ORIGINAL PAPER

Green Catalysis with Alternative Feedstocks

Nikolaos Dimitratos Æ Jose Antonio Lopez-Sanchez ÆGraham J. Hutchings

Published online: 7 January 2009

� Springer Science+Business Media, LLC 2009

Abstract The use of renewable feedstocks, derived from

biomass, for the chemical industry is discussed. The

modern chemical industry is based around platform

chemicals, e.g. ethene, propene, benzene and xylenes,

which are readily derived from oil, and using these inter-

mediates a broad range of finished products can be derived.

While it is feasible that biomass can be converted to syngas

and hence to existing key platform chemicals, this loses all

of the chemical complexity that is inherent in bio-derived

molecules. In this paper some of the options are considered

and, in particular, the oxidation of glucose and glycerol

using gold nanoparticles supported on carbon is described.

We also contrast the oxidation of glycerol using supported

gold and gold–palladium alloys prepared using an

impregnation technique, since the gold–palladium alloys

have been shown to be highly effective for the oxidation of

alcohols and the synthesis of hydrogen peroxide.

Keywords Gold catalysis � Green chemistry �Renewable feedstocks � Glucose oxidation �Glycerol oxidation

1 Introduction

For the last 30 years the chemical and energy industries

have been operating under the threat that oil supplies are

both finite and becoming increasingly scarce. The geo-

graphical disposition of the major oil supplies has made the

price of oil a political variable for the majority of recent

years. In the early 1970’s the huge increase in the price of

oil precipitated world financial chaos that fuelled massive

inflation. It also spurred scientists to find alternative strat-

egies in the design of new synfuel processes. Most notable

of these being the methanol to gasoline process [1] com-

mercialised by Mobil in New Zealand using the then newly

discovered zeolite ZSM-5. The relatively high oil prices of

the 1970’s then abated. Additional major oil and gas dis-

coveries were made and so the existing infrastructure and

methods for chemicals and energy production still persists.

However, there has been a recent surge in the price of oil

and this makes alternative carbon sources such as coal and

gas financially viable. Indeed, a number Fischer–Tropsch

plants based on natural gas have been commissioned, partly

as a route to sulphur-free diesel and gasoline, and we can

expect similar processes based on coal-derived syngas to

become a reality in economies where coal dominates, e.g.

India and China.

The consumption of fossil fuels using existing pathways

is likely to continue for some time. However, there are

distinct sustainability issues with the availability of these

fossil resources and there is now considerable interest in

utilising bio-renewable feedstocks, which do not have

sustainability issues associated with them. In this paper we

consider some of the possibilities and, in particular, the

oxidation of glucose and glycerol using gold nanoparticles

supported on carbon is described. We also contrast the

oxidation of glycerol using supported gold and gold–pal-

ladium alloys prepared using an impregnation technique.

We have previously shown that supported gold–palladium

alloys are highly effective catalysts for the selective oxi-

dation of alcohols [2] and the direct synthesis of hydrogen

peroxide [3–9] under mild green reaction conditions. We

now seek to determine if they can be equally effective for

glycerol oxidation. This paper is based on a presentation

N. Dimitratos � J. A. Lopez-Sanchez � G. J. Hutchings (&)

School of Chemistry, Cardiff University, P.O. Box 912,

Cardiff CF10 3AT, UK

e-mail: hutch@cardiff.ac.uk

123

Top Catal (2009) 52:258–268

DOI 10.1007/s11244-008-9162-4

given at the ‘‘Green Chemistry for sustainable Energy

Supply and Conversion’’ at the ACS meeting in New

Orleans in April 2008.

2 Availability and Diversity of Biomass

More than 80% of the world’s energy consumption and

production of chemicals originates from fossil resources

(oil, gas and coal). The finite nature of these fossil

resources coupled with concerns for global warming has

spurred a drive to develop new technologies for the gen-

eration of energy, chemicals and materials supply from

renewable resources. In this respect, it has recently been

considered that biomass could become a major source for

the production of energy and chemicals. Biomass derived

from the plants, is generated from carbon dioxide and

water using sunlight as the energy source and producing

oxygen as an important by-product. The total current bio-

mass production on the planet is estimated at around

170 billion tonnes and consists of roughly 75% carbohy-

drates (sugars), 20% lignins and 5% of other substances in

minor amounts such as oils, fats, proteins, terpenes, alka-

loids, terpenoids and waxes. Based on this biomass

production, only 3.5% is presently being used for human

needs, where the major part of this amount is used for

human food (around 62%), 33% for energy use, paper and

construction needs and the rest (about 5%) is used for

technical (non-food) raw materials such as clothing,

detergents and chemicals. The rest of biomass production is

used in the natural ecosystems [10–12].

At the current time, the available biomass materials,

and, therefore, the renewable raw materials, are almost

entirely provided by agriculture and forestry. Important

renewable raw materials such as carbohydrates can be

supplied by the sugar, starch and wood processing sectors.

Specifically, plant raw materials, such as sugar beet, sugar

cane, wheat, corn, potatoes and rice can be used for the

production of sucrose, starch, whereas cellulose and

hemicellulose could be derived from wood processing.

These basic products (starch, cellulose and hemicellulose)

could then further converted into a very broad range of

products, employing physical and chemical processes.

These processes are based on partial or complete acid or

enzymatic hydrolysis for breaking down the polysaccha-

rides into sugar monomers. Starch and cellulose are

polysaccharides with glucose monomer unit and a-1,4 and

b-1,4 glycoside linkages, respectively, therefore hydrolysis

will break them into glucose, whereas sucrose hydrolysis

will lead to the formation of fructose and glucose [13].

Hemicellulose contains five different sugars, of which two

of them are 5-carbon sugars (xylose and arabinose) and 3

of them are 6-carbon sugars (galactose, glucose and

mannose). In addition, hemicellulose contains xylan

(a xylose polymer), which consists of xylose monomer

units linked at the 1 and 4 positions and it is the most

abundant building block of hemicellulose. Therefore,

hydrolysis of hemicellulose will provide a variety of 5-

carbon and 6-carbon monomer sugars. In addition, other

sugars, such as maltose and lactose can be produced from

biomass (barley and whey, respectively) and their hydro-

lysis will break down to glucose for the former and to

glucose and galactose for the latter. There is therefore a

very wide range of sugars that can be made available from

the utilisation of biomass. Many of these can be generated

from biomass resources that cannot be utilised as foodstuffs

and can be harvested on land not suitable for crop gener-

ation, and it is these materials that need to be the focus of

future research attention.

3 Pathways for Utilisation of Bio-Renewable

Feedstocks

As noted previously, throughout the world there are huge

resources of bio-renewable feedstocks, e.g. starch, cellu-

lose, vegetable oils and attention has been turning to

considering if these materials can be utilised more effec-

tively. Until now the production of liquid fuels from

biomass is based on the use of processes such as acid

hydrolysis of biomass for sugar production, thermochemi-

cal liquefaction and/or pyrolysis for bio-oils production

and gasification of biomass to produce syngas (CO ? H2)

[13]. Further treatment of the sugars produced, using a

fermentation process leads to the production of ethanol,

whereas using a dehydration process it is possible to pro-

duce aromatic hydrocarbons [14, 15]. In the case of

liquefaction and pyrolysis, further treatment of these

products by refining produce liquid fuels [16, 17]. Finally,

the Fischer–Tropsch process can be used for the synthesis

of alkanes from syngas, whereas methanol production is

also possible from syngas [18].

An alternative strategy is the use of carbohydrates as a

liquid fuel in fuel cell systems and this has recently

attracted significant attention. For example, it has been

demonstrated that carbohydrates can be used for produc-

tion of electricity in a fuel cell consisting of a vanadium

flow battery [19].

At the current time, the primary technology for the

generation of liquid fuels using renewable biomass

resources is based on the fermentation of carbohydrates for

the production of ethanol (bio-ethanol). However, this

process is still not economically viable, due to the high cost

of the process. Consequently, research is focused on the

use of more plentiful and inexpensive sugars, such as lig-

nocellulose, as this will enable a reduction in the cost.

Top Catal (2009) 52:258–268 259

123

Production of bio-oils by liquefaction or pyrolysis, based

on thermochemical treatment of biomass, is a simpler

process, but it produces a wide range of products (aromatic

compounds, CO, CH4, H2, tars, chars, alcohols, aldehydes,

ketones, esters and acids). Therefore, improvement in

selectivity is considered essential. Research is currently

being conducted to upgrade bio-oils to more valuable

products [18]. Finally, the gasification of biomass can

produce syngas (CO and H2); however, this process

requires volatilisation of water, thus decreasing the overall

energy efficiency [20]. It can be concluded that the

majority of these methods need complex processing

requirements and the efficiency of liquid fuel synthesis

remains low.

There has been much attention to the concept of a

biorefinery that is capable of producing either syngas or a

new raft of platform chemicals (Fig. 1). Biomass from any

source can indeed, in principle, be gasified, but this, as

noted earlier, can incur an energy penalty. The production

of H2 and CO by vapour-phase and aqueous-phase

reforming of biomass-derived oxygenated hydrocarbons

has recently been described [13]. Vapour-phase reforming

of biomass-derived oxygenated hydrocarbons (with high

volatility, such as methanol, glycerol, ethylene and pro-

pylene glycol) is advantageous over the use of alkanes

derived by petrochemical feedstock. However, for less

volatile biomass-derived oxygenates such as glucose and

sorbitol it is advantageous to use the aqueous phase

reforming process to avoid using excessively high

temperatures.

Routes using bio-renewable feedstocks that generate

syngas can be used to interface with the need for the

existing platform chemicals to be sustained. For the last

70 years the petrochemical and energy industries have

focussed their process technology on oil and hence huge

processes have become entrenched making key platform

chemicals, e.g. ethene, propene, benzene, xylenes, from

which many finished products are derived (Fig. 2). There is

therefore a desire to maintain this infrastructure and tech-

nology; after all it works very well. The initial new

technologies based on methanol conversion [1, 21] were

also aimed at providing these platform chemicals, as we

can expect the new coal-based Fischer–Tropsch processes

also to be. Hence there is a major driving force to convert

everything to syngas and start from there. While techno-

logically sound, such an approach loses all of the in-built

complexity and functionality of bio-derived molecules. It

has been the hallmark of the chemical industry for the last

century to strip every initial feedstock down to, or almost

to, its elemental components, thereby incurring massive

energy penalties and potential loss of selectivity. For

example, the synthesis of ammonia involves a catalytic

pathway on the iron-based catalyst whereby the nitrogen–

nitrogen triple bond is initially broken and then the atomic

nitrogen hydrogenated, whereas in nature the biosynthetic

pathway sequentially hydrogenates molecular nitrogen

requiring the final fission of a nitrogen–nitrogen single

bond and this represents a much lower energy pathway

[22]. It would be unfortunate that a similar approach is now

taken and routes for the use of bio-renewable feedstocks

are based on synthesis gas chemistry, since this would lose

all the chemical complexity of the biological molecule.

The alternative to gasification and pyrolysis of biomass

is fermentation. Using fermentation (Fig. 1) a broad range

of carbohydrates is formed, principally sugars, as well as

ethanol. Ethanol can be used as bio-renewable gasoline. A

major feedstock produced from fermentation of biomass is

glucose, after all cellulose is essentially polymeric glucose

(Fig. 3). Glucose represents a highly functionalised mole-

cule with a number of stereogenic centres. It can be used to

prepare a range of useful products, which find applications

in pharmaceuticals and foodstuffs (Fig. 4) including glu-

conic acid, and this will be discussed in Sect. 3.

Alternative bio-renewable materials are triglycerides

which are the major component of vegetable oils and ani-

mal fats. For many years, these materials have been used as

the basis for soap manufacture by saponification with

aqueous sodium hydroxide. This derives bio-product

glycerol which represents a new highly functionalised bio-

renewable feedstock. The quantities of glycerol made from

saponification are readily used in the manufacture of

refined glycerol for use in pharmaceuticals. However, most

recently interest has focussed on glycerol as the triglycer-

ides can be reacted with methanol to produce biodiesel

(Fig. 5). Each mole of triglyceride reacts with 3 mol of

methanol producing 3 mol of biodiesel and 1 mol of

glycerol. Roughly, 10 wt% of the product is glycerol. This

process is likely to transform the availability of glycerol.

At present the global production of glycerol is [1 m

Hydrogen

syngas Bio-oil

Fuel

Energy

Hydrolysis (enzymatic/chemical)

Fermentations

Ethanol Platformmolecules

Chemicals

Carbohydrates/ lignocellulosic materials

Pyrolysis/gasification

Glucose,xylose, arabinose,galactose,mannose,maltose,lactose

Fig. 1 Scheme of biorefinery processes

260 Top Catal (2009) 52:258–268

123

tonnes, and this is growing at a rate of ca. 10% per annum

[23] as the quest to introduce biodiesel continues. There are

two drivers underpinning this surge in biodiesel production

and hence glycerol production. First, within Europe there is

the European Directive 2003/30 which requires that 5.75%

of all transport fuels is derived from renewable sources by

CO + H2

Gasification

Steam Reforming

Methanol Synthesis

MTGMethanol to

Gasoline

MTOMethan

ol to Olefins

MOGD Olefin to Gasoline

andDistillate

Dehydrogenation

COAL

BIOMASS

NATURAL GAS

NATURAL GAS

Gasoline

Gasoline

Distillate

Fig. 2 Scheme for the

production of gasoline and

distillate

OH

OHOH

OH

OO

O

OHOH

OH

OH

OOH

OH

O

O

O

OHOH

OH

OH

n

Glucose unit

Fig. 3 Polymeric structure of cellulose

CHO

OH

OH

OH

OH

OH

HOOC

COOH

OH

OH

OH

OH

HOOC

CHO

OH

OH

OH

OH

COOH

OH

OH

OH

OH

OH

COOH

OH

OH

OH

O

OH

COOH

O

OH

OH

OH

OH

COOH

O

OH

OH

O

OH

CHO

OH

OH

OH

O

OH

Glucaric acid

Glucuronic acid

Gluconic acid

2-keto-Gluconic acid

5-keto-Gluconic acid

2, 5-diketo-Gluconic acid

2-keto-Glucose

Glucose

Fig. 4 Oxidation products from glucose

Top Catal (2009) 52:258–268 261

123

2010. Secondly, the Kyoto Agreement requires that emis-

sions of CO2 are 8% below 1990 levels by 2012, for those

countries that are signatories to this agreement. There is

indeed much political consensus on the need to dramati-

cally decrease green house gas emissions and the reduction

in CO2 is a major goal to which the use of bio-renewable

feedstocks can contribute significantly. To achieve this it is

essential that biorenewable sources of energy are used

effectively, since, for example, it is known that green house

gas emissions are ca. 50% for biodiesel when compared

with fossil fuel-derived diesel on a per kilometre travelled

basis. However, these drivers also produce a tension since

many of the biorenewable resources can, and indeed are,

used as foodstuffs. The tension between the need for

energy and foodstuffs is something that has yet to be

resolved. However, it should be noted that cellulose and

many triglycerides are not used as precursors to foodstuffs

and this is where we should focus attention.

Glycerol is a highly functionalized molecule; of course,

it too can be gasified and thereby can be used as a feed-

stock to generate existing platform chemicals. However, it

can be used to make a broad range of potential chemicals

(Fig. 6). For example, it can be used to produce glyceric

acid and this will be discussed in Sect. 3. Hence it is

apparent that there is great scope for the use of biore-

newable feedstocks for the generation of a new range of

platform chemicals.

4 Utilisation of Glucose and Glycerol Using Gold

Catalysts

Selective oxidation using gold catalysts continues to make

significant progress. Early observations concerning CO

oxidation at low temperatures [24, 25] and ethyne hydro-

chlorination [26–28] demonstrated that gold catalysts could

be the catalysts of choice for these two reactions. Since

then, gold catalysts have been found to be highly effective

for alkene epoxidation [29–32] and alcohol oxidation

[2, 33] as well as being highly selective for of the hydro-

genation of a range of olefins [34–37] and in the

hydrogenation of a,b-unsaturated aldehydes to the corre-

sponding unsaturated alcohols: acrolein [38, 39] and

crotonaldehyde [40–42].

The observation that gold could be a selective catalyst

for alcohol oxidation, was first clearly demonstrated by

Rossi, Prati and co-workers [43–45] in their seminal

studies. They showed that supported gold nanoparticles can

O

O

R''

OO

R'

OO

R'''

+ CH3 OH3 + OH

OH

OH

O

O

R'CH3

O

O

R''CH3

O

O

R'''CH3

Fig. 5 Biodiesel manufacture

and formation of glycerol as a

by-product

OH OH

OH

Triglycerides

saponification

CO + H2

diesters

Highly branched polymers OH OH

CH2

O

OH OH

O

OO

ketomalonic acid

fermentation

dehydration

etherification

selective oxidation

esterification

OH OH

O

Sonora (Dupont)

dihydroxyacetone

acrolein

1,3-propandiol

OH OH

ORHighly branched polymers

OH OH

O

OH

glyceric acid

fuels oxygenate

Fig. 6 Main pathways for the utilisation of glycerol

262 Top Catal (2009) 52:258–268

123

be very effective catalysts for the oxidation of alcohols,

including diols to the corresponding acid, for example 1,2

propane diol formed lactic acid by oxidation of the primary

alcohol group rather than the secondary alcohol group,

which might have been expected to be more reactive. The

presence of base (typically NaOH) was found to be

essential for the observation of activity, and consequently

sodium salts of the acids were formed as products. The

base was considered to be essential for the first hydrogen

abstraction, and this is a significant difference between the

supported gold catalysts and Pd and Pt catalysts that are

effective in acidic as well as basic conditions.

Christensen and co-workers [46, 47] have made a

number of significant advances in the direct oxidation of

primary alcohols using supported gold nanocrystals and

they have concentrated on decreasing the amount of base

present in these oxidations. They have shown that gold can

catalyse the oxidation of aqueous solutions of ethanol to

give acetic acid in high yields [46]. This provides a

potential new route to a key commodity chemical that is

based on a bio-renewable feedstock using a substantially

green technology approach. Recently, this group have

shown that methyl esters of a broad range of primary

alcohols can be produced using a similar approach [47].

One of the most significant advances in the field of

alcohol oxidation has been the observations of Corma and

co-workers [33, 48] showing that an Au/CeO2 catalyst is

active for the selective oxidation of alcohols to aldehydes

and ketones and the oxidation of aldehydes to acids. Most

significantly, they showed that the oxidation could occur in

the absence of base under very mild reaction conditions,

without the addition of solvent and, consequently, the

aldehyde rather that the acid was formed as the selective

product. The omission of both the base and the solvent for

this reaction is most significant since this permits the use of

green reaction conditions for these oxidation reactions. The

results were shown to be comparable to, or higher than, the

highest activities that had been previously observed with

supported Pd catalysts [49]. Subsequently, Enache et al. [2]

showed that addition of Pd to Au significantly enhanced the

activity of the supported gold nanoparticles for alcohol

oxidation under these mild reaction conditions.

4.1 Oxidation of Glucose to Gluconic Acid

Rossi, Prati and co-workers have extended their earlier

[43–45] studies to the oxidation of sugars and similar high

catalytic efficiency for the oxidation of glucose and sorbitol

has been observed [50, 51]. Rossi and co-workers [52] have

shown that 1 wt% Au/C is a highly effective catalyst for

the selective oxidation of glucose to D-gluconic acid with

selectivities of [99%. In contrast, supported Pt and Pd

catalysts were rapidly poisoned during the course of the

reaction. The activity of the gold catalysts increased with

increasing pH (Table 1) and the activity of the gold cata-

lysts increased to ca. 3,000 h-1 at pH 9.5.

Rossi and co-workers [53] subsequently showed that a

process based on the supported 1 wt% Au/C catalyst could

compete with the current enzyme catalysed process

(Table 2) in which hyderase is used to convert glucose to

D-gluconic acid. Indeed, the gold catalyst shows potentially

higher activity. These are significant results, since they show

that green catalysis using very active gold oxidation catalysts

can potentially replace enzymes as catalysts in the synthesis

of key chemical intermediates from glucose. This observa-

tion opens up the possibility to change the concept of the

biorefinery for the processing of bio-renewable feedstocks.

In recent studies Prati and co-workers have reported

the synergistic effect of the addition of Pd or Pt to the

Au/carbon catalysts for the selective oxidation of D-sorbitol

to gluconic and gulonic acids [54]. The results (Table 3)

show that the supported gold catalysts can be used effec-

tively for the oxidation of sugars other than glucose. The

observation of a synergistic effect for the addition of Pd to

Au is in line with the observations of Enache et al. [2] for

alcohol oxidation.

Table 1 Effect of pH on TOF in the selective oxidation of glucose

from reference [52]

Catalyst TOF (h-1)

pH = 7 pH = 8 pH = 9.5

5%Pd-5%Bi/C 0 325 740

1%Pt-4%Pd-5%Bi/C 0 390 710

1%Au/C 900 1220 3000

Table 2 Comparative data for enzymatic and gold catalysed processes from [53]

Catalyst [C6H12O6] (mol L-1) Cat/glucose (g/kg) pH Stirring

rate (rpm)

T (oC) TOF (h-1) Productivity

(kg m-3 h-1)1st run Total

Au/C 3 5 1.3a 9.5 39000 50 217.7b 514.3b

Hyderase 1 6 – 5–7 1700 30 144.6b 121.7b

a Mean cat/glucose value considering 4 runs as life time of the catalystb Referred to a single run. Productivity data are referred to the catalytic reactor stage

Top Catal (2009) 52:258–268 263

123

4.2 Oxidation of Glycerol to Glyceric Acid

Oxidation of glycerol can lead to the formation of a number

of valuable oxygenated compounds, such as dihydroxyac-

etone, hydroxypyruvic acid, glyceric acid, glycolic acid,

oxalic acid and tartronic acid, control of product selectivity

is therefore crucial. To date these products have a limited

market because they are produced mainly using costly and

non-green stoichiometric processes (e.g. using potassium

permanganate or chromic acid as oxidants) or low pro-

ductivity fermentation processes [55, 56]. However, in the

last 10 years, the selective oxidation of glycerol has been

extensively studied using mainly supported noble metal

nanoparticles such as Pd, Pt and Au using molecular oxy-

gen. Kimura and co-workers demonstrated that by using

acidic conditions, secondary alcoholic groups were oxi-

dized mainly to dihydroxyacetone using Pt and Pd catalysts

[57] and hydroxypyruvic acid [58] were formed. Changing

the pH of the solution and moving forward to basic con-

ditions, the primary alcoholic groups were preferentially

oxidized and glyceric acid was obtained [59] and the use of

palladium supported catalysts led to increased selectivity in

the formation to glyceric acid [59, 60]. Furthermore, using

Bi as a promoter on a Pt/C catalyst gave the highest

selectivity to dihydroxyacetone indicating the change on

the direction of the reaction pathway towards to the sec-

ondary alcoholic group [59, 61].

The use of Au/carbon catalysts were extended by Car-

rettin et al. [62–64] for the oxidation of glycerol to

glycerate with 100% selectivity using dioxygen as the

oxidant under relatively mild conditions with yields

approaching 60% as long as base was present. Under

comparable conditions, supported Pd/C and Pt/C always

gave other C3 and C2 products in addition to glyceric acid

and, in particular, also gave some C1 by-products including

large amounts of carbon oxides. With the Au/carbon cat-

alyst, the selectivity to glyceric acid and the glycerol

conversion were very dependent upon the glycerol/NaOH

ratio (Table 4). In general, with high concentrations of

NaOH, exceptionally high selectivities to glyceric acid can

be observed. However, decreasing the concentration of

glycerol, and increasing the mass of the catalyst and the

concentration of oxygen, leads to the formation of tartronic

acid via consecutive oxidation of glyceric acid. Interest-

ingly, this product is stable with these catalysts. It is

apparent that, with careful control of the reaction condi-

tions, 100% selectivity to glyceric acid can be obtained

with 1 wt% Au/C. The activity is dependent on the gold

loading. For catalysts containing 0.25 or 0.5 wt% Au

supported on graphite, lower glycerol conversions were

observed (18% and 26%, respectively as compared to 54%

for 1 wt% Au/graphite under the same conditions) and

lower selectivities to glyceric acid were also observed. This

was observed to be consistent with the earlier studies for

diol oxidation by Prati and co-workers [43–45] which have

also shown that the conversion is dependent on the Au

loading upon the support. Characterisation of the selective

1 wt% Au/C catalysts demonstrated that they comprised

Au particles as small as 5 nm and as large as 50 nm in

diameter. The majority, however, were about 25 nm in size

and were multiply twinned in character. Decreasing the

loading to 0.5 wt% or 0.25 wt% did not appreciably

change the particle size distribution; the particle number

density per unit area was observed to decrease propor-

tionately, however, which may be correlated to the

decrease in glycerol conversion and selectivity to glyceric

acid. Subsequently, cyclic voltammetry was used to study

Au catalysts supported on graphite [65], since in this case

the support is conducting and this very incisive technique

can be used effectively under in situ conditions. The con-

version has been found to correlate with specific features in

the cyclic voltamogram, particularly those associated with

electro-oxidation of surface intermediates.

5 Effect of the Addition of Pd to Au for the Oxidation

of Glycerol

The studies using 1 wt% Au/C catalysts for the selective

oxidation of glucose to gluconic acid and glycerol to gly-

cerate using mild reaction conditions, highlighted in Sect.

3, demonstrate that there are significant opportunities for

catalysis to play a role in the utilisation of bio-renewable

feedstocks. We have previously shown that the addition of

Pd to Au in supported nanoparticle catalysts can signifi-

cantly enhance the catalytic performance for the oxidation

of alcohols as well as the direct synthesis of hydrogen

peroxide [2, 3]. The catalysts have been prepared using a

Table 3 Comparison of 1% (Au–Pd)/C and 1% (Au–Pt)/C in the

oxidation of D-sorbitol

Catalyst TOF (h-1) Selectivity (50%)c

1% (Au–Pd)Ca 154 60

1% (Au–Pt)Ca 149 71

1% Au/Ca 54 55

1% Pd/Ca 23 60d

1% Pt/Ca 62 40

1% (Au–Pd)Cb 333 57

1% (Au–Pt)Cb 266 58

1% Au/Cb 158 38

Reaction conditions: aD-sorbitol (0.3 M) at 50 �C, pH 11, Sorbitol/

metal = 1,000, flowing O2 at atmospheric pressure (25 mL/min)b

D-sorbitol (0.3 M) at 50 �C, pO2 3 atm, Sorbitol/metal = 1,000,

sorbitol/NaOH = 1c Selectivity at 50% conversion

264 Top Catal (2009) 52:258–268

123

wet impregnation technique using a mixed solution of

chloroauric acid and palladium chloride. This is in contrast

the 1 wt% Au/C catalysts that were formed by reduction of

an aqueous solution of chloroauric acid with formaldehyde

in the presence of the support. These impregnated gold

palladium catalysts have been extensively well character-

ised and the Au–Pd/TiO2 materials have been shown to

comprise core-shell structure with a gold-rich core and a

palladium-rich shell [2]. Whereas, the Au–Pd/C catalysts

comprise homogeneous alloys [66]. The composition of the

individual Au–Pd nanoparticles varies with size, and the

concentration of gold increases as the particle size

increases. We have now used these well-characterised

catalysts to study the effect of the addition of Pd to Au for

the oxidation of glycerol under mild conditions.

5.1 Catalyst Testing and Characterization

5.1.1 Autoclave Reactor Studies

Catalytic test reactions were carried out in a 50-mL Parr

autoclave. The catalyst was suspended in an aqueous

solution of glycerol (0.6 M, 20 mL). The autoclave was

pressurised with oxygen (10 bar pressure) and heated at

60 �C. The reaction mixture was stirred at 1,500 rpm for

4 h. The reactor vessel was cooled to room temperature and

the reaction mixture was analysed.

5.1.2 Product Analysis

Analysis was carried out using HPLC with ultraviolet and

refractive index detectors. Reactants and products were

separated using a Metacarb 67H column. The eluent was an

aqueous solution of H3PO4 (0.01 M) and the flow was

0.45 mL/min. Samples of the reaction mixture (0.5 mL)

were diluted (5 mL) using the eluent. Products were

identified by comparison with authentic samples. For the

quantification of the amounts of reactants consumed and

products generated, the external calibration method was

used.

5.2 Results and Discussion

Two supported gold–palladium catalysts were prepared

using the impregnation method and evaluated for the liquid

phase oxidation of glycerol, namely 2.5wt%Au-2.5%Pd/

TiO2 and 2.5%Au-2.5%Pd/C. The conversion as a function

of time for the two bimetallic supported catalysts is illus-

trated in Fig. 7, and it is apparent that the catalyst

supported on titania is more active. As a consequence, the

2.5%Au-2.5%Pd/TiO2 catalyst displays higher activity

based on turn over frequency values (TOF) (662 h-1) than

the 2.5%Au-2.%Pd/C catalyst (380 h-1) (Table 5). It is

possible that the differences in morphology of the Au–Pd

nanocrystals present on the two different supports may be

important for the origin of this activity difference. The

core-shell structure with a gold-rich core and a palladium-

rich shell found on the titania surface previously reported

could be responsible for enhancing the activity with respect

to the (Au–Pd) homogeneous alloy presented on the carbon

surface. In terms of selectivity (Table 5), comparison at

similar conversion level can give us a direct and mean-

ingful comparison between the product distributions

obtained with each catalyst. At around 45% conversion, the

2.5wt%Au-2.5wt%Pd/TiO2 gives high selectivity to gly-

ceric acid (80%). The minor by-products are glycolic,

formic and tartronic acids. At similar conversion level the

2.5wt%Au-2.5wt%Pd/C gives a slightly lower selectivity

Table 4 Oxidation of glycerol using Au/C catalysts: effect of reaction conditions on product distributiona

Catalyst Glycerol

(mmol)

pO2

(bar)

Glycerol/metal

(mol ratio)

NaOH

(mmol)

Glycerol

conversion (%)

Selectivity (%)

Glyceric acid Glyceraldehyde Tartronic acid

1% Au/activated carbon 12 3 538b 12 56 100 0 0

1% Au/graphite 12 3 538b 12 54 100 0 0

12 6 538b 12 72 86 2 12

12 6 538b 24 58 97 0 3

6 3 540c 12 56 93 0 7

6 3 540c 6 43 80 0 20

6 3 214d 6 59 63 0 12

6 3 214d 12 69 82 0 18

6 6 214d 6 58 67 0 33

a 60 �C, 3 h, H2O (and 20 mL), stirring speed 1,500 rpmb 220 mg catalystc 217 mg catalystd 450 mg catalyst

Top Catal (2009) 52:258–268 265

123

(76%), followed by formic, tartronic and glycolic acids. At

higher conversion (94%), the selectivity to glyceric acid

observed with 2.5wt%Au-2.5wt%Pd/TiO2 decreases from

80% to 73%, and an increase in the selectivity to tartronic

acid was observed. The selectivities of glycolic and formic

acid were not observed to change. These results indicate

that glyceric acid was mainly being slowly overoxidised to

tartronic acid, since the sum of selectivities for glyceric and

tartronic acids is similar at 45% (85%) and 94% (84%)

conversion, respectively. Thus, the main reaction pathway

involves the oxidation of the primary alcoholic group to

glyceric acid. The selectivity profile as a function of

reaction time is illustrated in Fig. 8. As the reaction pro-

ceeds, the selectivity to glyceric acid progressively

decreases, whereas the selectivity to tartronic acid

increases, confirming that consecutive oxidation of the

primary reaction product is occurring.

When comparing the use of 2.5wt%Au-2.5wt%Pd/C

with 2.5wt%Au-2.5wt%Pd/TiO2 at similar conversion

levels of ca. 45% the major product obtained was glyceric

acid (selectivity up to 76%) followed by the glycolic,

formic and tartronic acids. At higher conversion level

(around 90%) a slow overoxidation of glyceric acid (73%)

and an increase in the selectivity to formic acid (12%) was

observed. The selectivity profile as a function of reaction

time is shown in Fig. 9. It is very notable that the selec-

tivity to glyceric acid is decreasing more slowly than in the

case of the titania supported catalyst, possibly indicating

that the glyceric acid is rather stable under these reaction

conditions and very limited overoxidation to tartronic acid

was observed. These preliminary results indicate that

0

10

20

30

40

50

60

70

80

90

100

0 1 2 3 4 5

Reaction time (h)

Con

v.%

Fig. 7 Selective oxidation of glycerol using 2.5wt%Au-2.5wt%Pd/

TiO2 and 2.5wt%Au-2.5wt%Pd/C. Reaction conditions: water

(20 mL), 0.6 M glycerol, glycerol/M = 500, NaOH/glycerol = 2,

T = 60 �C, pO2 = 10 bar, time of reaction = 4 h, stirring rate

1,500 rpm. Key: r glycerol conversion 2.5wt%Au-2.5wt%Pd/TiO2,

j glycerol conversion 2.5wt%Au-2.5wt%Pd/C

0

10

20

30

40

50

60

70

80

90

100

0 1 2 3 4 5

Reaction time (h)

Sel

.%

0

2

4

6

8

10

12

14

Sel

.%

Fig. 8 Selective oxidation of glycerol using 2.5wt%Au-2.5wt%Pd/

TiO2. Reaction conditions: water (20 mL), 0.6 M glycerol, glycerol/

M = 500, NaOH/glycerol = 2, T = 60 �C, pO2 = 10 bar, time of

reaction = 4 h, stirring rate 1,500 rpm. Key: r Glyceric acid selec-

tivity (%), j Oxalic acid selectivity (%), m Tartronic acid selectivity

(%), D Glycolic acid selectivity (%), h Formic acid selectivity (%); the

arrows indicates the appropriate scale for glyceric acid and tartronic

acid, all other products are on the smaller scale opposite

Table 5 Liquid phase oxidation of glycerol using Au–Pd supported catalysts, comparison of selectivity at equivalent conversionsa

Catalyst Conv.% Selectivity %

Glyceric acid Oxalic acid Tartronic acid Glycolic acid Formic acid bTOF (h-1)

2.5%Au2.5%Pd/TiO2 44 79.7 1.5 5.8 6.8 6.2 662

94 73.2 2.9 10.5 6.8 6.5

2.5%Au2.5%Pd/C 47 75.7 1.4 5.7 9.0 8.3 381

88 72.5 1.8 7.0 6.3 12.4

a Reaction conditions: water (20 mL), 0.6 M glycerol, glycerol/M = 500, NaOH/glycerol = 2, T = 60 �C, pO2 = 10 bar. Stirring rate

1,500 rpmb TOF (h-1) at 20 min of reaction. TOF numbers were calculated on the basis of total loading of metals

266 Top Catal (2009) 52:258–268

123

Au–Pd supported catalysts prepared by the impregnation

method are active and highly selective to glyceric acid at

alkaline conditions.

6 Concluding Comments

Bio-renewable feedstocks can provide a valuable new route

to the generation of platform chemicals on which the

generation of finished products can be based. Although

these materials can be gasified and pyrolysed to produce

syngas and oil-like materials, that can be processed by

existing technology, retention of the chemical complexity

of the bio-derived molecules provides the basis for new

catalytic pathways to be explored. The oxidation of glucose

to gluconic acid and glycerol to glycerate using supported

gold catalysts clearly demonstrates that this can be

achieved under mild reaction conditions using oxygen. In

addition, we have demonstrated that a very simple prepa-

ration technique can be used to prepare highly active and

selective Au–Pd catalysts for the liquid phase oxidation of

glycerol producing glyceric acid as the major product.

Acknowledgement We acknowledge the European Union (Project

AURICAT; Contract HPRN-CT-2002-00174) for financial support.

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