Chemical hydrolysis lignocellulose

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. 

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 ... 

While there has been much progress in the study of cellulases, the applications of such technology have been limited by a lack of economical pretreatment of the lignocellulose. Without such pretreatment hydrolysis is slow and incomplete. The value of enhanced enzymatic attack on lignocellulose is not limited to the production of sugar and chemicals. The same procedures would be applicable to the increased digestibility of coarse fodder by ruminants. 

Chemical hycbolysis involves exposure of lignocellulosic materials to a chemical for a period of time at a >ecific temperature, and results in sugar monomers fiom cellulose and hemicellulose polymers. Acids are dominandy applied in chemical hydrolyses. Sulfuric acid is the most investigated acid (27), altoough other acids such as HCl (28) have also been used. Acid hydrolyses can be divided into two groups (a) concentrated-acid hydrolysis and (b) dilute-acid hydrolysis. 

A physical, chemical, or biological pretreatment will break down the lignocellulosic network and make the substrate better available for enzymatic hydrolysis. 

Considerable research has been directed at converting crops and plants into industrial chemicals. The major stumbling block in this effort has been fractionation of lignocellulose into its constituents, lignin and cellulose, and hemicellulose. Research in the past has focused on acid pretreatments and enzymatic hydrolysis, which have proven too costly to date for commercial applications. 

Dilute acid pretreatment opens up the lignocelluloses structure as well as enhances the hydrolysis process, if right conditions are employed. It is a very popular method in which a very low concentration of acid (e.g., 0.1-1.0%) is used. Much research has explored the use of acid pretreatments of different lignocellulosic materials, focusing on chemical composition of the materials, operating time, concentration of acid. 

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