Applications and Preparation Methods of Copper Chromite Catalysts: a Review Ram Prasad *, and Pratichi Singh

Available online at BCREC Website: http://bcrec.undip.ac.id

Bulletin of Chemical Reaction Engineering & Catalysis, 6 (2), 2011, 63 - 113

Review Article

Applications and Preparation Methods of Copper Chromite Catalysts: A Review Ram Prasad *, and Pratichi Singh

Department of Chemical Engineering & Technology, Banaras Hindu University, Varanasi 221005, India

Received: 19th March 2011, Revised: 03rd May 2011, Accepted: 23rd May 2011

Abstract

In this review article various applications and preparation methods of copper chromite catalysts have been discussed. While discussing it is concluded that copper chromite is a versatile catalyst which not only ca talyses numerous processes of commercial importance and national program related to defence and space research but also finds applications in the most concerned problem worldwide i.e. environmental pollution control. Several other very useful applications of copper chromite catalysts are in production of clean en ergy, drugs and agro chemicals, etc. Various preparation methods about 15 have been discussed which de picts clear idea about the dependence of catalytic activity and selectivity on way of preparation of catalyst. In view of the globally increasing interest towards copper chromite catalysis, reexamination on the impor tant applications of such catalysts and their useful preparation methods is thus the need of the time. This review paper encloses 369 references including a wellconceivable tabulation of the newer state of the art. Copyright © 2011 by BCREC UNDIP. All rights reserved.

Keywords : Copper chromite, Applications, Preparation methods, Review

Citation Guide : R. Prasad, and P. Singh. (2011). Applications and Preparation Methods of Copper Chro mite Catalysts: A Review. Bulletin of Chemical Reaction Engineering & Catalysis , 6 (2): 63113

Contents 3.3 Solid state reaction (ceramic method) 1. Introduction 3.4 Thermal decomposition of ACOC 2. Applications of copper chromite catalysts 3.5 Hydrothermal method 2.1 Commercial applications 3.6 Nanocasting method (Template technique) 2.2 Hydrogen production 3.7 Hydrolysis of Some soluble salts 2.3 Clean energy production 3.8 Microemulsion method 2.4 Vehicular Pollution control 3.9 Combustion synthesis 2.5 Desulphurization of hot coal gas 3.10 Flame spray pyrolysis method 2.6 Mercury capture from hot coal gas 3.11 Electroless method 2.7 Removal of aqueous organic waste 3.12 Sonochemical method 2.8 Burning rate catalyst for solid propellants 3.13 Metal organic chemical vapour deposition 2.9 Electrodes and Sensors 3.14 Chemical reduction method 2.10 Semiconductors 3.15 Solgel process 2.11 Drugs and agrochemicals 4. Conclusion 3. Preparation methods of copper chromite catalyst Acknowledgement 3.1 Coprecipitation method References 3.2 Coimpregnation method

* Corresponding Author, Email: [email protected] (R. Prasad) Tel.: +91 542 2367323, fax: +91 542 2368092.

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1. Introduction understanding the progress of preparation methods and applications of copper chromite The copper chromite (CuCr 2O4) is one of the catalysts. most efficient materials, has wide commercial

application as catalysts being used in the unit 2. Application of copper chromite catalysts processes of organic synthesis such as hydrogenation [1], dehydrogenation [2], 2.1 Commercial application hydrogenolysis [3], oxidation [4], alkylation [5], 2.1.1 Hydrogenation cyclization [6], etc. It can be used in the pollution abatement as the catalyst to remove aqueous Catalytic hydrogenation is undoubtedly the organic wastes [7], volatile organic compound most useful and widely applicable method for the (VOC) [8] and vehicular primary emissions [9] reduction of chemical substances, and has found such as CO, unburned hydrocarbon, NO x and numerous applications in organic synthesis in soot. In addition it has been used in various research laboratories and industrial processes composite solid propellants as one of the efficient [42]. Copper chromite is an industrially important combustion supporting catalysts [10] in the catalyst because of its ability to hydrogenate domain of space vehicles (rockets) and weapon functional groups in aliphatic and aromatic industries (high explosives, ballistic missiles). compounds selectively. It is employed in both Furthermore, copper chromite has been proved as vapourphase (e.g. hydrogenation of nitrobenzene promising catalyst for the production of H 2 a and nitrotoluenes to their corresponding amines) clean energy carrier, by photocatalytic and liquid phase (e.g. hydrogenation of carbonyl phenomena [1113], conversion of alcohols [14], group in aldehydes, ketones and esters to the water gas shift reaction [15], through sulphur corresponding alcohol) commercial processes [43]. based thermochemical water splitting cycles [16], Copper chromite catalyst was first reported by etc. The next application of CuCr 2O4 is catalyst Adkins et al. [44] to be active for the for alternative fuels preparations, synthesizing hydrogenation of a wide range of organic methanol [17], an important hydrogen carrier; compounds. They tested the catalyst in the high alcohol synthesis (HAS) by hydrogenation of hydrogenation of a group of twentyone organic CO or CO 2, and fast pyrolysis of biomass [18] to compounds in the liquid state at temperature biooil products. The catalyst is also helpful in the varying between 150220 0C and pressure 100150 production of drugs and agro chemicals [19]. In atm, out of twentyone compounds sixteen have fine chemicals industry for perfumery and been successfully hydrogenated in a batch reactor synthesis of fragrances [20] CuCr 2O4 is used as with 100% yield and 100% selectivity. catalysts. The CuCr 2O4 catalyst is useful in The catalysts of copper chromites (chromium desulphurization sorbents for hot coal gas in wt.% > 25) have found extensive use in industrial integrated gasification combined cycle (IGCC) processes for reducing furfural (C 4H3OCHO) to power plants [21,22]. Several other uses of furfuryl alcohol (C 4H3OCH 2OH), butyraldehyde CuCr 2O4 are electrodes and sensors [23], or crotonaldehyde to 1butanol, partially reducing semiconductors [24], heatresistant pigment [25], conjugated dienes to monoenes, and selectively etc. reducing carbonyl group in vegetable oils and Many research projects sanctioned [12, 2629], fatty acid with nonconjugated carbonyl and several Ph.D. theses approved [3034], a number ethylenic bonds [45]. of patents granted [3539] and numerous studies The selective hydrogenation of [14,15,17,19,4042] have been conducted on polyunsaturated organic compounds [46] attracts innovative preparation methods and utilizations great interest from both industrial and academic of the copper chromite catalysts. However, point of view. In fine chemicals industry, we often preparation methods and utilizations of such need a semihydrogenation, for example with catalysts have hardly been reviewed so far. Owing industrial foodstuffs and partial hydrogenation of to the recurrently expanding interest the world edible oils and fatty acids [47] for perfumery and over on the application of copperchromite synthesis of fragrances which require the catalysts, this brief article is an attempt to selective formation of allylic alcohols [48]. summarise the applications of the copper chromite catalytic systems and to review their 2.1.1.1 Hydrogenation of edible oils various useful preparation methods. This review Hydrogenation of edible oils is an important paper will be beneficial to the research process because of its wide applications to community as well as industries and national produce margarine, frying oils, etc. Vegetable oils programs related to defence and space research in

Bulletin of Chemical Reaction Engineering & Catalysis, 6 (2), 2011, 65 contain a mixture of saturated, monounsaturated, odour evaluations, copper chromite hydrogenated and polyunsaturated fatty acids. The mono and soybean oil gave higher scores and lower fishy polyunsaturated fatty acids have double bonds, all responses than nickelhydrogenated soybean oil in the normal “cis” formation. These bonds can after both had been exposed to fluorescent light easily be broken down by oxygen. This produces [68]. compounds that make the oil rancid. Rancidity The catalysts are usually charged into the oil in produces offflavours in foods. To control this edible the oxidized form and are partly reduced to Cu(I) oil is hydrogenated in the food industry to produce and/or Cu(0) during use. Prereduced copper fats and oils with desirable melting properties and chromites have found to be strongly deactivated in an improved shelf life. soybean oil hydrogenation due to disappearance of Cu(II) and Cu(I) species and to the decrement of Edible oil + H 2 → Margarine (1) Cu/Cr ratio on the catalyst surface [69]. Capece and coworkers [70] determined the oxidation Beside the desired hydrogenation reaction states and surface composition of copper chromite (eqn.1), transisomers of fatty acids are formed as at various stages of catalytic use and after well [49]. The transisomer has been reported to be reductive pretreatments, and they concluded that undesirable for human diet due to adverse health Cu 1 is the active species for doublebond effects [50]. It has similar effects as saturated fats isomerization while Cu 0 is required for increasing serum cholesterol levels in the blood, hydrogenation of conjugated dienes. According to believed to be a major cause of heart disease [51]. Rieke et al. [71], activity and selectivity correlate The reduction and/or elimination and content of well with the crystallinity of the copper chromite trans fatty acids in the food supply has attracted surface; they increase with decreasing worldwide interest [5256]. The options to reduce crystallinity. the trans levels in the hydrogenation of an edible Szukalska and Drozdowski [72] hydrogenated oil are changing process conditions and applying rapeseed oils with different erucic acid contents selective low trans heterogeneous catalysts [57]. with Adkins type copperchromite catalyst. The The copper chromite [44] has been extensively tested rapeseed oils, after the elimination of studied due to the high selectivity shown by this linolenic acid by selective hydrogenation showed catalyst in the partial hydrogenation of vegetable several times higher oxidative stability than the oils [58,59]. In particular, many experiments have initial raw material and retained the liquid state been done to correlate catalytic properties with at ambient temperatures. operational parameters such as temperature [60], hydrogen pressure [60,6163], hydrogen flow [64], 2.1.1.2 Hydrogenation of aromatic compounds catalyst concentration [61,63], substrate Adkins copper chromite CuO.CuCr 2O4 catalyst composition [62] and activation procedures [65]. [73] is a rugged one commonly used in Copper chromite catalysts have long been hydrogenations of ethylenic bonds, esters amides known in edible oils hydrogenation as the most under high pressures and temperatures, but rarely selective for the reduction of linolenate C 18:3 to employed to reduce aromatic compounds. It is less oleate C 18:1 leaving unaffected linoleate C 18:2 , susceptible to poisons. With this catalyst valuable component from the nutritional point of phenanthrene [74] and anthracene [75] are view [59]. The major factor responsible for the converted to their dihydroderivatives, whereas relative instability of soybean oil and other naphthalene [76] was converted to tetralin. In vegetable oils for food uses is widely recognized as general, copper chromite catalyst is employed in the linolenate present in the oil [66]. A particular hydrogenations of compounds where reducible feature of the copper chromites is their high groups other than aromatic nucleus are to be selectivity which has been used to advantage in the hydrogenated in preference, e.g. hydrogenation of hydrogenation of edible oils and fats, where nitrobenzene to aniline [75]. stronger hydrogenation catalyst, such as nickel can Selective hydrogenation of aromatic nitrogroups lead to excessive saturation and inferior to the corresponding aromatic amines is one of the nutritional quality of the final product [67]. most important reactions. There are three main Commercially employed Ni catalysts have limited categories of aromatic nitrogroup hydrogenations linolenate selectivity in comparision to copper depending on the presence of other functional chromite catalyst. Consequently, soybean oil groups at the aromatic ring [77]: hydrogenated over Ni catalyst to an iodine value a) The first category includes high volume (IV) of 110 contains 3% linolenate, while copper products, such as aniline and toluenediamines. chromite catalyst reducing it to 0.1% [66]. In room

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The largest end user of these products is the polyurethane industry; b) Category two includes the hydrogenation of halonitroaromatic compounds. The corresponding haloanilines are used in the life science and specialty chemicals industries as intermediates for the production of pesticides, rubber chemicals, dyes, pigments and pharmaceuticals; c) Category three nitrogroup hydrogenations include the selective hydrogenation of Fig.1. Hydrogenation of citral at atmospheric pressure [20] nitroaromatic compounds to anilines without hydrogenation of other functional groups present in the aromatic system [77]. Aniline is an important raw material and intermediate for the production of dyes, medicine, agriculture pesticides, antioxidants and vulcanisation accelerators. It is usually manufactured by the reduction of

C6H5NO 2 + 3H 2 → C 6H5NH 2 + 2H 2O (2) Fig. 2. 2campholenylidenbutanol [83]

Copper chromite is known for its ability to hydrogenate functional groups in aromatic compounds selectively without affecting the which require the selective formation of allylic benzene nucleus [79]. Fang et al. concluded that alcohols [20]. During citral (I) hydrogenation at hydrogenation of nitrobenzene is enhanced by atmospheric pressure, citronellal (II) nerol (III) addition of CrCu/SiO 2 catalysts [80]. Keki et al. appears simultaneously at the initial stage of the [81] found that the unreduced copper chromite is reaction (Fig. 1). The ratio of the amounts of these the stable active catalyst for hydrogenation of two primary products is about 5 in favour of the nitrobenzene. saturated aldehyde (II). Citronellol (IV), the The hydrogenation of nitrobenzene to aniline saturated alcohol appears before complete over reduced Cu(Fe xCr 2−x )O 4 series of catalysts consumption of the starting material but remains (where x=0, 0.2, 0.4, 0.6, 0.8 and 1.0) has been a secondary product [46]. The higher amounts of studied by Jebarathinam et al. [82] at 250 0C in a both primary products (II) and (III) are reached fixed bed flow type reactor. The conversion of for the same conversion value which roughly nitrobenzene to aniline is optimum over the corresponds almost to the total disappearance of catalysts with composition x=0.4. They compared citral. the results of reversible and irreversible Furfuryl alcohol is an important compound in adsorption of carbon monoxide with the fragrance industry. The hydrogenation of hydrogenation activity and concluded that furfural with copper chromite is the industrial univalent copper at octahedral sites is more means of producing furfuryl alcohol given by the active for hydrogenation than metallic copper. eqn. (3) [45]. The second cations [Cr(III) or Fe(III)] develop their catalytic activity by sharing anionic C4H3OCHO + H 2 → C 4H3OCH 2OH (3) vacancies.

2.1.1.3 Perfumery and synthesis of It is well known, a need exists for synthetic fragrances substances which are precious, in demand and having limited supply such as sandalwood There is a continuing search for synthetic substitute or extenders. It would be most materials having desirable fragrance properties. desirable to be able to synthetically provide the Such materials are used either to replace costly major odorant compound of such natural natural materials or to provide new fragrances of sandalwood oils such as, αsantalol and β perfume types which have not theretofore been santalol. Weigers et al. [83] described an available. For perfumery and synthesis of economical and novel process for preparing a fragrances semihydrogenation is often needed, mixture containing 2campholenylidenbutanol

Bulletin of Chemical Reaction Engineering & Catalysis, 6 (2), 2011, 67

a) the addition of ethyl methacrylate to mycrene under the conditions of a DielsAlder type reaction and the treatment of the resulting reaction mixture with an acidic cyclization catalyst; Fig. 3. Compound formed by hydrogenation of 2 b) the reduction of the obtained ester by Ni campholenylidenbutanol [83] CuCr 2O4 hydrogenation, and c) the reduction of the resulting by means of a usual reduction reagent of the ester formation.

2.1.1.4 Hydrogenation of Alcohols Longchain alcohols can be converted directly to N,Ndimethylalkylamines by the reaction with dimethylamine at 36 0C in the presence of of CuCr

Fig. 4. Bicyclic alcohol [84] catalyst and hydrogen at elevated temperatures and pressure as shown by eqn. (4) [85,86].

CuCr

RCH 2OH + HN(CH 3)2 → RCH 2N(CH 3)2 + H 2O (4)

H2 Ethoxylated tertiary amines can be produced by the reaction of primary or secondary amines with ethylene oxide. The asymmetrical tertiary amines Fig. 5. Preparation of bicyclic aliphatic alcohols [84] are used exclusively as starting materials for the manufacture of quaternary ammonium compounds, cationic and amphoteric surfactants, and amine having the structure shown in Fig. 2, by oxides. Quaternary ammonium compounds used as hydrogenating the compound having structure bactericides and algaecides are produced by the given in Fig. 3, in the presence of copper chromite reaction of tertiary amines with benzyl chloride, catalyst. Such mixtures are used in augmenting or methyl chloride, or dimethyl sulphate. Of these, enhancing the aroma of perfume compositions, the benzyl ammonium chloride salt is the most colognes and perfumed articles including fabric widely used [87]. softener compositions, cosmetic powders and solid or liquid anionic, cationic, nonionic and 2.1.1.5 Hydrogenation of aldehydes zwitteronic detergents. Giersch and Ohloff [84] discovered the bicyclic Copperchromium oxide catalyst is effective for alcohol of formula as shown in the Fig. 4. The the hydrogenation of aldehydes [88] at a 0 alcohol possesses a natural woody odour with an temperature of 125150 C. The hydrogenation of ambary character. The woody note is reminiscent benzaldehyde over copperchromium gives a high 0 in particular of cedar wood without however yield of benzyl alcohol even at 180 C without possessing the “sawdust” character of latter. The hydrogenolysis [44] to give toluene (eqn. 5). ambary note, on the other hand, is reminiscent of certain aspects presented by precious materials C6H5CHO + H 2 → C 6H5CH 2OH (5) such as grey amber. Owing to their odour properties, the alcohols find an utilisation of wide Vapourphase hydrogenation of furfural over scope, both in alcoholic perfumery and in technical copper chromite catalyst is perhaps the best applications such as, in the perfuming of soaps, method of producing furfuryl alcohol [1,89]. powder or liquid detergents, fabric softeners, Furfuryl alcohol is an important fine chemical for household materials, cosmetics, shampoos, beauty polymer industry. It is widely used in production of creams, body deodorizers or air fresheners. various synthetic fibres, rubbers, resins, e.g., dark A process for the preparation of bicyclic thermostatic resins resistant to acids, bases and aliphatic alcohols comprises the following reaction resins used for strengthening ceramics. It is also steps shown in Fig. 5 [84]: used as solvent for furan resin, pigment, varnish

Bulletin of Chemical Reaction Engineering & Catalysis, 6 (2), 2011, 68 and as rocket fuel. Pramottana et al. [39] observed Unlike other primary alcohols which are Cu 0 as the active phase in the copper chromite dehydrogenated to aldehydes, the dehydrogenation particles for the selective hydrogenation of furfural of methanol forms methyl formate over copper to furfuryl alcohol. chromite catalysts [94,95]. Methyl formate is used as larvicide and fumigant. It is a starting material 2.1.1.6 Hydrogenation of ketone in the synthesis of formic acid, acetic acid, N, N dimethylformamide, formamide, hydrogen cynide, Copperchromite catalyst is also effective for the methyl cellulose and high purity carbon monoxide hydrogenation of ketones to corresponding [96]. alcohols. Yurieva [90], reported maximum yield of Ethanol dehydrogenation to acetaldehyde over isopropanol on hydrogenation of acetone (eqn. 6) copper chromite catalysts [97,2,98] is highly over copper chromite catalyst at 300350 0C, selective ( selectivity > 95%) represented by the prepared by thermal decomposition of basic copper eqn. 10: ammonium chromate at 900 0C.

C2H5OH ↔ CH 3CHO + H 2 ∆H 0 = 12.51 Kcal/mole CH 3COCH 3 + H 2 ↔ C 3H7OH (6) (10)

Kang et al. [91] carried out hydrogenation of Acetaldehyde is an important intermediate for methyl dodecanoate for the synthesis of 1 the production of a number of industrial chemicals dodecanol in the presence of a copper chromite such as acetic acid, acetic anhydride, nbutanol, catalyst. The catalysts used were synthesized by pentaerythritol, pyridines, peracetic acid, ethyl ceramic method, coprecipitation, and improved co acetate, 2ethylhexanol, aldol, chloral, 1,3butylene precipitation method. The highest yield of glycol, trimethylolpropane, vinyl acetate, perfumes, dodecanol in the hydrogenation reaction was 95.5% aniline dyes, plastics and synthetic rubber [99]. It when copper chromite synthesized in the PEG is used in silvering mirrors and in hardening solution was used as a catalyst in the optimized gelatin fibers. It is also a starting material for the reaction condition. 1dodecanol is also known as polymer paraldehyde, phenol, aldehyde lauryl alcohol (C 12 H25 OH), is a fatty alcohol. It has condensation products, dyes, synthetic flavouring a floral odour. Dodecanol is used to make substance and finds its use as a hardener in surfactants, lubricating oils, and pharmaceuticals. photography. In cosmetics, dodecanol is used as an emollient. Isopropyl alcohol dehydrogenation to acetone

involves a secondary alcohol, whereas both R1 and 2.1.2 Dehydrogenation of alcohols R2 are methyl groups in eqn. (7). Copper chromite The dehydrogenation of alcohols to aldehydes or catalysts possess high selectivity and satisfactory ketones is a wellknown industrial process, and activity [27,100] for isopropyl alcohol these reactions are primarily carried out on copper dehydrogenation (eqn. 11): catalysts because of their high selectivity to the dehydrogenation product [27]. Catalytic Isopropanol: C 3H7OH ↔ CH 3COCH 3 + H 2 (11) dehydrogenation of alcohols plays a key role in the chemical industry particularly in the synthesis of Acetone is an excellent solvent for a wide range various pharmaceuticals and fine chemicals apart of gums, waxes, resins, fats, greases, oils, from bulk chemicals. The reaction can generally be dyestuffs, and cellulosics. It is used as a carrier for described by the eqn. 7: acetylene, in the manufacture of a variety of coatings and plastics, and as a raw material for the R1–CHOH–R2 →R 1–CO–R2 + H 2 (7) chemical synthesis of a wide range of products [101] such as ketene, methyl methacrylate, where, R 2 = H for primary alcohols or an alkyl or bisphenol A, diacetone alcohol, methyl isobutyl aryl group for secondary alcohols. ketone, hexylene glycol (2 methyl2,4 Methanol dehydrogenation to formaldehyde [92] pentanediol), and isophorone. or methyl formate over copper chromite catalysts The use of dehydrogenation for obtaining proceeds via successive reactions (eqn. 8 and 9) butyraldehyde from 1butanol is of interest [93]: because it does not allow any side reaction and yields pure hydrogen as a byproduct. The CH 3OH ↔ HCOH + H 2 (8) stoichiometric equation for the dehydrogenation of HCOH + CH 3OH ↔ HCOOCH 3 + 2H 2 (9) 1butanol is given by eqn. 12 [102]:

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is a promising route to increase the profitability of C4H9OH → C 3H7CHO + H 2 (12) biodiesel plant. PG is an important chemical used to make unsaturated polyester resins, functional According to Rao [1], 90% copper, 8% chromia, fluids (antifreeze, deicing, and heat transfer), and 2% carbon supported on pumice was best personal care, paints, animal feed, food industry catalyst for dehydrogenation of 1butanol to coolants, nonionic detergents, pharmaceuticals, butyraldehyde, with high activity and selectivity. cosmetics, flavours and fragrances, plasticisers and The CuZnOCr 2O3/SiO 2 catalysts prepared by hydraulic brake fluids [107,109]. It is also an impregnation method, exhibits high activity for the excellent solvent and extractant, and used as a dehydrogenation of 2butanol to 2butanone [103]. tobacco humectant. Dasari et al. [110] reported the A copper catalyst with chromium addition, efficiency of prereduced copper chromite catalysts prepared by the electroless plating method, was for the hydrogenolysis of glycerol to PG (85.0% investigated by Shiau et al. [104] for selectivity, 54.8% conversion and 73% yield) at 473 dehydrogenation of 1butanol. K and 1.4 MPa (a mild hydrogen pressure). Butyraldehyde is used in organic synthesis, The reduced CuCr catalysts show significant mainly in the manufacture of rubber accelerators, catalytic activity and selectivity in glycerol and as a synthetic flavouring agent in foods. hydrogenolysis, i.e. above 51% conversion of Isobutyraldehyde is an intermediate for rubber glycerol and above 96% selectivity to 1,2 antioxidants and accelerators. It is used in the propanediol in 4.15 MPa H 2 at 210 0C. The CuCr synthesis of amino acids and in the manufacture of catalysts with low Cu/Cr molar ratio present high perfumes, flavourings, plasticizers and gasoline conversion of glycerol, which is different from the additives. conventional copperchromite catalyst [107]. Crivello et al. [105] prepared hydrotalcitelike Chiu et al. [111,112] performed dehydration of materials containing Cu 2+ , Mg 2+ and Cr 3+ cations in glycerol in the presence of copperchromite catalyst the layers and carbonate in the interlayer by the co to obtain acetol in a single stage semibatch precipitation method with different Cu/Cr/Mg reactive distillation unit under mild conditions. molar ratios. The synthesized catalysts with 40% of The acetol from this reaction readily hydrogenates Cu show a high conversion of isoamylic alcohol and to form propylene glycol providing an alternative selectivity to isovaleraldehyde. The authors route for converting glycerol to propylene glycol. proposed that the presence of small percentages of They achieved high acetol selectivity levels ( ＞90%) magnesium contributes in a significant extent to using copperchromite catalyst. The conversion of the dispersion of entities of oxidized copper on the glycerol to propylene glycol is achieved through a surface of the calcined samples. Isovaleraldehyde reactive intermediate (acetol). First, glycerol is is an important industrial intermediary in the dehydrated to form acetol, and then, this acetol is manufacturing of synthetic resins, special hydrogenated in a further reaction step to produce chemicals and isovaleric acid which is widely used propylene glycol as illustrated by reaction eqn. 13. in the medical industry.

2.1.3 Hydrogenolysis of glycerol to propylene OH OH OH OH O OH OH │ │ │ H O │ ║ + H │ │ glycol 2 2 CH 2CHCH 2 → CH 2CCH 3 ↔ CH 2CHCH 3 (13) Glycerol is a byproduct from biodiesel glycerol acetol propylene glycol industry. As the biodiesel production is increasing exponentially, the crude glycerol generated from Kim et al. prepared copper chromite catalysts the transesterification of vegetables oils has also using methods involving impregnation and been generated in a large quantity. For every 9 kg precipitation, and evaluated for the hydrogenolysis of biodiesel produced, about 1 kg of a crude glycerol of glycerol [28]. Catalyst (10I and 50I) prepared by byproduct is formed [106]. The rapidly increased the impregnation method contained a mixed phase production of biodiesel has led to a drastic surplus of both individual copper and chromium oxide of glycerol in the chemical markets. For this structures, while the catalyst (50P) prepared by reason, many catalytic processes had been reported precipitation showed a single phase, with a copper to convert glycerol into valueadded chemicals [107 chromite spinel structure (CuCr 2O4). XPS data 109] by means of oxidation, hydrogenolysis, indicated that, after the reduction step, the copper dehydration, esterification, carboxylation and species in the impregnated catalyst was reduced to gasification. Among those routes, selective Cu 0, but the catalyst prepared by the precipitation hydrogenolysis of glycerol to propylene glycol (PG)

Bulletin of Chemical Reaction Engineering & Catalysis, 6 (2), 2011, 70 method retained a spinel structure evidenced by the large amount of Cu 2+ species. In hydrogenolysis reactions, the precipitated catalyst showed a higher catalytic activity than the impregnated catalyst. Thus, the reduced copper chromite spinel structure, which constitutes a single phase, appears to be responsible for the high catalytic activity in the hydrogenolysis of glycerol to propylene glycol. Copper chromite catalysts are useful for a variety of chemical reactions in the processing of oleochemical feedstocks. Major oleochemical applications include hydrogenolysis of fatty esters to fatty alcohols including both methyl ester and wax ester processes, alkylation of alcohols with amines and amination of fatty alcohols. The catalysts have unique performance for selective Fig. 6. Reaction scheme of ethylbenzene oxidation hydrogenation of vegetable oils and can be used in [117]. the conversion of biorenewable feedstocks into industrial chemicals. Dovell and Greenfield [113] used copper chromite as a catalyst for the Copper chromite catalyst also converts any preparation of alkylaryl secondary amines by the unsaturated carbon double bonds so that only reductive alkylation of a primary aromatic amine saturated fatty alcohols are formed [87]. The with an aliphatic ketone in the presence of hydrogenation process is carried out at 2530 MPa hydrogen (eqn. 14). and a temperature of 250300 0C in a tubular column. ArNH 2 + ORCR′ + H 2 → ArNHRCHR′ + H 2O (14) 2.1.4 Oxidation reactions The noble metals cause both nuclear Oxidation of ethylbenzene (liquid phase) with t hydrogenation and formation of alkylamines [114] butyl hydroperoxide (TBHP) as an oxidant is by hydrogenolysis of the carbonnitrogen bond feasible over nickel substituted copper chromite between the alkyl group and the nitrogen atom in catalysts [4]. Effective utilization of ethylbenzene, the secondary amine i.e.: ArNHR + H 2 → ArH + available in the xylene stream of the petrochemical RNH 2. Copper chromite catalysts avoid these industry, for more valueadded products is an undesirable side reactions, but a large amount of interesting option. Oxidation of ethylbenzene is of ketone is reduced to the corresponding alcohol. much importance for the production of the Fatty alcohols are an important raw material aromatic ketone, acetophenone, one of the key for surfactants as well as constitute one of the products in the industries. It is used as a largest groups within the oleochemicals. The component of perfumes and as an intermediate for fraction of natural fatty alcohols, i.e. fatty alcohols the manufacture of pharmaceuticals, resins, based on natural fats and oils, is steadily growing alcohols and tear gas. The oxidation pathways of [115]. The fatty alcohols can be produced by ethylbenzene are presented in fig. 6. hydrogenation of fatty acid methyl esters, a Benzaldehyde is used in perfumery and product from natural abundant coconut and palm pharmaceutical industries. Choudhary et al [117] kernel oils, to form high alcohol in the presence of prepared benzaldehyde in liquid phase oxidation of a CuCr 2O4 catalyst [116]. The hydrogenation of benzyl alcohol by tertbutyl hydroperoxide using methyl esters and of fatty acids to form fatty CuCr containing layered double hydroxides and/or alcohols is given by the following general eqns. (15) mixed hydroxides selectively. The reaction is given and (16) respectively: by eqn. (17): CuCr

RCOOCH + 2H 2 ↔ RCH 2OH + CH 3OH (15) C6H5CH 2OH + (CH 3)3COOH → C 6H5CHO + Methyl ester Fatty alcohol (CH 3)COH + H 2O (17)

CuCr George and Sugunan [118] prepared spinel RCOOH + 2H 2 ↔ RCH 2OH + H 2O (16) system with the composition of [Cu 1xZn xCr 2O4] by Fatty acid Fatty alcohol

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coprecipitation method and reported cyclohexane 2C 2H2(NH 2)2 → C 4H4(NH) 2 (19) oxidation at 273 K using TBHP as oxidant. 69.2% Ethylene diamine Piperazine selectivity to cyclohexanol and cyclohexanone at 23% conversion of cyclohexane. Oxidation of Moss and Bell [125] used NiCuCroxide for the cyclohexane is one of the important bulk processes amination of ethanolamine to a mixture of for the production of polyamide fibres and plastics, ethylenediamine and cyclic product, piperazine. such as nylon6 and nylon6,6. They found that addition of water increased the selectivity of a NiCuCroxide catalyst to the cyclic 2.1.5 Alkylation product. Wang et al. [6] found that the CuCrBa Al 2O3 catalyst was suitable for highly selective Alkylation reactions are of great interest in the synthesis of homopiperazine. Cu 0 is believed to be petrochemical industry as they lead to several the active site of the catalyst and the addition of commercially important alkyl aromatics. Cumene Ba to the Cubased catalyst improves the is one such alkyl aromatic produced by dispersion of copper and prevents it from sintering. isopropylation of benzene. The commercial The cyclization of Nβhydroxyethyl1,3 importance of cumene is felt by the world’s growing propanediamine to homopiperazine proceeded with phenol demand, 90% of which is met through more than a 90% yield under optimum reaction cumene. In the cumene route for the production of conditions. phenol, acetone is produced as a low value by product. Barman et al. [119] synthesised cumene with 100% selectivity, by reductive alkylation of 2.2 Hydrogen production benzene with acetone in the presence of a bifunctional catalyst system comprising a solid Hydrogen is used in massive quantities in the acid material, Hmordenite (HM), as alkylation petroleum and chemical industries [126]. In a functional and nanocopper chromite as petrochemical plant, hydrogen is used for hydrogenation functional, eqn. (18). hydrodealkylation, hydrodesulfurization, and hydrocracking, all methods of refining crude oil for C6H6 + CH 3COCH 3 + H 2 → C 6H5CH(CH 3)2 + H 2O wider use. Ammonia synthesis plants accounted for (18) 40% of the world's consumption of H2 in making fertilizer. In the food industry, hydrogen is used to Copper chromite has been reported as a catalyst hydrogenate oils or fats, which permits the for the reductive Nalkylation of aniline with production of margarine from liquid vegetable oil. acetone [120,100]. Pillai [121] prepared different Hydrogen is used to produce methanol and aliphatic secondary amines by reductive alkylation hydrochloric acid, as well as being used as a of methylamine and ethanolamine with carbonyl reducing agent for metal ores. Since H2 is the least compounds over copper chromite catalyst. Almost dense of gases, meteorologists use hydrogen to fill 100% selectivity was observed in all cases. Under their weather balloons. The balloons carrying a optimum conditions of reaction the yield of N load of instruments float up into the atmosphere, isopropylaniline was 91% and that of Nbenzyl for recording information about atmospheric ethanolamine was 94%. conditions. Hydrogen has the highest combustion energy 2.1.6 Cyclization release per unit of weight of any commonly occurring material (eqn. 20). Nitrogencontaining heterocyclic compounds are

pharmaceutically important [122,123]. Intensive H2(g) + 1/2O 2(g) → H 2O(l) ∆H 0 = 286 kJ/mol (20) attention was concentrated on the manufacture of

this type of compounds over a half century. Bai et This property makes it the fuel of choice for al. [124] employed CuCrFe/γAl 2O3 catalysts for upper stages of multistage rockets. Much has been the intramolecular cyclization of N(2 said about hydrogen being the "energy carrier of hydroxyethyl)ethylenediamine to piperazine the future" due to its abundance and its non which showed excellent activity, selectivity and polluting combustion products. When it is long service life under the optimum reaction combusted, heat and water are the only products. conditions (eqn. 19). Further they reported that The use of hydrogen as a fuel for fuel cellpowered satisfactory results were obtained for cyclizations vehicles can greatly reduce green house gas of other alkanolamines, such as N(2hydroxyethyl) emissions from internal combustion engines. 1,2diaminopropane and 5amino1pentanol. Moreover, development of affordable hydrogen fuel

Bulletin of Chemical Reaction Engineering & Catalysis, 6 (2), 2011, 72 cells will help reduce the nation’s dependence on foreign oil, leading to an increased national energy security. Thus, hydrogen offers a potentially non polluting, inexhaustible, efficient, and cost attractive cleanest fuel for today’s rising energy demands [127]. Hydrogen is not found in free state but it is abundantly available in nature as compounds of oxygen (water) or carbon (alcohols, hydrocarbons, carbohydrates, etc.). Energy must be supplied to generate hydrogen from either water or carbonaceous materials. Thus, note that hydrogen is not an energy source, as energy is needed to produce it (21). As an energy carrier, hydrogen is Fig. 7. Schematic illustration of photocatalytic the most attractive option with many ways to evolution of hydrogen produce and utilize it [128].

H2O + energy → H 2 + 0.5O 2 (21) Yan et al. [12] synthesized CuCr 2O4/TiO 2 hetero

junction via a facile citric acid assisted solgel Processes using copper chromite as catalysts for method for photocatalytic H 2 evolution. The nano the production of hydrogen are as follows: composite of CuCr 2O4/TiO 2 is more efficient than a). From water splitting their single part of CuCr 2O4 or TiO 2 in producing • Photoelectrolysis of water hydrogen. CuCr 2O4 is a ptype semiconductor with • Sulphur based thermochemical water a small band gap [129]. A possible reaction model splitting cycles (eqns. 22,23,24) of the CuCr 2O4/TiO 2 hetero b). Catalytic conversion of alcohols junction is proposed by Yan, et al. [12]. For both • Dehydrogenation of alcohols pure CuCr 2O4 and CuCr 2O4/TiO 2 heterojunction, • Decomposition of methanol mainly CuCr 2O4 can be activated under simulated • Reforming of alcohols sunlight irradiation: • Methanol reforming • Ethanol reforming CuCr 2O4 + hν → CuCr 2O4 (e , h +) (22) c). Water gas shift reaction The photogenerated electrons and holes 2.2.1 From water splitting migrate in opposite directions according to the p 2.2.1.1 Photoelectrolysis of water type conductivity of CuCr 2O4, i.e. the electrons migrate in the direction of the illuminated side of In the past two decades, the photo the particle to react with adsorbed water and electrochemical (PEC) processes at semiconductor produce H 2. Meanwhile holes move in the opposite (SC)/electrolyte junctions have been intensively direction (the particles dark side) to oxidize investigated [11]. The search of new materials to adsorbed oxalic acid. Because of the Schottky achieve the photochemical conversion has led to a barrier formed between the interface of CuCr 2O4 great deal of work on CuCr 2O4 [1113,129130] and and TiO 2, the heterojunction can improve the remains the best possible way of solar energy separation of the photogenerated electrons and storage in hydrogen form. Photochemical H 2 holes. When coupled with TiO 2, the photo evolution based on a dispersion of CuCrO 2 powder generated electrons are injected from excited in aqueous electrolytes containing various reducing CuCr 2O4 into the conduction band of TiO 2 and agents (S 2, SO 23 and S 2O23) has been studied by reduce adsorbed water into H 2: Saadi [129]. The powder dispersion has the advantage of a liquidjunction solarcell where the CuCr 2O4 (e ) + TiO 2 → CuCr 2O4 + TiO 2 (e ) (23) two half reactions take place simultaneously on the CuCr 2O4 (e , h +) + TiO 2 → CuCr 2O4 (h +) + TiO 2 (e ) same particle that behaves like a microphoto (24) electrochemical cell and it is cost effective too. It has a long lifetime, a pH insensitive energetic and As a result, CuCr 2O4/TiO 2 heterojunction with absorbs a large part of the sun spectrum. appropriate CuCr 2O4/TiO 2 ratio can enhance the Schematic photocatalytic evolution of H 2 is separation of the photogenerated electrons and illustrated in Fig. 7.

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holes, and therefore improve the H 2 photocatalytic propanol, butanol) can promote the use of fuel cells activity [130]. Boumaza et al. [13] reported that and other clean technologies as a source of energy the best catalytic performance over CuCr 2O4 for H 2 for mobile applications. Alcohols can serve as H 2 photoproduction was obtained in (NaOH 0.5 M, carriers that are compatible with current Na 2S2O3 0.025 M) with an average rate of 0.013 infrastructures for liquid fuels and can be cm 3 h1/mg catalyst and a quantum efficiency of catalytically converted onsite in order to minimize 0.2% under polychromatic light. energy input requirements and operating temperatures [134]. Methanol and ethanol have 2.2.1.2 Sulphur based thermochemical water the highest H/C ratios among the alcohols and splitting cycles gasolinerange alkanes (e.g., heptane). That is, more hydrogen can be extracted from their Thermochemical watersplitting cycle is a molecular frameworks [135]. Production of promising process to produce hydrogen using solar hydrogen from alcohols can be accomplished by or nuclear energy [131]. Ginosar et al. [16] their dehydrogenation and reforming processes. evaluated the activity and stability of several complex metal oxides: two ABO 3 structures 2.2.2.1 Dehydrogenation of alcohols (FeTiO 3 and MnTiO 3) and three AB 2O4 structures (NiFe 2O4, CuFe 2O4, and NiCr 2O4) and copper Hydrogen as byproduct is obtained from chromite (i.e., 2CuO.Cr 2O3) (was also selected due dehydrogenation of alcohols over copper chromite to the high activities of Cr 2O3 and CuO [132]) for catalysts as discussed in section (2.1.2). The the atmospheric decomposition of concentrated dehydrogenation of alcohols is a reversible and an sulfuric acid in sulfurbased thermochemical water endothermic process which implies that heat must splitting cycles. Catalyst activity was determined be supplied to the system. Of course very pure at temperatures from 725 to 900 0C. Catalytic hydrogen is obtained on dehydrogenation of stability was examined at 850 0C for up to 1 week ethanol over copper chromite catalyst [136]. of continuous operation. The results were compared to a 1.0 wt% Pt/TiO 2 catalyst. Over the 2.2.2.2 Decomposition of methanol temperature range, the catalyst activity of the Methanol is easy to transport over long complex oxides followed the general trend: distances and to store. The catalytic decomposition 2CuO.Cr 2O3 > CuFe 2O4 > NiCr 2O4.NiFe 2O4 > of methanol to CO and H 2 can provide a clean and MnTiO3. FeTiO3. Tagawa and Endo [132] observed efficient fuel (eqn. 25): the order of the activities for the decomposition of sulfuric acid in thermochemical water splitting CH 3OH ↔ CO + 2H 2 ∆H = 90.2 kJ/mol (25) process at the initial concentration of SO 3 of 4.0 mol% as follows: Pt ≈ Cr 2O3 > Fe 2O3 > (CuO) > For example, methanol decomposition on board CeO 2 > NiO > Al 2O3. The activity of Cr 2O3 above of a vehicle (Fig. 8) provide a fuel which is cleaner 700°C was nearly the same as the Pt catalyst. and 60% more efficient than gasoline and up to

34% better than undecomposed methanol [137 2.2.2 Catalytic conversion of alcohols 139]. Methanol decomposition is an endothermic In recent years, the catalytic decomposition/ reaction. The reaction heat can be provided by the reforming of alcohols has gained particular interest engine coolant and exhaust gas. This recovers the due to growing environmental, economic, and waste heat and increases the heating value of the political concerns regarding energy production fuel. The decomposed methanol may also be used [133]. Safe and efficient in situ hydrogen as a clean fuel for gas turbines at times of peak generation from alcohols (i.e. methanol, ethanol, demand of electricity [137140]. Lean and complete

Fig. 8. Methanol decomposition on board a vehicle

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CH 3OH Catalytic refor Shift conve Preferential 0 ming (250 C) rsion (200 Oxidation H2 0 H2O 500 C)

Air + Fuel Water Air CO 2

Fig. 9 . Flowsheet for a conventional SRM process

combustion will ensure low CO and formaldehyde modelled as occurring in two stages by two emissions. NOx emission will be greatly reduced different pathways, the first one involves the because of lower combustion temperatures. decomposition of methanol into CO and hydrogen Although the potential gain from (eqn. 26): implementation of methanol decomposition related technology is evident, successful development of CH 3OH ↔ CO + 2H 2 (26) efficient catalysts and reaction processes is crucial for the implementation of the technology. Cubased Followed by a water gas shift reaction (eqn. 27): catalysts such as Cu/Cr/Mn are active catalysts in 0 1 the decomposition of methanol to CO and H 2. CO + H 2O ↔ CO 2 + H 2 (H = ±41.2 kJmol ) (27) Cheng et al. [138] concluded that Cu/Crbased catalysts are much more active than the The second mechanism for methanol steam conventional Cu/Zn catalysts in methanol reforming consists of the reaction of water and decomposition. The acidic nature of the Cu/Cr methanol to CO 2 and hydrogen (eqn. 28): based catalysts, which leads to the decreased selectivity, can be greatly reduced by passivating CH 3OH + H 2O ↔ CO 2 + 3H 2 (H0 = ±49 kJmol1) the catalysts with alkali metal ions such as (28) potassium. which can be followed by a reverse shift reaction to 2.2.2.3 Reforming of alcohols establish the thermodynamic equilibrium (eqn. 29):

Alcohols and more specifically methanol are CO 2 + H 2 ↔ CO + H 2O (29) convenient storage systems for hydrogen [141]. These liquids can easily be transported through The advantage of SRM process is the amount of the existing infrastructure and have lesser CO generated along CO 2 is low. This is important constraints concerning safety than for instance since it acts as a poison for fuel cells. The accepted gaseous hydrogen storage. Alcohols transformation levels of CO are between 10 and 100 ppm [134]. into gaseous mixtures enriched with hydrogen can The main disadvantage of SR is its endothermicity be achieved by several reactions such as steam requiring external heating, which makes short reforming (SR) [142,20], partial oxidation (PO) startup and fast transient behaviour difficult to [135] and oxidative steam reforming (OSR) [143]. achieve [143]. The schematic flow sheet for a conventional SRM process is shown in Fig. 9. A. Methanol reforming Copperbased catalysts (CuMn 2O4 and CuCr 2O4 Catalytic reforming of methanol is a well spinels or CuO/CeO 2 and CuO/ZnO) exhibit a high established technology, mainly used for small activity and selectivity for the steam reforming hydrogen plants. This technology is promising for process. Small amounts of Cr 2O3 in skeletal copper energy feeding in portable electronic devices, for catalysts significantly enhance the copper surface decentralized refuelling units for hydrogenbased area and thus promoted the activities for SRM automobiles or as onboard generation systems for [142,145]. Supported CuCr on yttriadoped ceria hydrogenbased internal combustion engines or (YDC)/Al 2O3 showed the most pronounced PEM fuel cells [144]. Methanol is considered as an enhancement of the catalyst activity in the SRM at appropriate source of hydrogen due its safe reaction temperatures of 200250 0C, the CO handling, low cost and high storage density. concentration in the products was smaller than Moreover, it can be produced from renewable and 0.1% [146]. Valde´sSolı´s et al. [14] prepared nano fossil fuels [14]. sized CuCr 2O4 catalysts by nanocasting The steam reforming (SRM) of methanol can be techniques for the production of hydrogen by

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CH 3OH Partial oxidation, Preferential catalytic reforming Oxidation H2 0 H2O (250 C)

Air Air CO 2

Fig. 10. Flowsheet for a conventional OSRM process methanol steam reforming. They observed that the energy requirement is zero (∆H 0=0). The schematic activity values of the catalysts are similar to or flow sheet for a conventional OSRM process is greater than those found in the literature for shown in Fig. 10. common methanol steam reforming catalysts Horny et al. [151] developed a compact micro including noble metalbased catalysts. structured string reactor with catalytic brass wires Deactivation of the catalysts seems to be for OSRM to produce hydrogen in autothermal independent of the preparation method and can be mode for fuel cells. CuCr 2O4 catalyst used in attributed to the concomitant effect of coke OSRM showed high activity, and selectivity to deposition and sintering. carbon dioxide and hydrogen. The selectivity Partial oxidation of methanol (POM) is an towards carbon dioxide was 98% for a methanol exothermic reaction (eqn. 30) and can be used for conversion of 91.5% being higher than predicted for fast dynamic in methanol reforming [147]. the watergas shift equilibrium. The string reactor presents nearly isothermal profile. The reactor CH 3OH + 0.5O 2 = CO 2 +2H 2 ∆H 0 = 192 kJ mol 1 presents a short startup and a fast transient (30) behaviour showing a rapid temperature change when adjusting the oxygen amount introduced into But the formation of hotspots in the catalytic bed the reactor. is the main drawback, leading to catalyst sintering resulting in the activity loss [148]. Cubased catalysts have been the most generally studied B. Ethanol reforming catalysts for partial oxidation of methanol [149]. Hydrogen is essentially produced by steam Wang et al. [150] showed that Cu 60 Cr 40 catalyst reforming (SR) of hydrocarbon fractions (natural exhibits high CH 3OH conversion and H 2 selectivity gas, naphtha,) on an industrial scale. Replacing as compared with other binary catalysts and the fossil fuels by biofuels for H 2 production has introduction of Zn promoter not only helps to attracted much attention with an increased increase the activity of the catalyst but also interest for bioethanol reforming [152]. Reforming improves the stability of the catalyst, the highest of of ethanol is a potential way to generate renewable the activity of the ternary Cu/Cr/Zn is obtained hydrogen. Bioethanol can be easily produced in with a relative composition of Cu/Cr (6:4)/Zn (10%). renewable form from several biomass sources, Oxidative steam reforming of methanol (OSRM) including plants, waste materials from agro combined SRM and POM which gives fast industries (molasses, corn, bole, etc.) or forest dynamics and generates high hydrogen residue materials [153]. Moreover, a bioethanolto concentrations (eqn. 31). H2 system has the advantage of being CO 2 neutral [154]. Steam reforming, partial oxidation and auto CH 3OH + 0.75 H 2O + 0.125 O 2 → CO 2 + 2.75 H 2 thermal reforming of ethanol can be illustrated by ∆H 0 = 0 kJ mol 1 (31) eqns. 32, 33 and 35 respectively. Steam reforming is an endothermic process and Reactors for this process operate autothermally, requires energy input to initiate reactions [155]: i.e. it does not require any external heating or cooling once the operational temperature is C2H5OH + 3H 2O ↔ 2CO 2 + 6H 2 ∆H 0 = 174 kJ mol 1 reached. For fast transient response, the methanol/ (32) oxygen ratio can be varied as in the case of the Hot Spot reformer [151]. OSRM is combination of Alternatively, hydrogen can be obtained by partial steam reforming and partial oxidation processes. oxidation of ethanol at a temperature of about 500 Reactions are balanced in such a way that net

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0C according to the following reaction [156]: temperature rise of 810 0C per percent (wet gas) CO converted to CO 2 and H 2. Industrial WGS C2H5OH + 1.5O 2 = 2CO 2 +3H 2 ∆H 0 = 552 kJ mol 1 reaction is carried out using two different catalysts (33) in two stages (high temperature WGS and low temperature WGS) with interbed cooling [162] as However, hydrogen selectivity of ethanol partial shown in Fig. 11. oxidation is generally low. In order to enhance Recently, lowtemperature WGS reaction has hydrogen production, autothermal reforming can attracted renewed interest due to the increasing be applied. Autothermal reforming, also called demands for highpurity hydrogen for application oxidative steam reforming, is a combination of in fuelcell systems [162]. Cubased spineltype ethanol oxidation and steam reforming. The total oxides [163,1415] have received much attention in reaction of autothermal reforming of ethanol can the last years owing to their high catalytic activity be written as eqn. 34 [157,158]: and durability at low temperatures to that of conventional, Cu/ZnO/Al 2O3 for WGS reaction. C2H5OH + 2.25 H 2O + 0.375 O 2 = 2CO 2 + 5.25 H 2 Boumaza et al. [15] prepared a series of AB 2O4 ∆H 0 = 30 kJ mol 1 (34) spinel catalysts by the coprecipitation method where B is commonly Al, Zn, Mn, Co, Cr and Fe. This reaction indicates that the autothermal The catalysts were tested in the water gas shift reforming not only attains thermally sustained reaction at atmospheric pressure. They ranked the operation, but also maximizes hydrogen oxide systems in the following order follows: Cu production. Cr>CuFe>>ZnAl>CuCo at 250 0C. Significant Dolgykh et al. [159] investigated the catalytic improvements to the activity of skeletal copper activity of copper chromite catalysts in the process catalysts for the water gas shift reaction were of ethanol steam reforming at low temperatures achieved by adding small amounts of Cr 2O3 to the using 12 wt.% C 2H5OH in water mixtures. They surface of copper [145]. found that without catalyst appreciable hydrogen production starts at the temperatures above 300 0C 2.3 Clean energy production and reaches hydrogen yield 0.3 g Н2 (kg cat) 1 (h) 1 In the last few years new extremely stringent at 400 0C. Use of catalysts allows reaching the standards in emission legislation have required the hydrogen productivity 925 g Н2/(kg cat.)/(h) at 250 development of new technologies for both clean 300 0C. energy production and emission control, either for

industrial or for household appliances. Among the 2.2.3 Water gas shift reaction processes of energy production, high temperature The production of hydrogen mainly relies on the combustion of fossil fuels is the most liquid fuels reforming processes. Reformate gas conventionally used [164165]. During combustion, (CO/H2 mixture) contains significant amount of the formation of pollutants, in particular NO x, is CO (1015%) [160162]. Water gas shift (WGS) favoured due to the high temperatures attained reaction (eqn. 35) is used to enrich hydrogen in the [164,166]. Catalysis can offer the possibility to reformate stream and decrease the CO content. realize clean combustion in an interval of The water gas shift reaction is presented by: temperature in which the formation of given pollutants, e.g. NO x, is depressed [164,165,167]. CO + H 2O ↔ CO 2 + H 2 ∆H = 41.1 kJ/mol (35) The choice of the appropriate catalyst is a fundamental step for improving the combustion, in is mildly exothermic, with an adiabatic terms of both activity and selectivity, limiting the formation of hazardous byproducts [164,165]. The performances of Pt or Pd with respect to hydrocarbons combustion, in particular methane, have been largely studied by many authors [168,169]. The use of noble metals has stringent effects on the commercial cost of the whole catalyst, so that a great interest has turned to oxidebased catalysts [170]. Various oxides containing Co, Cr, Mn, Fe, Cu, and Ni are Fig. 11. Water gas shift reaction process potentially interesting for catalytic combustion

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appliances [171]. Mixed oxide compositions show CO + 2H 2 → CH 3OH (36) better activity than the single oxide ones [164]. The copper chromite catalyst offered interesting The stability of Cu/ZnO catalysts has been performances for CH 4 combustion [170] in view of discussed in view of the sintering of Cu via brass such application. Comino et al. [170] studied formation [175178]. It is suggested that the complete oxidation of methane over two sintering of Cu via brass formation could be unsupported over copper chromite (CuOCr 2O3) prevented in CuZnOCr 2O3 catalysts obtained and aluminasupported one (CuOCr 2O3/Al 2O3). from CuZnCr hydrotalcitelike precursor [179]. The unsupported catalyst showed better activity Venugopal et al. [180] synthesized a series of Cu while supported one was very stable. Kinetic study ZnOCr 2O3Al 2O3 (Zn/Cr = 1.26.8) catalysts using provided a value of the activation energy about110 hydrotalcite like layered double hydroxide kJ/mol. precursors for conversions of synthesis gas to To increase the combustion efficiency of dimethyl ether in a single step process. Single step gasoline, several additives such as antioxidants, dimethyl ether (DME) synthesis from syngas has oxygenates and other functionalities are added. technological advantages over the two step process Most prominent additives among them are of methanol synthesis (eqn. 37) and dehydration of oxygenates, which are a class of oxygen containing the methanol (eqn. 37). compounds used as blending components to improve the combustion efficiency of the fuel [172]. Blending of oxygenates with the gasoline results in 2CH 3OH → CH 3OCH 3 + H 2O (37) complete combustion, thereby reducing the CO and hydrocarbon (HC) emissions to a large extent The advantages being high onethrough under full load operating conditions [172]. Alcohols conversion in thermodynamic point of view [181] such as ethanol and methanol were recognized as and low operating cost in the economic point of octane boosters in 1920. Tertiary alkyl ether based view [182]. Venugopal et al. [180] concluded that oxygenates such as methyl tertiary butyl ether the high syngas conversion over Cu:Zn:Cr = (MTBE), ethyl tertiary butyl ether (ETBE), tertiary 7:6:1.88 (mol ratios) catalyst was attributed to the amyl ethyl ether (TAEE), tertiary amyl methyl high copper metal surface area as compared to the ether (TAME), isopropyl tertiary butyl ether other catalysts and higher yields of DME was (IPTBE) and diisopropyl ether (DIPE) are other obtained in single step over a CuZnCr (Zn/Cr = possible candidates for gasoline blending [173]. 3.3) with γAl 2O3. Methyltertbutylether or isooctane (MTBE) is a Ohyama and Kishida [183] prepared slurry clean energy resource and an alternative fuel. The phase methanol via methyl formate at 100 0C and selective synthesis of isobutyl alcohol (iBuOH) has an initial pressure of 5 MPa using alkoxide and a recently gained an increasing interest because this physical mixture of CuO and Cr 2O3 as a catalyst. substrate is a potential precursor of gasoline Methanol synthesis is a highly exothermic reaction additives such as methyltertbutylether or and thermodynamically favourable at lower isooctane. iBuOH may be directly synthesized from temperatures. Thus, if a catalyst which is highly syngas through the highermolecularweight active at low temperatures is available and the alcohols synthesis carried out at high temperature heat of reaction is efficiently removed, methanol and pressure over copper chromite catalysts [174]. production with high perpass conversion could be Another most promising clean energy source is achieved. Slurryphase methanol synthesis using methanol. Methanol decomposition or reforming alkali metal alkoxide and copperbased compound can provide clean fuel for fuel cells, automobiles, as a catalyst has been investigated by several power generation, chemical processes and material researchers [184188]. In this system, methanol is processing (see section 2.3.1 and 2.3.3.1). The use considered to be formed through the formation of of methanol as a fuel additive and in MTBE methyl formate by the following two reactions production has renewed interest in the search for (eqns. 38 and 39) which occur concurrently: improved methanol processes. CH 3OH + CO → HCOOCH 3 (38) 2.3.1 Methanol synthesis HCOOCH 3 + 2H 2 → 2CH 3OH (39)

Commercially methanol is synthesized from Here, the alkali metal alkoxide is a wellknown synthesisgas using Cu/ZnO/Al 2O3 catalysts (eqn. catalyst for carbonylation of methanol to methyl 36) [164]: formate (reaction 3.2) [189190], whereas copper

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based materials such as copper chromite reported to function as catalysts in the hydrogenolysis of methyl formate to methanol (reaction 3.3). Therefore, employing a mixture of both catalysts, methanol can be produced in a single reactor from H2 and CO according to the above sequence of reactions [190195]. Ohyama [196] reported excellent activities of copper chromite catalyst for the production of methanol in liquid phase in Fig.12. Flow diagram of H 2 purification by CO presence of potassium methoxide, which involves PROX carbonylation of methanol to methyl formate and consecutive hydrogenation of methyl formate to methanol. The effects of reaction variables on the catalytic performance are investigated under the direct catalytic conversion in the vapour phase at conditions of 373423K temperature and 1.5 low pressure via vapour cracking and reforming. 5.0Mpa pressure. Huang and Wainwright [145] Incorporating a catalyst into the pyrolysis unit is observed significant improvements to the activity expected to enhance deoxygenation, cracking and of skeletal copper catalysts for the methanol reforming reactions. The selection of appropriate synthesis achieved by adding small amounts of catalysts plays a vital part in biooil upgrading. Cr 2O3 to the surface of copper. Slurry phase The severity of these catalytic reactions often concurrent synthesis of methanol has also been influences the liquid product distribution and described by Palekar [192] using a potassium depends largely on types of catalyst and other methoxide/copper chromite mixed catalyst which processing parameters. Pattiya et al. [198] used operates under relatively mild conditions (100180 copper chromite catalyst for cassava rhizome as 0C, 3065 atm). the biomass feedstock for upgrading biooil. They observed that the copper chromite was selective to 2.3.2 Fast pyrolysis of biomass the reduction of most oxygenated lignin Biomass, a form of renewable sources, can be derivatives. transformed via thermochemical processing such as fast pyrolysis into liquid biooil, which is a 2.3.3 Preferential oxidation of CO storable and transportable fuel as well as a The preferential oxidation (PROX) process is potential source of a number of valuable chemicals one of the most effective methods for the removal that offer the attraction of much higher added of CO trace from the reformate stream. PROX of value than fuels. Biooil is successfully used as CO is a reaction to convert CO in a H 2rich gas boiler fuel and also showed promise in diesel mixture to CO 2 with minimal H 2 consumption. engine and gas turbine applications. Upgrading bio Therefore, preferential oxidation process is an oil to a quality of transport liquid fuel still poses indispensable step to reduce the concentration of several technical challenges and is not currently CO to 10 ppm level in a H 2 generation process economically attractive. Some chemicals, especially [199]. The flow diagram of hydrogen purification by those produced from the whole biooil or its major COPROX is shown in Fig. 12. fractions offer more interesting commercial The following reactions (eqns. 40 and 41) can opportunities. The main properties and occur in the PROX system. applications of biooil have been reviewed by Czernik and Bridgwater [197]. CO + ½ O 2 → CO 2 ∆H 0 = 282.98 kJ/mol (40) Biooils are complex hydrocarbon mixtures known to contain significant amount of oxygenated H2 + ½ O 2 → H 2O ∆H0 = 241.82 kJ/mol (41) compounds including lignin derivates which ultimately lead to low heating values, low CO is a typical byproduct in the production of stabilities, high viscosity, low volatility and low hydrogen by reforming of alcohols or hydrocarbons pH. Therefore, in order to improve the quality of and must be reduced down to ppm levels in order the biooils in terms of heating values, viscosity to be used as feed for protonexchange membrane and storage stability, the oxygenates and the large fuel cells. Up to date, several options for CO molecules derived from lignin need to be reformed removal have been studied and the selective into more useful products. Catalytic pyrolysis is a oxidation (COPROX) is considered one of the most promising approach for upgrading biooil involves

Bulletin of Chemical Reaction Engineering & Catalysis, 6 (2), 2011, 79 straightforward and costeffective methods to spark ignited engine [207] and catalytic filter for achieve acceptable CO concentrations (eqn. 42). diesel engine [41] exhaust have been documented.

H2 + CO + O 2 (air) → CO 2 + H 2 (42) 2.4.1 Purification of spark ignited engine exhaust Many industrial applications using H 2 as a Automotive catalytic converters having TWCs feedstock require a H 2 stream containing a little consisting of noble metals (Pt, Pd, Rh) are used for CO. These include ammonium synthesis, fuel cell purification of exhaust gases of gasoline driven and semiconductor processing etc. because CO is a vehicles [208]. Catalysts simultaneous carry out poison in these processes. Cu/Cr/Ba based catalysts both reduction and oxidation reactions that convert were found to be nonprecious metal catalysts that exhaust pollutant to normal atmospheric gases. can selectively oxidize CO in a H 2containing The reactions can be presented by following eqns. stream [200]. 44, 45 and 46:

The oxidation of CO and HC : 2.4 Vehicular Pollution control

Vehicles play a very important role in the 2CO + O 2 → 2CO 2 (44) development of a nation since they are widely used HC + O 2 → CO 2 + H 2O (45) to transport goods, services and people. But there The reduction of NOx with CO : is a major negative effect of the emissions of pollutants from the exhaust of the vehicle engines NO x + xCO → 1/2N 2 + xCO 2 (46) [201]. The complex reactions occurring in the internal combustion engine can be represented by Because of the abatement of all the three eqn. 43: primary pollutants catalyst used is known as TWC. Although noble metalbased catalysts have Fuel (HC) + Air (O 2+N 2) → CO 2 + H 2O + N 2 + O 2 + dominated this area, efforts were always put in CO + HC (unburned) + NO x (43) towards development of low cost nonnoble metal based catalysts [209] and modification of TWC to Engine exhausts consist of a complex mixture, decrease precious metal percentages by means of the composition depending on a variety of factors nanotechnologies [210] and in combination with such as: type of engine (two or fourstroke, spark base metals [211]. Among such catalysts copper or compression ignited), driving conditions, e.g. chromite has been observed to be an attractive urban or extraurban, vehicle speed, acceleration/ alternative to presently used noble metals in view deceleration, etc. Table 1 reports typical of availability, lower cost and comparable activity compositions of exhaust gases for some common [212216,9] for purification of vehicular exhaust. engine types [202]. CuCr catalyst system has been studied It is clear from the table 1 that there are three extensively for reactions such as the oxidation of main primary pollutants, CO, HC and NO x from CO [217], hydrocarbons [213], alcohols and the spark ignited engines while NO x and aldehydes [218], sulfurated hydrocarbons [219], particulate matter (PM) are the major composition and chlorinated hydrocarbons [220] as well as NO of the diesel engine exhausts. Because of the large reduction [214]. Kapteijn et al. [214] reported that vehicle population, significant amounts of HC, CO, CuCr catalysts showed higher CO oxidation and NO x and PM are emitted in the atmosphere. These NO reduction activity than single oxide catalysts pollutants have harmful effects [203205] on living based on Cu, Ni, Co, Fe, Mn, Cr. Stegenga et al. beings, materials, environment and climate [212] have found that a monolithsupported 10 change. Morover once emitted into the atmosphere, wt% Cu.Cr/Al 2O3 catalyst showed a threeway primary pollutants could be the precursors for the catalytic activity comparable to that of noble metal formation of more dangerous secondary pollutants catalysts operated under the same conditions. such as smog, aerosol, ground level ozone, peroxy acetylnitrate (PAN), polycyclic aromatic 2.4.1.1 Oxidation of carbon monoxide hydrocarbon (PAH) etc. by photochemical reactions Carbon monoxide (CO) can prove perilous [206]. without giving any physical indication because it is The catalytic control of these pollutants has a colorless, tasteless and odourless gas. It is been proposed as an end of the pipe treatment chemically inert under normal conditions with technology. The benefits of catalytic purification of estimated atmospheric mean life of about 2.5 primary pollutants to atmospheric gases (CO 2, N 2, months [221]. This, poisonous gas can seriously O2 and H 2O) using three way catalyst (TWC) for

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Table 1. Exam ple of exhaust conditions for two and fourstroke, diesel and leanfourstroke engines [202]

Exhaust components Diesel engine Fourstroke spark Fourstroke leanburn Twostroke spark a and conditions ignitedengine spark ignitedengine ignitedengine NO x 3501000 ppm 1004000 ppm ≈1200 ppm 100200 ppm HC 50330 ppmC 5005000 ppmC ≈1300 ppm C 20,00030,000 ppmC CO 3001200 ppm 0.16% ≈1300 ppm 13%

O2 1015% 0.22% 412% 0.22%

H2O 1.47% 1012% 12% 1012%

CO 2 7% 1013.5% 11% 1013% b SO x 10–100 ppm 1560 ppm 20 ppm ≈20 ppm

PM 65 mg/m 3

0 0 0 c 0 0 Temperatures r.t.650 C(r.t.–420 C) r.t.1100 C r.t.850 C r.t.1000 C (test cycle) GHSV (h −1 ) 30,000–100,000 30,000–100,000 30,000–100,000 30,000–100,000 d e λ (A/F) ≈1.8 (26) ≈1 (14.7) ≈1.16 (17) ≈1 (14.7)

a N2 is remainder. b For comparison: diesel fuels with 500 ppm of sulphur produce about 20 ppm of SO 2. c Closecoupled catalyst. d λ defined as ratio of actual A/F to stoichiometric A/F, λ = 1 at stoichiometry (A/F = 14 .7). e Part of the fuel is employed for scavenging of the exhaust, which does not allow to define a precise definition of the A/F.

affect human aerobic metabolism because of its noble metal catalysts and exhibits comparable affinity to haemoglobin being 210 times greater activity for CO oxidation to that of precious metals than that of oxygen and so, if its concentration based auto exhaust purification catalyst [224]. The becomes higher than 0.0998% by volume in catalyst converts CO to harmless product CO 2, atmosphere, it can lead to the failure of the process found in the atmosphere which is useful to the of combination of oxygen with the haemoglobin plants. Copper chromite catalysts have been resulting ultimately in respiratory failure and studied in detail for CO oxidation. The studies consequent death. It forms a very stable compound have been attributed to the catalytic activity and called carboxyhaemoglobin (COHb) in blood whose stability tests [225,226], postulating mechanisms 12% concentration can cause headache, fatigue, [227], characterization of catalysts [228,229], rapid drowsiness and may have effect on human evaluation of the catalysts [230], search for the behavioural performance, 25% concentration may active sites on the catalysts [231,232], kinetics of cause impairment of time interval discrimination, the reaction [228,233], catalysts deactivation visual acuity, brightness discrimination and [229,234], etc. Pantaleo et al. [235] reported the certain other psychomotor functions which may catalytic tests in CO oxidation indicated a lead to accident on roads. 510% concentration can synergetic effect between copper and chromium in lead to changes in cardiac and pulmonary the mixed oxides supported on silica and calcined functions and 1080% can cause coma, respiratory at 500 0C. Xavier et al [236] found that CuCr failure and death [222]. It not only affects human based catalysts are effective for CO oxidation at beings but also vegetations by interfering with lower temperatures. Park and Ledford [228] plant respiration and nitrogen fixation. The reported that the Cu/Cr/Al 2O3 catalyst of Cr/Al = vehicle emission is the major source of CO in urban 0.054 showed the highest CO oxidation activity due air, accounting for slightly over half of all the to the formation of CuCr 2O4 which was more active anthropogenic air pollutants. than the CuO phase. Higher loadings yielded The catalytic oxidation of carbon monoxide with higher activities per amount of catalyst, but above the object of reducing air pollution is actually an 10 wt.% Cu+Cr the intrinsic activity per metal important consideration when one thinks in terms atom decreased. Li et al. [229] synthesized of automobile emission control [223]. Copper monodispersed spherical CuCr 2 O 4 .CuO chromite is found to be most promising among non

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encapsulated Bi 2O3 nanoparticles with a automotive exhaust purification catalysts. Shelef homogeneous core/shell structure by using et al. [236] have documented the effects of various monodispersed Bi 2O3 nanospheres as templates poisons on the durability of various catalysts. with a precoating of NH 4+ on the surfaces of Bi 2O3 Mechanisms of catalyst deactivation have been nanoparticles. The prepared core/shell nano reviewed by Bartholomew [237]. SO 2 can be severe particles exhibit promising catalytic activities inhibitor for the oxidation of CO and hydrocarbons towards the oxidation of CO. Therefore, the Bi 2O3/ [238] in the temperature range (400600 0C) of use CuCr 2O4CuO core/shell nanomaterials provide of base metal catalysts in automobiles. Kim et al. promising for pollution treatment in car exhaust [239] studied the deactivation of supported copper purification. chromite catalyst by SO 2 and water vapour and Prereduced copper chromite catalysts have found that the degree of deactivation of the been shown to be more active than unreduced ones catalyst due to SO 2 was more severe than water [232]. Prereduction with H 2 produced higher vapour. Lauder [240] reported that small amounts activity than prereduction with CO, which can be of noble metals present in base metal perovskites attributed to a higher surface concentration of significantly improve their resistance to poison. active species after the former treatment. Activity General Motors company preferred copper results, together with TPR and XPS chromite catalyst for exhaust emission control in characterization point out that both Cu 0 and Cu + the beginning due to its much lower cost in species are the primary active sites of CO oxidation comparision to platinum [208]. It was later shown while surrounding Cr 2O3 phase can prevent Cu 0 to have inferior activity and was susceptible and Cu + from being oxidized to Cu(II), which will poisoning by sulphur present in the fuel. By the limit the catalytic activity. The activated carbon time catalysts were introduced in 1974, all car impregnated with Cu/Cr/Ag is a good catalyst for companies had adopted platinum based systems CO oxidation at low temperature (<70 0C) [234]. for emission control. Stegenga et al. [235] reported that CuCr oxide The main reason for withdrawal of base metals based converters are superior to precious metal from catalytic converter formulations was their based device for CO oxidation. undesirable behavior in cycled transient conditions Hertl and Farrauto [227] employing infrared and their high susceptibility to deactivation by spectroscopic, gravimetric, and kinetic techniques lead, sulphur and water [207]. The huge decrease studied the mechanism of oxidation of CO on a in sulphur content in the lead free fuels and copper chromite catalyst. They showed that surface progress in oxide catalyst synthesis, have allowed species involved in reaction are either a copper copper chromite to be reconsidered for practical carbonyl, at temperatures above 80 0C or a applications [241] in the catalytic converter. carbonate associated with chromium at higher temperatures above 200 0C. A thorough kinetic 2.4.1.2 Oxidation of hydrocarbon study [228] of the two reactions, including the Hydrocarbons such as benzene, formaldehyde, phenomena of N 2O formation and the reduction of hazardous polycyclic aromatic hydrocarbons N2O with CO, provided a kinetic model that (PAHs), and nitropolycyclic aromatic hydrocarbons accounts for the observed reaction behaviour of the (NPAHs), methane, butane, hexane, soot, propene, catalyst, and which can serve to model a converter decane, toluene etc. are mainly originated from based on the Cu/Cr catalyst. The experimental imperfect combustion of fossil fuels such as results indicate that these oxidic Cu/Cr catalysts petroleum and coal [242]. (PAHs) and (NPAHs) have a three way performance and that they may have been identified as potent mutagens and find application in the purification of exhaust gases possible carcinogens. Both PAHs having 4 rings or of low sulphur contents. Prasad and Rattan [233] more and NPAHs are detected in particulates studied kinetics of oxidation of CO over a novel exhausted from diesel and gasolineengine copper chromite catalyst in a compact bench scale vehicles, while PAHs of lower molecular weights (3 reactor. Preparation details of the catalyst are rings or below) were detected in the gaseous phase given elsewhere [232]. On the basis of the in the atmosphere and unburned diesel fuel [243 experimental findings they proposed the following 245]. Copper chrmomite is the active catalyst for empirical rate expression (eqn. 47): the pyridine oxidation and NO x control [246].

Methane is a potent greenhouse gas. It has a Rate = 2.02x10 4 exp(12290/RT)(C CO )0.7 (47) global warming potential (GWP) 23 times greater

than carbon dioxide. Reducing methane emissions Phosphorous and sulphor originating from fuel would lead to substantial economic and and engine oil may lead to degradation of

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environmental benefits. Methane is emitted from mixed SnCuCrO catalysts, exhibit comparable a variety of anthropogenic activities, such as fossil activity to conventional platinum catalysts for CO fuel production, biomass combustion, operation of and hydrocarbon oxidation. natural gas vehicles (NGVs), and waste management [247]. For example, in mine 2.4.1.3 NO X abatement ventilation air, the methane concentration is 0.1 Oxides of nitrogen (NO x) consist of N 2O, NO and 1vol% [248]. In naturalgasfuelled vehicles, which NO 2 pollutants which are emitted from both are increasingly popular in some countries because natural and anthropogenic sources in large of their low emissions of NO x and particulates [249 quantities. Enormous concern over the role of NO x, 251], the unburned methane concentration in the as one of the primary pollutants for the green exhaust is around 0.3 vol.% [252]. It is estimated house effect, on ozone formation and depletion in that 60% of global methane emissions are related stratosphere, on formation of photochemical smog, to humanrelated activities [253]. Catalytic and acidic precipitation has received increasing combustion (eqn. 48) is considered to be an attention since 1970s because of its detrimental effective approach to methane emissions effect on life on earth in general [258]. If NO x is abatement [254]. prevented from entering the atmosphere, most of

the downstream effects of pollution can be CH 4 + 2O 2→CO 2 + 2H 2O (48) eliminated. Copper chromite has been recognised

as the active catalyst for the NO x pollution control Hui et al. [255] showed that multitransition [259,260]. metal(Cu, Cr, Ni, and Co)ionexchanged zeolite Nitrous oxide (N 2O) is a compound that during 13X catalysts outperformed singleionexchanged the last decade has been recognized as a potential and acidified 13X catalysts and that lengthening contributor to the destruction of the ozone in the the residence time led up to 100% conversion at a stratosphere and acknowledged as a relatively relatively low temperature of 500 0C. The enhanced strong greenhouse gas [261,262]. The continuous catalytic activity in the multiionexchanged increase of its concentration, both due to natural catalysts was attributed to the presence of and anthropogenic sources (adipic acid production, exchanged transition ions instead of acid sites in nitric acid production, fossil fuels, biomass the catalyst. burning) and longer atmospheric residence time Comino et al. [170] found higher activity of (150 years), entails the need of developing efficient CuCr 2O4 for CH 4 combustion in the temperature catalysts for its decomposition (into nitrogen and range 300700 0C at constant CH 4/air ratio of 1:30 oxygen). In recent years, spineltype oxides such as and constant methane content, 1.2%. Ismagilov et CuCr 2O4 have been the subject of increasing al. [256] reported higher catalytic activity of CuCr fundamental and applied research because of their O catalyst for complete oxidation of nbutane in a good stability and intrinsic catalytic activity [259]. microchannel reactor. According to Solovev [41] The two most toxic oxide gases of nitrogen are NO the catalysts for nhexane conversion are arranged and NO 2 of which NO 2 is known to be more toxic. in the order of activities are expressed as follows: NO x are These oxides produced by the high CuCr 2O4 > CuCo 3O5 > CuMnO 3 > Co 3MnO 6. Chien temperature combustion of fuels that use air as an et al. [213] repoted that the CuCr/γAl 2O3 catalyst oxidant, through endothermic oxidation of N 2 by is highly active for oxidation of CO as well as for O2. Automotive vehicles as well as industries are propene also (eqn. 49). also the major sources of NO x emission [263,264].

NO plays a major role in the photochemistry of 2C 3H6 + 9O 2 → 6CO 2 + 6H 2O (49) the troposphere and the stratosphere [265]. The

photochemical complex HCNO xOx is formed The activity of propene oxidation over CuCr/γ during the HC interactions in the photolytic cycle Al 2O3 catalysts increases with increasing heat of NO; the mixture of products generated is called treatment temperature in an oxidizing “photochemical smog” and contains O 3, CO, atmosphere. The activity of propene oxidation on peroxyacetyl nitrates (PAN), alkyl nitrates, the CuO catalyst reaches a maximum at a copper ketones, etc [265]. The chemical depletion of ozone, content of about 10 wt.% and that on the Cr 2O3 in an important part due to nitrogen oxide species, catalyst reaches a maximum at a chromium is a prolonged phenomenon [266]. Carcinogenic content of about 25 wt.%. Harrison et al [257] products are also formed during these reactions. studied the preparation, characterization, and NO 2 is linked to causing bronchitis, pneumonia, catalytic activity of Cr(VI)and Cu(II)doped tin(IV) oxide catalysts. The catalysts, particularly the

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susceptibility to viral infection, and alterations to and Health Administration Permissible Exposure the immune system [267]. It also contributes to Level (OSHA PEL) for toluene is 100 ppm in the acid rain, urban smog, and ozone [268]. Therefore, workplace [277]. Therefore, the proper control, it is important to control the emission of such gases removal, and reduction of the emission of VOCs are to comply with stringent emission standards. The the crucial tasks for the protection of the three way catalytic (TWC) reduction of NO in the environment. presence of CO, HC and/or hydrogen is typical of Catalytic combustion is a promising way to automotive pollution control [265]. Shelef and control the emissions of the VOCs [278]. Catalytic Gandhi [269] first investigated the reduction of NO combustion processes (Fig.13), typically achieve with hydrogen on copper chromite catalysts. NO complete destruction at temperatures around 350 reduction by CO has been examined over 500 0C, which are much lower than those used in supported CuCr systems by sevaral workers thermal combustion (8001200 0C) [279]. The [209,235,270]. Lee at al. [271] observed maximum lowering of reaction temperatures leads to several activity for NO decomposition over coimpregnated advantages in terms of environmental impact as copper chromite on mordenites at 450 0C. For the well as energy saving: very low production of NO + CO reaction, the catalytic performance of secondary pollutants (i.e., NO x and micropollutants coppersupported catalyst was significantly such as dioxins from chlorinated compounds) and affected by the addition of Cr. The CuCr system reduction of fuel costs [275]. Moreover catalytic exhibited an overall better performance than the combustion exhibits advantages such as high single metal catalytic system (Cu, Co, and Ni) and efficiency, ultra low pollutant emissions, stable threeway catalytic converter [209,235]. The combustion, and so on [280]. The coupling of a low characteristics of CO and NO molecules at Cu 2+ temperature oxidation catalyst with an improved and Cr 3+ ion sites on the CuCr 2O4 (100) surface combustion technology can lead to further decrease have been studied by Xu et al. [272]. They found of reaction temperatures at which the complete that CO/NO mixture adsorb selectively at the Cu 2+ destruction of VOCs is obtained [280]. ion site and simultaneously at the Cr 3+ ion site, Among VOC, ethyl acetate is a key component respectively. Stegenga et al. [235] investigated a Zr in ink manufacture. The catalytic combustion of stabilized Degussa monolith prepared by using ethyl acetate has been investigated by Mazzocchia washcoats corresponding to 10 wt% CuCr loading and Kaddouri [9] using undoped and Mn or Ba on Al 2O3 or 2.5 wt% Lastabilized Al 2O3. These doped copper chromite catalysts. Good conversion monoliths had behaviour typical of threeway reported even at low temperatures (180240 0C). catalysts. While in the presence of molecular oxygen the only product observed was CO 2, when the reaction has 2.4.1.4 Volatile organic compounds abatement been carried out in the absence of oxygen (with lattice oxygen alone) noticeable amounts of CO Volatile organic compounds (VOCs) are the were also detected [281]. The synergic effects of the carbon containing compounds which have combustion catalyst BaCuOCr 2O3/Al 203 and significant vaporization due to high vapour ozone, used as strong oxidant species in the pressure at ambient conditions [273]. Being combustion of various VOCs such as, acrylonitrile, volatile they enter the atmosphere and are harmful to human beings and environment because of their toxicity and malodorous nature. These are emitted from many industrial processes, transportation and household activities and are considered as an important class of air pollutants [274]. Many VOCs have been proved to be carcinogenic and mutagenic and contribute to stratospheric ozone depletion [275]. Moreover, once emitted into the atmosphere, VOCs could be the precursors for the formation of secondary pollutants such as smog, aerosol, ground level ozone, peroxyacetylnitrate (PAN), and polycyclic aromatic hydrocarbon (PAH) etc. by photochemical reaction [276]. Because of these negative effects, legislation has been introduced in Fig. 13. Schematic drawing of catalytic oxidation many countries setting very low emission limits for of VOCs VOCs in process exhaust e.g. Occupational Safety

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methanol, styrene, toluene and 1,2dichloroethane butanal and methyl mercaptan [100 ppm in air]. [278] . The life of copper chromite could be extended by With the Clean Air Act amendments requiring washing after partial deactivation by retained increased use of oxygenated compounds such as sulphur. Odour removal efficiencies (measured by alcohols and ethers in motor fuels, the problem of olfactometry in terms of dilution to detection effectively controlling the emissions caused by threshold values) at 340415 0C was found mostly burning these substances has become a more in the range 98.599.5%. pressing issue. Alcohols, both ethanol and Organic Ion Exchange (IX) resins used to purify methanol, are currently in use. Supported catalysts primary coolant of nuclear power plants contain containing mixed oxides of copper and chromium high 137 Cesium and 60 Cobalt radioactivities. prepared by a wet impregnation technique and Direct fixation of spent IX resin wastes in polymer supported on γalumina shows activity for complete or slag cement matrices leads to increase in waste oxidation of ethanol, acetaldehyde, and methanol form volumes. For organic wastes, incineration ethanol mixture [213]. appears to be most logical route to reduce waste Driven by the need for decreasing form volumes. Pyrolysis of IX resins leads to manufacturing cost and increasing resistance to release of radioactivity, SO 2, styrene, toluene, ethyl poisoning of commercial catalysts for volatile benzene and trimethyl amine to the off gases. The organic compounds (VOCs) elimination, efforts organic matter in off gases could be successfully have been made to develop metal oxide catalysts, oxidised by using CuO.CuCr 2O4 catalysts at which can exhibit activity similar or higher than of temperature above 475 0C and 22500 hr1 space noble metal catalysts [281]. Among these catalysts velocity [283]. the copperchromite seems to be promising for the VOCs total combustion and it was also 2.4.2 Diesel exhaust gas purification comparatively tested with noble metal catalysts It is expected that the world market for diesel [278,281]. Ismagilov et al. [256] fabricated engines will grow significantly in the near future microreactors with alumina coating impregnated because of their superior thermal efficiency, with an aqueous solution of copper dichromate durability, and reliability in comparison to the followed by drying and calcination at 450 0C to gasolinepowered engine. Exhausts of gasoline produce active copper chromite catalysts. They fueled engines operated near stoichiometric air/fuel studied the effect of a copper dichromate ratio have been successfully cleaned up by three concentration, number of impregnation cycles (1 or way catalytic systems, while the pollution by diesel 2), and different aftertreatments on catalytic engine exhausts has become more and more activity and stability in complete oxidation of n serious in the last decade. For diesel emission butane. They observed that even at much lower control, three possibilities, i.e., oxidation, soot loadings of active metals, the catalytic activity of trapping and NO x reduction, have been the prepared coatings, related to the volume of the investigated [284]. Nitrogen oxides (NO x) and alumina layer, is superior to that of pelletized particulate matters (PM) are main objectives to be catalysts. They concluded that the microreactors removed from diesel exhausts. Since made according to their procedures can be used for concentrations of CO and hydrocarbons are lower a variety of smallscale total oxidation processes and that of O2 is higher in diesel exhausts than in involving combustion of toxic and hazardous gasolineengine exhausts, threeway catalysts chemicals. cannot remove NO x from the diesel exhausts [285]. The industrial formaldehyde production all over The everincreasingly stringent exhaust the world produce huge amount of dusts and emissions legislation requires an everincreasing exhaust gases, which contains CO, dimethyl ether degree of efficiency of a catalytic converter. and methanol as main components. Therefore, CuCr 2O4 catalysts are found active for their complete oxidation is of high importance for simultaneous catalytic removal of NO and diesel environmental protection. CherkezovaZheleva et soot particulates [286]. Catalytic performance of al. [282] reported total oxidation of CO, dimethyl CuCr 2O4 for the simultaneous NO xsoot removal ether and methanol over copper chromite catalysts reaction was investigated by Teraoka and Kagawa supported on γAl 2O3. Copper chromite catalysts [285]. Selective catalytic reduction of NO x with were evaluated by Heyes et al. [214] for the control hydrocarbons (SCRHC) has the potential in of organic air pollutants responsible for eliminating NOx emission from the oxygen rich malodorous process emissions. In laboratory tests, (lean conditions) exhaust. CuCr 2O4 is identified as catalyst shows the ability to achieve and maintain one such catalyst active for SCRHC [287]. The high efficiency for the complete oxidation of n

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conversion of NO over CuCr/ γAl 2O3 catalyst in the energy sources, such as coal, new sources, as well SCR of NO by NH 3 in the presence of sulfur dioxide as improvement of the efficiency of energy was reported to be very high (95100%) [288]. production technologies. However, the growth in Recent studies have demonstrated a positive energy production need not take place to the effect on the reduction of diesel emissions by detriment of the environment, so that strict adding oxygenated compounds [289]. Light regulations on pollutant emissions have been catalytic gas oil (LCO) hydrotreating was imposed around the world. An integrated performed by Tailleur and Caris [290] on a gasification combined cycle (IGCC) systems are laboratoryscale trickle bed reactor to obtain a 15 being advanced today, due to their higher thermal ppm sulfur fuel. The fuel was then selectively efficiency and superior environmental performance oxidized using a CuCr/IP(4PVP) catalyst in air at and economics, compared to conventional plants different operating conditions on a laboratoryscale [293]. The development of these systems depends continuously stirred tank reactor. The oxygenated on the ability to remove sulphur compounds, and polyoxygenated compounds formed were mainly H 2S, from the coal gas. During gasification measured and fuel stability was also measured. process, about 95% of the sulphur contained in the The results show a decrease in emissions by low coal is turned into H 2S and rest of them are sulfur diesel oxidation, as well as the benefits of converted into SO 2 [294]. The produced H 2S should having a high selectivity toward ketone formation be lowered from the typical gasifier output of 5000 when using a CuCr/IP(4PVP) catalyst [290]. ppm to the tolerable limit of 150 ppm [295]. H 2S being a very corrosive gas, could be damaging to 2.4.2.1 Diesel soot oxidation the mechanical parts and the construction material at high temperatures and pressures under IGCC The diesel engines find widespread applications system conditions. as power source in both automotive and stationary Although thermal efficiencies of 3035% are applications but their emissions of particulate typical for conventional pulverized coal plants, matter (soot) and NO x are responsible of severe efficiencies of 4346% may be easily attainable by environmental and health problems. Therefore, IGCC with HGC. This improvement in efficiency special attention has been paid to the reduction of would represent a significant decrease in the cost soot particulate emitted from diesel engines. Soot of electricity along with a reduction in emissions of can be effectively trapped by diesel particulate sulphur compounds [296]. In addition, the method filters [291]. Traditional soot filters are made from offers potential improvements for hydrogen fuelled metallic wires, ceramic foams, inorganic fibers, solid oxide fuel cells (SOFC) and molten carbonate etc., which have a number of unsatisfactory fuel cells (MCFC) technologies [297]. features connected, in the first place with the In order to have high efficiency in the IGCC complexity of regenerating the filters. The most system, removal of H 2S without cooling the coal effective solution to the problem is the combination gas is a very important task. These facts in a single element of the functions of the soot filter emphasises the importance of hot gas and soot oxidant [292]. desulphurization. Hot gas desulphurization may be In recent years there have been attempts to accomplished by using sorbents such as metal develop catalytic coatings for soot filters, capable of oxides that forms stable sulphides. Typically, these improving their effectiveness and of oxidizing metal oxides are converted to sulphides during a carbon particles directly in the soot filters. Solov’ev sulphurloading stage under reducing hot fuel gas et al. [41,292] showed that the most reactive conditions [298]. This reaction is represented catalyst both in the model reaction of oxidising CO generically by the following equation 50: and hexane and in the burning of diesel soot present in the gaseous exhaust of internal Sulfidation: MO + H 2S → MS + H 2O (50) combustion engines is copper chromite. Further they reported that regeneration of soot filters with The ultimate capability of removing H 2S catalytic coatings of copper chromite is possible depends on the thermodynamic properties of the even at the temperatures of exhaust gases from metal oxide. In addition, H 2S capacity, which is internal combustion engine. defined as grams of sulphur removed by 100 grams

of sorbent, is an important parameter. Economic 2.5 Desulphurization of hot coal gas considerations require that the sulphided sorbent The energy demand of the world, which be regenerable according to the following generic increases in accordance with the rise in economic reaction (eqn. 51): development, will have to be met by the prevalent

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refineries, coke ovens, organic compounds Regeneration: MS + 1.5O 2 → MO + SO 2 (51) production plants, pharmaceuticals factories, pulp and paper mills, textile, leather industries, etc. SO 2 must be captured and ultimately be Waste water from these industrial and agricultural converted to elemental sulphur or sulphuric acid. activities contains refractory organic pollutants Copper oxide is able to achieve low levels of H2S in such as plaguicides, organochloride compounds, the clean fuel gas provided the sorbent is not phenols, detergents, polycyclic aromatic reduced to elemental copper. This is because hydrocarbons, etc. that must be treated before copper oxide readily reduces in high temperature discharge to comply with the environmental reducing atmospheres, and elemental copper is an regulations. Water treatment technologies require order of magnitude less active for sulfidation than revision, modernization and enriching by utilizing Cu 2O and CuO [299]. more effective methods and catalyst to achieve Binary CuCrO oxides were studied as required standards for drinking water along with regenerable sorbents for high temperature fuel gas keeping in mind the conomic aspects [302]. desulphurization by Li and Stephanopoulos [21]. Therefore, it is important to have efficient catalyst CuOCr 2O3 sorbents can remove H 2S from at our disposal for removal and oxidation of simulated coalderived fuel gas to less than 510 pollutants present in wastewater in reduced forms ppmv in the temperature range of 650850 0C. The NO 2, NH 4+ , phenols etc. presence of stable CuCr 2O4 in CuOCr 2O3 solids retains some copper in the Cu +2 or Cu +1 oxidation 2.7.1 Wet oxidation of phenol state, which can account for the high H 2S removal Among the harmful organic compounds in efficiency. A regenerable copper chromite sorbent industrial wastewaters, phenol and phenolic with superior hot fuel gas desulfurization substances have deserved more attention in the performance has been developed for IGCC last two decades, because of their toxicity and the applications by Abbasian and Slimane [300] in the frequency of industrial processes producing waters temperature range of 550650 0C. This sorbent is contaminated by phenol. Moreover, phenol is capable of achieving less than 5 ppmv H 2S considered to be an intermediate product in the concentration in the cleaned fuel gas. oxidation pathway of higher molecular weight

aromatic hydrocarbons, thus it is usually taken as

a model compound for advanced wastewater 2.6 Mercury capture from hot coal gas treatments [303]. The conventional process of Coalfired utilities are the largest source of biological oxidation fails to destroy toxic and anthropogenic mercury emissions. Because of its recalcitrant pollutants like phenol. high volatility, almost all the mercury present in Santos et al. [304] have reported the high coal is transformed into gas phase during detoxification of phenolic wastewater about 77% combustion or gasification of coal. Control of using a heterogeneous catalyst based on mercury emissions from coalfired power plants is CuO.Cr 2O3 at basic pH, obtained by adding sodium a difficult task, in part due to its high volatility bicarbonate as buffer reactive (pH 8). Further, they and its much lower concentration (520 g/m3) in a observed that the catalyst was chemically and large volume of flue gas. In addition, depending on mechanically stable during long periods of time on the type of coal and combustion conditions, a stream, this point being of great importance for its majority of mercury in the flue gas can exist in the industrial application. The oxidation of phenol in elemental form (Hg 0), which is more difficult to water over 26% CuO + 74% CuChromite in a capture than its oxidized (Hg 2+ ) or particulate batch autoclave was investigated by Akyurtlu et al. (Hg p) forms. Jadhav [301] evaluated Nano Active [305]. They found copper chromite to be most sorbents (CuOCr 2O3) for Hg capture. He active catalyst and concluded that depending on explained the role of Cr, which would suppress the the operating conditions, the catalyst can facilitate reduction of Cu in the presence of H 2 and would complete phenol conversion within 90 min. maintain Cu in Cu 2+ state [4], which would have enhanced reactivity towards Hg. This experimental 2.7.2 Ammonia removal study has demonstrated that supported forms of The formation of ammonia is inevitable during binary oxides of Cu and Cr have showed their industrialscale wet oxidation of wastewater if potential as effective Hg sorbents. nitrogencontaining compounds (like: production of 2.7 Removal of aqueous organic waste soda, nitric acid, urea; metallurgical industry; coal The main polluting branches of industry are

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or biomass gasification) are present. The major term chemical stability in the nitrite conversion source of ammonia emission has been attributed to almost complete in less than ∼5 h [314]. CuCrO 2 the intensive farming areas and notably to exhibits a long term chemical stability with a livestock manure [306]. Ammonia is toxic for fishes corrosion rate of 0.34 mol m 2 year 1 in KCl (0.5 and other marine organisms. It causes M). eutrophication in ponds and lakes. It has been recently studied for its role in the formation of a 2.8. Burning rate catalyst for solid rural version of urban smog [307]. The ammonia propellants containing waste produced in industries is usually In recent years the CuCrO composites are characterized by high concentration and high found great promising in application as burning temperature, and is not treatable by biological rate catalysts (ballistic modifier) for solid methods directly [308]. Huang et al. [308] propellants used in defence (high explosives, suggested wet oxidation as a promising means of ballistic missiles) [315] and space vehicles (rocket pretreating wastewater containing ammonia at propellants) [316318]. Solid composite propellants concentrations of up to 600 mg/L. are mixtures of prepolymer (binder), aluminum As most of the wet oxidation processes run at fuel, oxidizer salts (e.g. ammonium perchlorate), elevated pressure and temperature, running the and other components, including curatives, heterogeneously catalysed oxidation of ammonia in plasticizers, bonding agents, stabilizers and the gas phase in a downstream reactor could catalysts [319]. Even though added at few percent protect the catalysts mainly from leaching and of the propellant binder, the catalysts used to offers an economic alternative by avoiding loss of control the burn rate are of high importance, since unused oxygen after depressurisation. Noble metal they allow improving the ballistics of rockets. containing catalysts [309] are used for wet Combustion of this system involves the oxidation processes, but they are very costly and decomposition of AP(ammonium perchlorate) and insufficient stability against hydrothermal impact. the binder and mixing and oxidationreduction of Martin et al [310] reported oxidation of ammonia the decomposition products. with air in steam atmosphere using Cu,Cr Rajeev et al. [316] prepared CuCrO composite containing supported and bulk copper chromite oxides via thermal decomposition of copper catalysts at 235305 0C and 3060 bar. Bulk copper ammonium chromate and found that the chromite catalyst gave the best performance (86% propellant burning rate is enhanced by the conversion) at 305 0C, 45 bar and contact time of 1 addition of CuCrO composite oxides. Li et al. [10] second. It shows only slight changes in their used CuCrO nanocomposites as additives for the structural phase composition, high contents of Cr catalytic combustion of APbased solidstate (III) in a CuCr 2O4 spinel phase and higher propellants, synthesized via a citric acid (CA) stability. complexing approach. They showed that well

crystallized CuCrO nanocomposites could be 2.7.3 Nitrite Removal produced after the CACuCr precursors are Nitrite is a common pollutant in water whose calcined at 500 0C for 3 h. Addition of the as principal source arises from fertilizers and synthesized CuCrO nanocomposites as catalysts industrial rejects causing ecological problems enhances the burning rate as well as lowers the [311]. The world health organization guidelines pressure exponent of the APbased solidstate required the levels of NO 2 and NO 3 in drinking propellants considerably. Noticeably, catalyst with water less than or equal to 3.3 and 44 mg L 1 a Cu/Cr molar ratio of 0.7 exhibited promising respectively. The nitrite removal from water is a catalytic activity with high burning rate and low topic of great concern owing to its toxicity to the pressure exponent at all pressures, due to the environment and detrimental to human health. effective phase interaction between the spinel NO 2 coming from industrial activities disturbs the CuCr 2O4 and delafossite CuCrO 2 contained in the ecological system and beyond a threshold assynthesized CuCrO nanocomposites. Patil, et concentration, it becomes an increasing problem al. [320] synthesized ptype nanoCuO and [312]. High concentrations of nitrates in drinking CuCr 2O4 by an electrochemical method and water are harmful due to their reduction to nitrites investigated the catalytic effect on the thermal (NO 2) that combine with haemoglobin in the decomposition behaviour of ammonium perchlorate human blood to form the toxic compound of (AP) as a function of catalyst concentration using methaemoglobin [313]. The photoactive CuCrO 2 is differential scanning calorimetry. The nanocopper low cost, easy to synthesize and exhibits a long chromite (CuCr 2O4) showed best catalytic effects as

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compared to nanocupric oxide (CuO) in lowering single substance and therefore have attracted the high temperature decomposition by 118 0C at 2 much interest owing to their unique magnetic, wt.%. They observed high heat released of 5.430 magnetooptical and magnetoelectrical effects. and 3.921 kJ g 1 in the presence of nanoCuO and Possible spintronic devices are spinvalve CuCr 2O4, respectively. The decrease in the transistors, spin lightemitting diodes and activation energy and the increase in the rate nonvolatile logic devices [24]. Generally, oxide constant for both the oxides confirmed the semiconductors have wide band gaps, i.e. enhancement in catalytic activity of AP. They transparent for visible light. This feature serves an proposed a mechanism based on an electron important role as transparent conducting oxides transfer process for AP in the presence of nano (TCOs) that are used for various applications such metal oxides. as transparent electrodes in flat panel displays and window layers in solar cells. Li et al. [24] 2.9. Electrodes and Sensors successfully prepared Mndoped CuCrO 2 polycrystalline semiconductors by the solgel NO x, are the primary pollutants produced by method which exhibit FM transitions at about 120 the high temperature combustion processes, K. automotive vehicles as well as industries. There is Among the physical properties of solid materials a need to detect NO x in the concentration range which have been used as heterogeneous catalysts between 10 and 2000 ppm for a greater control of in a variety of chemical industries, the electrical fuel combustion processes and for monitoring conductivity and thermoelectric power are of automobile exhaust gases [321]. NO x fundamental importance [324]. The electrical electrochemical sensors based on ceramic materials transport of catalysts is of basic importance in the are suitable in high temperature application and in determination of the relationship between chemically harsh environment since they are electronic structures and catalytic properties of mainly based on semiconducting oxides and solid semiconductors. The electrical conductivity and electrolytes. Xiong et al. [322,323] studied a mixed thermoelectric power of CuCr 2O4 are reported in potential sensor based on CuO + CuCr 2O4 which the temperature range 295 to 815 K [325]. exhibited high selectivity and sensitivity toward

NO 2. They found the opencircuit electromotive 2.11. Drugs and agrochemicals force (EMF) of the NO 2 sensor to be stable, reproducible and, the sensor responded rapidly Nitrogencontaining heterocyclic compounds, (t90 < 8 s) to change in the NO 2 concentration such as quinoline, indoles, pyrroles, pyrrolidines between 100 and 500 ppm at 659 0C [323]. Porous (and their alkylated homologues), piperazine and Pt reference electrode was used in the research pyrazine are of high industrial interest, for [321,322]. Recently, the same workers [23] tested a applications as intermediates to produce solidstate planar sensor consisting of scandia pharmaceuticals, agro chemicals (herbicides, stabilised zirconia (8 mol% Sc 2O3ZrO 2) solid fungicides), dyes, etc. [326328]. Pyrazine is electrolyte sandwiched between CuO + CuCr 2O4 synthesized by passing ethylenediamine (ED) mixedoxide and Au foil reference electrodes in vapor over a copper oxide/copper chromite catalysts NO 2 containing gaseous atmosphere in the range of in the temperature range of 340440 0C with a very 100500 ppm NO 2 at 611 and 658 0C. They found high selectivity (98100%) [19]. The reaction that the response time of the sensor was less than proceeds by intermolecular deamination and 6 s and the recovery time was approximately 6 s at cyclization of ED to form piperazine, which 658 0C, respectively. The response of the sensor undergoes subsequent dehydrogenation to form was found to be logarithmically dependent on the pyrazine (Fig. 14). concentration of NO 2 between 100 and 500 ppm at Propylene glycol (PG) is used in 611 0C whereas it was found to vary linearly with pharmaceuticals, such as: a carrier of active the concentration of NO 2 at 658 0C. The response of ingredients in vaccines, cough relief syrups or gel the sensor was highly reproducible to change in capsules to help deliver these substances within concentration of NO 2 and also showed negligible the body for treatment and prevention of diseases. crosssensitivity to O 2, CO and CH 4 in the gas It is also used as a solvent in many stream. pharmaceuticals, including oral, injectable and topical formulations. Notably, diazepam, which is 2.10. Semiconductors insoluble in water, uses PG as its solvent in its clinical, injectable form [329]. PG is synthesized Diluted magnetic semiconductors (DMSs) efficiently by hydrogenolysis of glycerol over copper involve charge and spin degrees of freedom in a

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chromite catalysts [110]. prepare CuCr 2O4, for instance chromium and A novel environmentfriendly vapour phase copper doubly promoted ceria catalysts have been synthesis of different classes of nitrogencontaining prepared by Harrison et al. [331] by three routes: heterocyclic compounds was developed by (A) coprecipitation from aqueous solutions Campanati et al. [330] using nonhazardous, containing cerium(IV), copper(II), and chromium commercially available and low cost feeds. 2 (III) ions, (B) sequential impregnation of CeO 2 by Methyl8ethylquinoline (MEQUI) was synthesized using an aqueous CrO 3 solution followed by an from 2ethylaniline (2ETAN) and ethylene glycol aqueous solution of copper(II) nitrate, and (C) (EG) or chloroethanol (CE), operating at high impregnation of Cu(II)/CeO 2 prepared by temperature in the presence of acidtreated K10 coprecipitation by using an aqueous CrO 3 solution. montmorillonite or ZnCl 2/K10 montmorillonite. At They found that at low processing temperatures, lower temperatures and using copper chromite copper is present as (polymeric) Cu(OH) 2 in both catalysts, 7ethylindole (7ETI) or 5ethylindole (5 the Cu/Cr/CeO 2cop and Cu/Cr/CeO 2cop/imp ETI) were obtained from 2ETAN or 4ethylaniline materials. Chromium is present as adsorbed (4ETAN), respectively, and EG; excess of alkyl (Cr 2O72) ions in the Cu/Cr/CeO 2imp/imp and Cu/ aniline was required to avoid the formation of Cr/CeO 2cop/imp materials, but a variety of polyalkylated byproducts. Mixing SiO 2 with the chromium species in oxidation states +III, +V and best copper chromite, made it possible to operate +VI are present in the Cu/Cr/CeO 2cop material. with higher LHSV values, thus improving the yield Brief descriptions of synthesis principles, in alkylindoles. Finally, N(2ethylphenyl) pyrrole typical processes, important aspects that influence (EPP) and N(2ethylphenyl) pyrrolidine (EPD) the characteristics, specific advantages and some were synthesised using a commercial copper experimental data are presented for the following chromite catalyst and feeding 2ETAN and 2,3 methods for the preparation of CuCr 2O4 catalysts: dihydrofuran (DHF), EPP being favoured by high temperatures and absence of water in the feed • Coprecipitation method (Fig. 15). • Adkins’ method • Complexingcoprecipitation method • Coimpregnation method 3. Preparation methods of copper chromite • Thermal decomposition (ACOC) catalyst • Thermal decomposition of ammoniacal Catalysts, when prepared via different routes, copper oxalate chromate (ACOC) would demonstrate different properties, even with • Hydrothermal method the same starting compositions. Crystallinity, • NonCasting method: Template technique surface properties, and specific surface area, three • Hydrolysis of some soluble salts of the most important parameters determining the • Microemulsion method catalytic activity of the product are highly dependent on the synthesizing routes [14]. Several preparation methods have been established to

Fig. 15. Reaction pathway for the synthesis of N(2ethylphenyl) pyrrole (EPP) and N Fig. 14. A generalized reaction pathway for the for (ethylphenyl) pyrrolidine (EPD) using CuCr 2O4 mation of pyrazine and other products. catalysts [331]

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• Combustion synthesis 2Cu(OH)NH 4CrO 4 → CuO.CuCr 2O4 + N 2 + 5H 2O • Selfpropagating hightemperature (54) synthesis CuO.CuCr 2O4 + 2CH 3COOH → CuCr 2O4 + • Solution combustion synthesis (CH 3COO) 2Cu + H 2O (55) • Electroless method • Sonochemical method Adkins et al. [44] have hydrogenated a group of • Metal organic chemical vapour deposition 21 organic compounds using above prepared (MOCVD) catalyst and reported that 16 have been • Flame Synthesis of Nanostructured successfully hydrogenated with 100% yield and • Chemical reduction method 100% selectivity. • Solgel process Kawamoto et al. [315] prepared the catalyst • Citric acid (CA) complexing approach following the coprecipitation method from aqueous • Pechini Method solution of K 2Cr 2O7 and CuSO 4.5H 2O in the • Nonalkoxide solgel route presence of ammonia at Cu/Cr molar ratios of 0.3 and 0.5. The fine precipitates of BCAC were filtered, washed with distilled water and dried at 3. 1 Coprecipitation method 110 0C to a constant weight. Then, they were powdered to a 250 mesh and heated at 500 0C for 2 3.1.1 Adkins′ method hours. The formation of the catalyst can be The preparation procedure [44] is as follows: represented by equations 56 and 57: Ammonium hydroxide was added to a solution of 126 g. (0.5 mole) of ammonium dichromate in 500 2CuSO 4 + K 2Cr 2O7 + 4 NH 3 + 3H 2O → K 2SO 4 + ml. of water until the colour of the solution 2Cu(OH)NH 4CrO 4 + (NH 4)2SO 4 (56) changed from orange to yellow. The volume of the solution was now 653 ml. The solution was allowed 110500 oC to come to room temperature and a solution of 2Cu(OH)NH 4CrO 4 → CuO.CuCr 2O4 + 5H 2O 241.6 g. (1.0 mole) of cupric nitrate trihydrate in (57) 300 ml. of water was added with stirring. The red brown precipitate of basic copper ammonium They concluded that the samples synthesized by chromate (BCAC) was filtered with suction and the coprecipitation method present a better dried overnight in an oven at 100110 0C. It was granulometric distribution than solid state then finely powdered, transferred to a porcelain reaction and this has led to propellants with higher casserole, and decomposed by heating the casserole burning rate and higher pressure exponent. in the flame of a Bunsen burner. After decomposition had begun, the heat of reaction was 3.1.2 Complexingcoprecipitation method sufficient almost completely to decompose the The most common method for mixedoxide chromate. When spontaneous decomposition had catalyst preparation is crystallization or ceased, the casserole was heated with the free precipitation or coprecipitation in solution of a flame until fumes ceased to be evolved and the precursor form (hydroxide, oxide, insoluble salt) of contents was black and so finely powdered as to be the catalyst [332]. Other specific steps, for example almost like a liquid. During this heating, care was either addition of an extra component or its taken to keep the material well stirred and to removal by partial extraction, may sometimes be rotate the flame in order to avoid local necessary to adjust the final catalyst composition superheating. The product was allowed to cool and and ensure homogeneity [333]. Adkins et al. [44] then suspended in 200 ml. of a 10% solution of described for the first time preparation of copper acetic acid. The suspension was filtered with chromite catalyst for hydrogenation of organic suction, washed thoroughly with water, dried for compounds following this method. Subsequently, twelve hours in an oven at 100110 0C, and finely several workers used this method for the powdered. The yield of catalyst was 113 g. The preparation of copper chromite catalysts using reactions involved can be represented by the different compounds of copper and chromium with following equation 5255: aqueous ammonia solution as precipitating agent.

(NH 4)2Cr 2O7 + 2NH 4OH → 2(NH 4)2CrO 4 + H 2O 3.2. Coimpregnation method (52) (NH 4)2CrO 4 + Cu(NO 3)2 + NH 4OH → Cu Coimpregnation method is the easiest method (OH).NH 4CrO 4↓ + 2NH 4NO 3 (53)

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of preparation of copper chromite catalysts. In this 2CuNH 3C2O4NH 4CrO 4 + 2O 2 → CuO.CuCr 2O4 + method catalysts are obtained [291,334] by mixing 4CO 2 + 2N 2 + 7H 2O (highly exo.) (59) of support (γAl 2O3) with an aqueous solution of chromium anhydride and copper nitrate in The overall decomposition of ACOC in argon was required proportion. The amount of solution taken mild exothermic, can be represented as follows is sufficient for incipient wetness to be observed. (eqn. 60): The samples are dried and calcinated at appropriate temperatures to get the catalysts. 2CuNH 3C2O4.NH 4CrO 4 → Cu.CuCr 2O4 + 2NH 3 + 4CO 2 + N 2 + 4H 2O (mild exo.) (60) 3.3 Solid state reaction (ceramic method) The environments of calcinations of ACOC have The vast majority of the powder synthesis of strong influence on mechanism of decomposition high temperature superconducting oxides to date and also on the chemical composition and physico has been carried out using the traditional “solid chemical properties of the resulting Adkins’ state reaction route” [335]. Copper chromite catalysts. Violent exothermic oxidative spinels are usually synthesized by the conventional decomposition of ACOC in air leads to bidispersed high temperature method of solid state reaction bigger particles and consequently lowers specific (eqn. 58): surface area of resulting catalyst, CuO.CuCr 2O4 in 900 oC comparison to finelydivided monodispersed CuO(s) + Cr 2O3(s) → CuCr 2O4 (s) (58) particles of active catalyst, Cu.CuCr 2O4 obtained 6h through mild exothermic decomposition in argon Kawamoto et al. [315] prepared copper chromite environment. Thus, decomposition of ACOC in by ceramic method, mixing copper (II) and argon produces novel active catalyst directly, chromium (III) oxides at 3 different ratios Cu/Cr = minimizing a process step of catalyst reduction, 0.61; Cu/Cr = 1.0; Cu/Cr = 1.5. The mixtures of leading to saving of not only reduction time but oxides were homogenized with acetone followed by also reducing and diluent’s gases. subsequent calcination at 900 0C for 6 hours. This method results in spinel particles with low surface 3.5 Hydrothermal method areas. In order to synthesize spinels with high surface area, they attempted different wet George and Sugunan [4] prepared the catalysts chemistry techniques. as follows: a mixture consisting of 10% solutions of copper nitrate, nickel nitrate and chromium 3.4 Thermal decomposition of ammoniac nitrate for Cu 1xNi xCr 2O4 (x = 0, 0.25, 0.5, 0.75 and copper oxalate chromate 1) series were taken in appropriate mole ratios and the mixture was heated to 343353 K. To this hot Prasad [2] synthesized a novel precursor, mixture a 15% ammonia solution was added drop ammoniac copper oxalate chromate (ACOC) and wise with constant and uniform stirring to suggested the stoichiometric formula of the maintain a constant pH of 6.58. The precipitate precursor closely approximating, was maintained at this temperature for 2 h and CuNH 3C2O4NH 4CrO 4. ACOC was used for the aged for one day. The precipitate was filtered, production of active copper chromite catalysts. The washed and dried at 353 K for 24 h and calcined at precursor, ACOC was prepared by mixing 923 K for 8 h. ammoniacal solutions of copper oxalate and Arboleda et al. [336] synthesized a new single ammonium chromate in a Cu/Cr atomic ratio of 1:1 phase copper chromate compound, (NH4) followed by evaporation on a steam bath and 1.5Cu 2Cr 2O8(OH) 1.5 H2O by the hydrothermal finally dried at 378 K in an oven for 5 h. No method. The synthesized compound decomposed precipitation occurred during mixing. The dried into CuCr 2O4 and CuO after being calcined at 600 greenish colored complex was stored in a desiccator 0C. The CuZnCr catalysts derived from over anhydrous calcium chloride. The hydrotalcite (HT) structures were prepared by the homogeneous complex precursor is soluble in hydrothermal method by Venugopal et al. [337]. aqueous ammonia. The catalysts were prepared by Catalyst precursors were prepared by the co calcination of the prepared complex precursor in precipitation method with mol% ratios of Cu:Zn:Cr air and in argon atmosphere around 623 K. The = 44:44:12 and Cu:Zn:Cr = 47:38:15 with overall decomposition of ACOC in air was highly hydrothermal treatment. About 0.6 L aqueous exothermic and can be exemplified by the following solution of copper, zinc, chromium nitrates equation 59:

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(solution A) and a solution B containing 2 M NaOH selectivity to 1,2propanediol in glycerol and 1 M Na 2CO 3 (1:1 = v/v) were added hydrogenolysis. The method is as follows: about 3 g simultaneously to 2.0 L of distilled water under of the carbon materials were impregnated at room vigorous stirring. The rate of addition of solution A temperature with about 15 ml aqueous solution of was approximately 0.5 L/h while maintaining a copper nitrate and chromium nitrate mixture of constant pH ~8.0 ± 0.1 by adjusting the flows of different molar ratios for 30 min. Alternatively, solutions A and B. The coprecipitation is carried impregnation was performed with a slight excess of out at room temperature. The precipitate was kept solution. The slurry was then filtered and squeezed at 70 0C for 1 h subsequently washed several times to remove the liquid on the carbon surface that until the pH of the gel reached the pH of the could cause the formation of large oxide particles. distilled water that has been used for the After drying in air at 70 0C overnight, the resulting preparation. The precipitates were oven dried at sample was transferred to a quartz reactor inside a 100 0C for 24 h and calcined in static air at 400 0C tubular resistance furnace. The furnace was then for 3 h. They found the presence of hydrotalcite ramped at 1 0C/min to final temperature and was structure and the finely dispersed copper species in held for 120 min. The atmosphere was of argon the calcined form. before 300 0C, and then shifted into a mixture gas of 20% O 2 in Ar at a flow rate of 120 ml/min. 3.6 Nanocasting method (Template Finally, carbon template was removed at 550 0C in technique) air for 8 h. The conversion is almost 100% from copper nitrate and chromium nitrate to final The common preparation methods of mixed copper and chromium oxide catalyst, and the ratio oxides and mixtures of oxides usually require the of Cu to Cr was obtained from the initial molar unconfined precursors to be subjected to high ratio of copper nitrate to chromium nitrate. temperature calcinations steps. This provokes the ValdésSolís, et al. [14] prepared high surface sintering of the resulting particles, with the area spineltype complex oxides using a porous concomitant reduction of active surface area, silica xerogel as template. To synthesize the metal typically in the range of a few square meters (<10) oxides, a solution of the hydrated metal nitrates in per gram. Thermal treatments can be avoided by the appropriate ratio was prepared in ethanol (0.4 the use of ‘‘ chimie douce ’’based nanotechnological 0.6 M). The silica xerogel was stirred with this approaches, such as microemulsion techniques solution at moderate temperature (60 0C) until [338], but these are expensive procedures which, complete removal of the solvent. The impregnated moreover, must integrate stages for the separation sample was dried at 80 0C overnight and calcined of the organic solvents and surfactants. These in air at 5 0C/min up to the temperature required disadvantages, when using thermal methods for for the formation of the oxide and then left for 4 h. unconfined precursors or liquid phase The mixed oxides were obtained after the preparations, have been recently overcome by dissolution of the silica framework in a NaOH means of nanocasting procedures, such as template solution (2 M) and final washing with distilled methods with hard templates (porous solids such water to remove impurities. Copperbased as silica gel and active carbon) [339341]. Using catalysts (CuMn 2O4 and CuCr 2O4 spinels or CuO/ this method the synthesis of the nanoparticles CeO 2 and CuO/ZnO) exhibit a high activity and takes place in a confined space formed by the selectivity for the methanolsteam reforming porosity of the template. The attraction of these process. methods is the confined calcination process that allows an effective control of the particle size of the 3.7 Hydrolysis of some soluble salts resulting materials by adjusting the conditions of synthesis, thus favouring the formation of Patron et al. [317] developed the hydrolysis nanostructures [341]. method for obtaining chromium based oxides. It Generally, the nanocasting methods involve two consists in the hydrolysis of the metal salts in main steps: (a) preparation of a nanostructured presence/or absence of a complexant agent. The template; and (b) filling the template nanopores potential advantages of this synthesis route lies in with the desired precursor, followed by the accessibility and moderate costs of the starting crystallization in the voids of the template and materials, in the low temperatures required by the then removing the template framework. Liang et procedure and the enhanced homogeneity and al. [106] firstly demonstrated that the carbon purity of the end products. They obtained a pure template route allows synthesizing highsurface tetragonal CuCr 2O4 with mean crystallite sizes area CuCr catalysts, which show high activity and

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varying in the range 133210 Å between the as important technique for the synthesis and temperatures limits 823 K1073 K. The precursors processing of advanced ceramics (structural and were prepared by hydrolysis of Cr(NO 3)3.9H 2O and functional), catalysts, composites, alloys, Cu(NO 3)2.3H 2O and Cu(CH 3COO) 2.H 2O. Working intermetallics and nanomaterials. In CS, the in different conditions two polynuclear exothermicity of the redox (reductionoxidation or coordination precursors [Cr 2Cu(NH 3)2(OH) 6](NO 3)2 electron transfer) chemical reaction is used to and [Cr 2Cu(OH) 8].4H 2O were obtained. The produce useful materials [345]. Depending upon following reaction equations 61 and 62 may be the nature of reactants: elements or compounds written for precursors’ synthesis: (solid, liquid or gas); and the exothermicity NH 4 (adiabatic temperature, T), CS is described as: self Cu 2+ → [Cu(NH 3)4]2+ → [Cr 2Cu(NH 3)2(OH) 6](NO 3)2] propagating high temperature synthesis (SHS); ~1011 (Precursor A) (61) lowtemperature CS, solution combustion synthesis (SCS), gelcombustion, solgel combustion, Cr 3+ → Cr(OH) 3 → [Cr 2Cu(OH) 8].4H 2O emulsion combustion, volume combustion (thermal (Precursor B) (62) explosion), etc. Combustion synthesis processes are characterised by hightemperatures, fast heating Copper chromite was obtained through thermal rates and short reaction times. These features decomposition of the precursors. Following make CS an attractive method for the manufacture decomposition mechanism (eqns. 63 and 64) has of technologically useful materials at lower costs been predicted for the two compounds: compared to conventional ceramic processes. Some other advantages [346] of CS are: Precursor A : (i) Use of relatively simple equipment [Cr 2Cu(NH 3)2(OH) 6](NO 3)2 → [Cr 2Cu(OH) 6](NO 3)2 (ii) Formation of highpurity products → Cr 2CuO(OH) 6 → Cr 2O2(OH) 2.CuO → (iii) Stabilization of metastable phases and Cr 2O8/3 .CuO → Cr 2O3.CuO → CuCr 2O4 (63) (iv) Formation of virtually any size and shape products. Precursor B: Combustion synthesis has been extensively [Cr 2Cu(OH) 8].4H 2O → Cr 2Cu(OH) 8 → Cr 2O2(OH) 2 used to prepare a variety of catalysts. Patil et al. CuO → Cr 2O3.CuO → CuCr 2O4 (64) [346] reviewed the recent developments in the field with special emphasis on the preparation of ‘Catalysts’ and ‘Nanomaterials’ by solid state 3.8 Microemulsion method combustion and solution combustion. The use of an inorganic phase in waterinoil 3.9.1 Selfpropagating hightemperature microemulsions has received considerable synthesis (SHS) attention for preparing metal particles. This is a new technique, which allows preparation of The SHS method is being developed for the low ultrafine metal particles within the size range 550 cost production of engineering and other functional nm particle diameter [342]. materials, such as advanced ceramics, Nanoparticles of copper chromium hexacyanide intermetallics, catalysts and magnetic materials. with varying particlesize are prepared by Kumar The method exploits selfsustaining solidflame et al. [343] using the microemulsion method and combustion reactions which develop very high Poly (vinylpyrrolidone) (PVP) as a protecting internal material temperatures over very short polymer. Two separate microemulsions of Cu(NO 3)2 periods. It therefore offers many advantages over and K 3Cr(CN) 6 with PVP are prepared and traditional methods, such as much lower energy subsequently mixed together to get the precipitate costs, lower environmental impact, ease of of copperchromium hexacyanide nanoparticles. manufacture and capability for producing The nanoparticles are separated out by adding materials with unique properties and acetone in the resultant mixture and are washed characteristics [347]. many times with acetone and demineralized water. Xanthopoulou and Vekinis [348] prepared the The different mixing ratios of PVP to Cu ion SHS catalysts and carriers from initial batch concentration (20 to 200) are used to control the mixtures consisting of nitrates and sulphates, size of the nanoparticles. metals and oxides, compacted under a pressure of 5 10 MPa in the form of rods of diameter 15 cm 3.9 Combustion synthesis and, in some cases, by extrusion as honeycomb Combustion synthesis (CS) [344] has emerged carrier blocks with diameter 15 cm and channel

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size of about 5 mm. The samples were preheated in precursors used for making the sorbents are copper an electric furnace at temperatures of 700900 0C (II) nitrate trihydrate, chromium (III) acetate, for a few minutes prior to initiation of SHS. The copper (II) acetate monohydrate, chromium (III) specific area was increased by depositing a second nitrate nonahydrate. A series of 0.3 molar aqueous oxide layer (washcoat) on the surface of SHS precursor solutions have been made with deionized carriers of about 0.94.9% aluminium oxynitrate water and mixed in the stoichiometric ratio. The (denoted as OX, 3.2% for the CuCrO catalyst). Pd solutions are then atomized with the help of was then deposited on the SHS carriers using compressed air in a medical nebulizer. The standard aqueous impregnation followed by droplets from the nebulizer are then sent directly calcination and reduction. Standard 0.05% or 0.5% into a flame rector. The flame in the reactor is Pd/Al(NO 3)3/Al 2O3 catalyst systems produced by a generated with methane, oxygen and nitrogen conventional impregnation/ calcination/ reduction gases. The total flow rates for each gas including process were used for comparison [348] . The the contributions from the air used to atomize the author reported that the CuCrO catalyst precursor were methane 0.90 l/min, oxygen 2.61 l/ prepared by SHS is resistant to fuel impurity min and nitrogen 5.95 l/min. The maximum poisoning and used as carrier for 0.05% Pd, temperature of the flame using these flow rates achieved 50% conversion (lightoff) at was in the range of 14001500 0C, which was temperatures about 50 0C lower than conventional measured by using Stype (Pt/10%RhPt) 0.5% Pd/Al 2O3 catalysts for CO oxidation [348]. thermocouple. In the reactor, the droplets undergo a series of physical and chemical steps including 3.9.2 Solution combustion synthesis solvent evaporation and precipitation, intraparticle reaction and densification, to form the final A highly exothermic and selfsustaining product, a dense particle. The particles move from reaction, the socalled ‘‘solution combustion the reactor to a watercooled surface placed synthesis’’ method is described by Russo et al. directly over the reactor. The temperature gradient [349]. Particularly, a concentrated aqueous caused by placing the cooled surface above the solution of various precursors (metal nitrates and flame reactor causes the particles to move from a urea) was located in an oven at 600 0C for a few hotter region to the cooler surface a minutes in a crucible, so as to ignite the very fast thermophoresis process. The authors reported that reaction. Under these conditions nucleation of the flamesynthesized sorbent has higher sulfur metal oxide crystals is induced, their growth is loading capacities than one synthesized by co limited and nanosized grains can be obtained. The precipitation. innovative solution combustion synthesis technique was adopted successfully because it was possible to produce in an easy and lowcost ‘‘one 3.11 Electroless method shot’’ way catalysts [259] with a rather high surface area and pureness. The same technique A critical comparison of those methods is could be used to deposit the catalyst on ceramic needed to make the best choice for given boundary carriers with a very high surface area. They conditions of targeted eventual material properties, studied the catalytic decomposition of N 2O to N 2 raw materials, investment, processing and waste and O 2 over CuCr 2O4 and reported that the disposal costs. Preparing catalysts using different conversion of N 2O reached 50% in the absence and methods also abet to compare the suitability of in the presence of oxygen at 630 0C and 745 0C catalysts for specific applications for instance, respectively. Shiau and Tsai [352] showed that the electroless deposition method provides more uniform copper 3.10 Flame spray pyrolysis method distribution than the impregnation method. The electroless method is an oxidationreduction Flame synthesis is an easy, single step method chemical deposition reaction, which can deposit for the preparation of CuCr 2O4 powder. The certain metals on a substrate without an external process is shorter than most wet chemical methods electrical source [352]. and is cost efficient [350]. The flamesynthesized Shiau and lee [104] prepared copper chromite sorbents have higher surface areas than particles catalyst via electroless plating process using Al 2O3 synthesized by coprecipitation. as substrate. Before conducting electroless Akhuemonkhan et al. [351] produced high deposition, Al 2O3 was pretreated with nitric acid to surface area CuCr 2O4 sorbent for desulfurization in remove any impurities, and activated by palladium fuel cells using the flame spray pyrolysis method. They described the procedure as follows: The

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chloride solution to provide palladium nucleating ambient temperature during the sonication centres on surface of Al 2O3. The activated Al 2O3 process. The mixture is centrifuged after was finally contacted with copper solution for sonication. The product is washed two times with copper plating. In the copper solution, water and once with acetone, and then dried at formaldehyde was added as reducing agent for the room temperature under vacuum. Samples S3, S4 oxidationreduction reaction. The plating bath was and S5 are prepared in the same way of preparing maintained at 70 0C and the pH was adjusted to S2 besides that 10, 15 and 20 ml of titanium (IV) 12.5. The plated Al 2O3 was filtered and washed isopropoxide (Me 2CHO) 4Ti are injected into the with distilled water, then dried at 110 0C for 24 h. mixture as soon as the sonication starts, The copper content was controlled at 15%. respectively. They concluded that comparision with Chromium was then added on the catalyst by a Adkins catalyst (S1) prepared by convenient traditional incipient wet impregnation method. method, the Adkins catalyst (S2) prepared via Chromium contents varied from 1 to 5%. They ultrasound has higher catalytic activity for reported that nbutanol dehydrogenation activity hydrogenation of furfural. reaches a maximum at 2 wt. % of chromium addition, which corresponds to the saturated 3.13 Metal organic chemical vapour chromium loading that can penetrate into the bulk deposition (MOCVD) of copper layer. This penetrated chromium can Chemical vapor deposition (CVD) is a chemical reduce the particle size of copper crystallites and process used to produce highpurity, high therefore can enhance the copper dispersion on γ performance solid materials. The process is often Al 2O3. used in the semiconductor industry to produce thin

films. In a typical CVD process, the wafer 3.12 Sonochemical method (substrate) is exposed to one or more volatile Sonochemistry is a very useful process for precursors, which react and/or decompose on the preparing novel materials with unusual properties substrate surface to produce the desired deposit. [353], and its utilization in the synthesis of Frequently, volatile byproducts are also produced, catalysts has been reported [354,355]. The which are removed by gas flow through the prominent advantages of catalysts prepared by reaction chamber [356]. sonochemistry are their high activity and Chang et al. [22] prepared for the first time nanosized particles. The driving force for copper chromite films by metal organic chemical sonochemistry is acoustic cavitation that is formed vapour deposition (MOCVD) with temperature in liquid. During cavitation, bubble collapse below 500 0C. The processing window was: produces intense localized hot spots (˜5000 K) and deposition temperature above 420 0C, partial high pressure (˜1000 atm). The microjets of liquid pressures of oxygen, above 190 torr, copper and shockwave created by ultrasound can make acetylacetonate(Cu(acac) 2), 0.21 torr, and great effect on solids in liquid. chromium acetylacetonate (Cr(acac) 3), 0.4 torr, Huang et al. [89] studied selective respectively. Keeping the more volatile reactants hydrogenation of furfural to furfuryl alcohol over as Cr(acac) 3 in excess amount is essential for CuCr 2O4 catalysts prepared via sonochemical depositing stoichiometric films. Higher deposition method. The method of preparation described is as temperature is essential for depositing oxide films follows: Preparation of catalyst S1: K 2Cr 2O7 (6 g) with complex crystalline structure. SEM results and CuAc 2.H 2O (6 g) is dissolved in NH 3.H 2O (30 showed CuCr 2O4 films exhibited different ml) solution. The solution is kept in a thermostatic morphologies such as equiaxed fine grain, bath (65 0C) under stirring. After NH 3 is removed truncated polyhedron, and hillocks, depending on completely, the formed precipitate is centrifuged the process condition. Both substrate temperature and washed two times with water and once with and precursor partial pressures have significant acetone, and then dried at room temperature under impact on film morphology and reflect the basic vacuum. Preparation of catalyst S2: K 2Cr 2O7 (6 g) natures of film growth mechanism. XRD patterns and CuAc 2.H 2O (6 g) are dissolved in water (20 ml), indicate that CVD films are polycrystalline, which respectively. Then 1 ml of NH 3.H 2O is added into exhibit highly textured, normal spinel structure. the solution of K 2Cr 2O7. Two solutions are mixed up and the mixture is sonicated for 3 h by 3.14 Chemical reduction method employing a direct immersion titanium horn The chemical reduction is a convenient method (Sonics and Materials model VCX600, 20 kHz, with simple procedures for preparing amorphous 100W/cm 2). The reaction cell is cooled by water at

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nanosize metal particles for catalysis [357]. The process [360]. The resulting processes involved and ultrafine copper chromite catalyst is obtained the properties of the precursor molecules have a using this method [45]. Liaw and Chen [45] decisive influence on the resulting material prepared the catalyst by reducing copper nitrate properties [360]. On addition of water, the metal (0.1 M) in an aqueous solution with sodium alkoxides [M(OR)n] readily hydrolyze as borohydride (1 M). The aqueous sodium represented by Eqn. 65. borohydride solution was added into the copper nitrate solution by a micropump under a flow of M(OR)n + H 2O → M(OR) n1OH + ROH (65) nitrogen. Cr introduced to copper by reducing/co precipitating with sodium borohydride in an Hydrolysis is followed by condensation to form aqueous solution of copper nitrate and an MOM bonds via either dehydration or appropriate amount of Cr salt. The resulting black dealcoholation as described in Eqs. (66) and (67), precipitate was thoroughly washed with distilled respectively: water three times to remove the residual ions and then washed with ethanol to remove water. The (RO) mMOH + HOM(OR) m → (RO) mMOM(OR) m precipitate are then dried and calcined. They + H 2O (66) studied catalytic properties of catalyst prepared by MOH + ROM → MOM + ROH (67) this method for hydrogenation on monofunctional (olefinic and carbonyl) and bifunctional (conjugated In this manner, inorganic polymeric oxide and nonconjugated) compounds and compared with networks are built up progressively. The those of a commercial catalyst of copper chromite. hydrolysis, condensation and polymerization The ultrafine catalysts of CrCuB containing a reactions are governed by several factors, including much lower content of chromium (Cr<5mol%) were the molar ratio of water to alkoxides, choice of more active than the commercial copper chromite solvents, temperature and pH (or concentration of (Cr > 50mol%). The authors proposed that these Cr acid or base catalysts). There are essentially three CuB catalysts are highly promising for replacing different kinds of solgel or gel technology for copper chromite for liquidphase hydrogenation preparation of catalysts. reactions. 3.15.1 Citric acid complexing method 3.15 Solgel method Citric acid (CA)assisted solgel method (namely The solgel methods show promising potential Pechini approach) is a facile synthesis for for the synthesis of mixed oxides catalysts. The producing homogeneous nanocomposites [10], in versatility of the solgel techniques allows control which the use of citric acid as chelating agent of the texture, composition, homogeneity, low ensures the formation of homogeneous transparent calcination temperatures (minimizing the metalcitrate gels, and the intimate mixing of undesired aggregation of the particles), and components ensures homogeneity of the final structural properties of solids, and makes possible product. Li et al.[10] have prepared CuCrO production of tailored materials such as dispersed nanocomposites by citric acid (CA) complexing metals, oxidic catalysts and chemically modified approach in which 0.01 mol Cu(NO 3)2 and 0.02 mol supports [358]. Cr(NO 3)3 are dissolved in 100 mL deionized water Such methods are used primarily for the to obtain a mixed metal nitrate solution. Then fabrication of materials (typically a metal oxide) citric acid is added to this solution and the molar starting from a chemical solution (sol) which acts ratio of citric acid to the total metal ions is fixed to as the precursor for an integrated network (or gel) be 2:1. After stirring for 30 min, the solution is of either discrete particles or network polymers heated at 95 0C for several hours to evaporate the [359]. Typical precursors are metal alkoxides and water solvent to produce dark brown transparent metal nitrates, which undergo hydrolysis and viscous gels. The gels are then dried at 160 0C for 2 polycondensation reactions to form either a h to obtain the foamy dark powders, which are network ‘elastic solid’ or a colloidal suspension (or denoted as precursors of CuCrO nanocomposites dispersion) a system composed of discrete (often (CACuCr). After grinding, the precursors are amorphous) submicrometer particles dispersed to successively heated at 600 0C for 3 h to obtain the various degrees in a host fluid. final black CuCrO nanocomposites. The hydrolysis of precursor molecules and the Li and cheng [229] have prepared Bi 2O3/ condensation between the resulting reactive CuCr 2O4 core/shell nanomaterials following the species, are the essential reactions of the solgel

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facile synthesis and show that the nanomaterials heterobimetallic alkoxide precursors [367], or by demonstrate high catalytic activities towards the nonhydrolytic solgel processes [368]. oxidation of CO. Yan et al. [12] also synthesized CuCr 2O4/TiO 2 heterojunction via a facile CA 3.15.3 Nonalkoxide solgel method assisted solgel method for photocatalytic H 2 Nonalkoxide solgel process, involving evolution. The optimized composition of the hydrolysis and condensation of metal salts, avoids nanocomposites has been found to be the disadvantage of alkoxide solgel process (high CuCr 2O4.0.7TiO 2. And the optimized calcination sensitivity to moist environment), however, has temperature and photocatalyst mass concentration still the disadvantage of different hydrolysis are 500 0C and 0.8 g/l, respectively. susceptibilities of the individual components [25].

One of the advantages of this method is the 3.15.2 Alkoxide solgel method (Pechini important reduction of the required calcination Method) temperatures, minimizing the undesired The Pechini method [361,362] based on aggregation of the particles. This method was polymeric precursors, is used to prepare spinels found to be an effective route to synthesize mixed and it does not require high temperature oxide nanoparticles with narrow size distribution calcinations and permits good stoichiometric [25]. Ma et al. [369] presented a nonalkoxide sol control as well as reproducibility. This method gel route to synthesised highly active and selective consists of the formation of a polymeric resin CuCr catalysts for glycerol conversion. The between a metallic acid chelate and polyhydroxide synthesis involves dissolving 3.3 g of Cr alcohol by polyesterification. The metal nitrate (NO 3.3.9H 2O and 1.0 g of Cu(NO 3)2.3H 2O in 16 mL solution is mixed with a stoichiometric amount of of ethanol at 60 0C to give a clear dark blue citric acid. The resulting solution is stirred for solution. After adding 5.0 mL of propylene oxide, a about 1 hour on a hot plate and the temperature is dark green transparent gel is formed within a few stabilized at 70 0C. The mixture is heated to 900 minutes under stirring. After drying overnight in 0C, at which point ethylene glycol is added at a air at 70 0C, the resulting xerogel is transferred to mass ratio of 40:60 with respect to citric acid. The a quartz reactor inside a tubular resistance temperature is maintained constant up to resin furnace. The furnace is then ramped at 1 0C/min to formation, which polymerizes at 300 0C. The a final temperature and is held for 120 min under precursor powders are then calcined for 4 hours at 20% O 2 in Ar at a flow rate of 120 ml/min. The various temperatures, ranging from 500 to 900 0C, yield of copper and chromium in the oxide catalyst or at 900 0C for 8 hours [363]. The crystallization of is about to 100%, and the ratio of Cu to Cr could be the spinel structure starts upon calcining at 700 varied by the initial molar ratio of copper nitrate 0C. Cu 0.8 Ni 0.2 Cr 2O4 is the only phase present upon and chromium nitrate. calcination at 900 0C. The process of the Pechini The results show that the surface area of the Cu method is almost the same as that of the citrate gel Cr catalyst is adjusted by the hydrolysis method, except that metal nitrates are dissolved in conditions, Cu/Cr molar ratio, and treatment alcohols, instead of water [364]. conditions (such as gas atmosphere and final The major disadvantages of using the metal temperature). For the sample with Cu/Cr = 0.5, the alkoxide based solgel process are its moisture surface area of CuCr xerogel can reach 94 m 2/g sensitivity and the unavailability of suitable and decreased to only 31 m 2/g after calcination at commercial precursors especially for mixedmetal 500 0C. The catalysts show significant catalytic oxides. The solgel synthesis of mixed oxides from activity and selectivity in glycerol conversion, i.e. alkoxide mixture usually suffers from the different above 52% conversion of glycerol and above 88% hydrolysis susceptibilities of the individual selectivity to 1,2propanediol at 210 0C and 4.15 components and the benefits of improved MPa H 2 pressure. CuCr 2O4 supported Cu catalysts homogeneity can be lost during the hydrolysis of are much more active than Cr 2O3 supported Cu the alkoxides, which may ultimately lead to catalysts. This indicates a strong interaction component segregation and mixed phases in the between Cu and CuCr 2O4 that is significantly final materials. To achieve homogeneous mixed improving the effectiveness of the catalyst for oxides with predetermined compositions, the glycerol conversion. difference in reactivity has been minimized by controlled prehydrolysis of the less reactive precursor [365], by chemical modification of the precursors [366], by using singlesource

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4. Conclusions produced by thermal decomposition of ammoniac copper oxalate chromate. Mater. Lett. 59: 39453949. Depending on the ratio of oxides of CuCr and [3] Ma, Z.; Xiao, Z.; Bokhoven, J.A.V.; and Liang, C. 2010. A preparation technique the final products are nonalkoxide solgel route to highly active and selective usually called copper chromite (CuCr 2O4) and CuCr catalysts for glycerol conversion. J. Mater. Chem. Adkins’ catalyst (CuO.CuCr 2O4). This review 20:755760. summarized the potential applications of [4] George, K.; Sugunan, S.; 2008. Nickel substituted unsupported and supported CuCr catalysts and copper chromite spinels: Preparation, characterization advances in their preparation methods. The CuCr and catalytic activity in the oxidation reaction of Catal. 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There [13] Boumaza, S.; Bouarab, R.; Trari, M.; Bouguelia, A. 2009. is a significant increase in the thermal stability of Hydrogen photoevolution Over the spinel CuCr 2O4. the material by the addition of chromia with other Energ. Convers. Manage. 50: 6268. refractory materials like alumina, silica gel, etc.; [14] Valde´sSoli´s, T.; Marba´n, G.; Fuertes, A.B. 2006. because of the formation of copper chromite and Nanosized catalysts for the production of hydrogen by higher dispersion, inhibiting the sintering at high methanol steam reforming. Catal. Today 116: 354360. temperature. [15] Boumaza S.; Auroux, A.; Bennici, S.; Boudjemaa, A.; Trari, M.; Bouguelia,A.; Bouarab, R. 2010. Water gas Acknowledgement shift reaction over the CuB 2O4 spinel catalysts. Reac Kinet Mech Cat 100:145151. The authors are gratefully acknowledging the [16] Ginosar, D. M.; Rollins, H. W.; Petkovic, L. M.; Burch, financial support given for the project by the K. C.; Rush, M. J.; 2009. 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Table 2: Recent literature review at a glance on CuCr catalysts development for various reactions

Ref. Catalyst prepn. Expmnt. Operating conditions Remarks No method 1. Hydrogenation a) Hydrogenation of edible oils

69 CuCr 2O4 Soybean oil hydrogenation, 200 0C Prereduced catalyst deactivates due to disappearance of and 6 atm Cu(II) and Cu(I) species and to the decrement of Cu/Cr ratio on the catalyst surface b) Hydrogenation of polyunsaturated organic compounds 20 CuCr O, coppt. of Citral hydrogenation, 300 mg of At high pressure, unsaturated alcohol competes with hydroxide, caln 370 catal, 0.5 mL of substrate in 5 mL of initial reagent on hydrogenation site; at low pressure, 0C decalin at 140 0C, atm & subatm this competition decreases, but the allylic alcohol is pressure, batch reactor isomerized on another active site. c) Hydrogenation of allylic alcohol

20 CuCr/Al, coppt. of Crotyl alcohol, βmethallyl alcohol, Allylic alcohols react with H 2 and lead to several primary Cu and Al hydroxides flow apparatus, reagentconstant pp products, Cu +1 promote hydrogenation, Cr +3 /Al +3 isomeri 20 Torr, atm. pressure sation and hydrodeoxygenation d) Hydrogenation of fatty acids

71 CuCr 2O4 Fatty methyl ester hydrog. to fatty Both activity and selectivity correlate well with the crys alcohol, semibatch reactor, low H 2 tallinity of the copper chromite surface; they increase press elevated temp. with decreasing crystallinity. e) Hydrogenation of Furfural

1 CuCr 2O4 prereduced Furfural/ crotonaldehyde , fixed bed, Both activity & TOF for furfural and crotonaldehyde cat at 473, 573, or 15 g cat, temp. 423 & 473 K, press hydrogenation reached their highest values after reduc 673 K for 4 h 730 Torr H 2 & 10 Torr furfural tion at 573 K, compared to 473 and 673 K, f) Selective hydrogenation of furfural

3 9 CuCr 2O4 Selective hydrogenation of furfural to Cu 0 is the active phase, reduction with H 2 at 573 K for 4 89 furfuryl alcohol h produced the highest specific activity of catalyst due to the existence of maximum accessible Cu 0 species g) Selective hydrogenation of nitrobenzene to aniline 0 +1 82 Cu(Fe xCr 2−x)O 4 250 C, fixed bed flow reactor Optimum composition, x=0.4, Cu at octahedral sites is more active than Cu 0 h) Perfumery and synthesis of fragrances

84 CuCr 2O4 Partial hydrogenation of vegetable Selective formation of allylic alcohols for perfumery & oils & fatty acids synthesis of fragrances i) Hydrogenation of ketones

91 CuCr 2O4 Treatment of CuCr 2O4 in H 2 at 180 Exhibit activity in hydrogenation reactions including 370 0C ketone & olefin

2. Cyclization 124 CuCrFe/γ Al 2O3, sol Cyclization of alkanol amines, fixedbed Excellent activity, selectivity & long life Modified sol gel reactor gel method > common impregn. or coppt. method

6 CuCrBaAl 2O3 Cyclization of Nβhydroxyethyl1,3 Ba improved the dispersion of Cu and prevented it propanediamine to homopiperazine, from sintering, conversion93.2%100%, selectivity fixedbed 90%

3. Dehydrocyclisation 329 CuCr 2O4, CuCr 2O4/SiO 2 Low temperature, 2ETAN/ EG molar Dehydrogenation of EG to 2hydroxy acetaldehyde, ratios ≥10.0 followed by Nalkylation of 2ETAN to form a Schiff’s base, that cyclises to 7ETI

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Table 2: (cont’d)

4. Dehydrogenation a) Dehydrogenation of Methanol

92 CuCr 2O4, CuO CH 3OH dehydrog. to HCOH, slurry CuCr 2O4 deactivate slowly than Cucatalyst at 673 K 75%, CuCr 2O4 reactor, temp. 598673 K 25% 40 CuCr 2O4 Dehydrog. of CH 3OH to HCOOCH 3, Order half with respect to the partial pressure of CH 3OH, microreactor operated at atm. press. follow LH model

b) Dehydrogenation of Ethanol 93 CuCr 2O4, thermal Methanol dehydrogenation to for Highly selective to methyl formate ( > 95%) decomp. of coppt. maldehyde or methyl formate CuCr hydroxyl carbonate

c) Dehydrogenation of Iso-propanol 27 CuCr 2O4, wet im Isopropanol dehydrogenation to High selectivity and satisfactory activity pregnation acetone

d) Dehydrogenation of butanol 104 CuCr 2O4/electroless Dehydrogenation 5 wt.% addition of chromium to Cu have higher activity plating method of nbutanol than the 2 wt.% Cr, especially at higher chromium loading.

e) Dehydrogenation of isoamylic alcohol 105 CrCuMg layered Dehydrogenation of isoamyl alcohol Show a high conversion and selectivity, catalytic activity double hydroxides to isovaleraldehyde attributed to copper content, with hydro talcite structure, coppt.

5. Dehydration a) Dehydration of Glycerol 32 CuCr 2O4 Dehydration of glycerol, semibatch > 90 % acetol selectivity, hydrogenation of acetol to propyl reactive distillation ene glycol 106 CuCr catalyst, High surface area CuCr catalysts Reduced CuCr, selectivity 51% in glycerol conversion, carbon materials as 96% to 1,2 propanediol templates & ni trates precursors

6. Hydrogenolysis a) Hydrogenolysis of glycerol to propylene glycol

110 CuCr 2O4, Temperature 200 0C, Pressure 200 psi, Yield >73% achieved at moderate reaction conditions. Cu +1 specially designed stainless steel multi as active sites for hydrogenation clave reactor capable of performing eight reactions simultaneously

111 CuCr 2O4 Convert glycerol to PG, Lowpressure At lower temp. & pressures, 50100% conversion, little vapourphase processing, Semibatch selectivity towards ethylene glycol and other byproducts, reactive distillation Acetol as transient intermediate with selectivity levels ＞90%

28 CuCr 2O4, Co Hydrogenolysis of glycerol to propylene Precipitated catalyst >impregnated catalyst ppt., impregn. glycol, Prereduced cat at 320 0C, flow method reactor 3 CuCr, non Temp. 210 0C, H 2 Press 4.15 MPa, Glycerol conversion > 52%, 88% selectivity to 1,2 alkoxide solgel propanediol, CuCr 2O4 supported Cu catalysts are much route, Cal 500 0C more active than Cr 2O3 supported Cu catalysts, highly active and selective for glycerol conversion

7. Alkylation a) Alkylation of amines

119 BaO promoted The red. alkyl. of C 6H5NH 2 with The active sites of CuCr 2O4 used for red. alkyl. of amines CuCr 2O4 & un (CH 3)2CO to C 9H13 N, CuO44%, are Cu(I) species, catalyst reduced at 573 K showed the promoted CrO45%, BaO9%, catalyst10 g, pre highest activity CuCr 2O4 reduced

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Table 2: (cont’d)

8. Hydrogen Production a) Sulphur based thermo-chemical water splitting cycles 16 AB 2O4 structures H2 production, through Sbase Highest activity at temp. >850 0C, Activity order: thermochemical water spliting cy CuCr 2O4> CuFe 2O4 > NiCr 2O4≈NiFe 2O4 > MnTiO 3≈FeTiO 3. cles, temp.725900 0C 16 Nanocomposit 5 wt% catalyst, burning rate cata Cu/Cr molar ratio of 0.7 exhibits the most stable combus CuCrO, Pechini lysts for AP solid propellants tion at all pressures method, b) Photocatalytic H 2 evolution 12 CuCr 2O4 /TiO 2, CuCr 2O4/TiO 2 0.7, calc.500 0C catalyst H2 evolution from oxalic acid solution under simulated facile CAsol–gel mass conc.0.8 g/l sunlight irradiation, consistent with the Langmuir model method 13 CuCr 2O4, coppt. H2 evolution, solutions containing Thiosulfate gives best performance, excellent chemical sta NaOH0.5 M, Na 2S2O30.025 M with an bility, corrosion potential of 0.775 V SCE and an exchange average rate of 0.013 cm 3h1 (mg cata current density of 16 A cm 2, exhibits ptype extrinsic con lyst) 1 and a quantum efficiency of 0.2% ductivity. 129 CuCr 2O4, solid H2 evolution based on a dispersion of Low cost H 2 photocathode, inertness to corrosion, prolonged state reaction, CuCrO 2 powder in aq. electrolytes con irradiation (>80 min) leads to a pronounced decrease of the delafossite taining various reducing agents (S 2, photoactivity CuCr 2O4, SO 32 & S 2O32) c) Water Gas Shift Reaction and Methanol Steam Reforming 145 skeletal copper Role of additives in MSR and WGSR Enhance the activity for the WGSR, MSR and methanol catalysts+ Cr 2O3 synthesis reactions d) Water Gas Shift Reaction 15 Different cats. co Production of H 2 via WGSR, 150 & 250 CuZnAl>CuAl>CuMn, CuCr>CuFe>>ZnAl>CuCo ppt. method 0C e) Reforming of alcohols Methanol Steam Reforming 14 CuCr 2O4 nano Using template technique silica xero High activity and selectivity particulate ternary gel , 250 0C, WHSV = 5257 h 1 oxides Partial oxidation of methanol

150 Cu/M/Cr (M = Zn, Production of hydrogen by Partial oxi Exhibits high CH 3OH conversion and H 2 selectivity Ce, Fe, etc), coppt. dation of methanol method Oxidative steam reforming of methanol 151 Cr/Cuspinel Compact microstructured string reac High activity, and selectivity to carbon dioxide (98%) and tor hydrogen, methanol conversion 91.5% Steam reforming of ethanol

159 CuCr 2O4 Ethanol steam reforming at low temps H2 productivity 925 g Н2 (kg cat.) 1(h) 1 at 250300 0C. using 12 wt.% C 2H5OH in water mixtures

9. Pollution Abatement a) Abatement of CO/Soot 280 CuCr 2O4/ Al 2O3 COnC6H14 airoxid: CO 1.6% ,nC6H14 CuCr 2O4 most active catalyst coating of soot filters in CO Impregn. method 1.1%; reduction : 0.8% NO and 0.9 % and nhexane oxidation and in combustion of soot. CO in a flow reactor; S.V. 20000 h 1 229 Bi 2O3/CuCr 2O4 Oxidation of CO, precoating of NH 4+ on High catalytic activities CuO, complexing the surfaces of Bi 2O3 nanoparticles coppt. method b) Hydrocarbon abatement

213 CuCr 2O4/Al 2O3, Co COpropene oxidation, Temp. 300 Mech.: adsorbed propene transforms to a unidentate CO 32 impregn, metal 900 0C; prereduced cat. and liberates H 2O vapour 20%; Cu/Cr=1 calc. 500 0C

170 CuCr 2O4, impre CH 4 oxidation, Temp. range 300700 0C Activation energy 110 kJ/mol, reaction rates between 10 3 method, calc. 600 0 at constant, CH 4 : air ratio of 1 : 30, and 10 4 (mol CH4 )/(g h) C constant CH 4 1.2%.

254 CuCr 2O4/ Al 2O3 CH 4 oxidn. Mechn.: adsorb. O 2 on active sites, oxid.→red. form RE or LH model