Conversion of methyl radicals to methanol and formaldehyde over vanadium oxide catalysts

Methyl radicals, formed by the thermal decomposition of azomethane, react with V2O5 to form methoxide ions. Infrared results confirm the presence of methoxide ions, and ESR spectra demonstrate that the reaction involves the reduction of V5+ to V4+. The methoxide ions may either decompose to formaldehyde or react with water or surface protons to form methanol. In temperature-programmed reaction experiments, both methanol and formaldehyde reached a maximum concentration in the gas phase at 200-250 degrees C over V2O5, but over V2O5/SiO2, the methanol and formaldehyde maxima were shifted to ca. 300 and 400 degrees C, respectively. The production of CH3OH at the lower temperature over V2O5/SiO2 suggests that methoxide ions may react with water or surface protons to form methanol more quickly than they decompose to formaldehyde. Nevertheless, in continuous flow experiments at 300 and 500 degrees C, formaldehyde was the principal oxygenated product because of the rapid oxidation of CH3OH at this temperature, and, in the case of pure V2O5, the direct conversion of methoxide ions to formaldehyde appears to compete favorably with their reaction with water. (C) 1997 Academic Press, Inc.