Preparation Of Modified ZSM-5Catalysts And Its Application In Conversion Of Methanol To Aromatics

Aromatics are important chemical products，including benzene, toluene,xylene (BTX), which are the main raw materials of many fine chemical products.With the market demand increasing and the amount of fossil fuel reducing, it isunable to meet the growing demands of aromatics. Catalytic conversion ofmethanol to aromatics (MTA), an important process to utilize the surplusindustrial methanol, has been widely concerned. And it is a kind of clean,efficient, practical aromatics production process whose aromatic products donot include phosphorus, sulfur and other harmful ingredients. Currently, toexplore suitable ZSM-5catalysts with high activity, high BTX selectivity andlong life has become a key of MTA technology development.In this thesis, we were committed to resolve the two main issues in thecatalytic conversion of methanol to aromatics, i.e., limited lifetime of catalystand low selectivity to liquid hydrocarbon. The characterization methods such asXRD, N2adsorption-desorption, NH3-TPD, FT-IR, SEM and TG were used toinvestigate the structure and physicochemical properties of modified ZSM-5zeolite.The shape of ZSM-5zeolite, silica alumina ratio(Si/Al), weak base modification, La impregnation and reaction condition were investigated toimprove ZSM-5catalyst activity. The specific conclusions are as follows:(1) We first studied the effect of shape over ZSM-5catalysts on MTAreaction. The results showed that ZSM-5molecular sieves could be effectivelydispersed to prevent carbon accumulation when extruded with adhesive. So thecoke deposition resistance capacity of strip ZSM-5had significantlyenhancement comparing with flake ZSM-5. The ZSM-5zeolite of Si/Al=80showed the best performance on MTA reaction with higher aromatic productsand higher BTX yield.(2) The effect of weak base (NaHCO3, Na2CO3and NH3H2O) modificationover ZSM-5catalyst was studied on the catalytic performance in MTA reaction.It was found that catalysts treated with weak base (NaHCO3, Na2CO3andNH3H2O) exhibited a hierarchical structure with interconnected micropores andmesopores. The existence of mesopore was benefit to improve the diffusion ofreactants and products, the lifetime of catalyst. Compared to traditional ZSM-5zeolite, the ratio of Bronsted to Lewis (B/L) acid sites of ZSM-5/NH3H2O (B/L=7.35) zeolite slightly increased but the amount of the acid sites reduced. whilethose of ZSM-5/NaHCO3(B/L=0.127) and ZSM-5/Na2CO3(B/L=0.107)significantly reduced. When the weak base concentration was0.6mol/L, theZSM-5catalyst modified by NH3H2O showed the best performance in MTAreaction. The aromatics yield was56.12%and the time-on-stream stability ofZSM-5/NH3H2O catalyst was126h. The selectivity of Benzene, Toluene and Xylene (BTX) increased to40.28%.(3) LaZSM-5/NH3H2O zeolite was prepared by the method of incipientwetness impregnation and the appropriate percentage of La content per gram ofZSM-5/NH3H2O catalyst was investigated. The results showed that there wereno other characteristic peaks observed except that of MFI structure from XRDpatterns, and the pore property of LaZSM-5/NH3H2O was similar to that ofZSM-5/NH3H2O. Compared with ZSM-5/NH3H2O, some changes wereobserved on the surface of LaZSM-5/NH3H2O zeolite samples according to theSEM images, which exhibited the deposition of amorphous La phases oncatalyst surface. The La impregnation not only improved the yield of aromatics,but also enhanced the stability of the catalyst. When the weight fraction of Lawas1%, aromatics yield increased to68.86%, and the BTX selectivity increasedto60.50%. The time-on-stream stability of the catalyst also slightly increased to137h.(4) The MTA reaction condition, including reaction pressure of fixed-bedreactor, weight hourly space velocities (WHSV), were systematicallyinvestigated to explore the optimum condition for MTA reaction over1.0LaZSM-5/NH3H2O catalyst. Considering the catalyst activity and stability,the yield of aromatics and the selectivity to BTX, the optimum conditions wereas follows: reaction temperature410℃, WHSV=1.0h-1,1.5MPa, puremethanol feed.