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Acid-catalyzed dehydration of carbohydrate monomers eventually leads to the formation
of hydroxymethylfurfural (47) from hexoses as the starting material
(Scheme 4.48). In a similar manner, the removal of 3 mol water from pentoses
results in the formation of furfural, which can be distilled off the pulping liquor.
The amount of furfural produced from degraded hemicelluloses during sulfite
pulping is sufficiently large to sustain commercial usage.
OH
HC OH
CH2OH
H O
HC
CH
HC
OH
HO
HC OH
CH2OH
O
HO CH
HC OH
CH2OH
HC OH
CH2OH
CHOH
C
CH
HC
HO
OH
OH OH
HC OH
CH2OH
H O
C
CH
HC
OH
-H2O
HC OH
CH2OH
H O
C
CH
CH
O O
HOH2C CHO
-H2O
O
OH
CH2OH
OH
CHOH
O
OH
CH2OH
CHO
-H2O
-H2O -H2O
-H2O
-H2O
O
OH
CH2OH
OH
CH2OH
OH
O
OH
CH2OH
OH
H
CHO
38 39 40 41
42 43 44 45
Scheme 4.48 Formation of hydroxymethylfurfural (HMF)
from glucose by acid-catalyzed dehydration.
Hydroxymethylfurfural (47) can be further converted to levulinic acid (c-oxo
pentanoic acid, 72) with 5-hydroxymethylfurfural as an intermediate (cf.
Scheme 4.49).
Furfural and its derivatives are highly prone to condensation reactions, either
with other furfural molecules or with lignin fragments present in the liquor
(Scheme 4.49). An example of such a Friedel-Kraft acylation-type condensation is
also shown in Scheme 4.49.
The formation of other aromatic compounds under acidic conditions has been
studied extensively by Theander et al. [36–38]. Scheme 4.50 shows aromatic compounds
resulting from the dehydration, degradation, and re-condensation of carbohydrates
under acidic solutions. The first step comprises the formation of anhydro-
sugars with intramolecular glycosidic linkages, resulting in elimination of
another water molecule from two hydroxyl groups (levoglucosan). The glycosidic
linkages are hydrolyzed, and further processes lead to aromatic and condensed
units.
4.3 Sulfite Chemical Pulping 419
O
HO
HO
OH
OH
O
CHO
O
OH
HO
HO
OH
OH
O
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
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