Depolymerization, cellulose

Magnesium compounds retard cellulose depolymerization by deactivating the transition metal catalysts. In alkaline media where hydroxy-acids or products of polysaccharide degradation are present, magnesium forms stable complexes with transition metals. The formation of iron-magnesium complexes in particular is supported by interactions approximating the coordination number of iron (6) and one half the coordination number (3). 

Section B is representative of C-C and C-O cleavage at the intrapolymer level which cannot be recovered. Examples of C-C bond breakage are lignin-hemicellulose copolymer separation, hemi-cellulose depolymerization, and amorphous cellulose depolymerization. 

In an environment without adequate humidity, the initial effect of heating wood is dehydration. As temperatures approach 55-65 °C for extended periods (2-3 months), hemicellulose and cellulose depolymerization begins (28), Pyrolysis and volatilization of cell wall polymers occur at about 250 °C followed by char formation in the absence of air and combustion in the presence of air. 

This initial oxidative depolymerization of cellulose evidently opens up the wood cell wall structure so that cellulolytic and hemi-cellulolytic enzymes can reach their substrates despite the presence of lignin. Solubility of wood in 1% NaOH increases markedly on brown-rot attack IS) and reflects cellulose depolymerization and the opening up of the wood structure. 

One possible explanation for these different modes of cellulose depolymerization in the same species of wood is that the cellulolytic enzyme molecules of Poria monticola are smaller than those of Polyporus versicolor and for that reason would be able to penetrate and act in regions of the fine structure of the fibers that are not accessible to those of the latter fungus. This hypothesis has led to efforts (as yet incomplete) to determine the molecular size of the cellulolytic enzyme proteins of these two organisms. Another possible explanation is that the initial dissolution of cellulose and other cell-wall polysaccharides is accomplished by catalysts that are not enzyme proteins and therefore could be substantially smaller in molecular size. Halliwell (21) has described experiments on the... 

An alternative strategy is to establish more robust covalent bonds between the G sheet and the sulfonic group. This can be achieved by attaching benzene sulfonate groups to the G sheet with the creation of robust, non-hydrolizable C-C bonds by electrophilic addition of suitable precursors containing benzene sulfonate moieties into C=C double bonds on the G sheet. In this way acid catalysts that can have activity for biomass transformation and particularly for cellulose depolymerization can be obtained [36]. 

Physico-chemical pretreatment is a synergistic effect of chemical reagent and physical approach. Lignin hydrolysis using NaOH is made easier with the assistance of physical treatment. Physico-chemical treatment of ligno-celluloses depolymerizes and hydrolyzes hemicelluloses to monosaccharide. On continuous degradation with uncontrolled or prolonged treatment. 

A second degradation process is oxidation, often photo-induced especially by exposure to light not filtered for uv. The radicals resulting from this reaction promote depolymerization of the cellulose, as well as yellowing and fa ding of paper and media. Aging causes paper to become more crystalline and fragile, and this can be exacerbated particularly if the paper is subjected to poor conditions. 

Since polysaccharides are the most abundant of the carbohydrates, it is not surprising that they comprise the greatest part of industrial utiliza tion (9,22). Most of the low molecular weight carbohydrates of commerce are produced by depolymerization of starch. Polysaccharide materials of commerce can be thought of as falling into three classes cellulose, a water-insoluble material starches, which are not water-soluble until cooked and water-soluble gums. 

By using this technique acrylamide, acrylonitrile, and methyl acrylate were grafted onto cellulose [20]. In this case, oxidative depolymerization of cellulose also occurs and could yield short-lived intermediates [21]. They [21] reported an electron spin resonance spectroscopy study of the affects of different parameters on the rates of formation and decay of free radicals in microcrystalline cellulose and in purified fibrous cotton cellulose. From the results they obtained, they suggested that ceric ions form a chelate with the cellulose molecule, possibly, through the C2 and C3 hydroxyls of the anhy-droglucose unit. Transfer of electrons from the cellulose molecule to Ce(IV) would follow, leading to its reduction... 

Representative condensation polymers are listed in Table I. The list is by no means exhaustive, but it serves to indicate the variety of condensation reactions which may be employed in the synthesis of polymers. Cellulose and proteins, although their syntheses have not been accomplished by condensation polymerization in the laboratory, nevertheless are included within the definition of condensation polymers on the ground that they can be degraded, hydrolytically, to monomers differing from the structural units by the addition of the elements of a molecule of water. This is denoted by the direction of the arrows in the table, indicating depolymerization. 

Depolymerization, e.g., polyethylene terephthalate and cellulose hydrolysis Hydrothermal oxidation of organic wastes in water Crystallization, particle formation, and coatings Antisolvent crystallization, rapid expansion from supercritical fluid solution (RESS)... 

Figure 1.15 gives an overview of the main constituents of non food biomass. There are three components Cellulose, hemicellulose and lignin. Cellulose and hemicellulose are built form sugar-type monomers, but their cost-effective isolation through enzymatic depolymerization remains a challenge. 

Various solvents are being investigated to dissolve lignocellulosic materials. Some approaches focus on the selective depolymerization and extraction of lignin and hemicellulose as pre-treatment to produce clean cellulose fibers for subsequent fermentation or for pulping. Other approaches attempt to dissolve the whole lignocellulose with or without depolymerization. The liquefaction processes that are carried out at high temperature (>300 °C), and produce a complex oil mixture, are discussed above with the pyrolysis processes. 

Full dissolution has been reported to proceed in ionic liquids such as butyl- or allyl-methyl-imidazolium chloride under microwave irradiation [59, 60], The Clanton is claimed to be essential to favor the de-agglomerization of the cellulose by breaking its H-bonds that hold it together [61]. The cellulose can subsequently be precipitated from the ionic liquid upon addition of, for example, water, without significant depolymerization. 

---