Hydrogenolysis of methyl formate over copper on silica: I. Study of surface species by in situ infrared spectroscopy

The hydrogenolysis of methyl formate to methanol over silica-supported copper has been studied using in situ infrared spectroscopy coupled with simultaneous determination of rate. Under flow reaction conditions two forms of adsorbed methyl formate exist. One has a carbonyl absorption at 1726 cm^-1 and is bound to the support by hydrogen bonding with a heat of adsorption of 65 kJ mol^-1. The second absorbs at 1666 cm^-1 and is bound to copper with an approximate heat of adsorption of 140 kJ mol^-1. At 457 K the hydrogenolysis rate is directly proportional to the band intensity of the latter and hence it, or another species in equilibrium with it, is involved in the rate-determining step. Adsorption of CO from CO H₂ mixtures gives rise to a single infrared band at 2117 cm^-1, the corresponding heat of adsorption being 60 kJ mol^-1. Competitive measurements under hydrogenolysis conditions show that methyl formate will partially displace adsorbed CO and not vice versa. Nonetheless CO does reversibly inhibit the rate and this is attributed to its adsorption displacing hydrogen from the surface. The lower concentration of surface hydrogen also reduces the rate of hydrogenation of a formaldehyde intermediate leading to its deposition as a polymer as revealed by infrared bands at 1483 and 1375 cm^-1. The same polymer accretes more rapidly during the reverse methanol to methyl formate reaction for which CO is a substantial by-product and hydrogen pressures are much lower than used for hydrogenolysis. Continuous deactivation of the catalyst is then observed.