STRUCTURAL CHANGES IN KRAFT PULP RESIDUAL LIGNIN UPON PERACETIC ACID TREATMENT Størker T. Moe Norwegian University of Science and Technology (NTNU), Dept. Chemical Engineering, Sem Saelands vei 4, N-7491 Trondheim, NORWAY Email:
[email protected] Arthur J. Ragauskas Institute of Paper Science and Technology, 500 10th Street NW, Atlanta GA 30318-5794, USA
ABSTRACT There is still some uncertainty about the mechanism of increased bleachability by interstage activation of twostage oxygen delignification (the OxO process). To possibly help in understanding the chemistry of the OxO process, residual lignin in kraft pulp bleached by oxygen and peracetic acid was extracted by the dioxane/water acidolysis method. The content of functional groups in the extracted lignin was determined by 13Cand 1H-NMR spectroscopy. For isothermally cooked kraft pulp, no significant difference could be seen in the chemical properties of residual lignin extracted with the dioxane/water acidolysis method. These data do not provide a chemical rationalization of the differences in bleachability between oxygen delignified (O) and peracetic acid treated (OPa) pulp. For polysulfide/ AQ kraft pulp cooked to high kappa numbers, the changes in functional group content is in accordance with studies performed on lignin model compounds. The results suggest that the increased bleachability of peracetic acid treated kraft-oxygen pulps is not a consequence of the content of functional groups in the lignin.
INTRODUCTION The negative environmental effects of bleach plant effluents is well known today. In principle, filtrates from bleach stages preceding the first chlorine-based bleach stage can be used for countercurrent wash of pulp and treated in the chemical recovery cycle, reducing the need for effluent treatment (system closure). Oxygen-based bleach chemicals have thus been investigated as alternatives to chlorine-based bleaching chemicals. Oxygen itself is a powerful bleaching agent, but oxygen has unfortunately too low selectivity to be used for extensive delignification for bleached pulp production. The limitations of oxygen as a bleach chemical has prompted investigations into methods for extending the oxygen delignification beyond 50%, which is regarded as the practical limit today1. Oxygen delignification beyond 50% seems to be a promising technology for reducing the lignin content of
pulp going to the bleach plant, thus reducing the amounts of organic effluents needing treatment before discharge into the recipient. Several principles for extending oxygen delignification has been investigated. The principal methods investigated for extending oxygen delignification are optimization of bleaching conditions in two-stage oxygen bleaching2, pretreatment of the pulp before singlestage oxygen delignification3-6 and two-stage oxygen delignification with interstage activation (the OxO process)6-11. Interstage activation can be performed using either chlorine-containing chemicals like chlorine dioxide or elemental chlorine7,8 or chlorine-free chemicals like peroxyacids or ozone6,9-11. System closure of the two-stage oxygen delignification puts some limitations on which chemical to use in the interstage activation. Chlorine-containing compounds may induce corrosion in the recovery system, and sulfur-containing compounds may offset the chemical balance in the recovery cycle. Thus, peracetic acid (peroxyacetic acid, Pa) and ozone initially seem like the most suitable activation agents for two-stage oxygen delignification with interstage activation, since they contain neither chlorine nor sulfur. Comparing peracetic acid and ozone for interstage treatment, pulp properties seem to be best preserved by the use of peracetic acid11. Although the beneficial effect of peracetic acid on pulp bleachability in a second oxygen stage seems to be well established11-12, the reason for the increased reactivity of pulp residual lignin towards oxygen after peracetic acid treatment seems somewhat unclear. One model compound study13 has indicated that peracetic acid is able to oxidise phenolic biphenyl compounds of the 5-5’ type and muconic acid-like structures may be formed. The degradation of so-called “condensed” structures may be one reason for the increased rectivity of pulp residual lignin after peracetic acid treatment. An increase in the content of carboxylic acid groups of the lignin may be expected when bleaching pulp with peracetic acid. Another model compound study14 has indicated that Pa is able to demethylate 3-methoxy-4-hydroxy type phenolic compounds. This leads to the formation of o-quinone type structures which may react further with the peroxyacid. Persulfuric acid (peroxymonosulfuric acid or Caro’s acid, Ps) seems to be able to introduce new phenolic hydroxyl groups on the aromatic ring of model compounds 13. In terms of lignin reactivity, this would generally be taken as an indication of increased bleachability by oxygen. Investigations of residual lignin15 have supported the conclusions drawn from model compound studies, however it was concluded that Pa and Ps are not very active on residual lignin.
Table 1: Characteristics of pulps used in this study Pulp
Kappa no. (TAPPI T236)
Process
PS/AQ
45.4
Simulated conventional continuous, 5 g/l PS. AQ charge: 0.05% on o.d. wood. Unbleached.
PS/AQ-O
25.7
PS/AQ pulp, single-stage oxygen
PS/AQ-OPa
16.0
PS/AQ pulp, single-stage oxygen, 6% Pa
PS/AQ-OO
15.7
PS/AQ pulp, double-stage oxygen
Kraft
22.4
ITC simulation. Unbleached
Kraft-O
10.4
ITC simulation pulp, single-stage oxygen
Kraft-OPa
10.7
ITC simulation pulp cooked to kappa no. 26.6, single-stage oxygen, 3% Pa Table 2: Conditions for bleaching
Stage
Time
Temp.
Chemical conditions
O
60 min.
110 °C
O2 pressure: 70 psi, decreasing to 0 psi in 1 hr. Alkali charge was 0.7-1.5% NaOH. 0.2% MgSO4 was added.
Pa
60 min.
80 °C
Equlibrium peracetic acid was used. The charge was 6% (PS/AQ-O) and 3% (Kraft-O), respectively. 0.2% DTPA, 0.25% MgSO4 was added. ing chemical composition may exhibit similar bleachability16. The scope of this work is thus to investigate the effect of peracetic acid treatment of oxygen delignified kraft pulp containing chemically different lignins.
50
Kappa no.
40 30
EXPERIMENTAL Pulping and Bleaching The raw material for all pulps was Norway spruce (Picea abies) chips supplied by Norske Skog Folla in Follafoss, Norway. The chips were cooked in a CRS forced-circulation laboratory digester described pre-
20 10 0 PS/AQ kraft
Kraft
Unbleached kappa Post O1 kappa Post Pa kappa Post O2 kappa Fig. 1
Bleaching strategy of pulps used
A recent study on oxygen delignification of kraft pulps containing apparently chemically rather different residual lignins has indicated that there is not necessary a direct correlation between the content of some functional groups in the residual lignin and the reactivity of the lignin towards oxygen and chlorine dioxide. Thus, the choice of cooking process also greatly influences the bleachabililty of the pulp in a second oxygen stage and pulps containing lignins of apparently widely vary-
viously16,17. Oxygen delignification was performed at 10% consistency in a peg-mixer as described previously16. Kappa number target (approximately 50% of brownstock kappa) was obtained by varying the NaOH charge at constant O 2 charge. Characteristics of the unbleached and oxygen delignified pulps are given in Table 1. After oxygen delignification, the pulps were bleached with equilibrium peracetic acid at 10% consistency in plastic bags. Parameters used during peracetic acid bleaching are given in Table 2. Bleachability with oxygen before and after peracetic acid activation was determined as (∆ kappa number)/(amount alkali charged). A summary of the bleaching results in terms of kappa number changes is given in Fig. 1. Lignin Isolation Residual lignins were isolated by refluxing the acetone-extracted pulp sample with 0.1M HCl in dioxane/ water (82:18) for two hours according to previously
Fig. 2
1
H- and 13C-NMR spectra of isothermal kraft pulp, oxygen delignified and peracetic acid treated
published procedures16,20. Yields were 43-46% of total lignin in pulp (as calculated from kappa numbers). Analyses Quantitative 1H-NMR and 13C-NMR spectra of underivatized lignin were recorded as decribed previously16,18-22 on a Bruker 400 MHz spectrometer. For the 1H-NMR spectra pentafluorobenzaldehyde (PFB) was used as a quantitative standard. The spectra were analyzed as described in the literature20-23.
Determination of peracetic acid in bleach liquor was performed as follows: In a 300 ml Erlenmeyer flask, 100 ml water (0 °C) was added to 25 ml TiOSO4 solution (2 g/l as TiOSO4) and 5 ml KI solution (1M). A predetermined amount of bleach liquor samle was added and the sample was diluted with water (0 °C) to a total volume of 150 ml. The sample was titrated to endpoint with 0.1N Na2S2O 3 solution using starch indicator. The titration volume gave the concentration of peracetic acid in the solution. When the endpoint was reached, 10 ml of NaF solution (5% by weight)
6.0 Unbleached O OPa OO
5.0
mmoles/g
mmoles/g
1.0
0.5
4.0 3.0 2.0 1.0
0.0
0.0
PS/AQ kraft Fig. 3
ITC kraft
Content of carboxylic acid groups determined by 1H-NMR spectroscopy
and three drops of saturated (NH4)MoO4 soution was added, and the sample was incubated at 35 °C for 55 min. The sample was again titrated with 0.1N Na2S2O3 solution until endpoint. The titration volume gave the concentration of free H2O2 in the bleach liquor. Pulp kappa numbers were measured according to TAPPI T236, and pulp viscosities were measured according to TAPPI T230. RESULTS AND DISCUSSION Two samples of NMR spectra are given in Fig. 2. The figure shows the NMR spectra of residual lignin extracted from oxygen delignified polysulfide/AQ pulp. As noted in the “Experimental” section, peak assignments in the 1H-NMR spectrum are as reported in the literature20,22. Carboxylic acid group content As has been described earlier16,18, oxygen delignification of kraft and PS/AQ kraft pulps will introduce
mmoles/g
3.0
2.0
Unbleached O OPa OO
1.0
0.0
PS/AQ kraft Fig. 4
ITC kraft
Content of phenolic hydroxyl groups determined by 1H-NMR spectroscopy
PS/AQ kraft Fig. 5
ITC kraft
Content of methoxyl groups determined by in Fig. 3
1H-NMR spectroscopy. Figure legends as
significant amounts of carboxylic acid groups on the residual lignin in the fiber. For kraft pulp residual lignins, the content of carboxylic acid groups does not seem to be significantly different irrespective of the bleaching agent (oxygen only or oxygen/peracetic acid) used for reaching a given kappa number. For PS/AQ kraft pulps, the post-oxygen Pa treatment does not lead to the same increase in carboxylic acid group content as a second oxygen stage. This may indicate that tpostoxygen he highly oxidised lignin is more readily extracted by a Pa stage than by a second O stage without Pa interstage treatment. Phenolic hydroxyl content It is well known that bleaching agents which react through a peroxide type reaction (e.g. oxygen and peroxide) primarily attacks lignin at the free phenolic groups. This can be seen for the PS/AQ kraft pulp. Again, it seems that for kraft pulp the content of free phenolic groups seems to be independent of the pathway chosen to reach a given kappa number (Fig. 4). Methoxyl group content A decrease in methoxyl group content is observed after both peracetic acid and oxygen bleaching of pulps (Fig. 5). This is in accordance with results reported in model compound studies. Condensation Measurement of C5 condensation by 13C-NMR spectroscopy (Fig. 6) is in accordance with the indirect measurement of condensation through aromatic proton content (Fig. 7), as a high degree of condensation should be reflected in a lower content of aromatic protons. Generally, a slight increase in the content of condensed phenolic units can be observed on peracetic acid treatment. The effect is smaller than what is observed from oxygen delignification.
14.0
0.5
12.0 10.0
0.4
mmoles/g
Fraction condensed
0.6
0.3 0.2
8.0 6.0 4.0
0.1
2.0
0.0
0.0
Fig. 6
ITC kraft
Condensation of C5 determined by 13CNMR spectroscopy. Figure legends as in Fig. 3
Reactivity of in situ lignin Comparing the reactivity of the lignin in the fiber, here taken as the kappa number decrease per % NaOH charged, one can see the well-known increased bleachability of Pa treated kraft-oxygen pulp. However, this increased bleachability does not correlate with any of the reported chemical features of the lignin. It is thus suggested that the increase in reactivity if in situ lignin towards oxygen after Pa interstage activation may not be a feature of the chemical properties of the lignin. Results from the studies over the lignins extracted from the polysulfide/AQ kraft pulps suggest that the increased bleachability may be a feature of the accessibility of the reactive lignin structures rather than a feature of the overall chemical properties of the lignin. Lignin-carbohydrate complexes (LCCs) in the pulps have not been studied, and one cannot exclude the possibility of chemical bonding between lignin and carbohydrate as the main cause for differences in bleachability.
CONCLUSIONS For isothermally cooked kraft pulp, no significant difference could be seen in the chemical properties of residual lignin extracted with the dioxane/water acidolysis method. These data do not provide a chemical rationalization of the differences in bleachability between oxygen delignified (O) and peracetic acid treated (OPa) pulp. For polysulfide/AQ kraft pulp cooked to high kappa numbers, the changes in functional group content is in accordance with studies performed on lignin model compounds. The results suggest that the increased bleachability of peracetic acid treated kraft-oxygen pulps is not a consequence of the content of functional groups in the lignin.
PS/AQ kraft Fig. 7
ITC kraft
Content of aromatic protons determined by in Fig. 3
1H-NMR spectroscopy. Figure legends as
5.0
Reactivity, kappa/%NaOH
PS/AQ kraft
4.0
3.0
D 2.0 1.0
0.0
Kraft-OPa Fig. 8
Kraft-O
Reactivity of in situ lignin towards oxygen
REFERENCES 1. McDonough, T.J. Oxygen Delignification, in Pulp Bleaching: Principles and Practice, Dence, C.W., Reeve, D.W., ed, p. 213 (1996) 2. Steffes, F., Bokström, M., Nordén, S. Breaking the Pulp Yield Barrier Symposium, TAPPI Press, Atlanta GA, p.183 (1998) 3. Kleppe, P.J., Storebråten, S. U.S. pat. 4,560,437. (1985) 4. Samuelson, H.O. U.S. pat. 4,602,982 (1987) 5. Fossum, G., Marklund, A. TAPPI J. 71(11), 79 (1988) 6. Liebergott, N. Proc. TAPPI Pulping Conference, San Diego, p. 357 (1994) 7. Lachenal, D., Choudens, C. De. Cellulose Chem. Technol. 20, 553 (1986) 8. Lachenal, D., Muguet, M. 1989 CPPA Annual Meeting, CPPA, Montreal, p. B187 (1989) 9. McGrouther, K., Allison, R.W. Appita J. 47(3), 238 (1994)
10. McGrouther, K. Allison, R.W., Appita J. 48(6), 445 (1995) 11. Li, X.-L., Fuhrmann, A., Rautonen, R. SCAN Forsk-rapport 658, KCL, Finland (1995) 12. Minja, R.W.A., Moe, S.T., Kleppe, P.J. Proc. Breaking the Pulp Yield Barrier Symposium, TAPPI, Atlanta GA, p.213 (1998) 13. Kawamoto, H., Chang, H.M., Jameel, H., Proc. 8th ISWPC, Helsinki, Vol. 1, p. 383 (1995) 14. Ni, Y., d’Entremont, M., Proc. 9th ISWPC, Montreal, Vol. 1 (Oral presentations), p. D6-1 (1997) 15. Lachenal, D., Delagoutee, T., Proc. 4th EWLP, Milano, p. 203 (1996) 16. Moe, S.T., Ragauskas, A.J., McDonough, T.J. 1998 Pulp Bleaching Conference Proceedings, KCL, Helsinki, p. 39 (1998) 17. Minja, R.J.A., Kleppe, P.J., Karlsen, T., Proc. TAPPI Pulping Conference, Atlanta, p.721 (1997) 18. Moe, S.T., Ragauskas, A.J. Holzforschung, in press (1999) 19. Gellerstedt, G., Pranda, J., Lindfors, E.L. J. Wood Chem. Technol. 14(4), 467 (1994) 20. Froass, P., Ragauskas, A.J. J. Wood Chem. Technol. 16(4), 347 (1996) 21. Robert, D. In: Methods in lignin chemistry, Lin, S.Y. and Dence, C.W. (eds), Springer Verlag, Berlin, p. 251 (1992) 22. Li, S., Lundquist, K. NPPR J. 3, 191 (1994) 23. Kringstad, K.P., Mörck, R. Holzforschung 37, 237 (1983)
