Metal Ligand-driven Construction Of Ionic Liquidbased Heterogeneous Catalysts And Performance Of CO₂ Cycloaddition Reaction Under Mild Conditions

Carbon dioxide（CO₂）,one of the greenhouse gases,has caused severe challenges to the survival and development of human beings due to the sharp increase of CO₂ concentration.Low energy consumption and high value resource utilization of CO₂ has become one of the important ways to reduce carbon emissions in the fossil energy field.Among them,the CO₂/epoxides cycloaddition reaction for the synthesis of cyclic carbonates have 100%atom economy and the products are widely used,which have become a CO₂ conversion route with great application potential.However,due to the high activation energy barrier of the reaction,most of the current catalysts still suffer from harsh reaction conditions or require the addition of cocatalyst.Construction of Lewis acid-base sites efficient and synergistic heterogeneous catalysts,which utilize the dual nucleophilic-electrophilic sites to attack the ring opening of epoxides and activate CO₂ molecules simultaneously to promote the reaction under mild conditions such as normal temperature and pressure,has become the key to promote the development and application of this reaction.Ionic liquids have unique designability and Lewis basic counter ions,and effective compounding of ionic liquids with metal organic framework materials（MOFs）compensate for the high viscosity and high mass transfer resistance of homogeneous ionic liquids,and couple the high specific surface area and inherent Lewis acidic metal sites of MOFs materials.It is expected to construct heterogeneous catalysts with efficient synergy of Lewis acid-base sites.In view of the shortcomings of current composite methods（impregnation,covalent grafting,etc.）such as easy loss of ionic liquids,complex synthesis process,low site utilization and clogging of pore channels.Exploring a green and controllable universal catalyst construction method,directional introduction of multiple active sites such as Lewis acid/base under mild conditions,and active regulation of site strength,content and spatial arrangement at the molecular level,it has become a key scientific issue in construction of highly active heterogeneous catalysts.Herein,carboxylic ionic liquids,as modulator and functional agent,were one-pot introduced into MOF framework structure as monodentate ligands by in-situ assembly method.The method does not cause pore blockage and can simultaneously induce a large number of unsaturated metal sites to synthesize a series of heterogeneous catalysts with Lewis acid-base sites(Cr³⁺,Cl^-,imidazolium nitrogen),open pore channels and high sites utilization,which can efficiently catalyze the CO₂/epoxide cycloaddition reaction under mild conditions such as normal temperature/pressure.Based on,the mechanism of the influence of ionic liquid acidity/configuration on the structure and properties of the composites was explored.The main research contents are as follows:（1）Carboxyl functionalized imidazolium-based ionic liquids with electrophilic cations and nucleophilic halogen anions and stable coordination with metal ions.In this paper,four carboxyl ionic liquids with different acidity and rigid/flexible were selected to explore their performance in catalyzing CO₂/epichlorohydrin to form cyclic carbonates under mild conditions（40°C,0.1 MPa,24 h）.The results show that as the acidity of ionic liquids decreased,their catalytic performance gradually increase.This is because the ionic liquids can produce intramolecular or intermolecular hydrogen bonds with Cl^-,which greatly reduces the flexibility of Cl^-,thus affecting the nucleophilic ability of Cl^-.This discovery has important guiding significance for the construction of efficient CO₂ conversion catalysts.（2）Homogeneous ionic liquid catalysts have high dosage and difficult recycling.Preparation of four ionic liquid-base metal complexes with Lewis acid-base sites for catalytic CO₂/epichlorohydrin cycloaddition reaction performance studied by coordination of carboxyl ionic liquids with Cr（NO₃）₂·9H₂O in water.The results show that the catalyst has good catalytic performance under relatively mild conditions（80°C,0.1 MPa,24 h）due to the synergistic effect of Lewis acid-base sites.Meanwhile,Cr-IBCIL has a high specific surface area and pore volume(1273 m²·g^-1and 0.91 cm³·g^-1),which are more conducive to the free approach of reaction substrate molecules to the active site and promotes the free diffusion of products,could reach 99%yield at 16 h,and exhibited excellent adsorption performance for CO₂.（3）Based on the above study,the method of direct coordination of Cr³⁺with carboxyl ionic liquids still failed to achieve the normal temperature and pressure catalytic conversion of CO₂ cycloaddition reaction.Carboxyl ionic liquids were one-pot further introduced into MIL-101 by in-situ assembly method,and a series of hierarchically porous MOFs（HP-MOFs）with defects were successfully prepared.Ionic liquids as monodentate ligands can promote the formation of unsaturated metal sites,meanwhile,by changing the structure/content of ionic liquids,the pore structure of the material can be effectively adjusted,thus significantly improving the Lewis acid-base site synergy effect and achieving efficient conversion of CO₂ to epoxides with epoxides under mild conditions（40°C,0.1 MPa,24 h）was achieved by the synergy of acid-base sites（the yield of chloropropene carbonate could reach 95%）.The method is novel,simple and flexible,and provides a new idea for the construction of efficient CO₂ conversion catalysts.