物理化学学报 >> 1998, Vol. 14 >> Issue (05): 394-400.doi: 10.3866/PKU.WHXB19980503

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甲烷脱氢芳构化Mo/HZSM-5基催化剂表征

曾金龙, 熊智涛, 林国栋, 于腊佳, 张鸿斌   

  1. 厦门大学化学化工学院 固体表面物理化学国家重点实验室，厦门 361005
  • 收稿日期:1997-07-15 修回日期:1997-12-02 发布日期:1998-05-15
  • 通讯作者:

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

Zeng Jin-Long, Xiong Zhi-Tao, Lin Guo-Dong, Yu La-Jia, Zhang Hong-Bin   

  1. Institute of Chemistry Chemical Engineering and State Key Laboratory for Physics Chemistry of the Solid Surfaces,Xiamen University,Xiamen 361005
  • Received:1997-07-15 Revised:1997-12-02 Published:1998-05-15
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摘要：

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．

关键词: 甲烷, 非氧化脱氢, 芳构化, 苯, 促进型Mo/HZSM-5基催化剂

Abstract:

 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 catalysts were investigated. The results of H2-TPR showed that the proportion of the Mo-species reducible to lower valence state(s) in the catalyst prepared at high calcination temperature (973K) was pronouncedly 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 indicated that there existed Mo-species with mixed valence-states at the surface of the functioning catalyst, with Mo4+ in major (ca. 85mol%) and Mo5+ in minor (ca. 15mol%) in quantities. A strong EPR signal (g|| =1.88, g⊥=1.93 ) assignable to Mo5+ appeared in the EPR spectrum taken on the Mo/HZSM-5 catalyst prepared at the calcination temperature of 973K; whereas the intensity 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 indicate the absence of proportional correlation between high activity of methane 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.

Key words: Methane, Non-oxidative dehydrogenation, Aromatization, Benzene, Promoted Mo/HZSM-5-based catalysts