As an alternative to petroleum, biomass derived carbon-oxygenates represent an emerging class of compounds with potential as a carbon feed stock for the production of fuels and chemicals. Typically derived from lignocellulosic materials, these compounds offer a range of chemical functionality, but remain less viable than petroleum due to a higher oxygen content and lower molecular weight. Electroductive coupling offers a promising option to address both issues and upgrade biomass oxygenates to higher value products. Driven by an applied electrical potential, this technique simultaneously reduces the oxygenate species while growing molecular size through new C-C bond formation. In particular, electroreductive coupling shows promise for upgrading many biomass derived carbonyl species, such as furfural and benzaldehyde derivatives, which show relatively facile coupling. Despite this promise, a lack of mechanistic understanding currently hinders further electroreductive catalyst development. In particular, the strong variation in carbonyl coupling ability between metals remains poorly understood, with some metals (Cu, Pb, Ti, Sn) able to perform coupling while others (Pt group, Ni, Ag) cannot. To address this gap in understanding, we have investigated electroreductive carbonyl coupling on Au, Cu, Pd, and Pt using benzaldehyde reduction as a model reaction. Reactivity tests show that, of the metals tested, Cu has a unique ability to perform benzaldehyde coupling. Subsequent operando spectroscopy suggests this unique coupling results from an ability of Cu to better stabilize a key reaction intermediate, i.e. the ketyl radical. Spectroscopic features corresponding to the ketyl radical are identified on both the Au and Cu surfaces, with a higher radical concentration likely allowing the coupling on Cu. Pt and Pd do not display these radical peaks, and instead show CO formation due to decarbonylation, suggesting instability of benzaldehyde adsorbates on Pt and Pd. Combining the reactivity and spectroscopy, we suggest kety radical stability as key descriptor in benzaldehyde coupling ability.

Electroreductive coupling of aldehydes on metal surfaces: Importance of ketyl radical stability for coupling activity

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.