8542
Ind. Eng. Chem. Res. 2008, 47, 8542–8549
Influence of Lignin Structural Features on Eucalyptus globulus Kraft Pulping Anderson Guerra* and Juan Pedro Elissetche Geno´mica Forestal SA, Concepcio´n, Chile
Marcela Norambuena, Juanita Freer, Sofı´a Valenzuela, and Jaime Rodrı´guez Centro de Biotecnologı´a, UniVersidad de Concepcio´n, Concepcio´n, Chile
Claudio Balocchi Bioforest SA, Coronel, Chile
Clones of Eucalyptus globulus Labill. (5- to 7-year-old), from a common geographic area, were evaluated for chemical pulping easiness. Significant variations were observed in the pulp yield and specific wood consumption to produce pulps with similar kappa numbers, as well as in the strength properties of the resulting kraft pulps. Comprehensive lignin analyses were undertaken in an attempt to rationalize the observed differences in these clones’ pulping performance. While lignin content did not correlate with pulp yield, the data reported here provides evidence of the influence of lignin features on the pulping response of different eucalyptus clones. Significant correlations were observed between pulp yield and specific wood consumption and the content of syringyl-type arylglycerol-β-aryl structures (β-O-4 linkages). Furthermore, eucalyptus woods with a greater content of uncondensed β-O-4 linkages were found to require more PFI revolutions to obtain pulps with a given drainability. In contrast, no relationship between pulping efficiency and the other lignin structural features evaluated was apparent, including syringyl/guaiacyl ratio (S/G), total aliphatic and phenolic hydroxyl groups, syringyl and guaiacyl units bearing free phenolic hydroxyls, and the erythro-to-threo ratio of β-O-4 structures. These findings support the use of the content of syringyl-type arylglycerol-β-aryl structures as a selection parameter in clonal breeding programs for pulpwood production. Introduction Remarkable efforts have been dedicated to select clones or to develop genetically improved trees with emphasis on increasing pulp yield and/or reducing pulping chemicals consumption.1-3 When pulp yield increase is the main goal, the focus has been on reducing lignin content to reduce pulping severity, which minimizes cellulose degradation and hemicelluloses solubilization and consequently improves pulpwood2 efficiency. However, different groups have recently indicated that in addition to lignin content, lignin chemical reactivity (which is associated with lignin structure) is also a critical barrier to pulp production.1,3-5 In this way, pulping and bleaching performances are expected to be highly dependent on the abundance, structure, and relative reactivity/stability of the lignin biopolymer.4,6 Despite evidence that lignin reactivity is a major obstacle to pulpwood efficiency, in eucalyptus breeding programs few efforts have been conducted to included lignin structure as a parameter to select the best clones (giving the highest pulp yield or the lowest specific wood consumption). When lignin structural features are considered, the emphasis has been on determining the effect of the syringyl (S; 4-hydroxy-3,5dimethoxyphenyl) to guaiacyl (G; 4-hydroxy-3-methoxyphenyl) ratio on the pulping performance estimated as alkali consumption3,4 or pulp yield.1,7 Although the relative wood pulping and pulp bleaching easiness of different hardwood species is related to the S/G ratio,4 the selection of clones of the same species based on this parameter is still a matter of discussion. Actually, the low correlation levels observed between pulp yield and S/G ratio determined by Py-GC/MS1 or nitrobenzene oxidation8 * To whom correspondence should be addressed. Tel.: +5641 220 3850. Fax: +5641 2247517. E-mail:
[email protected].
argue against the utilization of such a ratio as a unique factor to predict the pulp yield of different eucalypt clones. The evaluation of the influence of other lignin structural features, besides the S/G ratio, on the pulping performance of clones of the same eucalyptus species is thus warranted. A primary problem in incorporating lignin traits as a selection parameter in eucalyptus breeding programs is the lack of knowledge on the specific chemistry of the macromolecular components of this genus of hardwood.9-11 Consequently, the effects of eucalyptus lignin structure and reactivity on its removal from the polysaccharide matrix during alkaline pulping are not completely understood.4,9,10 Recent reports dealing with pulping performance of hardwoods have shown that different species of eucalyptus and even clones of the same species may require different pulping conditions to attain desired delignification extent and pulp yield.4,7,12 However, progress toward understanding the reasons behind such different behaviors has been hindered by the lack of knowledge on the chemistry and structure of the eucalyptus wood biopolymers, i.e., lignin, cellulose, and hemicelluloses.4 In this study, the influence of lignin structural features on the pulping performance of wood from Eucalyptus globulus plantations was evaluated. E. globulus Labill. wood samples from clonal tests established in similar geographic locations were selected to cover a wide range of pulp yields upon kraft cooking. Lignins from the different wood samples were characterized by chemical methods and 31P NMR in an attempt to identify parameters to predict the pulping efficiency of clones of the same wood species growing under similar conditions. The
10.1021/ie800320d CCC: $40.75 2008 American Chemical Society Published on Web 10/23/2008
Ind. Eng. Chem. Res., Vol. 47, No. 22, 2008 8543
effects of eucalyptus lignin features on the strength properties of the resulting kraft pulps are also discussed. Experimental Section Woods and Pulping. Wood chips (average size 30 × 20 × 0.5 mm) prepared from 15 E. globulus trees (5-7 years old) growing in a clonal trial established in southern Chile (37°21′S; 73°29′W) were submitted to kraft pulping. Each pulping experiment was performed in batch digesters (MK Systems, USA) equipped with an external electric heating system and temperature control. The conditions were as follows: liquor-towood ratio (L/kg), 4:1; sulfidity, 30%; active alkali, 16% (measured as Na2O); initial temperature, 80 °C. The reactor heating time was 120 min, and the cookings were carried out at 165 °C for different periods of time (17-30 min) to reach a kappa number of 15 ( 1. After pulping, the residual material was filtered, washed with flowing water for maximum black liquor removal, and disintegrated with 2 L of water in a TAPPI standard defibrillator. The pulp was classified in a 0.15 mm slot screen (Somerville screener), and the pulp yield was calculated after measuring the moisture content on the screened pulp. Kappa number was determined in all screened pulps according to a standard protocol (TAPPI T 236). Five replicates were performed for each clone in the conditions adjusted to attain a kappa number of 15 ( 0.5. The maximum relative standard deviation observed was 0.5%. Specific Wood Consumption. Specific wood consumption was calculated from eq 1: specific wood consumption ) 1000/(WBD*(PYK15/100)) (1) where WBD is the wood basic density (measured according to method described by Miranda and Pereira)13 and PY is the pulp yield (Kappa 15 ( 0.5). Five replicates were performed for each clone in the conditions adjusted to attain the kappa number of 15 ( 0.5. The maximum relative standard deviation observed was 2.9%. Strength Properties of Kraft Pulps. Screened kraft pulps prepared from the 15 clones were refined in a PFI mill (TAPPI T 248) from 350 to 2000 revolutions. A Schopper-Riegler test was performed to measure the drainage rate of refined pulps (ISO 5267-1). Handsheets prepared from these pulps (TAPPI T 205) were analyzed for their tensile (TAPPI T 494), tear (TAPPI T 414), and burst (TAPPI T 403) indexes. Opacity of the handsheets was also measured according to standard procedures (TAPPI T 425 om-01). Chemical Analysis of Wood Chips. Wood chips were milled to pass through a 0.5 mm screen. About 1.5 g of milled samples was extracted with ethanol-toluene (1:2, v/v) for 6 h followed by extraction with 95% ethanol for 6 h (TAPPI T 204 cm-97). The lignin content of the extracted wood samples was determined by acid hydrolysis with 72% sulfuric acid as described elsewhere.14 Each sample was analyzed in triplicates. Thioacidolysis. Thioacidolysis was performed on 20 mg of extracted milled woods in 10 mL of reagent according to a published method.15 The reagent was prepared by introducing 2.5 mL of BF3 etherate (Aldrich) and 10 mL of ethanethiol EtSH (Aldrich) into a 100 mL flask and adjusting the final volume to 100 mL with dioxane. The reagent and 1 mL of a solution of GC internal standard (tetracosane in CH2Cl2, 0.6 mg/mL) were added to the lignin sample in a glass tube closed with a Teflon-lined screw cap. Thioacidolysis was performed at 100 °C (oil bath) for 4 h. The cooled reaction mixture was diluted
with 30 mL of water and the pH adjusted between 3.0 - 4.0 (aqueous 0.4 M NaHCO3) before extraction with 3 × 30 mL CH2Cl2. The combined organic extracts were dried over Na2SO4, and the solvent was evaporated under reduced pressure at 40 °C. The final residue was dissolved in approximately 1 mL of CH2Cl2 and analyzed by GC after silylation. The response factor was the same as given by Rolando et al.15 The analysis results had a repeatability of (3%. Isolation and Characterization of Lignins. Lignins were isolated from 9 of the 15 eucalypt samples by acidolysis.16 To avoid extensive lignin degradation, the modifications introduced by Evtuguin et al.9 were applied. Quantitative 31P NMR spectra of all lignin preparations were obtained using published procedures.17,18 Approximately, 40 mg of dry lignin were placed into a sample vial, dissolved in 400 µL of pyridine and deuterated chloroform (1.6:1, v/v), and left at room temperature overnight with continuous stirring. N-hydroxynaphtalimide (200 µL, 11.4 mg mL-1) and chromium(III) acetylacetonate (50 µL, 11.4 mg mL-1) were used as internal standard and relaxation reagent, respectively. Finally, 100 µL of 2-chloro-1,3,2-dioxaphospholane or 2-chloro-4,4,5,5-tetramethyl-1,3,2-dioxaphospholane were added, and the mixture was transferred into a 5-mm OD NMR tube. When reagent II was used, cholesterol (100 µL, 40 mg mL-1) was added into the NMR tubes as a second internal standard. The spectra were acquired using a Bruker 250 MHz spectrometer. Pulp Yield, Specific Wood Consumption, and Strength Properties of Kraft Pulps. The eucalyptus woods selected for this study corresponded to 15 clones of E. globulus Labill. grown in a clonal test in Arauco province, Bio-Bio Region, Chile. The woods (5-7 years old) were kraft pulped to similar degrees of delignification (Kappa 15 ( 0.5) by adjusting the H-factor. Results and Discussion The resulting pulps were refined in a PFI beater to obtaining a target beating degree. Results of the various pulping and beating experiments as well as the strength properties of the resulting kraft pulps obtained at the same beating degrees are summarized in Table 1. The pulping yields (weight of classified unbleached pulp/weight of initial wood on dry base) were significantly different among the clones evaluated, ranging from 48.0 (sample 14) to 56.5% (sample 1). The pulp yield variations obtained are within the range reported for E. globulus of similar ages. For example, pulp yields of 48.0, 52.0, and 54.4% were reported for 6-, 8-, and 10-year-old eucalyptus trees,7,19 respectively. It is important to note that the aforementioned 8.5 percentage point difference (from 48.0% to 56.5%) in pulp yield could lead to a substancial economic and environmental benefit to any pulp mill. The clones also showed a considerable spread in the specific wood consumption (Table 1), which reflects variation in both pulp yield and wood basic density. A small reduction in the specific wood consumption may represent a significant improvement in pulp production per unit of planted area. The selection of the best clones (lowest specific wood consumption) offers a significant opportunity for maximizing fiber production. For example, changing a fast-rotation plantation based on clone 14 (5.0 m3 of wood/t of pulp) to one based on clone 5 (3.6 m3 of wood/t of pulp) would represent a significant reduction in the amount of wood required to produce each ton of unbleached pulp. Besides pulp yield and specific wood consumption, the strength properties of the resulting kraft pulps were also found to vary among the 15 clones evaluated (Table 1). The number
8544 Ind. Eng. Chem. Res., Vol. 47, No. 22, 2008 Table 1. Pulp Yield,a Specific Wood Consumption,a and Strength Propertiesb of Kraft Pulps Obtained from Different Clones of Eucalyptus globulus Labill. Wood clone
pulp yield (%)
specific wood consumption (m3/t of pulp)
beating energy (rev.)
tensile index (Nm/g)
tear index (mN · m2/g)
burst index (kPa · m2/g)
opacity (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
56.5 53.6 53.6 52.3 51.8 52.3 49.8 51.8 51.5 49.2 50.4 50.4 48.4 48.0 48.8
3.7 3.8 3.8 4.1 3.6 4.5 4.6 3.7 4.3 4.6 4.6 4.2 4.7 5.0 4.6
1700 1000 1100 1430 1220 610 780 1500 1000 630 820 340 650 620 660
97.0 98.7 66.0 96.0 94.7 84.0 100.4 89.1 95.9 93.0 95.0 74.0 94.1 101.5 100.2
6.6 6.7 6.0 8.7 8.4 6.9 7.2 7.4 7.4 6.2 7.4 6.6 6.4 6.3 6.8
4.9 6.4 7.7 6.1 6.4 5.1 6.4 6.4 6.3 5.8 6.3 4.3 6.2 6.5 6.4
95.0 95.8 95.0 97.5 96.0 98.8 97.8 97.9 96.7 98.0 96.2 98.0 97.2 98.9 97.0
a
Kappa 15 ( 0.5. b Pulps with similar beating degree °SR (Shopper Riegler degree).
Figure 1. Acid-insoluble (black filled bars), acid-soluble (gray bars), and total lignin content (open bars) of the 15 eucalypt clones from Bio-Bio Region, Chile. Error bars represent the standard deviation of three replicates.
of revolutions in a PFI beater required to reach a target fibrillation degree ranged from 340 (clone 12) to 1700 revolutions (clone 1). While the fibrillation easiness in clone 12 was obtained at the expenses of some strength properties, clones 7, 14, and 15 required less PFI revolutions than the average (949 rev.) without reducing tensile, tear, and burst indexes. Conversely, the evaluated clones did not show a significant variation in opacity, which ranged from 95.0 to 98.9%. Tensile, tear, and burst indexes varied between 66.0-101.5 Nm/g, 6.0-8.7 mN · m2/g, and 4.3-7.7 KPa · m2/g, respectively. Overall, these results indicate that substantial, genetically diverse, eucalyptus resources exist in Chile, creating a significant opportunity for selecting tailored fiber. Lignin Content and Structural Features. To elucidate whether the differences in pulp yield, specific wood consumption, and strength properties of the resulting kraft pulps are related to lignin content and/or structural features, the wood samples were analyzed by acid hydrolysis and thioacidolysis. Furthermore, lignin samples isolated from selected clones were characterized by quantitative 31P NMR. The eucalyptus clones showed a considerable variation in the total lignin contents (Figure 1), ranging from 24.0% (sample 1) to 28.6% (sample 11). The differences were due mainly to the acid-insoluble lignin portion rather than the acid-soluble fraction. As shown in Figure 1, the contents of acid-insoluble lignin differed by as much as 4.6% (from 19.4% in sample 12 to 24% in sample 11), while the acid-soluble lignin contents ranged from 3.7% to 4.9% for the entire sample group. In general, the average lignin content observed for the examined clones did not substantially differ from those of E. globulus
wood at the normal harvesting age of 10-13 years for pulpwood production.7,13,20 Comprehensive studies of the mechanisms that govern lignin depolymerization during kraft pulping have shown that ether bonds, such as R- and β-aryl, are primarily cleaved, whereas the carbon-carbon resistant linkages accumulate in the residual lignin.21 Therefore, the relative frequency of β-aryl-linked units within the lignin macromolecules is an important structural feature influencing lignin depolymerization and its consequent solubilization in the cooking liquor.5 Among the different degradative methods designed for selective cleavage and quantification of β-aryl ether structures in lignin, thioacidolysis is one of the most effective due to its sensitivity and demonstrated lack of artifacts.22 In this technique, the thiol group displaces the R-hydroxyl or R-ether group and the β-aryl ether to form an episulfide-type intermediate that is further converted in either syringyl or guaiacyl trithioethyl phenylpropane stereoisomers (erythro and threo forms). The quantification of these diagnostic monomers estimates the total amount of uncondensed β-aryl structures and may be useful to rationalize the differences in the pulping efficiency of different wood samples.5,22 The examined eucalyptus clones showed a considerable spread in the monomer yields from thioacidolysis (Table 2), indicating that these clones have significantly different amounts of uncondensed β-aryl structures despite the fact that they are of the same species, have similar ages, and come from a common environment site. The highest and lowest values of 2330 and 1675 µmol of uncondensed β-aryl per gram of lignin obtained for samples 1 and 15, respectively, correspond to 49.4% and 35.5% uncondensed β-aryl structures in these wood samples, respectively, considering an average molecular weight for one phenylpropane unit (C9) in E. globulus of 212 g/mol.9 The lowest value is in agreement with 36.5% uncondensed β-aryl structures within E. globulus lignin determined by DFRC/31P NMR,10 a value clearly above the average found in other hardwoods.10 These high monomer yields from thioacidolysis (Table 2) support previous observations that eucalyptus lignins are more linear than lignins from softwoods23 and other typical hardwoods.4,9,10 The absence of degradation products derived from p-hydroxyphenyl type structures (H) in the thioacidolysis products from E. globulus (Table 2) is in agreement with the data reported by Evtuguin et al.,9 and it has been attributed to the low yields of H-type lignin structures in the thioacidolysis.15 Although the lower lignin content and the highest monomer yield from thioacidolysis were found in the same sample, clone 1 (Figure 1 and Table 2), no relationship was apparent between
Ind. Eng. Chem. Res., Vol. 47, No. 22, 2008 8545 a
Table 2. Main Lignin-Derived Monomers Obtained by Thioacidolysis of Extractive-Free Eucalyptus globulus Labill. Wood
a
sample
G units involved only in uncondensed β-O-aryl bonds (µmol/g of L)
S units involved only in uncondensed β-O-aryl bonds (µmol/g of L)
total uncondensed β-O-aryl bonds (µmol/g of L)
S/G ratio
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
480 475 435 295 410 380 410 395 340 295 415 350 330 310 295
1850 1770 1830 1620 1710 1600 1510 1530 1610 1525 1585 1560 1500 1550 1380
2330 2245 2265 1915 2120 1980 1920 1925 1950 1820 2000 1910 1830 1860 1675
3.85 3.73 4.20 5.49 4.17 4.21 3.68 3.87 4.73 5.17 3.81 4.46 4.54 5.00 4.68
The samples were analyzed in replicates, and the maximum error was less than 3%. L: total lignin.
Figure 2. Relationships between uncondensed β-O-4 structures and total lignin content (A), syringyl units (B), and S/G ratio (C) determined by thioacidolysis. L: total lignin.
total lignin contents and composition in terms of uncondensed β-aryl structures when the entire sample group was considered (Figure 2A). This contrasts with the linear relationship between the total amount of monomeric units released through cleavage of the alkyl aryl ether linkages (i.e., the uncondensed β-O-4 bonds) in lignin from Arabidopsis stems and the overall content of lignin determined by the acetyl bromide method reported by Patten et al.24 The observed difference may reside in variations of the chemical nature of the lignin macromolecules on a species-by-species basis.
The relative importance of each monomer from thioacidolysis was also measured to evaluate the ratio of S to G units involved only in β-O-4 bonds (Table 2). The molar percentage of uncondensed syringyl-type units in the lignin of the different eucalyptus clones varied from 78.6 (S/G ) 3.68) to 84.6% (S/G ) 5.49). Such high syringyl contents have been used to explain the E. globulus pulping and pulping bleaching easiness compared with other hardwoods.4 Furthermore, while a linear relationship is apparent between the syringyl units and uncondensed β-O-4 bonds (Figure 2B), no correlation could be established between the S/G ratio, and the monomers yield from thioacidolysis (Figure 2C). Such behavior has been explained on the basis of the widely accepted theory for the formation of β-O-4 structures in lignin.25,26 According to this theory, lignification involves coupling reactions between a monolignol and a growing lignin polymer or between two lignin oligomers.26 Due to the nature of the molecules involved, there are two possible pathways for coupling of a hydroxycinnamyl alcohol at its β-position with the guaiacyl phenolic endgroup in the growing lignin polymer (at its 4-O- or 5-positions) and only one for coupling with a syringyl one (at its 4-O-position).26 Therefore, it becomes clear that high syringyl lignins have a higher relative content of uncondensed β-O-4 bonds (there is essentially one coupling pathway and that is β-O-4 coupling).26 Conversely, carbon-carbon linkages (at the free C5 position) are more frequent between the guaiacyl units due to the two available pathways for monolignol coupling with a guaiacyl phenolic endgroup. However, because the coupling process is essentially combinatorial,26 the two pathways are possible, but not equally probable.26-28 Besides the nature of the reactants, the nature of the postcoupling reactions, the matrix, pH, and ionic strength are also expected to affect the course of these couplings.26 For unknown reasons, some of the examined eucalyptus clones have a large amount of monolignols coupled at the guaiacyl 4-O-position (clones 1 and 2, for example). Such favored couplings are reflected in the high frequency of uncondensed β-O-4 bonds, regardless of the relatively high content of guaiacyl units and the consequent low S/G ratio observed in these clones. It is worth emphasizing that the aforementioned S/G ratio are valid for units linked by a β-O-4 linkage through both the β- and the 4-position, rather than the whole S/G ratio in E. globulus. Kraft pulp delignification begins with deprotonation of the phenolic hydroxyl groups.29 Therefore, β-O-4 ether bonds in units containing free phenolic hydroxyl groups are considered easier to cleave than its etherified counterparts.5,30 The relative
8546 Ind. Eng. Chem. Res., Vol. 47, No. 22, 2008
Figure 4. Erythro-to-threo ratio of lignins isolated from the selected eucalypt clones. The maximum standard error of the reported data was 1 × 10-2 mmol/g.
Figure 3. Total aliphatic hydroxyl (A), phenolic hydroxyl (B), syringyl and guaiacyl units bearing free phenolic hydroxyl groups (C), and S-OH/ G-OH ratio (D). The maximum standard error of the reported data was 1 × 10-2 mmol/g. L: total lignin.
proportion of free phenolic groups within the lignin macromolecule is therefore an important structural feature to be considered as far as kraft pulping performance is concerned. For the purpose of analyzing the various hydroxyl groups (phenolic and aliphatic), 9 of the 15 eucalyptus samples were studied by quantitative 31P NMR.17,18 The pulp yield was used as a ranking parameter, and the eucalyptus clones with a broad range of pulp yields were selected. Lignin samples were isolated from the nine selected wood samples according to the method of Pepper et al.16 modified by Evtuguin et al.9 The aliphatic (Figure 3A), phenolic (PhOH) (Figure 3B), syringyl (S-OH) and guaiacyl (G-OH) phenolic hydroxyl groups (Figure 3C), as well as the S-OH/G-OH ratio (Figure 3D) were determined by phosphitylating the lignins with 2-chloro-4,4,5,5-tetramethyl-1,3,2-dioxaphospholane.17 Quantification was then carried out via peak integration using cholesterol as internal standard. Details of signal acquisition, assignment, and integration can be found elsewhere.17,18 While a slight variation was observed in the aliphatic hydroxyl contents (Figure 3A), the total amount of phenolic hydroxyl groups (Figure 3B) were found to decrease in the order 12 = 2 > 9 = 4 > 14 = 5 > 6 = 3 > 1, according to a one-way ANOVA test performed on such data (p < 0.05). As shown in the Figure 3C,D, sample 4 has the greatest S-OH group content, whereas samples 5 and 14 have the lowest S-OH/G-OH ratio. Furthermore, the range of S/G ratios determined by thioacidolysis (from 3.66 to 5.41, Table 2) were somewhat different from the S-OH/G-OH ratios estimated by quantitative 31P NMR (from 1.96 to 3.19, Figure 3D), indicating that the syringyl units present in E. globulus are primarily of the nonphenolic type.9,10,31 Both erythro and threo stereoisomeric forms of β-O-4 structures can also be determined using quantitative 31P NMR after derivatization of lignin with 2-chloro-1,3,2-dioxaphospholane by integrating the regions from 135 to 134.2 and from 134 to 133.4 ppm, which have been attributed to CR-OH in erythro and threo forms of β-O-4 structures, respectively.18 Figure 4 shows the erythro-to-threo (E/T) ratio in the different eucalyptus clones. As expected, the erythro form predominates over its threo counterpart for all eucalypt samples, corroborating previous findings reported by Akiyama.32 These authors evaluated the E/T ratio in different species of wood and found
Figure 5. Relationships between pulp yield and lignin content (A), S/G ratio (B), uncondensed β-O-4 (C), and syringyl units in β-O-4 structures (D). L: total lignin.
equivalent amounts of both stereoisomers in softwoods, while for hardwood species the erythro form of β-O-4 structures was found to predominate. The extension of such predomination among clones of the same eucalyptus species ranged from 1.72 (sample 5) to 2.11 (sample 4). Moreover, the higher and lower E/T ratio observed in sample 5 and 4, respectively, are consistent with their syringyl contents determined by thioacidolysis (Table 2). According to model experiments simulations, the erythro form is preferred in syringyl-type aromatic rings33 due to a selective water addition to one of the two faces of the quinone methide formed during the formation of β-O-4 structures.26 Overall, these results are in agreement with previous findings10,23 and illustrate the significant variation in the functional group contents within the lignin isolated from clones of the same wood species. Relationship between Lignin Structural Features and Pulping Performance. As anticipated, tremendous efforts have been conducted to select clones or to develop genetically improved trees with emphasis on reducing lignin content.2 However, as far as E. globulus wood is concerned, there is no clear correlation between lignin content and pulp yield (Figure 5A) or specific wood consumption (data not shown) to produce pulps with similar kappa numbers. Such lack of correlation indicates that lignin contents cannot explain the significant differences observed in the pulping performance of different clones of E. globulus. This result agrees with the work of Pinto et al.,4 who evaluated the effects of lignin structural features on hardwood pulping and bleaching performances. These authors observed that the relative wood pulping and pulp bleaching easiness were determined by differences in the lignin structures rather than by their relative contents in wood and pulps. Rı´o et al.1 have also evaluated the influence of eucalyptus
Ind. Eng. Chem. Res., Vol. 47, No. 22, 2008 8547
lignin composition on pulping performance and found that lignin structure has a greater effect on pulp yield than the lignin contents do. Among the lignin structural characteristics considered to affect the pulping efficiency, the syringyl to guaiacyl (S/G) ratio is by far one of the most important.1,3,4,7,34 The higher reactivity of the S units with respect to the G counterparts has long been reported.35 Furthermore, the extent of lignin recondensation taking place in alkaline conditions is hindered in the S units by the presence of the additional methoxyl group.36 Consequently, high relative contents of S units are expected to favor lignin degradation and removal from wood fibers. Strikingly, however, the data of Figure 5B show that for different clones of the same wood species, the pulp yield may not be directly related to the S/G ratio. Actually, the two higher pulp yields (Table 1) were obtained from eucalypt woods containing middle-to-low S/G ratios (sample 1 and 2, Table 2). The observed difference between the results showed in Figure 5B and those reported elsewhere may reside in the nature of the wood samples examined. For example, Pinto et al.4 compared five different hardwood species with significant variations in the S/G ratio (from 1 to 6) and found a tendency between S/G ratio and active alkali consumption to produce pulps with similar kappa numbers. In the same way, Gonza´les-Vila et al.3 showed a correlation between lignin S/G ratio and the ease of delignification estimated as active alkali consumed to produce pulps from different species of eucalyptus. Conversely, low correlation levels between pulp yield and S/G ratio determined by pyrolysisGC/MS1 and nitrobenzene oxidation8 have been obtained for different wood samples of the same eucalypt species, corroborating our finding that other lignin structural features may prevail over the S/G ratio for samples of the same species. When the total amount of uncondensed β-O-4 structures were plotted against the pulp yield (Figure 5C), a significant correlation (R ) 0.86; P < 0.001) was observed, i.e., the higher the total amount of uncondensed β-O-4 structures, the higher is the pulp yield. This result is explained by the preferential removal of uncondensed units during kraft pulping and the consequent enrichment of condensed units in residual lignins.4,5 As described in lignin model studies, carbon-carbon linkages are resistant in kraft conditions, whereas the phenolic and nonphenolic R- and β-ether bonds are labile and easier to remove from the fibers. Therefore, the higher the amount of uncondensed β-O-4 structures within the lignin macromolecule, the lower is the alkali consumption during pulping, resulting in less cellulose degradation and hemicelluloses solubilization and consequently higher pulp yields are obtained. Note that β-O-4 linkages commented here refer to uncondensed structures. Besides affecting pulping performance, high β-aryl ether contents have also been found to contribute to better kraft pulp bleachability, especially when a hydrogen peroxide stage is involved.37 Despite the lack of correlation between S/G ratio and pulp yield (Figure 5B), a direct relationship was apparent between pulp yield and the total amount of syringyl units determined by thioacidolysis. As shown in Figure 5D, the higher the syringyl units involved only in uncondensed β-O-4 bonds, the higher the pulp yield is. These results are in line with those reported in Figure 2, indicating that among the many factors that may contribute to pulping easiness, the content of uncondensed β-O-4 structures is most influential. Accordingly, since the S units are primarily linked through uncondensed ether bonds (Figure 2B), it is expected that high contents of this unit facilitate the pulping process and,
Figure 6. Relationships between syringyl units in uncondensed β-O-4 structures and specific wood consumption (A) and PFI revolutions (B) to obtain pulps with a similar drainability.
consequently, impact pulp yield (Figure 5D). On the other hand, the pathway for coupling of a monolignol with a guaiacyl-growing polymer during the lignin biosynthesis seems to be more important for pulping purposes than the relative content of guaiacyl units. As anticipated, clones 1 and 2 have high content of monolignols coupled at the guaiacyl 4-O-position (G units in β-O-4 structures, Table 2). Such high frequency of uncondensed β-O-4 bonds is reflected in high pulp yields, regardless of the relatively low S/G ratio observed in these clones. Therefore, the type of coupling (βO-4 versus β-5) has a greater effect on pulp yield (Figure 5C) than the relative guaiacyl contents (Figure 5B). Based on these findings and also in the significant correlation levels observed in Figure 4D (R ) 0.86; P < 0.001), it may be concluded that the pulp yield of different clones of the same eucalypt species can be predicted by the content of syringyl units involved only in uncondensed β-O-4 bonds rather than by the S/G ratio determined by degradative methods, such as thioacidolysis. The plot of the specific wood consumption versus uncondensed syringyl units determined by thioacidolysis is shown in Figure 6A. The correlation levels observed (R ) -0.86; P < 0.001) indicate that this lignin feature could be used to predict the specific wood consumption to produce pulps with the same kappa number, if new validation experiments are made. Furthermore, it can be observed that wood samples having greater uncondensed syringyl units generally require more PFI revolutions to reach a defined beating degree (Figure 6B). However, the correlation levels obtained in this case (R ) 0.63; P < 0.05) do not allow use of uncondensed syringyl units as the unique parameter to predict the beating time required to obtain pulps with a given drainability. On the other hand, no relationships were apparent between uncondensed syringyl units and the other strength properties evaluated. Therefore, other factors such as fiber length, cell wall thickness, and vessel frequency could be more appropriate to predict pulp quality. The influence of the different hydroxyl groups on the pulping performance of different eucalyptus clones was also evaluated. In general, no correlation could be established between pulp yield and the relative abundance of any phenolic (syringyl or guaiacyl) or aliphatic hydroxyl group within the lignin macromolecule. Although it could be expected that lignin units bearing free phenolic hydroxyl groups would be more reactive than their etherified counterpart, the lack of correlation between the total content of phenolic hydroxyl groups and pulp yield is not surprising. As anticipated,4 despite the higher reactivity of the phenolic structures in kraft conditions, the pulping and bleaching performances of different hardwoods species, including E. globulus, are determined by other lignin structural features rather than by the content of free phenolic hydroxyl groups.
8548 Ind. Eng. Chem. Res., Vol. 47, No. 22, 2008
Since the erythro diastereomers cleave somewhat faster than their threo counterparts under alkaline treatment,38 the stereochemistry of the arylglycerol-β-aryl structures within the lignin macromolecule is also expected to affect their removal from the cell wall. Despite the significant variation observed, as shown in Figure 4, no relationship was apparent between pulp performance (estimated as pulp yield or specific consumption) and E/T ratio of the isolated lignins. Conclusions The chemical structure and relative lignin contents varied significantly among eucalypt clones of similar age and growing in a common geographic area. While lignin contents did not correlate with pulping performance, the data reported here provides evidence of the effect of the lignin structure on the pulp yield, specific wood consumption, and fibrillation easiness. On the basis of the correlation levels observed, the pulp yield and specific wood consumption to produce kraft pulps with similar kappa numbers can be predicted by the content of syringyl units involved only in uncondensed β-O-4 bonds rather than by the S/G ratio. This result argues in favor of including the uncondensed syringyl units as a selection parameter in clonal breeding programs for pulpwood production. Further efforts are required to predict this specific lignin feature by a more practical and less labor intensive technique, such as near-infrared spectroscopy. No relationship was observed between pulp yield and specific consumption and the lignin phenolic group content and the stereochemistry of the arylglycerol-β-aryl structures. Acknowledgment Financial supports from FONDEF (DO3i1103) and Consorcio Geno´mica Forestal SA - PROGRAMA INNOVACHILE and FONDECYT (1080151) are gratefully acknowledged. Literature Cited (1) Rı´o, J. C.; Gutie´rrez, A.; Hernando, M.; Landı´n, P.; Romero, J.; Martı´nez, A. T. Determining the influence of eucalypt lignin composition in paper pulp yield using Py-GC/MS. J. Anal. Appl. Pyrolysis 2005, 74, 110. (2) Li, L.; Zhou, Y.; Cheng, X.; Marita, J. M.; Ralph, J.; Chiang, V. L. Combinatorial modification of multiple lignin traits in trees through multigene cotranformation. Plant Biol. 2003, 100 (8), 4939. (3) Gonza´les-Vila, F. J.; Almendros, G.; Rı´o, J. C.; Martı´n, F.; Gutie´rrez, A.; Romero, J. Ease of delignification assessment of wood from different Eucalyptus species by pyrolysis (TMAH)-GC/MS and CP/MAS 13C-NMR spectrometry. J. Anal. Appl. Pyrolysis 1999, 49, 295. (4) Pinto, P. C.; Evtuguin, D. V.; Neto, C. P. Effects of structural features of wood biopolymers on hardwood pulping and bleaching performances. Ind. Eng. Chem. Res. 2005, 44, 9777. (5) Lapierre, C.; Pollet, B.; Petit-Conil, M.; Toval, G.; Romero, J.; Pilate, G.; Leple´, J.-C.; Boerjan, W.; Ferret, V.; De Nadai, V.; Jouanin, L. Structural alterations of lignins in transgenic poplars with depressed cinnamyl alcohol dehydrogenase or caffeic acid o-methyltransferase activity have an opposite impact on the efficiency of industrial kraft pulping. Plant Physiol. 1999, 119, 153. (6) Huntley, S. K.; Ellis, D.; Gilbert, M.; Chapple, C.; Mansfield, S. W. Significant increases in pulping efficiency in C4H-F5H-transformed poplars: improved chemical savings and reduced environmental toxins. J. Agric. Food Chem. 2003, 51 (21), 6178. (7) Wallis, A. F. A.; Wearne, R. H.; Wright, P. J. Analytical characteristics of plantation eucalypt woods relating to kraft pulp yields. Appita J. 1996, 49, 427. (8) Gomide, J. L.; Colodette, J. L.; Oliveira, R. C.; Silva, C. M. Technological characterization of the new generation of Eucalyptus clones in Brazil for kraft pulp production. ReVista A´rVore 2005, 29 (1), 129. (9) Evtuguin, D. V.; Neto, C. P.; Silva, A.; Domingues, P.; Amado, F.; Robert, D.; Faix, O. Comprehensive study on the chemical structure of
dioxane lignin from plantation Eucalyptus globulus wood. J. Agric. Food Chem. 2001, 49, 4252. (10) Guerra, A.; Lucia, L.; Argyropoulos, D. S. Isolation and characterization of lignins from Eucalyptus grandis Hill ex Maiden and Eucalyptus globulus Labill. by enzymatic mild acidolysis (EMAL). Holzforschung 2007, 62, 24. (11) Capanema, E. A.; Balakshin, M. Y.; Kadla, J. F. Quantitative characterization of a hardwood milled wood lignin by nuclear magnetic resonance spectroscopy. J. Agric. Food Chem. 2005, 53, 9639. (12) Neto, C. P.; Evtuguin, D.; Pinto, P. A.; Silvestre, A.; Freire, C. Chemistry of plantation eucalypts: specificities and influence on Wood and fiber processing. In Proceedings of the 13th International Symposium on Wood, Fiber Pulping Chemistry, Auckland, New Zealand, 2005; APPITA: Auckland, New Zealand, p 431. (13) Miranda, I.; Pereira, H. Variation of pulpwood quality with provenances and site in Eucalyptus globulus. Ann. For. Sci. 2002, 59, 283. (14) Ferraz, A.; Baeza, J.; Rodrı´guez, J.; Freer, J. Estimating the chemical composition of biodegraded pine and eucalyptus wood by DRIFT spectroscopy and multivariate analysis. Bioresour. Technol. 2000, 74, 201. (15) Rolando, C.; Monties, B.; Lapierre, C. Thioacidolysis. In Methods in Lignin Chemistry; Lin, S., Dence, C. W., Eds.; Springer-Verlag: Heidelberg, 1992; pp 334-349. (16) Pepper, J. M.; Baylis, P. E. T.; Adler, E. The isolation and properties of lignins obtained by the acidolysis of spruce and aspen woods in dioxanewater medium. Can. J. Chem. 1959, 37, 1241. (17) Granata, A.; Argyropoulos, D. S. 2-Chloro-4,4,5,5-tetramethyl-1,3,2dioxaphospholate, a reagent for the accurate determination of the uncondensed and condensed phenolic moieties in lignins. J. Agric. Food Chem. 1995, 43, 1538. (18) Argyropoulos, D. S. Quantitative phosphorus-31 NMR analysis of lignin: a new tool for the lignin chemist. J. Wood Chem. Technol. 1994, 14, 45. (19) Valente, C. A.; Souza, A. P.; Furtado, F. P.; Carvalho, A. P. Improvement program for Eucalyptus globulus at PORTUCEL: Technological component. Appita 1992, 45, 403. (20) Farrington, A.; Hansen, N. W.; Nelson, P. F. Utilization of young plantation of Eucalyptus globulus. Appita 1977, 20, 313. (21) Gierer, J. The chemistry of delignification. A general concept. Holzforschung 1982, 36, 43. (22) Lapierre, C.; Monties, B.; Rolando, C. Thioacidolysis of lignin: comparison with acidolysis. J. Wood. Chem. Technol. 1985, 5, 277. (23) Guerra, A.; Filpponen, I.; Lucia, L.; Argyropoulos, D. S. Comparative evaluation of three lignin isolation protocols for various wood species. J. Agric. Food Chem. 2006, 54, 9696. (24) Patten, A. M.; Cardenas, C. L.; Cochrane, F. C.; Laskar, D. D.; Bedgar, D. L.; Davin, L. B.; Lewis, N. G. Reassessment of effects on lignification and vascular development in the irx4 Arabidopsis mutant. Phytochemistry 2005, 66 (17), 2092. (25) Sarkanen K. V. Precursors and their polymerization. In Lignins, Occurrence, Formation, Structure and Reactions; Sarkanen K. V., Ludwig C. H., Eds.; Wiley-Interscience: New York, NY, 1971; pp 95-163. (26) Ralph, J.; Lundquist, K.; Brunow, G.; Lu, F.; Kim, H.; Schatz, P.; Marita, J.; Hatfield, R.; Ralph, S.; Christensen, J.; Boerjan, W. Lignins: Natural polymers from oxidative coupling of 4-hydroxyphenylpropanoids. Phytochemistry 2004, 3, 29. (27) Sederoff, R. R.; MacKay, J. J.; Ralph, J.; Hatfield, R. D. Unexpected variation in lignin. Current Opin. Plant Biol. 1999, 2 (2), 145. (28) Boerjan, W.; Ralph, J.; Baucher, M. Lignin Biosynthesis. Ann. ReV. Plant Biol. 2003, 54, 519. (29) Sjo¨strom, E. Wood Chemistry: Fundamentals and applications; Academic Press: San Diego, CA, 1993. (30) Stewart, J. J.; Kadla, J. F.; Mansfield, S. D. The influence of lignin chemistry and ultrastructure on the pulping efficiency of clonal aspen (Populus tremuloides Michx.). Holzforschung 2006, 60, 111. (31) Adler, E. Lignin chemistry - past, present and future. Wood Sci. Technol. 1977, 11, 169. (32) Akiyama, T.; Goto, H.; Nawawi, D.; Syafii, W.; Matsumoto, Y.; Meshitsuka, G. Erythro/threo ratio of β-O-4 structures as an important structural characteristic of lignin. Part 4: Variation in the erythro/threo ratio in softwood and hardwood lignins and its relation to syringyl/guaiacyl ratio. Holzforshung 2005, 59, 276. (33) Brunow, G.; Karlsson, O.; Lundquist, K.; Sipila¨, J. On the distribution of the diastereomers of the structural elements in lignin: the steric course of reactions mimicking lignin biosynthesis. Wood Sci. Technol. 1993, 27, 281. (34) Ona, T.; Sonoda, T.; Ito, K.; Shibata, M.; Tamai, Y.; Kojima, Y. Studies on decision of selection indexes for quality breeding of eucalypt pulpwood
Ind. Eng. Chem. Res., Vol. 47, No. 22, 2008 8549 (VI) -Relationship between wood and pulp properties brought by their withintree variations on E. camaldulensis. Jpn. Tappi J. 1995, 49, 1347. (35) Tsutsumi, Y.; Kondo, R.; Sakai, K.; Imamura, H. The difference of reactivity between syringyl lignin and guaiacyl lignin in alkaline systems. Holzforschung 1995, 49, 432. (36) Fengel, D., Wegener, G., Eds. Wood chemistry, ultrastructure and reactions; Walter de Gruyter: Berlin, 1989; 613p. (37) Gellerstedt, G.; Wafa Al-Dajani, W. Bleachability of alkaline pulpsPart 1. The importance of beta-aryl ether linkages in lignin. Holzforschung 2000, 54, 609.
(38) Ahvazi, B. C.; Argyropoulos, D. S. Thermodynamic parameters governing the stereoselective degradation of arylglycerol-β-aryl ether bonds in milled wood lignin under kraft pulping conditions. Nordic Pulp Paper Res. J. 1997, 12, 282.
ReceiVed for reView February 26, 2008 ReVised manuscript receiVed August 7, 2008 Accepted September 1, 2008 IE800320D
