Ind. Eng. Chem. Res. 2004, 43, 5181-5186
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Experimental Assessment and Kinetic Modeling of Cellulose Carboxymethylation Sonia Dapı´a, Beatriz Gullo´ n, Valentı´n Santos,* and Juan C. Parajo´ Department of Chemical Engineering, University of Vigo (Campus Ourense), Polytechnical Building, As Lagoas, 32004 Ourense, Spain
Carboxymethylation of mercerized cellulose was carried out in heterogeneous media by reaction with monochloroacetic acid at temperatures in the range of 30-70 °C. The time courses of monochloroacetic acid concentration, total degree of substitution (DS), mole fractions of mono-, di-, and trisubstituted anhydroglucose units, and partial DS at positions C2, C3, and C6 of the anhydroglucose units were measured. The presence of soluble decomposition products was assessed by analysis of the liquid phase. The kinetic interpretation of data was carried out by assuming negligible diffusional resistances and a bimolecular, nucleophilic substitution mechanism for carboxymethylation, with the participation of an additional parallel, pseudo-firstorder, parasitic reaction leading to monochloroacetic acid decomposition. The proposed model provided a close interpretation of the experimental concentration profiles for both reagents and products. Introduction Carboxymethylcellulose (CMC), the most important cellulose ether at the industrial scale, finds applications in a variety of fields. For example, CMCs are useful in systems where hydrophilic colloids are involved, and they show an ability to suspend solids in aqueous media, stabilize emulsions, absorb moisture from the atmosphere, solubilize proteins (milk proteins, egg proteins, etc.), thicken solutions (sugar solutions, paints, etc.), and form films.1-4 In recent years, the synthesis of cellulose derivatives with different patterns of functionalization and properties using unconventional media was investigated.5 In heterogeneous media containing alkali cellulose and monochloroacetate ions in water-alcohol mixtures, the carboxymethylation reaction follows the Williamson ether synthesis:
Cell-OH‚NaOH + ClCH2COO- f Cell-OCH2COO- + NaCl + H2O Carboxymethylation can take place at positions C2, C3, and/or C6 of each anhydroglucose unit (AGU), where available hydroxyl groups exist. Despite the important world production of CMC, scarce scientific research has been devoted to assessing the basic kinetic aspects of the carboxymethylation reaction. Xiquan et al.6 studied the carboxymethylation of cellulose in media containing 2-propanol at 45-65 °C using molar ratios of NaOH: monochloroacetic acid (MCA):cellulose ) 2.3:1.3:1 and reaction times up to 180 min, determined the average DS by a conductivimetric method, and concluded that the carboxymethylation reaction followed a monomolecular, nucleophilic mechanism. Salmi et al.7 studied the carboxymethylation kinetics of birch-derived cellulose at 30-80 °C in media containing 2-propanol and mixtures NaOH-MCA-cellulose in molar ratios of 8:4:1 for reaction times up to 120 min, determined the * To whom correspondence should be addressed. Tel.: +34 988 387047. Fax: +34 988 387001. E-mail:
[email protected].
average DS by a titrimetric method, and proposed a bimolecular, nucleophilic mechanism in which the reaction rate depended on the concentrations of both monochoroacetate ions (MCAI) and unsubstituted hydroxyl groups. In this work, the interpretation of experimental data was carried out by considering the participation of diffusional effects as well as different relative reactivities for C2, C3, and C6 positions. Olaru et al.8 considered the effects of different organic solvents (ethanol, acetone, and acetone-ethanol mixtures) on the carboxymethylation kinetics using NaOH-sodium monochloroacetate-cellulose mixtures in molar ratios of 2.11: 0.7-1.9:1 for reaction times up to 90 min and determined the average DS by a titrimetric method. The highest DS (about 0.75) was obtained in media 7:3 (w/ w) acetone-ethanol, whereas the carboxymethylation rate depended on both the structural modification of cellulose provoked in the various reaction media and the crystallinity of the starting cellulose. The experimental DS was interpreted by means of a monomolecular mechanism. The present work deals with the kinetic modeling of carboxymethylation in a heterogeneous medium containing 2-propanol and mixtures NaOH-MCA-cellulose at fixed molar ratios. In carboxymethylation experiments, the temperature (in the range of 30-70 °C) and reaction time were considered as operational variables. The average DS, the relative reactivities of the different hydroxyl groups, and the mole fractions of mono-, di-, and trisubstituted AGUs were determined in experiments. Additionally, the concentrations of MCA and soluble reaction byproducts (glycolic, diglycolic, and acetic acid) were determined. A kinetic model providing a close interpretation of the results was developed. Experimental Section Cellulose Carboxymethylation. Carboxymethylation was carried out under optimized operational conditions.9 Cellulose (Avicel PH-101, Fluka, 6.00 g) was suspended in 360 mL of 2-propanol under mechanical stirring (1000 rpm), 16 mL of aqueous NaOH [45.15%
10.1021/ie034204a CCC: $27.50 © 2004 American Chemical Society Published on Web 07/08/2004
5182 Ind. Eng. Chem. Res., Vol. 43, No. 17, 2004
(w/v)] was added dropwise, and the mixture was stirred for 1 h at room temperature to yield mercerized sodium cellulose. After the mercerization stage, 7.06 g of MCA dissolved in 30 mL of 2-propanol was added to the reactor. Owing to its exothermic character, this step was performed at a temperature slightly lower than the one desired for the selected experiment (30, 35, 40, 50, 60, or 70 °C). Once MCA was added, the reaction system reached the target temperature in about 30 s and was maintained with the reactor immersed in a thermostatic bath. Time zero for carboxymethylation was set when MCA was added. Samples of the suspension (20 mL, including solid and liquid phases) were withdrawn from the reactor at preset reaction times and filtered. Analytical Characterization of CMC Samples. The solid phases from the above filtration were suspended in 40 mL of aqueous methanol [70% (v/v)], neutralized with acetic acid [90% (v/v)], washed with 50 mL of aqueous ethanol [80% (v/v)], washed again with 50 mL of pure methanol (to reduce the concentrations of glycolic and diglycolic acid generated as reaction byproducts), and dried at 60 °C for 24 h under vacuum. The DS was determined by both 1H NMR spectroscopy and high-performance liquid chromatography (HPLC). Vacuum-dried CMC samples (150 mg) were mixed first with 1 mL of D2O (ensuring good impregnation) and then with 2 mL of 50% D2SO4 (added slowly). The mixture was kept in a bath at 90 °C for 5 h. Treated samples (homogeneous, with a yellow color and low viscosity) were analyzed by 1H NMR (using a Bruker ARX400 instrument operating at 400 MHz at room temperature) to yield information on the partial DS at positions C2, C3, and C6 (denoted as x2, x3, and x6, respectively).10 These data allowed the direct calculation of the total average DS. CMC samples (0.1 g) were prehydrolyzed following a modification of the procedure reported by Heinze et al.11 Samples were treated for 10 min at room temperature with 2 mL of 70-72% HClO4, diluted with 20 mL of distilled water, and then subjected to a posthydrolysis step at 120 °C for 1 h. The resulting solution was neutralized with 2 M aqueous KOH and kept at 4 °C for 1 h to precipitate KClO4. The salt was filtered off and washed with distilled water. The posthydrolyzed, neutralized solution was vacuum-evaporated together with the washing water, and the resulting solid was redissolved in 5 mL of distilled water. This solution was analyzed by HPLC (Hewlett-Packard 1100 series fitted with an IR detector HP1047A thermostated at 55 °C) using a two-column Interaction ION-300 apparatus eluted with 0.4 mL of 0.006 N sulfuric acid/min to obtain information on the mole fractions of unsubstituted AGU, monosubstituted AGU (2-, 3-, and 6-monocarboxymethylglucose), disubstituted AGU (2,3-, 2,6-, and 3,6-dicarboxymethylglucose), and trisubstituted AGU (2,3,6tricarboxymethylglucose). This information provides an alternative method for calculating the total average DS. The same HPLC method provided information on the amounts of glycolic and diglycolic acid remaining in the CMC samples. To assess the reproducibility of the entire experimental protocol (including carboxymethylation and analysis), duplicate experiments were carried out. Excellent repetitivity was found in all of the cases, with mean deviations of 0.6% in the case of 1H NMR data and 2.8% in the case of HPLC data. On the other hand, the DS values calculated by 1H NMR and HPLC were in close
agreement (mean deviation between both techniques in corresponding analyses, 1.1%). Analytical Characterization of Reaction Liquors. To follow both MCA decomposition and byproduct formation (including glycolic, diglycolic, and acetic acid), samples from the reaction media were weighed and filtered. The solid phases from filtration were washed with ethanol and methanol, dried, and weighed. The liquid phases from filtration and washing were neutralized with dilute, aqueous HClO4. To avoid interferences, no acetic acid was employed for neutralization. The solutions were vacuum-evaporated, and the solid phases were redissolved in 5 mL of distilled water and directly analyzed by HPLC. Fitting of Data. The set of differential equations was solved by a fourth-order Runge-Kutta method. A commercial optimization routine dealing with Newton’s method was used to calculate the kinetic parameters by minimizing the sum of the squares of the deviations between experimental and calculated data. Results and Discussion The experimental work described in the above section enabled the determination of the concentration profiles of MCA and reaction products (mono-, di-, and trisubstituted AGU units), as well as the time dependence of the average and partial DS values at all of the temperatures considered (30, 35, 40, 50, 60, or 70 °C). For calculation purposes, dimensionless concentrations of MCA (CMCA, defined as the ratio between the mole of MCA and the mole of initial cellulose present in the reaction media) were used in calculations. Dimensionless concentrations defined in the same way were used to express the concentrations of unreacted AGU (denoted as C0), monosubstituted AGU in position 2, 3, or 6 (denoted as C2, C3, and C6, respectively), disubstituted AGU in position 2,3, 2,6, or 3,6 (denoted as C23, C26, and C36, respectively), and mono-, di-, and trisubstituted AGU units (denoted as Cm, Cd, and Ct, respectively). The partial DS in position 2, 3, or 6 was denoted as x2, x3, and x6, respectively. As representative examples, Figures 1-3 present experimental data obtained for variables CMCA, average DS, Cm, Cd, Ct, x2, x3, and x6 operating at 50, 60, and 70 °C. At a qualitative level, the experimental results of Figures 1a, 2a, and 3a show that the maximum DS (about 1.26) was similar at all of the temperatures assayed and accounted for about 62% of the initial MCA-cellulose mole ratio (2.03). The fact that MCA was depleted at the end of the experiments confirmed the participation of parasitic reactions leading to MCA consumption. On the other hand, HPLC data confirmed the purity of the CMC samples, which showed a low content of glycolic acid resulting from reaction (
