New Mechanistic and Reaction Pathway Insights for Oxidative Coupling of Methane (OCM) over Supported Na2WO4/SiO2 Catalysts.

The complex structure of the catalytic active phase, and surface-gas reaction networks have hindered understanding of the oxidative coupling of methane (OCM) reaction mechanism by supported Na 2 WO 4 /SiO 2 catalysts. The present study demonstrates, with the aid of in-situ Raman and chemical probe (H 2 -TPR, TAP and steady-state kinetics) experiments, that the long speculated crystalline Na 2 WO 4 active phase is unstable and melts under OCM reaction conditions, partially transforming to thermally stable dispersed surface Na-WO x sites. Kinetic analysis via temporal analysis of products (TAP) and steady-state OCM reaction studies demonstrate that ( i ) surface Na-WO x sites are responsible for selectively activating CH 4 to C 2 H x and over-oxidizing CH y to CO and ( ii ) molten Na 2 WO 4 phase is mainly responsible for over-oxidation of CH 4 to CO 2 and also assists in oxidative dehydrogenation of C 2 H 6 to C 2 H 4 . These new insights reveal the nature of catalytic active sites and resolve the OCM reaction mechanism over supported Na 2 WO 4 /SiO 2 catalysts.