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

• Phenolic hydroxyl-functionalized imidazolium ionic liquids were synthesized from reaction of imidazole derivatives and alkyl bromides.
• The high catalytic activity of ionic liquids in the reaction of CO2 and epoxides attributed to synergistic effect of phenolic hydroxyl and C2-H.
• The TOF of IL-2 was 900 h-1 in the reaction of CO2 and epichlorohydrin (in 90% yield) under mild reaction conditions.

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.

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