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Hoh2c CHO
|
|
HCOOH
CH2
CH2
CH3
CO2H
O
Condensation products
H O 2
O
CHO
OR
R
OMe
H
+
OR
R
O OMe
OH
OR
OMe
OR
R
OMe
OR
R
MeO
O
H+
-3H2O
H+
-3H2O
+
+ lignin fragments
Pentoses
Hexoses
+
+
47 72
71 73 74
Scheme 4.49 Degradation of pentoses and hexoses under
acidic conditions and condensation of degradation products
with lignin units.
R
OH
OH O
HO
HO
CH3
OH
OH
O
O
O
HO
HO
OH
OH
COOH
OH
OH
O
OH
R = H, OH, CH3,
CO2H, or COCH3
R = CH3 or CHO
48 49 50 51
52 53 54
Scheme 4.50 Aromatic compounds formed from dehydration
of sugars under acidic conditions [38,39].
420 4 Chemical Pulping Processes
Reaction of Hexenuronic Acid under Acidic Conditions
Hexenuronic acids are only formed under alkaline pulping conditions (cf. reaction
in kraft pulping). However, in two-stage sulfite processes with a first stage at a
higher pH, their reactions in the subsequent acidic stage must be considered.
According to Teleman et al. [40], hexenuronic acids are degraded under acidic conditions
to 5-formyl furancarbonic acid (61) according to Scheme 4.51.
O OXyl
OH
O
HO
OH
-XylOH
O
OH
O
O
OH
H
OH2
+
O
OH
O O
HO
O
O
O
HO OH
O
H
H
H
O
O
HO O
H
O
H
H
O
O
HO
O
O
HO O
H2-HCO O 2H
-H2O
-H2O
-H2O
55 56
60 59
+
+
Scheme 4.51 Formation of 5-formylfurancarboxylic acid from hexenuronic acid [40].
Дата публикования: 2015-01-23; Прочитано: 379 | Нарушение авторского права страницы | Мы поможем в написании вашей работы!
