Swimming upstream – photocatalytic depolymerization of lignosulfonate in seawater

The valorization of lignin, a major component of lignocellulosic biomass, is essential for reducing reliance on fossil-based resources. Lignosulfonates, water-soluble lignin derivatives produced in large volumes by the pulp and paper industry, remain underutilized due to lignin's structural complexity and the harsh conditions typically required for its depolymerization. Here, we present a mild and sustainable photocatalytic method for depolymerizing sodium lignosulfonate (LS) using sodium anthraquinone-2-sulfonate (AQ2S), a non-toxic, water-soluble, and commercially available photocatalyst. The reaction proceeds under UV LED irradiation at room temperature and ambient pressure in saline aqueous media, including untreated Baltic Sea seawater, without the need for organic solvents, acids, bases, or hazardous additives. Salinity is crucial for stabilizing AQ2S and maintaining its activity during prolonged irradiation. Computational methods at the density functional theory (DFT) level provide insight into the reaction mechanism and catalyst behavior, showing that in the absence of salt, AQ2S is deactivated via hydroxylation reactions. Conversely, in saline media it remains catalytically active in mediating LS depolymerization. The process is successful while yielding lower-molecular-weight lignin fragments, as confirmed by SEC, UV-vis spectroscopy, NMR, and FTIR, with evidence of decreased aromaticity and preserved structural motifs. This study demonstrates the utility of seawater as a green reaction medium and introduces an eco-friendly, mechanistically informed strategy for lignin conversion.