Effect of diffusion limitations of surface-produced radicals on the C2 selectivity in the oxidative coupling of methane

Abstract

In the literature (1), the existence of concentration profiles of oxygen and surface-produced methyl radicals in the catalyst pores and in the film surrounding a single pellet was reported. In this study, the effects of these profiles on C2 selectivity during fixed bed operations were investigated by integration of the mass balances in the interstitial and intraparticle phase. In both phases, concentration gradients for the reactive intermediates, e.g., oxygen. The resulting higher average methyl-radical concentration in the catalyst pellet favors their coupling towards ethane since this is the only second-order reaction with respect to the methyl-radical concentration. Increasing the pellet size can thus lead to a higher C2 selectivity by increasing the volume where a high methyl-radical concentration exists. Experiments with a tin-promoted lithium magnesia catalyst using pellets of different diameters indeed showed a small selectivity increase with increasing pellet size.