Cellulose wood, constituents

Generally, no cellulose is lost in the acid sulfite process. The losses of hemicelluloses are higher for hardwood than for softwood. Typical yield values for various wood constituents have been collected in Table 7-3 in which comparative data for the kraft process are also shown. 

Cu / lignin complexes physically adsorbed on wood constituents Cu / cellulose complexes... 

Effect of Wood Species and Extractives. The sorption isotherms of all woods are generally similar in shape. However, there may be considerable variations among them with respect to the absolute values of hygroscopicity. This variation may be because of differences in the proportion of the primary wood constituents, such as cellulose, hemicellulose, and lignin in different woods or more importantly, because of differences in the kind and quantity of ex-... 

Any treatment of wood must take account of the differing accessibilities and reactivities of the principal wood constituents. Further, any chemical or microbial method of breaking down wood has to devise conversion pathways for cellulose, the hemicelluloses and lignin, and if necessary consider ways of isolating the individual reaction products so that they can be processed separately. Hydrolysis has proved to be a most effective method of opening up the wood structure for subsequent treatments. The expression hydrolysis of wood is used rather loosely. It is not technically correct since the reactions affect primarily the carbohydrate fraction of wood. Lignin is largely unaffected. 

In the future, more attention will undoubtedly be devoted to the molecular architecture of wood, including the exact physical state of the hemicelluloses in the cell wall and their relation to other wood constituents, especially cellulose and lignin. Little is currently known concerning the bios3mthesis of cellulose and hemicelluloses, and their role in cell development. The numerous hemicelluloses occurring in the phloem have only recently attracted attention. It is probable that, in many species, some of them differ considerably from those present in the xylem. 

The different main polymers in wood appear to have structural roles that are related to this microarchitecture. Cellulose, an intrinsically rigid crystalline material, provides the reinforcing framework of the cell walls. Theoretical treatments of stress-strain relationships in wood under load indicate that the stiflhess of the material is imparted primarily by cellulose fibrils 10). The main function of hemicellulose and lignin is to buttress the fibrils. Degradation of any of these wood constituents results in a decrease in the strength of the material. 

The spectrum of aspen wood (Figure 7.1) shows the presence of the three fundamental wood constituents. A band for cellulose is at 895 cm (g-anomer in pyranose ring) <22). xhe band at 1740 cm" is due to uronic acid and acetyl groups in hemicellulose. The bands from 1235 to 1605 cm, specially the one at 1505 cm", are representative of lignin while those from 950-1100 cm" are due, in part, to carbohydrates (C-0 bonds in alcohols). 

Besides cellulose, wood has several other constituents, the most important of which are hemi-celluloses and lignin. The chemical composition of wood varies widely according to source but a typical hardwood analysis might be as follows ... 

All wood chemical components (lignin, cellulose, hemicellulose, and extractives), which contain internal chemical groups such as carboxyl, carbonyl, aldehyde, unsaturated double bonds, phenolic hydroxyl, and external entities such as fats, waxes, and metal ions, are capable of absorbing sunlight or UV light. The absorbed energy can cause the dissociation of bonds in the molecules of the wood constituents. Similar to thermoplastics, the photodegradation of wood also involves a serials of radical-based reactions. 

Surface-active compounds can be produced from all wood constituents. For instance, water-soluble cellulose derivatives and lignin, in the form of lignosulfonates, can be used as dispersing agents in many applications. This chapter focuses on the production of resin and fatty acids as well as sterols and sterol ethoxylates as forest-industry by-products, and their use as surface-active components. Moreover, the potential production of hemi-celluloses and their possible utilization as steric stabilizers of oil-in-water emulsions is also outlined. 

Cellulose is the most abundant of naturally occurring organic compounds for, as the chief constituent of the eell walls of higher plants, it comprises at least one-third of the vegetable matter of the world. The cellulose eontent of such vegetable matter varies from plant to plant. For example, oven-dried cotton contains about 90% cellulose, while an average wood has about 50%. The balance is composed of lignin, polysaccharides other than cellulose and minor amounts of resins, proteins and mineral matter. In spite of its wide distribution in nature, cellulose for chemical purposes is derived commerically from only two sources, cotton linters and wood pulp. 

Wood is a composite material that is made, up basically of a mixture of three main constituents, cellulose, hemicellulose, and lignin (see Textbox 54), all of them biopolymers synthesized by the plants, which differ from one another in composition and structure (see Textbox 58). The physical properties of any type of wood are determined by the nature of the tree in which the wood grows, as well as on the environmental conditions in which the tree grows. Some of the properties, such as the density of wood from different types of trees, are extremely variable, as can be appreciated from the values listed in Table 71. No distinctions as to the nature of a wood, whether it is a hardwood or a softwood, for example, can be drawn from the value of its specific gravity. 

The two most important chemical constituents of wood are cellulose and lignin. Through the efforts of many investigators over the past... 

The two most important natural pentoses, 1 -arabinose and 1 -xylose, occur in nature as polymeric anhydrides, the so-called pentosans, viz. araban, the chief constituent of many vegetable gums (cherry gum, gum arabic, bran gum), and xylan, in wood. From these pentapolyoses there are produced by hydrolysis first the simple pentoses which are then converted by sufficiently strong acids into furfural. This aldehyde is thus also produced as a by-product in the saccharification of wood (cellulose) by dilute acids. Furfural, being a tertiary aldehyde, is very similar to benzaldehyde, and like the latter undergoes the acyloin reaction (furoin) and takes part in the Perkin synthesis. It also resembles benzaldehyde in its reaction with ammonia (p. 215). 

Many of the physical, chemical and biological properties of wood can be understood by referring to the polymeric chemical constituents. In many cases of wood modification, these polymeric components are modified to some extent. The three structural polymeric components of the wood cell wall are cellulose, hemicellulose and lignin. There are many excellent texts describing the structure and function of these components, and only a brief account is given here. 

Biorefinery includes fractionation for separation of primary refinery products. The fractionation refers to the conversion of wood into its constituent components (cellulose, hemicelluloses and lignin). Processes include steam explosion, aqueous separation and hot water systems. Commercial products of biomass fractionation include levulinic acid, xylitol and alcohols. Figure 3.3 shows the fractionation of wood and chemicals from wood. 

The more stable carbohydrate constituent of wood is assumed to be almost pure alpha cellulose. This is shown by the fact that 94-96% of the reducing materials, obtained in the later stages of hydrolysis, is fermentable. High temperatures and longer periods of time are required for the hydrolysis of the stable cellulose when dilute acid is used. If strong acid (72% sulfuric, 45% hydrochloric, 85% phosphoric) is used, the cellulose dissolves and then is converted into smaller units. Both of these procedures have been studied by many experimenters. 

CeHeOB, 126.11, occurs in pine needles and the bark of young larch trees. It is produced when cellulose or starch are heated and is a constituent of wood tar oils. It forms crystals mp 162 164 °C) with a caramel-like odor, reminiscent of freshly baked cakes. 

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