2 research outputs found

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

    No full text
    The detailed mechanism by which ethylene polymerization is initiated by the inorganic Phillips catalyst (Cr/SiO<sub>2</sub>) 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<sup>VI</sup> ions dispersed on silica. A lower oxidation state, generally accepted to be Cr<sup>II</sup>, is required to activate ethylene to form an organoCr active site. In this work, a mesoporous, optically transparent monolith of Cr<sup>VI</sup>/SiO<sub>2</sub> 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<sup>VI</sup> first to Cr<sup>IV</sup>, then to Cr<sup>II</sup>. 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<sup>IV</sup> sites are inactive toward ethylene at 80 °C. The Cr<sup>II</sup> 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<sup>III</sup> active sites, characterized by X-ray absorption and UV–vis spectroscopy, which initiate polymerization

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

    No full text
    The detailed mechanism by which ethylene polymerization is initiated by the inorganic Phillips catalyst (Cr/SiO<sub>2</sub>) 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<sup>VI</sup> ions dispersed on silica. A lower oxidation state, generally accepted to be Cr<sup>II</sup>, is required to activate ethylene to form an organoCr active site. In this work, a mesoporous, optically transparent monolith of Cr<sup>VI</sup>/SiO<sub>2</sub> 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<sup>VI</sup> first to Cr<sup>IV</sup>, then to Cr<sup>II</sup>. 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<sup>IV</sup> sites are inactive toward ethylene at 80 °C. The Cr<sup>II</sup> 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<sup>III</sup> active sites, characterized by X-ray absorption and UV–vis spectroscopy, which initiate polymerization
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