Mechanochemical Treatment Facilitates Two-Step Oxidative Depolymerization of Kraft Lignin

Mechanochemical treatment of Indulin AT kraft lignin by ball-milling with KOH and toluene produces significant carbonyl functionality, among other changes. The chemical reactivity of the lignin is increased, resulting in greater lignin degradation from porphyrin oxidation followed by Baeyer–Villiger oxidation. The mechanochemical treatment produces a level of lignin oxidation that is similar to that produced by porphyrin-catalyzed oxidation. Combining mechanochemical treatment with porphyrin oxidation has a synergistic positive effect on the depolymerization of lignin, as demonstrated by a significantly higher yield of monomers. The methyl ester of vanillic acid was obtained as the main monomeric product (after methylation), along with of methyl 5-carbomethoxyvanillate, in a 10% yield, which is among the highest reported from kraft lignin.