CRITICAL PERFORMANCE EVALUATION OF CATALYSTS AND MECHANISTIC IMPLICATIONS FOR OXIDATIVE COUPLING OF METHANE

Global performance data of catalysts used for oxidative coupling of me thane are evaluated with the object of identifying good catalysts and correlating the observed activity trends with the catalyst composition s. Existing surface mechanisms are then examined to see if they provid e grounds for understanding these trends. For performance comparison o nly those catalysts are considered which satisfied a simple performanc e criterion of the sum of the methane conversion and selectivity to hi gher hydrocarbons of 80 or more, for a methane conversion of at least 5%. Performance evaluation completed on this basis clearly shows that only the basic oxides constitute good catalysts. However, strongly bas ic oxides easily form carbonates with high thermal stability requiring the use of suitable additives to destabilize the carbonate. These obs ervations are consistent with the mechanism originally proposed by Sto ne and his co-workers. According to this mechanism, the methane activa tion proceeds via hydrogen abstraction on a basic O2- site producing C H3- which, in a secondary reaction with oxygen, produces CH3., simulta neously converting O2 to O2-. For a sustained catalytic process, O2 ne eds be transformed to O2- by some complex equilibrium which is apparen tly favoured by high electrical conductivity. Thus, basicity and condu ctivity are the key features in a good catalyst.