Active sites formation and their transformations during ethylene polymerization by the Phillips CrOx/SiO2 catalyst

作者：Gierada Maciej; Handzlik Jaroslaw^*

来源：Journal of Catalysis, 2017, 352: 314-328.

DOI：10.1016/j.jcat.2017.05.025

• Phillips catalyst
• Chromium oxide
• Silica
• Surface
• Ethylene polymerization
• Active site
• Defect site
• Mechanism
• DFT
• Cluster model

The nature of the active sites and the mechanism of ethylene polymerization over the Phillips CrOx/SiO2 catalyst are still under debate. In this work, a number of potential initiation, propagation and termination mechanisms for ethylene polymerization are investigated using density functional theory and cluster models. Cr(II), Cr(III), Cr(III)-OH and Cr(V) oxide species are considered as the active site precursors. It is predicted that the oxachromacycle ring expansion mechanism is more probable than the routes involving equivalent to Si(OH)Cr(II)-vinyl or equivalent to Si(OH)Cr(III)-vinyl sites. We also show that the hydroxylated Cr(III)-OH species, further transforming into the active Cr(III) vinyl sites, can be effective for ethylene polymerization. Additionally, we propose that defect sites in the amorphous silica framework can play a role in low-temperature transformation of the Cr(II) species into the active Cr(III) sites. A potential polymerization mechanism involving minor Cr(V) oxide species is also calculated.

• 出版日期2017-8