Mechanism of fixation of CO2 with an epoxide catalyzed by ZnBr2 and a choline chloride co-catalyst: a DFT study

Abstract

To explore the reason for the high activity of the cycloaddition reaction of PO (propylene oxide) with CO₂ catalyzed by ZnBr₂/CH (choline chloride) co-catalyst, the mechanism has been constructed using a DFT (density functional theory) method. The combination of CH and ZnBr₂ will generate three new stable complexes, i.e., [Ch]₂[ZnBr₂Cl₂], Ch⁺ZnBr₂Cl⁻, and Ch⁺ZnBrCl₂⁻. The latter two are derived from the dissociation of [Ch]₂[ZnBr₂Cl₂]. The detailed mechanism of a coupling reaction catalyzed by the more stable complex Ch⁺ZnBrCl₂⁻ is explored. It has been elucidated that the attack from the Zn complex and the Br⁻ anion is the major factor in promoting the cleavage of the C–O bond of PO. Finally, the performance of [Ch]₂[ZnBr₂Cl₂] is also investigated, providing less activity, indicating that it should dissociate to gain better catalytic effect.