Research On Effect Of HZSM-5 Alkali Treatment And Ga Modification On Coaratization Performance Of Hexane-methanol

Aromatics(benzene,toluene and xylene)are important organic chemical in chemical industry and are manufactured from petroleum for a long time.Recently,with the rapid development of polyester industry in China,the demand for aromatics increases year by year.The methanol to aromatics(MTA)process is from the coal chemical route and methanol is sufficient,reducing the dependence on petroleum resources.The co-aromatization coupling byproduct hydrocarbons with excess methanol from coal chemical industry not only improves the value of chemicals but also balances the thermal effect of MTA.In this paper,aimed at the improvement of catalytic performance of co-aromatization,clarity of the relationship between surface acidity and the catalyst activity,and the effect of process conditions on product distribution,the surface acidity and textural properties were adjusted by modification of Zn,Ga and Cr elements and alkali treatment,and the inlfuences on the co-aromatization reaction of n-hexane/methanol were studied.1.The hierarchical zeolite catalysts(M/AT-HZ-x)were prepared by alkaline treatment and impregnation method.The characterizations of XRD,N2 adsorption/desorption,TEM,NH3-TPD,Py-FTIR,FT-IR,ICP-OES,Raman and GC-MS were performed.The results show that Zn,Cr,and P modified HZSM-5 modulates the catalyst acidity,reduces BAS while increases LAS.The diffusion and accommodation of coke precursors are improved after alkaline treatment,which enhanced the ability to resist carbon deposition and resulted in the increase of catalyst life,i.e.,the life of M/AT-HZ-30 reached 2.5 times of M/HZ.2.The effect of reaction temperature on product distribution of co-aromatization was investigated.It is found that increasing the reaction temperature is beneficial to the formation of aromatics.Under the conditions of methanol/n-hexane feed molar ratio of 7:3,500℃,and WHSV of 1.0 h-1,the yield of aromatics was 49.6%and xylene yield was 16.3%,The effect of methanol feed position on the product distribution of co-aromatization was also studied.The optimal methanol feeding position is the 0.3 of the catalyst bed,indicating that methanol feeding in the appropriate position not only promotes the aromatization but also enhanced the alkylation of benzene and toluene into xylene.3.The ZnGa/HZ catalyst was prepared by impregnation and its physical and chemical properties were characterized by using XRD,N2 adsorption/desorption,TEM,py-FTIR,ICP-OES,XPS,and GC-MS.The results showed that Zn and Ga,as active components with excellent dehydroaromatization performance,mainly changed the acidity of the zeolite.Zn species strongly interacts with BAS to form ZnOH+,which reduces the amount of BAS.After Ga modification on ZSM-5,some extra-framework Ga species replace the hydroxyl protons of Si(OH)Al groups,further leading to a decrease in the concentration of BAS,i.e.,changing the S/W ratio from 0.55 to 0.20 and L/B from 0.09 to 1.10.Under the conditions of methanol/n-hexane feed ratio of 7:3,430℃ and 1.0 h-1,the aromatics yield reached 65.7%showing excellent catalytic performance.The yield of aromatics increases with the increase of reaction temperature.Taking 1Zn7Ga/HZ catalyst as an example,the highest yield of aromatics reached 73%(the benzene,toluene and xylene yields reached 26%,34%and 13%,respectively)at 550℃ and the catalytic stability is so good with the increase of reaction time.