Hemicellulose reactivity

It has been stated mat me reactivity of the wood cell wall polymers to acetic anhydride decreases in me order lignin > hemicelluloses > cellulose, both within me wood cell wall (Rowell, 1982) and with me isolated polymers (Callow, 1951 Rowell etal., 1994 Efanov, 2001). A comprehensive series of NMR studies has been performed investigating me substitution of me cell wall polymeric OH groups at various WPGs (Ohkoshi and Kato, 1992, 1993, 1997a,b). These have shown that not all of me lignin OH groups are... 

Reactive organic chemicals can be bonded to cell wall hydroxyl groups on cellulose, hemicelluloses, and lignin. Much of our research has involved simple epoxides (1 3) and isocyanates (4), but most of our recent effort has focused on acetylation. Acetylation studies have been done using fiberboards (5f6), hardboards (7 11) particleboards (12-20), and flakeboards (21-23), using vapor phase acetylation (8,2 257, liquid phase acetylation (, ), or reaction with ketene (28). 

H. Chemical Modification. The chemical modification of wood involves a chemical reaction between some reactive part of a wood component and a simple single chemical reagent, with or without catalyst, to form a covalent bond between the two. The wood component may be cellulose, hemicellulose, or lignin. The objective of the reaction is to render the wood decay resistant. The mechanism of the effectiveness is not known, but some possible explanations were given earlier. 

By far the most abundant reactive chemical sites in wood are the hydroxyl groups on cellulose, hemicellulose, and lignin. The types of covalent chemical bonds of the carbon-oxygen-carbon type that are of major importance are ethers, acetals, and esters. 

This chapter discusses major factors affecting the reactivity of cellulose, hemicelluloses, and lignin under both acidic and alkaline modifications. 

It is possible to thermoplasticize only the lignin and hemicellulose polymers of the cell wall using succinic anhydride [31]. If a nondecrystallizing reaction condition is used, it is possible to chemically modify the lignin and, possibly, the hemicelluloses but not the cellulose. This selective reactivity has been shown to occur if uncatalyzed anhydrides are reacted with wood fiber [25]. 

An endo-1,4-j8-xylanase (xylanase A), obtained from Schizophyllum commune, is rapidly deactivated using l-(4-azonia-4,4-dimethylpentyl)-3-ethylcarbodiimide iodide, indicating that the reactive site in the enzyme is a carboxyl group. " Xylanase A is of importance for the conversion of hemicellulosic biomass into fermentable products. 

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. 

Many factors influence the reactivity and digestibility of the cellulose fractions of lignocellulose materials. These factors include Hgnin and hemicellulose content, crystalhnity of cellulose, and the porosity of the biomass materials. Pretreatment of Hgnocellulosic materials prior to utiHzation is a necessary element in biomass-to-ethanol conversion processes. The objective of the pretreatment is to render biomass materials more accessible to either chemical or enzymatic hydrolysis for efficient product generation. The goals of the pretreatment are ... 

Because the hemicellulose fraction of biomass materials can be separated from lignin and cellulose by dilute acid treatment, cellulose becomes more reactive towards cellulase. Hemicellulose hydrolysis rates vary with acid concentration, temperature, and solid-to-liquid ratio. With most lignocellulosic materials, complete hemicellulose hydrolysis can be achieved in 5-10 min at 160°C or 30-60 min at 140 °C. Dilute acid hydrolysis forms the basis of many pretreatment processes for example, autohydrolysis and steam explosion are based on high-temperature dilute acid catalyzed hydrolysis of biomass. 

Cellulose, the hemicelluloses, and lignin are distributed throughout the wood cell wall (Figure 1), and the hydroxyl groups they contain are the most abundant reactive chemical sites. 

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