Effects of zinc addition to silica supported copper catalysts for the hydrogenolysis of esters

Abstract

When using silica supported copper-zinc oxide catalysts in the hydrogenolysis of methyl acetate, a profound effect of zinc on the activity and selectivity of the catalysts has been observed. The activity and selectivity improve with increasing zinc content. The increase in activity is likely to be an effect that originates either from finely dispersed copper particles in close contact with the zinc oxide phase or from the promotion of copper metal crystallites by zinc oxide particles. A decrease in ethane formation with higher zinc content is attributed to the elimination of dehydrating surface sites. This lowering of ethane selectivity goes hand in hand with a higher production of diethyl ether, which is explained in terms of a less severe dehydration on acid and basic surface sites. Catalysts supported on aerosil are more selective than catalysts supported on sol-gel silica, probably because of differences in the contents of impurities and surface density of hydroxyl groups. In addition to this, a hysteresis has been observed in the conversion as a function of the reaction temperature. The effects have been studied in relation to the role of adsorbed hydrogen and the nature of the adsorbed phase. The increase in the hysteresis with higher zinc content provides indirect evidence for the participation of the zinc oxide component in the activation of the reactants.