文献类型：期刊文献

中文题名：Mechanistic Study on the Dominant Promotion Effect of Al-/Ti-/Zr-modifications over the VOx/SiO2 UHMWPE Catalysts

英文题名：Mechanistic Study on the Dominant Promotion Effect of Al-/Ti-/Zr-modifications over the VOx/SiO2 UHMWPE Catalysts

作者：Yu-Long Jin[1];Lin Liu[2];Yu-Jie Wang[1];Zhen Liu[2];Bo-Ping Liu[1]

机构：[1]College of Materials and Energy,South China Agricultural University,Guangzhou 510642,China;[2]School of Chemical Engineering,East China University of Science and Technology,Shanghai 200237,China

年份：2019

卷号：37

期号：10

起止页码：995

中文期刊名：Chinese Journal of Polymer Science

外文期刊名：高分子科学（英文版）

收录：CSTPCD;;Scopus;CSCD:【CSCD2019_2020】；

基金：financially supported by the National Natural Science Foundation of China (No. 21801079)

语种：英文

中文关键词：Density functional theory;Vanadium-oxides;Ethylene polymerization;Support modification;Mechanism

外文关键词：promotion;insertion;catalyst;

摘要：Recently, we reported the first VOx/SiO2 ethylene polymerization catalyst for making Cl-free UHMWPE, and found the dominant promotion effects of Al-/Ti-/Zr-modifications over this catalyst system(Macromol. Chem. Phys. 2017, 218, 1600443). In this work, density functional theory is applied to investigate the underlying mechanism of this remarkable promotion effect of Al-/Ti-/Zrmodifications on a molecular and atomic level. The cluster model with V(Ⅲ) is found to be the most possible active site due to its lowest overall energy barrier for monomer insertion, though the process of C2H4 coordination and the subsequent formation of transition state are most energy favored for V(Ⅱ) species. By modifying one of or both V―O―Si in the active model with V―O―M(M = Al, Ti, or Zr),the energy barrier for the binding of the upcoming C2H4 gets lower(particularly for Al-and Zr-modified catalysts), and the transition state also becomes more stable. Generally, the insertion process of C2H4 gets easier after support Al-/Ti-/Zr-modifications. This dominant promotion effect is partially ascribed to the more enriched positive charge distribution on or nearby the V center, and the narrower energy gap between the LUMO of model catalysts and the HOMO of C2H4 for these modified catalysts also contributes much. In addition, the decreased steric hindrance around the V center should be taken into account for the modified models as well. Furthermore, the Br?nsted acidity of the catalysts is investigated by introducing a pendent hydroxyl group to the model catalysts, which has a close contact with the V center. Similar promotion effect of support modification by Al, Ti, and Zr could still be observed.
Recently, we reported the first VOx/SiO2 ethylene polymerization catalyst for making Cl-free UHMWPE, and found the dominant promotion effects of Al-/Ti-/Zr-modifications over this catalyst system（Macromol. Chem. Phys. 2017, 218, 1600443）. In this work, density functional theory is applied to investigate the underlying mechanism of this remarkable promotion effect of Al-/Ti-/Zrmodifications on a molecular and atomic level. The cluster model with V（Ⅲ） is found to be the most possible active site due to its lowest overall energy barrier for monomer insertion, though the process of C2H4 coordination and the subsequent formation of transition state are most energy favored for V（Ⅱ） species. By modifying one of or both V―O―Si in the active model with V―O―M（M = Al, Ti, or Zr）,the energy barrier for the binding of the upcoming C2H4 gets lower（particularly for Al-and Zr-modified catalysts）, and the transition state also becomes more stable. Generally, the insertion process of C2H4 gets easier after support Al-/Ti-/Zr-modifications. This dominant promotion effect is partially ascribed to the more enriched positive charge distribution on or nearby the V center, and the narrower energy gap between the LUMO of model catalysts and the HOMO of C2H4 for these modified catalysts also contributes much. In addition, the decreased steric hindrance around the V center should be taken into account for the modified models as well. Furthermore, the Br?nsted acidity of the catalysts is investigated by introducing a pendent hydroxyl group to the model catalysts, which has a close contact with the V center. Similar promotion effect of support modification by Al, Ti, and Zr could still be observed.