Effect of Water on Methane and Ethane Oxidation in the Conditions of Oxidative Coupling of Methane Over Model Catalysts

The effect of water onto the rate and selectivity of methane and ethane oxidation in the conditions of oxidative coupling of methane (OCM) is studied. The effect of water strongly depends on the OCM catalyst composition: whereas PbOX/Al2O3 undergoes an irreversible deactivation, no effect of water is observed on both reaction rate and OCM selectivity over La/MgO. Over NaWMn/SiO2 catalyst both rate of reaction and selectivity are enhanced by water addition to the feed at low conversions during methane oxidation. In the case of ethane oxidation, the rate of reaction is strongly affected by water addition, whereas selectivity to ethylene does not change at equal conversions. At increasing concentration of water in the feed gas its relative effect onto the methane oxidation substantially decreases, so the increasing concentration of water above similar to 8 vol% does not further enhance the rate of methane oxidation. The observed effects are explained by the participation of water in the active site turnover, namely in its re-oxidation by shifting the prevailing re-oxidation route from filling surface oxygen vacancies to oxidative dehydrogenation of surface hydroxy-groups.