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

Oxidative coupling of methane (OCM) is one of the most promising approaches to produce ethylene and ethane (C 2 -hydrocarbons) in the post-oil era. The MnO x -Na 2 WO 4 /SiO 2 system shows promising OCM performance, which can be further enhanced by cofed steam. However, the positive effect of steam on C 2 -hydrocarbons selectivity practically disappears above 800 °C. In the present study, we demonstrate that the use of SiC as a support for MnO x -Na 2 WO 4 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 C 2 -hydrocarbons is due to the inhibition of the direct CH 4 oxidation to carbon oxides because of the different enhancing effects of steam on the rates of CH 4 conversion to C 2 H 6 and CO/CO 2 . 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.