Oxidation of cellulose under controlled conditions

Polymer Degradation and Stability 77 (2002) 25–27 www.elsevier.com/locate/polydegstab Oxidation of cellulose under controlled conditions A.J. Varma*, M.P. Kulkarni Chemical Engineering Division, National Chemical Laboratory, Pune-411008, India Received 22 February 2002; accepted 7 March 2002 Abstract Detailed studies on the sodium metaperiodate oxidation of cellulose to yield 2,3-dialdehyde cellulose were carried out to ascertain the effects of concentration of periodate relative to cellulose, temperature of reaction, pH of the medium, effect of morphology of the cellulose, and effect of homogenous versus heterogenous reaction conditions. Microcrystalline cellulose had slightly higher reactivity then cellulose due to its greater purity and lower molecular weight, which gave rise to more reactive end groups. There were no significant changes in the reactivity of cellulose with periodate in buffer solutions of pH 2–5, or of homogenous oxidation of methylcellulose and carboxymethylcellulose as compared to heterogenous oxidation of cellulose powder. It was found that only controlling the concentration of periodate used and the temperature could easily control the rate and extent of oxidation of cellulose. The conclusion is that in order to achieve higher extent of oxidation of cellulose it is preferable to use higher concentration of periodate at 55  C for short reaction time, instead of stoichiometric periodate concentration for longer reaction times. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Periodate oxidation; Cellulose; Heterogeneous; Homogeneous 1. Introduction Oxidation of cellulose using sodium metaperiodate has been extensively investigated in the literature, since it leads to selective cleavage at the C-2 and C-3 vicinal hydroxyl groups to yield a product with 2,3-dialdehyde units along the polymer chain [1–4]. The latter is an important functional polymer for further derivatisation to specialized products, such as 2,3-dicarboxycellulose, and specialized applications. In a detailed investigation of some factors regulating the reactivity of cellulose, Maekawa and Koshijima [3] found that the kinetics of the reaction depended on the physical form of the cellulose, such as film, fiber, powder, etc. They reported the initial pH of their system (cellulose powder, water, periodate) to be 4.45, and the final pH to be 3.2. We report here a detailed study of the effect of temperature, pH (maintained with the help of buffers), and the concentration of periodate, on the progress of the reaction. The reaction was followed by sodium thiosulfate titration, as reported in our earlier publication [4]. * Corresponding author. Fax: +91-20-5893041. E-mail address: ajvarma@che.ncl.res.in (A.J. Varma). One important observation was that beyond a certain level of cellulose oxidation for a particular temperature– pH–Periodate concentration, the white color of the reaction mixture changes to pink or orange due to the liberation of iodine. This agrees with the chemical and spectral analyses of the progress of the reaction. Therefore, in our studies we report the reaction only upto the point where the coloration appears in the reaction mixture. The pH of the reaction medium at each data point was also measured and is reported. These detailed observations have not been reported previously. 2. Experimental 2.1. Materials Hardwood cellulose powder (CP-100, obtained from Cellulose Products of India, Ltd., Ahmedabad, India) of  150 mesh was used in this study. It contained  85% alpha-cellulose and an ether extract of 0.2% maximum. Microcrystalline cellulose ( 96% alpha-cellulose), methyl cellulose and carboxymethyl cellulose were obtained from Aldrich. 0141-3910/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S0141-3910(02)00073-3 26 A.J. Varma, M.P. Kulkarni / Polymer Degradation and Stability 77 (2002) 25–27 2.1.1. Preparation of 2,3-dialdehyde cellulose These we prepared by the method published earlier [4]. Titrimetry used to calculate the consumption of metaperiodate. Table 1 Reaction of sodium metaperiodate with cellulose power or microcrystalline cellulose power at 55  C without buffer solution, with 2.0 times the theoretical amount of NaIO4 (I2 liberated after last reading in each case) 2.1.2. Buffer solutions Buffer solution of pH 2–5 was prepared as follows [5]: Ratio of oxidant/ cellulose Reaction time (h) 0.8 NaIO4/cellulose 3 6 9 12 24 14.3 25.1 26.5 30.1 48.0 0.8 NaIO4/microcrystalline cellulose 3 6 12 24 23.76 (pH=3.24) 30.32 (pH=2.95) 38.4 (pH=2.87) 57.5 (pH=4.43) 2.0 NaIO4/cellulose 3 6 10 24 For pH=2, 25 ml 0.2 M KCl solution and 6.5 ml 0.2 M HCl solution were mixed and diluted to 100 ml with distilled water. For pH=3, 50 ml 0.1 M potassium hydrogenphthalate solution and 22.5 ml 0.1 M HCl solution were mixed and diluted to 100 ml with distilled water. For pH=4, 50 ml 0.1 M solution potassium hydrogenphthalate and 0.1 ml 0.1 M HCl solution were mixed and diluted to 100 ml by distilled water. For pH=5, 50 ml 0.1 M potassium hydrogenphthalate solution and 22.6 ml 0.1 M NaOH solution were mixed and diluted to 100 ml with distilled water. % Conversion of cellulose to 2,3-dialdehyde cellulose 30.4 51.9 66.0 100.0 (pH=3.24) (pH=3.12) (pH=3.02) (pH=5.50) (pH=4.60) (pH=3.15) (pH=3.15) (pH=2.83) (pH=5.05) 3. Results and discussion Most studies dealing with periodate oxidation of cellulose to yield 2,3-dialdehyde cellulose are carried out at temperature at or below 55  C. At 55  C and above, the periodate is unstable and decomposes to liberate iodine after some time, which makes it difficult to estimate accurately the amount of periodate consumed. At room temperature (< 35  C), the oxidation reaction proceeds very slowly. Hence, it is difficult to choose appropriate reaction conditions for obtaining a particular oxidation level of cellulose. We therefore present here some data on the effects of periodate concentration, temperature of reaction, and effect of buffer on the heterogeneous oxidation of cellulose in aqueous media. For comparison of morphological effects (extent of crystallinity), we also carried out some reactions with microcrystalline cellulose powder. Finally, to ascertain whether water soluble derivatives of cellulose in aqueous solution react at enhanced rates as compared to the heterogeneous reaction of cellulose suspended in aqueous media, we also studied the periodate oxidation of methyl cellulose and carboxymethylcellulose in homogeneous aqueous solution. Table 1 shows data on effect of periodate concentration at 55 C in water without use of buffers, on the extent of oxidation of cellulose. Increase in concentration of periodate dramatically increases the rate of oxidation. For example, after 3 h the conversion was only 14% for periodate/cellulose ratio of 0.8, which more than doubles to 30.4% when the periodate/cellulose ratio is 2.0. Surprisingly, microcrystalline cellulose Table 2 Reaction of sodium metaperiodate with cellulose power or microcrystalline cellulose power at 35  C without buffer solution, with 2.0,1.0 and 0.8 times the theoretical amount of NaIO4 (I2 liberation after last reading in the case NaIO4/cellulose and 1.0; in case of NaIO4 /cellulose 2.0 the reaction was stopped after 47 h) Ratio of NaIO4/cellulose Reaction time (h) % Conversion of cellulose to dialdehyde cellulose 0.8 7 19 25 36 47 52 90 6.77 (pH=3.35) 5.73 (pH=3.17) 21.3 (pH=3.09) 23.3 (pH=2.99) 28.1 (pH=2.90) 28.7 (pH=2.89) – (pH=5.03) 1.0 2 4 12 22 27 36 47 52 73 10.1 (pH=3.39) 13.6 (pH=3.30) 17.6 (pH=3.31) 23.3 (pH=3.11) 26.4 (pH=2.94) 32.0 (pH2.98) 34.2 (pH=2.74) 39.5 (pH=2.62) – (pH=5.01) 2.0 2 4 12 22 27 36 47 13.9 (pH=3.39) 23.9 (pH=3.35) 47.9 (pH=3.16) 5235 (pH=3.01) 57.70 (ph=2.97) 57.70 (pH=2.97) 58.40 (pH=2.95) 27 A.J. Varma, M.P. Kulkarni / Polymer Degradation and Stability 77 (2002) 25–27 Table 3 Reaction of NaIO4 with cellulose powder at 55  C in water at different pHs using buffer solutions. (Ratio of NaIO4/cellulose: 0.8) Time (h) 3 6 9 12 24 a % Conversion of cellulose to 2,3 dialdehyde cellulose Buffer of pH=2 Buffer of pH=3 Buffer of pH=4 Buffer of pH=5 No Buffer 17.3% 24.6% 28.75% 34.17% 38.68%a 19.83% 23.93% 28.30% 31.84% 36.81%a 19.85% 25.26% 29.68% 32.49% 35.26%a 15.09% pH=4.69 22.78% pH=4.60 24.91% pH=4.60 27.77%a pH=4.56 – 14.3% pH=3.24 25.13% pH=33.12 26.46% pH=3.02 30.07%a pH=5.05 – pH=1.89 pH=1.86 pH=1.86 pH=1.86 pH=1.86 pH=2.95 pH=3.05 pH=3.05 pH=3.05 pH=3.24 pH=3.79 pH=33.69 pH=3.61 pH=3.65 pH=4.16 I2 liberation in reaction started after the last reading in each case. powder was slightly more oxidisable compared to the cellulose used. This may be due to its lower molecular weight (greater number of oxidisable end groups) and greater purity (cellulose powder was  85% pure, while microcrystalline cellulose is estimated to be over 96% pure). At 35  C the reaction rates are considerably slowed down (Table 2). Thus, after 25 h at 35  C the oxidation level is only 21.3% at a periodate/cellulose ratio 0.8 whereas at 55  C the corresponding reaction level was more than double at 48%. At higher periodate concentrations, at 35  C, the reaction rate tapers off as the reaction proceeds. Thus, at a periodate/cellulose ratio 2.0, after 22 h the oxidation was 52.35%, which marginally increased to 58.4% after 48 h. This may be contrasted with the reaction at 55  C at this ratio (Table 1), where the reaction had gone to 100% completion in 24 h. When the reaction was carried out in the presence of buffer solution (pH of 2, 3, 4, and 5), there was no change in the rate of reaction (Table 3). Similarly, there was no significant effect of the homogenous versus heterogeneous reactions (Table 4). Thus, methylcellulose and carboxymethylcellulose had approximately similar reactivities to cellulose, though it was noticed that methylcellulose was slightly more sluggish and carboxymethylcellulose was slightly more reactive than cellulose. Thus, it seems that the rate and extent of periodate oxidation reaction can easily be controlled by controlling only the concentration of periodate and the temperature. Since liberation of iodine due to periodate decomposition is time and temperature dependant, in order to achieve greater extent of oxidation it is preferable to utilize higher concentration of periodate at 55  C for short reaction period, instead of stoichiometric periodate concentration for longer reaction times. Table 4 Reaction of sodium metaperiodate with cellulose powder, methyl cellulose powder, and carboxymethyl cellulose powder without buffer at 55 C with 1.0 times the theoretical amount of NaIO4 in each case Sample Cellulose (heterogenous system) Methyl cellulose (homogenous system) Carboxymethyl cellulose (homogenous system) Reaction time (h) % Conversion of cellulose 2 8.7 (pH=3.31) 4 12 14.5 (pH=3.24) 30.5 (pH=3.00) 2 11.1 (pH=3.71) 4 6 12 25.3 (pH=3.40) 32.3 (pH=3.18) 25.8 (pH=2.83) 2 4 7 10 9.4 24.4 38.2 39.4 (pH=3.8) References [1] Guthrie RD. In: Wolfram ML, editor. Advances in carbohydrate chemistry. New York: Academic Press; 1961. p. 105. [2] Nevell TP. In: Whistler RL, editor. Methods in carbohydrates chemistry. New York: Academic Press; 1963. p. 164. [3] Maekawa E, Koshijima T. J Appl Polym Sci 1984;29:2289–97. [4] Varma AJ, Chavan VB. Polym Degrad Stab 1995;49:245. [5] Dawson RM, Elliott M, Elliott WH, Jones KM, editors. Data for biochemical research, 2nd ed. Oxford: Clarendon Press; 1969.