Catalytic Behavior of Different Amounts of WO4 Active Sites on Na-W-Mn/SiO2 Catalysts for Oxidative Coupling of Methane

Oxidative coupling of methane OCM served as a direct pathway for producing C-2 C2H6 and C2H4 has received widespread attention. Among the conventional catalysts, Mn/Na2WO4/SiO2 Na-W-Mn/SiO2 catalysts have many advantages like high methane conversion, high C-2 selectivity, and relatively high stability even at high temperature. In this work, a series of Na-nW-Mn/SiO2 catalysts with different W loadings were prepared using silicotungstic acid as the precursor. In order to further investigate the effects of W loadings on the structure and OCM performance of the catalysts, X-ray diffraction XRD , transmission electron microscopy TEM , hydrogen temperature programmed reduction H-2-TPR , oxygen temperature programmed desorption O-2-TPD , and X-ray photoelectron spectra XPS characterizations and the activity evaluation for oxidative coupling of methane were performed on all samples. The results demondtrated that there is a strong interaction among the crystal phases of Na2WO4, Mn2O3 and alpha-cristobalite, which enhances the OCM performance of these catalysts through synergistic catalysis. It is interesting that with the continuous increase of W loading, the W species maintained the form of tetrahedral WO4 on the surface of catalysts. Structure-activity relationship analysis shows that tetrahedral WO4 is the active center for the OCM reaction on this series of catalysts. Therefore, the Na-10.0%W-Mn/SiO2 catalyst with high tetrahedral WO4 content has a lower reduction temperature from W6+-> W4+ and a higher proportion of surface lattice oxygen, which may be the main reason for its optimal methane conversion and C-2 yield. When the temperature reached 775 degree celsius, the conversion of methane over Na-10.0%W-Mn/SiO2 catalyst is 44.2%, the yield of C-2 is 24.1%, and the yield of ethylene is 18.7%.