(763f) Operando spectroscopy during Ethylene Polymerization By Supported CrOx/SiO2: Role of Promoters

Supported catalysts for the ethylene polymerization reaction have been studied since the discovery of the CrO_x/SiO₂ catalyst system in the 1950s. The addition of small amounts of promoter oxides, such as TiO_x, ZrO_x, or AlO_x, has been seen to have such effects as increasing the catalytic activity or altering polymer properties. For example, the inclusion of a few percent TiO_x has been observed to broaden the molecular weight index. There have been only a few studies reported over the years, so very little is understood about the fundamental structural and mechanistic details. The current studies extend previous work done on the un-promoted catalyst and are focused on elucidating the role of the promoter oxides used in ethylene polymerization.

To address the molecular details of ethylene polymerization by supported and promoted 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 (high UV-vis edge energy value) 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 differ depending on the promoter oxide. 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 study revealed a higher reactivity of the dioxo surface CrO₄ species and 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. The catalysts promoted with TiO_x and ZrO_x exhibited the best polyethylene production, and they were about equal, while the production of polyethylene by the catalyst promoted with AlO_x was even less than the un-promoted catalyst. This poster will address the role of the promoters for the ethylene polymerization reaction.