Study On The Regulation Of Nanosheet ZSM-5 Zeolite B-Axis Thickness、Optimal Synthesis And Their Catalytic Performance

ZSM-5 zeolite has a series of characteristics such as flexible and adjustable acidity,and excellent thermal stability.It has been widely used in catalysis,adsorption and other fields.Conventional ZSM-5 zeolite has only micropores cause macromolecular reactants to be diffusion-limited.Hierarchical porous ZSM-5 zeolite has both micropores and mesopores,which can enhance the diffusion of molecules and make the product molecules diffuse to the bulk phase in time.The nanosheet ZSM-5 zeolite has a short b-axis thickness and abundant external specific surface area,which can achieve stronger reaction diffusion by shortening the b-axis straight channel diffusion distance.However,currently there is lack of systematic research on the regulation of the b-axis thickness of nanosheet ZSM-5 zeolite,and the synthesis of this zeolite requires use of expensive Gemini quaternary ammonium surfactant,its synthesis cost is high.In view of the above problems,the work of this paper mainly focuses on:develop the synthesis method of nanosheet ZSM-5 zeolite with different b-axis thickness,and study the influence of the b-axis thickness of zeolite on the catalytic cracking performance;Low-cost optimization of the synthesis method of nanosheet ZSM-5 zeolite;Mesoporous-rich sheet ZSM-5 zeolite were synthesized by adding CTAB to the synthesis system.In order to obtain nanosheet ZSM-5 zeolite with different b-axis thicknesses and study their catalytic cracking properties,this paper based on the previous laboratory research,prepared Gemini quaternary ammonium surfactant with different carbon chain length.Among them,C_16-6-6Br₂、C_18-6-6Br₂、C_22-6-6Br₂ three template agents can synthesize nanosheet ZSM-5 zeolite,and the zeolite synthesized with C_12-6-12Br₂ as template agent is spherical.By adding seed crystals to assist induction,the synthesis time can be shortened to 1 day.By adjusting the conditions of the synthesis system,such as template-silicon ratio,alkali-silicon ratio,crystallization time,and crystallization method achieving control of the stacking method,lamella morphology,and b-axis thickness of the nanosheet ZSM-5 zeolite（about 10-100nm）.The n-heptane catalytic cracking performance of ZSM-5 zeolite with different b-axis thicknesses was evaluated,and the zeolite with b-axis thickness of 30 nm showed better comprehensive catalytic performance:When the cracking temperature was550°C,the conversion of n-heptane was 94.3%,and the yield of light olefins was63.7%.Aiming at the high cost of synthesizing nanosheet ZSM-5 zeolite with Gemini quaternary ammonium surfactant,a low-cost optimization study on the synthesis method was carried out.By reducing the water-silicon ratio of the synthesis system,the purpose of maintaining a high template agent concentration in the reaction system can be achieved without increasing the amount of template agent.After the template-silicon ratio is reduced from 0.1 to 0.05,nanosheet ZSM-5 zeolite with relatively regular morphology can still be synthesized.Thin layer ZSM-5 molecular sieve.By introducing a small amount of CTAB,the template-silicon ratio can be further reduced to 0.02,indicating that CTAB can partially replace the gemini quaternary ammonium salt template and play a role in assisting crystallization.When the silicon source was replaced by tetraethyl orthosilicate with water glass,the molecular sieve still showed a nanosheet morphology,but the degree of regularity of the lamellae decreased slightly.The n-heptane catalytic cracking performance of the nanosheet ZSM-5 zeolite synthesized by the optimized synthesis method was evaluated.When the reaction temperature was 550℃,the n-heptane conversion could reach 94.8%,and the light olefine yield could reach 50.8%,still having good catalytic cracking performance.On the basis of the seed-assisted synthesis of ZSM-5 zeolite,by adding different amounts of CTAB,the morphology of the silica-alumina micelles formed in the synthesis system can be controlled,and then the crystallization process of molecular sieves can be affected to obtain different morphologies.Including nano-sheet,nano-needle and nano-crystal hierarchical porous ZSM-5 zeolite are finally obtained.The catalytic cracking performance of n-heptane of the hierarchically porous ZSM-5zeolite with different morphologies was evaluated.The nano-needle molecular sieves showed the best catalytic performance,the n-heptane conversion was 93.2%at the reaction temperature was 550℃,and the yield of light olefine was 62.4%.Because of its low cost and easy preparation,it has good practical application prospects.