Chemical Fixation of CO₂ by Using Carbon Material-Grafted N-Heterocyclic Carbene Silver and Copper Complexes

Chemical fixation of CO2 is investigated by using heterogeneous catalysts (CNT-NHC-M and GN-NHC-M; M = Ag, Cu) of N-heterocyclic carbene (NHC)–silver complex and NHC–copper complex grafted on carbon materials such as multiwalled carbon nanotubes (CNTs) and graphene (GN). These carbon-based versatile catalysts efficiently promote carboxylative cycloaddition of CO2 into propargylic alcohols to produce cyclic carbonates; catalyze three-component coupling of CO2, propargyl alcohols, and amines to afford carbamates and oxazolidinones; and contribute cycloaddition of CO2 with epoxides to yield cyclic carbonates. Under optimized reactions, CNT-NHC-Ag efficiently promotes the cycloaddition of CO2 with propargyl alcohol under atmospheric pressure of CO2 by affording cyclic carbonate yield of 99%, while the CNT-NHC-Cu efficiently catalyzes three-component coupling of CO2, propargyl alcohol, and amines by giving β-oxopropylcarbamate in 99% yield and oxazolidinone with 96% yield, respectively. In the case of cycloaddition of CO2 with epoxide, a quantitative yield of cyclic carbonate is obtained by using a combination of CNT-NHC-Ag and 4-dimethylaminopyridine. The developed catalysts thus provide a typical platform for a rational combination of homogeneous and heterogeneous catalysts for the utilization and transformation of renewable carbon resource CO2 in a green and efficient way.