Syntheses And Catalytic Performance Of Hierarchical MFI Zeolite And Its Composites

Zeolites with regular micropores are important industrial heterogeneous catalysts because of their excellent hydrothermal stability,suitable acidity,and unique shapeselective catalytic ability.It is a great challenge to improve the catalytic efficiency of zeolites from both academic and industrial points of view.The synthesis of hierarchical zeolites is a milestone in the development of zeolite-based catalysts.In the hierarchical zeolites,the micropores play a role in shape-selective catalysis or/and confinement,the mesopores improve the mass transfer capacity and the accessibility of the active center,and the macropores allow the unimpeded diffusion of various molecules.Therefore,the hierarchical structures with effective improvement of the catalytic activity have received extensive attention.However,the size and distribution of mesopores and macropores are difficult to precisely control.It is important to precisely construct hierarchical zeolites with high mass transfer capacity based on the anisotropy of the crystals.In addition,it is difficult for a single-component acidic zeolite catalyst to meet the complex industrial production needs.Therefore,it is necessary to develop new zeolite composites with multiple active sites and reasonable space configuration.The current construction of hierarchical zeolite composites is mainly based on the core-shell structure.Although mesoporous shells can reduce diffusion limitations,they may lead to unavoidable shielding of the inside core particles,partially inhibiting the functionality of the cores.Thus,zeolite composites with regioselective shells can fully take advantage of the functions of both cores and shells.In this thesis,taking the most widely used MFI zeolite as a model,the synthesis methodology and catalytic performance of hierarchical MFI zeolite and its composites are studied.By investigating the feature of MFI zeolite crystals and regulating the crystal growth kinetics,ZSM-5 nanocages with open structure,multil-shelled hollow silicalite-1 zeolites and zeolite@meso-carbon composites have been successfully prepared.These materials exhibit excellent catalytic performance in the methanol-tohydrocarbons reaction,the borane ammonia hydrolysis reaction,and the shapeselective hydrogenation reaction of nitro-compounds under the Pickering emulsion system.These research works are devoted to developing the synthesis methodologies of hierachical zeolites and composites with controllable pore size and distribution,which afford important guidance for the preparation of zeolites or other inorganic materials and their composite materials.Main contents of this thesis are as follows:1.Rational construction of single-crystalline zeolites with precisely controllable open nanostructures and tunable framework compositions is of significant importance for catalytic applications.Herein,we report an anisotropic-kinetics transformation strategy to prepare single-crystalline aluminosilicate MFI zeolites(ZSM-5)with highly open nanoarchitectures and hierarchical porosities.The methodology relies on the cooperative effect of in situ etching and recrystallization under hydrothermal conditions.The strategy enables a controllable preparation of ZSM-5 nanostructures with diverse open geometries by tuning the relative rate difference between etching and recrystallization processes.Meanwhile,it can also be extended to synthesize other heteroatom-substituted MFI zeolite nanocages.The ZSM-5 nanocages with singlecrystalline nature,highly open nanoarchitectures,and hierarchical porosities exhibit remarkably enhanced catalytic lifetime and reduced coking rate in the methanol-tohydrocarbons(MTH)reaction.2.Multi-shelled nanomaterials are potential materials with complex structures.We prepared multil-shelled hollow silicalite-1 zeolites through multi-step growth and selective etching strategy.The hollow multi-shelled MFI zeolites have large specific surface areas,which is beneficial to the dispersion of Rh nanoclusters.Due to the effectively improvement of the hydrophilicity and the mass transfer efficiency,the Rh supported triple-shelled hollow catalysts exhibit excellent catalytic activity and cycle stability in borane ammonia hydrolysis reaction.3.Precise control of porous nanocomposites with precise encoding growth is a long-standing challenge.Herein,we report a controllable interfacial energy barrier strategy to direct the regioselective surface encoding of mesoporous polydopamine on anisotropic silicalite-1 nanocrystals.This strategy leverages the energy barriers of sitespecific growth and homogeneous nucleation energy of the dopamine to obtain controllable accessibilities to the surfaces of silicalite-1 crystals.Moreover,not only controlling where growth occurs but also adjusting pore size and structures of mesoporous polydopamine are achieved.After loading Pt nanoparticles and carbonization,the obtained Pt-loaded zeolite/mesoporous carbon composites show excellent catalytic efficiency and shape-selective ability in biphasic hydrogenation reaction.