Kinetic Study of Binuclear Manganese−Tris(2-methyl pyridyl)amine

A kinetic study about the bleaching of Kraft pulp by hydrogen peroxide in a reaction catalyzed by binuclear manganese complexes is presented. The vari...
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Ind. Eng. Chem. Res. 2001, 40, 3435-3444

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Kinetic Study of Binuclear Manganese-Tris(2-methyl pyridyl)amine Complex Used as a Catalyst for Wood Pulp Bleaching T. Tzedakis,* Y. Benzada, and M. Comtat Laboratoire de Ge´ nie Chimique, UMR CNRS 5503, Universite´ Paul Sabatier, 118, Route de Narbonne, 31062 Toulouse, France

A kinetic study about the bleaching of Kraft pulp by hydrogen peroxide in a reaction catalyzed by binuclear manganese complexes is presented. The various kinetic parameters (initial reaction orders, activation energy, and integrated kinetic laws) were determined for various reactions involving the action of the complex on lignin and its regeneration by hydrogen peroxide. The regeneration rate of the oxidized form of the complex is faster than the lignin oxidation rate. The influence of stirring on the rate of the reaction complex/Kraft pulp suggested an oxidation process limited by the transport of the complex from the solution into the wood pulp and into the pulp fibers. For low reaction duration, the rate-limiting step is the diffusion of the complex into the pulp fibers, a step unaffected by stirring. Introduction The oxidation of lignin is currently performed on the industrial scale with chlorine and increasingly with chlorine dioxide and is responsible for the production of polluting effluent that is not easily biodegradable.1-4 Because of the ramified structure of lignin,1,2 defining a single mechanism for its oxidation is unrealistic; there are many types of bond breaking and lignin oxidation liquors contain several products such as aldehydes, quinones, and many chloroaromatics. In some instances, such as Eucalyptus globulus pulp, for example, treatment of the pulp with various blends of solvents5-9 or reagents10 can give efficient bleaching. Although this treatment reduces the amount of pollution, the strength of the fibers obtained is not as high as that of Kraft pulp (KP). In nature, microorganisms such as Phanerochaete chrysosporium can oxidize lignin by means of a manganese peroxidase,11-14 which catalyzes lignin oxidation by hydrogen peroxide (H2O2). A biomimetic approach to lignin degradation shows that it is possible to replace the enzyme by mixed valency complexes of transition metals (Fe, Mn, and Cu).11,15-19 These complexes with manganese in high oxidation states (particularly Mn(IV) and Mn(III)) can oxidize many substrates such as phenols and ethers15 as well as water in cell membranes.11,20-21 Each atom of manganese is bound to oxygen and the complex is stabilized by amino-pyridyl multidentate type ligands.22-25 The ability of these complexes to bleach wood pulp depends on their redox properties, their stability, and the ligand stability. The ability of diethylenetriamine pentaacetic acid (DTPA), ethylenediamine tetraacetic acid (EDTA), and β-alanine diacetic acid (ADA), separately and in combination, to bind and transport metals such as Fe or Mn were studied by Ramo et al.19 in four pilot-plant recirculation systems simulating alkaline hydrogen peroxide bleaching solutions. DTPA was able to bind * To whom correspondence should be addressed. Tel.: 33 (0)5 61 55 83 02. Fax: 33 (0)5 61 55 61 39. E-mail: tzedakis@ chimie.ups-tlse.fr.

60% of manganese, EDTA 100%, and β-ADA 40%. EDTA could bind only 10-20% of iron. The ligands appeared more soluble in the presence of iron and manganese probably because of the significant solubility of the manganese complexes. The redox behavior and catalytic properties of various binuclear manganese-ligand complexes was examined by electrochemical techniques in previous work.26 The complex the best suited for wood pulp bleaching is {(TPA)Mn(IV)O2Mn(III)(TPA)}2(SO4)3 (Mc ) 1019 g‚mol-1). TPA represents the ligand tris(2-methylpyridyl)amine. This complex can be used from pH 3 to pH 5 and over the temperature range 25-80 °C. Its catalytic behavior was confirmed26 by measurements of the zero current potential of a carbon electrode during Kraft pulp bleaching and by current-potential curves obtained by a thin layer electrochemical technique. The results show that after several successive scans in the 0-1-V range the complex can be oxidized or reduced without irreversible degradation; it can be used in catalytic amounts (0.1 g of complex/g of Kraft pulp) for lignin oxidation by hydrogen peroxide. For the sake of simplicity, the complex {(TPA)Mn(IV)O2Mn(III)(TPA)}2(SO4)3 is designated as Mn(IV)-Mn(III) and the product of its reduction by two electrons as Mn(II)-Mn(III). The objective of this paper is to determine the kinetic laws for the reaction of the complex on various substrates as well as its regeneration reaction and, also, to understand the action of the complex in the bleaching of wood pulp by hydrogen peroxide. A kinetic study was performed to determine the initial kinetic laws of the various reactions steps. 1. First, the action of the complex on substrates such as guaiacol (o-CH3O-C6H4-OH, reaction (2)), free lignin (noted L, reaction (3)), and lignin contained in Kraft pulp (noted KP, reaction (4)) was determined (Table 1). Guaiacol is a representative molecule of softwood lignin monomer, so any mediator that oxidizes guaiacol has some potentiality to be a lignin degradation agent. The validity of the initial kinetics laws obtained was examined theoretically and experimentally. 2. To examine the ability of the complex to be regenerated, the reaction of H2O2 in its reduced form was studied.

10.1021/ie0100840 CCC: $20.00 © 2001 American Chemical Society Published on Web 07/11/2001

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Table 1. Kinetic Measurements for Oxidation Reactions by the Mn(IV)-Mn(III) Complex on Various Substratesa

rate constants at 25 °C for (1) and (2):

L∑orders-1/(s-1‚mol∑orders-1); no.

reaction

(1) 3H2O2 + 4TPA + 3SO42-+ 4Mn2+ f {(TPA)Mn(IV)O2Mn(III)(TPA)}2(SO4)3 + 2H2O + 2H+ (2) Mn(IV)-Mn(III) + C7H8O2 f Mn(II)-Mn(III) + products of guaiacol oxidation (3) n(Mn(IV)-Mn(III)) + L f n(Mn(II)-Mn(III)) + products of lignin oxidation (4) q(Mn(IV)-Mn(III)) + KP f q(Mn(II)-Mn(III)) + bleached pulp + products of lignin oxidation

for (3) and (4): (s-1‚g∑orders-1)

L∑orders-1/

initial rate

activation energies Ea (kJ)b

calcd initial ratec for (1) and (2): mol‚L-1‚s-1; for (3) and (4): g‚L-1‚s-1 at 25 °C

at 85 °C

r0S ) kS[TPA]00.5[MnSO4]00.25[H2O2]00.5

kS ) 2.0 × 10-3

110 ( 16 3.6 × 10-7

2.2 × 10-2

r0G ) kG[Mn(IV)-Mn(III)]00.5 [guaiacol]01.8

kG ) 1.0 × 10-5

116 ( 17 1.25 × 10-12 1.2 × 10-7

r0L ) kL(L)00.5 [Mn(IV)-Mn(III)]00.5

kL ) 1.9 × 10-5

35 ( 1

1.9 × 10-5

20 × 10-5

r0KP ) kKP(KP)00.5 [Mn(IV)-Mn(III)]00.9

kKP ) 3.3 × 10-3

16 ( 2

3.3 × 10-3

9.8 × 10-3

a Measurements of the initial rate performed after a reaction time between 0.5 and 3 min; pH 3.5; θ 25 °C; initial mass concentration of Kraft pulp: 0.01 g‚L-1 e (KP)0 e 0.50 g‚L-1; initial mass concentration of lignin: 0.1 g‚L-1 e (L)0 e 0.5 g‚L-1; 0.05 × 10-3 mol‚L-1 e [soluble species] e 100 × 10-3 mol‚L-1. q and n are the stoichiometric factors of reactions (3) and (4), expressed as the electron equivalent per kilogram of substrate. b All the reagents were at the same temperature before mixing: 15 < temperature < 90 °C; [soluble species]0 )10-3 mol‚L-1; mL ) mKP ) 0.5 g. cCalculated rate for fixed experimental conditions at two temperatures. Concentration of all the soluble species: 10-3 mol‚L-1; mass concentration of lignin or Kraft pulp 1 g‚L-1.

3. The effect of stirring on the oxidation rate of the lignin contained in Kraft pulp was also studied. Remark: Reaction (1), leading to the formation of the Mn(IV)-Mn(III) complex from Mn2+, the free ligand (TPA) and H2O2 were assumed to be identical to the regeneration reaction by H2O2 of the Mn(IV)-Mn(III) complex used beforehand for Kraft pulp bleaching (Table 1). This would mean that the catalyst of lignin oxidation by H2O2 was the system Mn(IV)-Mn(III)/Mn(II)-Mn(II). However, a previous electrochemical study26-28 showed that the mediator of lignin oxidation by H2O2 is the redox system Mn(IV)-Mn(III)/Mn(II)-Mn(III) and reaction (1) does not take place during the catalytic cycle of Kraft pulp bleaching. The study of this reaction is, however, of interest especially for optimum complex synthesis. Experimental Section Preparation of the TPA Manganese Complex in a Sulfate Medium. The complex Mn(IV)-Mn(III) was synthesized using the method described by Suzuki23 by the action of a large excess of hydrogen peroxide on a one-to-one stoichiometric mixture of MnSO4 and free ligand (reaction (1), Table 1). After reaction, the excess of hydrogen peroxide was eliminated by decomposition in contact with a platinum grid under low-pressure distillation and the homogeneous mixture was obtained containing manganese in the form {(TPA)Mn(IV)O2Mn(III)(TPA)}2(SO4)3}. Previous works27 have shown that Mn(IV)-Mn(III) is stable in aqueous media over a pH range of 3-5. For pH values lower than 3, the spectrum in the visible revealed modifications indicating a change in the complex’s structure linked to ligand protonation. At pH 2, the complex decomposed to manganese(II) rapidly; its concentration decreased from 10-3 to 10-5 mol‚L-1 in 2 days. For pH values higher than 6 the complex decomposed and manganese oxides precipitated. Furthermore, the reactivity of the complex on the wood pulp was optimum for 3 < pH < 5. Most of the tests were performed in a buffered solution at pH 3.5 (Na2SO4/ H2SO4). Kinetic Measurements. All the chemicals used were Normapur (99.5%). The wood pulp was a washed and dried hardwood Kraft pulp with a Kappa number of 20

(about 3% of lignin in KP). The lignin was “alkali lignin” from Aldrich (ref 37,095-9, 1997). The kinetic study was performed by visible spectrophotometry using a Hewlett-Packard diode array spectrophotometer. The ultraviolet-visible spectra of the various reagents were obtained in 0.1 mol‚L-1 Na2SO4 acidic solution (pH 3.5). The spectra showed nonbands for wavelengths higher than 300 nm for the free ligand, Mn2+, and H2O2. Dissolved lignin does not absorb at wavelengths above 400 nm. Only the complex absorbs in the range 400-750 nm, thus enabling its appearance or disappearance to be followed by spectroscopy. The initial rate r0 of the complex/substrate reaction was calculated according to the following relationship:

r0 ≈

∆A (l∆t )

0

) k([substrate]0)R([complex]0)β

(5)

The homogeneous reactions ((1), (2), and (8)) were performed in a 1-cm3 spectrophotometer cell. The pH of all solutions was adjusted with Na2SO4/H2SO4 solution. Absorbance was measured 1 min after the reagents were mixed and no decrease of more than 10% was observed. For reagent concentrations between 10-5 and 10-3 mol‚L-1, the plot of absorbance versus time was linear from 3 to 10 min. For heterogeneous reactions ((3) and (4)) the mixture was introduced into a thermoregulated tank with a maximum capacity of 75 cm3 vigorously stirred (dispersion of the mixture by a magnetic bar (0.8 × 0.3 cm) at 104.7 rd‚s-1). After reaction times of 0.5 and 3 min, a sample of the suspension was filtered and analyzed by spectrophotometry. Because the conversion was low, it is assumed that the concentration of the species, for which the reaction order was being studied, was constant during this period. For both homogeneous and heterogeneous reactions, the order was determined from 10 to 20 measurements of the initial reaction rate with various concentrations of the species; the concentrations of all other species remained constant. For the reaction of the complex on lignin or on Kraft pulp, runs were made varying the mass concentration over the range 0.01-0.5 g‚L-1 for Kraft pulp and 0.1 to

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0.5 g‚L-1 for free lignin. The concentration of the complex was held constant and the pH of all solutions was adjusted to 3.5 with Na2SO4/H2SO4 solution. The order with respect to the Mn(IV)-Mn(III) complex was determined by similar experiments. The mass concentration of lignin or wood pulp was kept constant and the concentration of the complex varied between 5 × 10-5 and 10-2 mol‚L-1. The influence of temperature was studied from 15 to 90 °C with several measurements being performed at the various temperatures. An experimental verification was made of the empirical laws for a high conversion of the complex using a thermoregulated stirred reactor with a capacity of 75 cm3. The procedure was identical to that used to determine the initial rate laws. A sample of the suspension was filtered and analyzed by spectroscopy. In the study of the reaction between a Mn(IV)-Mn(III) complex and H2O2, the reduced form (Mn(II)-Mn(III)) was obtained by two methods: (1) By chemical reduction of the {(TPA)Mn(IV)O2Mn(III)(TPA)}2(SO4)3 complex by Kraft pulp (KP) according to global reaction (4) (Table 1). A solution of 50 cm3 containing the Mn(IV)-Mn(III) complex at 6.25 × 10-3 mol‚L-1 and 1 g of KP was vigorously stirred for about 40 h and filtered with a Millipore filter (pore size, 0.22 µm). The solution became colorless and the visible spectrum of the filtrate showed no peaks between 450 and 800 nm. (2) By electrochemical reduction according to Mn(IV)Mn(III) + 2e- f Mn(II)-Mn(III). A 37-cm3 solution of {(TPA)Mn(IV)-O2-Mn(III)(TPA)}2(SO4)3 complex (5.5 × 10-3 mol‚L-1) was reduced at a constant potential of 0.0 V/SCE in a glassy carbon tank, which was used as the cathode. The auxiliary electrode was made of platinum. Electrochemical reduction was stopped when the current intensity reached zero and the visible spectrum showed no peaks between 450 and 800 nm. An inert atmosphere was maintained above the Mn(II)-Mn(III) solution because the complex is oxidized by dissolved oxygen to give products other than Mn(IV)Mn(III). After a week, any colorless Mn(II)-Mn(III) complex solution that was not maintained under nitrogen at a pH of 3.5 turned bright blue and had an absorbance peak at 610 nm. Influence of Stirring on the Bleaching Rate of the Kraft Pulp by the Manganese Complex. The influence of stirring on the reaction rate of lignin oxidation by the manganese/TPA complex was followed by UV-visible spectroscopy at 558 nm, using the experimental setup shown in Figure 1. A rotating disk with a cavity in which the wood pulp was confined while immersed in a Mn(IV)-Mn(III) complex solution was used to study the effect of masstransport limitation on the lignin oxidation rate. Results I. Study of the Mn(IV)-Mn(III) Stability. The manganese complex Mn(IV)-Mn(III) is stable in aqueous media over a pH range of 3-5. The solubility of {(TPA)Mn(IV)O2Mn(III)(TPA)}(ClO4)3} in acidic solution (pH 3.5) of NaClO4 was about 10-3 mol‚L-1. At the same pH, the solubility of this complex increased to approximately 0.01 mol‚L-1 in a solution of Na2SO4. Although the shape of the absorbance vs wavelength spectra was not modified in the temperature range

Figure 1. Schematic representation of the experimental setup used for bleaching Kraft pulp by the manganese TPA complex; residence time of the solution in the UV-visible cell: 34 s.

examined (24 < θ < 80 °C), the molar absorptivity coefficients measured at wavelengths characteristic of the complex (438, 560, and 652 nm) increased by approximately 5% when the temperature increased from 24 to 80 °C in Na2SO4 0.1 mol‚L-1 acidic solution (pH 3.5): 438nm ) 1115 + 1.6θ - 0.008θ2; 560nm ) 452 + 1.3θ - 0.008θ2; 652nm ) 437 + 1.7θ - 0.015θ2;  and θ are expressed in (mol‚L-1)-1‚cm-1 and °C, respectively. The time dependence of the complex’s stability at 80 °C and pH 3.5 was studied during a period of about 1 day. For the three characteristic wavelengths the variation of the absorbance observed did not exceed 5%, allowing us to conclude that a rise in temperature to 80 °C does not affect the stability of the complex. Therefore, the stability of the complex in an aqueous medium depends essentially on the pH. The results show that, at pH 3.5 and at room temperature, the complex remains stable for several days. The temperature rise causes an increase in the complex’s molar absorptivity coefficients but does not favor its decomposition. II. Determination of the Initial Kinetic Laws. A.1. Action of the Complex on Various Substrates. The initial rate laws for the formation of complex (1), its action on guaiacol (2), lignin (3), and Kraft pulp (4) were determined and are presented in Table 1. The results show that the manganese complex can oxidize the guaiacol (a lignin model compound), the free lignin in solution, and the lignin in Kraft pulp. A comparison of the rate constants shows that kKP is higher than kL (kKP/kL ) 179), which could signify that the manganese complex preferentially oxidizes the cellulose instead of free lignin. Nevertheless, as the following results indicate, this conclusion seems unlikely. Indeed, the literature reports many works12,29-31 about the oxidation of lignin in aqueous solution or in softwood Kraft pulp using enzymes such as laccase or manganese peroxidase. The mechanism of oxidation of lignin by laccase and mediators such as 2,2′-azinobis(3-ethylbenzthiazoline-6-sulfonate) has been investigated by Bourbonnais.29 Lignin oxidation at the redox potential of laccase, by the mediator, was shown to be possible, but at a very slow rate. Reid and Paice30 have studied the effects of manganese peroxidase on residual lignin of softwood Kraft pulp. The results show that manganese peroxidase treatment lowered the Kappa number of Kraft pulp and increased the alkali extractability of the residual lignin

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but did not directly solubilize it. This indicates that manganese peroxidase partially oxidizes the lignin in the pulp but does not degrade it to soluble fragments. Hunt31 used a very efficient and selective delignifying agent such as dimethyldioxirane, and a process has been developed for in situ generation of the chemical for Kraft pulp bleaching. Similarly, oxidative enzymes such as laccase and manganese peroxidase show some potential for bleaching of Kraft pulps. Sequential enzyme and dimethyldioxirane bleaching of oxygen-delignified softwood pulps followed by alkaline extraction resulted in significantly lower Kappa and higher brightness pulps than those with enzyme or dimethyldioxirane alone. The mechanical strengths of the pulps were comparable to those bleached by dimethyldioxirane alone. Oxidation of lignin in aqueous solution using enzymes such as manganese peroxidase may cause lignin to polymerize; depolymerization has also been observed. Unstable radicals are formed in the reaction, which may couple, resulting in an increase in the size of the lignin polymer. In contrast, when the lignin is located within pulp fibers, recombination of radicals is less likely. Therefore, oxidation of lignin to give soluble fragments is more likely. The same behavior was observed with the manganese complex that has a analogous activity to certain manganese peroxidase isozymes. In fact, experiments into bleaching of Kraft pulp by the manganese complex provided good quality pulp:26,32 (polymerization number for cellulose 800 to 1500; Kappa number ∼5). The activation energy (Ea values, Table 1) for the complex’s synthesis reaction in a homogeneous medium was 110 kJ, a value close to that of the complex/guaiacol reaction (116 kJ). Activation energies were guaiacol > lignin > Kraft pulp (116 > 35 > 16 kJ). For the heterogeneous reactions (3) and (4) the activation energies are low. A low activation energy reflects the influence of the temperature on the global rate constant; its values are relative to the chemical reaction as well as a physical phenomenon such as adsorption or diffusion. In fact, assuming that adsorption of the complex on the fibers take place, the experimental activation energy includes both values: the activation energy of the chemical reaction and the activation energy of the complex adsorption. Moreover, the low activation energy can too be attributed to a limitation of the reaction rate including a limitation due to mass transfer of the complex. Indeed, the experimental value of the activation energy for the heterogeneous reaction (4) is very close to the value obtained in aqueous media for KOH (0.1 mol‚L-1) diffusion (16.3 kJ33). In the case of the Kraft pulp bleaching, the complex must diffuse to the internal part of the fibers to oxidize the lignin. Nevertheless, the experimental order with respect to the complex was not zero. This is probably due to the occurrence of a several step process: first, a global reaction between the complex and easily accessible lignin (located at the surface of the cellulose fibers) took place and then the lignin confined between the fibers of Kraft pulp was oxidized. Other experimental results,28 obtained at 25 °C, show that the reduced (Mn(II)-Mn(III)) and the oxidized (Mn(IV)Mn(III)) forms of the complex can be adsorbed on cellulose and Kraft pulp fibers. Because of the reaction of Mn(IV)-Mn(III) and unbleached Kraft pulp, direct study of the adsorption

seems difficult. A pure cellulose paper was used to study adsorption of the complex (oxidized and reduced form) on the fibers and to compare this with adsorption on unbleached Kraft pulp. (1) In the case of mixtures of suspensions of pure cellulose (0.05 g) with manganese complex solutions (2.5 cm3), concentrations of complex were varied from 0.2 to 4 mmol‚L-1. The results show that the oxidized form was adsorbed on cellulose in quantities that varied from 40 to 10% with respect to the initial concentration, while the quantities of the reduced form of the complex varied from 30 to 20%. Note that the results obtained show that the Mn(III)-Mn(IV) complex can be adsorbed on the cellulose fiber, but it does not react with it; indeed, at 25 °C the concentration of free complex present in the suspension of cellulose decreased for about 10 min and after that remained constant. (2) For Kraft pulp suspensions, only the adsorption of the reduced form was examined in the same operating conditions as used previously. In this case the quantity of the adsorbed complex was higher than the previous quantity, reaching from 90 to 75%, for the same range of complex concentrations. These results show strong interactions between the reduced form of the complex and the lignin of the Kraft pulp, probably because of the high electron density of the aromatic rings and the electron deficiency of the manganese. Cellulose does not contain aromatic rings but the high oxidation state of the Mn(III)-Mn(IV) might cause interactions that would explain the adsorption of the complex on cellulose fibers. Assuming that, it is very probable that there is a strong interaction between the oxidized form of the complex and unbleached Kraft pulp. The reaction orders are fractional, indicating a lignin oxidation process that involves several steps. Rigorous explanation of these orders is difficult and needs further experiments. Nevertheless, a comparison of the experimental orders with respect to the complex for reactions (3) and (4) {respectively, 1 and 0.5} show that (a) with Kraft pulp one molecule of the complex reacts in the rate-limiting step and (b) in contrast, the order obtained in reaction (3) (i.e., 0.5) could signify that, during the rate-limiting step, there are two simultaneous interactions between one molecule of the complex and the lignin. This is in accordance with the adsorption results. A.2. Verification of the Validity of the Experimental Kinetic Laws. Analytical integration of the rate laws for reactions (3) and (4) gives the variation of the concentration of the Mn(IV)-Mn(III) complex with respect to time. The stoichiometric factors n and q of these reactions, expressed as the electron equivalent per kilogram of substrate, were estimated by analogy with the technique used to determine the Kappa number.28,34 A quantity of Kraft pulp or lignin was added to a solution containing an excess of permanganate. After the reaction, the quantity of residual permanganate was determined by means of the potentiometric method using a thiosulfate-calibrated solution. The results show that 1 mol of MnO4- or 5 equivalent electrons are required to bleach 1 kg of Kraft pulp with a Kappa number of 20, whereas 13 mol of MnO4- or 65 equivalent electrons are required to oxidize 1 kg of pure lignin. In the case of lignin oxidation by the Mn(III)-Mn(IV) complex, the reduced form is Mn(III)-Mn(II) and two electrons are transferred per mole of complex; if the reaction leads to the same products as those obtained by permanganate oxidation, the bleaching of 1 kg of

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Figure 2. Theoretical (s) and experimental (x x x) evolution of Mn(IV)-Mn(III) complex absorbance as a function of time. Suspensions with 50-cm3 total volume contained Kraft pulp (0.5 g‚L-1) or lignin (0.5 g‚L-1) in Na2SO4 0.1 mol‚L-1 (pH 3.5) aqueous solution and Mn(IV)-Mn(III) complex at various concentrations; θ ) 85 °C.

Kraft pulp (Kappa number ) 20) requires 5/2 mol of complex, and for the oxidation of 1 kg of lignin, 65/2 mol of complex are required. These values show that the number of moles of Mn(IV)-Mn(III) complex required to oxidize the lignin is 13 times greater than the number of moles of complex required to bleach Kraft pulp with a Kappa number of 20. However, the products of lignin oxidation by manganese complex are certainly different from those obtained by its oxidation by MnO4-. Indeed, because this Kraft pulp contains 3% pure lignin, the theoretical quantity of the complex required to oxidize 1 kg of pure lignin would be 83 mol instead of 65/2 mol. Consequently, measured values cannot be used rigorously, but do provide an approximate estimation of the stoichiometric factors to integrate the initial kinetic laws. In the reaction of the complex with Kraft pulp (4), assuming that the order with respect to manganese complex is 1, integration of the kinetic law (see Appendix) leads to the theoretical expression of the variation of the complex mass concentration versus time:

Cc ) C0c -

(

KP0 -

(

{

( ) ((

) [{ ( )) ))} { ( )) ) ))}] } 0

McCc q KP0 - KP0 × Mc q McC0c q

((

0.5

+

exp - KP0 0.5

(KP0)0.5 - KP0 -

)

McC0c q

) (( ((

McC0c KP0 q

(KP0)0.5 +

0.5

kKPqt

McC0c q

higher than the theoretical concentration. This indicates that the Mn(IV)-Mn(III) complex does not react with lignin oxidation products; if so, the theoretical concentration would be higher than the experimental concentration. This indicates a diffusion limitation of the complex toward the lignin located between the fibers of the Kraft pulp. Indeed, this term is not taken into account in the integration of the empirical laws and consequently the theoretical concentration decreases faster than the experimental concentration. The influence of the stoichiometric factor (q ) 5/2) on absorbance was examined for a complex concentration of 0.125 × 10-3 mol‚L-1 (curves 1a-1c). Similar absorbance profiles were obtained for values of q of 2 and 3 (variations of (20% around 5/2). Increasing the stoichiometric factor (from 2.5 to 3) led to a rapid decrease in absorbance (at 50 s, A decreased about 12%). For this concentration an increase of 20% on the experimental values of q seems to improve the agreement between the theoretical and the experimental evolution of the absorbance profile. For the complex/lignin reaction (3), integration of the kinetic law (see Appendix) led to the theoretical expression of the variation of the mass concentration of the Mn(IV)-Mn(III) complex with respect to time:

0.5

/ (KP0)0.5 +

McC0c 0.5 - (KP0)0.5 - KP0 q 0 0.5 2 McCc exp - KP0 kKPqt (6) q

Figure 2 shows results obtained with a suspension of Kraft pulp and complex at various concentrations. There is good agreement between the theoretical and the experimental values for complex concentrations of the order of 10-4 mol‚L-1 (curve 1b). When the complex concentration increases to 5 × 10-4 mol‚L-1, a discrepancy appears with the experimental concentration being

{

Cc ) C0c n Mc

(

nL0 + Mc

( )) ( ( )) (

(L0)0.5 +

McC0c n

2 (L0)0.5 +

0.5 2

exp(-n0.5kLt) + L0 -

McC0c n

0.5

exp -

)

n0.5 k t 2 L

}

McC0c n

2

(7) Curves 4a and 5 (Figure 2) show the calculated variation of the complex’s absorbance versus time in a reactor initially containing a suspension of lignin (0.5 g‚L-1) in 50-cm3 aqueous solution of Mn(IV)-Mn(III) at two concentrations (0.25 and 0.5 m mol‚L-1). The graph also shows the experimental results obtained with lignin. The agreement between theoretical and experimental curves is good for low concentrations of complex. At

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Table 2. Kinetic Measurements for the Oxidation of Mn(II)-Mn(III) by Hydrogen Peroxide at Various Concentrations, pH Values, and Temperaturesa [Mn(II)-Mn(III)], [H2O2], m mol‚L-1 m mol‚L-1 pH θ (°C)

105*r0 b (mol‚L-1‚s-1)

105*r0 c (mol‚L-1‚s-1)

0.10 0.25 0.37 0.50 0.62 0.75

3.25 5.34 1.84 1.98 2.45 2.73

10.0 3.50

21

15.6 21.4

1.0 1.5 2.5 5.0 7.5

10.0 1.00 0.5 0.1

19.0 3.51 3.03

19.9 20.5

3.25 3.47 4.37 5.07 5.85 5.00 5.00 5.00

32 32 32 32 32 32 32

4.13 8.14 12.5 37.4 11.6 3.24 1.84 r0(mol‚L-1‚s-1) ) r0(mol‚L-1‚s-1) ) (2.8 × 10-3)([H2O2]0)0.08([Mn(II)-Mn(III)]0)0.6 4.3 × 105 exp(-5283/T)([H2O2]0)0.05([Mn(II)-Mn(III)]0)0.8 the concentrations are expressed in the concentrations are expressed in mol‚L-1; mol‚L-1; calculated initial rate at 25 °C: calculated initial rate at 25 °C: 242 × 10-7 255 × 10-7 mol‚L-1‚s-1 d mol‚L-1‚s-1 d order with respect to H+ ) -0.4; 2.5 < pH < 5.5 order with respect to H2O2 ) 0.4 at pH ) 5.0

a The initial rate was obtained by UV-visible spectroscopy measurements; the pH was adjusted using Na SO /H SO solution. b Complex 2 4 2 4 Mn(II)-Mn(III) was obtained by chemical reduction of Mn(IV)-Mn(III) by Kraft pulp (1 g‚L-1). c Complex Mn(II)-Mn(III) was obtained by (IV) (III) d electrochemical reduction of Mn -Mn . Calculated rate for fixed experimental conditions; concentration of all the soluble species: 10-3 mol‚L-1.

higher concentrations and conversion rates for complex (i.e., approximately 70%), the correlation is poor and the theoretical variation of absorbance is lower than the experimental. This implies the following: (1) All the complex is used in the lignin oxidation reaction and the resulting products do not react with the Mn(IV)-Mn(III) complex in side reactions; in fact, if the products were reactive, the theoretical concentration would be higher than the experimental concentration. (2) The experimental values of the stoichiometric factor (n ) 65/2) could be responsible for discrepancy. Curves 4a-4c show the influence of n (variations of (20%) on the theoretical absorbance profile; decreasing n by 20% seems to improve the agreement between theoretical and experimental results. Consequently, the initial kinetic laws are not valid for high concentrations and conversions of the complex. Nevertheless, in the process of bleaching Kraft pulp by hydrogen peroxide, the complex would be used in catalytic amounts26 and these empirical kinetic laws could give a good approximation of the evolution of its concentration. B. Regeneration of Mn(IV)-Mn(III) Complex by H2O2. The overall oxidation reaction of Mn(II)-Mn(III) by H2O2 can be written

{(TPA)Mn(II)O2Mn(III)(TPA)}2SO4 + 2H2SO4 + 2H2O2 f {(TPA)Mn(IV)O2Mn(III)(TPA)}2(SO4)3 + 4H2O (8) where the initial reaction rate equation is given by relationship (5). The results of all the experiments of Mn(II)-Mn(III) complex oxidation by H2O2 are presented in Table 2 and lead to the initial kinetics laws.

The temperature dependence of the initial rate constant leads the estimation of the activation energy of the reaction; its value (43.9 kJ‚mol-1) is lower than those of the complex synthesis from free Mn2+ and TPA (reaction (1)) and confirm that in reaction (4) the manganese complex is not degraded to Mn2+ and free ligand. The rate of oxidation of the Mn(II)-Mn(III) complex was almost independent of the hydrogen peroxide concentration (order near to zero) (Table 2). One possibility is that before oxidation an intermediate is formed and reacts with the hydrogen peroxide to give Mn(IV)-Mn(III). In this case, the formation of the intermediate would be the rate-determining step. Another possibility may be the dependence of the oxidation rate on the pH, in agreement with Manchanda who proposed that the reduction of the Mn(IV)-Mn(III) complex is accompanied by protonation of the µ-oxo bonds.35 The results of the dependence of the pH on the oxidation rate of the Mn(II)-Mn(III) complex by H2O2 are presented in Table 2. An experiment at pH 2.7 showed no change in absorbance over time. The oxidation reaction was very slow and the initial rate at this pH was too slow to be determined. However, it increased with pH and the order with respect to the H+ concentration being -0.4. An explanation may be given by equilibria between the double-protonated Mn(II)OH-HOMn(III) complex, an intermediate {Mn(II)OOHMn(III)}2+, and a nonprotonated form. -H+

} {Mn(II)OH-HOMn(III)}3+ {\ + {Mn OOHMn (II)

+H (III) 2+ -H+

}

{\ } Mn(II)O2Mn(III) (9) + +H

Ind. Eng. Chem. Res., Vol. 40, No. 16, 2001 3441

The intermediate form of the complex {Mn(II)OOHMn(III)}2+ would be oxidized by H2O2; at low pH values its concentration and the concentration of the nonprotonated form are very low and the reaction rate almost zero. When the pH increases, reaction (9) moves to the right and the rate of the oxidation reaction increases. At pH 5, the reaction rate is dependent on the H2O2 concentration with an order of 0.4, while at pH 3.5 the initial rate is almost independent of the peroxide concentration. At a pH lower than 3.5 the deprotonation reaction (9) is the rate-determining step and at pH 5 the oxidation reaction is the rate-determining step. The overall oxidation reaction (10) is the sum of the two following half reactions:

{Mn(II)O-HOMn(III)}2+ S {Mn(IV)O2Mn(III)}3+ + H+ + 2eH2O2 + 2H+ + 2e- S 2H2O {Mn(II)O-HOMn(III)}2+ + H2O2 + H+ S {Mn(IV)O2Mn(III)}3+ + 2H2O (10) In fixed experimental conditions (concentration of all the soluble species: 10-3 mol‚L-1; mass concentration of Kraft pulp: 1 g‚L-1) the rate of the regeneration of the complex is about 8 times higher than the rate of its action on the Kraft pulp, and the corresponding step is not limiting. 0 rreaction(4) 0 rreaction(10)

3.3 × 10 ) 7.9 (2.6 × 10-5) × 1019

transport of the complex from the solution to the confined Kraft pulp. For runs carried out with Kraft pulp dispersed or confined, a strong decrease of the complex concentration was observed at the beginning of the experiment (time of about 1 h). After 30% conversion of the complex (time of about 5 h) a linear decrease of the concentration and an almost constant reaction rate were obtained. time (h) -103 × dA/dt (s-1)

-3

)

Figure 3. Influence of the angular speed (ω) of a rotating disk containing the Kraft pulp, on the lignin/complex reaction rate. Mass of Kraft pulp: 0.1 g; [Mn(IV)-Mn(III)]°: 6 × 10-4 mol‚L-1. I: A558nm ) f(t); Kraft pulp dispersed (a) or enclosed (b) in the cavity of a rotating cylinder (1000 rpm). II: A558nm ) f(t) for various angular speeds. III: reaction rate ) f(ω0.5) for various reaction durations.

(11)

This observation reveals that the experimental rate of the action of the complex on the lignin also includes other phenomena, such as the adsorption of the complex on the pulp, as well as its transport toward the lignin located between the pulp fibers. In contrast, the complex regeneration reaction takes place in solution. III. Influence of Stirring on the Lignin/Complex Reaction Rate. The influence of the angular speed (ω) of a rotating disk in which the wood pulp is confined within a cavity while immersed in a Mn(IV)-Mn(III) complex solution on the mass transport of the complex was examined. In the case of a rotating disk, the resolution of the equation of diffusion/convection at the steady state (Von Karman and Cochran36) leads to the profile of the flow rate of the solution on the surface of the disk. In the case of a homogeneous mixture the flow rate is proportional to the square of the angular speed (ω) of the disk and the mass-transport coefficient is given by the following relationship:36 k ) constant × D2/3ω1/2ν-1/6. An experiment was done (Figure 3, I) where the Kraft pulp was dispersed in the stirred solution (curve a). The results (A ) f(t)) are compared to those obtained with Kraft pulp enclosed in the rotating support (curve b). The initial rate of the reaction, measured with the dispersed Kraft pulp, was 13 times higher than that observed with the confined Kraft pulp. This is in agreement with the kinetic study, which shows that the reaction rate depends on the mass concentration of Kraft pulp accessible and reveals a limitation of mass

0.16 1.32

1.0 0.35

5.0 0.12

6.0 0.11

This is in agreement with (1) a first step in which primarily the lignin at the solid-solution interface is oxidized by the manganese complex and (2) a second step in which the transport of the complex from the surface through the pulp fibers becomes the ratedetermining step of Kraft pulp bleaching. The influence of the rotational velocity of the support on the oxidation rate of lignin is shown in Figure 3II,III. Similar results were obtained in that: (1) At times less than 1 h, the rate of the reaction complex/Kraft pulp increases with the angular speed of the support. This confirms that the oxidation process is limited by the mass transport of the complex from the solution into the wood pulp. However, the observed variation of the reaction rate with the square of the speed of rotation is not linear because the surface of the Kraft pulp in contact with the solution (pulp absorbs the solution) cannot be considered as a perfect disk. (2) For times around 2 h, the reaction rate increases with the speed of rotation of the support but not linearly, suggesting an oxidation process limited both by the transport of the complex in the solution and into the pulp fibers. (3) After 5 h, the curves show that the oxidation rate becomes practically independent of the speed of rotation of the support, the rate-limiting step being the diffusion of the complex into the pulp fibers, a step unaffected by stirring. Conclusion This kinetic study shows that the manganese complex can be used as a catalyst in the bleaching of Kraft pulp

3442

Ind. Eng. Chem. Res., Vol. 40, No. 16, 2001

by hydrogen peroxide. Its stability was satisfactory in the acidic conditions (pH 3.5) and relatively high temperatures (80 °C) used. Fractional rate orders were obtained for the Kraft pulp bleaching by the complex, suggesting a multistep process. Comparisons of the activation energies show a limitation of the bleaching rate by the diffusion of the complex into the Kraft pulp; this limitation was confirmed by the study of the influence of stirring of a disk containing the Kraft pulp on the lignin oxidation rate; after 5 h of reaction the bleaching rate became independent of the stirring rate. In agreement with a previous study, these data show that oxidation of lignin in pulp can be performed by manganese complex without significant oxidation of cellulose. After bleaching, the catalyst can easily be regenerated by H2O2 more rapidly than the rate of the Kraft pulp bleaching. The regeneration rate increased with the pH of the solution because a high concentration of H+ favors the protonation of the reduced manganese complex, which is a nonreactive species. The elevated cost of the catalyst relative to currently used bleaching chemicals and concerns with contamination of wastewater streams with heavy metals would require the manganese complexes to be used at catalytic concentrations. To increase the global rate of the bleaching, only the temperature remains the operative parameter. However, results show temperature exerts little influence on the reaction of the complex with the lignin contained in the Kraft pulp (rate increases 3-fold for a ∆T ) 60 °C). Furthermore, the operating conditions used must facilitate the mass transport of the complex toward the lignin in the Kraft pulp, located within the pulp fibers. Significant improvements can be obtained if the size of the catalyst particle is reduced. The complex can then diffuse in the pulp fibers more easily and increase the rate of the reaction with the lignin. Acknowledgment We would like to thank the Total-Elf-Fina Company (France) for financial support during this work. Thanks are due to Dr. J. L. Seris from the “Lacq Research Group” and Dr. J. J. Girerd (Laboratoire de Chimie Inorganique, Universite´ Paris Sud) for helpful discussions.

Greek Letters R, β ) reactional orders  ) molar absorptivity coefficient, (mol‚L-1)-1‚cm-1 θ ) temperature, °C ν ) kinematic viscosity, m2‚s-1 ζ, λ, ξ, y, χ ) variables used to integrate the kinetic laws ω ) angular speed, rpm or rd‚s-1 Additional Sub-/Superscripts 0 ) initial value c ) relative to manganese complex G, L, KP ) relative to guaiacol, lignin, and Kraft pulp S ) relative to the synthesis of the manganese complex

Appendix In the case of reaction of the complex with lignin (3):

-

-

Mc dCc d(Complex) dL ))) dt n dt n dt kLL0.5Complex0.5 ) kLMc0.5L0.5Cc0.5 Mc dCc McCc Mc C0c dL )SL) L0 ) dt n dt n n constant ) p -

)

0.5

Cc0.5

When the variable (p + (McCc/n)) ) x2 is changed, the previous equation can be written

dx n0.5 k dt ) )0.5 2 L (x - p) if L is in excess with respect to the complex ) p > 0 f p ) ζ2 dx (x2 - ζ2)0.5 2

whose integration with x2 > ζ2 give the following solution:

ln

Nomenclature A ) absorbance Complex ) mass concentration of Mn(IV)-Mn(III) complex, kg‚L-1 C ) molar concentration, mol‚L-1 D ) diffusion coefficient, m2‚s-1 Ea ) activation energy, kJ‚mol-1 KP or KP ) Kraft pulp or mass concentration of Kraft pulp (for equations) ki ) rate constant of reaction i l ) UV-visible cell length, cm L or L ) lignin or mass concentration of lignin (for equations) M ) molar weight, g‚mol-1 Mn(IV)-Mn(III) ) {(TPA)Mn(IV)O2Mn(III)(TPA)}2(SO4)3 n, q ) an estimation of the stoichiometric factors for reactions (3) and (4) r ) rate of the complex/substrate reaction, g‚L-1‚s-1 or mol‚L-1‚s-1 SCE ) saturated calomel electrode TPA ) tris (2-methylpyridyl)amine ligand t ) time, s

(

Mc dCc McCc ) kLMc0.5 p + n dt n

x)

[

x + (x2 - ζ2)0.5

x0 + ((x0)2 - ζ2)0.5

]

)-

n0.5 k t 2 L

(x0 + (x2 - ζ2)0.5)2 exp(-n0.5kLt) + ζ2

(

)

n0.5 k t 2(x + ((x ) - ζ ) ) exp 2 L 0

0 2

2 0.5

)

(

p+

)

McCc n

0.5

The theoretical expression of the variation of the mass concentration of the Mn(IV)-Mn(III) complex with respect to time is

{

Cc ) C0c n Mc

(

nL0 + Mc

( )) ( ( )) (

(L0)0.5 +

McC0c n

2 (L0)0.5 +

0.5 2

exp(-n0.5kLt) + L0 -

McC0c n

0.5

exp -

)

n0.5 k t 2 L

}

McC0c n

2

For the complex/Kraft pulp reaction (4), assuming that the order with respect to manganese complex is equal

Ind. Eng. Chem. Res., Vol. 40, No. 16, 2001 3443

to 1,

-

-

Mc dCc dKP dComplex ))) dt q dt q dt kKPKP0.5 Complex ) kKPMcKP0.5Cc dComplex dKP )S dt q dt KP -

McCc

) KP0 -

q constant ) λ

McCc0

)

q

98 S McCc dCc ) kKPqCc λ + dt q

(

)

0.5

When the variable (λ + (McCc/q)) ) y2 is changed, the previous equation can be written

q dy ) - kKP dt ) 2 y -λ if KP is in excess with respect to the complex ) λ > 0 f λ ) ξ2 dy 2 y - ξ2 2

Those integrations with y2 > ξ2 give the following solution:

| |

qkKPt 1 y-ξy )ln 2ξ y + ξ y0 2 y0 + ξ + (y0 - ξ) exp(-qξkKPt) y)ξ 0 y + ξ - (y0 - ξ) exp(-qξkKPt) The theoretical expression of the variation of the complex mass concentration versus time obtained is given by

) [{ ( ( )) (( ) ))} { ( ) (( ( )) (( ) ))}] }

Cc )

C0c

{

( ) ((

0

McCc q KP0 - KP0 × Mc q

KP0 -

McC0c q

0.5

+

exp - KP0 -

KP0 -

McC0c q

(KP0)0.5 +

(KP0)0.5 - KP0 -

McC0c q

0.5

0.5

kKPqt

/ (KP0)0.5 +

0.5

-

McC0c q

(KP0)0.5 - KP0 -

exp - KP0 -

McC0c q

McC0c q

0.5

0.5

2

kKPqt

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(6) Sarkanen, K. V. Method and System For Selective Alcaline Defiberization and Delignification. U.S. Patent 155,244, 1982. (7) Pereira, H.; Olivera, M. F.; Miranda, I. Kinetics of EthanolWater Pulping and Pulp Properties of Eucalyptus Globulus. Appita 1986, 39 (6), 455. (8) Puech, J. L.; Sarni, F. Delignification of Oak Wood with an Ethanol-Water Solution in a Flow-Through Reactor. Holzforschung 1990, 44 (5), 367. (9) De Hass, G. G.; Lang, C. J. Non-Catalytic Process for the Production of Cellulose from Lignocellulosic Materials Using Acetic Acid. U.S. Patent 3,553,076, 1971. (10) Ljunggren, S. Kinetic Aspects of Some Lignin Reactions in Oxygen Bleaching. J. Pulp Pap. Sci. 1986, 12 (2), J54. (11) Wieghardt, K. The Active Sites in Manganese-Containing Metalloproteins and Inorganic Models Complexes. Angew. Chem. Int. Ed. Engl. 1989, 28, 1153. (12) Boe, J. F. Roˆle du Mangane`se et de la Mangane`se Peroxydase de Phanerochaete Chrysosporium, dans la Biode´gradation de la Lignine. Ph.D. Thesis, Universite´ Paul Sabatier Toulouse, France, 1992. (13) Vares, T.; Niemenmaa, O.; Hatakka, A. Secretion of Ligninolytic Enzymes and Mineralization of 14C-Ring-Labelled Synthetic Lignin by Three Phlebia Tremellosa Strains. Appl. Environ. Microbiol. 1994, 60 (2), 569. (14) Kirk, T. K.; Farrell, R. L. Enzymatic Combustion: The Microbial Degradation of Lignin. Annu. Rev. Microbiol. 1987, 41, 465. (15) Gref, A.; Balavoine, G.; Riviere, H. Electrochemical Oxidation of Benzyl Ethers and Alcohols Mediated by the System [(L2)2MnO2Mn(L2)2]4+/[(L2)2MnO2Mn(L2)2]3+. New J. Chem. 1984, 8 (10), 615. (16) Pal, S.; Chan, M. K.; Armstrong, W. H. Ground Spin State Variability in Manganese Oxo Aggregates. Demonstration of an S)3/2 Ground State for [Mn3O4(OH)(bpea)3]. J. Am. Chem. Soc. 1992, 114 (16), 6398. (17) Labat, G.; Meunier, B. Factors Controlling the Reactivity of a Ligninase Model Based on the Association of Potassium Monopersulfate to Manganese and Iron Porfyrin Complexes. J. Org. Chem. 1989, 54 (21), 5008. (18) Walker, C. W.; McDonough, J. T.; Dinus, J. R.; Eriksson, K. E.; Catalytic Reactions in a Polymeric Model System for hydrogen peroxide Delignification of Pulp. AIChE Symp. 1998, 94 (319), 57. (19) Ramo, J.; Sillanpaa, M.; Orama, M.; Vickackaite, V.; Niinisto, L. Chelating Ability and Solubility of DTPA, EDTA and beta-ADA in Alkaline Hydrogen Peroxide Environment. J. Pulp Pap. Sci. 2000, 26 (4), 125. (20) Renger, G.; Hanssum, B. Studies on the Deconvolution of Flash-Induced Absorption Change into the Difference Spectra of Individual Redox Steps within the Water-Oxidizing Enzyme System. Photosynth. Res. 1988, 16 (3), 243. (21) Hansson, O.; Aasa, R.; Vanngard, T. The Origin of the Multiline and g)4.1 Electron Paramagnetic Resonance Signals from the Oxygen-Evolving System of Photosystem II. Biophys. J. 1987, 51 (5), 825. (22) Harriman, A.; Porter, G. Photochemistry of Manganese Porphyrins. J. Chem. Soc., Faraday Trans. 1979, 75, 1532. (23) Suzuki, M.; Tokura, S.; Suhara, M.; Uehara, A. Binuclear Manganese(III,IV) and Manganese(IV,IV) Complexes with Tris(2-pyridylmethyl) Amine. Chem. Lett. 1988, 477. (24) Towle, D. K.; Botsforfd, C. A.; Hodgson, D. J. Synthesis and Characterization of the Binuclear Mixed Valence Complex Diµ-oxobis[tris(2-methylpyridyl)amine] Dimanganese(III,IV) Dithionate Heptahydrate. Inorg. Chem. Acta 1988, 141, 167. (25) Goodson, P. A.; Glerup, J.; Hodgson, D. J.; Michelsen, K.; Peedersen, E. Binuclear Bis(µ-oxo) Dimanganese(III,IV) and (IV,IV) Complexes with N,N′-Bis(2-pyridylmethyl)-1,2-ethanediamine. Inorg. Chem. 1990, 29 (3), 503. (26) Tze´dakis, T. Electrochemical Study of Binuclear Manganese Complexes as Catalysts in Kraft Pulp Bleaching. Electrochim. Acta 2000, 46, 99. (27) Tze´dakis, T.; Durliat, H.; Comtat, M. Electrochemical Characterization of the {(TPA)Mn(III)O2Mn(IV)(TPA)}2(SO4)3 Complex. J. Electroanal. Chem. 1997, 421, 187. (28) Benzada, Y. Apport de l'e´lectrochimie dans la mise au point d'un proce´de´ de blanchiment de la paˆte a` papier par oxidation catalytique au peroxyde d'hydroge`ne. PhD Universite´ Paul Sabatier, No. 2665, Toulouse, France, 1997.

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(29) Bourbonnais, R.; Leech, D.; Paice, M. G. Electrochemical Analysis of the Interactions of Laccasse Mediators with Lignin Model Compounds. Biochim. Biophys. Acta 1998, 1379 (3), 381. (30) Reid, I. D.; Paice, M. G. Effects of Manganese Peroxidase on Residual Lignin of Softwood Kraft Pulp. Appl. Environ. Microbiol. 1998, 64 (6), 2273. (31) Hunt, K.; Lee, C. L.; Bourbonnais, R.; Paice, M. G. Pulp Bleaching with Dimethyldioxirane and Lignin-Oxidizing Enzymes. J. Pulp Pap. Sci. 1998, 24 (2), 55. (32) Devic; Michel; Schirmann; Jean-Pierre, Process for the Preparation of Delignified and Bleached Chemical Paper Pulps. U.S. Patent 6,019,870, 2000. (33) Horvath, A. L., Ed. Handbook of Aqueous Electrolyte Solutions; Ellis Horwood Limited: Chichester, 1985; p 296 (ISBN 0-85312-894-4).

(34) Norme franc¸ aise, De´termination de l’indice Kappa, 1987, NF ISO 302: T 12-018. (35) Manchanda, R.; Thorp, H. H.; Brudvig, G. W.; Crabtree, R. H. An Unusual Example of Multiple Proton-Coupled Electron Transferts in a High-Valent Oxomanganese Dimer, [(phen)2MnIIIO2MnIV(phen)2](ClO4)3 (phen ) 1, 10-Phenanthroline). Inorg. Chem. 1992, 31, 4040. (36) Bard, A.; Faulkner, L. R. Electrochimie: principes, me´ thodes et applications; Masson: Paris, 1983.

Revised manuscript received April 23, 2001 Resubmitted for review January 18, 2001 Accepted May 8, 2001 IE0100840