Hydrogenolysis of methyl formate over copper on silica: II. Study of the mechanism using labeled compounds

Deuterium labeling has been used to investigate processes occurring during the conversion of methyl formate to methanol over silica supported copper at 393 to 419 K. Unlabeled and labeled methyl formates (CH₃OCDO and CD₃OCHO) react with hydrogen and deuterium at identical rates. The product distribution is that expected for successive simple additions without exchange at methyl or aldehyde positions (e.g., the initial products of the reaction between CH₃OCHO and D₂ are CH₃OD and CHD₂OD). The only complication is a rapid transesterification equilibrium (e.g., CH₃OCHO + CHD₂OD CHD₂OCHO + CH₃OD). Control experiments show that this process, and a similarly rapid CH₃OH + D₂ CH₃OD + HD equilibration, are catalyzed by copper and not the silica support. The mechanism suggested for hydrogenolysis involves formation of a surface hemiacetal species which, on cleavage of the CO bond, gives surface methoxy groups and formaldehyde. It is unclear if the slow step is this bond cleavage or the addition step which forms the intermediate. During reaction between CH₃OCHO and H₂ D₂ mixtures equilibration to HD is 10 to 30 times faster than hydrogenolysis. Added CO inhibits both hydrogenolysis and equilibration but the latter is inhibited to a greater extent. Both effects can be explained in terms of the displacement of adsorbed hydrogen by adsorbed CO with equilibration being affected more because of its higher kinetic order in adsorbed hydrogen. The lower concentration of surface hydrogen concentration in the presence of CO also reduces the rate of hydrogenation of the formaldehyde intermediate causing its diversion into a polymeric by-product which poisons the catalyst.