Photocatalytic Production of Methyl Formate by Methanol Self-Coupling: From Oxidative Dehydrogenation to Direct Dehydrogenation

Photocatalytic production of methyl formate (MF) by methanol direct dehydrogenation under an inert Ar atmosphere with Pd/TiO2 as photocatalyst was realized in the present work. For comparison, experiments of the oxidative dehydrogenation under a CO2 atmosphere were also conducted. The production rates of H2 in the gaseous product and MF in the liquid-phase product under different atmospheres were compared. Intermediates involved in the dehydrogenation processes were characterized by in situ DRIFTS analysis; on the basis of the results, a new mechanism of MF generation from methanol dehydrogenation was proposed: methanol successively breaks the O–H bond and C–H bond to form adsorbed formaldehyde, which then reacts with undissociated methanol to form MF rather than dimerize with another formaldehyde. In addition, it was evidenced that during the dehydrogenation in CO2, the intermediate formate derived from the reduction of CO2 further reacts with methanol to increase the production rate of MF.