Mechanistic study of vanadium-modified and sulfation-modified Phillips catalyst

Abstract

• Differed from “naked” chromium center, two reduction routes with methyl formate derivant attaching on chromium center are put forward. • Compared with traditional Phillips catalyst, vanadium-modified Phillips catalyst is more exergonic during reduction process. • Alkylation with assistance of cocatalyst is meaningful to Phillips catalyst and assistance of TiBA with larger steric hindrance shows lower Gibbs free energy than dimeric TMA, which is in accordance with experimental evidences. • The addition of sulfur atom directly bonding with chromium center reduces energy barrier of chain propagation significantly. Phillips catalyst is one of the most significant industrial catalyst for polyolefin. Various modified methods have been developed to promote the polymerization activity and regulate properties of polyolefin, including vanadium-modification, sulfation-modification, etc. Vanadium-modified Phillips catalyst can produce bimodal polyethylene with sectional ultra-high molecular weight polyethylene to improve the mechanical property. Compared with traditional Phillips catalyst, sulfation-modified Phillips catalyst exhibits higher polymerization activity and wider molecular weight distribution. However, lack of knowledge still puzzles researchers in reduction, alkylation and modification for Phillips catalyst. In addition, complex chemical surrounding for heterogeneous catalyst is still a big challenge to carry out explicit analysis of this catalyst. In this work, fundamental insight into two modified methods towards Phillips catalyst has been garnered. The possible active metal centers existing on the surface of silica support and feasible precursors with reductant attaching on the chromium center were investigated. Moreover, based on two alkylation precursors with assistance of different alkyl aluminum, we proposed a probable six-membered ring mechanism through the Tischenko reaction with formation of methyl formate derivant. Interestingly, the addition of sulfur atom reduces energy barrier of chain propagation significantly. These results account for the advantages of vanadium-modification, sulfation-modification and cocatalyst for traditional Phillips catalyst.