Spectroscopic investigation of a polymerization catalyst: Tetra(neophyl)zirconium reacted with alumina

Abstract

Tetra(neophyl)zirconium (TNZ) reacted with alumina, one of a family of high activity, innocuous, and thermally stable polymerization catalysts, was studied using transmission infrared and uv-visible diffuse reflectance spectroscopy (DRS) with the goal of understanding the synthesis of the catalyst and its evolution at the high temperatures of use. (Neophyl is the 2-methyl-2-phenylpropyl group.) Reaction of TNZ with the free hydroxyls present at the surface of alumina to bind the zirconium was observed. Hydrogen bonded hydroxyls were unreactive during synthesis but can serve as catalyst poison at elevated temperatures. High neophyl contents of catalysts prepared with thermally dehydroxylated aluminas imply other modes of TNZ binding are possible on these aluminas. No organic species other than intact neophyl ligands were observed on the untreated catalyst. Heat treatment of the catalyst in vacuum caused loss of neophyl ligands and reduction of the Zr oxidation state. An absorption at 360 nm is seen in the DRS of reduced catalysts. Heat treatment in hydrogen caused hydrogenation of the phenyl group of the ligand, loss of ligands, but no appearance of DRS features characteristic of catalyst reduction. An infrared absorption at 1900 cm −1 is produced on hydrogenation of the catalyst and is assigned to a Zr hydride species on the basis of its frequency and response to exposure to air, water, and ethylene. Catalysts containing reduced Zr or Zr hydride sites were active in the polymerization of ethylene. The high temperature activity is suggested to result from the preserved activity of the decomposition products of the original catalyst.