Supported catalysts for the ethylene polymerization reaction have been studied since the discovery of the CrOx/SiO2 catalyst in the 1950s. The addition of small amounts of promoter oxides, such as TiOx, ZrOx, or AlOx, has been seen to have such effects as increasing the catalytic activity or altering polymer properties. For example, the inclusion of a few percent TiOx 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 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 (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 than 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 operando IR spectroscopy and key surface reaction intermediates were detected. The catalysts promoted with TiOx and ZrOx exhibited the best polyethylene production, while that of the AlOx-promoted catalyst was even less than the un-promoted catalyst. This presentation will address the role of the promoters for the ethylene polymerization reaction.

Operando spectroscopy during ethylene polymerization by supported CrOx/SiO2 catalysts: Role of promoters

253rd National Meeting (2017)

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