Читать книгу Handbook of Biomass Valorization for Industrial Applications - Группа авторов - Страница 32

Structural cell walls of the entire elevated vascular land plants are a major component of lignin (Figure 2.2) and provide resistance to biological and chemical degradation. This is because of its hydrophobic character that prohibits access to decaying chemicals and species in the aqueous environment. Via various kinds of bonds, the monomeric parts of phenylpropane present in lignin polymers are connected into a composite arrangement. In plant cell walls, lignin typically associated with lignocellulosic resources is used to remove cellulose fibers from paper or composite applications or to generate dissolving cellulose. It is chemically degraded or modified in extreme environments to extract and dissolve hydrophobic lignin. The biochemical life phase of lignin carbon is a significant component in completion of the full cycle of greenhouse gases and mineral deposits of plant biomass carbon. Degradation by microbial enzymes [43] requires the normal processes of lignin decomposition.

Figure 2.2 Sources of lignocellulose.

In the cell wall, lignin is an essential factor and a most common large-molecule polymer in cell wall, except for cellulose. Family of plants, from the chemical point of view, lignin enfolds the package units, such as wood fibers and sclerenchyma cells; phenylpropanoid derived materials are the essential composition of lignin and which incorporate C–C bonds through higher molecular substances by ether bonds. Lignin is hard as per physical characteristics, which improves the strength of the wall of the cell. Frequently, high lignin content is always present in plants cell wall along with a supporting function and mechanical treatment. In woody plants, the constituent of lignin is approximately 27–32% whereas herbaceous plants consist around 14–25% [44].

Organosolv lignin and soda lignin are known as lignin without sulfur and processed on an industrial range, and another new resource of sulfur-free lignin with various uses is the second generation biorefinery method. Strong demand commodities on the market [45]. The presence of ash content Organosolv lignin is about 1.75% & lignin, owing to its hydrophobicity, is soluble in organic solution whereas impossible to solve in water [46]. It is derived by precipitation from the solvent. Organosolv’s most general techniques are focused on ethanol/water pulping and acetic acid pulping [47]. Soda lignin emerges from the pulping phase of soda. There is no sulfur content in soda lignin, but the presence of ash composition is about 0.7 to 2.3%, which is comparatively more than organosolv lignin [48]. In soda-based cooking methods [49], annual plants such as straw, flax and hardwood are used. The significant applications of soda lignin are employed in manufacture of phenolic resin, animal nutrition, as well as polymer manufacture.

Lignin is present in the most of vascular plants and second to cellulose in quantity between polymers in general. Because, like several other constituents of biomass, the photosynthesis reaction is formed. It is sustainable and the produced annually of lignin on earth has been estimated to be in the range of 5–36 * 10⁸ tons. The composition of lignin is in the range of about 15–40% in the woody plants of gymnosperm and angiosperm phylum [5, 50]. In annual plants, low lignin content is also commonly found (Figures 2.3a, b, c). Many deciduous and coniferous species, along with certain types of animals, are given significant values for both the composition of lignin in different forms of plants which are having commercial importance; plants have a personal investment as a source of lignocellulosic materials to produce paper and board products.

Lignin is an essential component of cell walls in developing plants, along with chemical bonds to the monosaccharide components present. The key component of lignin has been found to be covalently bound to hemicellulose in spruce wood [51]. Some mechanical disintegration of the material must precede any efforts to separate lignin from wood or other forms of biomass. Typically, intense substance milling is used whereby structural integrity, cell layers and some homogeneity in macromolecule level are calculated as well as only the typical lignin structure can be derived from such materials.

Figure 2.3a Lignin content in various types of plants (Gymnosperms).

Figure 2.3b Composition of Lignin in different plants (Eudicotyledons).

Figure 2.3c Presence of Lignin in different plants (Monocotyledons).