Phenolic hydroxyl-functionalized imidazolium ionic liquids: Highly efficient catalysts for the fixation of CO₂ to cyclic carbonates

A series of imidazolium ionic liquids were designed and synthesized by the reaction of imidazole derivatives with alkyl bromides. These ionic liquids were applied to catalyze the reaction of CO₂ and epoxides. A detailed investigation was carried out on the relationship between catalytic activities and catalyst structures. The result showed that phenolic hydroxyl-functionalized imidazolium ionic liquids containing both phenolic hydroxyl and C2-H were highly efficient catalysts. Meanwhile, the spatial positions of the phenolic hydroxyl and C2-H in the imidazolium also exhibited great effect on the catalytic efficiencies, and the order of catalytic activity was meta-isomer > ortho-isomer > para-isomer. NMR titration and DFT calculations showed a synergetic effect, which C2-H and phenolic hydroxyl cooperatively activated epoxides by hydrogen bonds, was crucial for the reaction to proceed smoothly under mild conditions. In addition, the TOF of the most active catalyst 3-(3-phenolic hydroxyl)-1-butyl-imidazolium bromide (IL-2) reached to 900 h^-1 in the reaction of CO₂ and epichlorohydrin under the reaction conditions of 0.1 mol% catalyst, 1 MPa CO₂, 120 °C and 1 h.