Preparation And Catalytic Performance Of Hierarchical ZSM-5 Zeolites

Hierarchical ZSM-5 aggregates were fabricated using a two-stage steam-assisted crystallization process without mesoporous template.The nonuse of sodion avoid the loss of acidity caused by the incompleteness of ammonia exchange.The effects of crystallization temperature on the synthesis,morphology and properties of ZSM-5 zeolite were investigated in details.The porous structural diameter,acid properties and catalytic performance of the composites were characterized by XRD,SEM,NMR,NH₃-TPD,FT-IR and nitrogen adsorption-desorption.A possible formation mechanism was proposed based on the investigation of the crystallization process.The obtained hierarchical ZSM-5 aggregates were well crystallized,possessing large external surface area,mesopore volume and regular mesoporous size distributions.And they were proved to be more efficient in 1,3,5-triisopropylbenzene and heavy diesel oil cracking due to improved accessibility to reactant macromolecules.Hierarchical ZSM-5 molecular sieve was prepared by a crystallization method using silicalite-1 seeds without mesoporous template.Hierarchical ZSM-5 was synthesized during the merely crystallization time of 2 hours.The effects of different templating agents,seed addition amounts and crystallization times on the pore structure,acidity and catalytic performance of the synthesized zeolites were investigated.The pore structure and acid properties of ZSM-5zeolite were characterized by XRD,SEM,NMR,NH₃-TPD,FT-IR and nitrogen adsorption-desorption.The effect of different synthesis conditions on the structure and porous properties of molecular sieve was studied by using H-ZSM-5 molecular sieve loaded Pd O for the hydrogenation reaction of guaiacol.The catalytic performance of H-ZSM-5 for the cracking reaction for macromolecular was studied using heavy diesel oil as a probe molecule.The results show that the catalytic performance of H-ZSM-5 in heavy diesel oil cracking is closely related to its porous structure and acid properties.Correspondingly,the catalytic performance of Pd-ZSM-5 for the hydrogenation of guaiacol is related to its porous structure.In summary,with the short the crystallization time,it is feasible to prepare hierarchical ZSM-5 zeolite crystals by adding seeds without the use of mesoporous template.Hierarchical ZSM-5 molecular sieve was synthesized by mixing different proportions of phosphorus-containing template TBPOH and TPAOH.Phosphorus entering into ZSM-5 zeolite spoils part of the acidity sites.The experiment turns a positive significance to the research of the regulation of acid properties by using phosphorus.The synergistic effect of TPAOH and TBPOH was investigated.It is expected that phosphorus-containing hierarchical ZSM-5with fairly high degree of crystallinity can be obtained.The physico-chemical properties were characterized by XRD,BET,SEM,TEM,NH₃-TPD and MAS NMR.The results show that the morphology of ZSM-5 is regular and it shows higher specific surface area and pore volume when the molar ratio of TBPOH and TPAOH is 1:1 with TEOS as the silicon source.The average pore size distributes at 4¹0 nm and 50⁷0 nm.Hierarchical ZSM-5 zeolite was constituted by the accumulation of small grains and showed rich mesoporous structure.And compared with conventional ZSM-5 molecular sieve,hierarchical ZSM-5 shows greater weak acidity and greater total acidity.In condition of same conversion rate in the cracking reaction of 1,3,5-triisopropylbenzene,the microporous ZSM-5 zeolite shows better selectivity for benzene and diisopropylbenzene,while the hierarchical ZSM-5 with mesopores shows more excellent selectivity for propylene and cumene.