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Pulping and BleachingPSE 476/Chem E 471

Lecture #9Kraft Pulping Lignin Reactions

Lecture #9Kraft Pulping Lignin Reactions

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Agenda

• Lignin Reactions» Cleavage of -O-4 linkages

- Effect of phenolic group- Effect of nucleophile (OH-/HS-)

» -O-4, -carbonyl» -1 linkages» -5 linkages» 4-O-5, 5-5, - linkages» Condensation Reactions

• Lignin Reactions» Cleavage of -O-4 linkages

- Effect of phenolic group- Effect of nucleophile (OH-/HS-)

» -O-4, -carbonyl» -1 linkages» -5 linkages» 4-O-5, 5-5, - linkages» Condensation Reactions

3

Kraft Reactions of -O-4 Linkage (Free Phenolic Hydroxyl)

Vinyl Ether

For

mal

dehy

de

Notice that the -O-4 bond isnot cleaved.

HCHO+

O

OCH3

CH

HC O

CH3O

-

H

-O

OCH3

CH

C O

CH3OH2COH

H

O

OCH3

CH

HC O

CH3OH2CO

O

OCH3

CH

C O

CH3OH2COH

OH-

O

OCH3

CH O R

HC O

CH3OH2COH

-

HO-

HO-

Notes

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Reaction of Phenolic -aryl Ether: HS-

This reaction is very rapid resultingin the cleavage of the -O-4 linkage.

Notes

O

OCH3

HC

HC

CH2OH

O

CH3O

O

OCH3

HC

HC

CH2OH

O

CH3O

O

OCH3

HC

CH2OH

O

CH3O

OH HSCHS

O

OCH3

HC

HC

CH2OH

CH3OS

O+

OR

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Reactions of -O-4 Linkages: Etherified Phenolic Hydroxyl/Benzyl hydroxyl

• Reaction with OH-

» The -O-4 linkage is cleaved but only very slowly. This reaction requires elevated temperatures.

» Cleavage of the -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 with OH-

» The -O-4 linkage is cleaved but only very slowly. This reaction requires elevated temperatures.

» Cleavage of the -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.

H2COH

O

OCH3

C

R

H

HC

+

CH3O

O-

H2COH

O

OCH3

C

R

H O R

HC O

CH3O

OH-

H2COH

O

OCH3

C

R

H O R

HC O

CH3O

OH-

HS- No Reaction

* Reaction mechanisms on following pages

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Reaction of Nonphenolic -aryl Ether: OH-

In this reaction, the -O-4 linkage is cleaved. This is,however, a very slow reaction.

Notes

O

OCH3

HC

HC

CH2OH

O

CH3O

OHOR

O

OCH3

HC

CH2OH

O

CH3O

CHO

O

OCH3

HC

HC

CH2OH

O

O

CH3O

+

Further Degradation

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Reaction of Nonphenolic -aryl Ether: HS-

OR

CH3O

CH OH

HC O

H2COH

HS(-)

No Reaction

No phenolic hydroxyl = no reaction

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A Quick Review

OH- (alone) HS-(with OH-)

Phenolic Hydroxyl

No cleavageFormation of vinyl ethers.

Rapid cleavage

Non Phenolic

Slow cleavage No cleavage

Reaction of -O-4 ether linkedstructure with benzyl hydroxyl

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Reactions of -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

• 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

HC

O

OCH3

CH

HC

CH2OH

HC

HOH2C

OR

OCH3

O

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Alkaline PulpingActivation of Carbon by Carbonyl

O

OCH3

C

C

O

OR

CH2OH

H

HO-

O

OCH3

C

C

O-

OR

CH2OH

O

OCH3

C

C

O

OR

CH2

A: Keto-enol tautomerism

A

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Alkaline Pulping -O-4 , -carbonyl

Keto-enolisomerization

O

OR

OCH3

CH

HC

H2COH

O

OH(-)

C O

OH

OR

OCH3

C

H2COH

(-)

SH

O

OR

OCH3

C

H2C

OC

O

OR

OCH3

C

HC O

CH2S(-)

+O

OR

OCH3

C

CHS

CH2

O(-)

HS-

OH-

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Alkaline Pulping Mechanismsβ-1/free phenolic (OH- alone)

O-

OCH3

HC OH

HC

OCH3

OH

H2C OH

OH-

O

OCH3

HC

HC

OCH3

OH

H2C OH

O-

OCH3

HC

HC

OCH3

OH

O-

OCH3

HC

C

OCH3

OH

H2C OH

Stilbene structures

No fragmentation of dimer

+ CH2O

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-1 Reaction Mechanisms (OH- alone)

O

OCH3

HC

HC

OCH3

OH

H2C O H

OH-

O-

OCH3

HC

HC

OCH3

OH

+ CH2O

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-1 Reaction Mechanisms (OH- alone)

O

OCH3

HC

C

OCH3

OH

H2C OH

H

HO-

O-

OCH3

HC

C

OCH3

OH

H2C OH

Formation of Stilbene

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Alkaline Pulping Mechanisms-5 /free phenolic (OH- alone)

OH

OCH3

HC

HC

HOH2C

OCH3

R

OHO-

O

OCH3

HC

HC

HOH2C

OCH3

R

O-

O-

OCH3

HC

C

HOH2C

OCH3

R

O-

O-

OCH3

HC

HC OCH3

R

O-

Formation ofStilbenes

No fragmentation of dimerMechanism similar to -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.

• 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.

HC

C O-

+ H+

- H+

C

C+ H2O

- H+

H2C

C O

OCH3

O

C

+ H2O

- H+

O-

OCH3

O

CH

O-OCH3

C O

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Alkaline Pulping Mechanisms-1 /free phenolic (OH- /HS-)

O

OCH3

HC

HC

OCH3

OH

H2C OH

HS-

HO-

O-

OCH3

HC

HC

OCH3

OH

H2C OH

S-

O

OCH3

HC

C

OCH3

OH

H2C

2HS-

OH-

O-

OCH3

HC

HC

OCH3

OH

H2C S-

S-

No fragmentationof dimer

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Alkaline Pulping Mechanisms-5 /free phenolic (OH- /HS-)

OH

OCH3

HC

HC

HOH2C

OCH3

R

O

OH-

HS- Same reactions as forThe -1 linkage

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Alkaline Pulping Mechanisms-1/etherified phenolic OH (HS-/OH-)

OCH3

OCH3

HC OH

HC

OCH3

OH

H2C OH

OH-

OCH3

OCH3

HC OH

HC

OCH3

OH

H2C OH

OH-

HS-

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Alkaline Pulping Mechanisms-5/etherified phenolic OH (HS-/OH-)

OH-

HS-

OH-

OCH3

OCH3

HC

HC

HOH2C

OCH3

R

O

OCH3

OCH3

HC

HC

HOH2C

OCH3

R

O

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Alkaline Pulping Mechanisms-5 (HS-/OH-)

OCH3

O

OCH3

CH3O

R

R

O-

O

OCH3

CH3O

R

R

HS-

OH-

HS-

OH-

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Alkaline Pulping Mechanisms-5 (HS-/OH-)

O- O-

OCH3CH3O

R R

OCH3 OCH3

OCH3CH3O

R R

HS-

OH-

HS-

OH-

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Alkaline Pulping Mechanisms Methyl Aryl Ether Cleavage

+ HS- + HO

-

O

OCH3

[O-] O

O-

[O-]

+ CH3S- + H2O

O

OCH3

[O-] O

O-

[O-]

+ CH3SCH3+ CH3S-

2CH3S- + 1/2 O2 + H2O CH3S SCH3 + 2HO

-

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.

• Esters and glycosidic linkages easily cleaved.

• Ether linkages.» Free phenolic

hydroxyl: easily cleaved.

» Etherified phenolic hydroxyl: no reaction.- Residual lignin

problem.

OH

C

OCH3

O CH

CHO

CH2OH

O

O

OH

OHCH3O

Xyl-Xyl-Xyl

O

Benzyl Ester Linkage

O

OH

OH

OH

C

OCH3

O CH2

O

H

CHO

CH2OH

Xyl-Xyl-Xyl

Benzyl Ether Linkage

OCH2OH

OH HOOMan-Glu-Man

O

CHO

CH2OH

C

OCH3

H

Phenolic Glycosidic Linkage

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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.

• 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.

O-

OCH3

CH2OH

O

OCH3

CH2OH

-

O

OCH3

CH2OH

-

O

OCH3

CH2

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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).

• 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).

CH2

O

CH3O

O

OCH3

CH2OH

-

O-

CH3O

H2C

O

OCH3

CH2OH

H

O-

CH3O

H2C

O-

OCH3

CH2OHOH-

CH2

O

CH3O

O

OCH3

CH2OH

-

O-

CH3O

H2C

O

OCH3

CH2OH

H

O-

CH3O

H2C

O-

OCH3

CH2OHOH-

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Condensation Reactions (C1)

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

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

O

OCH3

HC

O

OCH3

HC O H(-)

+

HC

OCH3

OH

OH

OCH3

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Condensation with Formaldehyde

• Formaldehyde, which is formed from 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 -1 & -5).

• Formaldehyde, which is formed from 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 -1 & -5).

CH3O

O

R

(-) C

H

O

H

CH2

R

O

CH3OCH3O

O

R

CH2 OH

(-)

O

OCH3

R

(-)

O

OCH3

R

CH2

R

O

CH3OH

(-)

CH3O

O

R

CH2

O

OCH3

R

CH2

R

O

CH3O

(-) (-)

(-)

OH

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Reactivities of Lignin Linkages

Free Phenolic OH Etherified Phenolic OH

Linkage OH- HS- OH- HS-

-O-4 No Yes Yes No/Yes*

-O-4 Yes No No No

-5, -, -1, 5-5, 4-O-5

No No No No

Free Phenolic OH Etherified Phenolic OH

Linkage OH- HS- OH- HS-

-O-4 No Yes Yes No/Yes*

-O-4 Yes No No No

-5, -, -1, 5-5, 4-O-5

No No No No

Short Note

* No if hydroxyl, yes if carbonyl