A Mechanistic study : gas phase radical formation over metal oxides

Abstract

The formation of gas phase organic radicals during the reactions of methane, ethane, ethylene, propylene and toluene over a variety of metal oxides was investigated. Radicals were produced on the metal oxide surface in a low pressure flow reactor, released into the gas phase and trapped downstream in a solid argon matrix where they were analyzed by EPR spectroscopy. The formation of gas phase allylic radicals during the reaction of propylene and propylene-d6 over Bi2O3 was examined. Direct detection of allyl and allyl-d5 radicals indicated that a primary kinetic isotope effect was present and this provides conclusive evidence that allyl radical formation is the rate-limiting step in the partial oxidation of propylene over Bi2O3. The formation of gas phase methyl radicals during the reaction of methane over pure and lithium-doped MgO was investigated. Relatively large amounts of gas phase methyl radicals were produced over both catalysts, with Li/MgO being the most active. Radical production was found to reach a maximum at a doping level of 13.5 wt% lithium. Reaction in the presence of molecular oxygen was found to be catalytic; whereas, use of N20 as the oxidant led to a continuous decline in activity. Two pathways are believed to be responsible for radical formation. Over pure MgO, intrinsic cation vacancies react with molecular oxygen to produce an O- center which can abstract a hydrogen atom from methane to produce the methyl radical. In the doped materials, substitutional Li+ ions react with molecular oxygen to form a [Li+O-] center which is also capable of abstracting a hydrogen atom from methane. Gas phase allyl radicals were also detected during the reaction of propylene over [gamma]-Bi2O3 *MoO3, PbO, MgO and Li/MgO. Large amounts of gas phase methyl and ethyl radicals were produced from methane and ethane, respectively, over MgO and Li/MgO, although Bi2O3, [gamma]-Bi2O3 *MoO3 and PbO were essentially inactive. The differences in activity were probably due to differences in reactivity of surface O- and O^-2 ions. The formation of gas phase vinyl and benzyl radicals from ethylene and toluene, respectively, was not observed over any of the oxides examined.