Oxidative Activation of Methane over MnNaW/SiO2 Catalysts: Models for Active Sites
The oxidative coupling of methane (OCM) is a promising process for direct production of ethane and ethylene. In the present study quantum-chemical modeling of the active center of MnNaW/SiO2 catalyst were performed at the B3LYP level using the LANL2TZ(F) and 6-311G basis sets for tungsten and lighter atoms, respectively. The optimization of mono- and polynuclear W and Mn species on the surface of α-cristobalite has been performed and values of barrier energy for the abstraction of hydrogen from methane were determined. The barriers of the methane dissociation at oxo-centers of closed-shell configuration provided by the W(+4) and W(+6) species on α-cristobalite are 40-70 kcal/mol. The barrier of C-H activation drops to about 5 kcal/mol for WO(OH)4 + model center where oxygen center possesses radical character. It is proposed that the true OCM active center is that associated with Mn (+5) complex having terminal oxygen ligand with a radical character.