Understanding Silica-Supported Group 4 and 5 Metal Oxide Catalysts for Selective Oxidations with Hydrogen Peroxide and for Epoxide ActivationPublic Deposited
Supported metal oxides are an important class of catalysts for chemical synthesis and manufacturing, particularly for selective oxidation reactions. This dissertation seeks to understand broad relationships among families of silica-supported Group 4 and 5 metal oxide catalysts for the synthesis of epoxides with hydrogen peroxide (H2O2) and to further extend these trends to two other related oxidation reactions, thioether sulfoxidation and the ring-opening of epoxides. Here, silica-supported niobium(V) catalysts are emphasized, given their intrinsically high activity and selectivity for these applications and their relative paucity in the oxidation literature. Specifically, rigorous trends are established across (i) synthesis techniques that generate highly active sites of Nb and other oxides, (ii) spectroscopic evaluation of those sites, and (iii) kinetic assessments of catalytic performance. These quantitative relationships map predictive niobia-silica descriptors that link synthesis to structure to function for these under-appreciated catalysts, providing rational design criteria for next-generation oxidation systems of industrial and academic significance. The first part of this thesis explores the epoxidation of alkenes by H2O2 oxidant over highly dispersed Group 4 and 5 oxides on silica. Olefin epoxidation is extensively studied for organic synthesis and is of value for both fine and commodity chemical industries. There is a desire to understand supported oxide catalysts for use with H2O2, an environmentally advantageous oxidant. Hydrogen peroxide has high active oxygen content and generates H2O as a byproduct, obviating the need to sell or dispose of alcohol coproduct as in processes for alkyl hydroperoxides. Catalysts for the synthesis of epoxides and other oxygenates have predominantly focused on TiOx supported on or co-condensed with SiO2, whereas most other Group 4 and 5 metals have seen less research. The performance of these catalysts is dependent on both oxide identity and structure. The dependence of the latter on preparation methods can confound attempts at comparative studies across the periodic table, thus requiring a systematic catalyst synthesis approach to understand intrinsic behaviors. Here, a periodic family of molecular precursors designed to give the same active site geometry and nearly 100% kinetically active metal allows first comparisons of the intrinsic epoxidation activity across all of Groups 4 and 5. From this study, niobium(V)-silica (Nb-SiO2) catalysts are seen to be >2× as active and more intrinsically selective to non-radical epoxidation pathways relative to the more standard Ti-SiO2 – but only when Nb-SiO2 is synthesized to give highly dispersed active sites. Next, this high-performing class of materials is studied exhaustively, aided by an in situ chemical titrant approach developed to quantify the fraction of kinetically active Nb when catalysts are synthesized by other methods, connecting synthesis to structure to reactivity in benchmark epoxidation reactions. The difficulty in counting participating active sites has long troubled supported oxide catalysis, and key insights here have the potential to be touchstone criteria for developing Nb-SiO2 materials to displace incumbent Ti-SiO2 catalysts used in important industrial chemical processes. The second part of this dissertation extends these quantitative, predictive synthesis-structure-function relationships to study two related selective oxidation applications: thioether sulfoxidation and the ring-opening of epoxides. Sulfoxidation of thioethers with H2O2 is important for both the synthesis of medicinally relevant compounds and as a potential route to control sulfur contamination in transportation fuels; here, oxide identity and modification of Nb site Lewis acidity are explored as handles to control rate and selectivity. These design tools further enable catalyst discovery and benchmarking in the Lewis acid-catalyzed ring-opening of epoxides, as is relevant to the industrial production of polyether alcohols. Overall, this dissertation takes steps towards rational catalyst design for future sustainable chemical production.
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