Chapter 5
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Methods for the Selective Oxidation of Cellulose: Preparation of 2,3-Dicarboxycellulose and 6-Carboxycellulose 1
1
A. C. Besemer , A. E. J. de Nooy , and H. van Bekkum 1
2
TNO Nutrition and Food Research Institute, P.O. Box 360, 3700 AJ, Zeist, Netherlands Delft University of Technology, Julianalaan 136, 2628 BL, Netherlands 2
Three methods for the selective oxidation of cellulose are described. The classical method consists of consecutive oxidation with sodium periodate, leading to 2,3-dialdehyde cellulose and sodium chlorite, giving 2,3dicarboxy cellulose. This material, which is obtained in high yield and has a high carboxylate content (7.6 mmol COONa/g; 90% of the theoretical value), has a very good calcium sequestering capacity. The second method is by oxidation of the substrate, dissolved in concentrated phosphoric acid with nitrite/nitrate, leading to the selective oxidation of the substrate at the 6-position of the glucose unit. Generally, the yields are higher than 80%, and the degree of oxidation is 80-90%. However, the reaction is not completely specific, since some oxidation at the secondary hydroxylic groups occurs. Borohydride reduction of the product restores the diol configuration and also ß-elimination is avoided and thereby depolymerization. Oxidation with sodium hypochlorite and bromide as a catalyst and TEMPO as a mediator appears also to be applicable to cellulose. Selectivity of oxidation at the 6-CH OH group is somewhat lower than that obtained earlier for glucans like starch and pullulan. Products with a degree of oxidation of 80% are obtained in 90% yield or higher. 2
Oxidation of (poly)saccharides has been studied in detail by numerous investigators, but, because of the presence of several reactive groups, it is not easy to attain high selectivity, and only a limited number of reagents are available for this purpose (1). Because it is insoluble in water and most common organic solvents, there are especially difficulties with cellulose. In view of the structure of an anhydroglucose unit in glucans such as cellulose and starch, one has to account for the presence of three reactive groups: one primary and two secondary OH-groups. Usually the oxidation of the secondary hydroxyl ©1998 American Chemical Society In Cellulose Derivatives; Heinze, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.
73
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74 groups in glucans results in ring cleavage. Well-known methods for this conversions are reactions with sodium periodate or with lead(IV) tetraacetate, which lead to the formation of the corresponding 2,3-dialdehyde derivatives (2-4). Upon subsequent oxidation of this material with sodium chlorite the corresponding dicarboxy derivatives are obtained. The conversion of starch has been studied in detail because the oxidation products have excellent calcium binding properties and therefore may be used as a substitute for builders in laundry detergents (3,4). Good results can also be obtained with cellulose. Floor et al. (4) improved the procedure by using hydrogen peroxide in the second step. For the quantitative conversion of dialdehyde derivatives 6 mol instead of 2 mol of sodium chlorite are required: Dialdehyde polysaccharide + 2 NaC10 - Dicarboxy polysaccharide + 2 NaOCl 2
because the reaction product (NaOCl) decomposes sodium chlorite according to NaOCl + 2NaC10 + H 0 - 2 C10 + NaCl + 2 NaOH. 2
2
2
Use of hydrogen peroxide has two advantages: less reagent is needed, since sodium hypochlorite reacts faster with hydrogen peroxide than with sodium chlorite according to NaOCl + H 0 2
2
- NaCl + H 0 + 0 2
2
and, better products are obtained. About twenty years ago the most attractive way to selectively oxidize polysaccharides such as starch and cellulose at the 6-position of the anhydroglucose unit was by exposing them to (gaseous) N 0 or N 0 (5). Satisfactory results can be obtained with cellulose and starch; i.e., the corresponding 6-carboxy polysaccharide can be prepared with a high carboxylate content and with a satisfactory yield. An alternative, which has been studied by a few authors, is oxidation with nitrous acid (6,7). In this system, the substrate is dissolved in concentrated phosphoric acid and allowed to react with sodium nitrite. In the highly viscous solution a foam develops in which various oxidizing species like N 0 and N 0 are present. A drawback of these methods is that considerable depolymerization occurs and that the degree of oxidation is no higher than approximately 80% (6). Recently, we improved the latter oxidation and we developed a new method for oxidation of primary alcohol groups in polysaccharides using hypochlorite as the primary oxidant and bromide and 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) as the catalysts. In (8-11) we presented the results of this reaction with respect to selectivity, depolymerization, and scope. Mainly water soluble-polymers, such as starch, inulin, and pullulan, were investigated. In this study we describe results obtained by applying these oxidation methods to poorly water-soluble polymers such as cellulose. A comparison is made with some model compounds. 2
2
4
2
3
2
4
In Cellulose Derivatives; Heinze, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.
75 Results and discussion
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a. Periodate/clorite/hydrogen peroxide oxidation of cellulose Table I shows the results of the glycol cleavage of starch and cellulose, using 2 mol of sodium chlorite and 2 mol hydrogen peroxide. For comparison we have also presented data on the method in which 6 mol of sodium chlorite are used (and no hydrogen peroxide). Generally, in polysaccharide oxidation, method A gives the best results. However, it is seen that oxidation of cellulose according to method Β can give somewhat better results.
Table I. Dicarboxy polysaccharides obtained by sodium periodate/sodium chlorite oxidation Yield(%)
CC(%)
SC (mmol Ca/g)
A
93
82
2.20
Cellulose
Β
91
86
2.29
Starch
A
98
79
2.51
Starch
Β
93
86
2.39
Amylose
A
87
74
2.39
Amylose
Β
95
80
2.29
Substrate
Method
Cellulose
3
b
a
Method A: 2 mol of sodium chlorite and 2 mol of hydrogen peroxide per anhydroglucose unit Method B: 6 mol of sodium chlorite per anhydroglucose unit SC = sequestering capacity defined as the mmol of Ca bound by 1 gram of material until the concentration is below 10" M Ca(H). C C = carboxylate content (at 100 % conversion the C C = 8.4 mmol COONa/g) b
5
b. 6-carboxy cellulose (oxidation in phosphoric acid with sodium nitrate/nitrite) The results of the oxidation of cellulose in the oxidation with sodium nitrate are shown in Table Π. Generally, satisfactory results are obtained. Good results were also obtained in the oxidation of amylose (yield 80%; carboxylate content 95%). It can be concluded that this method is very suitable for the oxidation of poorly soluble biopolymers in water. Yield and carboxylate content are high. Although some may occur (see Figure 1), depolymerization appears to be very modest, especially in comparison to former procedures (5) . From the kinetic experiments
In Cellulose Derivatives; Heinze, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.
76 (see Figure 2) it can be seen that nitrite has a catalytic effect. The reduction of nitric acid by NO, leading to the formation of N 0 , is thought to play an important role. An important advantage of this method over the TEMPO method is that no glycolic oxidation can occur. Table ΙΠ shows some results of the oxidations of βcyclodextrin using the phosphoric acid/nitrate/nitrite method. 2
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From the results presented in Table IV, it appears that only the stoichiometric amount of nitrate is needed, according to the theoretical reaction equation 3 -CH OH + 4 H N 0 2
3
- 3 -COOH + 4NO + 5H 0, 2
indicating 1.33 N a N 0 per CH OH-group. 3
2
Table II. Oxidation of cellulose with sodium nitrate/sodium nitrite in phosphoric acid
a
a
Substrate
T(°C)
Time (hours)
DO (%)
Yield(%)
cellulose
20
3
23
77
cellulose
20
6
72
85
cellulose
20
9
88
82
cellulose
20
20
99
74
cellulose
4
6
