Mechanistic insights and optimization of lignin depolymerization into aromatic monomers using vanadium-modified Dawson-type polyoxometalates

Lignin, as the largest renewable aromatic resource, has significant opportunities for producing high-value products via catalytic depolymerization. However, its complex structure and stable chemical bonds present challenges to its transformation. This study explores the catalytic depolymerization of lignin to aromatic monomers by means of Dawson-type phosphomolybdovanadate polyoxometalates (POMs), understanding the underlying mechanisms. Furthermore, vanadium modification is employed to adjust the catalyst's oxidative and acidic properties, demonstrating that the vanadium content in Dawson-type POMs greatly influences monomer yield. The highest yield is achieved with H₈P₂Mo₁₆V₂O₆₂ (V2). Optimal conditions include a reaction temperature of 150 °C, 4 h, an oxygen pressure of 1 MPa, a methanol-to-water ratio of 8:2, and a mass ratio of the catalyst to the wood powder being 1:1, which leads to a total yield of aromatic monomers at 24 %. POMs with high REDOX potential selectively oxidizes benzyl hydroxyl groups in lignin to benzyl carbonyl groups under the combined action of acidity and oxidation of polyacids. This weakens the βO4 bond and promotes CO bond cleavage. This approach, utilizing the tunable oxidative acidity of polyoxometalates to degrade lignin and produce various aromatic monomers, shows promising potential for lignin valorization and advancing a bio-economy based on lignocellulosic resources.