Synthesis And Catalytic Performance Of Hierarchical ZSM-5 Zeolite In Toluene Alkylation With Methanol

p-Xylene is an important intermediate species for the production of polyesters.The demand for p-xylene has been increasing enormously in recent years.The alkylation of toluene uses the less-expensive reactants of toluene and methanol as feedstocks and p-xylene can be obtained in one step.Toluene methylation is an economical route with the advantages of high utilization rate and less byproducts.ZSM-5 zeolite exhibits great properties,such as excellent hydrothermal stability,shape-selectivity and catalytic activity.However,ZSM-5 zeolite suffers the problems of low activity and rapid deactivation in the alkylation of toluene with methanol.Hierarchical ZSM-5 zeolite with micropores and mesopores/macropores solves the problems of the diffusion of bulky molecules,which is benificial for improving the activity and the lifetime of catalysts.In the present work,hierarchical ZSM-5 zeolite was prepared by a hydrothermal method and a vapor-phase transport（VPT）method.The influence of synthesis paramaters on the properties of the final ZSM-5 products and the catalytic performance of the synthesized ZSM-5 zeolites in toluene alkylation with methanol were systematically investigated.Firstly,commercial H-ZSM-5 zeolites were treated with alkali and used as the seeds.Hierarchical ZSM-5 nano-sized aggregates were synthesized through a one-step crystallization process.The influences of crystallization temperature and crystallization time on the textural properties of synthetic samples were investigated,and the possible synthesis mechanism was proposed.The results showed that ZSM-5zeolites with high crystallization could only be synthesized by using the commercial H-ZSM-5 zeolites with the similar SiO₂/Al₂O₃ ratio to that of the synthetic samples as seeds.Cetyltrimethylammonium bromide（CTAB）may promote the formation of ZSM-5 primary crystals and inhibit the intergrowth of the primary crystals.The primary crystallites were assembled together to generate hierarchical ZSM-5aggregates.Secondly,CTAB was used as mesoporogen to synthesize hierarchical ZSM-5aggregates.The effects of CTAB amount on the textural properties and catalytic performances of the synthetic samples synthesized by one-step crystallization were studied.The results showed that a trace amount of CTAB significantly promoted the crystallization of ZSM-5 zeolite,with the morphology changing from hexagonal-shape crystals to uniform spherical aggregates.When CTAB/SiO₂ ratio was 0.03,the ZSM-5 zeolite with the smallest primary particles of 50–80 nm exhibited large mesopore volume and micropore surface area.The toluene conversion and lifetime of the hierarchical nano-sized ZSM-5 aggregate were significantly enhanced with high p-xylene selectivity in toluene alkylation with methanol.Finally,Silicalite-1（S-1）was used as seeds and hierarchical binder-free ZSM-5zeolites were prepared by a VPT method.The effects of aluminum amount on the textural properties and catalytic performance on toluene alkylation with methanol were studied.The results showed that the synthesized samples with different amount of aluminum added were all hierarchical ZSM-5 zeolites with high crystallinity.The morphologies and microporous/mesoporous properties had no appreciable difference between different samples.The acid amounts of ZSM-5 zeolites gradually increased with increasing the amount of aluminum.When SiO₂/Al₂O₃ ratio was 125,ZSM-5zeolite showed suitable acid amounts and exhibited high p-xylene selectivity,high toluene conversion and long lifetime.