The Study Of Modified HZSM-5 Zeolite On Methanol To Aromatics

Methanol to aromatics(MTA) is a very positive way to get aromatics from methanol.Via this process it could effectively alleviate the methanol dilemma of excess capacity, but also could explore a new technology for obtaining aromatic and expanding the source of aromatic hydrocarbons. The ZSM-5 zeolite exhibite high stability, activity and good shape selectivity during the chemical reaction process because of its special skeleton construction, so it is widely studied in the methanol to aromatization reaction.However, a lot of water is produced in the process which will damage the structure of ZSM-5 zeolite. As a result, the catalyst structural stability and the lifetime reduce quickly. So, the key for MTA reaction is to improve the hydrothermal stability of the catalyst by some modification methods. In this paper, the influences of zeolite species, hydrothermal treatment, Zn, P, Si modification and recrystallization of ZSM-5 by TPAOH were investigated,and the characterization methods such as BET, NH3-TPD, SEM were used.The results show that:The kind of zeolite(Mordenite, MCM-22, ZSM-5 and β zeolite) has remarkable influence on aromatic yeild and products distribution. Because of its special three-dimensional channel and suitble acide sites, the ZSM-5 is better than other zeolite on the aromatics yield, distribution. So, it will be used well in the MTA reaction.The grain size and Si O2/Al2O3 ration also have obvious influence on the MTA reaction. With the Si O2/Al2O3 ration increasing from 31 to 150, the amount of olefin will increase from 1.69% to 4.42%, but the amount of aromatic slightly down. The increased of zeolite grain size could lower the yield of aromatics and olefins. Meanwhile, the amount of alkanes slghtly increased, it showed that the little crystal ZSM-5 zeolite was sutible for the MTA reaction due to its short diffusion channel and low acid site density.The ZSM-5 zeolite(Si O2/Al2O3=100) after hydrothermal treament with few acide site show good hydrothermal stability in the MTA reaction process,and could improve the selectivity of ethylene and proylene, but the aromatics yield was reduced at the same time. Compared with the untreated ZSM-5zeolite, the catalyst acid site decreased significantly as the hydrothermaltreament temperature reached 600℃. So the high temperature is harmful to raise the aromatics yield.The introduction of Zn, P can significantly improve the ZSM-5 zeolite acidity. With the increasing of Zn, P, catalyst acidity, especially, the number of strong acid sites is significantly reduced. When supported amount of Zn O reach to 1.38%, the aromatic content in the product reduced from 46.01% to41.60% compared with unmodified ZSM-5 zeolite. When P up to 2.04%, the aromatics content in the product is 38.51%, which indicated that too much Zn,P loaded on ZSM-5 is not benefit for improve the catalyst acivity.The modification of Si can reduce the amount of active acid sites on the catalyst surface as a result inhibite the olefin ringing into aromatics hydrocarbons, so the selectivity of olefins is increased, but the amount of aromatics is not increased. As the deposition amount further increases to 0.15,the surface acid sites has been completely covered, as a result, olefins and aromatics content in product are almost constant, the suitable supported amount of Si is 0.04.