Understanding the characteristics of LB is necessary for the effective application of a conversion technology or a pretreatment method. LB is the most abundant organic and renewable resource on the planet (Klass,

1998) . Man has been producing chemicals, materials, and energy from LB since his origin. These activities continue to be the most promising activities to pursue in order to address our contemporary challenges. For
example, we currently look to use LB to reduce depen­dence on fossil fuels.

There are three families of LBs: grassy plants, shrubs, and trees, each possess four primary components: cellu­lose, hemicellulose, lignin and extractives. Each species possesses these components in different proportions. Hardwood trees (angiosperms), shrubs and grassy plants (graminoids) usually possess less lignin than soft­wood trees (gymnosperms) (Liu, 2012).

Figure 27.1 illustrates the variation in cellular struc­ture between hardwood and softwood. Notice that in both images the structure is porous, where the pores are empty spaces and run longitudinally. Figure 27.2 is a compilation of sketches of wood cells. The cellular structure of each plant species is of different sizes and shapes and varies in the size and number of pores. From Figure 27.1, one can note that there is no space between the cells. Regardless of size and shape, each cell is glued tightly to its neighbors. The intercellular spaces are called middle lamellae. The great majority of the middle lamellae, over 80%, contain lignin (Liu, 2012). Lignin is the glue that binds the cells together and pro­vides the rigid structure of wood. The remaining volume of the middle lamellae consists of hemicellulose and extractives. Conversely, the cell wall is mostly made of cellulose and to a much lesser degree contains lignin and hemicellulose. The great majority of biomass dry weight is derived from the cell wall. A much smaller portion of the biomass dry weight comes from the middle lamellae. Most of the total lignin content of LB comes from the cell wall. Despite the high concentration of lignin in the middle lamellae, over 60% of the lignin from LB comes from the cell wall portion of the material (Liu, 2012).

Table 27.1 shows that cellulose is the largest portion of LB. Cellulose, which represents between 40% and 50% of the dry weight of wood, is a homopolymer of

b-D-glucopyranose where dehydration of the b-D — glucose units forms a linear chain with a degree of poly­merization (DP) between a few hundred and several thousand b-D-glycosidic bonds. The dehydration occurs between the one and four carbons of b-D-glucopyranose units and leaves an oxygen atom to join the two units, which is written as, b-1-O-4 glycosidic bonds. The for­mula for cellulose is H—(СбН^05)п—OH, where "n" rep­resents the DP. This highly ordered, tightly bound pattern is made of bonds that are quite strong and are difficult to break.

Cellulose grows into microfibrils with crystalline and amorphous regions. The crystalline portions of the molecule line up side by side. Hydrogen bonds, between the hydroxyl groups, provide strong, sturdy and stable links between and within these crystalline units. When these microfibrils form macrofibrils and interact with noncellulosic material in the cell walls of plants, the result is strength and rigidity.

While the crystalline regions are stable and strong, the amorphous regions provide an opportunity to break down the large structure into smaller saccharides. Sol­vents, reagents and enzymes may be used to penetrate and hydrolyze the structure. Hydration requires the addition of energy or a strong acid. Alternatively, en­zymes, such as cellulase, may facilitate the conversion. Enzymatic hydrolysis tends to be much slower than acid hydrolysis. Reducing the chip size or increasing
the exposed surface area of LB increases the effective­ness of these solvents, reagents and enzymes.

Hemicelluloses compose another large portion of LB, between 20% and 30% of the dry weight of wood, see Table 27.1. These are heteropolymers, or heterosacchar­ides of five — and six-carbon sugars. They are found mostly in the cell walls of LB. Common hemicellulose sugars are D-glucose, D-mannose, D-galactose, D-xylose, L-arabinose, and to a lesser degree, L-rhamnose. Hemi — cellulose has a low DP, around 100—200, and thus is more easily hydrolyzed into their monomeric sugar components (Glaudemans and Timell, 1958; Goring and Timell, 1960; Koshijima et al., 1965; Timell, 1960).

The structure of a hemicellulose tends to possess a primary backbone, off of which might hang a variety of residual units. These residual units are nonpolymeric acids and sugars. The degree of branching or number of residual units depends on the origin or species of the biomass. For hardwood, the backbone is xylan, contain­ing b-linked bonds at carbons one and four, like cellu­lose. Unlike cellulose, residual units can hang off from the other carbon positions. These residues may include those of acetic acid, glucuronic acid, mannose, arabinose and galactose. Softwood is even more vari­able in that the backbone may be made of more diverse materials. The backbone is typically made of galacto — glucomannan units or arabinoglucuronoxylan units. Galactoglucomannan is a polymer that is a primarily

TABLE 27.1 Major Components of Wood

Lignin

Phenolic —OH

Aliphatic —OH

Methoxyl —OCH3

Carbonyl >C=O

Hemicellulose

Galactoglucomannan

(1:1:3)

(Galacto)glucomannan

(0.1:1:4)

The OH groups in the xylose units were partially substituted by OAc on the C-2 or C-3 positions, i. e. R=CH3CO (Ac) or H

OH in the xylo-units were partially substituted by OAc on the C-2 or C-3 positions (about 7 in 10 xylo-units), i. e. R=CH3CO (Ac) or

5—8 2—4

Terpenes, terpenoids, esters, fatty acids, alcohols, etc.

Phenols: p-cresol, p-ethylphenol, guaiacol, salicyl alcohol, eugenol, vanillin, coniferyl aldehyde, acetovanillone, propioguaiacone, salicylic acid, ferulic acid, syringaldehyde, sinapaldehyde, and syringic acid; stilbenes: pinosylvin, pinosylvin monomethyl and dimethyl ethers, 4-hydroxystilbene, 4-hydroxystilbene monomethyl ether; lignans; hydrolyzable and condensed tannins; flavonoids; isoflavones or isoflavonoids

Arabinose, galactose, glucose, xylose, raffinose, starch, pectic material

Ca, K, Mg, Na, Fe, SO4~, CL, etc.

Cyclitols; tropolones; amino acids, protein, alkaloids, etc.

0.2—0.5 0.2—0.8

Source: Fengel, 1989.

linear and perhaps mildly branched chain. In hemicel — lulose, the residual units take the place of the strong hydrogen bonding that occurred with cellulose components.

Recall that cellulose is highly ordered and tightly bound and thus resistant to hydrolysis. Hemicellulose is not. Hemicellulose tends to be more randomly orga­nized with a more variable and loosely bound structure (amorphous). Therefore, it can be hydrolyzed by weaker or more dilute acids and bases, or at milder conditions.

Lignin is the third largest component of LB at 25—35% of the LB dry weight (Boerjan et al., 2003). Lignin is a het­eropolymer with methoxylated phenylpropylene alcohol units. Its structure tends to be amorphous and variable. These units are interconnected by stable ether and ester linkages. It is hydrophobic and aromatic. It covalently links to hemicelluloses and cross-links different plant polysaccharides giving mechanical strength to the cell wall (Mielenz, 2001). Additionally, lignin is highly resis­tant to biological degradation and thus it protects cellulose and hemicellulose from decay.

Lignin from different plant families vary in their alcohol content and composition. These lignins are thus defined by these components into different types. The lignin precursor in gymnosperms is coniferyl alcohol. The precursor in angiosperms is p-coumaryl alcohol and sinapyl alcohol. The corresponding lignins are guaiacyl (G), p-hydroxyphenyl (H) and syringal

(S), respectively. Grasses tend to contain G while palm trees contain mostly S (Sjostrom, 1993).

The next largest component of LB is the extractives. These make up between 2% and 8% of the total dry weight (Table 27.1). Extractives are compounds found in LB that are soluble in neutral organic solvents or water at standard temperatures and atmospheric condi­tions. Extractives vary in solubility. Some are lipophilic and others are hydrophilic. Lipophilic extractives that are soluble in nonpolar organic solvents are called resins. There is a large diversity in the number of extrac­tives. Additionally, the concentrations of extractives are highly variable throughout the plant depending on the tissue type, i. e. root, stem, bark, branch, needle or leaf. It is important to note that over 70 metal, earth elements, and inorganic compounds may be found in LB. The extractives are the first components that can be extracted from wood. This is advantageous for using LB as a biore­mediation for toxic soil and wastewater in addition to being a source for biofuel and other products.