4.1 Lignocellulose and Its Utilization

4.1.1 Lignocellulose

The structural materials that are produced by plants to form cell walls, leaves, stems, and stalks are composed primarily of three different types of biobased macromolecular chemicals, which are typically classified as cel­lulose, hemicellulose, and lignin. These biobased chemicals are collectively called lignocellulose, lignocellulosic biomass, or lignocellulosic materials. As shown in Figure 4.1 here and also in Chapter 3, a generalized plant cell wall structure is like a composite material in which rigid cellulose fibers are embedded in a cross-linked matrix of lignin and hemicellulose that binds the cellulose fibers.

Generally speaking, the dry weight of a typical cell wall consists of approx­imately 35-50% cellulose, 20-35% hemicellulose, and 10-25% lignin [1]. Others claim that cellulose typically accounts for 40-50% of woody biomass, whereas lignin and hemicellulose each account for about 20-30%. Although lignin comprises only 20-30% of typical lignocellulosic biomass, it provides 40-50% of the overall heating value or total available energy of the biomass, due to its higher calorific value (CV) than cellulose and hemicellulose. This explains why chemical conversion or beneficial use of lignin is very impor­tant in fuel/energy utilization of lignocellulosic resources.

Lignocellulosic biomass structures also contain a variety of plant-specific chemicals in the matrix; these include extractives (such as resins, pheno — lics, and other chemicals) and minerals (calcium, magnesium, potassium, and others) that will leave behind ash when the biomass is combusted. The trace minerals and major elements in lignocellulosic materials display a high degree of variability for most of the elements between different spe­cies, between different organs within a given plant, and also depending on the growing conditions including the soil characteristics [2]. In addition to their potential health and environmental effects, trace minerals can exhibit nontrivial effects on the next stage chemical treatment, including catalytic conversion of thermochemical intermediates of lignocellulose.

Cellulose is a large polymeric molecule composed of many hundreds or thousands of monomeric sugar (glucose) molecules, and in this regard it may be considered a polysaccharide. The molecular linkages in cellulose form linear chains that are rigid, highly stable, and resistant to chemical attack. Due to its linear polymeric structure, cellulose exhibits crystalline proper­ties [3]; for example, cellulose may be somewhat soluble in a suitable solvent. However, cellulose molecules in their crystalline form are packed so tightly that even small molecules such as water cannot easily permeate the struc­ture. Logically, it would be even more difficult for larger enzymes to perme­ate or diffuse into the cellulose structure. Cellulose exists within a matrix of other polymers, mainly hemicellulose and lignin, as illustrated in Figure 4.1.

On the other hand, hemicellulose consists of short and highly branched chains of sugar molecules. It contains both five-carbon sugars (usually D-xylose and L-arabinose) and six-carbon sugars (such as D-galactose, D-glucose, and D-mannose) as well as uronic acid. For example, galactan, found in hemicellulose, is a polymer of the sugar galactose, whose solubility in water is 68.3 g per 100 grams of water at room temperature. Uronic acid is a sugar acid with both a carbonyl and a carboxylic function. Hemicellulose is amorphous due to its highly branched macromolecular structure [3] and is relatively easy to hydrolyze to its constituent simple sugars, both five-car­bon and six-carbon sugars. When hydrolyzed, the hemicellulose from hard­woods releases sugary products high in xylose (a five-carbon sugar), whereas the hemicellulose contained in softwoods typically yields more six-carbon sugars. Even though both five-carbon and six-carbon sugars, illustrated in Figure 4.2, are simple fermentable sugars, there is a discerning difference

OH

H

OH

in their fermentation chemistry and process characteristics with regard to specific yeasts and enzymes involved.

Humans have had far more extensive and successful experience in the fer­mentation of six-carbon sugars (hexoses) than five-carbon sugars (pentoses or xyloses), as well evidenced by a long history of manufacturing alcoholic beverages throughout the world. This statement is still valid for fuel ethanol fermentation as well. Many years ago, it was believed that xylose could not

CH2OH

OH O OH O c

OH

HOft,,..

HOH2C OH

P-D-Galactofuranose

FIGURE 4.2 (CONTINUED)

Molecular structures of five-carbon and six-carbon sugars.

be fermented by yeasts, but in recent years, a number of yeasts have been found to be capable of fermenting xylose into ethanol [4, 5]. Genetic engi­neering of xylose fermentation in yeasts has also been carried out with suc­cessful outcomes [6].

Lignin is a complex and highly crosslinked aromatic polymer that is cova­lently linked to hemicellulose, as shown in Figure 4.1. Lignin contributes to the stabilization of mature cell walls. Lignin yields more energy than cel­lulose when burned due to its higher calorific value. Lignin is a macromol­ecule whose typical molecular weight exceeds 10,000. Due to its crosslinked structure, lignin is generally more difficult to process, extract, hydrolyze, or react than cellulose or hemicellulose. Therefore, degradation or biodegrada­tion of the crosslinked structure becomes the first step for biofuel production from the cellulosic feedstocks. Needless to say, efficient conversion of lignin would result in a substantial increase in fuel yield as well as an enhanced economic outlook with utilization of lignocelluloses.