The systematic incorporation of Cr ions in a phase-pure silicalite-2 lattice was accomplished through a hydrothermal synthesis using 3,5-dimethylpiperidinium as a templating agent. The chromium ions, after calcination to remove the template, were in the 6+ oxidation state, with their incorporation into the lattice verified by the systematic expansion of the unit cell as a function of Cr loading. The structures of these materials as revealed by electronic spectroscopy and x-ray absorption near-edge spectroscopy (XANES) were consistent with the dioxo-structure typically exhibited by Cr6+ in an amorphous silica matrix. These materials were highly luminescent, with the emission spectra showing an unusually well-resolved vibronic structure characteristic of an emissive site with little in-homogeneous broadening. The site was reduced under flowing CO to Cr4+, as characterized by XANES. The reduction of the Cr from 6+ to 4+ resulted in unit cell volumes that are systematically smaller than those observed with Cr6+, even though the ionic radius of Cr4+ is larger. This is attributed to the fact that the Cr6+ site is not a simple metal ion but a significantly larger [CrO2]2+ unit, requiring a larger lattice expansion to accommodate it. Through analysis of the XANES pre-edge and assignment of the ligand-field spectrum of the Cr4+ ions, it is possible to establish isomorphic substitution into the silicalite lattice Cr6+-silicalite-2 at a low percent Cr loading exhibited a remarkably well resolved vibronic emission which consists of a high frequency progression of 987 cm-1, which is assigned to the fundamental symmetric stretching mode of the (-O-)2Cr(=O)2 groups. A low frequency progression at 214 cm-1, which was assigned to a symmetric O-Cr-O bending mode is built on each band of the 987 cm-1 progression. Studies of the vibronic structure of the emission spectrum as function of temperature and Cr ion concentration reveals an abrupt change in the Franck-Condon factor of the emission at 20 K for sample with very low Cr concentrations (0.03 mol%). The change in the Franck-Condon factor in was attributed to a temperature induced structural change in the coordination sphere of the metal ion. This structural change was found to be accompanied by a concomitant structural change in the lattice structure of the silicalite-2. This structural change, as studied by temperature dependent X-ray diffraction, did not involve a crystallographic phase change but an abrupt decrease in the unit cell volume caused specifically by a decrease in the c axis. This structural change was not observed in pure silicaite-2 indicating that it is not intrinsic to the silicalite lattice. Moreover, no similar structural change is observed at higher Cr loading (> 0.5 mol %). This suggests that the presence of the Cr ions, and the changes in the coordination geometry they undergo at low temperature, induced the observed contraction in the silicalite-2 lattice, in effect acting as a thermal switch that decreases the unit cell volume In spite of its long commercial use, extending over 50 years, the origin of the active sites in the Phillips' ethylene polymerization catalyst (Cr/SiO2) has never been satisfactorily explained. Specifically, what is not fully understood is structure of the organometallic active site that results from initial interactions of the ethylene with the chromium site. Using sol-gel chemistry, the Stiegman's group prepared the Phillip's catalyst as a microporous, optically transparent monolith. These studies have lead to the conclusion that the key organometallic intermediate is a Cr3+ site terminated in an intact vinyl group, Cr3+-CH=CH2.This structure was initially elucidated with solid-state 13C NMR. Further elucidation and confirmation of the presence of the terminal vinyl group was carried out by FT-IR. The spectra collected during the very early stages of polymerization reveals two peaks assignable to the terminal vinyl group. These peaks arise first followed, after several additions of ethylene, by the bands typically assigned to the methylene groups in polyethylene (-CH2-CH2-) which arise from insertion of ethylene between the Cr-vinyl bond. / A Dissertation submitted to the Department of Chemistry and Biochemistry in partial fulfillment of the requirements for the degree of Doctor of
Philosophy. / Fall Semester, 2011. / October 25, 2011. / fluorescence, luminescence, redox active sieves, silicalite, single-site catalysts, vibronic structure / Includes bibliographical references. / Albert E. Stiegman, Professor Directing Dissertation; Peng Xiong, University Representative; Susan Latturner, Committee Member; John Dorsey, Committee Member.
| Identifer | oai:union.ndltd.org:fsu.edu/oai:fsu.digital.flvc.org:fsu_183149 |
| Contributors | Tao, Yuchuan, 1980- (authoraut), Stiegman, Albert E. (professor directing dissertation), Xiong, Peng (university representative), Latturner, Susan (committee member), Dorsey, John (committee member), Department of Chemistry and Biochemistry (degree granting department), Florida State University (degree granting institution) |
| Publisher | Florida State University, Florida State University |
| Source Sets | Florida State University |
| Language | English, English |
| Detected Language | English |
| Type | Text, text |
| Format | 1 online resource, computer, application/pdf |
| Rights | This Item is protected by copyright and/or related rights. You are free to use this Item in any way that is permitted by the copyright and related rights legislation that applies to your use. For other uses you need to obtain permission from the rights-holder(s). The copyright in theses and dissertations completed at Florida State University is held by the students who author them. |
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