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Comparative Evaluation of Oxygen Delignification Processes for Low- and High-Lignin-Content Softwood Kraft Pulps Lucian A. Lucia,* Arthur J. Ragauskas, and Fadi S. Chakar† Institute of Paper Science and Technology, Georgia Institute of Technology, 500 10th Street, NW, Atlanta, Georgia 30318-5794
The present study explores the efficiency of delignification achieved during standard oxygen, double-oxygen, and mini-oxygen [(E + O)Dkf)0.05(E + O)] delignification of high- (kappa ) 56.2) and low- (kappa ) 26.6) lignin-content softwood (SW) kraft pulps in the context of the structural changes occurring in the lignin as measured by nuclear magnetic resonance (NMR) spectroscopy. The relative bleachability of the high-kappa pulps was determined to be superior to that of the lower-kappa pulps during the oxygen bleaching experiments. The general trend of increasing ease of bleachability was double oxygen > oxygen > mini-oxygen for both the low- and highlignin-content pulps. NMR spectroscopy demonstrated that part of the rationale for the higher levels of delignification in the high-kappa pulps was due to higher contents of β-O-4 and methoxy lignin functional groups. In addition, the high-kappa pulp contained a lower number of resistant 5,5′-condensed lignin units and diphenylmethane structures. The NMR data also provided strong evidence for the presence of p-hydroxyphenyl units, a relatively unique resistant structure whose elimination was approximately the same for both series of pulps. These new structures might potentially function as end-capping termini that hinder access to bulk lignin clusters, thus limiting the overall efficiency of oxygen delignification. Introduction The development of oxygen delignification systems occurred in response to environmental concerns and impending future U.S. Environmental Protection Agency (EPA) regulations on bleaching operations in the U.S. pulp and paper industry. Oxygen delignification was intended to remove lignin from kraft pulp through application of high-pressure oxygen under alkaline conditions and to offset downstream bleaching chemical requirements. This technology evolved as a millcompatible operation that primarily off loads bleaching chemical demands, while simultaneously providing well delignified pulp without compromising pulp physical properties. The pulp from the pulping operations, mainly consisting of carbohydrates (cellulose and various hemicelluloses) and a small lignin fraction (known as residual lignin, typically less than 5% of the total mass), is subjected to relatively high-pressure oxygen conditions (600-700 kPa) under a hot (90-105 °C) alkaline environment for 30-90 min.1 Unfortunately, despite process and chemical parameter modifications, the oxygen delignification operation cannot remove more than 35-50% of the residual lignin before sustaining detrimental oxidative carbohydrate degradation manifested as a loss in pulp CED (chain entanglement density) viscosity and fiber strength.2 The loss in selectivity (change in overall lignin concentration/change in pulp viscosity) incurred during oxygen delignification is cited as a major drawback to its continued implementation and therefore necessitates renewed efforts to provide the benefits of this environmentally benign technology. A theme in current oxygen delignification research is to achieve a more fundamen* To whom correspondence should be addressed. † Current address: Appleton Paper Inc., 825 E. Wisconsin Avenue, P.O. Box 359, Appleton, WI 54912-0359.
tal understanding of the chemical factors governing delignification and carbohydrate damage involved in the mill. Beyond the milestone 50% oxygen delignification, kraft pulp incurs a significant loss in pulp viscosity and strength properties.2,3 A number of approaches that focus on manipulating the chemistry of an oxygen delignification stage have been explored, including the double-oxygen delignification stage.4,5 Yet, double stages have yet to reach idealized limits; in fact, Chirat and Lachenal demonstrated in an elegant study of multistage oxygen delignification that a significant fraction of the residual lignin remains even after four stages.6 Early efforts used model reactions to explore the basis of the inactivity of residual lignin in oxygen delignification.7,8 It was found that moieties bearing free (noncondensed) phenolic groups exhibited a higher reactivity than their etherified counterparts.9-11 On the basis of the accumulated information, Gierer proposed a reaction in which the phenolate residues are oxidized to phenoxy radicals and the oxygen-based radicals convert lignin to organic hydroperoxides and muconic acids.12 The latter structures have been observed in structural studies associated with lignin isolation using an acidolytic method developed by Gellerstedt and popularized in fundamental chemistry research circles.13 The search for the inactivity of lignin during an oxygen delignification stage has focused recently on “condensed” aromatic structures.14,15 Many of these structures (diphenyl methanes, phenylcoumarans, 5,5-biphenyls) have been determined by model studies to be fairly resistant to oxygen delignification conditions.14 In fact, research efforts by Gellerstedt and Lindfors,16 Liu and Argyropoulos,17 Moe et al.,18 and Lucia et al.,19 Chen and Lucia,20 and Chen and Lucia21 have investigated the fundamental nature of these limiting reactions occurring in lignin during oxygen delignification. One of the insights obtained from these studies is the importance
10.1021/ie011026h CCC: $22.00 © 2002 American Chemical Society Published on Web 09/24/2002
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Figure 1. Interunit linkages that typically occur in softwood kraft lignin. Structures such as β-O-4 are referred to as noncondensed structures, whereas 5,5- and diphenylmethane are condensed structures.
of several structural components known as condensed and noncondensed phenolic structures (Figure 1) in delignification. The noncondensed phenolics appear to encourage delignification, whereas condensed phenolics tend to be more resistant. The basic chemical reactions that occur in delignification, however, do not change. For example, the typical oxidative reactions occurring during oxygen delignification are shown in Figure 2. Nonetheless, many competing oxidative reactions occur during the process, thereby modifying the chemical state of lignin in addition to promoting carbohydrate damage through radical intermediates such as hydroxy (generated as shown in Figure 2 in addition to diproprotionation from organic peroxides). The purpose of the present effort was to define the relative bleachability of high- and low-lignin-content pulps as a function of the oxygen delignfication system and specific conditions employed and to provide a fundamental basis for the results observed through the use of 13C and 31P NMR spectroscopies. Experimental Section Materials. All chemicals and reagents were commercially purchased from Aldrich, Fischer, or EM Science and used as received, except for the ACS-grade p-dioxane, which was refluxed for 1 h over sodium borohydride and distilled. In-house-distilled water was used for the oxygen delignification studies. The lowkappa (LK) kraft SW pulp (P. taeda)was generously supplied by a member company of the Institute of Paper Science and Technology, whereas the high-kappa (HK) kraft softwood (SW) pulp was manufactured in-house
through controll of the H-factor (time/temperature cooking profile) in a kraft batch cook. Oxygen Delignification Trials and Chlorine Dioxide Bleaching. The metal ion contents (iron, copper, magnesium, and manganese) of the brownstock starting pulps were determined using ICP (inductively coupled plasma) to assess qualitatively the effects of nonprocess elements in the reactions. All oxygen delignification experiments were conducted in a temperature-controlled, pressurized reactor bomb under controlled mixing. Each trial was performed as follows: A pulp was added to a preheated solution of water at a selected temperature containing a concentration of 2.5% NaOH and 0.1% MgSO4 relative to dry pulp mass. The amount of water for the experiments was generally added in a 9:1 m/m ratio to pulp (10% pulp consistency). After the pulp had equilibrated to temperature (typically 5-10 min), a constant pressure of 620 kPa of high-purity (>99.99%) oxygen was applied, and the reaction “clock” was started and allowed to run for up to 60 min. Afterward, the reaction was stopped by removal of the pulps from the reactor and isolation of the straw-colored effluents through a Whatman #4 filter paper. The pulps were thoroughly washed with excess distilled water until the filtrate was clear. They were allowed to airdry or used in a subsequent oxygen delignification trial with or without effluent from the prior run (carryover). The various oxygen delignification experimental conditions are given in Table 2. All pulps were tested for lignin content using TAPPI Test Method T256c and for CED viscosity using TAPPI Test Method T258c. As part of a mini-oxygen run [(E + O)D(E + O)], fresh solutions of chlorine dioxide were prepared before use and applied
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Figure 2. Relevant oxidation reactions occurring during oxygen delignification are shown: (a) oxidation of a canonical lignin structure to muconic acid and methyl ester derivative, (b) four-electron reduction process where dioxygen is transformed to water.
to the pulps as a 0.05 kappa factor (number of equivalents of chlorine multiple/kappa number of pulp; 1 equiv of ClO2 ) 2.63 equiv of Cl2) in a KayPak polyethylene bag. ClO2 bleaches were conducted at 10% consistency and 70 °C for 30 min. Residual Lignin and Effluent Lignin Isolation. The pulps were acetone-extracted in a Soxhlet extractor for 24 h, thoroughly washed with water, and air-dried before isolation of the residual lignin. The air-dried pulp was placed in a three-necked round-bottom flask and diluted with a 9:1 v/v portion of p-dioxane and 0.10 M HCl to yield a final consistency of 4%. The pulp slurry was refluxed for 2 h under argon positive pressure and subsequently allowed to cool to room temperature. The slurry was filtered to remove the hydrolyzed pulp and worked up according to established procedures.16 Yields for lignin isolation ranged from 45 to 55% based on the original lignin content of the pulp. Effluent lignin from the trials was isolated by concentration of the effluent (50% reduction in volume) and acidulation of with 1 M HCl until the pH was 2.0. The precipitated lignin was frozen to coalesce low-molecular-weight fragments, thawed, and centrifuged to yield a thick brown paste. The paste was rinsed with pH 2.0 water, centrifuged, and collected. This latter process was repeated twice. All of the subsequent lignin fractions were collected into one flask, freeze-dried, and oven-dried at 40 °C overnight. Selected Klason lignin runs confirmed the kappa measurements for the range of kappa values obtained in the series of experiments in this study.
NMR Spectroscopy. All nuclear magnetic resonance (NMR) spectra were collected on a Bruker DMX 400MHz NMR spectrometer. The reproducibility and statistical significance of the acquired data were confirmed by duplicate and triplicate runs for most of the spectra obtained, in addition to acquisition of spectra of the same lignin sample from independent oxygen delignification runs. 13C NMR Spectroscopy. 13C NMR spectra were obtained under quantitative conditions through the use of an inverse-gated pulse sequence with a π/2 pulse, a sweep width of 330 ppm, and a 10-s delay. In general, 100-200 mg of lignin/mL was dissolved in 0.4 mL of d6-DMSO (dimethyl sulfoxide) and heated to 50 °C. All output signals were Fourier transformed using 10-Hz line-broadening and analyzed using reported chemical shifts for lignin functional groups.22 All integrations were calculated relative to an integration value of six carbons assigned for the aromatic functionalities.23 The accuracy of this method is typically within 5%, which was confirmed by our experiments. 31P NMR Spectroscopy. Lignin isolates were phosphitylated with 2-chloro-4,4,5,5-tetramethyl-1,3,2-dioxaphospholane following the procedure of Granata and Argyropoulos and subsequently analyzed by 31P NMR spectroscopy.24 All standard deviations are
