Fabrication Of Mof@oxide Core-shell Nanostructures And Their Catalytic Performance

Because of their large surface area,controllable pore size,and pore volume as well as functionable pore surface,metal-organic frameworks（MOFs）have been widely used in various fields,such as adsorption and separation,catalysis,proton conduction,drug delivery and so on.As MOFs are constructed on the basis of coordination bonds between metal ions and organic ligands,this leads to their poor chemical and thermal stability.Therefore,fabrication of high stable MOF materials is of great value for their further applications.On the other hand,with the increasing demand for new multifunctional materials,single MOF component can no longer meet diverse application needs.To this end,encapsulating MOF nanoparticles in a stable oxide shell to prepare MOF@oxide core-shell nanostructures can not only improve the stability of MOF components,but also bring new functions by constructing multifunctional MOF composites.Moreover,derivatives such as metals,metal oxides,carbon materials and so on,prepared by calcining MOF precursors show high dispersibility,homogeneous particle size,and large specific surface area.Inspired by this,MOF@oxides can also be calcined to prepare metal@oxide or metal oxide@oxide core-shell derivatives.However,research on the design and synthesis of MOF@oxides is still in its early stage,more efforts should be devoted into this field.In this doctoral thesis,we focused on the design and synthesis of multifunctional MOF@oxide core-shell nanostructures and their derivatives.MOF@mesoporouSiO₂yolk-shell nanoreactors（MOF@mSiO₂-YS）with protective and permeable mesoporous SiO₂ shell were prepared via a mesoporous silica coating followed by selective water-etching strategy.MOF@mSiO₂-YS showed improved stability for catalytic cycloaddition of CO₂ and styrene oxide（SO）.By coating MIL-101 with functional TiO₂ shell,MOF@TiO₂ multifunctional core-shell nanostructures were obtained,which can adsorb and photodegrade organic dyes,as well as exhibit high stabilities.Co-MOF@TiO₂ derivatives-Co₃O₄@TiO₂ hollow structures were also prepared by calcining the MOF@TiO₂ precursors in the air.With Co₃O₄-TiO₂ heterojunctions generated during calcination,Co₃O₄@TiO₂ hollow heterojunctions showed excellent catalytic activity and stability for photocatalytic CO₂ reduction.The main research results of this dissertation are as follows:1.MOF@mesoporous SiO₂ yolk-shell nanoreactors were designed and synthesized to improve the stability of MOF crystals via a mesoporous silica coating followed by selective water-etching strategy.Different from conventional alkali-or acid-etching methods,water etching of MOF surface presents a green and cost-effective way to form yolk-shell structures.The yolk-shell nanoreactors exhibited higher catalytic stability than bare MOF crystals in CO₂ cycloaddition with product yield remaining unchanged upon three cycles,owing to their permeable mesoporous SiO₂ shells,exposed active sites on MOF surfaces,and protective shells.The design concept and synthetic strategy via selective water etching may be used to construct other highly stable MOF-based nanocatalysts,broadening their applications in diverse catalytic reactions.2.MOF@TiO₂ multifunctional core-shell nanostructures were designed and synthesized via an amorphous TiO₂ coating followed by water-assisted crystallization strategy.In order to prevent MOF structures from collapse,a water-assisted crystallization method was used to crystallize the amorphous TiO₂ shell in a hydroalcoholic solution at relatively low temperature.The obtained MOF@TiO₂ coreshell nanostructures were multifunctional,in which MOF cores with high surface area and mesopores can adsorb dye molecules,anatase TiO₂ shells can catalyze the photodegradation of adsorbates.As a result,MOF@TiO₂ multifunctional core-shell nanostructures exhibited excellent adsorption and photodegradation performance for removal of congo red,as well as high recycling stabilities.3.Co₃O₄@TiO₂ hollow heterojunctions were designed and synthesized by calcining MOF@TiO₂ precursor in the air.As Co source and sacrificial template,ZIF-67 dodecahedrons were encapsulated in amorphous TiO₂ to prepare ZIF-67@TiO₂ coreshell nanostructures.After calcination in the air,Co₃O₄@TiO₂ hollow heterojunctions were obtained via simultaneous ZIF-67 decomposition and TiO₂ crystallization.The hollow heterojunctions exhibited high activity for photocatalytic reduction of CO₂ compared to their component parts and componential mixtures.This excellent performance was owing to their unique structures: Co₃O₄-TiO₂ p-n junctions generated during calcination can facilitate the separation of photogenerated electrons and holes;the hollow structures can not only enhance the absorption of dye molecules because of high surface area,but also improve light utilization due to the light scattering and reflection effects.