Adrianna G. Kirkman

Biobleaching of pulp with dioxygen in laccase-mediator system: effect of variables on the reaction kinetics

Comparative studies were carried out on the kinetics and mechanism of pulp biobleaching with laccase-mediator system (LMS) with two different mediators, 1-hydroxybenzotriazole (HOBT) and N-hydroxyacetanilide (NHAA). The optimal NHAA and laccase charge was found to be 0.1 mmol and 10 U per gram of pulp with pulp consistency of 10%, at the reaction temperature of 40 ◦ C for 8 h under atmospheric pressure, respectively. The kinetic studies on Kappa number reduction and dioxygen uptake suggest that a very fast rate of delignification with NHAA at the beginning of the process is the result of fast formation of the oxidized mediator species. However, a very slow delignification rate after the initial phase (0.5–1 h) could be caused by low stability of the mediator species. After the reaction time of 2 h, the degree of delignification is higher when HOBT is used as mediator. In contrast to the delignification with NHAA, the formation of the oxidized mediator species is the rate-determining step of the pulp biobleaching with dioxygen in the LMS using HOBT as mediator. Increase in temperature increases the rate of chemical reactions, but decreases the laccase stability. The optimal temperature for pulp biobleaching with HOBT and laccase from Coriolus versicoloris 40 ◦ C. Increasing oxygen pressure improves the efficiency of delignification due to better penetration of the reagents, but does not affect the rate of chemical reactions. The reaction mechanism is discussed based on the kinetic data.

Biobleaching of pulp with dioxygen in the laccase-mediator system — reaction mechanisms for degradation of residual lignin

Abstract Pine Kraft-AQ pulp was biobleached with pressurized dioxygen at 40°C in laccase-mediator system (LMS), i.e. in acetate buffer (pH 4.5) containing Coriolus-laccase and 1-hydroxy-benzotriazole (HOBT), the latter being as a mediator. The LMS-treatment was followed by alkaline extraction (E) under standard conditions. The structures of the residual lignins before and after the biobleaching did not differ appreciably. This indicates that only a part of the residual lignin in the pulp undergoes oxidative degradation in the LMS treatment. In contrast, the treatment resulted in strong changes in the structure of the lignin isolated from E-effluents. The 2D HMQC (1H13C correlation) spectra showed the disappearance of β-O-4′, β-β′ and β-5′ bonds in the structure of the alkaline soluble lignin (ASL) from E-effluents, which are present in the 2D spectrum of the original residual lignin (RKL). In addition, the spectra exhibited new signals that are assigned to ArCOOH in biphenyl (5-5′) moieties. This implies that oxidative cleavage of side chains plays an important role in the delignification of pulp. The NMR studies also indicated that intensive degradation of aromatic ring has occurred in the biobleaching. However, premethylation of neither benzyl alcohol nor phenolic hydroxyl groups of the residual lignin in pulp before the biobleaching affected the rate of delignification. The latter indicates that phenolic moieties participate not only in oxidative degradation but also dehydrogenative polymerization reactions in the biobleaching. This is consistent with an appreciable increase in the proportion of fractions with higher molecular mass in lignin isolated from E-effluents.

Laccase-catalyzed oxidation of 1-(3,4-dimethoxyphenyl)-1-propene using ABTS as mediator

The fungal laccases catalyzed oxidation of 1-(3,4-dimethoxyphenyl)-1-propene (2) with dioxygen in acetate buffer (pH 4.5) producing 1-(3,4-dimethoxyphenyl)propane-1,2-diol (4) and its 1-O-acetyl and 2-O-acetyl derivatives 5 and 6, and 3,4-dimethoxybenzaldehyde (7). However, in phosphate buffer (pH 5.9), the same reaction produced only 4 and 7. When 4 was treated in the same fashion in the phosphate buffer, it was converted into 7 with more than 95 mol% yield. This, together with the formation of 5 and 6 in the acetate buffer, showed that 2 is converted into 3–5 via 1-(3,4-dimethoxyphenyl)propane-1,2-epoxide (3) in the acetate buffer in the presence of ABTS. The major reaction of fungal laccase-catalyzed oxidation of 2 with dioxygen in the presence of ABTS is epoxidation of the double bond conjugated to the aromatic ring.

Kinetic studies on oxidation of veratryl alcohol by laccase-mediator system. Part 2. The kinetics of dioxygen uptake

Summary The kinetics of dioxygen uptake in the laccase-catalyzed oxidation of veratryl alcohol with dioxygen in the presence of ABTS, the mediator, was studied. The kinetics of dioxygen uptake consists of two phases: (1) the initial phase up to a reaction time of one hour, and (2) the second phase, after a reaction time of one hour. In the initial phase, ABTS is mainly oxidized to the corresponding cation radical. The kinetics of dioxygen uptake follows a pseudo-zero order rate law. The dioxygen uptake under the reaction condition correlates with the initial ABTS concentration according to the stoichiometric relationship of 0.25 moles dioxygen per mole ABTS. In the second phase, veratryl alcohol is mainly oxidized to veratraldehyde. The kinetics of the dioxygen uptake follows a pseudo-first order rate law. The dioxygen uptake correlates linearly with the yield of veratraldehyde. The stoichiometric ratio between the formation of veratraldehyde and the consumption of dioxygen differs slightly at different M/S ratios. On average, however, it is 0.42 moles of dioxygen per one mole of veratraldehyde formed. The reaction mechanism is discussed on the basis of the kinetic data.

Biobleaching of Pulp with Dioxygen in the Laccase-Mediator System. Part 1. Kinetics of Delignification

Summary Kinetics of pine kraft-AQ pulp delignification with the laccase-mediator system (LMS) and the effects of variable factors on the delignification were studied. The delignification was conducted in acetate buffer solution at pH 4.5 and at 40°C under atmospheric pressure. Only a part of the residual lignin could be removed in one-stage processes. Kinetics of kappa number reduction follows a pseudo-second order rate law with pulp consistency of 10 %, mediator charge of 0.1 mmole HOBT/g pulp and laccase charage of 10 U Coriolus laccase/g pulp. Kinetics of dioxygen uptake follows a pseudo-first order rate law up to first 8 hours of the reaction and a pseudo-zero order rate law at the reaction time of 8–24 hours. The amounts of dioxygen consumed per removal of one C9-unit equivalent of residual lignin is rather high, 1.5–2.5 mole, and increases with increasing reaction time. Experimental data show that side reactions between the Laccase-Mediator System and products of oxidative degradation of lignin strongly inhibit the delignification either by chemical or physical means or both. Removal of the degraded lignin fragments by alkaline extraction effectively restores the delignification of pulp with LMS. A four-stage process consisting of consecutive treatment of pulp with dioxygen-laccase-HOBT (LMS) followed by alkaline extraction (E), (LMS-E)4, decreased kappa number of a pine kraft-AQ pulp from 21.8 to less than 5. On the basis of the kinetic data, the mechanism of the pulp delignification with LMS is discussed.

Kinetic studies on oxidation of veratryl alcohol by laccase-mediator system: Part 1. Effects of mediator concentration.

Summary Kinetics of the laccase-catalyzed oxidation of veratryl alcohol with dioxygen in the presence of 2,2′-Azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) diamonium salt (ABTS), the mediator, were studied to elucidate the possible reaction mechanism and the role of the mediator in this reaction. The reaction follows a pseudo-first order reaction law. The first order rate constant (κ) is dependent on the Mediator/Substrate (M/S) ratio and has a maximum at M/S molar ratio of 0.15. The kinetic studies show that the mechanism of veratryl alcohol oxidation with dioxygen-laccase-ABTS is rather complex and includes different reaction pathways. The mediator is involved in competitive reactions. It has been suggested that at low mediator concentration, the veratryl alcohol is oxidized via the laccase redox cycle. The mediator acts mostly as a laccase activator at a M/S ratio lower than 0.15. With increasing ABTS concentration with respect to the substrate concentration, ABTS acts increasingly as a cosubstrate competing with the original substrate for active centers of the laccase. This results in inhibition of veratryl alcohol oxidation in the enzyme cycle and increases the role of substrate oxidation by an oxidized mediator.

Oxidative ammonolysis of technical lignins. Part 1. Kinetics of the reaction under isothermal condition at 130°C

Summary Investigations were conducted on the oxidative ammonolysis of REPAP organosolv lignin at 130 °C in 0.8M NH4OH solution under oxygen pressure of 12 bar. The lignin was completely solubilized at the reaction time of 165 min. The kinetics of the nitrogen incorporation consists of two phases. The first phase is up to the reaction time of approximately 35 min including 15 min heating up period. The rate of nitrogen incorporation in the first phase is 2.3 times higher than that in the second phase: κ1 = 4.58 × 10−4 s−1 versus κ2 = 1.90 × 10−4 s−1. The oxygen uptake and CO2 formation in the reaction is rather high. When the nitrogen incorporation was ceased after reaction for 255 minutes, more than 4 moles of oxygen/C9-unit of lignin were consumed and approximately 1.5 moles of carbon dioxide/C9-unit of lignin were released. In addition, extensive O-demethylation of methoxyl groups occurred. The molar ratio of the nitrogen incorporation to the methoxyl group eliminated is approximately 1.4 and 0.7 for the soluble and insoluble N-modified lignins, respectively. Structural analyses of the soluble N-modified lignins by FTIR and 1H NMR spectroscopic techniques showed only quantitative differences in the spectra obtained at different reaction times. This indicates that the reaction pathways do not change in the course of the oxidative ammonolysis. Possible reaction mechanisms of the oxidative ammonolysis are discussed on the basis of the experimental data.

Kinetic study on delignification of kraft-AQ pine pulp with hydrogen peroxide catalyzed by Mn(IV)-Me4DTNE.

Summary The kinetics of delignification of a kraft-AQ southern pine pulp with hydrogen peroxide catalyzed by [LMn(IV)(μ-O)3Mn(IV)](ClO4)2 (1), where L = 1,2-bis(4,7-dimethyl-1,4,7-triazacyclonon-1-yl)ethane was studied. The degree of delignification was significantly improved by using the catalyst. The pulp was bleached for 2 hours at 80°C, in 10% consistency with 2% NaOH, 4% H2O2 and 60 ppm catalyst charges on pulp (O.D.). Kappa number of the pulp was reduced from 31.6 to 16.8 corresponding to a degree of delignification of approximately 4%, while GE brightness was increased from 24.2 to 44.7. At the same time, viscosity of the resulting pulp was reduced from 31.1 mPa•s to 20.1 mPa•s compared to the reduction from 31.1 mPa•s to 20.1 mPa•s in the uncatalyzed bleaching under the same reaction condition. This indicates that the degradation of the carbohydrates was moderate in the catalyzed bleaching compared to the uncatalyzed bleaching. The delignification was found to follow pseudo first order kinetics with respect to kappa number, i.e., residual lignin, in the initial phase and quickly slowed down after 30 minutes (residual phase) under all the reaction temperatures investigated. The delignification rate constants in the initial phase were 0.17, 0.18, and 0.21 min−1 at 50, 60, and 80°C, respectively. Degree of delignification at the delignification time of 30 minutes is approximately 40% at 80°C. The possible delignification mechanism was discussed on the basis of the kinetic studies and lignin model compound experiments.

Oxidative Ammonolysis of Technical Lignins Part 2. Effect of Oxygen Pressure

Summary Investigations were conducted on the effects of oxygen pressure on the oxidative ammonolysis of REPAP organosolv lignin at 130 °C under oxygen pressure of 5, 8 and 12 bar. The rates of reactions monitored, such as nitrogen incorporation, lignin solubilization, oxygen uptake and CO2 formation, increase with increasing oxygen pressure. Kinetics of nitrogen incorporation under different oxygen pressure consists of two phases and follows a first order law in each phase. Linear correlation between the rate of nitrogen incorporation and oxygen pressure implies that the reaction is first order with respect to oxygen concentration. This indicates that oxygen participates directly in the rate-determining step of nitrogen incorporation. The rate of lignin solubilization also linearly increases with increasing oxygen pressure, implying that the rate of lignin degradation directly depends on oxygen pressure. The nitrogen incorporation is linearly correlated with the oxygen uptake, CO2 formation, oxygen incorporation into lignin, loss of carbon and methoxyl group content under all values of oxygen pressure and during the entire reaction period. This suggests that the reactions in the oxidative ammonolysis of lignin proceed via the same pathways in the different kinetic phases. In addition, the changes in the oxygen pressure were found to have only minor effect upon the coefficients of these linear correlations. This is in good agreement with the structures of N-modified lignin elucidated from FTIR and indicates that oxygen pressure affects only the reaction rate, but not the reaction mechanism.

Oxidative Ammonolysis of Technical Lignins Part 3. Effect of Temperature on the Reaction Rate

Summary The effect of the reaction temperature on the kinetics and the reaction mechanism of oxidative ammonolysis of Repap organosolv lignin have been studied. The reaction was conducted in 0.8 M Nh4oh solution under oxygen pressure of 12 bar and at three different temperatures, 70 °C, 100 °C and 130 °C. The resulting N-modified lignins were analyzed by elemental and methoxyl group. About 20–25% of maximum nitrogen content is incorporated into the lignin very fast, in 1–2 min of the reaction. The reaction kinetics then follows a pseudo-first order reaction law and consists of two phases. The activation energies for nitrogen incorporation and lignin solubilization are rather low, in the range of 33–34 kJ/mol. Linear correlation between nitrogen incorporated into the lignin and molecular oxygen uptake, oxygen incorporation, CO2 formation, O-demethylation and total carbon loss was analyzed at the different reaction temperatures. On the basis of kinetic data obtained so far, we have postulated that the reaction temperature affects the reaction rate, but not the reaction pathways. The reaction temperature also affects the ratios between different reaction pathways, though the effect is not very strong. The results obtained are discussed in the terms of competitive reactions of lignin oxidation followed by nitrogen incorporation and lignin deactivation involving nitrogen