Synthesis Of Imidazolium Ionic Liquid/MIL-101 Composites For Catalyzing CO₂ Cycloaddition

The CO₂ emission is rapidly increased with the improvement of industrial production scale and human living quality,and it’s a crucial problem that how to reduce the CO₂ concentration in atmosphere.Making CO₂ convert to high value-added industrial product through chemical reactions is regarded as an effective solution.Thereinto,the chemical fixation of CO₂ and further synthesizes cyclic carbonate is the one of research focus due to 100%atom economy.However,the chemical inertness of CO₂ causes this reaction to occur under high temperature,high pressure,and catalyst existed condition.In a variety of catalysts,ionic liquids as a kind of efficient homogeneous catalyst,the difficulty in separation and recycle is limited their utilization.And metal organic frameworks（MOFs）as heterogeneous catalysts,exhibit high surface area and abundant Lewis acid sites,but it’s required to add soluble cocatalyst for further enhanced catalytic performance in CO₂ cycloaddition.To solve these above issues,making the ionic liquid included nucleophile incorporate into the MOF to synthesize composite might be an efficiently strategy.Selecting the classical MOF material MIL-101 as carrier,the dicationic imidazolium ionic liquid@MIL-101 composite（Mim-6@MIL-101）was facilely prepared by in-situ synthesis.Due to the incorporation of ionic liquid,the CO₂ adsorption isosteric heat of composite was increased to 45.8 k J/mol from 26.0 k J/mol that of MIL-101,meaning the composite existed better CO₂ affinity than MIL-101.In addition,Mim-6@MIL-101 could catalyzed the cycloaddition of CO₂ with propylene oxide under 100 ^oC,1.0 MPa,2.5 h,cocatalyst-and solvent-free conditions,finally,the propylene carbonate product yield reached 92.5%,and this composite also could be recycled for 3 runs after simple centrifugation.Furthermore,the experiments indicated the Lewis acid Cr sites could activate epoxides,and the Br^-in ionic liquid and the porous structure of MIL-101 are also beneficial to the catalyzation.The reaction mechanism of composite catalyzing CO₂-epoxides cycloaddition was proposed:the epoxide occurred ring-opening reaction facilitated by the synergistic effect between Cr centers and Br^-,and the good CO₂ affinity of composite increased the activity of CO₂,thus,the composite efficiently catalyzed the cycloaddition reaction.It’s worth that the PC yield significantly reduced to 72.4%after Mim-6@MIL-101recycled for 4 runs,indicated the reusability of composite could be further improved.Hence,1-vinylimidazole and 1,4-dibromobutane were incorporated into MIL-101framework by impregnation,then synthesized dicationic vinylimidazolium-based ionic liquid,finally,the poly（ionic liquid）@MIL-101（PIL-4@MIL-101）composite was obtained by free-radical polymerization.This composite exhibited high catalytic activity for the epichlorohydrin（ECH）and CO₂ cycloaddition,the 99.6%reactant conversion could be reached under 90 ^oC,1.0 MPa,2.5 h,and remained 87.4%conversion after recycled for 5 runs.Functionalization could increase the amount of active reaction site in materials,therefore,different composites were prepared through modifying MIL-101 with amino group or incorporating hydrogen bond donor（HBD）groups into poly（ionic liquid）.Thereinto,selecting amino functionalized MIL-101 as a carrier,poly（ionic liquid）was grafted on MIL-101 via covalent bond to obtain MIL-101-NH-PIL composite.It catalyzed cycloaddition reaction with 95.2%ECH conversion at 100 ^oC,1.0 MPa,2.5 h.In addition,MIL-101-NH-PIL exhibited better reusability with 92.4%ECH conversion after recycled for 5 runs due to the stronger interaction between MIL-101 and poly（ionic liquid）.Furthermore,based on the functional modification for poly（ionic liquid）,the HBD functionalized poly（ionic liquid）s@MIL-101 composite（PIL-COOH@MIL-101）were prepared.PIL-COOH@MIL-101 not only exhibited good CO₂ adsorption capacity（46.8cm³/g at 0.1 MPa,273 K）,but also the carboxyl as HBD group,so it efficiently catalyzed and converted ECH under milder condition（70 ^oC,1.0 MPa,2.5 h）,the conversion reached 92.7%,and selectivity was 99%,by contrast,those foregoing composites only obtained around 60%at same conditions.In addition,PIL-COOH@MIL-101 also obtained 81.6%conversion under 30 ^oC,1.0 MPa for 24 h.These were verified the useful synergistic effects among multiple active sites.The multi-functionalized PIL-COOH@MIL-101 composite shows good CO₂ adsorption and excellent catalytic activity,hence,it possesses further industrialized application for CO₂ capture and conversion.