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Agenda

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. The Goal of Lignin Reactions in Kraft Pulping. Kraft Pulping. Soluble Fragments.

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Agenda

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  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 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 Notes

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

  5. Reactions of a-O-4 LinkagePhenolic and Etherified • Inkraft 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 * No if a hydroxyl, yes if a carbonyl

  12. Lignin Removal during Kraft Pulping

  13. Lignin Removal:Effect of Effective Alkali 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

  15. Kraft Pulping:Reaction Phases of Lignin Removal 70°C 70°C Initial Phase 137°C 170° C Bulk Phase Residual Phase

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