71
5.3 Preparation of Lignin With High Antioxidant Activity
As indicated above, the high antioxidant activities of lignin are determined by several factors, such as large amounts of certain functional groups (carbonyl, carboxyl,
methoxyl and phenolic hydroxyl), high homogeneity, low Mw and a small content
of carbohydrate impurities. These factors depend on the origin, pretreatment,
Table 5.2 Bond dissociation enthalpy (BDE) values of various phenolic compounds
Compounds
Structures
BDE
(kJ/mol) Compounds
Structure
BDE
(kJ/mol)
4-(1-Hydroxyethyl)
phenol
OH
HC
CH 3
OH
87.0
p-Coumaric
alcohol
OH
HC
CH
CH2OH
84.7
4-Hydroxybenzaldehyde
OH
C
O
H
90.3
p-Cresol
OH
CH 3
85.3
4-Hydroxybenzyl
alcohol
OH
CH 2 OH
87.2
Phenol
OH
88.0
4-Methylsyringol
OH
OCH3
H3CO
CH3
81.7
Sinapic alcohol
OH
OCH3
H3CO
HC
CH
CH2OH 80.9
a-Methylsyringyl alcohol
OH
OCH3
H3CO
HC
CH3
OH
82.6
Syringaldehyde
OH
OCH3
H3CO
C
O
H
85.0
Apocynol
OH
OCH 3
HC
CH 3
OH
86.3
Syringol
OH
OCH 3
H 3 CO
83.3
Coniferol
OH
OCH 3
HC
CH
CH 2 OH
83.5
Syringyl alcohol
OH
OCH 3
H 3 CO
CH 2 OH
82.7
Creosol
OH
OCH 3
CH 3
84.8
Vanillin
OH
OCH 3
C
O
H
89.9
Guaiacol
OH
OCH 3
86.6
Vanillyl alcohol
OH
OCH 3
CH 2 OH
86.1
5 Lignin as a Natural Antioxidant: Property-Structure Relationship and Potential…