Imidazole Ionic Liquid Modified MOFs Material And Its Application In CO₂ Fixation

Among the many reactions of carbon dioxide fixation,the reaction between carbon dioxide and epoxides is considered to be able to accomplish 100%atom conversion,and the product（cyclic carbonate）can be widely used in industry.However,CO₂ is inert and not easily activated.Therefore,the search for efficient and stable catalyst becomes the key to achieve CO₂ rapid conversion.Imidazole ionic liquids have excellent catalytic performance in cycloaddition reaction because of nucleophile（e.g.halide anion）,but they are difficult to recover.Meanwhile,the metal organic frameworks（MOFs）with unsaturated metal sites and porosity have been used as heterogeneous catalysts for CO₂ cycloaddition reaction,but the reaction conditions are harsh.In this paper,for combining the merits of MOFs and imidazole ionic liquids,two kinds of MOFs materials（ZIF-9-NH₂ and Cr-MIL-101）were modified by covalent modification and coordination covalent modification,respectively,and two kinds of MOFs supported imidazole ionic liquids were prepared.（1）The amino modified ZIF-9-NH₂ material was prepared by utilizing2-aminobenzimidazole as a partial substitute for ZIF-9 ligand to coordinate with cobalt ions.Partial substitution ligands can increase the alkaline sites on MOFs and provide the modifiable functional groups（-NH₂）at the same time.[BrBMIM]Br was synthesized from 1-methylimidazole and 1,4-dibromobutane.After that[BrBMIM]Br was Fixed on the ZIF-9-NH₂ by covalent bonding,and the ZIF-9-[BMIM]Br catalyst was obtained.ZIF-9-[BMIM]Br could provide both Co unsaturated metal sites,alkaline site and nucleophilic Br.The catalyst was used for cycloaddition reaction of CO₂ and propylene epoxide.Under the mild conditions（120^oC,2 MPa,3.5 h）,the yield and the selectivity were 93.8%and 98.9%,respectively.The results showed that the catalyst（ZIF-9-[BMIM]Br）prepared by ZIF-9-NH₂ supported imidazole bromide ionic liquid possessed good catalytic activity.In addition,the possible reaction mechanism of carbon dioxide cycloaddition catalyzed by the catalyst（ZIF-9-[BMIM]Br）was discussed.（2）More unsaturated metal sites of Cr were exposed by removing coordination water molecules on Cr-MIL-101 materials at high temperature in vacuum.2-mercapto-1-methylimidazole with the electron-rich group mercapto（-SH）can form coordination covalent bond with Cr and therefore be easily immobilized on Cr-MIL-101.Moreover,N on 2-mercapto-1-methylimidazole can react with 2-bromoethanol to form ionic liquids（[SHIMOH]Br）.Thus,a Cr-MIL-101-supported[SHIMOH]Br catalytic material（Cr-MIL-101-[SHIMOH]Br）was constructed.Based on simple synthetic modification,this material can provide both Cr unsaturated metal sites and nucleophilic Br and hydroxyl groups.Therefore,under mild conditions（120 ^oC,2 MPa,1.5 h）,the cycloadditionofCO₂andpropyleneepoxidewascatalyzedbythe Cr-MIL-101-[SHIMOH]Br.The results showed that the catalyst exhibited better catalytic performance with the yield of 96.7%and the selectivity of 100%.The catalytic activity remained virtually unchanged after 5 cycles.The possible mechanism of carbon dioxide cycloaddition catalyzed by the catalyst was also discussed in this paper.