Do Mono-oxo Sites Exist in Silica-Supported Cr(VI) Materials? Reassessment of the Resonance Raman Spectra

The monomeric, single-atom oxochromium species present on the surface of silica-supported Cr­(VI) catalysts was characterized in detail using resonance Raman (RR) spectroscopy over a range of excitation wavelengths corresponding to the primary electronic transitions of Cr­(VI)/SiO₂. The findings resolve a long-standing controversy regarding the possible contribution of mono-oxoCr­(VI) sites, (SiO)₄CrO, postulated to coexist with the well-established dioxoCr­(VI) sites, (SiO)₂Cr­(O)₂. Density functional theory (DFT) calculations and a normal coordinate analysis conducted using a chromasiloxane model cluster confirm prior assignments of bands in the nonresonant Raman spectrum at 986 and 1001 cm^–1 to the symmetric and antisymmetric stretching modes, respectively, of the dioxoCr­(VI) sites. For all excitation energies, the symmetric stretch shows apparent resonant enhancement. Since all of the electronic transitions are strongly allowed, this finding is consistent with A-term enhancement. UV excitation at 257 nm (into the high energy electronic transition centered at 271 nm) also results in modest resonant enhancement of the antisymmetric stretch, due to the low average symmetry of the surface sites. Excitation at 351 nm (into the electronic transition centered at 343 nm) results in a strong increase in the relative intensity of the antisymmetric stretch, which is likely caused by B-term enhancement. Previously reported evidence for a mono-oxoCr­(VI) site consists of a vibrational band observed at ca. 1011 cm^–1 and assigned to its CrO stretch. However, the band is observed only upon excitation into the lowest-energy electronic transition, at 439 nm. We show that excitation into this electronic transition causes photoinduced decomposition. The process depends on the laser power and duration of exposure, and it yields the band previously assigned to a mono-oxo species. The resonance Raman study reported here, in combination with our recent rigorous analysis of the corresponding electronic spectra, lead us to conclude that there is no credible spectroscopic evidence for the existence of mono-oxochromate species in highly dispersed Cr/silica materials

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

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

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

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