The Catalytic Performance For Methane Non-oxidative Aromatization Over Mo/HZSM-5 Catalyst

The catalytic dehydro-aromatization of methane to aromatics and hydrogen is of great scientific importance and industrial interest in the direct conversion of natural gas. Up to now, Mo/HZSM-5 has been regarded as the most promising catalyst for nearly equilibrium conversion of CH4 under 973 K with high selectivity to aromatics. However, it is also observed that methane conversion decreases drastically with time-on-stream due to heavy carbonaceous deposits on the catalyst, which becomes a serious barrier to the industrial application of the reaction. In this thesis, the effect of various pH of the impregnation solution and pretreatment of commercial HZSM-5 zeolite in the presence of different acid on the catalytic performance of Mo/HZSM-5 for dehydro-aromatization had been investigated. The results are summarized as follows:(1) Ammonia was applied to alter the pH of the impregnation solution. It was found that the pH of the impregnation solution greatly influenced the catalytic performance of Mo/HZSM-5 for methane dehydro–aromatization. When the pH of the impregnation solution was 10, the catalyst showed good performance for methane non-oxidative aromatization. The possible reason was that large anions, such as Mo₇O₂₄^6- or H₂Mo₇O₂₄^4-, were mainly presented in ammonium heptamolybdate (AHM) aqueous solution at low pH, while small monomer (MoO₄^2-) was predominated species at high pH, and MoO₄^2- species could diffuse into the micropores of HZSM-5 than Mo₇O₂₄^6- species more easily.(2)The dealumination of commercial HZSM-5 zeolite was carried out using citric acid and nitric acid. The results showed that modified catalysts improved the catalytic performance of Mo/HZSM-5 for methane dehydro-aromatization. Acid treatment of HZSM-5 zeolite, the framework compositions and acidity of the HZSM-5 zeolite would be significantly altered, the origin strong Br?nsted acid would be destroyed and sufficient weak acidic sites would be produced. This treatment not only effectively restrained the carbonaceous deposits on the catalyst, but also satisfied the requirement of the aromatization of the medium strong Br?nsted acid during the MDA reaction. Temperatured-programmed reduction (TPR) showed that the Mo species, which located in the channels of HZSM-5 zeolite and combined with Br?nsted acid sites, were more active and stable for the formation of one-ring aromatics in the methane dehydroaromatization.