Acetic Acid Production Using Au Catalysts

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321 Dedicated to the memory of Da/id T Thompson Gold Bulletin Volume 42 Mo 4 2009

Acetic acid production by selective oxidation of ethanol using Au catalysts supported on various metal oxide
Steton M. Tembe', Gary Pat^ick^ Mike S. ScurrelM*
www. gotdbultetin.org

Abstract
The liquid-phase oxidation of ethanol to acetic acid using Au catalysts supported on various metal oxides was studiedat 150°C using molecular oxygen as stoichiometric oxidant. Catalysts containing 1 wt% Au supported on TiO^, AljO^, and ZnO were examined for ethanol oxidation. The results showed that ZnO and TiO^ gave higher initial activities as supports for gold in ethanol oxidation, followed by AI^03. Ethanol conversions of >90% and selectivities to acetic acid of >95% were achieved when using ZnO and TiO^ assupports under conditions where a slight oxygen deficiency was used. With a slight excess of oxygen present initially, ethanol conversions of 99.4%, and a selectivity to acetic acid selectivity of 99.8% could be achieved. Gold leaching seemed to be very apparent with alumina as support and also, after continued use with titania-based catalysts. The use of higher initial concentrations of ethanol(range studied 5 - 40 mass% ethanoi in water) led to higher ethyl acetate selectivities. High acetic acid selectivities were seen for relatively low (5-10 mass%) initial ethanol concentrations.

Introduction
The cost of various fuels and chemicals is largely governed by the cost of the raw material and the efficiency of the processes involved. Oil resources are becoming far more expensivecompared with biomass resources. It is potentially possible to convert biomass (e,g. bioethanoi) to essentially all high-value commodity chemicals and fuels currently available from fossil fuel resources. There could be additional advantages in using biomass as a feedstock compared with fossil resources, for example, in a chemical industry the introduction of oxygen functionalities into hydrocarbons canbe rather difficult, whereas many products derived from biomass already contain some oxygen [1], However, this entails an increase in production costs for some of the non-oxygen-containing products, such as gasoline [2]. It is possible to produce some chemicals more easily and in fewer steps from biomass than from fossil resources [3]. The use of biomass as a sustainable resource for productionof fuels and commodity chemicals should offer an improved security in supply, since biomass can be grown in most parts of the world. Bioethanoi could be one of the examples of biomass that can be used in future as a feedstock in this context.

Since the pioneering discovery by Haruta and his co-workers that gold nanoparticles can catalyze CO oxidation effectively at low temperatures [4] there hasbeen a tremendous interest in gold-catalyzed oxidation reactions [5,6]. In particular, the oxidation of alcohols has received significant attention, and nghtfully so since the reaction satisfies most of the ' Molecular Sciences Institute. School of Chemistry. University of demands for a modern, sustainable, and "green" the Witwatersrand. Private Bag 3. PO Wits. 2050. South Africa reaction [7].The selective oxidation of alcohols to '^ Advanced Materiais Division. Mintek. PrivateBag X3015, carboxylic acids is a fundamental transformation in Randburg. 2125. South Africa organic chemistry as these compounds are widely ' For correspondence: michae!.scurrell@wits.ac.za used as intermediates for fine chemicals [7-9]. The Tel+27 11 7176716. Fax+2711 7176749 high activity of supported goldnanoparticles allows the use of air as the stoichiometric oxidant, leading Keywords: gold, ethanol. bioethanoi. acetic acid, oxidation, to a relatively high atom economy Furthermore, the catalysis

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Gold Bulletin volume 42 No 4 2009

reaction has the potential to play an important role in the synthesis of bolh commodity and fine chemicals, where the use of a heterogeneous catalyst allows the...
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