A Study Of The Copolymerization Mechanism Of CO₂with Epoxides Over Zinc-based Catalysts

Due to the growing concern about the greenhouse effect and the economical benefits arising from the utilization of renewable sources, catalytic transformation of CO2into useful organic compounds has attracted much attention. Since the copolymerization of epoxides and carbon dioxide was first reported by Inoue in the1960s using the heterogeneous ZnEt2/H2O catalyst system, a significant amount of research has been directed towards the development of catalysts with improved activity and selectivity. However, compared with the development of the catalyst, the research on the copolymerization mechanism of CO2and epoxide still lags far behind. In this study, we investigated the reaction mechanism of CO2and epoxides over zinc-based catalysts through combined experimental and theoretical approaches.First of all, the mechanisms of the reaction of CO2with propylene oxide (PO) catalyzed by the diethylzinc-water and diethylzinc-methanol catalyst systems have been studied by means of density functional theory (DFT). Our calculations indicate that both reactions follow monometallic mechanism and are initiated by PO insertion into the Zn-OR bond of the catalyst. PO insertion process is calculated to be exoergic, and needs to overcome a large Gibbs free energy barrier, while CO2insertion process is endothermic, and only needs to overcome a low Gibbs free energy barrier. Therefore, the rate-determining step for the reaction of CO2and PO is the second PO insertion into the Zn-carbonate bond for both catalyst systems. In addition, the consecutive PO insertion to give polyether and consecutive CO2insertion to give dicarbonate linkages have been found to be disfavored. However, the most possible active species for the diethylzinc-water catalyst system is likely to be the condensed species with repeated Zn-O unit, which shows high activation barrier to the formation of cyclic propylene carbonate. While the most possible active species for the diethylzinc-methanol catalyst system is the non-condensed species, which shows low Gibbs free energy barrier for cyclic propylene carbonate formation. These results are consistent with experimental reports in literature.Secondly, the alternating copolymerization of CO2and PO catalyzed by ZnEt2-glycerine-Y(CCl3COO)3ternary catalyst has been investigated through combined experimental and theoretical approaches. The ternary catalyst shows an increased activity by introducing a support of SiO2and Al2O3modified SiO2. For the supported ternary catalyst using Al2O3modified SiO2, the catalyst activity increases with increasing the amount of Al2O3loading, and the highest activity is achieved at3wt%Al2O3. NH3-TPD measurement confirms that the surface acidity of the Al2O3modified SiO2increases with increasing the amount of Al2O3. However, with further increasing the amount of Al2O3loading over the silica surface, the simultaneously decreasing of surface area and pore diameter due to the Al2O3modification, which would create negative effects for the catalyst activity, might become dominant effect when5wt%Al2O3modified SiO2is used as the support, which could rationalize well the decreased catalytic activity. Moreover, the mechanisms for the alternating copolymerization of CO2and PO over binary catalyst and ternary catalyst system have been studied by DFT. The insertion of PO into Zn-carbonate bond is rate-determining and the corresponding activation barrier decreases with increasing of the natural bond order (NBO) charge of the zinc species. The rare earth compound in the ternary catalyst could increase the electron deficiency around zinc catalytic center and decrease the Gibbs free energy barrier of the rate-determining step, thus improve the catalytic activity.Finally, the catalyst efficiency for different zinc phenoxides and the different behaviour between cyclohexene oxide (CHO), PO, styrene oxide (SO) and epichlorohydrin in the reaction with CO2over zinc phenoxide catalysts have been investigated using DFT. Our calculations show that the reaction of CHO with CO2produces polycarbonate. The substituents on the phenoxide ligands affect the catalyst efficiency as a function of the phenolate ligands on zinc is:’Pr<Ph<tBu<Me, and the activity of zinc phenoxide catalyst is a balance between an electron-deficient metal center and the steric hindrance around the metal center. These theoretical results are consistent with Darensbourg’s experiment. The reaction of PO, SO and epichlorohydrin with CO2solely gives cyclic carbonate. The difference might result from the steric hindrance of the epoxide. The catalyst efficiency of zinc phenoxide catalysts increases as reaction monomer varies as SO> PO> epichlorohydrin proportional to the electron deficiency of the zinc center.