期刊
ACS CATALYSIS
卷 5, 期 9, 页码 5574-5583出版社
AMER CHEMICAL SOC
DOI: 10.1021/acscatal.5b00927
关键词
Phillips catalyst; ethylene polymerization; active site; heterogeneous catalyst; sol-gel; reaction mechanism; redox activation; in situ spectroscopy
资金
- Catalysis Science Initiative of U.S. Department of Energy, Basic Energy Sciences [DE-FG02-03ER15467]
- National High Magnetic Field Laboratory
- NSF [DMR 1157490]
- State of Florida
- U.S. Department of Energy
- U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences [DE-AC02-76SF00515, DE-AC02-05CH11231]
- U.S. Department of Energy (DOE) [DE-FG02-03ER15467] Funding Source: U.S. Department of Energy (DOE)
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 degrees 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.
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