Assessment of catalysts for oxidative coupling of methane and ethylene

Abstract

Oxidative coupling of methane (OCM) presents a direct route to upgrade methane to higher value hydrocarbons in the presence of an oxidant. There have been extensive efforts dedicated to studying the initial coupling reactions to form ethylene, but there has been much less emphasis on further molecular weight growth to C3+ species since it was first noted in the 1990′s. Here, catalysts were first screened for OCM activity and production of C3+ during operation at high methane conversion, especially within the more-recently developed family of A2WO4-MnOx/SiO2 (A=none, Li, Na, K, Rb, Cs) catalysts. Within these, K and Rb presented a similar OCM performance to that of the more popular Na-system. Ag- and La- co-doping were also assessed to follow up on recent reports of high performance, but the latter had minimal impact on C2+ formation under these conditions. Ethylene and propylene concentrations rose in proportion, independent of catalyst composition, suggesting that C3+ formation was a gas-phase, radical process, not occurring directly on the catalyst surface. As such, ethylene-methane co-feeds were investigated over a range of reactor conditions for Na2WO4-MnOx/SiO2, which was relatively stable over multiple days at high conversion and the most active under conventional OCM conditions. Ethylene co-feeds increased the C3+ selectivity over a range of reactor conditions, but it also promoted COx formation. Nonetheless, this work shows that further growth of C3+ species under OCM conditions can be achieved, which may be desirable under certain scenarios.