Compositional Variability of Lignin in Biomass

Abstract

The objective of this chapter is to provide a concise overview of lignin composition and structure in different species and materials (wood, barks and nonwood plants). It includes a brief review on the lignin precursors and their polymerization as well as of the analytical tools used for lignin characterization from wet chemical to spectroscopic methods. Wood of gymnosperms is characterized by high lignin content (25-35%) and a HG-type of lignin with more guaiacyl (G) units and a small portion of p-hydroxyphenyl (H) units. Wood of angiosperms has a lignin content of 15-28%, with a GS-lignin having different proportions of syringyl (S) units. Nonwoody monocotyledon species have different lignin content (9-20%) and a HGS type of lignin, characterized by a high proportion of H units. Bark lignin content ranges from 13 to 43% and is of HGS-type with species-specific composition and different in the bark components, phloem and cork. Lignin composition and macromolecular structure are key issues to understand the properties of lignocellulosic materials and to design a lignin-based pathway within biomass biorefineries. The available information on lignin composition is still limited to a few species and plant components. This is certainly an area where more research is needed.

Deconstruction of Lignin: From Enzymes to Microorganisms

“…Softwood lignins are mainly composed of guaiacyl units, and thus are classified as G-type lignin. Hardwood lignins are mainly composed of different proportions of guaiacyl-syringil (GS-type) [ 24 , 25 ]. Grass lignins have a mixture of the three aromatic units (HGS), and contain a higher proportion of p -coumaryl alcohol (H)-derived units than other lignin types [ 26 , 27 ].…”

Willow Lignin Recovered from Hot‐Water Extraction for the Production of Hydrogels and Thermoplastic Blends

“…As a hardwood lignin, WL is a less condensed, S/G lignin (S/G=1.25,), expected to be closer in structure to the native lignin with more β‐O‐4 linkages, due to the relatively mild conditions (P factor ∼691) in processing. On the other hand, as a softwood G‐lignin, KL has been documented to have a highly modified condensed structure, with notably less β‐O‐4 linkages, and it is abundant in atypical structures, such as stilbenes and aryl enol ethers . Additionally, our results show that the free phenolic hydroxyl group content (PhOH) of KL is much higher (4.93 mmol g −1 ) as compared to WL (2.28 mmol g −1 ).…”

Production of Aromatic Compounds by Catalytic Depolymerization of Technical and Downstream Biorefinery Lignins

“…The monomer distribution showed that, as expected, the major monomers were guaiacyl monomers (81% and 88%, respectively) followed by syringyl monomers (10% and 5%, respectively). As a thumbnail rule, syringyl monomers are not present in pine lignins; however, it has been reported that small amounts can be found in pine barks [ 57 ]. 31 P NMR spectrum of derivatized SeolA confirmed the presence of syringyl units by the presence of a shoulder at 143.1 ppm.…”