The adsorption and hydrogenation of benzene and toluene on alumina- and silica- supported palladium and platinum catalysts

Abstract

The adsorption and hydrogenation of benzene and toluene has been studied in the temperature range 378–608 K over alumina- and silica-supported platinum and palladium catalysts, using a microreactor technique. Over each of the catalysts, in the absence of hydrogen, adsorption of both hydrocarbons leads to the formation of retained species, which, as shown by [ 14c]labelling experiments, are not active in either hydrogenation or molecular exchange. Retention is also observed when the aromatic hydrocarbon is hydrogenated over the platinum catalysts, but not when palladium catalysts are used. In the hydrogenation of benzene and toluene over both the palladium and platinum catalysts, the activity of the catalyst passes through a maximum as the temperature is increased, the position of the maximum depending upon the catalyst and the hydrocarbon. Hydrogenation of [ 14c]benzene in the presence of cyclohexane shows that this behaviour is a kinetic, rather than a thermodynamic, effect. No hydrogenolysis products are observed with either catalyst. Platinum has a higher specific activity than palladium for both benzene and toulene hydrogenation. The results are interpreted in terms of dissociative adsorption leading to the formation of retained secies, whilst the species active in hydro-genation is associatively adsorbed. Hydrogenation of mixtures of benzene and toluene shows that the latter is more readily hydrogenated over both metals. It is suggested that the sites for the adsorption and hydrogenation of benzene are probably distinct from those involved in toluene adsorption and hydrogenation.