Density functional theory study on the cycloaddition of carbon dioxide with propylene oxide catalyzed by alkylmethylimidazolium chlorine ionic liquids.

The cycloaddition of carbon dioxide (CO2) with propylene oxide (PO) in the absence and presence of alkylmethylimidazolium chlorine ([C(n)mim]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(n)mim]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(n)mim]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(n)mim]Cl promotes the reaction via modifying the reaction mechanism and thereby remarkably decreases the barrier. The origination of the catalytic activity of [C(n)mim]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(n)mim]Cl.