Understanding the electrochemical reduction and coupling of biomass derived carbonyl species
Date
2021
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Publisher
University of Delaware
Abstract
Establishing a sustainable future requires the replacement of petroleum as the primary carbon source for modern industry. Biomass derived species offer a promising alternative to petroleum derived species for fuel and chemical production. However, such species are often low molecular weight and heavily oxygenated, and require reductive upgrading or coupling to form more valuable species. Electrochemistry offers a promising technology for biomass upgrading. Easily utilizing renewable energy sources, such as wind and solar power, electrochemical reduction uses an applied electrical potential to drive the reduction of biomass species. This technique applies to a wide range of functional groups, including aromatic rings, C=C bonds and carbonyls. Despite these advantages, electrochemical biomass upgrading largely remains unviable due to poor catalysts and a lack of mechanistic understanding. This work seeks to advance the mechanistic understanding of electrochemical biomass upgrading by investigating the electrochemical reduction of carbonyl species. ☐ The first chapter investigates benzaldehyde reduction on four different metals: Cu, Au, Pt and Pd. Reactivity tests show a large difference in reduction selectivity between metals, with Cu showing benzaldehyde coupling ability, while the other metals do not. In situ infrared spectroscopy experiments suggest this difference in coupling ability results from the relative ability of the metal surface to stabilize the ketyl radical reaction intermediate. Spectroscopic features related to the ketyl radical appear on Au and Cu surfaces, but not on Pt or Pd. The appearance of radicals on both Au and Cu suggests the difference in Cu and Au coupling ability results from a lower radical concentration on Au, likely due to lower radical stability. On Pt and Pd, CO appears under reduction conditions, suggesting the general instability of benzaldehyde adsorbates limits surface coverage and coupling ability. Combined, the spectroscopic and reactivity evidence suggest ketyl radical stability acts as a key descriptor of benzaldehyde coupling ability. ☐ Subsequently, the second chapter extends the analysis to the reductive coupling of benzaldehyde and furfural on Cu and Pb electrodes. Under simultaneous reduction, reactivity tests show both the self-coupling and cross-coupling of the aldehyde species on the two metal surfaces, but with different selectivities. Cu shows greater selectivity for cross-coupling, whereas Pb favors furfural coupling. Comparison with a stochastic model suggests both metals deviate from stochastic coupling control, with greater deviation on Pb, likely due to a larger difference in aldehyde binding energies. Cyclic voltammetry and in situ spectroscopy further support stronger benzaldehyde adsorption compared to furfural on both metals, with a larger difference in binding energy for Pb. Combined, the reactivity, cyclic voltammetry and spectroscopy experiments suggest that the cross-coupling of two aldehydes follows a two reactant Sabatier rule, with optimum cross-coupling for electrodes with similar reactant binding energies. ☐ Finally, the third chapter investigates the effect of structure on reduction activity for aliphatic ketone reduction on Pb and Au electrodes. Specifically, reduction kinetics are investigated for acetone, 2-butanone, 2-pentanone, 2-hexanone, cyclopentanone and cyclohexanone. Reactivity tests show only an alcohol product, with reduction activity decreasing with size for the linear ketones. Cyclic species show higher activity than the corresponding linear species, with activity increasing with ketone size. Similar Tafel slopes suggest a common reduction mechanism for all ketones on both metals. A change in Tafel slope with potential suggests a change in the ketone reaction network. Comparison with a simple model suggests this change likely results from increased hydrogen competition at lower potentials. Rate order and pH dependent measurements further support this explanation. Temperature dependent measurements suggest that rate decreases with ketone size result from a smaller pre-exponential factor. Comparison with a kinetic model suggests the decrease in pre-exponential factor results from weaker orbital overlap for larger ketones, with hydraulic radius offering a good descriptor for ketone size. Cyclohexanone proves the exception, likely due to a different binding orientation or higher binding strength. Activation energy measurements suggest similar intrinsic activation energies for all ketone species, with variation in observed activation energy resulting from different adsorption energies.
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Keywords
Biomass Upgrading, Electrochemical Reduction, Electrochemistry, Sustainability