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

详细信息    查看全文

• 作者：Carole Brown ; J. Krzystek ; Randall Achey ; Adrian Lita ; Riqiang Fu ; Robert W. Meulenberg ; Matthew Polinski ; Nathan Peek ; Youhong Wang ; Lambertus J. van de Burgt ; Salvatore Profeta ; Jr. ; A. E. Stiegman ; Susannah L. Scott
• 刊名：ACS Catalysis
• 出版年：2015
• 出版时间：September 4, 2015
• 年：2015
• 卷：5
• 期：9
• 页码：5574-5583
• 全文大小：508K
• ISSN：2155-5435

文摘

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鈥揼el chemistry in order to monitor the reduction process spectroscopically. Using in situ UV鈥搗is 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鈥搗is spectroscopies. The intermediate Cr^IV sites are inactive toward ethylene at 80 掳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鈥搗is spectroscopy, which initiate polymerization.