Proposal of a reaction mechanism for the thermal carbonation of ethylene oxide by CO2 in ionic liquids

Abstract

We revisit a possible mechanism for the thermal conversion of the title reaction, which appears to be driven by solvent effects by the ionic liquid (IL) dihydroxyethyldiethyl ammonium bromide Net2HE2Br. The effect of the solvent is discussed following two complementary models, namely, an explicit solvation approach, suggesting that the IL can be acting as a catalyst, and an implicit solvation model showing that stabilization/destabilization of the stationary points in the potential (free) energy, confirming that the epoxide ring opening is the rate determining step. The IL plays a key role in the activation of ethylene oxide, where the cation of the IL provides an efficient hydrogen bond to the epoxide moiety, together with a concerted approach of the Br− counterion. The activation step of the proposed mechanism is rationalized on the basis of a second order Fukui response function within the conceptual density functional theory. The proposed mechanism suggests a global third order reaction rate: order one in the cation and anion forming the IL structure and order one with respect to ethylene oxide.