INFLUENCE OF BASICITY ON THE CATALYTIC ACTIVITY FOR OXIDATIVE COUPLING OF METHANE

Basicity is shown to be a fundamental catalytic property of the alkaline earths and the rare-earths oxide (REO) catalysts to promote the oxidative coupling reaction of methane (OXCO). The basicity was measured by thermogravimetry from the decomposition temperatures of the carbonates. At an OXCO reaction temperature that allows at least part of the catalyst to be in the oxide form, the C2+ selectivity is directly proportional to the basicity of the alkaline earth oxide catalysts. A similar basicity-C2+ selectivity relationship can also be seen among the REO's. The difference in basicity between two REO's is not as great as the alkaline earth oxides, nor does basicity change uniformly across the periodic table. Moreover, the REO's contain sites of various basic strengths. Some of these sites are active for OXCO reaction through the rupture of C-H bond whereas others promote deep oxidation of the product hydrocarbons, giving rise to a low apparent selectivity.