Catalytic reduction of esters and carboxylic acids to alcohols over silica-supported copper

The catalytic reduction of acetic acid, methyl acetate, and ethyl acetate over silica-supported copper was investigated through a combination of microcalorimetric, spectroscopic, and reaction kinetics measurements with quantum-chemical calculations based on density-functional theory. Initial heats for the dissociative adsorption of methyl acetate, ethyl acetate, acetaldehyde, methanol, and ethanol on silica-supported copper are estimated to be 124, 130, 130, 128, and 140 kJ mol-1 , respectively. Turnover frequencies are measured to be less than 60 hr-1 at atmospheric pressure and temperatures between 500 and 600 K. The overall rate of conversion of the probe molecules decreases in the order from ethyl acetate to methyl acetate to acetic acid. Silica is proven to be inert under reduction conditions, and therefore it is believed to act only as a support of metallic particles. Quantum-chemical and kinetic models of the surface chemistry indicate a trend in the rates of dissociation of carbonyl-containing organic molecules that correlates with their reducibility. The rate of dissociation on copper increases in the order from acetic acid to methyl acetate to ethyl acetate to acetaldehyde; activation energies are estimated to be 125, 119, 104, and 63 kJ mol-1, respectively. On the basis of a proposed reduction scheme, we suggest that the rate of reduction of acetic acid, methyl acetate, and ethyl acetate over silica-supported copper is determined by the dissociative adsorption of these molecules and by the surface hydrogenation of acyl species.