The Reactions of Peroxides with Lignin and Lignin Model Compounds

Chapter 6

The Reactions of Peroxides with Lignin and Lignin Model Compounds Downloaded by NORTH CAROLINA STATE UNIV on September 17, 2012 | http://pubs.acs.org Publication Date: March 26, 2001 | doi: 10.1021/bk-2001-0785.ch006

John F. Kadla and H-m. Chang Department of Wood and Paper Science, North Carolina State University, Raleigh, NC 27695

The reactions of peroxides with lignin and lignin model compounds have been extensively investigated. It is generally accepted that peroxides react via two reaction mechanisms depending on the structure of the peroxide and the reaction conditions employed. In alkaline conditions peroxides react nucleophilically with electron deficient carbonyl and conjugated carbonyl structures. In neutral to acidic media, most peroxides react with electron rich aromatic and olefinic structures via electrophilic pathways. In this paper, the reactions of peroxides with lignocellulosic materials, concentrating on lignin and lignin model compounds are reviewed. Emphasis is placed on the reactions of hydrogen peroxide and peroxy acids. However, owing to their chemical structures, and susceptibility to thermal and transition metal catalyzed decomposition, the reactions of hydroxyl, H O ·and superoxide ·O2- radicals with lignin model compounds will also be discussed.

With an ever-increasing demand by governmental as well as environmental agencies for environmentally benign bleaching technology, the pulp and paper industry is being forced to reduce the amount of chlorine containing bleaching reagents it employs. Processes utilizing peroxygen-based reagents such as hydrogen peroxide and peroxy acids are thus being implemented. As a result, an exorbitant amount of literature exists on the chemistry of peroxides and lignin model systems under the conditions utilized during the bleaching of chemical pulps (1-14).

108

© 2001 American Chemical Society In Oxidative Delignification Chemistry; Argyropoulos, D.; ACS Symposium Series; American Chemical Society: Washington, DC, 2001.

Downloaded by NORTH CAROLINA STATE UNIV on September 17, 2012 | http://pubs.acs.org Publication Date: March 26, 2001 | doi: 10.1021/bk-2001-0785.ch006

109 Traditionally, hydrogen peroxide has been classified as a lignin-preserving bleaching reagent. It preferentially removes chromophoric structures present in residual lignin, but is incapable of degrading the lignin network (14-17). To degrade and remove the residual lignin, hydrogen peroxide must be activated. Conventionally this is accomplished via the in situ formation of a peroxy acid by the addition of acetic, formic or sulfuric acid or cyanamide into a hydrogen peroxide solution (5J 8-28). The peroxy acid then reacts as an electrophile with lignin, leading to oxidation and subsequent degradation (29-34). Similarly the use of metalloporphyrins (35-41) and other biomimetic systems (42-47) have been explored as means to increase the reactivity of hydrogen peroxide. In the following chapter, a review of the recent literature of hydrogen peroxide, and peroxy-acids in aqueous solutions will be given. Emphasis will be on the oxidation of lignin model compounds, lignin preparations and pulp. In addition, the reactions of the oxygen-based radicals generated in these peroxide systems with similar materials will be mentioned.

Properties of Aqueous Peroxide Solutions An important aspect of hydroperoxides and peroxy acids is the acidity of the peroxide proton. Hydroperoxides, ROOH, are generally stronger acids than the corresponding alcohols, ROH (48). This shift in pK has been attributed to (i) the RO group which has a greater electronegativity than the R group, causes a relative decrease in the electron density on the O-H bond in ROOH as compared to ROH and (ii) the greater the electron withdraw by RO more effectively stabilizes the resulting peroxy anion. On the other hand, peroxy acids are generally weaker than their parent acids (49). Two plausible explanations exist, (i) intramolecular hydrogen bonding in the peroxy acid stabilizes the neutral form relative to the anion, (ii) the parent acid has a resonance stabilized anion form making the acid relatively strong, whereas the peroxy anion has only the inductively delocalized electronic configuration. When undissociated, hydroperoxides and peroxy acids are relatively stable. e

R 0 H + HO" 2

ROO" + H 0 2

(1)

Peroxides are powerful oxidizing agents reacting as both electrophiles and nucleophiles. In nucleophilic reactions, it is generally accepted that the conjugate base, peroxyl anion, is the active species, since conditions for its formation parallel those required for attaining a maximum reaction rate (50,51). Due to its chemical structure, the peroxyl anion (ROO') is a strong nucleophile. This enhanced nucleophilicity is characteristic of molecules with unshared pair of electrons on the atom adjacent, or alpha, to the nucleophilic

In Oxidative Delignification Chemistry; Argyropoulos, D.; ACS Symposium Series; American Chemical Society: Washington, DC, 2001.


110 atom, and is known as the alpha effect (52-54). As a result, peroxides are modest two-electron transfer agents (25,55-55). In alkaline conditions, the primary reaction is alkaline or nucleophilic epoxidation involving electron deficient alkenes (59). Depending on the peroxide usai, the stereospecificity of the epoxidation will differ (60). In general stereospecificity increases (i) with increasing nucleofiigacity (i.e. leaving group ability) of the peroxide, and (ii) with decreasing electron-deficiency of the alkene. The most characteristic electrophilic reactions of peroxides are oxygen transfer reactions, particularly epoxidation (61-63) and hydroxylation reactions (64-71). The oxygen atoms of the peroxides are transferred to substrate nucleophiles with lone-pair or π-bonding electrons. It has been shown that ionic intermediates are not present, and that the transition state involves the coordination of the π-electrons with the peroxide-oxygen. Again, peroxide and olefin structure dramatically effect the reactions. Both electron-donating substituents on the alkene/aromatic ring and electron withdrawing groups on the peroxide accelerate the reactions. Peroxy acids show enhanced reactivity relative to hydroperoxides due to their increased nucleofiigacity, which is apparentfromthe acidities of the departing acids (pK ~4-5) relative to those of alcohols (pK ~ 15) and increased electrophilicity (72). Thermodynamically, peroxides are potentially unstable, decomposing exothermically according to equation 2. a

a

ROOH

ROH + V 0 2

(2)

2

1

The facile decomposition is a result of the weak O-O bonds, ~ 31 kcal mol" for CH C(0)0-OH vs 51 kcal mol for HO-OH, which are easily cleaved by light and heat (73). In both peroxy acids and hydroperoxides, heating above a critical temperature (peracetic acid ~80°C, H 0 ~120°C) initiates homolysis of the OO bond, leading to the formation of radical species. What ensues is the kinetic decomposition of the peroxide to a variety of radical intermediates. Fortunately, the activation energy of this decomposition is high and not readily reached in aqueous solutions (74). The decomposition is however easily catalyzed by trace amounts of transition metal ions and other easily oxidizable materials. It is well established that peroxides undergo facile alkaline decomposition in which the rate of decomposition increases with increasing pH and hydroxide ion concentration (75-77). In hydrogen peroxide, this decomposition proceeds via a disproportionation reaction, with a maximumrateat the pH of its pK (11.6 @25°C) (48). 1

3

2

2

a

HOOH + HOOHOOH + HOO*

* - H 0 ' + H 0 0 + HO' HO" + 0 ( 0 ) + H 0 #

3

2

1

2

2


(3) (4)

Ill Disproportionation then leads to a rapid decomposition of hydrogen peroxide through an autocatalytic process. In aqueous media, the kinetics and thermodynamics of a number of 'active oxygen" species have been evaluated by pulse radiolysis (78-80). 4

k = 3x10 M"V

HOO* + HO · — • Ό " + H 0 2


HOOH + HO ·

+ Η*

*-H0" + 0

2

(5)

1

(6)

1

pKa = 4.8@25°C

(7)

k=1x10 M"V

(8)

10

2

HOOH + · 0 - - ^ Η Ο · + H O ' + 0 2

7

2

2

2

k = 3x10 M"V

—•HOO** H 0

HOO- ^ = ^ · 0 HO · + · 0

7

2

1

k = 1x10" M ' V 1

1

(9)

From the above information, reactions 3,5,6 and 8 appear to represent a viable mechanism for the base-induced decomposition of hydrogen peroxide to oxygen and water. A similar series of reactions occurs in peroxy acids in aqueous conditions. In addition, as hydrogen peroxide is also produced in the alkaline hydrolysis of peroxy acids, the series of reactions outlined above will also occur. A kinetic chain reaction can be catalyzed by traces (10-20 ppm) of transition metal ions, particularly iron, cobalt, manganese and copper, and is often referred to as Fenton's chemisfcy (81,82). In such conditions the peroxide acts as both a reducing and oxidizing reagent with the transition metal ions in the higher and lower valence states respectively.

As these transition metal ions are insoluble under alkaline conditions, a heterogeneous surface-catalyzed reaction caused by colloidal transition metal oxides/hydroxides has been proposed (83). To minimize the effect of these transition metal induced decomposition reactions, many techniques have been employed, which include the addition of sequestering agents (84-88% and/or inorganic salts, (89-91) as well as acid washing (92-94) or other pretreatments (95-103) to remove the majority of these metal ion contaminants prior to peroxide bleaching (104-107). Thus, peroxygen bleaching involves a series of oxygen containing compounds that are formed and consumed dependent on pH, temperature and organic/inorganic contaminants. Therefore, depending on which of these


112 9

'active oxygen compounds are present, the outcome and extent of lignin degradation will vary (/).


Reactions of hydrogen peroxide with lignin model systems Under the conditions representing those used for commercial chemical pulp bleaching, alkaline hydrogen peroxide has been shown to react with both aliphatic and aromatic structures in lignin (108-117). However, the majority of the work in this area and the conclusions drawn therein has been a result of reactions of hydrogen peroxide degradation products, i.e. oxidative radicals, and not hydrogen peroxide itself (116). Fortunately, through the careful control of reaction conditions, pH, temperature, metal management etc., the predominant reaction pathways of alkaline hydrogen peroxide with lignins have been ascertained. Prevalent functionalities in residual lignins are the α-carbinol and acarbonyl structures (118). In the degradation of aryl-a-carbonyl structures, both free and etherified phenolic hydroxy! compounds undergo oxidation by alkaline hydrogen peroxide (119,120). The initial step is nucleophilic attack of hydrogen peroxide at the α-caibonyl carbon. In the presence of a free-phenolic hydroxyl compound R=H, the reaction proceeds by way of the Dakin-reaction (121). This reaction involves, as a rate-determining step, the formation of an intermediate epoxide, which under alkaline conditions is rapidly hydrolyzed accompanied by cleavage of the C C bond. r

a



113 In the absence of afree-phenolichydroxyl group, the initially formed hydroperoxide is precludedfromundergoing the Dakin reaction. Instead the hydroperoxide undergoes intramolecular nucleophilic attack leading to the formation of a new phenolic hydroxyl group and a dioxetane intermediate, which subsequently decomposes to the corresponding benzaldehyde, which is susceptible to further oxidation (122). It has been reported that in the decomposition of such β-aryl ethers, thefree-phenoliccompounds react an order of magnitude faster than the etherified counterpart (113,123). For α-caibinol structures, whether β-aryl ethers, β-l or β-5 diols, only free-phenolic lignin moieties react with alkaline hydrogen peroxide, even at extreme reaction conditions (122,124). The reaction proceeds by way of a quinone methide intermediate followed by the Dakin-like reaction.

In this reaction, the quinone methide intermediate rapidly reacts with the hydroperoxyl anion to produce the corresponding hydroperoxide. It then rearranges in a Dakin reaction like fashion, resulting in the cleavage of the CiC«bond. Compared to the Dakin reaction, the Dakin-like reaction is extremely slow and does not occur at any appreciable rate at temperatures below 50°C (113,122,125). This decreased reactivity is likely due to the decreased rate of formation of the necessary quinone methide intermediate. As a result the corresponding hydroperoxide is not formed, thereby making side-chain displacement not possible. Although the predominant reaction of a-carbinol containing phenolic compounds is the Dakin-like reaction, numerous



114 investigators have reported that the oxidation to the corresponding a-caibonyl compound also takes place (126,127). However we have recently shown that in the absence of peroxide decomposition, this oxidation does not occur, even at extreme temperatures (123). By suppressing thermal and metal induced decomposition of hydrogen peroxide, the reactions of lignin and lignin model compounds can be studied at temperatures higher then those previously employed (123,124). Thus, in the presence of DTMPA (diethylene-triamine-pentamethylene phosphonic acid) hydrogen peroxide was found to be completely stable at 90°C under peroxide bleaching conditions. While reaction pathways remained essentially the same, the rate of some reactions greatly increased when the reaction temperature was increasedfrom50°C to 90°C. For example, apocynol, which reacts exclusively via the Dakin-like reaction, was almost completely degraded after 1 hour at 90°C, whereas only a fraction was reacted at 50°C. Similarly, several nonphenolic lignin structures, known to be stable to alkaline hydrogen peroxide underwent oxidation to varying degrees. Other lignin structures containing carbonyl and ethylenic groups in the side chain have also been studied (110,128). In these systems peroxide oxidation occurs via conjugate addition to form the corresponding oxirane followed by hydrolysis and carbon-carbon bond cleavage. Finally, extended carbonyl structures such as quinoids play an important role in the oxidation of phenolic lignin model compounds, and likely residual lignin, by alkaline hydrogen peroxide. Recall that quinone methides enable side-chain displacement in the Dakin-like reaction (discussed above). Para- and ortho-quinoids, which constitute chromophoric structures found in residual lignins, are also rapidly degraded by alkaline hydrogen peroxide. Primarily produced in the electrophilic reactions of previous bleaching stages, or during peroxide bleaching in which peroxide decomposition has occurred, these simple quinoid-type chromophoric structures are rapidly degraded to mono/di fimctional carboxylic acids (129-132). O- and /7-quinoidringsare comprised of dual enone structures offering multiple sites for attack by hydroperoxyl anion (133). Para-quinoids reactions start through nucleophilic attack of one of the electron deficient carbon atoms in the quinoid ring by hydroperoxyl anions, followed by either elimination of a hydroxyl ion (formation of an epoxide) or ring closure to a dioxetane structure. These intermediates are then further attacked by perhydroxyl and hydroxyl anions in one or several steps to yield the final carboxylic acid products.


115 ο

ο

OCH

3

οοσ


OCH

3

In the hydrogen peroxide oxidation of o-quinoid-type structures (shown below) the mechanism involves epoxidation of one or both of the double bonds followed by cleavage between the carbonyl groups (129). The resulting monoepoxide dicarboxylic acids are subsequently converted by intramolecular nucleophilic attack, to the corresponding lactonic acids. Although the reaction with conjugated enone structures is quite facile, it has been observed that hydroxylated quinones, are more resistant to oxidation by hydrogen peroxide than the corresponding non-hydroxylated analogues, requiring higher temperatures and longer reaction times. lignin

Even at temperatures as high as 90°C, alkaline hydrogen peroxide is not reactive towards most nonphenolic lignin structures (122). This is due to the fact that hydrogen peroxide is a very weak electrophile and reacts mainly as a nucleophile under alkaline conditions. One way to improve the electrophilicity of hydrogen peroxide is to react it under acidic conditions. Acidic hydrogen peroxide has been used as a pretreatment to enhance deligniiication in subsequent oxygen stages (134,135). Gierer has proposed that under acidic conditions, hydrogen peroxide would be protonated, making it a strong electrophile, leading to direct hydroxylation of the aromatic ring (136). However, Kishimoto et al. have shown that in the reaction with creosol no hydroxylated products were found, in fact creosol was almost completely


116


unreacted (137). However in the reactions with apocynol and methyl vanillyl alcohol, side chain replacement products similar to those obtained in the Dakin-like reaction in alkaline media were obtained. The reaction appears to go through the formation of a benzylic carbocation followed by nucleophilic addition of hydrogen peroxide. Subsequent protonation facilitates a Dakin-like reaction mechanism, resulting in side chain displacement. Interestingly, both free and etherified phenolic compounds reacted under these acidic conditions. CH3 H-C-OH

VÔCHa OR

CH3 CH3 H H - C + H2Q2 H - C - O - Ô - H

VÔCH OR

3

YÔCHa OR

1^ ^

CH3 OH

Q

YÔCHa

^ o C H a OR

+OR

Unfortunately, problems associated with the practical applications of hydrogen peroxide in acidic media exist. First, a substantial amount of lignin condensation products occur, suggesting that under acidic conditions lignin will condense with its degradation products. Second, hydrogen peroxide decomposes rapidly under acidic conditions with even a trace amount of transition metal ions. Therefore, unless transition metal ions can be completely removed from the pulp, acidic hydrogen peroxide treatments will result in significant deterioration of pulp strength (138). Thus the most effective way to increase the electrophilicity of hydrogen peroxide is through the conversion to a peroxy acid.

Reactions of peroxy acids with lignin model systems Peroxy acids react primarily as electrophiles oxidizing olefinic structures to epoxides. Owing to their electrophilic nature, peroxy acids, in particular peracetic acid (CH3CO3H) (29,112,139-143), peroxymonosulfuric acid (H S0 ) (29,144-147), performic acid (HCO3H) (148,149% peroxymonophosphoric acid (H3PO5) (150,151) and peroxyimidic acid (NH C(NH)0 H) (24,25,30,31,152) have been extensively studied with regard to their reactions toward lignin model systems (4,5,27,28,29,135,153). The reactions involved include hydroxylation of the aromaticring,ring opening via o-and /xjuinone formation, side-chain oxidation/elimination and Baeyer-Villiger reactions. Unlike hydrogen peroxide, peroxy acids have been shown to react with etherified phenolic lignin model compounds, although their rate of oxidation is much slower than that observed for their free-phenolic counterparts (144,154). 2

2

2


5

117 In general, the initial reactions of peroxy acids with lignin can be classified into three main types: • •


•

Eleetrophilic substitution resulting in ring hydroxylation, Nucleophilic addition to a benzylic caibocation followed by side chain displacement, and Nucleophilic addition to an α-caibonyl followed by Baeyer Villiger Rearrangement.

1. Eleetrophilic aromatic substitution Aromatic hydroxylation is the dominant reaction for lignin structures with saturated side-chains. The reactions are initiated by eleetrophilic hydroxylation ortho and/or para to an oxygen bearing substituent (143). The reaction involves the formation of a π-eomplex followed by a rate-determining σ-complex formation (155). The rate-determining step is affected mainly by the ability to stabilize the developing positive charge leading to the formation of the σcomplex. Consequently, the rate of the reaction is mostly affected by the leaving ability of the corresponding acid anion (XO) (72).

ft-complex

σ-oomplex

These initial hydroxylated products, now more reactive towards the peroxy acids, are further oxidized to the corresponding quinoids and aromatic ring cleavage products (32,143,J45). The initial hydroxylation results in either demethoxylation to the o-benzoquinone followed by rapid oxidation to a muconic acid derivative, (top pathway) or hydroxylation and subsequent oxidation/demethoxylation to a methoxy-/?-benzoquinone or a hydroxy-obenzoquinone (lower pathway). The latter under going further oxidation to the hydroxy- muconic acid derivative and the corresponding lactones.


118


Ugnin

Ugnin

In addition to hydroxylation/ring-opening type reactions involving the aromatic ring, direct cleavage of inter-aryl linkages, particularly in β-aryl ether compounds have also been observed (156). Three major routes are believed to be involved, one involves side-chain displacement (C), while the other two (A, B) are due to cleavage of the β-aryl ether bond. Cleavage of the β-Aryl ether bond can be attributed to: (A) oxidative cleavage through the hydroxylation of the β-aryl ether ring ipso, or ortho to, the inter aryl linkage followed by either reaction with water, or neighboring group participation and oxirane formation. The involvement of a carbonium ion intermediate has been proposed (157), however the observed retention of stereochemistry would not be satisfied by such a mechanisms, or (B) hydroxylation of the β-aiyl ether ring ipso to the inter aryl linkage followed by oxidative hydrolysis and re-aromatization to the corresponding products.

HÇ-O-/ HCOH

\

Demethoxylated and/or Hydroxylated β-Aryl ether type oxidation products OChb HC-O-f CHO + OH

VcHa OCH3

0CH3 OCH3



119 In the reactions of type (C), ipso hydroxylation of the non-aryl ether ring is followed by oxidation of the α-hydroxyl group and subsequent degradation of the aiyl side-chain (32,136). However based on the recent work of Kishimoto et al (137) and Zhu et al (151) the possible involvement of a benzylic caibocation followed by nucleophilic addition of the peroxy acid and subsequent Baeyer Villiger rearrangement seems most likely. Finally, numerous investigators have also reported the oxidation of the a-hydroxyl group to the corresponding keto compound (D). Although the mechanism is not known, it is thought to involve afreeradical process (29,144). 2. Nucleophilic addition to a benzylic carbocation Peroxy acids are good nucleophiles and as such, add readily to caibocations or quinone methides. Under acidic conditons, caibocations can be formed from either ot-carbinol or α-ethers of both phenolic and non-phenolic units, though the α-ether requires much lower pH (158). Nucleophilic addition of the peroxy acid gives a peroxy intermedate, which may rearrange via two different pathways, both resulting in side-chain displacement (discussed above). In pathway I rearrangement gives the α-caibonyl product, which is further degraded through the Baeyer-Villiger oxidation (vide infra). In pathway II, the side-chain is displaced through epoxide formation, which is subsequently hydrolyzed via an unstable hemiacetal intermediate, analogous to the DakinLike reaction under alkaline conditions. c=o

The transition state again involves the development of a positive charge and it is likely that the leaving ability of the peroxy acid is the main factor determining the rate (72). Thus, the rate of the reactions among the various peroxy acids follows the same order as the eleetrophilic substitution reactions.


120


3. Baeyer Villiger rearrangement The reaction of ketones with peroxy acids is well known to go through the Baeyer Villiger rearrangement (159). The initial step is nucleophilic addition of the peroxy acid to the ketone followed by a rate-determining rearrangement to the peroxyester intermediate via a concerted aryl migration to form the acetate ester. The transition state of the rate-determining step likely involves neither the development of carbocation nor the tentative loss of the aromatic structure as in the other two reactions described above. Thus the rate-determining step is influenced conceivably more by the electrophilicity than the leaving ability of the peroxy acid adduct (72).

Recently, Chang and coworkers investigated the kinetics of several peroxy acids with various lignin model compounds (72). They propose that two main factors affect the reactivity of peroxy acids (XOO*H): 1) the leaving ability of XO" and 2) the electrophilicity of the peroxide oxygen O*. The leaving ability of the peroxy acids is estimated by the pKa values for the conjugated acids, ΧΟΗ, whereby the relative rank of the three peroxy acids they studied were H S 0 " > H3PO5 > CH3CO3H > H2PO5" » H P 0 . The electophilicity of the peroxide oxygen O*, or the ability of the peroxide oxygen bond to become polarized and form a partial positive charge on the peroxide oxygen O* is dependent on the inductive effect of the XO group. Since Ο in the XO group is common to all peroxy acids, the inductive effect of the X group is then responsible for the electrophilicity of the various peroxy acids. The relative rank of the electrophilicity of the peroxy acids investigated was H3PO5 > 5

5

CH3CO3H > HSO5- > H P0 ~ » 2

5

s

HP0 . 5

=

Finally, peroxyimidic acid, which is generated in the activation of hydrogen peroxide by cyanamide (NH CN), has been reported to be isoelectronic with peroxyacids in the bleaching of chemical pulps (24>25,160). However, the peroxyimidic acid intermediate has not been isolated nor has there been any evidence that it is the only reactive species formed in this reaction system. Recently, utilizing electron paramagnetic resonance (EPR) spectroscopy and spin trapping, Kadla et al have shown free radical involvement in the cyanamide-activated hydrogen peroxide reaction system 2


121 (29,30,31). In contrast to previous literature, a predominately free-radical mechanism exists. On the basis of the reactions carried out with superoxide dismutase (SOD), the presence of superoxide (·0 ") has been determined. 2


Peroxide decomposition products: hydroxyl and superoxide radicals As discussed above, peroxides are susceptible to thermal and transition metal induced homolyticfragmentationreactions in which hydroxyl (HO ) and superoxide ( 0 ) radicals are generated. The hydroxyl radical (HO) is the strongest one-electron oxidant that can exist in aqueous conditions (161J62). As a short-lived but extremely powerful oxidizing agent, the hydroxyl radical is capable of oxidizing a variety of organic and inorganic compounds. Generally, the reaction proceeds by hydrogen abstraction (78-80,163% the driving force being the difference between thefreeenergy of bond formation (-AG F), and the enthalpy of bond dissociation for R - H (AH E). In general most molecules react exothermally with hydroxyl radical, generating a newfreeradical and water. The newly generated organicradicalthenrapidlyreacts with molecular oxygen to yield the corresponding peroxyl radical, as outlined below. 2

B

DB

HO · + RH R- + 0 2

ΗΟ·+ RX

- R . + Hp

(10)

R0 -

(H) (12)

2

-

RXÎ + Ησ

These reactive intermediates initiate the chain reactions of oxidative degradation, leading ultimately to carbon dioxide and water. Although Η-atom abstraction is the common path for most substrates, electron transfer constitutes another mechanism of oxidative degradation (48). The large bond dissociation energy of aromatic G-H bonds precludes direct Hatom abstraction, but addition to the conjugated π-electron system is energetically favorable, the result being hydroxylation and thus further activation of the aromatic substrate.

Ph. + HOH HO. PhOH + HOH

From the above reaction schemes it is clear that the rate and efficacy of the oxidative degradation process depends on the bond energy of the substrate and


122 the reactivity of the radical intermediates. Thus in the bleaching of wood pulps, which are made up of aromatic (lignin) and carbohydrate (cellulose and hemicellulose) components, the generation of hydroxyl radicals leads to the degradation of both substrates. In fact, studies into the reactions of hydroxyl radicals generated by γ-irradiation, UV photolysis or Fenton's chemistry have shown just this (163-168). Superoxide anion radical ( · 0 ' ) also plays an important role in free-radical reactions and has been extensively studied and documented (169). It is now clear that the superoxide displays four basic types of reactions depending on the reaction conditions, deprotonation, Η-atom extraction, nucleophilic attack and electron transfer. The majority of research into the behavior of superoxide has been carried out in aprotic solvents. This has been necessary due to the fact that in the presence of a proton or protic source, superoxide rapidly disproportionates to molecular oxygen and perhydroxyl anion (78-80).


2

2O2+H2O — ^ HQ2-+Q2+HO-

K~9.1x10

8

(13)

In fact in aqueous media, superoxide functions primarily as a strong Bransted base, deprotonating weakly acidic organic compounds. Although the equilibrium favors molecular oxygen formation, therateof disproportionation is pH dependent, having a maximum rate at a pH equal to the pKa of superoxide (-4.5 @ 25°C) and decreasing with increases in pH. As a result, only those reactions that can compete with therapiddisproportionation will be observed in protic media. Superoxide has been reported to undergo hydrogenabstraction reactions (31 170) however, as with molecular oxygen, substrate activation is required, i.e. heat, ionization, etc. Kadla et al. have demonstrated that even at 90°C in alkaline conditions, nonphenolic lignin structures are unreactive towards superoxide (30 31). Numerous researchers have also proposed aradical-radicalcoupling mechanism for superoxide with other generatedradicals(171,172) to yield the corresponding hydroperoxide anion, and the accompanying ring-opening reactions. Although this is quite typical offree-radicals,it is not necessarily common forradicalanions (173). There are also several documented cases where superoxide does not react withfree-radicalsby coupling, but rather reduces them (174-176). Due to the complex nature of superoxide chemistry, it is important to note that a variety of autoxidation reactions can take over after the initial superoxide reaction; therefore caution must be taken when attempting to analyze the mechanism of superoxide by simply examining the generated products. t

9

t



123 Cited References 1. Several review articles have been published on peroxide bleaching chemistry: (a) Gierer, J. Svensk. Papperst. 1977, 16, 510; (b) Gierer, J. Holzforschung, 1990, 44, 395; (c) Gierer, J. Holzforschung, 1990, 44, 387; (d) Gratzl, J. S., Oxygen Delignification, TAPPI Symp Notes, 1990, pp. 1; (e) Dence, C. W.; Reeve, D. W. Pulp Bleaching - Principles and Practices, TAPPI Press, Atlanta GA, 1996, Chapters III-V. 2. Xu, C.; Jameel, H.; Chang, H-m.; Hoekstra, P. M. TAPPI Pulping Conference Proceedings. 1994, 1331. 3. Sundara, R. P.; Kronis, J.D. TAPPI Pulping Conference Proceedings, 1998, 863. 4. Jaaskelainen, Α.; Poppius-Levlin, K. Nordic Pulp Pap J., 1999, 14, 116. 5. Jaaskelainen, Α.; Poppius-Levlin, K. J Pulp Pap Sci , 1999, 25, 37. 6. Delagoutte, T.; Lachenal, D.; Ledon, H. Paperi Ja Puu, 1999, 81, 506. 7. Heuts L., Gellerstedt, G. Nordic Pulp and Paper J., 1998, 13, 107. 8. Gellerstedt, G.; Heuts, L.; Robert, D. J Pulp Pap Sci. 1999, 25, 111. 9. Sun,Y; Argyropoulos, D. S. Holzforschung, 1996, 50, 175. 10. Yuan, Z.; Ni, Y.; Van Heiningen, A. Can. J. Chem. Eng., 1997, 75, 37. 11. Yuan, Z.; Ni, Y.; Van Heiningen, A. Can. J. Chem. Eng., 1997, 75, 42. 12. Gronroos, Α. J.; Pitkanen, M.; Vuolle, M. J. Pulp Pap. Sci., 1998, 24, 6. 13. Perez, D.D.; Terrones, M.G.H.; Grelier, S.; Nourmamode A; Castellan, A. J. Wood Chem. Tech., 1998, 18, 333. 14. Chang, H.M.; Allan, G.G. Lignins, Occurrence, Formation, Structure and Reactions, Sarkanen, K.V. Ludwig, C.H. Eds, Wiley-Interscience, NY, 1971, pp.433. 15. Gratzl, J. S. Papier, 1987, 41, 120. 16. Gratzl, J. S. Papier, 1986, 40, 614. 17. Wiberg, K. B. J. Am. Chem. Soc. 1955, 77, 2519. 18. McIsaac Jr, J. Ε; Ball, R. E; Behrman, E. J.. J. Org. Chem., 1971, 36, 3048 19. Payne, G. B. J. Org. Chem. 196126668. 20. Breger, H; Bickel, A.F. Trans. Faraday Soc. 1961, 57 1325. 21. Denney,D. B.; Rosen, J. D. Tetrahedron, 1964, 20, 271. 22. Bouillon, G.; Lick, C.; Schank, K. In The Chemistry ofFunctional Groups, Peroxides. S. Patai Ed., John Wiley and Sons Ltd., NY, 1983, pp. 279. 23. Strum, W.; Kuchler, G. Non-Chlorine Bleaching Conference Proceedings. 1993, 31. 24. Strum, W. Wochenblatt der Papierfabrikation, 1990, 10, 423. 25. Ben, H. B.; Gorsane, M.; Geerts-Evrard, F.; Pecher, J.; Martin, R. H.; Castelet, D. Bull. Soc. Chem. Belg, 1986, 95, 547. 26. Obrocea, P.; Cimpoesu, G. Cell. Chem. Tech., 1998, 32, 517.


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