Glucose Oxidation into Gluconic Acid:

From Batch to Trickle Bed Reactor

Dissertation for the academic degree of Doctor of Science

Faculty of Chemistry and Biochemistry of Ruhr-Universität Bochum

Alessia Padovani

Born on 13.12.1988 in Verona, Italy

Bochum

December 2016

The present work was made in the period from December 2012 to December 2015 in the Department of heterogeneous catalysis at Max Planck Institute für Kohlenforschung in Mülheim an der Ruhr , headed by Prof. Dr. Ferdi Schuth .

Supervisor: Prof. Dr. Ferdi Schüth

Co-supervisor: Prof. Dr. Wolfgang Grünert

For my Parents

“Above all, don't fear difficult moments. The best comes from them.”

“The body does whatever it wants. I am not my body; I am my mind”.

Rita Levi-Montalcini

“Nothing in life is to be feared, it is only to be understood.”

Marie Curie

Acknowledgements

At the end of this challenging experience, I would like to thank all the people who were involved in this PhD research work.

First of all, I am really thankful to Prof. Dr. Ferdi Schüth for the great opportunity to work in his Group, for his supervision on this PhD work and for the academic independence and autonomy he gave me.

Thanks to Prof. Dr. Wolfgang Grünert for the co-supervision and for the interest in my research.

I would like to thank the HPLC department, especially Heike Hinrichs and Marie Sophie Sterling for the many measurements, for their evaluation and useful discussion. Thanks to Inge Springer for the ICP analysis and Silvia Palm for the EDX measurements. Big thanks go to Bernd Spliethoff for patiently teaching me how to use the TEM and for the help in evaluating the images.

Thanks to Wolfgang Kersten and Knut Gräfenstein from the Workshop for the great technical support, especially in the construction and repair of the batch reactor used in this PhD work. Thanks to the Glassblowing, especially for the trickle bed reactors.

I would like to thank also Andre Pommerin and Laila Sahraoui for the practical support in the laboratory and for their effort in keeping the laboratories clean and functional.

Thanks to Annette Krappweis and Kirsten Kalischer for helping me in my move to Germany and for all the support in all the organizational matters.

In the success and enjoyment of the work, my officemates played an important role. Heartfelt thanks to Valentina Nese, Dr. Mariem Meggouh, Dr. Tobias Grewe, Jean Pascal Schulte and Xiaohui Deng for our daily chats and leisure activities outside the Institut. Thanks to Vale and Mariem for our friendship, to Tobi for all the laughs and also for helping me with the design of the trickle bed reactor. Thanks to JP, for sharing his precious fume hood with me and for the time we spent together in the laboratory.

Infinite thanks go to my parents Antonio and Cristina, for all the support during both my academic studies and my PhD work, for their trust and for their encouragement to always pursue and achieve my goals.

Last but not least, I would like to deeply thank my boyfriend Daniel for the endless patience and fondness he always showed me and for all the support he gave me.

Big thanks the Max Planck Society for financial support.

Abstract

The central point of this work is the metal catalyzed liquid phase oxidation of glucose to gluconic acid. During recent years, this reaction has indeed received much attention, since gluconic acid is a fine chemical which finds many industrial applications, mainly as water soluble cleansing agent and as additive for food and beverages.

In this study, the glucose oxidation is performed starting from an alkaline sugar solution. However, no basic solution (NaOH for example) is added to the reaction mixture to maintain the pH at a fixed value; the reaction is therefore carried out at uncontrolled pH.

The reaction is first performed in a batch reactor. Au, Pd and Pt nanoparticles immobilized on metal oxides, resins and porous carbons are used as catalysts; among them, carbon supported metal materials, mainly prepared according to the sol immobilization procedure [1], are the most used ones. After performing the glucose oxidation with varying temperature, pressure and oxidizing agent (pure O2 or air), it can be observed that, at 70°C and 3 bar pure O2, SX carbon supported Au(1wt%) catalyst shows the best performance. Indeed, already after 30 minutes, glucose is almost fully converted into gluconic acid (98% yield).

As the maximum gluconic acid formation is achieved in a very short time, the carbon supported Au catalyst might be successfully used also in a continuous system, i.e. in a trickle bed reactor (TBR). However, as powdered catalysts like Au(1wt%)/SX are difficult to handle in TBRs, a “in home” carbon (IHC) in grain form is chosen as support for the Au(1wt%) catalyst for use in the TBR. The glucose oxidation is performed with varying liquid and gas flow rate, temperature and initial glucose concentration; the optimal reaction conditions, which allow to achieve 81.5% yield of gluconic acid, are 20 ml/h (1.2 minutes as average residence time) and 575 ml/min as liquid and gas flow rate, respectively, 70°C and 5wt% starting concentration of glucose.

Both under batch conditions and in the trickle bed reactor, with carbon supported Au catalysts, high gluconic acid yields were obtained in a very short time and without pH control during the reaction.

Contents

1. Introduction ...... 1

1.1. The importance of biomass conversion and catalysis ...... 1

1.2. Gluconic Acid ...... 5

1.3. Metal Catalysed Liquid Phase Glucose Oxidation to Gluconic Acid ...... 9

1.3.1. State of the Art ...... 9

1.3.2. Supported Metal Catalysts: Preparation Methods ...... 17

1.3.3. Batch and Trickle Bed Reactors ...... 20

2. Motivation and Aim ...... 22

3. Results and Discussion ...... 25

3.1. Glucose Liquid Oxidation in Batch Reactor – Finding the Best Catalyst for the Trickle Bed Reactor ...... 25

3.1.1. Batch Reactor - Metal Oxides as Metal Nanoparticles Supports ...... 26

3.1.2. Batch Reactor - Resins as Metal Nanoparticles Supports ...... 36

3.1.3. Batch Reactor - Carbon as Metal Nanoparticles Support ...... 45

3.1.3.1. IHC-1 and IHC-2 Carbons as Metal Nanoparticles Supports ...... 47

3.1.4. Non Powdered Catalyst: from Batch to Trickle Bed Reactor ...... 50

3.2. Glucose Liquid Oxidation in Batch Reactor – Optimal Catalyst and Reaction Conditions ...... 52

3.2.1. Commercial Carbon Supported Metals ...... 53

3.2.2. Ordered Mesoporous CMK-5 Carbon ...... 57

3.2.3. Mesoporous SX carbon ...... 62

3.3. Glucose Liquid Oxidation under Oxygen Flow ...... 93

3.4. Glucose Liquid Oxidation in Trickle Bed Reactor ...... 97

3.4.1. Trickle Bed Reactor – Preliminary Tests ...... 103

3.4.2. Trickle Bed Reactor - Effect of the Liquid Flow Rate ...... 104

3.4.3. Trickle Bed Reactor – Effect of the Gas Flow Rate ...... 110

3.4.4. Trickle Bed Reactor - Effect of the Temperature ...... 112

3.4.5. Trickle Bed Reactor - Effect of the Initial Glucose Concentration ...... 114

3.4.6. Trickle Bed Reactor - Effect of the Reactor Diameter ...... 117

4. Conclusions ...... 121

5. Experimental Part ...... 128

5.1. Chemicals ...... 128

5.2. Catalyst Synthesis ...... 130

5.2.1. Synthesis of Metal Nanoparticles Supported on Commercial Resins ...... 130

5.2.2. Synthesis of Au Nanoparticles supported EGD/DVB Resin ...... 132

5.2.3. Synthesis of Metal Nanoparticles supported on Different Carbons ...... 133

5.2.4. Evaluation of the Metal Content of the Catalysts ...... 138

5.3. Reaction Set-ups ...... 141

5.3.1. Glucose Oxidation performed in Batch Reactor ...... 141

5.3.2. Glucose Oxidation performed in Continuous Mode ...... 143

6. Appendix ...... 146

7. References ...... 154

Introduction ▪ The importance of biomass conversion and catalysis

1. Introduction

1.1. The importance of biomass conversion and catalysis Catalysis has become a very significant and important field of chemistry, as, currently, more than 60% of chemical syntheses and 90% of the chemical transformations in chemical industries are using catalysts [2]. Moreover, because of environmental issues, catalysts will become even more important than in the past and will be one of the major drivers of improvements in our society [3]. Nowadays, the development of new processes is based on driving forces which correspond more to a market-driven strategy. Preferred are less-capital-intrusive processes and the use of cheaper feedstocks. Society issues have also become an important modern motivation since the end of the twentieth century. Intensive research of new catalytic materials and more efficient processes was indeed devoted to convert by-products to useful products and to treat all kinds of wastes, with the aim to preserve and protect the environment [2]. More efficient catalytic processes require improvements in the catalytic activity and selectivity, which can be enhanced by tailoring catalytic materials with the desired structure and dispersion of active sites. Different kinds of solid catalyst are available; these include metals, oxides, carbons, etc., which can be used as bulk materials or immobilized on a more or less catalytically active support like silica, alumina, titania, carbons, etc. These materials may possess specific chemical properties, such as acid-base, redox, dehydrogenating, hydrogenating or oxidizing, and physical properties like porosity, high surface area, thermal and/or electrical conductivity, etc. The largest family of catalysts correspond to oxides, which are used both as catalysts and supports. Currently, petroleum and natural gas represent more than 60% of the primary energy worldwide supplied; coal remains an important source of energy mainly in Asia. The oil, natural gas and coal consumption is expected to rise in the near future, whether used in the field of energy production, transport, heating or as a source of chemical raw materials. By increasing the production of fossil resources, it was possible to satisfy the significant increase in energy demand over the past 20 years. Because of this situation and the problem of CO2 induced global warming, several governments approved new laws aiming at CO2 emissions reduction by promoting the use of renewable sources and biofuel. 10% of the world energy is derived from biomass and 7% from nuclear power. For the production of nuclear power the isotope uranium-235 (235U) is employed. Unfortunately, the by-

1 Introduction ▪ The importance of biomass conversion and catalysis product of this reaction is the very hazardous element plutonium-239 (239Pu, 235U  239Pu). Moreover, since the disaster following the tsunami in Japan in 2011 and the collapse of the nuclear plant in Fukushima, the construction of nuclear plants has been strongly questioned in many countries, although nuclear power is essentially CO2 free and does not consume fossil resources [2]. Although many challenges relative to the competition between uses and the management of local natural resources, the biomass introduction into energy systems presents some advantages, such as the reduction in greenhouse gas emission, as its synthesis uses CO2 and water. This aspect is in agreement with the principle of green chemistry, by which only chemical processes which are environmentally benign should be used [4]. In future energy scenarios, biomass, i.e. lignin, cellulose and hemicellulose, has emerged as an important source of energy and raw chemicals in the replacement, at least partially, of oil, natural gas and coal. [5]. The European Union receives approximately 66% of its renewable energy from biomass; this surpasses the total combined contribution from hydropower, wind power, geothermal energy and solar power. There are three main strategies for biomass valorisation, as shown in Fig. 1.1; the components from cellulose and hemicelluloses streams are integrated within the lignin conversion framework [6]. In the first strategy, biomass is gasified to syngas or degraded by pyrolysis to a mixture of small molecules, which can be used to produce chemicals using the technologies developed for petroleum feedstocks. The second strategy consists in the extensive removal of the functional groups present in the lignin monomers; this results in simple aromatic compounds, such as phenol, benzene, toluene and xylene. In the third strategy, biomass is converted directly to valuable chemicals in a one-pot process, which requires highly selective catalysts able to eliminate functionalities and linkages. However, product separation and purification is an important step of each process, as none of the three strategies is expected to generate a single product in high yield. Since cellulose is the main constituent of the most abundant renewable lignocellulosic feedstock and it is non-edible, its transformation had attracted significant attention in recent years. Unlike other conversion routes, like high-temperature gasification, pyrolysis and enzymatic fermentation, for the transformation of cellulose a low-temperature and selective process is desirable. This process should be preferably carried out in a water medium and it should be able to produce platform molecules, which can be converted into valued chemicals and fuels. Currently, glucose, polyols, organic acid and 5-hydroxymethylfurfural (5-HMF) are the most promising platform molecules [2].

2 Introduction ▪ The importance of biomass conversion and catalysis

Analogous to the history of the petroleum refinery, with the development of catalytic technology the biorefinery may become, too, an efficient and highly integrated system to meet the chemical and fuel requirements of the twenty-first century.

Figure 1.1. Lignocellulosic bio refinery scheme with particular emphasis on the lignin stream [5]. Reprinted (adapted) with permission from [5]. Copyright (2010) American Chemical Society.

The development of catalytic technologies is an important step towards the realization of this system, by which, in addition to the catalytic conversion of cellulose and hemicellulose, the lignin fraction of biomass can be transformed from a low-quality and low-price waste product into high-quality and high-value feedstocks for bulk and specialty. However, there are still some scientific, environmental, economic and energy challenges for the future. The scientific challenges consist mainly in the design, preparation, evaluation and optimization of new catalytic materials and the probing/understanding of catalyst behaviour in terms of activity and selectivity. From an environmental point of view, by-products should be minimized by converting them into useful products, replacing multistep processes by direct schemes, in order to avoid the exposure to dangerous intermediates, and by using sustainable sources of raw materials and energy supplies. Economic challenges correspond to the use of cheaper and readily available raw materials,

3 Introduction ▪ The importance of biomass conversion and catalysis but also in increased productivity and decreased lag-time between discovery and commercialization and development of more selective processes and of new catalysts. The reduction of the energy consumption remains the main energy challenge.

In the future, solar, geothermal and presumably nuclear power plants will probably be used for generation of electricity, while biomass, oil, natural gas and coal predominantly for the production of syngas and chemicals. Hydrogen will be used for GTL and hydroprocessing, and refineries will produce heat, electricity, transportation fuels and bulk chemicals [2].

4 Introduction ▪ Gluconic Acid

1.2. Gluconic Acid Organic acids represent the third largest category after antibiotics and amino acids in the global market of fermentation. The market of organic acids is dominated by citric acid due to its application in various fields. The market of gluconic acid is comparatively smaller; however 60000 tonnes are produced worldwide annually [7].

Fig. 1.2. (a) Gluconic acid and (b) glucono-δ-lactone.

Gluconic acid (Fig. 1.2a) is a noncorrosive, non-volatile, nontoxic, mild organic acid. It is a natural constituent in fruit juices and honey, and is used in the pickling of foods. Its inner ester, glucono-δ-lactone (Fig. 1.2b), imparts an initially sweet taste which later becomes partly acidic. It is used in meat and dairy products, especially in baked goods, and as flavouring agent. Generally speaking, gluconic acid and its salts are used in the formulation of food, but also of pharmaceutical and hygienic products. Different salts of gluconic acid find various applications based on their properties. Gluconic acid derives from glucose through a simple oxidation reaction. Microbial production of gluconic acid (by the enzymes glucose oxidase and glucose dehydrogenase) is the preferred method. The most studied fermentation process (FDA approved) involves the fungus Aspergillus Niger, which allows to covert nearly 100% of the glucose to gluconic acid under the appropriate conditions [7].

Gluconic acid production started back in 1870 when it was discovered by Hlasiwetz and Habermann. Ten years later, Boutroux found for the first time that acetic acid bacteria are

5 Introduction ▪ Gluconic Acid capable of producing sugar acids, and in 1922 gluconic acid was detected in Aspergillus Niger by Molliard [7]. Gluconic acid production has been extensively studied [8] and Currie et al. filed a patent employing submerged culture using Penicillium lautem, giving yields of gluconic acid up to 90% in 48-60 h. Later, Moyer et al. used A. Niger in pilot plant studies, producing 95% yields from glucose solution of 150 to 200 g/L in 24 h [7]. Different approaches are possible for the production of gluconic acid, namely, electrochemical, biochemical and bioelectrochemical [9] [10] . There are several different oxidizing agents available, but these processes appear to be more expensive and less efficient compared to the fermentation processes. Although the conversion is a simple one- step reaction, the chemical method is not favoured. This is the reason why fermentation, involving fungi and bacteria, is one of the most efficient and dominant technique for manufacturing gluconic acid. Among various microbial fermentation processes, the method utilising the fungus A. Niger is one of the most widely used. This method is based on the modified process developed by Blom et al. [11], which involves fed-batch cultivation with intermittent glucose feeding and the use of sodium hydroxide as neutralising agent. The pH is held at 6.0-6.5 and the temperature at about 34°C. The productivity of this process is very high, since glucose is converted at a rate of 15 g/(L∙h). Irrespective of the use of fungi or bacteria, the importance lies on the product which is produced (sodium gluconate or calcium gluconate, for example). As the reaction leads to an acidic product, neutralization is required by the addition of neutralising agents; otherwise the acidity inactivates the glucose oxidase, resulting in the arrest of gluconic acid production [7]. In the production of calcium gluconate and sodium gluconate, the conditions for the fermentation processes differ in many aspects, i.e. glucose concentration (initial and final) and pH control. The process for sodium gluconate (readily soluble in water, 39.6% at 30°C) is highly preferred, as glucose concentrations up to 350 g/L can be used without any problems, and the pH is controlled by the automatic addition of NaOH solution. In contrast, in the calcium gluconate production process, pH control is achieved by calcium carbonate slurry addition. The calcium gluconate solubility in water (4% at 30°C) is lower than the sodium gluconate one. At high glucose concentration (>15%), supersaturation occurs and, if it exceeds the limit, the calcium salt precipitates on the mycelia, with oxygen transfer inhibition as a consequence [7]. The main product among the gluconic acid derivatives is the sodium gluconate, which has a high sequestering power and is a good chelator at alkaline pH. Aqueous solutions of

6 Introduction ▪ Gluconic Acid sodium gluconate are resistant to oxidation and reduction at high temperatures. It is an efficient plasticizer and a highly efficient set retarder, but it is easily biodegradable (98% at 48 h). Calcium gluconate is mainly used in the pharmaceutical industry as a source of calcium for treating calcium deficiency [7].

Although the gluconic acid production is a simple oxidation process, which can be carried out by electrochemical, biochemical or bioelectrochemical methods, production by fermentation process involving fungi and bacteria is commercially well established. However, development of novel and more economical processes for glucose conversion to gluconic acid with longer shelf life would be promising [7]. A chemical process, consisting in the aerobic liquid phase glucose oxidation involving the use of metal catalysts, could be a valid and alternative method. Different products can be obtained, depending on the functional group that is oxidized (Fig. 1.3).

Figure 1.3. Possible products obtained from glucose oxidation.

7 Introduction ▪ Gluconic Acid

If the oxidation process involves only the aldehyde group in the glucose molecule, gluconic acid is formed; from further oxidation of gluconic acid, 2-keto gluconic acid and 5-keto gluconic acid are produced. When only the primary alcohol function is oxidized, glucuronic acid is formed; further oxidation of glucuronic acid produces glucaric acid. Additional side products can result from glucose isomerization, i.e. fructose, and from C-C bond cleavage, i.e. formic acid and glycolic acid.

8 Introduction ▪ Metal Catalysed Liquid Phase Glucose Oxidation to Gluconic Acid

1.3. Metal Catalysed Liquid Phase Glucose Oxidation to Gluconic Acid

1.3.1. State of the Art In very recent years, the aerobic oxidation of glucose to gluconic acid has gained much consideration due to gluconic acid´s application as food and beverage additives and in detergents [12]. Biochemical pathways are used in the glucose oxidation reaction; however, these routes are cumbersome, multistep processes and expensive [13]. In addition, the catalysts are not recyclable.

In the last decade, metal nanoparticles (NPs) have received substantial interest due to their unique properties, finding potential application in the catalysis of glucose oxidation. In particular, over the last twenty years, gold nanoparticles have established an important role after Haruta [14] and Hutchings [15] discovered the peculiar activity of this metal in CO oxidation and ethylene hydrochlorination. Gold has shown promising behaviour in both selectivity and resistance to deactivation, compared to Pd and Pt catalysts. Although the employment of gold in catalysis has been widely expanded [16], since the beginning of its application, the use of this metal in creating new catalytic systems was affected by the high variation in the catalytic performance, depending on the preparation method employed and the support used [17] [18] [19] [20].

In 1995, Besson et al. [13] reported the use of palladium catalysts supported on active charcoal in the oxidation of a water solution of glucose, with air at 313 K. They obtained high gluconate yields (99.3%) in the presence of a bismuth promoted catalyst; bismuth was deposited via a surface redox reaction on Pd/C catalysts containing 1 to 2 nm Pd particles. Bismuth adatoms were able to prevent oxygen poisoning of the palladium surface by acting as co-catalyst in the oxidative dehydrogenation mechanism. Via STEM-EDX, it was shown that bismuth atoms were selectively and homogeneously dispersed on the palladium particles. The catalyst was recycled without activity or selectivity loss and without bismuth leaching during both the reaction and the recycling. In 2002, the selective oxidation of D-glucose to D-gluconic acid in the presence of a carbon supported gold catalyst, prepared by metal sol immobilization procedure, was investigated by Biella et al. [21]. The reaction was performed at both controlled (7-9.5) and free pH in an aqueous solution using dioxygen as the oxidant under mild conditions (323-

373 K, pO2= 100-300 kPa). No glucose isomerization to fructose was observed during the

9 Introduction ▪ State of the Art reaction and total selectivity to D-gluconate was reached. In comparison to commercial palladium and platinum-derived catalysts, supported gold showed unique properties, i.e. it was active at low pH (2.5). At a buffered higher pH (9.5), carbon supported gold and bismuth-doped platinum-palladium catalysts showed comparable selectivity, although gold had a higher activity. Furthermore, upon recycling, gold was found to be more stable toward deactivation (although this also depended on the pH). Also Önal et al. [22] studied the activity of Au/C catalysts in the heterogeneously catalysed oxidation of D-glucose to D-gluconic acid. They prepared a series of Au/C catalysts by the sol immobilization method, using different reducing agents and different kinds of carbon support. The materials with Au mean particle diameters in the range of 3-6 nm prepared on Black Pearls and Vulcan type carbons were shown to be active in the liquid phase glucose oxidation to gluconic acid. The best results were obtained at 50°C and pH 9.5; the reaction was described by an oxidative dehydrogenation mechanism in the aqueous phase. From kinetic tests, carried out excluding mass transfer limitations by intensive stirring and high volumetric air flow rate, Önal and co-workers [22] showed that the rate-limiting step was the surface reaction. Rossi et al. [23] reported that both carbon-supported and naked colloid, i.e. in the absence of common protectors (PVA, PVP or THPC), Au nanoparticles with a mean diameter of 3.6 nm exhibited very high activity in converting D-glucose to D- gluconic acid [24]. Unfortunately, the unsupported Au colloids rapidly deactivated within several hundred seconds; this was attributed to the increasing particle size over 10 nm due to the agglomeration of Au nanocrystallites. Therefore, in order to improve the catalytic performance, catalyst supports, such as carbon, are needed to stabilize the structure and activity of colloidal Au nanoparticles. In the work of M.B. Zhang et al. [25], the Au/C catalysts used for the glucose oxidation was prepared following a standard wet impregnation method; one portion of the sample was reduced by hydrogen and the other by plasma using argon as the plasma-forming gas. The samples reduced by plasma showed highly dispersed gold nanoparticles on carbon and a better catalytic performance than their hydrogen-reduced counterparts. The plasma reduced the metal leaching and increased the hydrophilicity of the samples by enhancing the amount of oxygen groups on the surface.

Especially in liquid phase oxidations, when dioxygen or air is used as the oxidant, the industrial application of metal-supported catalysts is limited by their durability [26]. Furthermore, the presence of a base in the gold catalysed reactions is a serious drawback

10 Introduction ▪ State of the Art for the industrial exploitation of Au catalysts [27]. It must also be taken into account that monometallic gold catalysts suffer from some intrinsic defects that sometimes limit the application of gold nanocatalysts to a great extent. There are two main limitations of these kinds of catalysts: 1) upon heat treatment, gold NPs tend to aggregate; 2) gold NPs are highly sensitive to moisture [28], often resulting in poor reproducibility of the catalytic performances. One of the most promising approaches to overcome the problems related to monometallic Au catalysts is the addition of a second metal to gold [29]. Bimetallic materials can combine the properties associated with the two constituent metals resulting in a great enhancement in their specific physical and chemical properties, due to a synergistic effect. According to their mixing pattern, bimetallic systems may have one of the four structural types shown in Fig. 1.4 [30]. Based on the chemical properties of the second metal, gold-based bimetallic catalysts are classified into two types. The first type is Au- BM catalysts, where BM refers to a base metal, and the second type is Au-PGM catalysts, where PGM refers to platinum group metal [29]. In the Au-BM bimetallic catalysts, BM is much more susceptible to oxidation than gold. Phase segregation tends to occur upon treatment in an oxidizing atmosphere and, as a consequence, the BM will be enriched on the surface and may form BMOx patches or shells, decorating the gold-rich core. Depending on the ratio of the two metals [31] [32], the base metal can act as a promoter to provide reactive oxygen. Since BM can participate directly in oxidation reactions by providing reactive oxygen, only a small amount of BM is required to achieve significant [33] synergy . In the Au-PGM catalysts, the PGM is much more active than Au toward H2 dissociation and, at the same time, it is typically far less selective toward activation of only one functional group in polyfunctionalized substrate molecules [34]. When Au-PGM is used in oxidation reactions, surface enrichment might take place, forming an Au-rich core and a PGM-rich shell. In this case, the catalytic performance is actually dominated by the chemical composition of the PGM -rich shell, and Au behaves more like a promoter of PGM to prevent over-oxidation and poisoning of PGM by intermediates or products. In this way, the Au-PGM are often performing better than the monometallic counterparts [29]. In addition, the presence of the second metal may also limit the growth of gold nanoparticles. This anti-sintering effect is common in Au-PGM bimetallic systems due to a higher melting point of PGM than of gold [29].

11 Introduction ▪ State of the Art

Figure 1.4. Schematic representation of possible mixing patterns: core–shell (a), subcluster segregated (b), mixed (c), three-shell (d). The pictures show cross sections of the clusters. Reprinted (adapted) with permission from [30] - Published by The Royal Society of Chemistry.

Among the gold-based bimetallic systems, AuPd catalysts are the most extensively studied. Au is miscible with Pd in all compositions; this facilitates obtaining AuPd alloys and limits segregation of the single metals. Venezia et al. [35] prepared AuPd catalysts on silica using polyvinyl pyrrolidone (PVP) as the protective agent. Au and Pd were reduced in the presence of PVA either simultaneously or by sequential reduction, usually using NaBH4 as the reducing agent. Polyvinyl alcohol (PVA) is the most employed stabilizer for the generation of AuPd nanoparticles [36]. In order to obtain bimetallic nanoparticles, the fundamental step is the control of the reduction and the nucleation processes of the two metals, because of their different redox potentials and the different chemical nature. To avoid any segregation of the two metals, a proper reducing agent and/or reaction system should be selected. Prati et al. [37] were among the first to prepare PVA-protected AuPd nanoparticles in a liquid phase reaction (selective oxidation of glycerol). By co-reduction of Au and Pd, an alloy was obtained even though partially segregated palladium was detected. In contrast, Hutchings´ group [38] obtained pure alloys. This difference was ascribed to the different amounts of PVA used: a higher amount of protective agent

12 Introduction ▪ State of the Art probably limited the diffusion of Pd on the gold nanoparticles and segregation of Pd was observed. The role of the protective agent for the Au precursor on the formation of an alloy of uniform composition has been investigated [39]. A uniform AuPd alloy could only be obtained when the Au-PVA system was used. With unprotected Au or weakly stabilized Au, the NPs underwent reconstruction during the deposition/reduction of Pd, not providing efficient seeds for alloying the Pd. In these latter cases, the segregation of the two metals or the formation of different alloy compositions have been observed [30].

In 2006, mono- and bimetallic catalysts (Au, Pt, Pd and Rh) in form of supported particles or colloidal dispersion were tested in the aerobic glucose oxidation, in water solution and under mild conditions, by Comotti et al. [40]. They found that the activity of bimetallic particles was enhanced by combining Au with Pd or Pt (TOF = 924 h-1), while the activity of single metals under acidic conditions was low in the case of Au and Pt (TOF = 51-60 h- 1) and very low in the case of Rh and Pd (TOF < 2 h-1). The great synergistic effect of platinum was observed working at low pH, whereas almost no effect was present at pH 9.5. In the presence of alkali, bimetallic colloidal particles appeared more stable towards agglomeration than monometallic gold particles, resulting in higher conversions. H. Zhang et al. [41] prepared unsupported AuPt bimetallic nanoparticles (BNPs) with an Au-rich core and a Pt-rich shell, and investigated their catalytic activity in the aerobic glucose oxidation.

The materials were prepared using simultaneous reduction with rapid injection of NaBH4, simultaneous reduction with dropwise addition of NaBH4, and simultaneous alcohol reduction. By the use of the first reduction method, highly active PVP-protected AuPt BNPs of about 1.5 nm in diameter were obtained. These materials were characterized by higher and more durable catalytic activity for aerobic oxidation compared to Au nanoparticles (NPs) with nearly the same particle size. The higher catalytic activity of AuPt BNPs was ascribed to two main factors; (1) the small average diameter (1.5 nm) and (2) the presence of negatively charged Au and Pt atoms due to electron donation from the protecting polymer (PVP) by electronic charge transfer effects to the catalytically active sites. In contrast, AuPt BNPs prepared by dropwise NaBH4 addition and alcohol reduction were characterized by large mean particle sizes and, therefore, they showed a low catalytic activity. Beside the metal sol immobilization procedure, the impregnation method has widely been used for the preparation of AuPd on titania and carbon, in particular by Hutchings´ group

13 Introduction ▪ State of the Art

[42]. On carbon supports random AuPd alloys were formed, whereas for oxidic supports core-shell structures were obtained with a gold-rich core and palladium-rich shell [43]. AuPd/C catalysts prepared by incipient wetness method were evaluated in the glucose oxidation by Hermans et al. [44]. These materials showed superior performance compared to the corresponding monometallic Pd/C and Au/C, and no metal leaching was observed. The AuPd/C catalysts were characterized by high Pd:C surface ratios, by full Pd reduction, and by small Pd particles (to which the high activity was connected). The presence of small amounts of Au in contact with Pd was used to explain the bimetallic cooperative effect, as the synergistic effect seems to require an interface between the two metals to form.

Beside carbon, also metal oxides have been used in the preparation of supported catalysts for use in the glucose oxidation reaction. In 2013, Delidovich et al. [45] studied the aerobic glucose oxidation in the presence of Au/Al2O3 catalysts with different dispersion of supported gold and Au/C catalysts containing highly dispersed gold nanoparticles. The aim of the work was to determine the contribution of the mass-transfer processes to the overall reaction kinetics in different regimes. The glucose:Au molar ratios were varied. At high glucose:Au molar ratios, the Au/Al2O3 catalysts showed higher activity than the Au/C catalysts, with the highest TOF reached with Au/Al2O3 materials characterized by metal particles of 1-5 nm in size. The Au/Al2O3 catalysts were most effective, if the gold distribution through the catalyst grains was uniform. For the Au/C materials with a non- uniform gold nanoparticle distribution, the apparent reaction rate was affected by internal diffusion, while the interface gas-liquid-solid oxygen transfer influenced the overall reaction kinetics as well. At a low glucose:Au ratio, the reaction rate was limited by oxygen dissolution in the aqueous phase. In this mass transfer regime the rate of glucose oxidation over the carbon-supported catalysts exceeded the reaction rate over the alumina- supported catalyst, which was attributed to a higher adhesion of the hydrophobic carbon support to the gas–liquid interface facilitating the oxygen mass transfer towards catalytic sites. When the reaction rate was determined by oxygen dissolution, hydrophobic materials were the supports of choice for the aerobic glucose oxidation. In 2010, M. Rosu and A. Schumpe [46] focused their study on the chemical enhancement of gas absorption into catalyst particles employed in slurry systems. They prepared a silanized palladium on alumina catalyst; they showed that, for the glucose oxidation to gluconic acid on suspended

Pd/Al2O3 particles, the particle adhesion at the gas-liquid interface was promoted by

14 Introduction ▪ State of the Art moderate hydrophobization with trichloromethylsilane (TMS). Different from Pd/C catalysts, the hydrophobized Pd/Al2O3 catalyst was not able to adsorb surface active contaminants. They found that the silanization had no effect on the catalyst activity, when the reaction was studied under kinetic control. In the mass transfer controlled regime, they observed that an enhancement of the absorption rate by the hydrophobized Pd/Al2O3 catalyst particles occurred at very low catalyst loadings. The enhanced gas absorption was ascribed to interfacial adhesion leading to a locally higher catalyst concentration in the film. In 2011, Wintonska et al. [47] studied the effect of tellurium introduction on the activity and selectivity of home-made supported palladium catalysts in the oxidation of glucose to gluconic acid. The catalysts for which the presence of PdTe was proven showed high activity and selectivity. The modification of the catalytic properties of PdTe /support bimetallic systems was ascribed to the strong mutual interaction between atoms of active [47] Pd and Te. Wintonska et al found that bimetallic PdTe /SiO2 and PdTe /Al2O3 catalysts containing 5wt% of Pd and 0.3-5 wt% of Te were characterized by both high activity and selectivity towards gluconic acid. As well as carbons and metal oxides, also polymers were successfully used as metal nanoparticle supports. Gold nanoparticles were deposited directly onto ion-exchange resins by reducing HAuCl4 or Au(en)2Cl3 (en = ethylenediamine) with NaBH4 or with surface amine and ammonium groups in anion-exchange resins. The catalytic activity for the oxidation of glucose with molecular oxygen was more strongly influenced by the nature of the polymer supports than by the size of the Au NPs, and it was observed to increase in the order of the basicity of the ion-exchange resins. Strongly basic anion-exchange resins, such + as quaternary ammonium salt (-N Me3) functionalized resins, exhibited a TOF as high as 27.000 h-1 for glucose oxidation at 60°C and at pH 9.5. Organic polymers have been used as supports to efficiently stabilize small Au nanoparticles (2-10 nm in diameter) and clusters (below 2 nm) [48] [49]. Biffis et al. [49] investigated the catalytic performance of Au NPs (2.4 nm) supported on polymer gel, prepared from N,N-dimethylacrylamide (DMAA) as the main comonomer, ethylene dimethacrylate (EDMA) as the cross-linker, and N,N- dimethylamino-ethylmethacrylate (DMAEMA) as the functional metal-binding comonomer, in the aerobic oxidation of alcohols. These materials showed higher catalytic activity than Au/C for the oxidation of hydrophobic alcohols but lower activity for glucose oxidation. Except for Au NPs supported on strongly basic anion-exchange resin [50], there are not many polymer supported Au catalysts showing high catalytic activity in the glucose

15 Introduction ▪ State of the Art oxidation. Ishida et al. [12] have explored renewable polymeric materials obtained from natural feedstocks as possible supports for gold catalysts. Cellulose appeared to be a promising candidate, since, besides being the most abundant and easily obtained organic compound in nature, it has three significant features: (1) chemical stability and resistance to degradation by acids or bases, (2) oxygen-rich structure (i.e. hydroxyl groups, which are expected to interact with the metal ion precursors and stabilize the metal NPs), and (3) hydrophilic nature, which seems to be suitable for reactions in aqueous media. Ishida et al. [12] have attempted to properties of the nanoparticles, i.e. shape, size and stability, on the cellulose support by depositing Au NPs onto the cellulose directly from gold complexes by a deposition-reduction method and by a solid grinding method. Au NPs of around 2 nm in diameter could be deposited onto the cellulose by the solid grinding method (with

Me2Au(acac)). Surface OH groups of cellulose acted as stabilizers to keep Au particles small, although the deposited Au amount was limited (0.23%) because of the low specific surface area. They also evaluated the catalyst performance in the aerobic oxidation of glucose in aqueous media, observing that small Au NPs supported on cellulose showed higher catalytic activity for glucose oxidation compared to large ones [12].

16 Introduction ▪ Supported Metal Catalysts: Preparation Methods

1.3.2. Supported Metal Catalysts: Preparation Methods As suggested from the literature reported in Section 1.3.1., the most often used synthetic strategies for the preparation of metal catalysts, especially gold-based materials to be applied to liquid phase oxidations, are the metal sol immobilization procedure and the direct impregnation of metal salts [30]. The activity and/or the selectivity of gold catalysts are correlated with many parameters, such as morphology, dispersion and interaction between gold particles and support. Due to the low melting point of gold, traditional catalyst synthesis methods, like incipient wetness and impregnation, often fail to produce high metal dispersion, depending on the type of support employed.

The immobilization of a pre-formed metallic sol allows a pretty good control of metal particle size, reducing the influence of the support on metal dispersion [51]. This method is based on the preparation of metallic systems through the reduction of the metal precursor in the presence of a stabilizing agent (polymer, surfactant, polar molecule, etc.), and their subsequent immobilization on a support [52]. The crucial point for obtaining good metal dispersions using this technique is the immobilization step which depends on the surface properties and morphology of the support [53].

For the catalyst preparation by immobilization of metal colloids, it is very important to be able to separate the nucleation and the growth into different steps, as suggested by Lamer et al. [54]. The reducing and the protective agent play both an important role in the structure formation of the final catalyst. The use of a strong reducing agent, such as NaBH4, is needed in order to reduce the metal; however, a quick metal reduction makes the nucleation and growth process difficult to control. It is for this reason that the use of an appropriate protective agent is important. It can passivate the nanoparticles´ surface and prevent them from aggregation making the process easier to control [30]. The stabilization against agglomeration can be achieved by electrostatic, steric or electrosteric effects [30]. Electrostatic stabilization is based on the mutual repulsion of electrical charges. When two similar particles are close to each other, van der Waals forces, resulting from an electromagnetic effect, are always attractive. The addition of a protective agent, such as citrate or tetrakis (hydroxymethylphosphonium chloride (THPC), to the metal precursor generates an electrical double layer of cations and anions. The adsorbed layers result in coulombic repulsions between the particles with the stabilization of the colloid as effect. As THPC is an electrostatic stabilizer, the positive part of THPC molecules coordinates

17 Introduction ▪ Supported Metal Catalysts: Preparation Methods with the negatively charged metal precursor. During the formation of the metallic sol, a huge excess of NaOH is present. THPC/NaOH acts as the reducing agent via formation of formaldehyde, a well-known reducing agent for gold under basic conditions, following the equation [55]:

+ - P(CH2OH)4 + OH  P(CH2OH)3 + CH2O + H2O

Sodium citrate has also been widely used as electrostatic protective agent, in particular for monometallic Au. Sodium citrate acts as a stabilizer as well as a reducing agent. During the metal reduction, it becomes oxidized to the intermediate ketone (acetone dicarboxylic acid), which in return is an even better reducing agent. The steric effect was investigated by adsorption of polymers of a sufficiently high molecular weight, forming a protective layer and keeping the nanoparticles at a distance too large to show van der Waals interactions and, therefore, avoiding agglomeration. Among them, polyvinyl pyrrolidone (PVP) is the most one used. Electrosteric stabilization is achieved by the adsorbed polymers having non-negligible electrostatic charges on the metal precursors, resulting in significant double-layer repulsion. Polyvinyl alcohol (PVA) is a typical example and the most employed stabilizer for the generation of Au nanoparticles. Besides for monometallic catalysts, the metal sol immobilization procedure is also used for the preparation of gold bimetallic catalysts. In the case of bimetallic systems, the method is based on co-reduction or consecutive reduction of the metal precursors in the presence of the stabilizing agent, which passivates the nanoparticles´ surface and prevents them from aggregation, and their subsequent immobilization on a support [35]. When the second metal, with a lower redox potential, is reduced, it can deposit on the surface of the preformed nucleus of the first metal symmetrically with a core-shell structure. If the two metals are completely miscible, as in the case of Au and Pd, an alloy can be formed [30]. The impregnation method consists in the direct impregnation of the support with an aqueous solution containing the metal precursors in the absence of any protective agent, followed by evaporation of the water. The dried material is further reduced, normally using [56] high temperature treatment or gas phase reduction under a H2 flow . The characteristics of the catalysts obtained with this method strictly depend on the post-treatment conditions and on the type of supporting material. Even though impregnation was shown to produce less active gold catalysts than sol immobilization, the simplicity of the methodology makes impregnation still attractive for industrial scale-up purposes.

18 Introduction ▪ Supported Metal Catalysts: Preparation Methods

The advantage of using the sol immobilization procedure lies in its applicability, regardless of the type of support employed; moreover, it is possible to control the particle size/distribution obtaining normally highly dispersed metal catalysts. In contrast, direct impregnation is affected by the surface properties of the support [30].

19 Introduction ▪ Batch and Trickle Bed Reactors

1.3.3. Batch and Trickle Bed Reactors Glucose oxidation, and generally metal catalysed oxidation reactions, is usually carried out in batch reactors containing the solution of the organic substrate to be oxidized and a suspension of the catalyst in powder form. Typically, these reactions are run at atmospheric pressure under continuous stirring with air bubbling through the suspension maintained at constant temperature in the range of 20-80°C. Oxidation reactions are carried out with pH ranging from 2 to 13, but in many instances from 7 to 9. The pH is regulated by the addition of dilute alkali solutions under the control of a pH regulator. The reaction kinetics are followed by monitoring the addition of alkali solution required to maintain a constant pH or by chromatographic analysis of the reaction medium taken at time intervals [57]. Generally, no difficulties and obstacles are encountered when the reaction is carried out in a batch reactor.

In contrast, performing the glucose oxidation in a trickle bed reactor might be more challenging: indeed, in trickle bed reactors, different transport processes occur at different time and length scales. On a reactor scale, gas and liquid phases are introduced from the top and they flow through the voids of the catalyst bed. Several different flow regimes may therefore exist in trickle bed reactors, because of different levels of interphase interactions. The distribution of liquid phase reactants in the bed depends on the quality of liquid distribution at the inlet, overall variation in bed porosity, wetting and capillary forces. Though liquid is distributed uniformly in the top region of the column, non-uniformities in bed porosity and uneven wetting may cause further non-uniformities, as the liquid flows along the length of the reactor. Packing configuration significantly influences possible channeling and maldistribution within the reactor. The nature of voids formed between particles affects the flow structure inside the void and hence controls the mixing, heat and mass transport rates. It also affects the dynamic liquid holdup and the stagnant liquid holdup (corresponding to stagnant liquid pockets) in the bed. The exchange between these two quantities often determines the effective residence time distribution in trickle bed reactors. For exothermic reactions, as oxidation processes, this phenomenon is important where dry out of particles may lead to the formation of local hotspots which may result in temperature runaway. Solvent evaporation adds further complexities in heat and mass transfer rates. On a single catalyst particle scale, wetting of particle and intraparticle mass and heat

20 Introduction ▪ Batch and Trickle Bed Reactors transfer play an important role in the overall rate. Though wetting of particles is a result of global operations, particle-scale parameters also determine the degree of wetting of each particle. Flowing liquid forms a film over the external surface of the catalyst and partial wetting may occur at lower liquid flow rates. The wetted part of the catalyst surface gets exposed to the liquid phase reactants and the dissolved gas phase reactants. The non-wetted part, instead, is exposed to the gas phase reactant. In most cases, however, due to capillary effects, catalyst particles get completely filled with the liquid phase. However, this condition is not always true if liquid phase is evaporating or pores are larger such that capillary effects are negligible. Partial wetting condition affects the reaction rates in various ways, influencing mass transfer from gas and liquid phases to catalytic sites available on and in the particle. Gas-particle mass transfer rates are significantly enhanced due to direct access of gaseous reactants through the non-wetted surface. The analysis of the reaction rates under partial wetting condition is extremely complex, due to solvent and substrate condensation/evaporation and local temperature variation; often, this leads to an increase/decrease of catalyst effectiveness and in some cases to multiplicity of conversion and temperature. Furthermore, most of the trickle bed reactions are exothermic in nature and careful account of intraparticle, interphase and even bed-to-wall heat transfer is crucial in understanding the overall performance [58].

21 Motivation and Aim

2. Motivation and Aim In the chemical industry, oxidation reactions play a relevant role for the production of many significant and crucial compounds [59]. New “green” catalytic oxidation approaches must meet both health and environmental standards, and at the same time aim at a reduction of cost and time [60]. Homogeneous catalysis has been widely used in oxidative processes for the manufacturing of bulk and fine chemicals [61]. The main advantage of molecular catalysts is that they dissolve in the reaction medium; hence all catalytic sites are available, resulting in high reaction rates and in a reaction time reduction. However, homogeneous catalysts are rather difficult to separate from the reaction mixtures and they may also cause corrosion to the industrial materials with possible deposition on the reactor walls [62]. Heterogeneous catalysis is considered a better alternative for the synthesis of commodity materials. Different materials, i.e. silica, carbon, clay, zeolite, metal oxide, polymers and other materials are currently used as solid supports [63]. In heterogeneous catalytic reactions, the catalyst and the reactants exist in different phases; actually, the majority of heterogeneous catalysts are solids and the reactants are usually either liquids or gases [64]. Solids catalysts are easier to prepare and handle, as they are stable, reusable, and easy to separate and they can also be used as fixed beds. The catalyst is often a metal to which chemical and structural promoters or poisons are added in order to enhance the efficiency and/or the selectivity. Currently, heterogeneous catalysis dominates the industries of chemical transformation and energy generation [62].

Among the heterogeneously catalysed oxidation reactions of potential industrial interest, the metal catalysed liquid-phase oxidation of glucose to gluconic acid has recently gained much consideration, due to the wide gluconic acid applicability [12]. Although the gluconic acid production by fermentation process is commercially well established, the development of novel, more economical and one-step processes for the glucose conversion to gluconic acid might be a valid and successful alternative [7]. The aim of this PhD research work is to find the optimal catalyst and reaction parameters in order to successfully perform the metal catalysed liquid phase glucose oxidation under batch and continuous conditions, i.e in a trickle bed reactor (TBR); furthermore, high gluconic acid yields should be achieved in a short time and without pH control.

22 Motivation and Aim

The selective catalytic oxidation of glucose with molecular oxygen is an environmentally benign process for the production of gluconic acid, widely used in the food, detergent and pharmaceutical industries [24]. In this work, pure molecular oxygen will therefore be used as oxidizing agent to perform the reaction; however, some experiments will be carried out with air, in order to verify if this is a valid and more convenient alternative to the use of pure oxygen. Different solid catalysts will be evaluated in the reaction. The procedures based on supported palladium and platinum materials have been intensively investigated in the past years [26] [65] [66]; however, since these catalysts often suffer from drawbacks of low catalyst durability and relatively low selectivity, many studies have recently been devoted to employ gold as catalyst to achieve selective aerobic oxidation of glucose under mild conditions; moreover, superior selectivity, high catalytic activity and long term stability were observed [21] [67] [68] [23].

Contrary to the majority of the studies on glucose oxidation, in the present work no alkaline solution will be added to the reaction mixture to maintain the pH at a fixed value during the reaction performed in the batch reactor; the glucose oxidation will be indeed performed starting directly from an alkaline sugar solution. This is due to the intended use of the trickle bed reactor; in this kind of reactor, pH adjustments during the reaction are indeed difficult.

Since performing the reaction in a trickle bed reactor might be particularly challenging, due to the different transport processes occurring at different time and length scales, the glucose oxidation will be first carried out in a batch reactor. In this system, the reaction will be performed under mild conditions, under variation of the reaction temperature (25- 90°C) and oxygen pressure (1-4 bar). Different commercial and self-prepared supported Au, Pd and Pt materials will be evaluated in the glucose oxidation in order to find the catalyst and the reaction conditions by which maximum glucose conversion into gluconic acid is achieved in the shortest time.

This catalyst will be later used to perform the reaction in continuous mode instead of under batch conditions, which is the typical mode of operation described in the literature. As the trickle bed reactor (TBR) is the most industrially used reactor to treat continuously three- phase systems, a lab-scale TBR set-up will be assembled. Here, the glucose oxidation will be carried out under variation of several reaction parameters, such as liquid and gas flow

23 Motivation and Aim rate, temperature and glucose solution concentration, with the aim of obtaining the highest gluconic acid yields in the shortest time.

24 Results and Discussion ▪ Glucose Liquid Oxidation in Batch Reactor – Finding the Best Catalyst for the Trickle Bed Reactor

3. Results and Discussion

3.1. Glucose Liquid Oxidation in Batch Reactor – Finding the Best Catalyst for the Trickle Bed Reactor

In the batch reactor, the reaction was carried out at 70°C, 3 bar O2 and under mechanical stirring; high stirring rate (1000 rpm) was applied in order to exclude external mass transfer limitations. Oxygen, due to its low solubility, is the deficit compound in the glucose oxidation. Therefore, a sufficient concentration of dissolved oxygen has to be ensured during the course of the reaction [69]. The use of a closed vessel under oxygen pressure might increase oxygen dissolution, with the additional effect of speeding up the reaction [21].

The glucose oxidation was performed starting from an alkaline (pH 13.5) sugar solution (5wt% glucose alkaline solution) with glucose:metal ratio = 1000. No basic solution (NaOH for example) was added to the reaction mixture to maintain the pH at a fixed value; the glucose oxidation was therefore carried out at uncontrolled pH. Alkaline conditions are apparently necessary to increase the reaction rate and to avoid drastic catalyst deactivation; conversely, however, such conditions are also responsible for side reactions that reduce gluconate productivity [21]. Moreover, glucose starts to decompose at pH above 11 [21].

The following tests performed in the batch reactor were aimed to find the catalyst by which maximum gluconic acid yield is achieved in the shortest time, and which might therefore be used in a trickle bed reactor. In order to be used in a trickle bed reactor, the catalyst should be in non-powder form; generally, powder catalysts are difficult to handle in TBRs, mainly because of low bed porosity and difficulties in flow distribution. Gold nanoparticles, as well as palladium and platinum, supported on metal oxides, resins and carbons, were employed as catalysts. The metal oxides, the resins and the carbon used as supports consist of pellets, spheres and grains, respectively; for this reason, catalysts supported on these kinds of materials might be packed as a bed through which gas and liquid can flow. Furthermore, by using different materials as supports, also the effect of the support on the catalytic activity of the dispersed metal nanoparticles for the reaction in the batch reactor could be investigated.

25 Results and Discussion ▪ Batch Reactor - Metal Oxides as Metal Nanoparticles Supports

3.1.1. Batch Reactor - Metal Oxides as Metal Nanoparticles Supports The glucose oxidation was performed using commercial gold supported on metal oxides.

Au(1wt%)/ ZnO, Au(1wt%)/Al2O3 and Au(1wt%)/TiO2 were evaluated in the reaction at

70°C, 3 bar O2, 1000 rpm stirring and for 7h. The conversion and gluconic acid yield profiles obtained are reported in Fig. 3.1. After 420 minutes, the highest glucose conversion (89.3%) was achieved with Au(1wt%)/ZnO. With Au(1wt%)/Al2O3 and

Au(1wt%)/TiO2, 85.4% and 80.4% of initial glucose was converted. However, the amount of gluconic acid detected in the three reaction mixtures was very low. After 7 h of reaction,

13.7% of gluconic acid was achieved using Au(1wt%)/ZnO, while with Au(1wt%)/Al2O3 and Au(1wt%)/TiO2 the amount of gluconic acid detected in the corresponding reaction mixture was 7.5% and 9.5% respectively.

The catalytic performance of alumina supported gold in the glucose oxidation was already studied by Baatz et al. in 2007 [70] [71]. For the preparation of gold catalysts, they used the deposition-precipitation methods, with NaOH (DP NaOH) or urea (DP urea) as precipitation agents, as described by Haruta [17] and Dekkers [72]. The catalysts prepared by DP urea showed a strong dependence of specific activity on the gold content. Very high activity was observed at very low gold content. Increasing the gold content led to a decrease in the catalyst activity [71]. Baatz et al. [70] [71] prepared gold catalysts also by the incipient wetness method. They found that these catalysts had an activity-gold content relationship similar to the one observed for the materials prepared by the DP urea method. In addition, the alumina-supported catalysts, prepared either by DP urea or incipient wetness, showed 100% selectivity towards D-gluconate. Although the preparation method of the commercial gold supported on metal oxides used in this PhD work is unknown, it can be assumed that they were synthetized by deposition-precipitation (either with NaOH or urea) or by incipient wetness impregnation. These are indeed the most frequently used preparation methods for metal oxide supported materials. The most active catalysts prepared by Baatz et al. [70] [71] by DP urea and incipient wetness impregnation had gold contents of 0.1 wt% and 0.3 wt%, respectively, and both showed extremely small gold [71] particles between 1.2 and 3 nm . In this PhD work, the commercial Au/Al2O3, as well as

Au/ZnO and Au/ TiO2, had a higher gold content (1wt%) with an average gold crystallite size of ~ 2-3 nm. However, it should be also taken into account that the Au/Al2O3, Au/ZnO and Au/ TiO2 were used in the form of extrudates, and that they were evaluated in the

26 Results and Discussion ▪ Batch Reactor - Metal Oxides as Metal Nanoparticles Supports glucose oxidation without previously being crushed to powder. Therefore, mass transfer limitations might have been significant for the reaction in the batch reactor, with the effect of further decreasing the catalytic performance of these materials.

Au(1wt%)

100 Conversion

80

60 ZnO

(%) TiO 40 2 Al2O3

20 Yield Gluconic a.

0 0 50 100 150 200 250 300 350 400 Time (min)

Figure 3.1. Conversion and gluconic acid yield profiles for the glucose oxidation in the batch reactor performed with commercial gold (1%) supported on ZnO, Al2O3 and TiO2 as catalysts (70°C, 3 bar O2, 1000 rpm stirring, 5wt% glucose alkaline solution (pH 13.5)). 1 run for each test.

The product distribution of the glucose oxidation performed with Au(1wt%)/Al2O3,

Au(1wt%)/ZnO and Au(1wt%)/TiO2, corresponding to the end of the reaction (7h), is shown in Fig. 3.2. Glucuronic acid was the glucose oxidation side-product detected in major amounts in all the reaction mixtures corresponding to the three catalysts. The highest amount of glucuronic acid (36.4%) was formed when the reaction was performed with

Au/Al2O3. With Au/ZnO and Au/ TiO2, the amount of gluconic acid detected in the corresponding reaction mixtures was 28.0% and 28.3% respectively. Beside these acids, also other products were found in the three reaction mixtures.

27 Results and Discussion ▪ Batch Reactor - Metal Oxides as Metal Nanoparticles Supports

89.3% 85.4% 80.4% Conversion 10.7% 14.6% 19.6% 100 Unreacted glucose

80

60 11.4 11.9 15.0 Fructose 40 28.0 36.4 Glucuronic a.

20 28.2 Yield (%) after 420 min 420 after (%) Yield 13.6 9.5 Gluconic a. 0 7.5

ZnO Al2O3 TiO2 supports

Figure 3.2. Product distribution after 420 minutes for the glucose oxidation performed in the batch reactor with Au(1wt%)/ZnO, Au(1wt%)/Al2O3 and Au(1wt%)/TiO2 as catalysts (70°C,

3 bar O2, 1000 rpm stirring, 5wt% glucose alkaline solution (pH 13.5)).

The presence of fructose was observed during the glucose oxidation, indicating the isomerization of glucose. It is known that by treating monosaccharides with concentrated alkaline solutions, the sugars are destroyed [73] and that alkaline media with lower pH induce an isomerisation reaction of glucose to fructose, resulting in an equilibrium mixture of the two sugars [73]. Unreacted glucose was found in all three reaction mixtures, with the highest amount in the case of Au(1wt%)/TiO2 (19.6%). Other oxidation side-products, like 2- and 5-keto gluconic acid and decomposition products like formic, glycolic and acetic acid were also detected in minor amounts (<5.0%).

Since the highest amount of gluconic acid was achieved with Au(1wt%)/ZnO, this material was used to perform further tests with variation of reaction conditions, such as the reaction temperature and the pH of the initial glucose solution. The results are reported in Fig. 3.3. When the reaction was performed at 70°C starting from a neutral glucose solution, gluconic acid was formed in 41.6% yield. With respect to the reaction carried out at the same temperature from an alkaline sugar solution (89.3% conversion and 13.7% gluconic acid yield), the conversion was lower (67.5%) but the gluconic acid yield obtained was higher.

28 Results and Discussion ▪ Batch Reactor - Metal Oxides as Metal Nanoparticles Supports

pH 13.5, RT 100 pH 13.5, 70°C 89.3 80 Neutral pH, 70°C 60 100.0 40 89.3 67.5 Conversion (%) (%) Conversion 20

0 420 min

100 pH 13.5, RT 80

60 Neutral pH, 70°C 86.5 40

20 pH 13.5, 70°C 41.6 Yield Gluconic a. (%) (%) a. Gluconic Yield 13.7 0 420 min

Figure 3.3. Conversion and gluconic acid yield values after 420 minutes for the glucose oxidation performed in the batch reactor with Au(1wt%)/ZnO as catalyst with variation of reaction temperature and pH (3 bar O2, 1000 rpm stirring, 5wt% glucose solution (pH 13.5)). 1 run for each test.

Starting from an alkaline glucose solution, the glucose oxidation was also carried out at room temperature (RT). In this case, the highest conversion and gluconic acid yield were obtained, with 86.5% gluconic acid detected in the corresponding reaction mixture. Minor amounts of fructose and 5-Keto gluconic acid were also found. A reaction temperature of 70°C might be too high to perform the glucose oxidation with gold supported on metal

29 Results and Discussion ▪ Batch Reactor - Metal Oxides as Metal Nanoparticles Supports oxides. This is also in agreement with the above mentioned work of Baatz et al. [70], who tested the performance of all catalysts in a thermostat glass reactor at the lower temperature of 40°C. Also Mirescu et al. [73] obtained the best results with 0.45 wt% Au supported on titania at a reaction temperature between 40-60°C and a pH value of 9. When, in this PhD work, the glucose oxidation was performed at 70°C, starting from a high alkalinity glucose solution (pH = 13.5) with the commercial gold on metal oxide catalysts, the final solution had a brown/caramel colour and a caramel odour. The high temperature induced a degradation of glucose with fragmentation of the molecule, resulting in short chain carboxylic acids, aldehydes, etc.; this phenomenon is known as caramelization [74] . As the process occurs, browning of the sugar is observed and volatile chemicals are released, producing the characteristic caramel colour and odour. The caramelization consists of different type of reaction, such as dehydration and fragmentation reactions, unsaturated polymer formation, isomerization of aldoses and ketoses and condensation reactions. The process is temperature dependent and different sugars have their specific point, at which the reaction begins to proceed. Usually, glucose caramelization occurs at 160°C [75]. However, caramelization reactions are also sensitive to the chemical environment. The reaction rate or the temperature at which the reaction occurs may be altered by controlling the pH of the sugar solution. In general, the caramelization rate is lowest around pH 7 and accelerated under both acidic (especially pH < 3) and basic (especially pH > 9) conditions. When performing the reaction at 70°C with respect to RT, both in alkaline and neutral pH, the caramelization process might be the main reason for the lower gluconic acid yield. Since the caramelization rate is higher at pH > 9, the process occurs in greater extent when the reaction is performed starting from an alkaline solution than from a neutral one. Indeed, 41.6% gluconic acid is detected in the reaction mixture at neutral pH, while only 13.7% gluconic acid yield is formed starting from an alkaline solution. The lower conversion observed performing the reaction at neutral pH with respect to basic pH at 70°C might instead be due to the pH itself. The reaction rate increases with increasing pH; indeed, in alkaline solution, the deactivation of the catalyst, due to gluconic acid blocking the active centres on the catalyst surface, is prevented. Considering instead the reactions performed starting from an alkaline glucose solution, the lower conversion observed at 70°C might be related to the effect of the temperature on the solution pH. The pH of a solution decreases with increasing temperature; this could lead to a small extent of catalyst deactivation by

30 Results and Discussion ▪ Batch Reactor - Metal Oxides as Metal Nanoparticles Supports gluconic acid poisoning. However, this phenomenon remains more significant in neutral solution than at lower, but still alkaline, pH. Indeed, the difference in conversion observed at 70°C and RT in alkaline solution is lower than the difference in conversion observed at alkaline and neutral pH, at 70°C. The higher yield of gluconic acid obtained at RT with respect to 70°C, at basic pH, might still be explained by the absence of caramelization process. According to the obtained results, in order to achieve a significant amount of gluconic acid using metal oxides supported gold, the glucose oxidation should be carried out with Au(1wt%)/ZnO at room temperature starting from an alkaline glucose solution.

Pd(5wt%)/Al2O3 Pt(5wt%)/Al2O3 100 Conversion 100 Conversion 80 80

60 60

Gluconic a.

(%) (%) 40 40 Gluconic a. 20 Fructose 20 Fructose 2-Keto gluconic a. 2-Keto gluconic a. 0 0 0 50 100 150 200 250 300 350 400 0 50 100 150 200 250 300 350 400 Time (min) Time (min)

Figure 3.4. Conversion and products yields profiles for the glucose oxidation performed in the batch reactor with commercial Pd(5wt%)/Al2O3 and Pt(5wt%)/Al2O3 as catalysts (70°C, 3 bar O2, 1000 rpm stirring, 5wt% glucose alkaline solution (pH 13.5)). 1 run for each test.

Beside gold, also palladium and platinum were tested on a metal oxide support.

Commercial Pd(5wt%)/Al2O3 and Pt(5wt%)/Al2O3 were used as catalysts to carry out the glucose oxidation in the batch reactor. Fig. 3.4 shows the conversion and yield profiles for the products detected in the highest amounts. With Pd, higher conversion and gluconic acid yield was achieved with respect to Pt. At the end of the reaction (7h), 98.0% and 79.0% conversion was reached with Pd and Pt respectively. Simultaneously, the gluconic acid

31 Results and Discussion ▪ Batch Reactor - Metal Oxides as Metal Nanoparticles Supports yield obtained with Pd (51.0%) was twice higher than the amount detected in the reaction mixture resulting from use of the Pt catalyst (~23.0%). Similar amounts of fructose were detected with both metals, while a higher quantity of unreacted glucose was found in the reaction mixture corresponding to Pt. With both Pd and Pt, 7.0-8.0% of 2-Keto gluconic acid was formed, and also smaller amounts (<3.0%) of glucaric, glycolic and 5-Keto gluconic acid were detected in the two reaction mixtures.

The conversion profiles obtained for Au(1wt%)/ZnO, Au(1wt%)/Al2O3 and

Au(1wt%)/TiO2 reported in Fig. 3.1 are all characterised by a plateau reached within 30 minutes of reaction. A possible explanation might be a product poisoning of the catalyst. In order to verify this hypothesis, possible products were individually added to the starting glucose solution (mmol added product:mmol glucose = 1:4); Au(1wt%)/ZnO was used as catalyst. The aim was to observe the effect of these additions on the initial reaction rates. Since the oxidation of glucose to glucuronic acid is in competition with the oxidation of glucose to gluconic acid (Fig. 1.3), glucuronic and gluconic acid were considered possible sources of catalyst poisoning. Furthermore, from further oxidation of glucuronic and/or gluconic acid, glucaric acid is obtained; therefore, glucaric acid was also added to the initial glucose solution in order to study its effect on the reaction rate. The influence of glycolic acid, as possible degradation product, was also investigated. From the results reported in Fig. 3.5, it is clear that the addition of gluconic acid, the target product of the glucose oxidation, did not have any significant effect on the reaction rate. Indeed, after 5 minutes, around 78.0% conversion was reached with or without gluconic acid addition. In contrast, both glucuronic acid and glucaric acid addition to the initial glucose solution resulted in a conversion decrease. After 5 minutes, 57.7% and 68.5% conversion was obtained with glucuronic acid and glucaric acid addition, respectively. An interesting effect on the initial rate was observed in the case of glycolic acid addition; indeed, higher conversion was obtained with respect to the reaction performed without any product addition (89.4% and 77.6%, respectively).

32 Results and Discussion ▪ Batch Reactor - Metal Oxides as Metal Nanoparticles Supports

No product addition 100 + Glycolic a.

80 + Glucaric a.

60 + Gluconic a. + Glucuronic a.

40 Conversion(%) 20

0 0 5 10 15 20 25 Time (min)

Figure 3.5. Effect of the individual addition of possible products to the starting glucose solution on the initial reaction rate. Au(1wt%)/ZnO used as catalysts (70°C, 3 bar O2, 1000 rpm stirring, 5wt% glucose alkaline solution (pH 13.5)). 1 run for each test.

The addition of possible products influenced also the yields of the main glucose oxidation products. The higher conversion obtained after adding glycolic acid to the initial glucose solution is due to the higher gluconic acid amount produced (Fig. 3.6a). After 300 minutes, while only 14.3% gluconic acid is formed when the reaction is performed without any product addition, 63.2% gluconic acid is detected in the reaction mixture corresponding to glucose+glycolic acid as initial solution. 32.1% and 22.5% is the gluconic acid yield obtained with glucuronic and glucaric acid addition, respectively. Contrary to what is observed for gluconic acid, lower glucuronic acid amounts were produced after adding glucaric (12.1%), gluconic (11.8%) and glycolic (2.5%) acid (Fig. 3.6b). When the reaction was performed without any product addition, 28.8% glucuronic acid was formed. When no product was added, the amounts of glucaric, glycolic. 5- and 2-keto gluconic acid were generally lower (<5%) than gluconic and glucuronic acid. The highest glucaric acid yield was detected when glucuronic acid was added to the initial glucose solution (4.5%); without any product addition, basically no glucaric acid was produced (0.4%) (Fig. 3.6c).

33 Results and Discussion ▪ Batch Reactor - Metal Oxides as Metal Nanoparticles Supports

a) b)

70 + Glycolic a. 30 60 14.3 25 50 20 40 + Glucuronic a. 63.2 15 30 28.8 + Glucaric a. + Gluconic a. + Glucaric a. 10 20 32.1 12.1 11.8 Yield Gluconic a. (%) a. Gluconic Yield 10 22.5 5 14.3 (%) a. Glucuronic Yield 2.5 0 0 No product No product + Glycolic a. addition addition

c) d) + Glucuronic a. + Gluconic a. 3 + Glucaric a. 4

3 2 3.7 4.5 2 2.9 2.7 2.5 + Glycolic a. 1 1 + Gluconic a.

1.3 (%) a. Glycolic Yield Yield Glucaric a. (%) a. Glucaric Yield 0.4 0.7 0 0 No product No product + Glucuronic a. addition addition

e) + Gluconic a. : 0% + Glycolic a. : 0% 4 + Glucaric a.

3

2 3.9 + Glucuronic a.

1 2.3 1.6

Yield 5-keto gluconic a. (%) a. gluconic 5-keto Yield 0 No product addition

Figure 3.6. Effect of the individual addition of possible products to the starting glucose solution on the product yields after 300 minutes (70°C, 3 bar O2, 1000 rpm stirring, 5wt% glucose alkaline solution (pH 13.5)).

1.3% and 0.7% yield were obtained by addition of glycolic and gluconic acid, respectively. The glycolic acid yield increased by addition of gluconic acid (3.7%) and slightly

34 Results and Discussion ▪ Batch Reactor - Metal Oxides as Metal Nanoparticles Supports decreased when the reaction was performed starting from a glucose+glucaric acid (2.7%) and glucose+glucuronic acid (2.5%) solution (Fig. 3.6d). By addition of glucaric acid, the amount of 5-keto gluconic acid was higher (3.9%) than the one obtained without any product addition (2.3%) (Fig. 3.6e). In contrast, the addition of glucuronic acid had the effect of decreasing the formation of 5-keto gluconic acid (1.6%). The addition of possible products did not have any effect on the 2-keto gluconic acid yield.

According to the results reported in Fig. 3.5, the catalyst poisoning by glucuronic and glucaric acid might be the reason for the inhibition of the catalytic activity, which corresponds to a plateau in the conversion profile. Although the glucuronic and glucaric acid addition to the initial glucose solution resulted in lower glucose conversion, higher gluconic acid yields were obtained. The highest conversion and gluconic acid amount produced was observed by adding glycolic acid to the starting sugar solution.

35 Results and Discussion ▪ Batch Reactor - Resins as Metal Nanoparticles Supports

3.1.2. Batch Reactor - Resins as Metal Nanoparticles Supports Metal nanoparticles supported on resins, both commercial and “home-prepared”, were evaluated in the glucose oxidation performed in the batch reactor. Equilibrium reactions taking place within resins can be conveniently shifted to the right if the products have a low compatibility with the resin. Using hydrophobic polymer matrices as metal nanoparticle supports could be a strategy to favour the expulsion of the glucose oxidation products, mostly polar, from the resin. Furthermore, the application of supported polymers in catalytic oxidation has gained much attention because of their inertness, nontoxicity, non-volatility, and recyclability [76]. Commercial porous resins Amberlyst A35 and A70, sulfonated styrene/divinylbenzene (PS-DVB) copolymers (Fig. 3.7), were both used as supports for Pt, Au and Ru nanoparticles (5wt% metal loading). Although both resins belong to the macroreticular type (DVB > 4%), they differ in the cross-linker content (DVB) which is 20% and 8% for the A35 and A70 resin, respectively. According to ICP-analysis, the actual metal content matched the theoretical one (Section 5.2.4).

Figure 3.7. Example of vinyl monomer polymerization: co-polymerization of styrene and divinylbenzene to a polystyrene resin and further sulfonation.

36 Results and Discussion ▪ Batch Reactor - Resins as Metal Nanoparticles Supports

Pt(5wt%)/A70 100 Pt(5wt%)/A35

80 Conversion

60 (%) 40

20 Yield Gluconic a. 0 0 50 100 150 200 250 300 350 400

Time (min)

Figure 3.8. Conversion and gluconic acid yield profiles for the glucose oxidation performed in the batch reactor with Pt(5wt%)/A35 and Pt(5wt%)/A70 as catalysts (70°C, 3 bar O2, 1000 rpm stirring, 5wt% glucose alkaline solution (pH 13.5)). 1 run for each test.

The resin supported catalysts were evaluated in the reaction at 70°C, 3 bar O2, 1000 rpm stirring and for 7h The conversion and gluconic acid yield profiles obtained for Pt(5wt%) are shown in Fig. 3.8; with Au and Ru similar trends were observed. In general, the conversion reached at the end of the reaction (7h) was around 60-70% while the gluconic acid yield was close to zero. However, when Pt(5wt%)/A35 was used as catalyst in the glucose oxidation, slightly higher conversion was obtained. Although the reaction was carried out for 7 h, already after 5 minutes a plateau around 65.0% with A35 and 62.0% with A70was observed in the conversion profile. The reason for the very low gluconic acid formation observed with all the resin supported catalysts might be found in the product distributions corresponding to the respective reaction mixtures. As reported in Fig. 3.9, fructose and gluconic acid were detected in major amounts. For all catalysts, a significant quantity of glucose did not react. Only a negligible amount of gluconic acid (2%) was formed when the glucose oxidation was performed with Pt(5wt%)/A70.

37 Results and Discussion ▪ Batch Reactor - Resins as Metal Nanoparticles Supports

65.5 72.4 66.5 70.8 66.2 67.4 Conversion 100 34.5 27.6 33.6 29.2 33.8 32.6 Unreacted glucose

80 A70 A35 60

40

22.1 Fructose 20 22.9

24.8 17.3 20.1 16.6 Yield (%) after 420 min 420 after (%) Yield 8.6 7.9 8.7 13.3 Glucuronic a. 0 5.6 6.4 Pt(5wt%) Au(5wt%) Ru(5wt%)

Figure 3.9. Product distribution after 420 minutes for the glucose oxidation performed in the batch reactor with Pt, Au and Ru nanoparticles (5wt% metal loading) supported on A35 and

A70 resins (70°C, 3 bar O2, 1000 rpm stirring, 5wt% glucose alkaline solution (pH 13.5)).

Other side-products detected in all the reaction mixtures in similar amounts (<5.0%) were glucuronic, formic, glycolic, 2-Keto and 5-Keto gluconic acid. With Pt, Au and Ru supported on A70 and A35 resins the carbon balance does not close. The reason might be found in the chromatograms of the reaction mixtures corresponding to the resin supported catalysts. Indeed, in all of them, the presence of many peaks, some of which even overlapped, was observed. Beside the difficult quantification of the known peaks, many other peaks could not be assigned to the known molecules and quantified. Thus, the acidic resins supports induce many side reactions, which render them overall unsuitable for glucose oxidation. Furthermore, with all catalysts supported on the commercial resins, the final solution had a brown/caramel colour and a caramel odour. As in the case of gold supported on metal oxides (Section 3.1.1), this was a sign of the degradation of glucose with fragmentation of the molecule, resulting in short chain carboxylic acids, aldehydes, etc. (caramelization) [74]; this phenomenon might explain the low yield of glucose oxidation products. The rapid deactivation of the metal nanoparticles supported on resins, which results in a plateau in the conversion profile, might instead be due to products poisoning, as it was observed for the Au/metal oxides catalysts.

38 Results and Discussion ▪ Batch Reactor - Resins as Metal Nanoparticles Supports

TEM images of Pt(5wt%)/A70and Pt(5wt%)/A35 samples are shown in Fig. 3.10. In both cases, the diameter of the metal nanoparticles diameter was around 30-40 nm. It is reasonable to assume that the low gluconic acid production with Pt, Au and Ru supported on A70 and A35 resins was due to large dimensions of the metal nanoparticles.

Figure 3.10. TEM images of Pt(5wt%)/A70 and of Pt(5wt%)/A35.

The A70 and A35 supported Pt, Au and Ru materials were prepared by reducing the metal nanoparticles with gaseous hydrogen. In order to investigate, if different metal reduction methods had any effect on the catalytic performance, the same materials were prepared, but, instead of H2, they were reduced with a freshly prepared NaBH4 solution. As an example, Fig. 3.11 shows the product distributions for the glucose oxidation performed with Ru(5wt%)/A70 prepared with the H2 reduction method (Ru(5wt%)/A70-H2) and via

NaBH4 solution as reduction method (Ru(5wt%)/A70-NaBH4). The amounts of un- converted glucose and of oxidation products detected in the reaction mixtures corresponding to Ru(5wt%)/A70-H2 and to Ru(5wt%)/A70-NaBH4 were very similar. Only

39 Results and Discussion ▪ Batch Reactor - Resins as Metal Nanoparticles Supports

the yield of fructose was higher in the case of Ru(5wt%)/A70-NaBH4. This suggested that the type of reduction method used in the preparation of resin supported materials did not have a significant influence on the catalytic performance.

66.5 66.3 Conversion 33.6 33.7 Unreacted 100 glucose

80

60

40 5.5 Formic a. 5.4 Fructose 20 20.1 30.2

8.6 6.0 Gluconic a. Yield (%) after 420 minutes 420 after (%) Yield 0

H2 reduction NaBH4 reduction

Figure 3.11. Product distributions after 420 minutes for the glucose oxidation performed in the batch reactor with Ru(5wt%)/A70-H2 and Ru(5wt%)/A70-NaBH4 (70°C, 3 bar O2, 1000 rpm stirring, 5wt% glucose alkaline solution (pH 13.5)). 1 run for each test.

Different metal distribution would be expected to be generated by the two different reduction protocols, as shown for Au nanoparticles by Calore et al. in 2012 [77]. According to their work, there are mainly two reasons: 1) the different nature of the reducing agent, and 2) the difference in the expansion of the polymer matrix between the semi-dried resin reduced by gaseous H2 and the fully swollen resin reduced by aqueous NaBH4 solution. The penetration of small hydrogen molecules into the interior of the resin beads is allowed by the eventual residual water content in partially dried resins. However, the collapsed polymer structure inhibits the mobility of the metal ions, helping to maintain the homogeneity of the metal nanocluster distribution. According to Calore et al. [77], the presence of at least small residual water amounts is highly important for the reduction by molecular hydrogen. In fact, metal redistribution during the reduction with gaseous H2 might probably effectively be blocked by the partial wetness of the polymer. When the

40 Results and Discussion ▪ Batch Reactor - Resins as Metal Nanoparticles Supports

reduction is carried out with aqueous NaBH4 solution, the water-saturated environment of the swollen polymer matrix promotes the fast penetration of Na+ into the resin beads, + – allowing the displacement of metal ions with Na . However, the BH4 anion (reducing agent) penetration is expected to be hindered by repulsion electrostatical forces between – - BH4 and the SO3 groups.

Figure 3.12. TEM images of Pt(5wt%)/A70-H2 and of Pt(5wt%)/A70-NaBH4.

Hence, the transformation of metal ions into M0 atoms proceeds more quickly at the periphery of the polymer matrix than in its interior. Aggregation of M0 atoms into nanoclusters is more likely to occur to a significant extent at the very border of the swollen polymer matrix [77]. As an example, in Fig.3.12, the TEM images of two samples of

Pt(5wt%)/A70 prepared by H2 and NaBH4 reduction methods, respectively, are shown. The presence of a cluster at the periphery of the polymer matrix was observed in the image corresponding to Pt(5wt%)/A70-NaBH4, while in the Pt(5wt%)/A70-H2 sample the metal nanoparticles distribution looked more homogeneous. This homogeneity is probably the

41 Results and Discussion ▪ Batch Reactor - Resins as Metal Nanoparticles Supports consequence of the nanoparticles formation inside the partially swollen polymer matrix, where their size is controlled by steric effects imposed by the polymer framework [78].

Figure 3.13. Model for the generation of size-controlled metal nanoparticles inside resins. Reprinted (adapted) with permission from [78].

According to this model for the generation of size-controlled metal nanoparticles inside resins, the metal ion is first homogeneously dispersed inside the polymer framework. In the second step, the metal ion is reduced to M0. Later, M0 atoms start to aggregate in subnano- clusters and as result a single nanocluster is formed and blocked “inside“ of the largest mesh present in that fraction of a given polymer framework [79] (Fig. 3.13). It should be taken into account that the A70 and A35 supported materials were in the form of small spherical pellets and that they were evaluated in the glucose oxidation without previously being ground to powder. Therefore, as in the case of catalysts supported on metal oxides, mass transfer limitations might have been significant for the reaction in the batch reactor, with the effect of decreasing the catalytic performance of these materials.

42 Results and Discussion ▪ Batch Reactor - Resins as Metal Nanoparticles Supports

Besides commercial resins, also “home-made” polymer supports were used in the preparation of metal catalysts for the aerobic glucose oxidation. Instead of increasing the hydrophilicity of the ST/DVB copolymers by introducing a hydrophilic component, such as carboxylic and sulfonic acid groups, the new approach proposed by Zhao et al. [80] was used. Knowing that the cross-linking density is an important element for the hydrophobic porous resins to acquire the water-swelling ability [81], they provided a new strategy to obtain a water-wettable or –swellable polymer material with a higher cross-linking degree by copolymerization of two kinds of cross-linkers, divinylbenzene (DVB) and ethylene glycol dimethacrylate (EGDM). In this PhD work, Au(3wt%)/EGD64 (60% EGDM and 40% DVB) was used to perform the glucose oxidation in the batch reactor. The results are shown in Fig. 3.14. Although the reaction was carried out for 7 h, after 15 minutes the conversion profile reached a plateau at around 76.0%. At the same time, unreacted glucose and fructose concentrations stabilized at ~24.0% and ~15.0% respectively. No gluconic acid was produced during the reaction; instead ~30.0% glucuronic acid was formed within 420 minutes. The formation of 5-keto gluconic acid reached its peak at 1 h with 16.0% yield, after which a slow decrease was observed till the end of the reaction with 3.0% yield. Both glucuronic acid and 5-keto gluconic are oxidation side products. However, while glucuronic acid derives from a glucose oxidation side reaction, 5-keto gluconic is a side product of the gluconic acid oxidation. Low amounts (<5%) of formic, glycolic, 2- keto gluconic and acetic acid were also detected in the reaction mixture. Although gluconic acid was not formed in the reaction carried out with Au(3wt%)/EGD64, this material showed good catalytic performance for the generation of glucuronic acid (industrially used in the production of pharmaceuticals). However, the reaction proceeded slowly and only ~30.0% gluconic acid was obtained after 7 h. The activity of Au(3wt%)/EGD64 is reasonably correlated to its water-swelling ability. Two conditions are necessary for a hydrophobic porous copolymer to directly swell in water [81]. Zhao et al. [80] observed that over a wide range of DVB contents (64% or less) in the EGDM/DVB resins, the value of water uptake was greater than the corresponding pore volume, indicating that the water not only filled the existing air-filled (permanent) pores, but also penetrated into the gel phase of the resins. The swelling of the EGDM/DVB resins is a very complex phenomenon, which involves the expansion of the permanent pores and the “reopening” of the collapsed pores [82]. The gel phase of the porous copolymers is composed of highly cross-linked

43 Results and Discussion ▪ Batch Reactor - Resins as Metal Nanoparticles Supports microgel particles closely linked together by fewer cross-linked networks to form a continuous phase.

100

80 Conversion

60 (%) 40 Glucuronic a.

20 Fructose 5-Keto gluconic a. Gluconic a. 0 0 50 100 150 200 250 300 350 400 Time (min)

Figure 3.14. Conversion and product yield profiles for the glucose oxidation in the batch reactor performed with Au(3wt%)/EGD64 as catalysts (70°C, 3 bar O2, 1000 rpm stirring, 5wt% glucose alkaline solution (pH 13.5)). 1 run for each test.

Any expansion of the permanent or the collapsed pores must be accompanied by either the deformation of the gel phase or the relaxation of the cross-linked network chains. Therefore, some driving forces must cause the volume increase of the hydrophobic porous resins on absorbing water. Wei et al. [81] considered these driving forces to result from the inner stresses in the strained polymer network, and the weak interaction between polymer and water, and the polymer polarity of the resins. Moreover, Zhao et al. [80]showed that at 60% DVB the resin became water-swellable, as the value of the water uptake was much greater than the pore volume. The water swelling ability of the 60% DVB resins was attributed to the increased cross-linking density (rigidity) of the polymer network, leading to an increase in inner stresses in the dry resin products. As in the case of the catalysts supported on the metal oxides (Section 3.1.1), also with Au(3wt%)/EGD64, rather than the oxidation of glucose to gluconic acid, the competitive oxidation reaction of glucose to glucuronic acid was favoured.

44 Results and Discussion ▪ Batch Reactor - Carbon as Metal Nanoparticles Support

3.1.3. Batch Reactor - Carbon as Metal Nanoparticles Support Metals supported on carbon have been considered over the last decades for their utilization in several processes involving heterogeneous catalytic reactions. In particular, the exploration of supported gold nanoparticles has grown, due to the discovery of high catalytic activity and selectivity towards a variety of hydrogenation and oxidation reactions. Many reports on supported gold catalysts involve the use of oxide materials like

TiO2 and Al2O3, but there is a consistent section of literature where carbon based materials are used as supports, especially for liquid phase applications, such as the glucose oxidation reaction, where typically mild temperature conditions (RT-100°C) are used [18].

The role of the carbon support is not only to maintain the catalytic phase in a well dispersed state, but also to affect the catalytic activity, for example, by favouring the interactions between active phase and support. Indeed, although the catalytic effect is mainly a result of the chemical properties of the active phase, the dispersion and the local distribution of the active phase across the carbon support, as well as the active phase- support interaction, are highly important. These are the main aspects of carbon supports, which make them highly attractive for heterogeneous catalysis. Beside the easily tailorable pore structure and surface chemistry, carbon materials present other advantages: (i) metals on the support can be easily reduced; (ii) the carbon structure is resistant to acids and bases; (iii) the structure is stable at high temperatures; (iv) porous carbon catalysts can be prepared in different physical forms (granulates, pellets, etc.); (v) the active phase can be easily recovered; (vi) the cost of conventional carbon supports is usually lower than that of other conventional supports, such as alumina and silica [83]. Although several kinds of carbon materials have been studied, activated carbons (AC) are the most commonly used carbon supports. The term activated carbon (also known as activated charcoal) defines a group of materials with highly microporous structure and high surface area; therefore, these materials are able to adsorb chemicals from gases and liquids. The adsorption on the surface is essentially due to van der Waals or London dispersion forces. This force is strong over short distances, equal between all carbon atoms, and not dependent on external parameters such as pressure or temperature. Thus, adsorbed molecules will be held most strongly where they are surrounded by the highest number carbon atoms. The most commonly used raw materials are coal, coconut shells, wood, peat

45 Results and Discussion ▪ Batch Reactor - Carbon as Metal Nanoparticles Support and petroleum based residues. The preparation of AC involves two main stages, the carbonization of the starting material (thermal treatment of the raw material that implies dehydration and elimination of non-carbon elements) and the activation of the resulting char. There are two main activation methods, i.e. physical and chemical activation. The physical activation is the partial gasification of the char with steam, carbon dioxide and air, or a mixture of these, at temperatures around 1073–1473 K (lower for air).Contrary to the physical activation method, during the chemical activation, carbonization and activation are accomplished in a single step by carrying out thermal decomposition of the raw material impregnated with certain chemical agents (H3PO4, H2SO4, HNO3, NaOH, KOH or

ZnCl2). The choice of the activation method is dependent upon the starting material, and whether a low or high density, powdered or granular carbon is desired [83].

In order to verify if the carbon support affects the catalytic activity of the dispersed metal nanoparticles with respect to metal oxide and polymer supports, carbon supported materials were evaluated in the liquid phase glucose oxidation.

46 Results and Discussion ▪ Batch Reactor - Carbon as Metal Nanoparticles Support

3.1.3.1. IHC-1 and IHC-2 Carbons as Metal Nanoparticles Supports Two samples of micro-mesoporous carbon differing in the grain dimension, surface area and pore volume (Table 3.1) were used to immobilize Au, Pd and Pt nanoparticles (1wt% metal loading). These carbons, which were prepared in house by an aerogel method, are referred to as IHC-1 and IHC-2 (IHC = “in-house-carbon”).

IHC-1 IHC-2 Grain dimension (µm) < 600 > 600 Surface area (m2/g) 1230 1735 Pore volume (cm3/g) 1.47 1.87

Table 3.1. Properties of the two in-house prepared sample of the micro-mesoporous carbon used as support for Au, Pd and Pt nanoparticles.

The materials were evaluated in the glucose oxidation under batch conditions. The gluconic acid yield profiles obtained as a function of the reaction time are reported in Fig. 3.15.Higher gluconic acid amounts were detected in the reaction mixtures corresponding to the gold catalysts. Au(1wt%)/IHC-1 and Au(1wt%)/IHC-2 showed similar gluconic acid yield trends. Already after 30 minutes, slightly higher gluconic acid yield (69.0%) was obtained with Au(1wt%)/IHC-2 than with Au(1wt%)/IHC-1 (63.3%). When palladium and platinum were supported on IHC-1, the gluconic acid yield obtained was in the range 12.0- 14.0%. Similar results were observed when the glucose oxidation was performed with Pd(1wt%)/IHC-2. The lowest gluconic acid amount was detected in the reaction mixture corresponding to Pt(1wt%)/IHC-2. Regarding the IHC-1 supported catalysts, according to ICP analysis, 0.5% Au and 0.8% Pd was present in the corresponding catalysts, while 1.0% Pt was immobilized on the carbon. Although the actual gold amount was lower than palladium and platinum, the highest gluconic acid yield was obtained with Au(1wt%). A similar trend was observed for the IHC-2 supported catalysts (Section 5.2.4).

47 Results and Discussion ▪ Batch Reactor - Carbon as Metal Nanoparticles Support

100 IHC-1 *IHC-2 80 Au(1wt%)

60

40 Pd(1wt%)

20 Pt(1wt%) Yield Gluconic a. (%) (%) a. Gluconic Yield

0 0 20 40 60 80 100 120 140 160 180 Time (min)

Figure 3.15. Gluconic acid yield profiles for the glucose oxidation performed in the batch reactor with Au(1wt%), Pd(1wt%) and Pt(1wt%) supported on IHC-1 and on IHC-2 (70°C, 3 bar O2 and1000 rpm stirring, 5wt% glucose alkaline solution (pH 13.5)). 1 run for each test.

1.2 6.4 19.5 22.9 23.0 39.8 Unreacted 100 glucose

19.3 80 20.2

60 Glucuronic a. 5.4 2-keto 5.2 gluconic a. 40 5.4 Formic a. 66.9 73.0 32.9 26.6 27.9 20 33.4 Fructose

6.5 Yield (%) after 180 min 180 after (%) Yield 13.2 12.9 14.0 Gluconic a. 0 Au(1wt%) Pd(1wt%) Pt(1wt%) IHC-1 IHC-2

Figure 3.16. Product distribution after 180 minutes for the glucose oxidation performed in the batch reactor with Au(1wt%), Pd(1wt%) and Pt(1wt%) supported on IHC-1 and on IHC-

2 (70°C, 3 bar O2 and1000 rpm stirring, 5wt% glucose alkaline solution (pH 13.5)).

At the end of the reaction (3h), different product distributions were obtained for the Au, Pd and Pt supported on the two carbons (Fig. 3.16). Fructose was the side product detected in

48 Results and Discussion ▪ Batch Reactor - Carbon as Metal Nanoparticles Support all reaction mixtures in significant amounts resulting from the glucose isomerization in alkaline solution. Around 20.0% fructose was found in the reaction mixtures corresponding to the Au catalysts. The fructose yield for Pd and Pt catalysts was in the range 28.0-33.0%. Unreacted glucose was significantly present in the reaction mixtures corresponding to Pd catalysts and to Pt(1wt%)/IHC-1 at around 20.0-22.0%. Almost 40.0% glucose was not converted when the reaction was performed with Pt(1wt%)/IHC-2. Glucuronic, formic, glucaric, glycolic, 2- and 5-keto gluconic acid were present in small amounts (<7.0%) in the reaction mixtures corresponding to the Pd and Pt catalysts and to Au(1wt%)/ IHC-1. The highest gluconic acid yield (73.0%) and the lowest side product amounts were obtained with Au(1wt%)/ IHC-2.

In the TEM images of the samples corresponding to the two gold catalysts (Fig.3.17), smaller metal nanoparticles were observed in the IHC-1 supported material than in the IHC-2 catalyst. When gold was supported on the IHC-2 carbon with higher surface area and pore volume (Table 3.1), bigger particles were obtained. Furthermore, no homogeneity in the particles size was observed, as small and big particles were present in the Au(1wt%)/IHC-2 sample. Surprisingly, the largest particles, which should be expected to be the least active, were actually the most active, since for the Au(1wt%)/IHC-2, higher gluconic acid yield was obtained.

Figure 3.17. TEM images corresponding to Au(1wt%)/IHC-1 and Au(1wt%)/IHC-2 samples.

49 Results and Discussion ▪ Non Powdered Catalyst: from Batch to Trickle Bed Reactor

3.1.4. Non Powdered Catalyst: from Batch to Trickle Bed Reactor By performing the reaction in the batch reactor with different non-powder supported metal catalysts, different main products were obtained, in different yields and within different times of reaction. The results discussed in Section 3.1.1-3.1.3 are summarized in Tables 3.2.

Main Product Au/ZnO Au/Al2O3 Au/TiO2 Glucuronic a. yield (%) 28.0 36.4 28.2 after 420 min.

Main Product Pd/Al2O3 Pt/Al2O3 Gluconic a. yield (%) 50.3 28.0 after 60 min.

Main Product Pt/A70 Pt/A35 Au/A70 Au/A35 Ru/A70 Ru/A35 Au/EGD64 Glucuronic a. yield (%) 5.6 6.4 8.6 7.9 8.7 13.3 30.0 after 420 min.

Main Product Au/IHC-1 Au/IHC-2 Gluconic a. yield (%) 69.0 63.3 after 30 min.

Tables 3.2. Main product yield obtained with Au(1wt%)/metal oxides, Pd and Pt (5wt%)

supported on Al2O3, Pt, Au and Ru (5wt%) nanoparticles immobilized on A70 and A35 resins, Au (3wt%)/EGD64 and with Au(1wt%) supported on IHC carbons.

With gold catalysts supported on metal oxides, the product detected in the highest amount

is glucuronic acid. Around 28.0% was obtained with Au/ZnO and Au/TiO2, while 36.4%

glucuronic acid was formed using Au/Al2O3 as catalyst. Also by performing the glucose

50 Results and Discussion ▪ Non Powdered Catalyst: from Batch to Trickle Bed Reactor oxidation with Pt, Au and Ru nanoparticles supported on Amberlyst resins, glucuronic acid was the oxidation product detected in major amounts. However, in this case, lower yields were obtained (> 13.3%). When gold was supported on a different resin, EGD64 (60% EGDM and 40% DVB), 30.0% glucuronic acid was formed. For both Au/metal oxide and metal nanoparticles supported on resins, the highest glucuronic acid yield was reached at the end of the reaction, i.e. after 7 h.

The target product of the glucose oxidation, i.e. gluconic acid, was produced when the reaction was performed with Pd and Pt supported on Al2O3 and with Au nanoparticles immobilized on IHC carbons. However, using Pd/ Al2O3 and Pt/ Al2O3, gluconic acid yields lower than with Au/IHC catalysts were achieved. Furthermore, with the carbon supported gold materials, higher gluconic acid amounts were reached in a shorter time (30 minutes rather than 1 h).

Therefore, IHC carbon supported materials were chosen as potential catalyst to successfully perform the glucose oxidation in the trickle bed reactor (Section 3.4).

51 Results and Discussion ▪ Glucose Liquid Oxidation in Batch Reactor – Optimal Catalyst and Reaction Conditions

3.2. Glucose Liquid Oxidation in Batch Reactor – Optimal Catalyst and Reaction Conditions After having identified the most suitable catalyst which might successfully be used to perform the glucose oxidation in the trickle bed reactor, the aim was to find the best catalyst and reaction conditions by which the highest glucose conversion into gluconic acid could be achieved, in the shortest time, in the batch reactor. As observed from the results reported in Section 3.1, the highest gluconic acid yields were reached with the porous carbon supported catalysts; these materials were evaluated in the glucose oxidation without previously being crushed to powder. However, for reaction kinetics in the batch reactor, mass transfer limitations have to be excluded. In case of catalysts consisting of metals supported on porous materials, a fine powder should be used in order to exclude pore diffusion [69]. Therefore, further tests in the batch reactor were performed using different powder carbon supported materials, in order to investigate if the carbon structure has any effect on the catalytic activity of the dispersed metal nanoparticles.

Some commercially available disordered and microporous Pd/C or Pt/C were directly used as catalysts. Pt nanoparticles were immobilized on an ordered mesoporous carbon (CMK- 5), while gold nanoparticles, as well as palladium and platinum, were supported on a commercial disordered mesoporous activated carbon (SX).

With the SX porous carbon supported catalysts, as they showed the best catalytic performance, several reaction and catalyst preparation parameters were investigated. The glucose oxidation was performed varying temperature, pressure and oxidizing agent; the catalysts were prepared with different metal loadings, metal precursors (in the case of Pd and Pt), with addition of Pd to Au and in different metal amounts.

52 Results and Discussion ▪ Commercial Carbon Supported Metals

3.2.1. Commercial Carbon Supported Metals Samples of commercial microporous carbon-supported Pt(1wt%), Pt(5wt%) and Pd(5wt%) were used as catalysts in the glucose oxidation in batch reactor; the reaction was performed starting from both an alkaline and from a neutral glucose solution, in order to investigate the effect of the pH conditions on the reaction using carbon supported materials. The results are shown in Fig. 3.18. Commercial carbon supported ruthenium and iridium were also evaluated; however, since no oxidation products were formed in significant amounts in both reaction mixtures, the results for Ru and Ir are not shown.

The commercial carbon-supported catalysts were evaluated in the reaction at 70°C, 3 bar

O2, 1000 rpm stirring and for 7h. When the reaction was performed starting from an alkaline glucose solution, at the end of the reaction (7h) the highest amount of gluconic acid (66.7%) was detected in the reaction mixture corresponding to Pd(5wt%). Lower gluconic acid amounts were formed with Pt(5wt%) and with Pt(1wt%), with 46.4% and 37.1% yield, respectively. Using the same materials, but starting from a neutral glucose solution, a decrease in both conversion and gluconic acid amount obtained was observed. 26.7% and 14.8% gluconic acid was detected in the reaction mixture corresponding to Pt(5wt%) and Pt(1wt%), respectively. No gluconic acid was formed when Pd(5wt%) was used as catalyst. By performing the glucose oxidation starting from a neutral glucose solution, already in the first minutes, the pH of the reaction mixture was around 3; since alkaline conditions are necessary to increase the reaction rate and to avoid drastic catalyst deactivation, the low catalytic activity of Pt(1wt%), Pt(5wt%) and Pd(5wt%) at pH~3 might therefore be due to the inhibition of the reaction, due to low reactions rates, and to catalysts deactivation. This phenomenon is in agreement with what Abbadi et al. [84] observed already in 1993 and 1994 [85]. After testing Pt and Pd based materials in the glucose oxidation under different pH values, they concluded that the pH has a profound effect in the platinum- and palladium-catalysed glucose oxidation. In neutral and acidic medium, catalyst poisoning by the reaction products was observed, and the inhibition of the catalytic activity was pH dependent. The main inhibiting species of platinum and palladium catalysts during glucose oxidation in non-alkaline medium was considered to be gluconic acid in its “free” form. For Pt, it was observed that the inhibition effect was delayed when the reaction temperature was increased. Furthermore, the presence of bismuth as platinum promoter

53 Results and Discussion ▪ Commercial Carbon Supported Metals suppressed the poisoning of the catalysts while promoting selective oxidation of the gluconic acid formed in situ to 2-keto gluconic acid [85].

Pt(1wt%)/C Pt(5wt%)/C Pd(5wt%)/C 100 Alkaline pH Neutral pH 80

60 99.1 92.2 90.4 40

Conversion (%) (%) Conversion 92.2 20 22.8 31.2 0 420 min

Pt(1wt%)/C Pt(5wt%)/C Pd(5wt%)/C 100 Alkaline pH Neutral pH 80

60

40 66.8 20 46.4 Yield Gluconic a. (%) (%) a. Gluconic Yield 37.1 26.7 14.7 0 420 min

Figure 3.18. Conversion and gluconic acid yield after 420 minutes for the glucose oxidation performed in the batch reactor with commercial Pt(1wt%)/C, Pt(5wt%)/C and Pd(5wt%)/C as catalysts in alkaline (pH 13.5) and neutral conditions (70°C, 3 bar O2, 1000 rpm stirring, 5wt% glucose alkaline solution). 1 run for each test.

Considering the catalysts with the same metal content, Pd(5wt%) and Pt(5wt%), over which the gluconic acid amount produced was higher than over Pt(1wt%), different product distributions were obtained at the end of the reaction. As reported in Fig. 3.19a,

54 Results and Discussion ▪ Commercial Carbon Supported Metals more fructose and 2-keto gluconic acid was formed with the Pt catalyst. Small amounts of glycolic and glucaric acid (<3%) were also detected. The presence of higher amounts of side products might therefore explain the lower gluconic acid yield reached with Pt(5wt%) than with Pd(5wt%).

a) Pd(5wt%) Pt(5wt%) 100

80 66.766.7 2-keto 13.5 gluconic a. 7.9 Glycolic a. 60 16.0 Fructose

40 Yield (%) Yield 66.7 20 46.4 Gluconic a.

0 420 min

b) Pd(5wt%) 100

80 Pt(5wt%)

60

40 Conversion (%) Conversion 20

0 0 10 20 30 40 50 Time (min)

Figure 3.19. a) Product distribution after 420 minutes and b) conversion profiles in the first 30minutes for the for the glucose oxidation performed in the batch reactor with commercial

Pt(5wt%)/C and Pd(5wt%)/C as catalysts in alkaline conditions (70°C, 3 bar O2, 1000 rpm stirring, 5wt% glucose alkaline solution (pH 13.5)).

55 Results and Discussion ▪ Commercial Carbon Supported Metals

Although the conversion values reported in Fig. 3.18 correspond to the end of the reaction, Pd(5wt%) showed better catalytic performance already in the first 30 minutes of reaction. As reported in Fig. 3.19b, the reaction rate was faster with the Pd catalyst than with the Pt material; already after 15 minutes, higher conversion was reached with Pd(5wt%) (97.2%) than with Pt(5wt%) (84.8%). The reason might be found in the different particles size. From the TEM images of the Pd and Pt catalysts (5wt%) (Fig. 3.20), smaller nanoparticles (1.7-3.0) nm were observed for Pt(5wt%). In contrast, the nanoparticles in the palladium materials had very different diameter in the range 2.0-10 nm. The presence of nanoparticles smaller than 2 nm in Pt(5wt%) might be the reason for the lower catalytic activity with respect to Pt(5wt%), due to metal oxidation and following deactivation. Metals of high reduction potentials, such as platinum and palladium, are less prone to oxidation; however, small metal particles (<2 nm) deactivate more readily because of their stronger affinity to oxygen [86].

Figure 3.20. TEM images of commercial carbon supported Pd(5wt%) and Pt(5wt%).

56 Results and Discussion ▪ Ordered Mesoporous CMK-5 Carbon

3.2.2. Ordered Mesoporous CMK-5 Carbon The carbon support of the commercial Pd and Pt materials discussed in the previous section (3.2.1) are characterized by a disordered and microporous structure. From the results reported in Fig. 3.18, higher gluconic acid yields were obtained with palladium with respect to platinum. In order to verify if the carbon structure had any effect on the catalytic activity of platinum, metal nanoparticles were supported on CMK-5. CMK-5 (surface area= 1852 m2/g, total pore volume = 1.73 cm3/g) is a mesoporous carbon material characterized by an ordered tubular structure (Fig. 3.21a) and obtained via a template synthetic strategy, where mesoporous silica SBA-15 was used as the hard template.

b) 100 Conversion 78.3% 80

60 Yield Gluconic a.

69.7% (%) 40 Pt(1wt%)/CMK-5 20 Pt(1.8wt%)/CMK-5

0 0 50 100 150 200 250 300 350 Time (min)

Figure 3.21. a) TEM image of the ordered and tubular CMK-5 structure and b) conversion and gluconic acid yield profiles for the glucose oxidation in the batch reactor, with

57 Results and Discussion ▪ Ordered Mesoporous CMK-5 Carbon

Pt(1wt%)/CMK-5 and Pt(1.8wt%)/CMK-5 as catalysts (70°C, 3 bar O2, 1000 rpm stirring, 5wt% glucose alkaline solution (pH 13.5)). 1 run for each test.

Ordered mesoporous materials have been proven to be ideal catalyst supports due to their three-dimensional open pore network structures, high surface area and porosity, high reusability and heat stability, and uniform and interconnected pores [87] [88] [89] [90]. Pt(1wt%)/CMK-5 and Pt(1.8wt%)/CMK-5 were evaluated in the batch reactor at 70°C,

3bar O2, 1000 rpm stirring and for 3h; the results are shown in Fig. 3.21b. Although the same conversion trend was observed for the two materials, the amount of gluconic acid formed with Pt(1wt%)/CMK-5 was higher than the one detected in the reaction mixture corresponding to Pt(1.8wt%)/CMK-5. After 60 minutes, the maximum amount of gluconic acid was achieved, corresponding to 78.3% for Pt(1wt%)/CMK-5 and 69.7% for Pt(1.8wt%)/CMK-5. After 1 h of reaction, a slight drop in the gluconic acid yield profiles was observed for both catalysts. The reaction was completed in a very short time, and a plateau in the conversion was already reached after 15 minutes, although the reaction was run for 7 h. The catalytic performance of Pt(1wt%)/CMK-5 was compared with a catalyst consisting of Pt nanoparticles supported at 1wt% loading on disordered carbon. The aim was to investigate the influence of the carbon support structure on the glucose oxidation. The results are shown in Fig. 3.22a. After 60 minutes of reaction, although the conversion reached was similar for both materials (95.9% and 90.4% with Pt(1wt%)/CMK-5 and Pt(1wt%)/disordered carbon, respectively), higher gluconic acid formation was observed with Pt(1wt%)/CMK-5 (78.3% compared to 50. 6%). The ordered structure of CMK-5 obviously enhanced the gluconic acid formation. Pt(1.8wt%)/CMK-5 was also evaluated starting from a neutral pH glucose solution instead of an alkaline one (Fig. 3.22b); in this case, after 60 minutes, lower conversion and gluconic acid yield were obtained. The pH value of the reaction mixture has a decisive effect on the reaction rate of D-gluconic acid formation.

58 Results and Discussion ▪ Ordered Mesoporous CMK-5 Carbon

a) Pt(1wt%)/CMK-5 Pt(1wt%)/disordered C 100 100 Pt(1wt%)/CMK-5 80 80

60 60 Pt(1wt%)/disordered C 95.9 40 40

90.4 78.3 Conversion (%) Conversion

20 20 50.6 Yield Gluconic a. (%) a. Gluconic Yield

0 0 60 min 60 min

b) Pt(1.8wt%)/CMK-5, 100 Alkaline pH 100 Pt(1.8wt%)/CMK-5, 80 80 Alkaline pH

60 60

95.5 40 40 69.7

Conversion (%) (%) Conversion 20 Pt(1.8wt%)/CMK-5, 20 Pt(1.8wt%)/CMK-5,

Neutral pH (%) a. Gluconic Yield Neutral pH 0 0 60 min 60 min

Figure 3.22. Conversion and gluconic acid yield after 60 minutes for the glucose oxidation performed in the batch reactor with a) Pt(1wt%)/CMK-5 and Pt/disordered carbon and with b) Pt(1.8wt%)/CMK-5 in alkaline (pH 13.5) and neutral conditions (70°C, 3 bar O2, 1000 rpm stirring, 5wt% glucose alkaline solution). 1 run for each test.

From the time profiles of conversion and gluconic acid yield shown in Fig. 3.23, the reaction rate accelerates with increasing pH value. This phenomenon might be explained by the absence of catalyst deactivation by self-poisoning in alkaline solution, as gluconic acid is then deprotonated and no longer able of blocking active centres on the catalyst surface [57, 86] [57]. This is in agreement with the reaction mechanism of the aldehyde oxidation to acids proposed by Mallat and Baiker [91] (Fig.3.24).

59 Results and Discussion ▪ Ordered Mesoporous CMK-5 Carbon

Conversion Yield Gluconic a. 100 * Pt(1.8wt%)/CMK-5, 80 Alkaline pH

60 (%) 40

Pt(1.8wt%)/CMK-5, 20 Neutral pH

0 0 50 100 150 200 250 Time (min)

Figure 3.23. Conversion and gluconic acid yield profiles for the glucose oxidation performed in the batch reactor with Pt(1.8wt%)/CMK-5 in alkaline (pH 13.5) and neutral conditions

(70°C, 3 bar O2, 1000 rpm stirring, 5wt% glucose alkaline solution). 1 run for each test.

Figure 3.24. Reaction mechanism proposed by Mallat and Baiker [91]for the heterogeneously catalysed glucose oxidation. Reprinted (adapted) with permission from [22].

In the aqueous phase, glucose exists to 99.9% in the pyranose form. As glucose is a very unstable aldehyde, the open-chain glucose (0.1%) converts in aqueous solution into the hydrate which should be more stable. After adsorption on the catalyst surface, the hydrate is dehydrogenated and desorbed. If the product is deprotonated, at pH>7, the gluconate desorption is faster; at pH<7, catalyst deactivation by self-poisoning cannot be neglected. Furthermore, the hydrate formation is faster in weakly alkaline or acidic media. Since in

60 Results and Discussion ▪ Ordered Mesoporous CMK-5 Carbon acidic media the overall reaction rate should be limited by gluconic acid desorption, a mild alkaline solution should be preferred. Hydrides bound to the surface react with adsorbed

OHad to form water, which desorbs, too. The hydrate formation is therefore another reason for the pH dependence of the glucose oxidation reaction rate [22].

61 Results and Discussion ▪ Mesoporous SX carbon

3.2.3. Mesoporous SX carbon Sugar conversion processes based on supported palladium and platinum materials have been intensively investigated in the past years [26] [65] [66]; however, since these catalysts often suffer from drawbacks of low catalyst durability and relatively low selectivity, many studies have recently been devoted to gold as catalyst to achieve selective aerobic oxidation of glucose under mild conditions; moreover, superior selectivity, high catalytic activity, and long term stability were observed [21] [67] [68] [23]. However, Au/C catalysts are not commercially available; therefore, gold nanoparticles were prepared and supported on carbon. Since mesoporous carbons are very attractive for applications in many areas, such as catalyst supports, the commercial mesoporous carbon Norit SX ultra (powder, surface area= 947.1 m2/g, pore volume = 0.8 cm3/g, average pore size = 3.4 nm) was used to immobilize gold nanoparticles. While the most common methods for the preparation of commercial heterogeneous catalysts (including the Pt/C and Pd/C materials used in Section 3.2.1) are impregnation and deposition-precipitation, a different procedure was chosen for the preparation of carbon supported gold materials. Gold catalysts supported on carbon were indeed prepared according to the metal sol immobilization method. In order to compare the catalytic activity of the three metals, also Pd and Pt nanoparticles were supported on the SX carbon. The actual metal content of the M(1wt%)/SX (with M = Au, Pd and Pt) corresponded to the theoretical one, according to ICP analysis (Section 5.2.4).

The catalysts were evaluated in the glucose oxidation in the batch reactor for 3 h at 70°C, 3 bar O2 and 1000 rpm and the results are reported in Fig. 3.25. After 30 minutes, the presence of a plateau was observed in the conversion and gluconic acid yield profiles corresponding to the 1wt% gold catalyst. 98.1% and 75.0% conversion was achieved with Au and Pt catalysts respectively. 73.0% as maximum conversion was obtained with palladium after 60 minutes. The same trend was observed in the gluconic acid yield profiles. After 30 minutes, the gluconic acid yield with gold was 97.0% and appreciably less gluconic acid (27.0%) was formed with platinum. The highest amount of gluconic acid detected in the reaction mixture corresponding to palladium was only 20.0% after 60 minutes of reaction. After 60 minutes, a significant drop in the gluconic acid production was observed for Pt.

62 Results and Discussion ▪ Mesoporous SX carbon

30 min Au(1wt%) 30 min Au(1wt%) 100 100

80 30 min Pd(1wt%) 80

Pt(1wt%) 60 60 min 60

40 40 30 min Pd(1wt%) Conversion (%) Conversion 20 20 60 min

Yield (%) Gluconic acid acid Gluconic (%) Yield Pt(1wt%) 0 0 0 20 40 60 80 100 120 140 160 180 0 20 40 60 80 100 120 140 160 180 Time (min) Time (min)

Figure 3.25. Conversion and gluconic acid profiles for the glucose oxidation performed in the batch reactor with Au(1wt%)/SX, Pd(1wt%)/SX and Pt(1wt%)/SX (70°C, 3 bar O2, 1000 rpm stirring, 5wt% glucose alkaline solution (pH 13.5)). 1 run for each test.

Pt catalyst 40

30 6.9 5-keto 20 9.8 gluconic a.

Yield (%) Yield 22.6 4.4 Glucaric a. 10 4.3 Glucuronic a.

6.5 Gluconic a. 0 60 min 180 min

Figure 3.26. Product distribution after 60 and 180 minutes for the glucose oxidation performed in the batch reactor with Pt(1wt%)/SX (70°C, 3 bar O2, 1000 rpm stirring, 5wt% glucose alkaline solution (pH 13.5)).

As shown in Fig. 3.26, at the end of the reaction (180 min), the amount of gluconic acid was only 6.5%, but other glucose oxidation products, which were not detected in the sample corresponding to 60 minutes, were present, i.e. 5-keto gluconic acid (9.8%) and

63 Results and Discussion ▪ Mesoporous SX carbon glucuronic acid (4.4%). While glucuronic acid is a glucose oxidation side product, 5-keto gluconic acid derives from the further oxidation of the gluconic acid. Glucaric acid can be formed either by glucuronic or gluconic acid oxidation (Fig. 1.3).

TEM images of SX supported Au(1wt%), Pd(1wt%) and Pt(1wt%)samples (Fig. 3.27) show that the Au nanoparticles are ~6.9 nm in diameter. The metal nanoparticle diameter observed in the Pt(1wt%) sample was ~4.0-5.0 nm, while in the case of the Pd material the particles has a diameter in the range of 30.0 nm. The big metal nanoparticles might be the reason for the low catalytic performance of Pd(1wt%). Since Pt and Au particles dimensions were not significantly different, the presence of other oxidation products beside gluconic acid might be confirmed as the main factor causing the drop in gluconic acid formation observed at the end of the reaction over the Pt catalyst.

Figure 3.27. TEM images of samples corresponding to SX supported Au(1wt%), Pd(1wt%) and Pt(1wt%).

64 Results and Discussion ▪ Mesoporous SX carbon

The reason for the better catalytic performance of gold with respect to palladium and platinum lies in a different reaction mechanism. The classic oxidative dehydrogenation mechanism was proven for alcohol and carbohydrates oxidation on other noble metals, like Pd and Pt, by Besson and Gallezot [57]. According to this mechanism, glucose is first hydrated to the geminal diol that dissociatively adsorbs on the catalyst surface. This leads to adsorbed gluconic acid, which desorbs, and adsorbed hydrogen atoms. The reaction between adsorbed hydrogen and dissociatively adsorbed oxygen leads to adsorbed water, which then desorbs. In 2011, a new mechanism for the gold catalysed glucose oxidation was proposed by Pruesse et al. [92]. According to this mechanism, the reaction starts with a nucleophilic attack of a hydroxide ion on glucose, leading to a deprotonated geminal diol of glucose which adsorbs on the catalyst surface leading to electron-rich gold. In an oxygen atmosphere, linearly adsorbed oxygen and the glucose species are adsorbed simultaneously. The adsorbed glucose species decomposes by hydrogen transfer to the gold surface. After gluconic acid desorption, both the hydrogen species and the linear O2- species are co-adsorbed on the catalyst surface. Those two species react in the final step to an adsorbed peroxide species, which subsequently desorbs. If oxygen is not present, or if oxygen adsorption is slow compared to glucose adsorption, hydrogenated products are produced. The main difference of this mechanism to the classic oxidative dehydrogenation is the fact that the gold catalyst adsorbs oxygen not dissociatively but in a linear manner. This difference in oxygen chemisorption of the gold catalyst compared to palladium and platinum catalysts might be the reason for the unusually good catalytic performance of gold in carbohydrate oxidation compared to palladium and platinum [92].

In order to verify that 70°C, 3 bar O2 and 1wt% metal loading were the optimal reaction conditions by which the highest gluconic acid yield could be reached, different reaction parameters were varied. Furthermore, also the effects of the metal loading, the metal precursor, the Pd addition to Au, and the PVA amount were investigated.

65 Results and Discussion ▪ Mesoporous SX carbon

3.2.3.1.1. Effect of the Temperature

SX carbon supported Au(1wt%), Pd(1wt%) and Pt(1wt%) were tested as catalysts in the glucose oxidation at 50 ºC. Au(1wt%) showed the best catalytic performance with respect to Pd and Pt catalysts (Fig. 3.28). Although the reaction was carried out for 3 h, in the presence of Au catalyst the maximum conversion and gluconic acid yield values, 97.0% and 88.0%, respectively, were reached already after 60 minutes; both profiles reached a plateau already after 30 minutes. In contrast, the reaction was appreciably slower with Pd and Pt.

30 min Au(1wt%) 100 100 30 min Au(1wt%) 80 Pt(1wt%) 80

60 Pd(1wt%) 60

40 40 Pt(1wt%)

Conversion (%) Conversion Pd(1wt%)

20 20 Yield (%) Gluconic acid acid Gluconic (%) Yield 0 0 0 20 40 60 80 100 120 140 160 180 0 20 40 60 80 100 120 140 160 180 Time (min) Time (min)

Figure 3.28. Conversion and gluconic acid profiles for the glucose oxidation performed in the batch reactor with Au(1wt%)/SX, Pd(1wt%)/SX and Pt(1wt%)/SX (50°C, 3 bar O2, 1000 rpm stirring, 5wt% glucose alkaline solution (pH 13.5)). 1 run for each test.

Au(1wt%)/SX, being the most active catalyst both at 50°C and 70°C, was used to perform the glucose oxidation also at room temperature (RT). Around 90.0% conversion and 67.0% gluconic acid yield were reached after three hours (Fig. 3.29). Comparing the conversion and gluconic acid yield profiles obtained with Au(1wt%) at the three reaction temperatures, it appears that the reaction rate is enhanced by increasing the operating temperature. Indeed, high conversion and gluconic acid yield values were reached in

66 Results and Discussion ▪ Mesoporous SX carbon shorter time (30 min) when the reaction was performed at 50°C (97.0% conversion and 88.0% yield) and 70°C (98.1% conversion and 97.0% yield).

a) b) 30 min 70 °C 70 °C 100 100 30 min 50 °C 80 80 50 °C

60 60 RT

40 40 RT Conversion (%) Conversion

20 20 Yield (%) Gluconic acid acid Gluconic (%) Yield 0 0 0 20 40 60 80 100 120 140 160 180 0 20 40 60 80 100 120 140 160 180 Time (min) Time (min)

Figure 3.29. Comparison of (a) conversion and (b) gluconic acid yield profiles for the glucose oxidation performed in the batch reactor at room temperature (RT), 50°C and 70°C with

Au(1wt%) (3 bar O2, 1000 rpm stirring, 5wt% glucose alkaline solution (pH 13.5)). 1 run for each test.

The influence of the reaction temperature on the reaction rate of glucose oxidation was studied, for example, by Önal et al.[22]. They found that the optimal temperature range was around 50°C for pH 9.5. They also observed a decrease in the reaction rate of gluconic acid formation at higher temperatures, which was explained by the side reactions of glucose in the aqueous phase. These reactions were indeed found to be favoured at higher temperatures and pH values. According to previous works [22],[93], temperatures higher than 60°C are generally not advisable for carbohydrate oxidation in order to achieve a high selectivity.

However, in the present study, it was observed that by increasing the temperature, the gluconic acid yield increased while the amount of fructose decreased (Fig. 3.30). At room temperature, only 28.7% gluconic acid was produced and fructose was the main side

67 Results and Discussion ▪ Mesoporous SX carbon product with 16.9% yield. At 50°C and 70°C, lower amounts of fructose, (<6%) were detected.

48.4 1.4 0.4 Unreacted glucose 100 Fructose 80

60 98.1 Formic a. 88.2 40 16.9 Gluconic a.

20

28.7 Yield (%) after 60 min 60 after (%) Yield 0 RT 50°C 70°C

Figure 3.30 Product distribution after 60 minutes for the glucose oxidation performed in the batch reactor at room temperature (RT), 50°C and 70°C with Au(1wt%)/ SX (3 bar O2, 1000 rpm stirring, 5wt% glucose alkaline solution (pH 13.5)). 1 run for each test.

RT 30

25 Fructose 20

15

Yield (%) Yield 10 Gluconic a. 5

0 0 10 20 30 40 50 60 Time (min)

Figure 3.31. Gluconic acid and fructose in the first minutes of reaction for the glucose oxidation performed in the batch reactor at room temperature (RT) with Au(1wt%)/ SX (3 bar O2, 1000 rpm stirring, 5wt% glucose alkaline solution (pH 13.5)).

68 Results and Discussion ▪ Mesoporous SX carbon

The reason for the low catalytic activity of Au(1wt%) at RT might be related to the fact that, at low temperatures, the reaction is limited by kinetics and the major product formed is the one deriving from the fastest reaction. Indeed, as reported in Fig. 3.31, in the first 30 minutes of reaction, higher amounts of fructose, deriving from the fast glucose isomerization, than gluconic acid were produced. In contrast, at higher temperatures, the reaction is under thermodynamic control; therefore the major product is the most stable one. Indeed, when the glucose oxidation was performed at 70°C, even during the first minutes, higher gluconic acid amounts than fructose were formed.

From these results, it appears that running the reaction at 70°C is the best way to achieve high gluconic acid yield and low fructose amount. Therefore, the following experiments were carried out at this temperature.

69 Results and Discussion ▪ Mesoporous SX carbon

3.2.3.1.2. Effect of the Pressure In order to investigate the pressure influence on the reaction, the glucose oxidation was carried out varying the O2 pressure inside the reactor while keeping the other parameters constant. Beside at 3 bar, the reaction was performed also at 1 and 4 bar O2; SX supported Au(1wt%), Pd(1wt%) and Pt(1wt%) were used as catalysts.

Au(1wt%) Pd(1wt%) Pt(1wt%) 100

80

1 bar O2 3 bar O 60 100.0 2 4 bar O2 96.0 99.7 40 84.3 85.2

81.9 81.7 79.9 80.6 Conversion (%) Conversion 20

0 180 min

Au(1wt%) Pd(1wt%) Pt(1wt%) 100

80

1 bar O2

60 3 bar O2

4 bar O2 92.5 40 80.5 43.4 20 41.4 39.8 44.6

Yield Gluconic a. (%) a. Gluconic Yield 31.9 24.9 21.8 0 180 min

Figure 3.32. Conversion and gluconic acid yield after 180 minutes for the glucose oxidation performed in the batch reactor at 1, 3 and 4 bar O2 with Au(1wt%)/SX, Pd(1wt%)/SX and Pt(1wt%)/SX (prepared with 2 mL PVA) (70°C, 1000 rpm stirring, 5wt% glucose alkaline solution (pH 13.5)). 1 run for each test.

70 Results and Discussion ▪ Mesoporous SX carbon

In the preparation of these materials, a higher amount of PVA with respect to the standard procedure (2 ml instead of 0.4 ml) was used. The results are shown in Fig. 3.32. For all the catalysts, the use of different oxygen pressure did not seem to significantly influence the value of the final conversion. At 180 minutes, the conversion reached with Au at the three pressures was in the range 96.0-100.0%; lower final conversions were obtained with the Pd (82.0-85.0%) and Pt (~81.0%) catalysts.

3 bar O 100 2

80 1 bar O2 4 bar O2 60 Au(1wt%)

40 Conversion (%) Conversion 20

0 0 20 40 60 80 100 120 140 160 180 Time (min)

100 3 bar O2

80 4 bar O2 60 1 bar O 40 2

20 Yield Gluconic a. (%) a. Gluconic Yield

0 0 20 40 60 80 100 120 140 160 180 Time (min)

Figure 3.33.Conversion gluconic acid yield profiles for the glucose oxidation performed in the batch reactor at 1, 3 and 4 bar O2 with Au(1wt%)/SX (prepared with 2 mL PVA) (70°C, 1000 rpm stirring, 5wt% glucose alkaline solution (pH 13.5)). 1 run for each test.

71 Results and Discussion ▪ Mesoporous SX carbon

Concerning the gluconic acid amount produced, it was observed that, with Pd(1wt%), increasing yields were obtained by increasing the operating pressure; for the Au and Pt catalysts, the highest gluconic acid amount was formed at 3 bar O2, 92.5% and 44.6%, respectively. For all the catalysts, the lowest gluconic acid yield was obtained when the reaction was performed at atmospheric pressure.

With Au(1wt%)/SX, gluconic acid was formed in higher amounts with respect to Pd and Pt at each operating pressure. Fig. 3.33 shows the conversion and gluconic acid yield profiles of the reaction mixtures corresponding to the different operating pressures obtained with

Au(1wt%) as catalyst. By raising the operating pressure from 1 to 3 bar O2, the oxygen concentration in the liquid phase increases leading to higher reaction rates and, hence, higher gluconic acid yields. Prüße et al. [92] examined the glucose oxidation at elevated oxygen pressures at a constant pH value using an Au/Al2O3 catalyst. They found that higher oxygen partial pressure had a positive effect on the catalytic activity. The highest activity was observed at an oxygen partial pressure of 9 bar, and at an initial glucose concentration of 20 wt% and at 40°C [69]. In this PhD study, by further increase of the pressure from 3 to 4 bar O2, a decrease in the catalytic activity could be observed. Indeed, already after 15 minutes, lower conversion was reached at 4 bar than at 3 bar O2, 93.4% and 99.1%, respectively. Moreover, at the same time, the reaction produced less gluconic acid when performed with 4 bar O2, 70.5%, than 3 bar O2, 96.0%.

Different product distributions were observed at the end of the reaction by performing the glucose oxidation at different oxygen pressure. As shown in Fig. 3.34, at atmospheric pressure the lowest amount of gluconic acid (56.7%) was produced while the highest yield of fructose (22.0%) was obtained. Furthermore, small amounts of formic (2.9%) and glycolic acid (3.8%) were formed. Traces of 2-keto gluconic and glucaric acid were also detected. Performing the reaction at 3 bar O2, the highest gluconic acid (92.5%) and lowest fructose (7.2%) amounts were produced; basically, no side products were formed. In the reaction mixture corresponding to 4 bar O2, the yield of gluconic acid and fructose were 80.5% and 17.65, respectively; the amounts of glycolic and formic acid were around 1.0%.

72 Results and Discussion ▪ Mesoporous SX carbon

Au(1wt%)

100 Glycolic a. 7.2 Formic a. 17.6 Fructose 80 22.0 60 92.5

40 80.5 Yield (%) Yield 53.7 20 Gluconic a. 0

1 bar O2 3 bar O2 4 bar O2

Figure 3.34. Product distribution after 180 min for the glucose oxidation performed in the batch reactor at 1, 3 and 4 bar O2 with Au(1wt%)/SX (prepared with 2 mL PVA) (70°C, 1000 rpm stirring, 5wt% glucose alkaline solution (pH 13.5)).

73 Results and Discussion ▪ Mesoporous SX carbon

3.2.3.1.3. Effect of Pure Oxygen and of Air as Oxidizing Agent In order to investigate if the reaction could be more conveniently carried out with air without any disadvantage for the gluconic acid formation, SX carbon supported Au(1wt%),

Pd(1wt%) and Pt(1wt%) were evaluated in the glucose oxidation using, instead of pure O2, air as oxidizing agent; the conversion and gluconic acid yield profiles obtained are shown in Fig. 3.35. In the preparation of these materials, a higher amount of PVA with respect to the standard procedure (2 ml instead of 0.4 ml) was used.

a) b) pure O2 Au(1wt%) * O in Air 100 100 2

Au(1wt%) 80 80 Pt(1wt%) 60 Pd(1wt%) 60 Pt(1wt%) 40 40 pure O Conversion (%) Conversion 2 20 20 * O2 in Air Yield (%) Gluconic a. a. Gluconic (%) Yield Pd(1wt%)

0 0 0 20 40 60 80 100 120 140 160 180 0 20 40 60 80 100 120 140 160 180 Time (min) Time (min)

Figure 3.35. (a) Conversion and (b) gluconic acid yield profiles for the glucose oxidation in the batch reactor performed with pure O2 and air using Au(1wt%)/SX, Pd(1wt%)/SX and Pt(1wt%)/SX as catalysts (prepared with 2 mL PVA) (70°C, 3 bar gas, 1000 rpm stirring, 5wt% glucose alkaline solution (pH 13.5)). 1 run for each test.

In both cases, the gas pressure applied was 3 bar. With Au(1wt%), higher conversions were achieved performing the glucose oxidation under pure O2 atmosphere (Fig. 3.35a). In this case, full glucose conversion was obtained already after 15 minutes while with air the maximum conversion (89.4%) was reached at the end of the reaction (3h). In the case of

Pd(1wt%), the conversion values at 180 minutes were very similar using pure O2 and air (81.9% and 80.3% respectively). However, already after 15 minutes, the glucose conversion was higher in pure O2 atmosphere than in air (~73.0% and ~63.0% respectively). When the glucose oxidation was carried out with Pt(1wt%), the conversion

74 Results and Discussion ▪ Mesoporous SX carbon

after 3 h was slightly higher using air than pure O2 with 82.6% and 79.9% respectively. With all three metal catalysts, higher gluconic acid yields were obtained performing the reaction in pure O2 atmosphere than in air (Fig. 3.35b). With Au(1wt%), ~96.0% gluconic acid was formed after 15 minutes of reaction, while with Pd(1wt%) and Pt(1wt%) the maximum gluconic acid amount was detected at the end of the reaction (39.8% and 44.6% respectively).

O2 as oxidizing agent 18.1 20.1 Unreacted 100 7.2 glucose 2-keto 80 13.9 8.4 gluconic a. Glycolic a. 60 28.6 24.8 Fructose 92.5 40

44.6

20 39.8 Gluconic a. Yield (%) after 180 min 180 after (%) Yield 0 Au(1wt%) Pd(1wt%) Pt(1wt%)

Air as oxidizing agent 10.6 19.7 17.4 Unreacted 100 glucose

80 24.8 5-keto 2-keto gluconic a. 11.6 60 gluconic a. 6.6 20.9 Fructose 40 13.9 7.6 20

36.7 14.9 36.9 Gluconic a. Yield (%) after 180 min 180 after (%) Yield 8.6 0 Au(1wt%) Pd(1wt%) Pt(1wt%)

Figure 3.36. Product distributions after 180 minutes for the glucose oxidation performed in the batch reactor with pure O2 and air with Au(1wt%)/SX, Pd(1wt%)/SX and Pt(1wt%)/SX (prepared with 2 mL PVA) (70°C, 3 bar gas, 1000 rpm stirring, 5wt% glucose alkaline solution (pH 13.5)).

75 Results and Discussion ▪ Mesoporous SX carbon

By performing the glucose oxidation in air, similar gluconic acid yields (~36.0%) were obtained with the Au and Pt catalysts at the end of the reaction (Fig. 3.36); the lowest gluconic acid amount (~8.0%) was detected in the reaction mixture corresponding to

Pd(1wt%). For each metal catalyst, after performing the glucose oxidation with pure O2 or air different product distributions were obtained at the end of the reaction. With Au(1wt%), only fructose was detected besides gluconic acid by running the reaction in pure O2 atmosphere while, under air pressure, a broader product distribution was observed. With Pd(1wt%) and Pt (1wt%), similar product distributions were obtained performing the glucose oxidation under pure O2 and air atmosphere.

5 bar Air Conversion * 1 bar O 100 2

80 Gluconic a.

60 (%) 40

20 Fructose Glucose 0 0 20 40 60 80 100 120 140 160 180 200

Time (min)

Figure 3.37. Glucose oxidation performed in the batch reactor at 1 bar O2 and 5 bar air with Au(1wt%)/SX (70°C, 1000 rpm stirring, 5wt% glucose alkaline solution (pH 13.5)). 1 run for each test.

As already mentioned, 3 bar of gas were loaded in the reactor both using pure O2 and air. 3 bar of air corresponds to a partial pressure of 0.6 bar. In order to have the same oxygen concentration in both atmospheres, the glucose oxidation was performed at 5 bar air and 1 bar O2. Au(1wt%)/SX was used as catalyst. As shown in Fig. 3.37, the profiles of conversion, and of yields of gluconic acid, fructose and glucose obtained with 5 bar air and

1 bar O2 are very similar. In both cases, at the end of the reaction, full glucose conversion

76 Results and Discussion ▪ Mesoporous SX carbon was achieved with different gluconic acid yields, and the yields for the different products are basically identical within the error margins.

Thus, by performing the glucose oxidation at higher air pressure (pAir = 5∙pO2), it is possible to reach comparable conversions and gluconic acid yields. Therefore, the reaction can be more conveniently carried out with air without any disadvantage for the gluconic acid formation, at the expense of applying higher pressures.

.

77 Results and Discussion ▪ Mesoporous SX carbon

3.2.3.1.4. Effect of the Metal Loading In order to study the influence of the catalyst metal amount on the glucose oxidation, different loadings of gold, palladium and platinum were immobilized on SX carbon. Besides 1wt%, also 3wt% and 5wt% metal loading materials were prepared and evaluated in the reaction under batch conditions. The actual metal content of the SX supported M(1wt%), M(3wt%) and M(5wt%) (with M = Au, Pd and Pt) prepared from chloride precursors corresponded to the theoretical one, according to ICP analysis (Section 5.2.4). The gluconic acid yield profiles obtained are shown in Fig. 3.38. With the Au catalysts, the highest gluconic acid amount (~100.0%) was detected at the end of the reaction (3h) in the reaction mixture corresponding to Au(1wt%). With Au(3wt%) and Au(5wt%), 86.8% and 91.5% gluconic acid yields were obtained.

100 Au

80 1wt% 3wt% 60 *5wt% 40 Pt

20 Yield Gluconic a. (%) a. Gluconic Yield Pd 0 0 20 40 60 80 100 120 140 160 180 Time (min)

Figure 3.38. Glucose oxidation performed in the batch reactor with SX supported Au, Pd and

Pt catalysts in 1wt%, 3wt% and 5wt% metal loadings (70°C, 3 bar O2, 1000 rpm stirring, 5wt% glucose alkaline solution (pH 13.5)). 1 run for each test.

According to the gluconic acid yield profiles obtained with the Au catalysts (Fig. 3.38), in the first 15 minutes of reaction, higher gluconic acid yields were reached with Au(1wt%) and Au(5wt%) (~82.0); at the same time, 74.4% gluconic acid was formed with the 3wt% catalyst. Although with Au(1wt%) and Au(5wt%) similar gluconic acid amounts were produced, the 5wt% material deactivated faster than the 1wt%. These results might be

78 Results and Discussion ▪ Mesoporous SX carbon related to the metal nanoparticles size. With respect to the Au(1wt%) sample (6.9 nm), smaller metal nanoparticles were indeed observed in the TEM images of Au(3wt%) and Au(5wt%) samples (particles diameter around 3.0 nm) (Fig. 3.39).

Figure 3.39. TEM images of Au/SX with 1wt%, 3wt% and 5wt% metal loading.

Lower catalytic performances in the glucose oxidation to gluconic acid were observed with Pd and Pt catalysts, with all the three metal loadings. In the case of the SX supported Pd materials, nanoparticle agglomerates were observed in the TEM images (Fig. 3.40), hence no activity-particle size dependence could be established. Nevertheless, 21.8% and 17.4 % gluconic acid was detected after 3 h in the reaction mixtures corresponding to Pd(1wt%) and Pd(3wt%), respectively. Among the Pd catalysts, the highest gluconic acid yield was reached with Pd(5wt%), with 45.9% at the end of the reaction. In the case of the Pd materials, it appeared that the gluconic acid formation was influenced only by the metal amount immobilized on the carbon support.

79 Results and Discussion ▪ Mesoporous SX carbon

When the glucose oxidation was performed using the Pt materials as catalyst, a similar gluconic acid yield profile was observed for Pt(1wt%) and Pt(3wt%) within the first 60 minutes.

Figure 3.40. TEM images of Pd/SX with 1wt%, 3wt% and 5wt% metal loading.

At the end of the reaction, a drop in gluconic acid formation was observed with Pt(1wt%). After 3 h, 6.5% and 23.2% gluconic acid was obtained for 1wt% and 3wt% Pt loading respectively. As for the Pd materials, also in the case of the Pt catalysts the highest gluconic acid amount was achieved with 5wt% metal loading. At the end of the reaction, the gluconic acid amount detected in the reaction mixtures corresponding to Pt(5wt%) was 28.1%. As shown in Fig. 3.41, the metal nanoparticles observed in the samples corresponding to Pt(1wt%), Pt(3wt%) and Pt(5wt%) had similar dimensions (5.5-7.0 nm). Same particle size at different metal loading means higher overall surface for higher loading, which correlates well with the gluconic acid yield.

80 Results and Discussion ▪ Mesoporous SX carbon

Figure 3.41. TEM images of Pt/SX with 1wt%, 3wt% and 5wt% metal loading.

81 Results and Discussion ▪ Mesoporous SX carbon

3.2.3.1.5. Effect of the Metal Precursor The SX supported catalysts were generally prepared starting from chloride precursors,

Pd(NH3)4Cl2∙H2O and PtCl4 for Pd and Pt, respectively. In order to investigate if the use of different precursors had any influence on the catalysts and hence on the reaction, Pd and Pt materials in 1wt% metal loading were synthetized using nitrate precursors,

Pd(NH3)4(NO3)2 and Pt(NH3)4(NO3)2, respectively. The preparation method used with the chloride and nitrate precursors was the same (metal sol immobilization procedure). The actual metal content corresponded to the theoretical one, according to ICP analysis (Section 5.2.4). The catalysts prepared from the nitrate precursors were evaluated in the glucose oxidation in the batch reactor, at 70°C, 3 bar O2 and 1000 rpm. The results and the comparison with the materials synthetized using the chloride precursors are shown in Fig. 3.42.

a) b)

40 40

30 30

PtCl4 Pd(NH3)4Cl2H2O 20 20

Pd(NH ) (NO )

10 3 4 3 2 10 Yield (%) Gluconic a. a. Gluconic (%) Yield Yield (%) Gluconic a. a. Gluconic (%) Yield Pt(NH3)4(NO3)2 0 0 0 20 40 60 80 100 120 140 160 180 0 20 40 60 80 100 120 140 160 180 Time (min) Time (min)

Figure 3.42. Gluconic acid time profiles for the glucose oxidation performed in the batch reactor with SX supported a) Pd and b) Pt catalysts (1wt%) (70°C, 3 bar O2, 1000 rpm stirring, 5wt% glucose alkaline solution (pH 13.5)). The precursor used in the materials preparation were Pd(NH3)4(NO3)2 and Pd(NH3)4Cl2∙H2O for Pd and Pt(NH3)4(NO3)2 and PtCl4 for Pt. 1 run for each test.

In the case of the Pd catalysts, similar gluconic acid profiles were obtained for the materials prepared from the chloride (Pd-Cl2) and the nitrate precursors (Pd-NO3). At the

82 Results and Discussion ▪ Mesoporous SX carbon

end of the reaction, comparable gluconic acid amounts were formed with Pd-Cl2 and Pd-

NO3, 21.8% and 18.9%, respectively. In the case of the Pt catalysts, the highest gluconic acid yield (26.6%) was reached after 30 minutes when the glucose oxidation was performed with Pt(1wt%) prepared with the chloride precursor (Pt-Cl2); at the same time, only 8.5% gluconic acid was formed using the Pt catalyst prepared from the nitrate precursor (Pt-NO3). However, at the end of the reaction, the gluconic acid amount produced with Pt-NO3 (10.5%) was slightly higher than the one obtained with Pt-Cl2 (6.5%).

The gluconic acid yield profiles of the Pd and Pt catalysts might be related to the metal particle sizes. From the TEM images of Pd-Cl2 and Pd-NO3 (Fig. 3.43), it was observed that the catalysts had similar particles diameter; indeed, the Pd nanoparticles in Pd-Cl2 and

Pd-NO3 were around 30.0 nm and 22.0 nm, respectively. The comparable particle size for the materials prepared from the chloride and the nitrate precursor might therefore be the reason for the similar gluconic acid yield profiles.

Figure 3.43. TEM images of Pd/SX (1wt%) prepared from Pd(NH3)4Cl2H2O (left) and from

Pd(NH3)4(NO2)2 (right).

Compared to the Pd catalysts, smaller nanoparticles were observed in the samples corresponding to Pt-Cl2 and Pt-NO3 (Fig. 3.44). The Pt catalysts were characterized by similar particles size, 4.0-5.0 nm and around 7.0 nm for Pt-Cl2 and Pt-NO3, respectively. Since the particles diameters were not significantly different, the presence of other oxidation products beside gluconic acid could be responsible for the drop in gluconic acid

83 Results and Discussion ▪ Mesoporous SX carbon formation observed at the end of the reaction over the Pt catalyst prepared from the chloride precursor.

Figure 3.44 TEM images of Pt/SX (1wt%) prepared from PtCl4 (left) and from

Pt(NH3)4(NO2)2 (right).

28.12 25.73 Unreacted 100 glucose 5.1 80 5-keto 2-keto gluconic a. Glucaric a. gluconic a. 60 Glycolic a. Acetic a. 5.3 9.8 5.1 Formic a. 40

Yield (%) Yield 5.9 4.4 34.7 Fructose 20 17.2 Glucuronic a. 10.5 Gluconic a. 0 6.5

Pt-Cl2 Pt-NO3

Fig. 3.45. Product distribution for Pt-Cl2 and Pt-NO3 after 180 minutes for the glucose oxidation performed in the batch reactor (70°C, 3 bar O2, 1000 rpm stirring, 5wt% glucose alkaline solution (pH 13.5)).

The product distributions for Pt-Cl2 and Pt-NO3 after 180 minutes are reported in Fig. 3.45. 5-keto gluconic acid (9.8%) and glucaric acid (4.4%), both deriving from further oxidation

84 Results and Discussion ▪ Mesoporous SX carbon

of gluconic acid, were detected in the reaction mixture corresponding to Pt-Cl2. In contrast, only 3.7% 2-keto gluconic acid was formed when the reaction was performed with the Pt catalyst prepared from the nitrate catalyst.

85 Results and Discussion ▪ Mesoporous SX carbon

3.2.3.1.6. Effect of Pd Addition The synergistic effect between gold and palladium in many liquid phase oxidation reactions is well known [94]. The fact that Au is miscible with Pd in all compositions facilitates the formation of AuPd alloys and limits the segregation of the single metals [30], as observed by Prati et al. [94] while investigating the glycerol oxidation reaction. They observed a strong synergistic effect by using AuPd bimetallic catalysts, and they ascribed the strong enhancement in activity to the presence of an alloy phase. They noticed that binding energy of oxygen to palladium decreases when palladium atoms are surrounded by gold [95]. It was also found, in the acetoxylation of ethylene to vinyl acetate, that Au acted as promoter to isolate Pd monoatomic sites, preventing undesirable pathways to CO, CO2 and surface carbon [96]. Prati et al. [94] concluded that the same mechanism could be responsible for avoiding deactivation, which is severe for the pure Pd catalyst. However, as improved resistance to self-poisoning was not sufficient to explain the strong synergistic effect, they suggested a modification also in the interatomic distances and therefore in the electronic structure [97]. According to Prati et al. [94], the number of Pd and Au monoatomic sites on the bimetallic catalyst surface was greatly reduced, and the highly improved activity could be attributed to the AuPd bifunctional sites. The improvement in the activity could then be attributed to a positive combination between electronic and geometric effects [98]. Also Zhang et al.[99] suggested that the synergistic phenomenon between two metals has its physical origin both in electronic (ligand) and geometric (ensemble) effects. Considering the electronic effect, the added metal can electronically affect the electron density of the metal at the catalyst site, with the effect of either improving or reducing the catalytic activity and selectivity. The reaction substrates interact only with the catalytic metal atoms, without direct interaction with the added metal atoms (which should be adjacent to those of the catalytic metal). In contrast, in the case of a geometrical effect, the substrate should interact both with the catalytic and with the added metal atoms. In this way, both kinds of metal atoms can directly interact with a substrate and affect activity and selectivity [99]. As gold is the most electronegative metal (2.54) among the transition metals, in gold bimetallic catalysts the electron transfer from the second metal to gold occurs. This affects the catalytic performance of gold by electronic modifications. By tuning the structure, the surface composition and the particle size and size distribution of the bimetallic systems (all dependent on the preparation procedure), electronic and geometric modifications can be accomplished [29]. The synergistic effect was also observed

86 Results and Discussion ▪ Mesoporous SX carbon by using bimetallic catalysts in other liquid phase oxidation reactions. Dimitratos et al. [100] investigated the performance of bimetallic catalysts in the oxidation of sorbitol. They observed that carbon supported monometallic Au, Pt and Pd showed poor activity, whereas by using bimetallic catalyst (AuPd or AuPt) a great enhancement in the activity was achieved. This was once again ascribed to the synergistic effect between Au and Pd or Pt, as the physical mixture of monometallic catalysts presented poor activity. Furthermore, bimetallic catalysts were found to be more resistant to poisoning by dioxygen and by reaction products [100]. Also Bianchi et al. [37] observed that bimetallic catalysts (AuPd, AuPt) were more active than the monometallic materials in the oxidation of glycerol. They also found that, by using bimetallic catalysts, the product distribution could be controlled. Comotti et al. [40] prepared and successfully evaluated carbon supported AuPd catalysts in the aerobic glucose oxidation; they found that the activity of bimetallic particles was enhanced by combining Au with Pd.

In this PhD work, Au(1wt%)Pd(1wt%)/SX and Au(3wt%) Pd(1wt%)/SX were thus prepared and evaluated in the glucose oxidation in the batch reactor, at 70°C, 3bar O2 and 1000 rpm stirring. Fig. 3.46 shows the conversion and the yield of gluconic acid obtained with the two bimetallic catalysts, and the comparison with the corresponding monometallic gold materials. Although the reaction was performed for 180 minutes, the maximum conversion was reached within 1 h for both for the mono- and the bimetallic materials. In the case of AuPd catalysts, full conversion was observed already after 15 minutes; at the same time, 89.7% and 94.2% conversion was achieved with Au(1wt%) and Au(3wt%), respectively. With the gold monometallic materials, the maximum conversion was indeed reached after 1h. Therefore, the synergistic effect of palladium had a positive influence on the catalytic activity of gold catalysts, by enhancing the conversion and speeding up the reaction. After 60 minutes of reaction, the highest gluconic acid amounts were produced with the monometallic catalysts; 98.1% and 90.6% gluconic acid yields were obtained using Au(1wt%) and Au(3wt%), respectively. The gluconic acid amounts detected in the reaction mixtures corresponding to Au(1wt%)Pd(1wt%)/SX and Au(3wt%) Pd(1wt%)/SX were 87.8% and 69.3%, respectively.

87 Results and Discussion ▪ Mesoporous SX carbon

15 min 60 min Au(1wt%)Pd(1wt%) 100

Au(1wt%) Au(3wt%)Pd(1wt%) 80

Au(3wt%) 60

40 Conversion (%) Conversion 20 Bimetallic Monometallic

0 0 20 40 60 80 100 120 Time (min)

60 min Au(1wt%) 100

80 Au(1wt%)Pd(1wt%) Au(3wt%) Au(3wt%)Pd(1wt%) 60

40

20 Yield Gluconic a. (%) a. Gluconic Yield

0 0 20 40 60 80 100 120 Time (min)

Figure 3.46. Conversion and gluconic acid yield profiles for glucose oxidation performed in the batch reactor with monometallic Au catalysts and bimetallic AuPd catalysts (70°C, 3 bar

O2 and 1000 rpm stirring, 5wt% glucose alkaline solution (pH 13.5)). 1 run for each test.

These results could be related to the metal nanoparticles size. TEM images (Fig. 3.47) revealed that the diameter of the metal nanoparticles in the Au(1wt%) sample were around 6.9 nm; the metal nanoparticles in Au(3wt%) and those in the bimetallic catalysts had a smaller diameter of around 3.0-4.0 nm. Surprisingly, higher gluconic acid yields were obtained using the catalyst with bigger nanoparticles.

88 Results and Discussion ▪ Mesoporous SX carbon

Figure 3.47. TEM images of AuPd catalysts supported on SX and CA1 carbon.

89 Results and Discussion ▪ Mesoporous SX carbon

3.2.3.1.7. Effect of the PVA Amount Au, Pd and Pt materials (1wt% metal loading) were synthetized using a different polyvinyl alcohol (PVA) amount during the metal sol immobilization procedure, in order to investigate, if this had any effect on the catalytic performance. 2 ml PVA, instead of 0.4 ml, were added to the metal precursor solutions during the sol immobilization procedure. According to ICP analysis (Section 5.2.4), the actual metal amount corresponded to the theoretical one. The obtained materials were then evaluated in the glucose oxidation under batch conditions (3 h). The gluconic acid yield profiles obtained with catalysts prepared for 0.4 ml and 2 ml PVA are compared in Fig. 3.48.

0.4 ml PVA 2 ml PVA Au(1wt%) 100 *

80

60 Pt(1wt%) 40 Pd(1wt%)

20 Yield Gluconic acid (%) (%) acid Gluconic Yield 0 0 20 40 60 80 100 120 140 160 180 Time (min)

Figure 3.48. Gluconic acid yield profiles for glucose oxidation performed in the batch reactor with Au, Pd and Pt (1wt%) catalysts prepared with 0.4 ml and 2 ml PVA (70°C, 3 bar O2 and1000 rpm stirring, 5wt% glucose alkaline solution (pH 13.5)). 1 run for each test.

At the end of the reaction, while full glucose conversion into gluconic acid was achieved for the Au catalyst prepared with 0.4 ml PVA, around 92.0% yield was obtained when the reaction was performed with Au(1wt%)-2 ml PVA. With Pd(1wt%) and Pt(1wt%) prepared with 2 ml PVA, higher gluconic acid yields were reached with respect to the corresponding catalysts prepared using 0.4 ml PVA (39.8% and 44.6% instead of 21.8% and 6.5% respectively). The effect of the PVA protecting group on the activity of a supported gold catalyst was investigated by Villa et el. [51] in the glycerol oxidation.

90 Results and Discussion ▪ Mesoporous SX carbon

Au/TiO2 catalysts with different Au:PVA ratios were synthetized by a metal sol immobilization procedure and then evaluated as-prepared (without washing treatment) in the reaction. The authors found that the activity decreased upon increasing the relative amounts of PVA, and that reducing the PVA concentration resulted in a slight increase in particle size, meaning that the largest particles were the most active. Washing the catalyst at 60°C, that caused the complete removal of PVA, led to a drastic activity drop. Villa et al. [51] demonstrated that small amounts of PVA enhanced the activity of the catalyst. In this PhD study, all the catalysts were evaluated in the glucose oxidation after washing treatment with warm water (50°C) in order to completely remove PVA from the samples. Although bigger Au nanoparticles were obtained preparing the Au catalyst with 2 ml PVA (13.0-14.0 nm) with respect to the Au(1wt%)-0.4 ml PVA (~9.5 nm), at the end of the reaction the difference in the gluconic acid amounts detected in the two reaction mixtures was not significant. In the case of Au, contrary to what was observed by Villa et al. [51], with higher PVA amounts slightly bigger nanoparticles were obtained. Concerning Pd and Pt catalysts, bigger metal particles were observed in TEM images of Pd(1wt%) and Pt(1wt%) prepared with 0.4 ml PVA with respect to the corresponding catalysts synthetized using 2 ml PVA (Fig.3.49). This is in agreement with the observations of Villa et al. [51], who reported that the reduction of the PVA concentration led to a slight increase in particle size. However, contrary to what they observed for the glycerol oxidation, in the case of the glucose oxidation the smallest particles were the most active, when using Pd and Pt catalysts. For both Pd and Pt catalysts, higher gluconic acid amounts were indeed detected in the reaction mixtures corresponding to the materials prepared with 2 ml PVA. Although all the samples were washed before being tested in the glucose oxidation, PVA traces could still have been present in the catalysts and therefore directly participated in the reaction mechanism. However, it is still unknown how it could mediate catalyst activity and stability in the glucose oxidation. In the case of the glycerol oxidation, Villa et al. [51] suggested that a possible explanation of the effect of PVA on the selectivity toward this reaction could lie in the ability of PVA to direct the adsorption mode of glycerol. According to them, the PVA arrangement on the surface of Au NPs could create a sort of porous structure interacting with the OH groups of glycerol and, hence, directing the contact between the active site and OH functionality [51].

91 Results and Discussion ▪ Mesoporous SX carbon

Figure 3.49. TEM images corresponding to Pd(1wt%) and Pt(1wt%) samples prepared with 0.4 ml and 2 ml PVA.

92 Results and Discussion ▪ Glucose Liquid Oxidation under Oxygen Flow

3.3. Glucose Liquid Oxidation under Oxygen Flow Among the non-powder supported metal catalysts, the highest gluconic acid yield was reached with Au nanoparticles immobilized on IHC carbons, already after 30 minutes of reaction. Therefore, these materials might be successfully used to perform the reaction in the trickle bed reactor. However, the glucose oxidation was first carried out in a different continuous set-up, i.e. in a three neck round glass flask bubbling pure oxygen, at atmospheric pressure, directly into the solution. Since the difference in the gluconic acid yield obtained in the batch reactor with Au/IHC-1 and Au/IHC-2 was not significant (69.0% and 63.3%, respectively), Au(1wt%)/IHC-1 was used as catalyst. The reaction was carried out at 70°C; for comparison, the gluconic acid yield profiles and the product distribution obtained under O2 pressure (batch reactor) and with bubbling O2 are shown in Fig. 3.50. The glucose oxidation was faster when performed in the batch reactor; furthermore, at the end of the reaction, higher gluconic acid yields (66.9% rather than 22.4%) were reached (Fig. 3.50a). Higher amounts of side products were produced by carrying out the reaction under bubbling O2 (Fig. 3.50b); furthermore, glucuronic, 2- and 5- keto gluconic acid were detected in the corresponding reaction mixture.

a) Au(1wt%)/IHC-1 3 bar O (batch) b) 2 5-keto bubbling O 100 2 100 gluconic a. 2-keto Glycolic a. gluconic a. Formic a. 80 80 6.2 20.2 Fructose 6.3 60 60 70°C 36.5 40 40 66.7 Gluconic a. 6.2

20 20 Yield Gluconic a. (%) a. Gluconic Yield Yield (%) after 180 min 180 after (%) Yield 22.4 36.5 0 0 0 20 40 60 80 100 120 140 160 180 bubbling O2 3 bar O2 (batch) Time (min)

Figure 3.50. a) Gluconic acid profiles and b) product distributions for the glucose oxidation with Au(1wt%)/IHC-1 performed at 70°C with bubbling O2 (three neck round glass flask , atmospheric pressure, 1000 rpm stirring, 5wt% glucose alkaline solution (pH 13.5)) and 3 bar

O2 (batch reactor, 1000 rpm stirring, 5wt% glucose alkaline solution (pH 13.5)). 1 run for each test.

93 Results and Discussion ▪ Glucose Liquid Oxidation under Oxygen Flow

Among the powder carbon supported catalysts, Au/SX allowed reaching the highest gluconic acid yield; therefore, this material was also evaluated performing the glucose oxidation in the three neck round glass flask set-up. The aim was to verify if this material might be successfully used in a continuous system instead of a batch one. With Au/SX, the reaction was carried out at 70°C and 50°C; the results are shown in Fig. 3.51a-left. It was observed that in the first 15 minutes of reaction more gluconic acid was formed at 70°C (88.0%) than at 50°C (76.3%). However, while the gluconic acid production reached a plateau for the reaction carried out at 50°C after 15 minutes, a drop in the gluconic acid yield profile was observed for the glucose oxidation performed at 70°C. At the end of the reaction, 79.2% and 28.3% yields were obtained at 50°C and at 70°C, respectively. This is due to the formation of side products, as shown in Fig. 3.51a-right; indeed, after 15 minutes, the formation of fructose and glucuronic, glycolic and 5-gluconic acid started, lowering the gluconate production. Önal et al. [22] performed the glucose oxidation by bubbling air through the reaction mixture at a constant flow rate at atmospheric pressure with a Au/C catalyst. They found an optimal temperature range at which the reaction rate was at a maximum. This range was around 50°C at pH 9.5 and 60°C at pH 7.0. At alkaline pH values, the reaction rate and selectivity to gluconic acid depended stronger on temperature than at lower pH. In this PhD work, the pH of the reaction mixture after 180 minutes was ~8-9. Corresponding to the observations of Önal et al. [22], a higher gluconic acid yield was achieved at 50°C than at 70°C. In contrast to the reaction with bubbling O2, higher gluconic acid yields were obtained when the glucose oxidation was performed under 3 bar O2 pressure at 70°C than at 50°C (Fig. 3.51b-left). Already after 30 minutes, where a plateau was observed, 97.3% and 88.3% gluconic acid was produced at 70°C and at 50°C, respectively. Because of the plateau, for both temperatures, the product distribution after 30 minutes was basically the same as the one corresponding to the end of the reaction reported in Fig. 3.51b-right; indeed, after 180 minutes, the gluconic acid yields obtained at 70°C and at 50°C were 98.1% and 88.3%, respectively.

94 Results and Discussion ▪ Glucose Liquid Oxidation under Oxygen Flow

a) Au(1wt%)/SX, 70°C bubbling O 100 2 100

50°C 80 80

60 60 5-keto gluconic a. Glucuronic a. 40 40 70°C (%) Yield Gluconic a. Fructose

20 20 Yield Gluconic a. (%) a. Gluconic Yield Glycolic a. 0 0 0 20 40 60 80 100 120 140 160 180 0 20 40 60 80 100 120 140 160 180 Time (min) Time (min)

b) 98.0 70°C 88.3 100 100

50°C 80 80 Fructose

60 60

98.1 88.3 40 40

Yield Gluconic a. Gluconic Yield Gluconic a. 20 Au(1wt%)/SX, 20

3 bar O2 (batch) min. 180 after (%) Yield 0 0 0 20 40 60 80 100 120 140 160 180 70°C 50°C Time (min)

c)

100

80

60 70°C 50°C 40

20

3 bar O2 (batch) Yield Gluconic a. (%) a. Gluconic Yield

bubbling O2 0 0 20 40 60 80 100 120 140 160 180 Time (min)

Figure 3.51. Gluconic acid profiles and product distributions for the glucose oxidation with

Au(1wt%)/SX performed at 70°C and at 50°C with a) bubbling O2 (three neck round glass flask , atmospheric pressure, 1000 rpm stirring, 5wt% glucose alkaline solution (pH 13.5)) and b) 3 bar O2 (batch reactor, 1000 rpm stirring, 5wt% glucose alkaline solution (pH 13.5)) .

In c) gluconic acid profiles at 50°C and 70°C, in batch reactor and with bubbling O2. 1 run for each test.

95 Results and Discussion ▪ Glucose Liquid Oxidation under Oxygen Flow

In Fig. 3.51c, the gluconic acid yield profiles at both temperatures for the reaction performed with bubbling O2 and under 3 bar O2 are plotted for comparison. Generally, the glucose oxidation is faster when performed at 70°C than at 50°C, for both systems; higher gluconic acid yields are more rapidly reached by carrying out the reaction in the batch reactor rather than in the flask with bubbling O2.

From the results obtained for the IHC-1 and SX supported gold catalyst, the highest gluconic acid amounts were obtained when the reaction was carried in the batch reactor. Oxygen, due to its low solubility, is indeed the deficit compound in the glucose oxidation. Therefore, a sufficient concentration of dissolved oxygen has to be ensured during the course of the reaction [69]. The use of a closed vessel under oxygen pressure might increase oxygen dissolution, with the additional effect of speeding up the reaction [21]. In contrast, performing the glucose oxidation by bubbling O2 directly into the solution seemed to decrease the gluconic acid production, by favouring the formation of oxidation side- products, i.e. 2- and 5-keto gluconic and glucuronic acid.

96 Results and Discussion ▪ Glucose Liquid Oxidation in Trickle Bed Reactor

3.4. Glucose Liquid Oxidation in Trickle Bed Reactor A better continuous reaction system, where the glucose oxidation might be successfully performed, is the trickle bed reactor (TBR), which is also the most often used industrially reactor to treat in continuous way three-phase systems.

A trickle bed reactor (TBR) consists of a column with a fixed bed of catalytic particles through which the liquid flows. The gas moves usually concurrently, but sometimes also counter-current flows are used; usually one reactant is introduced in the liquid phase and the other in the gas phase. A trickle bed reactor can be visualized as a bed of catalyst particles with interstitial space among them forming a complex pattern of interconnecting and randomly distributed pores [101]. When gas and liquid reactants flow over these catalyst particles, complex interactions between the flowing liquid phases and the stationary solid particles lead to different flow pattern or regimes. Four distinct regimes were identified in the trickle bed reactors: 1) trickle flow regime, 2) pulse flow regime, 3) spray flow regime and 4) bubbly flow regime. The names of these flow regimes indicate their typical characteristics, which are schematically shown in Fig. 3.52. Traditionally, the different flow regimes have been experimentally studied by varying either gas or liquid flow rates. At low gas and liquid flow rates, gas-liquid interaction is small, and the liquid flows in the form of rivulets over the packed particles, as shown in Fig. 3.52a. This flow regime is known as trickle flow regime or low interaction regime. At moderate gas and liquid flow rates, the interaction among the phases increases, leading to the formation of alternate gas- liquid-enriched zones, as shown in Fig. 3.52b. The corresponding regime is classified as pulse flow regime or high interaction regime. Trickle and pulse flow regimes occur at low- to-moderate flux of gas and liquid flow rates; industrial reactors are commonly operated in these flow regimes. At higher gas or liquid flow rates, the other two, even if less common, flow regimes may occur. In the spray flow regime, taking place at low liquid and high gas flow rates, the gas phase becomes the continuous one, while the liquid phase becomes dispersed in the form of droplets (Fig. 3.52c). On the other hand, at low gas and high liquid flow rates, the liquid phase occupies entire portions of the bed and becomes the continuous phase; in contrast, the gas phase flows in the form of bubbles in the downward direction. This flow regime is known as bubbly flow regime (Fig. 3.52d) [101].

97 Results and Discussion ▪ Glucose Liquid Oxidation in Trickle Bed Reactor

Figure 3.52. Flow regimes in trickle bed reactors: a) trickle flow regime, b) pulse flow regime, c) spray regime and d) bubbly regime. Reprinted (adapted) with permission from [102]. Copyright (2005) American Chemical Society.

Different hydrodynamics parameters might influence the performance of trickle bed reactors. The liquid holdup, defined as the volume of liquid per unit of bed volume (total liquid holdup), is highly sensitive to particle diameter [102]; indeed, the specific area of solid particles is higher for the smaller-sized particles, leading to higher liquid phase retention and holdup. Gas and liquid phase throughputs have significant effect on the liquid holdup. In the case of low interaction regime (both gas and liquid flow rates are low), liquid holdup is insensitive to the gas flow rate. At moderate and high gas and liquid flow rates, liquid holdup decreases with increase in gas flow rate. In contrast, the liquid holdup increases with liquid flow rate because of displacement of gas phase by the liquid. In a trickle flow

98 Results and Discussion ▪ Glucose Liquid Oxidation in Trickle Bed Reactor regime, this displacement occurs till liquid occupies the maximum possible volume. Therefore, the rate of increment in holdup with liquid flow rate is higher compared to the pulse flow regime. Liquid holdup is sensitive to the density and viscosity of the gas and liquid phases with varying extent; liquid holdup is higher for viscous liquids, and in such cases liquid-solid shear plays a greater role than the gas-liquid interactions. Many other parameters of trickle bed reactors, like wetting efficiency and heat and mass transfer coefficients, are dependent on liquid holdup [101]. The wetting efficiency is defined as the extent of wetting of the catalyst particles. Two types of wetting phenomena are generally observed: external wetting and internal wetting of the catalyst particles. External wetting of the particle is indispensable and is a measure of fraction of catalyst surface covered by the liquid film. Internal wetting is the fraction of catalyst surface covered by the liquid phase and in many situations is not complete in spite of capillary effects. In general, in trickle bed reactors three different wetting behaviours are observed, complete wetting, partial wetting and incomplete internal wetting of particles. Wetting efficiency of trickle bed reactors is defined as percent wetting of catalyst external surface area[103] [103b]. External wetting can affect some of the hydrodynamic properties. The presence of liquid film over the catalyst surface, like in the trickle flow regime, restricts access of the gas phase reactants to the active sites. If the limiting reactant is present in the liquid phase, reaction rates are directly proportional to the extent of wetting of the bed, and therefore partial wetting has negative effect on the performance of the reactor. In case the limiting reactant is in the gas phase, reaction rates get enhanced due to direct contact of gas phase reactants with active sites over the unwetted surface, since the catalyst particle is completely wetted internally due to capillary effects. In this case, particle wetting gives positive effect on the reactor performance. Particle diameter has a significant effect on wetting efficiency; the decrease in wetting efficiency with increasing particle diameter can be attributed to capillary pressure and liquid holdup. With smaller- sized particles, larger solid-liquid interaction leads to better spreading, resulting in a considerable enhancement of the wetting at the expense of increase in pressure drop [101]. Wu et al. [104] have suggested the use of inert fine particles along with catalyst particles to improve wetting efficiency without much increase in pressure drop. Besides particle diameter, external wetting efficiency is sensitive to the gas-liquid flow rates, as it will be explained in Section 3.3.1.

99 Results and Discussion ▪ Glucose Liquid Oxidation in Trickle Bed Reactor

Mass transfer rates are lower than in other reactors and often become rate-limiting in the performance of the trickle bed reactors; three types of mass transfer rates are relevant for trickle bed reactors, i.e. gas-liquid, liquid-solid and gas-solid mass transfer rates [101]. Gas- liquid mass transfer rates are dependent on particle size; gas-liquid mass transfer rate increases with decrease in particle size. For smaller-sized particles, the interaction among the flowing phases is considerably higher, and therefore the gas-liquid mass transfer rates are higher as well. The gas-liquid mass transfer coefficient is also sensitive to gas-liquid throughput; gas-liquid flow rate enhances the interaction between gas and liquid phases, leading to an increase in gas-liquid interfacial area [101]. Liquid-solid mass transfer rate is quite sensitive to particle diameter and increases with decrease in particle size [105] [105b]. Furthermore, the liquid-solid mass transfer rates depend more on the liquid mass flow rates compared to the gas mass flow rates; the mass transfer rate increases with liquid mass flow rates, and significant enhancement is observed at lower liquid flow rates than higher. Gas- solid mass transfer coefficient is required in modelling of trickle bed reactors when the catalyst particles are partially wetted. On the dry surface of the catalyst particle, there is a direct contact between gas phase reactants and the external catalyst surface [101]. Most of the reactions carried out in trickle bed reactors are exothermic in nature. Removal of heat from the reactor liberated due to chemical reactions becomes necessary to avoid catalyst deactivation/sintering and for safe operation. Trickle bed reactors are prone to temperature runaway conditions due to poor heat transfer rates. Heat transfer in trickle bed reactors occurs at various levels: i) inside the catalyst pellets where reactions occur, i.e. intraparticle heat transfer, ii) from pellet to the surrounding fluid, i.e. particle-fluid heat transfer, iii) from pellets to pellets, i.e. interparticle heat transfer, and iv) from bed to the wall of the reactor. However, considering the internal wetting and presence of liquid in the pores, in many cases, temperature is reasonably uniform inside the catalyst particles [101].

In this PhD work, a small lab scale TBR set-up was assembled; a schematic representation of the system and a picture of the reactor are shown in Fig. 3.53. A liquid-gas concurrent flow was chosen and the glucose and the oxygen were introduced through the liquid and the gas phase, respectively. The glucose oxidation was performed under the same conditions as used in the batch reactor, but at 1 bar O2 instead of 3 bar.

100 Results and Discussion ▪ Glucose Liquid Oxidation in Trickle Bed Reactor

Figure 3.53. At the top, schematic representation of the trickle bed reactor set-up used to perform the glucose oxidation reaction in continuous. It consists of 1) column and 2) heating mantle made of glass, 3) thermostat, 4) gas flow controller, 5) O2 line, 6) syringe pump, 7) solution exit after passing through the catalytic bed. At the bottom, photograph of the TBR.

Under batch conditions, with Au(1wt%)/SX, Au(1wt%)/IHC-2 and Au(1wt%)/IHC-1 catalysts the highest gluconic acid yields (~98.0%, 69.0% and 63.3%, respectively) could

101 Results and Discussion ▪ Glucose Liquid Oxidation in Trickle Bed Reactor be achieved already after 30 minutes. Therefore, these catalysts could be theoretically used in a continuous system, i.e. in the trickle bed reactor set-up. However, only the metals immobilized on the micro-mesoporous carbon (IHC-2 and IHC-1) could be used in the TBR, since, unlike the SX materials, these materials consist of larger grains which can be packed as a bed through which gas and liquid can flow. Generally, powder catalysts are indeed difficult to handle in TBRs, mainly because of their consistency. The IHC-2 supported catalysts, characterized by bigger grain sizes (Table 3.1) which make them easier to pack as bed, were chosen to perform the glucose oxidation in the TBR.

102 Results and Discussion ▪ Glucose Liquid Oxidation in Trickle Bed Reactor

3.4.1. Trickle Bed Reactor – Preliminary Tests In preliminary tests performed in the trickle bed reactor of 1 cm diameter, using Au(1wt%)/IHC-2, almost full conversion and 81.5% yield of gluconic acid were reached. This result was achieved at 1.2 minutes average residence time (a.r.t.), which corresponds to 20 ml/h liquid flow rate. The residence time is defined as time the liquid phase spends in contact with the catalytic bed and it can be calculated as the ratio between the volume of the catalytic bed and the liquid flow rate. As observed for the glucose oxidation carried out in the batch reactor, carbon-supported Pd and Pt showed lower catalytic performance also in the trickle bed reactor. Compared to the Au catalyst, at the same average residence time (1.2 min), both the conversion and yield of gluconic acid were lower (Table 3.3).

Conversion (%) Yield Gluconic a. (%) Au(1wt%)/IHC-2 98.3 81.5 Pd(1wt%)/IHC-2 55.1 6.9 Pt(1wt%)/IHC-2 50.4 4.6

Table 3.3. Conversion and gluconic acid yield obtained with Au-, Pd- and Pt/C (1wt%) as catalyst at 1.2 minute a.r.t. (20 ml/h liquid flow rate, 575 mL/min oxygen flow rate, 70°C, 1 bar O2, 5wt% initial glucose solution (pH 13.5)). 1 run for each test.

These results were achieved performing the reaction at 20 ml/h liquid flow rate, 575 mL/min oxygen flow rate, 70°C and starting from a 5wt% initial glucose solution in a 1 cm diameter TBR. In order to verify that these were the optimal reaction conditions under which the highest gluconic acid amount could be reached, different reaction parameters were varied. Since with the gold catalyst higher gluconic acid yield with respect to Pd and Pt was obtained, only Au(1wt%)/IHC-2 was used to perform further tests varying the liquid and gas flow rate and the temperature. The glucose oxidation was also carried out starting from a different initial glucose concentration. In the end, the Au(1wt%)/IHC-2 material was tested in the glucose oxidation performed in 2 cm diameter TBR.

103 Results and Discussion ▪ Glucose Liquid Oxidation in Trickle Bed Reactor

3.4.2. Trickle Bed Reactor - Effect of the Liquid Flow Rate Table 3.4 reports the liquid flow rates used to perform the glucose oxidation in the TBR, keeping the gas flow rate and the temperature constant (575 ml/min, 70°C). To each liquid flow rate corresponds a different average residence time. The results obtained using Au(1wt%)/IHC-2 are shown in Fig. 3.54.

Liquid flow rate (ml/h) 10 15 20 40 80 Residence time (τ, min) 2.36 1.57 1.18 0.59 0.29

Table 3.4. Liquid flow rates and corresponding average residence times. The radius and the height of the catalytic bed was 0.5 cm.

98.2 99.2 98.3 82.8 49.6 Conversion 1.8 0.8 1.7 17.2 50.5 Unreacted 100 glucose 6.6 6.6 80 8.8 60 7.9

82.1 81.5 Formic a. 40 Yield (%) Yield 62.9 13.5 Fructose 59.8 20 29.3 Gluconic a. 0 10 15 20 40 80 Liquid flow rate (ml/h)

Figure 3.54. Product distribution obtained for the glucose oxidation performed in the TBR with Au(1wt%)/IHC-2 as catalyst at different liquid flow rates (575 ml/min oxygen flow rate,

70°C, 1 bar O2, 5wt% initial glucose solution (pH 13.5)). 1 run for each test.

When the reaction was performed at 10, 15 and 20 ml/h, the glucose was almost fully converted. At 40 ml/h, the conversion decreased to 82.8%, and the lowest value was reached at 80 ml/h, with 49.6%. Also the product distribution varied with the liquid flow

104 Results and Discussion ▪ Glucose Liquid Oxidation in Trickle Bed Reactor rate. At 10 ml/h, 59.8% gluconic acid yield was obtained. For both 15 and 20 ml/h liquid flow rates, the gluconic acid yield was around 82.0%. With further increase in the liquid flow rate, a decrease in the gluconic acid yield was observed; at 40 ml/h and 80 ml/h, the gluconic acid yield detected in the corresponding reaction mixtures was 62.9% and 29.3%. At 80 mL/h the highest amount of fructose (13.5%) and unreacted glucose (50.5%) was observed. Formic, glucaric and glycolic acid were found in very low amounts (<5.0%) in the reaction mixtures corresponding to 10, 15 and 20 ml/h liquid flow rate.

100 6.6 8.6 7.2 Fructose 80

60 88.7 89.0 89.7 Gluconic a. 40

Relative yield (%) yield Relative 20

0 15 30 60 Time on stream (min)

Figure 3.55. Relative yields for the glucose oxidation in the TBR corresponding to samples taken at different times on stream with Au(1wt%)/IHC-2 as catalyst (20 ml/h liquid flow rate,

575 mL/min oxygen flow rate, 70°C, 1 bar O2, 5wt% initial glucose solution (pH 13.5)).

The conversion and product yield values shown in Fig. 3.54 refer to the end of the glucose oxidation. During the reaction, unfortunately, in all the experiments carried out in the TBR some water evaporated, hence leading to inaccurate results. However, the final product concentrations could be corrected by determining the amount of water lost during the reaction. The initial glucose solution and the reaction mixture collected at the bottom of the reactor were weighted before and after the reaction respectively. From the weight difference, the water loss was known and this was used to correct the concentrations of the final products, which are those reported in Fig. 3.54. The same procedure could not be

105 Results and Discussion ▪ Glucose Liquid Oxidation in Trickle Bed Reactor applied to the samples taken during the course of the reaction. However, in this case, the relative product yields could be considered in order to evaluate the product distributions during the reaction. As reported in Fig. 3.55, already after 15 minutes (time on stream), the relative gluconic acid yield was 88.7% and it remained unchanged also after 30 and 60 minutes. The gluconic acid concentration stayed constant till the end of the reaction, where the absolute (corrected) yield was 81.5% (Fig.3.54, 20 ml/h liquid flow rate). Therefore, it can be assumed that the product yields recalculated for the final samples did not significantly differ from the ones corresponding to the 15 minutes (time on stream) samples, which would be expected for a trickle bed reactor operating at steady state. The correction of the final product concentrations was applied to all the experiments performed in the TBR.

The fact that, with the Au(1wt%) catalyst, the highest conversion and yield of gluconic was reached at 15-20 mL/h might be explained taking into consideration some hydrodynamic parameters. Trickle bed reactors operate under a variety of flow regimes ranging from gas- continuous to liquid-continuous patterns [101]. They usually fall into two broad categories referred to as low interaction regime (trickle flow regime) and high interaction regime (pulse, spray and bubbly flow regime). The trickle flow regime exists at low gas and liquid flow rates, where the inertial forces are weaker compared to the local surface forces and the liquid spreading is mainly controlled by capillary pressure. Therefore, the liquid phase flows in the form of rivulets. At higher flow rates, the inertial forces become important with respect to the interfacial forces, leading to film formation over the catalyst surface [101]. The flow regime map reported by Sie and Krishna [106] for the air-water system showed that the trickle flow regime exists till 12-15 kg/m2s of liquid mass flux and ~1.25 kg/m2s of gas mass flux [101]. In this flow regime, the gas represents the continuous phase while the liquid is the semi-continuous phase. Since both the liquid and gas flow rates used in the present work fell into the range proposed by Sie and Krishna [106], it is reasonable to assume that the trickle bed reactor used in this PhD work was operating in trickle flow regime (Table 3.5 and 3.6).

106 Results and Discussion ▪ Glucose Liquid Oxidation in Trickle Bed Reactor

Liquid Flow Rates (ml/h) 10 15 20 40 80 (kg/m2s) 3.67E-02 5.50E-02 7.33E-02 1.47E-02 2.93E-01

Table 3.5. Conversion of liquid flow rates (ml/h) to mass flux (kg/m2s) (1.036 g/mL as density of the initial glucose solution, 7.85E-05 m2 as cross-sectional area of the catalytic bed).

Gas Flow Rates (mL/min) 339 575 1000 (kg/m2s) 1.36E-03 2.31E-03 4.02E-03

Table 3.6. Conversion of gas flow rates (ml/h) to mass flux (kg/m2s) (1.136E-03 g/ml as density of gas calculated through the equation of state for gas at 70°C, 7.85E-05 m2 as cross-sectional area of the catalytic bed).

In a trickle bed reactor, the liquid holdup, i.e. the volume of liquid per unit bed volume, increases with increasing liquid flow rate because of the gas phase displacement by the liquid. In a trickle flow regime, this displacement occurs till the liquid occupies the maximum possible region. The increase in the liquid holdup assures that the amount of liquid phase covering the catalyst surface is sufficient. The liquid holdup in the bed also controls the liquid phase residence time and therefore conversion of the reactants. By increasing the liquid flow rate from 10 to 20 ml/h, the liquid holdup increased; this might be the reason for the increase in the gluconic acid formation. With liquid flow rates higher than 20 ml/h, lower gluconic acid yields were obtained, due to a decrease in the residence time, i.e. the time the liquid phase spends in contact with the catalytic bed. Furthermore, another parameter should be taken into account in the trickle bed reactor operation, i.e. the wetting efficiency. The wetting efficiency, i.e. the extent of wetting of the catalyst particles, is indeed highly sensitive to the liquid flow rates. Usually, most of the trickle bed reactors operate at very low liquid mass flux (0.01-2 kg/m2s), as in the case of the TBR used in the present work. At such low liquid flow rates, the amount of liquid is not

107 Results and Discussion ▪ Glucose Liquid Oxidation in Trickle Bed Reactor sufficient to cover the catalyst surface and, therefore, partial wetting is inevitable. In general, when porous catalysts are used, two types of wetting can exist:

i) Internal wetting: the internal area of catalyst particles wetted by the liquid and is a measure of the active surface area available for the reaction. In most cases, the internal catalyst is almost completely wetted even if there is incomplete wetting of the external surface due to capillary forces. Under conditions of poor liquid distribution near the entrance of the trickle bed or under conditions of channelling of the liquid flow, some of the catalyst bed is poorly irrigated by the liquid resulting in ineffective internal wetting of the catalyst. ii) External effective wetting: the fraction of external area of catalyst wetted by flowing liquid. Although the catalytic reaction occurs over the internal surface area of the particles, inefficient external wetting can significantly influence the overall performance of a trickle bed reactor; a schematic illustration of increase in effectiveness factor with wetting efficiency is shown in Fig. 3.56.

Fig. 3.56. Schematic illustration of increase in effectiveness factor with wetting efficiency. Reprinted (adapted) with permission from [107].

The incomplete wetting conditions also correspond to significant stagnant liquid pockets due to lower liquid superficial velocities. The wetting efficiency initially changes at higher rate with the liquid flow rate, mainly because of the formation of new rivulets, till it reaches a saturation point. Here, instead of formation of new rivulets, the size of existing rivulets increases, resulting in an enhancement of the wetting. Therefore, the relative rate of increase of the wetting efficiency is low at higher liquid throughputs [101]. In the TBR

108 Results and Discussion ▪ Glucose Liquid Oxidation in Trickle Bed Reactor used to perform the glucose oxidation, the saturation point of the wetting efficiency might have been reached at 15 ml/h and 20 ml/h. After this point, the wetting efficiency started to decrease, as a consequence of the residence time decrease, leading to a drop in the gluconic acid formation. It should also be considered that in the trickle flow operation no rigorous mixing mechanism is present. Therefore, mass transfer rates are lower compared to other reactors and often become rate-limiting in the performance of TBRs. In particular, liquid-solid mass transfer rates are dependent on the extent of the contact of the liquid with the available solid surface. By increasing the liquid flow rate, the wetting behaviour improves and, therefore, also the liquid-solid mass transfer rates. Mass transfer rate increases with liquid mass flow rates and a significant enhancement is usually observed at lower liquid flow rates than higher [101]. This is in agreement with the results obtained performing the glucose oxidation at increasing liquid flow rates (Fig. 3.54). The amount of gluconic acid increased with increasing liquid flow rate from 10 to 20 ml/h and decreased at the higher liquid flow rate values of 40 ml/h and 80 ml/h.

Au(1wt%)/IHC-2 showed the best catalytic performances at 15 ml/h and 20 ml/h liquid flow rates. Since the difference in the product distribution and in the gluconic acid yield was not significant, 20 ml/h liquid flow rate was chosen to carry out further glucose oxidation experiments varying other reaction parameters.

109 Results and Discussion ▪ Glucose Liquid Oxidation in Trickle Bed Reactor

3.4.3. Trickle Bed Reactor – Effect of the Gas Flow Rate Keeping the liquid flow rate and temperature constant (20 ml/h, 70°C), the glucose oxidation was performed varying the oxygen flow rate. 339 mL/min, 575 ml/min and 1000 mL/min were the chosen values; the results obtained are reported in Fig. 3.57. Almost 100% conversion was reached with the oxygen flow rate through the catalytic bed set at 575 ml/min. When the gas flow rate was either decreased or increased with respect to this value, lower glucose conversion in the oxidation reaction was observed. The final gluconic acid yield values followed the same trend. The gluconic acid amount detected in the reaction mixture corresponding to 575 ml/min oxygen flow rate was 81.5%, as already shown previously in Fig. 3.54 (20 ml/h liquid flow rate). At the same time, very low gluconic acid yield was obtained performing the reaction at 339 ml/min and 1000 ml/min (5.8% and 3.3% respectively): Formic (12.0-14.0%) and glycolic acid (<5.0%), fructose (22.8.0-30.1%) and a significant amount of unreacted glucose (49.3-60.4%) were detected in the reaction mixtures.

39.6 98.3 50.7 Conversion 100 60.4 1.7 49.3 Unreacted glucose 6.6 80

60 Glycolic a. 40 81.5 14.0 Formic a. Yield (%) Yield 12.1

20 22.8 30.1 Fructose

Gluconic a. 0 5.8 339 575 1000 Gas flow rate (ml/min)

Figure 3.57. Product distribution for the glucose oxidation performed in the TBR with Au(1wt%)/IHC-2 as catalyst at different gas flow rates (20 ml/h liquid flow rate, 70°C, 1 bar

O2, 5wt% initial glucose solution (pH 13.5)). 1 run for each test.

110 Results and Discussion ▪ Glucose Liquid Oxidation in Trickle Bed Reactor

Although a correlation between liquid flow rate and hydrodynamic parameters could be established, this is less straightforward in the case of the gas flow rate. Indeed, not many studies have reported the effect of the gas flow rate on the wetting efficiency, the liquid holdup, etc. By increasing the gas flow rate from 339 ml/min to 575 ml/min, an increase in the gluconic acid formation was observed; the reason for the low conversion and gluconic acid yield observed performing the reaction at 339 ml/min gas flow rate might be oxygen depletion in the system. Therefore, increasing the oxygen flow rate, might result in higher conversion and gluconic acid production. On the contrary, when the oxygen flow rate was raised from 575 to 1000 ml/min, lower conversion and gluconic acid were obtained; this might be due the higher gas amount with respect to the liquid phase.

111 Results and Discussion ▪ Glucose Liquid Oxidation in Trickle Bed Reactor

3.4.4. Trickle Bed Reactor - Effect of the Temperature In three additional experiments, the temperature was set at RT, 50°C and 90°C respectively; the results are shown in Fig. 3.58. At 70 °C, almost 100% conversion and 81.5% yield of gluconic acid was achieved. Lower and higher temperatures did not appear to be advantageous for the oxidation reaction in the trickle bed reactor; indeed, significant amounts of unreacted glucose (69.8% and 70.8%, respectively) were detected in the reaction mixtures corresponding to room temperature and 50°C. Around 22.0% of glucose did not react when the reaction was performed at 90°C and, furthermore, side products such as formic, glycolic, 2-keto gluconic and acetic acid were found with amounts ≤ 15.0%.

30.2 29.2 98.3 77.9 Conversion 69.8 70.7 1.7 22.1 Unreacted 100 glucose

6.6 80

60

Yield (%) Yield 81.5 Glycolic a. 40 9.9 15.8 Formic a. 20 21.9 16.3 Fructose 11.5 Gluconic a. 0 25 50 70 90 Temperature (°C)

Figure 3.58. Product distribution for the glucose oxidation performed in the TBR with Au(1wt%)/IHC-2 as catalyst at different reaction temperatures (20 ml/h liquid flow rate, 575 ml/min oxygen flow rate, 1 bar O2, 5wt% initial glucose solution (pH 13.5)). 1 run for each test.

As already observed when running the reaction in the batch reactor, 70°C turned out to be the best temperature to carry out the glucose oxidation in order to achieve high gluconic acid yield. The glucose oxidation was always performed starting from an alkaline glucose solution. At alkaline pH values, the dependence of the reaction rate and selectivity to D- gluconic acid on temperature is higher compared to lower pH values. Furthermore,

112 Results and Discussion ▪ Glucose Liquid Oxidation in Trickle Bed Reactor degradation and side products formation is highly favoured when carbohydrate oxidation reactions are performed at temperature higher than 70°C. This might be the reason for the product distributions obtained in the reaction mixture corresponding to 90°C. RT and 50°C, on the contrary, might be not sufficient to start the reaction.

113 Results and Discussion ▪ Glucose Liquid Oxidation in Trickle Bed Reactor

3.4.5. Trickle Bed Reactor - Effect of the Initial Glucose Concentration As for industrial applications the system must be able to produce high gluconic acid amounts starting from high glucose solution concentrations, the oxidation of glucose was carried out starting from a 10 wt% initial glucose solution instead of 5 wt% (at 70°C, with 20 ml/h liquid gas flow rate and 575 ml/min gas flow rate). With double a glucose concentration, a decrease both in conversion and gluconic acid yield was observed (Fig. 3.59). Only a small amount of gluconic acid (1.9%) was produced and fructose (20.1%) and formic acid (10.6%) were the main side products formed. Small amounts of glucuronic, glycolic and 2-keto gluconic acid (<5.0%) were detected.

98.3 41.9 Conversion 1.7 58.1 Unreacted 100 glucose

80

60

40 81.5 Yield (%) Yield 10.6 Formic a. 20 Gluconic a. 20.1 Fructose 0 5wt% 10wt% Glucose concentration (%)

Figure 3.59. Product distribution for the glucose oxidation in the TBR performed with Au(1wt%)/IHC-2 as catalyst at 5wt% and 10wt% initial glucose concentration (20 ml/h liquid flow rate, 575 ml/min oxygen flow rate, 70°C, 1 bar O2, (pH 13.5)). 1 run for each test.

The low catalytic performance of Au(1wt%)/IHC-2 with the 10 wt% glucose solution might be due to the viscosity of the solution itself. Viscosity is a relevant property of fluids to take into account when they are used in flow systems and industrial processes [108]. A relatively small number of works report sugar solution viscosities in limited ranges of temperature and concentration. Among them, Telis et al. [108] determined experimental values of viscosities for glucose aqueous solutions as a function of temperature and

114 Results and Discussion ▪ Glucose Liquid Oxidation in Trickle Bed Reactor concentration in a wide range of temperatures and solute concentrations (10-60% w/w and 0°C-85°C, respectively). They found that glucose solutions viscosities decreased with temperature and increased with concentration (at 70°C, the viscosity value of a 10% w/w glucose was 0.46 mPa.s). A high viscosity might cause the slowing down of the glucose solution through the catalytic bed; this might lead to sugar accumulation in the bed and, hence, to no reaction completion.

Beside the enhanced viscosity of the 10wt% glucose solution as a consequence of the increased sugar concentration, the lower gluconic acid yield obtained might also be explained by possible oxygen depletion in the system. In order to verify this hypothesis, experiments were performed either reducing the liquid flow rate or increasing the gas flow rate.

(a) (b) (c) Glycolic a. 50 11.6 Formic a.

40 14.1 Fructose 30 8.7 10.6

Yield (%) Yield 20 10.6 28.3 Gluconic a. 20.1 10 13.3 0 Lower Standard Higher liquid flow rate reaction gas flow rate parameters

Figure 3.60. Product distribution for the glucose oxidation in the TBR performed with

Au(1wt%)/IHC-2 as catalyst with 10wt% initial glucose concentration at (70°C, 1 bar O2, (pH 13.5)) a) 10 ml/h liquid flow rate, 575 ml/min oxygen flow rate, b) 20 ml/h liquid flow rate,

575 ml/min oxygen flow rate, c) 10 ml/h liquid flow rate, 1000 ml/min oxygen flow rate. 1 run for each test.

The results, compared with the experiment carried out under standard reaction parameters (Fig. 3.60(b)), are reported in Fig. 3.60. By reducing the liquid flow rate (Fig. 3.60(a)) from 20 to 10 ml/h, higher gluconic acid (13.3% with respect to 1.9%) and lower fructose

115 Results and Discussion ▪ Glucose Liquid Oxidation in Trickle Bed Reactor

(10.6% with respect to 20.1%) amounts were formed. The increase in the gas flow rate (Fig. 3.60(c)) had a positive effect on the gluconic acid formation, too. 28.3% gluconic acid was indeed formed, together with 14.1% fructose. The amount of formic acid was similar in all three cases (8.7%-11.6%). These results seemed to confirm the hypothesis of oxygen depletion in the system.

Although by reducing the liquid flow rate or increasing the gas flow rate more gluconic acid was produced, even higher amounts could be obtained using longer beds which would indeed favour better fluid dynamics.

116 Results and Discussion ▪ Glucose Liquid Oxidation in Trickle Bed Reactor

3.4.6. Trickle Bed Reactor - Effect of the Reactor Diameter

Preliminary tests were performed in a second TBR of 2 cm diameter (TBR2).

Au(1wt%)/IHC-2 was used as catalyst. As in the TBR1 (1 cm diameter), also in the TBR2 liquid and gas flow rate, temperature and initial glucose concentration were varied in order to investigate the effect on the reaction; the results are shown in Fig. 3.61.

Liquid flow rate (ml/h) 10 20 40 80 Residence time (τ, min) 4.71 2.36 1.18 0.59

Table 3.7. Liquid flow rates and corresponding average residence times. The radius and the height of the catalytic bed were 1 cm and 0.25 cm respectively.

The liquid flow rates with the corresponding average residence time are reported in Table

3.7. With respect to the TBR1, lower gluconic acid amounts were produced in the TBR2; however, the optimal reaction conditions to apply in order to achieve the highest gluconic acid yield appear to be the same.

As already observed performing the glucose oxidation in the TBR1 with smaller diameter, the increase of the liquid flow rate from 10ml/h to 20 ml/h enhanced the gluconic acid formation (from 17.1% to 25.0%). When the liquid flow rate was further increased up to 40ml/h and 80ml/h, lower gluconic acid amounts were detected in the corresponding reaction mixtures (21.3% and 14.9%, respectively) (Fig. 3.61a). 25.0% gluconic acid yield was obtained performing the reaction with 575 ml/min as gas flow rate; with lower (339 ml/min) and higher (1000 ml/min) gas flow rate, less gluconic acid was produced (13.6% and 17.9%, respectively) (Fig. 3.61b). As observed in the case of the TBR1, in the TBR2 70°C was the optimal temperature in order to obtain the highest gluconic acid amount; in the reaction mixtures corresponding to 50°C and 90°C, only 12.5% and 6.4% gluconic acid was detected (Fig. 3.61c). When the reaction was performed starting from a 10wt% glucose solution, the gluconic acid yield obtained was 6.5% (Fig. 3.61d).

117 Results and Discussion ▪ Glucose Liquid Oxidation in Trickle Bed Reactor

a) 61.1 89.4 84.3 79.9 Conversion 38.9 10.6 15.7 20.1 Unreacted 100 glucose

80

60 Glycolic a.

40 Formic a. Yield (%) Yield 8.6 19.0 11.1 20 9.3 Fructose 25.0 21.3 17.1 14.9 Gluconic a. 0 10 20 40 80 Liquid flow rate (ml/h)

b) 79.3 89.4 80.8 Conversion 20.7 10.6 19.3 Unreacted 100 glucose

80

60

40 Glycolic a. Yield (%) Yield 7.5 Formic a. 11.1 8.6 15.0 Fructose 20 11.1 25.0 13.9 17.9 Gluconic a. 0 339 575 1000 Gas flow rate (ml/min)

c) 80.1 89.4 80.6 Conversion 19.9 10.6 19.3 Unreacted 100 glucose

80

60

40 Yield (%) Yield Glycolic a. 8.6 13.5 Formic a. 20 8.9 25.0 12.7 Fructose 12.5 0 6.4 Gluconic a. 50 70 90 Temperature (°C)

118 Results and Discussion ▪ Glucose Liquid Oxidation in Trickle Bed Reactor

d) 89.4 46.0 Conversion 10.6 54.0 Unreacted 100 glucose

80

60

Glycolic a. 40 Yield (%) Yield 1.6 Formic a. 8.6 20 20.2 Fructose 25.0 Gluconic a. 0 6.5 5 10 Glucose concentration (wt%)

Figure 3.61. Product distribution for the glucose oxidation in the TBR2 performed with Au(1wt%)/IHC-2 as catalyst varying the a) liquid flow rates (575 ml/min oxygen flow rate,

70°C, 1 bar O2, 5wt% initial glucose solution (pH 13.5)), b) gas flow rate (20 ml/h liquid flow rate, 70°C, 1 bar O2, 5wt% initial glucose solution (pH 13.5)), c) temperature (20 ml/h liquid flow rate, 575 ml/min oxygen flow rate, 1 bar O2, 5wt% initial glucose solution (pH 13.5)) and initial glucose concentration (20 ml/h liquid flow rate, 575 ml/min oxygen flow rate, 70°C, 1 bar O2, (pH 13.5)). 1 run for each test.

From the preliminary tests performed in the trickle bed reactor set-up, it appeared that, independently from the reactor diameter, the best reaction conditions which allow to reach the highest gluconic acid yield are 20 ml/h and 575 ml/min as liquid and gas flow rate respectively, 70°C and 5 wt% initial glucose solution.

For performing the experiments in the trickle bed reactor of 1 cm diameter (TBR1), the amounts of glucose, NaOH and catalyst were the same as those used in the batch reactor. A 20 ml glucose solution (~1g glucose, 5wt% concentration) was let flow through the column of the TBR1; the ratio catalyst:glucose was 1:10, so that the catalyst amount used was ~0.1 g. In the case of the TBR1, this resulted in a bed length of 0.5 cm. When the glucose oxidation was performed in the trickle bed reactor of 2 cm diameter (TBR2), the amounts of glucose solution and catalyst were doubled. A 40 ml glucose solution (~2g glucose) was let flow through the TBR2 column; the catalytic bed consisted of ~0.2 g of catalysts, so that the ratio catalyst:glucose was maintained at 1:10. As for the TBR1, also in the case of the

TBR2, the bad length was 0.5 cm. Such a bed is likely too short to ensure good fluid

119 Results and Discussion ▪ Glucose Liquid Oxidation in Trickle Bed Reactor

dynamics. To keep the ratio Db:Lb (Db = reactor diameter and Lb = catalytic bed length) used in the TBR1, the length of the catalytic bed in the TBR2 should be 2 cm. Therefore, the amount of catalyst used should be 0.4 g and, in order to keep the glucose:catalyst ratio equal to 10, a 80 ml glucose solution (~4g glucose, 5wt% concentration) should be let flow through the column.

Increasing the length of the catalytic bed by using more catalyst might therefore be a way to improve the performance of the TBR2. An alternative approach to favour the fluid dynamics might consist in increasing the length of the layers of inert material between which the catalytic bed is placed.

120 Conclusions

4. Conclusions The aim of this PhD work consisted in performing the glucose oxidation in the trickle bed reactor. For this, the reaction has to be explored first in the batch reactor, since here many of the difficult issues of the TBR do not occur. Different catalytic systems and reaction conditions were investigated in order to find the best catalyst and reaction parameters, which allow reaching the highest gluconic acid yield in the shortest time, both in the batch and in the trickle bed reactor. During the last twenty years, the metal catalysed liquid phase oxidation of glucose to gluconic acid has gained the attention of many research groups, being a reaction of high potential industrial interest. Gluconic acid and its salts are widely used in the formulation of food, but also in the field of pharmaceutical and hygienic products [7].

Molecular catalysts have been widely used in oxidative processes for the manufacturing of both bulk and fine chemicals [62]. Although molecular catalysts dissolve in the reaction medium resulting in high reaction rates, they are also rather difficult to separate from the reaction mixture and, moreover, can cause corrosion of industrial materials. Therefore, solid catalysts are considered a better solution for the synthesis of commodity materials [62]. They are easier to prepare, to handle and to separate and they can also be used as fixed beds. For these reasons, in the present PhD work, solid catalysts were chosen to perform the glucose oxidation to gluconic acid. Different solid catalysts were evaluated in the reaction. Since supported palladium and platinum materials often suffer from low catalytic durability and low selectivity [26] [65] [66], many studies have recently focused on gold as alternative catalyst for the aerobic oxidation of glucose under mild conditions. Commercial and self-prepared supported gold catalysts, but also palladium and platinum for comparison, were therefore evaluated in the glucose oxidation.

Pure molecular oxygen was mainly used as oxidizing agent, since the catalytic oxidation of glucose with O2 is an environmentally benign process. However, some tests were also performed with air, in order to verify if this might be a valid and more convenient alternative to pure oxygen.

In this PhD work, contrary to the majority of the studies on glucose oxidation, the pH of the reaction mixture was not adjusted during the reaction at a fixed value; the glucose oxidation was performed starting directly from an alkaline sugar solution. This is due to

121 Conclusions the intended use of the trickle bed reactor; in this kind of reactor, pH adjustments during the reaction are indeed difficult. Alkaline conditions are necessary to increase the reaction rate and avoid drastic catalyst deactivation; conversely, however, such conditions are also responsible for side reactions which reduce gluconate productivity [21]. Moreover, glucose starts to decompose at pH above 11 [21]. Nevertheless, a highly alkaline sugar solution (pH = 13.5) was used, in order to verify if the increase in the reaction rate could overcome the drawback of the formation of side products and in order to maintain relatively high productivity also at high conversion.

The initial series of tests performed in the batch reactor were aimed to find a catalyst by which maximum gluconic acid yield is achieved in the shortest time, and which might therefore be used in a trickle bed reactor, the most often used industrial reactor to treat continuously three-phase systems. However, in order to be used in a trickle bed reactor, the catalyst should be in non-powder form; generally, powder catalysts are difficult to handle in TBRs, mainly because of low bed porosity and difficulties in flow distribution. Gold nanoparticles, as well as palladium and platinum, supported on metal oxide, resin and carbon were employed as catalysts. The metal oxides, the resins and the carbons used as supports consist of pellets, spheres and grains, respectively; for this reason, catalysts supported on these kinds of materials might be packed as a bed through which gas and liquid can flow.

Using commercial gold catalysts (1wt%) supported on metal oxides, i.e. ZnO, Al2O3 and

TiO2, the glucose oxidation was performed at 70°C, 3 bar O2, under mechanical stirring (1000 rpm) and for 7 hours. Under these conditions, Au(1wt%)/ZnO showed the highest activity with 89.3% conversion after 420 minutes; however, the amount of gluconic acid detected in the reaction mixture was very low, 13.7%. Fructose and glucuronic acid were the side products present in the highest amounts, 11.9% and 28.0%, respectively; small amounts of glycolic, formic, 2-keto gluconic and acetic acid were also found. At 70°C, rather than gluconic acid, the product obtained in major amount with the metal oxide supported catalysts was glucuronic acid, which is industrially used for pharmaceutical formulations; while gluconic acid derives from the oxidation of the aldehyde group of the glucose molecule, glucuronic acid is produced when the oxidation involves the primary alcoholic function. The highest gluconic acid yield, 36.4%, was reached with

Au(1wt%)/Al2O3. When the reaction was performed with Au(1wt%)/ZnO at RT instead of

122 Conclusions

70°C, 86.5% gluconic acid was formed; this might be due to the absence, at room temperature, of caramelization reactions which induce glucose degradation and, hence, reduce the gluconate productivity. The conversion profiles obtained for the gold catalysts supported on metal oxides were all characterised by a plateau reached within 30 minutes of reaction. A possible explanation might be a product poisoning of the catalyst; therefore, in order to verify this hypothesis, possible products were individually added to the starting glucose solution (mmol added product:mmol glucose = 1:4). It was observed that by adding glucuronic or glucaric acid to the initial glucose solution, lower conversions were obtained. Therefore, the catalyst poisoning by glucuronic and glucaric acid might be the reason for the inhibition of the catalytic activity, which corresponds to a plateau in the conversion profile. Au nanoparticles, as well as Pt and Ru, were immobilized on commercial sulfonated Amberlyst A70 and A35; they are both macroreticular resins consisting of styrene/divinylbenzene copolymers (ST/DVB), but they differ in the cross-linker content

(DVB). The glucose oxidation was performed at 70°C, 3 bar O2, under mechanical stirring (1000 rpm) and for 7 hours. At the end of the reaction, no gluconic acid was produced with any of the metal catalysts supported on the Amberlysts. In all the reaction mixtures, side products and significant amounts of unreacted glucose were detected. Furthermore, very big particles (30.0-40.0 nm) were observed in the TEM images of the catalysts supported on A70 and A35; this might have also contributed to the low gluconic acid production. Instead of increasing the hydrophilicity of the ST/DVB copolymers by introducing a hydrophilic component, such as carboxylic and sulfonic acid groups, the new approach proposed by Zhao et al. [80] was used. In this PhD work, Au(3wt%)/EGD64 (60% EGDM and 40% DVB) was prepared by copolymerization of two kinds of cross-linkers, divinylbenzene (DVB) and ethylene glycol dimethacrylate (EGDM) and used to perform the glucose oxidation in the batch reactor (70°C, 3 bar O2, 1000 rpm, 7 hours). No gluconic acid was produced during the reaction; instead, around 30.0% glucuronic acid was formed after 420 minutes. Two samples of micro-mesoporous carbon, named ICH-1 and IHC-2, were used to immobilize Au, Pd and Pt nanoparticles (1wt% metal loading). Both carbons are in grain form, with IHC-2 characterized by bigger grain dimension, higher surface area and pore volume. The materials were then evaluated in the glucose oxidation at 70°C, 3 bar O2, 1000 rpm and for 3 hours. Higher gluconic acid amounts were detected in the reaction

123 Conclusions mixtures corresponding to the gold catalysts. Au(1wt%)/IHC-1 and Au(1wt%)/IHC-2 showed similar gluconic acid yield trends. Already after 30 minutes, slightly higher gluconic acid yield (69.0%) was obtained with Au(1wt%)/IHC-2 than with Au(1wt%)/IHC-1 (63.3%). There is a consistent section of literature where carbon based materials are used as supports, especially for liquid phase applications, such as the glucose oxidation reaction, where typically mild temperature conditions (RT-100°C) are used [18]. The carbon support does not only to maintain the catalytic phase in a well dispersed state, but also affects the catalytic activity, for example, by favouring the interactions between active phase and support. By using different materials as supports, it was observed that the carbon support has indeed a strong influence on the catalytic activity of the dispersed metal nanoparticles, in particular of gold.

In the batch reactor, the non-powder catalysts were evaluated in the glucose oxidation without previously being ground to powder; therefore, mass transfer limitations might have been significant, with the effect of decreasing the gluconic acid production. This performance in glucose oxidation should be thus checked with the corresponding powders.

For both Au/metal oxide and metal nanoparticles supported on resins, the product detected in major amounts was glucuronic acid, with the highest yield reached at the end of the reaction (7 h). The target product of the glucose oxidation, i.e. gluconic acid, was produced when the reaction was performed with gold immobilized on IHC carbons; furthermore, with these materials, higher gluconic acid amounts were reached already after 30 minutes. Therefore, IHC carbon supported materials were chosen as potential catalysts to perform the glucose oxidation in the trickle bed reactor; however, only the IHC-2 supported catalysts were used in the TBR, since they are characterized by bigger grain sizes (Table 3.1) and are, therefore, easier to pack as bed inside the column. In the TBR of 1 cm diameter, almost full conversion and 81.5% yield of gluconic acid were reached with 20 ml/h liquid flow rate (1.2 minutes average residence time) and 575 ml/min gas flow rate. The reaction temperature was set at 70°C and the concentration of the initial glucose solution was 5wt%. After varying different reaction parameters, i.e. liquid and gas flow rate, temperature, initial glucose concentration and reactor diameter, it appeared that the above mentioned conditions were the optimal reaction parameters under which the highest gluconic acid yield could be reached. These are, however, preliminary results for a system

124 Conclusions

which should be further studied and investigated. The glucose oxidation in the trickle bed reactor should be also performed using air instead of pure oxygen, to investigate if the reaction could be more conveniently carried out with air without any disadvantage for the gluconic acid formation.

Since the non-powder carbon catalysts showed good catalytic performance in the batch reactor, despite the possible mass transfer limitations, it seemed worth it to find the best catalyst and reaction conditions by which the highest glucose conversion into gluconic acid could be achieved, in the shortest time, in the batch reactor. For reaction kinetics in the batch reactor, mass transfer limitations have to be excluded. In case of catalysts consisting of metals supported on porous materials, a fine powder should be used in order to exclude pore diffusion [69]. Therefore, further tests in the batch reactor were performed using different powder carbon supported materials, in order to investigate if the carbon structure has any effect on the catalytic activity of the dispersed metal nanoparticles. Samples of commercial microporous carbon-supported Pt(1wt%), Pt(5wt%) and Pd(5wt%) were used as catalysts, at 70°C, 3 bar O2, under mechanical stirring (1000 rpm) and for 7 hours. Higher amounts of gluconic acid were obtained with Pd(5wt%) and Pt(5wt%) (66.7% and 46.4%, respectively) rather than with Pt(1wt%) (37.1%). The presence of higher amounts of side products in the reaction mixture corresponding to Pt(5wt%) might explain the lower gluconic acid yield with respect to Pd(5wt%); furthermore, the lower catalytic activity of Pt(5wt%) respect to Pd(5wt%) was correlated with the presence, in the Pt sample, of small nanoparticles (<2.0 nm), which easily deactivate. The carbon support of the commercial Pd and Pt materials are characterized by a disordered and microporous structure; with these catalysts, higher gluconic acid yields were obtained with palladium with respect to platinum. In order to investigate if the carbon structure had any effect on the catalytic activity of platinum, metal nanoparticles were supported on CMK-5, a carbon material characterized by an ordered tubular structure. The reaction was performed at 70°C, 3 bar O2, under mechanical stirring (1000 rpm) and for 7 hours; it was observed that the ordered structure of CMK-5 enhanced the gluconic acid formation, with 78.3% yield after 60 minutes. Since Pd and Pt catalysts often suffer from drawbacks of low catalyst durability and relatively low selectivity, many studies have recently been devoted to employ gold as catalyst to achieve selective aerobic oxidation of glucose under mild conditions. However,

125 Conclusions

Au/C catalysts are not commercially available; therefore, gold nanoparticles were prepared and supported on carbon. Since mesoporous carbons are very attractive for applications in many areas, such as catalyst supports, the commercial mesoporous carbon Norit SX ultra was used to immobilize gold nanoparticles. In order to compare the catalytic activity of the three metals, also Pd and Pt nanoparticles were supported on the SX carbon. The reaction was performed at 70°C, 3 bar O2, under mechanical stirring (1000 rpm) and for 3 hours; Au(1wt%)/SX showed better catalytic performance with respect to Pd and Pt catalysts (Fig. 4.1).

30 min Au(1wt%) 30 min Au(1wt%) 100 100

80 30 min Pd(1wt%) 80

Pt(1wt%) 60 60 min 60

40 40 30 min Pd(1wt%) Conversion (%) Conversion 20 20 60 min

Yield (%) Gluconic acid acid Gluconic (%) Yield Pt(1wt%) 0 0 0 20 40 60 80 100 120 140 160 180 0 20 40 60 80 100 120 140 160 180 Time (min) Time (min)

Figure 4.1. Conversion and gluconic acid profiles for the glucose oxidation performed in the batch reactor with Au(1wt%)/SX, Pd(1wt%)/SX and Pt(1wt%)/SX (70°C, 3 bar O2, 1000 rpm stirring, 5wt% glucose alkaline solution (pH 13.5)).

Already after 30 minutes, with Au(1wt%)/SX, glucose was fully converted into gluconic acid (~98.0% yield), without any pH control during the reaction. With Pd and Pt catalysts, more time was necessary to complete the reaction. After 3h, in the reaction mixtures corresponding to SX supported Pd and Pt catalysts, significant amounts of unreacted glucose and formic, glucaric, glycolic and 5-keto gluconic acid were detected. Fructose was also present, deriving from glucose isomerization under alkaline reaction conditions. After varying different reaction parameters, i.e. temperature, oxygen pressure, oxidizing

126 Conclusions

agent and metal amount, it appeared that 70°C, 3 bar O2 and gold in 1wt% metal loading were the optimal reaction conditions by which the highest gluconic acid yield could be reached.

Gold supported on carbon was found to be the best catalyst by which the highest gluconic acid amounts could be achieved, both under batch and continuous conditions. The amount of gluconic acid produced from the glucose oxidation in the TBR was about 10% higher than the concentration detected in the reaction mixture of the batch reactor. However, the reproducibility of all the obtained results should be confirmed. Although a highly alkaline sugar solution (pH = 13.5) was used to perform the reaction in both reactors, the resulting increase in the reaction rate could overcome the drawback of the formation of side products which typically occurs for pH > 11.0-12.0, leading to the formation of high gluconic acid amounts in short times. With the catalysts which showed the best catalytic performance in the batch and in the TBR, it would be interesting to investigate if high gluconic acid yields could be achieved under neutral reaction conditions. Indeed, an important economic drawback of using bases is the production of gluconate instead of the free acid. Recycling tests should be performed both under batch and continuous reaction conditions. Furthermore, the variation of the surface chemistry of Norit SX and IHC-2 carbons should be investigated, before and after the reaction, in order to better understand the role of these supports in the catalytic performance of the gold catalysts.

127 Experimental Part ▪ Chemicals

5. Experimental Part

5.1. Chemicals D-Glucose (Sigma Aldrich) and NaOH (VWR Chemicals) were used without further purification and pure oxygen and air (Air Liquide) were used as oxidizing agent. The catalysts evaluated in the glucose oxidation reaction consisted of metals nanoparticles immobilized on different kind of supports. All the metal oxide supported materials were purchased, Pd(5wt%)/Al2O3 and Pt(5wt%)/Al2O3 from Sigma Aldrich and Au(1wt%)/ZnO,

Au(1wt%)/TiO2 and Au(1wt%)/Al2O3 from AUROlite. Commercial PS/DVB (polystyrene/divinylbenzene) resins A70 and A35 (Sigma Aldrich) were used to immobilize Pt, Au and Ru nanoparticles using Pt(NH3)4(NO3)2, Au(en)2Cl3 and

Ru(NH3)6Cl3 (Sigma Aldrich) as precursors respectively and H2 (Air Liquide) or NaBH4 (Sigma Aldrich) as reducing agent. For the synthesis of the EGD/DVB resin (ethylene glycol dimethacrylate/divinylbenzene), the monomers and the toluene (used as porogen agent) were purchased from Sigma Aldrich, as well as the initiator 2,2´- azobis(isobutyronitrile), the hydroxyethyl cellulose and the NaCl. Au(en)2Cl3 from Sigma Aldrich was used as metal precursor for the metal immobilization on the EGD/DVB resin.

Carbon name Appearance Porosity type BET (m2/g) Pore Volume (cm3/g) Norit CA1 Powder Mesoporous 1313.5 1.1 Norit SX Ultra Powder Mesoporous 947.1 0.8 IHC-1 Grain Micro- 1230.0 1.57 (<600μm) mesoporous IHC-2 Grain Micro- 1735.0 1.9 (>600μm) mesoporous

Table 5.1. Porous carbons used as supports for the immobilization of metal nanoparticles. Textural data were determined by nitrogen adsorption.

While the carbon supported Pt(1wt%), Pt(5wt%) and Pd(5wt%) catalysts were purchased from Sigma Aldrich and the CMK-5 supported materials were available in the laboratory,

128 Experimental Part ▪Chemicals the majority of the carbon supported catalysts was prepared by supporting the metal nanoparticles on different types of carbons. All metal precursors, i.e. HAuCl4∙3H2O for gold, Pd(NH3)4Cl2∙H2O and Pd(NH3)4(NO3)2 for palladium , PtCl4 and Pt(NH3)4(NO3)2 for platinum, were purchased from Sigma Aldrich, as well as polyvinyl alcohol. Activated carbons Norit CA1 and Norit SX Ultra were purchased (Sigma Aldrich), while SCW-SF- 100-51 and SCW-SF-100-42 carbons were available in the laboratory (“home-prepared” by aerogel process) (Table 5.1). The resins, the carbons and the supported catalysts prepared were used after drying in vacuum oven at 50°C overnight. Silicon carbide (Sigma Aldrich) with 200-450 mesh particle size was used as an inert material for packing the catalyst in the column of the trickle bed reactor. D-glucose, fructose, D-gluconic acid sodium salt, D- glucuronic acid, formic acid, D-saccharic acid potassium salt, glycolic acid, 2-Keto-D- gluconic acid hemicalcium salt hydrate, 5-Keto-D-gluconic acid potassium salt and acetic acid (Sigma Aldrich) were used as reference compounds for the possible products of the reaction.

129 Experimental Part ▪ Catalyst Synthesis

5.2. Catalyst Synthesis Except for the commercial metal oxide supported Au, Pd and Pt materials (Section 5.1), all the catalysts used in the present work were synthetized in the laboratory. In particular, for the preparation of the carbon supported materials, the sol immobilization procedure was mainly followed; however, some samples were also synthetized according to the wet impregnation method.

5.2.1. Synthesis of Metal Nanoparticles Supported on Commercial Resins Metal nanoparticles of Pt, Au and Ru were supported on commercial resins A70 and A35 by the equilibrium adsorption (ion-exchange) procedure. The resin and metal precursor, together with 40 ml deionized H2O, were introduced in a 100 ml Erlenmeyer flask. The solution was mixed for 24 h by a magnetic stirrer. Subsequently, the polymer was filtered over a Gooch filter (G3) and washed several times first with H2O and then with MeOH. Around half of the total polymer supported metal catalyst amount was transferred in the

Teflon inlet of a steel autoclave. The autoclave was closed and pressurized 45 bar H2. The reduction procedure was performed at 100°C (~55 bar H2), 250 rpm stirring for 6 hours (t0 corresponded to T > 95°C). After the reduction process, the resin was washed with H2O and MeOH (3-4 times). The other half of the total polymer supported metal catalyst amount was suspended in 30 ml deionized H2O in a 100 ml Erlenmeyer flask. A freshly prepared NaBH4 solution was added to the polymer supported metal catalyst. After 3 hours of stirring (mechanic stirrer), the resin was filtered and washed with H2O and MeOH (3-4 times). The amounts of reagents used in the synthesis are reported in Tables 5.2. The filtration waters were kept and used for the metal content determination by ICP-MS.

Resin total Pt(NH3)4(NO3)2 H2 NaBH4 amount precursor reduction reduction

Pt(5wt%)/A70 1.0072 g 0.0999 g 0.5061 g 0.5011 g Pt/70 (+ 0.1315

Pt/A70 g NaBH4 in 5 ml H2O) Pt(5wt%)/A35 1 g 0.0995 g 0.5036 g 0.4964 g Pt/35 (+0.1308

Pt/A35 g NaBH4 in 5 ml H2O)

Resin total Au(en)2Cl3 H2 NaBH4

130 Experimental Part ▪Catalyst Synthesis

amount precursor reduction reduction Au(5wt%)/A70 2.0027 g 0.268 g 0.9525 g 1.0475 g Pt/70 (+ 0.4065

Pt/A70 g NaBH4 in 5 ml H2O) Au(5wt%)/A35 2.0002 g 0.2673 g 0.9418 g 1.0584 g Au/35 (+0.4065

Au/A35 g NaBH4 in 5 ml H2O)

Resin total Ru(NH3)6Cl3 H2 NaBH4 amount precursor reduction reduction Ru(5wt%)/A70 2.004 g 0.323 g 1.02 g 0.984 g Pt/70 (+ 0.781 g

Pt/A70 NaBH4 in 5 ml H2O) Ru(5wt%)/A35 2.001 g 0.3228 g 1.02 g 0.981 g Pt/35 (+0.7823 g

Pt/A35 NaBH4 in 5 ml H2O)

Table 5.2. Synthesis of Pt, Au and Ru nanoparticles supported on commercial A70 and A35 resins.

131 Experimental Part ▪ Catalyst Synthesis

5.2.2. Synthesis of Au Nanoparticles supported EGD/DVB Resin According to the procedure reported by Zhao et al. [80], the ethylene glycol dimethacrylate/divinylbenzene resin (EGD/DVB) was prepared by suspension polymerization in the presence of toluene as porogen. 2.530 g EGD (2.378 ml) and 2.454 g DVB (2.735 ml) were first mixed with toluene (10.22 ml) at a 2:1 ratio (vol/vol) to the monomers in order to form an organic phase. In a second step, the initiator AIBN (2,2´- azobis(isobutyronitrile)) was added in the amount corresponding to 1wt% of monomers (0.05 g). The organic phase mixture was added (at 1:3 volume ratio) to an aqueous phase containing 0.2% hydroxyethyl cellulose and 20% NaCl. The polymerization was performed first at 80°C for 14 h and then at 90°C for 4 h. The resulting resin was washed with hot water (~50°C), extracted with acetone in a Soxhlet apparatus (72 h) and finally dried in a vacuum oven at 50°C. The EGD/DVB resin obtained was used as support for the immobilization of Au nanoparticles. 0.5096 g were left to swell in 15 ml H2O in a 50 ml round flask, under stirring for 2 h. The gold precursor solution (0.0269 g HAuCl4∙3H2O in 2 ml H2O) was then added to the resin. After 3.5 h of stirring, the resin supported catalyst was filtered, repetitively washed with warm water and dried overnight in vacuum oven at 50°C.

132 Experimental Part ▪Catalyst Synthesis

5.2.3. Synthesis of Metal Nanoparticles supported on Different Carbons

5.2.3.1. Metal Sol Immobilization Procedure The carbon supported metal catalysts were prepared by a metal sol immobilization procedure, as reported by Prati et al. [1]. The first step consisted in the preparation of an aqueous metal precursor solution of the desired concentration. HAuCl4∙3H2O for gold,

Pd(NH3)4Cl2∙H2O and Pd(NH3)4(NO3)2 for palladium , PtCl4 and Pt(NH3)4(NO3)2 for platinum were used as metal precursors. Under vigorous stirring, a 2 wt% solution of the protective agent polyvinylalcohol (PVA) was added. In order to reduce the metal, a freshly prepared NaBH4 solution was added dropwise. By adding the carbon to the metal dispersion, the metal sol was immobilized on the support. The carbon amount was adjusted according to the final metal loading. The metal dispersion was vigorously stirred for about 1 h (RT), till the solution was clear. The obtained catalyst was then filtered, repetitively washed with warm water and dried overnight in a vacuum oven at 50°C. The amounts of all the reagents used in the preparation of the carbon supported metal catalysts are reported in the following tables (Tables 5.3 to 5.8). The filtration waters were kept and used for the metal content determination by ICP-MS.

Nitrate PVA solution NaBH4 solution Support precursor (2wt%) (0.1 M) CA1

Au(1wt%)* 0.0207 g 0.4 ml 4 ml 1.0538 g

(100 ml H2O) (0.0168 g NaBH4)

Au(3wt%)* 0.066 g 1.2 ml 12 ml 1.0257 g

(300 ml H2O) (0.0466 g NaBH4)

Au(5wt%)* 0.1075 g 2 ml 20 ml 1.0232 g

(500 ml H2O) (0.0745 g NaBH4)

Pt(1wt%)** 0.0246 g 0.4 ml 4 ml 1.0268 g

(100 ml H2O) (0.0161 g NaBH4)

Pt(1wt%)*** 0.0181 g 0.4 ml 4 ml 1.0473 g

133 Experimental Part ▪ Catalyst Synthesis

(100 ml H2O) (0.016 g NaBH4)

Table 5.3. Reagents used in the synthesis of CA1 carbon supported Au, Pd and Pt catalysts, with metal chlorides as precursors (*HAuCl4∙3H2O, ** Pd(NH3)4Cl2∙H2O and *** PtCl4).

Chloride PVA solution NaBH4 solution Support precursor (2wt%) (0.1 M) SX Au(1wt%)* 0.0226 g 0.4 ml 4 ml 1.0523 g

(100 ml H2O) (0.015 g NaBH4) Au(3wt%)* 0.0629 g 1.2 ml 12 ml 1.0058 g

(300 ml H2O) (0.0457 g NaBH4) Au(5wt%)* 0.1061 g 2 ml 20 ml 1.001 g

(500 ml H2O) (0.0765 g NaBH4) Pd(1wt%)** 0.0278 g 0.4 ml 4 ml 1.0568 g

(100 ml H2O) (0.0156 g NaBH4) Pd(3wt%)** 0.0812 g 1.2 ml 12 ml 1.0792 g

(300 ml H2O) (0.0457 g NaBH4) Pd(5wt%)** 0.1359 g 2 ml 20 ml 1.0002 g

(500 ml H2O) (0.0781 g NaBH4) Pt(1wt%)*** 0.0188 g 0.4 ml 4 ml 1.0586 g

(100 ml H2O) (0.0154 g NaBH4) Pt(3wt%)*** 0.055 g 1.2 ml 12 ml 1.0066 g

(300 ml H2O) (0.0462 g NaBH4) Pt(5wt%)*** 0.0911 g 2 ml 20 ml 1.0009 g

(500 ml H2O) (0.0779 g NaBH4)

Table 5.4. Reagents used in the synthesis of SX carbon supported Au, Pd and Pt catalysts, with metal chlorides as precursors (*HAuCl4∙3H2O, ** Pd(NH3)4Cl2∙H2O and *** PtCl4).

134 Experimental Part ▪Catalyst Synthesis

Nitrate PVA solution NaBH4 solution Support precursor (2wt%) (0.1 M) SX Pd(1wt%)# 0.0222 g 0.4 ml 4 ml 1.0012 g

(100 ml H2O) (0.0163 g NaBH4) Pd(3wt%)# 0.66 g 1.2 ml 12 ml 1.0007 g

(300 ml H2O) (0.045 g NaBH4) Pd(5wt%)# 0.1095 g 2 ml 20 ml 1.0012 g

(500 ml H2O) (0.0775 g NaBH4) Pt(1wt%)## 0.0212 g 0.4 ml 4 ml 1.001 g

(100 ml H2O) (0.015 g NaBH4) Pt(3wt%)## 0.0633 g 1.2 ml 12 ml 1.0009 g

(300 ml H2O) (0.0457 g NaBH4) Pt(5wt%)## 0.1047 g 2 ml 20 ml 0.9999 g

(500 ml H2O) (0.0767 g NaBH4)

Table 5.5. Reagents used in the synthesis of SX carbon supported Au, Pd and Pt catalysts, # ## with metal nitrates are used as precursors ( Pd(NH3)4(NO3)2 and Pt(NH3)4(NO3)2).

Chloride PVA solution NaBH4 solution Support precursor (2wt%) (0.1 M) SX Au(1wt%)* 0.0216 g 2 ml 4 ml 1.0019 g

(100 ml H2O) (0.0145 g NaBH4) Pd(1wt%)** 0.0256 g 2 ml 4 ml 1.0449 g

(100 ml H2O) (0.0146 g NaBH4) Pt(1wt%)*** 0.017 g 2 ml 4 ml 1.187 g

(100 ml H2O) (0.0144 g NaBH4)

Table 5.6. Reagents used in the synthesis of SX carbon supported Au, Pd and Pt catalysts, with metal chlorides as precursors (*HAuCl4∙3H2O, ** Pd(NH3)4Cl2∙H2O and *** PtCl4) and higher PVA solution amount (2 ml PVA instead of 0.4 ml).

135 Experimental Part ▪ Catalyst Synthesis

Chloride PVA solution NaBH4 solution Support precursor (2wt%) (0.1 M) SCW-SF-100-51 Au(1wt%)* 0.0206 g 0.4 ml 4 ml 1.0046 g

(100 ml H2O) (0.0143 g NaBH4) Pd(1wt%)** 0.0252 g 0.4 ml 4 ml 1.001 g

(100 ml H2O) (0.0146 g NaBH4) Pt(1wt%)*** 0.018 g 0.4 ml 4 ml 1.0482 g

(100 ml H2O) (0.0145 g NaBH4)

Table 5.7. Reagents used in the synthesis of IHC-1 carbon supported Au, Pd and Pt catalysts, with metal chlorides as precursors (*HAuCl4∙3H2O, ** Pd(NH3)4Cl2∙H2O and *** PtCl4).

Chloride PVA solution NaBH4 solution Support precursor (2wt%) (0.1 M) SCW-SF-100-42 Au(1wt%)* 0.0218 g 0.4 ml 4 ml 1.008 g

(100 ml H2O) (0.0156 g NaBH4) Pd(1wt%)** 0.0273 g 0.4 ml 4 ml 1.0284 g

(100 ml H2O) (0.0159 g NaBH4) Pt(1wt%)*** 0.0191 g 0.4 ml 4 ml 1.006 g

(100 ml H2O) (0.0151 g NaBH4)

Table 5.8. Reagents used in the synthesis of IHC-2 carbon supported Au, Pd and Pt catalysts, with metal chlorides as precursors (*HAuCl4∙3H2O, ** Pd(NH3)4Cl2∙H2O and *** PtCl4).

136 Experimental Part ▪Catalyst Synthesis

5.2.3.2. Wet Impregnation Procedure The appropriate amount of carbon (adjusted to the final metal loading) was transferred with 20 ml H2O in a 100 ml round flask. The palladium solution was prepared by dissolving Pd(OAc)2 in CH3COOH and then was poured into the aqueous carbon solution (previously prepared). After 3.5 h of stirring, the aqueous solution of the Au precursor was added. The Au/Pd precursor solution was then added to the carbon and let stirring for 3.5 h

(RT). The solid was filtered and transferred into a freshly prepared NaBH4 solution (0.1 M). After 3 h stirring, the obtained material was filtered, repetitively washed with water and dried overnight in vacuum oven at 50°C. The amounts of all the reagents used in the preparation of the carbon supported metal catalysts are reported in Table 5.9.

Precursors NaBH4 solution Support (0.1 M) SX 0.0228 g (Au)

Au(1wt%)Pd(1wt%) (5ml H2O) + 8 ml 1.0081 g (20 ml H2O)

0.0212 g (Pd) (0.0301 g NaBH4)

(5ml CH3COCH3) 0.0622 g (Au)

Au(3wt%)Pd(1wt%) (5ml H2O) + 16 ml 1.0057 g (20 ml H2O)

0.0210 g (Pd) (0.0608 g NaBH4)

(5ml CH3COCH3)

Table 5.9. Reagents used in the synthesis of SX carbon supported Au/Pd catalysts, with

HAuCl4∙3H2O and Pd(OAc)2 as precursors.

137 Experimental Part ▪ Evaluation of the Metal Content of the Catalysts

5.2.4. Evaluation of the Metal Content of the Catalysts The metal content of the prepared carbon supported catalysts was determined by ICP-MS (Inductively Coupled Plasma Mass Spectroscopy) or EDX (Energy-dispersive X-ray spectroscopy) analysis. The ICP-MS allows determinations of elements with atomic mass ranges 7 to 250 (Li to U), which includes also Au, Pd and Pt. After filtering and washing the catalyst, the filtrates were collected, transferred in a volumetric flask (the capacity depended on the amount of filtration waters). Few milliliters of aqua regia (HNO3 + HCl, 1:3 molar ratio) were then added to dissolve the metal; the filtration waters were then made up to the mark with distilled water. A fraction of these filtrates was analysed by ICP-MS. The actual metal content of the catalyst was calculated from the difference between the amount of the metal precursor used and the metal traces detected in the filtrates. The metal amounts determined by ICP-MS are reported in Tables 5.10 to 5.15.

A70 supported A35 supported

Pt(5wt%) 4.8 % (H2 reduction) 4.8 % (H2 reduction)

Au(5wt%) 4.7 % (H2 reduction) 4.7 % (H2 reduction)

Ru(5wt%) 4.8 % (H2 reduction) 4.8 % (H2 reduction)

4.6 % (NaBH4 reduction)

Table 5.10. Metal content of Pt, Au and Ru nanoparticles supported on commercial resins A70 and A35 determined by ICP.

. % metal Au(1wt%) 1.1 Pd(1wt%) 1.1 Pt(1wt%) 1.1 Au(3wt%) 3.0 Pd(3wt%) 2.9 Pt(3wt%) 3.0 Au(5wt%) 5.0

138 Experimental Part ▪Evaluation of the Metal Content of the Catalysts

Pd(5wt%) 5.0 Pt(5wt%) 4.9

Table 5.11. Metal content of SX carbon supported catalysts determined by ICP (0.4 ml, 1.2 ml and 2ml PVA was used in the synthesis of 1wt%, 3wt% and 5wt% metal loading materials respectively). Metal chlorides Pd(NH3)4Cl2∙H2O and PtCl4 were used as precursors for the Pd and Pt materials respectively.

% metal Pd(1wt%) 1.1 Pt(1wt%) 0.6 Pd(3wt%) 3.2 Pt(3wt%) 1.7 Pd(5wt%) 4.6 Pt(5wt%) 4.7

Table 5.12. Metal content of SX carbon supported catalysts determined by ICP (0.4 ml, 1.2 ml and 2ml PVA was used in the synthesis of 1wt%, 3wt% and 5wt% metal loading materials respectively). Metal nitrates Pd(NH3)4(NO3)2 and Pt(NH3)4(NO3)2were used as precursors for the Pd and Pt materials respectively.

% metal Au(1wt%) 0.6 Pd(1wt%) 0.9 Pt(1wt%) 0.8

Table 5.13. Metal content of SX carbon supported catalysts determined by ICP (2ml PVA was used in the synthesis).

% metal Au(1wt%) 0.5

139 Experimental Part ▪ Evaluation of the Metal Content of the Catalysts

Pd(1wt%) 0.4 Pt(1wt%) 1.0

Table 5.14. Metal content IHC-1 carbon supported catalysts determined by ICP. Metal chlorides Pd(NH3)4Cl2∙H2O and PtCl4 were used as precursors for the Pd and Pt materials respectively.

% metal Au(1wt%) 0.3 Pd(1wt%) 0.2 Pt(1wt%) 2.2

Table 5.15. Metal content of IHC-2 carbon supported catalysts determined by ICP. Metal chlorides Pd(NH3)4Cl2∙H2O and PtCl4 were used as precursors for the Pd and Pt materials respectively.

% metal Au(1wt%)Pd(1wt%) 1.4 (Au), 1.6 (Pd) Au(3wt%)Pd(1wt%) 3.1 (Au), 1.2 (Pd)

Table 5.16. Metal content of SX carbon supported catalysts determined by EDX.

The metal content of the bimetallic AuPd catalysts was determined by EDX analysis (Tables 5.16).

140 Experimental Part ▪Reaction Set-ups

5.3. Reaction Set-ups

5.3.1. Glucose Oxidation performed in Batch Reactor The batch reactor set-up used to perform the glucose oxidation is schematically represented in Fig. 5.1. A photograph of the reactor is also shown.

Figure 5.1. At the top, schematic representation of the batch reactor used to perform the glucose oxidation reaction. It consists of 1) glass vessel and steel lid with 2) tube dipping in

141 Experimental Part ▪ Reaction Set-ups the inner solution, 3) thermocouple connected to 4) temperature controller, 5) tube feeding oxygen in the reactor and 6) gas line. At the bottom, photograph of the batch reactor.

It consisted of a glass round vessel (50 ml) equipped with a steel net structure. The reactor lid was made of steel as well. Samples were taken through a valve connected to a tube dipping in the solution. The oxygen was fed in the reactor via a second valve in the lid. The reactor was heated by an oil bath placed on a heating-stirring plate and the inner temperature was measured with a thermocouple. Glucose and the catalyst (S/M = 1000) were mixed in distilled water (typically 5wt% glucose solution). Subsequently, NaOH was added (1 mol glucose:1 mol NaOH) and the mixture was stirred until the NaOH was completely dissolved. The reactor was closed, loaded with pure oxygen (or air) and the temperature of the oil bath was adjusted by using a thermostat. As soon as the reaction temperature was reached, the reaction was considered started. Samples were periodically taken and stored in a fridge until they were analysed via HPLC (300 ∙ 6.5 mm Polyspher OAHY column with precolumn 0.1% TFA in water, 10 μL injection volume, 0.8 mL/min, 15.8 MPa, 308 K, RI and UV at 220 nm detectors).

142 Experimental Part ▪Reaction Set-ups

5.3.2. Glucose Oxidation performed in Continuous Mode

5.3.2.1. Glass Flask Set-up The first glucose oxidation tests in continuous gas flow mode were performed in a set-up consisting of a three neck round glass flask (Fig. 5.2) in which the sugar solution (5wt% glucose) was mixed with the catalyst (S/M = 1000 as used in the batch reactor) and with NaOH (1 mol glucose:1 mol NaOH). The initial reaction mixture contained in the round flask was heated by an oil bath up to the reaction temperature. The temperature of the oil bath was measured with a thermocouple. A condenser was connected to the central neck of the round flask, while the other two necks were closed with a rubber septum. The gas was bubbled directly into the solution by a syringe with the needle going through the left rubber septum. The syringe was connected to the gas line. Samples were periodically taken with a syringe dipping into the reaction mixture through the right rubber septum. The samples were then analysed via HPLC (same method used to analyse samples taken from batch reactor).

Figure 5.2. Photograph of the Glass Flask Set-up used to perform the glucose oxidation reaction.

143 Experimental Part ▪ Reaction Set-ups

5.3.2.2. Trickle Bed Reactor (TBR) The glucose oxidation reaction was also carried out in a glass trickle-bed reactor (Fig. 5.3).

Figure 5.3. At the top, schematic representation of the trickle bed reactor set-up used to perform the glucose oxidation reaction in continuous mode. It consists of 1) column and 2) heating mantle made of glass, 3) thermostat, 4) gas flow controller, 5) O2 line, 6) syringe pump, 7) solution exit after passing through the catalytic bed. At the bottom, photograph of the TBR.

144 Experimental Part ▪Reaction Set-ups

The setup used consisted of a column (1 cm inner diameter and 15 cm in length) with a filter (100-160 μm nominal pore size) where the catalytic bed (S/M = 1000 as used in the batch reactor) was placed between two layers of silicon carbide as inert material. After that the catalytic bed was placed inside the column, it was heated up by running water in the mantel (4 cm diameter and 9 cm in length) around the column. The water temperature was adjusted by using a thermostat. The top of the column was closed with a cork and oxygen was let flow through the catalytic bed during the heating time. A gas flow controller was used to monitor the oxygen flux. Meanwhile, the alkaline sugar solution was prepared by mixing glucose and NaOH (1 mol glucose:1 mol NaOH) in 20 ml distilled water (typically 5wt% glucose solution). The amounts of glucose, NaOH and catalyst were the same as those used in the trickle bed reactor. As soon as the reaction temperature was reached, the oxygen flow was reduced and the cork was replaced by a rubber septum. The glucose solution was added to the catalytic bed by a syringe with the needle going through the rubber septum. The syringe was placed in a syringe pump. The position of the needle was adjusted in order to be around the central axis of the catalytic bed, paying attention that it never touched the upper layer of inert material. At this point, the oxygen flow was adjusted using the gas flow controller. The calibration of the gas flow controller was performed with a Definer 220-L device, under standard conditions, and with 50 measurements for each value. The solution was collected in round flak placed at the bottom of the reactor. The sample was taken with a syringe dipping in the flask, stored in the fridge and analysed via HPLC analysis (same method used to analyse samples taken from batch reactor).

145 Appendix

6. Appendix

Figure Lab Jornal Label Material Details

Batch

3.1, 3.2 PAC-PA-057 Au(1wt%)/ZnO 70°C, 3 bar O2, pHsolution=13.5

PAC-PA-058 Au(1wt%)/Al2O3 „

PAC-PA-059 Au(1wt%)/TiO2 „

3.3 PAC-PA-057 Au(1wt%)/ZnO 70°C, 3 bar O2, pHsolution=13.5

PAC-PA-066 Au(1wt%)/ZnO 70°C, 3 bar O2, pHsolution=7

PAC-PA-078 Au(1wt%)/ZnO RT, 3 bar O2, pHsolution=13.5

3.4 PAC-PA-107 Pd(5wt%)/Al2O3 70°C, 3 bar O2, pHsolution=13.5

PAC-PA-109 Pt(5wt%)/Al2O3 „

3.5, 3.6 PAC-PA-089 Au(1wt%)/ZnO 70°C, 3 bar O2,

pHsolution=13.5, glucaric a. addition

PAC-PA-090 Au(1wt%)/ZnO 70°C, 3 bar O2,

pHsolution=13.5, glycolic a. addition

PAC-PA-113 Au(1wt%)/ZnO 70°C, 3 bar O2,

pHsolution=13.5, glucuronic a. addition

PAC-PA-114 Au(1wt%)/ZnO 70°C, 3 bar O2,

pHsolution=13.5, gluconic a. addition

3.8 PAC-PA-099 Pt(5wt%)/A70 (H2) 70°C, 3 bar O2, pHsolution=13.5

PAC-PA-100 Pt(5wt%)/A35 (H2) „

3.9 PAC-PA-099 Pt(5wt%)/A70 (H2) 70°C, 3 bar O2, pHsolution=13.5

146 Appendix

PAC-PA-100 Pt(5wt%)/A35 (H2) „

PAC-PA-116 Ru(5wt%)/A70 (H2) “

PAC-PA-117 Ru(5wt%)/A35 (H2) “

PAC-PA-124 Au(5wt%)/A70 (H2) “

PAC-PA-128 Au(5wt%)/A35 (H2) “

3.10 PAC-PA-002-03 Pt(5wt%)/A70 (H2) TEM image

PAC-PA-001-03 Pt(5wt%)/A35 (H2) “

3.11 PAC-PA-116 Ru(5wt%)/A70 (H2) 70°C, 3 bar O2, pHsolution=13.5

PAC-PA-130 Ru(5wt%)/A70 (NaBH4) “

3.12 PAC-PA-002-03 Pt(5wt%)/A70 (H2) TEM image

PAC-PA-002-02 Pt(5wt%)/A70 (NaBH4) “

3.14 PAC-PA-073 Au(3wt%)/EGD64 70°C, 3 bar O2, pHsolution=13.5

3.15, 3.16 PAC-PA-138 Au(1wt%)/ICH-1 70°C, 3 bar O2, pHsolution=13.5

PAC-PA-141 Pd(1wt%)/ICH-1 “

PAC-PA-146 Pt(1wt%)/ICH-1 “

PAC-PA-244 Au(1wt%)/ICH-2 “

PAC-PA-247 Pd(1wt%)/ICH-2 “

PAC-PA-257 Pt(1wt%)/ICH-2 “

3.17 PAC-PA-004-00 Au(1wt%)/ICH-1 TEM images

PAC-PA-004-03 Au(1wt%)/ICH-2 „

3.18 PAC-PA-061 Pt(1wt%)/C 70°C, 3 bar O2, pHsolution=13.5

PAC-PA-104 Pt(5wt%)/C „

PAC-PA-105 Pd(5wt%)/C „

PAC-PA-102 Pt(1wt%)/C 70°C, 3 bar O2, pHsolution=7

147 Appendix

PAC-PA-103 Pt(5wt%)/C „

PAC-PA-106 Pd(5wt%)/C „

3.19 PAC-PA-104 Pt(5wt%)/C 70°C, 3 bar O2, pHsolution=13.5

PAC-PA-105 Pd(5wt%)/C „

3.20 PAC-PA-111-00 Pt(5wt%)/C TEM image

PAC-PA-444-00 Pd(5wt%)/C “

3.21 PAC-PA-101 Pt (1.8wt%)/CMK-5 70°C, 3 bar O2, pHsolution=13.5

PAC-PA-122 Pt (1wt%)/CMK-5 “

3.22 PAC-PA-122 Pt (1wt%)/CMK-5 70°C, 3 bar O2, pHsolution=13.5

PAC-PA-123 Pt (1wt%)/disordered C “

PAC-PA-101 Pt (1.8wt%)/CMK-5 “

PAC-PA-098 Pt (1.8wt%)/CMK-5 70°C, 3 bar O2, pHsolution=7

3.23 PAC-PA-101 Pt (1.8wt%)/CMK-5 70°C, 3 bar O2, pHsolution=13.5

PAC-PA-098 Pt (1.8wt%)/CMK-5 70°C, 3 bar O2, pHsolution=7

3.25 PAC-PA-205 Au(1wt%)/SX 70°C, 3 bar O2, pHsolution=13.5

PAC-PA-207 Pd(1wt%)/SX “

PAC-PA-209 Pt(1wt%)/SX “

3.26 PAC-PA-209 Pt(1wt%)/SX 70°C, 3 bar O2, pHsolution=13.5

3.27 PAC-PA-003-16 Au(1wt%)/SX TEM image

PAC-PA-003-17 Pd(1wt%)/SX “

PAC-PA-003-18 Pt(1wt%)/SX “

3.28 PAC-PA-302 Au(1wt%)/SX 50°C, 3 bar O2, pHsolution=13.5

PAC-PA-198 Pd(1wt%)/SX “

PAC-PA-200 Pt(1wt%)/SX “

148 Appendix

3.29, 3.30 PAC-PA-205 Au(1wt%)/SX 70°C, 3 bar O2, pHsolution=13.5

PAC-PA-302 “ 50°C, 3 bar O2, pHsolution=13.5

PAC-PA-076 “ RT, 3 bar O2, pHsolution=13.5

3.31 PAC-PA-076 Au(1wt%)/SX RT, 3 bar O2, pHsolution=13.5

3.32, PAC-PA-162 Au(1wt%)/SX 70°C, 1 bar O2, pHsolution=13.5 3.33, 3.34

PAC-PA-205 “ 70°C, 3 bar O2, pHsolution=13.5

PAC-PA-168 “ 70°C, 4 bar O2, pHsolution=13.5

3.32 PAC-PA-166 Pd(1wt%)/SX 70°C, 1 bar O2, pHsolution=13.5

PAC-PA-207 “ 70°C, 3 bar O2, pHsolution=13.5

PAC-PA-170 “ 70°C, 4 bar O2, pHsolution=13.5

3.32 PAC-PA-164 Pt(1wt%)/SX 70°C, 1 bar O2, pHsolution=13.5

PAC-PA-209 “ 70°C, 3 bar O2, pHsolution=13.5

PAC-PA-169 “ 70°C, 4 bar O2, pHsolution=13.5

3.35, 3.36 PAC-PA-150 Au(1wt%)/SX 70°C, 3 bar O2, pHsolution=13.5 (2ml PVA) PAC-PA-155 Pd(1wt%)/SX “

PAC-PA-157 Pt(1wt%)/SX “

PAC-PA-151 Au(1wt%)/SX 70°C, 3 bar Air,

pHsolution=13.5 (2ml PVA) PAC-PA-153 Pd(1wt%)/SX “

PAC-PA-159 Pt(1wt%)/SX “

3.37 PAC-PA-236 Au(1wt%)/SX 70°C, 1 bar O2, pHsolution=13.5

PAC-PA-237 Au(1wt%)/SX 70°C, 5 bar Air,

pHsolution=13.5

3.38 PAC-PA-205 Au(1wt%)/SX 70°C, 3 bar O2, pHsolution=13.5

149 Appendix

PAC-PA-203 Au(3wt%)/SX “

PAC-PA-174 Au(5wt%)/SX “

PAC-PA-207 Pd(1wt%)/SX “

PAC-PA-211 Pd(3wt%)/SX “

PAC-PA-258 Pd(5wt%)/SX “

PAC-PA-209 Pt(1wt%)/SX “

PAC-PA-213 Pt(3wt%)/SX “

PAC-PA-259 Pt(5wt%)/SX “

3.39 PAC-PA-003-16 Au(1wt%)/SX TEM image

PAC-PA-003-19 Au(3wt%)/SX “

PAC-PA-SX3-05 Au(5wt%)/SX “

3.40 PAC-PA-003-17 Pd(1wt%)/SX “

PAC-PA-003-20 Pd(3wt%)/SX “

PAC-PA-Pd5-S2 Pd(5wt%)/SX “

3.41 PAC-PA-003-18 Pt(1wt%)/SX “

PAC-PA-003-21 Pt(3wt%)/SX “

PAC-PA-Pt5-S2 Pt(5wt%)/SX “

3.42 PAC-PA-207 Pd(1wt%)/SX 70°C, 3 bar O2, pHsolution=13.5

PAC-PA-260 “ (NO3 precursor)

PAC-PA-209 Pt(1wt%)/SX 70°C, 3 bar O2, pHsolution=13.5

PAC-PA-261 “ (NO3 precursor)

3.43 PAC-PA-003-17 Pd(1wt%)/SX (Cl2) TEM image

PAC-PA-003-23 Pd(1wt%)/SX (NO3) “

3.44 PAC-PA-003-18 Pt(1wt%)/SX (Cl2) “

150 Appendix

PAC-PA-003-24 Pt(1wt%)/SX (NO3) “

3.45 PAC-PA-003-18 Pt(1wt%)/SX (Cl2) 70°C, 3 bar O2, pHsolution=13.5

PAC-PA-003-24 Pt(1wt%)/SX (NO3) “

3.46 PAC-PA-205 Au(1wt%)/SX 70°C, 3 bar O2, pHsolution=13.5

PAC-PA-203 Au(3wt%)/SX “

PAC-PA-082 Au(1wt%)Pd(1wt%)/SX „

PAC-PA-083 Au(3wt%)Pd(1wt%)/SX „

3.47 PAC-PA-003-16 Au(1wt%)/SX TEM image

PAC-PA-003-19 Au(3wt%)/SX „

PAC-PA-003-05 Au(1wt%)Pd(1wt%)/SX „

PAC-PA-003-06 Au(3wt%)Pd(1wt%)/SX „

3.48 PAC-PA-205 Au(1wt%)/SX 70°C, 3 bar O2, pHsolution=13.5

PAC-PA-207 Pd(1wt%)/SX

PAC-PA-209 Pt(1wt%)/SX

PAC-PA-150 Au(1wt%)/SX 70°C, 3 bar O2, pHsolution=13.5 (2ml PVA) PAC-PA-155 Pd(1wt%)/SX „

PAC-PA-157 Pt(1wt%)/SX „

3.49 PAC-PA-003-17 Pd(1wt%)/SX TEM image

PAC-PA-003-18 Pt(1wt%)/SX „

PAC-PA-003-08 Pd(1wt%)/SX (2ml “ PVA) PAC-PA-003-09 Pt(1wt%)/SX (2ml “

PVA)

3.50 PAC-PA-138 Au(1wt%)/ICH-1 70°C, 3 bar O2, pHsolution=13.5

151 Appendix

PAC-PA-140 “ 70°C, bubbling O2, pHsolution=13.5

3.51 PAC-PA-205 Au(1wt%)/SX 70°C, 3 bar O2, pHsolution=13.5

PAC-PA-302 “ 50°C, 3 bar O2, pHsolution=13.5

PAC-PA-199 “ 50°C, bubbling O2,

pHsolution=13.5

PAC-PA-206 “ 70°C, bubbling O2,

pHsolution=13.5

TBR1

3.54 PAC-PA-240 Au(1wt%)/ICH-2 70°C, 575 ml/min, 10 ml/h

PAC-PA-234 “ 70°C, 575 ml/min, 15 ml/h

PAC-PA-237 “ 70°C, 575 ml/min, 20 ml/h

PAC-PA-241 “ 70°C, 575 ml/min, 40 ml/h

PAC-PA-242 “ 70°C, 575 ml/min, 80 ml/h

3.55 PAC-PA-237 Au(1wt%)/ICH-2 70°C, 575 ml/min, 20 ml/h

3.57 PAC-PA-273 Au(1wt%)/ICH-2 70°C, 339 ml/min, 20 ml/h

PAC-PA-237 “ 70°C, 575 ml/min, 20 ml/h

PAC-PA-280 “ 70°C, 1000 ml/min, 20 ml/h

3.58 PAC-PA-279 Au(1wt%)/ICH-2 RT, 575 ml/min, 20 ml/h

PAC-PA-274 “ 50°C, 575 ml/min, 20 ml/h

PAC-PA-237 “ 70°C, 575 ml/min, 20 ml/h

PAC-PA-278 “ 90°C, 575 ml/min, 20 ml/h

3.59 PAC-PA-281 Au(1wt%)/ICH-2 70°C, 575 ml/min, 20 ml/h; 10wt% glucose 3.60 PAC-PA-282 Au(1wt%)/ICH-2 70°C, 10 ml/h; 10wt% glucose

152 Appendix

PAC-PA-281 “ 70°C, 575 ml/min, 20 ml/h; 10wt% glucose PAC-PA-294 “ 70°C, 1000 ml/min; 10wt% glucose

TBR2

3.61 PAC-PA-264 Au(1wt%)/ICH-2 70°C, 575 ml/min, 10 ml/h

PAC-PA-266 “ 70°C, 575 ml/min, 20 ml/h

PAC-PA-267 “ 70°C, 575 ml/min, 40 ml/h

PAC-PA-268 “ 70°C, 575 ml/min, 80 ml/h

PAC-PA-269 “ 70°C, 339 ml/min, 20 ml/h

PAC-PA-266 “ 70°C, 575 ml/min, 20 ml/h

PAC-PA-270 “ 70°C, 1000 ml/min, 20 ml/h

PAC-PA-271 “ 50°C, 575 ml/min, 20 ml/h

PAC-PA-266 “ 70°C, 575 ml/min, 20 ml/h

PAC-PA-272 “ 90°C, 575 ml/min, 20 ml/h

PAC-PA-283 “ 70°C, 10wt% glucose

153 References

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