Lignin Structure
Lignin contains an aromatic structure with an amorphous macromolecule of repeating phenylpropane units, making it a potential precursor to produce materials, fuels, and chemicals.
Lignin is the second major component of wood cell walls (20–30%) that serves to cover structural supports and transport water and nutrients, as cement between wood fibers, as a hardening agent in fibers, and as a barrier against chemical, biological, and enzymatic degradation of cell walls . It is a three-dimensional, irregular, and highly branched phenolic polymer with a highly complex chemical structure and formed through an irregular biosynthetic process.
A lignin structure consisting of basic phenylpropanoid monomers, guaiacyl (G), syringyl (S), and p-hydroxyphenyl (H) units, which are derived from coniferyl, synapyl alcohol, and p-coumaryl precursors, re-spectively . *ese macromolecules contain large amounts of C-C and ether-related compounds with the aryl glycerol-β-aryl ether (β-O-4) link predominating so far,followed by the biphenyl (5–5) and phenyl coumarin (β-5) bonds .
Lignin Extraction Method
Species and environment influence the content and composition of lignin. Hardwood lignin con-
sists mainly of G and S units and has traces of H units 2 Advances in Materials Science and Engineering
, while grasses contain G, S, and H units. Hardwoods have more lignin composition variability than softwoods. In contrast, softwood lignin consists mainly of G units, with low levels of H units.
*e proportions of S, G, and H units were 1:94:5 in spruce lignin, 56:40:4 in beech lignin , and 1:1:1 in grass
lignin. Even though there is variation in the content con-tingent on the sources, the typical lignin contents measured by the Klason method are 25–35% in softwoods, 20–25% in hardwoods, and 15–25% in herbaceous plants . Pre-cipitation of lignin begins in the primary wall’s middle la-mella and the secondary wall’s S1 layer.
Many con-version pathways for lignin utilization have been proposed. In some cases, the structure and properties of the extracted lignin (technical lignin) change due to the method of processing lignocellulosic biomass into pulp. In commercial applications such as fillers in composites,technical lignin can be extracted from biomass physically with or without chemical or biochemical treatments.
However, controlling the physical properties, reactivity, and chemistry of the lignin surface should be performed.Processes with sulfur such as kraft and sulfite pulping are discussed below, as are sulfur-free processes such as alkaline pulping (soda lignin) and solvent pulping (organosolv lig-nin). Sulfur-free lignin is lignin that has a low macromo-lecular size upon fractionation. In addition to low molar mass phenols or aromatic compounds, they have other desirable properties. presents some key features of
lignins related to their utilization with a detailed description of each lignin in the discussion.
