Partial oxidation of methane over oxide catalysts. Comments on the reaction mechanism

Abstract

A detailed investigation of the partial oxidation of methane to ethane and ethene is described. In particular a range of oxidants (O₂, N₂O, O₃) have been reacted with a series of catalysts (MgO, Li/MgO, La₂O₃, Li/La₂O₃, Sm₂O₃ and Li/Sm₂O₃) and these studies have been utilized to investigate the reaction mechanism. Methane oxidation at conversions of ca. 1 % have indicated that ethene as well as ethane is a primary reaction product, but at higher conversions the major fraction of the ethene formed is via oxidative dehydrogenation of ethane in a secondary reaction. Based on this study a reaction mechanism is proposed and discussed. In line with previous studies the results from used of N₂O as oxidant confirm that O^–_(s) is the oxidising species responsible for CH^˙₃ formation, from CH₄, which on dimerisation yields ethane. In addition it is proposed that primary formation of ethene results from the interaction of methane with a different surface oxygen species (e.g. O^2–_2(s)). The oxides studied, which can be considered as broadly representative of the wide range of oxides utilised as catalysts, become active only at high temperatures ( 700 °C) and at such temperatures the gas phase nature of the overall reaction is dominant. The consequences of this observation are discussed with respect to the design of improved catalysts.