Chemistry of Oxygen Delignification

Manfred Schwanninger

Among different pulping techniques, kraft pulping is the most important process,

consisting of wood treatment with a solution of sodium hydroxide and sodium

sulfide at high temperature. This results in wood delignification through the degradation

of lignin (and also carbohydrates) and its dissolution in pulping liquor.

Although a major fraction of wood lignin (~97%) can be removed in kraft pulping,

the remainder of the lignin (residual lignin) is rather resistant under the pulping

conditions. In order to remove the residual lignin from pulp, oxidative lignin degradation

with bleaching reagents such as dioxygen, hydrogen peroxide, ozone,

and chlorine dioxide is required.

According to the general concept of the chemistry of delignification [1,2], the reactions

of lignin during pulping and bleaching can be divided into two categories:

_ Nucleophilic additions and displacements, which are involved in

pulping processes, in later phases of lignin-degrading bleaching,

and in lignin-retaining bleaching.

_ Electrophilic additions and displacements, initiating the lignindegrading

bleaching processes.

Depending on the nature of the reagent(s), the reactions can be further divided

into categories of nucleophilic and electrophilic which frequently, but not always,

conform to a reduction-oxidation classification.

Carbonyl carbons or the vinylogous carbon atoms in intermediates of the enone

type (quinone methide intermediate; see Section 4.2.4, Chemistry of kraft pulping,

Scheme 3) are the sites where the nucleophiles, which are present in pulping

liquors, begin the attack [1,2]. Additionally, nucleophilic groups in the a– (or c-)

position of the side chain attack the b-carbon atom in a neighboring group participation-

type of reaction which, in b-aryl ether structures, leads to fragmentation

632 7Pulp Bleaching

[1,2]. The initial attack by electrophiles, which are present in bleaching liquors,

takes place on the aromatic rings and side chains, which are activated by free or

etherified phenolic hydroxyl groups [1–4].

In order to emphasize the principal difference in delignification during pulping

and bleaching, it should be stressed that delignification during pulping occurs

exclusively due to nucleophilic reactions [1,2,5], whereas delignification during

bleaching is primarily initiated by electrophilic reactions, which may be followed

by nucleophilic processes [6–9].

This initial step of oxygen-alkali bleaching will be briefly described here. In alkaline

media, the phenolic hydroxyl group (1) (Scheme 7.1) is deprotonated to produce the

phenolate anion (2) that furnishes the high electron density needed to initiate a oneelectron

transfer. The reactive electrophilic (d-) sites marked in Scheme 7.1(2) are

situated at alternating carbons. Scheme 7.2 (left) depicts theHOMOof the phenolate

ion of coniferyl alcohol. The size of the orbitals’ nodes correspond to centers of high

electron density, and hence to sites of preferred attack of electrophiles; thus, they

determine the pathway of the subsequent reaction. The resultant electron density distribution

is shown in Scheme 7.2 (right), where red zones denote centers of high electron

density. Oxygen attacks at an electrophilic (d-) site and abstracts an electron,

leaving a phenoxyl radical (3) and/or a mesomeric cyclohexadienonyl radical,

while oxygen itself is reduced to the superoxide anion radical.

OH

OCH3

CH

CH

CH2OH

ä- ä-

ä-

ä-

O-

OCH3

CH

CH

CH2OH

+ OH -, - H2O

O

OCH3

CH

CH

CH2OH

O2 O2

-

1 2 3

Scheme 7.1 The initial step of oxygen-alkali bleaching at

electrophilic (d-) sites.

Scheme 7.2 HOMO-distribution (left) and electron density

distribution (right) of the phenylpropene unit 2 shown in

Scheme 7.1 (PM3calculation with Spartan 4.0).

7.3 Oxygen Delignification 633

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