Active Site Transformation During the Induction Period of Ethylene Polymerization over the Phillips CrO_<i>x</i>/SiO₂ Catalyst

Abstract

Abstract In spite of the great importance of the Phillips catalyst in commercial polyethylene production and long‐term research efforts, the initiation mechanism of polymerization still remains unclear. The effect of formaldehyde desorption on the active site transformation during the induction period of the Phillips catalyst is investigated over cluster models by using DFT. No reaction can be initiated over the cluster model coordinated with two formaldehyde molecules, owing to steric hindrance and electronic donation. The first reaction over cluster models, on which either one or no formaldehyde molecule is adsorbed, follows the metallacyclic mechanism into chromacyclopentane. Subsequent dimerization to 1‐butene and metathesis to propylene and ethylene are more favorable over the cluster model adsorbed with one formaldehyde molecule. Only after a complete desorption of formaldehyde does further ring expansion to chromacycloheptane followed by 1‐hexene formation become preferential. Spin state crossing from quintet diethylene–Cr II complex to triplet chromacyclopentane with a spin acceleration effect is revealed.