Experimental and theoretical studies on CO2 and propylene oxide (PO) copolymerization catalyzed by ZnEt2–glycerine–Y(CCl3COO)3 ternary catalyst

Abstract

Alternating copolymerization of CO2 and propylene oxide (PO) catalyzed by ZnEt2–glycerine–Y(CCl3COO)3 ternary catalyst was investigated through combined experimental and theoretical approaches. The ternary catalyst showed an increased activity by introducing a support of SiO2. The catalytic activity was further improved when Al2O3 modified SiO2 was used as support. For the supported ternary catalyst using Al2O3 modified SiO2, the catalyst activity increased with increasing of the amount of Al2O3, and the highest activity with 69% of enhancement in catalytic activity compared with the non-supported ternary catalyst was achieved at 3 wt% Al2O3 followed by a further decrease of activity. NH3–TPD measurement confirmed that the surface acidity of the Al2O3 modified SiO2 increased with increasing the amount of Al2O3, which indicated that a proper increment of surface acidity of the SiO2 support was favorable for the improvement of the catalytic activity for the supported ternary catalyst. Moreover, the mechanism for the alternating copolymerization of CO2 and PO over ZnEt2–glycerine binary catalyst and ternary catalyst system has been studied by DFT. The insertion of PO into Zn-carbonate bond was rate-determining and the corresponding activation barrier decreased with increasing of the natural bond order (NBO) charge of the zinc species. The combined experimental and theoretical results suggested that electron deficiency of zinc centers of the ternary catalyst was crucial for the alternating copolymerization of CO2 and PO.