Efficient transformation of CO₂ to cyclic carbonates using bifunctional protic ionic liquids under mild conditions

A series of 1,8-diazabicyclo-[5.4.0]undec-7-ene (DBU) based bifunctional protic ionic liquids (DBPILs) were easily prepared by acid–base reactions at room temperature. They were used to catalyze the cycloaddition reaction of CO₂ with epoxides under mild conditions. As a metal free catalyst, the best DBPIL showed a 92% yield of products within 6 hours at 30 °C and 1 bar CO₂ without any solvents and co-catalysts. It could afford carbonates in good yields with broad epoxide substrate scope and CO₂ from simulated flue gas (15% CO₂/85% N2). IR spectroscopy and DFT studies were carried out to investigate the mechanism of the cycloaddition reaction. The results showed that the DBPILs could activate both CO₂ and epoxides by alkoxy anions and powerful hydrogen-bonding, which was well consistent with experiments.