载体硅胶表面氟改性对Phillips铬系催化剂诱导期内引发乙烯聚合行为的影响

Effect of fluorination of silica support on initiation of ethylene polymerization during induction period over Phillips catalyst

通过密度泛函理论考察了载体表面氟改性对吸附不同数目甲醛分子的Phillips催化剂诱导期内引发乙烯聚合反应的影响。结果表明,当乙烯还原六价铬酸酯形成的二价铬前驱体模型上吸附两分子甲醛时,其位阻效应阻碍了任何反应的进一步发生;当二价铬前驱体模型上吸附一分子甲醛时,只能通过先形成铬金属五元环进而发生乙烯二聚反应和易位反应,但是氟改性对两者的影响很小;当二价铬前驱体模型上吸附的甲醛分子完全脱附后,则可以进一步环增长生成铬金属七元环,并且氟改性对这一步反应有促进作用;而氟改性对铬金属七元环进一步开环生成1-己烯则是不利的。研究还表明,氟改性对于三价铬-烷基聚合活性中心模型上的链增长是有利的。

In spite of great importance of Phillips CrOx/SiO2 catalyst for commercial polyethylene procluctlon ana long term research efforts, polymerization mechanism, especially initiation mechanism, still remains unclear. The effect of surface fluorination of silica support on initiation of ethylene polymerization during induction period over Phillips catalyst adsorbed with different amounts of formaldehyde molecules was investigated with the density functional theory. It was demonstrated that no reaction could be initiated over cluster models adsorbed with two formaldehyde molecules on account of steric hindrance. For cluster models adsorbed with one formaldehyde molecule, ethylene dimerization to 1-butene and metathesis to propyIene took place via chromacyclopentane intermediate, and fluorination of silica support showed minor influence on both reactions. After a complete desorption of formaldehyde molecules, further ring expansion to chromacycloheptane occurred and surface fluorination of silica support showed improvement on this process. Fluorination of silica support was unfavorable to ring-opening of ehromacycloheptane to give 1-hexene. It was also demonstrated that fluorination showed positive effect on chain propagation over models of Cr^III-alkyl active sites during ethylene polymerization.