Construction And Catalytic Study Of Acidic Metal-Organic Frameworks With Multiple Active Sites

Solid acids with muti-active sites have been developed as important catalysts for modern industry.The types of active sites,chanel structure and proximity among active sites are the key factors that affect the catalytic activity and selectivity.The design and synthesis of solid acids with muti-active sites are important both in academia and inductry.The structure features of metal-organic frameworks（MOFs）allow researchers to introduce acidic sites and tune the active centers simultaneously,by decoration of ligands,regulation of metal coordination vacancies,post-synthetic modification,and introduction of polyoxometalates（POMs）.Those active sites not only have high density with fixed distances between each other,but also are endowed with local microenvironments which can be confirmed by single crystal diffraction analyses.The refined structure and tailorablility of MOFs are very beneficial for the design and synthesis of porous solid acid catalysts with multiple active sites.By virture of the spatial and electronic confinements of MOFs,synergies between acid sites and other multi-active sites are expected to optimize existing catalytic systems in terms of product selectivity and catalytic activity or even to develop new reaction systems,which are potentially applicable in CO₂ capture and conversion and green catalysis,helping to address the issues of global energy and environment.The main contents of this thesis are as listed as below:（1）A classical thermal and chemical stable MOF material,MIL-101（Cr）,was used as a template to synthesize MOF with both Br（?）nsted acid and Lewis acid sites by grafting sulfuric acid groups onto organic ligands through post-synthetic modification.The catalytic conversion characteristics of polar small molecules at the two kinds of active sites were studied by continuous gas-solid ethanol reaction.It was found that the type of adsorption site determined the reaction pathways of dehydrogenation or dehydration,and the concentration of adsorbed substrates determined the intra-or intermolecular reaction routes.The reaction pathways were controlled by means of material design and reaction kinetics,and the product selectivity of either 100%ethylene or 99%diethyl ether was finally achieved,respectively.In this section,the preparation,characterization,host-guest chemistry,adsorption behavior,and catalytic performance of acidic MOFs were investigated,which provided a perspective for the design of MOFs with high stability,multiple active sites to facilitate the study of structure-activity relationship.（2）Then,considering the chemical stability and modifiability of various POM precursors,a well-defined POM,PW₉O₃₄^9-,was utilized together with 4,4’-BPY,BPDO,and Co²⁺metal cation in the assembly of Co POMOF-500,a MOF with ultra-high thermal stability and synergistic action of multiple active sites.The parallel aromatic rings with a distance of 7.2（?）were elaborately assembled into the framework as CO₂ capture sites,achieving highly selective carbon capture indexes when dealing with CO₂/N₂（CO₂:N₂=7932:1）and CO₂/CH₄（CO₂:CH₄=13207:1）mixtures and also maintaining adsorption and separation performances under humid conditions.The acid,base and redox sites of the catalyst were well characterized by chemical adsorption and other experiments.The multi-functional acidic MOF was used as a catalyst with spatial confinement effect in heterogeneous tandem reaction system,efficiently conducting the epoxidation of olefin and CO₂ cycloaddition to generate cyclic carbonates.（3）Based upon the contents of previous chapters,it was clear that Keggin-typed POM featuring good thermodynamic stability and high structural symmetry made them the competent candidate of MOFs’backbone and simultaneously maintain the porosity of frameworks.To alleviate diffusion-limited photoinduced electron transfer（PET）in solution,a two-module construction method was developed by coupling a triphenylamine-derived dye and a Keggin POM-typed electron relay onto the MOF backbone,which gave rise to photoinduced long-lived charge-separation pairs with enough reductive/oxidative potential to pump multiple electrons unidirectionally from external electron donors to acceptors,thus furnishing photocatalytic radical couplings to afford value-addedα-amino C–H arylation products.