Mechanistic insights into CO_2 cycloaddition to propylene oxide over a single copper atom incorporated graphene-based materials: A theoretical study

摘要

Density functional theory (M06-L-D3) calculations have been employed to theoretically study the CO2 cycloaddition to propylene oxide (PO) on the copper-doped graphene with a di-vacancy defect (Cu-DV) and the Cu-N-4 moiety embedded into graphene (Cu-NG). A CO2 molecule is adsorbed on the Cu active site of two catalysts with the adsorption energies of about -5.5 kcal/mol. PO adsorbs strongly on the coordinatively-unsaturated Cu site with adsorption energies of -10.0 (Cu-DV) and -11.3 (Cu-NG) kcal/mol. The catalytic generation of cyclic carbonate from CO2 and PO by Cu-DV and Cu-NG is predicted to follow a similar multistep mechanism. The first step is the ring-opening of the adsorbed PO by nucleophilic attack of bromide. The second step is the insertion of CO2 into the Cu-O bond of alkoxide intermediate to form the linear carbonate intermediate. The third step is the transformation of linear carbonate via intramolecular cyclic S(N)2-type reaction to form the corresponding cyclic carbonate. Mechanism explorations reveal that the rate-limiting step lies in the formation of cyclic carbonate with activation barriers of 14.8 and 14.9 kcal/mol for the catalytic process over Cu-DV and Cu-NG, respectively. Therefore, our theoretical study suggests that Cu-DV and Cu-NG could possess catalytic activity for CO2 cycloaddition to PO as comparable to that of potential catalysts.