Synthesis And Performance Of Hierarchical ZSM-5 In CTAB-C₂H₅OH-H₂O System

Due to three-dimensional porous network(0.55 nm)and adjustable silicon-aluminum ratio(10??)of zeolite ZSM-5,it is suitable for catalysts for various reactions involved hydrocarbons.However,the well-known drawbacks of zeolite ZSM-5 from sole presence of micropores are the limitations for the accessibility for bulky hydrocarbons and diffusion in micropore channels for reactants.It is necessary to synthesis zeolite ZSM-5 containing a secondary pore system(?2 nm).Previous methods for preparing hierarchical zeolites mainly focused on the post-treatment of microporous zeolites,the hard template method and the soft template method.The intracrystalline mesopores produced by the post-treatment method are random and the hard template agent has poor compatibility with zeolite precursor.The synthesis process of the soft template method is simple and easy to control,which can effectively control the mesopore structure,pore size and porosity of the product.Generally,synthesis of hierarchical ZSM-5 was based on the use of micropore organic amine and hexadecyltrimethyl ammonium bromide(CTAB)simultaneously in one system or designed multiammonium surfactants composed of long hydrophobic chains.However,microporous organic amines are more toxic and expensive,and the synthesis of long-chain multiammonium template is very complicated.Therefore,it is necessary to seek no template or simple and safe template to synthesize zeolite ZSM-5 with high performance.With the aid of C₂H₅OH-H₂O system,high-silica zeolite ZSM-5 was prepared at lower temperature(130?)without the participation of organic amine.Then,three types of hierarchical ZSM-5 was prepared by using simple,inexpensive and safe template CTAB as a single template.The relative speed(degree)between mesopore phase and zeolite framework formation is controlled by adjusting the amount of CTAB and crystallization time in CTAB-C₂H₅OH-H₂O system,owning to the characteristics of the formation and stability of CTAB micelle.The plate-like high-silica ZSM-5 zeolite with uniform morphology was synthesized in C₂H₅OH-H₂O system at a lower crystallization temperature(130°C)in the absence of organic amine and seed.Results confirmed that the synthesized ZSM-5 zeolite(C₂H₅OH/Si O₂=2.8,Na₂O/Si O₂=0.14,Si O₂/Al₂O₃=150)exhibited high crystallinity of the product.The framework Si O₂/Al₂O₃molar ratio reached 96.4.The specific surface area and micropore volume were 409 m²/g and 0.14 cm³/g.It proved the possibility of synthesizing high-silica ZSM-5 zeolite without template.The hierarchical ZSM-5 was synthesized in C₂H₅OH-H₂O hydrothermal system by using CTAB(CTAB/Al₂O₃=3.4)as sole template at a low crystallization temperature(130°C).Ethanol molecules in synthesis system were beneficial to speed up crystallization.The existence of CTAB resulted in highly crystalline and pure phase hierarchical ZSM-5 microspheres assembled by nanocrystals.The traces of crystallization evolution by XRD,FT-IR,N₂adsorption/desorption,SEM/TEM,²⁹Si MAS NMR,²⁷Al MAS NMR spectra and TGA revealed that the hierarchical ZSM-5 microspheres were formed through the transformation of mesostructured amorphous aluminosilicates,in which the initial surfactant micelles were gradually excluded and thus acted as a molecule structure-directing agent to direct the crystallization of MFI nanozeolite.At the same time,the CTAB may disrupt nanocrystals further growth and aggregate them into hierarchical microsphere through CTA⁺.Pyridine(Py)and pivalonitrile(Pn)adsorbed FT-IR spectra showed that the hierarchical ZSM-5 microspheres possessed large number of accessible acid sites due to the introduction of mesopores into zeolite.Therefore,it displayed much higher catalytic activities for both large molecules 1,3,5-trimethylbenzene(TMB)transformation and 1,3,5-triisopropylbenzene(TIPB)cracking reactions than conventional MFI zeolite,and the high selectivity to isomerization products(1,2,3-TMB and 1,2,4-TMB)and benzene product from over-cracking.It proved the possibility of synthesizing hierarchical ZSM-5 by using CTAB as a single template in CTAB-C₂H₅OH-H₂O system.Ordered mesoporous aluminosilicate assembled by ZSM-5 microcrystals was successfully synthesized in CTAB-C₂H₅OH-H₂O system at a lower crystallization temperature(130°C).The crystallinity and acidity of zeolite ZSM-5 was adjustable by controlling crystallization time(6?8 d)and the amount of CTAB(CTAB/Al₂O₃=10.1?16.8).The existence of ZSM-5microcrystals increased the hydrothermal stability of the amorphous ordered mesoporous material.Therefore,the ordered mesoporous aluminosilicate assembled by ZSM-5 microcrystals displayed a significantly higher catalytic activity for the cracking of macromolecular 1,3,5-triisopropylbenzene(TIPB)than traditional MFI zeolites.The sample synthesized with prolonged crystallization time was highly selective for the deep cracking product(benzene).In the heavy cycle oil(HCO)cracking reaction,ordered mesoporous aluminosilicate assembled by ZSM-5 microcrystals pre-cracked distillate or heavy diesel into light diesel,realizing the pre-cracking of heavy oil.It showed that adjusting the amount of CTAB and the crystallization time balanced the mesoporous phase and the zeolite phase to avoid phase separation in CTAB-C₂H₅OH-H₂O system.The nanosheet-assembled flower-like hierarchical ZSM-5 was successfully synthesized by increasing the amount of CTAB(CTAB/Al₂O₃=16.8)in the system by using single surfactant CTAB.The ratio of C₂H₅OH/Si O₂,Na₂O/Si O₂,Si/Al and silicon source on the influence of hierarchical ZSM-5 was systematically investigated.Studies on the probe reaction for the conversion of macromolecules 1,3,5-TMB and the cracking reaction of 1,3,5-TIPB showed that flower-like hierarchical ZSM-5 owned more accessible acid sites.The flower-like hierarchical ZSM-5 possessed larger external specific surface area(280 m²/g).In the cracking reaction of heavy cycle oil(HCO),flower-like hierarchical ZSM-5 assembled with nanosheets realized the precracking of heavy oil components.The distillate and heavy diesel were pre-cracked into light diesel components.The flower-like hierarchical ZSM-5 displayed higher selectivity for C₃⁼,C₄⁼,C₅⁼in the gas-phase product distribution.