Critical Performance Evaluation of Catalysts and Mechanistic Implications for Oxidative Coupling of Methane

Global performance data of catalysts used for oxidative coupling of methane are evaluated with the object of identifying good catalysts and correlating the observed activity trends with the catalyst compositions. Existing surface mechanisms are then examined to see if they provide grounds for understanding these trends. For performance comparison only those catalysts are considered which satisfied a simple performance criterion of the sum of the methane conversion and selectivity to higher hydrocarbons of 80 or more, for a methane conversion of at least 5%. Performance evaluation completed on this basis dearly shows that only the basic oxides constitute good catalysts. However, strongly basic oxides easily form carbonates with high thermal stability requiring the use of suitable additives to destabilize the carbonate. These observations are consistent with the mechanism originally proposed by Stone and his co-workers. According to this mechanism, the methane activation proceeds via hydrogen abstraction on a basic O2− site producing CH−3 which, in a secondary reaction with oxygen, produces CH3·, simultaneously converting O2 to O−2. For a sustained catalytic process, O−2 needs be transformed to O2− by some complex equilibrium which is apparently favoured by high electrical conductivity. Thus, basicity and conductivity are the key features in a good catalyst.