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DOE OSTI · 2505032

Supported Organochromium Ethylene Oligomerization Enabled by Surface Lithiation

Kanbur, Uddhav·Hall, Jacklyn N.·Kim, Yu Lim·Niklas, Jens·Poluektov, Oleg G.·Liu, Cong·Kropf, A. Jeremy·Delferro, Massimiliano·Kaphan, David M.
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Abstract

Here, in this work, supported organochromium ethylene polymerization catalysts have been tuned to mediate ethylene oligomerization via surface lithiation, which provides a generalizable protocol to control stereoelectronics and redox states of surface organometallic active sites. The homoleptic chromium(IV) alkyl complex Cr(CH 2 SiMe 3 ) 4 was grafted on high-surface-area anatase titania (TiO 2 ) nanoparticles as well as on silica to produce Cr/TiO 2 and Cr/SiO 2 , respectively. Treatment of these materials with excess n-butyllithium led to the reduced chromium complexes Cr/Li x TiO 2 and Cr/Li/SiO 2 , each of which still retains one hydrocarbyl ligand on chromium. A set of heterogeneous complexes were studied by electron paramagnetic resonance and X-ray absorption spectroscopy, which indicate a reduction in the oxidation state of the major chromium species to Cr II upon lithiation. Cr/Li x TiO 2 converts ethylene to hexenes with a high selectivity (>80%), which was persistent over 10 days at 80 °C, achieving >950 turnovers. The exclusive formation of C 4 and C 6 olefins, preferring the trimerization product, without a statistical (Flory–Schulz) distribution is characteristic of the oxidative cyclization oligomerization mechanism rather than the traditional Cossee–Arlman mechanism, whereas Cr/Li/SiO 2 produced a mixture of trimerization and polymerization products, suggesting site heterogeneity in the silica-based material. On the other hand, the unreduced chromium(IV) materials as well as low lithium-containing Cr/Li x TiO 2 (x < 0.16) exclusively produced ultrahigh molecular weight polyethylene, determined by differential scanning calorimetry and gel permeation chromatography analysis, likely formed via a linear-insertion mechanism, with a crossover from the polymerization to oligomerization regime observed at ~16% Li intercalation.

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37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRYchromium catalysisethylene trimerizationlithium intercalationoxidative cyclizationsurface organometallic chemistry

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Kanbur, Uddhav, Hall, Jacklyn N., Kim, Yu Lim, Niklas, Jens, Poluektov, Oleg G., Liu, Cong, Kropf, A. Jeremy, Delferro, Massimiliano, Kaphan, David M.. 2024-05-20. Supported Organochromium Ethylene Oligomerization Enabled by Surface Lithiation. https://doi.org/10.1021/acscatal.4c01672

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