Study On Immobilized Ionic Liquids As Catalysts For Synthesis Of Cyclic Carbonates Via The Cycloaddition Of CO₂to Epoxides

CO2is the main green-house gas at present. Considering environmental protection and resource utilization, it is interesting to convert CO2into valuable chemicals. An important reaction of CO2fixation is the synthesis of five-membered cyclic carbonates via cycloaddition of CO2to epoxides, because of its remarkable advantages such as cheap materials, high utilization of atoms and little by-product. Among those catalysts used to catalyze the cycloaddition of CO2and epoxides, ionic liquids are very popular owing to their unique properties. This paper tried to develope highly efficient immobilized ionic liquids for the cycloaddition of CO2and epoxides to synthesis cyclic carbonates. Besides, we also proposed the plausible machanisms for the coupling reaction of CO2to epoxides.Carboxyl-functionalized imidazolium-based ionic liquid was grafted onto cross-linked divinylbenzene polymer (denoted as PDVB-CEIMBr) for the first time. The catalyst was characterized and used for the cycloaddition of CO2to epoxides under relatively mild reaction conditions without any use of cocatalyst and organic solvent. The effects of reaction conditions (temperature, initial CO2pressure, and time) and water addition on the yield of cyclic carbonate were investigated systematically. Under optimal conditions (140℃, initial CO2pressure2.0MPa and4h), propylene carbonate selectivity and yield were99.8%and96.1%, respectively. PDVB-CEIMBr also showed good activity towards the coupling reation of other epoxides and CO2to synthesize corresponding cyclic carbonates. Trace water (H2O/PO<0.6, mol/mol) had little effect on the yield of cyclic carbonate; but when more water was added into the reaction, the epoxide could hydrolyze into by-product glycol, thus leading to low yield and selectivity of cyclic carbonate. Moreover, the catalyst showed good stability and reusability. A plausible reaction mechanism was proposed for the catalytic reaction. It is suggested that synergetic effect among Bransted acid COOH, Lewis base Br and the support facilitates the coupling reaction.In order to obtain efficient catalyst which is low-cost, economic and simple in preparation, we selected the chloromethylated polystyrene as support, and the heterogeneous catalysts, chloromethylated polystyrene bearing quaternary ammonium salt ionic liquids (denoted as PS-QNS and PS-CQNS, respectively), were obtained by one-step reaction between the support and N,N-Dimethylethylamine, N,N-Dimethylglycine, respectively. Similarly, the effects of the amount of catalyst, reaction temperature, initial CO2pressure, and time on the activity of the catalyst were discussed systematically. Under the relatively mild reaction conditions (150℃, initial CO2pressure2.0MPa and5h), the yield and selectivity of propylene carbonate in the cycloaddition of CO2and propylene oxide catalyzed by PS-CQNS were96.2%and99.3%, respectively. Compared with PS-CQNS, PS-QNS showed higher catalytic activity (97.9%/99.5%, yield/selectivity) under the same condition. What’s more, the reusability of PS-QNS is excellent. From the viewpoint of industrial application, the catalyst PS-QNS is attractive because of its excellent catalytic efficiency, low cost and simplicity in preparation.