Catalytic hydrogenolysis of esters: a comparative study of the reactions of simple formates and acetates over copper on silica

Previous studies of the gas phase hydrogenolysis of simple esters on a copper-on-silica catalyst have indicated differences in the kinetics and mechanism of formate hydrogenolysis as compared to the reaction of larger esters. The present studies have focused on the reaction of methyl and ethyl formate compared to that of methyl and ethyl acetate in order to quantify these differences. Formates are found to react some 1000 times faster than acetates. The detailed kinetics of formate and acetate hydrogenolysis have been measured and the results correlated with predictions of Langmuir-Hinshelwood models. Experiments with deuterium have been used to explore the mechanisms of the reactions. The results suggest that acetate hydrogenolysis proceeds via dissociative adsorption of ester followed by slow hydrogenation of the acetyl fragment so formed. Formates, on the other hand, appear to involve the reaction of undissociated ester molecules with hydrogen. The latter reaction is controlled, at least in part, by the packing density of formates on the active surface.