Na-W-Mn/SiO_(2)催化剂上不同含量WO_(4)活性位点在甲烷氧化偶联中的催化行为

Catalytic Behavior of Different Amounts of WO_(4)Active Sites on Na-W-Mn/SiO_(2)Catalysts for Oxidative Coupling of Methane

甲烷氧化偶联(OCM)作为生产C_(2)(C_(2)H_(6)和C_(2)H_(4))的直接途径受到了广泛关注.在传统催化剂中,Mn/Na_(2)WO_(4)/SiO_(2)(Na-W-Mn/SiO_(2))类催化剂具有甲烷转化率高、C_(2)选择性高且在高温下也能保持相对高的稳定性等优点.本文利用硅钨酸作为前驱体,制备了一系列不同W载量的Na-nW-Mn/SiO_(2)催化剂,并进行了X射线粉末衍射、透射电子显微镜、氢气程序升温还原、氧气程序升温脱附和X射线光电子能谱表征以及甲烷氧化偶联活性的评价.研究发现,所有催化剂上的Na_(2)WO_(4)、Mn_(2)O_(3)和α-方英石晶相之间均存在较强的相互作用,并通过协同催化作用来提高催化剂的OCM性能.随着W载量的持续增加,以硅钨酸作为前驱体制备的催化剂表面的W物种一直保持以四面体WO_(4)的形式存在,而构效关系分析揭示了四面体WO_(4)是此系列催化剂上OCM反应的活性中心.具有高含量四面体WO_(4)的Na-10.0%W-Mn/SiO_(2)催化剂有较低的W^(6+)→W^(4+)还原温度和较高的表面晶格氧占比,这可能是其获得最佳甲烷转化率和C_(2)产率的主要原因.当温度为775℃时,Na-10.0%WMn/SiO_(2)催化剂上CH_(4)的转化率为44.2%,C_(2)的产率为24.1%,其中乙烯的产率为18.7%.

Oxidative coupling of methane(OCM)served as a direct pathway for producing C_(2)(C_(2)H_(6)and C_(2)H_(4))has received widespread attention.Among the conventional catalysts,Mn/Na_(2)WO_(4)/SiO_(2)(Na-W-Mn/SiO_(2))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/SiO_(2)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(O_(2)-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 Na_(2)WO_(4),Mn_(2)O_(3)andα-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 WO_(4)on the surface of catalysts.Structure-activity relationship analysis shows that tetrahedral WO_(4)is the active center for the OCM reaction on this series of catalysts.Therefore,the Na-10.0%W-Mn/SiO_(2)catalyst with high tetrahedral WO_(4)content has a lower reduction temperature from W^(6+)→W^(4+)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℃,the conversion of methane over Na-10.0%W-Mn/SiO_(2)catalyst is 44.2%,the yield of C_(2)is 24.1%,and the yield of ethylene is 18.7%.