The Conversion Of Methanol To Gasoline Over Modified HZSM-5Catalysts

With the development of society and the improvement of people’s living standards, gasoline is in great need both at home and abroad. However, the traditional gasoline production methods have not already met this demand for the shortage of world oil resources. Meanwhile, the Fischer-Tropsch (F-T) may not be the best approach because of the complex products containing a lot of heavy paraffin hydrocarbons, which must be post-processed to use. The methanol to gasoline (MTG) process is a simple technology and has high-octane gasoline, which contains no sulfur, chlorine or other impurities and has good anti-knock performance. Therefore, MTG has attracted a growing interest of researchers and has wide application and flourishing prospect. ZSM-5zeolite is mostly used as a catalyst in the MTG process, but the stability of the parent HZSM-5is not satisfactory because of easy coke deposition on the strong acid sites on the surface. It is therefore important to improve the catalyst stability. In this work, the effects of hydro thermal treatment, HNO3treatment and TPAOH treatment on the catalytic performance of MTG reaction were investigated in a fixed bed reactor, the catalytic performance of different types of catalysts were compared and the optimum process conditions were determined.The effect of hydrothermal treatment over HZSM-5on the catalytic performance in MTG reaction was studied. It was found that a decrease of weak acid amount and a faster decrease of strong acid amount with the rising of hydrothermal treatment temperature. The B/L ratio decreased, the aromatics in liquid products decreased and the stability of catalysts significant improved after hydrothermal treatment.600℃is determined as the optimal hydrothermal treatment temperature. HNO3treatment can remove the non-framework Al in the zeolite channel and expose part of strong acid sites again, causing the increase of B/L ratio, more aromatics and less olefins and the further improvement of catalyst lifetime.The effect of TPAOH treatment over HZSM-5for MTG reaction was investigated. The modification led to desilication, dealumination, and secondary crystallization of the zeolite, resulting in the migration of non-framework silicon and non-framework aluminum to the zeolite surface. The structures of the modified samples were basically unchanged, but the relative crystallinities increased and the crystal morphologies were more regular. The redistribution of framework silicon and framework aluminum caused an increase in the surface Si/Al ratio and the BET surface area, and the exposure of increased numbers of catalytic activity sites. The diffusion capacity and coke tolerance improved, which was attributed to an increase in the micropore volume and clearing of the channels. The stability of the catalyst was improved, and the total conversion capacity increased by a factor of3as a consequence of the most severe treatment. Hydrogen transfer reactions became faster and led to more isoparaffins and less olefins in the liquid products. Compared with the process of first extrusion then TPAOH treatment, the process of first TPAOH treatment then extrusion can more significantly improve stability of catalyst and can keep aromatics unabated.The catalytic performance in MTG reaction has been studied over HZSM-5, HMCM-22and HZSM-11. The results indicated that HZSM-11showed the best stability and the highest aromatics selectivity.The reaction conditions, including reaction temperature, the weight hourly space velocity (WHSV) of methanol, water and other factors were systematically studied to identify ideal conditions for MTG reaction over HZSM-5treated by steam at600℃. Considering the activity, stability and the selectivity of catalyst, the optimum reaction conditions were as follows:reaction temperature380℃-400℃, WHSV (MeOH)2h-1, Pure methanol feed.