Cellulose hydrolysis hemicelluloses

Processes need to be developed to expose LHC for effective penetration of chemicals that promote separation of LHC or which render cellulose accessible by swelling. These processes can have many objectives (1) to get at cellulose (2) to remove hemicellulose from the reaction site before cellulose hydrolysis (3) to remove lignin from the site and (4) to bring penetrating or hydrolysis agents into effective contact with cellulose. 

For the most part, cellulosic hydrolysis studies have been concerned with pure or at least relatively pure cellulose. However, cellulose in its natural state never occurs in a pure form. It always is associated with lignin and hemicellulose. The portions are indicated in Table II. 

In contrast to cellulose, which is crystalline, strong, and resistant to hydrolysis, hemicellulose has a random, amorphous structure with little strength. It is easily hydrolyzed by dilute acid or base, but nature provides an arsenal of hemicellulase enzymes for its hydrolysis. Hemicellulases are commercially important because they open the structure of wood for easier bleaching and thus support the introduction of ECF or TCF methods. Many different pentoses are usually present in hemicellulose. Xylose, however, is always the predominating sugar. The pentoses are also present in rings (not shown) that can be five- or six-membered. 

Unlike starch or cellulose, BSG hemicellulose has a complex structure, which is still present, in part, on its autohydrolysis products. Oligosaccharides from BSG hydrolysis consist mainly of branched arabino-xylo-glucurono oligosaccharides that are not highly acetylated when compared to other xylans, such as from Eucalyptus wood (31). The action of several enzymatic activities including endo-l,4-P-xylanase P-xylosidase and accessory activities such as acetyl xylanesterase, a-glucuronidase, and a-arabino-furanosidase is therefore required for the complete hydrolysis of OCL to monosaccharides. 

Carbohydrates would be the predominant raw materials for future biorefineries. The major polysaccharides found in nature are cellulose, hemicellulose and starch (see Chapter 1). These molecules would be mainly utilised after they are broken down to their respective monomers via enzymatic hydrolysis, thermochemical degradation or a combination of these two. Cellulose and hemicellulose, together with lignin, constitute the main structural components of biomass. Starch is the major constituent of cereal crops. This section would focus on the potential utilisation of carbohydrates and lignocellulosic biomass for chemical production. 

Moreover, lignocellulose is not edible and could theoretically be utilized without any impact on food production. The cellulose and hemicellulose fraction of lignocellulose may serve for the production of cellulosic ethanol, which could be produced via acid or enzymatic catalyzed hydrolysis of cellulose, followed by further fermentation to yield ethanol. Alternatively, the whole plant can be gasified to yield syngas, followed by methanol or dimethyl ether synthesis or Fischer-Tropsch technology that produces hydrocarbon fuels. Furthermore, controlled (bio-)chemical transformations to novel fuel compounds based on cellulose, hemicellulose, or lignin are possible, and numerous recent publications emphasize intense research in this direction. 

The hydrolysis of polysaccharides into monomeric sugars is a well-studied process its mechanism is still believed to be accurately described by the work of Saeman in 1945.432 Its significance is linked to the production of fermentable sugars for ethanol manufacture. Many of these processes are based on acid catalysis and overlap closely the pretreatment methods described earlier.363,364,373 Treatments with both concentrated and dilute acids are well known as methods for the hydrolysis of cellulose and hemicellulose, as is the use of organic dicarboxylic acids as alternative catalysts to mineral acids.433... 

Farone, W. A. and Cuzens, J. E., Methods of Producing Sugars Using Strong Acid Hydrolysis of Cellulosic and Hemicellulosic Materials. U S. Patent No. 5,562,777, October 8, 1996. 

Surface Activation. Acid Activation. Acid treatment of cellulose and hemicelluloses generally leads to hydrolysis to monosaccharides, which can subsequently dehydrate and condense to form furan-type compounds such as furfural and its 5-hydroxymethyl adduct. Further reactions lead to polymeric materials of dark color as well as to monomers such as levulinic acid, formic acid, and angelica lactones. Various condensation and solvolysis reactions also accompany the acid treatment of lignin 123). The hydrolysis, dehydration, and condensation reactions have been used to explain formation of covalent bonds between surfaces (85), increase in water resistance (85, 124), and weakening of wood (75) in nonconventional plywood or particle board production. However, very little factual information is available on how far, in terms of the consecutive reactions mentioned, and in what direction, in terms of the parallel reactions mentioned, does the surface of lignocellulosic materials actually change... 

---