Type of Document Dissertation Author Brown, Carole Elizabeth URN etd-07102006-210229 Title Studies of Silica Supported Metal Catalysts Degree Doctor of Philosophy Department Chemistry and Biochemistry, Department of Advisory Committee
Advisor Name Title A. E. Stiegman Committee Chair Ashby Plant Committee Member Ken Goldsby Committee Member Naresh Dalal Committee Member Vincent Salters Committee Member Keywords
- Metal Catalysts
Date of Defense 2006-06-21 Availability unrestricted AbstractSTUDIES OF SILICA SUPPORTED METAL CATALYSTS
Name: Carole Elizabeth Brown
Department: of Chemistry and Biochemistry
Major Professor: A. E. Stiegman
Degree: Doctorate of Philiosophy
Term Degree Awarded: Summer 2006
Many industrially important heterogeneous catalytic reactions are not well understood. This lack is mainly due to the high reactivity and transient nature of the reactive sites and chemical intermediates involved. Monolithic, sol-gel derived, metal-silica glasses are ideal for use in studying these processes in greater detail for several reasons. First, the sol-gel process homogeneously disperses the discrete metal sites along the silica support which allows the catalytic sites to act as single molecules. Second, the reactive metal oxide sites are accessible by small molecules and, therefore, specific reaction processes can be studied and active catalytic sites can be generated systematically.
In the studies described here, silica-supported vanadium oxide groups were used to investigate metal-ligand multiple bond formation. The existence of surface-supported transition metals containing multiply bonded ligand systems such as alkylidyne and imido groups have long been proposed as key intermediates heterogeneous catalytic reactions including polymerization and ammoxidation. Aniline was shown to convert cleanly to a phenyl-imido ligand group on the supported vanadium oxide with the elimination of water. Once generated, the metal-phenyl-imido site is catalytically active in the formation of benzylideneaniline from benzaldehyde. Further studies investigated more industrially important reactions such as the activation of the vanadium by nitrogen compounds, including ammonia and alkylated amines, for use in the NOx/NH3-catalytic reaction. Ammonia and alkylated amines react cleanly and result in reduction of the vanadium(V) to vanadium(IV).
The formation, activation, and catalytic reaction of the Phillips catalyst was studied using discrete silica-supported chromium dioxide groups. As with the vanadium catalysts, the reactivity of the discrete metal sites provide an ideal way to study the mechanisms of activation and catalysis on a molecular level. The metal in the chromium(VI) dioxide species is reduced, from chromium(VI) to chromium(IV) to chromium(II), and then react with ethylene to produce the Phillips active site. The active site of the Phillips catalyst was found to be a chromium(III) alkyl site. This site has not been previously identified.
The mechanisms of the activation and catalytic processes as well as the resulting complexes of both catalyst types were characterized using techniques such as GC, IR, and Raman spectroscopy.
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