The Structure Design Of Pt@ZIF-8/SiO₂ Composite With 3-Dimensional Ordered Macropores And Its Size-Selective Catalytic Properties

Three-dimensional ordered macroporous materials have higher mass transfer efficiency due to their unique pore structures,therefore showing great potential in macromolecular transportation and related fields.Metal-organic frameworks?MOFs?are a new kind of porous materials that are bonded by metal ions and organic ligands through coordination bonds.In recent years,MOFs have attracted increasing interest in a variety of fields due to their high specific surface area,high porosity,devisable structure and modifiability.However,most of the MOFs reported have only micropores which severely limit the mass transfer efficiency and thus the application of the materials in some special fields with large molecules involved.Giving these premises thesis,a self-template method was designed here for in-situ encapsulation of Pt nanoparticles in MOF composites with three-dimensional ordered macroporous structure,which were futher investigated as catalysts for the hydrogenation of olefins.In this thesis,polystyrene?PS?colloidal crystal was used as template,and3DOM-Pt/ZnO/SiO₂?3DOM is a shorthand for three-dimensional ordered macropores?was prepared by an impregnation method based on an ethylene glycol solvent system and a temperature programmed calcination method.The 3DOM-Pt@ZIF-8/SiO₂ material was successfully synthesized by the replica coordination nanocasting technology.The structure and composition of the materials were analyzed by X-ray powder diffraction,transmission electron microscopy,scanning electron microscopy,N₂ adsorption and desorption analysis.The results showed that the 3DOM-Pt@ZIF-8/SiO₂ had a highly ordered three-dimensional macroporous structure,and a ZIF-8 nanoshell was epitaxially grown on the surface of the skeleton.Moreover,all the Pt elements in the framework were reduced to metallic Pt nanoparticles.The materials prepared with different coordination times were further characterized.The structural changes of the materials during the preparation process and the growth of ZIF-8 shells on the three-dimensional skeleton were analyzed.It was found that the solvent had a crucial influence on the epitaxial crystallization of ZIF-8 by exploring the solvent components.A dense ZIF-8 nanoshell could form on the three-dimensional skeleton surface only by using a mixture of N,N-dimethylformamide?DMF?and water with a volume ratio of 3:1 as the solvent.In addition,the composition of zinc and silicon was regulated to prevent clogging of the macroporous structure and enable the uniform growth of ZIF-8.It was found that it was suitable to control the molar ratio of zinc to silicon in the precursor at1:1.In the olefin hydrogenation reaction,the 3DOM-Pt@ZIF-8/SiO₂ showed good molecular size selectivity and recycling performence.