HETEROHOMOGENEOUS PROCESSES INVOLVED IN OXIDATIVE CONVERSION OF METHANE, ETHANE AND HYDROCARBON MIXTURES OVER BASIC OXIDES

The conversion, selectivities and yields of methane-oxygen and ethane-oxygen mixtures over Li/MgO catalysts have been compared. The differences which are observed are likely to be due to the much shorter life time of ethyl radicals which are rapidly transformed into ethylene, thus preventing subsequent heterogeneous deep oxidation. Data on the oxidative dehydrogenation of ethane over rare earth oxides are also reported. The insensibility of this reaction to the chemical nature of these solids contrasts with what is observed for the oxidative coupling of methane. This is also accounted for by the short life time of ethyl radicals. When mixing methane with another hydrocarbon, cross coupling occurs only if the hydrocarbon C-H bond strength is relatively small (propene, toluene) suggesting that radicals are involved. (CH4)-C-13 experiments have shown that for the case of toluene, the reaction proceeds via the addition of a C1 block from CH4. The selectivity of cross coupling compared to homo-coupling is also explained in terms of life time of radicals.