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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 cellu- lose. 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 cel- lulose, 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 tem- perature, pH (maintained with the help of buffers), and the concentration of periodate, on the progress of the reaction. The reaction was followed by sodium thio- sulfate titration, as reported in our earlier publication [4]. 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. There- fore, in our studies we report the reaction only upto the point where the coloration appears in the reaction mix- ture. 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 Polymer Degradation and Stability 77 (2002) 25–27 www.elsevier.com/locate/polydegstab * Corresponding author. Fax: +91-20-5893041. E-mail address: [email protected] (A.J. Varma).

Oxidation of cellulose under controlled conditions

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Page 1: Oxidation of cellulose under controlled conditions

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 ascertainthe effects of concentration of periodate relative to cellulose, temperature of reaction, pH of the medium, effect of morphology ofthe 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. Therewere no significant changes in the reactivity of cellulose with periodate in buffer solutions of pH 2–5, or of homogenous oxidationof 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 cellu-lose. The conclusion is that in order to achieve higher extent of oxidation of cellulose it is preferable to use higher concentration ofperiodate 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 metaperiodatehas been extensively investigated in the literature, sinceit leads to selective cleavage at the C-2 and C-3 vicinalhydroxyl groups to yield a product with 2,3-dialdehydeunits along the polymer chain [1–4]. The latter is animportant functional polymer for further derivatisationto specialized products, such as 2,3-dicarboxycellulose,and specialized applications. In a detailed investigationof some factors regulating the reactivity of cellulose,Maekawa and Koshijima [3] found that the kinetics ofthe reaction depended on the physical form of the cel-lulose, such as film, fiber, powder, etc. They reportedthe 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 tem-

perature, pH (maintained with the help of buffers), andthe concentration of periodate, on the progress of thereaction. The reaction was followed by sodium thio-sulfate titration, as reported in our earlier publication [4].

One important observation was that beyond a certainlevel of cellulose oxidation for a particular temperature–pH–Periodate concentration, the white color of thereaction mixture changes to pink or orange due to theliberation of iodine. This agrees with the chemical andspectral analyses of the progress of the reaction. There-fore, in our studies we report the reaction only upto thepoint where the coloration appears in the reaction mix-ture. The pH of the reaction medium at each data pointwas also measured and is reported. These detailedobservations have not been reported previously.

2. Experimental

2.1. Materials

Hardwood cellulose powder (CP-100, obtained fromCellulose 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), methylcellulose and carboxymethyl cellulose were obtained fromAldrich.

0141-3910/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved.

PI I : S0141-3910(02 )00073-3

Polymer Degradation and Stability 77 (2002) 25–27

www.elsevier.com/locate/polydegstab

* Corresponding author. Fax: +91-20-5893041.

E-mail address: [email protected] (A.J. Varma).

Page 2: Oxidation of cellulose under controlled conditions

2.1.1. Preparation of 2,3-dialdehyde celluloseThese we prepared by the method published earlier

[4]. Titrimetry used to calculate the consumption ofmetaperiodate.

2.1.2. Buffer solutionsBuffer solution of pH 2–5 was prepared as follows [5]:

For pH=2, 25 ml 0.2 M KCl solution and 6.5 ml 0.2M HCl solution were mixed and diluted to 100 mlwith distilled water.For pH=3, 50 ml 0.1 M potassium hydro-genphthalate solution and 22.5 ml 0.1 M HCl solu-tion were mixed and diluted to 100 ml with distilledwater.For pH=4, 50 ml 0.1 M solution potassium hydro-genphthalate and 0.1 ml 0.1 M HCl solution weremixed and diluted to 100 ml by distilled water.For pH=5, 50 ml 0.1 M potassium hydro-genphthalate solution and 22.6 ml 0.1 M NaOHsolution were mixed and diluted to 100 ml with dis-tilled water.

3. Results and discussion

Most studies dealing with periodate oxidation of cel-lulose to yield 2,3-dialdehyde cellulose are carried out attemperature at or below 55 �C. At 55 �C and above, theperiodate is unstable and decomposes to liberate iodineafter some time, which makes it difficult to estimateaccurately the amount of periodate consumed. At roomtemperature (<35 �C), the oxidation reaction proceedsvery slowly. Hence, it is difficult to choose appropriatereaction conditions for obtaining a particular oxidationlevel of cellulose. We therefore present here some dataon the effects of periodate concentration, temperature ofreaction, and effect of buffer on the heterogeneous oxi-dation of cellulose in aqueous media. For comparisonof morphological effects (extent of crystallinity), we alsocarried out some reactions with microcrystalline cellu-lose powder. Finally, to ascertain whether water solublederivatives of cellulose in aqueous solution react atenhanced rates as compared to the heterogeneous reac-tion of cellulose suspended in aqueous media, we alsostudied the periodate oxidation of methyl cellulose andcarboxymethylcellulose in homogeneous aqueous solu-tion.Table 1 shows data on effect of periodate concentra-

tion at 55�C in water without use of buffers, on theextent of oxidation of cellulose. Increase in concentra-tion of periodate dramatically increases the rate of oxi-dation. For example, after 3 h the conversion was only14% for periodate/cellulose ratio of 0.8, which morethan doubles to 30.4% when the periodate/celluloseratio is 2.0. Surprisingly, microcrystalline cellulose

Table 1

Reaction of sodium metaperiodate with cellulose power or micro-

crystalline 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)

Ratio of oxidant/

cellulose

Reaction time

(h)

% Conversion of

cellulose to

2,3-dialdehyde cellulose

0.8 3 14.3 (pH=3.24)

NaIO4/cellulose 6 25.1 (pH=3.12)

9 26.5 (pH=3.02)

12 30.1 (pH=5.50)

24 48.0 (pH=4.60)

0.8 3 23.76 (pH=3.24)

NaIO4/microcrystalline

cellulose

6 30.32 (pH=2.95)

12 38.4 (pH=2.87)

24 57.5 (pH=4.43)

2.0 3 30.4 (pH=3.15)

NaIO4/cellulose 6 51.9 (pH=3.15)

10 66.0 (pH=2.83)

24 100.0 (pH=5.05)

Table 2

Reaction of sodium metaperiodate with cellulose power or micro-

crystalline 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 6.77 (pH=3.35)

19 5.73 (pH=3.17)

25 21.3 (pH=3.09)

36 23.3 (pH=2.99)

47 28.1 (pH=2.90)

52 28.7 (pH=2.89)

90 – (pH=5.03)

1.0 2 10.1 (pH=3.39)

4 13.6 (pH=3.30)

12 17.6 (pH=3.31)

22 23.3 (pH=3.11)

27 26.4 (pH=2.94)

36 32.0 (pH2.98)

47 34.2 (pH=2.74)

52 39.5 (pH=2.62)

73 – (pH=5.01)

2.0 2 13.9 (pH=3.39)

4 23.9 (pH=3.35)

12 47.9 (pH=3.16)

22 5235 (pH=3.01)

27 57.70 (ph=2.97)

36 57.70 (pH=2.97)

47 58.40 (pH=2.95)

26 A.J. Varma, M.P. Kulkarni / Polymer Degradation and Stability 77 (2002) 25–27

Page 3: Oxidation of cellulose under controlled conditions

powder was slightly more oxidisable compared to thecellulose used. This may be due to its lower molecularweight (greater number of oxidisable end groups) andgreater purity (cellulose powder was �85% pure, whilemicrocrystalline 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 oxidationlevel is only 21.3% at a periodate/cellulose ratio 0.8whereas at 55 �C the corresponding reaction level wasmore than double at 48%. At higher periodate con-centrations, at 35 �C, the reaction rate tapers off as thereaction proceeds. Thus, at a periodate/cellulose ratio2.0, after 22 h the oxidation was 52.35%, which mar-ginally increased to 58.4% after 48 h. This may be con-trasted with the reaction at 55 �C at this ratio (Table 1),where the reaction had gone to 100% completion in 24h.When the reaction was carried out in the presence of

buffer solution (pH of 2, 3, 4, and 5), there was nochange in the rate of reaction (Table 3). Similarly, therewas no significant effect of the homogenous versus het-erogeneous reactions (Table 4). Thus, methylcelluloseand carboxymethylcellulose had approximately similarreactivities to cellulose, though it was noticed thatmethylcellulose was slightly more sluggish andcarboxymethylcellulose was slightly more reactive thancellulose.Thus, it seems that the rate and extent of periodate

oxidation reaction can easily be controlled by control-ling only the concentration of periodate and the tem-perature. Since liberation of iodine due to periodatedecomposition is time and temperature dependant, inorder to achieve greater extent of oxidation it is prefer-able to utilize higher concentration of periodate at 55 �C

for short reaction period, instead of stoichiometric per-iodate concentration for longer reaction times.

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.

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) % 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

3 17.3% pH=1.89 19.83% pH=2.95 19.85% pH=3.79 15.09% pH=4.69 14.3% pH=3.24

6 24.6% pH=1.86 23.93% pH=3.05 25.26% pH=33.69 22.78% pH=4.60 25.13% pH=33.12

9 28.75% pH=1.86 28.30% pH=3.05 29.68% pH=3.61 24.91% pH=4.60 26.46% pH=3.02

12 34.17% pH=1.86 31.84% pH=3.05 32.49% pH=3.65 27.77%a pH=4.56 30.07%a pH=5.05

24 38.68%a pH=1.86 36.81%a pH=3.24 35.26%a pH=4.16 – –

a I2 liberation in reaction started after the last reading in each case.

Table 4

Reaction of sodium metaperiodate with cellulose powder, methyl cel-

lulose powder, and carboxymethyl cellulose powder without buffer at

55� C with 1.0 times the theoretical amount of NaIO4 in each case

Sample Reaction time

(h)

% Conversion of cellulose

Cellulose

(heterogenous system)

2 8.7 (pH=3.31)

4 14.5 (pH=3.24)

12 30.5 (pH=3.00)

Methyl cellulose

(homogenous system)

2 11.1 (pH=3.71)

4 25.3 (pH=3.40)

6 32.3 (pH=3.18)

12 25.8 (pH=2.83)

Carboxymethyl cellulose

(homogenous system)

2 9.4

4 24.4

7 38.2

10 39.4 (pH=3.8)

A.J. Varma, M.P. Kulkarni / Polymer Degradation and Stability 77 (2002) 25–27 27