(665g) Ethylene Polymerization By Supported CrOx/SiO2 Catalysts: Active Sites, Surface Intermediates and Structure-Activity Relationship

Although supported CrO_x/SiO₂ ethylene polymerization catalysts have been extensively studied over the years, the same fundamental structural and mechanistic questions are still being debated today with researchers being no closer to a common view. For example, the initial molecular structure of the oxidized surface Cr⁺⁶O_x site has been proposed to be present as isolated surface dioxo CrO₄, isolated surface mono-oxo CrO₅, and dimeric surface Cr₂O₇, while the chromia oxidation state during ethylene polymerization has been proposed to be reduced Cr⁺² and Cr⁺³.

To address the molecular details of ethylene polymerization by supported CrO_x/SiO₂ catalysts, modern in situ molecular spectroscopic techniques were applied before and during reaction conditions. The initial oxidized supported CrO_x/SiO₂ catalyst was found to only contain isolated chromia sites (very high UV-vis edge energy value), a significant fraction of CrO₄ coordinated sites (intense XANES pre-edge feature) and two distinct surface chromia sites (dioxo (O=)₂CrO₂ and mono-oxo O=CrO₄).The ratio of dioxo:mono-oxo chromia species was quantified by C₂H₄-TPSR and found to be ~2. In situ Raman spectroscopy during ethylene polymerization revealed that the surface dioxo CrO₄ site reduces and becomes activated more easily than the surface mono-oxo CrO₅ site. The C₂H₄-TPSR spectra revealed that the activation of dioxo surface CrO₄ species is ~400x faster than of the mono-oxo surface CrO₅ species. The corresponding UV-vis experiments verified the reduction of Cr⁺⁶ to Cr⁺³ for both the dioxo and mono-oxo surface chromia sites during ethylene polymerization. The condensed polyethylene product formed on the catalysts during the polymerization reaction was monitored with in situ IR spectroscopy and key surface reaction intermediates were detected. This presentation will address the catalytic roles of the two distinct surface CrO_x species on SiO₂ in the ethylene polymerization reaction and approaches taken to optimize the concentration of surface dioxo species on the silica support.