Synthesis Of Imidazole Ionic Liquide-based MIL-101-NH₂ Catalyst And Catalysis Of CO₂ And Epoxide Cycloaddition Reation

CO₂ is the main greenhouse gas,and it is also a renewable resource with abundant reserves,low cost and non-toxicity.Converting CO₂ into high value-added products is not only conducive to improving the ecological environment,but also the development of renewable energy.The conversion of CO₂to cyclic carbonate has attracted the attention of scientists.This reaction replaces the toxic phosgene in the traditional process,is environmentally friendly,and has 100%atomic economy.Due to the thermodynamic stability and kinetic inertness of CO₂,a catalyst needs to be designed to activate the reaction.The ionic liquid（IL）supported metal organic framework（MOF）catalytic system is designed based on the activation of CO₂ and epoxides.The unsaturated metal sites in MOF can be used as Lewis acid sites to activate epoxide ring opening,and the halogen anion in IL can nucleophilically attack the epoxide to activate the reaction substrate.The synergistic effect of Lewis acid and halogen anion catalyzes the CO₂ cycloaddition reaction.1,6-bis（N-methylimidazolium bromide）hexane（Mim-6）is physically encapsulated in the pores of MIL-101-NH₂（Mim-6@MIL-101-NH₂）.The physically encapsulated Mim-6@MIL-101-NH₂ can catalyze the cycloaddition of CO₂ under optimized conditions（100℃,1.0 MPa,2.5 h）without a co-catalyst,and the yield of propylene carbonate（PC）is 85.1%.Since IL is encapsulated in the pores of the MOF,the catalyst has excellent recyclability,and the catalytic activity does not change under multiple cycles.A trifunctional catalytic system（MIL-101-NHIM-NH₂）that connects ethylaminoimidazole ionic liquid and MIL-101-NH₂ through a covalent bond.Synthesis of MIL-101-NHIM-NH₂ by post-synthesis method under optimized conditions（120℃,2.0 MPa,4 h）catalyzed the cycloaddition reaction of CO₂ and propylene oxide（PO）with a yield of 93.1%without solvent And co-catalyst.The trifunctional system attributable to Br^-,Cr³⁺and amino groups synergistically catalyzes the cycloaddition reaction,and the presence of covalent bonds draws the active site closer to the reaction substrate,which can improve the catalytic activity.In order to increase the loading of ionic liquids,reducing the synthesis steps of the catalyst may be a way to solve this problem.The original method of introducing imidazole and then ionic liquid was changed to synthesize the ionic liquid first,and then directly graft the ionic liquid on MIL-101-NH₂（MIL-101-NH-Bu-IMOH）.This is equivalent to increasing the yield of the previous step of introducing the ionic liquid to 100%.Under the optimized conditions（120℃,2.0 MPa,4 h）,the PC yield was 77.55%,which was higher than the 59.4%PC yield of MIL-101-NH-Bu-IMOH synthesized by the stepwise method.