A kinetics study for the oxidative coupling of methane on a Mn/Na2WO4/SiO2 catalyst

This paper presents an experimental kinetic study for the oxidative coupling of methane (OCM) over a Mn/Na2WO4/SiO2 catalyst prepared by incipient wetness impregnation. Because the catalyst is a reducible metal oxide, the stability of the catalyst has been assessed by Thermo Gravimetric Analysis (TGA). These experiments show that the catalyst has to be pre-treated with oxygen in order to obtain high C-2 selectivity (around 85%) and that a low oxygen partial pressure during the OCM reactions is already sufficient to maintain the catalyst stable in the oxidized state. The catalyst has subsequently been tested in a micro-catalytic fixed bed reactor. The overall reaction orders and rate constants of the primary reactions were determined by measuring the intrinsic reaction rates at different methane and oxygen inlet concentrations. It was found that the reaction order in oxygen for the coupling reaction is 0.38, while the reaction order in oxygen for ethylene oxidation approaches unity, indicating that low oxygen concentration levels are beneficial for obtaining a high C-2 selectivity (up to 80-90%). Such a low oxygen concentration can be obtained with distributive feeding in a membrane reactor. Based on the experiments and least-squares minimization, a simplified reaction mechanism is proposed, where the dependency of the ethane (coupling) and carbon dioxide (oxidation) production rates and the secondary ethylene production and C-2 oxidation rates can be described with power-law type reaction rate expressions. (C) 2012 Elsevier B.V. All rights reserved.