An XAS investigation of the structure of silica-supported titanium, vanadium and gallium complexes.

摘要

This thesis is divided into 3 main sections. The first section examines the structure of the surface sites formed in the reaction of VOCl3 with the surface of Aerosil and Sylopol silicas. Each silica was dehydroxylated at two different temperatures. Vanadium K-edge XANES and EXAFS reveals that the sites have a uniform first coordination sphere regardless of the hydroxyl content. EXAFS curve-fitting confirmed that the sites are ≡SiOVOCl 2, but revealed an unexpected asymmetry in the V-Cl bond distances, which is suggested to be a manifestation of silicon-chloride interactions. The analogous reaction was performed with VO(OiPr)3 affording ≡SiOVO(OiPr)2. The EXAFS suggested a similar asymmetry of 2-propoxy ligands.; The second section describes the reaction of TiCl4 with Aerosil silica, dehydroxylated at two pretreatment temperatures. Metals analysis of the material show an initial 1:1 Ti:OH for each silica pretreatment temperature. The product of the reaction of TiCl4 with silica pretreated at low temperature (100°C) is unstable under dynamic vacuum. Cryogenic XANES and EXAFS show that the 5-coordinate [Cl3Ti(mu-OSi≡) 2] site, which is initially formed, transforms to a dipodal Cl 2Ti(OSi≡)2 species by heating or extended evacuation. Information confirming the dimeric titanium site is present only in EXAFS recorded at low temperature due to the lack of thermal motion. The site formed on silica pretreated at 500°C, ≡SiTiOCl3 does not evolve.; The final section explores the structure of sites formed when GaMe 3 reacts with the surface of a nonporous Aerosil silica pretreated at 100 or 500°C. Elemental and gas-phase analysis, in situ IR and solid-state NMR, reveal a uniform surface site with the empirical formula ≡SiOGaMe 2. Most of the grafting (85%) occurs on hydroxyls, but a small amount (15%) of siloxane (O3SiO-SiO3) bond cleavage is also observed. The Ga K-edge EXAFS of GaMe3-modified silica, recorded at 10 K, reveals that each Ga has a Ga neighbor at 2.97-2.99 A. Features apparent for the positive identification of a dinuclear site are enhanced with cryogenic EXAFS collection.{09}The EXAFS of two model complexes, which also provide dinuclear gallium sites, are presented. In all cases, gallium sites are bridged through siloxide ligands suggesting that silica hydroxyls are always predominately adjacent.