Synthesis Of Isomorphous/Polymorphous Core-shell Structured Zeolite-Zeolite Composite

Y,Beta and ZSM-5 zeolites are widely used in petrochemical field.Y zeolite exhibits excellent catalytic performance in the catalytic cracking,hydrocracking,hydroisomerization reaction with high activity,good selectivity and stability;ZSM-5 zeolite possesses a three-dimensional pore structure,acid strong and good thermal stability,so it shows good shape selective catalysis in the catalytic cracking reaction;Beta zeolite possesses large microporous structure with three-dimensional 12 membered ring straight channel system,and exhibits excellent performance in hydrothermal stability,molecular shape selectivity,anti coking.But,the application of conventional microporous zeolites with limited transfer rate,large diffusion resistance,long diffusion path and low utilization rate of active sites is limited in industry.In order to solve the problems,on one hand,researchers prepared nanometer-sized zeolites by reducing the size of the zeolite particles.On the other hand,the mesopore is introduced into the conventional microporous zeolites to prepare hierarchical porous zeolites.At present,the high cost of the preparation of nanometer-sized zeolites and hierarchical porous zeolites zeolites hinders application in industry.In this paper,the single zeolite was modified to prepare core-shell structured zeolite composites.The core-shell zeolite composites are composed of conventional microporous zeolite as a core and nanometer-sized zeolite a polycrystalline shell,and improve the external surface area of core zeolite effectively and resolve the problems of nanometer-sized zeolite filtration and recovery difficulty successfully.At the same time,the combination of the bi-phase zeolites can play a good synergistic effect and improve the catalytic performance and ensure the certain distance between the active sites of bi-phase zeolites.The nanometer-sized zeolites of the shell increase the external surface area,which benefits the industrial application.The zeolite composites with core-shell structure were prepared through modifying the single zeolite.Isomorphous Y-Y composite zeolites possessing commercial Y core and small particles Y zeolite polycrystalline shell were synthesized by hydrothermal crystallization procedure.The Y-Y zeolite composites can effectively increase the external surface area and reduce the size of shell particles.The effective factors including the added amount of core phase,crystallization time,alkalinity,silicon aluminum ratio and silicon source were investigated in the synthesis procedure of Y-Y zeolite composites.The framework structures and properties were characterized by XRD,SEM,TEM,FT-IR,NH₃-TPD,NMR,Pyridine-IR and N₂ adsorption.The results showed Y-Y zeolite composites possess obvious core-shell structure and the growth orientation of shell particles is not consistent,and the size of shell particles between 50 and 600nm can be adjusted.The external surface area of composite zeolites with mesoporous structure is 60m²/g.Polymorphous Beta/ZSM-5 composite zeolites were prepared by modifying single Beta.The synthesis procedure including amorphous silica deposited on Beta zeolite surface and a subsequent hydrothermal treatment taking amorphous silica as silicon source to prepare Beta/ZSM-5 composite zeolites possessing Beta core phase and small particles Y zeolite polycrystalline shell was carried out.The composite zeolites with different sediment amount of TEOS were characterized by XRD,FT-IR,SEM,N₂ adsorption.The results showed that the characteristic peaks corresponding to Beta and ZSM-5 zeolites can be observed.The peak intensities of Beta zeolite reduced and the peak intensities of ZSM-5 zeolite increased with the increasing sediment amount.The composite zeolites have obvious core-shell structure with the particle size of the shell between 200 and 800nm.The migration to shell of part of silicon and aluminum from destructive skeleton structure of Beta zeolite contributed to generation of small particles of ZSM-5 zeolite.Compared to pure Beta zeolite,the external surface area and mesoporous volume of Beta/ZSM-5 zeolite composites increase significantly.