1

Pulping and BleachingPSE 476

Lecture #8Kraft Pulping: Early Reactions

andKraft Pulping Lignin

Reactions

Lecture #8Kraft Pulping: Early Reactions

andKraft Pulping Lignin

Reactions

2

Agenda

• Basic Chemical Pulping Discussion• Loss of Components During Kraft

Pulping• Reactions in the Early Portion of the

Cook» Saponification» Neutralization of Extractives

• Initial Lignin Discussion• Kraft Pulping Lignin Reactions

• Basic Chemical Pulping Discussion• Loss of Components During Kraft

Pulping• Reactions in the Early Portion of the

Cook» Saponification» Neutralization of Extractives

• Initial Lignin Discussion• Kraft Pulping Lignin Reactions

3

Wood Chemistry

• For the students who do not recognize this molecule (did not take PSE 406), there is a short appendix at the end of this lecture to help you. Additionally, the class notes are available for review.

• For the students who do not recognize this molecule (did not take PSE 406), there is a short appendix at the end of this lecture to help you. Additionally, the class notes are available for review.

4

Pulping

• The goal of kraft pulping is to remove the majority of lignin from chips (or other biomass) while minimizing carbohydrate loss and degradation.

• Removal of lignin is accomplished through treatment of raw material with NaOH and Na2S at elevated temperatures.

• The goal of kraft pulping is to remove the majority of lignin from chips (or other biomass) while minimizing carbohydrate loss and degradation.

• Removal of lignin is accomplished through treatment of raw material with NaOH and Na2S at elevated temperatures.

5

The Goal of Lignin Reactions in Kraft Pulping

CH2 O

OH

OCH3

COHH

HC

CH2OH

OH CH2OHC

O

H3CO

C O

CH

CH

H

CH

CHOH2

HO

H

CH

OCH3

OH

C

OH2C

CHO

O

C

CH2OHH3C

O

O

COH

O CH

H3C

CH2OH

H

HCOH

1

2

3

4

5

6

7

H

HC CH

O

O CH

CH2O

C

OCH3

O

CHO

H2C

H3C

8

OHC CH CH2OH

CH2OH

O

O

C

OH

H3C9

10

O

HC CH

COHH2

CH2O

CH

O

OH

H3C

11

H3C

12

HO

CH2OHH3C

13

O

C

O

CH

O CH

O

H3C

H

CH3

CH

OH

O

CH

H3C

CH

H2COH

15

16

Carbohydrate

CH2OH

OH

OCH3

HC 14

H2COH

HC

CHO

17

HOCHO

O

C O CH2

H3C18

HCHO

O

H

H3C

19

O

CH

OCH

O

CH

O

COHH2

OH

OCH3

COHHCOHH2

20

H

CH

H2COH

OCH3

O

HC O

C

OCH3

CH

CH

CHO

22

21

O

H2COHCH2

CH2

H

C O

C

OCH3

24

25

26CH

28

27

O

CH2OH

H

CH3

CH

O

O

H2COH

H

H2COH

H3C

H3C

H2COH

O

CH

CH

OHC

O

O

O

H

23

COH

OCH3

Kraft Pulping SolubleFragments

During kraft pulping, thelarge insoluble ligninmolecules are converted into small alkali soluble fragments.

Carbohydrates are alsodegraded during pulping

6

Yield of Wood Components After Kraft Pulping

Pine Birch After* Before* After* Before*

Cellulose 35 39 34 40

Glucomannan 4 17 1 3

Xylan 5 8 16 30

Other Carb. - 5 - 4

Lignin 3 27 2 20

Extractives 0.5 4 0.5 3

Pine Birch After* Before* After* Before*

Cellulose 35 39 34 40

Glucomannan 4 17 1 3

Xylan 5 8 16 30

Other Carb. - 5 - 4

Lignin 3 27 2 20

Extractives 0.5 4 0.5 3

Notes

* Yields = % of wood (pulp) components

7

Initial Reactions: Low Temperature

• Carbohydrates» Alkaline hydrolysis of acetyl groups on xylan (see

next slide).» Removal of certain soluble carbohydrates.

- Certain galactoglucomannans.- Arabinogalactans.

• Extractives» Alkaline hydrolysis of fats (saponification), waxes,

and other esters.» Neutralization of extractives.

- There are a number of acidic extractives which consume NaOH.

• Carbohydrates» Alkaline hydrolysis of acetyl groups on xylan (see

next slide).» Removal of certain soluble carbohydrates.

- Certain galactoglucomannans.- Arabinogalactans.

• Extractives» Alkaline hydrolysis of fats (saponification), waxes,

and other esters.» Neutralization of extractives.

- There are a number of acidic extractives which consume NaOH.

8

Alkaline Hydrolysis:Example Using Acetyl Groups

• Esters are cleaved in alkaline solutions through hydrolysis reactions forming carboxylic acids and alcohols.

• Hydrolysis of acetyl groups occurs readily in alkaline solutions.» Reaction occurs rapidly even at room temperature.

• Reaction consumes alkali.

• Esters are cleaved in alkaline solutions through hydrolysis reactions forming carboxylic acids and alcohols.

• Hydrolysis of acetyl groups occurs readily in alkaline solutions.» Reaction occurs rapidly even at room temperature.

• Reaction consumes alkali.

O

OH

OHHO

CH2OH

O CO-

CH3

OH

O

OH

OHHO

CH2OH

O CO

CH3

HO-O

OH

OH

OHHO

CH2OH

CO

CH3HO

+

9

Saponification of Fats(Review slide from PSE 406)

C

O

OH2C

R

OH-

C

O-

OH2C

R OHH2O

C

O

O-

R

H2C OH

• Treatment of fats with alkali converts them to fatty acids and glycerol through saponification.

• Treatment of fats with alkali converts them to fatty acids and glycerol through saponification.

HOCH2CHCH2OH

OH

Glycerol (glycerine)

Once again this reaction consumes part of the alkalicharge.

10

Acidic Extractive Species

OH

OCH3

O

HO

CH3O

O

COOH

COOH

Resin Acids Lignans Monoterpenoids

11

Consumption of Alkali

0

1

2

3

4

0 50 100 150

Time (minutes)

Res

idu

al N

aOH

(m

ole

s/kg

wo

od

)

0

1

2

3

4

0 50 100 150

Time (minutes)

Res

idu

al N

aOH

(m

ole

s/kg

wo

od

)

Impregnation zone

12

Where Does All the Alkali Go?

• Spruce wood was soda pulped at a NaOH concentration of 19% (as Na2O).

• 12.5% (or 66% of alkali) consumed to lower lignin content of wood to 2.8%.» 2.3-3% used in dissolution of lignin.» 1.3% for hydrolysis of acetyl and formyl groups.» 8.2-8.9% for neutralization of acidic products

- Some extractives- Mostly carbohydrate degradation products

(discussed later).

• Spruce wood was soda pulped at a NaOH concentration of 19% (as Na2O).

• 12.5% (or 66% of alkali) consumed to lower lignin content of wood to 2.8%.» 2.3-3% used in dissolution of lignin.» 1.3% for hydrolysis of acetyl and formyl groups.» 8.2-8.9% for neutralization of acidic products

- Some extractives- Mostly carbohydrate degradation products

(discussed later).

13

Lignin Removal during Kraft Pulping

• This chart shows the lignin removal rate during a kraft cook. It is important to note that the rate of lignin removal is temperature dependent. What does this fact tell us about of lignin removal in this slide?

• This chart shows the lignin removal rate during a kraft cook. It is important to note that the rate of lignin removal is temperature dependent. What does this fact tell us about of lignin removal in this slide?

0

20

40

60

80

100

0 50 100 150 200 250

Time (minutes)

Lig

nin

Yie

ld (

%)

0

50

100

150

200

Tem

pera

ture

(C

)

Lignin

Temperature0

20

40

60

80

100

0 50 100 150 200 250

Time (minutes)

Lig

nin

Yie

ld (

%)

0

50

100

150

200

Tem

pera

ture

(C

)

Lignin

Temperature

14

Lignin Removal

• In the last slide, the rate of lignin removal appears to be linear over a large portion of the cook; even as the temperature increases.

• This means that lignin removal in the first portion of the cook is easier than as the cook proceeds.

• Lignin removal has been broken into three sections:» Initial Phase (fast lignin removal reactions)» Bulk Phase (slow lignin removal reactions)» Residual Phase (really slow lignin removal)

• In the last slide, the rate of lignin removal appears to be linear over a large portion of the cook; even as the temperature increases.

• This means that lignin removal in the first portion of the cook is easier than as the cook proceeds.

• Lignin removal has been broken into three sections:» Initial Phase (fast lignin removal reactions)» Bulk Phase (slow lignin removal reactions)» Residual Phase (really slow lignin removal)

15

Kraft Pulping:Reaction Phases of Lignin Removal

0

10

20

30

40

50

60

0 5 10 15 20 25 30

Yield of Lignin (%)

Eff

ecti

ve A

lkal

i (g

/l N

aOH

)

0

10

20

30

40

50

60

0 5 10 15 20 25 30

Yield of Lignin (%)

Eff

ecti

ve A

lkal

i (g

/l N

aOH

)

Bulk Phase

Initial PhaseImpregnation zone

Residual Phase

70°C

70°C137°C

170° C

Notes

16

Kraft Pulping Lignin Reactions

17

Dissolution of Lignin

• In review the goal in kraft pulping is the cleavage of lignin into alkali soluble fragments.

• Cleavage is affected by the following factors:» Type of linkage» Presence of free phenolic hydroxyl group» Functional groups (benzyl hydroxyl, carbonyl)» Type and amount of nucleophiles (OH-, HS-)» Reaction temperature

• We are going to first look at the chemical mechanisms of the reactions and then the kinetics.

• In review the goal in kraft pulping is the cleavage of lignin into alkali soluble fragments.

• Cleavage is affected by the following factors:» Type of linkage» Presence of free phenolic hydroxyl group» Functional groups (benzyl hydroxyl, carbonyl)» Type and amount of nucleophiles (OH-, HS-)» Reaction temperature

• We are going to first look at the chemical mechanisms of the reactions and then the kinetics.

18

• The cooking chemicals used in kraft cooking (NaOH and Na2S: OH- and HS-) both act as nucleophiles* because of their free pair of electrons.

• Sites for nucelophilic attack in lignin are those areas of reduced electron density (partially positive sites).

• The cooking chemicals used in kraft cooking (NaOH and Na2S: OH- and HS-) both act as nucleophiles* because of their free pair of electrons.

• Sites for nucelophilic attack in lignin are those areas of reduced electron density (partially positive sites).

Sites for Nucleophilic Attack

Alkaline Media

R1 = OH, OAr or OAlk

-

- -R1

O

OCH3

HC R1

O

OCH3

HC

* Notes

19

Formation of Quinone Methide

O-

HC

OCH3

OH

O

OCH3

HC

Quinone Methide(very reactive)

These arrows indicate that a pair of electrons are moving

Nucleophillicattacksite!

20

Formation of Nucleophilic Attack Sites

• A free phenolic hydroxyl group is needed for the formation of a quinone methide.

• The oxygen of the quinone group (carbonyl) attracts the electron density on the double bond thus making the carbon more positive. This in turn shifts the electron densities of the other bonds on this conjugated system.

• A free phenolic hydroxyl group is needed for the formation of a quinone methide.

• The oxygen of the quinone group (carbonyl) attracts the electron density on the double bond thus making the carbon more positive. This in turn shifts the electron densities of the other bonds on this conjugated system.

O

OCH3

HC

21

Two Additional Examples of Nucleophilic Addition Sites

R1 = OH, OAr or OAlk

- -R1

HC

H2C R1

-O

OCH3

HC

HC

H2C

O

OCH3

HC

Coniferaldehyde type structuresThis structure contains an-ketogroup. Notice that a free phenolichydroxyl groups is not needed!

O

OCH3

C O

CR

H2C R1

- R1

O

OCH3

C O

CR

H2C +

+

R = OAr, Ar or Alk, R1 = OH, OAr, or OAlk

Notes

22

Important Issues!!!!

• When learning about alkaline pulping mechanisms, remember to ask yourselves these questions!» Which reactant are we concerned with: OH-

or HS-?» Does the lignin structure have a free phenolic

hydroxyl group or is it etherified?» Which linkage are you hoping to cleave?» Is there an -carbonyl or benzyl hydroxyl?

• When learning about alkaline pulping mechanisms, remember to ask yourselves these questions!» Which reactant are we concerned with: OH-

or HS-?» Does the lignin structure have a free phenolic

hydroxyl group or is it etherified?» Which linkage are you hoping to cleave?» Is there an -carbonyl or benzyl hydroxyl?

23

Reactions of α-O-4 LinkagePhenolic and Etherified

• In kraft pulping, α-O-4 linkages do not react with HS-

• Reaction with OH-» Phenolic Units: α -O-4

are very rapidly cleaved by alkali. This is the fastest of the lignin degradation reactions. (Will occur at low temperatures)

» Etherified Units: α -O-4 linkages are stable (no reaction).

» Please work out reaction mechanism.

• In kraft pulping, α-O-4 linkages do not react with HS-

• Reaction with OH-» Phenolic Units: α -O-4

are very rapidly cleaved by alkali. This is the fastest of the lignin degradation reactions. (Will occur at low temperatures)

» Etherified Units: α -O-4 linkages are stable (no reaction).

» Please work out reaction mechanism.

OH

O

CH3O

C OH

R

(-)

(-)

O

CH3O

CH

R

O(-)

OR

CH3O

C OH

R

OH(-)

No Reaction

24

Reactions of -O-4 Linkages: Free Phenolic Hydroxyl/Benzyl Hydroxyl

• Reaction with OH- alone» The ether linkage is not

cleaved; a vinyl ether structures is formed.

» Vinyl ether linkages are difficult to cleave.

• Reaction with HS- (OH- present)» HS- is a very strong

nucleophile which cleaves the β-O-4 linkage.

» Reaction is very rapid even at lower temperatures.

• Reaction with OH- alone» The ether linkage is not

cleaved; a vinyl ether structures is formed.

» Vinyl ether linkages are difficult to cleave.

• Reaction with HS- (OH- present)» HS- is a very strong

nucleophile which cleaves the β-O-4 linkage.

» Reaction is very rapid even at lower temperatures.

OH-

O

OCH3

CH O R

HC O

CH3OH2COH

-Vinyl Ether Linkage

O

OCH3

CH

HC O

CH3O

-

H2COH

-O

OCH3

CH

HC

+

CH3O

O-HS-

-

H2COH

O

OCH3

CH O R

HC O

CH3O

OH-

* Mechanisms on following pages

25

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

Vinyl Ether

For

mal

dehy

de

Notice that the -O-4 bond isnot cleaved.

Notes

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-

26

Appendix

Basic Wood ChemistryBasic Wood Chemistry

27

What is the Chemical Makeup of Wood?

0

10

20

30

40

50

60

%

DouglasFir

Redwood YellowPine

Balsam Fir

Cellulose*

Hemicellulose*

Lignin*

Extractives

* Data for Cellulose, Hemicellulose & Lignin on extractive free wood basis

28

Cellulose

• Very long straight chain polymer of glucose (a sugar): approximately 10,000 in a row in wood. Cotton is nearly pure cellulose.» Think about a very long string of beads with each bead being a glucose molecule.

• Cellulose molecules link up in bundles and bundles of bundles and bundles of bundles of bundles to make fibers.

• Uncolored polymer.

O

O O

O

O

O

O

O

CH2OH

OHHO

OHHO

CH2OHOH

CH2OH

HO

OH

CH2OH

HO

O

Cellulose

Cellobiose Unit

29

Hemicelluloses

• Branched little uncolored sugar polymers (~ 50 to 300 sugar units)» Composition varies between wood species.

- 5 carbon sugars: xylose, arabinose.- 6 carbon sugars: mannose, galactose, glucose.- Uronic Acids: galacturonic acid, glucuronic acid.- Acetyl and methoxyl groups (acetic acid &

methanol).

• Major hemicelluloses:» Xylans - big in hardwoods» Glucomannans: big in softwoods

• Minor hemicelluloses: pectins, others.

30

Xylan Structure

4--D-Xly-14--D-Xly-14--D-Xly-14--D-Xly4--D-Xly

4-O-Me--D-Glc

-L-Araf

O O

O

O

O

O

OH

OHHO

HO

HO OH

OO

O

HO OH

O

O

CO2HH3CO

OHHOH2C

O

O

OH

31

Glucomannan Structure

4--D-Glc-14--D-Man-14--D-Man-14--D-Man-1

2,3

Acetyl

6

-D-Gal1

• There are different structured glucomannans in hardwoods and softwoods (and within softwoods)

• Glucomannans are mostly straight chained polymers with a slight amount of branching. The higher the branching, the higher the water solubility.

• There are different structured glucomannans in hardwoods and softwoods (and within softwoods)

• Glucomannans are mostly straight chained polymers with a slight amount of branching. The higher the branching, the higher the water solubility.

32

Lignin

• Phenolic polymer - the glue that holds the fibers together.

• Lignin is a very complex polymer which is connected through a variety of different types of linkages.

• Colored material.CH2 O

OH

OCH3

COHH

HC

CH2OH

OH CH2OHC

O

H3CO

C O

CH

CH

H

CH

CHOH2

HO

H

CH

OCH3

OH

C

OH2C

CHO

O

C

CH2OHH3C

O

O

COH

O CH

H3C

CH2OH

H

HCOH

1

2

3

4

5

6

7

H

HC CH

O

O CH

CH2O

C

OCH3

O

CHO

H2C

H3C

8

OHC CH CH2OH

CH2OH

O

O

C

OH

H3C9

10

O

HC CH

COHH2

CH2O

CH

O

OH

H3C

11

H3C

12

HO

CH2OHH3C

13

O

C

O

CH

O CH

O

H3C

H

CH3

CH

OH

O

CH

H3C

CH

H2COH

15

16

Carbohydrate

CH2OH

OH

OCH3

HC 14

H2COH

HC

CHO

17

HOCHO

O

C O CH2

H3C18

HCHO

O

H

H3C

19

O

CH

OCH

O

CH

O

COHH2

OH

OCH3

COHHCOHH2

20

H

CH

H2COH

OCH3

O

HC O

C

OCH3

CH

CH

CHO

22

21

O

H2COHCH2

CH2

H

C O

C

OCH3

24

25

26CH

28

27

O

CH2OH

H

CH3

CH

O

O

H2COH

H

H2COH

H3C

H3C

H2COH

O

CH

CH

OHC

O

O

O

H

23

COH

OCH3

33

Lignin Nomenclature

OH

OCH3

C

C

C

Methoxyl Group

Phenolic Hydroxyl

1

2

3

45

6

Phenylpropane UnitC9 } Common Names

Side Chain

Notes

34

Lignin Reactions:Linkage Frequencies

O

C

C

C

C

O

C

C

C

O C

O

C

C

C

O

C

C

C

O

C

C

C

O C

O

C

C

C

O

C

C

C

O

C

C

C

O

C

C

C

O

C

C

C

O

O

-O-4 -O-4 -1

- 5-5 4-O-5 -5

Linkage Softwood %

Hardwood %

-O-4 50 60

-O-4 2-8 7

-5 9-12 6

5-5 10-11 5

4-0-5 4 7

-1 7 7

- 2 3

Notes

35

Extractives

• The term extractives refers to a group of unique chemical compounds which can be removed from plant materials through extraction with various solvents.

• Typically these chemicals constitute only a small portion of the tree (<5%).» In some tropical species this can be as high as

25%.• Extractives are produced by plants for a variety of uses.

» The most common use by plants is protection.• Extractives can cause serious problems for processing.• Pitch is a term which is often used when describing some

groups of extractives.• Extractives are responsible for the characteristic color and

odor of wood.

• The term extractives refers to a group of unique chemical compounds which can be removed from plant materials through extraction with various solvents.

• Typically these chemicals constitute only a small portion of the tree (<5%).» In some tropical species this can be as high as

25%.• Extractives are produced by plants for a variety of uses.

» The most common use by plants is protection.• Extractives can cause serious problems for processing.• Pitch is a term which is often used when describing some

groups of extractives.• Extractives are responsible for the characteristic color and

odor of wood.