Novel Hydroxyl-Functionalized Ionic Liquid: An Efficient Catalyst for Chemical Fixation of CO2 into Cyclic Carbonates Under Mild and Additive-Free Conditions

The cycloaddition reaction of CO2 with epoxides is an important way to realize the chemical fixation of CO2. However, it often suffers from harsh reaction conditions, such as high pressure, high temperature and long reaction time. In order to explore an efficient catalyst for this reaction, 14 kinds of ionic liquid catalysts were designed, synthesized and used for the cycloaddition reactions of CO2 with epoxides. The results showed that the dihydroxyl ionic liquid 3-(2-hydroxy-ethyl)-1-(3-hydroxy-propyl)-3H-imidazolium iodide (IL9) exhibits the best catalytic activity under mild reaction conditions: 60–100% yields of cyclic carbonates were obtained within 2 h at 100 °C and 1 atm CO2 without any solvents and co-catalysts, thus alleviating the high-pressure-dependent limitation of reported hydroxyl-functionalized ionic liquid catalysts and significantly reducing the reaction time. The functional ionic liquid IL9 not only maintains its structure and activity unchanged after five runs; it also exhibits potential advantages in the conversion of CO2 from simulated flue gas (15% CO2/85% N2). Further density functional theory (DFT) calculations revealed that the IL9 could activate epoxides and CO2 by iodide anion, powerful hydrogen-bonding and the electrostatic force of the imidazole nucleus, consistent with the experimental results.