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NON-DESIRABLE CARBOHYDRATE REACTIONS IN PULPING AND BLEACHING
NON-DESIRABLE CARBOHYDRATE REACTIONS IN PULPING AND BLEACHING Goran Gellerstedt'. and Jiebiiig Li' 'Department of P u b and Paper Chernishy and Technologv Royai Instiiute of Technology,SE-100 44 Stockholm. Sweden
ABSTRACT
The remaining lignin content in chemical pulps is usually measured as the "Kappa Number" and, fiequently, this number has been transformed into a corresponding amount of residual lignin. In this paper it is demonstrated that the kappa number does not correlate exactly with the amount of remaining lignin Other oxidizable groups present in the pulp carbohydrates can also contribute and the extent of this contribution can vary largely depending on wood species and pulping procedure. In a subsequent bleaching operation these carbohydrate derived structures may or may not react depending on the bleaching agent(s) employed. This, in tim can result in bleached pulps still having considerable amounts of reactive but colourless structures being chemically attached to the fibre polysaccharides. The major contribution fiom the carbohydrates to the kappa number is hexenuronic acid which is formed under alkaline pulping conditions. In addition, other, still unknown, carbohydrate structures formed in the pulping process can contribute to various extents depending on the process and wood species. In the paper, a summary of our present knowledge concerning the structure of the carbohydrate derived reactive structures in haft pulp fibres, unbleached and bleached, is presented.
INTRODUCTION In haft pulping, wood is delignified at around 160-170 "(3 by the action of strong alkali which, together with hydrogen sulfide ions, promote cleavage of the lignin macromolecule into smaller alkali soluble hgments. The resulting unbleached kraft pulp is brownish in colour and still contains around 2-5% lignin attached to the fibres. In a subsequent bleaching operation, this lignin is removed leaving a colourless cellulosic fibre. The colour of haft pulps has usually been attributed to the presence of lignin which in the pulping process is modified such that chromophoric groups are introduced (1). In a model experiment with cellulose this view has, however, been modified (2). Thus, haft pulping of pure cellulose results in a certain formation of chromophores in the resulting "pulp" although the same experiment carried out in the presence of lignin gives an even more discoloured pulp. In the same investigation it was also shown that krafi pulping of cellulose in the presence of either xylan or glucomannan likewise gave rise to a discoloured cellulose. The results are summarized in Fig. 1 which also shows the UV-absorption curves of the pulping liquors corresponding to the various experiments described above. From the UV-spectra it can be seen that experiments carried out with only polysaccharides present result in an absorption maximum around 300 nm whereas the experiment with lignin gives a maximum around 280 nm. The latter
348
New polymers and materials
is well-known and attributed to the lignin aromatic ring while the identity of the former
Polymer
Brightness of "pulp"
UV-spectra of liouors Absorbance
A. Cellulose
80.1
B. Cellulose t Lignin (25%)
65.6
C. Cellulose + Xylan (25%)
69.5
r
\
D. Cellulose + Glucomnnnan (25%) 72.2
230
250
270
290
310
1.nm
Figure 1. Brightness of the resulting "pulp" as well as UV-spectra of the resulting pulping liquors after kraft pulping of pure cellulose (cotton linters; original brightness 90.5%) in the absence/presence of either lignin, xylan or glucomannan (2). is unknown. The figure also shows one further absorption maximum, viz. at around 260 nm,in the experiment with xylan (curve C). In fiuther work, Theander et a1 were able to demonstrate that simple sugar molecules like glucose or xylose to a small extent can be converted to a variety of aromatic and olefinc structures by treatment with alkali at an elevated temperature (3). These structures are shown in Fig. 2. Such and similar structures have also been found in the black liquor from kraft pulping of pine (4) and it was recently shown that at least the cyclopentenone structures are formed early in the h a f t cook (5), i.e. in that part of the cook when the majority of the hemicelluloses are degraded (Fig. 3). Treatment of birch cnm
won
on
Figure 2. Conversion of either D-glucose or D-xylose to aromatic and olefinic structures by the action of alkali at an elevated temperature (3).
Non-desirable carbohydrate reactions
Krnft rook
+ Black liquor
Pine wood +White liquor --->Pulp R!
349
,OH
Figure 3. Cyclopentenones found in the black liquor after kr& pulping of pine wood (4, 5).
kraft pulp with strong alkali at an elevated temperature has also been shown to result in the formation of cyclopentenone structures. In this case it is not known, however, if these compounds were present in the pulp or formed during the treatment (6). The ability of 4-0-methyl-uronic acids to yield the corresponding unsaturated hexenuronic acid by loss of methanol has been known b r a long time. In pulping, this reaction was fmt described in a model experiment by the use of 2-0-(4-O-methyLP-Dglucopyranosyluronic acid)-D-xylitol(7). When treating this compound with alkali at 150 OC, the corresponding unsaturated compound was found and identified as hexenuronic acid-D-xylitol. In its protonated form, this acid exhibits a UV-absorption maximum at around 230 nm whereas in alkali the maxinium is shifted to 260 nrn (8), i.e. the value found by Theander (see Fig. 1). More recently, the presence of hexenuronic acid as part of the xylan in krafi pulps has been thoroughly investigated by Buchert et al(9). By employing enzymatic techniques coupled with N M R the hexenuronic acid moiety could be identified and quantified directly on pulp samples. Contrary to the traditional sugar analysis based on acid hydrolysis, this type of analysis does not result in any degradation of the acid sensitive hexenuronic acid. Alternative analytical methods for the quantificationof hexenuronic acid in pulp samples have been developed and give results in good agreement with each other (10). In chemical pulping, the kappa number is fiequently used as a tool for process control. The method is based on the oxidation of a pulp .sample with an excess of acidic potassium permanganate, which, under specified conditiims, is allowed to react with the pulp. After 10 min, the unreacted permanganate is determined and the permanganate consumption is calculated. Although the method states that there is no direct relationship between kappa number and lignin content, a conversion factor is often used to calculate the lignin content of the pulp. Kappa number measurements are also used fiequently in technical investigations on pulping and bleaching processes in order to evaluate the degree of delignification. The results are used in e.g. comparisons of the influence of different pulping and bleaching parameters imd of different bleaching agents. As discussed above, there are, however, reasons to believe that not only lignin but also a variety of carbohydrate derived structures may contribute to the kappa number
350
New polymers and materials
measurement since permanganate is a powerful oxidant. Furthermore, the carbohydrate derived structures may have a different reactivity as compared to lignin thus giving rise to suboptimization of e.g. a bleaching stage if only the kappa number is used for process evaluation. Therefore, an attempt has been made to identify and quantifL different types of structures that can contribute to the kappa number in unbleached and bleached chemical pulps thereby facilitating a more thorough understanding of the chemical structures and changes that take place in the fibres when going from wood to bleached chemical pulp.
RESULTS AND DISCUSSION In order to determine the contribution to the kappa number h-om different types of structural units in chemical pulps, a series of oxidation experiments were carried out using lignin model compounds as well as isolated lignins and a variety of other compounds containing oxidizable functional groups. Based on these experiments, it was found that lignin, irrespective of type and origin, gave a consumption of permanganate in the kappa number method of approximately 11.6 equivalents of permanganate per mole of phenylpropane units. For hexenuronic acid, the corresponding value was found to be around 8.5 equivalents; a value based on both model experiments and on experiments with pulp samples (1 1). Other types of structuresKunctiona1groups can, however, also contribute as shown in Fig. 4. Although the presence of some of these in unbleached pulp fibres have not been unequivocally identified, they all constitute possible structures based on the discussion above. In a series of unbleached chemical pulps, the contribution to the kappa number was determined using the values for lignin and hexenuronic acid shown in Fig. 4.The amount of lignin in the pulps was determined as Klason lignin and recalculated as kappa number. For hexenuronic acid, the recalculation was based on the amount of hexenuronic acid, analysed as described in Ref 12. On all pulps, the kappa number was
1-
-t
1 1 ~ . polymeric lignin
~ ~ ~ _ _ ..._... _ . _ _ _ _ _ _ _ _
-0 0
-____
8
E
- 6
0
~
___ __ __
____....!?&8:6
1,
Main contributors
hexeneuronic acid
7.7
\
0 .0 05
0.22
0.14
0.081
a
Pentose hexose uronic acid
04
\
5.7
Small contributors
t
Noticeable contributors
:
n
2.0
\.
Figure 4. Different types of structuredfunctional groups that consume permanganate
when subjected to a kappa number determination.
Non-desirable carbohydrate reactions
35 1
0 other non-lignin mHaxaneuronic acid
Figure 5. The contribution to kappa number fiom Klason lignin and hexenuronic acid with all values re-calculated in kappa number units. The values for "other non-lignin" structures are taken by difference to the analysed kappa numbers in the pulps. also measured directly. As shown in Fig. 5, all pulps gave calculated values that were lower than the actual measured ones with a discrepancy in the order of 2-4 kappa number units, referred to in the figure as "other non-lignin" structures. It can also be seen that the contribution fiom hexenuronic acid varies widely depending on both pulping process and wood species. Thus, soda based processes seem to promote a dissolution or degradation of the hexenuronic acid which, consequently, is present in a very small amount. In birch kraft pulps, on the other hand, the contribution fiom hexenuronic acid is substantial and may, in fact, exceed that fiom lignin. In birch haft pulps, the total contribution to the kappa number fiom lignin is rather small and, consequently, the kappa number does not at all reflect the degree of delignification. Based on the results presented above, a m h e r series of pulp samples was analysed with respect to the contribution to kappa number fiom hexenuronic acid (Table 1). In agreement with the well-known fact that birch kr& pulps usually contain much more xylan and, consequently, more uronic acid groups than pine and spruce pulps it was found that the kappa number contribution was higher for the former. When these pulps
Table 1.
The contribution to kappa number fiom hexenuronic acid (HexA) groups present in some unbleached and bleached hiiftpulps. Bleaching with oxygen (0),ozone (Z) and hydrogen peroxide (P). Q denotes a treatment with chelating agent; n.d. = not determined.
Pulp Unbleached, pine 0-bleached, pine OZQP-bleached, pine OQPQ(PO)-bleached, pine Unbleached, birch OQP-bleached, birch
Kappa No 18.4 10.4 n.d. n.d.
14.5 4.5
HexA contribution 2.3 2.3 0.3 1.9 4.9 3.4
352
New polymers and materials
-+
Unbleached pulp bleaching
h,,H COOH
Equivalentslrnole
10.8
RxcooH
R
I d c o o H LOOH co
COOH
9.2
4.2
2.0
Figure 6. Consumption of permanganate in the oxidation of various structures assumed to be present in bleached chemical pulps.
were subjected to bleaching, none of the investigated sequences was able to completely eliminate the hexenuronic acid, however. Thus, despite being a powerhl oxidant for hexenuronic acid groups in addition to lignin (13), ozone bleaching (2)of a pine kraft pulp sample did not result in a complete elimination of these groups. For bleaching agents like oxygen (0)and hydrogen peroxide (P) which are used in alkaline medium, the kappa number reduction proceeds smoothly but the amount of hexenuronic acid in the pulp is only affected to a small extent (12). The fact that bleaching of krafi pulps does not result in a complete removal of hexenuronic acid (and possibly of other non-lignin structures) is supported indirectly by studies on the heat-induced yellowing tendency of such pulps. Thus, it has been demonstrated that the yellowing tendency of a variety of bleached pine and birch kraft pulps can be reduced after subjecting the pulps to treatment with xylanase (14). In agreement with the discussion above, the birch pulps showed the greatest reduction of yellowing after this treatment. The chemical reactions of lignin in bleaching have been elucidated in a large number of studies both with lignin model compounds and with isolated lignin samples (1 5 ) . In summary, it can be concluded that the aromatic rings generally are the most reactive structures present in the lignin. Chlorine and ozone as well as chlorine dioxide and oxygen are all oxidants which are able to degrade the aromatic ring albeit with very large differencies in reactivity. Hydrogen peroxide, on the other hand, does not react with aromatic rings, whether phenolic or not, unless the peroxide is allowed to decompose into radical species. This bleaching agent is, however, superior in eliminating chromophoric groups and it may also degrade lignin through oxidation reactions in the side chains. In the bleaching of chemical pulps, a successive oxidation of the remaining lignin will thus take place with formation of degraded and partly degraded aromatic rings and/or side chains. At the same time, it can be anticipated that oxidizable non-lignin structures and hexenuronic acid will or will not react depending on the chosen bleaching agent. In the kappa number determination, intact aromatic rings consume 1 1.6 equivalents of permanganate per mole of phenylpropane units as shown in Fig. 4. The contribution &om some other possible structures in unbleached pulps are also shown in that figure. Some other types of structures assumed to be present in bleached or partly bleached pulps give, when subjected to permanganate oxidation, the results shown in Fig. 6. The frst two of these structures are known to be formed when lignin is oxidatively degraded whereas the presence of the furan derivative is more uncertain. The a-ketoacid structure may be present in oxidized carbohydrates. All of these structures contribute to a
Non-de:sirable carbohydrate reactions
353
consumption of permanganate although the number of equivalents is different from that of aromatic rings. The fact that even fully bleached pulps, irrespective of the bleaching sequence, usually have a measurable kappa number can, however, be expIained. In order to further detail the contribution fiom various,structures to the kappa number in chemical pulps, a modified kappa number determination procedure has been developed (16). This is based on the fact that mercury (11) can be used to eliminate double bonds in the so-called oxymercuration reaction. Ifthis is followed by a demercuration step, i.e. a treatment with sodium borohydride, almost all interfering structures can be eliminated leaving the aromatic rings as the sole source of permanganate consumption in the kappa number determination. The reaction sequence, denoted Ox-Dem kappa number, is outlined in Fig. 7. In the first reaction step, hexenuronic acid is eliminated fiom the pulp and dissolved. Other types of double bonds also add mercury(T1)-ions and, after hydroxylation, the mercury(I1)-ions are reductively eliminated by the action of borohydride. At the same time, keto groups present in aldehydes and ketones are reduced to the corresponding alcohol groups. The effects of applying the Ox-Dem kappa number to some unbleached birch krafl pulps are shown in Fig. 8. In these two series of pulping experiments, birch wood was used and the krafi cooks performed under identical conditions with exception of the concentration of alkali. The reaction time at the maximum temperature was varied and is expressed as the H-factor. From both series of cooks, the normal as well as the OxDem kappa number was measured on the resulting pulps. It can clearly be seen that a
Llgnin containing pulp + exceaa of KMnO, Determination of unreacted KMnO,
Figure 7. The reaction sequence employed to selectively analyse the content of lignin in chemical pulps by kappa number determination;the "Ox-Dem" kappa number.
354
New polymers and materials
160 "C,(HO-]= 0.6 M
160 "C, [HO-J= 1.0 M
Figure 8. Normal and Ox-Dem kappa numbers of two series of birch kraft pulps prepared with variation of cooking time at two different alkalinity levels. large discrepancy exists between the two measured values in any given pulp. An increased alkalinity during the cook results, as expected, in an enhanced delignification rate but it is also obvious that, at a prolonged cooking time, there is a tendency for an increase of the apparent lignin content in the pulp. The reason for this is not known but reactions of the type discussed above (see Fig. 1 and 2) may well be responsible. Two of the birch pulps, both having a normal kappa number around 16, were chosen and subjected to bleaching in an OQP sequence. As before, both the normal and the OxDem kappa number was measured after each bleaching stage. The results are shown in Fig. 9 and demonstrate that the large difference between the two types of kappa number, 18
2 '
I
Unbleached
* 140 C,
(OH-)= 0 . a cook Kormal Kappa No.
* 160 C,
(OH-)= I.0M cook K o m l Kappa No.
1
Aftcr 0 2
I
After OQP 140 C, (OH-I=0.6M COOL
Ox-Dcm-Kappa No.
* Ox-Dcm-Kappa 160 C, (OH-]=1.OM COO^ No.
Figure 9. Normal and Ox-Dem kappa number of birch krafi pulps after bleaching in an OQP sequence.
Non-desirable carbohydrate reactions
355
observed after the cook, still remain after the subsequent bleaching operation. Thus, the oxygen (0)bleaching stage is an efficient delignification stage, particularly if the preceding cook is carried out at the lower alkalinity 1evt:l. The hydrogen peroxide (P) stage, on the other hand, does not give much lignin dissolution but it can be seen that the normal kappa number is lowered indicating the presence of structures in the pulp being non-aromatic but reactive towards hydrogen peroxide. These structures are still unknown since it has been shown in other work that hexenuronic acid does not react with hydrogen peroxide under bleaching conditions (1 1, 12).
CONCLUSIONS Based on the results presented in this work as well as the earlier work on conversion reactions of carbohydrates in alkaline pulping, it can be concluded that carbohydrate structures contribute to the kappa number and possibly also to the colour of unbleached chemical (haft) pulps. A major portion of the non-lignin related kappa number originates from hexenuronic acid, formed through elimination of methanol from the uronic acid moieties in xylan. Other non-lignin structures are, however, also present in the pulp in amounts that vary with pulp type and wood species. Indications have been obtained that pulping under non-optimal delignification conditions may result in a formation of new structures in the pulp which behave like lignin in the kappa number measurement. Oxygen is an efficient delignification agent but neither oxygen nor hydrogen peroxide is able to degrade hexenuronic acid thus giving bleached chemical pulps still containing a considerable amount of this structure.
ACKNOWLEDGEMENTS Financial support to one of us (JL) f?om The Swedish Pulp and Paper Research Foundation, Grants No 87 and 21 1, is gratefully acknowledged. The authors are also much indebted to professor Olof Theander for generously sharing his knowledge about carbohydrate reactions in pulping with us.
REFERENCES 1.
J Gierer, The reactions of lignin during pulping, Svensk Papperstidn, 1970 73 571-596.
2.
0 Theander, in S.S. Stivala, V. Crescenzi and I.C.M. Dea (Eds.), Industrial Polysaccharides. The Impact of Biotechnology and Advanced Methodologies, New York, Gordon and Breach Science Publishers, 1987, pp 481-492.
3.
I Forsskal, T Popoff and 0 Theander, Reactions of D-xylose and D-glucose in alkaline aqueous solutions, Carbohydr Res, 1976 48 13-2 1.
4.
K Niemelii,The formation of 2-hydroxy-2-cyclopenten-1-ones from - polysaccharides during haft pulping of pine wood, Carbohydr Res, 1988 184 13 1-137.
5.
F Berthold and G Gellerstedt, Reactive structures formed during the initial phase of a haft cook, 7" int conf Wood and Pulping Chemistry,Beijing 1993. Proceedings 3 160- 163.
356
New polymers and materials
6.
G Gellerstedt and J Li, Extraction and fractionation of residual lignin from birch kraft pulp, 3'* European Workshop on Lignocellulosics and Pulp, Stockholm 1994. Proceedings 2 15-218.
7.
M H Johansson and 0 Samuelson, Epimerization and degradation of 2-0-(4-Omethyl-a-D-glucopyranosyluronicacid)-D-xylitol in alkaline medium, Carbohydr Res. 1971 54 295-299.
8.
A Torngren and G Gellerstedt, The nature of organic bound chlorine fiom ECFbleaching found in kraft pulp, 9" int symp Wood and Pulping Chemistry, Montreal 1997. Proceedings 1M2- 1--4.
9.
J Buchert, A Teleman, V Harjunpw M Tenkanen, L Viikari and T Vuorinen, Effect of cooking and bleaching on the structure of xylan in conventional pine krafi pulp, Tappi J, 1995 78:ll 125-130.
10.
M Tenkanen, G Gellerstedt, T Vuorinen, A Teleman, M Perttula, J Li and J Buchert, Determination of hexenuronic acid in softwood krafi pulps by three different methods, J. Pulp Paper Sci. 1999 25 306-3 11.
11.
J Li and G Gellerstedt, The contribution to kappa number from hexenuronic acid groups in pulp xylan, Carbohydr Res, 1997 302 213-2 18.
12.
G Gellerstedt and J Li, An HPLC method for the quantitative determination of hexenuronic acid groups in chemical pulps, Carbohydr Res, 1996 294 41-51.
13.
N - 0 Nilvebrant and A Reimann, Xylan as a source for oxalic acid during ozone bleaching, 4h European Workshop on Lignocellulosics and Pulp, Stresa 1996. Proceedings 485-491.
14.
J Buchert, E Bergnor, G Lindblad, L Viikari and M Ek, The role of xylan and glucomannan in yellowing of krafi pulps, 8' int symp Wood and Pulping Chemisw, Helsinki 1995. Proceedings 3 43-48.
15.
C W Dence and D W Reeve, Pulp Bleaching. Principles and Practice, Atlanta, TAPPI PRESS, 1996.
16.
J Li and G Gellerstedt, Oxymercuration-demercuration-kappanumber; A more accurate estimation of lignin content in pulps, 5h European Workshop on Lignocellulosics and Pulp, Aveiro 1998. Proceedings 28 1-284.
ABSTRACT
The remaining lignin content in chemical pulps is usually measured as the "Kappa Number" and, fiequently, this number has been transformed into a corresponding amount of residual lignin. In this paper it is demonstrated that the kappa number does not correlate exactly with the amount of remaining lignin Other oxidizable groups present in the pulp carbohydrates can also contribute and the extent of this contribution can vary largely depending on wood species and pulping procedure. In a subsequent bleaching operation these carbohydrate derived structures may or may not react depending on the bleaching agent(s) employed. This, in tim can result in bleached pulps still having considerable amounts of reactive but colourless structures being chemically attached to the fibre polysaccharides. The major contribution fiom the carbohydrates to the kappa number is hexenuronic acid which is formed under alkaline pulping conditions. In addition, other, still unknown, carbohydrate structures formed in the pulping process can contribute to various extents depending on the process and wood species. In the paper, a summary of our present knowledge concerning the structure of the carbohydrate derived reactive structures in haft pulp fibres, unbleached and bleached, is presented.
INTRODUCTION In haft pulping, wood is delignified at around 160-170 "(3 by the action of strong alkali which, together with hydrogen sulfide ions, promote cleavage of the lignin macromolecule into smaller alkali soluble hgments. The resulting unbleached kraft pulp is brownish in colour and still contains around 2-5% lignin attached to the fibres. In a subsequent bleaching operation, this lignin is removed leaving a colourless cellulosic fibre. The colour of haft pulps has usually been attributed to the presence of lignin which in the pulping process is modified such that chromophoric groups are introduced (1). In a model experiment with cellulose this view has, however, been modified (2). Thus, haft pulping of pure cellulose results in a certain formation of chromophores in the resulting "pulp" although the same experiment carried out in the presence of lignin gives an even more discoloured pulp. In the same investigation it was also shown that krafi pulping of cellulose in the presence of either xylan or glucomannan likewise gave rise to a discoloured cellulose. The results are summarized in Fig. 1 which also shows the UV-absorption curves of the pulping liquors corresponding to the various experiments described above. From the UV-spectra it can be seen that experiments carried out with only polysaccharides present result in an absorption maximum around 300 nm whereas the experiment with lignin gives a maximum around 280 nm. The latter
348
New polymers and materials
is well-known and attributed to the lignin aromatic ring while the identity of the former
Polymer
Brightness of "pulp"
UV-spectra of liouors Absorbance
A. Cellulose
80.1
B. Cellulose t Lignin (25%)
65.6
C. Cellulose + Xylan (25%)
69.5
r
\
D. Cellulose + Glucomnnnan (25%) 72.2
230
250
270
290
310
1.nm
Figure 1. Brightness of the resulting "pulp" as well as UV-spectra of the resulting pulping liquors after kraft pulping of pure cellulose (cotton linters; original brightness 90.5%) in the absence/presence of either lignin, xylan or glucomannan (2). is unknown. The figure also shows one further absorption maximum, viz. at around 260 nm,in the experiment with xylan (curve C). In fiuther work, Theander et a1 were able to demonstrate that simple sugar molecules like glucose or xylose to a small extent can be converted to a variety of aromatic and olefinc structures by treatment with alkali at an elevated temperature (3). These structures are shown in Fig. 2. Such and similar structures have also been found in the black liquor from kraft pulping of pine (4) and it was recently shown that at least the cyclopentenone structures are formed early in the h a f t cook (5), i.e. in that part of the cook when the majority of the hemicelluloses are degraded (Fig. 3). Treatment of birch cnm
won
on
Figure 2. Conversion of either D-glucose or D-xylose to aromatic and olefinic structures by the action of alkali at an elevated temperature (3).
Non-desirable carbohydrate reactions
Krnft rook
+ Black liquor
Pine wood +White liquor --->Pulp R!
349
,OH
Figure 3. Cyclopentenones found in the black liquor after kr& pulping of pine wood (4, 5).
kraft pulp with strong alkali at an elevated temperature has also been shown to result in the formation of cyclopentenone structures. In this case it is not known, however, if these compounds were present in the pulp or formed during the treatment (6). The ability of 4-0-methyl-uronic acids to yield the corresponding unsaturated hexenuronic acid by loss of methanol has been known b r a long time. In pulping, this reaction was fmt described in a model experiment by the use of 2-0-(4-O-methyLP-Dglucopyranosyluronic acid)-D-xylitol(7). When treating this compound with alkali at 150 OC, the corresponding unsaturated compound was found and identified as hexenuronic acid-D-xylitol. In its protonated form, this acid exhibits a UV-absorption maximum at around 230 nm whereas in alkali the maxinium is shifted to 260 nrn (8), i.e. the value found by Theander (see Fig. 1). More recently, the presence of hexenuronic acid as part of the xylan in krafi pulps has been thoroughly investigated by Buchert et al(9). By employing enzymatic techniques coupled with N M R the hexenuronic acid moiety could be identified and quantified directly on pulp samples. Contrary to the traditional sugar analysis based on acid hydrolysis, this type of analysis does not result in any degradation of the acid sensitive hexenuronic acid. Alternative analytical methods for the quantificationof hexenuronic acid in pulp samples have been developed and give results in good agreement with each other (10). In chemical pulping, the kappa number is fiequently used as a tool for process control. The method is based on the oxidation of a pulp .sample with an excess of acidic potassium permanganate, which, under specified conditiims, is allowed to react with the pulp. After 10 min, the unreacted permanganate is determined and the permanganate consumption is calculated. Although the method states that there is no direct relationship between kappa number and lignin content, a conversion factor is often used to calculate the lignin content of the pulp. Kappa number measurements are also used fiequently in technical investigations on pulping and bleaching processes in order to evaluate the degree of delignification. The results are used in e.g. comparisons of the influence of different pulping and bleaching parameters imd of different bleaching agents. As discussed above, there are, however, reasons to believe that not only lignin but also a variety of carbohydrate derived structures may contribute to the kappa number
350
New polymers and materials
measurement since permanganate is a powerful oxidant. Furthermore, the carbohydrate derived structures may have a different reactivity as compared to lignin thus giving rise to suboptimization of e.g. a bleaching stage if only the kappa number is used for process evaluation. Therefore, an attempt has been made to identify and quantifL different types of structures that can contribute to the kappa number in unbleached and bleached chemical pulps thereby facilitating a more thorough understanding of the chemical structures and changes that take place in the fibres when going from wood to bleached chemical pulp.
RESULTS AND DISCUSSION In order to determine the contribution to the kappa number h-om different types of structural units in chemical pulps, a series of oxidation experiments were carried out using lignin model compounds as well as isolated lignins and a variety of other compounds containing oxidizable functional groups. Based on these experiments, it was found that lignin, irrespective of type and origin, gave a consumption of permanganate in the kappa number method of approximately 11.6 equivalents of permanganate per mole of phenylpropane units. For hexenuronic acid, the corresponding value was found to be around 8.5 equivalents; a value based on both model experiments and on experiments with pulp samples (1 1). Other types of structuresKunctiona1groups can, however, also contribute as shown in Fig. 4. Although the presence of some of these in unbleached pulp fibres have not been unequivocally identified, they all constitute possible structures based on the discussion above. In a series of unbleached chemical pulps, the contribution to the kappa number was determined using the values for lignin and hexenuronic acid shown in Fig. 4.The amount of lignin in the pulps was determined as Klason lignin and recalculated as kappa number. For hexenuronic acid, the recalculation was based on the amount of hexenuronic acid, analysed as described in Ref 12. On all pulps, the kappa number was
1-
-t
1 1 ~ . polymeric lignin
~ ~ ~ _ _ ..._... _ . _ _ _ _ _ _ _ _
-0 0
-____
8
E
- 6
0
~
___ __ __
____....!?&8:6
1,
Main contributors
hexeneuronic acid
7.7
\
0 .0 05
0.22
0.14
0.081
a
Pentose hexose uronic acid
04
\
5.7
Small contributors
t
Noticeable contributors
:
n
2.0
\.
Figure 4. Different types of structuredfunctional groups that consume permanganate
when subjected to a kappa number determination.
Non-desirable carbohydrate reactions
35 1
0 other non-lignin mHaxaneuronic acid
Figure 5. The contribution to kappa number fiom Klason lignin and hexenuronic acid with all values re-calculated in kappa number units. The values for "other non-lignin" structures are taken by difference to the analysed kappa numbers in the pulps. also measured directly. As shown in Fig. 5, all pulps gave calculated values that were lower than the actual measured ones with a discrepancy in the order of 2-4 kappa number units, referred to in the figure as "other non-lignin" structures. It can also be seen that the contribution fiom hexenuronic acid varies widely depending on both pulping process and wood species. Thus, soda based processes seem to promote a dissolution or degradation of the hexenuronic acid which, consequently, is present in a very small amount. In birch kraft pulps, on the other hand, the contribution fiom hexenuronic acid is substantial and may, in fact, exceed that fiom lignin. In birch haft pulps, the total contribution to the kappa number fiom lignin is rather small and, consequently, the kappa number does not at all reflect the degree of delignification. Based on the results presented above, a m h e r series of pulp samples was analysed with respect to the contribution to kappa number fiom hexenuronic acid (Table 1). In agreement with the well-known fact that birch kr& pulps usually contain much more xylan and, consequently, more uronic acid groups than pine and spruce pulps it was found that the kappa number contribution was higher for the former. When these pulps
Table 1.
The contribution to kappa number fiom hexenuronic acid (HexA) groups present in some unbleached and bleached hiiftpulps. Bleaching with oxygen (0),ozone (Z) and hydrogen peroxide (P). Q denotes a treatment with chelating agent; n.d. = not determined.
Pulp Unbleached, pine 0-bleached, pine OZQP-bleached, pine OQPQ(PO)-bleached, pine Unbleached, birch OQP-bleached, birch
Kappa No 18.4 10.4 n.d. n.d.
14.5 4.5
HexA contribution 2.3 2.3 0.3 1.9 4.9 3.4
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New polymers and materials
-+
Unbleached pulp bleaching
h,,H COOH
Equivalentslrnole
10.8
RxcooH
R
I d c o o H LOOH co
COOH
9.2
4.2
2.0
Figure 6. Consumption of permanganate in the oxidation of various structures assumed to be present in bleached chemical pulps.
were subjected to bleaching, none of the investigated sequences was able to completely eliminate the hexenuronic acid, however. Thus, despite being a powerhl oxidant for hexenuronic acid groups in addition to lignin (13), ozone bleaching (2)of a pine kraft pulp sample did not result in a complete elimination of these groups. For bleaching agents like oxygen (0)and hydrogen peroxide (P) which are used in alkaline medium, the kappa number reduction proceeds smoothly but the amount of hexenuronic acid in the pulp is only affected to a small extent (12). The fact that bleaching of krafi pulps does not result in a complete removal of hexenuronic acid (and possibly of other non-lignin structures) is supported indirectly by studies on the heat-induced yellowing tendency of such pulps. Thus, it has been demonstrated that the yellowing tendency of a variety of bleached pine and birch kraft pulps can be reduced after subjecting the pulps to treatment with xylanase (14). In agreement with the discussion above, the birch pulps showed the greatest reduction of yellowing after this treatment. The chemical reactions of lignin in bleaching have been elucidated in a large number of studies both with lignin model compounds and with isolated lignin samples (1 5 ) . In summary, it can be concluded that the aromatic rings generally are the most reactive structures present in the lignin. Chlorine and ozone as well as chlorine dioxide and oxygen are all oxidants which are able to degrade the aromatic ring albeit with very large differencies in reactivity. Hydrogen peroxide, on the other hand, does not react with aromatic rings, whether phenolic or not, unless the peroxide is allowed to decompose into radical species. This bleaching agent is, however, superior in eliminating chromophoric groups and it may also degrade lignin through oxidation reactions in the side chains. In the bleaching of chemical pulps, a successive oxidation of the remaining lignin will thus take place with formation of degraded and partly degraded aromatic rings and/or side chains. At the same time, it can be anticipated that oxidizable non-lignin structures and hexenuronic acid will or will not react depending on the chosen bleaching agent. In the kappa number determination, intact aromatic rings consume 1 1.6 equivalents of permanganate per mole of phenylpropane units as shown in Fig. 4. The contribution &om some other possible structures in unbleached pulps are also shown in that figure. Some other types of structures assumed to be present in bleached or partly bleached pulps give, when subjected to permanganate oxidation, the results shown in Fig. 6. The frst two of these structures are known to be formed when lignin is oxidatively degraded whereas the presence of the furan derivative is more uncertain. The a-ketoacid structure may be present in oxidized carbohydrates. All of these structures contribute to a
Non-de:sirable carbohydrate reactions
353
consumption of permanganate although the number of equivalents is different from that of aromatic rings. The fact that even fully bleached pulps, irrespective of the bleaching sequence, usually have a measurable kappa number can, however, be expIained. In order to further detail the contribution fiom various,structures to the kappa number in chemical pulps, a modified kappa number determination procedure has been developed (16). This is based on the fact that mercury (11) can be used to eliminate double bonds in the so-called oxymercuration reaction. Ifthis is followed by a demercuration step, i.e. a treatment with sodium borohydride, almost all interfering structures can be eliminated leaving the aromatic rings as the sole source of permanganate consumption in the kappa number determination. The reaction sequence, denoted Ox-Dem kappa number, is outlined in Fig. 7. In the first reaction step, hexenuronic acid is eliminated fiom the pulp and dissolved. Other types of double bonds also add mercury(T1)-ions and, after hydroxylation, the mercury(I1)-ions are reductively eliminated by the action of borohydride. At the same time, keto groups present in aldehydes and ketones are reduced to the corresponding alcohol groups. The effects of applying the Ox-Dem kappa number to some unbleached birch krafl pulps are shown in Fig. 8. In these two series of pulping experiments, birch wood was used and the krafi cooks performed under identical conditions with exception of the concentration of alkali. The reaction time at the maximum temperature was varied and is expressed as the H-factor. From both series of cooks, the normal as well as the OxDem kappa number was measured on the resulting pulps. It can clearly be seen that a
Llgnin containing pulp + exceaa of KMnO, Determination of unreacted KMnO,
Figure 7. The reaction sequence employed to selectively analyse the content of lignin in chemical pulps by kappa number determination;the "Ox-Dem" kappa number.
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New polymers and materials
160 "C,(HO-]= 0.6 M
160 "C, [HO-J= 1.0 M
Figure 8. Normal and Ox-Dem kappa numbers of two series of birch kraft pulps prepared with variation of cooking time at two different alkalinity levels. large discrepancy exists between the two measured values in any given pulp. An increased alkalinity during the cook results, as expected, in an enhanced delignification rate but it is also obvious that, at a prolonged cooking time, there is a tendency for an increase of the apparent lignin content in the pulp. The reason for this is not known but reactions of the type discussed above (see Fig. 1 and 2) may well be responsible. Two of the birch pulps, both having a normal kappa number around 16, were chosen and subjected to bleaching in an OQP sequence. As before, both the normal and the OxDem kappa number was measured after each bleaching stage. The results are shown in Fig. 9 and demonstrate that the large difference between the two types of kappa number, 18
2 '
I
Unbleached
* 140 C,
(OH-)= 0 . a cook Kormal Kappa No.
* 160 C,
(OH-)= I.0M cook K o m l Kappa No.
1
Aftcr 0 2
I
After OQP 140 C, (OH-I=0.6M COOL
Ox-Dcm-Kappa No.
* Ox-Dcm-Kappa 160 C, (OH-]=1.OM COO^ No.
Figure 9. Normal and Ox-Dem kappa number of birch krafi pulps after bleaching in an OQP sequence.
Non-desirable carbohydrate reactions
355
observed after the cook, still remain after the subsequent bleaching operation. Thus, the oxygen (0)bleaching stage is an efficient delignification stage, particularly if the preceding cook is carried out at the lower alkalinity 1evt:l. The hydrogen peroxide (P) stage, on the other hand, does not give much lignin dissolution but it can be seen that the normal kappa number is lowered indicating the presence of structures in the pulp being non-aromatic but reactive towards hydrogen peroxide. These structures are still unknown since it has been shown in other work that hexenuronic acid does not react with hydrogen peroxide under bleaching conditions (1 1, 12).
CONCLUSIONS Based on the results presented in this work as well as the earlier work on conversion reactions of carbohydrates in alkaline pulping, it can be concluded that carbohydrate structures contribute to the kappa number and possibly also to the colour of unbleached chemical (haft) pulps. A major portion of the non-lignin related kappa number originates from hexenuronic acid, formed through elimination of methanol from the uronic acid moieties in xylan. Other non-lignin structures are, however, also present in the pulp in amounts that vary with pulp type and wood species. Indications have been obtained that pulping under non-optimal delignification conditions may result in a formation of new structures in the pulp which behave like lignin in the kappa number measurement. Oxygen is an efficient delignification agent but neither oxygen nor hydrogen peroxide is able to degrade hexenuronic acid thus giving bleached chemical pulps still containing a considerable amount of this structure.
ACKNOWLEDGEMENTS Financial support to one of us (JL) f?om The Swedish Pulp and Paper Research Foundation, Grants No 87 and 21 1, is gratefully acknowledged. The authors are also much indebted to professor Olof Theander for generously sharing his knowledge about carbohydrate reactions in pulping with us.
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J Gierer, The reactions of lignin during pulping, Svensk Papperstidn, 1970 73 571-596.
2.
0 Theander, in S.S. Stivala, V. Crescenzi and I.C.M. Dea (Eds.), Industrial Polysaccharides. The Impact of Biotechnology and Advanced Methodologies, New York, Gordon and Breach Science Publishers, 1987, pp 481-492.
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New polymers and materials
6.
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J Buchert, A Teleman, V Harjunpw M Tenkanen, L Viikari and T Vuorinen, Effect of cooking and bleaching on the structure of xylan in conventional pine krafi pulp, Tappi J, 1995 78:ll 125-130.
10.
M Tenkanen, G Gellerstedt, T Vuorinen, A Teleman, M Perttula, J Li and J Buchert, Determination of hexenuronic acid in softwood krafi pulps by three different methods, J. Pulp Paper Sci. 1999 25 306-3 11.
11.
J Li and G Gellerstedt, The contribution to kappa number from hexenuronic acid groups in pulp xylan, Carbohydr Res, 1997 302 213-2 18.
12.
G Gellerstedt and J Li, An HPLC method for the quantitative determination of hexenuronic acid groups in chemical pulps, Carbohydr Res, 1996 294 41-51.
13.
N - 0 Nilvebrant and A Reimann, Xylan as a source for oxalic acid during ozone bleaching, 4h European Workshop on Lignocellulosics and Pulp, Stresa 1996. Proceedings 485-491.
14.
J Buchert, E Bergnor, G Lindblad, L Viikari and M Ek, The role of xylan and glucomannan in yellowing of krafi pulps, 8' int symp Wood and Pulping Chemisw, Helsinki 1995. Proceedings 3 43-48.
15.
C W Dence and D W Reeve, Pulp Bleaching. Principles and Practice, Atlanta, TAPPI PRESS, 1996.
16.
J Li and G Gellerstedt, Oxymercuration-demercuration-kappanumber; A more accurate estimation of lignin content in pulps, 5h European Workshop on Lignocellulosics and Pulp, Aveiro 1998. Proceedings 28 1-284.