The oxidative coupling of methane over Sm₂O₃ and La₂O₃

A comparison has been made of the behaviour in the oxidative coupling of methane of Sm₂O₃ in the cubic modification with that of Sm₂O₃ in the monoclinic form and with that of La₂O₃. Particular attention has been paid to the effect of residence time (W/F) on the conversion of methane and oxygen and on the rates of production of the various products. It is shown that the cubic and monoclinic forms of Sm₂O₃ have very different catalytic behaviour and that the change in surface area occurring during the phase change cannot account for this change. In contrast, experiments with La₂O₃ calcined at various temperatures showed that the rate of C₂ production was directly proportional to the surface area. The effect of residence time on the formation of products was studied for all three catalysts and the results are compared and contrasted with equivalent results for the Li/MgO and Ba/CaO systems. The primary C₂ product in all cases is ethane. The direct oxidation of methane (or of an intermediate methyl or CH_xO species) to give CO and CO₂ (i.e. a parallel route) is significant in the case of monoclinic Sm₂O₃ but is less important in the case of cubic Sm₂O₃; the activation energies for the production of ethane and CO_x are significantly different and the values also depend on the crystal modification. La₂O₃ and Ba/CaO also give substantial production of CO and CO₂ by a parallel route whereas Li/MgO has a high initial selectivity to ethane. A scheme for the reactions occurring on the various catalysts is proposed.