For elucidating the reaction mechanism of the oxidative gas phase carbonylation of methanol (MeOH) to dimethyl carbonate (DMC) over CuY zeolites a new operando DRIFTS/UV-Vis-DRS/MS setup was developed and used enabling simultaneous information concerning the nature of adsorbates and their changes during reaction (DRIFTS) and the changes of oxidation state and coordination sphere of the catalytically active Cu species (UV-Vis-DRS) where the gaseous products are analyzed at the same time by MS.
The interaction of the catalysts with different reactants and reactant mixtures (O2, CO, CO/O2, MeOH/O2, MeOH/CO, MeOH/CO/O2) was studied in detail. By using 16O2/18O2- and 12CO/13CO containing gas mixtures the transient isotopic exchange of reactants and products were monitored which allowed the discrimination between active and spectator species.
DMC is produced via a monodentate monomethyl carbonate (MMC) species as intermediate, forming by interaction of adsorbed methoxide and CO at the same Cu+ site, which consecutively reacts with methanol to DMC. Lattice oxygen supplied by CuOx species is involved in the required oxidation step for DMC formation while the gas phase oxygen is needed to re-oxidize the partially reduced catalyst. Adsorbed bidentate MMC species were found to be inactive. Unselective oxidation reactions of MeOH and CO cause the formation of by-products like methyl formate, dimethoxy methane and CO2.