A Principal Reaction Schema for Oxygen Delignification

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Over 30 years of research into the oxidation of lignin and lignin model compounds

with dioxygen has now elapsed, and has provided insights into the reactions

involved in the degradation, and their mechanisms. Based on the reaction

products formed from the degradation of lignin and lignin model compounds

with dioxygen and with ROS generated during bleaching, a number of mechanisms

have been proposed. Several excellent reviews have been produced on the

mechanisms involved in lignin degradation [1,2,4,6,7,72,101,122,138,146,147] and

the reactive species present in these reactions [3,9,90,129,130], including their

selectivity. The latter remains of interest [148,149], especially in connection with

protective systems and additives [94,150–156]. In addition, an excellent book on

oxygen delignification chemistry was published a few years ago [157]. It is impossible

to cover all of these mechanisms in detail within this chapter; thus, a general

summary with selected mechanisms will be provided.

Oxygen delignification is actually based on the competitive reactions of oxygen

or ROS within pulp lignin and carbohydrates [94]. Lignin removal under alkalioxygen

conditions is accompanied by a kinetically less favorable oxidation of carbohydrates,

whereas the oxidation of the carbohydrates becomes a more favorable

process when the kappa number decreases [94]. The reaction of phenolic compounds

with oxygen produces ROS, namely the hydroxyl radical (_OH), which

can degrade nonphenolic (model) compounds.

As shown previously (see Scheme 7.1), the initial step in oxygen-alkali bleaching

is the formation of the phenoxyl radical as a consequence of an electrophilic

attack by oxygen (Scheme 7.5A). Moreover, the hydroxyl radical formed during

oxygen treatment [Eqs. (5), (6), and (19)] is also capable of generating a phenoxyl

radical (Scheme 7.5B) being reduced to the hydroxide anion (OH–).

A principal reaction schema for oxygen delignification [3,6,7,138] starts with the

generation of hydroperoxides, which are key intermediates in the oxidation of lignins

and carbohydrates. They can be formed either by electrophilic or nucleophilic

reactions:

_ Formation of hydroperoxides [8,138]

_ Fragmentation of hydroperoxides (homolytic – forming radicals;

or heterolytic – forming hydrogen peroxide, singlet oxygen)

[8,138]

_ Involvement of the radicals in the bleaching process [3,9,129,130]

7.3 Oxygen Delignification 649

OCH3

CH2OH

ä- ä-

ä-

O-

OCH3

CH2OH

+ OH-, - H2O

OCH3

CH2OH

O2 O2

A 3

O-

OCH3

CH2OH

OCH3

CH2OH

B 3

OCH3

CH2OH

OH-

+ OH-, - H2O

+ HO

Scheme 7.5 Formation of the phenoxyl radical by oxygen (A)

and the hydroxyl radical (B).

R1 OCH3

C R

R1 OCH3

C R

R1 OCH3

C R

R1 OCH3

C R

R1 OCH3

C R

O -O

R1 OCH3

C R

OCH3

C R

O O-

A 6 7 8 9

R1 OCH3

C R

O -O

O-

R1 OCH3

C R

R1 OCH3

C R

B 10 11 12 13

14 15 16 17 18 19

-O

OCH3

C R

OCH3

C R

O O-

OCH3

C R

+ H2O, - OH-

R1 OCH3

C R

R1 OCH3

O C R -O

R1 OCH3

O C R -O

O-

R1 OCH3

O C R

R1 OCH3

O C R

+ O2

+ H2O, - OH-

R = H, OAr, Ar or Alk

Scheme 7.6 Formation of hydroperoxide intermediates in

alkaline media followed by an intramolecular nucleophilic

attack of the hydroperoxide anions (adapted from Ref. [6]).

650 7Pulp Bleaching

+ OH - (-)

+ O2, - HOO

+ HOO, - HOOH

+ HO, - HOH

R = H, OH, organic moiety

+ HOO

+ O2

OOH

20 21 22 23

O-

+ O2, + H+

OOH

O-

+ HOO-

24 25 26

Scheme 7.7 Formation of hydroperoxides in the autoxidation

of enolisateable and enediol structures, and the formation of

the hydroperoxy anion (adapted from Refs. [4,6]).

The abstraction of an electron from phenolate anions by oxygen (or the hydroxyl

radical) (Scheme 7.5) yields phenoxyl radicals (Scheme 7.6, 4 and 14) and the

mesomeric cyclohexadienonyl radicals (5a and 5b) or “quinone methide” radicals

(15). The superoxide anion radicals then form hydroperoxide intermediates (6 and

10) with the mesomeric cyclohexadienonyl radicals or the b-radical (15). A nucleophilic

attack by the peroxide anions on the carbonyl carbon (11) or a vinylogous

carbon of the cyclohexadienone- (7) or quinone methide (17) moieties yields the

corresponding dioxetane intermediates (8, 12 and 18). Intermediate 8 finally form

an oxirane structure (9). The rearrangement of 12 results in an opening of the peroxide

ring and heterolytic cleavage of the carbon–carbon bond, giving a “muconic

acid” ester (13), and 18 is fragmented by scission of the Ca–Cb bond of the former

conjugated double-bond forming the corresponding aldehydes (19) and/or a

ketone, depending on the nature of R.

The hydroperoxy intermediates formed during the autoxidation of phenolic

(Schemes 7.6 and 7.8) and enolic (Scheme 7.7) structures in lignin and carbohydrates

can be displaced by the hydroxide ion via a SN2 reaction (28 – 29), or the

bond can be cleaved heterolytically giving the hydroperoxy anion, which is

described elsewhere. Homolytic decomposition of hydroperoxy intermediates produces

phenoxy (31) and hydroperoxy (Scheme 7.8) radicals. The latter can be

reduced to the hydroperoxy anion.

The hydroxyl radical reacts with the main components of wood, and attacks

preferentially electron-rich aromatic and olefinic moieties in lignin. It also reacts

with aliphatic side chains in lignin and carbohydrates, but at a lower rate. Depending

on the pH, the hydroxyl radical is converted to its conjugate base, the oxyl

anion radical (see Scheme 7.4). The oxyl anion radical does not react with electron-

rich structures, but rather with aliphatic side chains in lignin and carbohydrates.

The first step in all reactions of the hydroxyl radical with aromatic substrates

(Scheme 7.9, 32) is a rapid addition to the p-electron system of the aromatic

ring forming a short-lived charge-transfer adduct (33) that decays under

alkaline conditions to give isomeric hydroxycyclohexadienyl radicals (34 and 37).

7.3 Oxygen Delignification 651

O-

R1 OCH3

+ O2, + H+

OOH

R1 OCH3

+ OH -

- HOO

R1 OCH3

- HOO

+ e-

HOO-

27 28 29 30

HOO-

Scheme 7.8 Formation of hydroperoxides in the autoxidation

of phenolic structures, and the formation of the hydroperoxy

anion (from Ref. [6]).

OCH3

or OH

Scheme 7.9 Formation of hydroxycyclohexadienyl radicals

(adapted from Ref. [6]).

The hydroxycyclohexadienyl radical can be oxidized by addition of oxygen

(Scheme 7.10) followed by alkali-promoted elimination of the superoxide anion

radical forming a cation radical (35, 38 and 42) and elimination of a proton (rearomatization)

leading to hydroxylation (Scheme 7.10, path A, 36) or, in combination

with elimination of methanol and cleavage of an alkyl-aryl ether bond, to dealkoxylation

(path B) with formation of ortho-quinonoid structures (39). From conjugated

structures (Scheme 7.10, path C), “quinone methide” intermediates (41,

42 and 43) are formed, giving glycolic structures (44) by adding hydroxide ions

that undergo oxidative cleavage of the glycolic C–C bond (44) [6].

The hydroxyl radical adducts (Scheme 7.11, 45) can undergo disproportionation

reactions from which the same oxidation products (46 and 47) arise, together with

the corresponding reduction products (48 and 49) [6].

652 7Pulp Bleaching

OCH3

R = H or alkyl

+ O2

- O2

OCH3

- H+

OCH3

+ O2

- O2

OCH3

- CH3OH, - H+

(- ROH)

O-

OCH3

O-

OCH3

C OH

O-

OCH3

C OH

+ O2

- O2

OCH3

C OH

+ OH -

O-

OCH3

OH cleavage

Scheme 7.10 Reactions of the hydroxyl radical adducts of aromatic

and ring-conjugated structures (adapted from Ref. [6]).

O-

OCH3

O-

OCH3

O-

OCH3

O-

OCH3

Disproportionation

Scheme 7.11 Disproportionation of hydroxyl radical adducts

(adapted from Ref. [6]).

Another reaction mode of the hydroxycyclohexadienyl radical (Scheme 7.12, 51

and 56) is the elimination of the hydroxyl radical as hydroxide anion (Scheme

7.12, paths A and B). This results in the formation of cation radicals (52 and 57)

followed by the generation of side-chain oxidation products and products of homolytic

Ca–Cb bond cleavage (58). The elimination of a proton leads to a re-aromatization

(59).

7.3 Oxygen Delignification 653

OCH3

- OH -

OCH3

CR1

R = H or alkyl

OCH3

+ - H+

OCH3

R = alkyl; R1 = aryl or aroxyl

further oxidation

OCH3

CR1

OCH3

CR1

- OH -

OCH3

CR1

OCH3

- H+

Scheme 7.12 Reactions of the hydroxyl radical adducts of

aromatic and side chain structures (adapted from Ref. [6]).

Elimination of the hydroxyl radical as hydroxide anion results in the formation

of a cation radical (62 and 63), followed by a phenolic coupling (64) (Scheme 7.13)

and elimination of two protons to form a diphenyl (5–5) structure (58). The formation

of diphenyl structures is an undesirable reaction, because the 5–5 bond is

very stable and can hardly be cleaved.

O-

H3CO

O-

H3CO

O-

H3CO

- OH -

H3CO

2 x

H3CO

OCH3

O-

H3CO

O-

OCH3

- H+

Scheme 7.13 Phenolic coupling of the hydroxyl radical

adducts of aromatic structures (adapted from Ref. [6])

Singlet oxygen that can be generated during oxygen bleaching in different ways

[Eqs. (13–16)] has been of growing research interest for the past few years

[140,142,158–180]. Both, lignin model compounds and pulp have been investigated.

However, in most of the studies photosensitizers, such as rose bengal

654 7Pulp Bleaching

[159,162,164–167,174,179], methylene blue [140,141,160,163,169,175,177] or titandioxide

(TiO2) [139,169,175,177] have been used to generate singlet oxygen using

light from the visible range to the UV, the latter also used for direct irradiation of,

for example, an a-carbonyl group-containing lignin. Alternatively, singlet oxygen

was produced from sodium hypochlorite (NaOCl) and hydrogen peroxide [142]

according to Eq. (14). Some of these studies were performed in organic solvents

[162,167,174,176] and others in aqueous alkaline solution [142,168,177,179,181],

with the latter category being of main interest for this chapter. The photo- and radiation

chemical-induced degradation of lignin model compounds have been

summarized in a very good review [171], including other ROS, and the photochemical

oxidation of lignin models in the presence of singlet oxygen has been

studied by using ab initio calculations [178].

As mentioned, singlet oxygen has a pronounced electrophilic character, and hence

reacts well with electron-rich groups such as olefinic or aromatic derivatives. These

electron-rich groups tend to form an intermediate exciplex as a result of charge transfer

reactions between the electron-rich substrate and the singlet oxygen. This exciplex

is able to later form dioxetanes, hydroperoxides, or endoperoxides.

Photosensitized degradation studies of a-carbonyl group-containing lignin

model compounds (Scheme 7.14) show that a hydrogen atom transfer from the

phenolic OH group (66) to 1O2 might occur, leading to a phenoxyl radical (67) and

subsequently to quinonoid species (path A). However, formation of an endoperoxide

(68) leading ultimately to p -quinones (70) is also possible (path B).

H OH

Scheme 7.14 Photodegradation of a-carbonyl group-containing

lignin model compounds (from Ref. [171]).

Moreover, a-carbonyl-containing b-O-4 lignin model compounds intensively

used in singlet oxygen degradation studies have been degraded to products deriving

from b-C–O bond cleavage. The main reactions were conversion of phenolic

aromatic units into carboxylic acids and cleavage of the b-O-4 ether bonds, leading

to a depolymerization of the lignin framework into smaller fragments [177]. Cleavage

of the b-O-4 aryl ether bond has been found for phenolic as well as nonpheno-

7.3 Oxygen Delignification 655

656 7Pulp Bleaching

lic derivatives [162]. Photochemical oxidation of the phenolic b-O-4 aryl ether gave

the same type of product, which confirmed that, in this case, the presence of the

carbonyl group is not indispensable for the cleavage reaction to occur [162]. When

the phenoxy portion of the molecule [1-(4-hydroxy-3-methoxyphenyl)-2-(2,6-

dimethoxyphenoxy)-3-hydroxy- 1-propanol] shows a lower reactivity towards singlet

oxygen, the oxidation of the phenol moiety to hydroquinone can occur. The

photochemical behavior of this model compound can be rationalized from a reaction

of singlet oxygen with the phenoxy part of the molecule [162].

Due to the unknown real contribution of singlet oxygen to lignin degradation

during oxygen bleaching, and the fact that in processes interconversions between

reactive species occur, this section of the text will be minimized.

One example of a rose bengal photosensitized degradation of loblolly pine

(Pinus taeda) kraft pulp, the final product of which contained 4% by mass of residual

lignin with the remainder being carbohydrates, is presented [179]. In this

study, the reactivity of singlet oxygen with kraft softwood substrates with respect

to the chemistry of lignin and cellulose has been investigated. The results revealed

that, despite the relatively high selectivity of singlet oxygen for lignin aromatic

units, degradation of the cellulose nevertheless occurred after approximately 50%

removal of the lignin. A decrease was observed in the number of aliphatic hydroxyls

(17%), condensed phenolics (4%), and guaiacyl phenolics (7%), and an

increase in carboxylic acids (54%). This result is typical of what is observed in the

reactions of ground-state oxygen with pulp or lignin, and suggests that despite the

initial electrophilic reactions of singlet oxygen with lignin, it is likely that ensuing

oxidations follow some of the typical reactions associated with ground-state oxygen

reactions, such as ring additions by hydroperoxide and oxygen followed by

ring openings to the muconic esters and acids. However, unlike ground-state oxygen

reactions, the levels of condensed phenolics (e.g., conjugated lignin monomers

at the C5 positions of the benzene moieties) were reduced during the singlet

oxygen reactions. Thismay be a consequence of the high electrophilic reactivity of singlet

oxygen, and was tested by subjecting substrates enriched in condensed phenolics

to singlet oxygen reactions [179]. The most salient difference between this systemand

a typical ground-state oxygen delignification system is the absence of condensed

phenolic units in the lignin. Subsequently, it was discovered that both the condensed

and noncondensed (guaiacyl) units react well with singlet oxygen [179].

This finding is important since 5-condensed phenolic subunits (5–5 and diphenylmethane;

DPM) in lignin are quite resistant. Their relative robustness does

not, however, appear to be the main rationale for the inactivity of lignin towards

oxygen delignification, but serves to suggest that the nature and reactivity of the

free phenolics deserve increasing scrutiny [182].

Residual lignins isolated from unbleached and oxygen-bleached eucalyptus

kraft pulps by acid hydrolysis and dissolved lignins in the kraft cooked and oxygen-

bleached liquors were studied, and the results compared with the corresponding

residual lignins. The data showed that etherified syringyl structures were

quite resistant towards degradation in the oxygen bleaching, causing little depolymerization

in residual lignin and a small increase in carboxylic acid content, but

producing appreciable amounts of saturated aliphatic methylene groups [105].

7.3 Oxygen Delignification 657

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