Degradation of Lignin

Ozone preferentially reacts with olefinic compounds, whereas the reactivity for

the electrophilic attack is increased by substituents with +M- and +I-effect. The

reactivity of lignin structures can therefore be ranked as follows: stilbenes > styroles

> phenolic substances > muconic acid-like substances > nonphenolic substances

> aldehydes >> a-carbonyls.

The reactions of ozone with aromatic compounds involve initial an electrophilic

attack by the oxidant, followed by the loss of oxygen that results in hydroxylation

of the aromatic ring (Scheme 7.30, pathway A) [29]. Formation of the hydroxyl

group increases the reactivity towards electrophilic substitution reactions. Therefore,

it is probable that in a subsequent step ozone reacts with the aromatic ring

by a 1,3-cycloaddition. Alternatively, the electrophilic substitution can be followed

by oxidative dealkylation giving an ortho quinone (Scheme 7.30, pathway B). 1,3-

Dipolar cycloaddition by a concerted mechanism proceeds at C3 and C4, because

these atoms have the highest negative charge density of the aromatic system.

Finally, the aromatic ring is cleaved (Scheme 7.30, pathway C) [29].

H3CO

O [OH]

H3CO

O [OH]

+ O3

+ H

O O O -

- O2

H3CO

O [OH]

OH

OH

OH

+ O3

+ OCH3

O O O -

- O2

O

O

A

B

OCH3

- CH3OH

O

O

+ O3

OCH3

O

O

O

+ H2O

O

O

C

OCH3

- H2O2

[OH]

[OH] [OH]

OCH3

O

Scheme 7.30 Reactions of ozone with aromatic moieties

(redrawn from Gierer [29]).

790 7Pulp Bleaching

An ionic [30] 1,3-dipolar cycloaddition is also opening across olefinic double

bonds (Scheme 7.31, pathway D) [29]. According to the generally accepted mechanism

of Criegee, the resulting “initial ozonide” (11), a 1,2,3-trioxacyclopentane, is

formed. This decomposes to a dipolar ion intermediate, a simple carbonyl compound

and a carbonyl oxide (12), that recombines further to give the “final ozonide”

(13) that is cleaved immediately into the ozonolysis products by hydrolysis

(14). 1,1-Cycloaddition of ozone to olefins can also occur via p- (16) and r-complexes

(17) (Scheme 7.31, pathway E) forming the corresponding epoxide (18)

after the loss of one molecule of oxygen (singlet state). The insertion of ozone into

carbon–hydrogen bonds in alcohol-, aldehyde- and ether-type structures is a

further reaction mode (Scheme 7.31, pathways F and G) [29]. In these reactions

the hydrotrioxide intermediate eliminates molecular oxygen (again singlet oxygen)

forming the corresponding oxidation products. In the case of aryl and alkyl ethers,

the reaction thus results in the cleavage of the ether bond [29].

In addition to the “ionic” ozone reactions, there is an appreciable number of

“radical reactions” which bear a resemblance to that of oxygen (and hydrogen peroxide

bleaching) [30]. This is due to the presence and involvement of molecular

oxygen (see Scheme 7.28), hydrogen peroxide (Scheme 7.31, path D) and the

same radical species, superoxide and hydroxyl radicals [30]. In addition to the radical

formation already described elsewhere (see Sections 7.3.2 and 7.6.4, Oxygen

bleaching and hydrogen peroxide bleaching), the initial step of radical reactions of

ozone may be regarded as a one-electron transfer from the substrate (SB) to the

oxidant, here with the formation of a hypothetical ozonide (_O3

–)/hydrotrioxy

(HO3_) radical and a substrate radical (SB_) (Scheme 7.32, path A) [30]. This step

may be followed by combination of these two radicals to give hydrotrioxides. Due

to its high reduction potential, ozone reacts with the substrate much more rapidly

and more comprehensively than does oxygen [30]. Thereby phenolic, nonphenolic,

olefinic and aliphatic structures may be attacked. Intermediary HO3_ may arise via

electron transfer (Scheme 7.32, path A), which is followed by combination of the

two resultant radical species. HO3_ can also form through the reduction of ozone

by superoxide radicals (Scheme 7.32, path B). Due to the high rate of ozone reduction

by superoxide radical, path B may compete with path A. The formation of

hydrotrioxides via direct insertion into C–H bonds (Scheme 7.32, path C) is a

further characteristic reaction of ozone [30]. The hydroxyl radicals formed by HO3_

decomposition, hydrotrioxides SB-O3H, and possibly also by transition metal ioncatalyzed

decomposition, enter the oxidation cycle and accelerate the initial step.

As shown above, the efficient starting bleaching reagent ozone can be expected to

compete with hydroxyl radicals for the substrate in the initial step much more successfully

than oxygen. Gierer provided a tentative scheme for the course of ozone

bleaching [30]. The higher efficiency and lower selectivity of ozone as compared to

oxygen is due not only to the greater reactivity of ozone but also to the facile generation

of hydroxyl radicals [30]. The reduction of O3 and H2O2 can be blocked by

scavenging O2

–

_/HO2_. By the way, in water it is likely that superoxide will primarily

disproportionate as it acts as a very strong lewis base. In this case, ozone

bleaching should be limited to the ionic part of the process (Schemes 7.30 and

7.5 Ozone Delignification 791

+ O3

D

C

C

R R

R R

R2C

R2C

O

O

O

R2C

R2C

O

O

O

+

-

R2C

O

O

C

R2

O

+ H2O

- H2O2 R2C O

R2C O

+

11 12 13 14

+ O3

E

C

C

16 18

C

C

O3

π-complex σ-complex

C

C

O

O O-

+ - O2 C

C

O

+ O3

F C

H

OR

20 21

C

O

OR

O OH

R = H, Ar or Alk

- ROH, - O2

C O

+ O3

G C

H

O

23 24

C

O

O

O OH

- O2

C OH

O

Scheme 7.31 Reactions of ozone with olefins, alcohols, aldehydes

and ethers (adapted from Gierer [29]).

7.31) and to the action of hydroxyl radicals arising directly by electron transfer

from the substrate to ozone. In this way, it could be expected that the process

becomes more selective without any severe loss in efficiency [30].

The formation of radicals in ozone reactions with lignin model compounds

have been extensively studied by Ragnar et al. (1999) [31]. Radical formation is

mainly due to a direct reaction between ozone and the substrate. By the way, in

water it is likely superoxide is the primarily formed radical in acidic solution. In

the presence of oxygen and ozone, superoxide is easily converted to the hydroxyl

radical and vice versa, according to Eqs. (97–99).

O3 _O__ 2 _H_

_____ k_109M_1s_1 HO_ _ 2O2 _97_

O3 _HO_

_____ k_104M_1s_1 HO_ _ 2O2 _98_

O__

2 _HO_

_____ k_109M_1s_1 HO_2 _O2

___ H_ H2O2 _99_

Radical formation cannot be prevented under process conditions (e.g., by the

elimination of transition metals), and they always occur, with yields being higher

for syringyl compounds than for the corresponding guaiacyls. An increase in radical

792 7Pulp Bleaching

O3 + SB

+ H+

electron

transfer

SB + HO3 SB-O3H

O2 OH SBO2

O3 + O2

- + H+

reduction

O2 + HO3

O2 OH

O3+ C H C O3H

insertion

C O2 OH

A

B

C

Scheme 7.32 Radical reactions of between ozone and the

substrate (SB) (redrawn from Gierer [30]).

yield was observed for all phenolic lignin model compounds, starting at pH 3. As

pH is the parameter that most profoundly affects the radical formation; pH 3 may

be regarded as optimal in ozone bleaching. Beside that is a self-evident impact of

temperature. Moreover, the rate of ozone addition may also affect the selectivity in

an ozone bleaching stage, particularly in hardwood pulp bleaching [31].

The reaction of ozonewith creosol and a nonphenolic compound (3,4-dimethoxytoluene)

in an aqueous medium proceeds via a charge-transfer state (Scheme 7.33) [32].

From the charge-transfer state, two reactions pathways are possible. In path A

(Scheme 7.33) a complete electron-transfer takes place, and this results in the formation

of an aromatic cation radical and an ozonide radical. After protonation, the ozonide

radical decomposes to oxygen and a hydroxyl radical, which is a direct route to

hydroxyl radical formation. In path B (Scheme 7.33), ozone adds to the aromatic ring,

preferentially to the oxygen-substituted carbons, and the resulting zwitterions subsequently

react via different routes.Homolytic cleavage of the trioxide yields superoxide

and a quinol radical (path B) [32]. Heterolytic cleavage of the aromatic ring (path

C) yields the same reaction products as does the ozonolysis, forming hydrogen

peroxide. For nonphenolic structures, heterolytic ozonolysis dominates. This is

supported by quantum chemical and thermochemical methods [32].

In reactions with hydroxyl radicals lignin is preferred compared to carbohydrates

[33]. After ozone treatment a significant increase in hydrophilicity [34] and

the number of carboxyl groups were observed in the residual lignin [34] as well as

the dissolved lignin [35]. Likewise, an increase in the number of carbonyl groups

was also observed [36]. In general, apart from the oxidation of double bonds of the

side chain and the ring, the structure of residual lignin after ozone treatment is

very close to that of the residual lignin after Kraft pulping [34]. Hoigne and Bader

found that lignin-like structures react much faster with ozone than carbohydrates

in the presence of radical-scavengers [27,28].

7.5 Ozone Delignification 793

OR

OH

+ O3

+

OCH3

O

O

-O

A

C

OCH3

OR

OCH3

δ+

δ−

O3

OR

+

OCH3

+ O3

- H+

O2 + OH

- H+

O

OCH3

O

O

-O

homolysis

O

OCH3

O

+ O2

-

O

OCH3

O

O

HO

B

heterolysis

H2O

O

OCH3

O

OH

+ H2O2

+ H+

Scheme 7.33 Proposed mechanism for the formation of

superoxide and hydrogen peroxide with ozone (adapted from

Ragner et al. [31]).

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