Mediated Electrocatalysis for Lignin Valorization: Iron-Based Catalysts for β-O-4 Cleavage

Abstract

Electrochemical lignin valorization provides a sustainable route to aromatics, yet its viability is limited by high overpotentials, electrode fouling, and indiscriminate bond cleavage. Herein, we present a low-cost electrocatalytic cycle, based on soluble iron complex mediators, that decouples electron transfer at the electrified interface from chemical oxidation to drive the sustained, targeted cleavage of recalcitrant β-O-4 linkages. Using a representative lignin model compound, 1-(3,4-dimethoxyphenyl)–2-(2-methoxyphenoxy)–1,3-propanediol, we contrast the low-spin [Fe(CN)6]3–/4– with the high-spin of the Fe(acac)3 system, demonstrating that the mediator's spin state, electrode material, and applied potential dictate reaction yield and product distribution. This enables tunable selectivity without requiring costly or synthetically demanding ligands. Mechanistic and computational studies reveal the distinct effects of outer-sphere vs inner-sphere processes of the two systems. Translating this methodology to technical lignosulfonate validates its robustness for macromolecular processing, leading to the formation of both aliphatic products (e.g., acetic acid, butanoic acid, and glycolic acid) and high-value aromatic products (e.g., o-cresol, 3,4-dihydroxy-5-methoxybenzaldehyde, and 2-hydroxyterephthalic acid).