COS Hydrolysis Using Zinc-promoted Alumina Catalysts
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Catalysis Letters Vol. 104, Nos. 1–2, October 2005 (Ó 2005) DOI: 10.1007/s10562-005-7430-5
COS hydrolysis using zinc-promoted alumina catalysts Hongmei Huanga, Nicola Youngb, B. Peter Williamsb, Stuart H. Taylora, and Graham Hutchingsa,* a
School of Chemistry, Main College, Cardiff University, Cardiff CF10 3AT, UK b Johnson Matthey Catalysts, PO Box 1Billingham, Teeside TS23 1LB, UK
Received 23 June 2005; accepted 24 June 2005
The effect of doping alumina catalysts with zinc oxide is investigated for the COS hydrolysis reaction (COS + H2O=CO2 + H2S) at 150 °C. The effect of the catalyst preparation method is described and discussed, and two methods are compared, namely: impregnation by incipient wetness of zinc nitrate followed by calcination to form the oxide and coprecipitation to form a hydroxide followed by calcination. The most effective zinc-promoted catalysts are prepared using the incipient wetness impregnation method. The promotional effect of zinc oxide on alumina is only observed on the basis of intrinsic activity and is not particularly significant at the initial time on stream, but becomes more marked with increased reaction time. The addition of the zinc oxide therefore decreases the deactivation and experiments using catalysts pretreated with H2S and H2O show that the alumina is deactivated by adsorption of these reactants. However, the effect is related to ZnO acting as a sulfur scavenger at 150 °C and we conclude that any promotional effect is likely to be relatively short lived. KEY WORDS: COS hydrolysis; c-alumina; catalyst promotion.
1. Introduction There is presently significant interest in environmental issues and in particular there is general concern about emissions of pollutants to the atmosphere. While much of the focus of this interest concerns carbon dioxide and global warming, there have been sustained efforts concerning the reduction of sulfur containing emissions for many years. The reason for this is sulfur emissions from the combustion of hydrocarbon fuels leads to the formation of sulfur dioxide which is a potent precursor of acid rain. Consequently there has been continued interest in removing sulfur from process streams in chemicals production. Most sulfur present in hydrocarbon feedstocks is removed by hydrodesulfurisation to form H2S that is subsequently absorbed onto ZnO. This process does not affect COS which is a potent catalyst poison, and consequently a different strategy is used to form H2S and this is COS hydrolysis (COS + H2O=CO2 + H2S). This process has been well studied and to date alumina and titania have been found to be promising catalysts [1–6]. There has been interest in improving the activity of catalysts by the addition of promoters and to date most of these studies have concerned the modification of alumina to affect its acid/base properties. George [7] and Fiedorow et al. [8] demonstrated that the rate of COS hydrolysis could be increased by the presence of a base and that basic sites were essential for the reaction. To date, a wide range of alkali metals (Li, Na, K,
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