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ISSN 1463-9262

Cutting-edge research for a greener sustainable future

CRITICAL REVIEW

Robert J. Davis et al.Selective oxidation of alcohols and aldehydes over supported metalnanoparticles

www.rsc.org/greenchem Volume 15 | Number 1 | January 2013 | Pages 12

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Green Chemistry

CRITICAL REVIEW

Cite this:Green Chem., 2013, 15, 17

Received 11th September 2012,

Accepted 12th October 2012

DOI: 10.1039/c2gc36441g

www.rsc.org/greenchem

Selective oxidation of alcohols and aldehydes oversupported metal nanoparticles

Sara E. Davis, Matthew S. Ide and Robert J. Davis*

Oxidation is a key reaction in organic synthesis and will likely play a signicant role in the development

of value-added chemicals from biomass. The application of heterogeneous catalysis and molecular

oxygen to oxidation reactions offers a green alternative to traditional, toxic chemical oxidants. However,

making comparisons of catalyst performance (reaction rate, product selectivity) between reports in the

literature is difficult because of inconsistencies in the ways results are reported. Herein, we examine the

literature on supported metal catalysts for the oxidation of molecules of interest in biomass conversion

(primary alcohols, polyols, 5-hydroxymethylfurfural, and various sugars). Reaction rates are calculated

and compared in a consistent manner and recommendations for avoiding common pitfalls in kineticinvestigations are made.

1. Introduction

The diminishing supply of oil has heightened interest in thedevelopment of more sustainable alternatives to petroleum-derived fuels and chemicals. Although much attention is paidto the transportation and energy sectors of the economy, 5%of current consumption of crude oil is used for the productionof chemicals.1 Renewable resources such as solar and windpower can meet some of the worlds energy demands, but

replacements for petroleum-derived chemicals require asource of carbon atoms, such as biomass. The U.S. NationalRenewable Energy Laboratory (NREL) describes a biorefineryfor the conversion of biomass to produce fuels, power, andchemicals that is analogous to the contemporary petroleumrefinery.2 A major challenge to the development of a bio-refinery, and thus the biorenewable chemicals industry, isidentifying key intermediates and platform chemicals. Whiletodays petroleum refinery benefits from years of optimizationand a well-defined set of platform chemicals for the pro-duction of value-added chemicals, the implementation of thebiorefinery is still under development.

In 2004, the US Department of Energy (DOE) sought toidentify the molecules with the greatest potential for use as

value added chemicals from biomass.3,4 In the years since theDOE report, the literature involving the production and trans-formation of these chemicals has grown substantially. While aconsensus on the key intermediates that will be utilized in a

biorefinery has yet to be reached, common reactions such asoxidation, reduction and dehydration will undoubtedly play arole in their development. As the oxidation of alcohols to theircorresponding ketones, aldehydes, and/or carboxylic acids isone of the most important transformations in organic syn-thesis, reviews of the transformations of alcohols by bio-catalysts5 and heterogeneous photocatalysts6 were recentlypublished. The use of Au catalysts for selective oxidation re-actions of sugars and alcohols was also recently discussed. 7

In the current review, we focus on the selective oxidation ofalcohols (and aldehydes) over supported metal catalysts,because of the ease of recovery of heterogeneous catalysts fromthe products and their potential for recyclability. In particular,this critical review aims to classify reaction rates on a commonbasis (the turnover frequency, TOF) so that catalyst perform-ance (rate and selectivity) can be compared in a consistentmanner.

We have chosen to examine the oxidation of several classesof alcohols, including monoalcohols, 5-hydroxymethylfurfural(HMF), and polyols. The monoalcohols ethanol, octanol,benzyl alcohol, and cinnamyl alcohol, have been used pri-

marily as model alcohols to test for oxidative activity and selec-tivity over a wide variety of supported metals and reactionconditions. The oxidation of HMF to 2,5-furan dicarboxylicacid (FDCA) was highlighted by the DOE as a target reaction toproduce a monomer for use in polyethyleneterephthalate(PET)-like plastics because HMF can be produced fromglucose.4 The polyol glycerol is a byproduct of the transesterifi-cation of vegetable oil and animal fats into bio-diesel. Whileglycerol can be utilized in pharmaceutical and cosmeticapplications, bio-diesel production provides low purity gly-cerol. The oxidation of glycerol to fine chemicals, such as

Department of Chemical Engineeri ng, University of Virgi nia, 102 Engineers Way,

Charlottesville, VA 22904-4741, USA. E-mail: [email protected];

Fax: +1 434-982-2658; Tel: +1 434-924-6284

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dihydroxyacetone (DHA), glyceric acid (GA), and tartronic acid(TA), might also help offset the cost of bio-diesel production.8

Finally, the direct oxidation of glucose and other simplesugars to acids for use as additives in food and beverages wasexamined. The oxidation of sugars is also interesting because,in aqueous solution, sugars exist in equilibrium between theirring-opened aldose form and their ring-closed cyclic form.9

As the depth of the literature on selective oxidation grows,

the wide variety of reaction conditions, catalyst characteristics,and methods for rate calculations make comparing the conver-sion, selectivity, and kinetic results difficult. Perhaps the mostproblematic area involves the rate, as the literature generallydoes not report turnover frequency calculations in a consistent

way. It is our belief that the most useful form of the turnoverfrequency (TOF) is reported at a low level of conversion, beforesubstantial deactivation of the catalyst takes place, and isbased on the amount of surface metal available to the sub-strate (eqn (1)).

TOFmoles of substrate convertedmoles of surface metaltime

1

Therefore, in this review, we discuss the literature that hasreported turnover frequency values in this way, or has reportedenough information for us to calculate the turnover frequencyourselves, so that various catalysts for oxidation reactions canbe compared. Because many of the reactions included in thisreview are not simple one step oxidations but are sequentialreactions, we emphasize the importance of comparing productselectivity at equal levels of conversion.

2. Alcohol oxidation mechanism

The oxidation of a primary alcohol proceeds first to an alde-hyde and subsequently to a carboxylic acid, as outlined inFig. 1. The oxidation of an alcohol to an aldehyde over aheterogeneous catalyst likely occurs in three steps: first, thealcohol adsorbs on the metal surface, producing an adsorbedmetal alkoxide. Second, -hydride elimination occurs toproduce a carbonyl species and a metal hydride. Last, themetalhydride is oxidized by dioxygen to regenerate the metalsurface. The oxidation of an aldehyde to carboxylic acid isbelieved to proceed through a geminal diol intermediate.

2.1 Metalalkoxide formation

As mentioned above, the mechanism of primary alcohol oxi-dation to aldehyde over a supported metal catalyst likelybegins with the formation of a metal alkoxide;1015 however,the nature of the metal or the nature of substrates adsorbed

on the metal may influence its formation. In the case of benzylalcohol and Ru/Al2O3or Ru(OH)x/Fe3O4catalysts, a Rualkoxidehas been hypothesized to form via ligand exchange betweenRuOH and the alcohol, producing a Rualkoxide and a watermolecule.16 In other work with Ru and Pd catalysts, activationof the OH bond in benzyl alcohol on the metal surface is pro-posed, yielding a metalalkoxide and a metalhydride.11,12,15,17

The selective oxidation of primary alcohols over secondary

alcohols was demonstrated for benzyl alcohol and phenyletha-nol oxidation over Ru catalysts.10,14 These results are consist-ent with formation of a metal alkoxide intermediate, as theformation of this species is well known for the selective oxi-dation of primary alcohols.10,16,18,19 In a seeming contra-diction, Mori et al. report similar reaction rates for benzylalcohol and 1-phenylethanol oxidation over a Pd catalyst, yetpropose the same metal alkoxide intermediate.11 Furthersupport for metal alkoxide formation was derived from 2-pro-panol oxidation with O2over Ru/Al2O3. Production of H2O andacetone were monitored during the reaction and revealed a1 : 1 molar ratio.10,18 Similarly, the uptake of O2was monitored

during benzyl alcohol oxidation, and a 1 : 2 molar ratio of O2to aldehyde was observed.11,14,15,18 These studies demonstratethat simple dehydrogenation of alcohol to aldehyde does nottake place.

2.2 -Hydride elimination

A-hydride elimination is widely accepted as the second stepof alcohol oxidation over metal catalysts, producing a carbonylgroup and a metalhydride.1012,14,15,18,20,21

The Hammett methodology has been employed by a fewgroups to probe the mechanism of benzyl alcohol oxidationand to confirm the -hydride elimination step.10,13 Comparingthe rates of para-substituted benzyl alcohols over Ru catalysts

yielded a Hammettvalue of0.461, which indicates the for-mation of a carbocation-type intermediate due to hydrideabstraction from the Rualkoxide.10 Similarly, a Hammettvalue of1.10 was found over a Au catalyst, indicating that asimilar mechanism operates despite the higher oxygen cover-age on Ru than on Au.13

The hydride abstraction step was also confirmed throughthe incorporation of deuterium in the -position of thealcohol. The deuterated alcohol should react significantlyslower than the non-deuterated alcohol and the kinetic isotopeeffect was found to be 1.82. Transfer hydrogenation was notedin a mixture of acetophenone and 2-propanol with a Ru cata-

lyst, further supporting the formation of a RuH species.10

Interestingly, the -hydride elimination step has been pro-posed as the likely rate-determining step in alcoholoxidation.10,11,14,15

2.3 Oxidation of metalhydride and regeneration of catalyst

surface

The third step in alcohol oxidation to aldehyde is the oxidationof the metalhydride species generated from the -hydrideelimination step to regenerate either the metalhydroxide10,14

or metal surface.11,12,15 The oxidation of the metalhydrideFig. 1 General oxidation scheme for primary alcohol oxidation to acid.

Critical Review Green Chemistry

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likely proceeds through a peroxide intermediate to yield awater molecule and one half of an O2molecule.

11

2.4 Oxidation of aldehyde to carboxylic acid

It is well known in organic chemistry that aldehydes in H2Oundergo reversible hydration to geminal diols and that the rateof hydration is accelerated at elevated pH. The geminal diol

will likely adsorb to a metal surface to form a metal alkoxide,

which will undergo-hydride elimination to form a carboxylicacid.22

2.5 Illustrative example: ethanol and glycerol oxidation

The oxidation of an alcohol to an acid over supported Au andPt was investigated both experimentally and computationally

with density functional theory (DFT).23 Specifically, the oxi-dation of ethanol to acetic acid and glycerol to glyceric acidover Au and Pt in an aqueous solvent requires dioxygen. Inaddition, Au requires added base for the reaction to proceed atlow temperature whereas the rate of reaction over Pt is signifi-cantly increased with the addition of base. The roles of dioxy-

gen and the hydroxide (high pH) in the oxidation mechanismwere explored to elucidate the most likely steps in alcohol oxi-dation. Since ethanol (alcohol) and glycerol (polyol) are oxi-dized by the same mechanism, it is likely that the mechanismcan be extended to most alcohol oxidations.

Table 1 (Steps 1 and 2) compares the energetics of the firststep of ethanol oxidation to form a metal alkoxide via theinitial deprotonation of the alcohol. The deprotonation of analcohol can occur in solution, where it is controlled by thesolution pH and pKaof the alcohol, or it can occur on a metalsurface where the deprotonation forms a metal alkoxide.

While the intrinsic barrier for deprotonation of the alcohol ona metal surface is high (Step 1), the presence of a surface

bound hydroxide decreases the activation energy significantly(Step 2). The activation energy over a Au(111) surface decreasesfrom 204 kJ mol1 over the bare metal surface to 22 kJ mol1

over one containing adsorbed hydroxide. Thus, adsorbedhydroxide enhances the deprotonation of an alcohol similar tothat in basic solution. It is important to note that Step 1 formsa metalhydride bond, while Step 2 results in the formation ofadsorbed water.

After the formation of the metalalkoxide, a-hydride elimi-nation can similarly happen on the metal surface alone or be

facilitated by surface bound hydroxide. While the energy ofactivation is again lower over Au(111) with an adsorbed hydroxide(Step 4) compared to just the bare metal (Step 3), the differ-ence in activation energy is not as significant as the initialalcohol deprotonation step. In addition, the -hydride elimi-nation has similar activation energies over Pt(111) alone and

with an adsorbed hydroxide. Once the -hydride eliminationhas been completed, an adsorbed aldehyde is formed on the

surface.The aldehyde can undergo reversible hydration to a

geminal diol in the solution phase. Under basic conditions,the aldehyde can also react over the metal surface with hydro-xide to form an adsorbed diol intermediate (Step 5). A second-hydride elimination of the geminal diol can form a car-boxylic acid over the metal surface (Step 6) or be facilitated byadsorbed hydroxide (Step 7). At this point, O2does not play arole in the mechanism, yet O2is required for the oxidation toproceed. Consistent with the above mechanism, when 18O2

was used to oxidize ethanol or glycerol, only unlabeled aceticacid and glyceric acid were produced, regardless of whether

the catalyst could easily dissociate 18

O2, such as Pt and Pd, ornot, such as Au. However, when H218O was used, 18O appeared

in the products. The appearance of 18O in the product whenusing H2

18O is strong experimental evidence that aqueous-phase alcohol oxidation most likely proceeds through thealkoxy intermediate of the geminal diol which can undergo-hydride elimination on the metal surface to produce the car-boxylic acid.

Because the dissociation of O2on the Au surface is unlikely(105 kJ mol1) and most metal surfaces, such as Pt and Pd, arelikely inhibited by water and hydroxide adsorbed on thesurface, O2 might adsorb associatively on the surface. Experi-mental evidence of peroxide production formation during the

reaction suggests that the associatively adsorbed oxygen isreduced by electrons on the metal catalyst before dissociationinto hydroxide.24 The energetics of O2reduction, summarizedin Table 2, show a low barrier of 16 kJ mol1 over Au(111) (Step 1).The adsorbed peroxide can then react with water or decom-pose on the surface. The dissociation of peroxide into atomicoxygen and hydroxide on Au(111) (Step 2), however, is lesslikely energetically compared to the formation of hydrogenperoxide and hydroxide (Step 3), which is consistent withexperimental results in which peroxide formation is reported

Table 1 Reaction energies and activation barriers for ethanol oxidation calculated over Au(111) and Pt(111) in water

Step # Step

Au(111) Pt(111)

HRXN EACT HRXN EACT

1 CH3CH2OH* + * CH3CH2O* + H* +196 204 +98 1162 CH3CH2OH* + OH* CH3CH2O* + H2O* +13 22 5 183 CH3CH2O* + * CH3CHO* + H* 40 46 62 154 CH3CH2O* + OH* CH3CHO* + H2O* 222 12 165 245 CH3CHO* + OH* CH3CHOOH* + * 33 5 5 56 CH3CHOOH* + * CH3COOH* + H* 151 21 154 137 CH3CHOOH* + OH* CH3COOH* + H2O* 334 29 258 17

The * represents a catalytic site on the surface. All values in kJ mol1.23

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over supported Au.24 The last step closes the catalytic cycle byremoving excess electrons from the metal surfacevia adsorbedhydroxide reacting with any metal hydrides found on themetal surface (Step 5). Thus, the role of associatively adsorbedoxygen is ultimately to remove electrons from the metalsurface, oxidize metalhydride bonds, and regeneratehydroxide ions.

It is important to note that hydroxide ions are not con-served during the reaction. Hydroxide ions are reacted duringthe oxidation of glycerol to glyceric acid and some are used toneutralize the acid product. While the reduction of O2 canregenerate some of the hydroxide ions (Table 2, Step 1), about0.6 mole of hydroxide was consumed for every mole of glycerolconverted, after the hydroxide used to neutralize the acidproducts was accounted for.25

2.6 Illustrative example. HMF oxidation

The oxidation of 5-hydroxymethylfurfural (HMF) involves boththe oxidation of an aldehyde and an alcohol. The oxidation tomonoacid hydroxymethylfurancarboxylic acid (HFCA) anddiacid 2,5-furandicarboxylic acid (FDCA) at ambient temp-

eratures over Pt and Au catalysts requires both a base (typicallyNaOH) and gaseous dioxygen and the roles of both base andO2in the mechanism were elucidated recently.

26 Similar to theglycerol study,23 isotopically-labeled H2

18O and 18O2 wereemployed to discern the source of oxygen atoms inserted intothe products. To ensure that the incorporation of labeledoxygen was due to the mechanism of reaction and not due tooxygen scrambling in solution, control experiments underreaction conditions were conducted. No oxygen scramblingbetween H2

18O and the products HFCA and FDCA wasdetected.

The majority product of HMF oxidation was found pre-

viously to be affected by the catalyst type and ratio of NaOH :HMF.26 Thus, three different scenarios for HMF oxidation wereexamined in that study: oxidation to FDCA over Pt, oxidationto HFCA over Au with a relatively low concentration of NaOH(NaOH : HMF 2 : 1), and oxidation to FDCA over Au with a rela-tively high concentration of NaOH (NaOH: HMF 20 : 1). In allof these studies, the incorporation of 18O was found in theproduct when the reaction was conducted with H2

18O, but noincorporation of 18O was seen in reactions utilizing 18O2.In the case of a Au catalyst with low NaOH concentration, two18O atoms were incorporated in the product, HFCA, when the

reaction was run in H218O, presumably both in the acid group.

In the case of Au with a high concentration of NaOH and inthe case of the Pt-catalyzed reaction, four 18O atoms wereincorporated in the diacid product, FDCA, when the reaction

was run in H218O. Thus, in all cases, oxygen insertion in the

acid products occurred through H218O. The proposed mecha-

nism can be seen in Fig. 2.In step one, the aldehyde side chain of HMF undergoes

reversible hydration in solution to a geminal diol through thenucleophilic addition of a hydroxide ion to the carbonyl andproton transfer from H2O to the alkoxy ion intermediate. Thesecond step is a dehydrogenation of the geminal diol to form acarboxylic acid, likely facilitated by hydroxyl ions on the metalsurface, as indicated in the previous section. This step pro-duces two molecules of water and deposits two electrons onthe metal surface. In the third step, the hydroxymethyl groupis dehydrogenated to an aldehyde. It is believed that basedeprotonates the alcohol group, likely in solution, to form analkoxy intermediate.23 Subsequently, hydroxide ions on themetal surface facilitate the activation of the CH bond in the

hydroxymethyl group to form an aldehyde, producing twomolecules of water and depositing two additional electrons onthe metal surface. In step four, this aldehyde undergoes thesame reversible hydration to a geminal diol as seen in stepone. Finally, a dehydrogenation step produces the second car-boxylic acid (step five), depositing two more electrons on themetal surface, analogous to step two. In total, six electrons aredeposited on the metal catalyst surface.

Though the isotopic labeling studies indicate that oxygenatoms from dioxygen are not directly incorporated in the acidproducts, it is essential for the oxidation of HMF to FDCA. It

was proposed in the ethanol and glycerol oxidation mecha-nism that O2 is an electron scavenger, removing excess elec-

trons from the metal surface by undergoing the oxygenreduction reaction to peroxide and hydroxide ions.23,26 A testalso revealed peroxide in solution during HMF oxidation reac-tions; therefore, it is believed that the role of O2in HMF oxi-dation is to scavenge electrons from the metal catalyst surface,regenerating hydroxide ions and closing the catalytic cycle.26

This mechanism is completely consistent with that describedfor ethanol and glycerol oxidation.

3. Alcohol oxidation review

3.1 EthanolProduction of ethanol from biomass has been the investigatedthoroughly due to the application of ethanol as a liquid fuel.Much of the literature focuses on electro-oxidation of ethanolin fuel cells, or on oxide materials as catalysts for ethanol oxi-dation, which is beyond the scope of this review. The oxidationof ethanol can lead to acetic acid, an important industrialchemical in the production of synthetic fibers and fabrics, as

well as a chemical reagent in the production of vinyl acetate.Consistent with other alcohol oxidation reactions and themechanism discussed earlier, the first oxidation product is

Table 2 The role of O2 in oxidation of alcohols. Activation barriers calculated

over Au(111) and Pt(111) in water

Step # StepAu(111) Pt(111)EACT EACT

1 O2* + H2O* OOH* + OH* 16 182 OOH* + * O* + OH* 83 523 OOH* + H2O* H2O2* + OH* 48 414 H2O2* + * OH* + OH* 71 29

5 OH* + H* H2O* + * 39 30The * represents a catalytic site on the surface. 23



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acetaldehyde and the second oxidation product is acetic acid.The general oxidation scheme for ethanol oxidation is seen inFig. 3.

Zope et al. reported on the oxidation of ethanol over sup-ported Au and Pt catalysts, both in presence and absence ofadded NaOH.23 The group found the turnover frequencies over

Au to be an order of magnitude higher than those overPt when a 2 : 1 ratio of NaOH : ethanol was used. In theabsence of NaOH, the Au catalysts were inactive for ethanoloxidation at 333 K, whereas Pt/C displayed some activity. Theturnover frequencies for ethanol oxidation are summarized inTable 3.

Jorgensen et al.27

investigated the oxidation of aqueousethanol over Au/TiO2 catalysts at temperatures between 363and 473 K, and a yield of 95% acetic acid was reached. Prelimi-nary investigations over Au/C did not demonstrate nearly ashigh a conversion of ethanol or selectivity to acetic acid.

Although attempts were made to recycle the catalyst, a signifi-cant decrease in activity was noted, likely due to Au particleaggregation (spent catalyst Au size 57 nm; unused 36 nm).The intermediates and byproducts noted in this reaction wereacetaldehyde, CO2, and ethyl acetate. The reaction profileshows that the oxidation of ethanol proceeds through

acetaldehyde before reaching acetic acid. Using acetaldehydeas the reactant demonstrated a rapid oxidation to acetic acid(acetic acid yield 98%); thus, it was determined that the de-hydrogenation of ethanol was the rate determining step.Because extending the reaction time showed no degradation ofacetic acid, formation of CO2 likely occurredvia over-oxidationof an intermediate species. One proposed intermediate forms

by the adsorption of ethanol to the catalyst surface and can beeither oxidized/dehydrogenated to acetaldehyde or cleaved toproduce CO2. An attempt was made to increase the yield ofethyl acetate by varying the concentration of ethanol and amaximum selectivity to ethyl acetate (50%) was seen at concen-trations between 80100% ethanol, indicating that water has asignificant limiting effect on the production of ethyl acetatefrom ethanol.

At moderate temperature and pressure (423 K and 0.6 MPaO2), the selective oxidation of aqueous ethanol into acetic acid

with 90% yield was demonstrated by Christensen et al.28

Fig. 3 Reaction scheme for ethanol oxidation.

Table 3 Turnover frequencies and reaction conditions for ethanol oxidationa

CatalystParticlesize (nm) T(K)

Pressure(kPa) Solvent

TOF(s1) Reference

Au/C 20 333 1035 H2O + 2 eq.NaOH

0.30 23

Au/TiO2 3.4 333 1035 H2O + 2 eq.NaOH

0.46 23

Pt/C 2.3 333 1035 H2O + 2 eq.NaOH 0.04 23

Pt/C 2.3 333 1035 H2O 0.01 23

aAll reactions were performed in batch mode with O2as oxidant.

Fig. 2 Proposed HMF oxidation mechanism, adapted from ref. 26.



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Ethanol concentrations corresponding to those obtainedduring fermentation were utilized. Comparing the yields from

Au/MgAl2O4 (conversion 97%, selectivity 86%), Pt/MgAl2O4(conversion 93%, selectivity 65%), and Pd/MgAl2O4(conversion82%, selectivity 20%) at the same reaction conditions, thehighest yield was obtained over the Au catalyst. No signs of sin-tering were seen for the Au catalyst (particle size 36 nm) overthe course of the reaction (30 h). Under optimized conditions

(453 K and 3.5 MPa O2), 92% yield of acetic acid was obtainedafter 8 h. Increasing the reaction temperature resulted in bothincreased conversion of ethanol and selectivity to aceticacid.27,28

Gold catalysts are among the most effective of the sup-ported metal nanoparticle catalysts for ethanol oxidation,though much of the literature seems to be focused on metaloxides or electrocatalysts for this transformation. The numberof reports of initial TOF is limited, and most reports onethanol oxidation are at too high conversion to calculate initialrate.

3.2 Octanol

Octanol is often used as a probe molecule to determine theeffectiveness of a catalyst in the oxidation of alkanols. Thegeneral reaction scheme for octanol oxidation is seen in Fig. 4.Low reactivity is characteristic of octanol. Accordingly, there isan abundance of low conversion results in the literature,

which facilitates the calculation of initial reaction rate.

Both primary and secondary octanols are investigated fre-quently, and the rates of 1-octanol oxidation are summarizedin Table 4. The highest rate was obtained over bimetallic cata-lysts containing Au and either Pt or Pd (0.3 s1 for PdAu/C and0.2 s1 for PtAu/C) in H2O solvent with 4 equivalents of NaOHat 323 K.29 Under the same conditions, but in absence ofadded base, the rates of oxidation were an order of magnitudelower for both bimetallic catalysts. An experiment utilizing a

AuPd bimetallic catalyst in water with 1-equivalent NaOHexhibited a TOF of 0.06 s1, higher than the base-free case butlower than the experiment with higher NaOH concentration. 30

Additionally, Au monometallic catalysts were inactive for1-octanol oxidation without addition of base in both water andtoluene solvents.29,30 Thus, the role of base in octanol oxi-dation over bimetallic PdAu and PtAu catalyst is significant.The role of base is also significant in oxidation over monome-tallic Au catalysts; in the absence of added base, the Au catalyst

was virtually inactive, though some activity was realized withthe addition of 4 : 1 NaOH : substrate. Interestingly, theaddition of base to the reaction mixture over Pt and Pd mono-

metallic catalysts had no significant eff

ect on the reaction rate.Aside from the bimetallic catalysts, the rates shown in Table 4are similar to each other and it would seem that the intrinsicTOF of 1-octanol oxidation in water at temperatures 323 K373 K over Au, Pt, and Pd metals is on the order of 0.01 s 1.

Dimitratos et al. have speculated that NaOH facilitates thefirst step of oxidation, H abstraction, which Au alone is unableto catalyze at low temperature.30 Moreover, the addition of

Fig. 4 Reaction scheme for octanol oxidation.

Table 4 Turnover frequencies and reaction conditions for 1-octanol oxidationa

Catalyst Particle size (nm) T(K) Pressure (kPa) Solvent TOF (s1) Reference

Au/CeO2 Not reported 373 200 None 0.007 31Au/C Not reported 323 310 H2O + 4 eq. NaOH 0.05 29Pd/C Not reported 323 310 H2O + 4 eq. NaOH 0.007 29Pd/C Not reported 323 310 H2O 0.007 29Pt/C Not reported 323 310 H2O + 4 eq. NaOH 0.008 29Pt/C Not reported 323 310 H2O 0.008 29PdAu/C Not reported 323 310 H2O + 4 eq. NaOH 0.3 29PtAu/C Not reported 323 310 H2O + 4 eq. NaOH 0.2 29PdAu/C Not reported 323 310 H2O 0.01 29PtAu/C Not reported 323 310 H2O 0.02 290.73%Au0.27%Pd/C 3.4 333 155 H2O 0.02 300.73%Au0.27%Pd/C 3.4 333 155 H2O + 1 eq. NaOH 0.06 30




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NaOH favors formation of Na-carboxylate, which is morestable than metal-carboxylate. Thus, the products are thoughtto desorb from the metal surface so the catalytic cycle can con-tinue uninhibited.

The addition of NaOH can enhance the reaction rate, butalso affects the product selectivity, presumably because of thesequential nature of the oxidation reaction. In the case of aPdAu/C catalyst, 17% conversion of aqueous 1-octanol with

70% selectivity to the aldehyde was realized after 8 h at 323 Kwithout base; adding 4 equivalents of NaOH without changingthe other conditions shifted the selectivity toward the acid(98% selectivity) with 93% conversion in just 4 h.29 Under thesame conditions but with the monometallic Pd/C catalyst, at2% conversion of 1-octanol the selectivity to the aldehyde was70% in absence of NaOH, but with 4 equivalents of NaOH, thereaction was 97% selective to the acid at 2% conversion. Thetrend continued over Pt and PtAu catalysts.

Prati et al. also noted a negligible effect of O2pressure on1-octanol reaction rate. Increasing the reaction temperatureenhanced activity but resulted in a decrease in selectivity to

aldehyde.29

In addition to 1-octanol, secondary octanols have also beeninvestigated. For example, the rates of oxidation of neat3-octanol over some supported catalysts are reported inTable 5. The highest rates for 3-octanol oxidation wereobtained over Au catalysts supported on nanocrystalline CeO2.Both supported Au catalysts in Table 5 exhibited high selec-tivity to the ketone product (99 and 97%, respectively). Table 5also presents a comparative study of Au/CeO2 and Pd/CeO2

demonstrating the enhanced activity of Au (TOFAu = 2.92 s1

compared to TOFPd= 0.09 s1 at 333 K), although supporting

Pd on apatite improved its activity. The oxidation of 2-octanolwas investigated over supported Ru(OH)x and Pd catalysts intoluene and trifluorotoluene, respectively11,14 and both metals

were shown to be active for this oxidation.The literature investigating octanol oxidation highlights the

superior activity (rate) of Au catalysts over Pd catalysts for

alcohol oxidation, typically exhibiting a TOF an order of mag-nitude greater. The role of additional homogenous base is sig-nificant in reactions utilizing bimetallic Pt or Pd and Aucatalysts or Au monometallic catalysts. The role of base isapparently much less important in reactions utilizing mono-metallic Pt or Pd catalysts.

3.3 Benzyl alcohol oxidation

Benzyl alcohol is often used as a model alcohol to test for cata-lyst reactivity. While the literature is extensive on this topic,quality rate data obtained at low conversion is lacking. The

popularity of benzyl alcohol as a probe molecule results fromits extremely high reactivity and the limited number of sideproducts (the intermediate benzaldehyde is a non-enolizablestructure).31 Like other alcohols, benzyl alcohol oxidation pro-ceeds through an aldehyde intermediate to the acid finalproduct, as depicted in Fig. 5. Typical metal catalysts forbenzyl alcohol oxidation are Pt, Pd, and Au.

3.3.1 Palladium catalysts. Nanoparticles of Pd have beensupported on a variety of materials, including hydroxyapatite(HAP),11 carbon,11,30Al2O3,

11 SiO2,11 pumice,34 and SiO2Al2O3

mixed oxide.15 Table 6 shows the turnover frequency resultsfrom selected works that provided appropriate information.

It is interesting to note that the TOFs vary by an order ofmagnitude from 0.19 s1 to 2.8 s1 over a fairly narrow rangeof experimental conditions.

The role of the support for Pd catalysts was investigated byMori et al.11 and Chen et al.15 Mori et al. found that benzylalcohol conversion and the benzaldehyde selectivity dependedon support, with the hydroxyapatite (HAP)-supported catalysthaving the highest conversion (99%) and selectivity to alde-hyde (99%). The other supports examined (Al2O3, SiO2, and C)produced the aldehyde with less than 50% selectivity; the mostselective was Pd/SiO2 (47%) though the conversion was only71%. The highest conversion was achieved over Pd/Al2O3,though the selectivity was the lowest (38%). The Pd/C pro-duced the lowest conversion (46%) and low selectivity (42%).Unfortunately, the reaction rates over these catalysts were notFig. 5 Reaction scheme for benzyl alcohol oxidation.

Table 5 Turnover frequencies and reaction conditions for 3-octanol oxidationa

Catalyst Particlesize (nm) T(K) Pressure(kPa) Solvent TOF(s1) Reference

Au/CeO2 4.5 353 101.3 None 0.54 32Au/CeO2 4.5 393 Flowing None 2.92 33Pd/CeO2 6 393 Flowing None 0.09 33Pd/Apatite 7.5 393 Flowing None 0.74 33


Table 6 Turnover frequencies and reaction conditions for benzyl alcohol oxidation over Pd catalystsa


Pd/C 2.9 363 101.3 Trifluorotoluene 0.19 11Pd/pumice Not reported 333 202.6 Acetonitrile 2.8 34Pd/SiO2Al2O3 4.3 353 Flowing None 2.5 15

aAll reactions were performed in batch mode.



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given, so no conclusions can be drawn about the effect ofsupport on reaction rate.

Chen et al. probed the effect of Pd particle size on BA oxi-dation by varying the ratio of SiO2 : Al2O3 in a mixed oxidesupport to tune the size of Pd particles in the range2.210 nm.15 Decreasing the SiO2 : Al2O3 ratio resulted in adecrease in the size of the Pd nanoparticles, due to therelative strength of interactions of the Pd precursor with

the support. Though varying the SiO2 : Al2O3ratio changed therelative acidity of the support, the variation in activitycorrelated only to Pd nanoparticle size. A maximum TOF wasfound at a mean metal particle size of 3.64.3 nm, suggestingthat BA oxidation over Pd might be structure sensitive,

which was also noted by Mori et al.11 in their work over Pdsupported on HAP. The optimized catalysts (Pd particle sizesof 4.3 nm and 3.6 nm) demonstrated high TOF (2.53 s1 and2.45 s1, respectively) as well has high selectivity to the alde-hyde (98% selectivity with 73% conversion and 98% selectivity

with 99% conversion, respectively, in 10 h) in solvent-freeconditions.

Uozumi et al. utilized a novel support, an amphiphilicresin, in the oxidation of aqueous alcohols.35 The resin ishighly hydrophobic in its pores, so organic molecules candiffuse from the aqueous media into the matrix where theanchored Pd nanoparticles catalyze the oxidation. A mixture ofcatalyst and benzyl alcohol was refluxed in water under atmos-pheric pressure of dioxygen to give 97% yield of the aldehydeproduct after 1.5 h. The high selectivity to the aldehyde inaqueous medium is interesting particularly because sequentialoxidation to the carboxylic acid readily occurs in water. Appar-ently the hydrophobic nature of the resin prevents the conver-sion of the aldehyde in water over Pd.

The effect of solvent was investigated by Mori et al.11 and

Villa et al.36 Mori found the most effective solvent to be tri-fluorotoluene (99% conversion in 1 h), although 1,2-diethoxy-ethane afforded 65% conversion under the same conditions.

Aprotic polar solvents were not effective at all, which theauthors hypothesized may be due to the coordination of het-eroatoms to Pd(II) pre-catalysts, preventing their reduction toPd0. The oxidation of BA in aqueous NaOAc over Pd/HAPresulted in 90% yield of aldehyde and 10% yield of benzoicacid at 383 K in an O2atmosphere after 24 h, which is unusual

given the typical sequential oxidation of aldehyde to carboxylicacid in high pH solution.

The investigation of role of solvent by Villaet al.used Pd onactivated carbon (AC) or on carbon nanotubes (CNT).36

An increased rate of reaction was noted in neat alcohol (TOF0.83 s1) as opposed to a mixture of 80% water and 20%alcohol (TOF 0.023 s1) over Pd/AC catalyst, though theproduct distribution remained unchanged. The rate of reaction

was higher in cyclohexane than in water. It is important tonote that the leaching of Pd from the support into the liquid

was extensive in this study, 28% for Pd/AC and 25% forPd/CNTs.

From these results, it appears that the support material forPd nanoparticles can have a substantial effect on the productselectivity. Interestingly, the appropriate choice of support cansuppress the sequential oxidation of aldehyde to carboxylicacid in aqueous solution. Nevertheless, it seems that theintrinsic turnover rate for benzyl alcohol oxidation over Pdcatalysts at modest temperatures (99% conversion of benzylalcohol and 97% selectivity to the aldehyde product after 6 hat mild conditions (308 K, 100 kPa O2). Over longer reactiontimes (>24 h), further oxidation of benzaldehyde was not

Table 7 Turnover frequencies and reaction conditions for benzyl alcohol oxidation over Au catalystsa


Au on block co-polymers 9.5 308 100 1 : 1 H2O : chloroform with KOH(1 : 1 KOH : substrate)

0.04 37

Au/CeO2 4.5 373 200 None 0.042 31Au foil n/a 333 500 H2O + 0.6 M NaOH 2.8 38Au foil n/a 363 500 Toluene + equimolar K2CO3 2.8 38Au foil n/a 363 500 Heptane + equimolar K2CO3 4.4 38Au foil n/a 333 500 Heptane + equimolar K2CO3 2.8 38Au foil n/a 383 500 Heptane + equimolar K2CO3 5.7 38Au/MgO 3.3 453 Unknown Trifluorotoluene 0.13 39Au/HT 3.1 393 Flowingb Toluene 0.22 21

aAll reactions were performed in batch mode. bAr atmosphere.



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observed; the performance of these materials is in stark con-trast to other reports which showed partial or complete oxi-dation to benzoic acid in aqueous media or in the presence ofbase.22,38,40,41 The authors ascribed this difference to thetemperature (407 K) and the hydrophobic nature of thepolymer phase in conjunction with the chloroform solvent,similar to what Uozumi and Nakao proposed in their Pd/resincatalysts.35 Thus, a support material may inhibit oxidation of

aldehyde to acid in high pH aqueous media. However, theinfluence of the base might also be limited by the hydrophobicsupport.

Very recently, Guo et al. reported successful aerobic oxi-dation of benzyl alcohol over unsupported bulk Au.38 The oxi-dation of BA was performed in aqueous NaOH, and thedistribution of products was a function of the reaction temp-erature and concentration of NaOH. For benzyl alcohol oxi-dation at 333 K, the major product was the ester, benzylbenzoate, for lower concentrations of base (0.6 M NaOH) andthe acid, benzoic acid, at higher concentrations (1.2 M NaOH).Increasing the temperature to 363 K resulted in a majority of

the aldehyde product over the range of NaOH concentrations.Investigation of other solvents revealed that bulk Au wasinert in trifluorotoluene but active for BA oxidation in heptaneand in toluene, producing the aldehyde as the majorityproduct. The authors hypothesize that the -electrons in BAenhance interactions with Au, facilitating the oxidation. Theyfound that the -activation for the CH2OH group by thephenyl group in BA is critical for oxidation over bulk Au. Nooxidation occurred in the presence of inert gas, demonstratingthe importance of O2as oxidant in this case, in contrast to pre-

vious work on BA oxidation over supported Au.21,42 Increasingthe temperature increased the conversion of BA in heptanesolvent, whereas conversion decreased with increasing temp-

erature in water solvent. The authors suggested that the oxi-dation in aqueous NaOH may be favored by H2O2 producedduring the reaction and that the H2O2may decompose readilyat the higher temperatures. At the lower temperature, H2O2

was readily detected, whereas at the higher temperature it wasnot. No peroxide was detected when heptane was the solvent.Previous work proposed that, in organic solvent, the role of O2

was to remove the hydride from the Au surface, or, in aqueousNaOH, that O2 regenerated catalytic sites by removing elec-trons from the surface and in the process regenerating hydrox-ide ions.23

As an alternative to adding homogenous base, some groups

investigated solid bases as catalyst supports. The effect of acidand base sites on the support in Au catalysis for BA oxidationhas been investigated in the absence of additional homo-geneous base.21,31,41 In summary, low conversion of benzylalcohol was observed after 3 h (maximum conversion 7%, over

Au/Fe2O3), which is typical of oxidation reactions in theabsence of homogeneous base. Selective oxidation to benz-aldehyde was achieved over Au on SiO2, CeO2, and TiO2, withthe best results obtained with Au/CeO2 (3.4% conversion,100% selectivity to aldehyde after 3 h at 373 K and 200 kPaO2). The lowest conversion was seen over Au/TiO2 (0.65%

conversion in 3 h; 100% selectivity to aldehyde). The Fe 2O3and C supported catalysts were the only samples to form theester product (selectivity to benzyl benzoate 12% and 10%,respectively); the authors hypothesized that ester formation isrelated to the acid sites on the catalyst surface. Indeed, temp-erature programmed desorption of NH3revealed the strongestacidic sites on Au/Fe2O3, and addition of a small amount ofHCl to a reaction with Au/SiO2supported this hypothesis.

Villaet al.examined the role of base sites on product selec-tivity in cyclohexane solvent in the absence of homogeneousbase and found a correlation between basicity of the supportand catalyst activity.41 Gold catalysts supported on nanosizedNiO (35 nm) were more active by an order of magnitude than

Au on commercial micrometer sized NiO (55% conversionvs.6% conversion, in 6 h), likely due to both an increase in basi-city of the support (basicity per square meter of nNiO:0.21 mmol g1; of commercial NiO: 0.12 mmol g1) and also acooperative effect of metal and support.

An inverse relationship between the basicity of the supportand the selectivity to aldehyde formation was also noted. The

more basic nanosized support showed decreased selectivity toaldehyde (66%) compared to the less basic microsized support(selectivity to aldehyde 75%); the increased basicity enhancedsequential oxidation to acid products. The basicity of thesupport, however, was not the only factor responsible for theactivity, as MgO was more basic than nNiO (basic site density0.42 mmol g1) but was a much less active support for Au (7%conversion vs. 55%). Investigation via CO adsorption and IRspectroscopy revealed that Au/nNiO did not adsorb CO mole-cules but Au/TiO2 did, indicating a change in the electronicproperties of Au.

Finally, Fang et al. investigated amphoteric materials andthe effects of their acidity/basicity on BA oxidation in p-xylene

solvent at 393 K in the absence of an oxidant. 21 Gold nano-particles of a similar size were supported on a variety ofmaterials to compare support effects. After 1 h of reactiontime, Au/Al2O3, Au/MgO, Au/HAP, and Au/hydrotalcite (HT)catalysts demonstrated BA conversion of about 2030%; theselectivity to the aldehyde over each catalyst was compared atthis conversion. The Au/Al2O3catalyst showed the highest con-

version (32%) but the lowest aldehyde selectivity (18%),whereas Au/MgO showed the highest selectivity (99%) but thelowest conversion (20%). The Au/HT exhibited similar selectiv-ity to the Au/MgO, but at approximately 10% higher conver-sion. Temperature programmed desorption (TPD) of NH3and

CO2 revealed that Au/HT and Au/MgO possessed higher con-centrations of basic sites than the other catalysts. It was alsoshown that Au/HT had both strong acid and strong base sites,

Au/SiO2had neither acid nor base sites, and Au/MgO had onlystrong base sites. The catalysts with neither acid nor base sites

were almost inactive, whereas the catalyst with the highestacidity and basicity (Au/HT) had the highest conversion ofalcohol and selectivity to aldehyde. Moreover, a catalyst withstronger basicity in the absence of acidity (Au/MgO) hadhigher selectivity to aldehyde, but a catalyst with strong acidity(Au/Al2O3) was more active. This is in direct contrast with Villa



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et al., who saw a lower selectivity to aldehyde over catalystswith more base sites because of sequential oxidation to car-boxylic acid. It should be noted, however, that the work ofFanget al.was in absence of an oxidant so the other productspresent were toluene and benzene rather than other oxidationproducts. The role of O2 in conjunction with catalyst supportmay be significant with respect to product selectivity.

The effect of particle size of Au on BA oxidation was exam-

ined by Boronatet al.39 using MgO as a support and by Fanget al. using HT as support.21 In general, the smaller the par-ticle diameter, the higher the TOF. Thus, the groups concludedthat alcohol dehydrogenation on Au is a structure-sensitivereaction, which was also supported by DFT calculations.39

Interestingly, a maximum TOF for Au on MgO was observedfor a particle size of 3.3 nm (0.09 s1) while Au/MgO with asmaller particle size (1.9 nm) exhibited lower activity (0.03 s1).39

The trend is reminiscent of the trend noted by Chen et al.15

who saw an optimum ratio of edge and corner sites to terracesites in the oxidation of BA over Pd nanoparticles. Others havealso noticed structure sensitivity for alcohol oxidation.11,15,43

In summary, the oxidation of benzyl alcohol exhibited thehighest rates (5 s1) over Au catalysts in the presence of homo-geneous base. Although the oxidation of alcohol to aldehydeto acid is greatly accelerated by the presence of homogeneousbase, the sequential oxidation of aldehyde to acid may beinhibited through careful selection of the solvent and catalystsupport. The incorporation of acid sites and/or basic sites oncatalyst support can influence the product selectivity of theoverall reaction.

3.3.3 AuPt and AuPd bimetallic catalysts. Alloying ofmultiple metals has the potential to enhance reaction rate,alter product selectivity, and/or help slow or prevent catalystdeactivation. In one report, the alloying of Pd and Au pre-

vented the leaching of Pd from the catalyst support.36 Syn-thesis of bimetallic catalysts for benzyl alcohol oxidation isfocused on Au, Pt, Pd, and Ag catalysts. The rates of oxidationover these bimetallic catalysts are summarized from selectedstudies in Table 8.

Enache et al. noted a very high TOF for BA oxidation(24 s1) at mild conditions (373 K, 0.1 MPa O2) over a bimetal-lic AuPd/TiO2 catalyst.

45 Although the addition of Au to Pdnanoparticles improved the selectivity to aldehyde (92% at75% conversion for bimetallic, 54% selectivity at 51% conver-sion for monometallic Pd), the initial rate of the bimetallic

catalyst was lower than that of the monometallic Pd. This indi-cates that the TOF for the monometallic Pd catalyst (notreported) was greater than 24 s1, which is much higher thanany other reports for Pd catalysts for BA oxidation.

Microscopy and XPS showed that the nanocrystals weremade up of an Au-rich core with a Pd-rich shell; the enhance-ment of activity may be due to Au altering the electronic struc-ture of Pd.45 Pure Au and Pd catalysts did not retain high

selectivity to the aldehyde at high conversion, whereas thebimetallic catalyst did (selectivity 96% at 100% conversion).Bimetallic catalysts were also synthesized on Al2O3and Fe2O3,though the selectivity to the aldehyde was lower (87% at 83%conversion on Al2O3, 67% at 63% conversion on Fe2O3) poss-ibly due to acid sites on the support promoting esterformation.

Meenakshisundaram et al. also synthesized bimetallic AuPd/TiO2 catalysts for BA oxidation, and found that the initialrate of BA oxidation was an order of magnitude higher over thebimetallic catalyst than over the monometallic Pd catalyst,

which showed a rate an order of magnitude higher than the

monometallic Au catalyst.44

This contrasts the report above, inwhich the bimetallic demonstrated a rate between those of thePd and Au monometallic catalysts.45 Meenakshisundaramet al.s materials were found to be homogeneous alloys ratherthan the shell-and-core configuration of Enache et al. Thisgroup also looked at the reaction under inert pressure andfound that the reaction rates were an order of magnitude lowerthan in the presence of O2. Moreover, the selectivity to alde-hyde was 50% with toluene making up the balance. By com-paring results from the monometallic catalysts, it appears thatall of the activity of the bimetallic alloys to produce aldehydeunder inert pressure was due to the Pd metal and the Au metaldid not contribute to the activity.

Dimitratos et al.30 saw an enhancement of the reaction rateover Au, Pd, and AuPt and AuPd bimetallic catalysts sup-ported on C when water was used as solvent in place oftoluene, in all cases. The Au catalysts were inert in H2O andtoluene. However, it is important to note that no homogeneousbase was added to the reaction so this is not surprising, par-ticularly at such moderate temperatures (333 K). The highestactivity was reached over 0.73%Au0.27%Pd/C catalyst intoluene (96% conversion, 94% selectivity to aldehyde in 3 h).

The reaction rate of BA oxidation over AuPd bimetallicswas found to be zero order in O2 pressure in the range

Table 8 Turnover frequencies and reaction conditions for benzyl alcohol oxidation over bimetallic AuPt and AuPd catalystsa


AuPd/TiO2 4 353 100 None 1.6 44AuPd/TiO2 4 353 100 He None 0.24 440.73% Au0.27% Pd/C 3.4 333 155 Toluene 0.15 300.73% Au0.27% Pd/C 3.4 333 155 H2O 0.01 300.6% Au0.4% Pt/C 3.2 333 155 Toluene 0.004 300.6% Au0.4% Pt/C 3.2 333 155 H2O 0.05 302.5% Au2.5% Pd/C Not reported 433 100 None 24 45

aAll reactions performed in batch mode with O2as oxidant.



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1003000 kPa,44,45 though a direct dependence was noted inpressures up to 100 kPa.44

In summary, the bimetallic catalysts discussed in thisreview, with the exception of that of Enache et al.,45 do notdemonstrate enhanced reaction rates for benzyl alcohol oxi-dation when compared to their monometallic counterparts. Infact, the highest rate for an Au-containing bimetallic catalyst

was 1.6 s1, which is lower than the highest rate reported for a

monometallic Au catalyst (5 s1). Although there does notseem to be an obvious synergistic effect on rate, alloying twometals for alcohol oxidation catalysts has been shown toinhibit the leaching of metal (Pd) from the support or toprevent catalyst deactivation.30

3.3.4 Other metal catalysts. Other metals or metal com-pounds have been investigated as catalysts for benzyl alcoholoxidation, including Ag and Co3O4. These materials arecommon dehydrogenation catalysts and are more economicallyattractive than Au or Pd. Ruthenium catalysts have also beeninvestigated to a limited extent.

Beieret al.investigated a number of supported Ag catalysts

for the oxidation of alcohols by utilizing a screening method.46

The supports for the Ag particles were SiO2, Al2O3, Celite,CeO2, kaolin, MgO, and activated carbon. Although, Ag/SiO2and CeO2 were nearly inactive (conversion 95% selectivity to aldehyde with 20% conver-sion). The ratio of CeO2 : Ag/SiO2was also investigated, and athigher loadings of CeO2, a higher selectivity to the aldehyde

was also achieved. Leaching of Ag and CeO2were ruled outvia

ICP analysis of the filtered reaction mixture and hot filtrationtests confirming no continued reaction after the catalyst wasremoved. A soluble cerium source in addition to Ag/SiO 2 didnot enhance the activity. The work suggests that the CeO2adsorbs both water and benzaldehyde, thus preventing theiradsorption and deactivation of the Ag surface, a phenomenonalso noted as a problem by Zotova et al.over Ru catalysts.47

The effect of the calcination temperature of Ag catalysts wasalso shown to influence activity.46 Calcination of Ag above773 K reduced both activity and selectivity. Silver-oxygenspecies in mostly metallic silver particles reportedly played animportant role and were greatly affected by the pretreatment

temperature. Liotta et al. noted similar results on their Ag/pumice catalyst.34

Although the Ag catalysts required O2 to be catalyticallyactive, the authors surmised that the role of CeO2 may be toact as an oxygen reservoir, reversibly storing oxygen.46 In thisscenario, benzyl alcohol adsorbs on the silver surface where itis dehydrogenated to benzaldehyde. The CeO2 activates O2,

which can react with hydrogen produced from benzyl alcoholdehydrogenation and thus regenerate the Ag surface.

The work of Liotta et al. also investigated the use of Ag forBA oxidation, both in monometallic form and as a bimetallic

with Pd, on a pumice support.34 The Pd monometallic catalystwas the most active, with the bimetallic catalyst having a TOFten times lower than the Pd, and the Ag monometallic catalystanother order of magnitude lower than the bimetallic. Theselectivity to the aldehyde product was 95% in all cases.

A physical mixture of the two monometallic catalysts resultedin a rate higher than the sum of the rates for the monometalliccatalysts used independently, suggesting a synergy between

Pd0 and Ag0 in the reactor. The lower activity of the bimetalliccatalyst calcined at 773 K may be due to the presence of PdO.

Although EXAFS indicated the presence of alloyed PdAg, alloy-ing did not seem to be necessary as the physical mixture of thetwo metals was adequate for enhanced activity. The authorssuggested that the oxidative dehydrogenation mechanism, in

which the substrate is dehydrogenated on the metal surfaceand oxygen removes the hydrogen to regenerate the site, canexplain this observation. The Pd can activate the alcohol whilethe Ag can activate the O2. Close proximity of the two com-ponents allows for the activated oxygen to hop to the Pd.The importance of the role of O2 was confirmed by testing

reactions under inert atmospheres. The reaction rate over bothmonometallic catalysts was much slower in the absence of O2.Mitsudomeet al.42 used Ag nanoparticles (mean diameter =

3.3 nm) supported on hydrotalcite in the absence of oxidantfor high yield of benzaldehyde after 10 h (>99% conversion BA,90% selectivity to aldehyde) at moderate conditions (403 K,

Ar atmosphere). In this way, the over-oxidation to carboxylicacid was avoided, and the only byproduct was H2.

The literature for supported nanoparticle ruthenium cata-lysts is somewhat sparse, which is perhaps an indication of itsineffectiveness for this particular reaction. Indeed, low ratiosof substrate to Ru (1040 mole substrate per mole metal) arecharacteristic of many studies. However, the oxidation of

benzyl alcohol to aldehyde was possible over Ru on a variety ofsupports, including Al2O3,

10,47 carbon nanotubes (CNT),12 andFe3O4.

14 High selectivity to benzaldehyde (>99%) with highconversion of benzyl alcohol (>99%) was seen at moderatetemperatures (356363 K) over Ru supported on Al2O3 in tri-fluorotoluene10,16 and toluene,47 as well as over Ru supportedon Fe3O4 in toluene.

14 Much of the work used toluene12,14,47

and trifluorotoluene10 as solvent, though ethanol, water, andmixtures of toluene and water12 have also been investigated.Unfortunately, none of those studies provided sufficient infor-mation to calculate an initial TOF for BA oxidation.

The effect of O2 pressure was also studied over Ru, and a

five-fold increase in O2pressure from 500 kPa to 2500 kPa sig-nificantly enhanced the reaction rate. In contrast, Yamaguchiet al.18 found no effect of O2pressure on reaction rate, thoughthe range of pressures in this case (20300 kPa) may not havebeen large enough to note the effect.

A study of the effect of solvent system was conducted byYang et al.,12 who utilized novel Ru catalysts supported oncarbon nanotubes (CNTs). Three different solvents were tested(water, toluene, and ethanol). Although the reaction in watersolvent demonstrated the highest conversion, the majorproduct was the acid. The reaction carried out in ethanol



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resulted in 100% selectivity to the aldehyde, but only 50% con-version of the substrate. The optimized solvent system of 2 : 1toluene : water exhibited 98% conversion of benzyl alcohol and100% selectivity to benzaldehyde after 3 h at mild conditions(358 K and 0.1 MPa O2). The authors suggested that theRu/CNTs stabilized the emulsion of toluene and water bydecreasing the interfacial tension, while also providing ahigher interfacial surface area for the oxidation of alcohol to

take place. Not only did the oxidation occur at a faster rate inwater, but BA also had a higher solubility in water than intoluene. The role of toluene was to selectively extract the alde-hyde (which is more soluble in toluene than in water), thuspreventing over-oxidation to carboxylic acid.

Alternatively, Zhu et al. investigated the use of variouscobalt oxides supported on activated carbon for BA oxidationin the absence of promoters or base.48 Testing of cobalt oxideon a variety of high-surface-area supports verified that highsurface area is advantageous to the activity of cobalt oxide, butalso demonstrated a synergistic effect on the conversion ofbenzyl alcohol over cobalt oxide supported on activated

carbon. The group proposed that the activated carbon off

ersthe sites for dioxygen adsorption/activation, whereas Co3O4catalyzes the dehydrogenation reaction.

Although explored to a lesser extent than Au and Pd, Ru, Agand Co3O4catalysts offer interesting options for benzyl alcoholoxidation catalysts.

3.4 Cinnamyl alcohol

Cinnamyl alcohol (CA) oxidation is often used together withbenzyl alcohol as a model reaction for alcohol oxidation. Likebenzyl alcohol, CA is highly reactive; however, the CvC bondin cinnamyl alcohol can undergo side reactions such as hydro-genation and hydrogenolysis, which add a degree of complex-

ity. The general oxidation scheme for CA oxidation is shown inFig. 6. Because many studies investigating the oxidation ofboth BA and CA, the reaction conditions here are similar tothose in the previous section. Commonly used catalysts are Au,Pd, and Ru, and the solvents are generally xylene, toluene, and

water. The TOF values for cinnamyl alcohol oxidation at temp-eratures less than 373 K range from 0.17 s1 to 4.4 s1, whichis of the same order of magnitude as those for benzyl alcoholoxidation. The highest rate was found over Au bulk foil withadded homogeneous base. It should be noted that the rate forPd/hydroxyapatite is not reported on the table and would likelybe much higher than that of Pd/SiO2; however, insufficientinformation was provided to calculate an initial rate for CA oxi-

dation over Pd/HAP (Table 9).11There are multiple reports of aldehyde production in the

presence of basic aqueous solutions over Au and Pd cata-lysts.11,37,38 In one instance, Au supported on block co-poly-mers in a 50% H2O50% chloroform solution with1 : 1 KOH : CA produced 99% conversion of CA with 95% selec-tivity to aldehyde in 6 h at 273 K. 37 Similarly, the Pd/HAPsystem was effective for 90% conversion of CA with 98% selec-tivity to aldehyde in aqueous NaOAc in 24 h at 383 K. 11 Unsup-ported bulk Au with 1 : 1 K2CO3 : CA produced similar results,70% conversion of CA with 100% selectivity to aldehyde,though the solvent was heptane and therefore the high alde-

hyde selectivity is not surprising. Likewise, Au/CeO2 couldconvert CA with 73% selectivity to aldehyde (66% conversion)in the absence of solvent and base; using water and Na 2CO3asthe solvent system resulted in >99% conversion and 98% selec-tivity to the expected carboxylic acid.32 The choice of solventappears to be critical in the bulk Au case, as CA oxidation didnot proceed in trifluorotoluene, toluene, or acetophenone. Itshould be noted that BA oxidation did proceed in heptane andtoluene under the same conditions.38

An influence of solvent was also observed over Pd, Pt, AuPt, and AuPd catalysts supported on C. For example, the CAoxidation rates over all of these catalysts were higher in waterrather than in toluene without added base. The Au mono-

metallic catalyst was inactive in both solvents. Bimetallic0.73 wt% Au0.27 wt% Pd/C produced the highest conversionand selectivity to aldehyde in both toluene (conversion 72%,selectivity to aldehyde 85%) and water (conversion 95%, selectiv-ity to aldehyde 83%) at 333 K, whereas Pd and Pt monometallic

Fig. 6 Reaction scheme for cinnamyl alcohol oxidation.

Table 9 Turnover frequencies and reaction conditions for cinnamyl alcohol oxidationa


Pd/SiO2 4.7 363 101.3 Toluene 0.20 11Au foil n/a 363 500 Heptane + equimolar K2CO3 4.4 380.73% Au0.27% Pd/C 3.4 333 155 Toluene 0.17 300.73% Au0.27% Pd/C 3.4 333 155 H2O 0.22 30

aAll reactions were performed in batch mode.



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catalysts both exhibited poor conversion (36% and 27%,respectively) but relatively high selectivity to aldehyde (86%and 100%, respectively) in H2O at 333 K.

A Pd/HAP catalyst was likewise effective in toluene solventin the absence of added base (91% conversion, 87% selectivityto the aldehyde) at a slightly lower temperature (363 K).11 ThePd/C catalyst was nearly as effective (90% conversion, 68%selectivity). Other supported Pd catalysts, under the same con-ditions, were less effective; Pd/Al2O3 converted 83% CA withonly 66% selectivity to aldehyde and Pd/SiO2 produced only

31% conversion with 30% selectivity to the aldehyde.Similar to what was observed during BA oxidation, a physi-cal mixture of Ag/SiO2 and CeO2 had high activity for CA oxi-dation (98% conversion in 2 h for BAvs.83% conversion in 3 hfor CA).46 Likewise, a magnetically-separable Ru(OH)x/Fe3O4catalyst active for BA oxidation was also active for CA oxidationin toluene without addition of base (95% conversion, >99%selectivity to aldehyde in 1.5 h at 378 K).14 Ruthenium sup-ported on carbon was also found to be active for CA oxidationin toluene, with a 79% yield of the aldehyde after 24 h at343 K.17

The investigation of cinnamyl alcohol oxidation highlightsthe importance of solvent choice in alcohol oxidation with

regards to product selectivity and catalyst activity. As with BAoxidation, the product selectivity is greatly influenced by thesolvent as well as the nature of the catalyst support. As in otheralcohol oxidations, the Au catalyst features the highest oxi-dation rate at modest temperature,

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diester formation. However, the TOFs were based on totalmetal rather than on surface metal, so the values have beenrecalculated and presented in Table 10. It should be noted thatproduction of diester is the result of subsequent oxidation

steps that are presumably slower than the initial oxidation ofHMF. According to Table 10, Au/C had the highest rate of HMFoxidation but, relative to the other catalysts in the study,demonstrated low final yield of the diester product. On theother hand, Au/CeO2had the highest activity of the group forthe formation of diester product (99% yield in 5 h). An experi-ment with Au/CeO2in the absence of O2yielded large amountsof the acetal product and 3.8% of the monoester product,

which suggests that CeO2acts as an oxygen donor. When non-nanometric ceria was used as support, the activity of the cata-lyst was significantly reduced, in agreement with results fromCeO2-based catalysts for benzyl alcohol oxidation.

32

The effect of temperature was also investigated in the range

353 K403 K over the Au/CeO2and yielded an overall activationenergy of 34 kJ mol1. Although other alcohols, such asethanol and butanol, were also tested for the oxidation-esterifi-cation, methanol was the most suitable.58 The effect of water

was probed, and a negative effect on the initial reaction ratewas noted. In fact, when 20% water was loaded, the reactiondid not even reach completion. Interestingly, no trace of car-boxylic acid was noted, possibly because of the presence of aLewis acid site on the catalyst support aiding in the rapid reac-tion of any formed carboxylic acid to methyl ester.

3.5.2 HMF oxidation in aqueous solutions. In aqueoussolutions, HMF oxidation has been thoroughly investigated over

Pt, Pd, and Au catalysts and some recent work has also lookedat AuCu bimetallics and Ru(OH) catalysts.26,4952,5457,5961

The effects of added base concentration, temperature, dioxy-gen pressure, and catalyst composition have been studied. Ingeneral, the oxidation of HMF at moderate temperatures(295368 K) over Au and Pt catalysts requires the addition of ahomogeneous base, though a consensus has not been reachedin the literature about the amount of homogenous base thatshould be employed. The degradation of HMF in presence ofbase is of concern, and most researchers seek to find abalance between high enough concentration of base to allow

the oxidation to proceed at a reasonable rate but low enoughto limit degradation. The work of Davis et al. showed that theintermediate HFCA is much more stable in NaOH than isHMF, so rapid oxidation of HMF to HFCA, through the use of

a high catalyst loading, can allow for higher concentrationsof NaOH to be employed to facilitate the subsequent oxidationof HFCA to FDCA.49

The work of Davis et al. also showed that the activity of Aucatalysts for HMF oxidation was an order of magnitude higherthan either Pt or Pd catalysts. The selectivity to the desireddiacid, however, was much higher over the Pt and Pd catalysts.

At their standard conditions of 295 K and 690 kPa O2 and2 equivalents NaOH, the major product over Pt and Pd catalysts

was FDCA (selectivity of 79% and 71%, respectively) while themajor product over the Au catalysts was HFCA (selectivity of92% over Au/TiO2 catalyst). Gorbanev et al., however, demon-strated high selectivity to FDCA over Au catalysts in water by

increasing the amount of catalyst and NaOH (20 equivalents).52

Over a Au/TiO2 catalyst, Gorbanev et al. investigated theeffect of NaOH concentration and demonstrated that at 303 Kand 2000 kPa O2, the selectivity to diacid did not change in therange 520 equivalents NaOH.52 Below 5 equivalents, the selec-tivity shifted more toward the mono-acid HFCA. In theabsence of base at the same conditions, only 13% conversionof HMF was seen, with the majority product HFCA (yield 12%)and little FDCA (selectivity 1%).

Casanova et al.58 and Pasini et al.50 also varied the amountof NaOH to determine the effect of base on product selectivity.Casanovaet al.found that using 4 equivalents of NaOH in the

presence of Au/CeO2 produced a 96% yield of diacid in 5 h,while 2 equivalents afforded 96% yield in 20 h.51 Using just1 equivalent of NaOH resulted in only 20% yield in 14 h (76%

yield of HFCA) and the oxidation did not proceed further.Pasini et al. found that, in the presence of AuCu/TiO2 or

Au/TiO2, the product selectivity was influenced by amount ofNaOH in the range of 14 equivalents.50 Increasing the NaOHconcentration in the range 410 equivalents did not changeproduct selectivity. The work also showed that using 20 equi-

valents of NaOH in the absence of catalyst resulted in com-plete degradation of HMF after 2.5 h.

Table 10 Turnover frequencies and reaction conditions for 5-hydroxymethylfurfural oxidationa


Au/TiO2 2.6b 295 100 Methanol + 8% Sodium Methoxide 0.14 56

Au/CeO2 3.5 403 1000 Methanol 0.31 58Au/Fe2O3 3.5 403 1000 Methanol 0.12 58Au/TiO2 3.5 403 1000 Methanol 0.30 58Au/C 3.5 403 1000 Methanol 0.47 58AuCu/TiO2 4.4 368 1000 Water + 4 : 1 NaOH : HMF 0.18 50

Au/TiO2 2.6 295 690 Water + 2 : 1 NaOH : HMF 1.6 49Au/C 10.5 295 690 Water + 2 : 1 NaOH : HMF 5.0 49Au/C 3.0 295 690 Water + 2 : 1 NaOH : HMF 2.3 49Pt/C 2.5 295 690 Water + 2 : 1 NaOH : HMF 0.08 49Pd/C 3.3 295 690 Water + 2 : 1 NaOH : HMF 0.15 49

Au/TiO2 2.6 295 2000 Water + 2 : 1 NaOH : HMF 1.2 49Au/TiO2 2.6 295 3000 Water + 2 : 1 NaOH : HMF 1.4 49

aAll reactions were performed in batch mode. b Not given; estimated from data from World Gold Council standard catalyst.



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The role of O2pressure in HMF oxidation over Au/TiO2cata-lysts was investigated by Gorbanev et al.52 and Davis et al.49

Over the range 690 kPa to 3000 kPa, no appreciable differencein the rate of HMF oxidation was noted. In contrast, Vinkeet al., reported that reaction rate over Pt/Al2O3was first order

with respect to O2 pressure; however, this assertion is madewithout showing any data.61

The effect of O2pressure on product selectivity was investi-

gated by Pasiniet al.,50 Daviset al.49 and Gorbanevet al.52 overAu catalysts. A positive correlation between O2 pressure andselectivity to diacid was noted. A control experiment conductedby Gorbanevet al.in the absence of O2but in the presence ofcatalyst and 20 equivalents of NaOH found full conversion ofHMF after 18 h, with 51% yield of HFCA and the balance com-prised of decomposition products. This demonstrates thenecessity of dioxygen in this reaction to prevent undesirableside reactions in the highly basic solution.52

Lilgaet al., used a flow reactor to investigate the acid/basenature of the medium on HMF oxidation over Pt/Al2O3,Pt/ZrO2, and Pt/SiO2 catalysts.

57 Under basic conditions

(Na2CO3) at 373 K and 1035 kPa air, complete conversion ofHMF to FDCA was observed. However, however, the selectivityshifted to FCA after 2 h of operating at 100% conversion ofHMF, which might be attributed to adsorption of productsonto the catalyst. Successive experiments in batch reactorsreport the same phenomenon, yet the original activity could bereturned by washing the catalyst with hot water.57 In neutralconditions, the reaction was substantially slower but the down-stream separations were easier.57 Product inhibition wasapparently much higher than that in the basic conditions,likely because of the low solubility of the reaction products inneutral solution. It was noted that inorganic supportsadsorbed less of the products, which also correlated with the

surface area of the supports.Acidic conditions were achieved through addition of acetic

acid.57While the solubility of FDCA is limited in neutral feeds,the solubility in 40% acetic acid and 60% water is twice that in100% water. At 373 K and with air as the oxidant, the dialde-hyde (DFF) is the major product, though at higher tempera-tures and with O2 as the oxidant, 100% conversion and 85%selectivity to FDCA was realized.

Verdeguer et al. investigated PtPb/C catalysts and foundthat hydroxyl bases were more effective than carbonate basesat promoting the reaction.55Vinkeet al.found the rate of HMFoxidation over Pt/Al2O3to be independent of pH between 811,

but at lower pH values, the rate was lower.61

It is quite clear that the oxidation of HMF in basic aqueoussolution is facile over metal catalysts at room tempera-ture.26,49,50,56,57,61 Increasing the temperature, however, canreduce the reaction time or change the product selectivity. Forexample, Pasini et al. showed that at 333 K over AuCu/TiO2catalysts, the major product was the monoacid HFCA.50

Raising the temperature by just 20 K shifted the selectivitytoward the diacid FDCA, and raising the temperature anadditional 15 K produced 99% yield of FDCA. This is consist-ent with a rapid oxidation of HMF to HFCA and a slow

oxidation of HFCA to FDCA. Vinke et al. found the observedactivation energy to be 37.2 kJ mol1 over Pt catalysts,61 whichis similar to the value found over Au catalysts in MeOH solvent(34 kJ mol1).58

Casanova et al. varied the temperature between 298 and403 K and found a positive effect on conversion over their

Au/TiO2 catalysts, as expected. However, higher temperaturescaused the formation of some undesired byproducts (degra-

dation products).51 Attempted reuse of the catalyst revealedsignificant deactivation, which was attributed to the buildupof 2.5 wt% C during the reaction. Extensive washing of thespent catalyst did not improve activity. The researchersclaimed that the used catalyst was still able to oxidize thealcohol group, but its ability to oxidize the aldehyde group wasdiminished. This was confirmed by carrying out a reaction

with HFCA and a used catalyst; the reaction was completed in4 h, yielding 93% FDCA. Since the catalyst can be used tooxidize the aldehyde at 298 K without loss of activity, a newreaction protocol was established wherein the first oxidationstep of HMF to HFCA was conducted at 298 K, and then the

temperature was raised to 403 K to allow for complete andefficient oxidation to the diacid product while limiting catalystdeactivation. With this method, the spent catalyst could besuccessfully reused 3 times.

A possible promotional role of the support (Fe2O3, C, CeO2and TiO2) on Au nanoparticles was examined by Casanovaet al.51 The Au/CeO2 and Au/TiO2 supported catalysts showedthe best activity (over Au/TiO2, 84% selectivity to diacid after8 h; over Au/CeO2, 96% selectivity to diacid after 5 h), while

Au/Fe2O3and Au/C produced lower yields of FDCA. Highlight-ing the important collaborative effect of CeO2 and Au, the Auon nanoparticulate CeO2 reached high yields of FDCA in halfthe time of the Au on non-nanoparticulate CeO2. The reductive

pretreatment of the catalyst was shown to increase the activity,likely because it increases amount of Ce3+, which has beenshown previously to be important in catalyst activity.32

Monometallic Au and Cu catalysts, as well as bimetallic AuCu/TiO2 catalysts, were synthesized by Pasini et al.

50 Interest-ingly, the reaction with Cu/TiO2 produced no FDCA whereasthe bimetallic AuCu yielded twice the FDCA yield of themonometallic Au catalysts, demonstrating a synergistic effectof alloying the two metals. The optimum metal loading wasfound to be 1.5 wt% and the preparation method played a rolein catalyst activity. Catalysts synthesized from preformed AuCu sols supported on TiO2 were more active than those syn-

thesized by post-deposition of a Au sol onto a monometallicCu/TiO2 catalyst. This may be indicative of the promotionalactivity of Cu, or because Cu aids the dispersion of Au. Thebimetallic catalyst was able to be recycled 5 times withoutlosing activity, whereas the Au monometallic catalyst lostactivity after just 1 recycle, highlighting the beneficial effect ofalloying Cu with Au.

Pasini et al.also found an effect of pretreatment on catalystperformance.50 The calcined catalyst produced a higher yieldof HFCA and a lower yield of FDCA (92% and 8%, respectively)than did the uncalcined catalyst (69% yield HFCA and 31%



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yield FDCA). In addition, the particle size of the calcined cata-lyst was larger than the uncalcined (6 nm compared to 4.4 nm,respectively). From this information, the group hypothesizesthat the HFCA oxidation may be more sensitive to size of metalnanoparticles than is the HMF oxidation.

3.6 Glycerol oxidation

The oxidation of glycerol by heterogeneous catalysts has botheconomic and academic appeal. Glycerol is a major byproductof biodiesel synthesis via transesterification of triglycerides

with alcohols. Glycerol oxidation to high value fine chemicals,such as glyceric acid, tartronic acid, or dihydroxyacetone,could help bio-diesel economics become more competitivecompared to diesel produced from non-renewable resources.

Academic interest in oxidative upgrading of glycerol stemsfrom glycerol being a model polyol with three hydroxyl groups,two primary and one secondary. Fig. 8 shows a reactionscheme of the major glycerol oxidation products. Glycerol oxi-dation is a sequential reaction that can follow multiple paths.The oxidation of a primary or secondary alcohol of glycerol

produces glyceraldehyde or dihydroxyacetone, respectively.These two products are often in equilibrium in aqueous sol-ution, depending on the pH. Glyceraldehyde can be sequen-tially oxidized to an acid, glyceric acid, and bothglyceraldehyde and dihydroxyacetone can be oxidized to analpha-keto acid product, hydroxypyruvic acid. The sequentialoxidation of the primary alcohol of glyceric acid typically pro-duces tartronic acid. Many authors have also reported thatglycerol oxidation products can undergo carbon cleavage toform two carbon acids such as glycolic acid and oxalic acid,24

which necessarily means that one carbon products can also beformed.62

The rate and selectivity of glycerol oxidation depends onthe choice of catalyst and the reaction conditions. As the selec-tivity during glycerol oxidation strongly depends on the pH ofthe solution, this discussion will focus on the influence ofreaction conditions of oxidation on supported Au, Pt, andPd catalyst performance.

3.6.1 Monometallic catalysts. Research on supported Aucatalysts for the oxidation of glycerol has increased signifi-

cantly over the past decade. Carretin et al. first published theremarkable activity and selectivity of a 1% Au/C catalyst for theoxidation of glycerol to glyceric acid in the presence of base.63

After addition of NaOH to the reaction medium, the Au catalystreportedly had 100% selectivity to glyceric acid at 56% conver-sion of glycerol, whereas without the addition of NaOH there

was no glycerol conversion over the Au catalyst. Basic con-ditions favor the initial deprotonation of the alcohol groupeither on the Au surface or in solution whereas this is unlikelyon the Au alone in neutral or acidic solution, which is consist-ent with the mechanistic description of alcohol oxidationdescribed earlier.23 Additional experiments by Carretin et al.

showed an increase in conversion with increasing concen-trations of base and increasing dioxygen pressure. Since therate depended on O2 pressure, it was not clear if the kinetics

were limited by mass transfer of the oxygen to the catalyst orthe rate of electron removal from the metal to the oxygen.

It is known that the rate of glycerol oxidation over sup-ported Au catalysts is enhanced by the addition of base, 64 buta true rate of oxidation is difficult to discern from the litera-ture. While the reaction conditions affect the rate of reaction,the initial TOF reported in the literature based on the available

Au surface metal can vary by almost three orders of magnitudefrom 0.2 to 17 s1. The true rate of reaction, in the absence ofexternal and internal mass transfer limitations, is most likely

found at the higher bound of reported TOFs. Thus, selectedexamples of glycerol oxidation with supported Au catalysts arereported in Table 11.

The initial TOF for glycerol oxidation over well-dispersedsupported Au catalysts at 333 K, presumably in the absence ofmass transfer limitations, appears most frequently betweenthe values of 26 s1. Four different research labs have pro-duced initial rates within this range and the supported Au cat-alysts vary in synthesis procedure, support type, and Au weightpercent. In addition, a wide range of Au loadings in the reactor

were utilized, with glycerol to Au ratios ranging from 200065 toover 50 000.24 It is noteworthy that several of the authors of the

higher glycerol oxidation rates have addressed the externalmass transfer limitations of O2

23,6569 and one addressedinternal mass transfer limitations as well.67

The two highest rates for glycerol oxidation were 17 s1 by a0.5% Au/C catalyst with a glycerol to Au ratio of 50 00024 and6.1 s1 by a 0.8% Au/C catalyst with a glycerol to Au ratio of8000, both at 333 K.23 Both authors calculated the maximumO2transfer rate for their semi-batch reactor configuration andpurposely chose catalyst loadings in the reactor to ensure gly-cerol oxidation rates fell below the maximum rate of dioxygentransfer. The oxidation of sodium sulfite to sodium sulfate by

Fig. 8 Commonly observed glycerol oxidation products. Dotted lines represent

possible carbon cleavage of three carbon products to form two carbon

products.



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O2 was used to calculate the maximum oxygen diffusion rateinto the liquid phase at 298 K. In addition, the authors usedtwo different titania-supported catalysts and reported that fora 1.47% Au/TiO2the TOF was 4.1 s

1 64 and over 1.8% Au/TiO2it was 4.9 s1.23 Interestingly, the 0.8% Au/C catalyst investi-gated had a particle size approximately three times larger thanboth Au/TiO2catalysts at 10.5 nm, yet still had an initial TOFof 6.1 s1.

An accurate initial rate can be determined regardless ofreaction conditions as long as they are not mass transferlimited. Demirel et al. investigated a semi-batch process in

which the pH was controlled at 12 by the addition of NaOH

and the dioxygen pressure was only 100 kPa.67

Instead ofincreasing the dioxygen pressure, the loading of Au in thereactor was limited to ensure there were no external masstransfer limitations. An initial TOF of 5 s1was reported with a1% Au/C catalyst. The external and internal mass transferlimitations of glycerol oxidation were subsequently modeledby Demirel et al. and shown to fall within the gasliquid andliquidsolid criterion established by Chaudhari et al.for liquidphase reactions of fine chemicals.68,70 The kinetic modeling ofglycerol oxidation was fitted with the LangmuirHinshelwoodmechanism and suggested that only the adsorption of glycerol,

glyceric acid, and tartronic acid is relevant and that oxalic acidis most likely the product of glycolic oxidation. In addition,an initial rate was calculated for the 1% Au/C catalyst tobe 4.8 s1.

A good example of an initial rate that is limited by externalmass transfer of O2 was presented by Rodrigues et al. forglycerol oxidation over a 0.59% Au/C catalyst.65 An increase inthe dioxygen pressure from 300 to 1000 kPa resulted in anincrease in the initial rate from 1.3 to 3.4 s1. In addition, theynoticed that modification of the carbon support to increasethe number of oxygenated acid groups six-fold severelydecreased the initial rate of reaction, while carbon supports

with a low content of oxygenated surface groups all had a simi-larly higher initial rate.

Finally, Ketchie et al. illustrated that bulk Au powder cancatalyze glycerol oxidation at a rate of 2.5 s1 in the presenceof base at 333 K.66 Even very large 20 nm and 40 nm sized par-ticles of Au on carbon supports had oxidation rates of 2.3 and2.2 s1, respectively. All evidence in the literature shows that agood guideline for the initial TOF of a well dispersed sup-ported Au catalyst is on the order of magnitude of1 s1 at333 K and O2 pressure that does not introduce mass transferlimitations.

Table 11 Turnover frequencies and reaction conditions for glycerol oxidation

Catalyst Size (nm) T(K) pO2(kPa) NaOH : G lycerol (mol : mol) TOF (s1) Reference

0.5% Au/C 7.3 333 1000 2 17 240.8% Au/C 10.5 333 1100 2 6.1 231% Au/C 4.3 333 100 pH = 12 5.0a 671.8% Au/TiO2(WGC) 3.5 333 1100 2 4.9 231% Au/C 3.7 333 1000 2 4.8a 681.47% Au/TiO2 3.7 333 1000 2 4.1 64

0.75% Au/C 3.7 333 1000 2 3.9a

690.59% Au/C 6.8 333 1000 2 3.4a 650.6% Au/C 7.7 333 500 2 2.6a 71

Au powder 333 1000 2 2.5 660.4% Au/MWCNT 5.5 333 300 2 1.7a 721% Au/C 2.5 323 300 4 0.8a 731% Au/TiO2 3.8 363 100 4 0.7

a 741% Au/C 3.5 323 300 4 0.7a 751.1% Au/C 13.3 333 500 2 0.5a 711.5% Au/TiO2(WGC) 4 323 300 4 0.2

a 763

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