TEMPO Electromediated Oxidation of Some Polysaccharides

May 17, 2010 - at pH 10. When the regenerate cellulose fiber was treated by the TEMPO electromediated oxidation for 45 h, carboxylate and aldehyde gro...
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Biomacromolecules 2010, 11, 1593–1599

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TEMPO Electromediated Oxidation of Some Polysaccharides Including Regenerated Cellulose Fiber Takuya Isogai, Tsuguyuki Saito, and Akira Isogai* Department of Biomaterials Sciences, Graduate School of Agricultural and Life Sciences, The University of Tokyo, Tokyo 113-8657, Japan Received March 9, 2010; Revised Manuscript Received May 5, 2010

Curdlan, amylodextrin, and regenerated cellulose fiber were subjected to electromediated oxidation with a 4-acetamido-TEMPO catalyst in a buffer at pH 6.8 without NaClO or NaClO2. More than 90% of the C6 primary hydroxyls of Curdlan and amylodextrin were converted to sodium carboxylate groups by this method. Molecular mass values of the oxidized products were much higher than those prepared by the TEMPO/NaBr/NaClO system at pH 10. When the regenerate cellulose fiber was treated by the TEMPO electromediated oxidation for 45 h, carboxylate and aldehyde groups of 1.1 and 0.6 mmol/g, respectively, were formed in the oxidized cellulose fiber. The original fibrous and fine surface morphologies were maintained, and nearly no weight losses by the oxidation were observed. Thus, the TEMPO electromediated oxidation is a characteristic and environmentally friendly chemical modification for regenerated cellulose fibers, films, and related forming materials, and ionexchangeable carboxylate and reactive aldehyde groups can be efficiently introduced into regenerated celluloses.

Introduction Position-selective reactions under aqueous conditions around room temperature at atmospheric pressure like enzymatic reactions have become more significant for chemical modifications or functionalizations of naturally occurring polysaccharides as environmentally friendly and low-energy-consumptive processes.1 TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl radical)mediated oxidation in water has, therefore, opened new fields of polysaccharide chemistry, matching the above concept.2,3 We have studied TEMPO/NaBr/NaClO oxidation at pH 104-12 and 4-acetamido-TEMPO/NaClO/NaClO2 oxidation at pH 4-713-18 for conversion of C6 primary hydroxyl groups of cellulose and other polysaccharides to carboxylate groups, where NaClO and NaClO2, respectively, are used as the primary oxidants. Water-soluble cellouronic acids, that is, sodium (1f4)-β-Dpolyglucuronates, have been prepared from regenerated, mercerized, and ball-milled native celluloses by complete carboxylation of C6-OH groups in the TEMPO/NaBr/NaClO system at pH 10.4,10 Other sodium polyuronates have also been prepared from various water-soluble and insoluble polysaccharides by the TEMPO-mediated oxidation at pH 10.5,11 When native celluloses were subjected to the TEMPO/NaBr/NaClO oxidation at pH 10, the C6 primary hydroxyl groups of cellulose microfibril surfaces were effectively converted to sodium carboxylate groups, maintaining the original fibrous morphologies, crystallinities, and crystal sizes.6 Fully individualized TEMPO-oxidized cellulose fibrils approximately 4 nm in width and at least a few micrometers in length were obtained from the TEMPO-oxidized native celluloses with carboxylate contents of more than about 1 mmol/g by mild mechanical disintegration in water.7,8 Dry films prepared by casting the TEMPO-oxidized cellulose nanofibril/water dispersions had high tensile strengths, high optical transparencies, low thermal expansion coefficients, and extremely high oxygen barrier properties.19 * To whom correspondence should be addressed. Phone: +81 3 5841 5538. Fax: +81 3 5842 5269. E-mail: [email protected].

However, remarkable depolymerization of cellulose and other polysaccharides was unavoidable during the TEMPO-mediated oxidation at pH 10.10,20-24 Degrees of polymerization of the obtained water-soluble polyuronates were generally lower than 100,5,10,11 and those of the TEMPO-oxidized native celluloses were lower than 300.5 Such remarkable depolymerization of cellulose and other polysaccharides were suppressible to some extent by the 4-acetamido-TEMPO/NaClO/NaClO2 oxidation at pH 4-7.13-18 However, partial depolymerization of cellulose and other polysaccharides still takes place during the oxidation because glycoside bonds are not so stable to NaClO2. Thus, side reactions caused by NaClO or NaClO2 cannot be avoided as long as the above TEMPO-mediated oxidation systems are adopted, irrespective of the oxidation pH or the primary oxidant used. Moreover, it may be better not to use chlorine-containing chemicals such as NaClO and NaClO2 from environmental aspects. On the other hand, development of suitable electrochemical methods for oxidation of organic compounds, that is, electroorganic oxidation, has attracted worldwide attention as green chemistry.25-29 The most distinctive feature of electro-organic reaction in TEMPO-mediated oxidation is that organic compounds are modified by electrochemical energy without any primary oxidant such as NaClO or NaClO2.3,30-36 Electric energy and oxygen provided by water are consumed during the TEMPO electromediated oxidation. Hence, depolymerization and other side reactions of polysaccharides occurring during the TEMPO-mediated oxidation in the presence of NaClO, NaClO2, or NaBr may be avoidable by the TEMPO electromediated oxidation. In this process, TEMPO, as the mediator, is continuously reoxidized to the corresponding nitrosonium salt at the anode by applying a suitable potential in the electrical cell. The electrochemical properties of the nitroxides were investigated by cyclic voltammetry.31,33 However, TEMPO electromediated oxidation has never been applied to organic polymers like cellulose or other polysaccharides. In this paper, therefore, Curdlan, amylodextrin, and commercial regenerated cellulose

10.1021/bm1002575  2010 American Chemical Society Published on Web 05/17/2010

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Figure 1. Schematic representation of the electromediated oxidation system with the 4-acetamido-TEMPO catalyst in 0.1 M buffer at pH 6.8 for conversion of primary hydroxyls of cellulose to carboxylate groups via aldehydes.

fiber (viscose rayon) were subjected to the TEMPO electromediated oxidation in a 0.1 M phosphate buffer at pH 6.8, and the oxidized products were characterized in terms of chemical structures and molecular-mass parameters. Because 4-acetamidoTEMPO was more effective in oxidation of regenerated cellulose under neutral conditions than TEMPO,13,37 the former catalyst was used in this study.

Experimental Section Materials. Commercial curdlan and amylodextrin (Wako Pure Chemicals Co., Japan) were used as polysaccharides. A commercial viscose rayon (regenerated cellulose fiber prepared from dissolving wood pulp using the xanthate system) was used as regenerated cellulose. 4-Acetamido-TEMPO and other chemicals and solvents (Wako Pure Chemicals, Co. Japan) were of reagent grade and used without any further purification. Amperometric System. Cyclic voltammetry was conducted using a card-type potentiostat/galvanostat (SDPS-511C, Syrinx Co., Japan). Cyclic voltammetry and electromediated oxidation were executed with a three-electrode/aqueous system in a dual-partitioning cell. Volumes of the reaction and counter cells were 280 and 120 cm3, respectively. A Hg|Hg2SO4|K2SO4 electrode was employed as a reference. A platinum counterelectrode (φ 3 mm × 23 cm) and a glassy carbon working electrode (φ 3 mm × 5 cm) were separated by a cation-exchange membrane (Nafion N424, 22 cm2). TEMPO Electromediated Oxidation. The substrate (Curdlan or amylodextrin of 0.5 g, or regenerated cellulose fiber of 1 g) was subjected to the electromediated oxidation. The substrate and 4-acetamido-TEMPO (436 mg, 2 mmol) were suspended and dissolved, respectively, in a 0.1 M phosphate buffer (200 mL) at pH 6.8. The electromediated oxidation was conducted for the substrate in the buffer at 0.50 V versus Hg|Hg2SO4|K2SO4 by continuous stirring with a

magnetic stirrer bar at room temperature (Figure 1). Although the original curdlan and amylodextrin were insoluble in water at room temperature, the suspensions became clear solution at least after oxidation for 1-2 days. After the treatment for 2 and 3 days, the oxidized curdlan and amylodextrin, respectively, were isolated from the clear reaction solutions, purified, and dried according to the reported method.10,11,13,18 The rayon fiber maintained the initial fibrous morphology even after oxidation for 45 h, and thus the oxidized cellulose fibers prepared by the treatment for 3-45 h were washed thoroughly with water by filtration and freeze-dried. During the TEMPO electromediated oxidation, electric current was continuously monitored. SEC-MALLS Analysis. The oxidized curdlan and amylodextrin (1 mg each) were dissolved in 0.1 M NaCl (1 mL). The original amylodextrin was dissolve in water by heating at about 80 °C. The solutions were filtered, and molecular mass parameters were measured by means of a size-exclusion chromatograph (SEC) furnished with a multiangle laser light-scattering detector (MALLS: DAWN EOS, λ ) 685 nm, Wyatt Technologies, U.S.A) using 0.1 M NaCl as an eluent. A polyhydroxymethacrylate gel (SB-806 M HQ, 8 mm φ × 30 cm, Shodex, Japan) was used as the SEC column. Details of the SECMALLS system and operation conditions were described elsewhere.10,11,13,38 The values of 0.125 and 0.142 mL/g were used as the specific refractive index increments (dn/dc) of the oxidized products and the original amylodextrin, respectively, in 0.1 M NaCl.10,11,13,38 Determination of Carboxylate and Aldehyde Contents. Carboxylate contents of the oxidized cellulose fibers were determined by an electric conductivity titration method.6-8,14,15 The TEMPO-oxidized cellulose fibers were further oxidized with NaClO2 at pH 4-5 and room temperature for 2 days to selectively convert aldehyde groups in the samples to carboxylate groups. Carboxylate contents were determined for the TEMPO-oxidized and then NaClO2-oxidized cellulose fibers by the above electric conductivity titration method. The carboxylate

TEMPO Electromediated Oxidation of Cellulose groups formed at the NaClO2 oxidation stage were regarded as aldehyde groups present in the TEMPO-oxidized cellulose fibers.6-8,14,15 Other Analyses and Observations. Viscosity average degrees of polymerization (DPv) of the original and oxidized cellulose fibers were determined using 0.5 M cupriethylene diamine (cuen), according to the reported method.14 When aldehyde groups were present in the oxidized cellulose fibers, DPv values became low by β-elimination during dissolution in the alkaline cuen solution. Thus, aldehyde groups were oxidized to carboxyl groups by the NaClO2 treatment at pH 4-5 and room temperature for 2 days before the DPv measurement.14 Solution-state 13C NMR spectra of the original amylodextrin and oxidized products were recorded on a JEOL Alpha 500 spectrometer (JEOL, Japan) using D2O and 3-trimethylsilyl-2,2,3,3-d4-propionic acid sodium salt (both from Wako Pure Chemicals Co., Japan) as the solvent and internal standard, respectively. The original and oxidized cellulose fibers (0.1 g each) were converted to pellets by pressing at 750 MPa for 1 min. The pellets were subjected to X-ray diffraction measurement from 5 to 35° of diffraction angle 2θ using the reflection method by means of a Rigaku RINT 2000 with Ni-filtered Cu KR radiation (λ ) 0.1548 nm) at 40 kV and 40 mA. Crystallinity indices of the original and oxidized cellulose fibers were calculated according to the reported method.10 The original and oxidized cellulose fibers in never-dried state were centrifuged at 1500 gravity and 20 °C for 15 min, and water retention values (WRVs) were calculated from the following equation.

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Figure 2. Cyclic voltammogram of 4-acetamido-TEMPO at room temperature and pH 6.8.

WRV(%) ) 100 × (Ww - Wd)/Wd where Ww is the mass of the wet sample after the centrifugation, and Wd is that after drying of the wet sample at 105 °C for 3 h. The original and oxidized cellulose fibers (0.01 g each) were suspended in a 0.1% congo red or toluidine blue solution (20 mL). After shaking the suspension at room temperature for 3 h, the stained fibers were washed thoroughly with distilled water (300 mL) for the congo red-treated samples or ethanol (300 mL) and then water (50 mL) for the toluidine blue-treated samples by filtration.14,15 The original and oxidized cellulose fibers in water before and after the staining treatments were observed with or without cross-polarizers by means of an optical microscope (BX-50 with a digital camera DP-20, Olympus, Japan). Differential interference-contrast microphotographs of the cellulose fibers in water were taken for observation of fine surface structures of the cellulose fibers using another microscope (BX-51 with a digital camera DP-20, Olympus, Japan).

Figure 3. 13C NMR spectra of the oxidized curdlan and amylodextrin dissolved in D2O.

Results and Discussion TEMPO Electromediated Oxidation of Curdlan and Amylodextrin. The electromediated oxidation cell system for curdlan, amylodextrin, and regenerated cellulose fiber with the 4-acetamido-TEMPO catalyst was designed by slight modification of the reported method (Figure 1).33 Hydrogen gas is generated from the platinum counter electrode, as the oxidation proceeds. In cyclic voltammetry, 4-acetamido-TEMPO was repeatedly and stably oxidized and reduced during the positive and negative sweeps (Figure 2), showing that this 4-acetamidoTEMPO catalyst is stable and repeatedly convertible to the corresponding cationic N-oxoammonium compound in the phosphate buffer at pH 6.8 by electrochemical reaction. When curdlan and amylodextrin were subjected to the TEMPO electromediated oxidation, the monitored electric current values decreased and became plateau levels after oxidation for 2 and 3 days, respectively, at room temperature. The initially water-insoluble curdlan and amylodextrin became water-soluble as the oxidation proceeded. 13C NMR analysis showed that the C6 primary hydroxyls of more than 90% were converted to carboxylate groups by the oxidation (Figure 3). The 13C NMR spectra of the original curdlan and amylodextrin-

Figure 4. SEC elution patterns and the corresponding molecularmass plots of the original amylodextrin and the oxidized curdlan and amylodextrin.

containing starches dissolved in DMSO-d6 were shown in another paper.18 Thus, the electromediated oxidation in the 0.1 M phosphate buffer at pH 6.8 using 4-acetamido-TEMPO as the catalyst can oxidize most of C6-OH groups of both waterinsoluble curdlan and amylodextrin, although complete oxidation of the C6-OH groups could not be achieved under the conditions used in this study. SEC-elution patterns and the corresponding molecular mass plots of the original amylodextrin and the oxidized curdlan and amylodextrin are depicted in Figure 4. The oxidized curdlan and amylodextrin had normal distribution patterns without any additional peaks or apparent shoulders, and linear molecular mass plots were obtained by the MALLS analysis. The SEC

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Table 1. Weight and Number Average Molecular Masses (Mw and Mn, respectively), the Corresponding Degrees of Polymerization (DPw and DPn, respectively), Mw/Mn Values and Weight Recovery Ratios of the Oxidized Curdlan and Amylodextrin, Prepared by Electromediated Oxidation with the 4-Acetamido-TEMPO Catalyst sample a

curdlan oxidized curdlan amylodextrin oxidized amylodextrin

Mw

(DPw)

Mn

(DPn)

Mw/Mn

1100000 268000 60000 53900

6790 1380