Composition of Lignin, Residual Lignin after Cooking and after Bleaching

The limitation of the extent to which dioxygen delignification can be used is well

known in practice and research, and many investigations have focused on an elucidation

of responsible lignin structures, namely those that are stable or only react

slowly under dioxygen bleaching conditions. Therefore, information on the structures

of the residual and dissolved lignins, isolated from pulp and the pulping solution

[70,71], is of primary importance for a better understanding of the underlying

mechanisms of, for example, kraft delignification and the reactivity of residual

lignins in bleaching. Although extensive investigations into the isolation [72–75]

and characterization of residual [76,77] and dissolved lignins using different analytical

techniques have provided valuable information, their structures are not yet

well established. An excellent review of the procedures used for residual lignin has

recently been published [78]. Further progress in the characterization of lignins and

lignin–carbohydrate complexes requires the application of advanced techniques such

as nuclear magnetic resonance (NMR) [12,35,79–89] and others [77].

Based on the results of different procedures [78] applied, theoretical models for

residual lignin structures have been developed, and upon these presumed structures

functional groups and linkages have been selected for the study of model

compounds. Although the rate of degradation in actual lignin systems is much

slower and the extent of degradation is much lower [90], this might be due to the

different matrices and accessibility [26,31,63,66,80,91–93]. Moreover, despite the

selectivity of the chemical agents used [94], the results obtained with model compounds

are valid when describing lignin degradation, as comparisons with lignin

studies have revealed [90].

An excellent review of lignin model compound reactions under oxygen bleaching

conditions has recently been published [90]. This presents a compilation of

published data relating to functional group contents in residual kraft lignins (Tab.

7.3) and the relative reactivity of functional groups of lignin model compounds

634 7Pulp Bleaching

with oxygen (Tab. 7.4). The content of free phenolic hydroxyl groups, which is

important for lignin solubility and reactivity, was increased to about 25/100 C9 in

residual lignin and to about 65/100 C9 in dissolved kraft lignins [90].

Tab. 7.3 Functional group content in residual lignin (from Ref. [90]).

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