Lignocellulosic fibers hemicellulose

The cellulose microfibril constitutes the basic structural unit of the plant cell wall each microfibril can be considered as a string of cellulose crystallites, linked along the chain axis by amorphous domains (Fig. 7.1). Their structure consists of a predominantly crystalline ceUulosic core which is covered with a sheath of paracrystalUne polyglucosan material surrounded by hemicelluloses (Whistler and Richards 1970). These microfibrils are conjoined by other polymers like lignin and hemicelluloses and aggregate further to form lignocellulosic fibers. As they are almost defect free, the modulus of these sub-entities is close to the theoretical limit for cellulose. 

Moisture may be present in lignocellulosic fibers in the cell voids or lutnina (free water) and adsorbed to the cellulose and hemicellulose in the cell wall (bound water). 

Like all the lignocellulosic fibers, the physical and chemical properties of jute fiber is also dependent on the three chemical components, viz., cellulose, hemicellulose and lignin. The chemical composition and structural parameters of jute fibers are represented in Table 20.3. 

Plants are very attractive and potential sources of cellulose primarily because they are abundant and relatively cheap to harvest. Cellulose can be extracted from lignocellulosic fibers, which are available aU over the world. A wide variety of plant materials like cotton, ramie, sisal, flax, wheat straw, tubers, sugar beet, soya bean, etc., are known for cellulose microfibril production. Wood is another main important source of extracting cellulose fibrils, where cellulose microfibrils were reinforced by intracellular amorphous materials made of hemicelluloses, lignin, resin, etc. Extraction of pure cellulose microfibrils from these lignocelluloseic materials involves chemical treatments such as alkali extraction and bleaching. 

Natural fibers can be classified according to their source vegetable, animal or mineral. In particular, vegetable fibers consist basically of cellulose, hemicellulose, lignin, and other amorphous constituents, in smaller amounts, and due to their chemical composition they are also known as lignocellulosic fibers [21],... 

The chemical composition and cellular structure of lignocellulosic fibers are complex and heterogeneous, each fiber is considered to be a natural composite formed by rigid cellulose microfibrils wrapped in an amorphous polymeric matrix, basically formed by hemicellulose and lignin [21,22]. 

Various solvents are being investigated to dissolve lignocellulosic materials. Some approaches focus on the selective depolymerization and extraction of lignin and hemicellulose as pre-treatment to produce clean cellulose fibers for subsequent fermentation or for pulping. Other approaches attempt to dissolve the whole lignocellulose with or without depolymerization. The liquefaction processes that are carried out at high temperature (>300 °C), and produce a complex oil mixture, are discussed above with the pyrolysis processes. 

Lignocellulosics are three-dimensional, polymeric composites made up primarily of cellulose, hemicelluloses, and lignin. Table 1 shows the chemical composition of several different types of natural fibers. It is interesting to... 

Decomposition rate constants are negatively correlated to the total fiber content of the organic snbstrate bnt not well correlated to individual components such as cellulose, hemicellulose, and lignin. A field and laboratory study by Moran et al. (1989) measured the decomposition rates of whole litter and the lignocellulose components of the litter for the emergent macrophytes S. alterniflora and C. walteriana. Decomposition of individual carbon fractions varied with the rate of decomposition in the order of cellulose > hemicellulose > lignin (Figure 5.38, Moran et al., 1989). 

Hemicelluloses are heterogeneous polysaccharides, which are located between the lignin and cellulose fibers. Depending on wood species, hemicelluloses constitute about 20-30% of the naturally occurring lignocellulosic plant biomass [19]. Lignocellulosic biomass represents a renewable, widespread, and... 

The pretreatment of any lignocellulosic biomass is cmcial before enzymatic hydrolysis. The objective of pretreatment is to decrease the crystallinity of cellulose which enhances the hydrolysis of cellulose by cellulases (17). Various pretreatment options are available to fractionate, solubilize, hydrolyze and separate cellulose, hemicellulose and lignin components (1,18-20). These include concentrated acid (27), dilute acid (22), SOj (25), alkali (24, 25), hydrogen peroxide (26), wet-oxidation (27), steam explosion (autohydrolysis) (28), ammonia fiber explosion (AFEX) (29), CO2 explosion (30), liquid hot water (31) and organic solvent treatments (52). In each option, the biomass is reduced in size and its physical structure is opened. Some methods of pretreatment of Lignocellulose is given in Table I. 

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