415090 Ethylene Polymerization By Supported CrOx/SiO2 Catalysts: Active Sites, Surface Intermediates and Structure-Activity Relationship

Thursday, November 12, 2015: 10:30 AM
355A (Salt Palace Convention Center)
Anisha Chakrabarti and Israel E. Wachs, Department of Chemical and Biomolecular Engineering, Lehigh University, Bethlehem, PA

Although supported CrOx/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+6Ox 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 CrOx/SiO2 catalysts, modern in situ molecular spectroscopic techniques were applied before and during reaction conditions. The initial oxidized supported CrOx/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 CrOx species on SiO2 in the ethylene polymerization reaction and approaches taken to optimize the concentration of surface dioxo species on the silica support.

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See more of this Session: Rational Catalyst Design I
See more of this Group/Topical: Catalysis and Reaction Engineering Division