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 Cr^VI ions dispersed on silica. A lower oxidation state, generally accepted to be Cr^II, is required to activate ethylene to form an organoCr active site. In this work, a mesoporous, optically transparent monolith of Cr^VI/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 Cr^VI first to Cr^IV, then to Cr^II. 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 Cr^IV sites are inactive toward ethylene at 80 °C. The Cr^II 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 organoCr^III active sites, characterized by X-ray absorption and UV–vis spectroscopy, which initiate polymerization