Kraft Lignin Reaction

Lecture Overview

  • Focus on Kraft fractionation and lignin reactions.

Agenda

  • Lignin Reactions:

    • Cleavage of various linkages (b-O-4, b-1, b-5, 4-O-5, 5-5, b-b).

    • Effects of phenolic group and nucleophiles (OH-/HS-).

    • Exploration of condensation reactions.

Lignin Linkages

Reactions of α-O-4 Linkages

  • Phenolic and Etherified:

    • α-O-4 linkages not reactive with HS- in Kraft fractionation.

    • Reaction with OH-: Rapid cleavage occurs; it is the fastest lignin degradation reaction (can occur at low temperatures).

    • Etherified Units: Stable and do not undergo reaction.

Reactions of β-O-4 Linkages

  • With Free Phenolic Hydroxyl/Benzyl Hydroxyl:

    • OH- Alone: Vinyl ether structures formed; ether linkages remain intact and are difficult to cleave.

    • With HS- (in presence of OH-):

      • Rapid cleavage of the β-O-4 linkage occurs, even at lower temperatures.

Kraft Reactions of β-O-4 Linkage

  • Vinyl Ether Formaldehyde:

    • β-O-4 bond remains unbroken under certain conditions.

Reaction of Phenolic b-Aryl Ether with HS-

  • Very rapid reaction leading to β-O-4 linkage cleavage.

Reactions of β-O-4 Linkages (Etherified)

  • Reaction with OH-:

    • Cleavage occurs very slowly, requires elevated temperatures.

    • Produces free phenolic hydroxyl.

  • Reaction with HS-:

    • No cleavage occurs as HS- only reacts with compounds having a free phenolic hydroxyl.

Nonphenolic β-Aryl Ether Reactions

  • Reaction with OH-:

    • Very slow cleavage of β-O-4 linkage.

  • Reaction with HS-:

    • No reaction due to absence of phenolic hydroxyl.

Summary of Reactions

  • With OH- Alone: Formation of vinyl ethers with no cleavage of phenolic structures.

  • With HS-: Rapid cleavage for phenolic hydroxyl; no cleavage for non-phenolic structures.

Reactions Involving Alpha Carbonyl Group

  • With OH- Alone: No reaction indicated.

  • With OH-/HS-: Rapid cleavage of linkages occurs.

Alkaline Fractionation and Mechanisms

  • Involves creating reactions within α-O-4 and β-1 linkages, with complex mechanisms varying between free phenolic and etherified phenolic structures.

Retro Aldo Cleavage of Carbon-Carbon Bonds

  • Occurs slowly under alkaline conditions for groups containing double bonds.

Alkaline Fractionation Mechanisms Summary

  • Identified mechanisms for reactions involving free and etherified phenolic units, with varying outcomes based on whether nucleophiles are present.

Lignin Condensation Reactions

  • Discusses the reactions of quinone methides with hydroxyl groups leading to increases in molecular weight and creation of difficult-to-cleave bonds.

  • Irreversible formation of new carbon-carbon bonds through interactions with C5 anions, resulting in higher molecular weight lignin.

Condensation with Formaldehyde

  • Cross-linking of two molecules to form diphenyl methane linkages, implicated in molecular weight increases due to lignin reactions.

Reactivities of Lignin Linkages

  • Free phenolic OH vs. etherified phenolic OH under different reactions:

    • B-O-4: Cleaved by HS- (yes), slow or no reaction with OH-.

    • a-O-4: Cleaved by OH- (yes), no reaction with HS-.

    • Other linkages: Various levels of reactivity depending on phenolic structure presence.