The Role of Hemicellulose in the Delignification of Wood1

Transcription

1 The Role of Hemicellulose in the Delignification of Wood1 A. J. KERR AND D. A. I. GORING Pulp and Pnper Research lt~stitute of Canada and Departmetlt of Chemistry, McCill Universify, Montreal, Qlrebec H9R 3J9 Received November A. J. KERR and D. A. I. GORING. Can. J. Chem. 53,952(1975). Hemicellulose was selectively removed from white birch by treatment with cold alkali. Untreated and alkali-treated samples were pulped to various stages of delignification by the acid chlorite process. The acid chlorite reagent was shown to be selective for lignin removal from both samples during the first 60% of delignification. In alkali-treated white birch, the median pore width increased considerably as acid chlorite delignification proceeded. In contrast, in untreated white birch, this parameter remained relatively constant during the first 60% of delignification. Alkali-treated white birch delignified at a much faster rate than untreated white birch. In addition, there was a topochemical preference for removal of lignin from the secondary wall of the fibers in the hemicellulose-deficient wood. Apparently, prior removal of hemicellulose caused larger pores to be produced in the fiber walls during pulping, thereby facilitating a more rapid removal of lignin. A. J. KERR et D. A. I. GORING, Can. J. Chem. 53,952 (1975). On a enlev6 d'une fa~on selective l'htmicellulose du bouleau blanc par le traitement avec de l'alcali a froid. On a transforme, divers tchantillons de bois non trait6 et trait6 par les alcalis, en pdte a papier jusqu'a divers stages de delignification, utilisant le processus au chlorite en milieu acide. On a demontre que durant les premiers 60% de la delignification, ce reactif enleve ~Clectivement la lignine provenant des deux types d'tchantillons. Dans le cas du bouleau blanc trait6 par les alkalis, la largeur mediane des pores augmente considerablement i mesure que la dtlignification au chlorite se poursuit. Par opposition, lorsque le bouleau blanc n'est pas traiti, ce parametre demeure pratiquement constant durant les premiers 60% de la delignification. Le bouleau blanc traite par les alcalis se delignifie B une vitesse beaucoup plus grande que le bouleau blanc non traitc. De plus il y a une preference topochimique pour l'enlevement de la lignine provenant des cellules secnndaires de la fibre dans le bois deficient en hcmicellulose. Apparernrnent 1'Climination prealable de l'hemicellulose amene la production de pores plus grandes dans la fibre au cours du processus de transformation en plte facilitant ainsi I'elimination plus rapide de la lignine. [Traduit par le journal] Introduction Hemicelluloses are dissolved from wood along with lignin in the early stages of kraft and acid sulfite pulping but are retained during acid chlorite pulping (1). In all three pulping processes, the sizes of lignin macromolecules removed from the wood cells correspond closely to the sizes of the cell pores at various stages of delignification (2). In kraft and acid sulfite pulping, the pore sizes (3) and the sizes of the extracted lignin macromolecules (4, 5) increase during the pulping, whereas in acid chlorite pulping both remain almost constant throughout (2) Thus, hemicellulose removal plays an important role in chemical pulping. When hemi- 'This paper is dedicated to Professor C. A. Winkler of McGill University on the occasion of his sixty-fifth birthday by one of his colleagues (D.A.I.G.). celluloses are retained in the wood during pulping, the cell pores and the extracted lignin macromolecules remain small and delignification is characteristically slow. However, when hemicelluloses are removed in the early stages of pulping, the pores and lignin macromolecules increase in size and delignification is much more rapid. To test whether the increase in pore size and the faster rate of delignification is really caused by hemicellulose removal, it would be instructive to compare a highly selective delignification of whole wood with an identical treatment of wood from which hemicelluloses have been removed. The hemicelluloses should be removed by a method that does not change the chemical properties of the lignin. It is difficult to remove significant amounts of hemicelluloses from softwoods without chemical modification or simultaneous removal of lignin

2 ' KERR AND GORING: DELIGNIFICATION OF WOOD 953 (6). Hardwood hemicelluloses are more soluble, however, and it has been shown that large amounts can be removed from white birch by a cold alkali treatment (7). Accordingly, hemi-, celluloses were removed from white birch with alkali and the size of the pores in the cell wall and the rates of acid chlorite delignification of untreated and alkali-treated white birch were compared. Dissolution of hemicellulose from wood also seems to affect the topochemistry of the delignification reaction. During kraft pulping of black spruce (8), there is an initial preferential removal of lignin from the tracheid secondary wall, followed at about 50% delignification by a rapid dissolution of lignin from the compound middle lamella. In contrast, during acid chlorite pulping of black spruce, lignin dissolution occurs at the same relative rate from the secondary wall and compound middle lamella regions of the tracheids (9). The topochemical preference for early lignin removal from the secondary wall, was found to be greatest in kraft pulping and decreased in the order I j acid neutral acid I kraft > sulfite > sulfite > chlorite 1 In seeking an explanation for the above trend, Wood et al. (9) noted that the order of topochemical preference is the same as the order of the extent of hemicellulose dissolution from the wood in the early stages of pulping. Hemicellulose removal is rapid initially and occurs to decreasing degrees in kraft, acid sulfite, and neutral sulfite pulping (10, ll), whereas in acid chlorite pulping there is no hemicellulose removal until more than 60% of the lignin has been dissolved (1). This parallel behaviour led Wood et al. (9) to propose that the topochemical effect is associated with the removal of hemicelluloses from the wood. The present investigation provided a good opportunity to test the hypothesis of Wood et al. (9). If the hypothesis is correct, the alkali-treated hemicellulose-deficient wood should show a topochemical effect whereas the untreated wood should not. Accordingly, the removal of lignin from the cell walls and middle lamellae by acid, chlorite delignification of both the untreated and alkali-treated birch wood was analyzed by the I techniques of ultraviolet microscopy recently I developed in this laboratory (12). Experimental A sample of 2040 mesh sawdust was prepared from the trunk of a 46 year old birch tree. Hemicelluloses were extracted by treatment with 3% NaOH for 24 h at 30 C to produce a sample at 78% yield with 48.4% of hemicelluloses removed. The acid chlorite pulping was carried out according to the procedure of Wise ef a/. (13) as adopted by Timell (14) and Ahlgren and Goring (I). The chemical charge was 0.3 g of sodium chlorite and 0.1 ml of acetic acid per gram of dry wood and the reaction temperature was 70 "C. A fresh charge of chemicals was added at hourly intervals without withdrawal of any liquor. The initial liquor-to-wood ratio was 13 : 1. Tappi standard methods were employed for the determination of "Klason" lignin in the wood (15) and in the acid chlorite pulps (16). The acid-soluble lignin contents were calculated from the U.V. absorbances at 205 nm of the Klason hydrolysates, using an absorptivity of g-i cm-'. This figure was determined by Schoning and Johansson (17) on isolated samples of white birch acid-soluble lignin. Its application to acid chlorite pulps is justified as, for both spruce and beech, Campbell and McDonald (18) found little difference between the absorptivities of acid-soluble lignins prepared from wood and acid chlorite pulp. The total lignin content of each wood and pulp sample was obtained by adding the Klason and acid-soluble lignin contents. The Tappi standard method was employed for the determination of 3-cellulose (19). The hemicellulose content was calculated by difference, yield minus lignin and a-cellulose. Cumulative volume distributions of pore size in waterswollen pulp samples were measured by the solute exclusion technique of Stone and Scallan (3, 20, 21). The technique measures the accessibility of the samples to a number of dissolved sugars and dextrans ranging in hydrodynamic diameter from 8 A (glucose) to 560 A (a dextran with a molecular weight of about two million). Further details of the method are given elsewhere (22). For specimen preparation in U.V. microscopy, matchstick-size chips about 1 rnm2 x 5 mm were cut from the 44th annual ring of the tree and were alkali extracted and delignified together with the sawdust. At each stage of delignification. chips were solvent exchanged from the water-swollen state to absolute ethanol and then to propylene oxide prior to embedding in Epon 812. Ultrathin transverse sections ( pm) were cut from the embedded samples with a diamond knife mounted on a Porter-Blum ultramicrotome and were photographed in a Leitz u.v. microscope. The microscopic methods used were essentially those of Scott ef a/. (12), although a lower wavelength of 240 nm was employed. Changes in the absorptivity of in sif~r lignin produced by the acid chlorite reagent were found to be smallest at this wavelength (22). Results Hemicellulose Removal The results of the alkali treatment are summarized in Fig. 1. The data show that the alkali extracts mainly the hemicelluloses, although

3 954 CAN. 1. CHEM. VOL. 53, 1975 LIGNIN HEMICELLULOSES CELLULOSE PERCENTAGE OF UKTREATEO WOOD ALKALI TREATED WHITE BIRCH MATERIAL REMOVED BY ALKALI TREATh4Ehl FIG. 1. Diagram illustrating the results of the cold alkali treatment of white birch. small amounts of lignin and cellulose are also removed from the wood. To check if the lignin has been modified chemically during the alkali treatment, U.V. spectra were measured on fiber and vessel walls of untreated and alkali-treated white birch. For both morphological regions, the spectra of the untreated and alkali-treated wood were identical which indicated that the cold alkali caused no major chemical modification of the lignin. Acid Chlorite Pulping Yields, lignin contents, and percentage delignification are given in Table 1 and the selectivity of the delignification is illustrated in Fig. 2. The results show that only lignin is removed from both untreated and alkali-treated white birch during the first 60x of delignification. Ahlgren > CARBOHYDRATES. CAR80HYDRATES o TOTAL YIELD NON-LIWIN YIELD FIG. 2. Lignin and carbohydrate contents of untreated (left) and alkali-treated (right) white birch at various stages during acid chlorite delignification. and Goring (1) previously observed similar behavior with black spruce. The present results, therefore, indicate that the acid chlorite reagent is selective for lignin removal not only from softwoods but also from hardwoods. Pore Size The cumulative pore size distribution curves for untreated and alkali-treated white birch at various stages of acid chlorite delignification are shown in Figs. 3 and 4, respectively. In all samples studied, the pore sizes were found to be distributed log normally to a good approximation (22). A similar distribution of pore sizes in various wood and pulp samples was reported previously by Ahlgren (23). The median pore TABLE 1. Chemical compositions of the acid chlorite pulps Pulping Klason Acid-soluble Total Total lignin time Yield lignin lignin lignin (% of original Delignification (min) (%) (%) (%I (%) wood) (%) U~ltreated ~11ite birch Alkali-treated white birch* (78% yield) *Yield Tor uncooked alkali-treated wood is taken as LOO"/..

4 width, which is the pore width at one half the cell wall pore volume (or fiber saturation point) was conveniently determined for each sample from a straight line plot of the data on a log-probability diagram (22). Median pore width is plotted against percentage delignification for both untreated and alkalitreated white birch in Fig. 5. The median pore width of alkali-treated white birch is 16.9 A which is significantly higher than the value of 9.2 A found for untreated white birch, indicating that hemicellulose removal causes the cell pores to increase in size. The median pore width of untreated white birch remains fairly constant during the first 60% of acid chlorite delignification but increases slightly thereafter. A similar trend occurs in the median pore width of black spruce during acid chlorite delignification (2). In contrast to this, the median pore width of alkali-treated white birch increases throughout acid chlorite delignification (see Fig. 5). A rapid increase of the median pore width with the extent of lignin PORE WIDTH (A) FIG. 4. Cumulative pore volume us. pore width for alkali-treated white birch at various stages of acid chlorite delignification. removal also occurs in kraft and acid sulfite pulping of black spruce (2, 3). Rates of Pulping Percentage delignification is plotted against pulping time in Fig. 6 for the acid chlorite pulps prepared from both untreated and alkali-treated white birch. It is clear that the rate of delignification of alkali-treated white birch is considerably greater than that of untreated white birch. Thus, the prior removal of hemicelluloses causes a marked increase in the rate of delignification. This result is consistent with the findings of Procter (24) who compared the rate of delignification for western hemlock pretreated with H,S with that for untreated wood. The H,S pretreatment protected the hemicelluloses from dissolution during the kraft process and at the

5 956 CAN. J. CHEM. VOL. 53, 1975 WHITE BIRCH FIBERS UNTREATED DELlGNlFlCATlON (%) FIG. 5. Median pore width of untreated and alkalitreated white birch at various stages of acid chlorite delignification. PULPING TIME (MIN) FIG. 6. Percentage delignification us. pulping time for the acid chlorite pulps. same time caused a reduction in the rate of delignification. Topochemical EfSects The progress of acid chlorite delignification of untreated and alkali-treated white birch is illustrated for the fibers in the U.V. micrographs displayed in Fig. 7 and for the vessels in Fig. 8. As in previous studies of kraft, sulfite, and acid chlorite pulping (8,9), no significant gradient of lignin concentration across the secondary walls of the cells can be detected at any stage of delignification. The U.V. absorbances for the cell walls and middle lamellae of the fibers and vessels at ALKALI TREATEQ FIG. 7. Photomicrographs at 240 nm showing the progressive acid chlorite delignification of fibers in untreated and alkali-treated white birch. The photomicrographs are all of 1.0 p sections. UNTREATED ALKALI TREATED WHITE BIRCH VESSELS FIG. 8. Photomicrographs at 240 nm showing delignification of the vessels. The photomicrographs are all of 1.0 sections. various stages of delignification are given in Table 2. The absorbances were measured at a wavelength of 240 nm and were corrected for changes in the dimensions of the morphological regions and the absorbtivity of the lignin which were produced by the chlorite treatment. Details of the correction procedures are given elsewhere (22). The percentage of lignin removed from any morphological region of a sample at a particular stage of delignification, Ri, is given by in which Ai is the absorbance at any particular degree of delignification and A, is the absorbance in the original sample. The percentage of lignin removed from the secondary wall and compound middle lamella regions of the fibers and vessels

6 KERR AND GORING: DELIGNIFICATION OF WOOD TABLE 2. Corrected 240 nm U.V. absorbances Corrected u.v. absorbances (A,) Delignification Fiber Fiber Vessel Vessel-f i ber Fiber cell (%) wall ML wall ML corner ML UNTREATED Untreated ivhite birch Alkali-treated white birch ALKALI TREATED I I I I I I I I I I DELIGNIFICATION (%) FIG. 9. Topochemistry of acid chlorite delignification of fibers and vessels of untreated and alkali-treated white birch: percentage of lignin removed from the middle lamella (ML, filled circles) and the secondary wall (SW. empty circles) us. percentage of lignin removed from the whole sample (i.e. % delignification). of untreated and alkali-treated white birch were calculated from the corrected U.V. absorbances using eq. 1 and are plotted against the percentage of lignin removed from the whole wood (i.e. percentage delignification) in Fig. 9. No topochemical effect is apparent in either the fibers or vessels of untreated white birch. A marked topochemical effect occurs in the fibers of alkali-treated, hemicellulose-deficient white birch but not in the vessels. The results therefore indicate that hemicellulose removal produces a topochemical effect in the chlorite pulping of birch fibers. Discussion The results confirm that the removal of hemicellulose from birch wood (i) increases the average pore size in the cell wall during all stages of acid chlorite delignification; (ii) increases the overall rate of delighification; and (iii) increases the rate of delignification of the fiber wall with respect to that of the middle lamella. What physicochemical mechanisms of delignification are indicated by the above trends? Lignin in wood appears to be a polymeric material made up of rather short chains of phenyl propane units cross linked in a variety of ways to give a network structure. Between one third and one quarter of the lignin is found in the middle lamella region in a fairly pure form. Most of the remainder is in the secondary wall associated with the cellulose and hemicellulose moieties (25, 26). When wood is pulped two things must happen. Firstly, the lignin network must be broken down to give macromolecules which are soluble in the

7 958 CAN. J. CHEM. VOL. 53, 1975 pulping solvent. Secondly, the soluble lignin must diffuse out of the secondary wall into the lumen cavity. If the pores in the cell wall are small, then the lignin network must be broken down into small fragments to allow diffusion through the wall to the lumen. This will require sustained chemical action on the lignin and will tend to result in a slow rate of delignification. If the pore size is large, the chemical breakdown of the lignin network need not be excessive because large lignin macromolecules will be capable of diffusing through the wall to the lumen. It should be noted that, regardless of diffusion constraints, there will be a limit to the size of lignin macromolecules which will be soluble in the pulping solvent. However, both kraft and acid sulfite pulping of isolated lignins were found to produce soluble lignins with molecular weights in excess of lo6 (27, 28) which is considerably greater than the molecular weights of soluble lignins obtained by the usual methods of delignifying wood (4, 5). Thus the constraint to lignin removal from wood does not seem to be the solubility of the isolated lignin macromolecules but rather the porous structure of the cell wall. What the present results confirm is that the hemicellulose governs the pore size in the cell wall and thus the rate of delignification. Removal of hemicellulose increases the pore size and therefore also increases the rate of delignification. Retention of hemicellulose imposes a constraint on the diffusion of soluble lignin molecules out of the wall and therefore slows up the delignification process. The explanation of the enhancement of the topochemical effect by the removal of hemicellulose is not as straightforward. An obvious suggestion is that the S1 layer in the secondary wall acts as a diffusion barrier to the escape of lignin dissolved from the middle lamella. However a recent study of the kraft pulping of pressure refined spruce wood showed that even when the middle lamella lignin was exposed directly to the kraft pulping liquor, it dissolved more slowly than the secondary wall lignin (29). Thus it seems that the rate of dissolution of the lignin in the middle lamella is an intrinsic property of this morphological region and is not affected by the presence of the S1 layer. Just as the dissolution of lignin from the cell wall is governed by the porous structure of the cell wall, the dissolution of the lignin in the middle lamella will be governed by the porous structure of the middle lamella. The true middle lamella is almost pure lignin and hence will contain less hemicellulose than the cell wall. Thus hemicellulose removal is likelv to have a greater effect on the pore size of the cell wall than on the pore size of the middle lamella. This may be the cause of the enhancement of the topochemical effect produced by hemicellulose removal. It is also possible that the chemical structure (e.g. degree of cross linking) of middle lamella lignin differs from that of cell wall lignin and that such structural differences contribute to the topochemical effect. It is interesting to observe in Fig. 9 that the vessels showed no topochemical effect even after removal of hemicellulose. The cold alkali extraction used in the present investigation would be expected to remove only the- xylan (30). Perhaps the vessels are rich in glucomannan which is not extracted by cold alkali and the presence of which would be expected to reduce the topochemical preference for lignin removal from the cell walls. Finally it should be noted that only one type of delignification (acid chlorite) and one type of wood (white birch) have been covered by the present investigation. However, previous results for the kraft, acid sulfite, neutral sulfite (8), and acid chlorite (2, 9) pulping of spruce wood and for the kraft and neutral sulfite pulping of birch wood (31) are all consistent with the results described herein. Therefore it seems likely that the trends noted and the principles elucidated in the present paper are of general applicability to the delignification of wood. 1. P. A. AHLGREN and D. A. I. GORING. Can. J. Chern. 49, 1272(1971). 2. P. A. AHLGREN, W. Q. YEAN, and D. A. 1. GORING. Tappi, 54,737 (1971). 3. J. E. STONE and A. M. SCALLAN. Pulp Paper Mag. Can. 69, T288 (1%8). 4. W. Q. YEAN and D. A. 1. GORING. Svensk Papperstidn. 71,739(1968). 5. J. G. MCNAUGHTON, W. Q. YEAN, and D. A. I. GORING. Tappi, 50,548(1967). 6. E. BOOKER and C. SCHUERCH. Tappi, 41,650 (1958). 7. R. NELSON and C. SCHUERCH. J. Polymer Sci. 22,435 (1956). 8. A. R. PROCTER, W. Q. YEAN, and D. A. 1. GORING. Pulp Paper Mag. Can. 68, T445 (1%7). 9. J. R. WOOD, P. A. AHLGREN, and D. A. I. GORING. Svensk Papperstidn. 71,15 (1972).

8 KERR AND GORING: DELIGNIFICATION OF WOOD J. H. KALISCH and W. BEAZLEY. Pulp Paper Mag. Can. 61, T452(1%0). 11. I. CROON. Pulp Paper Mag. Can. 66,l71(1965). 12. J. A. N. Scorr, A. R. PROCTER, B. J. FERGUS, and D. A. I. GORING. Wood Sci. Technol. 3,73 (1969). 13. L. E. WISE, M. MURPHY, and A. A. D'ADDIECO. Paper Trade J. 122,35 (1946). 14. T. E. TIMELL. Tappi, 44,88 (1961). 15. Tappi Standard. T13m Tappi Standard. T223m A. G. SCHONING and G. JOHANSSON. Svensk Papperstidn. 68,607 (1965). 18. W. G. CAMPBELL and I. R. C. MCDONALD. J. Chem. SOC. 3180(1952). 19. Tappi Standard, T203m L. G. AGGEBRANDT and 0. SAMUELSON. J. Appl. Polymer Sci. 8,2801 (1964). 21. J. E. STONE and A. M. SCALLAN. Cellulose Chem. Technol. 2,343 (1968). 22. A. J. KERR. Ph.D. Thesis, McGill University, Montreal P. A. AHLGREN. Ph.D. Thesis, McGill University, Montreal A. R. PROCTER. Tappi, 55,424 (1972). 25. B. J. FERGUS, A. R. PROCTER, J. A. N. Sco~~,andD. A. I. GORING. Wood Sci. Technol. 3, 117 (1%9). 26. B. J. FERGusand D. A. GORIN RING. Holzforschung.24, 118 (1970). 27. P. R. GUPTA and D. A. I. GORING. Can. J. Chem. 38, 270 (1960). 28. A. REZANOWICH and D. A. I. GORING. J. Colloid Sci. 15,452 (1960). 29. A. J. KERR and D. A. 1. GORING. In preparation. 30. T. E. TIMELL. Svensk Papperstidn. 63,472(1%3). 31. B. J. FERGUS and D. A. I. GORING. Pulp Paper Mag. Can. 70, T314(1%9).