1. Agenda Lignin Structure - Linkages Lignin Reactions
Cleavage of b-O-4 linkages
Cleavage of a-O-4 Linkages
Non Cleavable Linkages
Condensation Reactions
Kinetics of Lignin Removal

2. The Goal of Lignin Reactions in Kraft Pulping
Soluble
Fragments
The goal in kraft pulping
is to take large insoluble
lignin and turn it into small
alkali soluble fragments.

3. 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

4. Lignin Nomenclature } Side Chain Phenylpropane Unit Common Names C9
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. }
Phenylpropane Unit C9
Common Names 5

5. Reactions of a-O-4 Linkage Phenolic and Etherified
In kraft pulping, a-O-4 linkages do not react with HS-.
Reaction with OH-
Etherified Units: a-O-4 linkages are stable
Phenolic Units: a-O-4 are very rapidly cleaved by alkali. This is the fastest of the lignin degradation reactions. Characteristic reaction in initial delignificaton

6. Kraft Reactions of b-O-4 Linkage
This ether linkage accounts for 50-60% of the linkages in lignin
Unlike most carbon-carbon bonds, this linkage can be cleaved under kraft pulping conditions.
Major variables which affect whether linkage can be cleaved:
Free phenolic versus etherified phenolic hydroxyl.
Presence of HS-.

7. Reactions of b-O-4 Linkages: Free Phenolic Hydroxyl/Benzyl Hydroxyl
Reaction with OH-
The ether linkage is not cleaved; a vinyl ether structures is formed.
Vinyl ether linkages are difficult to cleave.
Reaction with HS-
HS- is a very strong nucleophile which cleaves the b-O-4 linkage.
Reaction is very rapid even at lower temperatures.

8. Reactions of b-O-4 Linkages: Etherified Phenolic Hydroxyl/Benzyl hydroxyl
Reaction with OH-
The b-O-4 linkage is cleaved but only very slowly.
Cleavage of the b-O-4 linkage generates a free phenolic hydroxyl. Which can undergo rapid cleavage in the presence of HS-
Reaction with HS-
HS- will only react with groups containing a free phenolic hydroxyl so therefore there is not cleavage.

9. Reactions of b-O-4 Linkages: Alpha Carbonyl Group
These reactions occur whether there is a free or etherified phenolic hydroxyl group.
Reaction with OH-.
No reaction
Reaction with OH-/HS-
Rapid cleavage of linkage.

10. Non Cleavable Lignin Linkages/Condensation
b-1, b-5, b-b, 4-0-5, 5-5 linkages do not cleave!
There are reactions which will occur with these linkages which consume NaOH and Na2S:
Formation of stilbenes
Addition of HS-
Condensation Reactions
Under alkaline conditions, there are reactions that will build molecular weight through bond formation. Potential source of residual lignin.

11. Review of the Kraft Pulping Reactivities of Lignin Linkages
The yes and no answers in this table refer to whether the listed linkages will be
cleaved with the specific ions. It is obvious that in a kraft cook that both OH- and HS- are present. Sometimes, however, there won’t be any HS- present because of diffusion issues.
* No if a hydroxyl, yes if a carbonyl

12. Lignin Removal during Kraft Pulping

13. Lignin Removal: Effect of Effective Alkali
Lignin removal is very slow at first. The rate of lignin removal is independent of alkali concentration initially; it is diffusion limited, not chemical dependent.
As temperature increases, the rate increases and becomes alkali dependent.
Increase in EA charge results in faster pulping

14. Kraft Pulping: Reaction Phases of Lignin Removal
Delignification is divided into three phases: Initial, Bulk, Residual
Each phase has different pulping characteristics and kinetics that are governed by the lignin chemistry
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. In the chart above, these phases are evident from changes in the level of effective alkali.
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, there is little lignin lost. Lignin reactions during this phase are limited to cleavage of a-O-4 linkages and those b-O-4 linkages with free phenolic hydroxyl groups.
Bulk Phase: This is a very long period when the consumption of alkali is slow as is lignin losses. During this period, the rate of lignin degradation is dependent upon the OH- cleavage of b-O-4 linkages with etherified phenolic hydroxyls. This is a very slow reaction.
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 the point at which pulping is stopped.

15. Kraft Pulping: Reaction Phases of Lignin Removal
Initial Phase
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. In the chart above, these phases are evident from changes in the level of effective alkali.
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, there is little lignin lost. Lignin reactions during this phase are limited to cleavage of a-O-4 linkages and those b-O-4 linkages with free phenolic hydroxyl groups.
Bulk Phase: This is a very long period when the consumption of alkali is slow as is lignin losses. During this period, the rate of lignin degradation is dependent upon the OH- cleavage of b-O-4 linkages with etherified phenolic hydroxyls. This is a very slow reaction.
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 the point at which pulping is stopped.
Residual Phase