Pentosans Chemical

The support for a chemical linkage between cellulose and lignin in wood, presented by Hibbert and co-workers (90), is based on their treatment of oak wood with acetic anhydride, glacial acetic acid and catalytic amounts of sulfuric acid. A fraction soluble in dioxane was found to have, after several precipitations, the same composition of lignin, cellulose and pentosan as that of the wood itself. 

Scheme 2.1 Furanic commodity chemicals derived from pentosans in agricultural wastes (corn cobs, oat hulls, bagasse, wood chips).

According to the present theory (e.g. Zherebov [1]) most of the hemicelluloses, pentosans and hexosans are removed during sulphitation, because calcium sulphite is capable of forming addition compounds with pentosans. Complete removal of hemicelluloses is only possible however after lignins have been removed, as lignin seems likely to be chemically combined with hemicellulose and thus with pentosans or hexosans. Only rupture of these bonds and the separation of lignins makes it possible to attack the non-cellulosic polysaccharides (Heuser and Haug [2]). 

Most of the cells of wood are long, narrow hollow fibers and tubular-shaped cells arranged with their long axes parallel to the axis of the tree trunk. Certain food storage cells lie in radial bands, termed wood rays, which are perpendicular to the tree axis. The walls of this complex system of plant cells form the basic framework and material of all wood substance, All wood substance is composed of two basic chemical materials, lignin, and a polysaccharidic system, which is termed holoceliulose. The latter embraces cellulose and the hemicelluloses, a mixture of pentosans, hexosans and polyuronides, and in some instances small amounts of pectic materials. Wood cell wall tissue also always retains small amounts of mineral matter (ash). 

Gravitis et al. (2001) also reported a novel technology consisting of a two-step selective catalysis of wood and other pentosan-containing raw materials for the production of various chemicals (i.e. furfural, levoglucosan, ethanol) by using small amounts of strong catalysts. Furfural could be used for the production of dyes, plastics and fumaric acid. 

While there are many chemicals possessing the furan nucleus, furfural and furfuryl alcohol are of major industrial significance. The chief source of furfural in plant materials is the pentosan fraction, predominantly xylan (Scheme 1). 

Hemoclar (SP 54) or pentosan polysulfate is a linear cationic polymer of beechwood tree origin which has been used for the prophylaxis of deep venous thrombosis. This agent is developed by Bene Chemical, Munich, Germany. Because of it origin, this agent can be obtained in relatively large quantities and may prove to be useful in various indications. 

Filtration and storage behavior of beer depend strongly on its chemical composition (Table 20.1). Beer production is based on natural ingredients and therefore beer contains a wide variety of chemical compounds. Most chemical components of beer have an influence on beer filtration in general and membrane hltration in particular. Carbohydrates, such as pentosans and (3-glucans, proteins and protein-polyphenol complexes, are of particular importance in membrane hltration of beer, as they are responsible for membrane fouling [3], which has negative consequences on both the hux and quality of the hltered beer, as it will be discussed in detail later on in this chapter. 

As discussed in chapter 11, the methyl pentosan content of the raw material is of considerable interest as it determines the level of 5-methyl furfural and furyl methyl ketone in the final furfural. Both 5-methyl furfural and furyl methyl ketone are unwanted by-products requiring a sizable expense if they are to be removed from the furfural, and if they exceed the critical level of 1 %, the furfural thus loaded loses the attribute of being a fine chemical , which is an important aspect for pricing the product. 

Scores of chemical compounds have been identified in unbumed tobacco. Basically it is cellulose. The cured product contains acids (citric, oxalic, formic), alkaloids (nicotine, anabasine, myosmine), and carbohydrates (lignin, pentosans, starch, sucrose), as well as tannin, ammonia, glutamine, and micro amounts of zinc, iodine, copper, manganese, and polonium-210. See cigarette tar smoke (4). 

If the chemical composition of southern pine (lignin, 27.9% holocellulose, 67.0% cellulose, 48.1% hemicellulose pentosans,... 

Cacao Shell, Cocoa shells. Shells ol the seed of Theobroma cacao L., Sterculiaceae. Habit. Brazil, Central America, Mexico, West Indies, and most tropical countries. Constit. Theobromine, caffeine, cacao red. protein, pentosans, pec tic acid, starch. Ref Dittmar, Ettgenharia e quim S, no. ], I (1953), C.A. 48, 2949e (1954). Monographs.- E. M, Chatt, Cocoa (Interscience, New York, 1953) D, H. Urquhart, Cocoa (Longmans, London, I960 Powell, Harris, Chocolate and Cocoa in Kirk-Ot(inter Encyclopedia of Chemical Teh nofogy. vol. 5 (Interscience, New York, 2nd ed., 1964) pp 363-402. 

Chowdhury et al. (7) reported on the results of a study of four samples of ancient wood from India. One of the samples, teakwood, was taken from a Buddhist cave, where it had been exposed to atmospheric conditions for 2200 years. Chemical analyses showed a reduced cellulose and pentosan content. Light microscopic studies suggested that the only difference between the ancient and recent wood was helical fissures in the fiber walls of the former. A significant difference, however, was found in the birefringence of the two samples. The birefringence, considerably lower in the ancient wood, indicated a loss in crystallinity of the cellulose. 

Clark and Wolff carried out the first studies on the changes in the chemical composition of kenaf as a function of the growing season [22]. They also studied the chemical differences along the stem and between leaves and stem. This data showed that the pentosans, lignin, and a-cellulose content increases with age, while the protein and hot water extractives content decreases with age. Data taken from the top part of the plant shows similar trends however, the top part has less cellulose, pentosans, and lignin, but higher hot water extractives and protein than the bottom part of the plant (Table 7.4). 

Materials. Alpha cellulose (4% pentosans) and amylose (70%) were obtained from Sigma France. Starches from all other sources were kindly supplied firom INRA Nantes, France. Vegetal oils were obtained fixrm Sidobre-Sinova (France). Other chemicals were of reagent grade and were purchased from usual providers and were used without further purification or treatment. 

In the conventional dilute sulfuric acid-catalyzed percolation process, the plant raw material containing pentosans is mixed with dilute sulfuric acid and, in the first step, pentosans are hydrolyzed to pentoses (C6Hio05)n, which are then cyclodehydrated to furfural in the second step. The frirfiirai formed is recovered by steam distillation and fractionation. The chemical reaction is shown in Figure I. The mechanism of the reaction (4) is shown in Figure 2. 

The production of aromatic chemicals from renewable materials is mainly concentrated on the exploitation of wood products together with recovery of furfural by converting pentosans. Pentosans are almost as widely distributed in nature as cellulose, although in lower quantities. Pentosan-rich materials are commonly used in the production of furfural, particularly waste from corncobs, grain chaff and cotton pod cases. 

Sisal varies in quality. The large variations in its chemical compositions are because of its different sources, age, extraction methods, etc. [23]. Eor example, Wilson [13] indicated that sisal fiber contains 78% cellulose, 8% lignin, 10% hemicelluloses, 2% waxes, and about 1% ash by weight, whereas Rowell et al. [24] found that sisal contains 43-56% cellulose, 7-9% lignin, 21-24% pentosan, and 0.6-1.1% ash. The work carried out by Chand and Hashmi [25] showed that the cellulose and lignin contents of sisal vary from 49.62 to 60.95% and 3.75 to 4.40%, respectively, depending on the age of the plant. 

---