Knowledge of the chemical structures of the major organic components in biomass is quite valuable in the development of processes for producing derived fuels and chemicals. Information on the chemical structures can often lead to methods of improving existing processes and to development of advanced conversion methods. Somewhat more detailed information on the chemical structures of the major components in biomass is presented here.

Alpha cellulose is a polysaccharide having the generic formula (C6H10O5)„ and an average molecular weight range of 300,000 to 500,000. Complete hydrolysis established that the polymer consists of D-glucose units. Partial hydrolysis yields cellobiose (glucose-/3-glucoside), cellotriose, and cellotetrose. These results show that the glucose units in cellulose are linked as in cellobiose (Fig. 3.6). Cotton is almost pure a-cellulose, whereas wood cellulose, the raw material for the pulp and paper industry, always occurs in association with hemicelluloses and lignins. Cellulose is insoluble in water, forms the skeletal structure of most terrestrial biomass, and constitutes approximately 50% of the cell wall material. Carefully purified wood cellulose contains a few carboxyl groups which are believed to be esterified in the natural state.

Starches are polysaccharides that have the generic formula (СбН10О5)п. They are reserve sources of carbohydrate in some biomass, and are also made up of D-glucose units as shown by the results of hydrolysis experiments. But in contrast to the structure of cellulose, the hexose units are linked as in maltose, or glucose-a-glucoside (Fig. 3.6), as indicated by the results of partial hydroly­sis. Another difference between celluloses and starches is that the latter can

ОНО I неон I HOCH I неон I неон I CH2OH D-glucose (aldehydic form)

он

СН2ОН ‘ — о. .он

но

СН2ОН

он

Cellulose

CHO I неон I HOCH I неон I неон I CH2OH D-glucose (aldehydic form)

Starch Chains

CHO I неон I HOCH I неон I CH2OH D-xylose (aldehydic form)

OH OH OH Xylan Chains

CH30

CH30 Some structural units of lignin

o II ch2ocr I 0 I II CHOCR’ I 0 I II CH2OCR" Triglyceride

FIGURE 3.6 Chemical structures of some biomass components.

be separated into two fractions by treatment with hot water: a soluble compo­nent called amylose (10 to 20%) and insoluble amylopectin (80 to 90%). Amylose and amylopectin have molecular weight ranges of 10,000 to 50,000 and 50,000 to 1,000,000, respectively. Both fractions yield glucose or maltose on hydrolysis, but amylopectin is believed to consist of branched chains. Starches occur in the form of minute granules in seeds, tubers, and other plant parts and are important constituents of corn, beans, potatoes, rice, wheat, and other biomass foodstuffs.

Hemicelluloses are complex polysaccharides that occur in association with cellulose in the cell walls. But unlike cellulose, hemicelluloses are soluble in dilute alkali and consist of branched structures, which vary significantly among different woody and herbaceous biomass species. Many have the generic for­mula (C5H804)„. They are termed pentosans and yield mainly pentoses instead of hexoses on hydrolysis. Some hemicelluloses, however, contain hexose units. Hemicelluloses usually consist of 50 to 200 monomeric units and a few simple sugar residues. The most abundant one, xylan, consists of D-xylose units linked in the 1- and 4-positions (Fig. 3.6). Xylan is closely related to polyglucuronic acid with which it is associated in the natural state, and from which it can be produced by decarboxylation. Other hemicelluloses include the glucomannans, which consist of D-glucose and D-mannose units in the polymeric chains in ratios of about 30: 70, and galactoglucomannans, which consist of D-galactose, D-glucose, and D-mannose in the polymeric chains in ratios of about 2:10:30. The pentosans can occur in large amounts (20 to 40%) in corncobs and corn stalks and in biomass straws and brans. The xylans have been found in soft­woods and hardwoods up to about 10% and 30% of the dry weight of the species, respectively, whereas mannans are generally present at about 15% of the dry weight in softwoods and only a few percent by weight in hardwoods.

The lignins are highly branched, substituted, mononuclear aromatic poly­mers in the cell walls of certain biomass, especially woody species, and are often bound to adjacent cellulose fibers to form what has been called a lignocellulosic complex. This complex and the lignins alone are often quite resistant to conversion by microbial systems and many chemical agents. The complex can be broken and the lignin fraction separated, however, by treatment with strong sulfuric acid, in which the lignins are insoluble. The precise structures of the polymers have not been determined because of their diverse nature and complexity. The dominant monomeric units in the polymers are benzene rings bearing methoxyl, hydroxyl, and propyl groups that can be attached to other units (Fig. 3.6). The lignin contents on a dry basis in both softwoods and hardwoods generally range from 20 to 40% by weight, and from 10 to 40% by weight in various herbaceous species such as bagasse, corncobs, peanut shells, rice hulls, and straws.

As previously mentioned, the triglycerides found in biomass are esters of the triol, glycerol, and fatty acids (Fig. 3.6). These water-insoluble, oil-soluble esters are common in many biomass species, especially the oilseed crops, but the concentrations are small compared to those of the polysaccharides and lignins. Many saturated fatty acids have been identified as constituents of the lipids. Surprisingly, almost all the fatty acids that have been found in natural lipids are straight-chain acids containing an even number of carbon atoms. Most lipids in biomass are esters of two or three fatty acids, the most common of which are lauric (C12), myristic (C14), palmitic (Ci6), oleic (C!8), and linoleic (C18) acids. Palmitic acid is of widest occurrence and is the major constituent (33 to 45%) of the fatty acids of palm oil. Lauric acid is the most abundant fatty acid of palm-kernel oil (52%), coconut oil (48%), and babassu nut oil (46%). The monounsaturated oleic acid and polyunsaturated linoleic acid comprise about 90% of sunflower oil fatty acids. Linoleic acid is the dominant fatty acid in corn oil (55%), soybean oil (53%), and safflower oil (75%). Saturated fatty acids of 18 or more carbon atoms are widely distributed, but are usually present in biomass only in trace amounts, except in waxes.

Other classes of organic materials, such as alkaloids, pigments, resins, ste­rols, terpenes, terpenoids, and waxes, and many simple organic compounds are often present in various biomass species, but are not discussed here because they are usually present in very small amounts. The peptides present in herba­ceous biomass are also not discussed here because, although the nitrogen and sulfur contents of the biomass should be assessed for certain microbiological processes, the amino acids that make up the proteins are generally not impor­tant factors in conversion processes.