Comparative study on oxidation of methane to ethane and ethylene over Na2WO4-Mn/SiO2 catalysts prepared by different methods

The 5 wt% Na2WO4-2 wt% Mn/SiO2 catalysts have been prepared by the incipient wetness impregnation method, mixture slurry method and sol-gel method, and their catalytic performances for the oxidative coupling of methane (OCM) are evaluated in a continuous micro-reactor. 30% of CH4 conversion and 70% of C-2 selectivity have been obtained in methane-oxygen co-feed without any dilutes over these catalysts. X-ray diffraction (XRD) studies indicate that the main crystallite phases of catalysts are Mn2O3, Na2WO4 and alpha-cristobalite, and different precursors of silica are transformed ultimately into highly crystallite alpha-cristobalite. XPS results show that Na, W and Mn are mainly distributed on the surface of catalyst prepared by the incipient wetness impregnation method, but more uniform between the surface and bulk on the catalysts prepared by other two methods. Comparing with the catalyst prepared by the incipient wetness impregnation method, that of prepared by the mixture slurry method has an excellent stability and can alleviate the loss of active components during a 500 h run. Structure changes indicate that a-cristobalite is not indispensable for effective Na2WO4-Mn/SiO2 catalysts. (c) 2005 Elsevier B.V. All rights reserved.