The most common biomass source, lignocellulose, is the principal component of plant matter and is the largest renewable resource available [1]. Lignocellulose is composed primarily of three biopolymers: cellulose, hemicellulose, and lignin. Cellulose, which is the most abundant biopolymer in lignocellulosic biomass, is composed of glucose monomers linked together through 1-4 glycosidic linkages. As shown in Fig. 8.1, these chains of glucose hydrogen bond with the hydroxyl groups of neighboring cellulose molecules, providing a stable, crystalline structure to cellulose fibers in cell walls [23]. Because cellulose is the single most abundant renewable resource available, there has been significant work in its utilization across a wide range of applications. Cellulose can be somewhat difficult to break into its component glucose units and there are a number of methods, such as enzymatic or acid catalyzed hydrolysis, that will convert cellulose into monosac­charides or short carbohydrate chains [24, 25].

Hemicellulose, which, like cellulose, is a polymer composed of monosaccha­rides, makes up 20-30 % of plant biomass. Unlike cellulose, however, hemicellu — lose is a branched carbohydrate that can be made up of multiple monosaccharides, bonded through a number of different glycosidic linkages. The structure of hemi- cellulose is composed of a polysaccharide backbone made from glucose, xylose, or mannose units connected through p—(1—3) or p-(1 —4) glycosidic bonds. From these backbones, there are side chains of glucose, glucuronic acid, 4-O-methyl- glucuronic acid, mannose, xylose, arabinose, or galactose [26]. The composition of the hemicellulose is dependent on the plant species that produced it [26, 27]. Compared to cellulose, hemicellulose is significantly easier to hydrolyze into small carbohydrate chains and monosaccharides. Currently, there are a number of methods for hemicellulose extraction and degradation, including steam explosion, dilute acid treatment, and ammonia explosion [9].

Lignin is the third kind of structural biopolymer, which composes 15-30 % lignocellulosic biomass by weight [28]. In the structure of the cell wall, lignin fills the space between cellulose/hemicellulose fibers. Unlike cellulose and hemicellu — lose, lignin is not made from carbohydrates but from phenylpropane units that are linked through enzymatic radical polymerization [29]. The monomers that plants employ to create lignin are cinnaminyl alcohol, sinapyl alcohol, and p-coumaryl alcohol. These monomers are bonded together through a number of different linkages that form a complex, amorphous structure. The most common of these bonds are the p-O-4, 5-5, P-5, P-1, and a-O-4 linkages (Fig. 8.2), which represent 45-50, 18-25, 9-12, 7-10, and 6-8 % of the linkages in softwood lignin, respec­tively [30]. Lignin is a major inhibitor of biological degradation of lignocellulose, as there are only a few species in nature that can effectively metabolize it [31, 32]. While lignin benefits living plants, it presents a significant challenge to the successful utilization of biomass in the production of fuels or other chemicals.

Currently, lignin is processed through a number of different techniques. The pulp and paper industry generally uses a process called kraft pulping, in which a strong bases and sulfur compounds depolymerize and extract lignin from wood pulp [30, 33]. Other methods, such as acid pulping, organosolve pulping, and high temperature ethanol/water have been used to degrade lignin have also been employed on an industrial scale [34—38]. In other cases, lignin depolymerization, along with disruption of cellulose crystallinity, has been used for biomass pretreatment for further processing [9, 14].

Other sources of biomass can be used as substrates. Starches and simple sugars are currently used in the production of fuel ethanol. These carbohydrates can be sourced from corn, sugar cane, beets, or as a product from the depolymerization of longer chain polysaccharides [20, 39]. Algae have also received significant atten­tion as a source of renewable energy feed stocks [15]. In research on catalysis of biomass, simple sugars are often used as a model or substitute for more complicated carbohydrates and biomass in general. Other sources of biomass may be similar to common lignocellulose, but have unique characteristics that warrant special

attention. Rice hulls, for example, are coated in a layer of silica that makes effective catalytic conversion difficult [40].