Functional group Amount relative to native lignina Amount Reference

Free phenolic hydroxyl ~20% higher 25–35/100 C9 50, 95

Methoxyl group ~20% lower Variable 60

Catechol Formed in pulping 3/100 C9 96

Aliphatic hydroxyls ~60% lower 40/100 C9b 81, 83

Aliphatic carbonyls Destroyed in pulping Negligible 82, 97

Aliphatic carboxyls Formed in pulping 5/100 C9b 81, 98

Aliphatic reduced units Higher Variable 97

a. Approximate differences for a residual kraft lignin from a 30 kappa pulp.

b. Converted literature values to groups/100 C9 using 185 g mol–1 as C9 unit.

Tab. 7.4 Relative reactivity of lignin model compounds with

oxygen (from Ref. [90]).

Functional group Relative reactivity to oxygen Reference

Phenolic hydroxyl

Methoxyl

Side chain

OH

R3 R2

R1

OCH3

R3 R2

R1

OH

OH

R

OCH3

H3CO OCH3

R

OCH3

OCH3

R

OH

R

OH

OCH3

CH2

OH

OCH3

HC

OH

OCH3

C

O

OH

C

OCH3

OH OH O

100–102

100–102

7.3 Oxygen Delignification 635

Due to lack of reactivity of nonphenolic compounds, studies on ring cleavage by

oxygen have focused on compounds with free phenolic hydroxyl groups. The former

can be degraded in the presence of a compound containing a free phenolic

hydroxyl group that produces oxygen radical species in the reaction with oxygen

[99]. Depending on the raw material (hardwood or softwood), the number of

methoxyl groups varies and is also affected by alkaline demethylation reactions

that form catechol groups during pulping. Although the catechol-containing compounds

are by far the most reactive, their number present in residual lignin following

oxygen bleaching is not significantly changed, indicating that these groups

are also formed during oxidation [90].

Carboxyl groups, not present in native lignin, are formed during kraft pulping

(Tab. 7.3) and oxygen bleaching through side chain and ring cleavage reactions. In

contrast, muconic acid structures are the primary ring cleavage products that

should be present in only minor quantities after oxygen bleaching, due to their

high reactivity [90,103].

The quantity of aliphatic hydroxyl groups in softwood kraft pulp (Tab. 7.5) [80]

increased after oxygen alkali treatment, while the carboxyl group content

decreased after an initial increase in the residual lignin, and the quantity of all

hydroxyl groups increased in the effluent lignin. Both were accompanied by a

drastic increase in carbohydrates. Others have also found comparable changes in

the hydroxyl group content [104,105].

Tab. 7.5 Quantities of reactive groups (e.g., aliphatic hydroxyls,

phenolic hydroxyls, carboxylic acid groups) and carbohydrate

content in lignin samples (from [80]).

Group RL

[mmol g–1]

Lig-1st

[mmol g–1]

Lig-2nd

[mmol g–1]

Lig-3rd

[mmol g–1]

Lig-L1st

[mmol g–1]

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