Gas phase hydrogenation of benzaldehyde over supported copper catalysts

Benzaldehyde hydrogenation over copper catalysts supported on Al2O3, SiO2, TiO2, CeO2 and ZrO2 has been studied at atmospheric pressure and 100-350 de greesC. The reaction competitively produced benzylalcohol, toluene and benz ene with yields depending on the nature of the support and reaction tempera ture. The obtained order of activity was attributed to metal and acid-base surface properties. The important gap in activity (multiplied by 2 to 23) b etween 300 degreesC and 350 degreesC, observed for all catalysts except Cu/ SiO2, was attributed to an adsorption phenomenon, i.e. base sites or low re action temperature inhibited the reaction course, whereas acidic sites or h igh reaction temperature enhanced the catalyst activity. Benzylalcohol was obtained with high selectivity (83% at 68% of conversion) from only 100 deg reesC with the SiO2 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 benzylalcoho l was governed by the adsorption strength the alcohol OH function on the ca talyst surface. High selectivity to benzene was attributed to the easiness of the catalyst to break the C-H aldehydic bond and stability of the surfac e organic entity formed. The parallel study of the reduction of benzaldehyd e under Nz flow threw some light on the relation between metal and acid-bas e surface properties of the catalysts and reaction paths in benzaldehyde hy drogenation. (C) 2000 Elsevier Science B.V. All rights reserved.