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