Density Functional Theory Study on the Cycloaddition of Carbon Dioxide with Propylene Oxide Catalyzed by Alkylmethylimidazolium Chlorine Ionic Liquids

Abstract

The cycloaddition of carbon dioxide (CO₂) with propylene oxide (PO) in the absence and presence of alkylmethylimidazolium chlorine ([C_nmim]Cl, n = 2, 4, and 6) ionic liquids has been studied in detail by performing density functional theory calculations at the B3PW91/6-31G(d,p) level. It is found that in the absence of [C_nmim]Cl the reaction proceeds via two possible channels (each of them involves one elementary step) and the corresponding barriers are found to be as high as 59.71 and 55.10 kcal mol^-1, while in the presence of [C_nmim]Cl there exist five possible reaction channels (each of them involves two or three elementary steps) and the barriers of the rate-determining steps are reduced to 27.93−38.05 kcal mol^-1, clearly indicating that [C_nmim]Cl promotes the reaction via modifying the reaction mechanism and thereby remarkably decreases the barrier. The origination of the catalytic activity of [C_nmim]Cl has been analyzed in detail. The present theoretical study rationalizes the early experimental findings well and provides a clear profile for the cycloaddition of CO₂ with PO promoted by [C_nmim]Cl.