Various (SiO2/MgO/MgCl2)·TiClx Ziegler-Natta catalysts modified by the third metal elements were synthesized by the co-impregnation of water-soluble magnesium and the third metal salts. Several key factors including the electronegativity of the third metal elements, catalyst performances in ethylene homo-polymerization, ethylene/1-hexene copolymerization and hydrogen response were systematically investigated. Both the catalyst performance and the polymer properties are influenced by the introduction of the third metal elements. Compared with the unmodified (SiO2/MgO/MgCl2)·TiClx Ziegler-Natta catalyst, activity and 1-hexene incorporation are enhanced by the introduction of zirconium, vanadium, aluminum and chromium, while deteriorated by the addition of ferrum, nickel, molybdenum and tungsten. Correlations of the catalyst activities and 1-hexene incorporation ability with the electronegativity of the third metal elements are discovered. It is found that the lower electronegativity of the third metal elements leads to the catalyst with higher activity and higher α-olefin co-polymerization ability. The polyethylene produced by a nickel modified catalyst showed broad molecular weight distribution (MWD) and the lowest average molecular weight (MW), while by using a ferrum modified catalyst, the resulting polyethylene had the highest MW, reaching the ultra-high MW area. Vanadium and chromium modified catalysts demonstrated the best hydrogen response.

•The type and amount of silanols on Phillips Cr/silica catalyst were identified by the high-resolution solid state NMR.•The amount of residual silanol groups of the catalysts treated at different temperatures was compared with the silica gel counterparts.•The presence of the grafted chromate species obstructed further removal of the residual single silanols.•The role of silanols on the formation of surface chromate species on the well-defined models was theoretically studied.Phillips Cr/silica catalyst is industrially important in ethylene polymerization. The high-resolution solid state 1H MAS NMR and 29Si CP/MAS NMR allowed the identification of the type and amount of silanols: geminal vs. single (isolated and vicinal) silanols on Phillips catalysts calcined at different temperatures, which were compared with those of the bare silica gel counterparts. The residual silanols on the catalyst and silica gel samples were all decreased with increasing calcination temperatures from 120 to 800 °C. For the catalysts treated at temperatures lower than 300 °C, the amount of residual silanol groups were much lower than those of the silica gel counterparts. It suggested that the chromium species were mainly grafted on the silica gel through esterification reaction with surface silanols below 300 °C. The geminal silanols almost disappeared on the catalysts at 120 °C, while that for the silica gel occurred at 300 °C. Further increasing the calcination temperatures from 300 to 800 °C, the amount of single silanols were slower decreased for the catalysts than that for the silica gel samples. It indicated that the presence of the grafted chromate species obstructed further removal of the residual single silanols. The role of silanols on the formation of surface chromate species on the well-defined polyoligomericsilsesquioxane (POSS) models containing various types of silanols was theoretically studied by density functional theory (DFT) method. It was shown that one silanol of the geminal pair and another adjacent single silanol was the most thermodynamically favored for grafting chromium species. The priority of the reaction between chromium species and different types of surface silanol groups during calcination for Phillips catalysts were experimentally and theoretically elucidated for the first time.Download high-res image (95KB)Download full-size image

•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.

•The copolymerization of CO2 with propylene oxide studied by experiment and DFT.•Supported ZnEt2–glycerine–Y(CCl3COO)3 ternary catalyst with SiO2 and Al2O3/SiO2.•Supported catalyst exhibited the highest activity with 3 wt% Al2O3/SiO2 as support.•The activity of the supported catalysts was related to the acidity of the support.•Appropriate electron density of zinc center of the ternary catalyst was crucial.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.The ZnEt2−glycerine−Y(CCl3COO)3 ternary catalyst has been supported on SiO2 and Al2O3/SiO2 to improve catalytic activity. Meantime, copolymerization mechanisms of CO2 and propylene oxide (PO) over five models of ZnEt2--glycerine binary catalyst and ternary catalyst system with different NBO charge on zinc centers are investigated using the DFT method.

•Novel ZnGA is achieved with small particle size and increased surface area.•ZnGA/DMC composite catalyst system presents synergistic effects on CO2/PO copolymerization.•ZnGA/DMC composite catalyst shows high catalytic efficiency and selectivity to polycarbonates.•The synthesized copolymer PPC has fairly high molecular weight (Mw = 3.9 × 105 g/mol).The high yield and high molecular weight poly(propylene carbonate) (PPC) was synthesized via copolymerization of propylene oxide (PO) and carbon dioxide (CO2) catalyzed over zinc glutarate/double metal cyanide (ZnGA/DMC) composite catalyst. A fine crystalline ZnGA component was prepared using the crosslinked restrain effect of nonionic surfactant. Combined with a small amount of DMC, the ZnGA/DMC (molar ratio = 10:1) composite catalyst system showed an excellent synergistic effect on CO2/PO copolymerization with higher activity, selectivity, and shorter reaction time than those of the traditional ZnGA catalyst. Under the optimized reaction conditions, the molecular weight of PPC was up to 3.8 × 105 g/mol at the highest yield of 508.0 gpolym/gcat, and the selectivity was over 97.7% towards polycarbonates rather than polyether linkages. The alternating copolymer PPC exhibited good thermostability, high glass transition temperature (42.0 °C) and high decomposition temperature (5% weight loss at 253.4 °C). A synergistic mechanism of ZnGA/DMC composite catalyst was speculated.Download high-res image (155KB)Download full-size image