Controllable Synthesis And Functionalization Of Hierarchical MFI Zeolites Nanocrystals

Zeolites are crystalline frameworks with ordered networks and precisely uniform micropores,which favour numerous applications in catalysis,separation,adsorption,ion exchange and so on.MFI type zeolites with tunable acidity,unique shape-selectivity and high thermal as well as hydrothermal stability were among the most widely used zeolites.However,the small pore size unfortunately hinders the diffusion rate of reactants and limited their performance in adsorption and catalysis.Given this situation,much effort has been devoted to the hierarchical architecture of zeolites with controlled size and porosity in nanoscale to overcome the diffusion limitations of bulk phase.Usually,post-synthetic modifications or templating strategies were applied to generate mesopores in zeolites.However,the multistep procedure and high cost template limited their practical application for large-scale production.Thus,nanostructured zeolites with high diffusion ability as well as good shape-selectivity are still highly desirable.Herein,Kirkendall effect was utilized to synthesize hierarchical MFI zeolite nanocrystals and metal-zeolites composites without the involvement of additional templates.The results are summarized as follows:(1)Hierarchical silicalite-1 zeolite nanocrystals were facially synthesized by using Kirkendall growth without the involvement of additional templates.The Kirkendall growth was carried out during the crystallization procedure with the assitancance of limited amount of water.The mesopore size as well as the particle size can be easily controlled by simply adjusting the usage of structure-directing agent and water.The Kirkendall growth method involving less structure-directing agent is thus suitable for large-scale synthesis of hierarchical MFI zeolites with very high yields but low cost for practical applications.Moreover,the introduction of mesopore increased the dissusion rate effectively and the as synthesized hierarchical silicalite-1 zeolite showed excellent adsorption efficiency than that of bulk phase.(2)The Kirkendall growth method was successfully extended to synthesize hierarchical ZSM-5 nanocrystals with tunable Si/Al ratios as well as mesopores.The introduction of mesopores as well as nanosize increased the diffusion efficiency of product and changed the property of acid sites,thus made hierarchical ZSM-5 showed excellent performance in methanol to propylene(MTP)reaction than that of bulk phase.The Si/Al ratios of hierarchical ZSM-5 were optimized as MTP catalysts.As a result,hierarchical ZSM-5 with high Al content offered a highest stability while the propylene selectivity was comparable than that of high silica ZSM-5.The effect of pore structure of hierarchical ZSM-5 on their MTP performance was also investigated,the result showed that large pores preferred low-carbon product and higher pore volume benifited to stability.Moreover,centrifuging or filtration procedure can be omitted due to the qusai-solid synthesis system which made much less or zero effluent.The simplified process can also reduce the loss of material effectively and give high yield nearly 100%,thus is suitable for industrial production.(3)A serious of hierarchical silicalite-1 nanocrystals including TS-1,Fe-silicalite-1,B-silicalite-1 were synthesized based on the method of hierarchical ZSM-5 mentioned above.The catalytic performance of TS-1 and Fe-silicalite-1 were further studied.The results showed that they offered better catalytic performance than that of bulk phase in epoxidation of olefin and hydroxylation of phenol,especially in the reaction containing large-size molecules.The hierarchical structure as well as the small particle size helped to exposed more active sites which can not only increase catalytic efficiency but be easier for substance with large size to close,thus expand the scope of catalysis.Moreover,small particle size TS-1 was obtained by using inorganic silicon with assistance of this general strategy,and thus the cost of synthesis was decreased effectively which is benefit for industry production.(4)Pd nanoparticles were successfully encapsulated inside hierarchical silicalite-1 nanocrystals(Pd@mnc-S1)via a one-pot method.The as-synthesized Pd@mnc-S1 with excellent stability functioned as an active and reusable heterogeneous catalyst.The unique porosity and nanostructure of silicalite-1 crystals endowed the Pd@mnc-S1 material general shape-selecvity for various catalytic reactions,including selective hydrogenation,oxidation and carbon-carbon coupling reactions.