Cellulose selective oxidation

The selective oxidation of cellulose to dialdehyde by sodium periodate is well known. It has been postulated by Criegee (74) and by Waters (73) that this reaction proceeds by a free radical mechanism. Toda (76) and Morimoto, Okada, Okada, and Nakagawa (77) have concluded that sodium periodate oxidation should initiate graft polymerization. They succeeded in grafting methyl methacrylate and acrylonitrile onto cellulose substrates, such as rayon and paper. A similar procedure is recommended in a patent of Chemische Werke Huels (78) to graft vinyl monomers onto cotton, polyethylene oxide, copolymers of vinyl chloride-vinyl acetate, and others. 

The use of nitrogen dioxide for the selective oxidation of polysaccharides to polyuronic acids was introduced by Kenyon and his coworkers13,63 in 1941. By this means extensive oxidation of the primary alcohol groups in cellulose was obtained, through the mechanism of preferential nitration followed by decomposition of the nitric acid ester with carboxyl forma-tion.68(0< > Apparently some undissociated nitration products also were formed, since infrared absorption studies54 indicated the presence of nitrate radicals in the polyuronic acid. Side reactions produced carboxyl,... 

Many other chemical redox systems have been reported as initiators of macrocellulosic radicals and as catalysts for graft polymerization. One variation has been to modify cellulose chemically to increase its reactivity with selected oxidizing and reducing agents which on reaction yield macrocellulosic radicals (14, 15). 

Aldehyde starches are prepared by treatment with periodic acid/periodate ions, which selectively oxidize the adjacent hydroxyl groups on carbon atoms 2 and 3 to aldehyde groups. Dialdehyde starch can react with cellulose by forming covalent hemiacetal and acetal bonds.40 It is primarily used as a wet strength agent in the production of tissue and other sanitary grades. 

The use of dilute permanganate in sulfuric acid at 0°C. as a selective oxidant for the estimation of reducing end groups in cellulosic materials has recently been suggested. Employing this procedure it was found that surgical cotton had a calculated D.P. of 290 cotton linters pulp 1650 and viscose rayon, 165. 

Selective oxidation of amylose at C3 has been achieved with dimethyl sulfoxide/phosphorus pentaoxide [119]. In contrast, the oxidation of unprotected cellulose with dimethyl sulfox-ide/acetic anhydride [120] or dicyclohexylcarbodiimide/dimethyl sulfoxide [121] produced a product containing modified 8-D-glucopyranosyl units - some with a carbonyl function at C2, some with a carbonyl function at C3, and some with carbonyl functions at both C2 and C3[122],... 

Oxidation of secondary hydroxyl groups at C-2 and C-3 leads to acyclic dialde-hydes and dicarboxylic derivatives. Selective oxidation of primary hydroxyl groups is more difficult, but can be achieved by stoichiometric as well as catalytic methods. Nitric acid oxidizes monosaccharides into aldaric acids. Oxidation of cellulose or starch by nitrogen dioxide (N2O4) yields 6-carboxy starch and 6-carboxycellulose respectively. Subsequent hydrolysis under well-controlled conditions (0.5-2 M HCl at 150 °C) of these materials yields o-glucuronic acid (o-GlcAp) [12]. 

The selective oxidation of chitosan and chitosan derivatives by TEMPO has been studied. The modifications on chitosan present difficulties due to the presence of the amine moieties [15]. Several types of cellulose have been submitted to... 

C. Bosso, J. Defaye, A. Gadelle, C. C. Wong, C. Pedersen, Homopolysaccharides interaction with the dimethyl sulpoxide-paraformaldehyde cellulose solvent system. Selective oxidation of amy-lose and cellulose at secondary alcohol groups, J. Chem. Soc. Perkin Trans. I, (1982) 1579-1585. 

In preliminary work, we have demonstrated that polyoxometalates can catalyze equations 8-10 where the substrate, R, is wood pulp and the oxidant, OX, is either H2O2 or O2. This approach is effective as the lignin is selectively oxidized and removed the cellulose is minimally affected. Figure 2 gives molecular representations of lignin and cellulose as well as the general selective oxidation process (equation 8) for bleaching wood pulp by... 

To obtain the modified ANP, the chemical pretreatment can be carried out, for instance by means of the selective oxidation with nitrogen tetroxide. The oxycel-lulose was prepared by the following method (Kumar, 2003) the starting cotton cellulose (100 g) was soaked with mixture of 85 wt% o-phosphoric and 70 wt% nitric acids (2 1), and after addition of 20 g of sodium nitrite the reaction system was kept at room temperature for 48 h. 

Selectivity in these devices can be optimized through selection of moderate operating potentials, the use of enzymes that will selectively oxidize or reduce the analyte, or through selective control of mass transport via an additional outer permselective membrane. Mechanisms that have been used for this purpose include size (hydrolyzed cellulose acetate) and charge (Nafion ) exclusion. 

Cellulose-based natural fiber can be oxidized using oxidizing agent with acidic catalyst. Oxidation reactions applied to cellulose in fiber for chemical modifications [43]. Oxidation reactions occur on cellulose selectively at particular position. The reaction of sodium metaperiodate with cellulose in wood fiber in the presence of sulfuric acid catalyst at 120°C and 85 KPa pressure yielded the oxidized product. Sodium metaperiodate reacts with hydroxyl groups of cellulose and produce 2,3-dialdehyde cellulose which improved the physical and mechanical properties of polymer composites [44]. 

Recently novel techniques for obtaining of cellulose nanocrystals have appeared in the literature. Cao et al. (2012) showed a controllable method to fabricate cellulose nanowhiskers from jute fibers with a high yield (over 80 %) via a 2,2,6,6-tetramethylpiperidine-l-oxyl radical (TEMPO)/NaBr/NaC10 system selective oxidization combined with mechanical homogenization. 

A modified form of cellulose acetate containing some periodate-oxidized D-glucopyranosyl residues has been prepared. Fibres of cellulose acetate have been used to immobilize enzymes. Selectively oxidized cellulose (dialdehydo-cellulose) has been treated with hydroxylamine, bisulphite, and acetic acid. Dialdehydocellulose provides a matrix suitable for coupling with trypsin, giving an active immobilized enzyme. Reduction of the unchanged aldehydo-groups on the matrix stabilized the enzyme. 

The reactivity of cellulose at the mild hydrolysis with diluted mineral acids is proportional to amorphicity degree (Fig. 7.35]. As known, the amorphous cellulose is accessible to cellulolytic enzymes, and with increase the cellulose crystallinity rate of enzymatic hydrolysis decreases (loelovich et al., 2011a]. The reactivity of cellulose at the selective oxidation is also a function of the amorphicity degree. 

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