Design,Synthesis And Catalytic Applications Of Hierarchically Porous ZSM-5 Materials

ZSM-5,as a famous catalyst with special porous structure of terrific selectivity and catalytic properties,was widely used in petrochemical and fine chemical industry.But the pore dimention of conventional ZSM-5 zeolite was only 0.55nm,which also gave greatly restrictions for its applications while bulky molecules involved in the catalytic process.The small pore dimention doesn't limit the transmission of object molecules within the zeolitic framework,thus causing too much waste of active sites,but easy generating coke deposition,leading to the zeolite inactivation.Therefore,it is necessary to introduce meso-porosity into zeolite skeleton and synthesize some hierarchically meso-microporous zeolite composites or meso-microporous zeolitic structure.Aiming at the main defects of industrial microporous zeolite,we have successfully synthesized hierarchical zeolite/porous metal oxide composite materials and hierarchically meso-microporous zeolite material.We systematically studied the diffusion properties and its'catalytic properties in corresponding reactions.Compared with conventional ZSM-5,as-prepared materials exhibited excellent performance.The following two works will give more details.In the first part,we have successfully synthesized hierarchically meso-microporous ZSM-5/Al2O3 composites,using aluminum tri-sec-butoxide?TBOA?and ZSM-5 powder as the raw materials in butanol solution system.The obtained porous Al2O3 is then hydrolyzed and condensed with TBOA throughout the synthesis,thus generating three-dimentional nanosheet-assembly mesoporous Al₂O₃material.In fact,the introduction of porous Al₂O₃ offers a wide range of mesoporosity for the whole materials,and lot's of Lewis acid sites to a certain extent.In the test of methanol to olefins?MTO?reaction,as-prepared ZSM-5/Al2O3composite presented better stability and selectivities of ethylene and propylene.Deeply,we studied the influence of distances of two components on the whole MTO reaction,and found the closer distance is accompanied by higher selectivities of ethylene and propylene.While two components have farther distance,selectivities of ethylene and propylene dropped rapidly,together with the increase amount of methane.We analyzed the formation mechanism of these influences carefully and built the relationship between the diffusivity and performance.This work has provided a fresh idea for the study of bifunctional catalysts,specially the distance effect brought by different components,and effectively built up the relationship between structure of diffusion and catalytic properties.In the second part,we combined the advantages of nano-zeolite and hierarchical system,and successfully synthesized hierarchically meso-microporous zeolite single crystals.Amorphous silica nanoparticles,as the only silicon source,were coated by a layer of porous carbon,thus forming SiO₂@porous C composite.Then SiO₂@porous C powder was evenly mixed with zeolite precursor solution,followed by aging and crystallization process by stream assisted conversion?SAC?strategy.After crystallization and calcinations,amorphous SiO₂ all crystallized into crystalline zeolite materials and large number of mesoporosity generated after carbon layer removed.Most noteworthy is the original shape of nanosized particles still remained.As-prepared meso-microporous zeolite was composed of numerous closely packed and connected nano-zeolite of being a whole.And heteroatoms?e.g.Al,Ti et al?were successfully introduced into the zeolitic skeleton.In the adsorption of m-xylene test,as-prepared meso-microporous ZSM-5 exhibited superior properties compared with nano-ZSM-5 and conventional microporous ZSM-5.In the cracking of cumene,meso-microporous ZSM-5 also showed a higher conversion and terrific stability.This research work effectively solved the transmission problems of microporous zeolite,and provided a new thought for the application of carbon materials in synthetic field of zeolite.