Читайте также:
|
|
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].
Дата добавления: 2015-10-21; просмотров: 126 | Нарушение авторских прав
<== предыдущая страница | | | следующая страница ==> |
Reaction Path C | | | Reactions of Carbohydrates |