Advances in value-added aromatics by oxidation of lignin with transition metal complexes

Lignin is the second most abundant component in biomass, considered as the most abundant and available renewable source of aromatics in Nature. The depolymerization of lignin presents a sustainable way for producing value-added aromatic chemical platforms. Among all the transformation strategies, the catalytic oxidation transformation of lignin toward the production of highly functionalized aromatic chemicals is an efficient way for high-value chemicals. Herein, we review various transition metal complexes used for the effective transformation of lignin and lignin model compounds to aromatic building blocks, which involves metal porphyrins, metal salens, vanadium complexes, methyltrioxo rhenium and other highly elaborated complexes. The corresponding mechanisms are included. Based on the catalytic systems, challenges and future directions are discussed.