Chemistry of Pulping: Lignin Reactions - ACS Symposium Series

Chapter 20

Chemistry of Pulping: Lignin Reactions Josef S. Gratzl and Chen-Loung Chen

Downloaded by PENNSYLVANIA STATE UNIV on June 11, 2012 | http://pubs.acs.org Publication Date: November 30, 1999 | doi: 10.1021/bk-2000-0742.ch020

Department of Wood and Paper Science, North Carolina State University, Raleigh, NC 27695-8005

The chemical reactions of lignins in both acidic and alkaline pulping processes are reviewed. The acidic pulping processes include the sulfite process in the pH range of 2-4, and the bisulfite process in the pH range of 5-6. The alkaline pulping processes include soda, kraft, polysulfide, alkaline sulfite, neutral sulfite, and A Q / A H Q processes. The reactions of lignins with the active chemical species in these pulping processes are discussed in terms of lignin solubilization, degradation, condensation, the redox cycle of A Q / A H Q processes, and stabilization of carbohydrates. Furthermore, the formation of volatile compounds is discussed.

Among the industrial chemo-technical processes, the manufacture of pulp and paper is rather unique. The industries producing basic and specialty chemicals and polymers, which are based on non-renewable resources, such as coal, oil and natural gas, have been developed exclusively employing chemical and physico-chemical principles. The pulp and paper industry, on the other hand, relying on renewable raw materials, evolved empirically, mainly by trial and error employing state of the art mechanical and process engineering. The objective of chemical pulping is the selective removal of lignin without extensive degradation of the polysaccharides, cellulose and hemicelluloses, as well as the removal of extractives and colored structural moieties (chromophores) in residual lignin or other wood components, originally present or generated during pulping. The pulping processes are conducted either in acidic or alkaline aqueous solutions. In the solvent pulping processes under consideration, organic solvents (alcohols) are used exclusively, or are added to the pulping liquor to enhance the penetration of the chips by the pulping chemicals, as well as the removal of lignin and lignin degradation products.

392

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

393 The conditions required for the degradation of lignins and the removal of their degradation products are drastic. The high temperatures and rather high charges of acids or alkali lead not only to the desired fragmentation but also to undesired condensations of lignins and their fragments that are produced during the pulping. In addition, substantial degradation and removal of polysaccharides occur, resulting in losses in yield and fiber strength. However, the degradation and removal of lignin and retention of polysaccharides can be enhanced by process modification and use of pulping additives.


Lignins Lignins are three-dimensional polymers consisting of phenylpropane or C9 units. There are three different types occurring in a wide range of proportions in the woody tissues of different plant species. These three C9 units are p-hydroxyphenylpropane, guaiacylpropane and syringylpropane units (Figure 1). The proportion of these three C9 units in lignin occurring in the woody tissues of plants depends on the nature of plant species. In general, gymnosperm (softwood) lignins consist mostly of guaiacylpropane units. Angiosperm (hardwood) lignins are mainly composed of guaiacyl- and syringylpropane units with varying ratios depending on the wood species. By contrast, grass (greamineae) lignins are comprised of p-hydroxyphenyl-, guaiacyl- and syringylpropane units and consist of lignin cores and peripheral units. The lignin cores are essentially lignins of the guaiacyl-syringyl type, while the peripheral units consist of p-hydroxy-cinnamic acid and ferulic acid groups that are bonded by ester linkages mostly to hydroxyl groups at C-γ in the Cç-units of the lignin cores. The phenylpropane units are joined together by ether (C-O-C) and carbon-carbon (-CC-) linkages. The type of linkages connecting the phenylpropane units (substructures) and their estimated frequencies are given in Figure 2 (2). Except for the diarylether type linkages, interunit ether linkages readily undergo both acid- and base-induced hydrolysis under the given conditions. Side-chains may be cleaved depending on the type of substructures, particularly under alkaline conditions. As a result of the structural complexity of lignins and of the drastic reaction conditions applied, an understanding of the principal chemical reactions has been established by experiments with model compounds featuring lignin substructures, and employing state-of-the-art analytical methodologies. In the following discussion, we would like to concentrate on the very basic, underlying chemical reactions rather than on topochemical phenomena involved in the removal of lignin.

Acid Processes The degradation and removal of lignin is accomplished with aqueous solutions of sulfur dioxide ( S O 2 ) in presence of bases, such as magnesium, sodium, potassium and ammonium hydroxide. Replacement of the originally used calcium oxide by the aforementioned bases allowed not only their recovery, but also provided flexibility in the selection of raw materials and pulping conditions for the production of a wide In Lignin: Historical, Biological, and Materials Perspectives; Glasser, W., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.


394

OCHj

H CO 3

IT

L'

© Guaiacylpropane Unit

y ο

0ch3

®

®

Syringylpropane Unit

p-Hydroxyphenylpropane Unit

L = H or Lignin Moieties ] Figure 1. Phenylpropane units in lignins.

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


395

?

5-0-4 Spruce Birch

β-β

4

2

15

7

Ç

9

y-(p-hydroxycinnamic acid ester)

Figure 2. Estimated frequencies of different types of bonds between phenylpropane (C9) units of milled wood lignins (MWLs) per lOO-Cç-units. Adapted with permission from reference 1. Copyright 1977, Wood Science and Technology.


396


variety of pulp types. Figure 3 shows the equilibria of the components in aqueous solutions of sulfur dioxide. The relative concentrations are functions of pH and temperature. Pulping temperatures are in the range of 130-170°C. Interactions between lignin substructures and the acid (hydrated proton; H$0*) with concomitant dehydrations lead to the formation of electron-deficient centers (carbonium ions). These centers react with electron-rich centers of chemical species (nucleophiles), such as hydrated sulfur dioxide (SC^rfeO; H2SO3) and corresponding chemical species, producing sulfonates. Alternatively, the centers react with certain electron-rich aromatic carbons generating carbon-carbon (-C-C-) interunit linkages (condensations). Benzyl alcohol groups in phenolic and nonphenolic subunits, and in the corresponding α-arylether subunits are very reactive towards acid sulfonation. Protonation of the othydroxyl group in the benzyl alcohol moieties and the oxygen atom in the a-arylethers leads to die formation of oxonium intermediates. Elimination of water from the former and elimination of the corresponding phenols from the latter, with concomitant cleavage of the ether bond, results in the formation of corresponding benzyl carbonium ions, stabilized by the aryl substituent with hydroxyl and alkoxyl functions. Addition of hydrated sulfur dioxide (SC^rfeO; H2SO3) or related chemical species, such as bisulfite ion (HSO3"), leads to the formation of corresponding α-sulfonic acid or related derivatives, as shown in Figure 4. Cleavage of the α-arylether bonds is mainly responsible for fragmentation of lignins. The more abundant nonphenolic β-aryl ether linkages are relatively unreactive (2). The same overall mechanism applies to phenylcoumaran (β-5) and pinoresinol substructures (β-β), where the furan rings are cleaved followed by sulfonation or condensation of the benzylium ion intermediate (i). Figure 5 shows the sulfonation of coniferyl aldehyde type subunits. Protonation at the carbonyl of the α,β-unsaturated aldehyde endgroup leads to formation of a benzyl carbonium intermediate with an enol endgroup. The addition of a bisulfite ion on C - a of the carbonium intermediate results in the introduction of a sulfonic acid group. Furthermore, addition of hydrated sulfur dioxide (H2SO3) to the aldehyde endgroup may occur, resulting in the formation of corresponding γ-hydroxysulfonic acid. Thus, sulfonic acid groups can be introduced in both a- and γ-positions of side-chains in the coniferyl aldehyde substructures. Although the 4-O-alklyated coniferyl aldehyde substructure is a minor component in lignin, the introduction of a- and γ-sulfonate groups increases appreciably the solubility of lignin fragments in the pulping process. Guaiacylpropane moieties with an α-carbonyl group are sulfonated at the terminal group via the corresponding γ-carbonium ion intermediates (Figure 6) (3). The intermediates are produced via keto-enol tautomerism of the α-carbonyl group to the corresponding 4-O-alkylconiferyl alcohol intermediates, protonation of the terminal hydroxymethyl group and subsequent dehydration. Thus, 4-O-alkylconiferyl alcohol substructures are produced as intermediates in pulping. As shown in Figure 7, a similar mechanism is operating in the reactions of substructures with conjugated


397

S0

2

+ H 0 2

*==^

H S0 (S0 »H 0) 2

3

2

2

pKa

H +

HS0 ~

2

^ =

3

~2

2 H+ + S 0 ~ 3

~7

Strongly Acidic

Acid Sulfite

Bisulfite

pH Sulfite ion (S0 ~) > Bisulfite ion (HS0 ~) > Hydrosulfide (HS") > hydroxide ion. 3

3


406

HO" (Base):

Carbohydrates and Lignin Degradation

HS~, S ~ (Reduced Sulfur Compounds in Kraft and Polysulfide Pulping):

Lignin Degradation

2

Sn ~ (Poiysulfides):

Carbohydrate Stabilization, (Lignin Degradation)

HS0 ", SO3 " (Bisulfite and Sulfite ions):

Lignin Solubilization, Lignin Degradation

AQ/AHQ;

Carbohydrate Stabilization, Lignin Degradation

2

2


3

Figure 13. Reactivity of pulping chemical species in alkaline pulping.

+ 'O-Ka

R = Η or OCH3 or Lignin Moieties; Ra = Η or Ar; Rp = O-Ar or Ar

Figure 14. Formation of quinone-methide intermediate.

CHjOH CH OH 2

HC-R.

\ l R"

f

H

HOH ,

Â

If Ο

0

C

H

3

R

H

2 °

ha XdL

H^CH-^-H

Hi Q)

/=o

H

- H *, [Η0Ί

0 C H 3

R"

^OCH

3

O" R = H o r OCH3 or Lignin Moieties; Rp * O-Ar or Ar

Figure 15. Reaction of quinone-methides with cleavage of side-chain via reversed aldol reaction.


407 Cleavage of the phenolic β-arylether bond is initiated by the addition of nucleophiles to the C-α of the corresponding quinone-methide restoring the aromatic structure. This is followed by interaction of this α-substituent with the C-β via neighboring group participation, resulting in the elimination of the aroxyl substituent at C-β.


Reactions with hydrosulfide/sulfide ions lead to an α-sulfide, followed by an intramolecular substitution (SNO reaction which cleaves the β-arylether bond and forms a thiirane intermediate. This intermediate reacts further to form a styrene structure and elemental sulfur (Figure 16, Pathway a). In the presence of the A Q - A H Q redox system, a quinone-methide undergoes nucleophilic attack on C-α by the tautomer of the reduced form (AHQ) to give an adduct, which leads to cleavage of the β-arylether bond via Grob-type fragmentation. This results in the formation of a p-hydroxy-styrene type substructure and A Q (19,20) [Figure 16, Pathway (b)]. The conjugated phenolic substructures thus formed may be subject to condensation reactions via extended quinone-methides (secondary condensation). Model compound studies have also shown enhanced base-induced cleavage of phenolic β-arylether bonds in the presence of reducing sugars, such as glucose. The reaction involves nucleophilic addition of the C-2 of the 1,2-enediol tautomer of glucose to the C-α of a quinone-methide. The resulting intermediate then undergoes a Grob-type fragmentation producing a p-hydroxystyrene type substructure and oxidized sugar (21). It was also found that this reaction peaks at temperatures around 150°C, suggesting instability of the reducing sugar. This mechanism is analogous to the one described above for the reductive cleavage of phenolic βarylether subunits by A H Q . A l l of the above processes constitute reductive cleavages of β-arylether substructures, whereas the additives are oxidized. In sulfite pulping, the quinone-methide with a β-Ο-4 bond will be first sulfonated at Ca. This is then followed by a nucleophilic substitution at the C-β by a sulfonate anion via neighboring participation of the C-α sulfonate group with concomitant cleavage of the β-ether bond. These reaction mechanisms were discussed previously for the reactions with sulfides and A H Q , as well as for the formation of an α,β-disulfonate, which is a minor reaction. Upon further reaction, the sulfonate group in C-α is eliminated to form a quinone-methide intermediate (Figure 17). A subsequent elimination of a proton in C-β produces a conifieryl alcohol β-sulfonic acid structure. Alternatively, an elimination of the terminal hydroxymethyl group as formaldehyde via reverse aldol condensation of the extended conjugate unsaturated ketone system gives a styrene^-sulfonic acid structure (4). As shown in Figure 18, cleavage of β-arylether bonds also may be accomplished by an electrophilic 1,4-oxygen-to-oxygen migration of the aryl group in β-Ο-4 type substructures with α-hydroxyl and α'-carbonyl groups in a temperature range of 2040°C. This arrangement was shown to proceed via a spiro-Meisenheimer complex that is formed via enolization of the α'-carbonyl group to the cyclohexa-2,5dinenylidenol structure, with concomitant nucleophilic attack of the corresponding a-



408

R = H or OCH or Lignin Moieties; Rp - Ar 3

Figure 16. Reaction of quinone-methides with hydrosuifide ion and anthrahydroquinone leading to cleavage of β-arylether bonds.

R = H or OCH3 or Lignin Moieties; Rp = Ar

O"

Figure 17. Reaction of quinonemethide with formation of lignosulfonates.


409


hydroxide anion on C-4 of the structure (22). Since the resulting cc-O-4 arylether linkage is more susceptible to base-catalyzed hydrolysis than the β-Ο-4 type arylether linkage, any pretreatment of wood chips that introduces the oc'-carbonyl groups would be beneficial to improving the pulping process. Cleavage of Nonphenolic β-Arylether Substructures Nonphenolic β-arylether substructures are much more stable than the corresponding phenolic subunits. Cleavage of the ether bonds usually occurs in the later stages of pulping, and depends primarily on the concentration of the base. The a-hydroxyl group in benzyl alcohol moieties is ionized, and reacts with the adjacent C-β via neighboring group participation resulting in elimination of the β-arylether bond with the accompanying formation of an oxirane intermediate (SNÎ reaction) which undergoes hydrolysis to the corresponding diol (4) (Figure 19). The mechanism is the same as the one operating in the base-induced hydrolysis of polysaccharides. Condensation Reactions Quinone-methides react with carbanions to generate C-C linkages between the reactants. One may distinguish between primary and secondary condensations. Examples of primary condensation are shown in Figure 20. A diarylmethane substructure will be formed via nucleophilic addition of an or/Ao-carbanion derived from a phenolate anion on C-α of a quinone-methide originating from a phenolic structure by elimination of the substituent at C-α (4). By contrast, when the quinonemethide intermediate undergoes a nucleophilic addition of a /?ara-carbanion derived from a phenolate anion on C-α, a diarylmethane substructure will be formed with the concomitant elimination of a lignin fragment with an aldehyde group involving reverse aldol condensation of a side-chain with an α-hydroxyl group. Figure 21 shows secondary condensation involving extended quinone-methides formed from conjugated phenolic moieties generated after initial cleavage reactions (e.g., coniferyl alcohol type structures). A n extended quinone-methide of a coniferyl alcohol substructure undergoes a nucleophlilic attack by a carbanion at C-β of a second quinone-methide to form a C-C linkage between C-γ and C-β in the side-chains of two coniferyl alcohol units, with a concomitant elimination of formaldehyde via reverse aldol condensation. As shown in Figure 22, nonphenolic β-arylether substructures may undergo condensation reaction via β-carbonyl intermediates (23). The rearrangement of the oxirane intermediate leads to homologues of etherified homovanillin (also see Figure 19). Aldol condensation between the keto and enol forms of the intermediates leads to the formation of 1,3-diary1-1 -propene structures. Cleavage of β-Arylether Bonds in the β-Ο-4 type Condensed Substructures As shown in Figure 23, the β-Ο-4 type para and ortho condensed diarylmethane substructures are susceptible to cleavage of the β-arylether bonds via neighbouring group participation (24). In both cases, the phenolate anions produced by condensation (also see Figure 20) initiate the cleavage, leading to stable aromatic



410

"Το

1,

OCH

OCH3

3

R = HorCH

Spiro-Meisenheimeer Complex Intermediate

3

Figure 18. Migration of β-Ο-aryl group in nonphenolic β-Ο-4 substructures with ahydroxyl and α'-carbonyl groups to a - 0 via spiro-Meisenheimer complex intermediate in alkaline solution at temperature range of 20-40°C. Adapted with permission from reference 22. Copyright 1997, American Chemical Society.

R = Η or OCH3 or Lignin Moieties; Rp Ar, L = Lignin Moieties e

Figure 19. Cleavage of nonphenolic β-arylether substructures by base.



411

R = H or OCH or Lignin Moieties; Ljand Lj = Lignin Moieties 3

Figure 20. Reaction of quinone-methides involving lignin condensation to diarylmethane substructures.

0*

Figure 21. Reaction of quinone-methides, secondary condensation.



412

R = H or CH OH; L = CH or Lignin Moieties 2

3

Figure 22. Condensation of homovanillin structures derived from rearrangement of oxirane intermediates from nonphenolic β-arylether substructures during alkaline pulping. Adapted with permission from reference 23. Copyright 1987.



413

Figure 23. Cleavage of β-arylether bonds in β-Ο-4 type condensed substructures. Adapted with permission from reference 24. Copyright 1983, Arbor Publishing.


414


structures. In the case of para diarylmethane substructures, the resulting cyclopropane intermediate undergoes nuclophilic attack by hydroxide anion on C-α, leading to aromatization of the cyclopropane-4'-spiro-cyclodienone moiety. In the case of ortho diaryl-methane substructures, by contrast, the or/Ao-phenolate anion attacks C-β as a nucelophile resulting in the formation of an a-6 type coumaran structure with the concomitant cleavage of β-aryl ether bond. Thus, some of the condensation reactions of lignin in the alkaline pulping processes are beneficial to delignification. Cleavage of Alkylaryl Ether Bonds Linkages of this type (methoxyl groups) are reasonably stable towards base. In soda pulping, slight cleavage of methoxyl groups occured with the resulting formation of methanol. However, in the presence of stronger nucleophiles, such as in kraft and alkaline sulfite pulping, appreciable cleavage of alkylaryl ether bonds does occur, including alkylaryl ether of the a-O-4 and β-Ο-4 type structures, and methoxyl groups attached to aromatic rings. In kraft pulping, sulfide and hydrosulfide ions react with the methoxyl group generating methyl mercaptan. Less than 5% of the methoxyl groups are cleaved. The mercaptide ion is a strong nucleophile and forms dimethyl sulfide, which is oxidized to dimethyl disulfide on exposure to air (oxygen). Although all of these sulfur compounds are nontoxic, they have a very low odor threshold and thus contribute substantially to the air pollution of a kraft mill. In the presence of sulfite or bisulfite anion, the cleavage of methoxyl groups is more pronounced and substantial amounts of methylsulfonic acid, soluble in the alkaline spent liquor, are generated. It should be noted that for each methoxyl group cleaved a phenolic hydroxyl group is introduced. Thus, the cleavage of methoxyl groups enhances the phenolic character of the lignin, which will affect significantly the reactivity of lignins toward the base. Figure 24 shows the general scheme of the reactions involved in the cleavage of a methoxyl group. Reactions with the AQ/AHQ Redox system Among the many compounds investigated for more than half a century for their potential as additives to improve the performance of alkaline processes, anthraquinone (AQ) and its derivatives have shown unique properties. Most intriguing is the fact that addition of only extremely small amounts results not only in substantial improvements in yield, but also in drastic acceleration of delignification (bulk delignification) and increased in lignin removal. The superior efficiency of A Q in improving both yield and lignin removal suggests that it is regenerated continuously and that its reduction product, the anthrahydroquinone (AHQ), is contributing in a major way to the process improvements. This means, we are dealing with a rather alkali-stable redox system with both the oxidant (AQ) and the reductant (AHQ) reacting throughout the cook. The amounts of additive required to achieve optimum results are extremely small, about 0.05-0.1 % based on wood. The recovery of about 30% of the initial charge suggests that the redox processes operate throughout the cook. With the discovery of the reductive cleavage of phenolic β-arylether substructures by A H Q and concomitant regeneration of A Q , a very important lignin fragmentation reaction that drives the cycle of redox processes was established (19,20) (Figure 16, Pathway b). Figure 25


415

CH S-SCH


3

Λ

C H 3 S H (CH3-S-CH3)

S" (CH S~) 3

+ OCH

3

CH3S03"

S so3 ' 2

3

CH30H lHO~

R = H or OCH3 or Lignin Moieties; Ljand Li = Lignin Moieties

Figure 24. Cleavage of arylmethylether bonds; Formation of phenolic hydroxyl groups.

H-Ç-OH

V"

r

r

γ

j

H . .OH

H „ .OH

2

2

V ç

?

Ar

-9H-Ç-OH Ar

r - ç Ar

(Carbohydrate Reducing Endgroups)

H._*0

I

r fr 9

-

Chemistry of Pulping: Lignin Reactions - ACS Symposium Series

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