Doctoral Dissertations

Date of Award

12-2020

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Chemistry

Major Professor

Craig E. Barnes

Committee Members

Ziling Xue, Sharani Roy, David J. Keffer

Abstract

Catalysts are an essential component in the chemical industry converting raw materials into essential products. Transition metal catalysts, more specifically titanosilicates, are effective in the selective oxidation of olefins producing monomers and chemical intermediates used in many industrial applications. A major challenge in the catalysis community is developing synthetic methodologies that produce robust heterogeneous catalysts with isolated, single type of active sites with targeted connectivities to the support matrix.

The Barnes’ research group has developed a building block synthetic methodology that produces a single-site heterogeneous catalyst. This methodology uses a molecular building block and titanium (IV) precursors to create site isolated, atomically dispersed titanium actives sites with identical connectivities to a porous silicate matrix. The primary focus of this family of catalysts has been synthesizing mononuclear titanium (IV) active sites supported within a silicate matrix. The goals of this dissertation aim at extending the building block methodology to synthesizing and characterizing a trinuclear titanosilicate single-site heterogeneous catalyst with targeted connectivities to the support matrix.

The highly active trinuclear catalysts synthesized in this work have a Ti3O2 core containing bridging carboxylate and oxo ligands between titanium centers with a well-defined number of connections to the silica building block matrix. Characterization techniques determined the connectivity of the active site to the silicate matrix as well as the surface area and porosity of the resulting materials. Gravimetric analysis, 1 HNMR, and infrared spectroscopy were used as frontline techniques to determine the initial connectivity of the active site to the support. XAS and DRUV conclusively showed that the titanium atoms in the Ti3O2-core contain higher coordination geometries (5- and 6-coordinate centers), with EXAFS results clearly supporting the existence of a single type of Ti3O2-core.

Catalytic test reactions involving the selective oxidation of cyclohexene to cyclohexene oxide was extensively studied. The catalysts synthesized in this work exhibit excellent activity and selectivity to the targeted product and are better than several leading candidates described in the literature. Furthermore, these Ti3O2-based titanosilicates possess a superior chemical robustness capable of turn-over numbers exceeding 7500 with turn-over frequencies ~95 min-1, with respect to oxide formation.

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