Biomedical Engineering Reference
In-Depth Information
would have great differences. Therefore, a structural model was usually used to
present the structure of lignin. This kind of structural model only describes a
hypothetical structure inferred from the average results. Further, different plant
sources, or even lignin isolated from the same plant but in different ways, would
have different categories of linkages and composition of functional groups, resulting
in the complicated lignin structure. Through nearly two decades of research on
lignin structure, a dozen structural models have been proposed. Figure
2.4
is a
structural model of lignin from softwoods. It can be seen that the lignin has a
complicated structure [
34
].
Through the study of various types of lignin structural models, lignin is a com-
plicated amorphous polymer with three-dimensional network, which is basically
composed of phenylpropane units linked to each other by the irregular coupling of
C-C and C-O. Lignin includes three basic structural monomers: p-phenyl monomer
(H type) derived from coumaryl alcohol, guaiacyl monomer (G type) derived from
coniferyl alcohol, and syringyl monomer (S type) derived from sinapyl alcohol
(Fig.
2.4
). The structural formula is as follows:
Although lignin only has three basic structures, the quantity proportions of these
basic structures vary greatly in different families of plants. Lignin of hardwood
includes large amounts of syringyl units. In the UV-photodegradated production
from
Eucalyptus urophylla
lignin,
¨
(syringyl-type compounds) is 58.10 %, and
¨
(guaiacyl-type compounds) is 18.75 %. Compared with eucalyptus lignin, the
pyrolysis products of sulfate pulp lignin are rich in guaiacol. In the sulfate pulp
lignin, M (syringyl)/m (guaiacyl) is 4.3:1; in the eucalyptus lignin, this ratio is
6.4:1. Structural units of softwood lignin are mainly guaiacyl-type units; a small
amount of p-hydroxyphenyl-type units remains. Wheat straw lignin mainly consists
of noncondensed guaiacyl units, noncondensed syringyl units, and other condensed
units; the ratio of n (noncondensed guaiacyl units) to n (noncondensed syringyl
units) to n (condensed units) is 1.44:1:3.24. The content of the -OCH
3
group in
bamboo lignin is similar to the content in hardwood lignin, for example, in Bai jia
bamboo (ph, nidularia Mu) lignin, the ratio of n (guaiacyl units) to n (syringyl units)
to n (hydroxy-phenyl units) is 1:1.15:0.54 [
35
].
The coupling modes between each basic unit include
“
-O-4,
“
-5,
“
-1, and so on.
Figure
2.5
is a partial section of a softwood lignin structure.
Ether bonds in lignin include phenol-ether bonds, alkyl-ether bonds, dialkyl
bonds, diaryl ether bonds, and so on. About two thirds to three quarter phenyl-
propane units of lignin are linked to the adjacent structural units by ether bonds;
only a small part is present in the form of free phenolic hydroxyl. Phenol-
ether bonds account for 70-80 % in these groups, guaiacyl glycerol-
“
-aryl ethers
(
-O-4) account for about half of phenol-ether bonds, followed by the guaiacyl
glycerol-
“
-O-4), also containing other types of ether bonds. Lignin in
softwood and hardwood mainly contains aryl glycerol-
'
-aryl ethers (
'
-O-4) ether bonds,
approximately reaching half of the lignin in softwood and more than 60 % in
hardwood. In the C-C bonds of lignin, the dominant coupling type is
“
-aryl-(
“
“
-5,
“
-
“
linkage, followed by
“
-1,
“
-2, 5-5, and so on [
36
].
