Origins of steam-mediated selectivity improvement in the oxidative coupling of methane over MnOx-Na2WO4/SiC

Abstract

Oxidative coupling of methane (OCM) is one of the most promising approaches to produce ethylene and ethane (C₂-hydrocarbons) in the post-oil era. The MnO_x-Na₂WO₄/SiO₂ system shows promising OCM performance, which can be further enhanced by cofed steam. However, the positive effect of steam on C₂-hydrocarbons selectivity practically disappears above 800 °C. In the present study, we demonstrate that the use of SiC as a support for MnO_x-Na₂WO₄ is beneficial for achieving high selectivity up to 850 °C. Our sophisticated kinetic tests using feeds without and with steam revealed that the steam-mediated improvement in selectivity to C₂-hydrocarbons is due to the inhibition of the direct CH₄ oxidation to carbon oxides because of the different enhancing effects of steam on the rates of CH₄ conversion to C₂H₆ and CO/CO₂. Other descriptors of the selectivity improvement are MnO_x dispersion and the catalyst specific surface area. The knowledge gained herein may be useful for optimizing OCM performance through catalyst design and reactor operation.