1 / 29

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

ivy-cervantes
Download Presentation

Pulping and Bleaching PSE 476/Chem E 471

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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

More Related