An Inelastic Electron Tunneling Spectroscopic Study of the Interactions of Cyclic Hydrocarbons with a Zirconium Polymerization Catalyst Supported on Aluminum Oxide

Citation

Forester, Lynn (1986) An Inelastic Electron Tunneling Spectroscopic Study of the Interactions of Cyclic Hydrocarbons with a Zirconium Polymerization Catalyst Supported on Aluminum Oxide. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/fj5a-m086. https://resolver.caltech.edu/CaltechETD:etd-04112008-133837

Abstract

Inelastic electron tunneling spectroscopy (IETS) has been used to investigate the temperature and exposure-dependent reactions of two cyclic hydrocarbons, cyclohexene and 1,3-cyclohexadiene, with Zr(BH₄)₄ supported on Al₂O₃, a known olefin polymerization catalyst.

The interactions of cyclohexene with the adsorbed catalyst have been studied at exposures of 150, 1200 and 6000 Torr s at temperatures ranging from 298 to 623 K. At 150 Torr s and 298 K, the adsorbed cyclohexene is clearly an unsaturated hydrocarbon and cyclohexene coordination to the Zr catalyst does not require an appreciable displacement of BH₄ ligands. There is evidence of both tridentate and bidentate BH₄ stretching vibrations. The initial complex formed by the adsorbed cyclohexene is probably a π-complex. The IET spectra support this conclusion as well as the possibility of conversion to a η³-allyl ligand. The unsaturated surface species are converted to a saturated hydrocarbon at intermediate and high exposures of cyclohexene. The saturated specie accumulates on the surface as a function of both exposure and substrate temperature and the spectra show no olefinic increases in intensity. The saturated specie has been identified as polycyclohexene, a saturated polymer consisting of cyclohexane rings formed by homopolymerization.

The interactions of 1,3-cyclohexadiene with Zr(BH₄)₄ supported on v have been studied at exposures of 150 and 6000 Torr s. Adsorbed 1,3-cyclohexadiene has been shown to form an unsaturated hydrocarbon similar to cyclohexene at moderate exposures which continues to accumulate on the surface as a function of temperature. At high exposures, evidence has been presented that the catalyst may initiate the polymerization of 1,3-cyclohexadiene to form poly-1,3-cyclohexadiene. As with cyclohexene, there is no apparent saturation limit as based on increases in the intensity of key hydrocarbon spectral features and on junction resistance. Adsorbed 1,3-cyclohexadiene continues to accumulate on the surface as a function of temperature at 6000 Torr s. The spectral intensity of the ring breathing modes increases dramatically and their variability in position provide evidence of different types of substituted rings. Adsorption of 1,3-cyclohexadiene perturbs the catalyst much more than cyclohexene, promoting the disassociation of the BH₄ ligands.

Design changes and improvements to a high vacuum system used for the fabrication of tunnel junctions, as well as a detailed account of the experimental procedures, are described. Programs for computer spectral analysis are presented.

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