Oxidative coupling of methane over Na +- and Rb +-doped MgO catalysts

Abstract

The promoter effect of Na + and Rb − on MgO catalysts was studied for the oxidative coupling of methane. The maximum yield and selectivity obtained for the formation of C 2 hydrocarbons (C 2H 6 + C 2H 4) from CH 4 and O 2 were 22.4% (1073 K) and 57% (1023 K), respectively, over 2 g of 15 mol% Na +MgO catalyst. Physical factors of the promoted MgO catalysts were investigated with BET, SEM, and XRD techniques and were compared with the activities. Alkali metal ions, the promoters, caused structural changes in MgO. Those changes, which are related to the activity of C 2 hydrocarbon formation, are classified in two categories: (1) surface activation through the lattice distortion, and (2) specific surface area reduction. When catalysts with the same specific surface areas were compared, alkali ion-doped MgO in which extensive line broadening is observed by XRD was more active than pure MgO. On the other hand, a sintered pure MgO with low specific surface area showed higher C 2 hydrocarbon yield than unsintered pure MgO. The two factors are discussed in relation to the reaction mechanism.