Developing Catalytic Oxygen Delignification for Kraft Pulp: Kinetic

The effect of pH, ionic strength, and temperature on the kinetics of oxidation reactions between residual lignin and polyoxometalate (POM) anions ...
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Developing Catalytic Oxygen Delignification for Kraft Pulp: Kinetic Study of Lignin Oxidation with Polyoxometalate Anions Kyo1 sti Ruuttunen* and Tapani Vuorinen Helsinki University of Technology, Laboratory of Forest Products Chemistry, P.O. Box 6300, FIN-02015 TKK, Finland

The effect of pH, ionic strength, and temperature on the kinetics of oxidation reactions between residual lignin and polyoxometalate (POM) anions ([AlMnIII(OH2)W11O39]6- and [SiMnIII(OH2)W11O39]5-) in a pulp suspension of unbleached softwood kraft pulp at varying conditions was investigated. According to the results, cation concentration and pH have a remarkable effect on reactivity of the POMs. At cation concentrations e0.10 mol L-1, no reaction was observed during the first 5000 s; at 0.30 mol L-1, the reactions were significantly more rapid, and the rate correlated strongly with increasing pH for both POMs. The activation energies (Ea) of the oxidation reactions at 0.30 mo L-1 cation concentration varied between ∼27 and 68 kJ mol-1 for [AlMnIII(OH2)W11O39]6-, depending on pH and reaction time; the respective values for [SiMnIII(OH2)W11O39]5- were ∼17 to 82 kJ mol-1. The reasons for the observed decrease in Ea along with increasing pH and cation concentration are discussed. Introduction Ideally, chemical wood pulping aims at destroying the lignin molecules in wood while leaving the carbohydrate molecules (cellulose and hemicelluloses) intact. In practice, this is never achieved: carbohydrate is always lost in the process and lignin is never completely removed. The most feasible contemporary pulping processes comprise two stages, cooking and bleaching. Most of the delignification (i.e., lignin removal) is achieved during cooking, while bleaching is required for removing the residual lignin, i.e.that is, the dark-colored chemical structures persistent even after prolonged cooking. After cooking, the pulp still contains ∼2 to 5% lignin, whereas the lignin content of native wood is ∼20 to 30%, depending on the wood species.1,2 Chlorine-containing chemicals (e.g., ClO2) are very effective in reacting with the residual lignin structures, and therefore, most of the bleaching techniques are chlorine-based. Still, due to economical issues and environmental problems caused by the chlorine chemicals, the wood pulping industry, especially in Europe, has an interest toward new, totally chlorine-free ways of performing bleaching. In addition to chlorine-based chemicals, gaseous oxygen and oxygen-based chemicals (e.g., H2O2, ozone, and peracids) are other delignifying agents used in the industrial processes. The unlimited availability and cheap price of oxygen gas makes its use more attractive than many other chemicals; yet, the chemistry of oxidation with molecular oxygen is rich in free-radical reactions that are rapid and often uncontrollable. As a result, the selectivity of oxygen bleaching, especially at high degrees of delignification, is poor; in addition to residual lignin, cellulose and hemicellulose molecules in wood fibers are damaged.2 This leads to deterioration in quality and yield of the final product. Due to the mentioned disadvantages, much effort has been put into improving the efficacy of oxygen delignification process. One of the techniques proposed is usage * To whom correspondence should be addressed. E-mail: [email protected].

of activating agents to increase the rate and specificity of the oxidation reactions.3 The activating substances suggested include enzymes (e.g., laccase-mediator systems4), metalloporphyrins,5 organic activator molecules,6 polyoxometalates,7-14 and other transitionmetal-based catalysts.15 Although many of these activators and catalysts have showed very good performance in selective delignification, most of them lack other essential features: they are not cost-effective, they are unstable under process conditions, or they cause problems in later stages of the bleaching process. Of the activating substances mentioned above, (RKeggin type) polyoxometalates (POMs) are probably the most promising candidates for catalysts in oxygen delignification due to their remarkable stability and activity at wide pH and temperature ranges.8,9,16,17 R-Keggin POMs are spherical anions composed of early transition metal oxide clusters. The general formula of the Keggin structure is [XZyM(12-y)O40]q-, where X is called the heteroatom and M and Z are addendum atoms. The structure is based on a central XO4 tetrahedron that is surrounded by 12 MO6 and ZO6 octahedra linked together with oxygen atoms. The heteroatom X is most commonly phosphorus or silicon; the addendum atoms M are early transition metal atoms in high oxidation states, principally WVI and MoVI, and Z can be V, Mn, or some other metal atom. More detailed structural characteristics of POMs are not discussed here; the interested reader can find this information in several recent publications and in the references thereof.7,8,11,18-22 The POM bleaching processes presented are based on the ability of POM to be readily reduced and subsequently reoxidized. In reaction I, POM oxidizes a lignin structure and is reoxidized by molecular oxygen in reaction II.7-9

POMox + lignin f POMred + lignin* + H+

(I)

POMred + 2H+ + 1/2O2 f POMox + H2O

(II)

10.1021/ie048836o CCC: $30.25 © 2005 American Chemical Society Published on Web 04/29/2005

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In the reactions above, lignin, lignin*, POMox, and POMred represent a residual lignin structure, an oxidized residual lignin structure, and an oxidized and a reduced POM anion, respectively. The reactions show that no net consumption of POM occurs; i.e., POM can be used to catalyze oxygen delignification of unbleached cellulose pulp. Moreover, it has been reported possible to create effluent-free POM bleaching processes in which the only byproducts of bleaching are CO2 (from lignin oxidation) and water.8,10 So far, POM bleaching has not been applied industrially, which is partly due to inefficiency of the processes presented. One reason that likely decreases the efficiency is the Donnan effect,23 which is a very important factor affecting the distribution of ionic species in pulp suspension.24-26 According to our investigations27 the Donnan effect may completely prevent the POM from penetrating the fiber wall: the highly anionic POM ion is strongly repelled by the negatively charged fiber at near-neutral pH when ionic strength of pulp suspension is low. It is obvious that oxidizing lignin molecules by POM anions is impossible if these reactive species cannot get into contact with each other; hence, elimination of the Donnan effect may be one of the key issues in improving the delignification efficiency of POMs. It is known that the extent of the Donnan effect can be controlled by controlling the pH and ionic strength of pulp suspension. Therefore, the aim of the present work is to rationalize, with simple kinetic experiments, the role of different conditions (ionic strength, pH, temperature) in the reaction rate of lignin oxidation with POM in a cellulose pulp suspension (reaction I). Experimental Section Materials. Selecting the POMs. Not all reduced POMs can be easily reoxidized with gaseous oxygen. For example, [SiVIVW11O40]6- is extremely stable toward oxidation.8 However, [SiMnII(OH2)W11O39]6- has shown some activity toward reoxidation. In addition, [AlMnIII(OH2)W11O39]6- has been suggested as a suitable candidate for POM bleaching.8,14 The experiments reported in this paper concentrate on the oxidation of lignin with oxidized POMs (reaction I). In the future, our intention is to perform experiments concerning the reaction kinetics of the reoxidation reaction (reaction II). Because of this, the most thoroughly studied POMs in this work are [SiMnIII(OH2)W11O39]6- and [AlMnIII(OH2)W11O39].6Chemicals. The POMs used in this work were potassium salts of [AlMnIII(OH2)W11O39]6- (1), [AlMnII(OH2)W11O39]7- (1red), [SiMnIII(OH2)W11O39]5- (2), [SiMnII(OH2)W11O39]6- (2red), [SiVVW11O40]5- (3), and [SiVIVW11O40]6- (3red), all of which were synthesized in Forest Products Laboratory, USDA (Wisconsin).28-30 Prior to the use of POMs in the experiments, they were dissolved in distilled water and stored as 10 mmol L-1 solutions under a nitrogen atmosphere in vials closed with a septum. The heavy water (D2O) was supplied by Sigma Chemical Company (St. Louis, MO), and copper sulfate (CuSO4‚5H2O) by Merck KGaA (Darmstadt, Germany). The buffer solutions of pH 3 and 5 were supplied by Reagecon Diagnostics Ltd. (Shannon, Ireland) and the buffer solution of pH 7 by FF-Chemicals (Finland). Pulp. An unbleached softwood kraft pulp from a Finnish pulp mill was used. Prior to experiments, the pulp was acid-washed and subsequently treated with NaOH. This was carried out to remove metal cations

other than sodium from the acid groups present in the fiber.26 The pulp was stored at ∼20% consistency at +5 °C. Methods. General Aspects. The κ number of the pulp was determined according to the Scandinavian standard SCAN-C 1:00. The content of hexenuronic acid (HexA) groups in the pulp was determined with a method described by Vuorinen et al.31 The exact compositions of the buffer solutions were elucidated (information kindly provided by the suppliers) as well as preliminary tests performed with POMs and the buffer solutions before starting the kinetics experiments. Knowing the compositions was important because the ionic strengths of the buffer solutions needed to be known to exactly adjust the ionic strengths of the samples; moreover, it was imperative to ensure that the materials (apart from the pulp) added to the samples were inert toward the POMs under the conditions and reaction times used in the experiments. The Nernst Equation. In this work, the analysis of the results of reaction kinetics experiments was based on the Nernst equation, which reads in its general form

E ) E° +

RT [electron acceptor] ln nF [electron donor]

(1)

where E depicts the oxidation potential of the system, E° is the standard oxidation potential of the redox pair, R is the gas constant, T is the absolute temperature, F is the Faraday constant, and n is the number of electrons transferred per molecule. In a system in which POM oxidizes lignin, oxidized POM (electron acceptor) and reduced POM (electron donor) form the redox pair. Both 1 and 2 are one-electron oxidants, which means that for them, n ) 1 (when oxidizing lignin with 1 or 2, each Mn atom in each POM anion can only accept one electron, thus changing its oxidation number from III to II). Moreover, E° is an experimental value determined for the redox pair at standard conditions; therefore, if the reaction is performed at different conditions, a different value has to be used. In our case, for the “nonstandard” oxidation potential, a symbol E1/2 was chosen. This is an experimental value that was determined for the POM redox pairs under all of the conditions in which the reaction kinetics experiments were to be performed. Considering all of the facts above, the Nernst equation for our POM bleaching system becomes

E ) E1/2 +

[POMox] RT ln F [POMred]

(2)

From eq 2, it is seen that at equimolar concentrations of POMox and POMred, the potential of the system measures E1/2. Moreover, the equation enables one to calculate the concentrations of oxidized and reduced POMs in a system if the potential of the system is measured and if E1/2 and the total amount of POM are known. Determination of E1/2 and Activation Energy. In determining the E1/2 and activation energy (Ea), the composition of the samples was different. In determining the Ea, the sample contained pulp (added at ∼20% consistency). Because chemical reactions during determination of the E1/2 values were undesirable, in these experiments, the pulp was omitted. The samples (V ) 100 mL) placed in the reaction vessel (Normschliff Gera¨tebau GmbH, Germany) were

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mixed with a magnetic stirrer. The temperature was kept constant (25 or 50 °C) with circulating water in the jacket of the vessel. The pH of the samples was adjusted with a buffer solution, and the ionic strength, with KCl, taking into account the sodium originating from the buffer solutions. When pulp was used, the final consistency of the samples was 1.0%. The vessel was closed with a Teflon lid and laboratory film. The redox potential of the samples was monitored with a combination redox electrode consisting of a platinum ring indicator electrode and a Ag/AgCl reference electrode (Mettler Toledo InLab501; Mettler-Toledo GmbH, Switzerland). Air was flushed out of the reaction vessel, and its re-entry was prevented by a continuous flow of argon gas. When the sample had reached the desired temperature, POM was added into the reaction vessel. The final concentration of POM in the reaction mixture was 0.5 mmol L-1; in experiments in which Ea was determined, either 1 or 2 was used; in determining E1/2, an equimolar mixture of 1 and 1red or 2 and 2red was added. After the addition of POM, the oxidation potential of the sample changed sharply and fluctuated for a short time before settling. During the fluctuation period, no reliable results for E were obtained. In determining the Ea, the sharp, initial change of oxidation potential happening instantly after POM addition was considered as the starting point of the reaction. The reaction times used in the experiments to estimate Ea were 500 and 5000 s, and the potential of the system was measured at 1- and 10-s time intervals, respectively. The longer reaction time was used for slow-reacting samples (low pH or low ionic strength). When E1/2 was determined, the measuring time was 1000 s with a 10-s interval between reading the potential. The equipment used for measuring and saving the data consisted of a Metrohm 751 GPD Titrino autotitrator (Metrohm AG, Switzerland), which was connected to the redox electrode and a computer using TiNet 2.4 software (Metrohm AG). The data acquired in the experiments showed the oxidation potential (E) of the samples as a function of time for 1 and 2 at varying conditions. The results of E1/2 values for 1 and 2 were calculated as average values of E obtained during the experiments; for the samples that contained pulp, reaction rate graphs were calculated from the data (eq 2). The graphs showed [POMred]/ [POMtot] ratio as a function of reaction time. From a reaction rate graph, the reaction rate (kobs) at a certain reaction stage was obtained by calculating the slope of the graph at a certain [POMred]/[POMtot] value. This was achieved by calculating the slope of an asymptote of the reaction kinetics graph at the desired point either by linear fitting of data points or by deriving the function obtained from logarithmic fitting of the data with Microsoft Excel (Microsoft Corp., Redmond, WA). In most cases, only a poor correlation with logarithmic fitting was obtained; therefore, the latter method was used less than the prior. From the values of kobs determined for the same reaction mixtures at two different temperatures (25 and 50 °C), it was possible to deduce the energy of activation (Ea) of the lignin oxidation reaction at certain reaction stages. The method based on the Arrhenius equation32,33, that is, plotting the values of ln kobs versus T-1 and determining Ea from the graphs, was used. Determining kobs values at only two temperatures in reaction kinetics experiments is somewhat unusual; however,

our experimental plan included experiments at three different ionic strength and pH values. Therefore, using only two temperatures for the experiments is justified: three or more temperatures would have substantially increased the number of experiments. Moreover, even though only two temperatures were used, it was still possible to perform a sufficient error analysis for the results, as will be shown below. After the experiments, the pulp used in the samples was separated with suction, washed, and oven-dried (105 °C). The dried pulp pads were weighed, and this information was used in calculating the exact consistencies of the samples. Preliminary Estimation of the Effect of Ionic Strength on the Reactivity of POM. Prior to the reaction kinetics experiments, the effect that ionic strength has on the reaction of POM with residual lignin in unbleached pulp was roughly estimated in preliminary experiments as follows. Unbleached kraft pulp was suspended at 5% consistency in vials with distilled water (no pH adjustment was made). Two sample series were prepared. To one, no extra salts were added ([K+] ) 0.006 mol L-1 after the POM addition), and to the other, KCl addition was made ([K+] ) 0.500 mol L-1, K+ originating from added KCl and the POM solution). In both series, in addition to a blank sample (to which no POM was to be added), five samples corresponding to reaction times 3, 5, 12, 30, and 60 min were prepared. The vials containing the pulp suspensions were closed with a septum, and the air in them was replaced with nitrogen gas. The reactions were performed at room temperature, and they were started by adding 3 into the vials as an aqueous solution with a syringe through the septum and shaking the vials vigorously (the final POM concentration of the samples was 1.00 mmol L-1). After reaching the desired reaction times, the vials were opened, and the pulp was filtered by suction. The light absorption of the filtrate at 496 nm (A496nm) was measured (Unicam 5625 UV/vis spectrometer; Thermo Electron Corp., Massachusetts) and the concentration of 3red approximated from a calibration curve (the curve showed A496nm values of standard samples containing known amounts of 3 and 3red). Determination of Water of Crystallization. To be able to calculate precisely the concentrations of POM solutions, the water of crystallization contained by the solid potassium salts of 1, 1red, 2, 2red, 3, and 3red had to be determined. This was achieved by using infrared spectroscopy. Copper sulfate (CuSO4‚5H2O) was used as a sample for testing the reliability of the method. Samples for measuring the infrared spectra were prepared by dissolving the POMs and CuSO4‚5H2O in D2O. The spectra of the solutions were measured as transmission through an IR cuvette (RIIC F-04, Beckman-RIIC Ltd., UK) containing the samples. The areas of the H2O and D2O absorption peaks (at 3390 and 2495 cm-1, respectively) were determined from the spectra. Finally, the results were obtained by comparing the peak areas determined for the samples to the peak areas of standard samples and calculating the amount of water of crystallization for the POMs from this information. The equipment used included a Bio-Rad FTS 600 FTIR spectrometer (Bio-Rad Laboratories, Hercules, CA) and Win-IR Pro software (Digilab, Massachusetts). Error Analysis of the Results. The error analysis was performed with Microsoft Excel software. For the E1/2 values, the error was calculated as the confidence

Ind. Eng. Chem. Res., Vol. 44, No. 12, 2005 4287 Table 1. Water of Crystallization of the POMs compd

mol H2O/1 mol compd

K61 K71red K52 K62red K53 K63red CuSO4

16.4 13.7 8.3 9.3 11.6 4.3 5.0

Table 2. E1/2 Values of POMs 1 and 2 at Various Conditionsa pH

cation concn (mol L-1)

3

0.30

5

0.30

7

0.01 0.03 0.10 0.30

7

E1/2 (mV)b T (K)

POM 1

POM 2

298 322 298 322 298 298 298 298 322

490.5 ( 0.3 483.6 ( 0.4 419.5 ( 0.5 402.5 ( 0.3 324.6 ( 0.3 340.3 ( 0.5 359.1 ( 0.6 392.7 ( 0.3 369.8 ( 0.4

504.4 ( 0.4 494.6 ( 0.3 499.3 ( 0.3 487.5 ( 0.3 412 ( 1 436.9 ( 0.4 458.7 ( 0.9 495.6 ( 0.3 479.9 ( 0.2

Figure 1. Redox potential (E) as a function of reaction time in reaction kinetics experiments (oxidation of lignin with 2 at 25 °C and pH values 3, 5, and 7; for more details, see the Experimental Section). For clarity, every tenth data point is shown.

a In each experiment, the total POM concentration was 0.5 mmol L-1, consisting of 0.25 mmol L-1 of both reduced and oxidized forms of POM. For further details, see the Experimental Section. b The numeric values presented after the ( sign show the confidence interval (at 95% confidence level) for the E1/2 values.

interval (95% confidence level) from the data points obtained during the 1000 s of the measurement. In other words, the error shows the limits in the range of which 95% of the measured values are. In most cases, the error for the Ea values was determined in the following manner. First, the confidence interval (95% confidence level) for the slope of the reaction rate graph at the desired point was calculated (linear fitting of data points in Microsoft Excel). This information gave the confidence interval of the reaction rate (kobs) at certain reaction stage. Thereafter, the kobs values with the error limits were plotted in a graph as ln kobs vs T-1. The error for Ea was estimated from this graph. In the experiments for which a good correlation was achieved by logarithmic fitting of the reaction rate data, instead of the error limits for Ea values, the R2 value for each graph fitting provided by the software is presented. For determination of the κ number and the amount of water of crystallization in POMs, no error analysis was performed.

Figure 2. Amount of POMox reacted as a function of reaction time during reaction kinetics experiments (oxidation of lignin with 2 at 25 °C and pH values 3, 5, and 7; for more details, see the Experimental Section) For clarity, every tenth data point is shown.

In Figure 2, examples of reaction rate graphs are presented. The data for Figure 2 was obtained through calculations using the Nernst equation from the data shown in Figure 1 (for further details, see the Experimental Section). The activation energies, along with error limits, of POM-lignin reactions at various conditions determined from the data in the reaction rate graphs are shown in Table 3. In Figure 3, the effect of cation (potassium) concentration in the reaction rate of 3 with unbleached pulp is shown. Discussion

Results The κ number determined for the pulp used in this work was 24.9, and the HexA content was 21.2 meq kg-1. Table 1 depicts the content of water of crystallization determined for compounds K61, K71red, K52, K62red, K53, K63red, and CuSO4‚5H2O. The E1/2 values along with error limits determined at different conditions for systems containing equal amounts of 1 and 1red (POM 1) as well as 2 and 2red (POM 2) are shown in Table 2. During the reaction kinetics experiments, the oxidation potential of the reaction mixture was measured as a function of reaction time. An example of a diagram in which this kind of data is shown is depicted in Figure 1.

Pulp. The κ number (24.9) and the HexA content (21.2 meq kg-1) determined for the pulp used in this work are typical values for unbleached softwood kraft pulp. The κ number is a technical value that correlates with the lignin content. Its determination is based on oxidation of the aromatic structures in the pulp with KMnO4. However, being a very powerful oxidant, KMnO4 reacts also with some unsaturated functional groups that originate from the carbohydrates (e.g., HexA groups). This has to be taken into account when calculating the lignin content of the pulp. It has been estimated that for every 10 meq kg-1 of HexA in pulp, the κ number increases by 1.05 units.31 On the basis of this information, the “HexA corrected κ number” of the pulp equals 22.7, corresponding to ∼3.5% lignin content.34

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Table 3. Activation Energy (Ea) for Reactions of POMs 1 and 2 with Unbleached Pulp at Varying pHs and Reaction Stages POM 1 pH 3 5 7

POM 2

[POMred]/ [POMtot]

Ea (kJ mol-1)a

[POMred]/ [POMtot]

Ea (kJ mol-1)a

0.15 0.25 0.40 0.15 0.25 0.40 0.15 0.25 0.40 0.60

48.7 ( 4.3 59.2b 68.0b 47.6 ( 7.0 66.9 ( 6.8 NA 27.0 ( 3.6 35.0 ( 5.2 37.0 ( 5.4 44.0 ( 3.5

0.25 0.40 0.50 0.40 0.50 0.70 0.25 0.40 0.50 0.70

53 ( 14 67.0c 81.8c 34 ( 12 51.4 ( 4.6 56.3( 8.5 17 ( 10 15.2 ( 4.3 29.1 ( 2.9 30.5 ( 4.4

a The numeric values presented after the ( sign show the confidence interval (at 95% confidence level) for the Ea values.b The R2 values in logarithmic graph fitting for [POMred]/[POMtot] as a function of reaction time at 25 and 50 °C were 0.82 and 0.89, respectively. c The R2 values in logarithmic graph fitting for [POMred]/[POMtot] as a function of reaction time at 25 and 50 °C were 0.87 and 0.93, respectively.

Figure 3. Reaction of 3 with unbleached kraft pulp at two different cation (potassium) concentrations, 0.006 and 0.500 mol L-1.

Water of Crystallization. Our results for the amount of water of crystallization of the POMs (Table 1) are somewhat different from the results reported by other authors. Cowan et al.28 determined in their work that K61 and K71red contained 14 and 12 mol of H2O per 1 mol of POM, respectively; for K52 and K62red, Tourne´ et al.29 found 21 and 25 molecules of H2O per 1 molecule of POM, respectively. Although the latter values, especially, differ markedly from the values shown in Table 1, it has to be borne in mind that the results in this kind of analysis depend on conditions where the POMs have been stored as well as on the time elapsed after their synthesis.29 In addition, determining the water content of an inorganic salt (CuSO4) with our method gave a result (see Table 1) that is very much in accordance with the formula of the compound reported by the supplier (CuSO4‚5H2O). Therefore, it can be said that the results obtained for the amount of water of crystallization are reliable. E1/2 Values. The E1/2 values obtained for 2 are higher than the corresponding values determined for 1 (Table 2). This is not surprising, since the E° values for the MnIII/MnII redox pair in 1 and 2 have been reported to be ∼45035 and 650 mV,29 respectively. For both POMs, the E1/2 values follow the same pattern: at higher alkali metal cation concentration, the oxidation potential is higher. The effect of alkali metal cations on the E1/2

values of POMs is most probably due to ion pairing. Grigoriev et al.36 have reported an increase in oxidation potential of POMs in solutions containing Li+, Na+, and K+ cations. Their experiments were performed in acetatebuffered 2/3 (v/v) H2O/t-BuOH solutions, but the ion pairing also occurs in pure water at high concentrations of POMs or alkali metal salts.37 Reaction Kinetics. General Aspects. The curves of oxidation potential vs reaction time in suspensions containing 1 or 2 and unbleached kraft pulp (Figure 1) show that E decreases as the reaction time increases, indicating that POM is reduced during the reaction. In particular, at pH 5 and 7, the decrease in E is rapid in the beginning and becomes slower at longer reaction times. The same information can be seen even more clearly in the reaction rate graphs (Figure 2): [POMred] increases rapidly right after addition of POM to the reaction mixture, but after the rapid reaction stage, the rate of reaction decreases. With our experimental setup, it was impossible to obtain reliable results for the reaction rate during the very first few seconds of the reaction because of the time required for the reaction mixture and the measuring system to equilibrate. Therefore, the [POMred]/[POMtot] values plotted in Figure 2 do not start at reaction time 0; for the curves depicting the reaction kinetics, the first reliable values were normally obtained at 5-15 s after POM addition, depending on the reaction conditions. On the basis of several previous papers, it can be concluded that the reaction between nonphenolic lignin model compounds and POM anions is negligible.7,14,38,39 Only Yokoyama et al.16 have reported experiments in which an equilibrated POM ensemble13 was able to oxidize non-phenolic lignin model compounds at a temperature range of 120-180 °C. Because in our experiments the reaction temperatures were substantially lower (25 and 50 °C), however, it is safe to say that the reactive chemical species in the reactions are most certainly exclusively those phenylpropane units of lignin that contain free phenolic hydroxyl groups. Compounds 1 and 2 are one-electron oxidants. Consequently, complete oxidation of a phenolic lignin structure has at least two stages. In the first, the lignin structure loses one of its electrons to a POM anion: a phenoxyl radical is formed. If the concentration of the radicals is small or if their coupling is otherwise hindered, the radical loses its unpaired electron to another POM anion in a subsequent reaction, forming, for example, a quinone. In addition, oxidation or cleavage reactions of the side chain may consume POM. For these reasons, at a minimum, 2 equiv of POM anions is required to fully oxidize one phenolic phenylpropane unit in residual lignin.38-40 As was indicated above, the pulp used in this work contained ∼3.5% lignin. Although many factors affect the structure of the residual lignin, one estimation is that ∼25% of the phenylpropane units present in unbleached kraft pulp contain a phenolic hydroxyl group; the approximate average molecular mass of a phenylpropane unit is 183.41 Because the consistency of the pulp suspension in our experiments was 1.0%, the concentration of phenolic lignin structures in the reaction mixture can be approximated.

0.035 × 0.25 × 1.0 g ) 100 mL × 183 g mol-1 0.000478 mol L-1 ≈ 0.5 mmol L-1

[phenolic lignin units] )

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This is roughly equal to the concentration of added POM in the reaction mixture. Since at least 2 equiv of POM is required to oxidize 1 equiv of phenolic lignin, it is not surprising that all of the POM was consumed in some of our experiments (e.g., Figure 2, pH 7). The Reactive Chemical Structures. When comparing the activation energies (Ea, Table 3), it is seen that at identical conditions (same pH and reaction stage), Ea values for 2 are always lower than for 1. This is expected, given the higher oxidation potential of the MnIII/MnII redox pair in 2, as compared to 1 (see above). Hence, the phenolic lignin structures are more easily oxidized by 2 than 1. The results also show that as the reaction proceeds, its rate becomes slower. In Figures 1 and 2, the decrease in the oxidation potential and the increase in the [POMred]/[POMtot] ratio, respectively, are at their fastest in the beginning of the reaction; in addition, an increase in the activation energy is seen in Table 3, along with increased POM reduction. One interpretation of these observations is that the oxidation reaction has two stages: a rapid “initial” stage, and a slower “final” stage. This interpretation is partly true; nevertheless, it has to be remembered that there are certainly more than two different chemical structures in residual lignin that cause this situation. In fact, residual lignin contains a wide variety of phenolic structures that have different reactivities. The most reactive chemical species are the catechol structures and those phenolic structures that contain stilbene moieties; the less reactive structures include, for example, phenolic structures containing β-O-4, β-5, and 5-5 linkages.42 Earlier results are in accordance with the reasoning above: Weinstock et al.8 determined Ea for POM oxidation of pulp in anaerobic conditions (100-125 °C, 0.05 M concentration of 3, pH 7, 3% pulp consistency) and detected a rapid and a slow stage in the lignin oxidation reaction. The activation energies for the reactions were ∼15.5 and 113 kJ/mol, respectively. These values resemble our results: low values of Ea were determined at pH 7 immediately after the POM addition (27 and 17 kJ/mol for 1 and 2, respectively; see Table 3), but the activation energy increased as the reaction proceeded (44 and 30 kJ/mol for 1 and 2, respectively). In our experiments, very short reaction times were used; hence, even the largest values for Ea determined at pH 7 are rather small. In other papers that cover kinetics of POM bleaching, catalytic delignification systems containing oxygen gas have been investigated.14,43 The results for Ea in these experiments include the effect of the reoxidation reaction of POM. For this reason, it is hard to compare these Ea values with the results obtained in our work. The very first reactions in our experiments were extremely rapid. During the “equilibrating period” of our system (i.e., the first 5-15 s after POM addition to the reaction mixture), up to 20% of 1 and 2 reacted (data for 2 at 25 °C shown in Figure 2), depending on the reaction conditions. Because, theoretically, more than 2 equiv of POM is required to oxidize one phenolic lignin structure, the results indicate that 6, it can be safely assumed that at pH 3, all of the phenols are protonated. At pH 5, however, already some of the phenolate is present, and at pH 7, the amount of phenolate is even greater. Table 3 clearly shows this effect, especially in the case of reaction including 2: the Ea values increase in the order pH 7 < pH 5 < pH 3. Since phenolates are more reactive in oxidation than phenols, it is probable that at pH 7, the phenolic structures that are most extensively dissociated react first. On the basis of this assumption, the effect of dissociation of phenolic structures on their activation energy can best be evaluated by comparing the Ea values at pH 7 and pH 3 in the beginning of the reaction (i.e., low [POMred]/[POMox] ratio). For 1, this comparison shows Ea value ∼22 kJ mol-1 lower at pH 7 than at pH 3 (0.15 [POMred]/[POMox] ratio, i.e., when 15% of 1 has reacted)sfor 2 the corresponding difference is ∼36 kJ mol-1 (25% of 2 reacted). The respective values increase as the reaction proceeds (31 kJ mol-1 for 40% reacted 1, and 53 kJ mol-1 for 2 at 50% reacted POM). The observation indicates that at pH 7, the concentration of highly reactive chemical species is larger than at pH 3. This shows that through ionization, some of the lignin structures, which probably would not react at all in the protonated form, are oxidized by the POMs as phenolates.

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When looking at the results for which the effect that the pH has on the reactivity of POM is shown, it has to be borne in mind that the ionic strength where the experiments were carried out (0.30 mol L-1) is very high. Because of this, the negative charges of the fiber and POM anions are screened by the mobile ionic species (mostly K+ and Cl-), and thus, the Donnan effect is almost completely eliminated. At lower ionic strength, however, the Donnan effect prevails: there are not enough ions present to “neutralize” the charges. Hence, repulsion between POM anions and the fiber is created. These facts suggest a conclusion that when the ionic strength in a pulp suspension is high, pH and reactivity of POM have a positive correlation; by contrast, at low ionic strength, the correlation is most probably negative. The results obtained in the experiments for which the effect of the ionic strength on POM residual lignin reaction was investigated are consistent with the reasoning presented above. In the experiments, it was seen that the effect of cation concentration is remarkable. Its importance is illustrated by the fact that for 1 and 2, only negligible reaction was observed during the first 500 s at 1% pulp consistency (0.05 mmol L-1 POM concentration, pH 7, 298 K) when cation concentration (K+ and Na+ ions) was 0.100 mol L-1 or lower. Because of the very low reaction rate, we were unable to estimate activation energies for these oxidation reactions; neither are the data shown here. In the experiments carried out prior to the reaction kinetics experiments, more reactive POM (3) was used at a higher concentration, ionic strength, and pulp consistency. Being preliminary, the aim of these experiments was only to evaluate with stripped-down procedures the role of ionic strength in the reactivity of POM. Therefore, mixing of the samples was far from thorough, and temperature and pH were not rigorously controlled. As a consequence, the extent of error in the results presented in Figure 3 is probably large. Nevertheless, the observations strongly support the idea that adding a simple electrolyte to the pulp suspension enhances the reactivity of POM. The results show that in the sample containing elevated cation concentration (0.500 mol L-1) about twice as much 3 reacted during the first three min than during 60 min in the experiment where cation concentration was low (0.006 mol L-1). In addition, the elevated ionic strength caused almost 100% of 3 to react within 60 min. The corresponding value for the experiment carried out at low cation concentration is ∼50%. Various mechanisms can account for the acceleration of the lignin oxidation caused by POM in pulp suspensions with high ionic strength. One mechanism, as mentioned earlier, is the elimination of the Donnan effect. Second, dissolved POM anions form ion pairs with alkali metal cations,36 which increases their oxidation potential (see Table 2) and, thus, their reactivity toward residual lignin. In addition, an attraction between fiber and POM has been detected in suspensions containing elevated concentrations of alkali metal cations, the mechanism of which is unknown.27 Finally, high ionic strength decreases the electronic repulsion between POM and phenolate anions; hence, the activation energy of the reaction decreases, since formation of the transition state requires less energy (so-called positive salt effect).33 Conclusions The results presented in this paper clearly show that reactivity of POM toward phenolic lignin structures in

unbleached pulp increases with increasing (alkali metal) cation concentration and pH. The most important reasons for the observed behavior of POM are elimination of the Donnan effect due to high ionic strength and the formation of phenolate ions through dissociation of phenolic groups in lignin. On the basis of the results, it can be said that a drastic decrease of the catalyst concentration in POM-catalyzed oxygen bleaching may be possible, provided pH and ionic strength of the process are rigorously controlled. In earlier papers published on POM bleaching, the concentrations of POM reported for the processes range from 1-210,17 up to 50-5008,11 mmol L-1. The feasibility of the process concepts is based on minimal loss of POM; our goal in investigating POM-catalyzed oxygen delignification is to decrease the concentration of POM in the process significantly below 1 mmol L-1. At such low levels, the recovery of the catalyst is no longer (economically) necessary, and there is no need for strict recovery techniques, which are often costly and technically demanding. In this work, some of the activation energies determined for oxidation reaction of lignin with POM were very low. Therefore, changes in the reaction temperature affect the rate of these reactions very limitedly. This may be problematic, since elevating the reactivity of POM by increasing the ionic strength may be difficult in industrial processes. Moreover, it is still unclear how the kinetics of the reoxidation reaction of POM (reaction II) depend on pH, ionic strength, and temperature. Our future work will concentrate on elucidating these problems. Acknowledgment The authors thank Mr. Richard S. Reiner (Forest Products Laboratory, Madison, WI) for synthesizing the POMs used in this work and Mr. Timo Pa¨a¨kko¨nen (Helsinki University of Technology, Laboratory of Forest Products Chemistry) for his skillful assistance in the laboratory. The financing provided by the National Technology Agency of Finland (TEKES) and the supporting industry (Andritz OY, Kemira Ltd., M-real Ltd., Stora Enso Ltd., and UPM-Kymmene Ltd.) is gratefully acknowledged. Literature Cited (1) Minor, J. L. Production of Unbleached Pulp. In Pulp Bleaching: Principles and Practice; Tappi Press: Atlanta, GA, 1996; Chapter II, p 25. (2) Sjo¨stro¨m, E. Wood Chemistry: Fundamentals and Applications; Academic Press: San Diego, CA, 1993. (3) Suchy, M.; Argyropoulos, D. S. Catalysis and activation of oxygen and peroxide delignification of chemical pulps: a review. In Oxidative Delignification Chemistry. Fundamentals and Catalysis; ACS Symposium Series 785; American Chemical Society: Washington, DC, 2001; Chapter 1, pp 2-43. (4) Call, H. P.; Mu¨cke, I. History, overview and applications of mediated lignolytic systems, especially laccase-mediator-systems (Lignozym(R)-process). J. Biotechnol. 1997, 53, 163. (5) Perng, Y.; Oloman, C. W.; Watson, P. A.; James, B. R. Catalytic oxygen bleaching of wood pulp with metal porphyrin and phthalocyanine complexes. Tappi J. 1994, 77, 119. (6) Argyropoulos, D. S.; Suchy, M.; Akim, L. Nitrogen-centered activators of peroxide-reinforced oxygen delignification. Ind. Eng. Chem. Res. 2004, 43, 1200. (7) Weinstock, I. A.; Atalla, R. H.; Reiner, R. S.; Moen, M. A.; Hammel, K. E. A new environmentally benign technology and approach to bleaching kraft pulp. Polyoxometalates for selective delignification. New J. Chem. 1996, 20, 269.

Ind. Eng. Chem. Res., Vol. 44, No. 12, 2005 4291 (8) Weinstock, I. A.; Atalla, R. H.; Reiner, R. S.; Moen, M. A.; Hammel, K. E.; Houtman, C. J.; Hill, C. L.; Harrup, M. K. A new environmentally benign technology for transforming wood pulp into paper. Engineering polyoxometalates as catalysts for multiple processes. J. Mol. Catal. A: Chem. 1997, 116, 59. (9) Evtuguin, D. V.; Pascoal Neto, C. New polyoxometalatepromoted method of oxygen delignification. Holzforschung 1997, 51, 338. (10) Evtuguin, D. V.; Pascoal Neto, C.; Pedrosa de Jesus, J. D. Bleaching of kraft pulp by oxygen in the presence of polyoxometalates. J. Pulp Pap. Sci. 1998, 24, 133. (11) Weinstock, I. A.; Atalla, R. H.; Reiner, R. S.; Houtman, C. J.; Hill, C. L. Selective transition-metal catalysis of oxygen delignification using water-soluble salts of polyoxometalate (POM) anions. Part I. Chemical principles and process concepts. Holzforschung 1998, 52, 304. (12) Evtuguin, D. V.; Pascoal Neto, C.; Rocha, J. Lignin degradation in oxygen delignification catalyzed by [PMo7V5O40]8polyanion. Part I. Study on wood lignin. Holzforschung 2000, 54, 381. (13) Weinstock, I. A.; Barbuzzi, E. M. G.; Wemple, M. W.; Cowan, J. J.; Reiner, R. S.; Sonnen, D. M.; Heintz, R. A.; Bond, J. S.; Hill, C. L. Equilibrating metal-oxide cluster ensembles for oxidation reactions using oxygen in water. Nature 2001, 414, 191. (14) Gaspar, A.; Evtuguin, D. V.; Pascoal Neto, C. Oxygen bleaching of kraft pulp catalysed by Mn(III)-substituted polyoxometalates. Appl. Catal. A: Gen. 2003, 239, 157. (15) Walker, C. C.; Dinus, R. J.; McDonough, T. J.; Eriksson, K. L. Evaluating three iron-based biomimetic compounds for their selectivity in a polymeric model system for pulp. Tappi J. 1995, 78, 103. (16) Yokoyama, T.; Chang, H.; Reiner, R. S.; Atalla, R. H.; Weinstock, I. A.; Kadla, J. F. Polyoxometalate oxidation of nonphenolic subunits in water: Effect of substrate structure on reaction kinetics. Holzforschung 2004, 58, 116. (17) Gaspar, A. R.; Evtuguin, D. V.; Pascoal Neto, C. Polyoxometalate-catalyzed oxygen delignification of kraft pulp: a pilotplant experience. Ind. Eng. Chem. Res. 2004, 43, 7754. (18) Hill, C. L.; Prosser-McCartha, C. M. Homogeneous catalysis by transition metal oxygen anion clusters. Coord. Chem. Rev. 1995, 143, 407. (19) Weinstock, I. A. Homogeneous-phase electron-transfer reactions of polyoxometalates. Chem. Rev. 1998, 98, 113-170. (20) Neumann, R. Polyoxometalate complexes in organic oxidation chemistry. In Progress in Inorganic Chemistry; John Wiley & Sons: New York, 1998; Vol. 47, pp 317-370. (21) Kozhevnikov, I. V. Catalysis by heteropoly acids and multicomponent polyoxometalates in liquid-phase reactions. Chem. Rev. 1998, 98, 171-198. (22) Shatalov, A. A.; Evtuguin, D. V.; Pascoal Neto, C. Cellulose degradation in the reaction system O2/heteropolyanions of series [PMo(12-n)VnO40](3+n)-. Carbohydr. Polym. 2000, 43, 23. (23) Donnan, F. G.; Harris, A. B. Osmotic pressure and conductivity of aqueous solutions of Congo Red and reversible membrane equilibria. J. Chem. Soc. 1912, 99, 1554-1577. (24) Farrar, J.; Neale, S. M. The distribution of ions between cellulose and solutions of electrolyte. J. Colloid Sci. 1952, 7, 186195. (25) Grignon, J.; Scallan, A. M. Effect of pH and neutral salts upon the swelling of cellulose gels. J. Appl. Polym. Sci. 1980, 25, 2829-2843. (26) Towers, M.; Scallan, A. M. Predicting the ion-exchange of kraft pulps using Donnan theory. J. Pulp Pap. Sci. 1996, 22, J332J337. (27) Ruuttunen, K.; Vuorinen, T. Donnan effect and distribution of the [SiVIVW11O40]6- anion in pulp suspension. J. Pulp Pap. Sci. 2004, 30, 9.

(28) Cowan, J. J.; Bailey, A. J.; Heintz, R. A.; Do, B. T.; Hardcastle, K. I.; Hill, C. L.; Weinstock, I. A. Formation, isomerization, and derivatization of keggin tungstoaluminates. Inorg. Chem. 2001, 40, 6666. (29) Tourne´, C.; Tourne´, G.; Malik, S. A.; Weakley, T. J. R. Triheteropolyanions containing copper(II), manganese(II), or manganese(III). J. Inorg. Nucl. Chem. 1970, 32, 3875. (30) Domaille, P. J. The 1- and 2-dimensional tungsten-183 and vanadium-51 NMR characterization of isopolymetalates and heteropolymetalates. J. Am. Chem. Soc. 1984, 106, 7677. (31) Vuorinen, T.; Fagerstrom, P.; Buchert, J.; Tenkanen, M.; Teleman, A. Selective hydrolysis of hexenuronic acid groups and its application in ECF and TCF bleaching of kraft pulps. J. Pulp Pap. Sci. 1999, 25, 155. (32) Carey, F. A.; Sundberg, R. J. Advanced Organic Chemistry; Kluver Academic: New York, 2000; p 202. (33) Sykes, P. The Search for Organic Reaction Pathways; Longman: London, 1979; pp 20-21 and 31. (34) Kyrklund, B.; Strandell, G. Applicability of the chlorine number for evaluation of the lignin content of pulp. Pap. Puu 1969, 51, 299. (35) Weinstock, I. A.; Reiner, R. S.; Atalla, R. H. Unpublished results. (36) Grigoriev, V. A.; Cheng, D.; Hill, C. L.; Weinstock, I. A. Role of alkali metal cation size in the energy and rate of electron transfer to solvent-separated 1: 1 [(M+)(acceptor)] (M+ ) Li+, Na+, K+) ion pairs. J. Am. Chem. Soc. 2001, 123, 5292-5307. (37) Grigoriev, V. A.; Hill, C. L.; Weinstock, I. A. Polyoxometalate oxidation of phenolic lignin models. In Oxidative Delignification Chemistry: Fundamentals and Catalysis; ACS Symposium Series 785; American Chemical Society: Washington, DC, 2001; Chapter 18, pp 297-312. (38) Kang, G.; Ni, Y.; van Heiningen, A. Polyoxometalate delignification: study of lignin model compounds. Appita 1997, 50, 313. (39) Evtuguin, D. V.; Daniel, A. I. D.; Silvestre, A. J. D.; Amado, F. M. L.; Neto, C. P. Lignin aerobic oxidation promoted by molybdovanadophosphate polyanion [PMo7V5O40]8-. Study on the oxidative cleavage of β-O-4 aryl ether structures using model compounds. J. Mol. Catal. A: Chem. 2000, 154, 217. (40) Weinstock, I. A.; Hammel, K. E.; Moen, M. A.; Landucci, L. L.; Ralph, S.; Sullivan, C. E.; Reiner, R. S. Selective transitionmetal catalysis of oxygen delignification using water-soluble salts of polyoxometalate (POM) anions. Part II. Reactions of R-[SiVW11O40]5- with phenolic lignin-model compounds. Holzforschung 1998, 52, 311. (41) Gellerstedt, G.; Lindfors, E. Structural changes in lignin during kraft cooking. Part 4. Phenolic hydroxyl groups in wood and kraft pulps. Svensk Papperstidn. 1984, 87, R115. (42) Northey, R. A. A review of lignin model compound reactions under oxygen bleaching conditions. In Oxidative Delignification Chemistry: Fundamental and Catalysis; ACS Symposium Series 785; American Chemical Society: Washington, DC, 2001; Chapter 2, pp 44-60. (43) Evtuguin, D. V.; Pascoal Neto, C.; Rocha, J.; Pedrosa de Jesus, J. D. Oxidative delignification in the presence of molybdovanadophosphate heteropolyanions: mechanism and kinetic studies. Appl. Catal. A: Gen. 1998, 167, 123. (44) Ragnar, M.; Lindgren, C. T.; Nilvebrant, N.-O. pKa-values of guaiacyl and syringyl phenols related to lignin. J. Wood Chem. Technol. 2000, 20, 277.

Received for review December 2, 2004 Revised manuscript received March 6, 2005 Accepted April 8, 2005 IE048836O