Preparation Of Hierarchical ZSM-5 Zeolite Based On Self-assembly Of Nano-sized Zeolite Crystals

Zeolites are widely used in many industrial process as catalysts,adsorbents and separation materials especially in there fields of petroleum refining,petrochemical,adsorption and separation,ion exchange because of their high surface areas,uniform micropores,changeable chemical compositions,notably adsorption capacities,adjustable acidity,high thermal and hydrothermal stabilities.However,the disadvantage of their limited micropore channels,larger diffusion resistance,longer molecular diffusion path and easily coked limit their applications in macromolecular reactant catalysis and separation.The are two methods to solve the problems mentioned above,one is reduce the size of conventional zeolites to synthesize nanosized zeolites,the synthesis of nanozeolites is equal to enhance surface areas of zeolites,increasing the entrance of zeolites,which are facilitate the accessibility of reactant molecules;the other is introduce mesopores or macropores in the micropores of conventional zeolites to form micro-mesopores,micro-meso-macropores hierarchical structure,the introduce of mesopores can efficiently shorten micropore channels of zeolites,therefore,accelerating the diffusion of reactant molecules.In this paper,we synthesized ZSM-5 composed of nano-sized polycrystalline zeolite particles via a hydrothermal method with a single template,and a flower-like hierarchical ZSM-5 composed of nanorod ZSM-5and a hierarchical ZSM-5 consisting of MFI nanocrystals were respecetively preapared..Compare to nanozeolites prepared by dual template,confined space and bond block methods,single template is used in this paper based on adjust and control synthesis system so that it is favour nucleation.Advantage of the method in this paper is low cost,good repeatability and the final product is micron-sized polycrystalline aggregate composed of preliminary nanozeolite particles which can be easy to be separated from the synthesis system by filtering,and solve the solid-liquid separation difficulties of monodispersed nanoparticles.In the chapter of nanoporous ZSM-5 zeolite,actors,such as aging time,aging temperature,crystallization time,Si/Al ratio in the gel,the amount of templates,which have an effect on the formation of hierarchical ZSM-5 zeolite,morphology and performance,are discussed in detail.The results suggest that,the best condition to prepare nanoporous ZSM-5 zeolite is:aging 3h,the addition amount of template is T/SiO₂=1,the range of Si/Al ratio in the gel is between 25 and 30.The structural performance and texture property of synthetic materials are characterized by XRD,SEM,TEM,NH₃-TPD,Zero Length Column?ZLC?,FT-IR,Pyridine-IR,N₂ adsorption-desorption.The results indicate that,nanosized hierarchical ZSM-5 is ellipsoid-like polycrystalline aggregates which are composed of many different orientations primary nanocrystals,the size of polycrystalline aggregates is between 500 and 1500nm,abundant pore structure is existed between primary nanoparticles.Compared to micron-sized pure micropore ZSM-5,nanoporous ZSM-5 zeolite with lower activation energy,faster diffusion properties and different acidic sites,cumene is used as probe molecule to explore pre-cracking performance of macromolecules due to dramatically increase of external surface area resulting from nanocrystallization and compared with reference catalyst.The results demonstrate that the intial conversion of nanosized hierarchical ZSM-5 is 96.4%,and after 408h consecutive reaction,the conversion can still maintained at about90%.However,the intial conversion of the reference ZSM-5 is only 56.2%and only after 12h the conversion is reduced to 30%.In the chapter of “flower-like" hierarchical ZSM-5,factors such as crystallization time,H₂O/SiO₂ ratio,Si/Al ratio in the gel,alkalinity,the amount of templates and the kinds of template which control the formation of objective product,were investigated in details.The structural,crystalline,and textural properties of the as-synthesized NZSM-5 and a reference ZSM-5?TZSM-5?were characterized by x-ray powder diffraction?XRD?,scanning electron microscopy?SEM?,FT-IR,in situ infrared?IR?spectrometry of pyridine,temperature-programmed desorption of ammonia?NH₃-TPD?,and N₂adsorption–desorption.The formation of hierarchical ZSM-5 zeolite composed of nano-sized MFI zeolite granules can be depicted as follows.In the present synthesis method,the crystallization conditions of ZSM-5 zeolite were controlled to favor the formation of nano-sized zeolite crystals.A high sodium ion content in the precursor gel attributes to depressing the growth of zeolite crystals and restricts the size of the formed particles.Indeed,despite very complex for the mechanisms of zeolite growth and nucleation,it is clear that alkali metal ion determines the size of the precursor particles.Specifically,during the initial polycondensation of the aluminosilicate precursor,the high concentration of sodium ion in the synthesis solution depresses the extent of the polymerization process and leads to the formation of smaller zeolite particles.In ZSM-5 synthesis as mentioned in the experiment section,the high Na⁺concentration of 2.9 mol/L and the low H₂O/SiO₂ ratio of about 20 lead to a higher degree of supersaturation and higher ratio of nucleation to growth rates.Hence,primary zeolite nanocrystals are created during the initial reaction period.In addition,a higher anion concentration?more than 2.5 mol/L?in the precursor solution yielding NZSM-5 may also depress the excessive growth of the primary zeolite nanocrystals.Simultaneously,because the dissociative zeolite nanocrystals are not stable in the hydrothermal reaction condition,they self-assemble into aggregates,for example,the 2¹0?m microspheres.The performances of the catalysts were respectively investigated during the catalytic cracking of isopropylbenzene and n-octane.As compared with the reference TZSM-5 catalyst,the hierarchical NZSM-5 zeolite displays an excellent catalytic performance with a higher conversion of isopropylbenzene and a longer catalytic life because of the dramatically increased external surfaces;the shortened diffusion path length contributes to enhancing the stability of the hierarchical NZSM-5 catalyst by depressing the coking deposit during the catalytic cracking of n-octane.