Xylans acidic, acid hydrolysis

Xyhtol is synthesized by reduction of D-xylose catalyticahy (40), electrolyticahy (41), and by sodium amalgam (42). D-Xylose is obtained by hydrolysis of xylan and other hemiceUulosic substances obtained from such sources as wood, com cobs (43), almond shells, hazelnuts, or oHve waste (44). Isolation of xylose is not necessary xyhtol results from hydrogenation of the solution obtained by acid hydrolysis of cottonseed hulls (45). 

Xylan has the general properties of insolubility in water, solubility in alkaline solutions, ease of acid hydrolysis, high negative optical rotation, and non-reducing action toward Fehling s solution. It can be placed in three general polysaccharide classes (1) pentosan, (2) glycan, and (3) hemicellulose. It is classed as a pentosan because it is principally a polymer of a pentose. It is by far the most abundant pentosan. 

Relatively pure xylan isolated from the holocellulose of aspen (Populus) wood is said to contain 85% of xylose residues.78 One of the characteristic properties of xylan is its ease of hydrolysis. Because it hydrolyzes much more readily than cellulose, mild acid treatment may be employed to bring about preferential hydrolysis of xylan from plant material. Xylose is ordinarily prepared in the laboratory by direct sulfuric acid hydrolysis of the native xylan in ground corn cobs.74 Hydrolysis in hydrochloric acid proceeds rapidly, but decomposition to furfural also occurs to some extent.76 A commercial method for the production of D-xylose from cottonseed hulls76 and straw77 and from corn cobs17 78 has been described. 

Xyloisosaccharinic acid [2,4-dihydroxy-2-(hydroxymethyl)butanoic acid] is one of the major, alkaline-degradation products of wood xylan, in particular, that of birch. The disaccharide, 2-O-D-xylopyranosyl-L-arabinose, which was isolated as a hydrolysis product of corn-cob hemicellulose, is readily degraded at 100° in 15 mM Ca(OH)2 to acidic products, primarily saccharinic acids. Xylan oligosaccharides from corn-cob hemicellulose produced 2,4-dihydroxy-2-(hydroxymethyl)buta-noic acid when exposed to 0.02 M Ca(OH)2 at 25°. However, it was noted that the xylan, itself, was stable at 100° in Af NaOH. The major acidic component of the hemicellulose fraction of slash pine Pinus el-liotti) after acid hydrolysis was identified as 4-O-methyl-D-glucuronic... 

Xylo-oligosaccharides. Samples of X4 through X7 of 98% purity or better and of X3 and X9 of 90% or better were prepared by acidic hydrolysis of larchwood xylan and preparative chromatography, as previously described (i). 

D-xylose on hydrolysis with dilute nitric acid. Percival and Chanda7 isolated a xylan from the same plant. They found that the methylated xylan produced on hydrolysis 2-methyl-D-xylose, 2,3-dimethyl-D-xylose, 2,4-dimethyl-D-xylose and 2,3,4-trimethyl-D-xylose. From this and from the results of periodate oxidation, Percival and Chanda considered that the polysaccharide contains 1 — 3 and 1 —> 4 linkages with a non-reducing endgroup (yielding the 2,3,4-trimethyl-D-xylose) for every 20-21 D-xylose units. They considered that this xylan was not a mixture of 1 —> 3 and 1 —> 4 linked polysaccharides because careful fractionation of its diacetate and dimethyl ether failed to establish any polymer heterogeneity. Barry, Dillon, Hawkins and O Colla74 confirmed the conclusion of Percival and Chanda. 

Only minor amounts of additional reducing sugars from the xylan decomposition were detected in the reaction solutions. Acid hydrolysis showed, however, that higher-molecular xylan fragments were liberated by the combined cellulase-mannanase treatment. At the end of the experi-... 

Hemicellulose (or polyose) is primarily composed of xylan, a branched polymer composed of five-carbon sugar, xylose. Typical polymerization degree of hemicellulose is 50 - 200, which is shorter than the cellulose molecules. The acid hydrolysis of hemicellulose, (C6H10O5)n, produces mainly xylose (C6H10O5), which can be converted to furfural, a chemical feedstock, or can be fermented to ethanol. 

Many polysaccharides of the xylan group contain residues of D-glucuronic acid or 4-0-methyl-D-glucuronic acid. Since glycosiduronic acids are particularly resistant to acid hydrolysis, aldobiouronic acids may be isolated... 

Wood xylans are characterized by the presence of residues of 4-O-methyl-D-glucuronic acid linked to position 2 of D-xylose in the main xylan chains (XXVI), and, accordingly, the aldobiouronic acid, 0-(4-0-methyl-a-D-glu-cosyluronic acid)-(l —> 2)-D-xylose (IX), is commonly isolated after the graded hydrolysis of such xylans. Partial acid hydrolysis of Monterey pine wood69 furnishes this compound together with the isomeric aldobiouronic acid, 0-(4-0-methyl-a-n-glucosyluronic acid)-(l—>3)-D-xylose, as a minor product, but the role of the (1 — 3)-linked aldobiouronic acid unit in the... 

The main factors affecting monosaccharide recovery in dilute-acid hydrolysis are catalyst concentration, reaction time, and temperature, whereas enzymatic hydrolysis is also dependent on additional factors such as substrate structure, and type and ratio of enzymatic activities present in the commercial enzyme preparations. The most important enzymes for xylan hydrolysis are endo-l,4-(3-xylanases (which attack the main chain) (3-xylosidases (which hydrolyze xylooligosaccharides to xylose) and accessory enzymes, such as acetyl xylanesterases, a-glucu-... 

Feedstock material and processed solids were analyzed for glucan, xylan, arabinan, and acetyl groups by quantitative acid hydrolysis according to standard methods (25). The acid-insoluble residue was considered as Klason lignin, after correction for the acid-insoluble ash (determined by igniting the contents at 575°C for 5 h). 

Table IV shows the pyrolysis products formed from xylan at 300° (9). The pyrolysis of xylan yields about 16% of tar which contains 17% of a mixture of oligosaccharides. Upon acid hydrolysis, they give an approximately 54% yield ofj-xylose. Structural analysis of the polymers shows that they are branched-chain...

Acid hydrolysis has been successfully employed for pretreatment of lignocellulosic materials. Dilute sulfuric acid, used at either low or high temperature, achieves high xylan to xylose conversion. This is favorable to the overall economics, as xylan accounts for a large part of the total carbohydrates in the lignocellulosic materials. 

---