甲烷脱氢芳构化Mo/HZSM-5基催化剂表征

利用H2-TPR、XPS、EPR等谱学方法，对Mo／HZSM－5基催化剂的可还原性以及工作态催化剂上钼物种的价态进行了表征研究．结果表明，随着焙烧温度的升高，前驱态催化剂中可还原钢物种在总钼量中所占比率明显下降；工作态催化剂表面存在混合价态的Mo物种：以Mo4 为主，Mo5 为次；673K焙烧制备并经DHAM反应操作的催化剂上检测到Mo5 的EPR讯号相当弱，而973K焙烧制备并经同样反应操作的催化剂，其Mo5 EPR讯号却强得多；酸促进的Mo／HZSM－5基催化剂上高的甲烷转化活性与工作态催化剂表面Mo5 物种的浓度并不存在顺变关系，而与表面酸位（尤其是B酸位）及Mo4 物种的浓度密切相关．

Spectroscopic Characterization of Mo/HZSM-5-based Catalysts for Dehydro-aromatization of Methane

By using H2-TPR, XPS, and EPR spectroscopic methods, the reducibility of theMo/HZSM-5 based catalysts and valence-states of the Mthspecies on/in the functioning catalystswere investigated. The results of H2-TPR showed that the proportion of the Mo-species reducibleto lower valence state(s) in the catalyst prepared at high calcination temperature (973K) waspronouncedly decreased, most probably due to aggregation of the Mo-species and formation ofMoO3 crystallites. XPS measurement on the acid-promoted Mo-ZnSO4/HZSM-5 catalyst indi-cated that there existed Mo-species with mixed valence-states at the surface of the functioningcatalyst, with Mo4 in major (ca. 85mol%) and Mo5 in minor (ca. 15mol%) in quantities- Astrong EPR signal (g =1.88, g1=1.93 ) assignable to Mo5 appeared in the EPR spectrum takenon the Mo/HZSM-5 catalyst prepared at the calcination temperature of 973K; whereas the in-tensity of this signal was pronouncedly decreased for both MthZnSO4/HZSM-5 and Mo/HZSM-5 catalysts prepared at the calcination temperature of 673K- The results of the present work in-dicate the absence of proportional correlation between high activity of rnethane conversion over the strong acid-promoted Mo/HZSM-5-based catalysts and the concentration of Mo5-sites at the surface of functioning catalysts, and are in favor of the following viewpoints, i.e., the crucial first step of activation-dehydrogenation of methane was more likely to be mainly via the pathway of B-acid-assisted dehydrogenation by heterolytic splitting of C-H bond, whereas the lower valence Mo-species (mainly Mo4 ) may play a key role in the stabilization of metal-carbene intermediates and in the sequent aromatization-