Insight into the reaction mechanisms between CO2 and epoxides over Zn(II) phenoxide catalytic system – A DFT study

• The reaction of CO2 with epoxides over zinc phenoxides studied by DFT.
• The activity of catalyst was related to electronic and steric effect of ligands.
• Reaction of cyclohexene oxide and CO2 on Zn phenoxides produced alternating copolymer.
• Propylene oxide, styrene oxide and epichlorohydrin with CO2 gave cyclic carbonate.
• The different behavior of epoxides was related to the steric hindrance of epoxide.

The reaction mechanisms between CO2 and different epoxides including cyclohexene oxide (CHO), propylene oxide (PO), styrene oxide (SO) and epichlorohydrin (ECH) over Zn(II) phenoxide catalytic system were investigated using density functional theory (DFT). It was revealed that the reaction between CO2 and CHO over the catalytic system produced alternating copolymer. The higher polymerization activity of the catalyst was usually associated with the higher electron-deficiency and the lower steric hindrance of the zinc center, and the formation of cyclic carbonate from CHO was a two-step elimination reaction process with high activation energy barrier due to the high steric hindrance of CHO. However, the reaction of CO2 with the other three epoxides (PO, SO and ECH) provided only cyclic carbonate via one-step elimination reaction with low activation energy barrier. The catalyst efficiency of zinc phenoxide catalyst was predicted to increase in the following sequence: ECH < PO < SO. In addition, the ring-opening of PO and SO tended to occur at the methine CCH-R–O bond, whereas, the ring-opening of ECH was preferred thermodynamically at the unsubstituted methylene CCH2CCH2–O bond. These theoretical results rationalized well the experimental reports by Darensbourg.

The reactivity of three epoxides in the reaction with CO2 over Zn(II) phenoxides were investigated by DFT method. The results showed that the reaction of cyclohexene oxide (CHO) and CO2 produced copolymer, while propylene oxide (PO), styrene oxide (SO) or epichlorohydrin with CO2 provided only cyclic carbonate.Figure optionsDownload as PowerPoint slide