Major Components of Lignocellulosic Biomass
Lignocellulose or lignocellulosic biomass refers to the dry plant matter, which is the most abundant organic substance on earth. The three major components in lignocellulosic biomass and their typical percent compositions are:
1. Cellulose 35-50%
2. Hemicellulose 20-35%
3. Lignin 15-30%
The exact composition can vary in a wide range depending on the plant family, species and part of the plant. In addition to these, there are minor components like minerals, proteins, fats and oils in all plant materials [2].
Cellulose is a linear polymer of D-glucose molecules linked with P(1^4)-glycosidic bonds. The repeating unit of the polymer is D-cellobiose, which consists of two D-glucose molecules as shown in Figure 4.5.
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Figure 4.6 Linear polysaccharide chains in cellulose microfibrils. Inter — and intramolecular hydrogen bonds are shown in dashed lines. |
The cellulose structure is composed of stacks of linear chains with D-cellobiose repeating units. These closely-packed chains form robust crystal structures with inter — and intramolecular hydrogen bonds, as shown in Figure 4.6.
This motif in cellulose contrasts with that of a(1^4)-glycosidic bonds present in starch, glycogen, and other carbohydrates. Unlike starch, no coiling or branching occurs in cellulose, and the molecule adopts an extended and rather stiff rod-like conformation aided by the equatorial conformation of all the D-glucose units in the linear chains as shown in Figure 4.6. The chain length of a polymeric cellulose molecule varies in a wide range depending on the plant source. However, a typical value of a number of glucose units in the polymer is in the range 100 to 14,000 [3]. Each cellulose molecule consists of a linear chain of glucose residues that are covalently linked to one another to form a ribbon-like structure, which is stabilized by hydrogen bonds within the chain. In addition, intermolecular hydrogen bonds between adjacent cellulose molecules cause them to adhere strongly, giving a high tensile strength to the material. The bundles of linear cellulose chains are stacked along the axial direction of the microfibril as shown in Figure 4.4.
Cellulose, which is the principle scaffolding component of all plant cell walls, exists in the form of a robust crystalline structure in solution or in solid state. This highly hydrogen-bonded complex molecular architecture of the cellulose molecules provides tensile strength to the primary cell wall. Such a cell wall polymer is neither soluble in water nor easily digestible in the gastrointestinal tract of humans. These cellulose microfibrils with a complex network of hydrogen bonding and van der Waals interactions resists deconstruction by solvent or by physical treatments.