An Overview of Chemical Degradation Methods for Determining Lignin

Chapter 10

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An Overview of Chemical Degradation Methods for Determining Lignin Condensed Units 1

Y.-Z. Lai, H. Xu , and R. Yang Empire State Paper Research Institute, Faculty of Paper Science and Engineering, State University of New York College of Environmental Science and Forestry, Syracuse, NY 13210

Although a variety of chemical degradation methods can be used to characterize the chemical nature of lignocellulosic substrates, it is still a great challenge to reveal quantitatively all type of lignin units in situ, especially the diphenylmethane (DPM)-type structures. Recent studies on lignin DPM-model dimers indicate that they were rather reactive under both nitrobenzene and permanganate oxidation conditions including the formation of oxidized dimers characteristic of the parent units. The extent to which these dimeric products maybe used to evaluate lignin condensation reactions is discussed

Lignin in general plays a negative role in the chemical utilization of lignocellulosic materials. It must be modified and either partially or totally removed depending on the desired quality of final products. The aromatic groups in lignin exist in either phenolic or etherified form with the phenolic units accounting for less than 15% of wood lignin in situ (i). In alkaline pulping, the etherified lignin components are gradually degraded to phenolic structures resulting in dissolution (2). The last 5-10% of residual pulp lignin is known to be very resistant to alkaline degradation and generally requires a multiple bleaching sequence for a complete delignification (2-4). Current address: Union Camp Corporation, P.O. Box 3301, Princeton, N J 08543-3301

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

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240


Despite extensive studies (5,6), the chemical nature of residual lignin which causes resistance to degradation is still not fully understood notably; the role of lignin condensation reactions in alkaline pulping play in the process. Major challenges encountered in lignin analysis (7) are the lack of a specific procedure for a quantitative isolation of lignin in a pure and unaltered form as well as the limitation of analytical methods which can reveal quantitatively all types of lignin units in situ. Figure 1 indicates the major substitution patterns of guaiacyl nuclei that may occur in softwood or technical lignins. These include the uncondensed 1, β-5 2,5,5' 3 and 4-0-5 4, - linked units. Additionally, the diphenylmethane (DPM)-type units 5-7 maybe formed during acidic or alkaline treatments, and are generally thought of being absent in the wood lignin except small amounts of the a-6 type 6. Although a variety of chemical degradation methods (8) have been used to determine the nature of lignin units in situ, most of the product yields were not quantitative and can only be used to analyze certain types of condensed units. Also, none of the existing methods is capable of determining specifically DPM-type structures. This paper reviews briefly the principle and limitations of available degradation methods used for revealing the lignin condensed units in situ, as well as our recent attempts to develop specific procedures for determining the DPM-type structures. Acidoly sis, Thioacidolysis and D F R C Methods Acidolysis is conducted in a 9:1 dioxane-water mixture containing 0.2 M HC1 (9), whereas thioacidolysis (10) is a solvolysis in ethandiol with boron trifluoride etherate. Both procedures are effective in the cleavage of α and β-aryl ether linkages, and have been used to determine the content of uncondensed β-0-4 aryl ether structures in wood or pulp (II, 12) lignins. The thioacidolysis method, however, has the advantage of providing simpler products in higher yields (Figure 2). In addition to monomeric products, several dimeric products representing the β-5,5,5', and β-1 linkages have also been identified by acidolysis or a combined thioacidolysis and desulfurization technique (13). Recently, a "DFRC" method based on Derivatization (with acetyl bromide) Followed by Reductive Cleavage (in an acidic medium) and acetylation has been proposed to determine the uncondensed β-0-4 units (14). This procedure, based on the yield of 4-acetoxycinnamyl acetate 12, is quite similar to the thioacidolysis method. It is evident that all these acidic procedures determine mainly the uncondensed β-0-4 structures, and can only provide an indirect estimation of the condensed units. Nitrobenzene Oxidation

Figure 3 illustrates the typical alkaline nitrobenzene oxidation products from the guaiacyl units of Norway spruce wood lignin (15). Traditionally, the yield of vanillin

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


241

Uncondensed

OR

ι

Condensed Units in the Wood Lignin

5

6

7

Figure 1. Major types of guaiacyl nuclei.


242

Acidolysis

Hibbert's Ketones 10 OCH

3


H2COH HCOR

2

HCOR3

Thioacidolysis

γ OR}

och3

0ch3

8 Rl=Hor Alkyl R2 = Aryl R3 = H, Alkyl or Aryl

H2ÇOAC HC

DFRC

0ch3 Figure 2. Major products from acidolysis, thioacidolysis,and "DFRC" methods.

CHO

S

COOH

OCH

V

3

OH

OCH

3

ÔH

14 (5%)

13 (28%)

CHO

HOOO

OCH3

OHO

OCH3

OH 15 (1%)

16 (0.1%)

H CO^Y"

ÔCH

3

OH

3

OH

17 (0.8%)

Figure 3. Nitrobenzene oxidation products from guaiacyl units of Norway spruce wood lignin (15).



243 13 and vanillic acid 14 has been used to estimate the content of uncondensed guaiacyl units (76,17). These products, however, may be also produced to a certain extent from the D P M units as determined in lignin model compound experiments (18,19). On the other hand, 5-carboxyvanillin 15 and 5-formylvanillin 16 are derived from the C-5 condensed guaiacyl units, whereas dehydrodivanillin 17 is from the biphenyl units. Since the yield of nitrobenzene oxidation products from uncondensed structures is considerably affected by chemical nature of the side chain units, it is not entirely quantitative. Also, the yield of condensed products 15-17 is relatively small, and therefore this method serves only as a qualitative identification procedure.

Permanganate Oxidation Permanganate oxidation involves an initial methylation or ethylation of the phenolic hydroxyl groups followed by sequential oxidation with permanganate and then hydrogen peroxide to yield a variety of mono- and di-carboxylic acid derivatives (20). Figure 4 illustrates the yield proportion as percentages of total oxidation products identified from Norway spruce wood lignin. The relative amount of veratric acid 18 is frequently used as an indication of the uncondensed guaiacyl units. This acid, however, may also be derived partially from the D P M units as determined through lignin model compound reactions (21,22). Although the isohemipinic 19 and metahemipinic acid 20 acids are derived from C-5 and C-6 condensed units respectively, the quantitative contribution of lignin sub-structures to these acids is still not fully understood. For example, model compound reactions (20) indicate that the source of isohemipinic acid 19 may include, in addition to β-5 related structures 2, a mono-phenolic biphenyl unit or an incomplete alkylation of phenolic biphenyl units 3 (Figure 1). On the other hand, the dimeric acids 21 and 22 are derived from the 4-0-5 and biphenyl structures, respectively. The proportion of dicarboxylic acids 19-22 has been used to estimate the extent of lignin condensed units among the phenolic structural units (23,24). However, this procedure reveals little information on the nature of the etherified lignin component, which accounts for the bulk of wood or pulp lignins (1,5). Recent findings (25) suggest that the phenolic and etherified components of wood lignins differ significantly in chemical characteristics as reflected in the yield of nitrobenzene oxidation products. The phenolic lignin units appears to be appreciably less condensed than the etherified counterpart. Nucleus Exchange Reactions The phenyl nucleus exchange technique (26), developed originally by Funaoka and Abe (27,28), is based on the degradation of lignin by boron trifluoride in the presence of excess phenol. They observed that the uncondensed and a-5 diphenylmethane model compounds are much more reactive than other types of lignin model units and can be selectively oxidized to guaiacol 23 and catechol 24 (Figure 5). This contention, however, was recently questioned by Chan et al. (29).


C0 CH 2

3


OR

18 R i = R2 = H

(67%)

19 R i - C 0 2 H , R2 = H (10%)

20 R i = H , R 2 = C 0 2 H

CO CH 2

(8%)

3

C0 CH 2

CO2CH3

°-V0CH OR

CO2CH3

3

H co 3

0ch3

3

21 (3%) Figure 4. Permanganate oxidation products from guaiacyl units of Norway spruce wood lignin (20).

Figure 5. Major products from nucleus exchange reaction of uncondensed and a-5 diphenylemethane-type guaiacyl units (2(5).


245


There are no apparent explanations for these inconsistent findings. However, it appears that the reaction conditions-used by Chan et al. (29) were much more drastic than that of Funaoka judging from the absence of guaiacol in the reaction mixture. Funaoka et al. (26-28) reported that the yield of guaiacol accounted for more than 30% of the total products. The extent to which the selectivity of nucleus exchange reactions toward different lignin units may vary with reaction conditions merits a careful evaluation. Determination of Condensed Units Although a variety of chemical degradation techniques have been used to identify certain types of condensed units, these methods, as noted earlier, are not suitable for a direct and quantitative determination of total condensed structures. Permanganate oxidation is the only method able to measure C-5 and C-6 condensed units of the phenolic type. This method, however, is unable to differentiate between the specific types of C-5 condensed structures, and also does not measure etherified lignin structures. Additionally, no existing procedures are available for identification of DPM-type units. Attempts to Develop Methods for Detecting DPM units The reactivity of guaiacyl DPM-model dimers was examined under both nitrobenzene (19) and permanganate (22) oxidation conditions as part of our studies aimed at understanding the significance of DPM-type condensed units in residual kraft pulp lignin (5,30). These unsubstituted D P M dimers were shown to be quite reactive under oxidative conditions resulting in the formation of monomelic and dimeric products. The dimeric products as shown in Figures 6 and 7 are characteristic of the corresponding parent D P M units. Nitrobenzene oxidation. A common reaction of the a-5 26, a-6 27, and a-1 28 dimers is an oxidation of the α-methylene groups leading to the formation of diguaiacylketone derivatives 31,33 and 34, which were obtained in 5-28% yield (Figure 6). In contrast, the methylene group of 5,5-DPM dimer 29 was fairly resistant to oxidation. This dimer gave two 5,5-DPM aldehydes 35 (5%) and 36 (53%) corresponding to the oxidation of one and two methyl groups, respectively. Similar reactions were also observed for the a-5 and a-6 dimers (26 and 27) giving the corresponding diguaiacylmethane monoaldehyde 30 (29%) and 32 (11%), respectively. Permanganate oxidation. Major products from the permangante oxidation of unsubstituted guaiacyl α-DPM model dimers were the diguaiacylketone compounds 37-41 (36-47%) (Figure 7). Thus, there is a close similarity between permanganate and nitrobenzene oxidation on the reaction of unsubstituted D P M dimers as evident in the formation of the diguaiacylketone products. The feasibility of using these products to detect the D P M units in modified lignins merits a further study. Detection of DPM Units in Lignin. Theoretically, the formation of diguaiacylketone derivatives from the nitrobenzene and permangante oxidation Glasser et al.; Lignin: Historical, Biological, and Materials Perspectives ACS Symposium Series; American Chemical Society: Washington, DC, 1999.


246

28

34 (28%)

29

35 R i = C H

3

(5%)

36 R i = C H O (53%)

Figure 6. Nitrobenzene oxidation products characteristic of the parent diphenylmethane units conducted in 2 M NaOH at 170°C for 3h (19).


247

CH

3

1

f *AôcHj

0 =

IMethylation

0 H

2.KMn0

jA^ CH 0

3

OCH

f\

4

3

H CO-^ OH Downloaded by HONG KONG UNIV SCIENCE TECHLGY on September 10, 2017 | http://pubs.acs.org Publication Date: November 30, 1999 | doi: 10.1021/bk-2000-0742.ch010

3

26

37 R i = C H (3%) 38 Ri= COOCH3 (33%) 3

Γ

CH

3

Rl

2

1 I Η

Y ^ O C H J i.Methylation ^N^ÔCHJ OH I II kru. ° 2.KMn0 i c o V ° H

* ° Ύ 0 h

C H 3

0ch3

4

27

39 R ! = C H

(29%)

3

40 R!=COOCH

H C—^\-OH 1

H™. OCHj

H co>y 0

· Methylation 1 „

2.KMn0

3

4

3

ÔCH

3

„

3 α )

Λ^ OCH3

H

28

41 (36%)

CH

CH

3

CH

2

3

CH

2

CH

2

œ CH 2

OCH

H CO' y 3

OCH3

3

^

^Y0CH

42

3

ÔCH

v

2

(18%)

° H -OCH S

R

3

1 ^

3

OCH

3

3

H ccr 3

ÔCH

3

38(5%)

Figure 7. Permanaganate oxidation products characteristic of the parent diphenylmethane units (22).


248


(Figures 6 and 7) should provide a direct identification of the D P M units in lignin. However, the yield of these diguaiacylketone products are expected to be substantially reduced with substituted D P M units. For example, the permanganate oxidation of an ethyl a-5 D P M dimer 42, as reported by Erickson et al. (21), gave only small amounts of the diguaiacylketone acid 38 (5% as compared 33% for an unsubstituted a-5 dimer 26. Our preliminary results on the nitrobenzene oxidation of kraft pulp lignin has detected the formation of a-5 diguaiacylketone aldehyde 31 indicating the presence of a-5 condensed units. The extent to which the nitrobenzene and permanganate oxidation techniques maybe used as analytical tool for condensed units are currently being pursued further. Conclusions The presence of diphenylmethane-type units in modified lignin can be proved by identifying characteristic diguaiacylketone derivatives in the reaction mixture on nitrobenzene or permanganate oxidation. However, it continues to be a major challenge to devise a viable technique for determining the total condensed units. Acknowledgments The financial support of this study by the Empire State Paper Research Associates, Inc., (ESPRA) and by the U.S. Department of Energy (Award No. DE-FC07-96 ID 134438) is gratefully appreciated. References 1. Lai, Y.Z.; Guo, X.-P., Wood Sci. Technol. 1991, 25, 467-472. 2. Lai, Y.-Z. In Wood and Cellulosic Chemistry; Hon, David N.-S., Shiraishi, N . , Eds.; Marcel Dekker: New York, N Y , 1990, pp 455-523. 3. Gierer, J., Wood Sci. Technol., 1985, 19, 289-312. 4. Gierer, J., Wood Sci. Technol., 1986, 20, 1-33. 5. Lai, Y.-Z.; Mun, S.P.; Luo, S.-G.; Chen, H.-T.; Ghazy, M.; X u , H . ; Jiang, J.E.; Holzforschung, 1995, 49, 319-322. 6. Gellerstedt, G . In Pulp Bleaching Principles and Practice, Dence, C.W.; Reeve, D.W., Eds.; TAPPI Press: Atlanta, G A , 1996, pp 93-111. 7. Dence, C.W.; Lin, S.Y. In Methods in Lignin Chemistry, Lin, S.Y.; Dence, C.W.; Eds.; Springer-Verlag:Berlin, 1992, pp 3-19. 8. Chen, C.-L. In Wood Structure and Composition, Levin, M.; Goldsteins, I.S., Eds.; Marcel Dekker: New York, N Y 1991, pp 183-261. 9. Lundquist, K . In Methods in Lignin Chemistry, Lin, S.Y.; Dence, C.W., Ed.; Springer- Verlag: Berlin, 1992, pp 289-300. 10. Rolando, C.; Monties, B.; Lapierre, C. In Methods in Lignin Chemistry, Lin, S.Y.; Dence, C.W.; Eds.; Springer-Verlag: Berlin, 1992, pp 334-349. 11. Gellerstedt, G.; Lindfors, E.L. Svensk Papperstidn. 1984, 87, R61-R67. 12. Pasco, M.F.; Suckling, D. Holzforshung 1994, 48, 504-508. 13. Lapierre, C.; Brigitte, P.; Monties, B. Holzforschung 1991, 61, 61-68. Glasser et al.; Lignin: Historical, Biological, and Materials Perspectives ACS Symposium Series; American Chemical Society: Washington, DC, 1999.

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th

14. Lu, F.; Ralph, J. In Proceedings of 9 International Symposium on Wood and Pulping Chemistry; Montreal, 1997, pp L3-1 - L3-4. 15. Leopold B. Acta Chem. Scand. 1952, 6, 38-39. 16. Chen, C.-L. In Methods in Lignin Chemistry; Lin. S.Y.; Dence, C.W.; Eds.: Springer-Verlag: Berlin, 1992, pp 301-321. 17. Chang, H.-M.; Allan, G.G. In Lignins; Sarkanen, K . V . ; Ludwig, C.H.; Eds.; Wiley-Interscience: New York, 1991, pp 301-321. 18. Chan, F.D.; Nguyen, K . L . ; Wallis, A.F.A. J. Wood Chem. Technol. 1995, 15, 329-347. 19. X u , H.; Lai, Y . - Z . Holzforschung 1998, 52, 51-56. 20. Gellerstedt, G . In Methods in Lignin Chemistry; L i n , S.Y.; Dence, C.W., Eds; Springer-Verlag: Berlin, 1992, pp 322-333. 21. Erickson, M.; Larsson, S.; Miksche, G.E. Acta. Chem. Scand. 1973, 27, 127-140. 22. Meguro, S.; X u , H.; Lai, Y . - Z . Holzforschung, 1998, 175, 175-179. 23. Glasser, W.G. Svensk Papperstidn. 1981, 84, R25-R32. 24. Glasser, W.G.; Barnett, C.A.; Sano, Y . J. Appl. Polym. Symp. 1983, 37, 441-460. 25. Chen, H.-T.; Funaoka, M.; Lai, Y . - Z . Wood Sci. Technol. 1997, 31, 433-440. 26. Funaoka, M.; Abe, I.; Chang, V . L . In Methods in Lignin Chemistry; Lin, S.Y.; Dence, C.W., Eds.; Springer-Verlag: Berlin, 1992, pp 369-386. 27. Funaoka, M.; Abe, I. Mokuzai Gakkaishi 1983, 29, 781-788. 28. Funaoka M.; Abe, I. Wood Sci. Technol. 1987, 21, 261-279. 29. Chan, F.D.; Nguyen, K . L . ; Wallis, A.F.A. J. Wood Chem. Technol. 1995, 15, 473-491. 30. Lai, Y.-Z.; Funaoka, M.; Chen, H.-T. Holzforschung 1994, 48, 355-359.


An Overview of Chemical Degradation Methods for Determining Lignin

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