Sono-Fenton-Assisted Depolymerization of Lignin to Valuable Phenolic Compounds

Lignin, a naturally occurring aromatic polymer, is often regarded as an underutilized resource within the expanding paper and pulp and bioethanol industries. The aim of the present work is to employ a nonconventional, benign method like sonication-assisted Fenton treatment to depolymerize alkali lignin, and optimize the reaction conditions like time (15-90 min), lignin concentration (1-6 g/L), ultrasound power (300, 450 W), dosage of H₂O₂ (0.01-0.04 mol), and amount of FeSO₄ (5-20 wt %) to maximize the oil yields and total phenolics. Kinetics of depolymerization of lignin at optimal conditions was also studied by tracking the molecular weight reduction of lignin with time. The highest yield of lignin oil and rate constant achieved were 37% and 4.7 × 10^-5 min^-1, respectively, at 60 min, 4 g/L lignin concentration, 10 wt % FeSO₄, 0.02 mol of H₂O₂, and 450 W ultrasound power. The total phenolic content in the liquid phase was 0.75 mmol g^-1. Free-radical reactions mediated by active hydroxyl (^•OH) species generated by the synergetic action of both the Fenton reagent and ultrasound aid in breaking down the complex lignin molecule into valuable, low-molecular-weight phenolic monomers like vanillin, acetovanillone, vanillic acid, phenol, and guaiacol. The selectivity to vanillin was high (36%) under the optimal reaction condition.