Residual Lignin Structure (see Section 4.2.5)

The structure of the residual lignin which still remains in the pulp after cooking

depends highly on the degree of delignification, as well as on the kraft process

conditions. To study this lignin, an isolation procedure must be applied, with currently

available approaches being enzymatic procedures and acid-catalyzed hydrolysis

[29–32]or combinations thereof [33,34]. It is important to know that the procedure

chosen (e.g., acidolysis) has an influence on the structure of the residual

lignin [35,36], and that the yield of dioxan lignin is rather low. Enzymatic methods

isolate the lignin less destructively, with a higher yield with still intact lignin carbohydrate

linkages. The yields of residual lignins, for both enzymatic and acid

hydrolyses, are lower for hardwood lignins [37]. Enzymatic methods introduce

protein impurities which must be removed at later stages [38], but improved

methods to remove the latter from lignin preparations are available [37]. An acid

hydrolysis may cleave ether linkages to a certain extent, but provides lower yields

and somewhat purer lignin preparations.

Residual lignin still contains intact b-O-4 ether structures, as well as a very

small amount of enol ethers which are indicative of elimination reactions. Lignincarbohydrate

complexes are also proposed to be present in residual lignin [30,39–

41], and this has been confirmed with 2D-NMR [10,55]. Residual lignins can generally

be characterized by changes in the functional group distribution, or by different

NMR techniques.

Condensed structures (5–5′, b-5, 5-O-4 and DPM) in residual lignins are either

enriched or generated during pulping, and can be roughly determined by degradation

methods or NMR techniques, or combinations thereof [42–47]. However, the

DFRC method showed a limited potential for quantification with lignins [48].

Phenolic groups determine to a large extent the reactivity of lignin, and increase

its solubility. Free phenolic groups are generated by cleavage of the different ether

linkages, hence the dissolved lignin contains more free phenols compared to the

residual lignin, whereas in MWL the value is even less (Tab. 4.17).

Table 4.17 Phenolic hydroxyl groups in different

lignin fractions according to Gellerstedt [49,50].

Lignin type OH-groups/100 C9

Wood 13

Residual 27

Dissolved 50–60

The same trends were subsequently confirmed later by Faix et al. [51]and Froass

et al. [52,53].

b -Aryl ether structures: In general, the amount of b-aryl ethers decreases as the

kraft cook proceeds, although a substantial number of noncondensed b-aryl ether

4.2 Kraft Pulping Processes 173

structures remains in the residual lignin. The numbers of these in the dissolved

lignin, and in the lignin remaining in the pulp, approach similarity towards the

final phases [21,50]. However, in most dissolved lignins the amount is rather low,

and it can be concluded that most of them are cleaved.

Quinoide structures: o -Quinones may result from the oxidation of ortho -dihydroxy

benzenes (catechols), which are formed upon demethylation of the aromatic

methoxyl groups (cf. Scheme 4.8 and Scheme 4.12). They exhibit a dark color, and

are therefore considered to form a major part of the chromophores of kraft pulp

[54]. An overview on the structure of residual and dissolved kraft lignins as visualized

by NMR has been provided by Balakshin et al. [55].