Ionic Liquid-based Supramolecular Polymers For The Conversion Of CO₂ To Cyclic Carbonates Under Mild Conditions

Excessive consumption of fossil fuels has resulted in increasing concentrations of CO₂ in the atmosphere,which in turn has led to a series of environmental problems.As a safe and cheap C1 resource,CO₂ can be transformed into high value-added chemicals,which will become a powerful measure to achieve carbon emission reduction of fossil energy and low-carbon transformation of energy structure.The cycloaddition reaction of CO₂ and epoxides to synthesize cyclic carbonates has become one of the effective ways to utilize CO₂ resources because of its 100%atomic utilization efficiency and wide applications of cyclic carbonates.Due to the inherent kinetic inertia and thermodynamic stability of CO₂,it is still difficult to realize the catalytic conversion of CO₂ under mild conditions.At present,the catalysts for CO₂ cycloaddition reaction are mainly divided into homogeneous catalysts（ionic liquids,quaternary ammonium salts,etc.）and heterogeneous catalysts（metal oxides,metal-organic frameworks,etc.）.Most of the catalysts have low activity,which often requires high temperature and high pressure or with the help of co-catalyst to achieve efficient catalytic conversion of CO₂.Ionic liquid has developed rapidly in the field of CO₂ conversion due to its advantages of special designability,high CO₂ solubility and high stability.However,as a kind of homogeneous catalysts,ionic liquid has disadvantages such as high viscosity,difficult recovery and high cost,which limits its wide application.Immobilized ionic liquid in nanoporous functional materials can overcome the shortage of homogeneous ionic liquid,and combine the characteristics of porous materials such as large specific surface areas,open channels,stable spatial structures,etc.,providing an effective approach for the construction of heterogeneous catalysts with excellent performance.At present,the main methods for immobilization of ionic liquids are covalent grafting,impregnation and ionic thermal synthesis,which usually have problems such as complicated synthesis process,loss of ionic liquids and blockage of channels.Supramolecular self-assembly has the advantages of simple synthesis,adjustable function and controllable structure.Herein,supramolecular assembly strategy has been adopted for preparing a variety of ionic liquid-based supramolecular heterogeneous catalysts under mild conditions,which will be used for CO₂ catalytic conversion under atmospheric pressure and low temperature.The main research results are as follows:（1）Based on the acidic-basic synergistic effect for the activation of CO₂ and epoxides,a series of ionic liquid-based supramolecular polymers with acid-base active sites were prepared by the metal coordination between poly（amino）ionic liquid and metal ions(Zn²⁺,Cu²⁺,Co²⁺).The catalytic performance of these polymers in the cycloaddition reaction of CO₂ and epichlorohydrin was studied.The optimized supramolecular catalyst were selected by adjusting the proportion and type of building blocks.It was found that under the synergistic effect of acidic and basic sites(Zn²⁺,Br^-),PZn-3 could efficiently catalyze the reaction of CO₂ and epichlorohydrin to achieve a high yield of propylene chloride（95%）under mild conditions（75℃,0.1 MPa,no solvent and no co-catalyst）.After 5 cycles,the catalytic activity of PZn-3 remained well.（2）Based on the dynamical adjustable and structure controllable characteristics of dynamic covalent bonding,amino ionic liquid-based supramolecular monomers were prepared by dynamic covalent bonding between amino ionic liquid and aldehyde compound.Then,a series of ionic liquid-based supramolecular polymers were prepared by polymerization of supramolecular monomers.The catalytic performance of these polymers in the cycloaddition reaction of CO₂ and epichlorohydrin was studied.The structures and properties of the supramolecular polymers were systematically controlled by adjusting the molar ratio and type of the building blocks and pH.With the increase of the molar ratio of amino ionic liquid,the yield of chloropropene carbonate on the supramolecular polymer increased at first and then decreased.When the molar ratio of amino ionic liquid to biphenyldiformaldehyde was 3:1,the yield of chloropropene carbonate over PBP-3 reached 99%under the conditions of 75℃,0.1MPa,no solvent and no co-catalyst.Moreover,the catalytic activity of the catalyst did not change significantly after 5 cycles.