Dual effects of supported W catalysts for dehydroaromatization of methane in the absence of oxygen
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Catalysis Letters Vol. 102, Nos. 1–2, July 2005 ( 2005) DOI: 10.1007/s10562-005-5205-7
Dual effects of supported W catalysts for dehydroaromatization of methane in the absence of oxygen Kusmiyatia,b and N.A.S. Amina,* a
Department of Chemical Engineering, Faculty of Chemical and Natural Resources Engineering, Universiti Teknologi Malaysia, 81310 UTM, Skudai, Johor, Malaysia b Department of Chemical Engineering, Muhammadiyah University of Surakarta, Pabelan Surakarta, 57102, Indonesia
Received 1 November 2004; accepted 7 March 2005
The screening of a series of W-based catalysts on different supports i.e. HZSM-5, Hb, USY and Al2O3 for the dehydroaromatization of methane (DHAM) revealed that HZSM-5 emerged as the best support. Next, the performance of W/ HZSM-5 and W-H2SO4/HZSM-5 catalysts for the DHAM reaction was compared to study the effect of acidic treatment in the impregnation method. The results showed that the optimum activity of W-H2SO4/HZSM-5 catalyst exceeded that of W/HZSM-5 catalyst. Finally, the influence of Si/Al ratio in the W-H2SO4/HZSM-5 catalyst was studied and the catalyst with Si/Al ratio = 30 was found to be the most promising for the DHAM reaction. The remarkable activity of the catalyst is attributed to the presence of dual effects: suitable content of octahedral polymeric and tetrahedral monomeric tungstate species accompanied by proper amount and strength of acid sites in the catalyst. KEY WORDS: dehydroaromatization; methane; W-based catalysts; tungstate species.
1. Introduction Dehydroaromatization of methane (DHAM) to aromatics have received considerable attentions [1–18] in the study of catalytic reactions. The most common catalysts reported to be promising for DHAM are HZSM-5-supported Mo and also W catalysts [2–18]. Some of the characteristics of an active DHAM catalyst include a highly dispersed active metal species on the surface and also a proper amount of acidity for the support [1–12]. Mo-based catalysts supported on HZSM-5 have been used for catalytic reaction of DHAM in the absence of oxygen. By using in situ FT-IR pyridine technique, the acid sites of Mo/HZSM-5 and the interaction between Mo species and HZSM-5 were investigated [2]. By combining FT-IR study with catalytic evaluation, it was concluded that Mo/HZSM-5, which had a 60% remaining number of original Bro¨nsted acid sites exhibited a good catalytic performance. In addition, Naccache et al. [3] reported that the formation of Mo2C species in Mo/HZSM-5 under methane stream was responsible for the formation of aromatics. The reaction mechanism for the production of aromatics proceeded via the formation of acetylene from methane on Mo2C and the acetylene subsequently oligomerized into aromatics. 27Al and 29Si MAS NMR were employed to investigate the interaction between Mo species and HZSM-5 [13]. The results revealed that strong interaction occurred between the metal species * To whom correspondence should be addressed. E-mail: [email protected]
and HZSM-5 on Mo/HZSM-5 with relatively higher amount of Mo species an
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