Lignin - American Chemical Society

Chapter 27

Chemical Characteristics of Lignins Extracted from Softwood TMP after O and CIO Treatment 3

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D. R. Robert , M. Szadeczki , and D. Lachenal Downloaded by STANFORD UNIV GREEN LIBR on June 8, 2012 | http://pubs.acs.org Publication Date: November 30, 1999 | doi: 10.1021/bk-2000-0742.ch027

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CERMAV/CNRS Domaine Universitaire, BP 53, 38041 Grenoble cedex 9, France E F P G Domaine Universitaire, BP 65, 38402 St. Martin d'Hères cedex, France 2

This study investigates the role of ozonation in improving the physical properties of softwood thermomechanical pulp (TMP). Lignin fractions were extracted from pulp before and after ozonation and characterized by IR, C N M R and GPC. Results show the influence of the extraction method on the lignin structure, with enzymatic hydrolysis of the pulp giving a lignin fraction richer in etherified units but more contaminated with polysaccharides than acidolysis. The number of carboxyl groups formed in lignin was also studied, since this parameter has been correlated with pulp mechanical properties. C l O treatment of TMP, compared to ozonation, resulted in an increase of COOR(H) groups and the formation of muconic structures clearly identified on N M R spectra. 13

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For economical and ecological reasons there has been a spectacular increase in the past few years in the use of recycled fibers. Consequently, upgrading recycled fibers has become necessary. Recycled pulps usually consist of a mixture of chemical and mechanical pulps. It has been shown that the overall quality of recycled pulp is governed by the properties of its lignin-rich fibers (mechanical pulp fraction). Any change in these properties by lignin modification (oxidation) or lignin removal has a direct impact on the quality of the entire pulp (/). Attempts to upgrade mechanical pulp by ozonation have been performed in the late 70* s, which included pilot scale operations (2). No commercial applications are currently practiced, however, mainly because of economics. The mechanism by which mechanical properties are improved

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© 2000 American Chemical Society In Lignin: Historical, Biological, and Materials Perspectives; Glasser, W., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.

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Downloaded by STANFORD UNIV GREEN LIBR on June 8, 2012 | http://pubs.acs.org Publication Date: November 30, 1999 | doi: 10.1021/bk-2000-0742.ch027

by ozonation is believed to be due to carboxyl groups formation on lignin and thereby an increase in hydrophilicity (3). Since it is known that ozone reacts 100 to 1000 times more rapidly with lignin than with cellulose (4), it is thought that ozonation of a recycled pulp will primarily react with the lignin in the mechanical pulp fraction and thus improve the overall strength properties. The purpose of this study was to investigate the structural changes in the lignin of a mechanical pulp pretreated by ozonation, and to correlate these changes to the strength improvement of the pulp. Results were compared with another bleaching chemical, chlorine dioxide. Lignins extracted from softwood thermomechanical pulps, before and after 0 and C10 treatments, were characterized by SEC (Size Exclusion Chromatography), BR and C N M R . Since lignin must be extracted from the pulp before analysis, the possible effects of the extraction procedure upon lignin structure was a major concern. 3

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Effect of 0 and C 1 0 treatment on mechanical strength of TMP 3

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Ozonation and C10 treatment of pulp. Ozonation was performed at high consistency (35-40%) at room temperature in a rotating spherical glass reactor. Ozone was produced from oxygen in a G21 type Ozonia generator. The 0 concentration in oxygen was 90 mg/L NTP with a gas flow of 0.8 L/min. The residual ozone concentration was determined by iodometric titration. 2

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C10 , containing no molecular chlorine, was produced by reaction of a concentrated solution of sodium chlorite with sulfuric acid. Treatment of pulp with C10 was performed at 5% consistency and 70°C in polyethylene bags. The pulp samples were preheated in a water bath before introducing the C10 solution. 2

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Improvement of TMP mechanical properties after ozonation and C10 treatment As expected, an improvement in the breaking length and burst index of T M P pulp was observed, with increasing levels of ozonation and chlorine dioxide giving better strength properties (Table I). 2

Table I. TMP mechanical properties after O, and C1Q treatments 5.0 4.27 2.5 %o. 0 0.73 5.12 4.74 5.20 break. 1., km 3.41 4.05 3.30 2.80 burst ind., kPa 2.75 2.10 2.20 mVg 2

% CIO, break. 1, km burst ind. kPa

0 3.41 2.10

2.0 3.92 2.20

4.0 3.78 2.30

7.0 4.32 2.60

10.0 4.84 2.80

In Lignin: Historical, Biological, and Materials Perspectives; Glasser, W., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.

522 In the case of ozonation (Figure 1), an improvement in strength was observed when the ozonated thermomechanical pulp was mixed with a chemical pulp, provided that the mechanical pulp represented a significant fraction of the mixture (at least 10%) (7).


Influence of extraction methods upon lignin structure The overall molecular and macromolecular structure of the native wood cell wall, and of the corresponding thermomechanical pulp, should be quite similar. In both cases the lignin macromolecule is partly covalently bonded to the carbohydrate matrix. Release of this lignin from the pulp involves the splitting of these bonds, which should also introduce chemical changes in the lignin. The chemical structure of the lignin recovered will thus reflect both its original structure in the pulp and the extractionderived changes. The choice of an extraction method which minimizes structural changes in lignin, or at least gives known changes, is important. Among the most efficient methods two were selected: enzymatic hydrolysis of carbohydrates and acidolysis. Extraction by enzymatic hydrolysis of carbohydrates. In brief, the cellulasehemicellulase mixture totally hydrolyses the cellulose-hemicellulose matrix. Following the method proposed by Pan et al. (5), 5g of industrial softwood TMP, some characteristics of which are given in Table 1, were ultraground at room temperature for 2 days and put in a 100 ml acetate buffer solution, 0.5M and p H 4.6, with the enzyme, 5mg per buffer ml. The enzyme was a commercial cellulases-hemicellulases mixture, ONOZUKA R-10, highly contaminated by carbohydrates moieties (43.3%). After hydrolyzing for 72 hours at 37.5 °C in darkness the residue was centrifuged and hydrolyzed for another 48 hours, and then water washed and extracted with a 9/1 dioxane/water mixture. The solution was concentrated under vacuum and the lignin precipitated in water with 2% sodium sulfate at 4°C. A n 70% extraction yield was achieved (after correction for proteins and carbohydrates) for both lignin extracted from untreated TMP, designated as TMPe, and lignin extracted from ozone bleached TMP, designated as TMPOe. Extraction by acidolysis. According to the method proposed by Gellersted et al. (6), 50 g of T M P were refluxed for 2 hours in 1.5 1 of dioxane/water solution, 82/18, containing 0.1 mole of HC1. Dilution with water and concentration at 40°C under vacuum was conducted until all dioxane was eliminated. The precipitated lignin was washed and vacuum dried. Extraction yield was 30% for lignin extracted from untreated pulp, designated as TMPa, 50% for lignin extracted from ozone treated pulp, designated as TMPOa, and 80% for lignin extracted from pulp treated with C10 , designated as TMPCla. Yields are based on the lignin content of the pulp, estimated by the Klason method. 2


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Breaking length (m)

Figure 1. Change in breaking length of a blend of chemical and thermomechanical pulps after ozonation of the T M P fraction.


524 Structural differences between TMPe and TMPa lignins. Structural differences were investigated by SEC, C N M R and IR spectroscopies. Sugars analysis showed a significantly higher sugar content in TMPe, 6.9%, compared to TMPa, 0.1%. 13

Size Exclusion Chromatography. SEC was carried out on two PL Gel Mixed D columns in series with a 200 to 400000 polystyrene molecular weight distribution. A solution of 10 mg of lignin in dimethylformamide containing 0.01M N a N 0 was injected. Detection was by U V at 280 nm. SEC curves reveal that TMPe lignin has a smaller elution volume than TMPa, which is consistent with an acidolysis mechanism cleaving ether bonds to give partial depolymerization (7). Conversely, the enzyme treatment likely degrades only the cellulose-hemicellulose matrix.


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IR spectra. The IR spectra are very similar and present the classical fingerprint of lignin structure, except for a more intense band at 1655cm" in TMPe assigned to protein contamination (8). 1

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NMR analysis. C N M R spectra were recorded at 75.48 M H z , 323 K, using DMSO-d* and qualitative and quantitative N M R analysis performed according to classical methods (9). The central peak of the solvent signal at 39.6 ppm was used as the chemical shift reference. An inverse gate sequence with 10 s pulse delay was used for quantitative analysis. For the DEPT sequence, which gives selectively C H , C H or C H signals, a 1/2J = 0.0033 s was chosen as the refocusing delay. Signal intensities were estimated by comparison of their integral to the integral of the six aromatic carbons, assuming one neglects the contribution of vinylic carbons. Some results from the quantitative analysis are given in Table Π. Spectra in Figure 2 show, as expected, that the carbon skeleton of both lignins exhibit far less structural modifications than lignins extracted in the same conditions from chemical pulps, such as kraft pulp lignin (10,11). In particular, C6-C3-0 oxygenated side chains, found mainly in β-Ο-4 structures, are well preserved as seen from the relative intensity of signals 17,18, and 21 in the range 60 to 90 ppm, assigned predominantly to the CP, C a and Cy carbons, respectively, in β-Ο-4 units. 2

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Table II. Quantitative estimation by C-13 NMR of chemical functions in lignins extracted from ' "MP (number per aromatic unit ')· TMPOa lignin ext. TMPe TMPOe TMPa C - O aliph. 1.85 2.64 2.80 1.80 60-90 ppm -OCH, 0.98 0.88 0.95 0.92 0.25 COOR(H) 0.16 0 0.28

TMPCla 2.80 0.76 0.60

• precision : +/- 5% There are two main structural differences between TMPe and TMPa lignins. The first one is the presence in TMPe of a broad signal centered at 170 ppm, signal 3, assigned



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Figure 2. C N M R spectra of: a) TMPa lignin extracted from T M P by acidolysis; b) TMPe lignin extracted from T M P by enzymatic hydrolysis of carbohydrates. S, solvent = D M S O - J .

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526 to carboxyl functions and representing 0.16 COOR(H) groups per aromatic unit. As there was no chemical pretreatment of the pulp the presence of signal 3 in TMPe, and the absence of peak 3 in TMPa, can only originate from the extraction procedure, the importance of which is thus clearly evidenced. These carboxyl groups are due to contamination from the pulp and/or from the enzyme, which contains carboxyl functions and is itself highly contaminated by carbohydrates. The other difference is of quantitative nature and concerns the higher relative intensity of the group of signals 17,18 and 21 assigned to C - 0 aliphatic carbons, estimated as 2.6 carbons per aromatic unit in TMPe and 1.8 carbons per aromatic unit in TMPa. The six cellulosehemicellulose carbons signals which range between 60 and 100 ppm and indicate carbohydrate contamination, contribute to only part of the C-O aliphatic signals between 60 and 90 ppm in TMPe (the sugar content in this lignin is limited to 6.9%). Thus, the higher quantity of aliphatic C - 0 carbons in TMPe compared to TMPa is probably due to a substantially higher amount of β-Ο-4 structures in TMPe, which supports the observation made on the GPC curves. Additional confirmation is found in the aromatic region of the spectra: the relative intensity of signal 6 (C3 in etherified guaiacyl) compared to signal 7 (C4 in etherified and non-etherified guaiacyl units and C3 in non-etherified guaiacyl units), and the relative intensity of signal 10 (CI in etherified β-Ο-4) compared to 11 (CI in β-Ο-4 non-etherified), confirms that there are more β-Ο-4 linkages in TMPe than in TMPa. Again, this indicates that cleavage of βΟ-4 linkages occured during acidolysis. Effects of ozonation and C102 treatment of T M P upon lignin structure TMPOe was obtained in 80% yield by the enzymatic method from ozonated pulp, and TMPOa and TMPCla were obtained by acidolysis respectively in 65% yield from ozonated pulp and in 80% yield from pulp treated with chlorine dioxide. Again, lignin yield represents the quantity of extracted lignin based on the Klason lignin content of the pulp. G P C curves. Figure 3 shows similar curves for TMPa and TMPOa. TMPCla curve is shifted to the right of the two previous ones. This indicates that ozonation of pulp does not depolymerize the lignin significantly whereas C10 treatment does. 2

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IR spectra. TMPOe, TMPOa and TMPCla give a larger band at 1713cm than is found in lignins from untreated pulp. This band can be assigned to muconic acids and quinones which are known to be formed during ozonation and C10 treatment of pulp (12). The strongest band is found in TMPCla lignin. 2

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C N M R analysis. Compared to spectra in Figure 2, N M R spectra in Figure 4 show that ozonation does not significantly alter the overall lignin structure, the main change being the creation of a substantial amount of carboxyl groups. Surprisingly,



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elution volume (ml) Figure 3. GPC curves of: TMPa (—); TMPOa (—); and TMPACla (...) lignins.


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Lignin - American Chemical Society

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