Mechanistic Studies of Methanol Oxidation to Formaldehyde on Isolated Vanadate Sites Supported on MCM-48

The mechanism of methanol oxidation to formaldehyde catalyzed by isolated vanadate species supported on silica has been investigated by in situ Raman and TPD/TPO experiments. Raman, XANES, and EXAFS were used to characterize the V-MCM-48 sample, prepared with a loading of 0.3 V/nm2, and it is concluded that the oxidized form of the vanadium is isolated VO4 units. The VO4 species consist of one VO bond and three V−O−Si bonds in a distorted tetrahedral geometry. Methanol reacts reversibly, at a ratio of approximately 1 methanol per V, with one V−O−Si to produce both V−OCH3/Si−OH and V−OH/Si−OCH3 group pairs in roughly equivalent concentrations. Formaldehyde is formed from the methyl group of V−OCH3, most likely by the transfer of one H atom to the VO bond of the vanadium containing the methoxide group. Formaldehyde is formed in nearly equal concentrations both in the presence and in the absence of gas-phase oxygen. CO and H2 are produced by the decomposition of CH2O at higher temperature. In the absence of O2, Si−OCH3 groups undergo hydrogenation to form CH4, and in the presence of O2, these groups are oxidized to COx (x = 1, 2) and H2O above 650 K. Under steady-state reaction conditions, CH2O is produced as the dominant product of methanol oxidation at temperatures below 650 K with an apparent activation energy of 23 kcal/mol. Schemes for the product flows during both TPD and TPO experiments, along with proposed surface intermediates, are presented.