Pulping and Bleaching PSE 476/Chem E 471

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Pulping and BleachingPSE 476/Chem E 471

• Lecture 9
• Kraft Pulping Lignin Reactions

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Agenda

• Lignin Reactions
• Cleavage of b-O-4 linkages
• Effect of phenolic group
• Effect of nucleophile (OH-/HS-)
• b-O-4, a-carbonyl
• b-1 linkages
• b-5 linkages
• 4-O-5, 5-5, b-b linkages
• Condensation Reactions

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Kraft Reactions of b-O-4 Linkage (Free Phenolic
Hydroxyl)
Formaldehyde
Notice that the b-O-4 bond is not cleaved.
Vinyl Ether
Notes
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Reaction of Phenolic b-aryl Ether HS-
This reaction is very rapid resulting in the
cleavage of the b-O-4 linkage.
Notes
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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. This reaction requires elevated
  temperatures.
• Cleavage of the b-O-4 linkage generates a free
  phenolic hydroxyl.
• Reaction with HS-
• HS- will only react with groups containing a free
  phenolic hydroxyl so therefore there is not
  cleavage.

Reaction mechanisms on following pages
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Reaction of Nonphenolic b-aryl Ether OH-
Notes
In this reaction, the b-O-4 linkage is cleaved.
This is, however, a very slow reaction.
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Reaction of Nonphenolic b-aryl Ether HS-
No phenolic hydroxyl no reaction
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A Quick Review
Reaction of b-O-4 ether linked structure with
benzyl hydroxyl
OH- (alone) HS-(with OH-)
Phenolic Hydroxyl No cleavage Formation of vinyl ethers. Rapid cleavage
Non Phenolic Slow cleavage No cleavage
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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- (alone).
• No reaction
• Reaction with OH-/HS-
• Rapid cleavage of linkage.
• Mechanism on next page

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Alkaline PulpingActivation of g Carbon by
Carbonyl
A
A Keto-enol tautomerism
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Alkaline Pulping b-O-4 , a-carbonyl
HS-
OH-
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Alkaline Pulping Mechanismsß-1/free phenolic
(OH- alone)
CH2O
No fragmentation of dimer
Stilbene structures
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b-1 Reaction Mechanisms (OH- alone)
CH2O
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b-1 Reaction Mechanisms (OH- alone)
Formation of Stilbene
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Alkaline Pulping Mechanismsb-5 /free phenolic
(OH- alone)
Formation of Stilbenes
HO-
No fragmentation of dimer Mechanism similar to b-1
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Retroaldo Cleavage of Carbon-Carbon Bonds

• Groups containing double bonds (vinyl ethers,
  stilbenes, QM) will cleave very slowly under
  alkaline conditions through a retro aldo
  reaction. This reaction requires elevated
  temperatures.

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Alkaline Pulping Mechanismsb-1 /free phenolic
(OH- /HS-)
No fragmentation of dimer
2HS-
OH-
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Alkaline Pulping Mechanismsb-5 /free phenolic
(OH- /HS-)
Same reactions as for The b-1 linkage
HS-
OH-
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Alkaline Pulping Mechanismsb-1/etherified
phenolic OH (HS-/OH-)
No Reaction
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Alkaline Pulping Mechanismsb-5/etherified
phenolic OH (HS-/OH-)
OH-
No Reaction
HS-
OH-
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Alkaline Pulping Mechanisms4-O-5 (HS-/OH-)
No Reaction
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Alkaline Pulping Mechanisms5-5 (HS-/OH-)
No Reaction
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Alkaline Pulping Mechanisms Methyl Aryl Ether
Cleavage
Notes
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Alkaline PulpingLignin Carbohydrate Bonds

• Esters and glycosidic linkages easily cleaved.
• Ether linkages.
• Free phenolic hydroxyl easily cleaved.
• Etherified phenolic hydroxyl no reaction.
• Residual lignin problem.

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Lignin Condensation

• We have been discussing the reactions of quinone
  methides with OH- and HS-. In solution, these
  structures along with resonance structures of the
  phenoxy anion can react with lignin structures.
  This type of reaction is known as condensation
  and is a problem because it increases the
  molecular weight of lignin and forms a difficult
  to cleave bond.

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Condensation Reactions (C5)

• As shown below, a quinone methide can react with
  a C5 anion to form a new carbon-carbon bond.
  Remember, this new bond is not cleaved during
  pulping. The result of this reaction is higher
  molecular weight lignin (this is bad).

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Condensation Reactions (C1)

• C1 is another site for condensation reactions
  forming an a-1 linkage. In was seen with the
  b-1 free radical coupling reaction, this requires
  side chain elimination.

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Condensation with Formaldehyde

• Formaldehyde, which is formed from g carbons
  during the formation of vinyl ethers and
  stilbenes, will cross link 2 molecules. The
  linkage formed is known as a diphenyl methane
  linkage (same linkage in a-1 a-5).

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Reactivities of Lignin Linkages
No if a hydroxyl, yes if a carbonyl
Short Note