Biomedical Engineering Reference
In-Depth Information
2.5.3
Physicochemical Properties of Lignin
2.5.3.1
Chemical Properties of Lignin
The chemical properties of lignin include halogenation, nitration, and oxidation
reactions on the phenyl ring; reactions on the benzyl alcohol, the aryl ether bond,
and an alkyl ether bond in the side chain; lignin-modified chromogenic reaction;
and so on. The chemical reactions of the lignin structural unit are divided into two
major categories: nucleophilic reactions and electrophilic reactions.
(1) Chemical reactions of lignin structural unit on the side chain
Reactions on the lignin side chains are associated with pulping and lignin modifi-
cation; the reaction is a nucleophilic reaction. The following reagents can conduct
nucleophilic reactions with lignin:
①
In alkaline medium, the effect of HO-, HS-, and S2-nucleophilic reagents leads
to the cleavage of the main ether bond (e.g.,
'
-aryl ether bond, a phenol-type
'
-
alkoxy ether bond, and phenol-type
-aryl ether bond) and fragmentation and
partial dissolution of macromolecule lignin. In alkaline medium, the phenol-
type structural unit is separated into phenolate anions, and an oxygen atom that
affects the benzene ring by induction and a conjugative effect, which activates
their
ortho
- and
para
- positions and thereby affects the stability of the CO
bond and breaks the
“
-aryl ether bond, then generating a methylene quinone
intermediate and resulting in the aromatization of methylene quinone to generate
a 1,2-diphenylethene structure.
'
In neutral medium, reaction with nucleophile HSO
3
or SO
3
2
leads to breaking
of the ether bond and brings SO
3
2
groups in the degradation of lignin fragments.
②
③
Acidic media mainly relate to the lignin fragmentation reaction of the acidic
sulfite pulping process. SO
2
aqueous solution is taken as an affinity reagent,
leading to the breakage of phenol-type and nonphenolic
'
-aryl ether bonds,
sulfonation of
'
-carbon, and increased lignin hydrophilicity. Phenol-type and
nonphenolic
-alkoxy ether bonds may also have a similar reaction. In addition,
C1, C5, and C6 on the high electron density centers of the aromatic ring
could also have a condensation reaction with methylene quinone intermediates
[
10
,
36
].
'
(2) Chemical reaction of the aromatic ring in the lignin structure
Chemical reactions of the aromatic ring in the structural unit of lignin are closely
related to the lignin-bleaching process and its modification and have been divided
into electrophilic and nucleophilic reactions.
①
Electrophilic substitution reaction: This mainly refers to substitution and oxida-
tion reactions. Electrophilic reagents include chlorine, chlorine dioxide, oxygen
molecule, ozone, nitro cation, nitroso cation, and so on. The electrophilic reagent
replacement breaks the side chains of lignin and leads to the oxidative cleavage of
“
-aryl ether linkages. The aliphatic side chain is oxidized into a carboxylic acid,
