Electrochemical Oxidation of Phenolic Compounds at Boron-Doped Diamond Anodes: Structure-Reactivity Relationships

Electrochemical oxidation of phenolic compounds using boron-doped diamond (BDD) anodes has been shown as an effective approach to remove these contaminants from water. However, the understanding of the reaction mechanisms of substituted phenolic compounds at the BDD anode remains incomplete. In the present work, we investigated the electrochemical oxidation of 12 representative phenolic compounds (with varied substitution groups (e.g., -CH₃, -OCH₃, -NH₂, -Cl, -OH, -COOH, -NO₂, -CHO) and positions (-ortho, -meta, and -para)) at the BDD anode. Our analysis shows that unlike previous studies the two parameters, the Hammett constants of the substituents and the highest atomic charge on the aromatic ring, fail to adequately describe the reaction rate change when the chemical structures become complicated (i.e., with increased steric effects). Instead, a quantitative structure-property relationship (QSPR) was established with 26 molecular descriptors and using a partial least-squares regression approach. The QSPR analysis shows that the energy gap between the lowest unoccupied molecular orbital and the highest occupied molecular orbital, E_LUMO - E_HOMO, which reflects the chemical stability of a molecule, is the predominant molecular descriptor determining the reaction rate constant. Furthermore, the predicated rate constants agree well with the observed ones. The findings are consistent with previous studies of SnO₂ anodes, suggesting that chemical structural parameters such as the molecular orbital energies are critical to consider when elucidating and predicating the electrochemical reactivity of phenolic compounds at these nonactive anodes.