Lignocellulose compounds

FIGURE 8 Lignocellulose compounds of higher plant structural tissues from terrestrial and land-water interface regions form a major source of RDOM within aquatic ecosystems, and a major metabolic coupling between the drainage basin and aquatic ecosystems. Chemical structure indicates a humic macromolecule (P. Hatcher, personal communication). 

Cellulose is closely related to other biopolymers foimd in wood and plants. Together, these compounds are the raw materials used in a number of products encountered in forensic chemistry. For example, paper is made from wood chips derived from soft or hard woods, plants, or recycled paper stocks. Of interest in paper production are the polysaccharides found in wood— lignin, hemicellu-lose, and cellulose, all classified as part of the lignocellulose complex found in biomass such as wood. The lignocellulose compounds impart strength and... 

Thorn K A, KR Kennedy (2002) N NMR investigation of the covalent binding of reduced TNT amines to soil humic acid, model compounds, and lignocellulose. Environ Sci Technol 36 3787-3796. 

A hydrolysis step is involved in the pulp industry in order to concentrate the cellulose from wood. This uses large-scale processes whereby a liquid fraction, the lignocellulose, is formed as a by-product in the process, and contains high levels of phenolic components and their derivatives. These compounds also constitute an environmental problem due to their possible introduction into rivers, lakes, and/or seas. Chlorophenols from the cellulose bleaching process have traditionally attracted most of the interest in the analysis of industrial waste because of their high toxicity. 

An examination of the other articles in this text serves as an excellent illustration of the diverse analytical methods that have been successfully applied to lignocellulosic materials. The practitioners of wood chemistry have rapidly assimilated and adapted modern instrumental chemistry to their specific problems. In contrast, the techniques of computational chemistry have not been widely used in such an environment. The current paper will attempt to describe the capabilities, opportunities, and limitations of such an approach, and discuss the results that have been reported for lignin-related compounds. 

The breakdown of furan aldehydes leads to the formation of formic and levulinic acid. Moreover, acetic acid is formed during the degradation of hemicellulose. Partial breakdown of lignin can generate a variety of phenolic compounds (23), which also inhibit S. cerevisiae (14,15). In contrast to furan aldehydes and aliphatic acids, the toxic effect of specific phenolic compounds is highly variable (15). Different raw materials and different approaches to prepare lignocellulose hydrolysates will result in different concentrations of the fermentation inhibitors (16,17). 

It is well known that phenolic compounds are important inhibitors in hydrolysates prepared from hardwood (27) as well as softwood (20). The hydrophobicity of the matrix is likely to be an important property that contributes to the ability of an ion-exchange resin to remove phenols. Although the knowledge regarding the suitability of different methods to determine phenolics in lignocellulose hydrolysates has increased (28), the quantification of phenolics and the correlation with the inhibitory effect is still a challenging task, because hydrolysates contain a wide variety of phenolic compounds that have very different toxic effects. 

Hydrolysis of lignocellulose is necessary to enable its use for ethanol production. However, when lignocellulosic materials are hydrolyzed with acid, compounds toxic to the yeast cells are released. The inhibitors are of three main types aldehydes, organic acids, and phenolic compounds. Among the aldehydes, furfural and hydroxymethylfurfural (HMF) are typically found in high concentrations—particularly in dilute-acid hydrolysates (1,2). These compounds have been shown to inhibit certain enzymes in the catabolism necessary for cell growth (3-9). 

The development of pretreatment technologies that are tuned to the characteristics of the biomass is still needed. Ideally, lignocellulose should be fractionated into multiple streams that contain valuable compounds in concentrations that make purification, utilization, and/or recovery economically feasible. Predictive pretreatment models should enable the design of this step to match both the biomass feedstock and the fermentation technology. 

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. 

Figure 2.1.1 Schematic illustration of the chemical structure of the major compounds of lignocellulose, namely, cellulose, hemicellulose, and lignin.

Biomass can be divided mainly into three compound classes - namely, lignocellulose, lipids, and proteins. These may come from different feedstocks and may differ in composition but should need only different steps in preprocessing to obtain a... 

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