Gas phase hydrogenation of benzaldehyde over supported copper catalysts

Abstract

Benzaldehyde hydrogenation over copper catalysts supported on Al 2O 3, SiO 2, TiO 2, CeO 2 and ZrO 2 has been studied at atmospheric pressure and 100–350°C. The reaction competitively produced benzylalcohol, toluene and benzene with yields depending on the nature of the support and reaction temperature. The obtained order of activity was attributed to metal and acid–base surface properties. The important gap in activity (multiplied by 2 to 23) between 300°C and 350°C, observed for all catalysts except Cu/SiO 2, was attributed to an adsorption phenomenon, i.e. base sites or low reaction temperature inhibited the reaction course, whereas acidic sites or high reaction temperature enhanced the catalyst activity. Benzylalcohol was obtained with high selectivity (83% at 68% of conversion) from only 100°C with the SiO 2 acidic support, whereas selective formation of toluene or benzene was observed at higher reaction temperature and depended on the nature of the support. The relative selectivity of toluene and benzylalcohol was governed by the adsorption strength the alcohol OH function on the catalyst surface. High selectivity to benzene was attributed to the easiness of the catalyst to break the CH aldehydic bond and stability of the surface organic entity formed. The parallel study of the reduction of benzaldehyde under N 2 flow threw some light on the relation between metal and acid–base surface properties of the catalysts and reaction paths in benzaldehyde hydrogenation.