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

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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. O2– 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.

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