Mechanism of Initiation in the Phillips Ethylene Polymerization Catalyst: Redox Processes Leading to the Active Site

Abstract

The detailed mechanism by which ethylene polymerization is initiated by the inorganic Phillips catalyst (Cr/SiO2) without recourse to an alkylating cocatalyst remains one of the great unsolved mysteries of heterogeneous catalysis. Generation of the active catalyst starts with reduction of CrVI ions dispersed on silica. A lower oxidation state, generally accepted to be CrII, is required to activate ethylene to form an organoCr active site. In this work, a mesoporous, optically transparent monolith of CrVI/SiO2 was prepared using sol–gel chemistry in order to monitor the reduction process spectroscopically. Using in situ UV–vis spectroscopy, we observed a very clean, stepwise reduction by CO of CrVI first to CrIV, then to CrII. Both the intermediate and final states show XANES consistent with these oxidation state assignments, and aspects of their coordination environments were deduced from Raman and UV–vis spectroscopies. The intermediate CrIV sites are inactive toward ethylene at 80 °C. The CrII sites, which have long been postulated as the end point of CO reduction, were observed directly by high-frequency/high-field EPR spectroscopy. They react quantitatively with ethylene to generate the organoCrIII active sites, characterized by X-ray absorption and UV–vis spectroscopy, which initiate polymerization.