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Computational Mechanistic Study of the Reactivity of Metal Oxide Catalyst towards Olefins


Two heterogeneous catalysts, the Phillips ethylene polymerization catalyst and supported rhenium olefin-metathesis catalyst, are computationally investigated.

The mechanism of ethylene polymerization by the Phillips catalyst remains unknown despite numerous hypotheses and studies since its discovery sixty years ago. This work uses density functional and small chromasiloxane cluster models to compare initiation, propagation, and termination pathways for several previously proposed mechanisms and for two newly proposed mechanisms. Where possible, complete catalytic cycles and predicted kinetics, molecular weights, and site abundances are compared to properties of the Phillips catalyst. Prohibitively high activation barriers for propagation rule out chromacycle ring expansion and Green-Rooney mechanisms (alternating alkylidene/chromacycle). A new oxachromacycle ring expansion mechanism has a favorable plausible propagation barrier, but initiation for the oxachromacycle expansion is prohibitively slow. Chain growth is fast on a recently proposed bridging hydroxyl Si(OH)CrIII-alkyl site that is initiated by proton transfer from ethylene. However, the initiation step is extremely slow and uphill, and termination is even faster than propagation, so essentially all sites remain trapped in a dormant state. A new Si(OH)CrII-alkyl site also has a small barrier for Cossee-Arlman type chain growth, but it forms only oligomers because termination by proton transfer back to the alkyl chain is faster than propagation, and initiation is slow. Only the monoalkylchromium(III) site (≡SiO)2Cr-alkyl is viable as an active site for polymerization, however the nature of the initiation mechanism remains unknown.

CH3ReO3 (MTO) interacting with the surface of extensively chlorinated γ-Al2O3 is a highly active catalyst for olefin metathesis. Its activity, selectivity and stability exceed that of any other reported MTO-based catalyst as measured in propene metathesis at 0 °C, including MTO/SiO2-Al2O3 and MTO/ZnCl2/-Al2O3, and are far higher than MTO/-Al2O3. DFT calculations support facile Cl-O ligand exchange between MTO and Cl-Al2O3. The calculations further suggest that chlorination at Re promotes active site formation via tautomerization.

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