khim., 50. S. G. Kulkarni, C. S. Subbanna, V. Venugopal, D. 1978, 52, (8), 2124; Transl. Russ. J. Phys. Chem., D. Sood and S. Venkateswaran, J. Less Common 1978, 52, (8), 1229 Metals, 1990, 160, (1), 133 30. T. P. J. H. Babeliowsky, Physica, 1962, 28, (11), 51. F. T. Ferguson, K. G. Gardner and J. A. Nuth III, J. 1160 Chem. Eng. Data, 2006, 51, (5), 1509

The Author

John W. Arblaster is interested in the history of science and the evaluation of the thermodynamic and crystallographic properties of the elements. Now retired, he previously worked as a metallurgical chemist in a number of commercial laboratories and was involved in the analysis of a wide range of ferrous and non-ferrous alloys.

www.technology.matthey.com

A Facile Green Tea Assisted Synthesis of Palladium Nanoparticles Using Recovered Palladium from Spent Palladium Impregnated Carbon Biosynthesising palladium nanoparticles using green tea as a reducing agent

Ansari Palliyarayil, Kizhakoottu among the three chosen acid or acid mixtures Kunjunny Jayakumar, Sanchita Sil* with an average absolute yield of 96%. Finally, an and Nallaperumal Shunmuga Kumar attempt was made towards one pot biosynthesis Defence Bioengineering and Electromedical of Pd NPs from the recovered extract by using Laboratory (DEBEL), Defence Research and green tea as a reducing agent. The synthesised Development Organization (DRDO), C. V. NPs were characterised using UV-vis spectroscopy, Raman Nagar, Bangalore-560 093, India SEM and XRD.

*Email: [email protected] Introduction

Activated carbon is a versatile adsorbent Palladium impregnated activated carbon (Pd/C) possessing high surface area, microporous filters play a major role in air quality management structure and a high degree of surface reactivity. by the removal of toxic carbon monoxide from As an adsorbent, activated carbon has found confined environments. However, Pd is an application in economic sectors as diverse as expensive metal and therefore, recovery and the food, pharmaceutical, chemical, petroleum, reuse of Pd from spent filter cartridges is highly nuclear, automobile and vacuum industries as well desirable. The objective of the present study was as for the treatment of drinking water, industrial to biosynthesise Pd nanoparticles (NPs) using and urban wastewater and industrial flue gases. green tea as a reducing agent. The source of Pd for According to the European Council of Chemical the NP synthesis was spent Pd/C. Three different Manufacturers’ Federation (European Chemical acid based Pd extraction protocols constituting of Industry Council or Cefic), it is defined as hydrochloric acid-hydrogen peroxide (HCl-H2O2), non-hazardous, processed, carbonaceous material 2 M HCl and aqua regia were systematically having a porous structure and a large internal explored. The Pd impregnated carbon was surface area (1). It is known as a ‘universal characterised using scanning electron microscopy adsorbent’ due to its efficiency and has been (SEM), energy dispersive X-ray spectrometry employed for the adsorptive removal of impurities (EDS), ultraviolet-visible (UV-vis) spectroscopy, from exhaust gas and wastewater systems (2–7). X-ray powder diffraction (XRD) and atomic One of the ways to improve the efficiency of absorption spectrometry (AAS) before and after activated carbon is by impregnation of metals Pd extraction. It was found that the aqua regia (8). Impregnated activated carbon has been based extraction protocol was the most efficient extensively used for gas purification, catalysis,

60 © 2018 Johnson Matthey https://doi.org/10.1595/205651317X696252 Johnson Matthey Technol. Rev., 2018, 62, (1) civil and military gas protection (9–15). It has In the present work, green tea extract was used also been successfully used in air cleaning filters as a reducing agent for the biosynthesis of Pd in confined environments for the removal of toxic NPs. Green tea, an infusion of Camellia sinensis pollutants. These filters act as the ‘lungs’ ofa leaves, is rich in polyphenols which are important closed system that ensure air purification, thereby biologically active components having antioxidative, maintaining a level of toxic pollutants under the antimutagenic and anticarcinogenic effects (30– permissible limits. Pd is one of the metals which 32). Chemically, green tea is a complex mixture is impregnated on carbon in air cleaning filters. It of polyphenols including flavonoids, caffeine, acts as a catalyst by oxidative removal of CO as amino acids, organic acids, proteins, volatiles carbon dioxide, as shown in Equation (i): (low molecular weight aldehydes and alcohols), Pd minerals like sodium, potassium and calcium and CO + ½O2 → CO2 (i) trace elements (aluminium, manganese, selenium Pd as a catalyst is present in the range 4–8% and iron) (30–46). Flavanols and flavonols are the by weight of total carbon (16, 17). Once the two major classes of polyphenols present in green filter becomes saturated, it must be replaced. tea (approximately 16–30% of the dry weight of However, this is a cost intensive process and the fresh leaf) (30, 33). The phenolic acid-type recovery of Pd is highly desirable so that it can be biomolecules which are present in the green tea recycled and used for subsequent impregnation. infusion play a major role in the conversion of Additionally, metal-containing wastes such as metal ions into metal NPs by reduction. During the spent filters cannot be disposed of directly due NP formation, these biomolecules form complexes to environmental concerns (18). According to a with metal ions present in the solution and review by Butler, in the year 2011 alone, about reduce them into corresponding metals through 22% of the Pd market consisted of recovery from an electron transfer mechanism. The catechins autocatalysts and jewellery scrap (19). (flavan-3-ols), which are colourless, astringent and Many processes have been reported in the water soluble compounds, are characterised by the literature for extraction of Pd by using different meta-5,7-dihydroxy substitution of the A-ring and solvents such as ortho-8-hydroxyquinoline, HCl, di- or trihydroxy substitution of the B-ring as shown a HCl-H2O2 mixture and aqua regia (20–23). The in Figure 1(a) and are the predominant species objective of this study was to identify a simple, which are easily oxidisable. These are responsible cost effective and scalable technique for Pd for the biological activity of green tea. Green tea recovery that allows reuse of the recovered metal catechins are composed of eight catechin monomers for various applications and can be easily adopted (30, 34, 45–49), the structures of which are given in by industry. In the present work, a systematic Figure 1. The polyphenol profile of green tea along study for the recovery of Pd from Pd impregnated with their percentage composition is tabulated in (6.5%) activated carbon was conducted using the Supplementary Information (SI1). three different extraction routes. The first method The Pd NPs biosynthesised from the recovered comprised of digestion with 2 M HCl, the second extract using green tea as the reducing agent consisted of digestion with a HCl-H2O2 mixture while were characterised using UV-vis spectroscopy, the third used aqua regia for digestion. The details SEM and XRD. of the processes are provided schematically in the Supplementary Information (SI) accompanying Materials and Methods this article online. Additionally, it is important to reuse the extracted Chemicals metal for various applications. As an extension of this study, biosynthesis of Pd NPs using a Spent Pd impregnated activated carbon (Pd/C) ‘green’ route was also conducted. It is known was received from Active Char Pvt Ltd, India. that metal NPs show unique properties different Analytical grade HCl, sodium hydroxide (NaOH), from their bulk counterparts and have therefore H2O2, sodium borohydride (NaBH4) and nitric acid found applications in the areas of optoelectronics, (HNO3) were purchased from Merck Specialities catalysis, photothermal therapy, surface enhanced Pvt Ltd, India. Ethanol was purchased from Raman spectroscopy (SERS) detection and Changshu Yangyuan Chemical Co Ltd, China. All biological labelling (24). Recently, the syntheses of these chemicals were used as received without metal NPs by following green chemistry principles further purification. All aqueous solutions were have attracted a great deal of attention (25–29). prepared in distilled water.

(a) Catechin (b) Epicatechin (c) Gallocatechin (d) Epigallocatechin

OH OH OH OH OH OH OH OH B HO O HO O HO O HO O OH OH A C OH OH OH OH OH OH OH OH Empirical formula: Empirical formula: Empirical formula: Empirical formula:

C15H14O16 (MW 290.27) C15H14O6 (MW 290.27) C15H14O7 (MW 306.27) C15H14O7 (MW 306.27)

(e) Catechin (f) Epicatechin (g) Gallocatechin (h) Epigallocatechin gallate gallate gallate gallate OH OH OH OH OH OH OH OH

HO O HO O HO O HO O OH OH O O O O OH OH OH OH OH OH OH O O O OH O OH OH OH OH OH OH OH OH Empirical formula: Empirical formula: Empirical formula: Empirical formula:

C22H18O10 (MW 442.37) C22H18O10 (MW 442.37) C22H18O11 (MW 458.37) C22H18O11 (MW 458.37)

Fig. 1. Two-dimensional (2D) and three-dimensional (3D) structure of various catechin monomers present in green tea

Instrumental Details during palladium(II) chloride (PdCl2) precipitation. Scanning between the wavelengths 200–900 nm The pH of the spent Pd/C was measured using was performed to measure the absorbance values a CyberScan pH 510, Eutech Instruments Pte using the Lambda 25 software. Quantitative Ltd, Singapore. SEM studies for carbon samples analysis was carried out with the help of an iCETM were carried out with a Quanta 200 ESEM, Icon 3500 AAS, Thermo Fisher Scientific, USA, in Analytical Equipment Pvt Ltd, India, and EDS with air-acetylene mode using the SOLAARTM software. a Genesis XM4, Icon Analytical. Field emission scanning electron microscope (FESEM) studies Characterisation of Palladium of the Pd NPs were performed using an Ultra-55, Impregnated Activated Carbon Carl Zeiss AG, Germany. The scanning was carried out using an in-lens secondary electron detector, All the physical and chemical properties of the the voltage was maintained at 2.0 kV and 5.0 kV, spent catalyst (Pd/C) were studied as per the and the working distance was kept at ~10 mm Indian standards IS 877:1989 and IS 2752:1995 and 5 mm respectively. The XRD analysis was (reaffirmed in 2016). For each analysis, moisture carried out with an X’Pert PRO diffractometer, free samples were used. PANalytical, Spectris Plc, The Netherlands, using CuKα radiation of wavelength l = 1.54 Å with Palladium Recovery 40 kV generator voltage and 30 mA tube current. The 2q value for the analysis was selected All the extraction studies were carried out with between 5° and 90°. A BELSORP-mini II surface moisture free Pd/C. The spent Pd/C was oven dried area analyser, MicrotracBEL Corp, Japan, was at 110°C for 4 h. To check the repeatability of the used to measure the surface area by a nitrogen results, the extraction studies were carried out in adsorption method at –196°C. A LambdaTM 25 duplicate. UV-vis spectrophotometer, PerkinElmer Inc, USA, In the first method, recovery of Pd from Pd/C was was used to carry out the study of pH variation achieved using a HCl-H2O2 (10% HCl and 5% H2O2)

62 © 2018 Johnson Matthey https://doi.org/10.1595/205651317X696252 Johnson Matthey Technol. Rev., 2018, 62, (1) mixture. The details of the process are provided in Calcination, O2 the Supplementary Information (SI2). The reaction 2Pd/C 2PdO + C (iv) occurs as shown in Equations (ii) and (iii) (23):

PdO + 4HCl → H2PdCl4 + H2O (v) Pd/C + 4HCl + H2O2 → H2PdCl4 + C + 2H2O (ii)

H2PdCl4 + 2NaOH → PdCl2 + 2NaCl + 2H2O (vi) 4H2PdCl4 + NaBH4 + 2H2O → 0 4Pd + NaBO2 + 16HCl (iii) In the final approach, Pd was extracted from Pd/C The obtained filtrate was made up to a definite using aqua regia. The details of the process are volume and analysed for Pd content using AAS. provided in the Supplementary Information (SI2). Reduction of the Pd extract was performed by The reaction proceeds according to Equations (vii) slow addition of 1% NaBH4 to the filtrate under and (viii) respectively. The filtrate was made up to magnetic stirring and the reaction temperature a definite volume and analysed for Pd using AAS. was maintained at 95–100°C (Equation (iii)) (23). Part of the filtrate was used to remove nitrate ions The completion of the reduction process was and the residue was redissolved in acid (20% HCl) monitored with the help of UV-vis spectroscopy. for precipitation of Pd as PdCl2 as described in The precipitated Pd was filtered out, oven dried at Equation (vi). 110°C and was analysed quantitatively by AAS. Pd/C + 3HCl + HNO3 → In the second approach the extraction of Pd from PdCl2 + C + NOCl + 2H2O (vii) Pd/C was achieved using 2 M HCl. The details of the process are provided in the Supplementary PdCl2 + 2HCl → H2PdCl4 (viii) Information (SI2). The filtrate was analysed for Pd content using AAS after making up to a definite The precipitated Pd was then quantitatively volume. The pH of the extract was raised by using analysed by AAS.

NaOH solution to precipitate Pd as PdCl2. The precipitation of PdCl2 usually occurs at pH 9.5 (20). Synthesis of Palladium Nanoparticles Initially the pH of the extract was raised to pH 6 to check for precipitation of any impurities such as Green tea extract used for the synthesis of Pd NPs chlorides of iron or other metals. No precipitation was prepared by heating 0.25 g of tea leaves (Tetley was observed in this range. The pH of the extract green tea from TATA global beverages) with 50 ml was then further raised up to pH 11 to ensure the of distilled water at 80–85°C for 2 min followed by completion of the precipitation of Pd as PdCl2. The filtration. Since the amount of Pd recovered using precipitated PdCl2 was filtered by using a previously the HCl-H2O2 method was less compared to the weighed G-3 crucible. The precipitate was then other two methods (see Table I), only extracts oven dried at 110°C and quantitatively analysed from the 2 M HCl and aqua regia methods were by AAS. Equations (iv), (v) and (vi) describe the used for the synthesis of NPs. In this case, 40 ml chemical reactions: of the Pd extract (pH adjusted to pH 5.5–6) was

Table I Recovery of Palladium Estimated by Atomic Absorption Spectrometry Measurement using Different Extraction Media

Average Standard deviation Sample 1, Sample 2, Method recovery, with respect to % % % absolute recovery

Extract 58.68 59.09 58.89 HCl-H2O2 0.5939 Absolute recovery 55.96 56.80 56.38

Extract 75.01 73.56 74.29 2 M HCl 0.6576 Absolute recovery 73.65 72.72 73.19

Extract 93.95 98.73 96.34 Aqua regia 3.4577 Absolute recovery 93.84 98.73 96.29

63 © 2018 Johnson Matthey https://doi.org/10.1595/205651317X696252 Johnson Matthey Technol. Rev., 2018, 62, (1) heated with 2 ml of green tea extract at 90–95°C ash may also interfere with the carbon adsorption for 15–45 min with continuous stirring at 400 rpm. by competitive adsorption and catalysis (51). The In the case of aqua regia extract, the extract was obtained ash content for the material used in the evaporated to dryness for the removal of nitrate ion. present study was 8.62% as it also contains Pd. The obtained residue was dissolved in a minimum Since the pH values of most commercial carbons quantity of 20% HCl followed by addition of 40 ml are produced by their inorganic components, the distilled water and this solution was used for NP ash content may affect the pH of the carbon. synthesis. The reaction was carried out in the dark The acidic or basic nature of an activated carbon to prevent the effect of light. The colour change of depends on its preparation and inorganic matter the solution from light to dark brown indicated the content along with the chemically active oxygen formation of Pd NPs. The formation of Pd NPs was groups on its surface. In addition, the kind of further monitored by using UV-vis spectroscopy. chemical treatment to which the activated carbon The synthesised Pd NPs were then washed with is subjected would also determine the pH. The pH deionised water by centrifugation at 10,000 rpm of the Pd/C used in this study was slightly alkaline for 10 minutes. The process was repeated three suggesting that the carbon surface was essentially times followed by dispersion in ethanol and the negatively charged (1). resulting dispersion was used for characterisation The iodine number provides an idea of the studies. The morphology of the biosynthesised Pd micropore content of the activated carbon by NPs was obtained by using FESEM. The Pd NPs adsorbing iodine from solution. Additionally, it also were also studied using XRD. provides qualitative information about the surface area (52, 53). From the obtained iodine number –1 Results and Discussion in the present study (1463.4 mg g ), it can be concluded that the surface area of the Pd/C carbon Palladium on Carbon is ≥1100 m2 g–1 which is also confirmed by the BET surface area measurement value (1550.5 m2 g–1). The results of the characterisation of Pd/C are Adsorption capacity, an important property of an compiled in Table II. adsorbent material, was calculated with respect to The Pd/C was observed to have 11.53% moisture decolourising power. The high value of decolourising content. Although moisture content does not have power (365.47 mg) indicates that the Pd/C is a any detrimental effect on the adsorption efficiency good adsorbent. Since methylene blue comes under of carbon, it essentially dilutes the carbon content the category of a cationic dye, a higher value for which necessitates additional weight compensation methylene blue adsorption indicates the presence during the filtering process (50). One of the quality of more negative charges on the Pd/C. This can be indicators for activated carbon is the ash content further corroborated from the pH value which is which is defined as the residue that remains slightly in the alkaline range (54, 55). when the carbonaceous portion is burnt off. Ash SEM and EDS studies were carried out for the essentially consists of minerals such as silica, Pd/C before and after extraction via the described aluminium, iron, magnesium and calcium. This routes. Figure 2 depicts the micrographs of Pd/C is generally not desirable and lower values are and Figure 2(a) depicts the SEM microstructure indicative of a good quality carbon. Sometimes of Pd/C before Pd removal. The Pd/C possesses pores of diameter in the micrometre range. On

treatment with acids, HCl-H2O2, 2 M HCl and Table II Characterisation of Pd/C Used in aqua regia as shown in Figures 2(b), 2(c) the Present Study and 2(d) respectively, a visible change in the Test Result morphology of Pd/C treated with aqua regia was observed compared to the morphology of Pd/C

Moisture content 11.53% treated with HCl-H2O2 and 2 M HCl. In the case

of HCl-H2O2 and 2 M HCl treated Pd/C (Figures Ash content 8.62% 2(b) and 2(c) respectively), there is no visible difference in surface morphology compared to the pH 9.30 pure Pd/C. The HCl-H2O2 treated Pd/C appears to Decolourising power 365.47 mg be flakier which may be due to grinding of Pd/C before the extraction. The aqua regia treated Pd/C –1 Iodine number 1463.4 mg g (Figure 2(d)) shows a more porous structure with

2.8 k 2.4 k C C 2.4 k 2.0 k 2.0 k 1.6 k 1.6 k 1.2 k 50 µm 1.2 k 50 µm 0.8 k 0.8 k Pd Pd 0.4 k Pd 0.4 k Cl O Pd O Cl

2.00 4.00 6.00 8.00 10.00 2.00 4.00 6.00 8.00 10.00

2.4 k C C 1.2 k 2.0 k

1.6 k 0.9 k

1.2 k 50 µm 0.6 k 50 µm Cl 0.8 k Pd Pd Pd 0.3 k Cl 0.4 k Pd O Cl Ni Al O Cl Ni

2.00 4.00 6.00 8.00 10.00 2.00 4.00 6.00 8.00 10.00

Fig. 2. SEM and EDS graphs of: (a) Pd impregnated carbon (Pd/C); (b) Pd/C treated with HCl-H2O2; (c) Pd/C treated with 2M HCl; (d) Pd/C treated with aqua regia. The SEM images were taken at magnifications 500× and 5000× respectively

an increase in the pore densities. The EDS study of of the reaction. There was a steady decrease in Pd/C reveals the presence of Pd on its surface. The the absorption band which finally disappeared at disappearance of the Pd peak in the EDS spectrum the end of the reduction. Similarly, in the case of aqua regia treated Pd/C reveals that aqua regia of recovery of Pd using 2 M HCl and aqua regia treatment removes Pd effectively (Figure 2(d)) respectively, the UV-vis spectra at different pH and is the most effective method for the extraction were obtained which showed a steady decrease of Pd from Pd/C. This was also confirmed from AAS in the absorption band as the solution became and XRD studies. depleted of Pd. At pH 11, the Pd(II) absorption UV-vis spectrophotometric studies were carried peak disappeared which marked the completion of out during the course of Pd recovery for all three the Pd(II) precipitation. acid digestion routes. The UV-vis spectra are The energy level ordering of the transition metal provided in Figure 3. The trend of the decrease d-orbitals is greatly influenced by the ligand which in absorption peaks during the progress of the is manifested in the spectroscopic transition. reaction towards Pd recovery was monitored and Additionally, the orbital interaction with ligands is shown as an inset in the figure. In the case of influences the magnitude of orbital splitting. In

Pd removal using HCl-H2O2 digestion as shown in the case of complexes with p-donor ligands such 2– Figure 3(a), absorption peaks centred at 231 nm as (PdCl4) , the energetic ordering is perturbed and 277 nm respectively were obtained from the by p-donating ligands such as chloride (56). Based Pd containing solution, which are characteristics of on the different acid treatments, the absorbance Pd(II). The UV-vis spectra were acquired at different spectra show little shifts in the values. The 2– stages of reduction: during and after completion absorbance values for the (PdCl4) complexes

(a) (b) Before reduction Initial (pH 1.05) In between pH 2 After reduction pH 4 pH 6 pH 7.6 pH 11 Absorbance, au Absorbance, au

200 250 300 350 400 450 200 250 300 350 400 450 Wavelength, nm Wavelength, nm

Initial (pH 0.81) (c) pH 2 pH 4 pH 6 pH 7.6 pH 8.2 pH 8.8 pH 11 Absorbance, au

200 250 300 350 400 450 Wavelength, nm

Fig. 3. UV-vis spectrophotometric studies of Pd impregnated carbon (Pd/C) at different stages of the reaction or pH variation extracted using: (a) HCl-H2O2; (b) 2M HCl; (c) aqua regia. The plots in the inset depict the trend in absorbance as the reaction progresses for Figure 3(a), and the PdCl2 precipitation occurring with the increase in pH in the case of Figures 3(b) and 3(c) respectively

formed using various acids show bands around solution in the case of the 2 M HCl and aqua regia

231 nm and 277 nm in the case of HCl-H2O2 based extracts. Finally, AAS studies were performed on extraction whereas there is an appearance of an the final recovered Pd. Theaqua regia extract gave additional band around 211 nm and 218 nm in the the highest recovery of Pd at 96%, followed by the case of 2 M HCl and aqua regia based extraction 2 M HCl (~73%) and HCl-H2O2 (~56%) extracts. routes respectively. All the observed absorbances All the extractions were carried out in duplicate and can be attributed to a ligand-to-metal (Cl– to Pd(II)) the results were reproducible within <5% variation 2– charge transfer transition for (PdCl4) (57, 58). as mentioned in Table I. The detailed calculations AAS was employed to quantitatively analyse the are included in the Supplementary Information. amount of Pd recovered using the three extraction An ash test was carried out on the Pd/C after routes. The initial estimation was carried out using aqua regia extraction. The amount of ash obtained the Pd(II) solution immediately after acid digestion. from the Pd/C treated with aqua regia was 0.71%. The AAS analysis results are provided in Table I. The ash content value also reflects that the aqua It is observed that aqua regia based digestion was regia treatment effectively removes Pd from the most effective in recovering Pd, followed by the spent Pd/C.

2 M HCl and HCl-H2O2 routes with an average yield XRD studies were carried out on Pd/C before ~96%, 74% and 59% respectively for the three and after treatment and the spectra are shown in acids. Pd was then precipitated from the three Figure 4. The XRD pattern of Pd/C (Figure 4(a)) extracts by using NaBH4 in the case of the extract exhibited two broad peaks at 2q corresponding to from HCl-H2O2, and by increasing the pH of the 25.56° and 43.89° respectively. The peaks are

(a) (b) 1000 1200 Pd(111) 900 1000 800 Pd impregnated Pd/C treated with 700 carbon (Pd/C) C(002) 800 HCl-H2O2 Pd(200) 600 C(100) C(100) Pd(311) C(002) 500 Pd(220) 600 Intensity, au Intensity, Pd(222) au Intensity, 400 400 300

200 200 20 40 60 80 2q, ° 20 40 60 80 2q, ° (c) (d) 2000 Pd(111) 700 C(002) 1800 600 1600 C(100) Pd/C treated with 1400 2M HCl Pd/C treated with 500 Pd(200) aqua regia 1200 1000 Pd(311) 400 C(002) Pd(220)

Intensity, au Intensity, 800 C(100) au Intensity, 300 600 Pd(222) 400 200 200 100 20 40 60 80 20 40 60 80 2q, ° 2q, °

Fig. 4. XRD diffraction pattern for: (a) Pd impregnated carbon (Pd/C); (b) Pd/C treated with HCl-H2O2; (c) Pd/C treated with 2 M HCl; (d) Pd/C treated with aqua regia

attributed to the (002) and (100) plane, respectively, of Pd on the surface of Pd/C, implying that the characteristic of the hexagonal diffraction pattern extraction with 2 M HCl was not effective and the of the graphitic structure. In addition to these Pd/C contained some Pd even after treatment. two broad peaks, five major diffraction peaks The observation was in agreement with the EDS, centred at 2q 40.51°, 47.00°, 68.47°, 82.38° and XRD and AAS results. For the Pd/C treated with

86.87° were also present in the XRD pattern for HCl-H2O2, all five peaks were absent and the Pd/C (Figure 4(a)). The appearance of these five pattern was similar to the spectrum obtained peaks indicates the presence of Pd on the surface for Pd/C treated with aqua regia. However, the of Pd/C possessing (111), (200), (220), (311) and recovery percentage was lower for the HCl-H2O2 (222) planes (Bragg reflection) of the face centred method compared to the aqua regia and 2 M HCl cubic (fcc) crystalline structure. Two broad peaks based extraction methods. characteristic of the hexagonal diffraction pattern of the graphitic structure were also observed in the Nanoparticles XRD pattern for acid treated Pd/C (Figures 4(b), 4(c) and 4(d)). In the case of Pd/C treated with As mentioned in the earlier section, it is important aqua regia, all five Pd peaks disappeared, indicating to develop processes for the reuse of the recovered the complete leaching out of Pd from the Pd/C. metal. In this direction, biosynthesis of Pd NPs The XRD pattern for 2 M HCl treated Pd/C shows was carried out with the obtained extracts using peaks corresponding to Pd indicating the presence green tea as the reducing agent. The flavonoid

(+)-catechin, an antioxidant present in green tea, a 1:1 reaction and the amount of Pd NPs formed is responsible for the formation of NPs. The reaction will be proportional to the amount of Pd(II) present follows an electron transfer mechanism involving in the solution. the release of both electrons and protons from The formation of NPs from the extracts (2 M the molecule (59). (+)-Catechin has two different HCl and aqua regia methods) was identified with pharmacophores, the catechol group in ring B the help of a UV-vis spectrophotometer. The and the resorcinol group in ring A (Figure 1(a)). UV-vis spectra are provided in Figures 6(a) Additionally, at position 3, it has a hydroxyl group and 6(b). The UV-vis spectrum of green tea in ring C (Figure 1(a)). Electrochemical studies shows an absorption peak centred at 273 nm. on flavonoids show the trends of their electron The absorption peaks centred around 211 nm, donating ability. It has been shown that compared 234 nm and 277 nm corresponding to Pd(II) to resorcinol ring A, catechol ring B is more easily extracted from 2 M HCl and aqua regia start oxidisable, the ring with lower redox potential. The disappearing as the reaction proceeds, indicating stability of various catechin radicals will follow the the formation of Pd NPs. The complete reduction of sequence: 4’-OH, 3’-OH, 7-OH, 5-OH (59, 60). Pd(II) to Pd(0) was confirmed by the appearance During the formation of the NPs, catechin forms of a broad continuous absorption band (62). NP a chelation complex with the Pd2+ ion, followed formation was further confirmed by FESEM images by an electron transfer that reduces Pd2+ to Pd0 as shown in Figures 7(a) and 7(b). It is evident and oxidises catechin to quinone (59, 61). The from the micrographs that fine NPs are formed which reduction mechanism for the conversion of Pd2+ are well separated from each other. The approximate to Pd0 by green tea is schematically expressed size of the synthesised NPs was calculated from the by using a representative catechin monomer in FESEM images. Although agglomerated particles are Figure 5. This can be stoichiometrically expressed also visible in the image, the majority (~94%) of as Equation (ix): the formed NPs fall within the size range of 1–25 nm (Figures 8(a) and 8(b)). nPd(II) + 2R–(OH)n → The crystalline nature of the biosynthesised Pd nPd(0) + 2nR=O + 2nH+ (ix) NPs was confirmed by XRD analysis. The spectra where R and n represent the heterocyclic or alkyl are shown in Figure 9. The XRD pattern for Pd groups and the number of the hydroxyl groups NPs prepared from the aqua regia extract shows oxidised by Pd(II) species, respectively. Therefore, five major diffraction peaks centred at 2q 39.95°, it is apparent that the formation of Pd NPs follows 46.4°, 67.92°, 81.70° and 86.19°. The observation

Pd2+ OH Pd2+ O Fig. 5. Mechanism for the formation of Pd NPs by using OH O catechin as a representative reducing agent HO O HO O

–2H+ OH OH (+)-Catechin Metal chelation OH OH

Electron transfer O

HO O

+ Pd0 OH

OH Quinone

(a) (b) Fig. 6. UV-vis spectra for PdCl 2 PdCl 0 min 2 the Pd NPs 0 min 15 min 10 min synthesised 30 min 15 min from: (a) 2 M 45 min 20 min Green tea HCl extract; Green tea (b) aqua regia extract Absorbance, au Absorbance, au

200 250 300 200 250 300 Wavelength, nm Wavelength, nm

Fig. 7. FESEM images (a) (b) for Pd NPs synthesised from: (a) 2 M HCl extract; (b) aqua regia extract

200 nm 200 nm

(a) (b) Fig. 8. Particle size 100 100 distribution of Pd NPs synthesised from (a) 80 80 2 M HCl extract; (b) aqua regia extract. 60 60 The inset shows the reaction mixture 40 40 before and after the Particles, % Particles, Particles, % Particles, formation of Pd NPs 20 20

0 0 1–25 25–50 50–75 75–100 1–25 25–50 50–75 75–100 Particle size, nm Particle size, nm

(a) (b)

900 550 (111) (111) 800 500

700 450 Pd NP from 2 M HCl Pd NP from aqua regia extract extract 400 600 (200) 350 500 (220) (200) Intensity, au Intensity,

Intensity, au Intensity, 300 (220) (311) 400 (311) 250 (222) 300 (222) 200 200 150 20 40 60 80 20 40 60 80 100 2q, ° 2q, °

Fig. 9. XRD diffraction pattern for Pd NPs synthesised from: (a) aqua regia extract; (b) 2 M HCl extract

is similar for the Pd NPs prepared from the 2 M ~4 h and ~5 h for the HCl-H2O2 and aqua regia HCl extract with peaks centred at 2q 40.07°, methods respectively. It can thus be concluded 46.65°, 68.03°, 82.02° and 86.00°. As mentioned from the material and process points of view that in the previous section, the appearance of the the three extraction routes are more or less similar, five peaks represents the fcc structure of Pd. even though the HCl-H2O2 and aqua regia routes The results observed here are in agreement with are slightly costly in terms of materials. However, previously published reports, thereby confirming a the major cost incurred for Pd/C catalysts is the nanocrystalline form of Pd (63–66). The crystallite amount of Pd used for impregnation. As mentioned size of the Pd NPs was calculated using the earlier, the percentage recovery of Pd was highest Scherrer equation (67). On the basis of the (111) for the aqua regia based extraction method peak, the average crystallite size of the Pd NPs at ~96%, followed by the 2 M HCl (~73%) and prepared from aqua regia and 2 M HCl extract was HCl-H2O2 (56%) extraction methods. This suggests calculated to be ~13 nm and ~23 nm respectively. that the cost for the development of a fresh Pd/C The synthesised Pd NPs can be effectively used as with 6.5% Pd derived from extracted Pd could be a potential impregnant for the preparation of fresh greatly reduced by using the aqua regia route, with Pd/C. Further studies on CO removal are being a ratio of ~1:7:11 corresponding to aqua regia, carried out with the Pd NPs impregnated Pd/C and 2 M HCl and HCl-H2O2 extraction routes respectively. shall be published later. From a commercial perspective, the aqua regia based extraction procedure is an attractive method Economic Comparison for the recovery of Pd as the method is economical, simple and highly effective. A tentative economic comparison was made for the three methods employed in this study towards Conclusion the extraction of Pd from 1 kg of spent Pd/C. Three criteria to estimate economic viability were Studies on Pd recovery from a spent activated considered: materials, process and Pd recovery. carbon filter impregnated with Pd (Pd/C) were The comparison method is summarised in the carried out. Recovery studies were performed using

Supplementary Information (SI2c). Considering three acid or acid mixtures: 2 M HCl, HCl-H2O2 and the cost of the chemicals involved in the extraction, aqua regia respectively. The absolute yield of the Pd the cost was slightly higher for the HCl-H2O2 and obtained by different extraction methods showed aqua regia methods compared to the 2 M HCl a decreasing trend with aqua regia (~96%) > extraction route. However, the 2 M HCl extraction 2 M HCl (~73%) > HCl-H2O2 (~56%). Biosynthesis takes ~9 h for completion of the recovery process of Pd NPs from the recovered Pd was successfully as compared to the other two methods which take carried out using green tea as the reducing agent.

The synthesised Pd NPs had a crystallite size of 11. A. Abdedayem, M. Guiza and A. Ouederni, Comptes Rendus Chimie, 2015, 18, (1), 100 ~13 nm (aqua regia extract) and ~23 nm (2 M HCl extract) and the particles were well separated. It is 12. J. C.-S. Wu and T.-Y. Chang, Catal. Today, 1998, 44, (1–4), 111 also clear from the XRD results that the synthesised Pd NPs had a fcc crystalline structure. The present 13. C.-Y. Lu and M.-Y. Wey, Fuel Process. Technol., study indicates that the recovered Pd could be 2007, 88, (6), 557 reused successfully for the ‘green’ synthesis of 14. S. K. Ryu and S. R. Choi, J. Ceram. Soc. Jpn., Pd NPs which in turn can be effectively used for 2004, 112, Suppl. 112-1, s1539 the development of fresh Pd/C for the removal of 15. H.-H. Tseng, M.-Y. Wey, Y.-S. Liang and K.-H. Chen, pollutants. Carbon, 2003, 41, (5), 1079 Further studies on impregnation and CO removal 16. L. Yin and J. Liebscher, Chem. Rev., 2007 107, efficiency of the biosynthesised Pd NPs are being (1), 133 carried out in our laboratory. The authors are 17. B. Singh, A. Saxena, A. K. Srivastava and R. hopeful that with further optimisation of the Vijayaraghavan, Appl. Catal. B: Environ., 2009, experimental parameters, the synthesised NPs 88, (3–4), 257 will have the potential to be reused for different 18. M. Athar and S. B. Vohora, “Heavy Metals and applications. Environment”, New Age International (P) Ltd, New Delhi, India, 1995 Acknowledgement 19. J. Butler, “Platinum 2012: Interim Review”, Johnson Matthey Plc, Royston, UK, 2012 The authors are grateful to Dr U. K. Singh, Director, 20. R. V. Jasra, P. K. Ghosh, H. C. Bajaj and A. B. DEBEL, for constant encouragement and support Boricha, Council of Scientific and Industrial to carry out the work. The authors would also Research, ‘Process for Recovery of Palladium from like to thank Dr Kadirvelu, Scientist, Centre for Spent Catalyst’, US Patent 7,473,406; 2009 Life Sciences, Bharathiar University, India and 21. G. P. Demopoulos, G. Pouskouleli and P. J. A. D. R. Nayak, IISc, Bangalore for conducting the Prud’Homme, Canadian Patents and Development SEM-EDS. Ltd, ‘Direct Recovery of Precious Metals by Solvent Extraction and Selective Removal’, US Patent Appl. 1987/4,654,145 References 22. K. J. Shanton and R. A. Grant, Matthey Rustenburg 1. R. C. Bansal and M. Goyal, “Activated Carbon Refineries Ltd, ‘Solvent Extraction Process for the Adsorption”, Taylor & Francis Group LLC, Boca Selective Extraction of Palladium’, US Patent Appl. Raton, USA, 2005 1982/4,331,634 2. T. Budinova, E. Ekinci, F. Yardim, A. Grimm, E. 23. Ş. Sarioğlan, Platinum Metals Rev., 2013, 57, Björnbom, V. Minkova and M. Goranova, Fuel Proc. (4), 289 Technol., 2006, 87, (10), 899 24. D. S. Sheny, D. Philip and J. Mathew, Spectrochim. 3. A. Bhatnagar, W. Hogland, M. Marques and M. Acta Part A: Mol. Biomol. Spect., 2012, 91, 35 Sillanpää, Chem. Eng. J., 2013, 219, 499 25. G. Zhan, J. Huang, M. Du, I. Abdul-Rauf, 4. Z. A. Al-Othman, R. Ali and M. Naushad, Chem. Y. Ma, and Q. Li, Mater. Lett., 2011, 65, (19–20), Eng. J., 2012, 184, 238 2989 5. J. M. Dias, M. C. M. Alvim-Ferraz, M. F. Almeida, 26. T. Wang, J. Lin, Z. Chen, M. Megharaj and R. J. Rivera-Utrilla and M. Sánchez-Polo, J. Environ. Naidu, J. Clean. Prod., 2014, 83, 413 Manage., 2007, 85, (4), 833 27. T. Sun, Z. Zhang, J. Xiao, C. Chen, F. Xiao, S. 6. M. A. A. Zaini, R. Okayama and M. Machida, J. Wang and Y. Liu, Sci. Rep., 2013, 3, 2527 Hazard. Mater., 2009, 170, (2–3), 1119 28. S. Iravani, Green Chem., 2011, 13, (10), 2638 7. D. Mohan and C. U. Pittman Jr., J. Hazard. Mater., 29. S. Lebaschi, M. Hekmati and H. Veisi, J. Colloid 2007, 142, (1–2), 1 Interface Sci., 2017, 485, 223 8. “Carbon Materials for Catalysis”, eds. P. Serp and 30. M. Reto, M. E. Figueira, H. M. Filipe and C. M. M. J. L. Figueiredo, John Wiley & Sons Inc, Hoboken, Almeida, Plant Foods Hum. Nutr., 2007, 62, 139 USA, 2009 31. J. V. Higdon and B. Frei, Crit. Rev. Food Sci. Nutr., 9. K.-D. Henning and S. Schäfer, Gas Sep. Purif., 2003, 43, (1), 89 1993, 7, (4), 235 32. L. H. Yao, Y. M. Jiang, J. Shi, F. A. Tomás-Barberán, 10. M. Zhang, B. Zhou and K. T. Chuang, Appl. Catal. N. Datta, R. Singanusong and S. S. Chen, Plant B: Environ., 1997, 13, (2), 123 Foods Hum. Nutr., 2004, 59, (3), 113

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The Authors

Ansari Palliyarayil completed an MSc in Chemistry in 2010 at Kannur University, India. Presently he is a Senior Research Fellow at Defence Bioengineering and Electromedical Laboratory (DEBEL)-Defence Research and Development Organization (DRDO), India. His research interests include air quality management, catalysis, carbon based materials, adsorption, impregnation and nanoparticle synthesis and characterisation.

Kizhakoottu Kunjunny Jayakumar is a Technical Officer at DEBEL-DRDO. He has more than 10 years of experience in the field of air quality management, high energy materials, toxic emission studies and carbon based materials.

Dr Sanchita Sil is a Scientist at DEBEL-DRDO. She received her PhD in Chemistry in the area of Raman spectroscopy from the Indian Institute of Science, India. Prior to joining DEBEL, she was a Scientist at High Energy Materials Research Laboratory (HEMRL), India. She has more than 13 years of research experience. Her research interests include Raman spectroscopy, SERS, development of novel Raman based instrumentation towards detection of materials, air quality management, catalysis, carbon based materials, adsorption and nanoparticles.

Dr Nallaperumal Shunmuga Kumar is a Senior Scientist at DEBEL-DRDO, Bangalore. He received his PhD in Biomaterial Science and Technology in 1991 at Sree Chitra Tirunal Institute for Medical Sciences and Technology, India. He has more than 30 years of research experience in the field of air quality management, catalysis, adsorption, biomedical devices, polymers and development of biosensors.

www.technology.matthey.com

“Food Packaging” Edited by Alexandru Mihai Grumezescu, Nanotechnology in the Agri-Food Industry, Volume 7, Academic Press, an Imprint of Elsevier, Oxford, UK, 2017, 796 pages, ISBN: 978-0-12-804302-8, US$140.00, £105.00, €107.11

Reviewed by Chun-tian Zhao mostly in the first 15 years of the 21st century Tracerco, Tracer Technology Centre, The Moat, in developing advanced polymer packaging Belasis Hall Technology Park, Billingham, materials for food, by adopting the approaches of TS23 4ED, UK nanotechnology and biomaterials, and progress made, particularly in the following areas: Email: [email protected]; • Barrier properties [email protected] • Ecofriendly and/or cost effective packaging • Active packaging • Intelligent packaging. Although the book itself is not structured into Introduction different sections, this review covers all 20 chapters by categorising them into the above “Food Packaging” is the seventh volume of listed themes. Nanotechnology in the Agri-Food Industry, a series aimed at bringing together the most recent Nanotechnology and Improvement and innovative applications of nanotechnology in of Barrier Properties the agriculture and food industries and to present future perspectives in the design of new or Good barrier properties are key requirements alternative foods. The volume “Food Packaging” for food packaging materials. Compared to presents the development of novel nano-bio- glass, metal and ceramics, the barrier properties materials and the enhancement of barrier of polymer materials to gases and liquids are performance of nondegradable and biodegradable inferior and enhancement is often needed. plastics in industrial packaging. Like the rest of the Nanotechnology has emerged as a successful series, the book is edited by Dr Alexandru Mihai approach. Improvement of barrier performance Grumezescu at the Department of Science and is the central topic of many of the chapters in Engineering of Oxide Materials and Nanomaterials, this book and nanotechnology is extensively Politehnica University of Bucharest, Romania, who discussed. is an experienced and widely published researcher Chapter 1 by Iman Soltani et al. (North and editor in the field of nano- and biostructures. Carolina State University, USA) concentrates on The book comprises 20 independent chapters the fundamental aspects of nanotechnological authored by academics from 17 countries. Each strategies for high barrier polymer food packaging chapter reviews recent developments in a chosen materials. Two strategies are presented: applying area of food packaging. Altogether, the book gives inorganic coatings to the surface of plastics a full account of the efforts made by scientists and developing nanocomposites. Starting with across the world starting from the 1990s but classifying coatings according to the number of

74 © 2018 Johnson Matthey https://doi.org/10.1595/205651318X696350 Johnson Matthey Technol. Rev., 2018, 62, (1) layers, a clear overview of the inorganic coating these drawbacks and focus on polymer clay approach is provided. The history and technology nanocomposites (PCNs). Methods to prepare of applying a single layer (10–100 nm) of metal and and characterise PCNs are summarised. The oxide/nitride coating to the surface of polymers with well-recognised four typical morphologies of physical (for example thermal or sputtering) vapour layered silicates polymer micro/nanocomposites: deposition (PVD), chemical vapour deposition phase separated, exfoliated, intercalated and (CVD) and atomic layer deposition (ALD) methods intercalated plus flocculated are cited and depicted are described. For multilayer coatings, the authors (Figure 1). These correlate directly to the barrier focus on relatively recently developed layer-by- enhancement effect. Following a theoretical layer assembly (LBL) coating. The evolution of discussion of molecular transport in polymers LBL technology, including the interaction and and nanocomposites, case studies are included layer building mechanism employed, the range of to provide a clear comparison of the barrier materials used for fabrication, from the original enhancement abilities of the routes discussed. multilayer of oppositely charged polymer to Typically, improvement of up to 3, 3, 4 and 1 order charged particles (for example globular protein) of magnitude (against O2) can be achieved relative and charged platelets (exemplified by exfoliated to the base polymer materials for PVD/CVD, ALD, natural clays such as montmorillonite, MMT), and LBL coating and PCNs, respectively. The authors the fabrication methods from simple dipping to highlight the commercial importance of PCNs for spraying and spin coating, is accounted. their facile processability, mechanical robustness The ability to improve barrier properties is and lengthy stability, despite their relatively presented and the shortcomings of coating modest barrier efficacy. methods, which are defects and adhesion issues, Nanocomposites are no doubt the most are explained. The authors introduce the polymer appreciated nanotechnology in this book and nanocomposites (PNC) strategy to overcome are discussed by many authors separately in the

(a) (b)

Layered silicate Polymer

Microcomposite Nanocomposites

Phase-separated Exfoliated Intercalated Intercalated and flocculated

Fig. 1 (a) Schematic illustration showing the results of incorporating layered silicates into polymeric matrices to yield either phase-separated microcomposites or different types of nanocomposites (labelled), depending on the level of dispersion achieved; (b) TEM image of a nanocomposite composed of dispersed clay platelets in a PP matrix. Here, A, B, and C identify intercalated stacks, disordered intercalated tactoids and exfoliated platelets, respectively. A schematic representation of an intercalated clay stack is included wherein parallel platelets are separated by PP chains. (A) Adapted from (1). Reproduced from (2), copyright the Materials Research Society

75 © 2018 Johnson Matthey https://doi.org/10.1595/205651318X696350 Johnson Matthey Technol. Rev., 2018, 62, (1) other 16 chapters, as a key solution to improve not processing, including film blown technology, only the barrier performance but other properties and the morphology, mechanical properties and as well. barrier performance of the final films, the authors Studies of a special type of polymer nano present the feasibility of using polypropylene/ composites, polymer/graphene nanocomposites talc nanocomposites as low cost improved barrier are specifically reviewed in Chapter 20 by Ahmad materials as well as the scalability challenge. Allahbakhsh (Tarbiat Modares University and The enhancement of barrier properties is further Islamic Azad University, Iran). In outlining the studied by other authors in depth in Chapters 2, concept of substituting nanoclay with graphene 5, 12, 13 and 20. In Chapter 5, Xueyan Yun et al. in polymer nanocomposites for barrier and (Inner Mongolia Agricultural University, China) reinforcement, the author underlines the use of discuss the fabrication of high barrier plastics and functionalised or oxidised graphene (graphene their application for food packaging. In addition oxide (GO)) instead of pure graphene in polymer to nanocomposites, coating and LBL assembly nanocomposies for achieving dispersibility and technologies, the authors provide an overview compatibility of the nanoparticles. The preparation of more traditional technologies such as polymer (through chemical oxidation and micromechanical orientation/stretching and blending. The authors exfoliation of graphite) and the properties of GO also discuss using organic coatings to enhance and its covalent and non-covalent interaction with barrier properties, for example polyvinylidene polymers are discussed, which pave the way to chloride on polyethylene terephthalate. Improved review studies of nanocomposites of GO with a preservation of various foods by enhanced polymer number of polymers: polypropylene, polyethylene, films, including meat products, chilled meat, cheese, polyethylene terephthalates, polyvinylalcohol, salad sauce, seafood and agricultural products, is polylactide and pullulan. The preparation of demonstrated by case studies. As the basis of the nanocomposites by melt compounding, in situ discussion, fundamentals of food packaging, such polymerisation and solution blending routes plus as the impact on the shelf life of factors such as LBL techniques, and the effective reduction of light, oxygen, carbon oxides and moisture through permeation to gases are described. the mechanisms of oxidation and photooxidation, Chapter 8 by Agustín González et al. (Universidad degradation of protein and vitamins, colouration, Nacional de Córdoba, Argentina) discusses the growth of microorganisms and other physical or structure-property relationship of nanocomposites chemical reactions are provided. from the viewpoint of reinforcement. The literature of nanocomposites composed of various Bio-nanocomposites and Ecofriendly nanoparticles (starch and cellulose nanocrystals, Packaging chitosan nanoparticles, inorganic lamellar fillers, cellulose nanofibres and nanowhiskers and Judging by the market size and the huge carbon nanotubes) are thoroughly examined for volume of materials consumed, the packaging the mechanical reinforcement of biopolymers. industry may be seen as a materials intensive Additional effects of nanocomposites such as industry. There has been extensive research on magnetic and electric functions are also briefly ecofriendly packaging to minimise the impact on reviewed. the environment. A solution is using bioplastics – Chapter 14 by J. Rodolfo Rendón-Villalobos et al. biodegradable polymers or polymers made from (Instituto Politécnico Nacional, Mexico) concisely renewable resources. This book follows the trend reviews the development of nanocomposites and with 14 chapters discussing natural, bio-polymer presents them as the ideal barrier material. The or biodegradable polymer packaging. authors provide an overview of the technology of Renewability is a big advantage in bioplastics. polymer multilayer (generally 3–9 layers) films and This is exemplified in Chapter 7 by Ana Sofia coatings based on a discussion of gas permeability Lemos Machado Abreu et al. (University of and material barrier properties and the barrier Minho, Guimarães, Portugal). The authors properties that multilayer films can offer, with a discuss bioplastics from agro-wastes for food focus on bioplastics. packaging applications. An overview of a number Chapter 18 by Karolth Espinosa et al. (UNS- of petroleum based biodegradable polymers CONICET, Argentina) deals specifically with and polymers from renewable resources is polypropylene/talc nanocomposites. Through provided. It is interesting that the authors enlist a detailed description and a review of their polyethylene as a biodegradable polymer for its

76 © 2018 Johnson Matthey https://doi.org/10.1595/205651318X696350 Johnson Matthey Technol. Rev., 2018, 62, (1) hydro/oxodegradation ability and the possibility to polysaccharide nanocomposite packaging materials blend it with well-known biodegradable polymers. is identified by the authors at the end of the chapter. Cellulose is highlighted as a polymer which is ZnO nanocomposites with polyhydroxyalkanoates available from plants, bacterial processes and are further discussed in Chapter 6 by Ana in particular from agro-wastes. In presenting M. Díez-Pascual (Alcalá University, Spain). renewable cellulose and its derivatives as potential The synthesis, structure, properties and food packaging materials, studies on extracting applications of polyhydroxyalkanoate polymers cellulose from agro-wastes, physical blending and ZnO nanoparticles are detailed. Studies of cellulose with other biodegradable polymers, on nanocomposites of ZnO with two specific chemical modification of cellulose in ionic liquids polyhydroxyalkanoates: polyhydroxybutyrate and and graft polymerisation of cellulose are reviewed. polyhydroxybutyrate-co-valerate are reported. Nanocomposites play a significant role in The barrier properties of bio-nanocomposites bio-polymer packaging. Chapter 10 by Ana of biodegradable polymers with nanofillers are Sofia Lemos Machado Abreu et al. (University thoroughly discussed in a number of chapters, of Minho, Guimarães, Portugal) discusses particularly in Chapters 2, 12 and 13. Chapter 2 by biodegradable polymer nanocomposites for Giulio Malucelli (Politecnico di Torino, Alessandria, packaging applications. The chemistry of various Italy) provides a literature survey of biocomposites natural polymers including soy, corn-zein, starch, containing nanofillers of different morphology cellulose, chitosan and other polysaccharides (three-dimensional (3D) spherical and polyhedral and synthetic or bacterial-based polyesters particles, two-dimensional (2D) nanofibers and including polycaprolactone, polylactic acid and one-dimensional (1D) platelets) and of different polyhydroxyalkanoates are summarised. Studies chemistry (for example colloidal silica, polyhedral on nanocomposites of these polymers are reviewed. oligomeric silsesquioxanes, carbon black, nanoclays A short discussion of the biodegradation process and starch nanocrystals). An improvement of and its limitations is provided. A few examples of barrier properties by tens of percent can be seen. commercial biodegradable polymer food packages The authors also dedicate one section to the are given by the authors. In Chapter 3, Sushama LBL method, highlighting its advantages such as Talegaonkar et al. (Jamia Hamdard, India; Mendel effective barrier property improvement, imparting University, Czech Republic, and University of of other additional functions, process simplicity Natural Resources and Life Sciences, Austria) and eco-friendliness. thoroughly review the research on the commonly Chapter 12 by Marina Patricia Arrieta et al. used nanoparticle fillers including nanoclay, nano- (Institute of Polymer Science and Technology, cellulose, silicon dioxide, carbon nanotubes, Madrid and Polytechnical University of Valencia, silver dioxide, zinc oxide, titanium dioxide and Alcoy, Spain) aims at elucidating the role of the nanocomposites of common natural polymer nanoparticles in improving the barrier properties of cellulose, starch and chitosan. The authors attempt bioplastics and assessing the impact of the shape to look at bio-nanocomposites from the angle of of nanoparticles to barrier properties. The research active and intelligent packaging. of bio-nanocomposites of biodegradable polymers Chapter 19 by Aleksandra R. Nesic and Sanja I. (polylactide, polycaprolactone, polyhydroxy Seslija (University of Belgrade, Serbia) focuses on butyrate, polyhydroxy butyrate-co-valerate, the influence of nanofillers on the properties of plasticised starch and polyvinyl alcohol) with MMT, polysaccharide based nanocomposites. Following cellulose nanocrystals and ZnO nanoparticles as an overview of food packaging materials (materials representative 1D, 2D and 3D geometry nanofillers type, criteria and trends), the authors review is described in detail. The additional influence of polymer nanocomposites research by type of factors such as surface properties of nanoparticles nanofillers. The characteristics of the nanofillers and their interaction with components of the and the properties and functions of the respective bioplastics, adhesion and crystallisation on barrier nanocomposites are summarised. Next the properties is discussed. The extra antimicrobial authors provide comprehensive knowledge about benefits of nanocomposites containing ZnO is also the sources of extraction, physical structure and specified. physical-chemical properties of nine commonly seen Chapter 13 by Kateryna Fatyeyeva et al. polysaccharides and their application including food (Normandie University and University of Rouen, packaging materials via a nanocomposites route. France) focuses on high barrier biopolymer/clay A further hurdle to overcome for commercialising nanocomposites. The authors clearly outline the

77 © 2018 Johnson Matthey https://doi.org/10.1595/205651318X696350 Johnson Matthey Technol. Rev., 2018, 62, (1) background on nanocomposites, government antimicrobial properties of a list of nanoparticles regulation, biopolymers, clays and biopolymer/clay including metals (silver, copper, iron), metal nanocomposites. Mechanistic understanding and oxides (titania, ZnO, copper oxide, ferrous oxide, the barrier properties of nanocomposites formed magnesium oxide), carbon nanotubes, nanoclays, by nanoclay with polysaccharides, proteins, biopolymers (chitosan, nisin, alginate) and their aliphatic polyesters and mixtures of polymers composites with polymers. Chapter 16 concentrates are reviewed in detail. As well as enhancing the on bio-nanocomposites. It is worth mentioning concept of improving barrier properties by clay that toxicological properties of nanoparticles and nanocomposites, a full picture of the influence the health, ethical and regulatory issues related of the biopolymer and nanoclays themselves to antimicrobial packaging are well discussed in as a component to the final nanocomposites is the two chapters. The antimicrobial properties for provided. The future development of biopolymer/ ZnO-based bio-nanocomposites are also discussed clay nanocomposites is discussed. The barrier in Chapters 6 and 12. properties of bioplastics are also a focus of Nanodiamond is a special class of bioactive Chapter 14. nanoparticles. Chapter 9 by Katarzyna Mitura et al. Despite the potentials of bioplastics and some (Koszalin University of Technology, PLASTMOROZ examples of commercial products, more research Sp. z o.o. Sp. and Koszalin University of Technology, and development is needed to make bioplastics Poland) presents a complete report of bioactive more comparable to traditional petroleum based packaging with nanodiamond particles. A full account polymers in properties and performance to achieve of nanodiamonds, including their sources of origin, full commercialisation success in food packaging. preparation, modification, morphology, properties, This is highlighted by many authors in the above antimicrobial, antioxidant and biomedical activity, chapters. The authors also look at future directions cyto-toxicology and their increasing application in for development. coatings and nanocomposites is provided. Readers are shown the ways in which nanodiamonds differ Active Packaging from metals and metal oxides deriving from their electronic structure and zeta potential. A significant step in food packaging is active An interesting type of active packaging is packaging, which can proactively extend the provided by flavour and aroma. Chapter 17 shelf life, maintain or even improve the condition discusses nanoencapsulation of flavour and of packaged food. Functional nanocomposites, aroma for food packaging. The science of flavour nanocomposites of physically, chemically or and aroma is summarised in the chapter. Natural biologically active nanoparticles play vital roles aromatic substances are often synthesised by and have become an important route for active living organisms as a defense mechanism or as packaging. This book records the progress in active secondary metabolites and have antioxidant, packaging, in particular nanocomposites for active antibacterial and antifungal properties. By packaging. incorporating these into food packaging they can Active packaging including antimicrobial reduce the most common spoilage pathogens and packaging and modified atmosphere control is prevent deterioration of food quality. A proven highlighted as a trend in Chapter 19. In Chapter effective process is for flavours and aromas to be 8, readers can learn how metals and metal oxides, micro- or nano-encapsulated into capsules and carbon nanotubes and biopolymer nanoparticles are incorporated as particles into packaging materials. incorporated into polymers to impart antimicrobial, Encapsulation technology and the materials used UV-blocking and anti-oxidant functionalities. In for encapsulation, and studies of largely successful Chapter 3, several commercial product examples incorporation of nanoencapsulated flavours into of atmosphere control packaging (for moisture, polymer films for food packaging, are reviewed. oxygen and carbon dioxide) are briefly described. In Chapter 11, Mythili Prakasam et al. (University Antimicrobial and antibacterial packaging is well of Bordeaux, France) discuss flexible packaging for addressed in this book. Both Chapter 15 by Iva nonthermal decontamination by high hydrostatic Rezić et al. (University of Zagreb, Croatia) and pressure, which may be classed as a special type Chapter 16 by Majid Montazer et al. (Amirkabir of active packaging. High hydrostatic pressure University of Technology, Iran) are dedicated to processing technology is described, including the antimicrobial nanocomposites. Together the two materials requirements, the working mechanism chapters present essential information about the and its impacts on physical, chemical, mechanical,

78 © 2018 Johnson Matthey https://doi.org/10.1595/205651318X696350 Johnson Matthey Technol. Rev., 2018, 62, (1) barrier and active migration properties of packaging technology are provided. Many readers, especially materials particularly multilayer packaging. those researchers from industry, may be interested to learn more about the commercial status of the Intelligent Packaging technologies discussed in this book. As the book is written by many groups of independent authors, Intelligent packaging communicates information inevitably readers may find that there is some on the state of the package and packed goods to overlap in content between some of the chapters. owners and customers. It has been a target of There appear to be some plain language errors research and development and several types of easily spotted in parts of the current edition. intelligent packaging are emerging. Chapter 4 by Nevertheless, the book has succeeded in presenting Viviane Dalmoro et al. (Federal University of Rio readers with the frontiers of food packaging Grande do Sul and Polymer Technology Center at materials science. The state-of-the-art in developing Triunfo, Brazil) describes some developments in high barrier and multifunctional food packaging the area of encapsulation of sensors for intelligent materials using nanotechnology and biodegradable food packaging. Commercial interest in intelligent or renewable polymers is well documented, as well as packaging is assessed by the authors at the fundamental knowledge, scientific background and beginning of the chapter and readers are shown insights into the future development of packaging that the market is growing in size. A few products materials. The book provides a comprehensive currently on the market or in development such reference for the design of food packaging materials, as freshness, integrity and time-temperature development of technology and the manufacturing indicators, radio frequency identification tags of food packaging products, and thus will certainly and embedded sensors are then described. appeal to professionals working on food packaging An overview of the chemistry and processing from both industry and academia. The book will technology of current and emerging packaging also interest general readers including graduate and materials is made in the context of encapsulating undergraduate students in materials science and sensors. The authors highlight the sol-gel method engineering. as a versatile way to encapsulate and develop packaged sensors. Sol-gel chemistry, in particular silane chemistry, is discussed and the development "Food Packaging" of sol-gel encapsulated sensors, for example dyes or indicators, and their application in packaging to detect amine, formaldehyde and pH changes for food quality monitoring are reviewed. A summary of the current regulation status of intelligent and active packaging in Europe and the USA is included. Intelligent packaging is also discussed in some other chapters of the book. Chapter 8 mentions the development of nanocomposites containing nanoparticles as intelligent indicators, for example

TiO2 as a UV-activated oxygen indicator, chiral nematic nanocrystalline cellulose as a humidity indicator, Ag/Cu nanoparticles and carbon nanotubes as freshness indicators and quantum dots as microorganism growth indicators. In Chapter 3, a commercial product indicating oxygen levels is briefly described. References

Conclusions 1. M. Galimberti, ‘Rubber Clay Nanocomposites’, in “Advanced Elastomers – Technology, Properties As with similar books, the volume is primarily a and Applications”, ed. A. Boczkowska, InTech, collection of reviews published in scientific journals Rijeka, Croatia, 2012 and little patent literature is included, although in 2. E. Manias, Mater. Res. Soc. Bull., 2001, 26, some chapters brief summaries of the commercial (11), 862

The Reviewer

Chun-tian Zhao is a Senior Polymer Scientist and Research Group Leader at Tracerco, a Johnson Matthey company. He has interests in a broad range of polymer based technologies including microencapsulation and controlled release, membrane materials, sensors and medical devices, biodegradable polymers and food packaging, water treatment and oil field chemicals.

www.technology.matthey.com

“Air Pollution Control Technology Handbook”, Second Edition By Karl B. Schnelle Jr., Russell F. Dunn (Vanderbilt University, Nashville, Tennessee, USA) and Mary Ellen Ternes (Crowe and Dunlevy, Oklahoma, USA), CRC Press, Boca Raton, USA, 2016, 429 + xxvi pages, ISBN 9781138747661, £60.00, US$79.95

Reviewed by Martyn V. Twigg with related titles and the same general layout TST Ltd, Caxton, Cambridge CB23 3PQ, UK with a focus on the situation in the USA.

Correpondence may be sent via Johnson Matthey Technology Review: US Air Quality Legislation [email protected] The first short chapter (12 pages) ‘Historical Overview of the Development of Clean Air Regulations’ gives a historical overview of the Introduction US clean air regulations and is similar to that in the first edition with a small difference being that When the first edition of this book by Karl B. the references, instead of being at the end of the Schnelle and Charles A. Brown was published (1) chapter, are now collected together in a separate some 16 years ago, there were a number of texts section before the index at the end of the book. available that covered various aspects of pollution The larger second chapter (25 pages) entitled emissions and their control, including “Practical ‘Clean Air Act’ delineates the development of Handbook of Environmental Control” by Conrad the US legislation. Tabular information has been P. Straub (2) that gave in tabular form a huge updated to February 2015 and some more recent amount of easily accessed relevant data, and the references are included. These two chapters monumental text by B. J. Finlayson-Pitts and J. N. provide a particularly readable and useful source Pitts (3). Schnelle and Brown’s book was a guide for those wanting to gain insight into the way focusing on the sources of air pollutants, their US air quality legislation has developed over measurement and the control of pollutants from several decades, resulting in major air quality industrial plants. improvements in many urban areas. The second edition of the “Air Pollution Control Technology Handbook” retains the original senior Air Emission Permits and author, K. B. Schnelle (now Emeritus Professor at Atmospheric Pollutant Modelling Vanderbilt University, Nashville, Tennessee, USA), joined by Professor R. F. Dunn from the same The third chapter ‘Air Permits for New Sources’ (12 university and Mary Ellen Ternes, a Vanderbilt pages) explains the procedures necessary when graduate now an environmental lawyer. Like the an air permit is required before the construction of first edition, the present book has 24 chapters plant in the USA that adds or modifies a source of

81 © 2018 Johnson Matthey https://doi.org/10.1595/205651318X696657 Johnson Matthey Technol. Rev., 2018, 62, (1) pollution. It goes through the procedural elements absorption solution. As a result unless there are and details them for applications that include special reasons for not doing so, catalytic ammonia secondary emissions and the primary emissions selective catalytic reduction (SCR) is now usually rates, although only one additional reference is chosen for such NOx control requirements and given compared with the first edition. The next this efficient process enables modern very low chapter (13 pages) called ‘Atmospheric Diffusion NOx emissions standards to be achieved. Catalytic Modelling for Prevention of Significant Deterioration ammonia SCR technology is briefly discussed later Permit Regulations and Regional Haze’ highlights in the book (pages 280–281). the need for appropriately tall stacks which, The ninth chapter (9 pages) returns to financial while not preventing emissions from entering the considerations and is called ‘Profitability and atmosphere, do reduce ground level pollution to Engineering Economics’ which in separate short low enough concentrations so as not to be harmful sections deals with profitability analysis, effect around the source. The approach is qualitative and of depreciation, capital investment and total nonmathematical in outlining atmospheric diffusion product cost. models and reference is made to several computer models, particularly those available through the Pollutant Removal Processes US Environmental Protection Agency (EPA) Air Quality Modelling Group. Strangely the original two Chapter 10 (8 pages) gives an introduction to references in this chapter were not extended or control of gaseous pollutants with a focus on updated. volatile organic compounds (VOCs) and their stripping and absorption. Little is said about VOC Pollutant Monitoring catalytic oxidation that is commonly used to meet stringent emissions standards today, although later The fifth chapter (8 pages) deals with ‘Source Testing’ in the book under ‘Catalytic Incineration’ (pages by which is meant determining the concentration 235–237) this topic is discussed. Chapter 11 and/or quantity of pollutant at its source, for deals with ‘Adsorption for Hazardous Air Pollutants example a stack. Sampling techniques are covered and Volatile Organic Compounds Control’ (58 in some detail and the following chapter (18 pages) pages). This is a major improvement over the ‘Ambient Air Quality and Continuous Emissions corresponding shorter chapter (42 pages) in the Monitoring’ includes details of the federal reference first edition that when it appeared was dated and analytical methods for a wide range of pollutants did not consider more efficient tower packings but and the calibration of continuous measurement concentrated on older types such as saddles, the procedures. No additional references were added use of which would result in considerably oversized to the two of the first edition. and costly towers that could be improved by the use of modern packings. The present chapter Financial Aspects of Environmental includes more recent generations of mass transfer packings that allow for much more practical and far Compliance less costly tower designs. A wide variety of modern The short seventh chapter (7 pages) called ‘Cost structured metal packing devices are illustrated. Estimating’ is concerned with financial aspects of The twelfth 35 page chapter considers ‘Absorption air pollution control and, for instance, examines for Hazardous Air Pollutants and Volatile Organic the ways of determining the trade-offs between Compounds Control’ using solid absorbents such as capital and operating costs. Chapter 8 (11 pages) activated carbons, activated aluminas, silica gels ‘Process Design and the Strategy of Process Design’ and molecular sieves (zeolites). Their operational outlines the decisions involved in the choice, advantages and disadvantages are detailed before design and implementation of pollution control examining ways of predicting breakthrough times processes. Illustrative examples are taken from and the necessary in situ regeneration methods the first edition, one of which is the elimination of available in different situations if charges are not brown NOx plume from a stack with the chosen to be replaced. control being a packed counter-flow absorption Chapter 13 ‘Thermal Oxidation for Volatile tower. Although this illustrates various intended Organic Compounds Control’ in 17 pages considers points the nature of the chemistries involved are three thermal oxidation processors and each is not considered nor are today’s difficulties and costs addressed in turn: thermal oxidation (flares), associated with transport and disposal of spent thermal oxidation and incineration and catalytic

82 © 2018 Johnson Matthey https://doi.org/10.1595/205651318X696657 Johnson Matthey Technol. Rev., 2018, 62, (1) oxidation, with most space given to various types interesting graph of NOx emissions from various of flaring. Only a couple of pages are given to sources starting in 1970 only extends to 1997, as ‘Catalytic Incineration’ although its use is common in the first edition, and it is a pity so much relevant today. The important role of heat recuperation in more up-to-date information was not included. many situations is stressed. While the following The lowering of NOx from a variety of burner rather longer Chapter 14 looks at the ‘Control of technologies is covered before flue gas treatment Volatile Organic Compounds and Hazardous Air techniques are considered. Selective noncatalytic Pollutants by Condensation’ in 20 pages, that might reduction, selective catalytic reduction, ozone be thought more appropriately placed before the oxidation prior to absorption of the nitric acid previous chapter because of the relative removal so formed (Scheme I), catalytic oxidation and efficiencies of the processes involved. Indeed absorption of the resulting NO2 in alkaline solution, condensation is often the preliminary process for and finally oxidation in a corona induced plasma removing high levels of volatile organic compounds are all discussed. Given the significance of NOx with the advantage of recovering what might be emissions from highway vehicles and other valuable material, followed by one of the processes transportation sources and the efficient catalytic discussed in the earlier chapter which can eliminate systems available to control them it is curious they organics to the desired low levels. Like the first were not considered in the context of relevance to edition, this chapter has two appendices: one on treating NOx emissions from other sources. the derivation of the area model for a mixture condensing from a gas and the second giving the → calculation method or ‘algorithm’. NO + O3 NO2 + O2 The short (8 pages) Chapter 15 ‘Control of 2NO + O → N O + O Organic Compounds and Hazardous Air Pollutants 2 3 2 5 2 by Biofiltration’ is in some ways a gas analogue N2O5 + H2O → 2HNO3 of reed bed treatment of effluent waters. Here process gas is passed through a moist porous Scheme I bed of material containing microorganisms (fungi, bacteria and actinomycetes) fed with oxygen and suitable nutrients maintained at an appropriate Chapter 18 ‘Control of SOx’ has 20 pages and temperature and pH. When operated correctly good like the first edition relies much on the 1998 paper performance can be obtained that is competitive by Charles Brown (5). The most significant sulfur with alternative technologies. The four page emission into the atmosphere is sulfur dioxide Chapter 16 entitled ‘Membrane Separation’ has that is produced in many combustion processes, been lengthened compared with the first edition often at levels higher than the corresponding but still cites only rather old references. What is NOx, with a major source being coal fired power covered is useful but could have been beneficially stations. When oxidised to sulfur trioxide (SO3) its expanded. The advantages, such as recovery of a reaction with water leads to formation of sulfuric pollutant in a fairly concentrated stream that is also acid often formed as fine nanoparticles. There possible with some processes previously discussed is an interesting graph showing sulfur dioxide in Chapter 12, could have been highlighted, and emission trends from various sources that starts indeed the present short chapter could well have from 1970 but only extends to 1997; so missing been cognated into the earlier one. almost two decades of key data. Several acid gas removal processes are discussed including the NOx and SOx Pollutant Removal calcium-based process that involves partial aerial oxidation of first-formed calcium sulfite to very Chapter 17 ‘NOx Control’ has 13 pages, and as in stable calcium sulfate (Scheme II). This process the first edition nitrous oxide (N2O) is included in is widely used in coal-based power stations. More the term NOx although it usually refers to molecules expensive sodium-based processes using sodium containing one nitrogen and one or more oxygen carbonate or hydroxide are then considered but atoms and generally is not meant to include N2O in all of these situations disposal of spent solution (4). Combustion processes account for most of or solid is required that can be expensive today. the atmospheric NOx with highway vehicles and Problems associated with the oxidation of sulfur other transportation being major contributors. The dioxide to sulfur trioxide and thence in the presence

83 © 2018 Johnson Matthey https://doi.org/10.1595/205651318X696657 Johnson Matthey Technol. Rev., 2018, 62, (1) of water to sulfuric mist are discussed, but not control, wet scrubbing can also selectively remove the deliberate conversion to sulfuric acid when certain gaseous components so serving a dual large amounts of sulfur dioxide are available. Here function. Mechanisms of particle growth in a mist of the clean-up costs are offset by a commercially liquid are reviewed before outlining different types saleable product. of wet scrubbers and the key factors needed for their design and implementation. Although there was some rearranging of the material from the first SO2 + Ca(OH)2 → CaSO3 + H2O edition, it remains effectively the same in the new edition and there is no significant updating of the 2CaSO3 + O2 → 2CaSO4 text or references. Scheme II Filtration of particles from a dry gas stream and especially from an air stream is frequently done using a variety of porous materials including Particulate Pollutant Removal from ‘ceramic candles’, paper cartridges and fabric bags. On the large scale fabric bags are convenient Gas Streams to operate, shake and empty with the important Particulate matter (PM) emissions down to attraction of being cost effective and traditionally nanoparticle sizes similar to those of bacteria units having several bags are called ‘baghouses’. and virus particles have in recent years attracted Chapter 23 called ‘Filtration and Baghouses’ in its growing attention because of their serious health 23 pages discusses some of the fundamentals of implications. Chapter 19 on ‘Fundamentals of filtration and bag fabric properties, before turning Particulate Control’ has 10 pages and usefully to factors such as pressure drop during use and covers some of the basic concepts in this area but the effect of removing the collected filter cake. does not include the various particle measuring The modes of bag failure include fibre weakening techniques nor emphasise the current importance during use caused by flexing or chemical attack. of nanoparticle control. Their design should provide high geometric surface ‘Hood and Ductwork Design’, covered in the and for optimal performance often a fresh bag 14 page Chapter 20, as its title suggests, deals is first treated with large particles to encourage with the removal of pollutants from specific the development of a filter cake on the surface environments, and like the first edition provides to prevent entraining small particles within the a useful guide to design a variety of hood types fabric that leads to high pressure drop. The last and the importance of smooth bends in ducting to section on design theory is an addition over that prevent erosion when high velocity particles are in the first edition and includes factors such as the present. Cyclones are widely used for removal of number of compartments as well as an example of relatively large particles from process gas streams, a baghouse design. and in recent years related technology has been Like elsewhere in the book, the final Chapter 24 employed in domestic vacuum cleaners. Chapter called ‘Electrostatic Precipitators’ closely follows 21 entitled ‘Cyclone Design’ provides an overview that in the first edition without updating the now of their design and applications in 11 pages. rather old references or highlighting the relevance of After outlining fundamental operating principles any recent developments. Notwithstanding this the typical dimensions are given with details such as coverage in 15 pages of this important technology pressure-drop, before the theoretical collection for removal of PM is very useful. In an electrostatic efficiency limitations are derived that correspond to precipitator a potential of several tens of kilovolts is internal gas rates of between 50 and 100 feet per applied to thin wires spaced a few inches apart that second. run through holes in collector plates. The applied Wet scrubbing usually involves spraying a mist of potential is sufficient to form a corona around the water into a gas stream containing PM so as to wires without flash-over or sparking, and ions increase particle size via one or more mechanisms from the corona collide with larger particles thus to facilitate their removal trapped in liquid. Chapter charging them so they migrate to the oppositely 22 deals with the ‘Design and Application of Wet charged collecting plate surfaces where they Scrubbers’ in 26 pages. With traditional micron- are retained until deliberately removed. More sized particles removal efficiency decreases important for smaller particles is diffusion charging with particle size and normally wet scrubbing is where a charge is acquired by particles moving in a employed for removing particles above about a sufficiently high potential gradient. The nature and micron in size. As well as providing particulate particularly the resistivity of the particles concerned

is important for efficient electrostatic precipitation, “Air Pollution and for example flue gas from power plants burning Control low sulfur coal has to be conditioned by introducing Technology Handbook”, into the gas small amounts of sulfuric acid or its Second precursor sulfur trioxide. A range of additives have Edition been used in different situations.

References and Index

As mentioned previously rather than have references listed at the end of each chapter as in the first edition, in the second edition they are collected together in eleven pages before a useful general index which occupies a similar number of pages.

Conclusions

The second edition of the “Air Pollution Control Technology Handbook” closely follows the outline and content of the first edition, with some updates in several places. The updated review of the Clean Air Act is well done and a notable feature is in the Raton, USA, 2001, 408 pp much enlarged Chapter 11 on the use of packed 2. C. P. Straub, “Practical Handbook of Environmental towers for absorption of volatile organic compounds Control”, CRC Press, Boca Raton, USA, 1989 that now includes modern metal structured tower 3. B. J. Finlayson-Pitts and J. N. Pitts Jr., “Chemistry packing designs. Overall this book provides a useful general reference for chemical engineers of the Upper and Lower Atmosphere: Theory, and others involved with the control of pollution Experiments, and Applications”, Academic Press, emissions from industrial plants and as such it San Diego, USA, 2000 should be available in appropriate libraries. 4. “Handbook of Diesel Engines”, eds. K. Mollenhauer and H. Tschöke, Transl. K. G. E. References Johnson, Springer-Verlag, Berlin, Heidelberg, Germany, 2010, pp. 445–446 1. K. B. Schnelle Jr. and C. A. Brown, “Air Pollution Control Technology Handbook”, CRC Press, Boca 5. C. Brown, Chem. Eng. Progr. A, 1998, 94, 63

The Reviewer

Martyn Twigg retired as Johnson Matthey’s Chief Scientist in 2010. Since retiring Martyn has maintained research activities with several universities in the UK and overseas, and has honorary positions at some. His consulting business is thriving with work in a variety of areas many of which are providing new and exciting challenges, additionally novel catalytic systems have been developed and put into production.

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Johnson Matthey Highlights A selection of recent publications by Johnson Matthey R&D staff and collaborators

A Novel Integrated Biorefinery Process for Diesel Quantitative 3D Visualization of the Growth Fuel Blendstock Production Using Lipids from the of Individual Gypsum Microcrystals: Effect 2+ 2– Methanotroph, Methylomicrobium buryatense of Ca :SO4 Ratio on Kinetics and Crystal T. Dong, Q. Fei, M. Genelot, H. Smith, L. M. L. Morphology Laurens, M. J. Watson and P. T. Pienkos, Energy M. M. Mbogoro, M. Peruffo, M. Adobes-Vidal, E. Convers. Manage., 140, 62 L. Field, M. A. O’Connell and P. R. Unwin, J. Phys. Chem. C, 2017, 121, (23), 12726 A technically feasible biological gas-to-liquid fuel (bio-GTL) process is demonstrated for the first The present study elucidates the influence 2+ 2– time. The aerobic methanotrophic bacterium, of the ratio of Ca to SO4 ions at constant Methylomicrobium buryatense, was used to supersaturation on the rate of growth at the major produce a maximum fatty acid content of 10% crystal faces of gypsum. The 3D time-evolution of dry cell weight from CH4 in batch cultures of microcrystals are measured by in situ AFM and in a continuous gas sparging fermentation these measurements are coupled to a diffusion system. Novel lipid extraction methodology with model. The growth rate at the {100} and {001} advanced catalyst design was then used to prove faces is found to be highly sensitive to solution the feasibility of upgrading phospholipids to stoichiometry, with needle-like crystals forming hydrocarbon fuels. Up to 95% of the fatty acids in Ca2+-rich solutions and plate-like crystals were recovered by two-stage pretreatment and 2– forming in SO4 -rich solutions. The highest hexane extraction of the aqueous hydrolysate. growth rate was found using a stoichiometric The extracted lipids were then upgraded by 2+ 2– solution of Ca :SO4 . hydrodeoxygenation using a palladium on silica catalyst. The final hydrocarbon mixture is 88% Unravelling the Mystery of Palladium Acetate pentadecane. W. Carole and T. Colacot, Chim. Oggi, 2017, 35, Heavy Duty Diesel Engine Emission Control to (3), 64 Meet BS VI Regulations Pd3(OAc)6, 1, is used to catalyse organic S. Chatterjee, M. Naseri and J. Li, SAE Technical transformations such as C–H activation and Paper 2017-26-0125, 2017 cross-coupling and is used for manufacturing From 2020 the BS VI emission regulations in pharmaceuticals and agrochemicals. There India will require both advanced NOx control and are two typical impurities found in commercial advanced PM control along with particle number samples. This study systematically investigates limitations. This will require implementation of full high purity palladium acetate, 1, and the two diesel particulate filter (DPF) and simultaneous NOx main impurities: palladium acetate nitrite, 2, control using SCR technologies. This presentation and polymeric palladium acetate, 3. 3 was found gives an overview of various DOC, DPF and SCR to be just as active as 1 or 2 under specific catalyst technologies and their applications, with conditions, but shows reduced performance respect to their implementation for BS VI HDD at lower temperatures and in the absence of a regulations. DPF technologies have already been phosphine ligand. In pre-catalyst manufacture, implemented in systems for the Euro VI and US pure 1 gave the best yield and purity, and hence 10 HDD regulations. may be the ideal choice for reliable results.

Me Me Me Me Me Me Me O O O O O O Me O O O O O O O O O Pd O Pd Pd Pd Me O Pd Pd O Pd O Pd Me O O O O O Pd O O O Me O O O O Pd O O N Me Me O O Me Me O Me Me

Pd3(OAc)6 Pd3(OAc)5(NO2) (Pd(OAc)2)n 1 2 3

W. Carole and T. Colacot, Chim. Oggi, 2017, 35, (3), 64

Full Deflection Profile Calculation and Young’s Ni from the PtNi catalyst. Formation of nanoporous Modulus Optimisation for Engineered High PtNi nanoparticles was promoted. Catalyst layers Performance Materials with narrower pore size distributions were formed. A. Farsi, A. D. Pullen, J. P. Latham, J. Bowen, M. Improved high current density performance was Carlsson, E. H. Stitt and M. Marigo, Sci. Rep., 2017, observed under reduced RH conditions especially 7, 46190 when using reactant gases with low relative humidity. The effects of microstructural design and sintering process used in the manufacture of engineered Reverse Monte Carlo Studies of CeO2 using Neutron materials for additive manufacturing applications in and Synchrotron Radiation Techniques medicine, engineering and technology are studied. A. H. Clark, H. R. Marchbank, T. I. Hyde, H. Y. A new methodology is presented which shows that Playford, M. G. Tucker and G. Sankar, Phys. Scr., the full deflection profile can be calculated from 2017, 92, (3), 034002 video recordings of bending tests, Young’s modulus The structure of a crystalline CeO was extracted can be characterised by an optimisation algorithm 2 by a reverse Monte Carlo analysis from neutron and optical distortions can be quantified. The total scattering (consisting of both neutron results are compared with other standard tests. diffraction and pair-distribution functions) and The new procedure allows the Young’s modulus of CeL3- and K-edge EXAFS data. A notable difference highly stiff materials to be evaluated with greater was observed between using short ranged X-ray accuracy than possible with previous bending tests absorption spectroscopy data and using medium and extends to this class of materials the possibility long range pair-distribution functions and neutron to evaluate both the elastic modulus and the tensile diffraction data regarding the disorder of Ce atoms. strength with a single mechanical test. This demonstrates the significance of studying Enhanced MEA Performance for PEMFCs under multiple length scales and radiation sources. Low Relative Humidity and Low Oxygen Content Understanding and Overcoming the Limitations of Conditions via Catalyst Functionalization Bacillus badius and Caldalkalibacillus thermarum L. Xin, F. Yang, J. Xie, Z. Yang, N. N. Kariuki, D. J. Amine Dehydrogenases for Biocatalytic Reductive Myers, J.-K. Peng, X. Wang, R. K. Ahluwalia, K. Yu, Amination P. J. Ferreira, A. M. Bonastre, D. Fongalland and A. Pushpanath, E. Siirola, A. Bornadel, D. Woodlock J. Sharman, J. Electrochem. Soc., 2017, 164, (6), and U. Schell, ACS Catal., 2017, 7, (5), 3204 F674 The kinetic and thermostability parameters A new way to enhance catalyst performance for the were investigated for a newly engineered next generation of catalytic materials for PEMFCs is amine dehydrogenase from a phenylalanine demonstrated. Functionalised annealed-Pt/Ketjen dehydrogenase from Caldalkalibacillus thermarum black EC300j (a-Pt/KB) and dealloyed-PtNi/Ketjen and were compared against an existing amine black EC300j (d-PtNi/KB) catalysts using p-phenyl dehydrogenase from Bacillus badius. The former sulfonic acid improved the performance of MEAs in showed an increased thermostability (melting

PEMFCs. The size of Pt and PtNi catalyst particles temperature, Tm) of 83.5ºC compared to 56.5ºC for was increased and there was additional leaching of the latter. This newly engineered enzyme was also

87 © 2017 Johnson Matthey https://doi.org/10.1595/205651317X696324 Johnson Matthey Technol. Rev., 2018, 62, (1) used in the reductive amination of up to 400 mM Combined In Situ XAFS/DRIFTS Studies of the of phenoxy-2-propanone (c = 96%, ee (R) <99%) Evolution of Nanoparticle Structures from Molecular in a biphasic reaction system using a lyophilised Precursors whole-cell preparation. The lower turnover E. K. Dann, E. K. Gibson, R. A. Catlow, P. Collier, number of the existing amine dehydrogenase T. E. Erden, D. Gianolio, C. Hardacre, A. Kroner, A. compared to their phenylalanine dehydrogenase Raj, A. Goguet and P. P. Wells, Chem. Mater., 2017, counterpart was studied by computational docking 29, (17), 7515 simulations. The formation of PdO nanoparticles from two Key Considerations for High Current Fuel Cell different impregnated Pd precursors, Pd(NO3)2 and Catalyst Testing in an Electrochemical Half-Cell Pd(NH3)4(OH)2 were studied, and was captured by a spectroscopic method for advanced in situ B. A. Pinaud, A. Bonakdarpour, L. Daniel, J. characterisation. The temperature assisted pathway Sharman and D. P. Wilkinson, J. Electrochem. Soc., for ligand decomposition was identified by time- 2017, 164, (4), F321 resolved diffuse reflectance infrared Fourier transform

An economical and novel half-cell method for spectroscopy which demonstrated that NH3 ligands – – swiftly and accurately measuring oxygen reduction are oxidised to N2O and NO species but NO3 catalysts in different practical electrode formats, ligands help join Pd centres via bidentate bridging including GDE and bonded GDE/membrane layers coordination. By using this spectroscopic method is shown. The challenges in developing such a in conjunction with simultaneous X-ray absorption test platform were highlighted with a concise fine structure spectroscopy, the following nucleation summary of key design considerations and fuel cell and growth mechanisms of the previous metal oxide current densities of ~1 A cm–2 were achieved. A nanoparticles were determined. The formation and simple, reproducible process of measuring catalyst growth of larger PdO nanoparticles at lower onset performance was provided by constant current temperature (<250ºC) was assisted by the bridging polarisations with IR drop measurement at each capability of Pd(NO3)2. However, impregnation from 2+ stage. At a substantially lower testing cost the ORR [Pd(NH3)4] results in well isolated Pd centres attached to the support which needs a higher activity of commercial products was accurately temperature (>360ºC) for migration to develop provided by the half-cell used in this study and observable Pd–Pd distances of PdO nanoparticles. showed good agreement with measurements made in fuel cell hardware. Many types of GDE and CCM Electrochemical and Spectroscopic Characterization with different catalyst layers can be characterised of an Alumina-coated LiMn2O4 Cathode with by this half-cell method. Enhanced Interfacial Stability M. Pasqualini, S. Calcaterra, F. Maroni, S. J. Rezvani, Mild sp2Carbon–Oxygen Bond Activation by an A. Di Cicco, S. Alexander, H. Rajantie, R. Tossici and Isolable Ruthenium(II) Bis(dinitrogen) Complex: Experiment and Theory F. Nobili, Electrochim. Acta, 2017, 258, 175 S. Lau, B. Ward, X. Zhou, A. J. P. White, I. J. Casely, A. The electrochemical performances and stability of A. Macgregor and M. R. Crimmin, Organometallics, the LiMn2O4 cathode at high temperatures were 2017, 36, (18), 3654 enhanced by pristine LiMn2O4 which was synthesised by solid state route and coated by an Al2O3 layer At temperatures below 40ºC the isolable through co-precipitation process. A pure phase ruthenium(II) bis(dinitrogen) complex and crystallised nanomaterial forming clusters [Ru(H)2(N2)2(PCy3)2] reacts with aryl ethers were observed by XRD, scanning and transmission (Ar–OR, R = Me and Ar) comprising a ketone electron microscopy. Galvanostatic cycles at several 2 directing group to achieve sp C–O bond activation. charge/discharge rates were used to study the The oxidative addition of the C–O bond to Ru(II) cycling performances of pristine and modified happens in an asynchronous manner with Ru–C materials. A combination of galvanostatic cycles at bond formation prior to C–O bond breaking in a 1C and electrochemical impedance spectroscopy at low energy Ru(II)/Ru(IV) pathway for C–O bond T = 25ºC and T = 50ºC show that the electrode/ 2 activation, was concluded by DFT studies. sp C–H electrolyte interface of Al2O3-modified LiMn2O4 bond activation was demonstrated to be more is stabilised by suppressing Mn dissolution which simplistic compared to sp2C–O bond activation by exhibits improved cycleability particularly at high experiments and DFT calculations. temperatures.

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Coordination Compounds of Hexamethylenetetramine with Metal Salts: A Review Properties and applications of a versatile model ligand

Reviewed by Jia Kaihua* and Ba chemical industry. It has been used in many fields Shuhong including as a curing agent for phenolic resins Military Chemistry and Pyrotechnics, Shenyang (1), as an accelerant in vulcanisation (2), in food Ligong University, Shenyang, 110186, China preservatives (3) and explosives (4) because of its useful properties including high solubility in water *Email: [email protected] and polar organic solvents (5). In addition, hmta can act as a multifunctional ligand, using its N atom to form coordination complexes with many Hexamethylenetetramine (hmta) was chosen as transition metals (6–10). It has been employed a model ligand. Each of the four nitrogen atoms to prepare complexes with metals, and has been has a pair of unshared electrons and behaves increasingly applied in chemical synthesis where it like an amine base, undergoing protonation and has received increasing attention due to its simple N-alkylation and being able to form coordination operation, mild conditions and environmental compounds with many inorganic elements. The friendliness (11, 12). ligand can be used as an outer coordination sphere modulator of the inner coordination sphere and as 1. Coordination with Metal Salts a crosslinking agent in dinuclear and multinuclear coordination compounds. It can also be used as A large number of complexes of hmta and a model for bioactive molecules to form a great metal salts have been studied and reports on number of complexes with different inorganic their synthesis, preparation, structure analysis salts containing other molecules. Studies of hmta and applications in medicine and military are coordination compounds with different metal summarised below. salts have therefore attracted much attention. The present review summarises the synthesis, 1.1 Coordination with Metal Salts of preparation, structure analysis and applications Main Group Elements of coordination compounds of hmta with different metal salts. The magnesium dichromate hmta complex was

first crystallised by Debucquet and Velluz with 5H2O Introduction in 1933 (13), but subsequent analysis showed the presence of six water molecules instead of Hexamethylenetetramine as a reagent has five and on the basis of former study, Dahan been used as a source of –CH=N– and –CH= put forward the topic of “the crystal structure of functions. It has a cage-like structure and is the magnesium dichromate hmta hexahydrate considered to be a crucial basic material for the complex” in 1973 (14). In Dahan’s study, the

89 © 2018 Johnson Matthey https://doi.org/10.1595/205651317X696621 Johnson Matthey Technol. Rev., 2018, 62, (1) structure of the Mg dichromate hmta hexahydrate lead to new materials such as polyoxometalates complex can be regarded as composed of two CrO4 (POMs) with different properties and more stable tetrahedra joined through a shared oxygen atom, structures. a slightly distorted octahedron around Mg and Chen et al. synthesised two new extended two hmta molecules. There were no coordination frameworks based on two different sandwich-type bonds between these groups. They are linked by polytungstoarsenates under routine conditions in hydrogen bonds, thus determining the packing and 2009. They found an advance on the sandwich- controlling the stability. type POMs, which had larger volumes and more Sieranski and Kruszynski (15) studied complexes negative charges than commonly used POMs, of Mg and hmta in 2011. Sieranski chose hmta allowing the formation of higher coordination and 1,10-phenanthroline as ligands complexed numbers with metal cations. Thus, sandwich- with Mg sulfate. Coordination compounds type POMs should be excellent building blocks for 2+ 2− [Mg(H2O)6] •2(hmta)•SO4 •5(H2O) and constructing extended networks (28).

Mg(C12H8N2)(H2O)3SO4 were synthesised and characterised by elemental and thermal analysis, 1.3 Coordination with Different Metal infrared (IR) spectroscopy, ultraviolet-visible Salts of Subgroup B Elements (UV-vis) spectroscopy, fluorescence spectroscopy and X-ray crystallography. The compounds were Allan et al. (30) studied transition metal halide found to be air stable and well soluble in water. complexes of hmta in 1970. Complexes of Mg and calcium have been most studied but hmta have been prepared with the halides of heavier alkaline earth metals have also been manganese(II), cobalt(II), nickel(II), zinc(II), investigated. For instance, after the development cadmium(II), iron(II) and copper(II) and also with of the strontium-based drug, strontium renelate, the thiocyanates of Co(II), Ni(II) and Zn(II). These which reduces the incidence of fractures in complexes have been characterised by elemental osteoporotic patients, there has been an increasing analysis, vibrational and electronic spectra and awareness of this metal’s role in humans. magnetic moments. Khandolkar et al. (16) studied the synthesis, Ahuja et al. (31) studied hmta complexes with crystal structure, redox characteristics and Mn(II), Co(II), Ni(II), Cu(II), Zn(II) and Cd(II) photochemistry of a new heptamolybdate thiocyanates. As a potentially tetradentate ligand, supported coordination compound hmta complexes with Mn(II), Co(II), Ni(II), Cu(II),

(hmta)2[{Mg(H2O)5}2{Mo7O24}]•3H2O in 2015. Zn(II) and Cd(II) thiocyanates were prepared and In their study, the synthesis, characterisation characterised by their elemental analyses, magnetic and photochemistry of a new heptamolybdate susceptibilities, electronic and IR spectral studies supported Mg-aqua coordination complex using down to 200 cm–1, 611 cm–1 as well as X-ray powder hmta as structure directing agent was reported. diffraction patterns in the solid state. It was shown Tanco Salas (17) studied a complex of aluminium that hmta, though a potentially tetradentate ligand, and hmta (‘Al-hmta’) in 2004. The Al-hmta complex acts only as a terminally bonded monodentate was found to have deodorant and therapeutic ligand or a bidentate ligand bridging between two properties with potential applications in cosmetic metal atoms, retaining the chair configuration of and pharmaceutical compositions. Furthermore, the uncoordinated molecule in all these complexes. the Al-hmta complex had the capacity to trap The tentative stereochemistries of the complexes substances which could subsequently be released, were discussed. therefore being useful as a carrier of those Agwara et al. (32) studied the physicochemical substances. properties of hmta complexes with Mn(II), Co(II) and Ni(II) in 2004, and in 2012 these complexes 1.2 Coordination with Salts of Main had been synthesised in water and ethanol (33). All the complexes were hydrogen-bonded, except the Group Elements and Subgroup B Co complex [Co(hmta) (NO ) (H O) ] which was Elements 2 3 2 2 2 polymeric. These complexes were characterised Different metal salts have different coordination by elemental analysis, IR and visible spectroscopy abilities and properties. Usually, the complex is as well as conductivity measurements. The results made up of the main group element as the central suggest octahedral coordination in which the atom, and the subgroup elements appear in the central metal ion is bonded to aqua ligands and ligands. The combination of two metals may the hmta is bonded to the aqua ligands through

90 © 2018 Johnson Matthey https://doi.org/10.1595/205651317X696621 Johnson Matthey Technol. Rev., 2018, 62, (1) Reference No. (19) (19) (20) (20) (8) (21) (21) (21) (21) (21) (21) (16) (14) (15) (18) (18) (18) CCDC No. 1003176 1003177 802360 833296 266846 859820 859821 859822 859823 859824 859825 1049780 – 813464 865230 865231 865232 F (000) 976 584 624 688 1212 2448 688 1376 1520 632 704 3152 654 322 1304 752 864 = 0.0964 = 0.0867 = 0.0900 = 0.0877 = 0.0777 = 0.0881 = 0.1321 = 0.0806 = 0.1192 = 0.0716 = 0.0639 = 0.0829 = 0.0761 = 0.1334 = 0.1214 = 0.0735 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 = 0.0384, = 0.0284, = 0.0378, = 0.0339, = 0.0268, = 0.0339, = 0.0546, = 0.0388, = 0.0450, = 0.0308, = 0.0234, = 0.0382, = 0.0317, = 1.0538, = 0.0401, = 0.0304, 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 R indices (all data) R wR R wR R wR R wR R wR R wR R wR R wR R wR R wR R wR R wR – R wR R wR R wR R wR –3 , d Z, Mg m 4, 1.416 4, 1.625 2, 1.366 4, 1.559 6 18, 1.289 4, 1.629 4, 1.522 4, 1.649 4, 1.913 4, 2.095 4, 2.493 2, 1.595 1, 1.446 4, 1.3782 2, 1.580 2, 1.733

(No. 33) (No. 1 /c (No. 14) /c (No. 14) /c (No. 14) /c (No. 14) /c (No. 14) /c (No. (No. 2) (No. 1 1 1 1 1 1 Crystal system, space group Orthorhombic, Pna2 Orthorhombic, 62) Pnma (No. Monoclinic, P2 Monoclinic, 15) C2/c (No. Trigonal, 161) R3c (No. Trigonal, 155) R32 (No. Monoclinic, P2 Orthorhombic, 61) Pbca (No. Orthorhombic, 61) Pbca (No. Monoclinic, 12) C2/m (No. Monoclinic, 12) C2/m (No. Monoclinic, 15) C2/c (No. Triclinic P Triclinic, 1) P1 (No. Monoclinic, P2 Monoclinic, P2 Monoclinic, P2 Serial No. 1a 1b 2b 1c 3b 2a 3a 4b 5b 2c 1d 1e 2e 3e 4e 1f 1g O 2 (1:1) – (1:2) 4H – • 2Br 2I 2− (1:1) • • 3 – (1:2) O O – 2 2 4 (1:1) 2NO n (1:1) 2H 2H • O (1:2) – 2SCN • •

(1:1) 2 ClO • 2 – • Cl I 2+ 2+ } 2(hmta) (1:1) (1:1) • • 2(hmta) (1:1) 2 2+ ]

(2:1) 5 • 6H • (1:1) 2 2 n n (1:1) ]

• ] ]

] 4 n 4 4 n O) 2 ) ) 2 O (1:2) 3 3 2(hmta) 2(hmta) 2 O) • • O)(hmta)] 2 2 O) (1:2) 2(hmta) (hmta) (hmta) 3H 2+ 2+ 2 • • • (hmta)] (hmta) (NO (NO • ] ] • • 6 + + + 6 6 6 2(hmta) 4 4 ] ] ] )(H • (hmta)] }] 4 4 4 4 [{Mg(H O) 5(H 7 • O) O) O) 2 O) O) O)(hmta)I] 2 • 3 24 2 2 2 O 2 2 2 O) O) O) O)(hmta)I] 2 O 2 2 2 2− 2 (H 7 (H 4 2 (hmta)(SCN) 2 2 SO Table I Major Crystal Data and Refinement for Compounds of hmta Salts with Main Group Elementsof hmta Salts with Main Group Compounds and Refinement for Data Crystal Table I Major Formula (M:hmta) [Li(H [Na(ClO [Na(hmta)(H [K [NaNO [Li(H [Li(H [Na(H [Na(H [K(H [Rb(H (hmta) {Mo MgCr [Mg(H • [Mg(H (1:2) [Ca [Sr

91 © 2018 Johnson Matthey https://doi.org/10.1595/205651317X696621 Johnson Matthey Technol. Rev., 2018, 62, (1) Reference No. (22) (23) (23) (24) (25) (26) (26) (27) (28) (29) CCDC No. – – – – – – – 660692 705356 – F (000) – – – – – – – 8504 11708 725 = 0.043 = 0.045 = 0.055 = 0.036 = 0.062 = 0.1743 = 0.2071 = 0.1141 2 2 2 2 2 2 2 2 = 0.044, = 0.042, = 0.055, = 0.038, = 0.066, = 0.057 = 0.0591, = 0.0863, = 0.0389, 1 1 1 1 1 1 1 1 1 R indices (all data) – R wR R wR R wR R wR R wR R R wR R wR R wR –3 , d Z, Mg m 2, 1.43 4, 1.50 4, 1.48 4, 1.72 4, 1.676 4, 1.83 2, 1.76 2, 4.047 4, 3.681 1, 2.329

(No. 29) (No. 1 (No. 4) (No. 137) /nmc (No. 14) /c (No. 1 2 1 Crystal system, space group Orthorhombic, 44) Imm2 (No. Orthorhombic, Pca2 Triclinic, 1) P1 (No. Monoclinic, P2 Monoclinic, 15) C2/c (No. Tetragonal, P4 Monoclinic, 14) P21/c (No. Triclinic, 1) P1 (No. Monoclinic, P2 Triclinic, 1) P1 (No. O 2 19H • ] 2 ) 39 O O 2 11

~36H • ] OH] 3 O O 2 2 } 2 O [Ce(AsVW O 2 O 2 7 2 2 O O 2 2 O) 11H 5H 18H Na 2 4H 6H • •

• 2 • • 5H 4H O • • 2 8.5 }] )(H 4 38 7H Na • O) O 2 3.5 10 2(hmta) 3(hmta) K (hmta)K 2(hmta) 2(hmta) • • O] [(hmta)-CH 2 2 • • 2 2(hmta) 2(hmta) 2 2 ] ] ] ] ] • • ] 3 3 6 6 6 ] H ] 6 6 • 6 {Na(H 2(hmta) • 28 O Ca 7 10 {FeCe(AsW [Fe(CN) [Fe(CN) [Fe(CN) V O [Fe(CN) 3 2 3 [Fe(CN) 3 ⊂ [Fe(CN) 2 3 3 3 Table II Major Crystal Data and Refinement for Compounds of hmta with Different Metal Salts Metal Different of hmta with Compounds and Refinement for Data Crystal Table II Major Formula (M:hmta) Li (3:1:2) Na (3:1:2) K (3:1:2) Ca (2:1:2) Cr (1:2:2) Sr (3:2:3) Ba (3:2:2) [(hmta)-CH [K [(hmta)-CH [(hmta)-H [H (10:1:2)

92 © 2018 Johnson Matthey https://doi.org/10.1595/205651317X696621 Johnson Matthey Technol. Rev., 2018, 62, (1) hydrogen-bonding. Antibacterial activities of the complex may exhibit different structures when ligand and its complexes show that the ligand was synthesised by different methods. For instance, active against one out of 10 tested bacteria species; the Co(II)-hmta complexes Co(hmta)2Cl2•10H2O the Co complexes [Co(H2O)6](hmta)2(NO3)2•4H2O and Co(hmta)2Cl2•6H2O could be obtained using and [Co(hmta)2(NO3)2(H2O)2] were the most active, solution synthesis and mechanical grinding, showing activity against all the microorganisms. respectively. The thermogravimetric analysis These Co complexes also show greater activity (TGA) result further confirmed that the structure of than the reference antibiotic gentamycin against the Co(II)-hmta complex obtained by grinding was

Klebsiella pneumoniae. Co(hmta)2Cl2•6H2O. In addition, there were also Kumar et al. (34) studied Cd hmta nitrate in studies on complexes of hmta with Cu and silver 2012. The crystal structure analysis of the complex ion salts. revealed a zig-zag polymeric network in which Stocker (38) studied the crystal structure of a novel the Cd(II) ions are linked via the nitrogen atom Cu(I) cyanide complex with hmta, (CuCN)3(hmta)2 of the hmta ligand. The complex has a network of in 1990. Grodzicki et al. (39) studied a complex hydrogen bonds. For isothermal thermogravimetry of Cu(II) chloride with hmta in 1991. Their study (TG) data, use of the isoconversional method was included spectroscopic and magnetic investigations an effective means of unmasking complex kinetics. which indicated that, in the crystal structure of this

Salem (35), of Tanta University, Egypt, studied compound, polymeric chains, 3CuCl•2hmta•2H2O the catalytic effect of some transition metal hmta and hmta•HCl groups occurred. complexes in hydrogen peroxide decomposition. Hazra et al. (40) studied four new The kinetics of the catalytic decomposition of Cu(II)-hmta complexes synthesised and

H2O2 with Wofatit KPS resin (4% divinylbenzene), structurally characterised by X-ray crystallography (40–80 μm) in the form of 1: l Cu(II)-, Mn(II)-, in 2013. The unique feature of the study was that Co(II)- and Ni(II)-hexamine complexes was it had been possible to construct all probable studied in an aqueous medium. The decomposition architectures that can be assembled by utilising reaction was first order with respect to [H2O2] linear dinuclear Cu(II) carboxylate as a node and the rate constant, k (per gram of dry resin) and hmta as a spacer (angular, pyramidal and increased in the following sequence: Mn(II) > tetrahedral) varying only slightly the nonfunctional Co(II) > Cu(II) > Ni(II). The active species, formed part of the aromatic carboxylic acids. as an intermediate at the beginning of the reaction, A thesis by Degagsa et al. (41) showed that had an inhibiting effect on the reaction rate. An [Cu(hmta)3(H2O)3]SO4 had been synthesised by anionic surfactant, sodium dodecyl sulfate (SDS), direct reaction of hmta and penta-hydrated Cu sulfate considerably inhibited the reaction rate. A probable (CuSO4•5H2O) and was characterised by magnetic mechanism for the decomposition process was susceptibility measurements, atomic absorption suggested, which was consistent with the results spectroscopy, IR and UV-vis spectroscopic analysis obtained. as well as conductivity measurements. Sulfate Paboudam et al. (36), reported the results of a ion and hmta composition of the complex were study on the influence of solvent on the electronic also determined using gravimetric and volumetric and structural properties of metal-hmta complexes analysis respectively. The structure of the complex in aqueous and non-aqueous solvents. Their goal was then proposed to be distorted octahedral in was to study the effect of aqueous and non-aqueous which a single Cu ion is bonded to three water and media on the coordination of hmta to metal ions. three hmta molecules. The antimicrobial activity The protometric studies of the hmta ligand had of hmta was enhanced upon complexation due confirmed that only one basic site was protonated to the increase in lipo-solubility of the complex in acidic medium and this ligand was decomposed upon coordination with the ligands. It can also in acidic medium. In aqueous medium, hmta ligand be concluded that the complexes show greater does not coordinate directly to the metal ions but antibacterial activity than the reference antibiotic, rather through the H-bonded species. In non- gentamycin, towards Staphylococcus aureus and aqueous solvents, hmta coordinates to metal ions Salmonella typhi. Thus, with further investigation displaying diversity in the resulting structures in and exploitation this complex could be developed which hmta can either be monodentate, bridged, as an antibacterial drug for the treatment of tridentate or tetradentate. infections caused by these bacteria. 6+ Cai et al. (37) studied a Co(II)-hmta complex Carlucci et al. (42) studied [Ag6(hmta)6] prepared using jet milling. An organic metal in 1997. When solutions of AgPF6 and hmta in

93 © 2018 Johnson Matthey https://doi.org/10.1595/205651317X696621 Johnson Matthey Technol. Rev., 2018, 62, (1) ethanol/dichloromethane were evaporated to analysis by TG and thermogravimetry coupled with dryness, a polymeric species was obtained, differential scanning calorimetry (TG-DSC), the

[Ag(hmta)](PF6)•H2O, containing a three-dimensional discovery that the complex is insensitive towards (3D) triconnected network topologically related to impact and friction and the evaluation of its kinetics the prototypal SrSi2. by model fitting. The preparation, characterisation Plotnikov et al. (43) studied antibacterial and and thermolysis of the La nitrate complex with immunomodulatory effects of hmta Ag nitrate in hmta were investigated. 2016. The antibacterial properties and influence Trzesowska et al. (86) studied the synthesis, on immune blood cells of the Ag-based compound crystal structure and thermal properties of a with general formula [Ag(hmta)]NO3 were mixed ligand 1,10-phenanthroline and hmta investigated. A noticeable inhibitory effect of the complex of La nitrate, providing insight into the drug was observed only at high concentration in coordination sphere geometry changes of La(III) phytohemagglutinin (PHA)-stimulated reaction 1,10-phenanthroline complexes. Geometry of blast transformation of lymphocytes (RBTL). optimisation was carried out using a density Based on this result, the tested Ag complex could functional theory (DFT) method using the Becke, be considered as a potential candidate for an three-parameter, Lee-Yang-Parr (B3LYP) functional antibacterial drug with low toxicity. by means of the Gaussian 03 program package. The minimal basis set CEP-4G was used on La 1.4 Coordination with Metal Salts of and 6-31G(d′) on oxygen, nitrogen, carbon and hydrogen atoms. Lanthanide Elements

Zalewicz (82) carried out thermal analysis of 1.5 Coordination with Metal Salts of complex salts of lanthanide chlorides with hmta Quaternary Nitrogen in 1989. Thermal decomposition of lanthanide chloride complex salts with hmta of the general In Table IV, the grey background compounds formula LnCl3•2hmta•nH2O (Ln = lanthanum, are made up of hmta coordinated with quaternary praseodymium, neodymium, samarium, nitrogen, while those on a white background are dysprosium, erbium; n = 8, 10, 12) was examined. formed by coordination of hmta molecules. In Mechanisms of the thermal dehydration reaction of other words, the hmta molecule links to other these salts were established and kinetic parameters small atoms or molecules directly, to form new of the first state of the dehydration reaction were molecules with different properties. This particular determined. The following year, the same survey mode of coordination makes it easier for molecules was carried out for lanthanide bromine salts (83). in such complexes to be connected and form mesh Zalewicz et al. (84) carried out coupled structures (Scheme I). thermogravimetry-mass spectrometry (TG-MS) Interestingly, it was found that when the hmta + investigations of lanthanide(III) nitrate complexes is changed into a [hmta-CH3] cation during with hmta in 2004. New transition metal compounds reaction, acid-catalysed decomposition of some of the general formula Ln(NO3)3•2[hmta]nH2O, hmta molecules resulted and led to a subsequent where Ln = La, Nd, Sm, gadolinium, terbium, alkylation of hmta in solution (27). Dy, Er, lutetium; n = 7–12, were obtained. The compounds and the gases evolved in the course 2. Discussion of the Coordination of their thermal decomposition were characterised Mode of hmta with Metal Salts by TGA. The measurements were carried out in air and argon environments in order to compare 2.1 Effect of Different Metal Cations the intermediate products, final products and the mechanism of the thermal decomposition. The The reaction of main group metal salts with hmta combined TG-MS system was used to identify the leads to the formation of coordination compounds, main volatile products of thermal decomposition mononuclear in case of lithium compounds, dinuclear and fragmentation processes of the obtained in other element compounds. The electronic compounds. properties of the cations strongly influence the Singh et al. (85) studied the kinetics of thermolysis coordination modes of both hmta molecules and of La nitrate with hmta in 2014. The highlights anions. Li ions are four-coordinated, Na, potassium were the confirmation of the crystal structure of and rubidium ions are six-coordinated, and Mg, this complex by X-ray crystallography, its thermal Ca, Sr and barium ions are six or more. Thus the

94 © 2018 Johnson Matthey https://doi.org/10.1595/205651317X696621 Johnson Matthey Technol. Rev., 2018, 62, (1) (44) (45) (46) (47) (47) (48) (48) (49) (45) (50) (51) (52) (52) (52) (52) (52) Reference No. (Continued) – – 729399 685560 746415 – – 211785 658325 – 293765 837612 837613 837614 837615 837616 CCDC No. – 976 1384 704 1368 1348 3112 1355.7 355 – 1068 1350 2764 4624 5184 4632 F (000) = 0.118 = 0.088 = 0.1068 = 0.0792 = 0.0932 = 0.0896 = 0.1052 = 0.126 = 0.1783 = 0.0997 = 0.15156 = 0.2047 = 0.1366 2 2 2 2 2 2 2 2 2 2 2 2 2 = 0.039, = 0.0396, = 0.0295, = 0.0352, = 0.0358, = 0.0573, = 0.046, = 0.056, = 0.0241 = 0.0702, = 0.0482, = 0.0867, = 0.1273, = 0.0731, 1 1 1 1 1 1 1 1 1 1 1 1 1 1 – R wR R wR R wR R wR R wR R wR R wR R wR – R R wR R wR R wR R wR R wR R indices (all data) –3 , d 1, 1.523 1, 1.562 2, 2.252 1, 2.294 2, 2.275 2, 2.247 4, 2.077 4, 1.42 1, 1.583 4, 1.416 4, 1.865 2, 1.224 4, 1.349 8, 1.300 8, 1.306 4, 1.342 Z, Mg m

(No. 33) (No. 33) (No. 1 1 /c (No. 14) /c (No. 14) /c (No. 14) /c (No. 176) /m (No. 14) /c (No. 14) /c (No. (No. 2) (No. 2) (No. 1 1 1 3 1 1 1 1 Triclinic, 1) P1 (No. Triclinic, P Monoclinic, 12) C2/m (No. Triclinic, 1) P1 (No. Monoclinic, P2 Monoclinic, 12) C2/m (No. Orthorhombic, Pna2 Monoclinic P2 Triclinic, P Monoclinic, P2 Orthorhombic, Pna2 Hexagonal, P6 Monoclinic, P2 Orthorhombic, 41) Aea2 (No. Orthorhombic, 60) Pbcn (No. Monoclinic, P2 Crystal system, space group ) 14 H 6 O (10:3) -C 2 ] n 2 ( ) • 6H 3 • O) H ] 2 2 ) CH 28 O 3 5 (H 2 OH) O 2 H 7 ) CCO 6 10 4 CH H 3 4H O O 5 3 V • O 2 2 (C O 5 ) 2 H • 2 3 6 Me -C ] • 6H 4H O [H n ] ] 4H • • 2 2 ( (C 4H 2 2 1.5 O-ClO • 2 2 • • • 2 ] ] ](NO 4H 3 3 2 ] • (H OH] 2 2 2 OH} OH} O 28 ) 2 2 2 2 3 O (hmta) (hmta) (hmta) O} 6 6 6 2 10 2(hmta) 8H ) ) ) 2(hmta) • • (hmta) (hmta) V 3 3 3 (NO (Br) • 2 6 6 2 6 2 2 ) ) ) ) ) 3 3 3 2 3 [H O) O-(hmta)} 4 2 2 ] CCMe CCMe CCMe 3 2 2 2 (H ](NO 2 ](NO {H 2(hmta) {(hmta)-CH {(hmta)-CH CCMe CCMe 6 6 4 • • • 2 2 Mn(OH O-(hmta)-H O) 28 28 28 O(O O(O O(O 2 2 O) O) 2 2 2 2 2 2 O O O O(O O(O 3 3 10 10 10 O (5:2) 2 V V V 6 4 4 0.5MeCN Table III Major Crystal Data and Refinement for Compounds of hmta with Metal Salts of Subgroup B Elements of Subgroup Salts Metal of hmta with Compounds and Refinement for Data Crystal Table III Major [Fe(H (1:2) [Fe(H (1:2) H (5:1) H (5:1) H (5:1) [(hmta)-CH 5H [(hmta)-H][(hmta)-CH Mn(hmta) (1:2) [Mn{H (1:1) [Mn(H (1:2) (hmta) (1:2) [Mn (1:1) [Mn (1:1) [MnFe • (1:2:2) [MnFe (1:2:2) [MnFe Formula (M:hmta)

95 © 2018 Johnson Matthey https://doi.org/10.1595/205651317X696621 Johnson Matthey Technol. Rev., 2018, 62, (1) (Continued) (53) (54) (49) (55) (52) (52) (56) (45) (57) (58) (55) (45) (45) (59) (60) (58) (61) Reference No. 1267/1319 256349 211784 – 905135 905136 – 639634 – – – 658326 639635 1267/1129 1267/1142 – 281949 CCDC No. – 500 328.9 – 485 518 896 341 – 924 – 358 342 – – – – F (000) = 0.086 = 0.0659 = 0.1550 = 0.1484 = 0.094 = 0.062 = 0.131 = 0.127 = 0.147 = 0.156 = 0.076 2 2 2 2 2 2 2 2 2 2 2 = 0.0255, = 0.032, = 0.0622, = 0.0751, = 0.044, = 0.036, = 0.054, = 0.055, = 0.056, = 0.074, = 0.028, 1 1 1 1 1 1 1 1 1 1 1 – R wR R wR – R wR R wR – R wR – R wR – R wR R wR R wR R wR R wR – R indices (all data) –3 , d – 2, 1.862 1, 1.50 1, 1.52 1, 1.685 2, 1.769 4, 1.944 1, 1.546 1, 1.640 1.123 1, 1.53 1, 1.638 1, 1.566 1 4 1 1, 1.619 Z, Mg m

(No. 2) (No. 2) (No. 2) (No. 2) (No. (No. 2) (No. 2) (No. 2) (No. 2) (No. (No.2) (No.2) 1 1 1 1 1 1 1 1 1 1 Triclinic, Triclinic, P Orthorhombic, 44) Imm2 (No. Triclinic, 1) P1 (No. Triclinic, P Triclinic, P Triclinic, P Orthorhombic, 59) Pmmn (No. Triclinic, P Triclinic, 1) P1 (No. Triclinic, 1) P1 (No. Triclinic, P Triclinic, P Triclinic, P Triclinic P Monoclinic 15) C2/c (No. P Triclinic 1) P1 (No. Crystal system, space group n } 2 OH) 5 O ] H 2 2 O 2 2 O) 6H (C 2 • • 6H 2 • O ) 2 2 (H n 4 ) 2 O } 4 ) 2 4H2O 2 4H 4 • • O O tfbdc) ) O O ) 2 2 4H 2 O) 3 2 2 3 • 2 O }](SO O O (H 2H 2 O 6 2H 2 2 }](SO 7H 2H (H • • 2 • 6 • • O-ClO • ] 2 2 2 ](NO 2 ] O) 4H ](NO 4H 4H 2 2 2 4H • O) 2 • • 2 • 2 (H O) 2 2 2 O) 2 2 2 ) 2 O) 2(hmta) 3 2 • (H ) O} (hmta) 2 (hmta)] 4 2(hmta) 2 • {Co(H ) 4 • {Ni(H (mal) 2 4 (NO 2 O ) 2 4 2 ) 6 (hmta) 4 2 (hmta) ) O 4 (hmta) • 6 4 O) 5 H 6 2 O) O) O-(hmta)} 2 O) 4 ) O-(hmta)} 2 2 H CO 2 2 3 (tfbdc)(H O) 2 7 2 O) 2 2 (H 2 (H 2 ](C (H {H ](ClO 2 (C 6 2 {H ](ClO 4 6 2 4 6 (NO CCH 6 O) 2 O) O) Co(H Ni(H O-(hmta)-H 2 O) O) 2 2 2 2 2 2 2 O) ) ) (O 2 3 3 (hmta)(H 4 tfbdc = 2,3,5,6-tetrafluoroterephthalic acid) tfbdc = 2,3,5,6-tetrafluoroterephthalic 2 2 [Mn(H (1:2) Mn (2:1) (mal = malonate) Co(hmta) (1:2) (NO (1:2) {[Co(hmta) (1:2) (H {[Co(hmta)(tfbdc)(H [Cu (4:1) [Co(H (1:2) [Co(H (1:2) [Co(hmta) (1:1) (NO (1:2) [Ni{H (1:2) [Ni(H (1:2) Ni(H (1:2) Ni(hmta) (1:2) [Ni(hmta) (1:1) [Ni(H (1:2) Formula (M:hmta)

96 © 2018 Johnson Matthey https://doi.org/10.1595/205651317X696621 Johnson Matthey Technol. Rev., 2018, 62, (1) (61) (62) (63) (62) (62) (64) (38) (54) (40) (40) (40) (40) (45) (65) (65) (66) Reference No. (Continued) 281950 969623 – 969622 969624 – – 256350 942350 942351 942352 942353 658327 963566 963568 849277 CCDC No. – – 1612 – – – – 516 – – – – 360 936 344 – F (000) = 0.062 = 0.1577 = 0.120 = 0.2011 = 0.0888 = 0.1165 2 2 2 2 2 2 = 0.048, = 0.0572, = 0.047, = 0.0792, = 0.0324, = 0.0714, 1 1 1 1 1 1 – – – – – R wR – R wR – – – – R wR R wR R wR R wR R indices (all data) –3 , d 1, 1.412 1, 1.557 4, 1.579 2, 1.639 2, 1.709 – 4, 1.817 2, 2.047 16, 1.624 2, 1.491 2, 1.581 8, 1.754 1, 1.639 2, 1.634 1, 1.547 2, 1.702 Z, Mg m

(No. 2) (No. 2) (No. 2) (No. /a (No. 88) /a (No.

2d (No. 122) 2d (No. 1 1 1 1 4 Triclinic 1) P1 (No. Triclinic, 1) P1 (No. Orthorhombic, 56) Pccn (No. Triclinic, 1) P1 (No. Triclinic, 1) P1 (No. Triclinic, 1) P1 (No. Monoclinic, 15) C2/c (No. Orthorhombic, 44) Imm2 (No. Tetragonal, I Triclinic 1) P1 (No. Trigonal 143) P3 (No. Tetragonal I4 Triclinic, P Triclinic P Triclinic P Triclinic 1) P1 (No. Crystal system, space group O 2 6H • ] 2 2– 4 O) 2 2SO (H • 2 ) O 2+ 4 2 ] 6 4H • n n O) – ] ] 2 2(3-nbzH) n 3 2 2 O-ClO O • ] 2 2 ] 2 O] 2 O) O) 2 n n 2 2 (H 4H 2NO H 2 2 O) • • • [Zn(H 2 O) • 2 n O} (H 2 2+ 2 (H ] (mal) 2 4 2 2 O)(hmta) -hmta)] -hmta)] 2 2 2 3 -hmta) O) -hmta)(H -hmta)(H 2(hmta) 4 2(hmta) O) 2 m m 2 2 • • 2 m ( ( (H -hmta)] m m 6 6 ( 4 l2 3 (H (hmta) ( ( 2 2+ 2 m 2 2 2 (hmta) ] (hmta) ]C ( 2 6 2 6 6 (hmta) 3 O-(hmta)-H O) 2 O) 2 2 btc) (2-nbz) (pa) (4-nbz) 2 (4-nbz) 3 3 3 (hmta)(H 2 2 [Ni(H (1:2) [Ni(3-nbz) (1:2) (3-nbz = 3-nitrobenzoate) Ni(H (1:2) (btc = benzene-1,3,5-tricarboxylate) [Ni(2-nbz) (1:1) (2-nbz = 2-nitrobenzoate) [Ni(4-nbz) (1:1) (4-nbz = 4-nitrobenzoate) Cu(NCO) (1:2) (CuCN) (1:2) Cu (2:1) (mal = malonate) [Cu (3:1) [Cu (3:1) (pa = phenylacetate) [Cu (3:1) [Cu(3-nbz) (1:1) [Zn{H (1:1) [Zn(hmta) (1:1) [Zn(H (1:2) [Zn (1:1) Formula (M:hmta)

97 © 2018 Johnson Matthey https://doi.org/10.1595/205651317X696621 Johnson Matthey Technol. Rev., 2018, 62, (1) (66) (66) (65) (65) (67) (68) (69) (70) (71) (71) (71) (71) (34) (61) (72) (72) Reference No. (Continued) 849278 849279 963567 963569 232121 684814 – 1025425 903838 903839 903840 903841 281951 782486 782487 CCDC No. – – 928 824 2192 40800 3936 – – – – – – – – F (000) = 0.0839 = 0.2172 = 0.1506 = 0.0391 = 0.0627 = 0.1003 2 2 2 2 2 2 = 0.0759, = 0.1640, = 0.0514, = 0.0162, = 0.0584, = 0.0362, 1 1 1 1 1 1 R wR R wR R wR R wR R wR R wR – – – – – – – – – – R indices (all data) –3 , d 2, 1.658 4, 1.782 4, 1.913 4, 2.094 4, 1.816 4, 2.072 8, 1.185 16, 1.727 4, 1.748 1, 1.641 2, 1.866 2, 1.785 4, 2.096 2, 1.824 – – Z, Mg m (No. 33) (No. 1 m (No. 227) m (No. /c (No. 14) /c (No. 14) /c (No. 14) /c (No. 14) /c (No. (No. 2) (No.

3 1 1 1 1 1 Triclinic 1) P1 (No. Monoclinic P2 Triclinic P Monoclinic C2/c (No.15) Orthorhombic, Pna2 Orthorhombic, 41) Aea2 (No. Cubic, Fd Orthorhombic, 43) Fdd2 (No. Monoclinic P2 Triclinic 1) P1 (No. Orthorhombic 35) Cmm2 (No. Triclinic 1) P1 (No. Monoclinic, 15) C2/c (No. Monoclinic, P2 Orthorhombic, 60) Pbcn (No. Monoclinic, P2 Crystal system, space group O 2 2H • O) 2 68DMF O • 2 O O H O 2 2 • 2 OH)(H ] H 5 2 O O n ) 2 2 H 2H 2 46H 2 2 • 2(3-nbzH) H • • • • O 2H )] ] 2 2 68 2– • 2 2 ] 4 ] (OH H n 2 ) 2 2 ] 2 • ) ]Cl O)] 2 4 SO 2 2 17 n ] O) O)] (OH 2 (H 2 O) 2 2+ 2 2 n (ClO 0.5 ] 2 (H -hmta) O) 4 2 (H 2 2 ) 2 m -hmta)(OH -hmta)] 3 (hmta) ) ( 2 2 -1,3,5-cyclohexanetricarboxylate) O) 3 -hmta)(DMF)(C 4 2 m m (hmta) (hmta)(OH) 34 2 ( (hmta) ( 4 4 } cis 2 2 4 3 , (hmta) (μ (hmta) 2 2 4 O) cis 2 O) 2 (4-nbz) )(hmta) )(hmta)(H 2 1 2 (2-nbz) (3-nbz) 2 3 (CTC) = ligand) = ligand 1) 3 1 2 [Zn [Zn [Cd(hmta)(H [Cd(hmta)(NO Cd [Cd(SCN) [{Cd(H [Cd(cdpc)(hmta)(H [Cd(2-nbz) [Cd(3-nbz) [Cd(3-nbz) {[Cd Cd(hmta)(NO [Cd(H Ag(L (1:1) (L Ag(L (1:1) (3:1) (1:1) (1:1) (3:1) = (CTC (2:1) (2:1) (1:1) (cdpc = 1-carboxymethyl-3,5-dimethyl-1H-pyrazole-4- carboxylic) (1:1) (1:2) (1:1) (1:1) (1:1) (1:2) (1:1) (L Formula (M:hmta)

98 © 2018 Johnson Matthey https://doi.org/10.1595/205651317X696621 Johnson Matthey Technol. Rev., 2018, 62, (1) (72) (72) (72) (72) (73) (73) (73) (74) (74) (75) (75) (75) (75) Reference No. (Continued) 782488 782489 782490 782491 169729 169730 169731 – – – – – – CCDC No. – – – – – – – – – – – – – F (000) = 0.1624 = 0.2012 = 0.1263 = 0.0987 = 0.0848 = 0.1110 2 2 2 2 2 2 = 0.0670, = 0.0617, = 0.0478, = 0.0376, = 0.0380, = 0.0414, 1 1 1 1 1 1 – – – – – – – R wR R wR R wR R wR R wR R wR R indices (all data) –3 , d – – – – 2, 1.863 4, 1.675 2, 2.248 4, 2.170 4, 2.148 4, 1.828 4, 1.885 4, 1.939 4, 1.577 Z, Mg m (No. 31) (No. 31) (No. (No. 58) (No. (No. 62) (No. (No.33) 1 1

(No. 29) (No. 29) (No. 1 1 1 /c (No. 14) /c (No. 4) (No. 4) (No. 14) /c (No. 14) /c (No. 1 1 1 1 1 Orthorhombic, Pca2 Monoclinic, P2 Monoclinic, P2 Monoclinic, P2 Orthorhombic, Pmn2 Orthorhombic, Pmn2 Orthorhombic, Pnnm Orthorhombic, Pna2 Monoclinic, 15) C2/c (No. Monoclinic, P2 Monoclinic, P2 Orthorhombic, Pnma Orthorhombic, Pca2 Crystal system, space group O 2 18H • ] 3 O) 2 O -H 2 O m 2 ( 3H 2 • 3H ) O • 3 2 2H O • -pma) O 2 ) 2 OH) 6 (pma) m 5 2 ( H O)](NO ] 2 H 6 2 O) O) 2 2 OH) 2.5H 2 2 ) 2.5H 3 • 3 • 0.5(C O) 18(H 16(H • 2 -hmta) 2(CH • • 4 • ] ] -nba)] (H ] 6 6 -nba)] -hna)](C [Ag(NH (μ-ssa)(H (μ • 2 2 p m α 2 2 (hmta) (hmta) 6 6 -hmta) -hmta) )(hmta) ) ) -hmta)](abs) -hmta)](ns) -hmta)] 4 3 -hmta)( -hmta)( -hmta)(dnba)] -hmta)( )(hmta)] 4 5 6 3 3 3 3 3 3 3 3 (L (L (L (μ (μ 2 6 6 3 8 = ligand) = ligand) = ligand) = ligand) -nba = 3-nitrobenzoate) -nba = 4-nitrobenzoate) -hna = 1-hydroxy-2-naphthate) 3 4 5 6 p m α [Ag(L (1:1) (L [Ag (1:1) (L [Ag (1:1) (L [Ag (1:1) (L [Ag(μ (1:1) (abs = 4-aminobenzenesulfonate) [Ag(μ (1:1) (ns = 2-naphthalenesulfonate) [Ag(μ (1:1) (pma = 1,2,4,5-benzenetetracarboxylate) [Ag (1:1) (ssa = sulfosalicylate) [Ag (2:1) (pma = 1,2,4,5-benzenetetracarboxylate) [Ag(μ (1:1) ( [Ag(μ (1:1) ( [Ag(μ (1:1) (dnba = 3,5-dinitrobenzoate) [Ag(μ (1:1) ( Formula (M:hmta)

99 © 2018 Johnson Matthey https://doi.org/10.1595/205651317X696621 Johnson Matthey Technol. Rev., 2018, 62, (1) (75) (75) (75) (75) (76) (76) (76) (76) (77) (77) (78) (79) (50) (80) (81) Reference No. – – – – – – – – – – – – – – – CCDC No. – – – – 1376 – – 1392 1648 2160 – – – – – F (000) = 0.1156 = 0.1490 = 0.1362 = 0.1264 = 0.1314 = 0.0957 = 0.1439 = 0.1149 = 0.054 = 0.1894 = 0.073 2 2 2 2 2 2 2 2 2 2 2 = 0.0461, = 0.0579, = 0.0499, = 0.0446, = 0.0451, = 0.1071, = 0.2275, = 0.0788, = 0.045, = 0.0676, = 0.060, 1 1 1 1 1 1 1 1 1 1 1 R wR R wR R wR R wR R wR – – R wR R wR R wR – R wR R wR R wR – R indices (all data) –3 , d 4, 1.576 8, 1.509 4, 1.965 2, 1.814 8, 1.970 2, 2.101 8, 2.022 4, 2.014 4, 2.103 4, 2.321 – 4, 2.33 8, 3.850 2, 2.830 8, 3.548 Z, Mg m

(No. 176) (No. (No. 36) (No. (No. 58) (No.

(No. 29) (No.

1 (No. 70) (No. (No. 60) (No. (No. 61) (No.

(No. 205) (No. /m 14) /c (No. 11) /m (No. 3 3 1 1 Orthorhombic, Pca2 Orthorhombic, Pbca Orthorhombic, Pbcn Orthorhombic, Pnnm Orthorhombic, 60) Pbcn (No. Hexagonal, P6 Cubic, Pa Monoclinic 15) C2/c (No. Monoclinic C2/c (No.15) Monoclinic P2 Orthorhombic Orthorhombic, Cmc2 Monoclinic, 15) C2/c (No. Monoclinic, P2 Orthorhombic, Fddd Crystal system, space group O 2 H • ) 3 SO 3 OH OH) 5 5 H H 2 2 3 OH) O)](CF ) 5 2 O 6 2 3C n 2 H • O 2 Cl 4 2 )(H 2 O)(C ) O] 8H 3 2 6 2 ](PF • 4 4H ] • SO 2 2H 3CH 3 O) • • 2 2 ] ) 3 6 0.5 ] -hna)](C O)](PF (fa)] (adp)] (CF (H 2 2 β 2 2 2 2 Br ](PF (H 6 3 0.5 (hmta)] • -toluenesulfonate) ) (hmta) 2Hg(SCN) 3 -hmta) -hmta) -hmta)(Tos) -hmta) -hmta) 2 p • -hmta)( -hmta)(noa)](H 3 3 4 3 3 ) 3 3 2 (μ (μ (hmta) (hmta) (μ (μ (μ 2 2 5 4 2 2 3 -hna = 3-hydroxy-2-naphthate) β [Ag(μ (1:1) ( [Ag(μ (1:1) (noa = 2-naphthoxyacetate) [Ag (1:1) (fa = fumarate) [Ag (1:1) (adp = adipate) [Ag(hmta)(fma) (1:1) (fma = furmaric acid) [Ag (5:5) [Ag (4:3) [Ag (2:1) = (Tos [Ag (1:1) [Ag (3:2) [Ag(NO (1:1) [H-(hmta)][HgCl (1:1) [Hg(hmta) (2:1) (hmta) (2:1) (HgCl (2:1) Formula (M:hmta)

100 © 2018 Johnson Matthey https://doi.org/10.1595/205651317X696621 Johnson Matthey Technol. Rev., 2018, 62, (1) (Continued) Reference No. (86) (85) (87) (88) (89) (90) (90) (90) (90) (90) (90) (90) (90) (90) CCDC No. 269659 993310 – 1003/4553 – 258316 266394 266395 – 266396 258317 266397 258318 266389 F (000) 1520 – – – 1544 1528 1532 1536 1580 1628 1632 1636 822 824 = 0.0396 = 0.1239 = 0.071 = 0.0623 = 0.0634 = 0.0879 = 0.0754 2 2 2 2 2 2 2 = 0.0504, = 0.0697, = 0.037, = 0.0261, = 0.0307, = 0.0333, = 0.0287, 1 1 1 1 1 1 1 R indices (all data) R wR R wR – R wR R wR R wR – – – – R wR – R wR – –3 , d Z, Mg m 4, 1.731 4, 1.686 2, 1.638 – 4, 1.655 4, 1.707 4, 1.712 4, 1.721 4, 1.793 4, 1.730 4, 1.740 4, 1.75 2, 1.756 2, 1.767 m (No. 225) m (No. /n (No. 14) /n (No. 14) /c (No. 14) /c (No. 14) /c (No. 14) /c (No. 14) /c (No. 14) /c (No. 14) /c (No. 14) /c (No. 3 1 1 1 1 1 1 1 1 1 Crystal system, space group Monoclinic, P2 Monoclinic, P2 Triclinic, 1) P1 (No. Orthorhombic, Pnma (No 62) Cubic, Fm Monoclinic, P2 Monoclinic, P2 Monoclinic, P2 Monoclinic, P2 Monoclinic, P2 Monoclinic, P2 Monoclinic, P2 Triclinic, 1) P1 (No. Triclinic, 1) P1 (No. O 2 2H • O O 2 O 2 O O O O 2 2 2 2 2 O 3H 2 3H (hmta) • 3H • 2H 2H • H 2H 2H • O O • • – • • • 2 2 3 – 3 2H 2H NO • • • NO O • 2(hmta) 2 2(hmta) ]+ • 2(hmta) • 2(hmta) 2(hmta) 4 2(hmta) – 2(hmta) • – • • • 3 4H • – 3 – – – • – 3 O 3 O) 3 3 3 2 2 2(hmta) 2(hmta) (hmta)[FeDy] • • 2NO • NO NO (H 2NO 2(hmta) NO – – 9H NO • ] • • 2NO • • • • 3 3 • • 6 • + 2 + + + + + 2+ ] ] + ] ] ] N ] ] 6 6 ] 7 6 6 8 6 7 7 2(hmta) 3NO 3NO • H O) O) O) • • O) O) – O) O) 2 2 O) 2 2 2 12 2 2 2 3+ 3+ C ]Cl (H ] ] 2(hmta) (H ][M(CN) • (H (H (H 9 2 8 8 • 2 )(H 8 2 2 2 2 )(H ) 3 ) )(H 3 ) ) ) ) 3 3 3 3 3 3 3 O) 3 O) O) O) 2 2 2 2 Table IV Major Crystal Data and Refinement for Compounds of hmta with Metal Salts for Compounds Crystal Data and Refinement Table IV Major Formula (M:hmta) [La(NO (1:1) [La(NO (1:2) [Nd(H (1:2) La(NCS) (1:2) [Ln(H (1:2) [La(NO (1:2) [Ce(NO (1:2) [Pr(NO (1:2) [Nd(NO (1:2) [Sm(NO (1:2) [Eu(NO (1:2) [Gd(NO (1:2) [Dy(H (1:2) [Ho(H (1:2)

Alkylation

Reference No. (90) (90) (90) (91) (92) reaction

HMTA HMTA-CH3

Scheme I. Schematic diagram of alkylation reaction CCDC No. 266391 266392 266393 – –

ability to form multinuclear species depends on the type of metal and increases with increasing atomic

F (000) 828 830 832 3192 – number.

2.2 Effect of Different Anions

In the studied compounds, unshared electrons were not only provided by ligands, but also by anions = 0.1256

2 – – – – – 2– – = 0.0659, such as Cl , Br ,I , NO , ClO , Cr O and SCN .

1 3 4 2 7 R indices (all data) – – – R wR – Different anions are mainly used for regulating the electric charge and sometimes make the overall molecule slightly distorted. Among the inorganic –3 –

, anions serving as ligands the most popular is SCN d due to its great tendency to link metal ions. Z, Mg m 2, 1.781 2, 1.801 2, 1.807 4, 2.798 4, 1.54178

2.3 Effect of Water Molecules

Among these compounds, water molecules also play a very important role, providing oxygen atoms or hydrogen bonds and making the overall structure more stable. Thermal analyses show that the water molecules in the investigated compounds were Crystal system, space group Triclinic, 1) P1 (No. Triclinic, 1) P1 (No. Triclinic, 1) P1 (No. Orthorhombic, 62) Pnma (No. Monoclinic, 12) C2/m (No. totally removed during thermal decomposition and further heating.

2.4 Coordination Positions and O O 2 2 Crystal System 2H 4H • • ] ] The hmta molecule is a tetrafunctional neutral 2 O 24 O O 2 2 2 O organic ligand which can be used as an outer 7 2H (OH) 2H 2H •

2 coordination sphere modulator of an inner • • Mo ) 5 5 coordination sphere or as a crosslinking agent O O) 4 2

H building block in di- and multinuclear compounds. 4 2(hmta) 2(hmta) 2(hmta) •

(C It has been used to assemble new supramolecular • • – 2 – – 3 ) 3 3 2 structures with metal ions, via various coordination O)[Ce(H 3

3NO modes, involving one to four N atoms of the hmta 3NO 3NO • • • (H [(UO 2 2 3+ molecule. 3+ 3+ ] ] ] 8 8 8 The ions and molecules appear in different O) O) O) 2 2 2 positions, called the inner and outer coordination spheres. For example, main group metal salts

Formula (M:hmta) [Tm(H (1:2) [Yb(H (1:2) [Lu(H (1:2) {H-(hmta)} (1:1) {H-(hmta)} (1:1) coordinate with hmta in the inner coordination

102 © 2018 Johnson Matthey https://doi.org/10.1595/205651317X696621 Johnson Matthey Technol. Rev., 2018, 62, (1) sphere in the case of Na, K and Rb compounds, spectral and magnetic features and ligand field and in the outer coordination sphere in the case of effects. Li, Mg, Ca and Sr compounds. This phenomenon Organometallic complexes of lanthanide metals may be affected by many factors, the electronic are mainly used to catalyse organic reactions. properties of the cation being the most important. Some other functional studies are still in progress. Under the action of hmta, different ions and water molecules form different crystal systems: 3. Applications of hmta with Metal orthorhombic, monoclinic, triclinic, hexagonal, cubic Salts or parallelepiped. Among them, monoclinic and triclinic crystal structures are the most common. 3.1 Military Applications There are also some rare 3D hybrid compounds, such as [Na(ClO4)(H2O)(hmta)]n, [NaNO3•hmta]n, Complexes of hmta have been investigated for

[K2(hmta)(SCN)2]n, [K(H2O)(hmta)I]n and military applications as explosives. Singh et al.

[Rb(H2O)(hmta)I]n. It is thereby demonstrated (45) characterised a complex of lanthanum nitrate that simple inorganic anions and organic molecules with hmta by X-ray crystallography. Thermolysis can be used to construct 3D networks possessing of the complex was undertaken using TG and high complexity (multinodal nets with complicated TG–DSC, a micro-destructive technique which stoichiometry) after proper selection of the does not require any pretreatment before analysis molecular species for net self-assembly. and can provide useful information about thermal and chemical reaction processes in a relatively 2.5 A Discussion on Synthetic Methods short time. In order to establish safe handling procedures, sensitivity of explosives towards As the ligands, salts and hmta are soluble in water mechanical destructive stimuli such as impact almost all of the synthetic methods are similar: and friction are measured. The development of make the solution at a certain proportion and stir highly energetic materials includes processability them together for a number of hours, followed by and the ability to attain insensitive munitions treatment for several days or weeks at either room (IM). The paper investigated the preparation of temperature or a specific temperature. So far, a lanthanum nitrate complex of hmta in water most complexes were prepared by virtue of this at room temperature. The complex, of molecular method. When the molecules are larger or ligand formula [La(NO3)2(H2O)6](2hmta)(NO3)(H2O), types are more complex, the synthesis conditions was characterised by X-ray crystallography. should manipulate a number of variables such as Thermal decomposition was investigated using TG, pH, ligand and cation size and type. TG–DSC and ignition delay measurements. Kinetic analysis of isothermal TG data was carried out 2.6 The Similarities and Differences using model fitting methods as well as model free iso-conversional methods. The complex was found of Transition Metal Complexes to be insensitive towards mechanical destructive Compared to transition metal coordination stimuli such as impact and friction. compounds, those of alkaline earth metals have In order to identify the end product of thermolysis, not been widely studied. Such materials should be the X-ray diffraction (XRD) patterns were environmentally friendly and their syntheses should examined which proved the formation of La2O3. be simple and economical. Thus, investigation The crystal structure reveals that the lanthanum of alkaline earth metal coordination chemistry is metal had a coordination number of 10. TG–DSC indispensable for obtaining new compounds and studies showed a three-step decomposition of new materials. the complex. The oxidation-reduction reaction

There has been much research on transition metal between oxidiser (NO3) and fuel (hmta) led to complexes with hmta as well as comparison of their ignition yielding La2O3 and gaseous products. The performance in a range of potential applications. oxide residue was confirmed by the XRD pattern. Among the most studied are complexes with Ni, The impact and friction sensitivity measurements Mn, Cu or Zn. As a transition metal centre, Ni is well showed that this compound exhibited remarkably suited for the construction of various coordination low sensitivity towards impact (>73 J) and friction polymers with N- and/or O-ligands on account of (>360 N). This nitrogen-rich metal complex may its variable oxidation states, different geometries, be a good candidate as a ‘green’ metal energetic

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The Authors

Jia Kaihua is a graduate student in Military Chemistry and Pyrotechnics at Shenyang Ligong University, China. Her interests are in the preparation and application of energetic materials. This review is a summary of coordination compounds of hexamethylenetetramine with different metal salts, which is connected to her research direction.

Ba Shuhong is an Associate Professor and master’s supervisor at Shenyang Ligong University, China. His main research directions are the design and preparation of new energetic materials, special effects ammunition and intelligent technology, digital pyrotechnics systems and applications.

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Recent Advances in Controlled and Modified Atmosphere of Fresh Produce Postharvest technologies to reduce food waste and maintain fresh produce quality

chain. This paper critically reviews the application By Natalia Falagán and Leon A. of new postharvest techniques to manipulate Terry* gaseous environments and highlights areas that Plant Science Laboratory, Cranfield University, require further study. Bedfordshire, UK, MK43 0AL Introduction *Email: [email protected] Over the past decades, the nature of food retailing has been transformed by worldwide World trade has transformed food retailing trade. The development of infrastructure, and driven the development of technology for facilities and technology across the supply chain, the transportation and storage of horticultural together with the liberalisation of the global products, providing year-round supply of fruit economy, have driven consumers’ expectations and vegetables. Horticultural produce is highly for year-round availability of fresh fruit and perishable, as fruit and vegetables continue vegetables (1). Maintaining freshness requires their metabolic processes that lead to ripening the efficient transport and storage of highly and senescence after harvest, making them perishable horticultural produce. After harvest, ultimately unmarketable. Advanced postharvest fruit and vegetables maintain their physiological technologies are essential for reducing food waste systems and continue with their metabolic while maintaining high standards of safety and activity. Respiration and transpiration lead to quality. Together with cold storage, controlled the consumption of substrates, such as sugars atmosphere (CA) and modified atmosphere and organic acids, and the loss of water, which packaging (MAP) have been applied to alter the accompanies ripening and senescence, eventually produce’s internal and external environment, making the produce non-marketable. Food waste decreasing its metabolic activity and extending is a global problem that has increased in the last shelf-life. Both CA and MAP have benefitted ten years. from technological innovation. Respiratory In developed countries, access to advanced quotient control has improved the management postharvest technology is essential for reducing of conventional and recently developed CA loss and waste while maintaining food safety and systems; gas scavengers have made MAP more quality. Historically, cold storage such as cellars, efficient; and the inclusion of natural additives basements, caves and ice houses have been used has enhanced food safety across the supply to preserve fresh produce (2). The technology has

107 © 2018 Johnson Matthey https://doi.org/10.1595/205651318X696684 Johnson Matthey Technol. Rev., 2018, 62, (1) advanced since the recognition of microbial food shown that CA alters the atmosphere surrounding spoilage in the 18th century. Fruit and vegetables the product and thus the internal gas composition, must be cooled to remove heat: before processing, reducing the fruit or vegetable metabolic activity transporting and storing (3). Nowadays, and delaying senescence. Some controversy refrigeration units are more cost-effective, exists around the use of CA. This is because the sustainable and consume less energy. They can consumer may think that CA storage confers a be used as centralised systems that operate at a counterfeit freshness to the produce they buy. The wide range of temperatures and respond quickly reality is that CA extends the seasonal availability to changes in working temperature. Reducing of produce, maintains the physicochemical and storage temperature decreases enzymatic activity, functional quality and can reduce the cost to respiratory and metabolic processes, and hence can the consumer. On top of these advantages, the extend shelf-life. Yet, current market requirements reduction of storage disorders such as chilling are more demanding, having longer postharvest injury (14–16) help reduce food waste, which periods where high quality and food safety lowers economic, social and environmental impact standards must be maintained. Because of this, (17). Furthermore, its potential as an alternative other techniques such as CA storage and MAP are to using postharvest chemicals is a subject of high used to enhance and augment cold storage. These, interest (18, 19). either actively or passively, alter the atmosphere The effectiveness of CA depends on: cultivar, composition surrounding and within the produce climacteric nature, storage temperature, selected in order to influence cellular metabolism, causing concentration of gases, stage of maturity, commodity a reduction in catabolism in climacteric fruit and quality at harvest and pre-storage treatments. If vegetables (4), and an inhibition of enzymatic the conditions are optimal for the chosen crop, reactions (5). Each commodity has its own optimal senescence will be delayed by: reducing respiration CA and MAP conditions which, together with rate and substrate oxidation, delaying ripening of controls on storage duration, relative humidity climacteric fruit and reducing the rate of ethylene and ethylene concentration, may influence production (20). Also, CA reduces the pathogen shelf-life and flavour-life. An important feature of respiration rate, and can maintain natural disease the technologies is that they are innocuous and resistance. In summary, CA prolongs storage life. can be applied to organic fruit and vegetables. CA However, inappropriate CA store management and MAP techniques have been evolving because of can provoke the development of off-odours, the development of new technology and improved off-flavours and physiological disorders. To obtain knowledge of fresh produce physiology. This the best results, it is essential to have a deep review outlines the most recent approaches in CA knowledge about the produce physiology and adapt and MAP techniques highlighting their advantages, the technology to each scenario. It is generally disadvantages and main applications. accepted that applying CA as soon as possible is the best option to maximise efficacy. Yet this causes a Controlled Atmosphere Storage dramatic change in the surrounding environment that can elicit abiotic stress in the product (21). CA technology is one of the most successful Recent studies propose CA scheduling as a means techniques developed by the postharvest industry to better adapt to produce metabolism. Chope et in the 20th century. However, ca. 100 BC the al. (22) reported that delaying the start of CA on Romans already stored grain in sealed underground onions for three weeks was as effective at controlling pits (6, 7). Jacques Etienne Berard observed in the sprout growth using continuous CA. Alamar et al. early 1800s in France that fruit did not ripen in (13) applied different CA timings on strawberry, a low oxygen atmosphere (8). In 1927, Kidd and finding that the application of CA for 2.5 days mid-

West found that a reduction in respiration rate in way through storage at 5ºC (2.5 days; 15 kPa CO2 apples was correlated to an extension of storage + 5 kPa O2 after 2 days in air) increased shelf- life (9). Since this time, postharvest scientists life by 3 days. Likewise, it is recommended to use have progressively studied the effect of different low-temperature conditions during pre-storage. atmospheres on most horticultural produce to The equipment and the methods used are under obtain optimal concentrations of gases (10–13). constant development. However, the following key The application of conventional CA generally components should be installed for an efficient CA consists of increasing carbon dioxide levels and facility: gas tight stores or cabins, a refrigeration decreasing the oxygen concentration. It has been system, gas control instrumentation and robust

108 © 2018 Johnson Matthey https://doi.org/10.1595/205651318X696684 Johnson Matthey Technol. Rev., 2018, 62, (1) monitoring systems (for example oxygen, carbon In order to achieve an accurate gas control, dioxide, ethylene, temperature and humidity CA rooms are continuously monitored to detect sensors). the aforementioned stress. Ethanol production The optimal gas concentrations should be adapted (dynamic control system (DCS)), chlorophyll to each commodity. Preferably, fruit and vegetables fluorescence (DCA-CF), and the assessment of the must be stored under low oxygen concentrations, respiratory quotient (RQ) are the main parameters close to the anaerobic compensation point (ACP); measured. DCS uses ethanol, the final product of taking into account that oxygen levels above the fermentation, as the stress signal for anaerobic ACP quickly increase respiration rate, and when conditions. It is determined in the headspace of a below, fermentation will adversely affect fruit sample box placed in the storage room with sensors metabolism (23). In the 1990s, it was demonstrated such as a quartz crystal microbalance (33, 34). The that fruit like apples can be stored at oxygen levels main issue with this method is that most of the as low as 0.5% (24–26). If the storage is carried ethanol produced during fermentation remains in out below 2.5 kPa of oxygen, it is considered ultra- the cells, making its detection difficult (23). low oxygen (ULO) storage. Although applying ULO DCA-CF is another non-destructive method for is more expensive than conventional CA methods, measuring the primary processes of photosynthesis its use has resulted in better firmness and quality such as light absorption, excitation energy transfer retention (27). and the photochemical reaction in photosystem Another option is reducing the initial oxygen II (35). These processes are affected by factors concentration with the objective of conditioning such as light intensity, temperature, humidity the fruit to resist further abiotic stresses. This and gas composition. In this sense, changes in CF technique is known as initial low oxygen stress measurements are indicators to stress, such that (ILOS) and it has been found to be effective against CF can detect cellular injury in advance of symptom superficial scald, avoiding the use of chemical development (36). It has been successfully used treatments (28). CA and ULO storage are static to perceive low oxygen stress in CA environments systems, which means that the atmosphere is set for storage of apple, avocado, pear and kiwifruit to an optimal level and does not vary according (30, 36, 37). The limitations of this system are: to the product response (29). This has several that sensors can only measure a small portion of disadvantages: the lowest optimal oxygen content an individual fruit, extrapolating the results; they must be adjusted for each produce and condition cannot repeatedly measure at the same point; (such as cultivar and seasonal variation) and it is the sensors are still expensive; they need to be difficult to access fruit within a container without calibrated; and peaks in CF can also be caused disturbing the atmosphere, which gives no access by other kind of stress, for example abiotic stress to real-time information (30). (drought, chilling injury). The most popular system The CA technique has evolved with the development for DCA-CF is based on fluorescence interactive of more accurate control systems, to dynamic response monitor (FIRM) sensors, which detect controlled atmosphere (DCA storage). DCA storage fluoresced light (Isolcell, s.P.a., Italy). aims for the lowest possible oxygen level, as per An alternative to these methods is the RQ ULO, but adapts the gas concentrations dynamically measurement of stored produce, which can be on the basis of the changing physiological response used as a stress signal to adapt gas levels in the of the produce (31). If the system detects storage facility (23). RQ is the ratio of the carbon low-oxygen stress, it increases the oxygen level dioxide production rate to the oxygen consumption until the commodity response is back to the optimal rate of the stored fruit or vegetable (Figure 1). threshold (23). This method is attractive because The RQ will remain under one in aerobic conditions it uses existing CA technology that is improved by and increase exponentially over unity if oxygen controlling parameters in near real-time, extending concentration approaches zero, caused by a shift the produce storage life longer than traditional CA. from aerobic respiration to fermentation, which It can also reduce the impact of storage disorders implies low oxygen stress (5, 38). In this case, such as superficial scald in apples and pears. Until the limitation when applied to DCA systems recently, superficial scald was prevented by using is the leakage of the storage facility, which the postharvest antioxidant diphenylamine (DPA) introduces noise to the results. A new automatic or ethoxyquin (only for pears), but their use is no DCA control system based on online real-time RQ longer permitted within the European Union (32). measurements has been recently developed that is

200 Fig. 1. Effect of oxygen concentration on oxygen Aerobic compensation consumption and carbon dioxide point production in fresh produce 150

RQ shift 100

Relative respiration, % respiration, Relative 50

O2 consumption CO2 production CO2 production (fermentation) 0 0 2 4 6 8 10 12

O2 concentration, %

integrated into the control system of the CA facility communication to a central operating panel. In it, (39). This enables the CA system to adjust the RQ is periodically and automatically measured and gas concentrations immediately according to RQ used to set the gas concentrations in the storage readings, avoiding the mentioned noise as it takes room. It is recommended for products that are into account the leakage in a predictive model (23). kept in long-term storage, such as apples, kiwifruit This technology can be applied in individual and pear, as, at this moment, it requires a capital sample containers that are representative of the investment and is expensive to operate (40). Novel conditions of the storage facility. An example of biosensors and photonics are now being developed this option is the LabPod (Storage Control Inc, to better understand physiologically-targeted CA USA), a hermetically water-sealed container with interventions to control ripening. They will also a stainless steel base and a transparent plastic allow real-time phenotyping, which offers new cover (Figure 2). Each pod has oxygen, carbon insight into fruit and vegetable quality and safety dioxide and temperature sensors with digital aspects (41). Apart from the factors already mentioned, the

action of ethylene (C2H4) has to be carefully considered. Ethylene is a natural plant hormone which works at trace levels stimulating or regulating fruit ripening (especially in climacteric fruit) (42). CA storage implies the increase of carbon dioxide and the reduction of oxygen. Low oxygen and/or elevated carbon dioxide concentrations inhibit the ethylene production rate by suppressing 1-aminocyclopropane-1-carboxylic acid synthase transcripts, the key enzyme in the synthesis pathway of ethylene (43). Another effective option to inhibit ethylene is the application of 1-methylcyclopropene (1-MCP) (Figure 3). 1-MCP is a gaseous cyclic olefin which binds irreversibly to ethylene receptors avoiding ethylene-dependent Fig. 2. LabPod (Storage Control Inc, USA) storing responses (44–46). 1-MCP is very efficient Gala apples because its affinity for the receptor is around ten

thickness) and the temperature of the surrounding environment (50). When the packaging technology Ethylene (gas) is adapted to the produce respiration rate, an equilibrium modified atmosphere (EMA) can be Ethylene binding site 1-MCP established in the package, leading to a reduction in the respiration rate and metabolic processes, Signal and with it, increased product shelf-life. The most Genetic processes used gases in MAP are oxygen, carbon dioxide and nitrogen. As previously mentioned, whilst oxygen Physiological response (e.g. ripening) is consumed during storage life, carbon dioxide is generated through respiration. This process, Fig. 3. Diagram showing the sequence of ethylene as well as the interchange with the surrounding action and 1-MCP interaction in one of the possible environment, will help to achieve EMA. sites Packaging systems delay senescence by decreasing respiration rate, metabolic activity and microbial growth (51). There are two types of MAP times greater than that of ethylene (44). Some based on gaseous transmission rates: passive and recent studies show that the effects of 1-MCP are active. The former uses the natural permeability comparable to CA in maintaining fresh produce and thickness of the packaging film to establish the quality (47). However, DCA is the solution that can desired atmosphere for the product as a result of its allow optimal results during postharvest storage respiration (52). Despite the promise of MAP, it is (48). not yet used ubiquitously in the food industry (53) Research has been focused on inhibiting ethylene for the following reasons: the cost of the technology action in the last decade. However, scrubbing packaging machinery and materials, the analytical technologies are also available and their efficiency equipment necessary to ensure the correct gas has been widely proven. These techniques include mixture, and the fact that some benefits of MAP high-temperature catalytic degradation, oxidation are lost once the package is opened or where there of ethylene through potassium permanganate are leaks. The most common polymers used are

(KMnO4)-based mechanisms, activated carbon polyamide (PA), polypropylene (PP, oriented or and impregnated zeolite (42). The most not), polyethylene (PE), low density polyethylene commercially used technique to remove ethylene (LDPE), linear low density polyethylene (LLDPE), is simple ventilation but it is not compatible with polystyrene (PS), polyester (PES), polyethylene environments which require sealing such as CA terephthalate (PET), ethylene vinyl alcohol (EVOH) or some MAP solutions and ethylene adsorption and polyvinylchloride (PVC) (54, 55). materials. The technique has been successfully applied to whole and fresh-cut products such as artichokes Modified Atmosphere Packaging (56), lettuce (57) and strawberry (58). In order for the produce to create the optimal atmosphere Packaging should be designed according to the the packaging material must be permeable. marketing and distribution needs of the product. These packaging films can be microperforated to It should do the following: protect the product enable gas interchange between the inside and from mechanical damage, avoid moisture loss the outside of the packaging. Xtend® packaging and modify the internal atmosphere to prolong (Johnson Matthey, UK) helps equilibrate the packed shelf-life. Physical injuries (vibration and produce atmosphere within the optimal range of compression bruises or abrasion damage) can be oxygen and carbon dioxide for a specific fruit or reduced by proper package design which acts as a vegetable. It is also able to retain humidity within shock-absorber. Packages must also allow the product the package, reducing weight loss during storage. to reach the optimal storage temperature quickly. Another example is PerfoTec® (PerfoTec BV, The MAP is a technology that alters the atmosphere Netherlands). The film permeability for a particular within the package according to the interaction product is determined and the PerfoTec® laser between the product respiration rate and the system carries out the required microperforations. transfer of gases through the package (49). New structural polymers are now available to Diffusion through the package depends on the improve packaging materials towards bio-based film characteristics (permeability, area and and bio-degradable, non-petroleum sustainable

111 © 2018 Johnson Matthey https://doi.org/10.1595/205651318X696684 Johnson Matthey Technol. Rev., 2018, 62, (1) packaging materials such as polylactic acid (PLA; for Krebs’ cycle (77), slowing down ripening processes example, NATIVIA®, UAE) made from corn or other and decay. However, their efficacy will depend on starch or sugar sources (59), polylactide aliphatic cultivar, maturity stage and storage conditions. copolymer (CPLA) (60) and polymers derived With respect to active inserts, oxygen scavengers from high proportions of recycled plastics (61). At are traditionally based on a metal powder (generally this point, these new materials have limitations iron, ferrous carbonate or metallic platinum), in terms of their cost and technical performance. ascorbic acid and enzymes (glucose oxidase and MAP is highly dependent on respiration and alcohol oxidase). temperature. To overcome this, membranes like Active inserts are defined according to their BreatheWay® contain thermoresponsive crystalline scavenging reaction (such as enzyme mediated polymers that allow high gas transmission rates at oxidation and oxidation speed), and their high temperatures (62). scavenging capacity (millilitres of oxygen One of the current challenges of MAP is the control removed). They can lower oxygen concentrations of transpiration rate (TR) in fresh produce storage within the sealed packs, slowing deterioration (63). TR is related to the mass transfer process from caused by oxidation (78). The use of sulfites, such the stored product to the surrounding atmosphere as potassium sulfite, and natural antioxidants, (64) and is affected by fresh produce factors such including tocopherols, lecithin, organic acids and as maturity stage, and environmental factors plant extracts, are currently being explored (79) such as water vapour pressure deficit gradient to reduce the oxidation of the fresh produce and (65). Water loss after harvest leads to weight delay denaturation of proteins (80). Currently loss and quality reduction of the produce while an they are applied to breads, nuts, candies and accumulation of water at the product surface will confectioneries, coffee and tea and processed, help the growth of spoilage microorganisms (63). smoked and cured meats, among others, to Nowadays macroperforations are used to diminish improve storage conditions. Generally these the impact of this problem, yet their presence scavengers are designed for oxygen removal from precludes the creation of a modified atmosphere. sealed food packaging and not semi-permeable Microbial growth within the package is one of the fresh produce EMA packaging. More research is challenges for MAP. Nanotechnology can enhance needed as existing oxygen scavenger formats are packaging functionality by adding antimicrobial, typically cumbersome and not appropriate for fresh structural and barrier properties (66). This produce storage conditions. technology can also improve mechanical properties Carbon dioxide scavengers (chemical absorbers of films and reduce oxygen transmission rates such as calcium hydroxide, sodium carbonate, (61). Other gases have also enriched MAP: helium, calcium oxide; physical absorbers such as argon, xenon and nitrous oxide (N2O). They are zeolite and activated carbon) can similarly delay also reported to reduce microbial growth and senescence and reduce browning and mould decay maintain quality (67–69), but are yet to be widely (81). This is particularly interesting for climacteric used commercially. products, which produce high concentrations Active MAP is based on the alteration of gases of carbon dioxide affecting their organoleptic within the package to achieve the ideal gas characteristics. equilibrium earlier than passive MAP. The techniques Another option is the removal of ethylene used include flushing pre-set gas mixtures into from the package. Ethylene scrubbers, such as the package; introducing gas scavengers, such as potassium permanganate pellets (Ryan Co, USA) oxygen and carbon dioxide scavengers, moisture and clay mineral coated strips (It’s Fresh, UK) absorbers, and ethylene scrubbers; and inserting (42), can slow down senescence and reduce decay gas emitters, such as carbon dioxide emitters (70). by neutralising the effect of the plant hormone.

In the case of flushing gas mixtures, it is proven Carbon dioxide emitters increase carbon dioxide that high initial concentrations of oxygen (above concentration within the package, helping achieve 70 kPa) have an antimicrobial effect on aerobic and the optimal gas mixture for each product (70). anaerobic microorganisms (71–73). This is also A recent trend, known as smart or intelligent effective for helping inhibit enzymatic browning packaging, is to fit packaging with sensors (74, 75) and avoiding loss of firmness (74, 76). able to monitor quality, microbiological growth However, operating in high oxygen environments or temperature along the supply chain (82, carries the risk of fire. High carbon dioxide 83). Intelligent packaging components include concentrations inhibit several enzymes of the radio frequency identification sensors (84),

112 © 2018 Johnson Matthey https://doi.org/10.1595/205651318X696684 Johnson Matthey Technol. Rev., 2018, 62, (1) time-temperature and ripeness indicators (for respond to consumers’ demand for ready to eat fruit example, ripeSense®, New Zealand), and biosensors and vegetables. The fresh-cut industry has to face (85). Also, carbon dioxide and oxygen gas sensors not only physiological issues that lead to ripening are being developed for real-time monitoring of and senescence of fresh produce, but also the likely produce quality (86). Some low cost intelligent microbial growth caused by the exposure of tissues packaging options are available to provide visual to the environment. Mechanical wounding, due to information on freshness: fluorescent dyes or minimal processing, damages cells making it easier molybdenum ions (87, 88). These can inform not for pathogens to contaminate the produce and for only about food quality, but also food safety (89). enzymes to catalyse non-desirable processes such It is possible to create physical barriers on the as browning. Hence, the application of the correct fruit surface which provide protection against gas mixture environment, edible coatings and moisture loss and can help control oxygen and natural antimicrobials are critical in this case (99). carbon dioxide concentrations, in a similar way Other postharvest technologies can complement to MAP as they are able to change the internal MAP. In order to control microbial growth, atmosphere of the produce. This technique is non-ionising, germicidal and artificial ultraviolet C known as edible coating (90). The ideal edible (UV-C) light (100) can be applied. Some studies coating should be able to extend storage life show an enhancement of bioactive compounds without causing anaerobiosis and reduce decay when this technique is used (100, 101). There are and water loss (90), acting also as antimicrobial no residues left in the fruit or vegetable after UV-C agents. The development of this technique began treatment, which is an advantage in meeting new with the application of wax coatings on fruit using consumer requirements. A promising technique dipping methods. The material used to formulate to improve food safety is cold plasma technology them has to be generally regarded as safe (NSW Department of Primary Industries, Australia). (GRAS) and has evolved with time. According to It is created by applying an electric current to Arvanitoyannis and Gorris (91), the edible coating normal air or a gas to generate reactive gaseous must: be water resistant and cover the product species with antimicrobial activity. It involves no completely when applied, reduce water vapour chemicals, and therefore, no residues. permeability, generate the optimal atmosphere, improve the produce appearance, melt over 40ºC Future Prospects without decomposition, dry with high efficiency performance, have low viscosity, be easily The growing demand to decrease postharvest use emulsifiable, be economical, be translucent and of chemicals and the need for more sustainable not interfere with produce quality. The composition technologies has led to the development of of edible coatings has advanced to be based on improved CA and MAP storage methods. More natural compounds. Some of the latest examples research is needed to understand the dynamic are: Aloe vera gel (92), alginate-based edible physiological responses of fresh produce to CA and coatings (93), shellac (94) or silk fibroin (95). MAP in order to determine the optimal conditions At a commercial level, AgriCoat NatureSeal Ltd, for each cultivar and scenario. Critically, a better UK, provides a sucrose ester based edible coating understanding of how flavour can be extended is for whole fruit, mainly melon (Semperfresh®, required; one of the most repeated consumers’ UK) and fresh-cut produce (NatureSeal®, UK), complaints is the lack of fresh fruit and vegetable which are sulfite-free (GRAS) and delay ripening flavour. With respect to MAP, the structure and effects. Edible coatings are able to extend the functionality of film polymers should be improved, shelf-life of perishable products, maintain initial and new sustainable materials developed and appearance, including colour and gloss, and delay deployed. Moreover, a reduction of cost will make decay. The correct formulation should not affect these technologies accessible to a wider number flavour or appearance. To maintain safety within of companies within the sector, improving the the packaging an application of solutions such as adoption of MAP and reducing food waste. natural antimicrobial like cinnamon or vanillin (93), Research should focus on optimising gas and essential oils (96) can be used within the edible concentrations by selection of appropriately coating or on their own. Films can also be coated permeable packaging materials, and on improving with inhibitors such as titanium dioxide (TiO2), which their interaction with active materials such as is able to inactivate pathogens like Escherichia coli scavengers, emitters, and nanoparticles. In this (97, 98). These packaging options are required to case, a modelling approach taking into account

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The Authors

Natalia Falagán graduated as an Agricultural Engineer at the Technical University of Valencia, Spain, and obtained her PhD in Food Engineering in 2015 at the Technical University of Cartagena, Spain. She conducted her research work at the University of California, Davis, USA, and Institut National de la Recherche Agronomique (INRA), Bordeaux, France. She was awarded the PhD extraordinary award. During this period, Falagán gained extended experience in both controlled atmosphere storage and modified atmosphere packaging, analytical methods for the targeted and untargeted analysis of secondary metabolites as well as vitamins and sugars in numerous matrices. Falagán was appointed as a Research Fellow in 2016 by Cranfield University, UK.

Professor Leon A. Terry is Director of Environment and Agrifood at Cranfield University. He is an appointed member of the UK Government’s Agriculture and Food Security Strategy Advisory Panel (AFS SAP) 2015–2018, and a long-standing member of the Postharvest Biology and Technology Editorial Board. He is a Fellow of the Royal Society of Biology, the Higher Education Academy and the Institute of Agricultural Engineers. Terry has been heavily involved with two of the UK Government’s Agritech Centres as a current Director of the Agri-EPI Centre and former interim Director of Crop Health and Protection.

www.technology.matthey.com

Challenges and Opportunities in Fast Pyrolysis of Biomass: Part I Introduction to the technology, feedstocks and science behind a promising source of fuels and chemicals

By Tony Bridgwater and adaptability to a wide variety of feedstocks Bioenergy Research Group, European and products. Pyrolysis operates in anaerobic Bioenergy Research Institute, Aston University, conditions where the constituents of biomass are Birmingham B4 7ET, UK thermally cracked to gases and vapours which usually undergo secondary reactions thereby Email: [email protected] giving a broad spectrum of products. There are a number of conditions and circumstances that have a major impact on the products and the Fast pyrolysis for liquids has been developed in process performance. These include feedstock, recent decades as a fast and flexible method technology, reaction temperature, additives, to provide high yields of liquid products. An catalysts, hot vapour residence time, solids overview of this promising field is given, with a residence time, and pressure. comprehensive introduction as well as a practical Pyrolysis has been applied for thousands of guide to those thinking of applying fast pyrolysis years for charcoal and chemicals production but liquids (bio-oil) in various applications. It updates it is only in the last 40 years that fast pyrolysis the literature with recent developments that have for liquids has been developed. This operates at occurred since the reviews cited herein. Part moderate temperatures of around 500°C and I contains an introduction to the background, very short hot vapour residence times of less science, feedstocks, technology and products than 2 seconds. Fast pyrolysis is of considerable available for fast pyrolysis. Part II will detail interest because this directly gives high yields some of the promising applications as well as of liquids of up to 75 wt% which can be used pre-treatment and bio-oil upgrading options. The directly in a variety of applications (1) or used as applications include use of bio-oil as an energy an efficient energy carrier. Intermediate and slow carrier, precursor to second generation biofuels, pyrolysis focus on production of solid char as the as part of a biorefinery concept and upgrading to main product with the liquids and gases usually fuels and chemicals. as byproducts, although increasing attention is being paid to maximising the value of these 1.Introduction byproducts. Pyrolysis has also been used for many years to reduce quantities of waste that 1.1 Background require disposal as well as making the residues less harmful to the environment. These processes Pyrolysis has become of major interest due to the have traditionally employed slow pyrolysis as the flexibility in operation, versatility of the technology core technology.

1.2 Science of Pyrolysis oil on a weight basis or 60% that of fuel oil on a volume basis due to the high density. This liquid is Pyrolysis is thermal decomposition occurring in the referred to as bio-oil and is the basis of the recent absence of oxygen. Lower process temperatures ASTM standard (2). A high yield of liquid is obtained and longer hot vapour residence times favour with most low ash biomass of up to 75 wt% on the production of charcoal. Higher temperatures dry biomass feed. The essential features of a fast and longer hot vapour residence times increase pyrolysis process for producing liquids are: biomass conversion to gas, and moderate • Feed moisture content of less than 10 wt% since temperatures and short hot vapour residence time all the feed water reports to the liquid phase are optimal for producing liquids. Three products along with water from the pyrolysis reactions. are always produced, but the proportions can High water contents in the liquid product can be varied over a wide range by adjustment of lead to phase separation. the process parameters. Table I indicates the • Very high heating rates and very high heat product distribution obtained from different modes transfer rates at the biomass particle reaction of pyrolysis, showing the considerable flexibility interface usually require a finely ground biomass achievable by changing process conditions. Fast feed of typically less than 3 mm as biomass pyrolysis for liquids production is currently of generally has a low thermal conductivity. As particular interest commercially as the liquid can fast pyrolysis for liquids occurs in a few seconds be stored and transported, and used for energy, or less, heat and mass transfer processes transport fuels, chemicals or as an energy carrier. and phase transition phenomena, as well as chemical reaction kinetics, play important roles. 2. Fast Pyrolysis The rate of particle heating is usually the rate- limiting step in most fast pyrolysis processes In fast pyrolysis, biomass decomposes very quickly other than ablative pyrolysis where biomass to generate mostly vapours and aerosols and some directly contacts the hot reactor surface (3) charcoal and gas. After cooling and condensation, • Carefully controlled fast pyrolysis reaction a dark brown homogenous mobile liquid is formed temperature of around 500°C for most biomass if wood or a low ash feed is used. The liquid has a maximises the liquid yield. Ash, particularly heating value about 40% that of conventional fuel alkali metals, catalyses secondary reactions

Table I Typical Product Weight Yields from Wood (Dry Feed Basis) by Different Modes of Pyrolysis

Mode Conditions Liquid Solid Gas

~500°C 12 wt% Fast Short hot vapour residence time <2 s 75 wt% (bio-oil) 13 wt% char Short solid residence time up to 10 s

~400°C Moderate hot vapour residence time 40 wt% in two 40 wt% Intermediate 5–20 s 20 wt% phases char Moderate solid residence time up to 20 minutes

~400°C Long hot vapour residence time up to Slow pyrolysis 30 wt% in two 35 wt% hours depending on technology 35 wt% (Carbonisation) phases char Long solid residence time depending on technology

~750–900°C Gasification Up to Up to Short hot vapour residence time 5 s Minimal (allothermal) 2 wt% char 98 wt% Short solid residence time

0 wt% unless Torrefaction ~250–300°C vapours are 70–80 wt% 15 wt% (slow) Solids residence time up to 30 mins condensed, then solid up to 15 wt%

of the pyrolysis vapours to carbon dioxide Ash can be managed to some extent by selection and water resulting in lower liquid yields with of crops and harvesting time especially with a higher water content. In extreme cases (at rhizome crops like Miscanthus which senesces over ash levels typically above around 2.5 wt%) so winter with alkali metals returning to the rhizome, much water is formed that phase separation of however it cannot be eliminated from growing the liquid occurs. Therefore, short hot vapour biomass. Ash can be reduced by washing in water residence times are required of typically less or dilute acid and the more extreme the conditions than 2 seconds to minimise secondary reactions in temperature or concentration respectively, the • Rapid removal of product char is necessary to more complete the ash removal. Recent work has minimise catalytic cracking of hot vapours since shown that surfactants are the most effective (11). all the biomass ash is retained by the char. However as washing conditions become more Failure to minimise contact with char results in extreme firstly hemicellulose and then cellulose is cracking as above lost through hydrolysis. This reduces liquid yield • Rapid cooling of the pyrolysis vapours to and quality. In addition, washed biomass needs to minimise thermal cracking to give the bio-oil have any acid removed as completely as possible product, for similar reasons as for effective and recovered or disposed of and the wet biomass char removal. This is usually achieved in a has to be dried. quench system often employing a non-miscible So washing is not often considered a viable liquid such as a hydrocarbon or cooled product possibility, unless there are some unusual bio-oil. circumstances such as removal of contaminants. Several comprehensive reviews of fast pyrolysis Another consequence of high ash removal is for liquids production are available (4–10). the increased production of levoglucosan and levoglucosenone which can reach levels in bio-oil 2.1 Feedstocks where recovery becomes an interesting proposition.

Biomass is usually made up of three main 2.2 Technology components – cellulose, hemicellulose and lignin with water and ash. Cellulose is a polymer of A conceptual fast pyrolysis process is depicted in glucose, a six-carbon molecule, which can be Figure 1 from biomass feed to collection of a liquid thermally and catalytically cracked to monomers product. Each process step has several alternatives and decomposition products. Hemicellulose is such as the reactor and liquid collection but the a polymer of five-carbon rings that can also be underlying principles are similar. cracked to smaller organic molecules. Lignin is At the heart of a fast pyrolysis process is the a complex polymer built up of phenolic units reactor. Although it probably represents only about that can be cracked to a wide range of phenolic 10–15% of the total capital cost of an integrated products. Other components of biomass include system, most research and development has water at up to 60 wt% in freshly grown biomass; focused on developing and testing different and ash, mostly alkali metals from nutrients, reactor configurations on a variety of feedstocks, which is catalytically active and causes cracking of although increasing attention is now being paid organic molecules. This is beneficial in gasification to improvement of liquid collection systems and where they help to crack tars, but not beneficial improvement of liquid quality. The rest of the fast in pyrolysis where they crack the organics in pyrolysis process consists of biomass reception, the vapour resulting in lower liquid yields with storage and handling, biomass drying and grinding, an adverse effect on liquid properties. The alkali product collection, storage and, when relevant, metals that form ash which are essential for upgrading. nutrient transfer and growth of the biomass are The byproduct char is typically about 15 wt% of significant in fast pyrolysis. The most active is the products but about 25% of the energy of the potassium followed by sodium and calcium. These biomass feed. In commercial processes, it is used act by causing secondary cracking of vapours and within the process to provide the process heat reducing liquid yield and liquid quality. The vast requirements by combustion or it can be separated majority of these alkali metals report to the char and exported, in which case an alternative fuel which results in the char byproduct acting as a is required to provide the heat for pyrolysis. cracking catalyst thus requiring rapid and effective Depending on the reactor configuration and gas removal of char within the fast pyrolysis process. velocities, a large part of the char will be of a

Short hot vapour residence time

Rapid char removal GAS

Cylcones Quench BIOMASS Reactor

Grinding Drying ~500°C

ESP

BIO-OIL <10 wt% CHAR water <5 mm process heat or export

Fig. 1. Conceptual fast pyrolysis process

comparable size and shape as the biomass feed. achieved by electrical heaters. At a commercial Fresh char is pyrophoric i.e. it spontaneously scale Dynamotive built two fluid bed systems in combusts when exposed to air so careful handling Canada, one of which ran for a few years and and storage is required. This property deteriorates has now been dismantled, and the second larger with time due to oxidation of active sites on the unit is not believed to have been commissioned char surface. before dismantling. Heat transfer to the reactor is understood to have been a cause for concern. 2.2.1 Bubbling Fluid Bed Reactors Vapour and solid residence time is controlled by the fluidising gas flow rate and is higher for char Bubbling fluid beds have the advantages of a well than for vapours. As char acts as an effective understood technology that is simple in construction vapour cracking catalyst at fast pyrolysis reaction and operation, has good temperature control and temperatures, rapid and effective char separation very efficient heat transfer to biomass particles is important. This is usually achieved by ejection arising from the high solids density. The usual and entrainment followed by separation in one fluidising medium is sand, but increasing attention or more cyclones so careful design of sand and is paid to catalysts acting as the fluidising medium, biomass/char hydrodynamics is important. The high but care is needed to manage deactivation of the level of inert gases arising from the high permanent catalysts. Heating can be achieved in a variety gas flows required for fluidisation result in very low of ways and scaling is well understood. However, partial pressure for the condensable vapours and heat transfer to the fluid bed at large scales of thus care is needed to design and operate efficient operation has to be considered carefully because heat exchange and liquid collection systems. In of the scale-up limitations of different methods addition the large inert gas flowrates result in of heat transfer. Bubbling fluid-bed pyrolysers relatively large equipment thus increasing cost. give good and consistent performance with high Liquid collection is either by indirect heat exchange liquid yields of typically 70–75 wt% from wood on or quenching in recycled bio-oil or an immiscible a dry-feed basis, and for this reason, are widely hydrocarbon such as Isopar – a proprietary mix of used for smaller scale or laboratory experiments, isoparaffins with a high boiling point to minimise such as the pioneering work at the University of evaporation and produce a high flash point. Waterloo (12). Small biomass particle sizes of less Aerosols are a significant part of the liquid yield and than 2–3 mm are needed to achieve high biomass are collected either by electrostatic precipitation or heating rates, and the rate of particle heating is coalescence on demisters. These are incompletely usually the rate-limiting step. This technology depolymerised lignin fragments which seem to is ideal for laboratory units due to the ease of exist as a liquid with a substantial molecular operation and control, when heating is usually weight. Evidence of their liquid basis is found in the

121 © 2018 Johnson Matthey https://doi.org/10.1595/205651318X696693 Johnson Matthey Technol. Rev., 2018, 62, (1) accumulation of liquid in the ESP which runs down surface. In ablative pyrolysis, heat is transferred the plates to accumulate in the bio-oil product. from the hot reactor wall to ‘melt’ wood that is Demisters for agglomeration or coalescence of the in contact with it under pressure. As the wood is aerosols have been used but published experience moved away, the molten layer then vaporises to suggest that this is less effective. a product very similar to that derived from fluid bed systems. Some of the extensive fundamental 2.2.2 Circulating Fluid Beds and work was carried out in Nancy, France (14) and this concept was scaled to laboratory processing (15). Transported Beds Reactors The pyrolysis front thus moves unidirectionally Circulating fluid bed (CFB) such as Empyro and through the biomass particle. As the wood is transported bed reactor systems such as Ensyn mechanically moved away, the residual oil film have many of the features of bubbling beds both provides lubrication for successive biomass described above, except that the residence time of particles and also rapidly evaporates to give the char is almost the same as for vapours and gas, pyrolysis vapours for collection in the same and the char is more attritted due to the higher gas way as other processes. There is an element of velocities. This can lead to higher char contents in cracking on the hot surface from the char that is the collected bio-oil unless more extensive char also deposited. The rate of reaction is strongly removal is included. An added advantage is that influenced by pressure of the wood onto the heated CFBs are potentially suitable for larger throughputs surface; the relative velocity of the wood and the even though the hydrodynamics are more complex, heat exchange surface; and the reactor surface as this technology is widely used at very high temperature. The key features of ablative pyrolysis throughputs in the petroleum and petrochemical are therefore as follows: industry. • High pressure of particle on hot reactor wall, Heat supply is usually from recirculation of achieved by centrifugal force or mechanically heated sand from a secondary char combustor, • High relative motion between particle and which can be either a bubbling or circulating fluid reactor wall bed. In this respect the process is similar to a • Reactor wall temperature less than 600°C. twin fluid-bed gasifier except that the reactor As reaction rates are not limited by heat transfer (pyrolyser) temperature is much lower and the through the biomass particles, larger particles can closely integrated char combustion in a second be used and in principle there is no upper limit to reactor requires careful control to ensure that the the size that can be processed. The process, in temperature, heat flux and solids flow match the fact, is limited by the rate of heat supply to the process and feed requirements. Heat transfer is a reactor rather than the rate of heat absorption by mixture of conduction and convection in the riser. the pyrolysing biomass, as in other reactors. There One of the unproven areas is scale up and heat is no requirement for inert gas, so the processing transfer at high throughputs. equipment is smaller and the reaction system is All the char is burned in the secondary reactor to thus more intensive. In addition the absence of re-heat the circulating sand, so there is no char fluidising gas substantially increases the partial available for export unless an alternative heating pressure of the condensable vapours leading to source is used. If separated the char would be a more efficient collection and smaller equipment. fine powder that would require careful handling However, the process is surface-area-controlled due to its pyrophoric nature. so scaling is less effective and the reactor is mechanically driven, and is thus more complex. 2.2.3 Ablative Pyrolysis The char is a fine powder which can be separated by cyclones and hot vapour filters as for fluid bed Ablative pyrolysis is substantially different in concept reaction systems. compared with other methods of fast pyrolysis (13). In all the other methods, the rate of reaction 2.2.4 Screw and Augur Kiln Reactors is limited by the rate of heat transfer through the biomass particles, which is why small particles are There have been a number of developments that required. The mode of reaction in ablative pyrolysis mechanically move biomass through a hot reactor is like melting butter in a frying pan: the rate of rather than using fluids, including screw and augur melting can be significantly enhanced by pressing reactors. Heating can be with recycled hot sand the butter down and moving it over the heated pan as at the Bioliq plant at Karlsruhe Institute of

Technology (KIT), Germany, (Forschungszentrum process by combustion of the char in air. Typically Karlsruhe (FZK) until 2009) (16), or with heat the char contains about 25% of the energy of the carriers such as steel or ceramic balls as in feedstock, and about 75% of this energy is required Haloclean, also at KIT (17), or external heating. to drive the process. The byproduct gas only The nature of mechanically driven reactors is that contains about 5% of the energy in the feed and very short residence times comparable to fluid and this is not sufficient for pyrolysis. The main methods circulating fluid beds are difficult to achieve, and of providing the necessary heat are listed below: hot vapour residence times can range from 5 to • Through heat transfer surfaces located in 30 seconds depending on the design and size of and/or on suitable positions in the reactor reactor. Examples include screw reactors and more such as in-bed heating tubes and/or concentric recently the Lurgi LR reactor at KIT (10, 11) and annular heater around the bed the Bio-oil International reactors which have been • By heating the fluidisation gas in the case studied at Mississippi State University, USA (18). of a fluid bed or circulating fluid bed reactor, Screw reactors are particularly suitable for feed although excessive gas temperatures may be materials that are difficult to handle or feed, or are needed to input the necessary heat that could heterogeneous. The liquid product yield is lower result in local overheating and reduced liquid than fluid beds and is usually phase separated due yield, or alternatively very high gas flows may to the longer residence times and contact with be needed resulting in unstable fluid dynamics. byproduct char. Also the char yields are higher. KIT Partial heating is usually satisfactory and has promoted and tested the concept of producing desirable to optimise energy efficiency a slurry of the char with the liquid to maximise • By removing and re-heating the bed material liquid yield in terms of energy efficiency (19), but in a separate reactor as used in most CFB and this would require an alternative energy source to transported bed reactors provide heat for the process. • By the addition of some air, although this can create localised hot spots and increase cracking 2.2.5 Microwave Pyrolysis of the liquids to tars • By use of microwaves (see Section 2.2.5). There is growing interest in microwave pyrolysis There are a variety of ways of providing the as a more direct way of heating biomass quickly process heat from byproduct char or gas or from (20, 21). This offers the advantage of avoiding or fresh biomass. This facet of pyrolysis reactor design reducing the low thermal conductivity of biomass and optimisation is most important for commercial encountered in conventional thermal pyrolysis, but units and will attract increasing attention as plants requires premium energy to drive the process and become bigger. Examples of options include: needs careful design to overcome the potentially • Combustion of byproduct char, all or part poor penetration of microwaves through organic • Combustion of byproduct gas which normally material. The uniform heating resulting from use of requires supplementation for example with microwaves is likely to reduce secondary reactions natural gas as reaction products are less likely to interact with • Combustion of fresh biomass instead of char, pyrolysed biomass. There are some interesting particularly where there is a lucrative market challenges in scale up and comparisons of products for the char between microwave and conventional fast pyrolysis • Gasification of the byproduct char and will be interesting. combustion of the resultant producer gas to provide greater temperature control and avoid 2.2.6 Heat Transfer in Fast Pyrolysis alkali metal problems such as slagging in the char combustor There are a number of technical challenges facing • Use of byproduct gas with similar advantages the development of fast pyrolysis, of which the most as above, although there is unlikely to be significant is heat transfer to the reactor. Pyrolysis is sufficient energy available in this gas without an endothermic process, requiring a substantial heat some supplementation input to raise the biomass to reaction temperature, • Use of bio-oil product although the heat of reaction is insignificant. Heat • Use of fossil fuels where these are available transfer in commercial reactors is a significant at low cost, do not affect any interventions design feature and the energy in the byproduct allowable on the process or product, and the charcoal would typically be used in a commercial byproducts have a sufficiently high value.

2.3 Products 80 The liquid bio-oil is formed by rapidly quenching 70 and thus ‘freezing’ the intermediate products of 60 flash degradation of hemicellulose, cellulose and Organics 50 lignin. The liquid thus contains many reactive 40 species, which contribute to its unusual attributes. Gas 30 Bio-oil can be considered a micro-emulsion in Char which the continuous phase is an aqueous solution 20 of holocellulose decomposition products, that Yield, wt% of dry feed 10 stabilises the discontinuous phase of pyrolytic lignin 0 Reaction water macro-molecules through mechanisms such as 400 450 500 550 600 650 Reaction temperature, °C hydrogen bonding. One theory is that a surfactant is formed in fast pyrolysis that creates a stable Fig. 3. Typical yields of the major products from micro-emulsion with the pyrolytic lignin. Aging or fast pyrolysis of biomass (3) instability is believed to result from a breakdown in this emulsion. Bio-oil typically is a dark brown, free-flowing results are obtained for most biomass feedstocks, liquid and approximates to biomass in elemental although the maximum yield can occur between composition. Depending on the initial feedstock 480°C and 525°C depending on feedstock. Grasses, and the mode of fast pyrolysis, the colour can for example, tend to give maximum liquid yields of be almost black through dark red-brown to around 55–60 wt% on a dry feed basis at the lower dark green, being influenced by the presence of end of this temperature range, depending on the micro-carbon in the liquid and chemical composition. ash content of the grass. Liquid yield depends on Hot vapour filtration gives a more translucent biomass type, temperature, hot vapour residence red-brown appearance owing to the absence of time, char separation and biomass ash content, the char. High nitrogen content can impart a dark last two having a catalytic effect on vapour cracking. green tinge to the liquid. It is important to note that maximum yield is not It is composed of a very complex mixture of the same as maximum quality, and quality needs oxygenated hydrocarbons with an appreciable careful definition if it is to be optimised. Bio-oil proportion of water from both the original moisture quality and quality management and improvement and reaction product. Some solid char may also have also been reviewed (23). be present. Typical organics yields from different The liquid typically contains about 25 wt% water feedstocks and their variation with temperature which forms a stable single-phase mixture, but it are shown in Figure 2 and Figure 3 shows the can range from about 15 wt% to an upper limit of temperature dependence of the four main products about 30–50 wt% water, depending on the feed from a representative feedstock (22). Similar material, how it was produced and subsequently collected. Above 50 wt% water (and sometimes lower) the liquid phase separates. A typical feed material specification is a maximum of 10% 80 moisture in the dried feed material, as both this 70 Poplar aspen feed moisture and the water of reaction from 60 Maple pyrolysis, typically about 12% based on dry feed, 50 Cellulose Bagasse both report to the liquid product. Pyrolysis liquids 40 can tolerate the addition of some water, but there is 30 a limit to the amount of water which can be added Pine bark 20 to the liquid before phase separation occurs, in other words the liquid cannot be dissolved in water. 10 Water addition reduces viscosity, which is useful; 0 Organics yield, wt% of dry feed 400 450 500 550 600 650 reduces heating value which means that more Reaction temperature, °C liquid is required to meet a given duty; and can improve stability. The effect of water is therefore Fig. 2. Variation of organics yield with different complex and important. Bio-oil is miscible with feedstocks (3) polar solvents such as methanol, acetone, but

124 © 2018 Johnson Matthey https://doi.org/10.1595/205651318X696693 Johnson Matthey Technol. Rev., 2018, 62, (1) totally immiscible with petroleum-derived fuels. drums without any deterioration that would This is due to the high oxygen content of around prevent its use in any of the applications tested 35–40 wt%, which is similar to that of biomass, and to date, it does change slowly with time, most provides the chemical explanation of many of the noticeably there is a gradual increase in viscosity. characteristics reported. Removal of this oxygen More recent samples that have been distributed by upgrading requires complex catalytic processes for testing have shown substantial improvements which are described in Part II. in consistency and stability, demonstrating the The density of the liquid is very high at around improvement in process design and control as the –1 1200 kg t , compared with light fuel oil at around technology develops. –1 0.85 kg l . This means that the liquid has about Ageing is a well known phenomenon caused by 42% of the energy content of fuel oil on a weight continued slow secondary reactions in the liquid basis, but 61% on a volumetric basis. This has which manifests as an increase in viscosity with implications for the design and specification of time. It can be reduced or controlled by the equipment such as pumps and atomisers in boilers addition of alcohols such as ethanol or methanol. and engines. In extreme cases phase separation can occur. It is Viscosity is important in many fuel applications exacerbated or accelerated by the presence of fine (24). The viscosity of the bio-oil as produced char. This has been reviewed by Diebold (25, 26). can vary from as low as 25 m2 s–1 to as high as Fast pyrolysis liquid has a higher heating value 1000 m2 s–1 (measured at 40°C) or more depending (HHV) of about 17 MJ kg–1 as produced with about on the feedstock, the water content of the bio-oil, 25 wt% water that cannot readily be separated. the amount of light ends collected and the extent to which the oil has aged. While the liquid is widely referred to as ‘bio-oil’, Pyrolysis liquids cannot be completely vaporised it will not mix with any hydrocarbon liquids. It is once they have been recovered from the vapour composed of a complex mixture of oxygenated phase. If the liquid is heated to 100°C or more to compounds that provide both the potential and try to remove water or distil off lighter fractions, challenge for utilisation. There are some important it rapidly reacts and eventually produces a solid properties of this liquid that are summarised in residue of around 50 wt% of the original liquid, some Table II and Table III. There are many particular distillate containing volatile organic compounds characteristics of bio-oil that require consideration that have been cracked and water. While bio-oil for any application (6). Oasmaa and Peacocke have has been successfully stored for several years reviewed physical property characterisation and in normal storage conditions in steel and plastic methods (27, 28).

Table II Typical Properties of Wood-Derived Crude Bio-Oil

Physical property Typical value

Moisture content 25%

pH 2.5

Specific gravity 1.20

Elemental analysis C 56%

H 6%

O 38%

N 0–0.1%

HHV as produced 17 MJ kg–1

Viscosity (40°C and 25% water) 40–100 mPa s

Solids (char) which also includes ash 0.1%

Vacuum distillation residue up to 50%

Table III Characteristics of Bio-Oil

Characteristic Cause Effects

Acidity or low pH Organic acids from biopolymer Corrosion of vessels and pipework degradation

Ageing Continuation of secondary reactions Slow increase in viscosity from secondary including polymerisation reactions such as condensation Potential phase separation

Alkali metals (ash) Virtually all alkali metals report to Catalyst poisoning char so this not a big problem Deposition of solids in combustion High ash feed Erosion and corrosion Incomplete solids separation Slag formation Damage to turbines

Char Incomplete char separation in Ageing of oil process Sedimentation Filter blockage Catalyst blockage Engine injector blockage Alkali metal poisoning

Chlorine Contaminants in biomass feed Catalyst poisoning in upgrading

Colour Cracking of biopolymers and char Discolouration of some products such as resins

Contamination of feed Poor harvesting practice Contaminants notably soil act as catalysts and can increase particulate carry over

Distillability is poor Reactive mixture of degradation Bio-oil cannot be distilled – maximum products of biomass 50% typically Liquid begins to react at below 100°C and substantially decomposes above 100°C

High viscosity Gives high pressure drop increasing equipment cost High pumping cost Poor atomisation

Low H:C ratio Biomass has low H:C ratio and Upgrading to hydrocarbons is more thermal degradation products difficult replicate this ratio

Materials Phenolics and aromatics Destruction of seals and gaskets incompatibility

Miscibility with Highly oxygenated nature of bio-oil Will not mix with any hydrocarbons so hydrocarbons is very integration into a refinery is more difficult low

Nitrogen Contaminants in biomass feed Unpleasant smell High nitrogen feed such as proteins Catalyst poisoning in upgrading in wastes NOx in combustion

Oxygen content is Biomass composition has high Poor stability very high oxygen so thermal degradation Non-miscibility with hydrocarbons products have high oxygen

Phase separation High feed water Phase separation or High ash in feed Partial phase separation inhomogeneity Poor char separation in process Layering Poor mixing Inconsistency in handling, storage and processing

Smell or odour Aldehydes and other volatile While not toxic, the smell is often organics, many from hemicellulose objectionable

(Continued)

Characteristic Cause Effects

Solids Particulates from reactor such as Sedimentation sand Erosion and corrosion Particulates from feed contamination Blockage Small particles of char

Structure The unique structure is caused by Susceptibility to ageing such as viscosity the rapid de-polymerisation and increase and phase separation rapid quenching of the vapours and aerosols

Sulfur Contaminants in biomass feed Catalyst poisoning in upgrading

Temperature Incomplete or ‘frozen’ degradation Irreversible decomposition of liquid into sensitivity reactions two phases above 100°C Irreversible viscosity increase above around 60°C Potential phase separation above around 60°C

Toxicity Biopolymer degradation products Human toxicity is positive but small Ecotoxicity is negligible

Viscosity Chemical composition of bio-oil Fairly high and tends to increase with time gives high viscosity, which tends to Greater temperature effect on viscosity increase with time due to ageing change than for hydrocarbons Increasing temperature to lower viscosity requires care due to thermal sensitivity

Water content Pyrolysis reactions Complex effect on viscosity and stability: Feed water increased water lowers heating value, density and stability; and raises pH Affects catalysts for example through hydrolysis

2.4 Liquids Collection partial pressure of condensable products due to the large volumes of fluidising gas, and this is an The gaseous products from fast pyrolysis consist important design consideration in liquid collection. of aerosols, true vapours and non-condensable This disadvantage is reduced in the rotating cone gases. These require rapid cooling to minimise and ablative reaction systems, both of which secondary reactions and condense the true exclude inert gas which leads to more compact vapours, while the aerosols require additional equipment and lower costs (29). coalescence or agglomeration. Simple indirect heat exchange can cause preferential deposition 2.5 Byproducts of lignin-derived components leading to liquid fractionation and eventually blockage in pipelines Char and gas are byproducts, typically containing and heat exchangers. Quenching in product bio-oil about 25% and 5% of the energy in the feed or in an immiscible hydrocarbon solvent is widely material respectively. The pyrolysis process itself practised. requires about 15% of the energy in the feed, Orthodox aerosol capture devices such as and of the byproducts, only the char has sufficient demisters and other commonly used impingement energy to provide this heat. The heat can be devices are not reported to be as effective as derived by burning char in orthodox reaction electrostatic precipitation, which is currently system design, which makes the process energy the preferred method at both laboratory and self-sufficient. More advanced configurations could commercial scale units. The vapour product from gasify the char to a lower heating value (LHV) gas fluid bed and transported bed reactors has a low and then burn the resultant gas more effectively

127 © 2018 Johnson Matthey https://doi.org/10.1595/205651318X696693 Johnson Matthey Technol. Rev., 2018, 62, (1) to provide process heat with the advantage that and safety aspects. A study was completed in 2005 the alkali metals in the char can be much better to assess the ecotoxicity and toxicity of 21 bio-oils controlled. The waste heat from char combustion from most commercial producers of bio-oil around and any heat from surplus gas or byproduct gas can the world in a screening study, with a complete be used for feed drying and in large installations assessment of a representative bio-oil (34). The could be used for export or power generation. study includes a comprehensive evaluation of An important principle of fast pyrolysis is that a transportation requirements as an update of well-designed and well-run process should not an earlier study (35) and an assessment of the produce any emissions other than clean flue gas biodegradability (36). The results are complex i.e. CO2 and water, although they will have to meet and require more comprehensive analysis but local emissions standards and requirements. the overall conclusion was that bio-oil offers no significant health, environment or safety risks. 2.5.1 Char Acknowledgement Char acts as a vapour cracking catalyst, so rapid and effective separation from the pyrolysis product This is a revised and updated version of an vapours is essential, although it is not clear to what original text published by Taylor and Francis (37) extent cracking is caused by the alkali metals in Reproduced with permission from Taylor and the char. Cyclones are the usual method of char Francis Group LLC Books. removal, however some fines always pass through the cyclones and collect in the liquid product where they accelerate ageing and exacerbate References the instability problem which is described below. 1. S. Czernik and A. V. Bridgwater, Energy Fuels, Some success has been achieved with hot vapour 2004, 18, (2), 590 filtration which is analogous to hot gas cleaning in 2. ‘Standard Specification for Pyrolysis Liquid gasification systems (30–33). Problems arise with Biofuel’, ASTM D7544-12, ASTM International, the sticky nature of fine char and disengagement West Conshohocken, Pennsylvania, USA, 2012 of the filter cake from the filter. 3. G. V. C. Peacocke, ‘Ablative Pyrolysis of Biomass’, Pressure filtration of the liquid for substantial PhD Thesis, Aston University, Birmingham, UK, removal of particulates (down to <5 µm) can be October, 1994 difficult because of the complex interaction of the 4. A. V. Bridgwater, Biomass Bioenergy, 2012, 38, 68 char and pyrolytic lignin, which appears to form a 5. D. Mohan, C. U. Pittman Jr. and P. H. Steele, gel-like phase that rapidly blocks the filter. Energy Fuels, 2006, 20, (3), 848 Modification of the liquid microstructure by addition of solvents such as methanol or ethanol that 6. S. R. A. Kersten, X. Wang, W. Prins and W. P. M. van Swaaij, Ind. Eng. Chem. Res., 2005, 44, (23), 8773 solubilise the less soluble constituents can improve this problem and contribute to improvements 7. A. V. Bridgwater, Chem. Eng. J., 2003, 91, (2–3), 87 in liquid stability. However the suitability of this approach depends on the application for the bio-oil. 8. A. V. Bridgwater, S. Czernik and J. Piskorz, ‘The Status of Biomass Fast Pyrolysis’, in “Fast Pyrolysis of Biomass: A Handbook”, ed. A. V. Bridgewater, 2.5.2 Gas Vol. 2, CPL Press, Newbury, UK, 2002, pp. 1–22

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The Author

Tony Bridgwater is a professor of chemical engineering at Aston University, Birmingham, UK. He has worked at Aston University for most of his professional career and is currently director of the European Bioenergy Research Institute (EBRI). He has a world-wide research portfolio focusing on fast pyrolysis as a key technology in thermal biomass conversion for power, heat, biofuels and biorefineries. He is a fellow of the Institution of Chemical Engineers and a fellow of the Institute of Energy.

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