Oxidative depolymerization of lignin enhanced by synergy of polyoxometalate and acetic acid

Lignin, a renewable aromatic biopolymer with complex three-dimensional networks and diverse functional groups, represents a promising sustainable feedstock for value-added chemicals and fuels. However, its structural heterogeneity and poor solubility pose significant challenges for efficient valorization. Oxidative depolymerization has emerged as a particularly effective approach for producing carbonyl-containing aromatic compounds under mild conditions. Polyoxometalates serve as ideal bifunctional catalysts, combining acidic and oxidative sites while enabling oxygen-mediated regeneration. Current limitations involve the high oxygen pressures (1.0-2.5 MPa) required in conventional systems. Research now focuses on developing advanced catalytic systems with enhanced lignin solubility, reduced oxygen demand, and optimized catalyst-lignin interactions to enable practical industrial applications under milder conditions.

Recently, a research team led by Prof. Feng Wang (Dalian Institute of Chemical Physics, Chinese Academy of Sciences, China) & Prof. Junyou Shi’s team (Beihua University, China), developed a catalytic system for efficient lignin depolymerization under low oxygen pressure, utilizing the synergistic effect between H3PMo12O40 and acetic acid. The study successfully determined the structure of the catalytic complex and identified the active oxygen species, while elucidating the mechanism by which acetic acid enhances the oxidative capability of H3PMo12O40. The results were published in Chinese Journal of Catalysis (DOI: 10.1016/S1872-2067(25)64737-1).

The scalable H3PMo12O40 catalyst in acetic acid/water solvent achieved over 20 wt% yield of carbonyl-containing aromatics under mild conditions (0.1 MPa O₂). The exceptional depolymerization efficiency was demonstrated by complete disappearance of β-O-4 linkages and significant molecular weight reduction in the products. Mechanistic investigations revealed that H3PMo12O40 exhibits dual acid-oxidation catalytic functions, while acetic acid not only enhances lignin solubility but also forms a stable complex with the catalyst through esterification between acetyl groups and surface oxygen atoms of the Keggin structure, as confirmed by theoretical calculations and UV-Vis spectroscopy. This coordination significantly enhances the system's oxidative capability, evidenced by decreased half-wave potentials in electrochemical measurements. Importantly, the catalyst maintains its intact Keggin structure throughout the reaction.

Further mechanistic studies identified the presence of superoxide radicals, demonstrating effective molecular oxygen activation and stabilization of reactive oxygen species through catalyst-solvent synergy. Importantly, the detection of acetylated intermediates coupled with Gibbs free energy calculations established the predominance of an acetylation pathway that simultaneously lowers the energy barrier and prevents condensation side reactions via α-OH protection, thereby suppressing recalcitrant C-C bond formation. The system's remarkable universality was validated using various lignin feedstocks differing in concentration, wood species, and extraction methods.

he results were published in Chinese Journal of Catalysis (DOI: 10.1016/S1872-2067(25)64737-1)

About the Journal

Chinese Journal of Catalysis is co-sponsored by Dalian Institute of Chemical Physics, Chinese Academy of Sciences and Chinese Chemical Society, and it is currently published by Elsevier group. This monthly journal publishes in English timely contributions of original and rigorously reviewed manuscripts covering all areas of catalysis. The journal publishes Reviews, Accounts, Communications, Articles, Highlights, Perspectives, and Viewpoints of highly scientific values that help understanding and defining of new concepts in both fundamental issues and practical applications of catalysis. Chinese Journal of Catalysis ranks among the top one journals in Applied Chemistry with a current SCI impact factor of 17.7. The Editors-in-Chief are Profs. Can Li and Tao Zhang.

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