Study On The Relationship Between Pore Structure And Morphology Of Zeolites And Catalytic Performance For Olefin Cracking Reaction

Propene is one of basic chemical materials in petrochemical industry,which exhibits a huge market demand.In the past,propene was mainly obtained as the by-product in ethyene by the fluid catalytic cracking and stream cracking process,whose yield mainly depends on the materials and the operating condition.Because of the shift to shale gas for producing ethene,the yield of propene produced using the thermal and catalytic cracking process is significantly decreased in recent years,leading that the propene production is unable to satisfy the growing need of the market.Therefore,on-purpose routes to propene are being increasingly implemented,such as propane dehrogenation reaction,methonal to propene reaction,olefin metathesis reaction and olefin catalytic cracking reaction.However,propane dehydrogenation reaction and methonal to propene reaction have a high investment and less materials,though potentially very selective to propene.olefin metathesis process requires various procedures and the cost of valuable ethene.By contrast,olefin catalytic cracking process has the high research value because of lower inversion requirement and raw materials availibility.In this paper,silicoaluminophosphate zeolites and aluminosilicate zeolites were prepared,respectively.Characterizations,evaluation tests and theoretical calculation are performed to elucidate the influence of pore structure and morphology of zeolites on the catalytic performance of the hexene cracking reaction.（1）SAPO zeolites with three different pore structures and morphologies,namely SAPO-5,SAPO-41/5 and SAPO-41zeolites,were synthesized for the hexene cracking reaction.The experimental results show that the three SAPO zeolites exhibited similar physicochemical properties but different catalytic performance.In detail,lower propene selectivity（69.6%）and poor catalytic stability were obversed over micro-sized spherical SAPO-5 zeolite with the large pore diameter.While nanosheet SAPO-41 zeolite with the smallest pore diameter showed the highest propene selectivity（88.5%）and catalytic stability,which is ascribed to the inhibition of the bimolecular cracking reaction and secondary reactions of cracking products by smaller channels and shorter diffusional length.The propene selectivity and catalytic stability for the SAPO-41/5 intergrowth zeolite were in-between the above two catalysts.Those results indicate that the pore structure and morphology of SAPO zeolites can significantly influence on the cracking routes and side reactions for the hexene cracking process,resulting in the difference in the selectivity of propene and the stability of SAPO zeolites.（2）SAPO-41 zeolite was applied to upgrade the Fischer-Tropsch products.The results evidence that H₂ facilitates the hydrogenation process of light alkenes to form the corresponding alkanes under high pressure,but H₂ has little influene on the catalytic performance of the hexene cracking reaction under atmospheric pressure.Especially,the molecular dynamic（MD）simulation was performed to investigate the effect of the chain length for olefins on the diffusional behavior in the channel of SAPO-41 zeolite.The synergistic effect between diffusion behavior and cracking reaction in the channel of SAPO-41 zeolite contributes to the superior catalytic performance for medium chain length olefins（C₆-C₈）cracking reaction.（3）ZSM-5 zeolites with three different morphologies and sizes,including large single crystal,nanosheet,and nanoparticle of ZSM-5 zeolites,were synthesized for the hexene cracking reaction.The experimental results show that the three ZSM-5 zeolites exhibited similar physicochemical properties but different catalytic performance.In detail,compared to the large crystal ZSM-5 zeolite,nanosheet ZSM-5 zeolite has a shorter diffusional length,which attributes to a higher propene selectivity（63.5%）and better catalytic stability.Moreover,compared to the nanosheet ZSM-5 zeolite,nanoparticle ZSM-5 zeolite consists of more sinusoidal channels,which can further enhance the propene selectivity（66.6%）by facilitating the diffusion of light alkenes and suppressing the formation and diffusion of large aromatic molecules in the sinusoidal channels.those results indicate that the morphology and size of ZSM-5 zeolite can significantly influence on the channal composition and the diffusional length,which can have a grate effect on the selectivity of propene and the stability of ZSM-5 zeolite for the hexene cracking process.