Conversion of Biomass into Chemicals over Metal Catalysts

Review pubs.acs.org/CR

Conversion of Biomass into Chemicals over Metal Catalysts Michèle Besson, Pierre Gallezot,* and Catherine Pinel Institut de Recherches sur la Catalyse et l’Environnement (IRCELYON), Université de Lyon/CNRS, 2 Avenue Albert Einstein, 69626 Villeurbanne Cedex, France 5.1. Design of Metal Catalysts 5.2. Oxidation of Glucose 5.2.1. Oxidation to Gluconic Acid 5.2.2. Oxidation to 2-Ketogluconic and Glucaric Acids 5.3. Oxidation of Arabinose, Galactose, and Lactose 5.4. Oxidation of Cellobiose and Cellulose 5.5. Oxidation of 5-Hydroxymethylfurfural to 2,5Furandicarboxylic Acid 5.6. Oxidation of Glycerol 5.7. Oxidation of 1,2-Propanediol 5.8. Oxidation of Wood Extractives 5.9. Concepts Guiding the Choice of Metal Catalysts 6. Concluding Remarks and Prospects Author Information Corresponding Author Notes Biographies Acknowledgments References

CONTENTS 1. Introduction 2. Hydrogenation Reactions 2.1. Hydrogenation of Monosaccharides 2.1.1. Hydrogenation of Glucose 2.1.2. Hydrogenation of Fructose 2.2. Hydrogenation of Furanic Compounds 2.2.1. Hydrogenation of Furfural 2.2.2. Hydrogenation of 5-Hydroxymethylfurfural (HMF) 2.3. Hydrogenation of Biomass-Derived Carboxylic Acids 2.3.1. Hydrogenation of Succinic Acid 2.3.2. Hydrogenation of Levulinic Acid 2.3.3. Hydrogenation of Lactic Acid 2.3.4. Hydrogenation of Itaconic Acid 2.3.5. Hydrogenation of Arabinonic Acid 2.3.6. Hydrogenation of Glutamic Acid 2.4. Concepts Guiding the Choice of Metal Catalysts 2.5. Hydrogenation of Fatty Compounds 2.5.1. Hydrogenation to Edible Fats and Oils 2.5.2. Hydrogenation of CC Bonds in Fatty Acids 2.5.3. Isomerization of CC Bonds in Fatty Acids 2.5.4. Hydrogenation of Fatty Acids and Esters to Fatty Alcohols 2.5.5. Hydrogenation of Fatty Nitriles to Fatty Amines 2.6. Hydrogenation of Wood Derivatives 2.6.1. Hydrogenation of Tall Oil Products 2.6.2. Hydrogenation/Dehydrogenation of Terpenes 2.6.3. Conversion of Phenolic Compounds Derived from Lignin 3. Dehydroxylation/Hydrogenolysis Reactions 3.1. Hydrogenolysis/Dehydroxylation of Sorbitol, Xylitol, and Erythritol 3.2. Dehydroxylation of Glycerol 3.2.1. Glycerol to 1,2-PDO 3.2.2. Glycerol to 1,3-PDO 3.3. Concepts Guiding the Choice of Metal Catalysts 4. Hydrolysis/Hydrogenation of Polysaccharides 5. Oxidation of Carbohydrates and Derivatives © XXXX American Chemical Society

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1. INTRODUCTION The catalytic conversion of biomass and derivatives to chemicals has been the subject of intense research efforts during the past decade resulting in a 20% annual increase in the number of publications on the subject. Many review articles on biosourced chemicals have been published, either dealing with a large variety of biomass feedstocks and reactions types,1 or focusing on specific feedstocks such as carbohydrates,2 triglycerides,3 glycerol,4 5-hydroxymethylfurfural,5 cellulose,6 hemicelluloses and pentoses,7 lignin,8 and lignocellulose.6c,9 Some reviews were dedicated to specific reaction types such as hydrogenolysis/dehydroxylation,10 telomerization,11 metathesis,12 and oxidation.13 Requirements to develop cost-effective catalytic processes adapted to the molecular structure of highly functionalized biomass molecules and needs for process intensification have been highlighted.14 Issues about the choice of starting feedstocks, their processing in a biorefinery scheme, and the type of chemicals to be targeted have been addressed.15 While many reviews have dealt with metal catalysts employed for the conversion of biomass to biofuels, comparatively less attention has been paid to catalysts adapted to the biomass-tochemical value-chain. Recent investigations and reviews have

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Special Issue: 2014 Chemicals from Coal, Alkynes, and Biofuels

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dx.doi.org/10.1021/cr4002269 | Chem. Rev. XXXX, XXX, XXX−XXX

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focused on the design and mechanism of action of multifunctional catalysts.16 Some of the challenges to improve the activity, selectivity, and stability of metal catalysts have been pinpointed.16i,17 The addition of a second metal acting as a promoter of activity, selectivity, and stability was reviewed.1f,16i,18 Theoretical modeling has been applied to understand the reaction mechanism of biomass-derived molecules on the surface of metal particles.16c,19 Progress has been achieved in the development of multifunctional catalysts allowing process intensification,20 in the combination of homogeneous and heterogeneous catalytic processes,21 and in cascade catalysis combining enzymatic and chemo-catalytic steps.16b,22 The influence of organic impurities contained in biosourced raw materials on catalyst deactivation,18,23 the effect of porosity, hydrophilic, and acidic properties of supports,24 and the role played by water25 were considered. The importance of new reaction media and activation methods employed to improve biomass conversion and selectivity such as ionic liquids,26 molten salt hydrates,27 supercritical fluids,28 microwave activation,29 and ultrasonication30 has been pinpointed. This Review puts focus on the catalytic conversion of biosourced feedstocks into chemicals in the presence of monometallic, multimetallic, and multifunctional catalysts. The production of hydrocarbons, biofuels, and fuel additives was not considered. The performances of metal catalysts will be examined in the conversion of carbohydrates, triglycerides, and terpenes, and some applications will be mentioned on amino acids derived from proteins and phenolic compounds derived from lignin. Starting feedstocks will be either pure platform molecules obtained from carbohydrates by chemical or enzymatic processes or more complex mixtures of molecular species such as those present in plant oils or polysaccharides. Metal-catalyzed reactions including hydrogenation, dehydrogenation, dehydroxylation/hydrogenolysis, and oxidation reactions will be considered possibly in combination with acid/ base-catalyzed reactions such as hydrolysis and dehydration reactions to achieve a multistep conversion in one-pot process. Selected examples of biomass conversion into chemicals, either those already produced by traditional synthesis routes or those without synthetic counterpart, will be highlighted. In addition to pure chemicals, metal-catalyzed reactions leading to a mixture of chemicals that could be employed in the manufacture of high tonnage end-products such as paper additives, paints, resins, foams, surfactants, lubricants, and plasticizers will be considered.1e,31

Scheme 1. Glucose Hydrogenation to Sorbitol

HCl medium, hydrogenation on Ru/C catalysts, and dehydration) without isolation of intermediate products.27 The dehydration of a 5 wt % sorbitol solution at 245 °C under flowing H2 in a continuous fixed-bed reactor loaded with a Pt/ Al2O3−SiO2 catalyst was 100% selective to isosorbide at 20% conversion.33 Although sorbitol is a high tonnage commodity product, industrial reactions are still mainly carried out discontinuously in stirred tank reactors at 100−180 °C under 5−15 MPa of H2 pressure in the presence of suspended catalyst powders, usually Raney-type nickel catalysts (sponge or skeletal nickel). However, the performances of monometallic catalysts were steadily improved by the addition of metal promoters and by adjustments in particle size and porosity.17b,34 Thus, glucose hydrogenation over Mo-, Cr-, and Fe-promoted Raney-type nickel catalysts prepared by soda attack on Ni−Al−M alloys experienced a 7-fold rate enhancement with respect to unpromoted catalysts, which was attributed to the polarization of the CO bonds of the aldehydic form of glucose by electropositive metal promoters acting as Lewis acid sites;35 however, the Fe-promoted catalysts deactivated very rapidly by promoter leaching, while a slower aging of Mo- and Crpromoted catalysts was attributed to the poisoning of active sites by adsorbed organic species. More specifically, the deactivation of Raney-type Ni-catalysts was attributed to the presence of gluconic acid formed by the Cannizarro reaction, which poisoned the catalytic sites and favored nickel leaching.36 However, after many recycles under industrial operation, a loss of active surface area due to metal sintering was also pinpointed as a cause of catalyst deactivation.18 A process based on a threephase flow airlift loop reactor over Raney-type nickel afforded a 98.6% sorbitol yield.37 Several attempts have been made to use supported nickel catalysts as substitute for Raney-type nickel.38 The hydrogenation of 40 wt % glucose solution was studied at 130 °C under 8 MPa of H2 in a trickle-bed reactor in the presence of kieselguhr-supported nickel catalysts containing 48.4 wt % nickel;38c the activity was low (5 mmol h−1 gNi−1) and decreased with time because of the progressive leaching of nickel and support in the reaction medium. Ni−B/SiO2 amorphous catalyst prepared by reduction with KBH4 aqueous solutions exhibited a higher activity (TOF: 0.024 s−1) than commercial Raney-type catalysts.38a Ni/SiO2 catalysts were prepared by various methods, but deactivated by metal leaching, metal sintering, and support degradation.38d Ni/ SiO2 catalysts prepared by impregnation with nickel ethyl-

2. HYDROGENATION REACTIONS 2.1. Hydrogenation of Monosaccharides

2.1.1. Hydrogenation of Glucose. More than 800 000 ton/y of sorbitol is produced by catalytic hydrogenation of Dglucose obtained by hydrolysis of starch-containing crops (Scheme 1). Sorbitol is used as additives in many industrial products, particularly in the food, cosmetic, and paper industries, and as a building block for the synthesis of various fine chemicals including vitamin C. Among these products, it is worthwhile to mention the recent industrial development of isosorbide (1,4:3,6-dianhydro-D-glucitol) obtained by double dehydration of sorbitol, which is used as a green solvent and as an intermediate in the synthesis of pharmaceuticals, personal care products, polymers, and derivatives that could substitute phtalates and bisphenol A.32 Isosorbide was obtained from cellulose in three successive steps (depolymerization in ZnCl2/ B


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Table 1. Glucose Hydrogenation over Ruthenium Catalysts catalysts 1.6 wt % Ru/C 0.27 wt % Ru/TiO2 0.9 wt % Ru/ACC 4.5 wt % Ru/SiO2 10 wt % Pt/ACC 5 wt % Ru/Cnanotubes 8 wt % Ru/monolith 6.4 wt % Ru/Cmicrofibers 1 wt % Ru/(5 wt % NiO−TiO2) 1 wt % Ru/TiO2

reaction conditions 40 40 40 50 40 40 40 40 20

wt wt wt wt wt wt wt wt wt

% % % % % % % % %

glucose, glucose, glucose, glucose, glucose, glucose, glucose, glucose, glucose,

100 100 100 100 100 100 100 100 120

°C, °C, °C, °C, °C, °C, °C, °C, °C,

8 MPa (trickle-bed reactor) 12 MPa 8 MPa 8 MPa 4 MPa 4 MPa 8 MPa 8 MPa 5.5 MPa

activity (mol h−1 gmetal−1)

select/% (conv/%)

ref

0.7 11.5 2.4 6.0 1.8 168 h−1 0.08 7.7

99.6 (98) 85.0 (30) 99.5 (99.7)

40a 38d 44a 47 44 40b 45 43 46

99.5 (100)

97.5 (95.0) 93.0 (92.5)

a long time on stream in a trickle-bed reactor, a Ru/Al2O3 catalyst deactivated because of the structural modifications of alumina, and of the poisoning of ruthenium surface by sulfur compounds, gluconic acid, and iron atoms leached from the reactor walls.23a The loss of conversion from 99.9% to 98% after 1080 h on stream experienced by a Ru/Al2O3 catalyst was also attributed to the poisoning of the catalyst surface by metallic species leached out from the reactor walls.38d A 1.6 wt % Ru/C catalyst was subjected to a 3.7% loss of activity after 596 h on stream in a trickle-bed reactor, but the selectivity to sorbitol remained stable at 99.3%, and no leaching or sintering of ruthenium was detected;40a at total glucose conversion the selectivity decreased because sorbitol was epimerized into mannitol as the time of contact of the catalyst with sorbitol solutions increased. A bimetallic catalyst 1.6 wt % Ru−0.2 wt % Pt/C exhibited a more stable activity with time than a commercial 1.2 wt % Ru/C catalyst.18 Ru/C catalysts were used extensively in the hydrogenation of other hexoses such as arabinose, galactose, and rhamnose48 and disaccharides such as maltose48 and lactose.49 2.1.2. Hydrogenation of Fructose. The hydrogenation of D-fructose leads to sorbitol and mannitol, which are formed via the α- and β-furanose forms of fructose, respectively (Scheme 2).50 The challenge was to maximize the selectivity to mannitol, which is a high value-added, low caloric sweetener, by an appropriate choice of bimetallic composition. The selectivity to mannitol (mannitol formed/fructose converted) observed in various investigations is given in Table 2. The selectivity was close to 50% on platinum-group metal catalysts and Raney-type nickel.17b Copper catalysts oriented the reaction toward the formation of mannitol; thus a 67% selectivity was obtained on a 20 wt % Cu/SiO2 catalyst, and a further increase to 85% was obtained by adding borate to the reaction medium.50a Because copper catalysts have a low hydrogenation activity, attempts have been made to use more active platinum catalysts modified by metal promoters favoring mannitol selectivity. Thus, the selectivity to D-mannitol increased from 47% to 63% on Pt/C catalysts promoted by deposition of 1 wt % tin on a commercial 5 wt % Pt/C catalyst (Table 2).50b Inulin, an oligosaccharide based on fructose units, was converted to mannitol and sorbitol by combined hydrolysis and hydrogenation reactions over a bifunctional Ru/C catalyst where the carbon support was oxidized with ammonium peroxydisulfate to generate acid sites;51 a 40% selectivity to mannitol, similar to that observed by hydrogenation of fructose over a Ru/C catalysts, was obtained. A kinetic and modeling study of fructose hydrogenation was achieved over an industrial CuO−ZnO catalyst (61 wt % CuO, 39 wt % ZnO) in a batch reactor operating at 3.5−6.5 MPa and between 90 and 130

enediamine complexes did not leach significantly after 5 h on stream, but they were slightly less active than commercial catalysts and less selective to sorbitol.38b Nickel catalysts supported on ZrO2, TiO2, and ZrO2/TiO2 mixtures were more active and stable than Ni/SiO2 catalysts because of the absence of support leaching.39 Because nickel catalysts were prone to leaching and sintering and weakly active, processes based on supported ruthenium catalysts were progressively developed. The specific activities measured on nickel and ruthenium catalysts under the same reaction conditions showed that ruthenium was up to 50 times more active than nickel.36b,40 Reaction rates and selectivities to sorbitol measured over various Ru-catalysts are given in Table 1. Rates measured in trickle-bed reactors were lower as compared to continuously stirred tank reactors because of mass transfer limitation between the solid, liquid, and gas phases. The activity of Ru-catalysts was proportional to the ruthenium surface area and independent of the particle size.36b,40a A detailed kinetic and modeling study of glucose hydrogenation over a Ru/C catalyst in a semibatch slurry reactor was achieved.41 A 5 wt % Ru/MCM-41 catalyst prepared by impregnation with RuCl3 and formaldehyde reduction afforded an 83% yield to sorbitol, which decreased to 63% after four recycles.42 Ru-nanoparticles embedded in mesoporous carbon microfibers exhibited higher activity and stability than Ru-catalysts supported on multiwalled carbon nanotubes and activated carbons;43 the catalytic activity was improved by the incorporation of nitrogen in the microfibers. Activated carbon clothes (ACC) presented significant advantages with respect to conventional activated carbons in powder form such as efficient mass transfer from the liquid phase, no need of decantation or filtration, and high flexibility to fit into any reactor geometry;44 a 0.9 wt % Ru/ACC catalyst was very active (2.40 mol h−1 gRu−1) and selective to sorbitol (99.5% at 99.7% conversion) and could be easily recycled. The catalytic performances were even better with a 10 wt % Pt/ACC catalyst (Table 1). Ru-catalysts supported on γ-Al2O3 were compared in a monolith and in a stirred tank reactor;45 the kinetic analysis revealed that monolith-supported catalysts suffered from internal mass transfer resistances. The yield to sorbitol was higher over a 1 wt % Ru/(5 wt % NiO−TiO2) catalyst, where nickel oxide was added to TiO2 by impregnation with nickel chloride and subsequent calcinations, than over the unmodified 1 wt % Ru/TiO2 catalyst.46 All investigations pointed out that Ru-particles were stable to leaching in reaction media whatever their size.38d,40,47 However, supporting materials could be subjected to leaching; thus, while carbon, TiO2, and ZrO2 supports were very stable in reaction media, alumina23a and silica47 supports were transformed. After C


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alloys stabilized with polyvinylpyrrolidone prepared by reduction of nickel and cobalt salts with sodium borohydride;54 the catalysts were more active than NiB, CoB, and Raney nickel, but the selectivity to mannitol remained low. The kinetic modeling of fructose hydrogenation reaction was carried out over ruthenium and nickel catalysts.55

Scheme 2. Hydrogenation of Fructose

2.2. Hydrogenation of Furanic Compounds

2.2.1. Hydrogenation of Furfural. The acid-catalyzed hydrolysis of xylan-type hemicelluloses present in soft woods and straw yields C5 sugars such as arabinose and xylose, which can be further dehydrated to furfural (Scheme 3). Reviews were published on the hydrolysis of hemicellulose56 and on the chemistry of xylose7a and furfural.57 Scheme 3. Hydrogenation of Xylose and Furfural

Table 2. Fructose Hydrogenation into Mannitol catalysts 20 wt % Cu/SiO2 2.5 wt % Pt/C 2.5 wt % Pt/C, 1 wt % Sn 61 wt % CuO, 39 wt % ZnO 20 wt % Cu/SiO2 70 wt % CuO, 25 wt % ZnO, 5% Al2O3 CoNiB, PVPstabilized

reaction conditions

select/%

ref

30 g of fructose, 250 mL of H2O, 0.5 g of catalyst, 60 °C, 2 MPa 1 g of fructose, 80 mL of H2O, 0.05 g of catalyst, 100 °C, 10 MPa

67% (85% with borate)

50a

47% 63%

50b

30 wt % fructose, 110 °C, 5 MPa 110 °C, 5 MPa

68%

52

63%

30a

Xylose, the second most abundant carbohydrate after Dglucose, was hydrogenated to produce xylitol, which is employed in food, cosmetic, and pharmaceutical industries. Raney-type nickel catalysts were used industrially for the hydrogenation of xylose, but they deactivated due to promoter leaching, surface poisoning, or metal sintering. Acoustic irradiation during xylose hydrogenation in a three-phase hydrogenation system loaded with Raney-Ni catalyst increased the catalytic activity and decreased the deactivation rate;58 the effect was attributed to cavitation effect cleaning and grinding the catalyst, thus exposing fresh, uncontaminated active sites. Ru/C catalysts exhibited a higher activity than Raney-type nickel and were less prone to deactivation. Thus, 40 wt % aqueous solutions of xylose were hydrogenated in a continuous reactor packed with Ru/SiO2 and Ru/ZrO2 catalysts affording a 99.9% yield to xylitol.59 The hydrogenation of 20 wt % xylose solutions carried out over a 1 wt % Ru/(5 wt % NiO−TiO2) catalyst, where the TiO2 support was modified by an impregnation method using nickel chloride precursor and subsequent calcination, afforded a 99.7% yield to xylitol;60 NiO incorporated in the TiO2 support played an important role to minimize the formation of byproducts; however, the final solutions contained 11 mg L−1 of nickel leached away from the catalyst. Furfural (FAL) is currently produced industrially by the combined hydrolysis and dehydration of agricultural wastes7a,56a,61 or by the cyclodehydration of xylose on acid catalysts.62 Depending upon catalysts and process conditions, the hydrogenation of furfural was oriented to different products (Scheme 3). Most investigations were aimed at tuning the selectivity either to furfuryl alcohol (FOL), which is currently employed in the manufacture of foundry resins, adhesives, and

66%

10.7 g of fructose, 50 g of water, 2 mmol of catalyst, 70 °C, 4 MPa

50%

54

°C;52 the mannitol selectivity was within 60−68% and improved slightly as the hydrogen pressure increased or the reaction temperature decreased, but the catalyst deactivated rapidly due to copper and zinc leaching in solution. Fructose hydrogenation was investigated in a stirred tank reactor at 110 °C under 5 MPa of H2 over various heterogeneous catalysts in the presence of ultrasound irradiation;30a the selectivities over Cu/ZnO/Al2O3 and Cu/SiO2 catalysts were 66% and 63%, respectively, while Raney-Ni catalyst yielded only 50% of mannitol. The sonication had no effect on reaction selectivity, but it enhanced the reaction rate of Cu/SiO2 catalyst and retarded the deactivation of Raney-type Ni catalysts, which was attributed to a cleaning of the nickel surface. Fructose hydrogenation was carried out over various Raney-type nickel and copper catalysts;53 copper was less active than nickel, but favored the production of mannitol over sorbitol by a 2:1 ratio. Fructose hydrogenation was carried over amorphous CoNiB D


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Table 3. Hydrogenation of Furfural (FAL) to Furfuryl Alcohol (FOL) catalysts Raney-Ni + 7.1 wt % Cu3/2PMo12O40 0.6 wt % Pt/SiO2 0.6 wt % Pt−Sn0.3/SiO2 2 wt % Ir/TiO2 (473 °C) 2 wt % Ir/TiO2 (773 °C) Ir−ReOx/SiO2 copper chromite MoNiB/γ-Al2O3 (Mo:Ni = 1:7) Cu11.2Ni4.7/MgAlO PVP-stabilized colloidal Ru(0) + cyclodextrin 16 wt % Cu/MgO

select/% (conv/%)

reaction conditions 10 mL of FAL, 0.5 g of catalyst, 2 MPa, 80 °C, 1 h solvent: 2-propanol, 10 MPa, 100 °C solvent: heptane/ethanol, 0.6 MPa, 90 °C solvent: H2O, 0.8 MPa, 30 °C, 6 h supercritical CO2 (1.0 mL min−1 of CO2, 0.05 mL min−1 of FAL, 15 MPa), 1.9 g of catalyst, 140 °C 10 g of FAL, 40 mL of methanol, 2 g of catalyst, 5 MPa, 80 °C, 3 h 30 mL of FAL, 90 mL of ethanol, 1.0 g of catalyst, 1.0 MPa, 200 °C, 2 h Ru(0) (3.8 × 10−5 mol), PVP-K30 (3.0 × 10−4 mol), substrate/Ru(0) = 50, H2O (12 mL), H2 (1.0 MPa), 30 °C, 1.5 h vapor phase, H2:furfural = 2:5, 180 °C, GHSV = 0.05 mol h−1 gcat−1

ref

98.5 (98.1) 98.7 (36) 96.2 21 (30) 100 (30) >99 (>99) 98 91 (99) 89 (93) 97 (37)

63 65

67 28a 64 68 69

98 (98)

71

66

Table 4. Hydrogenation of Furfural to 2-Methylfuran (MF), Furan (FU), and Tetrahydrofuran (THF) catalysts Cu:Zn:Al:Ca:Na = 59:33:6:1:1 Cu−Fe/SiO2 (Cu:Fe = 50:50) copper chromite 5 wt % Pd/C 1 wt % Pd/Al2O3 + K2CO3 (8 wt % of K) stabilized colloidal Pd-particles/C

reaction conditions

select/% (conv/%)

ref

vapor phase, H2:FAL = 25, 250 °C vapor phase, 0.6 mol of FAL in toluene, H2/FOL = 5, 252 °C supercritical CO2, 240 °C (1.0 mL min−1 of CO2, 0.05 mL min−1 of FAL, 15 MPa), 1.9 g of catalyst supercritical CO2 (1.0 mL min−1 of CO2, 0.05 mL min−1 of FAL, 15 MPa), 250 °C, 0.5 g of catalyst, no H2 feed vapor phase at atm pres., WHSV = 0.77 h−1, H2/FAL = 20 (molar ratio), 260 °C

87 to MF (99.7) 98 to MF (99) 90 to MF

80 81 28a

98 to FU

28a

99.5 to FU (100)

82

2 mmol of FAL, 1.5 mL of formic acid, 1.5 mL of water, 0.1 g of catalyst, microwave irradiation, 100 °C, 30 min

15 to FU, 80 to THF (90)

85

wetting agents,57 but recent interest arose on 2-methylfuran (MF) considered as a promising biofuel.61 The hydrogenation of FAL into FOL was carried out industrially over copper−chromite catalysts in the liquid or vapor phase, but recent studies aimed at finding more efficient and environmentally acceptable catalysts that could selectively hydrogenate the carbonyl group and preserve the CC bonds. Table 3 gives the selectivities to FOL obtained with different catalysts. The solvent-free hydrogenation of FAL over RaneyNi modified by impregnation with Cu3/2PMo12O40 heteropolyacid (HPA) afforded a 98.5% selectivity to FOL at 98.1% conversion, while only a 75% selectivity at 25% conversion was observed on unmodified Raney nickel;63 the Mo-based HPA was a better selectivity promoter than W-based HPA. An amorphous MoNiB alloy supported on Al2O3 prepared by reducing NiCl2, 6H2O, and (NH4)Mo7O24·4H2O with NaBH4, afforded a 91% yield to FOL for the optimum Mo:Ni = 1:7 atomic ratio;64 a highly dispersed amorphous NiMoB alloy on the Al2O3 surface was deemed responsible for the catalyst stability. The liquid-phase hydrogenation of FAL dissolved in 2propanol over a 0.6 wt % Pt/SiO2 catalysts afforded a 98.7% selectivity to FOL at 36% conversion;65 however, the bimetallic Pt−Sn0.3 catalyst obtained by controlled surface reaction of SnBu4 with surface Pt-atoms was more active, although it underwent a 25% deactivation after recycling. The selectivity to FOL of a 2 wt % Ir/TiO2 catalyst in a 1:1 mixture of nheptane−ethanol depended greatly upon the reduction temperature of the H2IrCl6 precursor;66 thus, a 100% selectivity was obtained for the catalyst reduced at 500 °C, while the selectivity was only 21% at lower reduction temperatures because the presence of residual chlorine and acidic sites on the support favored the formation of hemiacetal byproducts. The hydrogenation of FAL over a Ir−ReOx/SiO2 catalyst provided a 99% yield to FOL under mild reaction conditions (30 °C, 0.8

MPa);67 it was assumed that ReOx species were responsible for the promotion of substrate adsorption and the assistance of heterolytic dissociation of H2. A CuNi/MgAlO catalyst prepared from a hydrotalcite-like precursor yielded FOL with a 89% selectivity at 93% conversion.68 A colloidal suspension of 2.2 nm Ru-particles was prepared in aqueous medium by using controlled mixtures of PVP (polyvynilpyrrolidone) and various types of cyclodextrins acting as stabilizers;69 a 97% selectivity to FOL was obtained while the Ru-particles remained stable against aggregation. A switchable system based on two consecutive fixed-bed flow reactors loaded with copper chromite (R1) and 5 wt % Pd/C catalysts (R2), respectively, was designed to convert furfural in supercritical CO2 into various hydrogenated derivatives;28a depending upon the respective reactor temperature, the following selectivities were obtained: 98% of FAL with R1 at 140 °C, 96% of tetrahydrofurfuryl alcohol (THFOL) with R1 at 120 °C and R2 at 200 °C, 90% of MF (R1 at 240 °C), 82% of 2methyltetrahydrofuran (R1 at 240 °C, R2 at 300 °C), and 98% of furan with R2 at 250 °C (Tables 3 and 4). While in most studies a high selectivity to FOL was obtained in the absence of water, FAL dissolved in water was converted to cyclopentanone with a 76.5% yield over a 5 wt % Pt/C catalyst via a reaction mechanism involving acidic sites after 30 min of reaction at 160 °C under 8 MPa of H2.70 The vapor-phase hydrogenation of FAL at atmospheric pressure on a 16 wt % Cu/MgO catalyst prepared by coprecipitation yielded FOL with a 98% selectivity at 98% conversion (Table 3);71 the high catalyst activity was attributed to the simultaneous presence of Cu0 and Cu+ sites on the catalyst surface. The vapor-phase hydrogenation of FAL on a 10 wt % Cu/SiO2 catalyst at 290 °C yielded 65% of FOL with minor amounts of MF;72 a detailed kinetic analysis was performed, and according to DFT calculations and DRIFTS E


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furan was obtained. Furan was hydrogenated to tetrahydrofuran (THF) over Pd/zirconia and Pd/alumina catalysts.83 A continuous hydrogenation of furan in supercritical CO2 was achieved on a 5 wt % Pd-catalyst supported on aminopolysiloxane;84 a 96% selectivity to THF at 98% conversion of furan was achieved while 1-butanol was the only side product formed by hydrogenolysis reactions. The hydrogenation of FAL with hydrogen generated by formic acid decomposition induced by microwave irradiation was achieved at 100 °C in the presence of Pd-colloidal particles stabilized by trioctyl- or triphenyl-phosphine supported on an oxidized carbon;85 a mixture of furan (15%) and THF (80%) was obtained at 90% FAL conversion (Table 4). The mechanism of furfural hydrogenation on platinum surface was studied by surface science tools. The hydrogenation of furan to THF was investigated over Pt(111) and Pt(100) single-crystal surfaces and size-controlled Pt-nanoparticles.86 Colloidal Pt-nanoparticles with an average diameter of 1.9 nm were incorporated into mesoporous oxides by sonicationinduced capillary inclusion, and their catalytic properties were evaluated in the hydrogenation of FAL (70 Torr of FAL and 700 Torr of H2);87 while Pt-nanoparticles within the size range 1.5−7.1 nm exhibited strong structure-dependent selectivity, various supports loaded with an homogeneous distribution of 1.9 nm Pt nanoparticles produced furan as the major product. The selectivity to FOL was higher on a TiO2 support because the carbonyl group hydrogenation was enhanced by a charge transfer interaction between the Pt-particles and the acidic surface of the oxide. As FAL hydrogenation (1 Torr of FAL, 100 Torr of H2, 659 Torr of He) was performed over Ptnanoparticle monolayers deposited on oxide substrates, only TiO2 enhanced the selectivity to FOL, while other oxides produced furan. The vapor-phase hydrogenation of furfural was investigated at atmospheric pressure over 1.5−7.1 nm Ptnanoparticles of various shapes (sphere, cube, octahedron) encapsulated in poly(vinylpyrrolidone) (PVP) and dispersed on a MCF-17 mesoporous silica.88 Furan and FOL were the two primary products formed by FAL decarbonylation and hydrogenation reaction, respectively. Both reactions exhibited structure sensitivity evidenced by the changes in product selectivity, turnover rate, and apparent activation energy as a function of Pt-particle size and shape. Thus, an increase in Ptparticle size from 1.5 to 7.1 nm increased the selectivity to FOL from 1% to 66%, while smaller particles (1.5 nm) afforded a high selectivity (96%) toward furan formation. Octahedral particles were selective to FOL, while cube-shaped particles produced an equal amount of furan and FOL. The different selectivities were attributed to the presence of two catalytically active sites on the Pt-surface the ratio of which changed with particle size and shape. 2.2.2. Hydrogenation of 5-Hydroxymethylfurfural (HMF). Reviews on 5-hydroxymethylfurfural (HMF) production by acid-catalyzed dehydration of fructose and glucose or hydrolysis/dehydration of polysaccharides, and its use as platform molecule for organic synthesis, were recently published.5 The main products obtained by catalytic hydrogenation of HMF over supported metal catalysts under various conditions are given in Scheme 4, and the yields to different products are given in Table 5. Two chemicals were specifically targeted, 2,5-dihydroxymethyltetrahydrofuran (DHMTHF) finding applications as a solvent and a building block for polymer synthesis,89 and 2,5-dimethylfuran (DMF), which is a potentially attractive transportation fuel that could be obtained

studies the most likely specie adsorbed on Cu- surface was a top η1(O)-aldehyde and the hydrogenation of FAL occurred via either an alkoxide or a hydroxyalkyl intermediate. The vaporphase hydrogenation of FAL catalyzed by Ni/SiO2 catalyst consisting of Ni-particles smaller than 4 nm yielded 94% of THFOL.73 A combined process of xylose dehydration and furfural hydrogenation was achieved in a biphasic reactor in the presence of 1-butanol, 2-methyltetrahydrofuran, and cyclohexane as solvents;74 the dehydration was carried out over Amberlyst-15 in the aqueous phase and the hydrogenation over a hydrophobic Ru/C catalyst in the organic phase. An increase in the hydrophobicity of the solvent, in the order 1-butanol < 2methyltetrahydrofuran < cyclohexane, suppressed the hydrogenation of xylose to xylitol due to a decrease of xylose solubility and led to furfural hydrogenation products, mainly THFOL. The hydrogenolysis of THFOL was used to synthesize 1,5propanediol at 120 °C under 8 MPa of H2 over Rh/SiO2 catalysts modified with Re, Mo, or W promoters;75 the catalytic activity was enhanced by almost 1 order of magnitude, and a 80% selectivity to 1,5-propanediol was obtained by addition of an optimized amount of modifiers. A 94% yield to 1,5pentanediol was obtained in the presence of a Remodified Rh/ C catalyst.76 A 82% yield to 1,5-propanediol was achieved in the presence of a 1.5 wt % Ir−Re/SiO2 catalyst (Re/Ir = 2);77 a reaction mechanism was proposed whereby hydride species formed on the Ir-surface reacted with the carbon adjacent to the alcohol group adsorbed on ReOx clusters. 1,5-Propanediol was also obtained directly from furan derivatives in the presence of a Pt/Co2AlO4 catalyst with a 31% yield.78 The use of supercritical CO2 in the hydrogenolysis of THFOL was investigated over a 1 wt % Rh/MCM-41 catalyst.79 Without any additive, THFOL was converted to 1,5-pentanediol with high conversion (80.2%) and selectivity (91.2%) at 80 °C under 4 MPa of H2. The conversion increased with the CO2 pressure due to the enhanced solubility of THFOL and reached a maximum at 14 MPa as a single phase (CO2−H2−substrate) was formed, but the selectivity of 1,5-pentanediol remained unaltered. The H2 pressure changed the conversion as well as the selectivity. The conversion increased along with the temperature, but the selectivity to 1,5-pentanediol dropped after reaching 120 °C. As the reaction was carried out in H2O instead of CO2, the conversion and the selectivity of 1,5pentanediol decreased substantially; however, the addition of only 7 MPa of CO2 modified the conversion and the product selectivity. The hydrogenation of FAL to 2-methylfuran (MF) triggered interest because of its good fuel performances.61 FAL hydrogenation carried out at 250 °C in a fixed-bed reactor over a multicomponent commercial catalyst (Cu:Zn:Al:Ca:Na = 59:33:6:1:1) yielded 87% of MF (Table 4).80 The hydrogenation of FAL in vapor-phase at 252 °C over a Cu/ Fe catalyst, prepared by coprecipitation of salts on a silica support followed by calcination and H2 reduction, afforded a 98% yield to MF, but the activity dropped after 20 h of operation (Table 4).81 The hydrogenation of FAL in supercritical CO2 on a copper chromite catalyst at 240 °C resulted in a 90% selectivity to MF (Table 4).28a The vapor-phase decarbonylation of FAL to furan was performed in a fixed-bed reactor at 260 °C on a 1 wt % Pd/ Al2O3 catalyst loaded with potassium carbonate to promote the decarbonylation and suppress hydrogenation side reactions;82 for an optimum 8 wt % potassium loading, a 99.5% yield to F


Chemical Reviews

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hexanetriol was obtained after 24 h with a 73% selectivity to 1,6 hexanediol.96 2,5-Dimethylfuran (DMF) was obtained with a 79% yield by vapor-phase hydrogenolysis of 10 wt % HMF in 1-butanol solution in a flow reactor loaded with Cu−Ru/C catalyst (Cu:Ru = 3:2) prepared by incipient wetness impregnation with Cu(NO3)2 solutions of a 10 wt % Ru catalyst supported on a Vulcan XC-72 carbon (Table 5).97 The hydrogenation of HMF dissolved in [EMIM]Cl and acetonitrile at 120 °C under 6 MPa of H2 pressure over a 10 wt % Pd/C catalyst resulted in a 32% selectivity to 2,5-DMF at 47% conversion.98 A 95% yield to DMF was obtained by heating a solution of HMF in refluxing tetrahydrofuran in the presence of formic acid, H2SO4, and a Pd/C catalyst.99 The one-pot conversion of agar, obtained from microalgae carried out in a reaction medium invoving (DMA)+(CH3SO3)− (DMA = dimethylacetamide) as solvent, a 5 wt % Ru/C catalyst, and formic acid as the hydrogen source afforded a 27% yield to DMF.100 The hydrogenation of a 1:1 mixture of pure HMF and FAL in 1butanol solution over a Ru/C catalyst afforded a 60.3% yield to DMF and a 61.9% yield to MF at 100% conversion of HMF and FAL;62c lower yields were obtained by hydrogenation of a mixture of products obtained by catalytic dehydration of glucose and xylose unless purification treatments were carried out. The catalytic hydrogenation of HMF in supercritical methanol over a Cu-doped porous metal oxide obtained from an hydrotalcite-like material, synthesized by coprecipitation of a mixture of Al(NO3)3, Mg(CH3COO)2, and Cu(NO3)2 with Na2CO3 and NaOH, afforded 48% and 10% yields to DMF and DMTHF, respectively (Table 5);101 the hydrogen needed for the reduction of HMF originated from the reforming of the solvent. DMTHF was obtained with a 79% yield from fructose by hydrogenation in toluene/HI mixture over a soluble RhCl3 catalyst;102 the dehydration of fructose to HMF was catalyzed by HI, and the hydrogenation to DMTHF was catalyzed by rhodium particles formed by hydrogen reduction of RhCl3.

Scheme 4. Hydrogenation of 5-Hydroxymethylfurfural

economically from HMF providing cheap and efficient hydrogenation catalysts are developed.90 The catalytic hydrogenation of HMF in neutral aqueous medium yielded mainly dihydroxymethylfuran (DHMF) over Cu- and Pt-catalysts and DHMTHF over Ni-, Pd-, and Ru-catalysts, while acidic media favored hydrogenolysis reactions leading to C6 diols and tetrols.91 The hydrogenation of HMF over a silica-supported Ni−Pd alloy (Ni/Pd = 7) prepared by a coimpregnation method yielded 96% of DHMTHF.92 The hydrogenation of HMF over a 1 wt % Ru/CeO2 catalyst yielded 91% of DHMTHF with C6 polyols as major byproducts;16e other supports with isoelectric points >7 such as alumina and Mg−Zr oxide provided high selectivities to DHMTHF, while Pdcatalyst on silica support with an isoelectric point Rh > Ru. Some leaching of the active metal from the catalyst support was detected in supercritical condition. β-Myrcene hydrogenation was also performed at various hydrogen pressures in the range from 2.0 to 4.5 MPa at a fixed total pressure of 12.5 MPa.212d The reaction proceeded in two phases because the total pressure was below the critical pressure of the CO2+βmyrcene+H2 system. The rate-controlling factor of β-myrcene hydrogenation was identified as the H2 to β-myrcene ratio. 2.6.3. Conversion of Phenolic Compounds Derived from Lignin. Phenolic compounds obtained from lignin by various depolymerization/degradation treatments8b,213 were converted in the presence of metal catalysts and hydrogen into fuels and chemicals via various reaction pathways. Most investigations aimed at producing hydrocarbons, for example, by hydrodeoxygenation reactions in the presence of supported metals and acidic solids.214 The conversion into chemicals of various types of lignin or model compounds catalyzed by supported metals was reviewed.8a The conversion of model compounds representative of lignin and lignin-derived bio-oils (guaiacol, anisole, 4-methylanisole, and cyclohexanone), over a Pt/Al2O3 catalyst in the presence of H2 at 300 °C, was described by a complex reaction network involving hydrogenolysis and transalkylation reactions.215 Various types of lignins solubilized in ethanol−water mixtures were subjected either to a reduction at 200 °C under 3 MPa of H2 in the presence of carbon- or alumina-supported Pt-catalysts or to aqueous phase reforming in acidic or basic medium over Pt/ Al2O3 catalysts;216 these reactions yielded 6 and 17 wt % of guaiacol type products, respectively.

Scheme 17. Hydrogenation/Dehydrogenation of Limonene and Hydrogenation of β-Myrcene

3. DEHYDROXYLATION/HYDROGENOLYSIS REACTIONS Biomolecules are rich in oxygen-containing functionnalities such as hydroxyl and carbonyl groups. The conversion of these molecules into lower oxygenates requires a complex set of reactions such as C−OH hydrogenolysis, combined dehydration−hydrogenation, combined condensation−hydrogenolysis, as well as decarbonylation or hydrogenation of carboxylic acids. Modified ruthenium and copper catalysts were able to convert polyols such as sorbitol, xylitol, and glycerol to valuable diols and triols by various reaction pathways such as dehydration− hydrogenation, and retro-Michael and retro-Claisen reactions.217 The various mechanisms of dehydroxylation involve bi or multifunctional catalysts including acidic or basic sites and metallic sites.10a Because of the economic importance of biomass conversion to fuels, the complete deoxygenation of biomass feedstocks to produce hydrocarbons from carbohy-

hydrogen as carrier gas over a 0.5 wt % Pd/SiO2 catalyst;211 a 100% yield to p-cymene was obtained over more than 500 h on stream. Investigations were devoted to the hydrogenation of terpenes under high pressure of CO2 and in supercritical CO2 condition.212 The hydrogenation of limonene (Scheme 17) was carried out in supercritical condition over 1 wt % Pd/C and 1 wt % Pt/C catalysts at 50 °C under 4 MPa of H2 and 16 MPa of CO2;212a the trans- and cis-p-menthane were formed in 2:1 and 1:1 ratios over Pd- and Pt-catalysts, respectively, which was interpreted by two different mechanisms of adsorption involving π-allyl-absorbed species on palladium and 1,2-σ2 adsorbed entities on platinum. The effect of the flow rate of a biphasic reaction mixture under high CO2 pressure on the distribution of limonene hydrogenation products was studied in detail.212b The catalytic performance in four kinds of overall flow rate conditions was compared under fixed hydrogen pressure (2.5 MPa) and total pressure (12.5 MPa) over a 1 wt Q


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to 39%, while upon addition of Ca(OH)2, the conversion decreased only from 75% to 63%. The deactivation in the absence of Ca(OH)2 was attributed to the damage caused to the support by the pH decrease. Sorbitol hydrogenolysis carried out over a 3 wt % Ru/CNF catalyst prepared by incipient wetness impregnation of carbon nanofibers and promoted with calcium hydroxide yielded mainly EG (19.3%) and 1,2-PDO (31.9%).223 Because the hydrogenolysis activity of ruthenium, nickel, and platinum catalysts was too high, less active copper catalysts were employed to dehydroxylate sorbitol without C−C bond breaking into a mixture of C6 polyols suitable for the manufacture of alkyd resins;224 21 wt % sorbitol aqueous solutions were converted into a mixture of C4−C6 diols, triols, and tetrols containing 63% of deoxyhexitols at 180 °C under 13 MPa of H2-pressure over a commercial CuO−ZnO (33:65) catalyst usually employed for alcohol synthesis from syngas. Sorbitol was dehydrated into cyclic ethers such as isosorbide in the presence of bimetallic Ru−Cu catalysts under hydrogen atmosphere, which avoided the formation of degradation products.217c Sorbitol solutions acidified with propionic acid were cyclodehydrated in the presence of 3 wt % Pd/C catalyst into a mixture of cyclic ethers containing 38% of isosorbide and 58% of tetrols (2,5-anhydromannitol, 1,4-anhydrosorbitol, and 2,5-anhydroiditol).225 The mixture of polyols obtained either by dehydroxylation or by dehydration was successfully employed without any further separation or purification to substitute pentaerythritol and other synthetic polyols for the manufacture of polyesters and alkyd resins meeting the specifications required for coating applications. The dehydration of 5 wt % sorbitol solution at 245 °C under flowing hydrogen in a continuous fixed-bed reactor loaded with Pt/ Al2O3−SiO2 catalyst was 100% selective to isosorbide at 20% conversion.33 Xylitol widely available by combined hydrogenolysis/hydrogenation of hemicellulose is a promising feedstock for the production of smaller polyols. The hydrogenolysis of 10 wt % xylitol solution was carried out at 200 °C under 4 MPa of H2 on 4 wt % Ru/C catalysts in the presence of Ca(OH)2;226 the selectivities to EG, 1,2-PDO, and GLY were 32.4%, 24.9%, and 9.6%, respectively, at 20% conversion. It was proposed that xylitol hydrogenolysis to ethylene glycol and propylene glycol involves the dehydrogenation of xylitol to xylose on the metal surfaces, and subsequent base-catalyzed retro-aldol condensation of xylose to form glycolaldehyde and glyceraldehyde, followed by direct glycolaldehyde hydrogenation to ethylene glycol and by sequential glyceraldehyde dehydration and hydrogenation to propylene glycol. For a better understanding of the mechanisms involved during hydrogenolysis of polyols, the reactivity of nine C3−C6 polyols was evaluated in the presence of Ru/C catalysts at 205−240 °C under 10 MPa of H2;227 the reaction rate of the polyols was affected by the

drates, pyrolysis oils, and triglycerides has been the subject of many investigations.218 It was demonstrated that aqueous solutions of carbohydrates can be converted in a single step over PtRe/C catalysts to a mixture of alcohols, ketones, acids, heterocycles, the composition of which was controlled by adjusting process variables.20a Aqueous phase reforming of sorbitol under inert atmosphere at various temperatures resulted in 260 compounds in liquid phase plus hydrocarbons in the gas phase.219 However, because aqueous phase reforming was more directed at producing hydrocarbons and fuels, this process is out of the scope of this Review. The present section will focus on the conversion of polyols to chemicals under hydrogen pressure over metal catalysts. 3.1. Hydrogenolysis/Dehydroxylation of Sorbitol, Xylitol, and Erythritol

Sorbitol obtained by hydrogenation of glucose, or directly from starch by combined hydrolysis−hydrogenation reaction, is a cheap feedstock for further conversion to valuable polyols (Scheme 18). Nickel and ruthenium catalysts used in early Scheme 18. Hydrogenolysis/Dehydroxylation of Sorbitol

studies yielded C2−C3 hydrogenolysis products such as ethylene glycol (EG), 1,2-propanediol (1,2-PDO), and glycerol (GLY) (Table 9). A 40% selectivity to glycerol was obtained on a 50 wt % Ni catalyst supported on kieselguhr with a Ca(OH)2 additive.220 The hydrogenolysis of sorbitol over Ru-catalysts in basic medium proceeded via a reverse aldolization yielding a mixture of C2−C3 products, while in neutral medium at lower temperatures hydrogenolysis occurred at the middle of the carbon chain yielding 1,2-PDO and glycerol.217a Sorbitol hydrogenolysis on a 6 wt % Ni−NaY catalyst afforded 62% and 14% selectivities to 1,2-PDO and EG, respectively, while GLY was the major product over a Pt−NaY catalyst.221 The difference in product selectivity observed between Pt- and Nicatalysts was attributed to the different modes of adsorption of sorbitol as determined by DFT calculations.222 The 6 wt % Ni− NaY catalyst underwent a severe loss of conversion from 66% Table 9. Sorbitol Hydrogenolysis

a

catalysts

reaction conditionsa

50 wt % Ni/kieselguhr 5 wt % Ru/SiO2 3 wt % Ru/CNF+CaO 6 wt % Ni/NaY 1 wt % Pt/NaY

40; 215;14 10, 220, 8 (pH 12.5) 20, 220, 8 15, 220, 6

conv/% 90 85.7 66 43

EG/%

1,2-PDO/%

GLY/%

ref

16 36 19.3 7 3

16 14 31.9 62 6

40 18 9.5 14 62

220 217a 223 221

wt % sorbitol aqueous solution; temperature/°C; H2-pressure/MPa. R


Chemical Reviews

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Scheme 19. Dehydroxylation of Glycerol to 1,2-Propanediol and 1,3-Propanediol

Table 10. Glycerol Dehydroxylation to 1,2-Propanediol catalysts

reaction conditions

conv/%

select/%

ref

CuO/ZnO/Ga2O3 CuO/ZnO (60/40) Cu0.4Mg5.6Al2O8.6 Cu/Al2O3 6 wt % Cu/3 wt % ZnO/Al2O3 20 wt % Ni−15 wt % Cu/Al2O3 2 wt % Ag/Al2O3 3.6 wt % Ru/TiO2 5 wt % Ru/ZrO2 5 wt % Ru/C 5 wt % Ru/C 5 wt % Ru/Cs2.5H0.5[PW12O40] 5 wt % Ru/CaZnMgAl 5 wt % Ru/bentonite-TiO2 (1:2) 28 wt % Cu−5 wt % Ru/CNT 3 wt % Ru−0.2 wt % Cu/ZrO2 5 wt % Ru−2.5 wt % Fe/CNT 2 wt % Pt/hydrotalcite 2.7 wt % Pt/NaY 1 wt % Pt/SiO2−Al2O3

90 wt % GLY, 5 MPa, 220 °C 90 wt % GLY, 5 MPa, 220 °C 75 wt % GLY, 3 MPa, 180 °C 10 wt % GLY, H4SiW4O40, 6 MPa, 240 °C 11.4 wt % GLY; MeOH = 7.2 wt %, 3.5 MPa (N2), 250 °C 4 wt % GLY, 4.5 MPa N2, 220 °C, HCOOH 50 wt % GLY, 1.5 MPa, 220 °C 20 wt % GLY, 3 MPa, 170 °C 100 wt % GLY, 8 MPa, 240 °C 40 wt % GLY, Amberlyst 70, 8 MPa, 170 °C 20 wt % GLY, +HPA/ZrO2, 6 MPa, 180 °C 20 wt % GLY, 0.5 MPa, 150 °C 20 wt % GLY, 2.5 MPa, 180 °C 20 wt % GLY, 2 MPa, 150 °C

96 52 80 90 89 90 46 66 41 49 44 21 59 70 100 100 86 92 85 27

80 98 98 90 39 82 96 48 61 70 64 96 86 80 86 79 52 93 64 35

238 239 240 241 243 244 245 246 248a 249 250 251 252 253 254 255 256 257 258 259

Gly = 20 wt 20 wt 20 wt 20 wt

60 wt %, 8 MPa, 180 °C % GLY, 4 MPa, 200 °C % GLY, 3 MPa, 220 °C % GLY, 230 °C, N2 % GLY, 220 °C, N2

catalytic reactions such as oxidation, ammoxidation, dehydration, esterification, etherification, and hydrogenolysis.4a−c Glycerol is obtained as a coproduct of the transesterification of triglycerides to fatty acid esters used as a diesel fuel, but because of hundreds of other applications its availability for the economic production of high tonnage intermediates could be overestimated unless new abundant and cheap source of triglycerides become available, for example, from algae. Glycerol is also produced as a byproduct of ethanol production by fermentation of sugars; although the extraction of glycerine from this residual stream is not economically feasible today, the ethanol value-chain is potentially an additional source of glycerol.230 Many investigations on the dehydroxylation of glycerol to 1,2-propanediol (1,2-PDO) and 1,3-propanediol (1,3-PDO) catalyzed by metals have been reported, and the subject was reviewed.10a,231 Glycerol hydrogenolysis into propanediols using in situ generated hydrogen was also reviewed.232 3.2.1. Glycerol to 1,2-PDO. Depending on its purity, 1,2PDO could be used as industrial fluid (antifreeze agent, hydraulic fluid, solvent) or in cosmetic and food applications.

configuration of the stereoisomers and was almost independent of the carbon chain length. The hydrogenolysis of sorbitol and xylitol solutions in a mixture of water/dodecane at 170 °C under 8 MPa over a Ir-ReOx/SiO2 catalyst combined with HZSM-5 acting as cocatalyst yielded 95.3% of n-hexane and 95.9% of n-pentane, respectively, at 99.9% conversion;228 a combined hydrogenation/hydrogenolysis of glucose and cellobiose substrates was also achieved. The hydrogenolysis of 20 wt % aqueous solution of erythritol in acidic medium was performed at 200 °C under 8 MPa of H2 over a Ir−ReOx/SiO2 (Re/Ir = 1) catalyst for the production of butanediols.229 The maximum selectivity to 1,4- and 1,3butanediols reached 33% and 12% at 74% conversion, respectively, and was almost maintained during four repeating tests. The kinetics and catalyst characterization data suggested a mechanism whereby the hydride species formed on the metal surface reacted with the alkoxide species adsorbed on the ReOx clusters at the interface with the Ir-particles. 3.2. Dehydroxylation of Glycerol

Glycerol is a platform molecule from which many useful intermediates or specialty chemicals can be produced by S


Chemical Reviews

Review

additives. Ru-nanoparticles supported on nanotubes showed high performances for glycerol hydrogenolysis to 1,2-PDO (60% selectivity) at a moderate 40% conversion;197 the authors clearly showed that the mean size of Ru particles played a significant role in the glycerol conversion and product selectivities. A 48% selectivity to 1,2-PDO at 66% conversion after 12 h was obtained at 170 °C under 3 MPa of H2 over Ru/ TiO2 catalysts.246 Ru/TiO2 catalysts were still efficient when crude glycerol was used as feedstock.247 Ru/ZrO2 and Ru/SiO2 catalysts exhibited also a high selectivity to 1,2-PDO (61%), but at a lower conversion (41% and 21%, respectively).248 A 77% selectivity to 1,2-PDO at 21% conversion was obtained after 10 h at 120 °C under 8 MPa of H2 using highly dispersed Ruparticles deposited on low surface area active carbons in the presence of Amberlyst15 acidic resin;249 the reaction at 180 °C with Amberlyst70, a more thermally stable resin, resulted in a higher selectivity to 1,2-PDO (70%) at 49% conversion, and the catalytic system was stable over three cycles. Bifunctional ruthenium catalysts were designed to improve the selectivity to 1,2-PDO. Glycerol was converted over a commercial 5 wt % Ru/C catalyst at 180 °C under 6 MPa of H2 pressure in the presence of cocatalysts (niobium oxide, tungstophosphoric acid (TPA) on ZrO2), cesium-exchanged TPA (CsTPA), and CsTPA supported on ZrO2);250 whatever the cocatalyst, the selectivity to 1,2-PDO was in the range of 60−70%, and the conversion was slightly below 45%. The conversion was correlated with the density of acid sites of the cocatalyst, and a synergetic effect between the acid and the metal catalyst was put forward. The hydrogenolysis of glycerol carried out under low hydrogen pressure (0.5 MPa) in the presence of a 5 wt % Ru/Cs2.5H0.5PW12O40 catalyst afforded a 96% selectivity to 1,2-PDO at 21% conversion after 10 h;251 the catalyst underwent a deactivation due to the reduction of the cocatalyst resulting in a limited 30% conversion after 14 h. The hydrogenolysis of 20 wt % glycerol aqueous solution at 180 °C under 2.5 MPa of H2 over a 5 wt % Ru catalyst supported on hydrotalcite modified with Ca and Zn additives afforded a 85.5% selectivity to 1,2-PDO at 58.5% conversion.252 A 5 wt % Ru/bentonite-TiO2 catalyst prepared by adding bentonite to titania in a 1:2 ratio afforded a 80% selectivity to 1,2-PDO at 70% conversion after 7 h of reaction at 150 °C under 2 MPa of H2;253 the catalyst was active and selective over four successive runs. A 28.4 wt % Cu−4.8 wt % Ru-catalyst supported on carbon nanotubes (CNT) afforded a 86% selectivity to 1,2PDO at near total conversion and was 3 times more active than monometallic Cu-catalysts because Ru-particles favored H2dissociation.254 Similar results were achieved with Ru−Cu catalyst supported on zirconia.255 A 5 wt % Ru/CNT catalyst operating at 200 °C under 4 MPa of H2 afforded only a 22% selectivity to 1,2-PDO at 65% glycerol conversion because the catalyst was too active for C−C bond cleavage and produced ethylene glycol and methane;256 in contrast, the selectivity to 1,2-PDO was increased up to 52% at 86% conversion after 12 h of reaction over the bimetallic Ru−Fe/CNT catalyst (Ru:Fe = 2:1), which was reused five times without loss of activity and selectivity. Platinum-based catalysts were also employed to convert glycerol into 1,2-PDO. A 93.0% selectivity to 1,2-PDO at 92% conversion was obtained over highly dispersed Ptparticles of a 2 wt % Pt/hydrotalcite catalyst.257 Under inert atmosphere, a 64% selectivity to 1,2-PDO at 85% conversion was achieved over a 2.7 wt % Pt/NaY catalyst within 15 h at 230 °C;258 hydrogen was provided by the aqueous phase reforming of glycerol. Platinum catalysts supported on

Because of the increasing cost of propylene, 1,2-PDO could be advantageously obtained by hydrogenolysis of glycerol providing the selectivity of the reaction is high enough to minimize C−C bond ruptures. The mechanism of glycerol dehydroxylation does not involve C−OH bond rupture on metal surface, but rather proceeds via a dehydration step leading to acetol followed by an hydrogenation of acetol to 1,2PDO. This mechanism is generally accepted when the reaction proceeds under acidic conditions, but in a basic medium the first step was a dehydrogenation to glyceraldehyde followed by dehydration and hydrogenation steps (Scheme 19).10a A combined experimental and theoretical study of glycerol dehydroxylation on rhodium catalysts demonstrated that the initial reaction step was the dehydrogenation into glyceraldehyde.233 The selectivities to 1,2-PDO obtained on various catalysts are given in Table 10. Supported or unsupported copper catalysts were employed because they avoid C−C bond rupture leading to ethylene glycol, ethanol, and methanol.234 Commercial CuO−ZnO catalysts were almost 100% selective to 1,2-PDO at low glycerol conversion.235 CuO−ZnO catalysts prepared by homogeneous coprecipitation with varying Cu/Zn atomic ratios afforded selectivities over 93%.236 The selectivity of a CuO/ZnO catalyst prepared by an oxalate gel method was selective (90%) and more active than a CuO/ZnO catalyst prepared by coprecipitation.237 A Cu/ZnO/Ga2O3 catalyst prepared by coprecipitation afforded an 80% yield to 1,2-PDO, and no deactivation was observed after four consecutive runs.238 The liquid-phase hydrogenolysis of glycerol over CuO−ZnO (60:40) catalyst at 200 °C under 5 MPa of H2 pressure resulted in a 98% selectivity at 52% conversion.239 A 98% selectivity at 80% conversion was obtained at 180 °C under 3.0 MPa of H2 over a Cu0.4/Mg5.6Al2O8.6 catalyst synthesized by thermal treatment of hydrotalcite.240 Glycerol solutions (10 wt %) were converted with a 89.7% selectivity to l,2-PDO at 90.1% conversion in a continuous reactor over Cu/ Al2O3 catalysts containing different loadings of H4SiW12O40 acting as an acidic promoter, which enhanced Cu-reducibility and glycerol dehydration.241 Glycerol was converted to 1,2PDO with a 96% selectivity at 100% conversion over a Cu/ Al2O3 catalyst in a fixed-bed down-flow reactor operating with a temperature gradient, allowing one to carry out the dehydration of glycerol into acetol at 200 °C, and the hydrogenation of acetol into 1,2-PDO at 120 °C under atmospheric H2pressure.242 Gycerol was converted into 1,2-PDO without the need of external H2 in the presence of CuO/ZnO/Al2O3 catalyst because H2 was produced in situ from methanol and water.243 Ni−Cu/Al2O3 bimetallic catalysts prepared by a sol− gel method were used in glycerol hydrogenolysis under inert atmosphere using formic acid as the source of hydrogen;244 after optimization, a 82% selectivity to 1,2-PDO at 90% conversion was achieved at 220 °C under 4.5 MPa of N2. Silver catalysts supported on γ-Al2O3 afforded a 96% selectivity to glycerol at 46% conversion (220 °C, glycerol/Ag = 100/2, 1.5 MPa of H2, 10 h);245 the catalyst deactivated due to the sintering of Ag-particles, but spent catalysts recovered their activity after calcination. Many investigations were carried out over supported ruthenium catalysts.10a Ru-catalysts were much more active but less selective to 1,2-PDO than Cu-catalysts because of the higher hydrogenolysis activity of ruthenium for C−C bond breaking. In most investigations, the maximum yield to 1,2PDO was lower than 40%, but higher yields were obtained with T


Chemical Reviews

Review

Table 11. Glycerol Dehydroxylation to 1,3-Propanediol catalysts

reaction conditions

conv/%

yield/%

1,3-PDO/1,2-PDO

ref

0.3 wt % Rh/C 2 wt % Pt/WO3/ZrO2 2 wt % Pt/WO3/ZrO2 3 wt % Pt/WO3/ZrO2 1 wt % Pt/H4SiW12O40/SiO2 10 wt % Cu−15 wt % H4SiW12O40/SiO2 Pt/Al2O3 + H4SiW12O40 Pt/ZrO2−SO42− 2 wt % Pt/Ti90W10 Pt−AlOx/WO3 1 wt % Ir−Re/SiO2 1 wt % Ir−Re/SiO2 + H−ZSM-5

sulfolane, 5% H2WO4, 8 MPa, 180 °C DMI, 8 MPa, 170 °C DMI/H2O H2O, 4 MPa, 130 °C, LHSV = 0.02 h−1, continuous reactor H2O, 6 MPa, 200 °C, WHSV = 0.045 h−1, continuous reactor vapor phase, 0.54 MPa, 210 °C H2O, 4 MPa, 200 °C, 18 h DMI, 7.3 MPa, 170 °C H2O, 5 MPa, 180 °C, 12 h H2O, 3 MPa, 180 °C, 10 h H2O, 8 MPa, 120 °C, 12 h H2O, 8 MPa, 120 °C, 24 h

32 85.8 31.6 70 81 83 49 67 18 90 81 59

12 24 11 32 32 28 28 56 7.4 40 38 26

2 2 4 18 2 1.5 4 21 2.9 20 8 4.8

235 266 267 268 269 270 271 272 273 274 277a 278

amorphous silica−alumina at 220 °C were used under inert atmosphere or hydrogen pressure (4.5 MPa);259 similar conversions (20−27% after 24 h) and selectivities (32−35%) were attained regardless the atmosphere. A detailed mechanistic study of the reaction over a Pt/Al2O3 catalyst has been performed.260 The hydrogenolysis of 1,2-PDO at 120 °C under 8 MPa over a Rh−ReOx/SiO2 catalyst (Re/Rh = 0.5) resulted in a 85% yield to propanols (66% to 1-propanol).261 Starting from glycerol, the maximum yield in propanols was 92% (76% in 1propanol). The synergistic effect of Rh and Re in the 1,2propanediol hydrogenolysis was explained by the hydrogenolysis of the alkoxide species on ReOx with hydrogen species on the Rh−metal surface. The selectivity was switched from 1,2-PDO to lactic acid (LA) upon addition of boric acid onto a 5 wt % Pt/CaCO3 catalyst.262 Glycerol hydrogenolysis carried out over a Cu/SiO2 catalyst in basic condition (NaOH 1M) yielded LA as main product (85%), which did not form under neutral condition.234 Ruthenium catalyst modified with sulfur yielded 75% of 1,2-PDO and 13% of LA in the presence of NaOH at 240 °C after 2 h.263 A 64% selectivity to LA was obtained at low conversion over a Pt/C catalyst at 180 °C under 4 MPa in the presence of CaO or NaOH.264 Significant amounts of LA were obtained during the treatment of basic glycerol solutions under inert gas in the presence of supported metallic catalysts;265 thus, over an Ir/C catalyst under inert atmosphere a 50% yield to LA was obtained. 3.2.2. Glycerol to 1,3-PDO. 1,3-Propanediol (1,3-PDO), which is used to produce poly(trimethylene terephthalate) by copolymerization with terephthalic acid, is currently synthesized from ethylene oxide or by fermentation of carbohydrates. Attempts have been made to produce 1,3-PDO by hydrogenolysis of glycerol, but the selectivity was much lower than that of 1,2-PDO. The conversion and yield to 1,3-PDO obtained on various catalysts are given in Table 11. At low conversion of glycerol in sulfolane, a selectivity ratio 1,3-PDO/ 1,2-PDO = 2 was achieved in the presence of a 0.3 wt % Rh/C catalyst with tungstic acid added as a selectivity promoter;235 the mechanism proposed was a dehydration favored by the protonation of the secondary hydroxyl group and a subsequent hydrogenation of the resulting keto-group (Figure 19c), but a dehydroxylation of the secondary hydroxyl group on rhodium surface could also occur while the two primary functions remain protected by chelating metal additives. A higher yield to 1,3PDO (24% at 85.8% conversion) was obtained in 1,3-dimethyl2-imidazolidinone (DMI) at 170 °C, under 8 MPa of H2 over a 2 wt % Pt/WO3/ZrO2 catalyst prepared by coimpregnation of

ZrO2 with H2PtCl6 and (NH4)6(H2W12O40) solutions and calcination at 500 °C.266 A 1,3-PDO/1,2-PDO selectivity ratio of 4 (11.0% yield to 1,3-PDO at 31.6% conversion) was obtained with a Pt//WO3/ZrO2 catalyst using a mixture of DMI and water as solvent.267 A Pt/WO3/ZrO2 (3 wt % Pt, 10 wt % W) catalyst prepared from a mixture of WO3/ZrO2 calcined at 700 °C was used in a fixed-bed continuous-flow reactor at 130 °C under 4 MPa of H2;268 a 32% yield to 1,3PDO was obtained at 70% of conversion, but a slight decrease in glycerol conversion was observed after 24 h on stream. A 1 wt % Pt-15 wt % H4SiW12O40/SiO2 catalyst afforded a 32% yield to 1,3-PDO at 81% glycerol conversion in a fixed-bed reactor.269 The conversion of pure glycerol at 210 °C under 0.54 MPa of H2 in the presence of a 10 wt % Cu−15 wt % H4SiW12O40/SiO2 catalyst resulted in a 28% yield at 83% glycerol conversion;270 the acid sites present on the Cu− H4SiW12O40/SiO2 catalyst favored the selective dehydration into 3-hydroxypropanal, which was then hydrogenated into 1,3PDO over Cu-particles following a dehydration mechanism leading to the formation of 1,2-PDO through the formation of acetol. Aqueous solutions of glycerol were converted at 200 °C under 4 MPa of H2 in the presence of a 5 wt % Pt/Al2O3 catalyst and silicotungstic acid affording a 28% selectivity to 1,3PDO at 49% conversion;271 the selectivity was controlled by the initial acid-catalyzed dehydration step, and a rapid successive hydrogenation was essential to avoid the degradation of hydroxypropanal intermediate. Glycerol was converted at 170 °C under 7.3 MPa of H2 over a platinum catalyst supported on sulfated zirconia bearing a large number of Brønsted acid sites (804 μmol.g−1);272 after 12 h of reaction in water 1,2-PDO was the main product, but in DMI a 56% yield to 1,3-PDO was obtained at 67% glycerol conversion. After regeneration by air treatment at 300 °C for 1 h, the catalyst was reused five times without a significant loss of activity. Other metals (Ru, Ni, Cu, Ni/Cu, Fe, Mn, and Al) on the same support gave a lower selectivity. Glycerol hydrogenolysis carried out over Pt-catalysts supported on high surface area, mesoporous Ti−W oxides (2 wt % Pt/Ti90W10 and 2 wt % Pt/Ti80W20) afforded up to 40.3% selectivity to 1,3-PDO, which was ascribed to a synergy between well-isolated acidic WOx clusters and highly dispersed Pt-nanoparticles.273 An aqueous solution of glycerol without any additive was converted at 180 °C under 3 MPa over a Pt− AlOx/WO3 catalyst yielding 40% of 1,3-PDO at 90% conversion;274 the catalyst was reused four times without significant deactivation, and a mechanism was proposed involving aluminum oxide species. Rhenium has been used as a promoter to Pt-group metals (Pt, Ir, Ru, or Rh) to improve U


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4. HYDROLYSIS/HYDROGENATION OF POLYSACCHARIDES The depolymerization of polysaccharides by acid hydrolysis leads to hexoses and pentoses that can be hydrogenated to polyols on metal catalysts, but the two steps were combined in a one-pot process on bifunctional metal catalysts. Thus, starch was converted to sorbitol by combined hydrolysis−hydrogenation on metal catalysts supported on acidic support where Brønsted sites catalyzed the hydrolysis of starch chains to glucose, which was hydrogenated on metal sites (Scheme 20).

their activity and selectivity to 1,3-PDO. As compared to monometallic Ru-catalysts, the Ru−Re catalysts showed a much higher activity in the hydrogenolysis of glycerol yielding a mixture of 1,2-PDO and 1,3-PDO.275 The influence of several additives (Re, Mo, W, V, Cr, Mn, Ag) to Ir/SiO2 or Rh/SiO2 catalysts has been studied in detail.276 Re-additives improved both glycerol conversion and 1,3-PDO selectivity regardless the nature of metal. Thus, monometallic iridium or platinum catalysts were inactive at 120 °C under 8 MPa of H2, while 45% and 33% glycerol conversions were measured in the presence of Ir−Re/SiO2 and Pt−Re/C catalysts, respectively.276a,277 A selectivity ratio 1,3-PDO/1,2-PDO of 4 at 63% conversion and a 38% yield to 1,3-PDO at 81% conversion were obtained for an optimum Ir/Re ratio of 1 where iridium and rhenium were in close interaction.276a,277a The catalyst was reused three times without change in rate and selectivity, and a reaction mechanism was proposed involving as a first step the adsorption of glycerol on the surface of ReOx clusters to form 2,3-dihydroxypropoxide, which was subsequently hydrogenated into 3-hydroxypropoxide on metal sites and finally hydrolyzed into 1,3-propanediol. The addition of H-ZSM-5 as a solid acid cocatalyst improved the reusability of the Ir−Re/SiO2 catalyst.278

Scheme 20. Combined Hydrolysis/Hydrogenation of Starch and Cellulose

3.3. Concepts Guiding the Choice of Metal Catalysts

Unlike the hydrogenation of carbonyl and carboxylic functionalities in carbohydrates or of the CC bonds in fatty compounds that can be performed with a very high selectivity to the desired products, hydrogenolysis reactions catalyzed by metals usually lead to C−C and C−OH bond ruptures at various positions of the carbohydrate molecule giving a mixture of products, which may ultimately result in small hydrocarbon molecules. Separation processes need to be developed to isolate desired chemicals, or alternatively a suitable mixture of polyols could be valorized to manufacture end-products such as paper additives, paints, resins, foams, surfactants, lubricants, and plasticizers.1e The mechanism of dehydroxylation reactions is more complex than a mere C−OH bond rupture as shown by the proposed mechanisms of glycerol conversion to 1,2-PDO and 1,3-PDO.10a Combined dehydration/hydrogenation reactions and retro-Michael or retro-Claisen reactions play a decisive role by controlling the formation of final products. Additional experimental and theoretical investigations are needed in the future to understand better the reaction mechanisms of hydrogenolysis reactions that could serve as a guideline for choosing metal catalysts and additives acting as cocatalyst improving the selectivity to desired products. Indeed, the analysis of data from the literature given in previous sections shows that the control of reaction selectivity depends on the metal, the nature of cocatalysts, and the reaction conditions, particularly the pH of solutions. There are a high number of parameters involved, and their choice depends heavily on the molecule targeted. Thus, starting from sorbitol solutions, the choice of catalysts and reaction conditions was completely different whether C6 tetrols224 or smaller polyols such as 1,2-PDO were targeted (Table 9). In the first case, C−C bond rupture was avoided by using copper-based catalysts, while in other instances metals such as nickel and ruthenium known for their high hydrogenolysis activity of C−C bonds were employed.

A 95% yield to sorbitol was obtained from starch in the presence of a 3 wt % Ru/HY catalyst.279 The combined hydrolysis and hydrogenation of inulin to a mixture of sorbitol and mannitol was achieved at 100% conversion over a Ru/C catalyst where the carbon support was oxidized to generate acidic sites;51 the hydrolysis of long inulin chains was faster than short chains due to a multisite attack and a stronger adsorption of long chains on the carbon surface. Cellulose based on β-1,4-glycosidic linkages is organized in microcrystallite domains, which are difficult to depolymerize, but physical and chemical treataments combined with catalytic reactions can lead to platform molecules useful for chemical synthesis.280 Many investigations were directed at producing hexitols by one-pot hydrolysis/hydrogenation or hydrolysis/ hydrogenolysis reactions in the presence of metal catalysts on acidic supports or liquid acids.281 To overcome the low reactivity of cellulosic materials, most of the investigations used ball-milled cellulose because the mechanical pretreatment reduced the crystallinity of cellulose and improved the accessibility of the substrate to the catalyst and the use of active carbon improved further cellulose depolymerization.282 Table 12 gives the selectivity data obtained in the conversion of cellulose over various catalytic systems. Cellulose was converted at 160 °C under 5 MPa of H2, with an 84% selectivity to sorbitol at 72% conversion over a 5 wt % Ru/C catalyst suspended in 2.5 wt % sulfuric acid solution.6a Ball-milled cellulose suspended at 2 wt % concentration in a solution of H4SiW12O40 heteropolyacid was converted with a 100% yield to hexitols (85% sorbitol + mannitol, 15% sorbitan) in the presence of a Ru/C catalyst at 190 °C under 9.5 MPa of H2.283 Using slightly modified reaction conditions and the same catalytic system, a 50% isosorbide yield was obtained;284 the reaction applied to a delignified wheat straw pulp afforded a 63% yield to isosorbide after 1 h of reaction. In the presence of a cesium salt of the heteropolyacid and a Ru/C catalyst, a 90% yield to hexitols (70% alditol + 20% sorbitan) was obtained at V


Chemical Reviews

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Table 12. One-Pot Cellulose Conversion to Polyols catalysts

reaction conditions

conv/%

yield/%

ref

31% sorbitol + mannitol 34.6% sorbitol, 11.4% mannitol, 13.4% sorbitan 11% sorbitol, 2% mannitol, 6% erytritol 69% sorbitol, 4% mannitol, 5% erythritol, 5% glycerol 33.2% sorbitol, 13.6% sorbitan 85% sorbitol + mannitol, 15% sorbitan 13% hexitols, 52% isosorbide

288 289

2.5 wt % Pt/γAl2O3 4 wt % Ru/C

0.68 g of cellulose, 0.21 g of catalyst, 60 mL of H2O, 190 °C, 5 MPa, 24 h 1 g of cellulose, 0.05 mmol of Ru, 50 mL of H2O, 245 °C, 6 MPa, 30 min

1 wt % Ru/CNT

0.16 g of cellulose, 85% crystallinity, 0.05 g of catalyst, 20 mL of H2O, 5 MPa, 185 °C, 24 h 0.16 g of cellulose, 33% crystallinity, 0.05 g of catalyst, 20 mL of H2O, 5 MPa H2, 185 °C, 24 h 0.5 g of cellulose, 0.1 g of Ru/C, 10 mL of 2.5 wt % H2SO4, 160 °C, 5 MPa, 1 h 1 g of ball-milled cellulose, 0.25 g of Ru/C, 1.22 × 10−2 M H+, 190 °C, 9.5 MPa, 1 h

72 100

0.8 g of ball-milled cellulose, 0.20 g of Ru/C, 5.47 × 10−2 M H+, 210 °C, 5 MPa, 1 h

68

1 g of ball-milled cellulose, 0.25 g of Ru/C, 1.5 × 10−2 M H+, 170 °C, 5 MPa, 48 h

100

70% alditol, 20% sorbitan

285

0.1 mmol of [BMIM]Cl, 10.0 μmol of Ru, 4.5 mmol of [BMIM]Cl, 1 g of cellulose, 1.5 mmol of sodium formate, 80 °C, 5 h 6 g of cellulose, 1.5 g of catalyst, 200 °C, 5 MPa, 6 h

100

94% sorbitol

292 293

0.5 g of cellulose, 0.15 g of catalyst, 245 °C, 6 MPa, 30 min

100


100

32% sorbitol + mannitol, 8% glycerol 46.8% sorbitol + 11.5% mannitol 61% EG + 7.7% PG

0.5 g of cellulose, 0.15 g of catalyst, 0.05 g of H2WO4, 245 °C, 6 MPa, 30 min

100

54.4% EG + 6% PG

299

1 g of cellulose, 0.02 g of catalyst, 1 g of WO3, 245 °C, 6 MPa, 30 min

100

300

2 wt % Pt/AlW 3 wt % Ru/C + MCM-041-SO3H 15 wt % Ni−15 wt % W/SBA15 20 wt % Ni/ZnO

1.6 g of cellulose, 0.65 g of catalyst, 190 °C, 5 MPa, 24 h 0.1 g of cellulose, 2 mg of Ru/C, 0.1 g of MCM-41-SO3H, 230 °C, 6 MPa, 40 min

70 100

48.9% EG + 7.4% PG + 6.8% sorbitol 28% acetol + 20% PG 49% EG + 8% PG

1 g of cellulose, 0.3 g of catalyst, 245 °C, 6 MPa, 30 min

100

75.4% EG + 4.1% PG

302


100

303

5 wt % Ru/C + NaOH

0.1 g of alkali cellulose, 0.02 g of Ru/C, 160 °C, 5 MPa, 5 h

59

19.1% EG + 34.4% PG + 10.1% butanediol 4% EG + 11% PG

5 wt % Ru/C + H2SO4 5 wt % Ru/C + H4SiW12O40 5 wt % Ru/C + H4SiW12O40 5 wt %Ru/C + Cs3.5H0.5SiW12O40 Ru° stabilized by [BMIM]Cl 7.3 wt % Ni−0.9 wt % Pt/Beta 75 4 wt % Ir−4 wt % Ni/MC 2 wt % Ni−30 wt % W2C/C 1.2 wt % Ru/C + H2WO4 3 wt % Ru/C + WO3

170 °C under 5 MPa of H2;285 whereas the recovery of commercial HPAs was not successful, CsPW salts were fully retained after reaction at 170 °C by simple recrystallization at room temperature without the need of an organic solvent. High cellulose conversion and a 81% yield to C4−C6 polyols were obtained at 160 °C using a H4[Si(W3O10)4] heteropolyacid in combination with Ru/C catalysts.286 While in its infancy, the partial depolymerization of cellulose assisted by a nonthermal atmospheric plasma is a promising alternative technique to improve the reactivity of the biopolymer.287 Although the combination of diluted acidic solution and supported metal catalysts was efficient to produce polyols by a one-step conversion of cellulose, attempts have been made to replace homogeneous catalysts by acidic solids. The combined hydrolysis/hydrogenation of cellulose in the presence of 2.5 wt % Pt/γ-Al2O3 catalyst afforded a 31% yield to sorbitol.288 Most of the subsequent investigations were conducted with Rucatalysts, which were more active than Pt-catalysts for hydrogenolysis and hydrogenation reactions. Thus, high yields to hexitols (34.6% sorbitol, 11.4% mannitol, 13.4% sorbitan) at 85.5% conversion were obtained in 30 min over a 4 wt % Ru/C catalyst;289 the protons generated from water dissociation at high temperature were the source of acidity required for the hydrolysis reaction. This interpretation in term of increased ionization constant of water was supported by systematic studies of cellulose conversion with or without catalysts and hydrogen;290 thus, at 190 °C under 5 MPa of H2, a 45% conversion of cellulose was achieved without any catalysts. Cellulose with a 33% crystallinity obtained by pretreatment

85.5

50

291

6a 283 284

294 297

301 132

304

with phosphoric acid was converted to hexitols with a 73% yield over 1 wt % Ru/CNT catalyst where the carbon nanotubes were treated with nitric acid to generate acid sites.291 A 94% yield to sorbitol at 100% cellulose conversion was obtained with a complex catalytic system based on Ru-nanoparticles in 1-nbutyl-3-methylimidazolium chloride ([BMIM]Cl) using sodium formate as hydrogen donor.292 Monometallic nickel catalysts were not efficient (5% conversion after 6 h at 200 °C under 5 MPa), but a 50% conversion was achieved with a 7.3 wt % Ni− 0.9 wt % Pt/β-zeolite catalyst yielding 32% of sorbitol + mannitol.293 The conversion of microcrystalline cellulose at 245 °C under 6 MPa over a 4 wt % Ir−4 wt % Ni catalyst supported on mesoporous carbon afforded a 58.3% yield to hexitols, and the catalyst was stable over four runs.294 Hemicellulose contained in sugar beet fibers was converted by one-pot combined hydrolysis/hydrogenation in the presence of a 2 wt % Ru/C catalyst at 155 °C under 5 MPa of H2 yielding up to 83 wt % arabitol.295 The combined hydrolysis and hydrogenation of bleached kraft pulp containing cellulose and xylans was carried out under 2 MPa of H2 at 155 °C over Pt/MCM-48 and Ru/C catalysts;296 the yields to xylose, glucose, xylitol, sorbitol, furfural, furfuryl alcohol, and 5-hydroxymethylfurfural depended upon the acidity and structure of the mesoporous materials. Instead of targeting hexitols, investigations were directed at producing smaller polyols such as ethylene glycol (EG), propylene glycol (PG), or glycerol by increasing the hydrogenolysis activity of catalytic systems. Thus, cellulose was totally converted with a 61% yield to EG and a 7.7% yield to W


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PG after 30 min at 245 °C under 6 MPa of H2 in the presence of a 2 wt % Ni−30 wt % W2C/C catalyst.297 The direct catalytic conversion of raw woody biomass was carried out at 235 °C under 6 MPa of H2 over a carbon supported Ni−W2C catalyst (4 wt % Ni−30 wt % W2C/C);298 the carbohydrate fraction in the woody biomass, that is, cellulose and hemicellulose, was converted to EG and other diols with a total yield of up to 76%, while the lignin component was converted into monophenols with a 46.5% yield based on lignin. The conversion of cellulose at 245 °C under 6 MPa of H2 with a catalytic system based on a 1.2 wt % Ru/C catalyst with H2WO4 afforded a 58% yield to EG at 100% conversion, and the system was reused 20 times without significant decrease of EG yield;299 a mechanism was proposed whereby H2WO4 was reduced to water-soluble HxWO3 species, which were reoxidized to insoluble species upon cooling in air. A 100% conversion of cellulose into 48.9% EG and 7.4% PG was achieved at 245 °C over a catalytic system combining WO3 and a Ru/C catalyst.300 The conversion of non ball-milled cellulose in the presence of a 2 wt % Pt/AlW catalyst at 180 °C under 5 MPa yielded 48% of acetol and PG;301 a low deactivation was observed after the first run. Cellulose was converted with a system combining MCM-41-SO3H and a Ru/ C catalyst into a range of products depending on reaction conditions;132 thus, at 230 °C under 6 MPa, a 49% EG yield was obtained at 100% conversion, but due to irreversible change in the mesoporous structure and loss of acid groups, the catalyst deactivated rapidly. EG and PG were obtained with 75.4% and 4.1% yields, respectively, at 100% cellulose conversion over a 5 wt % Ni−15 wt % W/SBA-15 catalyst.302 Ball-milled cellulose was converted into 1,2-alkanediol (34.4% PG, 19.1% EG, and 10.1% 1,2-butanediol) in the presence of a 20 wt % Ni/ZnO catalyst after 30 min at 245 °C under 6 MPa of H2.303 Alkaline cellulose pretreatment enhanced the hydrogenolysis properties toward C2−C3 polyols;304 thus, in the presence of a Ru/C catalyst at 160 °C under 5 MPa of H2, 59% of the alkali cellulose was converted to PG and EG, whereas starting from untreated cellulose a 25% conversion was obtained and no C2−C3 polyols were produced.

to Pd- or Pt-particles by impregnation with a solution of promoter salt or by redox surface reactions. Thus, bismuth was selectively deposited on Pd- or Pt-particles by reducing with formaldehyde or glucose BiONO3 solutions added to the suspension of Pt/C or Pd/C catalysts.306 Pt, Pd, and Pd−Pt colloidal metal particles of ca. 3 nm stabilized by tetraoctylammonium chloride were prepared by reduction of the metal chlorides and adsorbed on active charcoal coated with 5 wt % bismuth;307 the surfactant used for the stabilization of colloids acted as a modifier of the catalyst surface. Pd−Bi/C catalysts were also prepared by deposition on activated carbon of palladium and bismuth acetates, which were decomposed upon heating under N2 at 500 °C.308 Pd−Bi catalysts with a wide range of bismuth concentration (0.25−8 wt % Bi) were also prepared from a 5 wt % Pd/SiO2 catalyst by repeated impregnation with an aqueous solution of bismuth nitrate.309 A renewed interest for the oxidation of carbohydrates and glycerol arose as it was discovered that nanoparticles of gold were efficient catalysts for the oxidation of glucose310 or glycerol.311 While the performances of Pd- and Pt-catalysts did not depend critically upon particle size, the preparation of supported gold catalysts was more demanding because very small particle sizes were required for an optimum catalytic activity. Gold catalysts were prepared by various synthetic routes.312 Gold catalysts (0.25, 0.5, and 1 wt %) supported on graphite or activated carbon were prepared by contacting the support with a solution of HAuCl4, which was reduced with formaldehyde;311 however, the distribution of particle size was heterogeneous. A 1 wt % Au/C catalyst was prepared by the sol-gold method;310 an aqueous solution of HAuCl4 mixed with a polyvinyl alcohol (PVA) solution was reduced by NaBH4 leading to PVA-stabilized 2−4 nm gold particles, which were subsequently immobilized on the support. A similar method based on PVA-protected gold particles adsorbed on a carbon support was employed by others.313 The common protective agents of colloids were polyvinyl alcohol, polyvinyl pyrrolidone, tetrahydroxymethyl phosphonium chloride, and citrate. Unprotected gold sols (∼3.6 nm) were also prepared by the same method in the absence of a protector, but the “naked” particles coalesced rapidly during glucose oxidation314 The preparation of gold catalysts on alumina by incipient wetness method resulted in Au-particles smaller than 2 nm even at high gold loadings.315 Gold catalysts prepared on alumina were prepared by two different deposition−precipitation (DP) methods using sodium hydroxide or urea as precipitating agent. With urea, catalysts up to 10 wt % Au-loading were prepared, while a loss of gold occurred during the preparation with NaOH.316 The preparation by ion exchange of the support with HAuCl4 resulted in a very sharp particle size distribution centered at 1−1.3 nm, while the method of deposition−precipitation with urea resulted in slightly larger particles (2−3 nm).317 Gold nanoparticles smaller than 2 nm were loaded on different types of supports by a solid grinding method using the volatile organometallic gold complex [Me2Au(acac)], thus avoiding any solvent in the preparation.318 Supported bimetallic gold nanoparticles (Au−Pd, Au−Pt, Au−Ag) were synthesized by immobilization of polymer-protected bimetallic sols.319 The direct deposition of gold onto a parent palladium catalyst was used.320 Single-phase bimetallic Au−Pd systems were synthesized in a two-step method by forming the Pd-sol in the presence of Au/C using H2 as reducing agent instead of NaBH4 in the second step to avoid the segregation of palladium.321

5. OXIDATION OF CARBOHYDRATES AND DERIVATIVES 5.1. Design of Metal Catalysts

Water solutions of carbohydrates were oxidized by oxygen or air at atmospheric pressure in the presence of Pd- or Ptcatalysts at 30−80 °C.17a,305 Primary and secondary alcohol functions were oxidized to carbonyl and carboxylic functions via an oxidative dehydrogenation mechanism whereby oxygen reacted with dissociated hydrogen adsorbed on metal surfaces. The metal particles were usually supported on active carbons, which present the advantage of a high stability under reaction conditions, particularly at low pH and in the presence of chelating carboxylates. Metals were usually loaded by impregnation (solvent evaporation or dry impregnation), anionic adsorption, or cationic exchange. This latter technique was the most reproducible to obtain carbon-supported metal particles smaller than 2 nm, uniformly distributed in the catalyst pores, and with the highest stability to sintering, because of their anchoring to functional groups present on the carbons.306 The reduction of catalyst precursors was performed with H2 or by reducing agents such as formaldehyde or glucose in alkaline solutions. Metal promoters such as bismuth or lead were added X


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5.2. Oxidation of Glucose

interpret the glucose oxidation at alkaline pH over 10 wt % Pd−Bi/C catalysts.308 Among the bimetallic Pd−Met catalysts (Met = Bi, Tl, Sn, Co) supported on carbon or silica, Bipromoted catalysts showed the best activity and selectivity.325 The formation of binary intermetallic Pd−Bi, Pd−Tl, and Pd− Te compounds enhanced the oxidation performances.309,326A Ru−Bi/C catalyst produced high yields of glucuronate and oxalate at 60−70 °C as compared to Ru−Pd/C catalysts, which produced mainly gluconate.327 The aerobic oxidation of glucose in 1 M NaOH solution at 80 °C over a Pt/C catalyst yielded 47% lactate and 46% gluconate at total glucose conversion.328 The impact of internal diffusional limitations on the overall kinetics of glucose oxidation was quantified over nanostructured carbon xerogel Pd−Bi catalysts.329 Slurry catalysts, pellets, and a solid foam based on 3 wt % Pt−BiAl2O3 (Bi/Pt = 1) catalyst were compared at 60 °C and pH 8.2, showing that the process conditions had a strong influence on the reactor performance.330 An increased interest has been given to supported gold catalysts as an alternative to palladium-based catalysts because of their higher activity and selectivity to gluconate. The reaction conditions, particle size effects, preparation methods, and stability to leaching and poisoning have been intensively investigated. Reaction rates using different supported gold catalysts were compared in a review.13 The oxidation of glucose was performed under flowing oxygen (0.1−0.3 MPa) at 50− 100 °C over a 0.9 wt % Au/C catalyst prepared by immobilizing 2−5 nm gold sol particles on active carbon;310,331 without pH control, a 99% yield to gluconic acid was obtained with a higher rate than on commercial Pd−Bi/C or Pd−Pt−Bi/C catalysts, but the gold catalyst deactivated upon recycling due to metal sintering and leaching. The oxidation of glucose in a semibatch reactor at atmospheric pressure over Au/C catalysts with particle sizes in the range 3−6 nm prepared by the sol method with different reducing agents and different carbon supports was found to proceed via a Langmuir−Hinshelwood rate law;313a the best results were obtained at 50 °C and pH 9.5. The kinetic data obtained with colloidal gold particles were interpreted by an Eley−Rideal mechanism whereby adsorbed glucose reacts with molecular oxygen coming from the liquid phase.332 In contrast, kinetic studies over a 0.3 wt % Au/Al2O3 catalyst in a wider range of glucose concentrations (1−57 wt %) and oxygen partial pressures (0.15−0.9 MPa) pointed to a Langmuir−Hinshelwood model.333 In a series of gold catalysts based on mesoporous carbon-confined Au-particles, the highest turnover frequency (TOF) at 40 °C and pH 9 was attained by 3.3 nm nanoparticles confined in the 5.4 nm mesopororous channels.334 Unsupported gold particles without protecting agent were initially as active as Au/C catalysts at 30 °C in basic solution, but the naked particles sintered from 3.6 to 10 nm, whereas supported particles remained unchanged;314 the activity was inversely proportional to particle size in the range 3−6 nm. The TOF of Au-particles supported on metal− oxide (Al2O3, ZrO2, TiO2, CeO2) prepared by deposition− precipitation from HAuCl4 or by solid grinding from Me2Au(acac) increased as the particle size decreased;318 a high TOF of 45 s−1 (ca. 16.2 × 104 h−1) was observed for Au/ ZrO2 at 50 °C and pH 9.5. Gold nanoparticles deposited on cellulose by the solid grinding method showed an activity comparable to that of Au/C with similar particle size.335 A 1.5 wt % Au/C catalyst prepared via incipient-wetness impregnation followed by plasma reduction showed a higher activity than conventional H2-reduced counterpart;336 the plasma reduction

Gluconic, glucaric, and 2-keto-gluconic acids and their salts can be obtained by oxidation with oxygen, air, or hydrogen peroxide of aqueous glucose solutions (Scheme 21) in the presence of Scheme 21. Oxidation of Glucose

supported metal catalysts, although their industrial production still relies on expensive fermentation processes.322 Most of the investigations were oriented toward the selective oxidation of glucose to gluconic acid and gluconate, which are used in pharmaceutical and food applications and as water-soluble cleaner for removing calcareous and rust deposits. 5.2.1. Oxidation to Gluconic Acid. The conversion of glucose on monometallic palladium and platinum catalysts was often limited by the strong adsorption of oxygen on the metal surface.17a However, a Pd/Al2O3 catalyst prepared by roomtemperature plasma reduction followed by a treatment under argon at 500 °C exhibited a higher activity than catalysts prepared by the hydrogen reduction of a metal precursor.323 The selectivity to gluconic acid over a Pt/C catalyst did not exceed 80%; however, under controlled O2-diffusion the selectivity attained 95% at 90% conversion.324 The use of promoters enhanced the activity and the selectivity in glucose oxidation. The oxidation of glucose solution (1.7 mol L−1) was performed in a batch reactor at 40 °C under normal air pressure at pH 9 over a 5 wt % Pd−Bi/C catalyst (Bi/Pt = 0.1) prepared by deposition of bismuth on the surface of 1−2 nm palladium particles via a redox surface reaction using glucose as reducing agent;306 the rate of glucose oxidation to gluconate was 20 times higher on Pd−Bi/C than on Pd/C catalyst, a 99% yield to gluconate was obtained, and the catalyst was recycled five times without loss of activity and selectivity and without bismuth leaching provided the oxidation reactions were stopped just before 100% conversion. The high activity of Pt−Bi/C catalysts was attributed to the promoting effect of bismuth atoms deposited on the surface of palladium particles acting as cocatalysts preventing the overoxidation of palladium. The use of higher amounts of bismuth should be avoided because bismuth oxide not alloyed with Pd-particles leached as bismuth gluconate in solution. A Bi-promoted Pd−Pt/C catalyst prepared by deposition of tetraoctylchloride-stabilized, Pd88Pt12 colloids on a bismuth-impregnated active carbon was more active than an industrial catalyst of the same composition and more stable upon recycling at pH 9.5.307 The formation of Bi-glucose and Bi-gluconate complexes was suggested to Y


Chemical Reviews

Review

Scheme 22. Oxidation of Lactose

ultrasounds in a 30 wt % H2O2 solution afforded an 85% selectivity to gluconate at 100% conversion.347 The impact of mass-transfer processes was analyzed, using Au/Al2O3 and Au/ C catalysts with different dispersions at different glucose/ catalyst ratios.348 5.2.2. Oxidation to 2-Ketogluconic and Glucaric Acids. The addition of lead promoter to Pt/C catalysts oriented the selectivity to 2-keto-D-gluconate as glucose oxidation was run at basic pH.349 The same effect was noticed with Bi-promoter; however, in both cases, the yield to 2-ketogluconate was limited by the formation of degradation products.350 A 98% yield to 2keto-D-gluconic acid was achieved over a 5 wt % Bi−5 wt % Pt/ C catalyst without pH regulation;351 the improved selectivity at pH < 6 was attributed to the formation of a complex between the promoter and the carboxyl and α-hydroxyl group of gluconic acid. There are very few studies on the oxidation of glucose to glucaric acid. The selectivity to glucaric acid was affected by side and consecutive C−C cleavage reactions. The oxidation of a gluconate solution (0.2 mol L−1) over a Pt/C catalyst (gluconic acid/Pt = 20) at controlled pH (8−11) at 45−65 °C resulted in a maximum glucarate yield of 55%.352 During the oxidation of a 2 mol L−1 glucose solution at 40 °C and pH 9 over a Pt/C catalyst (glucose/Pt = 787), gluconate was formed with a 80% yield at nearly complete conversion of glucose after 3 h, but was further oxidized to glucarate with a selectivity of 57% at 97% conversion.350 Patents were issued on glucaric production in the absence of an added base on supported Pt353 and Pt−Au catalysts;354 a 4 wt % Pt/SiO2 catalyst afforded a 66% yield to glucaric acid at 90 °C under 0.5 MPa of O2, and a 71% yield was obtained at 119 °C under 2.8 MPa of O2 over a 4 wt % Pt−4 wt % Au/TiO2 catalyst.

increased the amount of oxygen-containing functional groups, which significantly enhanced the hydrophilic property and the catalyst stability. Unsupported gold colloids stabilized with different polymers were highly selective to gluconate (>99%),313b but they were less active than a 0.45 wt % Au/ TiO2 catalyst prepared by a deposition−precipitation method;337 the supported catalyst yielded 95% of gluconate at 40− 60 °C and pH 9 and was reused 17 times without loss of activity and selectivity, or change in particle size. A 0.3 wt % Au/Al2O3 prepared by incipient-wetness impregnation in highly dispersed state was as active, selective, and stable upon recycling as catalysts prepared by deposition−precipitation with urea.315,333 Gold supported on alumina prepared by deposition−precipitation with sodium hydroxide or urea exhibited an excellent long-term stability.316 The stability of a 0.25 wt % Au/Al2O3 catalyst prepared by deposition− precipitation with urea was investigated in a continuous stirred tank reactor at 40 °C, pH 9, and 0.1 MPa oxygen partial pressure;338 the catalyst experienced no loss of activity and selectivity during 70 days of continuous operation. Glucose solutions were oxidized under acidic conditions at 70 °C under 0.3 MPa of O2 on mono- or bimetallic catalysts (Au, Pt, Pd) under the form of carbon-supported particles or colloidal dispersions;339 monometallic catalysts exhibited low activities (TOF = 51−60 h−1), whereas the activity of bimetallic Au−Pt/ C catalyst (Au/Pt = 2) was as high as 924 h−1. A Au−Pd/C bimetallic catalyst prepared on activated carbon by one-pot successive adsorption of Au- and Pd-salts at optimal pH followed by reduction with NaBH4 was as active as a Bi−Pd/C catalyst in glucose oxidation at 50 °C, pH 9.25 in flowing O2 and much more active than the monometallic catalyst.340 A series of nonsupported, PVP-protected colloids of bi- or trimetallic Au-nanoparticles were synthesized: Ag−Au,341 Au− Pd,342 Au−Pt,343 and Au−Ag−Pt.319b,c Trimetallic Au−Pt−Ag (70:20:10) colloidal nanoparticles with an average diameter of 1.5 nm were several times more active than mono- and bimetallic particles with nearly the same size. The presence of negatively charged Au-atoms evidenced by XPS measurements and DFT calculations acted as catalytic sites. Hydrogen peroxide provided a higher activity than oxygen at atmospheric pressure for the oxidation of glucose or maltose at 40 °C, pH 9, over a 0.3 wt % Au/Al2O3 catalyst;344 the conversion and selectivity to gluconate exceeded 99% using 1 equiv of H2O2 per glucose at high glucose concentrations (up to 50 wt %). The oxidation of glucose in a continuous reactor over a 0.23 wt % Au/Al2O3 catalyst prepared on γ-alumina beads with an eggshell texture confirmed that hydrogen peroxide was more efficient than oxygen.345 The activity of Au/CeO2−Al2O3 catalysts in glucose oxidation with oxygen measured in a continuously stirred batch reactor was higher than the activity of a monometallic Au/Al2O3 catalyst;346 a lower activity was obtained with a monolith reactor because of mass transfer limitations. Glucose oxidation with hydrogen peroxide at 25 °C, pH 9 over a 1 wt % Au/SiO2 catalyst suspended with

5.3. Oxidation of Arabinose, Galactose, and Lactose

Aldonic acids produced from arabinose, galactose, lactose, and their derivatives have many applications in the food, pharmaceutical, and cosmetic industries. Selective oxidation of these sugars over supported metal catalysts has been investigated, and the use of gold catalysts has been reviewed.355 A total selectivity to aldonic acids (>99.5%) at more than 90% substrate conversion was observed during the oxidation of various pentoses (arabinose, ribose, lyxose, and xylose) or hexoses (glucose, acetylated glucosamine, galactose, mannose, and rhamnose) over a 0.45 wt % Au/TiO2 catalyst at 40 °C, pH 9, under 0.1 MPa of O2.356 The activities of gold and palladium catalysts supported on alumina were compared in the oxidation of L-arabinose obtained from arabinogalactans hydrolysis;357 the Au/Al2O3 catalyst exhibited the highest activity at 60 °C and pH 8. In situ measurements of the catalyst electric potential allowed one to develop a kinetic model and to propose a mechanism.358 The oxidation of arabinose was carried out over Pd−Au/Al2O3 and Pd−Au/CeO2 catalysts (4 wt % Au, 1 wt % Pd) prepared by deposition−precipitation using HAuCl4 and urea with subsequent PdCl2 impregnation;359 the bimetallic catalysts were more active and selective than monometallic catalysts. Au-species were deemed responsible for the arabinose Z


Chemical Reviews

Review

acetic, and oxalic acids were produced at neutral or acidic pH, while under basic conditions acetic acid (20% yield) and glucose (15% yield) were the major products. Cellobiose, a dimer of glucose, was oxidized selectively to gluconic acid over Pt- and Au-catalysts on various supports (Scheme 23, Table 13). Oxidation of cellobiose under flowing

activation and Pd-species for oxygen activation. The Au−Pd/ CeO2 catalyst reduced by aqueous solution of formaldehyde at room temperature displayed the highest activity and selectivity at 60 °C and pH 8. The oxidation of D-galactose to galactonic acid was carried out with oxygen over Au/Al2O3 catalysts calcinated at different temperatures;360 the highest activity was obtained for a mean particle size of 2.6 nm at 60 °C and alkaline pH, but the selectivity decreased with decreasing pH, and galactolactone was formed as coproduct. Lactose extracted from milk is a disaccharide containing galactose and glucose units, which can be oxidized on the glucose moiety on metal catalysts to lactobionic acid (LBA) and subsequently to 2-keto-lactobionate (Scheme 22). LBA finds applications in the food industry and in the synthesis of biodegradable surfactants. A 100% selectivity to LBA at 95% conversion was achieved on Pd−Bi/C catalysts at 65 °C in the pH range 7−10 with a maximum reaction rate of 0.47 mol kg−1 s−1 for a composition Bi/Pd = 0.50−0.67;361 the catalyst was recycled 15 times without significant loss of activity and selectivity. The oxidation of lactose over a Pt−Bi/C catalyst at pH 7 yielded lactobionate, which was subsequently converted to 2-keto-lactobionate with a final yield of 80%. Starting from lactobionate without pH control, a 95% selectivity to 2-ketolactobionic acid was obtained, but the reaction stopped at 50% conversion due to catalyst poisoning.362 The activity of lactose oxidation to LBA on monometallic Pd, Pt, and Ru catalysts was particle size dependent with an optimum size in the range of 3−10 nm.363 LBA was obtained with a 100% selectivity at 96% lactose conversion by aerobic oxidation at pH 9 over a bimetallic 1.02 wt % Pd−0.64 wt % Bi (Bi/Pd = 3) catalyst supported on a mesoporous SBA-15 silica.364 Lactose oxidation carried out at basic pH over a 2 wt % Au/Al2O3 catalyst synthesized by deposition−precipitation method afforded lactobionate in high yields with less catalyst deactivation than over palladium catalysts.365 The acidity of β and MCM-22 zeolites support and the method of palladium incorporation influenced the catalyst performances.366 A 0.6 wt % Au/Al2O3 catalyst was successfully used in 10 runs for the oxidation of lactose without an important decrease of activity and with a 100% selectivity to LBA at 40 °C, pH 9 under O2 flow.356 Among different supported metal catalysts (Ru, Pt, Pd, Au, Ni), the highest LBA yield (99%) was obtained at 60 °C, pH 8, with a 2 wt % Au/CeO2 catalyst.367 2 wt % Au-catalysts synthesized by means of a deposition−precipitation method on various oxide supports afforded LBA selectivities within 90−95% at 20% conversion, and their activity depended upon the nature of the support.368 Lactose oxidation was performed over gold particles immobilized on a mesoporous silica using a silane coupling agent;369 a 0.7 wt % Au/SiO2 catalyst afforded a 100% yield to LBA for a catalyst/lactose ratio of 0.2 after 100 min of reaction at pH 9.0 and 65 °C. The catalytic activities of a Au/ Al2O3 catalyst in the oxidation of arabinose, galactose, and lactose with oxygen at 60 °C, pH 8 were 0.53, 0.34, and 0.37 mmol s−1 gAu−1, respectively.355

Scheme 23. Oxidation of Cellobiose

oxygen over a 0.45 wt % Au/TiO2 catalyst at 40 °C and pH 9 afforded a higher specific activity (50 mmol min−1 g−1) and selectivity (>99.5%) than 4.6 wt % Pd/Al2O3 and 5 wt % Pt/ Al2O3 catalysts.356 The oxidation of cellobiose was carried out at 145 °C under 0.5 MPa of O2 pressure without pH adjustment over a 0.5 wt % Au/CNT catalyst supported on carbon nanotubes (5.8 nm Au-nanoparticles) yielding 80% of gluconic acid at 98% conversion;373 the Au/CNT catalyst exhibited a higher selectivity than gold catalysts supported on titania, alumina, MgO, and carbon materials, which was attributed to the presence of acidic groups on CNT surface generated by pretreatment with HNO3. Cellobiose oxidation at 120 °C under 0.1 MPa air over a bifunctional Pt/AC−SO3H catalyst prepared by sulfonation of a Pd/AC catalyst afforded 38% and 46% yields to glucose and gluconic acid, respectively.374 Polyoxometalate-supported gold nanoparticles were highly active and selective bifunctional catalysts for cellobiose oxidation with oxygen into gluconic acid;375 the highly acidic Au/Cs1.2H1.8PW12O40 catalyst with a mean particle size of 2.7 nm afforded a 97% yield to gluconic at 145 °C under 0.5 MPa of O2 after 3 h. Upon recycling, the conversion and yield decreased slightly, but was kept over 90% after five cycles. The same catalyst afforded a 70% conversion of a ball-milled cellulose (crystallinity 33%) by oxidation at 145 °C under 1 MPa of O2, resulting in a 60% yield to gluconic acid after 11 h. After three cycles, the conversion decreased to 41% with a 32% yield to gluconic acid, but the deactivation was overcome by using a mixture of Au/Cs3.0PW12O40 catalyst and H3PW12O40. 5.5. Oxidation of 5-Hydroxymethylfurfural to 2,5-Furandicarboxylic Acid

Reviews were published on the preparation of 5-hydroxymethylfurfural (HMF) by acid-catalyzed dehydration of fructose and glucose or hydrolysis/dehydration of polysaccharides and on its use as platform molecule for organic synthesis.5 The hydroxyl and aldehyde functional groups of HMF were oxidized to produce valuable chemicals (Scheme 24), particularly 2,5furandicarboxylic acid (FDCA), which is a potential substitute for terephtalic acid in the production of polyesters with properties similar to those of polyethyleneterephtalate or for other polymer syntheses.89 The oxidation of HMF on supported metal nanoparticles has been reviewed.13 Selected data on the conversion of HMF and yield to FDCA are given in Table 14. Experiments of HMF oxidation were usually performed in aqueous solutions with variable amounts of liquid bases to neutralize the acidic functions. Relatively diluted solutions were

5.4. Oxidation of Cellobiose and Cellulose

The conversion of cellulose or hemicelluloses into polyols has quickly progressed by combining acid hydrolysis with hydrogenation or hydrogenolysis reactions (see section 4). In the same way, hydrolysis was combined with oxidation reactions to produce gluconic acid.6b,281a,370 The direct oxidation of cellulose was investigated over alumina-supported, platinum371 or palladium catalysts.372 Over Pd/alumina catalyst, malic, AA


Chemical Reviews

Review

Table 13. Cellobiose Oxidation into Gluconic Acid

a

catalysts

T/°C

P[O2]/MPa

t/h

conv/%

yield/%

ref

0.5 wt % Au/CNT 4 wt % Pt/AC−SO3H 1 wt % Au/Cs2HPW12O40 1 wt % Au/Cs1.2H1.8PW12O40 1 wt % Au/Cs1.2H1.8PW12O40

145 120 145 145 145

0.5 ambient air 0.5 0.5 1

6 24 3 3 11

98

80 46 96 97 60a

373 374 375a 375b 375b

97 70a

Ball-milled cellulose (33% crystallinity).

Scheme 24. Oxidation of 5-Hydroxymethylfurfural (HMF)

Table 14. Oxidation of 5-Hydroxymethylfurfural catalysts 5 wt % Pt/Al2O3 5 wt %Pt−Pb/C (Pb/Pt > 2) 9.6 wt % Pt/C 5 wt % Pt/ZrO2 5 wt % Pt/ZrO2 3 wt % Pt/C 2.9 wt % Pd/C 0.8 wt % Au/C (sol) 0.8 wt % Au/C (sol) 1 wt % Au/TiO2 2.6 wt % Au/CeO2 1.9 wt % Au/HT 2.4 wt % Ru/HT 2.4 wt % Ru/MgO 2.4 wt % Ru/MgO−La2O3 1.5 wt % (Au−Cu)/TiO2 Au/TiO2 Au/CeO2 a

reaction conditions

solvent

conv/%

0.1 M HMF, 60 °C, 0.02 MPa O2 0.15 M HMF, 25 °C, O2 flow continuous reactor 3% HMF, 100 °C, 1 MPa air 0.5% HMF, 100 °C, 1 MPa air 1% HMF, 140 °C, 1 MPa O2 0.15 M HMF, 22 °C, 0.7 MPa O2

KOH pH 9 1.25 M NaOH

0.15 M HMF, 22 °C, 2 MPa O2 0.1 M HMF, 30 °C, 2 MPa O2 0.15 M HMF, 65 °C, 1 MPa O2 0.17 M HMF, 95 °C, O2 flow 0.05 M HMF, 140 °C, 0.25 MPa O2

2 M NaOH 2 M NaOH 0.6 M NaOH H2O H2O

0.1 M HMF, 95 °C, 1 MPa O2 0.32 M, 130 °C, 0.4 MPa O2 130 °C, 1 MPa O2

0.025 M NaOH methanol + 8% MeONa MeOH

2.4 wt % Na2CO3 H2O CH3COOH/H2O 0.3 M NaOH

yield/%

100 100

>95 80

100 100 100 100 100 100 100 100 100 (8 h) 100 100

99 98 >85 79 71 7 72 71 >99 >99 >95

100 100 100

>99 98a >99a

ref 376a 377 378

384

385 379 380 381 382

388,389 392 393

Furan-2,5-dimethylcarboxylate.

FDCA at complete conversion.378 In the presence of Pt/C and Pt/Al2O3 catalysts and stoichiometric amounts of sodium carbonate with respect of HMF, the reaction was faster affording a quantitative yield to FDCA, but the selectivity rapidly shifted to 5-formyl-furan carboxylic acid (FFCA) upon additional time on stream due to the adsorption of reaction products onto the catalysts. In acidic conditions (40% acetic acid/60% water), at 100 °C under air, diformyl-furan (DFF) was the major product, while at 140 °C and using O2, a 85% selectivity to FDCA at 100% conversion was attained with a LHSV of 7.5 h−1. The oxidation of HMF in 2 M NaOH solutions over a commercial 1 wt % Au/TiO2 catalyst afforded a maximum yield of 71% to FDCA at 25 °C under 2 MPa of O2;379 at lower pressures or lower base concentrations, the FDCA yield decreased, while in the absence of NaOH the conversion was only 13% and low negligible amounts of FDCA were obtained.

implemented because of the low solubility of the FDCA diacid formed in aqueous solutions, and high catalyst loadings were employed to achieve a rapid oxidation, thus avoiding HMF degradation in the presence of strong bases. The oxidation of HMF in the presence of a Pt/Al2O3 catalyst at 60 °C and pH 9 under 0.2 MPa of O2 oxygen resulted in a complete conversion to FDCA, while a Ru/C catalyst was much less active.376 In the presence of a bimetallic Pb−Pt/C catalyst with a high molar HMF/Pt ratio (18−50), a FDCA yield higher than 80% was obtained in a highly alkaline medium (NaOH/HMF = 8);377 metallic salts were added to the reaction medium to act as promoters, but only PbCl2 improved significantly the catalytic activity and the hydroxyl bases were more effective than carbonate bases. The feasibility of performing oxidation reactions in neutral medium has been demonstrated using a Pt/ZrO2 catalyst in a fixed-bed continuous flow reactor (LHSV = 3 h−1) at 100 °C under 1 MPa of air with a 98% selectivity to AB


Chemical Reviews

Review

Scheme 25. Oxidation Products of Glycerol

Using 4 equiv of NaOH at 65 °C under 1 MPa of O2, a 99% selectivity to FDCA was obtained at total conversion over gold nanoparticles supported on ceria; however, a significant deactivation was observed.380 An oxidation reaction at two temperatures was performed whereby HMF was first oxidized at 25 °C to 5-hydroxymethyl-2-furan carboxylic acid (HMFCA) avoiding the HMF degradation in basic medium at high temperature, then the temperature was raised to 130 °C to oxidize the more stable HMFCA intermediate to FDCA. Gold catalysts supported on hydrotalcite yielded >99% of FDCA at 95 °C under atmospheric O2 pressure (HMF/Au = 40) without the addition of a soluble base;381 the yield decreased to 90% after two recycles, and no leaching of gold was detected in solution. Different Ru(OH)x catalysts on basic supports were used for HMF oxidation without addition of a liquid base. Thus, excellent selectivities and conversions were obtained with basic magnesium oxide, magnesium−lanthanum oxide, and hydrotalcite supports; however, magnesium cations dissolved at 140 °C in the reaction medium, which became alkaline,382 In the presence of Ru(OH)x/MgO or hydrotalcite, the reaction was complete within 6−20 h, while the use of less basic MgAl2O4 support yielded only 60% FDCA after 42 h. Basic solids reacted with the formed acid in the presence of a mixture of Au/TiO2 and hydrotalcite.383 Supported Pt, Pd, and Aucatalysts were compared under the same conditions at 22 °C under 0.7 MPa of O2 in basic conditions (NaOH/HMF = 2);384 Au/C and Au/TiO2 catalysts were less active for the oxidation of the alcohol group of HMF than Pt/C and Pd/C catalysts which afforded 79% and 71% yields to FDCA, respectively, while under the same conditions, the Au/TiO2 catalyst led to HMFCA as major product. Gold catalysts required higher oxygen pressures and higher base concentrations to obtain an 80% yield to FDCA at 22 °C. The mechanism of selective oxidation of HMF at high pH was studied over supported Pt and Au catalysts.385 Results from labeling experiments conducted with 18O2 and H218O indicated that water was the source of oxygen atoms during the oxidation of HMF to HMFCA and FDCA, presumably through the direct participation of hydroxide anion in the catalytic cycle. Molecular oxygen was essential for the production of FDCA and played an indirect role during oxidation by removing electrons deposited into the supported metal particles. Different supported metals were also compared at 50 °C, 1 MPa of O2, HMF/metal = 10 in strongly basic solutions (pH 13);386 under these conditions, an 80% yield was observed over Au/TiO2 and Pt/C catalysts, which were much more active than Ru, Rh, and Pd/C catalysts. In acetic acid, Au/TiO2 or Au/CeO2 afforded an 80% yield to HMFCA at 130 °C under 1 MPa of O2.387 The oxidation of HMF at 95 °C under 2 MPa of O2 in NaOH

solution (NaOH/HMF = 4) over Au−Cu colloidal nanoparticles immobilized on titania resulted in 99% yield to FDCA;388 the excellent performances were attributed to isolation effects of gold by copper in the alloyed particles. The catalyst exhibited a remarkable stability as compared to monometallic Au-catalysts and was recovered by filtration and reused several times without significant loss of activity. A catalyst containing 1.5 wt % of total metal with an Au/Cu atomic ratio of 3/1 allowed an efficient conversion of the HMFCA intermediate into FDCA, which is the rate-limiting step of the process.389 A one-pot conversion of fructose to FDCA was attempted with a catalytic system based on membrane technology separating the acidic dehydration of fructose to HMF and the HMF oxidation to FDCA in methyl-isobutylketone (MIBK) over a Pt−Bi/C catalyst;390 a 25% yield to FDCA was obtained at 80 °C under atmospheric pressure. A one-pot conversion of fructose to FDCA was achieved with a 99% selectivity at 72% conversion under air pressure in the presence of cobalt acetylacetonate encapsulated in sol−gel silica.391 The synthesis of furan-2,5-dimethylcarboxylate by oxidative esterification of HMF in methanol and 8% of sodium methoxide relative to HMF was achieved with a 98% yield over a 1 wt % Au/TiO2 catalyst at 130 °C under 0.04 MPa of O2.392 HMF was selectively converted with a 99% yield into furan-2,5-dimethylcarboxylate at 130 °C under 1 MPa of O2 in the absence of any base by using gold nanoparticles on nanoparticulated ceria.393 The catalyst did not leach and was reused several times without any loss of activity or selectivity. The rate-limiting step of the reaction was the alcohol oxidation into aldehyde, which was then converted into hemiacetal and by further oxidation into the corresponding ester. 2,5-Diformylfuran (DFF) was not obtained selectively by the oxidation of HMF with molecular oxygen over Pt-catalysts.376a In contrast, a 95% selectivity to DFF at 90% conversion was reported in the aerobic oxidation of HMF in toluene and methyl isobutyl ketone in the presence of V 2 O 5 /TiO 2 catalysts.394 The aerobic oxidation of HMF was studied over vanadyl complexes immobilized on PVP polymer and over SBA-15 mesoporous materials in the presence of pyridine additive;395 the first catalyst afforded a 99% selectivty at 82% conversion and the second a 98% selectivity at 50% conversion. The oxidation of HMF in DMSO at 125 °C under 1 MPa of O2 over a 1 wt % V2O5/H-beta catalyst afforded a 84% DFF yield (>99% selectivity) after 180 min of reaction time;396 however, the contribution of the leached species to the total activity could not be disregarded. The selective oxidation of HMF to DFF carried out in toluene over Ru/γ-alumina catalyst at 130 °C under 0.28 MPa of O2 afforded a 97% selectivity to DFF at AC


Chemical Reviews

Review

Table 15. Oxidation of Glycerol into Dihydroxyacetone catalysts

reaction conditions

conv/%

select/%

yield/%

ref

5 wt % Pt−1 wt % Bi/C 3 wt % Pt −0.6 wt % Bi/C 7.4 wt % Pt−2.9 wt % Bi/C 5 wt % Pt−5.4 wt % Bi/C

10 wt % GLY, 50 °C, air, pH 2−4, batch, 4 h 50 wt % GLY, 50 °C, air, fixed bed (LSHV: 0.06 h−1) 10 wt % GLY, 60 °C, air, pH 2, batch 120 °C, pH 4, batch, 0.1 MPa O2, 7 h 120 °C, pH 1.3, fixed bed (LSHV 0.15 h−1) 80 °C, 0.2 MPa O2, initial pH 2 1.5 M glycerol, semibatch reactor, pH 12 0.1 MPa O2, 60 °C 0.6 mol/L GLY, NaOH/GLY = 1, 60 °C, 0.28 MPa O2, bubble column reactor 10 wt % GLY, 60 °C, flowing O2 5 wt % GLY, 80 °C, 0.3 MPa O2 10 wt % GLY, 60 °C, flowing O2

30 75 75 59 90 80 50 50 30 91.5 52 90 90 90

67 70 49 51a 50b 60 26 36 53 49 85 13 36 51

20 52 37 30 45b 48

398 399 400 4d

16 45 44

404 408 405

conv/%

select/%

yield/%

90

77

70

400

56 54 100 90 90 90 100 100 50 30 83 32 90 90 76 91 28 67 50 90 50 48 70 90 70 42.5 30 86 47 63

100 100 92 49 69 62 39 45 83 75 61 37 74 72 36 38 28 47 61 60 47 66 70 68 83 85 67 71 78 68

56 54 92

311,409a

3 wt % Pt−0.6 wt % Bi/C Au/C Au−Pt/C (Pt 0.25−0.33) 1 wt % Au/C 5 wt % Pt−5 wt % Bi/C Pd−Ag/C (Ag/Pd = 1) 4.9 wt % Pt/C 4.6 wt % Pt-5.2 wt % Bi/C 3.8 wt % Pt−3 wt % Sb/C a

401 403

407

At 20% conversion. bAt 200 h on stream.

Table 16. Oxidation of Glycerol into Glyceric Acid catalysts 5 wt % Pd/C 5 wt % Pt/C 1 wt % Au/C (25 nm) 1 wt % Au/graphite 1 wt % Au/C (>20 nm) 1 wt % Au/C (2−3 nm) 1 wt % (Au−Pd)/C (2−3 nm) 1 wt % Pd/graphite 1 wt % (Pd−Au)/graphite 1 wt % (Pd−Au)/graphite 1 wt % Au/C (45 nm) Au/C 1.6 wt % Au/TiO2 2.5 wt % Au−2.5 wt % Pd/TiO2 2.5% Au−2.5% Pd/C 1 wt % Au/C 1 wt % Au/TiO2 1 wt % Au/Al2O3 1 wt % Au/Nb2O5 3.1 wt % Au/MgAl2O4 1 wt % Au/Dowex M-43 5 wt % Pt/C 5 wt % Pt/MWNTs 5 wt % Pt/H2O2-MWNTs 5% Pt/S-MWNTs 1 wt % Au−Pt(6:4)/mordenite 1 wt % Au−Pt(1:3)/MgO 1 wt % Au−Pd(1:3)/MgO 5 wt % Pt−Cu/C (Cu:Pt = 1:1) 0.64 wt % Pt/HT (reduction HCHO) 0.7 wt % Pt/HT (reduction soluble starch)

reaction conditions 10 wt % GLY, pH 11, air 5 wt % GLY, NaOH/GLY= 1, 60 °C, 0.3 MPa O2 10 wt % GLY, NaOH/GLY = 4, 30 °C, 0.3 MPa O2 0.3 M GLY, NaOH/GLY = 4, 30 °C, 0.3 MPa O2 0.3 mol/L GLY, NaOH/GLY= 4, 0.3 MPa O2

0.3 1.5 0.3 0.3 0.6

M M M M M

GLY, GLY, GLY, GLY, GLY,

NaOH/GLY = 2, 60 °C, 0.5 MPa O2 NaOH/GLY= 2, 60 °C, 0.1 MPa O2 NaOH/GLY = 2, 60 °C 1 MPa O2, batch NaOH/GLY = 1, 60 °C, 1 MPa, continuous fixed bed NaOH/GLY = 2, 60 °C, 1 MPa O2

0.1 M GLY, NaOH/GLY = 2, 60 °C, 0.6 MPa O2

0.3 M GLY, NaOH/GLY = 4, 50 °C, 0.3 MPa O2 0.3 M GLY, NaOH/GLY= 4, 50 °C, 0.3 MPa O2 10 wt % GLY, 60 °C, O2, 6 h, base free 10 wt % GLY, 60 °C, 0.1 MPa O2

0.3 M GLY, 100 °C, 0.3 MPa O2 0.3 M GLY, 25 °C, 0.3 MPa O2 10 wt % GLY, 60 °C, 1 bar O2 0.1 M GLY, 30 °C, O2 flow

99% conversion;397 DFF was isolated by simple evaporation of the solvent, and the catalyst was washed with NaOH solution to remove adsorbed polymeric impurities.

411 319a 413a

412c 412a 434 413b 417a,b

24 32 49 62

418 419 426 427,428

429 430 61

431 432 433

chelating agents and useful intermediates in organic synthesis, but they have presently a limited market because they are produced by costly stoichiometric or enzymatic processes. This triggered an interest for new processes based on the oxidation of aqueous glycerol solutions over metal catalysts at temperatures lower than 100 °C under atmospheric or low pressure of air or oxygen. Most of the studies focused on the selective formation of glyceric acid and dihydroxyacetone, but tartronic

5.6. Oxidation of Glycerol

The oxidation of glycerol could follow complex pathways yielding various C3 oxygenates together (oxalic and glycolic acids) and C1 (formic acid) (Scheme 25). The C3 oxygenates are potentially

ref

reaction with C2 products valuable AD


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at 90% glycerol conversion on a 5 wt % Pd/C catalyst (Table 16). Because catalysts based on Pt-group metals suffered from oxygen-poisoning at high oxygen partial pressures, gold catalysts that had a better resistance to oxygen poisoning were employed extensively during the last 10 years. The selective oxidation of glycerol to glycerate over carbonsupported Au-catalysts required operation at high pH and under oxygen pressure to reach the optimum performance. A 100% selectivity to glycerate at ca. 50% conversion, which decreased to 86% at 72% conversion, was obtained with 1 wt % Au/C or 1 wt % Au/graphite catalysts in NaOH solutions;311,409 it was suggested that the base facilitated the initial dehydrogenation via the H-abstraction of a primary OH group of glycerol, which is the rate limiting step. Isotopic labeling experiments with 18O2 and H218O coupled with DFT calculations demonstrated that molecular oxygen was not incorporated in the reaction product, but it regenerated hydroxide ions formed via the catalytic decomposition of a peroxide intermediate.410 Using a concentrated basic solution (NaOH/glycerol = 4), a 92% selectivity to glycerate at full conversion was obtained by oxidizing 0.3 M glycerol solution at 30 °C over 1 wt % Au/C catalyst.411 Large nanoparticles of ca. 20 nm were more selective to glycerate because the consecutive oxidation reaction to tartronate was minimized. It was later confirmed that the selectivity to glyceric acid increased with increasing gold particle size.320,412 However, gold particles larger than 50 nm were inactive, and a compromise between activity and selectivity was achieved around 20 nm.409b A recurrent formation of hydrogen peroxide over gold catalysts leading to C−C bond scission was observed;412c the selectivity to glycerate was higher and the formation of glycolate lower as the concentration of hydrogen peroxide was low. Bimetallic Au−Pd catalysts prepared by sol immobilization on active carbon or graphite were more active than the corresponding monometallic catalysts.319a,413 Single-phase Au− Pd bimetallic catalysts exhibited better performances than a physical mixture Pd- and Au-catalysts supported on activated carbon, which was interpreted as a synergistic effect due to an alloy formation.321,414 Catalysts with a Au-rich composition showed an increased durability as compared to Pd-rich alloys.413c Catalysts prepared by sol immobilization resulting in homogeneous Pd−Au alloys were more active than catalysts prepared by impregnation resulting in a higher concentration of palladium on the surface.413b Bimetallic Au−Pt catalysts were also very active; however, the selectivity to glycerate was lower due to the formation of glycolic acid.415 The supporting materials played an active role in determining the activity and selectivity of oxidation reactions. Carbon-supported gold catalysts were more active than those on oxide (TiO2, Al2O3, MgO, and CeO2) support.416 A 1 wt % Au/Nb2O5 catalyst prepared by gold-sol method on a niobia support in crystalline form was slightly less active than Au/C and Au/TiO2 catalysts, but more selective to glycerate.417 The selectivity to glycerate over gold particles supported on different MgAl2O4 spinels using different preparation routes was mainly determined by the Al/Mg ratio at the surface because Al-rich surfaces enhanced the C−C bond cleavage.418 Gold nanoparticles stabilized by THPC (tetrahydroxymethyl phosphonium chloride) deposited on the weak base Dowex M43 resin and on activated carbon provided a 60% and 50% selectivity to glycerate, respectively, at 90% conversion and exhibited a comparable activity.419 In continuous fixed-bed

acid attracted recently a significant interest. Complete reviews of glycerol oxidation reactions in the presence of metal catalysts were published.4a,e,13 The performances of metal catalysts for the oxidation of glycerol into dihydroxyacetone and glyceric acid are given in Tables 15 and 16, respectively. Dihydroxyacetone (DHA) used in cosmetic industry as a sunless tanning agent is obtained industrially via costly biocatalytic processes, which triggered attempts to synthesize it by oxidation of glycerol with oxygen over metal catalysts. The oxidation with air at atmospheric pressure over carbon supported Pt-catalysts operating at 50 °C at acidic pH yielded 4% of DHA, but the addition of bismuth promoter (1 wt % Bi-5 wt % Pt) increased dramatically the selectivity, leading to a 20% yield at 30% glycerol conversion.398 It was postulated that bismuth adatoms acted as site blockers on Pt-surface controlling the glycerol orientation toward dihydroxyacetone formation. The continuous oxidation of 50 wt % glycerol solutions at 50 °C in a fixed-bed reactor loaded with a bimetallic 0.6 wt % Bi−3 wt % Pt catalyst (Bi/Pt = 0.2) supported on granular carbon afforded a 52% yield to DHA;399 the catalytic performances were maintained over 1000 h on stream. The oxidation of 10 wt % glycerol solution over a 1 wt % Bi−5 wt % Pt/C (Bi/Pt = 0.2) catalyst prepared by coimpregnation and formaldehyde reduction carried out in batch reactor at pH 2 yielded 35% of DHA.400 High oxidation rate with a yield limited to 30% was obtained at 120 °C under 1 MPa of air with a 5 wt % Pt−5.4 wt % Bi/C catalyst prepared by coimpregnation and supported on AC7-type active charcoal;4d however, in continuous experiments in a tricklebed reactor, the initial selectivity of 80% at 90% conversion decreased to 40% after 1000 h on stream. Catalyst 3 wt % Pt−0.6 wt % Bi supported on a Norit Darco activated carbon, prepared by impregnation of a Pt/C catalyst and reduced with NaBH4, yielded 48% of DHA at 80% glycerol conversion as the reaction was carried out in a semibatch reactor at 80 °C, 0.21 MPa O2-pressure, and pH 2.401 The deactivation of Pt−Bi catalysts due to strong glyceric acid adsorption on the metal surface was a limitation for widespread use of Pt−Bi catalysts.402 An alternative to Pt−Bi catalysts operating under acidic conditions was found with gold catalysts operating in alkaline media. The presence of platinum acting as a promoter of gold catalysts increased the selectivity to DHA from 26% (Au/C) to 36% (Au−Pt/C) at 50% glycerol conversion.403 Gold catalysts in basic medium afforded a maximum 16% yield to DHA in a continuous flow structured reactor using an Au/C catalyst.404 A Pd−Ag/C (Ag/Pd = 1) catalyst based on a Pd−Ag alloy phase was more active than a Pd/C catalyst and allowed one to reach a 44% yield to DHA at 52% conversion.405 The oxidation mechanism of GLY to DHA over Pd−Ag catalysts was investigated;406 the reaction data and characterization results supported a mechanism where the terminal OH group of glycerol was adsorbed on Ag-sites and the neighboring secondary OH group (CH−OH) was attacked by the oxygen species dissociatively adsorbed on Pd-sites. A catalyst based on a Pt−Sb alloy under the form of nanoparticles homogeneously dispersed on thiolated, multiwalled carbon nanotubes was more active and selective to DHA than a Pt−Bi catalyst on the same support (selectivity of 36% and 51% at 90% conversion, respectively).407 The oxidation of glycerol (GLY) to glyceric acid (GLYAC) was carried out on palladium and platinum catalysts at basic pH under flowing air;400 the maximum yield to glycerate was 70% AE


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bimetallic 5 wt % Pt−Cu reached 71% at 86% conversion;431 the improved performance was attributed to the formation of a PtCu3 alloyed phase. Hydrotalcite-supported Pt-catalysts prepared by impregnation with H2PtCl6 and subsequent reduction with formaldehyde afforded a 78% selectivity at 47% conversion with a high activity at room temperature under atmospheric oxygen pressure in pure water.432 When prepared with soluble starch as a reducing and stabilizing agent, the hydrotalcite supported Pt-catalyst afforded a 68% selectivity at 63% conversion and was reused at least three times.433 The oxidation of glycerol may lead to tartronic acid, which finds applications in the pharmaceutical industry and as an anticorrosive and oxygen-scavenger agent, and to β-hydroxypyruvic acid, which is a chemical intermediate for the synthesis of L-serine and was used as a flavor component. Starting from calcium glycerate under basic conditions (pH 10−11) over a 5 wt % Pt−1.9 wt % Bi/C catalyst, an 83% yield to tartronate at 85% glycerate conversion was obtained, while the same catalyst at acidic pH (3−4) yielded 64% of β-hydroxypyruvic acid at 80% conversion.435 Starting from sodium glycerate at a more acidic pH over a 5 wt % Pt−5 wt %Bi/C catalyst, a 93% selectivity to β-hydroxypyruvic acid at 95% glyceric acid conversion was achieved;305a the selectivity to β-hydroxypyruvic acid was attributed to the bonding of Bi-adatoms on the platinum surface to the carboxyl and α-hydroxyl group of the glyceric acid, which favored the selective oxidation of the secondary alcohol function. Starting from glycerol, a 44% selectivity to tartronate at 100% conversion was observed over a 1 wt % Au/TiO2 catalyst.313c Gold nanoclusters stabilized by poly(1-vinylpyrrolidin-2-one) (PVP) afforded a 45% selectivity to tartronate at total glycerol conversion.436 A bimetallic 0.7 wt % Au−0.9 wt % Pd supported on a Mg−Al mixed oxide catalyst exhibited a 37% selectivity to tartronate and a 43% selectivity to glycerate at nearly total glycerol conversion.437 Using a Au− Pd/TiO2 catalyst prepared by deposition−precipitation with urea at 50 °C, a 55% selectivity to tartronate was attained at 99% conversion. In a continuous up-flow fixed-bed reactor loaded with a 1.6 wt % Au/TiO2 catalysts, the 30% selectivity to secondary oxidation diacids (oxalic and tartronic acids) was higher than in a semibatch autoclave reactor (5% to the diacids). The higher selectivity to the secondary oxidation products, oxalic and tartronic acid, in the fixed-bed system was likely the result of direct gas−solid contact, which allowed for a higher inventory of dioxygen on the catalyst as compared to a batch system.434 The enhanced formation of tartronate was confirmed as glycerol oxidation was performed in a fixed-bed reactor over 1.6 wt % Au/TiO2 catalyst.383 These conditions would slow or tolerate the formation of ketone impurities, which have been suggested to cause the inhibition of the reaction rate.438 The addition of 0.1 wt % bismuth to the Au− Pd/C catalyst decreased the catalyst activity, but promoted the consecutive reaction of glycerate to tartronate, which was produced with a yield of 78% at full conversion of glycerol.439 Mesoxalic acid (or ketomalonic acid), a chelating agent and synthon for organic synthesis, was obtained by oxidation of tartronic acid with a 65% yield at 80% conversion over a Pt− Bi/C catalyst at 60 °C without pH control;440 a total conversion of tartronic acid was obtained at 80 °C giving a 50% yield of mesoxalic acid without any other products in solution because they were totally oxidized into CO2. Mesoxalic acid was also obtained in two steps using first a Pt/CeO2 catalyst to convert glycerol to tartronic acid, and then a Pt−Bi/ C catalyst to convert the secondary hydroxyl group of

reactor, the selectivity to glycerate was increased by decreasing the contact time and by increasing the base amount. A strong effect of the surface chemistry of supporting carbon materials on the selectivity to glycerate was observed showing particularly that the presence of surface oxygenated groups is unfavorable. A selectivity of 85% to glycerate was observed at 66% conversion using Pt-particles supported on carbon spherules.420 Gold catalysts prepared by impregnation and gold-sol method on activated carbon, graphite, and carbon nanofibers exhibited higher activity and selectivity as the Auparticle sizes were smaller and the support crystallinity higher.421 Further studies with graphite, ribbon-type carbon nanofibers, and carbon nanospheres catalysts prepared by the gold-sol method confirmed that the highest activity was achieved with Au-nanoparticles on highly crystalline supports.422 Au-particles of 3−4 nm size were prepared by goldsol method on the surface of carbon nanotubes (CNTs) and carbon nanofibers (CNFs), which were modified by oxidative treatment with HNO3 to introduce oxygen-containing groups or by NH3 treatment to produce nitrogen-containing surface groups;423 the catalytic activity increased with the amount of basic sites, whereas oxidative treatments did not alter significantly the activity. Catalysts with hydrophobic surfaces enhanced the selectivity to C3 (glycerate and tartronate), probably because of the lower formation of H2O2, which was responsible for the C−C cleavage. Gold nanoparticles supported on activated carbon Norit ROX0.8 modified by chemical and thermal treatments to get different oxygenated surface groups were more active than Au-nanoparticles on basic and oxygen-free supports.424 Similarly, gold particles supported on oxygen-free multiwalled carbon nanotubes were very active and led to a 60% selectivity to glycerate, whereas a 40% selectivity was obtained with oxygen-rich supports.425 The catalytic activity of gold catalysts supported on carbonaceous materials was considerably decreased when using crude glycerol from biodiesel synthesis as a feedstock; 422 a simple neutralization procedure of the crude glycerol restored the activity. Most of the studies dealing with the oxidation of glycerol over Pt- or Au-catalysts were performed at high pH to increase the activity, selectivity, and stability of catalyst, but the product was glycerate, which required subsequent treatment to obtain glyceric acid. The oxidation of glycerol in base-free conditions over a Pt/C catalyst yielded 24% of glyceric acid at 50% conversion but with a low reaction rate.426 Pt/multiwall nanotubes (MWNTs) were more active than Pt-catalysts supported on active carbons and afforded a 70% selectivity at 70% conversion, which was attributed to the easier accessibility of the Pt-nanoparticles on the external walls of the nanotubes.427 Upon functionalization of the MWNTs by thiolation, free glyceric acid was obtained with a 68% selectivity at 90% conversion.428 Bimetallic Au−Pt (6:4) nanoparticles supported on mordenite were able to oxidize glycerol with a 83% selectivity to glyceric acid at 70% conversion and 81% selectivity at complete conversion without the use of basic conditions;429 the use of a mordenite support prevented H2O2 formation, and the alloyed Au−Pt particles were less prone to leaching. Au−Pt and Au−Pd colloidal nanoparticles immobilized on Mg(OH)2 were highly active at ambient temperature under base-free conditions. The Au−Pt (1:3) catalyst afforded a selectivity of 85% to glyceric acid at 42% conversion; in contrast, similar Au−Pd catalyst was less active, and the selectivity was lower.430 The selectivity to glyceric acid over a AF


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tartronate.441 The oxidative esterification of glycerol in methanol afforded dimethylmesoxalate with 79% and 89% selectivity at total conversion over Au/TiO2 and Au/Fe2O3, respectively.392 The oxidation of glycerol with the rupture of C−C bonds was used to prepare glycolic acid, a cleaning agent and chemical intermediate. A 60% yield of glycolate could be achieved over a 1 wt % Au/C catalyst using H2O2 as oxidant.313c Using hydrogen peroxide, the selectivity to glycolic acid was 75% at 95% glycerol conversion at 100 °C over a 1 wt % Pd catalyst supported on Starbon a mesoporous material derived from polysaccharides.442 The oxidation of glycerol carried out over a Au−Pt/TiO2 catalyst in alkaline aqueous solutions (NaOH/glycerol molar ratio 4) at 90 °C under flowing O2 at atmospheric pressure afforded an 86% yield to lactic acid;443 the reaction proceeded through the formation of glyceraldehyde and dihydroxyacetone.

esinol (oxoMAT), which find applications as health protecting agent due to its high superoxide scavenger activity and in cosmetics because of its UV-protection properties.454 Gold particles supported on Y-type zeolite did not have any activity in the oxoMAT synthesis most probably due to steric restrictions. A 3 wt % Au-catalyst supported on wide pores alumina yielded oxoMAT with a 100% selectivity at 94% conversion after 4 h in a mixture of water/propan-2-ol;455 a deactivation occurred because of the strong adsorption of organic impurities on the catalyst surface, but it was regenerated by calcination. DFT calculation of HMR oxidation on model Au28 clusters accounted for the different reactivity in the presence or absence of oxygen depending upon the conformation of the two HMR diastereoisomers.456

5.7. Oxidation of 1,2-Propanediol

The application of supported metal particles and molecular oxygen for the oxidation of oxygenated compounds offers a green alternative to classical chemical oxidants, and their use is relevant for the production of chemicals starting from sugars, glycerol, and HMF. The catalysts employed so far were either platinum-group metals under the form of mono- and multimetallic systems, or gold catalysts under the form of nanoparticles highly dispersed on specific supports. Efficient catalysts must fulfill important requirements in term of activity, selectivity, and long-term stability. Whereas issues pertaining to activity and selectivity have been well documented, the longterm stability issue, which is required for an industrial use of these catalysts, has been comparatively neglected. Catalyst deactivation needs to be addressed by reaction studies involving multiple recycling or continuous operation and by a detailed characterization of catalyst structure and texture at different stages of their utilization. A major disadvantage of supported Pt-group catalysts is their rapid deactivation by overoxidation of the metal surface and by poisoning of the active sites by strongly adsorbed products or byproducts. On the other hand, they are comparatively stable to sintering and leaching. Significant improvements have been achieved by using bimetallic catalytic systems involving the incorporation of a second non active oxophilic promoter (e.g., Bi, Pb) protecting platinum and palladium from overoxidation. However, the design of bimetallic systems and the reaction conditions should be adapted to avoid promoter leaching. Because the activity and selectivity of gold catalysts are highly dependent upon the size of gold particles, the stability toward sintering either by Ostwald ripening or by particles agglomeration requires more attention in the future, although improvements of catalyst stability have been achieved already by alloying gold with other metals. Bimetallic formulation of gold catalysts led to a strong synergistic effect in terms of activity with respect to monometallic systems, but catalyst design is essential to attain optimum performances. Catalysts prepared by sol-immobilization of the alloyed phases are more active than the same systems with segregated metals when prepared by conventional impregnation methods. Further studies are required to obtain a better control of the synthesis of alloyed particles particularly in large batches necessary for practical operations. The influence of the composition and internal structure of the alloy, and the understanding of the mechanism by which they give enhanced performances, also remain an open field of research. There are still other challenges to be solved such as a better control of the selective oxidation of polyoxygenated com-

5.9. Concepts Guiding the Choice of Metal Catalysts

1,2-Propanediol (PDO), which can be produced by catalytic hydrogenolysis of biomass-based polyols including glycerol and sorbitol (see section 3), contains a primary and a secondary alcohol function that can be selectively oxidized to hydroxyacetone or lactic acid. At 90 °C at pH 8, a Pd/C catalyst modified by Pb, Bi, or Te promoters was able to oxidize both functions, leading to a mixture of lactic acid, hydroxyacetone, and pyruvic acid.444 In alkaline solutions at 70 °C under 0.3 MPa of O2, lactate was formed with a 90% selectivity at high conversion (73−84%) over 5 wt % Pd/C and 5 wt % Pt/C catalysts.445 A 1 wt % Au/C catalyst prepared by deposition− precipitation yielded lactate with a 100% selectivity at 78% conversion.446 As compared to monometallic Au-catalysts, carbon or titania supported Au−Pd catalysts enhanced the activity at 60 °C under 1 MPa of O2 in alkaline conditions, and maintained a 96% selectivity to lactate at 94% conversion.447 A Au−Pt/C bimetallic catalyst allowed one to perform the reaction at ambient temperature with air and was more active than a Au−Pd catalyst under the same conditions.448 Over a 2.8 wt % Au/Mg(OH)2 catalyst prepared by sol immobilization, a 89.3% selectivity was attained at 94.4% conversion at 60 °C under 0.3 MPa of O2.449 A Pt-catalyst supported on carbon spherules was able to oxidize 1,2-PDO to lactate with 90% selectivity at 91.4% conversion.420 The oxidation over a Au−Pt catalyst at 100 °C under 0.3 MPa of O2 in the absence of a base yielded hydroxyacetone as major product in addition to lactate.450 5.8. Oxidation of Wood Extractives

The conversion of lignin to chemicals by combined depolymerization and oxidation8a was mainly achieved with homogeneous catalysts such as transition metal salts213a,451 or organometallic complexes.452 Only few investigations were carried out with supported noble metal catalysts resistant to leaching. Thus, a catalytic process of aldehyde production from alkaline lignin extracted from sugar cane bagasse was evaluated in a batch slurry reactor and in a continuous three-phase fluidized-bed reactor with a Pd-catalyst, at 100−130 °C under 0.2−1 MPa of O2;453 the oxidation at 120 °C under 0.5 MPa of O2 of a feed of 30 g L−1 of lignin at a flow rate of 5.00 L h−1 yielded 65.1 × 10−1 and 114.8 × 10−1 g of vanillin and syringaldehyde, respectively, after 2 h on stream. Hydroxymatairesinol (HMR), a natural lignan extracted from spruce knots as a mixture of two diastereoisomers, was selectively oxidized at 70 °C under atmospheric pressure of air over supported Au, Pd, Au−Pd catalysts into oxomatairAG


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to develop a new portfolio of products that have no equivalent among those presently manufactured by classical synthesis routes from hydrocarbons. Ultimately, the production of biosourced chemicals should contribute to improve the public confidence in the chemical industry. The development of bioproducts at a large industrial scale requires that their quality and cost meet consumer demand. The value-chain starting from biomass intended to duplicate chemicals currently produced by conventional synthetic routes that have a well-established market could face the risk to manufacture products that are not cost competitive.15e,f Interestingly, successful examples of industrial development concern bioproducts with unique properties such as absence of toxicity, biocompatibility, or biodegradability that have no synthetic counterparts, such as isosorbide, alkylpolyglucoside surfactants, or PLA polymers. Isosorbide provides an example of a new product without synthetic counterpart derived from cheap and widely available polysaccharides via very selective catalytic steps following all of the principles of green chemistry. Besides their biosourced origin, isosorbide and derivatives are attractive products because of their biocompatibility and their suitability to find a market in number of applications notably as a substitute for phthalates and bisphenol A.466 Process economy depends both on the cost of the platform molecules that have to be produced with enough purity from raw biomass by a combination of extraction, depolymerization, and fermentation processes and on the development of optimized catalytic pathways from oxygenated platform molecules. The cost associated with the purification of platform molecules, particularly those obtained by depolymerization and/or fermentation of cellulosic materials, could impact severely the overall process economy. Processes requiring too many conversion and separation steps affect the overall atom economy, energy demand, and waste emissions and decrease the proportion of renewable carbon in final products. An alternative value-chain based on one-pot processes starting from biopolymers or plant oils could be more effective for mass production and accelerate greatly the industrial development of chemicals and materials based on renewable carbon.1e Thus, pools of molecules with similar functionality such as polyol mixtures could be used without further separation in the manufacture of consumer products. For example, biosourced surfactants,158b,467 lubricants,468 solvents,469 foams,470 resins,471 and plasticizers32c already have a sizable market share. This value-chain is more likely to be cost-competitive because it drastically reduces the number of conversion, extraction, and purification steps. Also, the one-pot modification of biopolymers such as polysaccharides, lignin, or proteins to functional polymers to meet consumer demand should be more sustainable from both economic and environmental standpoints than the multistep value-chain consisting of degrading biopolymers to small molecules that could serve as monomers to synthesize polymers. Eventually, biobased products such as polymers can be converted to fuels after their end of life. The past decade experienced a 14-fold increase in the number of literature references dealing with metal catalysts employed in biomass conversion, thus illustrating the dramatic development of metal catalysis in this comparatively new field of application. However, many investigations were merely exploratory and factual and did not provide useful guidelines for future work and mechanistic interpretations. Supported metal catalysts involved in the traditional hydrocarbon-to-chemical value-chain have been the subject of many investigations for

pounds to desired products. For instance, in the case of glycerol, most studies have focused on glyceric acid and dihydroxyacetone, while it is important to find conditions and catalysts able to yield selectively tartronic lactic or glycolic acids. Basic reaction conditions are often essential to promote the initial step of dehydrogenation. It was already demonstrated that the oxidation of polyols may proceed in the absence of a base as in the case of Au−Pt supported on mordenite,429 or in the presence of a basic support;430 these results open an attractive research field for improving catalytic systems operating in neutral aqueous media, thus preventing the formation of carboxylates.

6. CONCLUDING REMARKS AND PROSPECTS The development of first generation biofuels derived from agricultural crops is presently a subject of controversy because of their competition with food needs and land use and in view of the low efficiency of photosynthetic processes.457 Expectations on the reduction of greenhouse gas (GHG) emissions have been a subject of debate.458 As far as the production of chemicals is concerned, these criticisms are less relevant because of the much lower tonnages at stake. Nevertheless, the present trend is to find catalytical routes to chemicals from lignocellulosic feedstocks.459 Few life cycle assessment (LCA) studies were completed on a biomass-to-chemical value-chain because of the variety of feedstocks and multitude of synthesis routes to produce a given chemical.460 Simple indicators such as atom economy or efficiency461 and E-factor,462 which were very useful for evaluating the green character of chemical processes, are not readily transposable to biomass conversion processes because of the many uncertainties in boundary limits.463 Evaluation methodologies including environmental, economic, and societal aspects have been set up early in industrial companies as decision-making tools for product development. Thus, the BASF company developed various assessment tools such as “Eco-efficiency” and “SEEbalance” for quantifying the sustainability of products and processes, which were applied to a number of biomass conversion processes.464 The Shell Co. developed a semiquantitative evaluation tool based on waste, hazard, and cost, which was used for assessing biobased process for the manufacture of polyether polyols.14d A method integrating various economic and environmental indicators for the quick preliminary assessment of biobased processes at the laboratory stage has been developed and tested for a biobased process of butadiene production.465 A number of biomass conversion processes based on supported metal catalysts meet most of the green chemistry principles, provided metal particles and supporting materials do not leach in reaction media. Thus, the aerobic oxidation of glucose in aqueous solutions into gluconic acid can be achieved with a high productivity and selectivity at atmospheric pressure close to room temperature with a 100% atom economy and over recyclable metal catalysts (see section 5.2.1). However, the green character of some conversion processes using complex catalytic systems involving toxic metals and reaction media is disputable. The interest of the chemical industry for biosourced chemicals was triggered by the benefits anticipated from the marketing of innovative, high value-added bioproducts that could preserve the competitiveness of the chemical industry in a global market economy. In addition to merely duplicating existing products synthesized from fossil resources, the chemistry based on biomass derivatives provides opportunities AH


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functionalities into alcohols. As compared to nickel catalysts, which were mostly used in the past for carbohydrate hydrogenation, ruthenium catalysts exhibit a much higher resistance to sintering and leaching in acidic and chelating aqueous solutions and a much higher specific or intrinsic hydrogenation activity. Further studies should be awaited to find the reasons why they are more active and selective for the aqueous phase reduction of aldehydes, ketones, and carboxylic acids than other Pt-group metals. DFT calculations have shown that water adsorbed on Ru(0001) surface was half-dissociated so that the surface was covered with H2O, OH, and H species.472 These findings were substantiated by scanning tunneling microscopy and X-ray absorption spectroscopy.473 The dissociation of water molecules on Ru-surface might possibly account for the high hydrogenation activity of ruthenium in aqueous media, but additional studies are required to support this interpretation. The selective oxidation of alcohols to aldehydes in aqueous solutions over supported gold nanoparticles was the major breakthrough in metal catalysis during the past decade. Further investigations are required to understand the mechanism of oxidation of alcohols on gold catalysts, but a recent study demonstrates that gold nanoparticles are capable of activating O2 at the solid−liquid interface because the presence of water doubles the adsorption energy of dioxygen at the edge sites of 4 nm nanoparticles.474 A surface science study on benzyl alcohol oxidation on Au(111) surface points to a mechanism whereby benzaldehyde is produced via the sequential dehydrogenation of benzyl alcohol via reaction with the adsorbed oxygen atoms, first from the hydroxyl group to form a benzyloxy intermediate and then from the α-carbon in this species to form benzaldehyde.475 However, the main drawback of gold catalysts comes from the fact that gold nanoparticles can easily sinter via coalescence or Ostwald ripening processes leading to a dramatic loss of activity, which is a major drawback for a widespread industrial application of gold catalysts. Although sophisticated supporting materials could delay excessive gold sintering, regeneration processes should be established for practical application of gold catalysis. The use of methyl iodide for redispersing gold particles was tested successfully on TiO2 and Al2O3 supports but failed on SiO2 support,476 and the method is not suitable for large scale application, for example, due the toxicity of CH3I. Further investigations are also required to optimize the activity, selectivity, and stability of metal catalysts for the conversion of biomass by using carefully designed and well-characterized bi- or trimetallic catalysts. The substitution of noble metals by base metals is a desirable field of investigation, although there is a high risk of metal leaching in solutions due to the chelating properties of carbohydrates and derivatives. Investigations are required to better characterize metal catalysts employed in nonconventional reaction media such as ionic liquids and supercritical fluids and in the presence of activation treatments such as ultrasounds or microwaves. Finally, one of the major issues for the industrial development of the biomass-tochemical value-chain is the need for catalytic systems and engineering processes able to cope with the presence of residual impurities of feedstocks that may poison the activity or modify the selectivity of metal catalysts. Investigations on catalyst deactivation and regeneration employing real biomass feedstocks instead of model molecules are strongly needed. Supported metal catalysts, which were instrumental in the production of chemicals from fossil resources, are once again at

more than a century to optimize their design and understand their mechanism of action. However, there are presently few guidelines to design metal catalysts and processes specifically adapted to the conversion of biomass-derived molecules to chemicals. There is a need for fundamental studies on the interaction of biomolecules with metal surfaces based on advanced characterization tools, micro kinetic studies, and theoretical modeling. Measurements of turnover frequencies should greatly facilitate the comparison of catalyst activity. Metals and supporting materials employed in conventional hydrocarbon processing and organic synthesis are often not adapted to convert biomass-derived feedstocks. Various challenges should be met for the design of catalysts employed in biomass processing: (i) Robust and easily regenerated metal catalysts should be developed because natural raw materials contain impurities, which may alter their selectivity and decrease their activity thus hampering catalyst recycling or continuous processes. Thus, despite costly purification technologies, the platform molecules obtained by fermentation of carbohydrates, which are used as starting feedstocks for chemical synthesis, may contain residual proteins from fermentation broth or oligomeric species derived from carbohydrates liable to poison catalyst surfaces. In the past, huge efforts have been made in petroleum refining processes to avoid metal catalyst poisoning by sulfur compounds, in the same way metal catalysts employed in biorefining precesses should be designed to avoid poisoning by nitrogen and phosphorus-containing compounds. (ii) Catalysts resistant to leaching should be developed because liquid reaction media often contain oxygenated molecules with high chelating properties that are liable to complex and extract atoms from metal particles and oxide supports resulting in a rapid catalyst leaching. Thus, supporting materials such as aluminas and silico-aluminas, which were widely used as supporting materials in hydrocarbon processing and organic synthesis in nonpolar solvents, should preferably be avoided in acidic and/or chelating reaction media and replaced by carbons or stable oxides such as CeO2, TiO2, or ZrO2. (iii) Catalyst porosity and hydrophilic/hydrophobic properties should be adapted to process a variety of molecular species differing in size and functionalities such as biopolymers, highly fuctionalized carbohydrates derivatives, and fatty compounds. (iv) Catalysts should be compatible with a wide variety of reaction media such as water, ionic liquids, or supercritical fluids employed in biomass processing. They should be structurally resistant to activation methods such as sonification. (v) Bimetallic or multimetallic catalysts should be designed to tune reaction selectivity and improve catalytic activity and stability to poisoning. (vi) Multifunctional catalysts should be designed to achieve one-pot conversion allowing process intensification. The combination of enzymatic, homogeneous, and heterogeneous catalytic processes allowing a multistep conversion of biomass by cascade catalysis is an additional challenge. A survey of the literature data on metal-catalyzed conversion of carbohydrates shows that two metals, ruthenium and gold, that were not much employed for traditional metal-catalyzed reactions in petroleum chemistry and organic synthesis take an important role in biomass conversion processes, that is, ruthenium for hydrogenation, hydrolysis/hydrogenation, and hydrogenolysis/dehydroxylation reactions and gold for oxidation reactions. Indeed, supported ruthenium catalysts were found more active and selective than other Pt-group metal catalysts to achieve an efficient reduction of carbonyl AI


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the forefront of converting renewable biomass to valuable chemicals and materials.

except for two stays in 1976 and 1982 as visiting scientist at the Department of Chemical Engineering of Stanford University with Prof. Michel Boudart and at the Department of Material Sciences at NASA Ames Research Center (Moffett Field, CA). He was Ipatieff Lecturer in 1992 at Northwestern University (Evanston, IL) with Prof. Wolfgang Sachtler and visiting scientist in 1997 at Lawrence Berkeley Laboratory with Prof. Gabor Somorjai. His research interests evolved from physical chemistry studies of organometallic species and metal clusters encapsulated in zeolites, to regio- and stereoselective organic reactions catalyzed by heterogeneous systems, and finally to catalytic conversion of biomass into chemicals. He has coauthored more than 250 scientific publications and review articles and delivered plenary lectures at major catalysis conferences worldwide.

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. Biographies

Michèle Besson is CNRS research director at the Institut de recherches sur la catalyse et l’environnement de Lyon (IRCELYON, France). She completed her undergraduate studies in chemistry in Strasbourg (France). While preparing her Ph.D., she obtained a tenure research position from the CNRS in 1981 in Lyon (France), and she received her Ph.D. in Kinetics and Catalysis from Université de Lyon in 1984 on catalytic coal hydroliquefaction using sulfided iron catalysts. After a stay at University Grenoble (France) from 1986 to 1990 where she studied Raney nickel-based catalysts with the Unité Mixte CNRSRhône-Poulenc, she returned to Lyon and joined the recently set up team on fine chemicals. Since then, she has been active in the field of heterogeneous catalysis for liquid-phase reactions under pressure, working with supported-metal nanoparticles and their application in fine chemistry and transformation of biomass platform molecules (selective hydrogenation or selective oxidation with air), and in water treatment by catalytic wet air oxidation. She has been coleading the fine chemicals or water treatment groups at IRC and IRCELYON. She has coauthored more than 130 papers.

Catherine Pinel is director of research at the Institute of Research on Catalysis and Environment in Lyon (IRCELYON) in the team BIOVERT (Biomass upgrading and green chemistry). After her Ph.D. in 1992 dealing with enantioselective hydrogenation with chiral ruthenium complexes under the supervision of Prof. J. P. Genet (Paris VI), she moved to Cambridge in the group of Prof. S. V. Ley where her postdoc was devoted to the synthesis of the C10−C17 fragment of Rapamycin. In1994, she joined the CNRS with a tenure position, and her research interests have been concentrated on the development of homogeneous and heterogeneous catalysts applied in fine chemistry including selective biomass transformation. In 2005, she was awarded by the French Division of Catalysis. She has coauthored more than 110 scientific publications and a dozen patents.

ACKNOWLEDGMENTS Michel Lacroix, Director of IRCELYON, is gratefully acknowledged for providing logistic support to P.G. after his retirement. European COST action UBIOCHEM is acknowledged by C.P. REFERENCES (1) (a) Corma, A.; Iborra, S.; Velty, A. Chem. Rev. 2007, 107, 2411. (b) Mäki-Arvela, P.; Holmbom, B.; Salmi, T.; Murzin, D. Y. Catal. Rev.-Sci. Eng. 2007, 49, 197. (c) Gallezot, P. Catal. Today 2007, 121, 76. (d) Serrano-Ruiz, J. C.; Luque, R.; Sepulveda-Escribano, A. Chem. Soc. Rev. 2011, 40, 5266. (e) Gallezot, P. Chem. Soc. Rev. 2012, 41, 1538. (f) Alonso, D. M.; Wettstein, S. G.; Dumesic, J. A. Chem. Soc. Rev. 2012, 41, 8075. (2) (a) De Oliveira-Vigier, D.; Jerôme, F. Top. Curr. Chem. 2010, 295, 63. (b) Climent, M. J.; Corma, A.; Iborra, S. Green Chem. 2011, 13, 520. (3) (a) Meier, M. A. R.; Metzger, J. O.; Schubert, U. S. Chem. Soc. Rev. 2007, 36, 1788. (b) Metzger, J. O. Eur. J. Lipid Sci. Technol. 2009, 111, 865. (c) Behr, A.; Perez-Gomes, J. Eur. J. Lipid Sci. Technol. 2010, 112, 31. (4) (a) Zhou, C. H.; Beltramini, J. N.; Fan, Y. X.; Lu, G. Q. Chem. Soc. Rev. 2008, 37, 527. (b) Behr, A.; Eilting, J.; Irawadi, K.; Leschinski,

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