Electrochemical depolymerization of lignin catalyzed by biomass-based magnetic-electrocatalyst

Abstract

The depletion of fossil resources has intensified the search for sustainable carbon feedstocks, placing lignin, the most abundant renewable aromatic macromolecule in nature, at the center of modern biorefineries and circular bioeconomy strategies. Despite its enormous potential, lignin valorization remains challenging because its highly cross-linked three-dimensional architecture and exceptional chemical stability hinder efficient depolymerization and selective conversion. Electrochemical oxidation has emerged as an attractive alternative to conventional thermochemical approaches by operating under mild conditions without requiring strong chemical oxidants. Nevertheless, practical implementation remains constrained by two persistent challenges: heterogeneous powder catalysts are difficult to separate from reaction media after electrolysis, while inefficient mass transfer at the solid–liquid interface limits catalytic performance. To address these limitations, this thesis develops a novel strategy in which waste maple wood (MW) and the metal-rich industrial residue red mud (RM) are co-pyrolyzed to produce a low-cost magnetic biochar (MBC) heterogeneous electrocatalyst. Systematic optimization of the synthesis conditions identified M-MBC–5/1–900, prepared at 900 °C with a MW-to-RM mass ratio of 5 : 1, as the most active catalyst. During high-temperature carbothermic reduction, the iron oxides originally present in RM are transformed in situ into highly active zero-valent iron (α-Fe0) and iron carbide (Fe3C) nanophases, which become uniformly embedded within a highly graphitized carbon framework possessing a hierarchical mesoporous architecture. At the same time, the silica network derived from the intrinsic Si–O species in RM provides robust structural support, effectively suppressing nanoparticle sintering and agglomeration throughout pyrolysis. The resulting catalyst combines abundant accessible active sites with excellent structural stability, features that were consistently reflected in its superior electrochemical performance during repeated experiments. To account for the remarkable catalytic activity observed in alkaline electrolytes, experimental evidence directs our attention to the interfacial characteristics of MBC. The graphitized carbon matrix is proposed to establish strong π–π stacking and hydrophobic interactions with the aromatic backbone of lignin, overcoming electrostatic repulsion and promoting efficient enrichment of lignin molecules at the catalyst surface. Once concentrated at the electrochemical interface, lignin appears to undergo a distinctly time-dependent oxidation pathway likely dominated by reactive oxygen species, particularly hydroxyl radicals (·OH). During the initial 0–2 h of electrolysis, selective depolymerization is inferred to preferentially cleave the susceptible β–O–4 aryl ether bonds and Cα–Cβ linkages, fragmenting the polymeric backbone into value-added aromatic compounds, including vanillin, together with a limited amount of acetate. Extending the electrolysis period to 4–6 h fundamentally changes the reaction trajectory. The accumulated intermediates are deduced to undergo extensive over-oxidation, during which irreversible ring-opening (de-aromatization) reactions likely convert aromatic monomers into low-molecular-weight organic acids (LMWOAs), including formate, acetate, and various dicarboxylic acids. This progressive transformation is strongly supported by gel permeation chromatography (GPC), whose refractive index (RI) profiles reveal continuous fragmentation of the lignin macromolecular network. After 6 h of electrolysis, the Mw decreases dramatically from 2759 to 530 g mol–1, corresponding to an 80.8% reduction and providing compelling evidence for the highly efficient degradation of lignin. These findings collectively demonstrate that MBC electrocatalytic system offers advantages extending well beyond catalytic activity alone. The co-pyrolysis strategy lowers the specific energy requirement for catalyst synthesis by more than 50%, while electrolysis conducted under ambient conditions at an optimized lignin concentration of 5 g L–1 further reduces the overall energy demand of the conversion process. Equally significant is the incorporation of in situ generated magnetic nanophases, which impart excellent room-temperature magnetic responsiveness and enable rapid separation of the catalyst from the reaction mixture using only an external magnetic field. By simultaneously addressing catalyst recovery, mass-transfer limitations, and catalytic efficiency, this work establishes an integrated and economically viable platform for electrochemical lignin valorization. The proposed MBC catalyst therefore provides both fundamental mechanistic insights and practical technological support for advancing the sustainable and industrial-scale utilization of lignin within future circular biorefineries.