Influence of the reactant concentration in selective hydrogenation of 1,3-butadiene over supported gold catalysts under alkene rich conditions: A consideration of reaction mechanism

Abstract

This article is about the reaction of selective hydrogenation of 1,3-butadiene performed in an excess of alkenes (propene/butadiene ratio = 100) over supported gold catalysts, so as to mimic the conditions of purification of light alkenes (C4 cuts) from the presence of impurities (highly unsaturated compounds, 1–3%). Gold was found highly selective (close to 100%, no propane or butane formation) for the hydrogenation of butadiene to butenes at 100% conversion of butadiene. The influence of the concentrations of the various reactants, i.e. hydrogen, butadiene and propene, on the rates of butadiene and propene hydrogenation was investigated, and the apparent reaction orders were deduced from the dependence of rate on partial pressure in the gas phase. The reason for the high selectivity of gold catalysts is that the reaction of hydrogenation of alkenes occurs at much higher temperature (0.1% conversion at 300°C) than that of selective hydrogenation of butadiene (100% at T <170°C). Moreover, the presence of propene has only a limited influence on the reaction of butadiene hydrogenation, while butadiene has no influence on propene hydrogenation; the two reactions of hydrogenation hardly influence each other. Hydrogen dissociation is the rate limiting step of the reaction (partial order of 0.7 and 1 with respect to H2 for butadiene and propene hydrogenation, respectively), responsible for the low activity of gold. The catalytic properties of gold were compared to those of palladium-based catalysts, and reasons for their different behaviour were discussed.