1. Pulping and Bleaching PSE 476/Chem E 471
Lecture #10
Kraft Pulping
Carbohydrate Reactions
PSE 476: Lecture 10

2. Agenda Carbohydrate Reaction Mechanisms Glucomannan Reactions
Glycosidic Cleavage
Peeling
Stopping
Glucomannan Reactions
Xylan Reactions
Other Hemicellulose Reactions
Cellulose Reactions
PSE 476: Lecture 10

3. 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.
During kraft cooking most of the hemicelluloses are dissolved. Among them, the galactoglucomannans are much more affected than the xylans. Dissolution of the hemicelluloses results from the combination of peeling and alkaline hydrolysis reactions. The peeling process starts already at low temperature (< 100°C) (primary peeling), whereas alkaline hydrolysis is significant at higher temperatures (> 140°C). After hydrolysis peeling is reactivated (secondary peeling). In principle, cellulose undergoes the same series of reactions. However due to the size of its molecular chains, yield loss of cellulose remains low.
Close examination of yield loss profile versus delignification in kraft cooking shows that most of the hemicelluloses are already removed before the start of bulk delignification i.e. during the heating-up period. Another severe carbohydrate dissolution, relative to delignification, is observed at the end of the cook, during the so-called final delignification. Consequently actions against yield loss must be advantageously taken either at the start or toward the end of the cook.
The extent of peeling at the start of the cook can be minimized by the oxidation or the reduction of the aldehyde (hemi-acetal) end-groups of carbohydrates. Polysulfide and anthraquinone can oxidize these end-groups, which are transformed into stable carboxyl end-groups. Yield can then be increased by up to 3 % (on wood) depending on the charge of the additives.
Polysulfide is much less efficient than anthraquinone, on a molecular basis, because it is decomposed at high temperature (T> 130°C) and is therefore ineffective against secondary peeling. Antraquinone is not only stable but also continuously maintained in its oxidized form due to reaction of its reduced form with lignin and finally behaves as a catalyst.
Reduction of the aldehyde end-groups is feasible but not economically viable. Very few reagents can be used for this purpose. Sodium borohydride is one candidate. Unfortunately, yield saving does not cover the extra chemical cost.
Blocking the peeling reaction remains a challenge and is still the subject of numerous investigations.
Another approach is to stop the cook earlier, so as to avoid the poorly selective final delignification. Kappa number of around 50 and 30 for softwood and hardwood kraft pulps respectively appear to be the appropriate levels to start the bleaching phase, provided that this latter phase is performed with chlorine-free chemicals with good selectivity. Potential yield saving is 2 to 3 % on wood.
Finally it seems that some of the latest technologies developed for kraft cooking such as ITC offer some benefits in terms of yield, likely because lower temperatures and alkali charges are used, which minimizes the secondary peeling.
Whatever the means used for yield increase it must be reminded that pulp strength will be adversely affected.
PSE 476: Lecture 10

4. 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.
The reaction is a representative of 1,2-rearrangements. These rearrangements usually have migrating carbocations but this reaction is unusual because it involves a migrating carbanion. The long established reaction mechanism updated with in silico data [2] is outlined in scheme 2.
A hydroxide anion attacks one of the ketone groups in 1 in a nucleophilic addition to the hydroxyl anion 2. The next step requires a bond rotation to conformer 3 which places the migrating group R in position for attack on the second carbonyl group in a concerted step with reversion of the hydroxyl group back to the carbonyl group. This sequence resembles a nucleophilic acyl substitution. Calculations show that when R is methyl the charge build-up on this group in the transition state can be as high as 0.22 and that the methyl group is positioned between the central carbon carbon at a separation of 209 pm.
PSE 476: Lecture 10

5. “Peeling Reaction” (B) (C) (D) (E) (A) A. Isomerization
Formation of new reducing end group
(B)
(C)
(D)
(E)
(A)
A. Isomerization
B enediol formation
C. b-alkoxy elimination
D. Tautomerization
E. Benzilic acid rearrangement
This is by far one of the most important reaction pathways you will learn (and I mean you need to learn this pathway). Peeling is the process in which sugars are peeled of carbohydrates from the reducing end. This can cause yield losses during pulping. The first step in the process is the alkaline induced isomerization of the aldose to a ketose. The second step is formation of an enediol through keto-enol isomerization. This enediol is in equilibrium with the ketone but can also go through b-alkoxy elimination (follow the arrows) thus cleaving the glycosidic linkage. Through another keto enol tautomerization a ketone is formed. The final step in this pathway is the benzylic acid rearrangement forming an isosaccharinic acid. This shows that as the monosaccharides are released they are converted to acids.
Notes
PSE 476: Lecture 10

6. Benzilic acid rearrangement
Bond rotation
Nuclephilic addition
A hydroxide anion attacks one of the ketone groups in 1 in a nucleophilic addition to the hydroxyl anion 2. The next step requires a bond rotation to conformer 3 which places the migrating group R in position for attack on the second carbonyl group in a concerted step with reversion of the hydroxyl group back to the carbonyl group. This sequence resembles a nucleophilic acyl substitution. Calculations show that when R is methyl the charge build-up on this group in the transition state can be as high as 0.22 and that the methyl group is positioned between the central carbon carbon at a separation of 209 pm
The carboxylic acid in intermediate 4 is less basic than the hydroxyl anion and therefore proton transfer takes place to intermediate 5 which can be protonated in acidic workup to the final alpha-hydroxy-carboxylic acid 6. Calculations show that an accurate description of the reaction sequence is possible with the participation of 4 water molecules taking responsibility for the stabilization of charge buildup. They also provide a shuttle for the efficient transfer of one proton in the formation of intermediate 5
α-hydroxy-carboxylic acid
Proton transfer
PSE 476: Lecture 10

7. “Stopping Reaction” (A) (B) (C) (D) (will not “peel”) Notes
A. 1,2 Enediol formation
B. b-hydroxy elimination
C. Tautomerization
D. Benzilic acid rearrangement
The stopping reaction does what its name implies; it stops the peeling reaction. The end group is converted to a metasaccharinic acid. This group does not undergo the peeling reaction. It is also not cleaved from the carbohydrate polymer.
Notes
PSE 476: Lecture 10

8. Cleavage of Glycosidic Bonds
A. Inversion of ring confirmation, C2 OH ionized
B. Ionized hydroxyl groups attacks C1 eliminating methoxyl group forming 3 membered epoxide (oxirane)
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
The above figure represents a segment of a carbohydrate such as cellulose or hemicelluloses. The glycosidic bond in this model connects the sugar to a methyl group instead of another sugar. This glycosidic linkage could be anywhere in the carbohydrate polymer. The first step in this process is the ionization of the hydroxyl group on C2 followed by inversion of the ring confirmation. This allows a nucleophilic attack of the C1 group by the ionized hydroxyl followed by elimination of the methoxyl group (in this case; in normal carbohydrates this results in the elimination of a portion of the polymer). When the epoxide opens, a new reducing end is produced on the polymer. Therefore, if you started with a single carbohydrate molecule with a reducing end, after alkaline hydrolysis you would have 2 molecules each with reducing ends. In the figure above, the epoxide opens into a 1,6 anhydride structure which opens into a hemiacetal in base. Alkaline hydrolysis is a slow reaction and only proceeds at elevated temperatures.
Notes
PSE 476: Lecture 10

9. Cleavage of Glycosidic Bonds
What is shown in this table is that glycosidic bonds linking different sugars have different susceptibilities to alkaline hydrolysis. By far the most labile of this group is a methoxyl group attached to a glucuronic acid. It is also apparent that the confirmation of the anomer is also of great importance as the β-anomers are significantly more susceptible to alkaline hydrolysis.
Notes
PSE 476: Lecture 10

10. 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
Enodiol
An aldehyde –endiol a ketone
Transfer of acidic hydrogen from the alpha carbon to carbonyl oxygen, this is not a resonance structure, because of the transfer of H. Resonance structure will have different position of e.
PSE 476: Lecture 10

11. Loss of Glucomannans During Kraft Pulping
As can be seen in this figure, almost 70% of glucomannans are lost during the kraft pulping of softwoods. This happens primarily through peeling reactions and dissolution. The majority of glucomannans are lost during the first seventy minutes of the cook. By time the reaction reaches temperature, the loss of glucomannans is nearly complete. It is not completely known why the remaining 30% of the glucomannans are not lost. It is assumed that it is because of physical restrains due to the bulky cellulose and perhaps because of linkages to residual lignin structures.
In a very interesting experiment, wood was reduced with sodium borohydride. The reducing end of the glucomannans was reduced to an alcohol group which would not peel (or would peel very slowly). When this material was subjected to kraft pulping, it was found that the large early loss of glucomannans was not due to peeling but rather to the NaOH dissolving part of the glucomannans (I.e. the rate of glucomannan loss was not changed). As the cook progressed, the rate of glucomannan loss significantly slowed (because the end groups had been reduced) showing that as the temperature increases in a kraft cook, glucommans are lost to peeling reactions.
Notes
PSE 476: Lecture 10

12. Loss of Xylans During Kraft Pulping
You can see in this figure that the loss of xylans is both much less than glucomannans and also much slower. In the initial portion of the cook, xylans are lost very slowly through dissolution. As the reaction proceeds, the xylans are solubilized to a greater extent. As the cook nears completion, the amount of alkali in the reactor is much lower. At this point, the xylans begin to precipitate out of solution and to some extent back onto the surface of the fibers.
Sodium borohydride reduction studies have shown that the amount of xylans lost to peeling reactions is minimal. This is because xylans posses a unique end group which protects the xylans. Peeling does occur, however, if the xylan undergoes glycosidic cleavage first forming a new reactive reducing end. It is important to note that glycosidic cleavage is a high temperature reaction and therefore doesn’t significantly occur until almost maximum cooking temperature is reached.
Notes
PSE 476: Lecture 10

13. 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

14. 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.
--Xly-14---D-Xly3--L-Rha-12--D-GalA-14-D-Xly
PSE 476: Lecture 10

15. 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--D-Xly-14--D-Xly-14--D-Xly-14--D-Xly4--D-Xly 
4-O-Me--D-Glc    
-L-Araf 
PSE 476: Lecture 10

16. 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)
Beta elimination of methanol
PSE 476: Lecture 10

17. 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

18. 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