Investigating the Kinetics of Methyl Acetate Hydrogenolysis over Copper: A Combined Experimental and Computational Study

Copper-based catalysts are known to be highly active for the hydrogenolysis of esters with high selectivity to alcohols; however, the nature of the active sites─particularly the importance of metallic (Cu0) vs oxidized Cu sites (Cu+)─is not completely understood, despite prior work suggesting that the balance between these sites is important for good catalytic performance. In this study, we combine experimental and computational analyses to explore the extent to which a Cu(111) active-site model can be successful in describing the kinetics for the hydrogenolysis of methyl acetate (a model ester). The kinetics of the reaction on a Cu/SiO2 catalyst made by the ammonia evaporation method, and thus containing both Cu0 and Cu+ sites, were compared with that on a Cu(111) surface model. Our computational results show that methyl acetate undergoes initial hydrogenation at the carbonyl carbon on Cu(111) and dissociates to yield methoxy and acetaldehyde species that are further hydrogenated to alcohols, consistent with experimental results. The Cu(111) surface is highly active and selective for the conversion of methyl acetate to alcohols at temperatures below 500 K. However, hydrogenation of aldehyde intermediates to alcohol products is less favored at higher temperatures (>500 K) on Cu(111) sites; hence, the product selectivity differs from those observed experimentally on the Cu/SiO2 catalyst in the temperature range of 523–573 K. A higher surface coverage of hydrogen, observed on Cu(111) at lower temperatures (<500 K), seems to be needed for promoting the aldehyde hydrogenation process and predicting product selectivities and reaction orders observed experimentally at higher temperatures. These findings suggest that additional sites, such as Cu+ or Cu–O–SiOx interface sites, available on the Cu/SiO2 catalyst may enable stronger hydrogen adsorption and could be responsible for the hydrogenation activity observed experimentally at higher temperatures.