Oxidative coupling of methane(OCM)served as a direct pathway for producing C2(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 C2 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 tempera-ture programmed reduction(H2-TPR),oxygen temperature programmed desorption(O2-TPD),and X-ray photoelec-tron 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 α-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 C2 yield.When the temperature reached 775℃,the conversion of methane over Na-10.0％W-Mn/SiO2 catalyst is 44.2％,the yield of C2 is 24.1％,and the yield of ethylene is 18.7％.
关键词
Na-W-Mn/SiO2催化剂/甲烷氧化偶联/不同W含量/乙烯收率/C2
Key words
Na-W-Mn/SiO2 catalyst/Oxidative coupling of methane/Different W contents/Yield of ethylene/C2
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