Transformation of CO2 incorporated in adducts of N-heterocyclic carbene into dialkyl carbonates under ambient conditions: An experimental and mechanistic study

Abstract The undesirable greenhouse gas CO2 is chemically also a sustainable carbon resource to synthesize value-added products. Herein we report a two-step strategy for the transformation of CO2 into dialkyl carbonates under ambient pressure and temperature. CO2 is initially captured and activated in 1-alkyl-3-methylimidazolium ionic liquids ([CmC1Im][An], m = 2, 4; An = [HCO3]-, [OAc]-, [Triz]-), forming the adducts of N-heterocyclic carbene (CmC1Im−CO2) and being loaded up to 26.9 wt%; the incorporated CO2 is then mixed with alcohols (ROH, R = CH3, C2H5) and alkyl halides (R*X*n, R* = R’, R’’; X* = X’, X’’; n = 1, 2, respectively) to synthesize dialkyl carbonates (RO(CO)OR’ in R’X’; RO(CO)OR in R’’X’’2), where high selectivity up to 99.9% and CO2 conversion of 40.2% are achieved. The reaction mechanism both with and without the intramolecular C−O bond cleavage is identified by 18O isotopic labelling experiments and further explored by combining 13C NMR characterization and DFT calculation considering solvation. The process integrated with regeneration cycles is promising for the energy-efficient conversion of CO2 under ambient conditions.