H₂O₂‑Mediated Kraft Lignin Oxidation with Readily Available Metal Salts: What about the Effect of Ultrasound?

The hydrogen peroxide mediated oxidative depolymerization of kraft lignin was investigated in the presence of different metal salt catalysts in acetone/water (1/1 v/v) at low temperature. Na₂WO₄·2H₂O appeared to be the best catalyst, yielding four vanillin-based monomers. Using the optimized conditions, monomers were obtained in 0.51 wt % yield from lignin. This result is among the best depolymerization yield obtained from lignin using hydrogen peroxide mediated oxidation. Optimization of the reaction conditions (temperature, catalyst loading, oxidant amount) revealed that the monomer formation is closely related to the cleavage vs recombination of the intermediates. In addition, the use of ultrasound irradiation involves higher oxidative coupling of phenoxy radicals resulting from lignin polymerization