Effects Of Pretreatment Of Silicon And Aluminum Source On The Synthesis And Catalytic Performance Of ZSM-5 Zeolite

ZSM-5 zeolite has excellent selectivity and suitable acid properties due to its unique two-dimensional channel and tunable silicon-alumina ratio.It shows excellent catalytic performance in the aromatization of methanol with light hydrocarbons and the alkylation of benzene with methanol.In this thesis,on the basis of hydrothermal synthesis,ZSM-5 zeolites with Si O₂/Al₂O₃=80 and 300 were prepared by conventional mixing method,silicon source pretreatment method and aluminum source pretreatment method.The differences in the preparation of zeolites by different synthesis methods were explored,and it was found that the polymerization state of silicon species and aluminum species had an important influence on the synthesis products.The effects of pretreatment conditions such as pretreatment alkalinity,water amount and temperature on the particle size characteristics and polymerization state of silicon species and aluminum species,as well as the differences in synthesis products were investigated.The synthesized zeolites with Si O₂/Al₂O₃ ratio of 80 and 300 were evaluated for the synergistic aromatization of methanol-C₅ fractions and the catalytic performance of benzene and methanol alkylation,respectively.The following conclusions were drawn:（1）The silicon source pretreatment method and the aluminum source pretreatment method can effectively release active silicon species and aluminum species,and Facilitate the polymerization of silica-alumina species.Under the low alkalinity of 0.05and 0.1,both the silicon source pretreatment method and the aluminum source pretreatment method can synthesize low Si O₂/Al₂O₃ ratio ZSM-5 zeolite with the solid yield reaches 90%,which cannot be synthesized by the conventional mixing method.The low-silicon ZSM-5 molecular sieve belongs to the aluminum-rich system,and the low-silicon 80-Al^0.1 sample synthesized by the aluminum source pretreatment method is more excellent,which has higher acid content,specific surface area,etc.And it also showed higher activity in methanol-C₅ fraction aromatization reaction.The ZSM-5zeolite with high Si O₂/Al₂O₃ ratio belongs to the silicon-rich but aluminum-poor system,and the silicon source pretreatment method can better utilize Si atoms.Also,the synthesized molecular sieve 300-Si^0.1 has higher solid yield,silicon-alumina ratio,weak acid content and mesopore volume.In addition,the activity is higher in the alkylation reaction of benzene and methanol,and the selectivity of xylene is better.（2）The liquid phase of the aluminum sol mother liquor is mainly composed of Al（OH）₄^-ions and its oligomers,and the solid phase gel is mainly composed of undissolved aluminum oxide tetrahedron and aluminum oxide hexahedron species.Appropriate amounts of Al（OH）₄^-ions and aluminum-oxygen tetrahedra are the keys to the synthesis of ZSM-5 zeolites with low Si O₂/Al₂O₃ ratio.The optimum pretreatment conditions for synthesizing ZSM-5 zeolites with Si O₂/Al₂O₃=80 by the aluminum source pretreatment method are:OH^-/Al₂O₃=8,H₂O/Al₂O₃=2000,and the pretreatment temperature is 40℃.Then,the effect of ZSM-5 zeolites with different grain sizes on the catalytic performance of methanol-C₅ fraction synergistic aromatization was investigated.The results show that the smaller the crystal size of the zeolite,the higher the conversion rate of C₅ fraction and the higher the content of aromatics in the liquid product.（3）Further,the effect of the dissolution state of silicon source on the synthesis of ZSM-5 molecular sieve was further investigated.The silica sol mother liquor is mainly active Q² and Q³ silicon species units in alkaline medium.Under low alkalinity,the mother liquor is mainly composed of Q³ silicon species with high degree of polymerization,and the synthesized zeolite has high solid yield and Si O₂/Al₂O₃ ratio.At high alkalinity,the mother liquor is rich in Q² silicon species units with high activity,but too many Q² units will polycondensate and nucleate to form amorphous gels,resulting in lower silicon-alumina ratio and solid yield of the final product.The optimum pretreatment conditions for synthesizing high-silicon ZSM-5 zeolite by silicon source pretreatment method are:OH^-/Si O₂=0.05,H₂O/Si O₂=25,and the pretreatment temperature is 70℃.The catalytic performance of benzene with methanol alkylation of high-silicon ZSM-5 zeolite with different grain sizes prepared by silicon source pretreatment was further evaluated.At the same WHSV,the smaller the grain size,the higher the benzene conversion and the higher the xylene selectivity.Using different WHSV,xylene selectivity decreased with increasing the space velocity,while toluene selectivity increased.