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Pulping and Bleaching
PSE 476/Chem E 471
Lecture #10
Kraft Pulping
Carbohydrate Reactions
PSE 476: Lecture 10
1
Agenda
• Carbohydrate Reaction Mechanisms
» Glycosidic Cleavage
» Peeling
» Stopping
•
•
•
•
Glucomannan Reactions
Xylan Reactions
Other Hemicellulose Reactions
Cellulose Reactions
PSE 476: Lecture 10
2
Carbohydrate Reactions
• Carbohydrates react slower than lignin under
alkaline conditions. Overall, however, just as
much carbohydrates react as does lignin.
• The main alkaline reactions of carbohydrates
are:
» Glycosidic cleavage.
» Peeling.
» Stopping.
PSE 476: Lecture 10
3
Alkaline Pulping : Carbohydrates
General Aspects
• There are considerable carbohydrate losses during
kraft pulping due to alkaline degradation reactions.
» Acetyl groups are very quickly cleaved.
» Carbohydrates undergo “peeling”
- Peeling is the process in which sugars are removed one by
one from the reducing end of the polymer.
- Hemicelluloses highly degraded through “peeling”
- Monosaccharide fragments from peeling are highly degraded
to acidic compounds.
- This reaction is stopped by “stopping” reactions.
• Glycosidic linkages in carbohydrates are cleaved
through hydrolysis reducing overall molecular
weight and creating new reducing ends.
PSE 476: Lecture 10
4
“Peeling Reaction”
Formation of new reducing end group
CHO
CH2OH
HCOH
C O
HOCH
HCOR
HCOH
+
-H
HOCH
(A)
CH2OH
HCOR
HCOH
CH2OH
C OHOC
(B)
CH2OH
HC OR
HCOH
CH2OH
-
- RO
(C)
CH2OH
CH2OH
C O
C O
HOC
CH
HCOH
CH2OH
(D)
C O
+ H2O
CH2
(E)
HCOH
C(OH)CH2OH
CH2
HCOH
CH2OH
CH2OH
glucoisosaccharinic acid
A. Isomerization
B enediol formation
C. b-alkoxy elimination
CH2OH
CHO
COOH
C O
C O
CHOH
CH2OH
CH3
CH3
D. Tautomerization
Lactic Acid
E. Benzilic acid rearrangement
Notes
CO2H
CHO
CHO
HCOH
COH
CH2OH
PSE 476: Lecture 10
CH2
5
Benzilic acid rearrangement
Bond rotation
Nuclephilic addition
α-hydroxy-carboxylic acid
Proton transfer
PSE 476: Lecture 10
6
“Stopping Reaction”
CHO
(A)
HCOH
HOCH
(B)
(-)
HC O
COH
HO CH
CHO
-
-HO
(C)
CHO
COH
C O
CH
CH2
(D)
CO2H
HCOH
+ H2O
CH2
HCOR
HCOR
HCOR
HCOR
HCOR
HCOH
HCOH
HCOH
HCOH
HCOH
CH2OH
CH2OH
CH2OH
CH2OH
CH2OH
glucometasaccharinic
acid
(will not “peel”)
A. 1,2 Enediol formation
B. b-hydroxy elimination
C. Tautomerization
D. Benzilic acid rearrangement
Notes
PSE 476: Lecture 10
7
Cleavage of Glycosidic Bonds
(B)
(A)
(C)
(-)
CH2OH
CH2OH O
HO
HO
OMe
+
-H
CH2
CH2 O
-
OH
- MeO
O
OH
OH
O
O
O
OMe
HO
OH
O
(-)
OH
O
OH
OH
A. Inversion of ring confirmation, C2 OH ionized
B. Ionized hydroxyl groups attacks C1 eliminating methoxyl group
forming 3 membered epoxide (oxirane)
C. Opening of epoxide forms new reducing end or if steric conditions
are correct a 1,6 anhydride. This compound is opened by alkali.
This reaction requires elevated temperatures
Notes
PSE 476: Lecture 10
8
Cleavage of Glycosidic Bonds
Methyl pyranosides
of
D-glucose
D-mannose
D-galactose
D-xylose
L-arabinose
D-glucuronate
Relative Rate
- Anomer b-Anomer
1
2.5
2.8
1.1
1
5.7
1.2
5.8
10
1
280
-
Notes
PSE 476: Lecture 10
9
Alkaline Reactions of
Glucomanans/Cellulose
• Glucomannans:
» Very unstable to peeling reactions
» Galactose side chains fairly resistant
• Cellulose
» Large Dp means that glycosidic cleavage more
important than peeling
» Cellulose loss small but viscosity (Mw) significantly
reduced
PSE 476: Lecture 10
10
100
200
80
150
60
Glucomannan
Temperature
40
100
50
20
0
Temperature (C)
Glucomannan Yield (%)
Loss of Glucomannans During
Kraft Pulping
0
0
50
100
150
200
250
Time (minutes)
Notes
PSE 476: Lecture 10
11
100
200
80
150
60
100
40
Xylan
Temperature
20
50
0
0
Notes
50
100
150
200
Temperature (C)
Xylan Yield (%)
Loss of Xylans During Kraft
Pulping
0
250
Time (minutes)
PSE 476: Lecture 10
12
Xylan peeling Reactions
• Xylans much more resistant to peeling than
glucomannans.
• Large % of xylans are dissolved during
pulping instead of peeled. Much of these
xylans precipitate on the fiber surface at the
end of the cook as alkali is consumed.
• There appears to be two different
temperature dependent mechanisms
responsible for the protection from peeling.
PSE 476: Lecture 10
13
Resistance of Xylan to Peeling Below 100°C
• Low temperature stability (below 100°C) is due to the
resistance of galacturonic acid to peeling at this
temperature.
» Galacturonic acid groups are found in unique end group.
(see below)
» Above 100°C, galacturonic acid groups undergo peeling.
OH
HO CH
3
O
O
OH
O
O
O
OH
OH
OH
O
O
OH
HO
OH
O
O
OH
OH
O
COOH
-b-Xly-14--b-D-Xly3--L-Rha-12--D-GalA-14-D-Xly
PSE 476: Lecture 10
14
Resistance of Xylan to Peeling above 100°C
• A theory has been presented that above 100°C
glucuronic acid side chains on xylans slow the
peeling reaction.
» Above 120°C, glucuronic acids are somewhat converted
to hexenuronic acids which are much more stable to
peeling. Hexenuronic acid formation is discussed in the
next slide.
4-b-D-Xly-14-b-D-Xly-14-b-D-Xly-14-b-D-Xly4-b-D-Xly
4-O-Me--D-Glc
-L-Araf
PSE 476: Lecture 10
15
Formation of Hexenuronic Acids
• Hexenuronic acids are formed from uronic acids under alkaline
conditions.
» Method for identifying these compounds in pulps just
developed.
» Interfere with Kappa (lignin concentration) determination.
» Attract metals (color problem/cause problems in bleaching)
CO2H
CO2H
O
CH3O
OH
OH
O
-
OH
OH
OH
OH
Uronic Acid
OH
Hexenuronic Acid
PSE 476: Lecture 10
16
Loss of Other Hemicelluloses
During Kraft Pulping
• The minor hemicelluloses such as pectins,
starches, etc. are supposedly completely
destroyed during kraft pulping.
• This happens through dissolution and
peeling.
• Many of these carbohydrates are water
soluble so removal is easy. Once they are in
the hot alkali solution they are easily
destroyed.
PSE 476: Lecture 10
17
Cellulose Reactions During
Kraft Pulping
• Cellulose undergoes peeling and glycosidic
cleavage reactions during kraft pulping.
» Because cellulose molecules are so long, peeling
reactions only cause small yield losses.
» Glycosidic cleavage is more of a problem because of
molecular weight losses that cause strength
problems. This reaction also increases the rate of
peeling through generation of new reducing end
groups.
• Because cellulose molecules are so large
dissolution is not an issue.
PSE 476: Lecture 10
18