Mechanistic Insights into Controlling the Molecular Transformations of Oxygenates
2019-07
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Mechanistic Insights into Controlling the Molecular Transformations of Oxygenates
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2019-07
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The detrimental environmental impact of excessive use of petroleum and fossil-fuel has motivated efforts to manufacture chemicals and fuels from biomass, a renewable feedstock. In addition, a large emphasis has been placed on developing chemistries that are not only atom-efficient, but also involve one-pot synthesis, green solvents, and easy separations, thereby, driving next generation technologies towards sustainability and green manufacturing. To that effect, this dissertation elucidates reaction mechanisms of such new chemistries, identifies catalytic features that enable fast, yet selective chemical transformations, as well as explores the complex role of greener solvents in mediating reactions, in a series of collaborative studies involving density functional theory (DFT) simulations and experiments. Biomass-derived precursors are shown to possess chemical and structural flexibility to generate a novel slate of structurally diverse intermediates that can be used as scaffolds in the synthesis of current chemical feedstocks as well as new molecules with advanced functionality for the design of new chemical and materials intermediates. Chemical diversification is demonstrated by using heterogeneous catalysts together with solvents to achieve high reactivity and selectivity. The results presented in this dissertation aim to guide the design of more active, selective, stable and cheaper catalytic materials, as well as synthesis of new molecules and establish new strategies to control the specific products that are formed. A significant part of this dissertation focusses on chemically diversifying coumalic acid (CMA)- a bio-privileged molecule, using a selective, one-pot Diels-Alder-decarboxylation-dehydrogenation domino sequence to generate a library of aromatic compounds. Not only petroleum-derived aromatics, but also a host of novel structures can be synthesized using this approach. These new aromatics have functionalities and structures inaccessible via petroleum and have applications as pharmaceuticals, insecticides, and antimicrobials. Here, we combine in-situ NMR, DFT, and solid-state NMR to guide one-pot synthesis of aromatics from biomass, thus, providing a sustainable means to access these compounds. Our efforts lie at engineering the reactivity by identifying the rate-limiting step in the domino sequence and then subsequently catalyzing it using a heterogeneous catalyst. Selectivity is fine-tuned by esterification of CMA as well as by screening different solvents. Finally, during synthesis of more complex aromatics, structure-reactivity relationships are developed to guide stereo-selective synthesis. This dissertation also discusses aldol condensation and esterification, reactions that are can be utilized to make longer molecules from bio-derived oxygenates to synthesize fuel-grade precursors and additives respectively. Previous studies have shown that these reactions can be catalyzed when pre-equilibrated mixture of alcohol-aldehyde-H2 is fed over metallic Cu catalysts. Here, we reveal the nature of active sites on metallic Cu that enable C-C and C-O bond formation reactions that traditionally been thought to require explicit base or acid assistance. DFT simulations, together with kinetic analyses show the prevalence of Lewis-basic alkoxide sites as well as Lewis-acidic aldehyde sites that co-operatively carry out proton and hydride transfer reactions that enable C-C and C-O bond formation reactions. The studies not only demonstrate the use of greener, earth-abundant transition metals such as Cu in biomass processing, but also opens opportunities to utilize active sites unique to Cu, and possibly other coinage metals, in reduction reactions that selectively require hydrides (for example, C-H formation). Reactions that allow for selective C-O bond hydrogenolysis are central to biomass upgradation. This dissertation also reports hydrogenolysis of fatty acid over bimetallic rhenium oxide catalysts promoted by Pd. Such bimetallic catalysts allow for near-atmospheric pressure operations, contrary to commercial processes. The role of the constituent metals is elucidated here through DFT and multiple experimental probes including steady state and transient rate measurements, extensive characterization, and kinetic isotope effects. The results demonstrate a synergistic role of Pd and Re; while Pd directly provides hydrides (electrons) for C-H formation, Re catalyzes dehydration reactions by providing protons via Brønsted acid sites (ReOH). These results can be extended to over oxophilic metals to screen more selective, cheap and durable materials in the processing of fatty acids and other oxygenates. The last part of this dissertation highlights the role of protic solvents in mediating reduction reactions of oxygenates. Reduction reactions are not only at the heart of biomass upgradation, but also are involved in a host of important applications such as organic and pharmaceutical synthesis. The use of greener solvents such as water, methanol or their mixtures instead of organic solvents is of critical importance from an environmental viewpoint. Here we combine DFT, kinetic isotope effects, flow reactor measurements, and in operando X-ray spectroscopy to show that protic solvents directly participate in hydrogenation reactions over transition metal surfaces. Protic solvents are shown to activate characteristically different routes that can enable transfer of hydrogens as protons and electrons in proton coupled electron transfer (PCET) reactions that favor the formation of O-H bonds on oxygenates. In such reactions, solvent molecules react with co-adsorbed hydrogen atoms on the metal catalyst as well as enable proton-shuttling, while the metal catalyst is found to conduct electrons. In addition, we present evidence for the prevalence of heterogeneous mediator-based chemistry that governs reduction reactions when the protic solvent can decompose on the host catalyst. The structure and reactivity of these heterogeneous surface mediators are found to resemble well-known solution-phase mediators such as TEMPOH (in organic synthesis) and NADPH (in biology). These results demonstrate the ability to impact selectivity during hydrogenation of oxygenates and perhaps other unsaturated compounds by tuning the metal and the protic solvent.
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University of Minnesota Ph.D. dissertation. July 2019. Major: Chemical Engineering. Advisor: Matthew Neurock. 1 computer file (PDF); xvii, 276 pages.
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Chemburkar, Ashwin. (2019). Mechanistic Insights into Controlling the Molecular Transformations of Oxygenates. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/224596.
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