An Investigation of the Oxygen Pathways in the Oxidative Coupling of Methane over MgO-Based Catalysts

Abstract

The oxidative coupling of methane to ethane and ethene has been investigated by admitting pulses of pure methane, pure oxygen, and mixtures of methane and oxygen to MgO, Li/MgO, and Sn/Li/MgO at temperatures ranging from 923 to 1073 K in a Temporal Analysis of Products (TAP) set-up. Moreover, pulses of oxygen followed by pulses of either methane, ethane, ethene, or carbon monoxide were applied to study the role of both adsorbed oxygen and surface lattice oxygen in the reaction mechanism. Two types of reversibly adsorbed oxygen are present on Sn/Li/MgO. The first type is strongly adsorbed oxygen, which desorbs from the surface on a time scale of 3 min at 973 K. This type of oxygen does not seem to be reactive toward methane. The second type of oxygen consists of weakly adsorbed oxygen species with a time scale of desorption amounting to 4 s at 973 K. The weakly adsorbed oxygen species are involved in the direct conversion of methane to carbon dioxide. Surface lattice oxygen is also interacting with the admitted reductants. The percentage of surface lattice oxygen reactive in the methane conversion is less than 0.1% of a theoretical monolayer on MgO at 1023 K. This value amounts to 27% for Li/MgO and 44% for Sn/Li/MgO at the same temperature. On Li/MgO and Sn/Li/MgO two different types of surface lattice oxygen are present. The first is active in methyl radical formation, while the second is involved in the direct conversion of methane to carbon dioxide. Weakly adsorbed oxygen and the second type of surface lattice oxygen are also involved in the nonselective reaction paths of ethane and ethene as well as in the consecutive oxidation of carbon monoxide. Strongly adsorbed oxygen is not involved in these reactions. The observations are consistent with the Lunsford mechanism [Ito, T., Wang, J.-X., Lin, C.-H., and Lunsford, J. H.,J. Am. Chem. Soc.107, 5062 (1985)] for the generation of methyl radicals over MgO-based catalysts. The increasing activity toward methane due to the addition of lithium and moreover tin to MgO can be explained by an increase in the amount of reactive surface lattice oxygen.