Effect of Structural Features of Wood Biopolymers on Hardwood

Ind. Eng. Chem. Res. 2005, 44, 9777-9784

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APPLIED CHEMISTRY Effect of Structural Features of Wood Biopolymers on Hardwood Pulping and Bleaching Performance Paula C. Pinto, Dmitry V. Evtuguin, and Carlos Pascoal Neto* CICECO and University of Aveiro, Department of Chemistry, 3810-193 Aveiro, Portugal

Industrial hardwoods such as Eucalyptus species, Betula pendula, and Acacia mangium required different chemical charges during kraft pulping and presented distinct profiles of polysaccharides removal. The corresponding kraft pulps showed different chlorine dioxide consumption during bleaching. Woods and corresponding kraft pulps were characterized by chemical methods, 1H and 13C NMR spectroscopy, X-ray diffraction analysis, and gel permeation chromatography. The ease of lignin degradation and dissolution was essentially determined by differences in the proportion of syringyl and guaiacyl units and in the degree of condensation. The bleaching response was shown to be related also to the content of β-O-4 structures in the residual lignin. The relative stability of xylans during the pulping was suggested to be associated with differences in structure and molecular weight. The higher retention of Eucalyptus xylans was attributed essentially to their peculiar structure, including O-2-substituted uronic acid groups linked to other cell wall polysaccharides. Introduction Mature wood is a natural biocomposite material constituted essentially of elongated cells that have lost their biological activity, commonly known as wood fibers. The fiber cell walls are composed of cellulose fibrils (40-50%) with a predominantly crystalline structure, embedded in an amorphous matrix of hemicelluloses (20-35%) and lignin (20-30%). Wood also contains variable amounts of low-molecular-weight nonstructural organic components, the so-called extractives, and mineral components (ashes). Lignin is present in higher concentration in the outer layer of cell wall (middle lamella) and is responsible for the cohesion of fibers in wood.1,2 Industrial woods can be divided into two groups: coniferous woods or softwoods, obtained from gymnosperm trees, and hardwoods, obtained from angiosperm trees.1,2 As all natural materials, woods are characterized by a high variability of composition and structure.1,2 Such variability is particularly pronounced in the case of hardwoods because the number of species is much higher than in softwoods, affecting wood properties and behavior during industrial processing, particularly in pulp and paper making.1-4 Hardwoods are important raw materials used in the production of pulp and paper. For example, Betula pendula (birch) is the dominant hardwood species for such applications in Northern Europe, whereas Eucalyptus species (such as E. globulus, E. grandis, and E. urograndis) represent the main fiber sources for the pulp and paper industry in the Iberian Peninsula and South America.5 Acacia species, especially A. mangium, are also becoming important wood sources for pulp and paper production in Asia.5,6 * To whom correspondence should be addressed. Tel.: +351234370693. Fax: +351234370084. E-mail: [email protected].

Kraft pulping is the process most widely used in the industrial production of wood chemical pulps.7 Wood chips are treated at high temperatures (150-170 °C) with a strongly alkaline solution composed mainly of sodium hydroxide and sodium sulfide. Under such alkaline conditions, lignin is extensively degraded and dissolved (90-95%), liberating the wood fibers, composed essentially by cellulose and hemicelluloses, and minor amounts (1-4%) of residual lignin. Wood polysaccharides can also be degraded, mainly by alkaline hydrolysis of glycosidic linkages and sequential elimination of the terminal reducing end groups (peeling), part of them being dissolved in the aqueous alkaline medium, thus decreasing the pulping yield.8 The unbleached pulp is then subjected to the bleaching process, aiming to remove residual lignin and chromophore structures from pulp. Currently, chlorine dioxide is the most widely used bleaching chemical.9 The pulping and bleaching performance is highly dependent on the relative abundance, structure, and relative stability/reactivity of the wood biopolymers, i.e., lignin, cellulose, and hemicelluloses. In particular, different hardwoods can require significantly different pulping and bleaching conditions to achieve the same extent of delignification and degree of brightness, respectively. The reasons behind such different behaviors are not completely understood. Additionally, there is a lack of knowledge on the chemistry of macromolecular components of Eucalyptus and Acacia woods, despite their growing importance as pulp and paper raw materials. The understanding of how the chemical and structural features of hardwoods affect their behavior during pulping and bleaching is crucial for the optimization of such processes, improvement of pulp and paper quality, and reduction of the environmental impact of pulp and paper industry.

10.1021/ie050760o CCC: $30.25 © 2005 American Chemical Society Published on Web 11/03/2005

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Table 1. Compositions of Woods and Unbleached Kraft Pulpsa E. globulus lignin (Klason)b 22.1 neutral anhydro monosaccharides Glc 53.4 Xyl 14.2 Rha 0.3 Ara 0.4 Man 1.1 Gal 1.5 lignin (Klason)b 1.3 neutral anhydro monosaccharides Glc 45.0 Xyl 10.6 Rha 0.1 Ara 0.1 Man 0.1 Gal 0.4 a

E. urograndis

E. grandis

B. pendula

A. mangium

Wood Composition, % 27.9 26.7

21.5

27.6

52.1 11.4 0.2 0.4 0.7 1.2

44.5 23.6 0.8 0.7 2.1 0.8

51.7 11.9 0.3 0.2 1.0 0.6

1.3

1.2

38.8 12.4 0.1 0.1 0.3 0.1

42.2 6.7 0.2 0.0 0.2 0.0

50.9 12.4 0.3 0.4 0.7 1.0

Unbleached Pulp Composition, % (Wood Basis) 1.0 1.2 40.2 6.8 0.2 0.1 0.1 0.1

40.5 6.6 0.2 0.0 0.1 0.1

Kappa number 16-19. b Uncorrected for polyphenolics content.

In the research reported herein, we compared the kraft pulping and bleaching performances of Betula pendula (used as a reference hardwood), Eucalyptus globulus, Eucalyptus grandis, Eucalyptus urograndis, and Acacia mangium species and carried out a deep characterization of wood and unbleached pulp biopolymers, aiming to relate their structural features and relative reactivity and stability to the observed different pulping and bleaching responses. Experimental Section Woods, Pulping, and Bleaching. The following industrial wood chips (average size 30 × 20 × 0.5 mm) were used: E. globulus from Portugal, E. grandis and E. urograndis from Brazil, B. pendula (birch) from Sweden, and A. mangium from Indonesia. Kraft pulping experiments were carried out in 5.8-L forced-circulation batch digesters (M/K model 409 MII) equipped with an external electric heating system and temperature control. The conditions were as follows: liquor-to-wood ratio (L/kg), 4:1; sulfidity, 28%; initial temperature, 40 °C; final temperature, 160 °C; heating rate, 1 °C/min. Active alkali (%, as Na2O) was varied (keeping pulping time as similar as possible in all experiments) in order to attain similar degrees of delignification, expressed as kappa number. Screened kraft pulps were bleached to 90% brightness by a conventional “elemental chlorine free” sequence (DEDED, where D stands for a chlorine dioxide oxidation stage and E for an alkaline extraction with aqueous NaOH) in polyethylene bags plunged in a water bath. The strategy for the distribution of the ClO2 charge (expressed as “active chlorine”) was as follows: in stage D0, a predetermined ClO2 charge was used to reach a final kappa number of ∼6 (with no residual ClO2 at the end of the stage); the remaining ClO2 charge, predetermined to achieve 90% brightness at the end of the bleaching sequence, was distributed between stages D1 and D2 in proportions of ∼75% and ∼25%. ClO2 consumption was expressed as weight percentage of active chlorine per dry pulp. ClO2 in solution was quantified by conventional iodometric titration. The bleaching conditions were as follows: for D0, 25 min, 50 °C, pH ≈ 3.0; for E1, 120 min, 70 °C, 2.1% NaOH; for D1, 240 min, 70 °C, pH ≈ 3.5; for E2, 120 min, 70 °C, 0.6% NaOH; for D2, 240 min, 70 °C, pH ≈

3.5. All stages were carried out at 10% consistency [kg(dry pulp)/kg(suspension)]. Chemical Analysis of Wood and Unbleached Pulps. Woods were extracted with ethanol/toluene and pulps with dichloromethane. Neutral monosaccharides were quantified by GC as alditol acetates as described elsewhere.10 Errors associated with monosaccharides analysis were in the range of 5-10%. The klason lignin content of woods and pulps and kappa number and brightness of pulps were determined by standard TAPPI methods.11 Isolation and Characterization of Cellulose. Wood and pulp celluloses were isolated by the Ku¨rschner-Hoffer methodology, followed by treatment with diluted peracetic acid (5%) for 3-5 min at 80 °C, to obtain less than 1% of residual lignin.12 Celluloses were analyzed by X-ray diffraction as textured samples in pressed pellets of 1.3-cm diameter.13 The degree of crystallinity was corrected to the content of noncellulosic polysaccharides. Experimental error in X-ray analysis was lower than 5%. Isolation and Characterization of Xylans. Wood and pulp xylans were isolated from chlorite hollocelulose by extraction with aqueous KOH or from peracetic acid holocellulose by soft extraction with DMSO.12,14 The yields of xylan extraction with aqueous KOH were in the range of 50-65% (woods and pulps); with DMSO, the yields were 50-60% for wood samples and 20-25% for pulps. The KOH-isolated xylans were subjected to linkage (methylation) analysis.15 Errors associated with this analysis were in the range 5-10%. The KOH- and DMSO-isolated xylans were subjected to 1H and 13C NMR spectroscopy and GPC analysis.12,14 Isolation and Characterization of Lignins. Wood and pulp lignins were isolated by mild acidolysis.16 Extraction yields ranged from 50% to 65% in woods and from 40% to 70% in pulps. The determination of methoxyl groups and analysis by potassium permanganate oxidation (PO), 1H NMR and 13C NMR spectroscopy, and GPC were performed as described elsewere.16-18 Results and Discussion Wood Chemical Composition. The five hardwoods investigated showed significantly different chemical compositions (Table 1). Lignin contents (without correction for polyphenolics content) were in the range of

Ind. Eng. Chem. Res., Vol. 44, No. 26, 2005 9779 Table 2. Kraft Pulping Conditions,a Pulp Yield, and ClO2 Consumption during Bleaching by a Chlorine Dioxide-Based Sequence (DEDED)

a

wood species

active alkali, % Na2O/wood

unbleached pulp kappa number

unbleached pulp yield, %/wood

ClO2 consumption, %/ pulp

E. globulus E. urograndis E. grandis B. pendula A. mangium

16 20 19 18 24

18.9 18.4 16.1 18.5 16.0

55.6 49.6 50.6 49.8 51.1

4.4 5.3 5.4 7.2 7.4

Kappa number 16-19.

Figure 1. Percentages of dissolved glucose and xylose (as anhydrous monosaccharides) in kraft pulping.

22-28%, with the lowest values being found in E. globulus and B. pendula woods. The monosaccharide analysis showed relevant differences in the relative abundance of the main polysaccharides. Glucuronoxylans, as suggested from the relative abundance of xylose, were, as expected, the main wood hemicelluloses. The xylan content of B. pendula was about twice that of the other hardwoods. The glucose content (taking into consideration the presence of minor amounts of glucomannans in woods) suggests that cellulose was more abundant in Eucalyptus and Acacia than in B. pendula wood. Pulping and Bleaching Performance. Woods were kraft pulped to similar degrees of delignification (kappa number 16-19) by adjusting the charge of alkaline chemicals, and pulps were then bleached to 90% brightness by adjusting the chlorine dioxide charge. Representative results of the various pulping and bleaching experiments carried out are summarized in Table 2. E. globulus wood was the easiest to delignify and to bleach, whereas A. mangium wood required the highest amounts of pulping and bleaching chemicals. Although the relative content of lignin in wood might contribute to the different pulping performances of Eucalyptus species, the differences observed between E. globulus and B. pendula or between E. urograndis and A. mangium are clearly related to other factors, as these woods contained similar lignin contents (Tables 1 and 2). Additionally, no correlation could be established between the lignin content in unbleached pulps and the amount of ClO2 required to fully bleach the kraft pulp (Tables 1 and 2). The pulping yields (weight of unbleached pulp/weight of initial wood) were significantly different between the different wood species, ranging from about 50% in the cases of E. urograndis and B. pendula to 56% in E. globulus (Table 2), suggesting different patterns of wood polysaccharide degradation/retention. Table 1 and Figure 1 show, respectively, the unbleached pulp composition and the amounts of dissolved glucose and xylose (wood basis) during the kraft pulping. The dissolved glucose varied between 8.4 and 11.9% (wood basis) in

the case of Eucalyptus and A. mangium woods, whereas with B. pendula, it represented 5.7% of the wood weight, suggesting higher cellulose degradation in the former species. However, recent data obtained for E. globulus has indicated the dissolution of wood starch during the pulping.19,20 Hence, the extent of cellulose removal should be lower than that estimated on the basis of glucose dissolution. The dissolved xylose represented 3.6-5.8% (wood basis) in the cases Eucalyptus and A. mangium woods, representing relative estimated xylan losses of 25% in E. globulus, 40% in E. urograndis, 47% in E. grandis, and 46% in A. mangium. In B. pendula, the amount of dissolved xylose was about twice that in the other woods (11.2%, wood basis), representing about 47% of initial xylan. Although the different extents of xylan removal in the case of Eucalyptus species can be explained by the different alkaline charges in the pulping, when all five woods were considered, no general correlation could be established between the extents of cellulose and xylan removal and the alkalinity used in the pulping, suggesting that structural features play a key role in the stability and retention of these polysaccharides during the pulping process. Aiming to explain the different wood pulping and bleaching performances and polysaccharide retention patterns, the biopolymers from the five woods and unbleached kraft pulps were extensively analyzed. The most relevant results are now presented and discussed. Cellulose. Native cellulose in wood is organized in a fibrilar assembly, where domains of highly organized molecules (crystallites) alternate with domains of less ordered/amorphous cellulose chains.1,2 The chemical reactivity of cellulose can be affected by its supramolecular structure, and it is generally accepted that amorphous regions are more easily accessed by pulping and bleaching chemicals than the crystalline domains in fibers. X-ray diffraction analysis allowed information on the supramolecular structure of wood and unbleached pulp celluloses to be obtained. The unit cell dimensions of crystalline domains, according to the Meyer-MarkMisch model21 (Figure 2), are quite similar for all wood celluloses, as expected for cellulose I polymorph (Table 3). After the pulping, the distortion of the monoclinic cell (γ) increases, which is compatible with a partial conversion of cellulose I into the cellulose II polymorph in the strong alkaline pulping solution. The degree of crystallinity of wood celluloses ranged from 63% to 68%, increasing to 71-74% after the pulping (Figure 3). Such an increase can be attributed to the dissolution/ degradation of part of the amorphous cellulose and to the crystallization of paracrystalline cellulose during the partial cocrystallization of crystallites in the alkaline pulping medium.13,22 The latter explanation is supported by the observed increase in crystallite width (d002) from 4.4-4.7 nm in wood to 5.4-5.9 nm in unbleached pulp (Table 3).

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Figure 2. Schematic representation of the unit cell of cellulose I (wood native cellulose) according to the Meyer-Mark-Misch model (adapted from Kra¨ssig21).

Figure 3. Degree of crystallinity in wood and pulp celluloses, assessed by X-ray diffraction analysis.

No clear correlation could be established between the degree of crystallinity or crystallite width and the relative stability of cellulose (as estimated by monosaccharide analysis of woods and pulps) in the kraft pulping process. The presence of starch19,20 and other glucans in woods, even in minor amounts, hinders the correct estimation of cellulose in woods, thus complicating attempts to relate cellulose structure to its retention in fibers during the pulping process. This point merits further investigation. Xylans. Hardwood xylans are typically composed of a linear chain made up of (1f4)-linked β-D-xylopyranosyl units partially O-2-substituted with 4-O-methylR-D-glucuronosyl units.1,2 Part of the xylopyranosyl units are randomly acetylated at O-2 and O-3 positions.1,2 The xylan structure of the five hardwoods was investigated by methylation/linkage analysis, NMR spectroscopy, and GPC. The analysis of methylation products (results not presented) showed the previously mentioned typical features of hardwood xylans in all samples, including the presence of the reducing terminal structural fragment [f3)-R-L-Rhap-(1f2)-R-D-GalpA-(1f4)D-Xylp]. The most relevant detected structural difference was the presence, in Eucalyptus species xylans, of 4-O-methyl-R-D-glucuronic acid ([f2)-GlcpA-(1f] or

sMeGlcA) moieties substituted at O-2 by galactosyl or glucosyl units, in addition to terminal MeGlcA ([GlcpA(1f] or tMeGlcA), linked to the xylan backbone, as confirmed by NMR analysis. Such structural specificity (not present in B. pendula and A. mangium wood xylans) was previously detected in E. globulus xylan.12,14,23 On the basis of 1H and 13C NMR results, it has been proposed that this moiety might constitute points of linkage between xylan and other cell wall polysaccharides, namely, rhamnoarabinogalactans and glucans.12 The contribution of the MeGlcA acid substituents to the overall xylan retention during pulping is hard to estimate, as opposite effects might occur. On one hand, the substitution of O-2 in xylopyranosyl units with MeGlcA might prevent the isomerization of the reducing xylopyranose to the corresponding 2-ulose derivative, thus retarding the peeling reaction; if MeGlcA groups are O-2-substituted ([f2)-GlcpA-(1f], sMeGlcA), as in the case of Eucalyptus, the linkage to other cell wall polysaccharides should additionally improve the stability and retention of xylan in fibers. However, on the other hand, a high degree of branching (particularly with tMeGlcA) might have an opposite effect in xylan retention because of the higher solubility of highly ramified xylans1, favoring their direct dissolution in the pulping liquor. The degree of branching (number of tMeGlcA and sMeGlcA substituents/100 xylose units) ranges from 7% in B. pendula to 12% in E. urograndis. In Eucalyptus, the sMeGlcA units represent 25-35% of the uronic acid substituents (Figure 4). After pulping, the degree of branching decreases to 1-4%. A significant fraction of sMeGlcA units, particularly in the case of E. globulus xylan, resisted the kraft treatment, supporting our explanation of the contribution of these groups to the higher stability of xylans in the fibers of Eucalyptus species when compared to B. pendula xylans. However, A. mangium xylan, with a general structure similar to that of B. pendula xylan and a higher degree of substitution, showed retention on the same order as E. grandis, suggesting that structural features other than the amount and type of uronic acid substituents should contribute to the stability of xylans in fibers during the alkaline pulping. During pulping, some of the MeGlcA groups are converted into hexenuronic acid (HexA) by elimination of methanol. These unsaturated moieties contribute to the consumption of bleaching chemicals such as chlorine dioxide, thus affecting the bleaching performance of the unbleached pulps.24 The relative contribution of tMeGlcA and sMeGlcA to the formation HexA and its impact on the bleaching performance of the different hardwood kraft pulps was not assessed in this work but will be the focus of our attention in future investigations.

Table 3. Unit Cell Dimensions and Crystallite Widths (d002) of Woods and Unbleached Kraft Pulp Celluloses, Determined by X-ray Diffraction Analysisa woods

E. globulus E. urograndis E. grandis B. pendula A. mangium a

unbleached pulps

a, Å

b, Å

c, Å

γ, deg

d002, nm

a, Å

b, Å

c, Å

γ, deg

d002, nm

8.1 8.0 8.0 8.0 8.1

7.9 7.9 7.9 7.9 8.0

10.3 10.3 10.3 10.3 10.3

97 96 96 96 98

4.62 4.60 4.73 4.43 4.74

8.2 8.0 8.2 8.0 8.1

8.1 8.0 8.1 8.1 8.0

10.3 10.3 10.3 10.3 10.3

102 97 102 101 100

5.59 5.67 5.93 5.37 5.94

See Figure 2 for assignments.

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Figure 4. Number of terminal 4-O-methylglucuronic acid (tMeGlcA) and O-2 substituted 4-O-methylglucuronic acid (sMeGlcA) units per 100 xylose units in (top) wood and (bottom) pulp xylans. Table 4. Weight-Average Molecular Weight (Mw, (1 kDa) of Woods and Kraft Pulp Xylansa Extracted with DMSO Mw, kDa

a

wood species

wood

kraft pulp

E. globulus E. urograndis E. grandis B. pendula A. mangium

31 31 25 24 28

16 14 13 14 13

Determined by GPC.

Gel permeation chromatography allowed an estimation of the average molecular weight (Mw) of xylans in woods and unbleached pulps (Table 4). The xylans from E. globulus, E. urograndis, and A. mangium had the highest Mw (28-31 kDa), whereas E. grandis and B. pendula had Mw values of 25 and 24 kDa, respectively. The higher Mw’s of the former xylans favor their retention in fibers during pulping, contributing to the explanation of the different retentions observed between A. mangium and E. grandis, as well has the highest removal of xylan observed in B. pendula. After the pulping, the xylan Mw’s decrease to about half that in the initial wood, showing the impact of peeling and alkaline hydrolysis reactions during the pulping. In addition to the structural features of xylans, the different alkaline charges used in the pulping certainly also help explain the different extents of xylan removal, specifically among the Eucalyptus species, and the highest retention of these polysaccharides in the case of E. globulus. Lignins. Hardwood lignins are complex macromolecules composed of dehydropolymerized units derived from syringylpropane (S), guaiacylpropane (G), and p-hydrophenylpropane.1,2 The relative proportions of S, G, and H units and the nature and relative abundance of linkages between them are highly variable, influenc-

Figure 5. Molar percentages ((2%) of syringyl (S), guaiacyl (G), and p-hydroxiphenyl (H) units (determined by 13C NMR spectroscopy) in (top) wood and (bottom) pulp lignins.

ing the reactivity of lignin during pulping and bleaching processes. 1,2 The five wood and unbleached pulp lignins were extensively characterized by wet-chemistry functional analysis, permanganate oxidation, 1H and 13C NMR spectroscopy, and GPC. The H/G/S proportion of wood and unbleached pulp residual lignins, obtained by 13C NMR analysis, are summarized in Figure 5. More than 80% of units of E. globulus wood lignin are syringylpropane-type, a figure clearly above the average found in typical hardwoods. On the opposite side, A. mangium showed approximately equal amounts of S and G units. The other Eucalyptus species and B. pendula lignins showed S proportions between 65% and 70%. H units represent less than 3% of the wood lignin monomers. After pulping, the relative content of S units decreases. However, in the case of Eucalyptus species, they remain as the more abundant units, whereas for the other wood species, guaiacyl-type units become the predominant units in the structure of residual lignin. Syringyl-type units, because of the presence of the two methoxyl groups in positions 3 and 5 of the aromatic nuclei, are known to be more reactive than their guaiacyl counterparts,1,2 favoring lignin degradation and removal from wood fibers. Additionally, the extent of lignin recondensation occurring in the alkaline or acidic reaction media is hindered by the presence of the additional methoxyl group in the S units, when compared to G units. When the S/G ratios of wood and kraft pulp lignins were plotted against the chemical charges used in the pulping and bleaching, respectively (Figure 6), an interesting tendency was observed, confirming the effect of the syringyl units in the pulping and bleaching ability. E. globulus, having the highest S/G ratio, showed the best pulping and bleaching performance. The alkyl aryl ether Cβ-O-C4 (β-O-4) structure is the dominant linkage between lignin monomers. The cleav-

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Figure 8. Relationship between the relative abundance of β-O-4 structures (per aromatic ring) in pulp lignins and ClO2 consumption in the bleaching. Table 5. Phenolic Group Content ((0.02) in Wood and Kraft Pulp Lignins, Determined by 1H NMR Spectroscopy phenolic groups/ppu

Figure 6. Relationships (top) between wood lignin syringyl/ guaiacyl (S/G) ratios and active alkali used in kraft pulping and (bottom) between pulp lignins syringyl/guaiacyl ratios and ClO2 consumption in the bleaching.

Figure 7. Frequency of β-O-4 ((0.02) structures per aromatic ring (determined by 13C NMR spectroscopy) in wood and pulp lignins.

age of this linkage is crucial for the depolymerization and removal of lignin from the fiber cell walls. The 13C NMR analysis of isolated wood lignins indicated about 50-55 β-O-4 structures per 100 aromatic units in Eucalyptus and B. pendula and 43 in A. mangium (Figure 7). Although the lower β-O-4 content of A. mangium is consistent with the higher alkaline charge required to pulp this wood, this lignin structural feature does not explain the different pulping performances of Eucalyptus and B. pendula woods. After pulping, the lignin β-O-4 content is reduced to 20-39/100 aromatic units, the lowest decrease being observed in the case of E. globulus, certainly because of the lower alkaline charge used in the pulping of this species. A plot of the number of β-O-4 structures in residual lignin against the consumption of chlorine dioxide during the bleaching (Figure 8) clearly shows that the ease of bleaching increases with the content of alkyl aryl ether linkages, suggesting the cleavage of β-O-4 linkages as a crucial step in the degradation and removal of residual lignin

wood species

wood

kraft pulp

E. globulus E. urograndis E. grandis B. pendula A. mangium

0.28 0.30 0.29 0.26 0.30

0.42 0.41 0.52 0.54 0.20

during chlorine dioxide bleaching. The same general tendency was reported for ClO225 and hydrogen peroxidebased bleaching.26 The phenylpropane units in lignin might be involved in linkages other than the ether structures Cβ-O-C4 (β-O-4) and CR-O-C4 (R-O-4). Those include C-C or C-O-C linkages leading to the formation of, among others, β-5′, β-6′, 5-5′, and 4-O-5′ condensed structures, which are more resistant to cleavage in most pulping and bleaching reaction media than, for example, β-O-4 moieties.1,2 Lignin permanganate oxidation followed by GC-MS analysis of the aromatic carboxylic acids obtained yields information on the structure and, particularly, on the type and abundance of condensed units. The most relevant oxidation products (as methyl esters) of wood and pulp lignins are shown in Figure 9. Products 1-3 originate from noncondensed units, whereas products 4-9 arise from different condensed structures. The ratio “noncondensed units/condensed units” (nC/C) is very different in the five investigated woods, ranging from 2.5 in A. mangiun to 4.3 in E. globulus (Figure 10). The most prominent condensed structures in Eucalyptus lignins are of type 4-O-5′ (product 9), whereas in B. pendula and A. mangium lignins, biphenyl-type units (product 7) predominate. During kraft pulping, the nC/C ratio decreases, indicating the preferential removal of noncondensed units and the consequent enrichment of condensed units in residual lignins. This nC/C ratio decrease is particularly significant in A. mangium pulp. The key role played by the extent of lignin condensation in the pulping and bleaching ability is demonstrated in Figure 11. A clear correlation is observed between the proportion of noncondensed units in lignin and the ease of wood pulping and pulp bleaching. Lignin units bearing a free phenolic group are known to be more reactive than etherified units in most pulping and bleaching processes, particularly in the case of chlorine dioxide bleaching.27 The 1H NMR analysis of

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Figure 9. Prominent products (as methyl esthers) of permanganate oxidation of lignins. Table 6. Weight-Average Molecular Mass (Mw, (50 Da) of Woods and Kraft Pulp Lignins, Determined by GPC Mw, Da

Figure 10. Relative proportion ((5%) of noncondensed/condensed (nC/C) structures in wood and pulp lignins.

wood species

wood

kraft pulp

E. globulus E. urograndis E. grandis B. pendula A. mangium

2360 2290 2160 2230 2230

1820 1400 1280 1360 1400

nolic groups in the A. mangium pulp is consistent with its higher ClO2 consumption during bleaching, no general correlation could be established between the ease of pulping and bleaching and the relative abundance of this functional group, in agreement with previous results.24 Hence, despite the higher reactivity of the phenolic structures, the pulping and bleaching performances of the different hardwoods are determined by other lignin structural features such as the S/G ratio and the degree of condensation, rather than by the phenolic unit content. The molecular weight of lignins isolated from wood is in the range of 2100-2400 Da, decreasing to 13001800 Da after pulping (Table 6). Although it could be expected that the sizes of lignin macromolecules influence their removal from cell walls, no relationship was observed between wood pulping and pulp bleaching performance and the isolated lignin molecular weight. Conclusions

Figure 11. Relationships (top) between ratio of noncondensed/ condensed (nC/C) structures in wood lignins and active alkali used in kraft pulping and (bottom) between nC/C ratio in pulp lignins and ClO2 consumption in the bleaching.

wood and kraft pulp lignins showed phenolic group contents (expressed per phenylpropane unit, ppu) in the ranges 0.26-0.30 and 0.20-0.54 ppu, respectively (Table 5). The general increase in phenolic group content after pulping is consistent with the cleavage of some β-O-4 linkages. However, although the lower content of phe-

The investigated hardwoods showed distinct kraft pulping and bleaching performances, as demonstrated by the notable differences in pulping yields and profiles of polysaccharides degraded and dissolved, as well as in the amounts of alkali and ClO2 required to achieve similar degrees of delignification and brightness, respectively. In general, Eucalyptus woods and kraft pulps are easier to delignify and to bleach, respectively. E. globulus showed the best pulping and bleaching performance. Although the dissimilar relative abundances of cellulose, xylans, and lignin in the woods could contribute to the differences observed, the pulping and bleaching performances were essentially determined by structural features of the wood and pulp biopolymers. The variation in the retention of xylan in fiber during hardwood kraft pulping was associated, at least partially, with differences in the abundance and structure of uronic acid side-chain moieties and xylan molecular weight. The higher retention of xylans in Eucalyptus species, when compared to B. pendula, was explained by the presence of [f2)-GlcpA-(1f] moieties in the xylan backbone (not present in B. pendula), which constitute points of linkage between xylan and other cell

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wall polysaccharides. The ease of delignification of E. globulus wood and, consequently, the lower alkalinity used to achieve the same delignification degree as for the other species also contributes to the higher retention of xylan during the pulping of this Eucalyptus species. No relationship could be established between the supramolecular structure of wood cellulose and its distinct retention in the hardwood fibers. Such an apparent lack of correlation was most likely attributable to the difficulty in ascertaining the relative contribution of cellulose and glucans to the glucose dissolved during wood kraft pulping. The relative ease of wood pulping and pulp bleaching was shown to be determined by the structural features of lignins rather than by their relative content in woods and pulps. The ease of pulping and bleaching was related to the ratio of syringyl/guaiacyl (S/G) units, as well as the proportion of noncondensed/condensed (nC/ C) units. No clear relationship could be established between the content of β-O-4 structures in wood lignin and the pulping performance; however, the β-O-4 content in residual lignins was shown to be determinant in the bleaching ability of kraft pulps. No relationship was observed between pulping and bleaching performance and the lignin phenolic group content and lignin molecular weight. Acknowledgment Thanks are due to FCT and ESF, within the European Community Support Framework III, for the financial support of Project POCTI/46124/EQU/2002 and for the awarding of a Ph.D. grant to P.C.P. (SFRH/BD/ 1096/2000); to the Portuguese Forest and Paper Research Institute RAIZ for supplying the Eucalyptus wood samples and for the pulping and bleaching experiments; and to Stora Enso and Kvaerner Pulping AB for supplying the Betula pendula and Acacia mangium woods, respectively. Literature Cited (1) Sjo¨stro¨m, E. Wood Chemistry, Fundamentals and Applications; Academic Press: New York, 1981. (2) Fengel, D.; Wegener, G. Wood, Chemistry, Ultrastructure, Reactions; W. de Gruyter: New York, 1984. (3) Garland, C. P.; James, F. C.; Nelson, P. J.; Wallis, A. F. A. Chemical analysis and oxidative studies of Eucalyptus regnans, E. diversicolor, E. marginata, and E. tetrodonta wood samples. Appita J. 1986, 39, 361. (4) Bland, D. E. The composition and analysis of Eucalyptus wood. Appita J. 1985, 38, 291. (5) Hillman, D. C. Single-species pulping: The world’s preferred market pulps. Solutions 2002, (Nov), 27. (6) Coleman, M. J. Tropical forestry: Acacia plantations in Indonesia. Tappi J. 1998, 81, 43. (7) Gullichsen, J. Chemical Pulping; Papermaking Science and Technology Series; Gullichsen, J., Fogelholm, C. J., Eds.; Fapet: Helsinki, Finland, 1999; Vol. 6A. (8) Clayton, D.; Einspahr, D.; Easty, D.; Lonsky, W.; Malcolm, E.; McDonough, T.; Shroeder, L.; Thompson, N. Pulp and Paper Manufacture; Alkaline Pulping Series; Grace, T., Leopold, B., Malcolm, E., Kocurek, M., Eds.; TAPPI/CPPA: Atlanta, GA, 1983; Vol. 5. (9) Dence, C. W.; Reeve, D. W.; Eds. Pulp Bleaching. Principles and Practices; TAPPI Press: Atlanta, GA, 1996.

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Received for review June 27, 2005 Revised manuscript received September 6, 2005 Accepted September 29, 2005 IE050760O