(660b) A Dft Study of the Structure of Isolated Molybdena Species Supported on Silica and Their Activity for Methane Oxidation to Formaldehyde

Recent experimental studies by Ohler and Bell [1-3] showed that isolated molybdena sites on silica can convert methane to formaldehyde efficiently. We have used density functional theory (DFT) calculations to understand the structure of the catalytic site and the elementary steps in the conversion of methane to formaldehyde. In its resting state the catalyst may be present as a mono-oxo or di-oxo species. The mono-oxo Mo(VI) species is penta-coordinated with four Mo-O-Si binds and one Mo=O bond which points away from the silica surface. The di-oxo species is tetrahedral with two Mo-O-Si bonds and two Mo=O bonds. We have investigated the thermodynamics of interconversion between these forms on three different types of silica clusters. The Delta-G for such conversions lie in the range of -0.2 kcal/mol to -13.6 kcal/mol depending on the type of silica cluster used. This indicates that both species maybe present in equilibrium with each other. We have also compared the vibrational frequencies for Mo=O stretching vibrations observed experimentally [1-3] and determined from DFT calculations. Both mono-oxo and di-oxo species exhibit Mo=O stretching frequencies in the range of 992+/-22 cm-1. The experimentally observed frequency for isolated MoVI=O on silica is 988 cm-1 and is relatively broad; therefore, we can not say conclusively whether the observed frequencies arise from a mono-oxo or di-oxo species. Since the thermodynamic calculations indicate that both species are likely to be in equilibrium, we take both species into account in our analysis of methane oxidation on supported molybdena.

Ohler and Bell [1] have proposed the involvement of a peroxide species and methoxy species in the oxidation of methane to formaldehyde. The structures of the intermediates and energetics of the scheme they proposed is shown below (along with the calculated energetics). Some of the Mo(VI) species are reduced to Mo(IV) species during the reaction. Molecular oxygen adsorbs on the Mo(IV) species to form a molybdenum-peroxide. This species reacts readily with methane to form a hydroxy-methoxy species. The activation free energy barrier for the methane activation reaction is +57.4 kcal/mol. The methoxy and hydroxyl groups rearrange to form water and formaldehyde. Upon desorption of the products the Mo(IV) species is regenerated and then the cycle continues. The calculated reaction energies and activation barriers are in agreement with the experimental observations. For example if we assume 0.4% of the molybdenum on the surface is reduced from Mo(VI) to Mo(IV), then the calculated reaction rates and the observed rates match exactly with each other. The thermodynamics and kinetics for the formation of Mo(IV) is currently being investigated.

1) Ohler, N; Bell, AT. Study of the elementary processes involved in the selective oxidation of methane over MoOx/SiO2, J. Phys. Chem. B, 110 2006 2) Ohler, N; Bell, AT. A study of the redox properties of MoOx/SiO2, J. Phys. Chem. B, 109 2005 3) Ohler, N; Bell, AT. Selective oxidation of methane over MoOx/SiO2: isolation of the kinetics of reactions occurring in the gas phase and on the surfaces of SiO2 and MoOx, J. Catalysis, 231 2005