SELECTIVE PHOTO-ASSISTED CATALYTIC-OXIDATION OF METHANE AND ETHANE TOOXYGENATES USING SUPPORTED VANADIUM-OXIDE CATALYSTS

Selective photooxidation of light alkanes, mainly methane and ethane, into the corresponding aldehydes was achieved using silica-supported v anadium oxide catalysts under UV irradiation at elevated temperature. Photooxidation of methane using the V2O5/SiO2-IW (incipient wetness) ( 0.6 mol% V) catalyst at 493 K for 2 h gave 68 mu mol of methanal, whic h corresponds to 76 mol% selectivity and 0.48 mol% one-pass yield. Pho tooxidation of ethane using V2O5/SiO2 (calcined at 1023 K) -IW (0.6 mo l% V) catalyst for 1 h gave 85 mu mol of methanal, which corresponds t o 90% selectivity and 1.1% one-pass yield. The catalysts prepared by t he sol-gel method also showed activity, especially for the reaction of ethane. Both UV irradiation and a reaction temperature as high as 500 K were essential. The photo-assisted catalytic reactions were very se nsitive to the reaction temperature, method of preparation of the cata lyst, and addition of water vapour. While the reaction of methane was inhibited by the addition of water vapour, the photooxidation of ethan e and propane was promoted in the presence of a controlled amount of w ater vapour. In addition, the reaction with methane required UV irradi ation at a wavelength <310 nm, whereas the reaction with ethane or pro pane proceeded by irradiation at longer wavelength. According to Raman and UV diffuse reflectance spectroscopic studies and XRD studies, onl y isolated four-coordinated vanadium oxide surface species were consid ered to show catalytic activity toward the photooxidation of methane, while accumulated vanadium surface species, not crystalline V2O5, were shown to be active for the photooxidation of ethane and propane.