The Study On The Preparation Of Imidazolium Ionic Liquids Modified Triazine-based COFs And Their Catalytic Conversion Of CO₂

The excessive consumption of fossil fuels has caused the concentration of carbon dioxide（CO₂）in the atmosphere to increase year by year,while CO₂ is an abundant and renewable carbon monoxide resource in nature.The cycloaddition reaction of CO₂ with epoxides is the most widely studied class of reactions,which can effectively solve the dual problems of energy crisis and environmental pollution.Covalent organic frameworks（COFs）is a new class of crystalline porous materials consisting of rigid organic monomers connected by covalent bonds.Among them,triazine-based COF materials have high specific surface area,controllable pore structure and high thermal/chemical stability,and thus have good application prospects.In this paper,two types of multifunctionalized ionic COF materials are constructed by the modification strategies of imidazole-based ionic liquids or polyionic liquids（PILs）using the structural advantages of two-dimensional triazine-based COFs to achieve efficient catalytic conversion of CO₂ cycloaddition reactions without co-catalysts and under mild conditions,and the main research contents are as follows:（1）A series of functionalized imidazolium ionic liquids were covalently grafted onto the hydroxyl-containing triazinyl COF backbone via bridging groups based on a post-synthetic modification strategy,and the metal-free multifunctionalized ionic COF material-IM-COF-Y-R（Y=chlorine/bromine,R=methyl/vinyl/benzyl）was successfully prepared.Using the cycloaddition reaction of CO₂ with epichlorohydrin as a template,the effects of bridging alkyl chains,imidazole functional groups,nucleophilic reagents and the grafting amount on their catalytic activities were investigated separately.It was shown that IM-COF-Br-Viny had better CO₂ catalytic conversion performance due to its superior CO₂ adsorption ability and the synergistic effect of pro-CO₂ bases,hydrogen bond donors and nucleophilic anions in the catalyst.After optimization of the reaction conditions,the conversion of IM-COF-Br-Vinyl was as high as 96.49%and the selectivity of cyclic carbonate was higher than 99%after 6 h of reaction under 0.5 MPa CO₂ pressure,100℃,solvent-free and without co-catalyst,which was better than other reported COF catalysts.Meanwhile,the catalysts were easily separated and the catalytic activity did not decrease significantly after four cycles of use,indicating that catalysts had excellent cycling stability.The universality of the catalyst was determined by substrate expansion experiments.Finally,the possible catalytic reaction mechanism was proposed.（2）Based on the strategy of self-polymerization of imidazolium-based ionic liquids containing carboxyl groups in the pore channels of COF materials,the polyionic liquid-modified triazine COF-IM-COF-PCAVIm Br was successfully prepared.After a series of characterization analyses,the structure,crystallinity,porosity,and CO₂adsorption of IM-COF-PCAVIm Br were determined,and it was confirmed that the poly ionic liquids were successfully formed in the pore channels of COF materials.The superior catalytic performance of 50%IM-COF-PCAVIm Br for the cycloaddition reaction of CO₂ and epichlorohydrin was determined by adjusting the grafting amount of the polyionomer.After optimization of the reaction conditions,the yield of cyclic carbonate could be as high as 88.16%after the reaction of 50%IM-COF-PCAVIm Br at 1 MPa CO₂ pressure,100℃,solvent-free and without co-catalyst for 24 h.Meanwhile,the catalytic activity of the catalyst did not decrease significantly after four cycles of use.The universality of the catalyst was determined by substrate expansion experiments.Finally,the possible catalytic reaction mechanism was proposed.