Lignine

Lignine, sometimes spelled lignin, is a complex phenolic polymer that fills the cell walls of vascular plants alongside cellulose and hemicellulose. It is formed by radical coupling of phenylpropanoid monomers derived from p-coumaryl, coniferyl, and sinapyl alcohols. Lignine is highly crosslinked and amorphous, and it is the main contributor to the rigidity and hydrophobicity of wood. It accounts for roughly 20–30% of dry plant biomass and varies by species and tissue.

In plant cells, lignine polymerization occurs in the cell wall during secondary-wall formation, mediated by oxidative enzymes such as peroxidases and laccases. The result is a heterogeneous, three-dimensional network characterized by linkages including β-O-4 ether bonds, as well as C–C linkages like β-5, β–β, and 4-O-5. The lack of defined repeating units makes lignine resistant to enzymatic attack and difficult to depolymerize, contributing to biomass recalcitrance.

Industrial processing typically involves pulping to remove lignine from lignocellulosic biomass. Kraft and sulfite processes, as well as organosolv methods, yield different lignine streams (kraft lignin, lignosulfonates, organosolv lignin). These materials are used as dispersants, binders, and precursors for carbon fibers or aromatic chemicals, but their highly heterogeneous structure poses challenges for controlled depolymerization.

Research in lignine valorization aims to develop methods for selective depolymerization and upgrading of lignin fragments, enabling higher-value products and integration into biorefineries. Strategies include chemical, catalytic, and biotechnological approaches to reduce condensation reactions and tailor functional groups.