Molecular structure and catalytic promotional effect of Mn on supported Na2WO4/SiO2 catalysts for oxidative coupling of methane (OCM) reaction

The structure and promotional effect of Mn in supported Mn-Na2WO4/SiO2 catalysts for the oxidative coupling of methane (OCM) reaction has been debated for a longtime in the literature. In the current investigation, with the aid of multiple in-situ characterization studies, we show that the freshly calcined supported 1.2Mn-5Na(2)WO(4)/SiO2 catalyst possesses crystalline Na2WO4, Mn2O3 and SiO2 (cristobalite phase) along with surface MnOx and Na-WOx sites at low temperature and oxidizing environments. Under the OCM reaction environment (T > 800 degrees C), the crystalline Na2WO4 phase melts and Mn2O3 phase reduces. In contrast, the surface MnOx and Na-WOx sites exhibit excellent thermal and chemical stability. Exposure of the 1.2Mn-5Na(2)WO(4)/SiO2 catalyst to the OCM reaction environment redisperses the molten Na2WO4 phase on the SiO2 support to form new surface WOx sites. Interestingly, the stable MnOx species interacts with both molten Na2WO4 phase and surface Na-WOx sites during OCM reaction. Controlled transient kinetic experiments in TAP and detailed steady state OCM fixed-bed reaction studies reveal the role and promotional effect of Mn in the 1.2Mn-5Na(2)WO(4)/SiO2 catalyst. The W-oxides (both molten Na2WO4 and surface Na-WOx sites) are the active sites for the catalytic OCM reaction and the MnOx species only function as promoters. The promotion of MnOx strongly depends on the gas phase O-2 partial pressure and the MnOx species act as mediators for oxygen exchange between the gas phase molecular O-2 and catalyst lattice oxygen. The temperature dependent MnOx promotion reveals that the MnOx species selectively promote the molten Na2WO4 phase at lower reaction temperature and the surface Na-WOx sites at higher temperature.

Automated summary

This study investigates the structure and promotional effect of Mn in supported Mn-Na2WO4/SiO2 catalysts for the OCM reaction. Multiple in-situ characterization studies reveal that the catalyst possesses crystalline Na2WO4, Mn2O3, and SiO2 along with surface MnOx and Na-WOx sites. Under the OCM reaction environment, the Na2WO4 phase melts and the Mn2O3 phase reduces, while the surface MnOx and Na-WOx sites remain stable. The stable MnOx species interact with the molten Na2WO4 phase and surface Na-WOx sites during the OCM reaction.