Heteroatom-participated lignin cleavage to functionalized aromatics

Lignin, the most abundant aromatic polymer in nature, enables sustainable supply of miscellaneous aromatics as green fuels and chemicals. Obtaining the value-added aromatics from lignin, though subjected to enormous research efforts, mainly relies on depolymerization induced by activated hydrogen species or oxygen species, delivering hydrocarbons and oxygenates. The future bio-refinery demands a broad spectrum of fine chemicals, especially those containing elements other than C, H and O. Heteroatom-containing compounds have emerged as powerful reagents to participate in the bond cleavage in lignin; meanwhile, the obtained heteroatom-containing aromatics, which could be used as dye precursors, pharmaceutical precursors, hydrogen storage materials,etc., extend the application of lignin-derived products. This tutorial review updates recent advances in the lignin C-C and C-O bond cleavages induced by heteroatoms X (N, Si, I and Li), which also lead to functionalized products containing C-X and O-X bonds. The representative reaction pathways and feasibilities in lignin models and extracts are summarized. Potential applications of functionalized monomers in synthetic transformations, pharmaceuticals, dyes and energy storage are also discussed.