Electroreductive C-C Coupling of Furfural and Benzaldehyde on Cu and Pb Surfaces

Biomass derived species represent an emerging alternative to petroleum as an industrial carbon feedstock. Effective utilization of biomass requires reductive upgrading of the raw carbon-oxygenates to more valuable compounds. Electroreductive coupling offers a promising strategy for this upgrading by reducing the oxygenated functional groups and increasing molecular weight through C-C bond formation. Despite this promise, elctroreductive coupling suffers from a lack of fundamental understanding. In particular, the cross-coupling of different species remains poorly understood. In this work, the electroreductive coupling of benzaldehyde and furfural on Cu and Pb electrodes is investigated. Reactivity studies show both self-coupling and cross-coupling of the two aldehydes on these 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 that both metals deviate from stochastic coupling control, with greater deviation on Pb, likely due to a larger difference in aldehyde binding energies. Cyclic voltammetry (CV) 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, CV, and spectroscopy experiments suggest that cross-coupling of the two aldehydes follows a two reactant Sabatier rule, with optimum cross-coupling for electrodes and similar reactant binding energies.