1. Pulping and Bleaching PSE 476
Lecture #8
Kraft Pulping: Early Reactions and
Kraft Pulping Lignin Reactions

2. Agenda Basic Chemical Pulping Discussion
Loss of Components During Kraft Pulping
Reactions in the Early Portion of the Cook
Saponification
Neutralization of Extractives
Initial Lignin Discussion
Kraft Pulping Lignin Reactions

3. Wood Chemistry
For the students who do not recognize this molecule (did not take PSE 406), there is a short appendix at the end of this lecture to help you. Additionally, the class notes are available for review.

4. Pulping
The goal of kraft pulping is to remove the majority of lignin from chips (or other biomass) while minimizing carbohydrate loss and degradation.
Removal of lignin is accomplished through treatment of raw material with NaOH and Na2S at elevated temperatures.

5. The Goal of Lignin Reactions in Kraft Pulping
During kraft pulping, the
large insoluble lignin
molecules are converted
into small alkali soluble
fragments.
Kraft Pulping
Soluble
Fragments
SH- delignification in the high pH envir, carbohydrates OH- peeling reaction (decomposition)
Carbohydrates are also
degraded during pulping

6. Yield of Wood Components After Kraft Pulping
Notes
From this table, you can see that the process of kraft pulping removes the majority of lignin from the pine and the birch chips. There is only small losses in cellulose from both. The majority of glucomannans are lost during pulping. This significantly reduces pulp yield. Xylans show much more stability. The other carbohydrates, pectins, other hemicelluloses, and sugars are all removed from the fibers. The majority of the extractives are removed from the fibers but are also somewhat modified. The kraft pulping process does the job of removing the majority of the lignin. All the lignin is not removed because at a certain point, the rate of carbohydrate degradation is much faster than lignin removal. Pulping is stopped at this point and the fibers are bleached.
* Yields = % of wood (pulp) components

7. Initial Reactions: Low Temperature
Carbohydrates
Alkaline hydrolysis of acetyl groups on xylan (see next slide).
Removal of certain soluble carbohydrates.
Certain galactoglucomannans.
Arabinogalactans.
Extractives
Alkaline hydrolysis of fats (saponification), waxes, and other esters.
Neutralization of extractives.
There are a number of acidic extractives which consume NaOH.

8. Alkaline Hydrolysis: Example Using Acetyl Groups
Esters are cleaved in alkaline solutions through hydrolysis reactions forming carboxylic acids and alcohols.
Hydrolysis of acetyl groups occurs readily in alkaline solutions.
Reaction occurs rapidly even at room temperature.
Reaction consumes alkali.
Hardwoods higher concentration of acetic acid

9. Saponification of Fats (Review slide from PSE 406)
Treatment of fats with alkali converts them to fatty acids and glycerol through saponification.
Once again this reaction
consumes part of the alkali
charge.

10. Acidic Extractive Species
Resin Acids
Lignans
Monoterpenoids

11. Consumption of Alkali Impregnation zone
Effective alkali charge is 4-5 moles NaOH or 16-20% of wood
Impregnation 45 min C
Impregnation
zone

12. Where Does All the Alkali Go?
Spruce wood was soda pulped at a NaOH concentration of 19% (as Na2O).
12.5% (or 66% of alkali) consumed to lower lignin content of wood to 2.8%.
2.3-3% used in dissolution of lignin.
1.3% for hydrolysis of acetyl and formyl groups.
% for neutralization of acidic products
Some extractives
Mostly carbohydrate degradation products (discussed later).

13. Lignin Removal during Kraft Pulping
This chart shows the lignin removal rate during a kraft cook. It is important to note that the rate of lignin removal is temperature dependent. What does this fact tell us about of lignin removal in this slide?

14. Lignin Removal
In the last slide, the rate of lignin removal appears to be linear over a large portion of the cook; even as the temperature increases.
This means that lignin removal in the first portion of the cook is easier than as the cook proceeds.
Lignin removal has been broken into three sections:
Initial Phase (fast lignin removal reactions)
Bulk Phase (slow lignin removal reactions)
Residual Phase (really slow lignin removal)

15. Kraft Pulping: Reaction Phases of Lignin Removal
Initial Phase
Impregnation zone
170° C
Bulk Phase
The reactions of lignin in a kraft cook can be broken down into three distinct phases: the initial phase, the bulk phase and the residual phase.. This is a somewhat confusing chart but it does show distinctly where the phases are. In this experiment, pulp and liquor samples were taken at different times during a kraft cook. Lignin content of the pulp is plotted against the amount of alkali left in the pulping liquor. Temperature readings are added to plot.
Initial Phase:In this phase, the effective alkali drops rapidly even at very low temperature. Much of this is due to things we have discussed previously such as saponification reactions, neutralization reactions, etc. During this phase, roughly 20% of the lignin is lost. This is very reactive lignin.
Bulk Phase: This is a very long period when the consumption of alkali is slow as is lignin losses. However, the majority of lignin is lost during this time.
Residual Phase: At this point, it is getting very difficult to remove additional lignin. The carbohydrates are being degraded at a fast rate. This is why the alkali is being consumed at a rapid rate. At this point at the pulping is stopped.
Residual Phase
Notes

16. Kraft Pulping Lignin Reactions

17. Dissolution of Lignin
In review the goal in kraft pulping is the cleavage of lignin into alkali soluble fragments.
Cleavage is affected by the following factors:
Type of linkage
Presence of free phenolic hydroxyl group
Functional groups (benzyl hydroxyl, carbonyl)
Type and amount of nucleophiles (OH-, HS-)
Reaction temperature
We are going to first look at the chemical mechanisms of the reactions and then the kinetics.

18. Sites for Nucleophilic Attack
The cooking chemicals used in kraft cooking (NaOH and Na2S: OH- and HS-) both act as nucleophiles* because of their free pair of electrons.
Sites for nucelophilic attack in lignin are those areas of reduced electron density (partially positive sites).
Nucleophile: in chemistry, an atom or molecule that in chemical reaction seeks a positive center, such as the nucleus of an atom, because the nucleophile contains an electron pair available for bonding. Examples of nucleophiles are the halogen anions (I-, Cl-, Br-), the hydroxide ion (OH-), the cyanide ion...
* Notes

19. Formation of Quinone Methide
Nucleophillic
attack
site!
Quinone Methide
(very reactive)
These arrows indicate that a pair
of electrons are moving

20. Formation of Nucleophilic Attack Sites
A free phenolic hydroxyl group is needed for the formation of a quinone methide.
The oxygen of the quinone group (carbonyl) attracts the electron density on the double bond thus making the carbon more positive. This in turn shifts the electron densities of the other bonds on this conjugated system.

21. Two Additional Examples of Nucleophilic Addition Sites
Notes
The example on the right is an example of keto-enol tautomerism. This concept is discussed in the next lecture. For those of you who can’t wait, here is a quick preview of this information.
This structure contains an a-keto
group. Notice that a free phenolic
hydroxyl groups is not needed!
Coniferaldehyde type structures

22. Important Issues!!!!
When learning about alkaline pulping mechanisms, remember to ask yourselves these questions!
Which reactant are we concerned with: OH- or HS-?
Does the lignin structure have a free phenolic hydroxyl group or is it etherified?
Which linkage are you hoping to cleave?
Is there an a-carbonyl or benzyl hydroxyl?

23. Reactions of α-O-4 Linkage Phenolic and Etherified
In kraft pulping, α-O-4 linkages do not react with HS-
Reaction with OH-
Phenolic Units: α -O-4 are very rapidly cleaved by alkali. This is the fastest of the lignin degradation reactions. (Will occur at low temperatures)
Etherified Units: α -O-4 linkages are stable (no reaction).
Please work out reaction mechanism.

24. Reactions of b-O-4 Linkages: Free Phenolic Hydroxyl/Benzyl Hydroxyl
Reaction with OH- alone
The ether linkage is not cleaved; a vinyl ether structures is formed.
Vinyl ether linkages are difficult to cleave.
Reaction with HS- (OH- present)
HS- is a very strong nucleophile which cleaves the β-O-4 linkage.
Reaction is very rapid even at lower temperatures.
* Mechanisms on following pages

25. Kraft Reactions of b-O-4 Linkage (Free Phenolic Hydroxyl)
Formaldehyde
Under alkaline conditions, groups containing free phenolic hydroxyls and an ether linkage on the a carbon readily form quinone methides. Hydroxyls will readily add to the quinine methide forming a benzyl hydroxyl. Therefore, an equilibrium is established when prohibits the cleavage of the b-O-4 linkage.
Notice that the
b-O-4 bond is
not cleaved.
Vinyl Ether
Notes

26. Appendix
Basic Wood Chemistry

27. What is the Chemical Makeup of Wood?
* Data for Cellulose, Hemicellulose & Lignin on extractive free wood basis

28. Cellulose
Very long straight chain polymer of glucose (a sugar): approximately 10,000 in a row in wood. Cotton is nearly pure cellulose.
Think about a very long string of beads with each bead being a glucose molecule.
Cellulose molecules link up in bundles and bundles of bundles and bundles of bundles of bundles to make fibers.
Uncolored polymer.

29. Hemicelluloses
Branched little uncolored sugar polymers (~ 50 to 300 sugar units)
Composition varies between wood species.
5 carbon sugars: xylose, arabinose.
6 carbon sugars: mannose, galactose, glucose.
Uronic Acids: galacturonic acid, glucuronic acid.
Acetyl and methoxyl groups (acetic acid & methanol).
Major hemicelluloses:
Xylans - big in hardwoods
Glucomannans: big in softwoods
Minor hemicelluloses: pectins, others.

30. Xylan Structure
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 

31. Glucomannan Structure
14--D-Glc-14--D-Man-14--D-Man-14--D-Man-1
2,3 
Acetyl 6
-D-Gal 1
There are different structured glucomannans in hardwoods and softwoods (and within softwoods)
Glucomannans are mostly straight chained polymers with a slight amount of branching. The higher the branching, the higher the water solubility.

32. Lignin Phenolic polymer - the glue that holds the fibers together.
Lignin is a very complex polymer which is connected through a variety of different types of linkages.
Colored material.

33. Lignin Nomenclature } Side Chain Notes Phenylpropane Unit Common Names
The lignin precursor shown in this figure is referred to by lignin chemists as a phenylpropane unit or a C9 unit. This is obviously because: a. it is a combination of a phenol ring and a propane (C3) side chain and b. because there are 9 carbons in the structure ignoring the methoxyl group. It is important to note the numbering of the ring and the naming of the side chain. This is something you need to know. The carbon on the side chain nearest the ring is the alpha carbon. Nest up the line is the beta carbon and finally the gamma carbon. Some lignin chemists have recently taken to numbering the entire structure so that the three side chain carbons are numbered C7, C8, and C9 instead of alpha, beta, and gamma.
Notes }
Phenylpropane Unit C9
Common Names 5

34. Lignin Reactions: Linkage Frequencies
In order to understand the reactions which break up lignin into soluble fractions, it is important to first understand lignin structure. We do not have time in this class to go over this in detail so here are the basics. When it comes to alkaline cleavage reactions, the most important things to know about lignin is:
The basic building block of lignin (the monomer) is a phenolic ring with a three carbon side chain.
The lignin molecule is linked through a variety of linkages as shown in the above figure. The most prominent linkage is an ether linkage which connects the phenolic position of one ring with the b-carbon (2nd carbon of the side chain) of the next monomer. This linkage (b-O-4) makes up approximately ½ of the linkages.
There are a large number of the units connected through carbon-carbon bonds which are difficult to cleave.
There are a certain number of functional groups which are important including:
Phenolic hydroxyl: This is the hydroxyl group on the aromatic ring (para to the 3 carbon side chain).
Benzyl hydroxyl: Hydroxyl group on the carbon attached to the ring.
Carbonyl groups:
Notes

35. Extractives In some tropical species this can be as high as 25%.
The term extractives refers to a group of unique chemical compounds which can be removed from plant materials through extraction with various solvents.
Typically these chemicals constitute only a small portion of the tree (<5%).
In some tropical species this can be as high as 25%.
Extractives are produced by plants for a variety of uses.
The most common use by plants is protection.
Extractives can cause serious problems for processing.
Pitch is a term which is often used when describing some groups of extractives.
Extractives are responsible for the characteristic color and odor of wood.