OXIDATIVE COUPLING OF METHANE .2. FORMATION OF ACTIVE-SITES BY LEAD AND TIN OXIDES ON MGO

The oxidative coupling of methane has been studied over SnMgO catalysts with varying amounts of Sn. The addition of small amounts of tin oxide to MgO increased both the rate of methane conversion and the C2 selectivity. A maximum C2 selectivity of 40% was obtained over 0.4 at.% SnMgO at 780°C at a methane conversion of 20%. The maximum in Cz selectivity can be explained by considering an "isolated site" type mechanism in methane oxidation. According to this mechanism, an active and selective catalyst should have highly oxidizing sites isolated from each other by relatively inactive sites. Such an arrangement would allow a high methane conversion rate and the highest Cz selectivity by limiting the total oxidation of intermediates on the catalyst. A simple kinetic model based on the isolated site mechanism, which explains most of the catalytic features of the SnMgO catalytic system, has been developed. The model predicts an optimum selectivity for C2 hydrocarbons as a function of Sn content. © 1990.