Hierarchical MFI Zeolite: Synthesis,Characterization And Catalytic Application

In recent years,the production of aromatics,such as,benzene,toluene and xylene(BTX)mainly relies on the catalytic reforming of naphtha.Other alternate processes,such as methanol-to-aromatics(MTA)as replacement for the fossil-based processes have grown to be a new focus for the production of aromatics,owing to the rapid diminishing of fossil resources.ZSM-5 Zeolite with MFI type structure is widely applied in the fields of petrochemical industry as a heterogeneous solid acid catalyst,due to it possesses several catalytically desirable properties such as high surface area,adjustable pore size,especially amount of acid sites on external surfaces acidity,high thermal and chemical stability.However,the size of micropores in these materials often confines intracrystalline diffusion,resulting in low utilisation of the zeolite active volume in MTA catalysed reactions.According to the above problem,this paper adopts the different synthetic methods,including the reprocessing method,soft template method,extension method for synthesis of different morphology and properties of the MFI multistage aperture molecular sieve materials,and the physical and chemical properties of the composites were characterized,relationship of physiochemical properties with the catalytic performance of these catalysts and its application in the methanol to aromatics catalytic reaction were discussed,and the results as followed.Firstly,hierarchical zeolite with MFI structure was synthesized by post-processing way of removing silicon modified HZSM-5 molecular sieve synthesis of multistage aperture,the particle size of ZSM-5 and channel structure by adjusting the concentration of alkali treatment to modulation,investigation of modified ZSM-5 zeolite catalyst before and after the change of physical and chemical properties,and the methanol aromatization reaction of catalytic performance.Increase the concentration of alkali treatment,MFI zeolite crystallinity reduced a large number of mesoporous generated at the same time.By ZSM-5 catalyst alkali treatment yield and reaction selectivity of MTA aromatics have varying degrees of increase.Employing conventional Nonionic surfactant block copolymers(F127)as soft templates,a direct synthetic route of mesoporous zeolites with MFI structure has been developed with the assistance of post-steaming treatment.Such hierarchical micro/meso structure demonstrated the greatly enhanced catalytic activity for probe MTA reactions,because of the increased external surface,and the diminished coke formation by the greatly shortened diffusion length in microporous networks.Compared to the untreated ZSM-5,ZSM-5 modified by zinc,Ga and other metal modification generate L acid center on the surface of the catalyst,center of catalyst surface acid concentration change,which accelerated the transformation process of methanol in the MTA reaction,the carbon deposit catalyst deactivation has been also effectively inhibitedMFI/CHA core–shell zeolites were synthesized by a two-step hydrothermal synthesis strategy(epitaxial growth).The growth of silicoaluminophosphate SAPO-34 creates crystalline layers over ZSM-5 nanocrystal crystallites.The samples were characterized by X-ray diffraction(XRD),scanning electron microscopy(SEM),energy-dispersive X-ray spectrometry(EDS)and Fourier transform infrared(FTIR)spectroscopy.The analyses indicate that the pretreatment step of the core phase was an important step during the synthesis.The growth kinetics of the MFI/CHA core–shell zeolites were investigated by analyzing the products obtained with different secondary crystallization times and temperatures.The core–shell zeolite shows good shape-selective catalytic performance for converting methanol to aromatic compounds.The hierarchical pore system with core-shell structured ZSM-5@meso-SAPO-34 is successfully prepared by the hydrothermal method via introducing mesoporous SAPO-34 system.The characterization of SEM,BET,XRD,FTIR,TEM,EDS and NH3-TPD indicated that the cetyltrimethylammonium bromide(CTAB)and TPAOH aided prepared ZSM-5@mesoporous SAPO-34 core-shell composites had large surface area,and high catalytic activity for the MTA.These excellent physicochemical properties for ZSM-5@mesoporous SAPO-34 come from the synergistic effects of ZSM-5 and mesoporous SAPO-34,which shorten diffusion path length within the composites zeolites.Under optimal conditions,the methanol to aromatics on the ZSM-5@mesoporous SAPO-34 achieved a power conversion efficiency of approximately 100% and higher yield of aromatics,which is comparable with the performance of the ZSM-5 and mixture of ZSM-5 and SAPO-34.These results indicate that the ZSM-5@mesoporous SAPO-34 is a promising composite material and provides a novel way for the production of high-value aromatics.