Topics in Catalysis Vol. 27, Nos. 1–4, February 2004 (# 2004)
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Oxidation of glycerol using supported gold catalysts Silvio Carrettina, Paul McMorna, P. Johnstonb, Ken Griffinb, Christopher J. Kielyc, Gary A. Attarda, and Graham J. Hutchingsa, a
Department of Chemistry, Cardiff University, P.O. Box 912, Cardiff CF10 3TB, UK b Johnson Matthey, Orchard Road, Royston, Herts SG8 5HE, UK c Department of Materials Science and Engineering, Lehigh University, 5 East Packer Avenue, Bethlehem, PA 18015-3195, USA
A series of supported gold catalysts (0.25, 0.5, 1.0 wt% Au/graphite) have been investigated for the oxidation of glycerol and propan-1,2-diol. The 1 wt% Au/graphite catalyst is found to give 100% selectivity to the mono acid product, isolated as the sodium salt, as long as NaOH is present. The catalysts are characterized by TEM and cyclic voltammetry. By TEM, active catalysts all comprise fairly broad-size distributions (5–50 nm diameter) for the gold nanoparticles, although most are ca. 25 nm in diameter. An inactive 1 wt% Au/graphite is shown to have considerably larger particle sizes ð>50 nmÞ and this indicates that there may be an optimum particle size for the desired catalysis. Characterization using cyclic voltammetry of active Au/graphite catalysts carried out in NaOH reveals the presence of an oxide species that may be responsible for the observed catalysis. In contrast, the inactive 1 wt% Au/graphite catalyst shows no oxidation in the cyclic voltammetry experiments. KEY WORDS: gold catalysis; oxidation of alcohols; glycerol oxidation; alcohol oxidation by gold.
1. Introduction The use of renewable feedstocks is of crucial importance for a sustainable society. In this respect, catalysis represents a key approach to green chemistry in the activation and utilization of biorenewable chemical feedstocks. Glycerol is a highly functionalized molecule that is readily available from biosustainable sources, i.e., by hydrolysis or methanolysis of triglycerides, such as rapeseed and sunflower oil. This makes glycerol a particularly attractive starting point that, amongst others via oxidation, can be transformed to a large number of products (scheme 1). Oxidation is a well-used means of activating molecules for the synthesis of chemical intermediates. However, at present, many oxidations are carried out using stoichiometric oxidants (e.g. permanganate, nitric acid or chromic acid) and these routes entail the production of significant amounts of undesired byproducts. As outlined by Sheldon and Dakka [1], there exists immense scope for the replacement of stoichiometric oxidants with catalytic processes using dioxygen since this will generate significant environmental benefits. In recent years, there have been many studies dealing with the oxidation of alcohols and polyols to chemical intermediates [2]. Supported palladium and platinum nanoparticles are effective catalysts for the oxidation of polyols, e.g., the oxidation of glucose to gluconic acid [3,4]. One of the problems associated with these catalytic oxidation reactions concerns product selectivity since a broad range of possible products often To whom correspondence should be addressed. E-mail:
[email protected]
exists. For example, for glycerol oxidation (scheme 2), there are six potential C3 oxygenated products together with C2 (oxalic acid) and C1 products. Selective versus nonselective oxidation is, therefore, the big challenge and requires the design of new effective catalysts. Glycerol oxidation using palladium and platinum catalysts has been studied in detail [5–9] and, in general, palladium catalysts are more selective than platinum catalysts. For this particular oxidation, dihydroxyacetone [9] and glyceraldehyde [5] could be obtained by applying bimetallic catalysts (Pt/Bi) [10] and by controlling the reaction conditions, in particular, the pH of the solution. It should be noted that the industrial synthesis of dihydroxyacetone and gluconic acid applies biocatalysis. Recently, we have shown that supported gold nanoparticles can give 100% selectivity in the oxidation of glycerol to sodium glycerate when the reaction is carried out in NaOH [11,12]. In this paper, we extend these studies and present results on the characterization of both active and inactive Au/graphite catalysts using transmission electron microscopy (TEM) and we also present a preliminary account of characterization of three catalysts using cyclic voltammetry.
2. Experimental 2.1. Catalyst preparation A 1 wt% gold catalyst supported on graphite was prepared as follows. The carbon support (graphite, Johnson Matthey, 113.2 g) was stirred in dematerialized water (1 L) for 15 min. An aqueous solution of HAuCl4 (41.94% Au, Johnson Matthey, 2.38 g) in water (70 mL) 1022-5528/04/0200–0131/0 # 2004 Plenum Publishing Corporation
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polyglycerols
OH
polyglycerol esters
O
HO
glyceraldehyde
dihydroxyacetone
polyols
glycerol
glyceric acid
OH
HO
OH
Dihydroxyacetone
Glycerol
glycerol carbonate O
Scheme 1.
OH OH
HO
was added dropwise over a period of 30 min. The slurry was then refluxed for 30 min, cooled and reduced with formaldehyde over a period of 30 min. The slurry was then refluxed for 30 min, and, following cooling, the catalyst was recovered by filtration and washed with water until the washings contained no chloride. The catalyst was dried for 16 h at 105 C. This method was also adopted to prepare 0.25 wt% Au/graphite and 0.5 wt% Au/graphite catalyst samples using proportionately smaller amounts of chloroauric acid.
O
HO
Glyceraldehyde
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OH
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HO
HO
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2.2. Catalyst testing and characterization 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 mol/L, 20 mL). The autoclave was pressurized with oxygen (3 bar pressure) and heated to 60 C. The reaction mixture was stirred (1500 rpm) for 3 h, following which the reaction mixture was analyzed. Product analysis. Analysis was carried out using HPLC with ultraviolet and refractive index detectors. Reactants and products were separated on an ion exclusion column (Alltech QA-1000) heated at 70 C. The eluent was a solution of H2 SO4 ð4 104 mol=LÞ. Samples of the reaction mixture ð10 LÞ were diluted with a solution of an internal standard (100 L, 0.2 mol/ L isobutanol) and 20 L of this solution was analyzed. Catalyst characterization samples were structurally characterized in a JEOL 2000 EX high-resolution electron microscope operating at 200 kV. The catalyst powders were made suitable for TEM examination by grinding them in high-purity ethanol using an agate pestle and mortar. A drop of the suspension was then deposited onto, and allowed to evaporate to dryness, on a holey carbon grid. Catalysts were also characterized using cyclic voltammetry. The equipment and electrochemical cell have previously been described [13].
3. Results and discussion 3.1. Oxidation of alcohols using Au/graphite catalysts The three supported gold catalysts were prepared and evaluated for the oxidation of propan-1,2-diol and
OH HO
OH
O
O
Tartronic acid
Scheme 2.
glycerol as representative examples of diols and triols respectively. Although the catalysis data has been published elsewhere [10,11], a brief summary of key results is included in this paper. In our previous studies [11,12], we have shown that, in the absence of NaOH, no conversion is observed, and the presence of a base is essential to observe the reaction. Hence, the results shown in table 1 were all obtained in the presence of an equimolar amount of NaOH. We have also shown previously [12] that supported palladium and platinum catalysts are not selective for these reactions and give rise to a broad range of C1 , C2 and C3 products; hence, these data are not repeated here. The three catalyst samples give significant differences in activity and selectivity for the two substrates. The selectivity to the mono acid, isolated as the sodium salt, increases with the gold loading for both substrates with 100% selectivity to the mono acid being observed for the 1 wt% Au/graphite catalyst. It should be noted that the carbon mass balance of the data presented in table 1 is 100%; consequently, the 1 wt% Au/graphite catalyst
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S. Carrettin et al./Oxidation of glycerol Table 1 Oxidation of propan-1,2-diol and glycerol using Au/graphite catalystsa Catalyst
1 wt% Au/graphite 0.5 wt% Au/graphite 0.25 wt% Au/graphite 1 wt% Au/graphite 1 wt% Au/graphite 0.5 wt% Au/graphite 0.25 wt% Au/graphite
Substrate (h)
Reaction time (%)
Conversion
Mono acid selectivity (%)b
glycerol glycerol glycerol propan-1,2-diol propan-1,2-diol propan-1,2-diol propan-1,2-diol
3 3 3 3 15 3 3
56 26 18 32 51 53 28
100 61 54 100 86 71 65
a
Reaction conditions: 60 8C, water 20 mL in an autoclave, stirring speed 1500 rpm, 12-mmol substrate, 12-mmol NaOH, 220-mg catalyst, oxygen: 3 bar pressure. b Other products, principally formic acid and CO2.
provides a route for the exclusive formation of the monoacid product. For the lower loadings of gold, lower selectivity to the monoacid is observed and the other products comprise CO2 and HCOOH and no other C3 products are obtained. Using the 1% Au/ graphite catalyst for a longer reaction time results in a higher glycerol conversion being observed but with a lower selectivity. The additional products, in this case, are also CO2 and HCOOH. The critical role of the OH species requires further comment. For supported palladium and platinum catalysts, activity is observed, albeit largely nonselective, for the oxidation of alcohols in the absence of NaOH. However, with Au/graphite, the presence of NaOH is essential as reaction is observed only when it is present. This indicates that the active sites present on the surface of the gold catalysts cannot activate the alcohol substrate. In view of this, it is proposed that the OH aids the initial hydrogen abstraction. This is expected to be the first step in the oxidation process and is not possible for Au/graphite alone and is slow for Pd/ graphite and Pt/graphite catalysts. In the presence of OH , the Hþ is readily removed from a primary alcohol –OH group, thereby overcoming the rate-limiting step of the alternative oxidative pathway. Hence, we suggest that the oxidation of alcohols using supported gold catalysts proceeds via an initial deprotonation step, as has been previously proposed for supported platinum and palladium catalysts [14,15]. For glycerol oxidation, the catalyst activity increases as the concentration of gold increases. However, this trend is not observed with propan-1,2-diol, for which the 0.5 wt% Au/graphite catalyst displays the highest conversion and activity. If selectivity is taken into account, then the yield of sodium glycerate is very similar for both the 0.5 wt% Au/graphite and 1 wt% Au/ graphite catalysts. This may indicate that, as the concentration of gold is increased, the sites capable of nonselective oxidation are gradually removed from the catalyst.
3.2. Characterization using transmission electron microscopy In previous pioneering studies for diol oxidation using supported gold catalysts, Prati and coworkers [16– 19] have shown that the catalyst activity is dependent upon the particle size and morphology of the gold nanoparticle. Previous studies concerning CO oxidation using gold catalysts have also shown that the size of the gold nanoparticles is crucial for many of the catalyst systems, and, in many cases, an optimum size of ca. 2– 4 nm is required [20]. In view of this, a representative 1 wt% Au/graphite catalyst displaying high activity and 100% selectivity to sodium glycerate was characterized using TEM and a representative micrograph is shown in figure 1. The sample shows a broad range of particle sizes, and sizes as small as 5 nm, and as large as 50 nm, in diameter are observed. The majority was observed to be ca. 25 nm in diameter and most particles were multiply twinned. Interestingly, decreasing the gold loading to 0.5 and 0.25 wt% did not markedly change these observed size distributions. The particle number density per unit area, however, did decrease proportionately as the gold
100 nm
Figure 1. Bright field transmission electron micrograph of 1 wt% Au/ graphite that displays high activity for glycerol oxidation.
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an inactive 1 wt% Au/graphite catalyst using TEM (figure 2) revealed that the particle sizes present were considerably larger, with most particles being greater than ca. 50 nm in diameter, and, once again, they were virtually all multiply twinned. On face value, this could be taken to indicate that the smaller gold nanoparticles in the active sample (figure 1) are the origin of the catalysis that is observed.
3.3. Characterization using cyclic voltammetry
100 nm
Figure 2. Bright field transmission electron micrograph of 1 wt% Au/ graphite that displays no activity for glycerol oxidation.
concentration was decreased, which correlates with the observed activity of these catalysts for glycerol oxidation. It is clear that these catalysts are considerably different from those considered to be optimal for the oxidation of CO to CO2 [20]. We have found that great care needs to be taken with respect to the catalyst preparation and small variations in the procedure can result in catalysts that display no activity at all for glycerol oxidation, even in the presence of NaOH. In particular, care needs to be taken with the temperatures to which the catalyst is exposed. However, in our studies, it should be stressed that reproducible catalyst results have been attained in a sustained manner. In addition, changing the reducing agent can have a marked effect and inactive catalysts can be prepared, for example, using hydrazine. Characterization of such
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Cyclic voltammetry is a useful technique for the characterization of metal surfaces. Cyclic voltammograms (CV) present signals characteristic of the oxidation and reduction processes occurring at a metal surface. In combination with catalytic reaction data, cyclic voltammetry can often provide new insights into the nature of the active surface. In view of this, cyclic voltammetry was used to characterize the three samples of active Au/graphite catalysts (0.25, 0.5, 1.5 wt% Au) and the one inactive 1 wt% Au/graphite catalyst. The potential was varied between 0 and 1.6 V against a Pd/H reference electrode with a sweep rate of 0.02 V/s during voltammogram acquisition. Initial experiments were conducted using a gold holder so that the signals associated with nonnanocrystalline metallic gold in the presence of NaOH could be determined (figure 3). The CV obtained in the presence of NaOH (figure 3) gave a signal in the forward (oxidation) sweep mode at ca. 1.2 V and an oxide desorption peak at 1.05 V in the reverse (reduction) sweep mode. The CVs of the active catalyst (figure 4) show additional features. In the
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E (Pd/H Reference) / Volts Figure 3. Cyclic voltammetry of gold sample holder in NaOH (0.5 mol/L).
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E (Pd/H Reference) / Volts Figure 4. Cyclic voltammetry of active Au/graphite catalysts in NaOH (0.5 mol/L), (a) (——) 0.25 wt% Au/graphite; (b) (– – – –) 0.5 wt% Au/ graphite; (c) ð Þ 1.0 wt% Au/graphite.
forward (oxidation) sweep mode, as observed with the metallic gold holder, there is no hydrogen adsorption, as shown by the absence of signals in the 0–0.4 V region of the CV. In the reverse (reduction) sweep mode, there are now two signals labelled A and B in figure 4. The first at 1.05 V, labelled B, is due to desorption of oxide from metallic gold, as observed for the gold holder alone in figure 3. The intensity of this signal increases proportionally with the gold loading. The second signal at 0.78–0.8 V, labelled A, is characteristic of the three Au/
graphite catalysts. We attribute this to the formation of an oxide species formed in the presence of NaOH. There are differences in intensity for this feature, which approximately correlate with the gold loading. This CV signal for the 1 wt% Au/graphite catalyst is significantly broader and this may be of importance with respect to the nature of the selective oxidant. Interestingly, the CV for the inactive 1 wt% Au/graphite catalyst (which contains larger gold nanoparticles) (figure 5) in the presence of NaOH, shows no features
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in either the forward (oxidation) sweep mode or the reverse (reduction) sweep mode). This is consistent with the total inactivity of this catalyst, since there is no apparent oxidation of the gold surface. These preliminary results show that cyclic voltammetry, combined with detailed catalytic reaction data, can provide new insights into the nature of the active oxidation species on gold nanocrystals. Further studies are in hand to determine the nature of the oxygen species present on the surface of the active and selective Au/graphite catalysts.
4. Conclusions A series of catalysts comprising gold nanoparticles supported on graphite with 0.25, 0.5 and 1.0 wt% gold are active and selective catalysts for the oxidation of diols and triols to the monoacid product as long as NaOH is present. Characterization using TEM reveals that the active Au/graphite catalysts comprise gold nanoparticles 50 nm in diameter with a broad size distribution, whereas an inactive Au/graphite catalyst displays significantly larger particle diameters ð>50 nmÞ with a narrower size distribution. The use of cyclic voltammetry has shown that the three active and selective Au/graphite catalysts exhibit a signal associated with an oxide species that is considered to be the active oxidation species within these catalysts.
Acknowledgment We thank Johnson Matthey and the EPSRC for financial support.
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