Synthesis Of Functionalized Bis-imidazole Ionic Liquids And Their Application In Catalytic Carbon Dioxide Cycloaddition Reactions

The conversion of CO₂ into compounds and polymers has recently attracted a lot of attention in order to reduce greenhouse gas emissions and fossil fuel consumption in the chemical industry.Cyclic carbonates are widely used as industrial raw materials for the production of various chemical products or polymeric materials,solvents,electrolytes for batteries and fuel additives.The catalytic coupling of CO₂ and different epoxy compounds to form cyclic carbonates is one of the important CO₂ conversion processes.However,although CO₂ is a cheap,abundant and non-toxic material,its sustainable use is limited by its thermodynamic stability and kinetic inertness.As a result,a great deal of research has focused on the synthesis of efficient catalytic systems for the chemical conversion of CO₂ into products of high industrial value.In order to synthesize cyclic carbonates efficiently using the cycloaddition reaction between carbon dioxide gas and epoxides,a series of new hydroxyl-functionalized bis-imidazole ionic liquids have been successfully prepared in this paper,namely 1,4-bis-（3-hydroxyethyl-1-imidazole）butane butyrate ionic liquid,1,4-bis-（3-hydroxyethyl-1-imidazole）butaneacetateionicliquid,1,4-bis-(3-hydroxyethyl 1,4-bis-（3-hydroxyethyl-1-imidazolyl）butane ethanolate ionic liquid,1,4-bis-（3-hydroxyethyl-1-imidazolyl）butane lactate ionic liquid,1,4-bis-（3-hydroxyethyl-1-imidazolyl）butane p-hydroxybenzoate ionic liquid,1,4-bis-（3-hydroxyethyl-1-imidazolyl）butane salicylate ionic liquid,and the ionic liquids were characterized by ~1H NMR spectroscopy.The chemical structure was verified by ~1H NMR spectroscopy of the species of hydrogen atoms in different chemical environments and their corresponding numbers;the characteristic chemical bonds of the compounds were probed by infrared（IR）and ultraviolet（UV）spectroscopy.In order to determine the catalytic reaction conditions required to achieve the highest catalytic activity of the synthesized catalyst,The effects of reaction time,pressure,temperature and catalyst dosage on the cycloaddition reaction were investigated by factor experiments,and the optimal reaction conditions were obtained:when the catalyst dosage was 0.8 mol%,the carbon dioxide pressure was 0.1 MPa,and the reaction temperature was 90℃,After 12 hours of reaction in the autoclave,the yield of chloropropene carbonate was 98.6%.The catalytic performance of the six ionic liquids under optimum conditions was investigated.In addition,the suitability of the catalysts for different substrates was examined,and the kinetic characteristics of the cycloaddition reaction were verified by experiments investigating the reaction steps with catalyst concentration and epoxide concentration.The reaction activation energy was further determined by determining the reaction rate constants at a series of temperature gradients,and a possible reaction mechanism was proposed.Finally,the catalytic performance of a range of imidazole-based ionic liquid catalysts from the literature was compared.The above experimental results show that these new hydroxyl-functionalized bis-imidazole ionic liquids can catalyze the cycloaddition reaction of carbon dioxide to synthesize cyclic carbonates under mild conditions and without the need for a solvent,co-catalyst.Finally,in this paper,conformational optimization,molecular front orbital and electrostatic potential surface quantum chemical calculations were performed for the six ionic liquids synthesized at the molecular and electronic levels,which provide a basis for further theoretical validation of the reaction mechanism.