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Pulping and Bleaching PSE 476/Chem E 471

Pulping and Bleaching PSE 476/Chem E 471. Lecture #9 Kraft Pulping Lignin Reactions. 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

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Pulping and Bleaching PSE 476/Chem E 471

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  1. Pulping and BleachingPSE 476/Chem E 471 Lecture #9 Kraft Pulping Lignin Reactions

  2. 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

  3. Kraft Reactions of b-O-4 Linkage (Free Phenolic Hydroxyl) Formaldehyde Notice that the b-O-4 bond is not cleaved. Vinyl Ether Notes

  4. Reaction of Phenolic b-aryl Ether: HS- This reaction is very rapid resulting in the cleavage of the b-O-4 linkage. Notes

  5. 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

  6. Reaction of Nonphenolicb-aryl Ether: OH- Notes In this reaction, the b-O-4 linkage is cleaved. This is, however, a very slow reaction.

  7. Reaction of Nonphenolicb-aryl Ether: HS- No phenolic hydroxyl = no reaction

  8. A Quick Review Reaction of b-O-4 ether linked structure with benzyl hydroxyl

  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- (alone). • No reaction • Reaction with OH-/HS- • Rapid cleavage of linkage. • Mechanism on next page

  10. Alkaline PulpingActivation of g Carbon by Carbonyl A A: Keto-enol tautomerism

  11. Alkaline Pulpingb-O-4 , a-carbonyl HS- OH-

  12. Alkaline Pulping Mechanismsβ-1/free phenolic (OH- alone) + CH2O No fragmentation of dimer Stilbene structures

  13. b-1 Reaction Mechanisms (OH- alone) + CH2O

  14. b-1 Reaction Mechanisms (OH- alone) Formation of Stilbene

  15. Alkaline Pulping Mechanismsb-5 /free phenolic (OH- alone) Formation of Stilbenes HO- No fragmentation of dimer Mechanism similar to b-1

  16. 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.

  17. HS- HO- Alkaline Pulping Mechanismsb-1 /free phenolic (OH- /HS-) No fragmentation of dimer 2HS- OH-

  18. Alkaline Pulping Mechanismsb-5 /free phenolic (OH- /HS-) Same reactions as for The b-1 linkage HS- OH-

  19. Alkaline Pulping Mechanismsb-1/etherified phenolic OH (HS-/OH-) No Reaction

  20. Alkaline Pulping Mechanismsb-5/etherified phenolic OH (HS-/OH-) OH- No Reaction HS- OH-

  21. HS- HS- OH- OH- Alkaline Pulping Mechanisms4-O-5 (HS-/OH-) No Reaction

  22. HS- HS- OH- OH- Alkaline Pulping Mechanisms5-5 (HS-/OH-) No Reaction

  23. Alkaline Pulping Mechanisms Methyl Aryl Ether Cleavage Notes

  24. 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.

  25. 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.

  26. 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).

  27. 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.

  28. 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).

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

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