Kraft Reactions

8/18/2019 Kraft Reactions

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Pulping and Bleaching

PSE 476

Lecture #8

Kraft Pulping: Early Reactions andKraft Pulping Lignin Reactions


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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


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Wood Chemistry

•For the students who donot recognize thismolecule (did not takePSE 406), there is ashort appendix at theend of this lecture tohelp you. Additionally,

the class notes areavailable for review.


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4

Pulping

•The goal of kraft pulpingis to remove the majorityof lignin from chips (or

other biomass) whileminimizing carbohydrateloss and degradation.

•Removal of lignin is

accomplished throughtreatment of raw materialwith NaOH and Na2S at

elevated temperatures.


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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

OO

C

OH

H3C 9

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

H3C

18

H

CHO

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

H2COH

CH2

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 Pulpin !oluble

"ra#ent$

During kraft pulping, the

large insoluble lignin

molecules are converted

into small alkali soluble

fragments.

Carbohydrates are also

degraded during pulping


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Yield of Wood Components

After Kraft Pulping %ote$

& 'ield$ ( ) of *ood +pulp, -o#ponent$


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Initial Reactions:Low Temperature

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

»Removal of certain soluble carbohydrates.

-Certain galactoglucomannans.

-Arabinogalactans.

•Extractives

»Alkaline hydrolysis of fats (saponification), waxes, and otheresters.

»Neutralization of extractives.

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


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Alaline !"drol"sis:#$ample %sing Acet"l Groups

• Esters are cleaved in alkaline solutions through hydrolysisreactions 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

OH

HO

CH2OH

O CO

. CH3

OH

O

OH

OH

HO

CH2OH

O C

O

CH3

HO.

O

OH

OH

OH

HO

CH2OH

C

O

CH3HO

/


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9

Saponification of Fats(Review slide from PSE 406)

C

O

OH2C

OH.

C

O.

OH2C

OHH2O

C

O

O.

H2C OH

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

HOCH2CHCH2OH

OH

1ly-erol +ly-erine,

Once again this reaction

consumes part of the alkalicharge.


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Acidic #$tracti&e 'pecies

OH

OCH3

O

HO

CH3O

O

COOH

COOH

e$in -id$ inan$ onoterpenoid$


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Consumption of Alali

0

1

2

3

4

0 50 100 150

Time (minutes)

R e s i d u a l N a O H

( m o l e s / k g w o

o d )

0

1

2

3

4

0 50 100 150

Time (minutes)

R e s i d u a l N a

O H

( m o l e s / k g w

o o d )

#prenation

one


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Where Does All the Alkali

Go?•Spruce wood was soda pulped at a NaOHconcentration of 19% (as Na2O).

•12.5% (or 66% of alkali) consumed to lowerlignin 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).


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Lignin Remo&al during Kraft

Pulping• This chart shows the lignin removal rate during a kraft cook. It isimportant to note that the rate of lignin removal is temperaturedependent. What does this fact tell us about of lignin removal inthis slide?

0

20

40

60

80

100

0 50 100 150 200 250

Time (minutes)

i g n i n ! i e l d ( " )

0

50

100

150

200

T e m #

e $ a t u $ e ( % )

ignin

Tem#e$atu$e

0

20

40

60

80

100

0 50 100 150 200 250

Time (minutes)

i g n

i n ! i e l d ( " )

0

50

100

150

200

T e m # e $ a t u $ e ( % )

ignin

Tem#e$atu$e


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Lignin Removal

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

•This means that lignin removal in the first portion ofthe 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)


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Kraft Pulping:Reaction Phases of Lignin Removal

0

10

20

30

40

50

60

0 5 10 15 20 25 30

!ield o& ignin (")

' & & e t i e * l k a l i ( g

/ l N a O H )

0

10

20

3040

50

60

0 5 10 15 20 25 30

!ield o& ignin (")

' & & e t i e * l k a l i ( g / l N a O H )

ul Pha$e

nitial Pha$e#prenation one

e$idual Pha$e

70C

70C

137C170 C

%ote$


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Kraft Pulping Lignin Reactions


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Dissolution of Lignin

•In review the goal in kraft pulping is the cleavage oflignin 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 mechanismsof the reactions and then the kinetics.


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•The cooking chemicalsused in kraft cooking(NaOH and Na2S: OH-

and HS-) both act asnucleophiles* because oftheir free pair ofelectrons.

•Sites for nucelophilicattack in lignin are thoseareas of reduced electrondensity (partiallypositive sites).

Sites for Nucleophilic Attack

l8aline 4edia

1 ( OH: Or or Ol

.

.. 1

O

OCH3

HC 1

O

OCH3

HC δ+

δ+δ+

δ+

* Notes


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19

Formation of Quinone Methide

O-

HC

OCH3

OH

O

OCH3

HC

+uinone ,et-ide

(e$. $eatie)These arrows indicate that a pair

of electrons are moving

Nucleophillic

attack

site!


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Formation of Nucleophilic AttackSites

• Afree phenolic hydroxylgroup is needed for theformation of a quinonemethide.

• The oxygen of the quinonegroup (carbonyl) attracts theelectron density on thedouble bond thus makingthe carbon more positive.

This in turn shifts theelectron densities of theother bonds on thisconjugated system.

O

OCH3

HC δ+

δ+δ+

δ+


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21

Two Additional Examples ofNucleophilic Addition Sites

1 ( OH: Or or Ol

. . 1

HC

H2C 1

.O

OCH3

HC

HC

H2Cδ

+

O

OCH3

HC δ+

δ+

δ+

δ+

Coniferaldehyde type structuresThis structure contains an α -keto

group. Notice that a free phenolic

hydroxyl groups is not needed!

O

OCH3

C O

C

HC "

. 1

O

OCH3

C O

C

HC δ/

δ/

( Or: r or l: 1 ( OH: Or: or Ol

Notes


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Important Issues!!!!

•When learning about alkaline pulpingmechanisms, remember to ask yourselves

these questions!»Which reactant are we concerned with:OH- orHS-?

»Does the lignin structure have a free phenolichydroxyl group or is it etherified?

»Which linkage are you hoping to cleave?»Is there anα-carbonyl or benzyl hydroxyl?


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Reactions of α-O-4 LinkagePhenolic and Etherified

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

•Reaction with OH-»Phenolic Units:α -O-4 arevery rapidly cleaved byalkali. This is the fastest ofthe lignin degradationreactions. (Will occur at lowtemperatures)

»Etherified Units:α -O-4linkages are stable (noreaction).

»Please work out reactionmechanism.

OH

O

CH3O

C OH

+.,

+.,

O

CH3O

CH

O

+.,

O

CH3O

C OH

OH+.,

%o ea-tion


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Reactions of ()(* Linages:

+ree Phenolic !"dro$"l,-en."l !"dro$"l

• Reaction with OH- alone»The ether linkage is notcleaved; a vinyl etherstructures is formed.

»Vinyl ether linkages aredifficult to cleave.

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

nucleophile which cleavesthe β-O-4 linkage.

»Reaction is very rapid evenat lower temperatures.

OH.

OOCH3

CH O

HC O

CH3OH2COH

.

;inyl


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Kraft Reactions of ()(* Linage/+ree Phenolic !"dro$"l0

#inyl $ther

% o r m a l d e

h y d e

Notice that the

β-O-& 'ond isnot cleaved(

Notes

HCHO/

O

OCH3

CH

HC O

CH3O

.

H

.OOCH3

CH

C O

CH3OH2COH

H

O

OCH3

CH

HC O

CH3OH2CO

O

OCH3

CH

C O

CH3OH2COH

OH.

O

OCH3

CH O HC O

CH3OH2COH

.

HO.

HO.


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Appendix

Basic Wood Chemistry


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What is the Chemical

Makeup of Wood?

0

10

20

30

40

50

60

"

/ouglas

i$

Redwood !ellow

ine

alsam i$

Cellulose1

!emicellulose1

Lignin1

#$tracti&es

1 2ata for Cellulose3 !emicellulose 4 Lignin on e$tracti&e free wood 5asis


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Cellulose

•Very long straight chain polymer of glucose (a sugar): approximately 10,000 in a row in wood. Cotton is nearlypure 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

β

ββ

β

β

Cellulo$e

Cellobio$e =nit


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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.


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6"lan 'tructure

→&-β-)-lyρ-"→&-β-)-lyρ-"→&-β-)-lyρ -"→&-β-)-lyρ −1→ &-β-)-lyρ −1→2

&-O-+e-α-)-,lc ρΑ

→1

2

3

→

1α--.raf

5

O O

O

O

O

O

OH

OHHO

HO

HO OH

OO

O

HO OH

O

O

CO2H

H3CO OHHOH2C

O

O

OH


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Glucomannan 'tructure

1→&-β-)-,lcρ-"→&-β-)-+anρ-"→&-β-)-+anρ-"→&-β-)-+anρ-"→

/3

→

.cetyl

0

→

α-)-,alρ

"

•There are different structured glucomannans in

hardwoods and softwoods (and within softwoods)•Glucomannans are mostly straight chained polymerswith a slight amount of branching. The higher the branching, the higher the water solubility.


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Lignin

•Phenolic polymer -the glue that holdsthe fibers together.

•Lignin is a verycomplex polymerwhich is connectedthrough a variety

of different types oflinkages.

•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

H3C 9

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

HO

CHO

O

C O CH2

H3C

18

H

CHO

O

H

H3C

19

O

CH

OCH

O

CH

O

COHH2

OH

OCH3

COHHCOHH2

20

H

CH

H2COH

OCH3O

HC O

C

OCH3

CH

CH

CHO

22

21

O

H2COH

CH2

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


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Lignin Nomenclature

OH

OCH3

C

C

C

etho>yl roup

Phenoli- Hydro>yl

1

2

3

45

6

α

β

γ

1henylpropane 2nit

C4 Common Names

5ide Chain

Notes


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Lignin Reactions:

Linage +re7uencies

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

β.O.4 α.O.4 β.1

β.β 5.5 4.O.5 β.5

inkage 5oftwood6

Hardwood6

β-O-& 78 08

α-O-& -9 :

β-7 -" 0

7-7 "8-"" 7

&-8-7 & :

β-" : :

β-β 3

%ote$


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Extractives

•The term extractives refers to a group of unique chemicalcompounds which can be removed from plant materials throughextraction with various solvents.

•Typically these chemicals constitute only a small portion of the tree

(

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