kraft reactions
TRANSCRIPT
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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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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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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:#$ample %sing Acet"l Groups
• Esters are cleaved in alkaline solutions through hydrolysisreactions forming carboxylic acids and alcohols.
• Hydrolysis of acetyl groups occurs readily in alkaline solutions.
»Reaction occurs rapidly even at room temperature.
• Reaction consumes alkali.
O
OH
OH
HO
CH2OH
O CO
. CH3
OH
O
OH
OH
HO
CH2OH
O C
O
CH3
HO.
O
OH
OH
OH
HO
CH2OH
C
O
CH3HO
/
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Saponification of Fats(Review slide from PSE 406)
C
O
OH2C
OH.
C
O.
OH2C
OHH2O
C
O
O.
H2C OH
•Treatment of fats with alkali converts them to fatty acidsand glycerol through saponification.
HOCH2CHCH2OH
OH
1ly-erol +ly-erine,
Once again this reaction
consumes part of the alkalicharge.
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Acidic #$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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Formation of Quinone Methide
O-
HC
OCH3
OH
O
OCH3
HC
+uinone ,et-ide
(e$. $eatie)These arrows indicate that a pair
of electrons are moving
Nucleophillic
attack
site!
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Formation of Nucleophilic AttackSites
• Afree phenolic hydroxylgroup is needed for theformation of a quinonemethide.
• The oxygen of the quinonegroup (carbonyl) attracts theelectron density on thedouble bond thus makingthe carbon more positive.
This in turn shifts theelectron densities of theother bonds on thisconjugated system.
O
OCH3
HC δ+
δ+δ+
δ+
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Two Additional Examples ofNucleophilic Addition Sites
1 ( OH: Or or Ol
. . 1
HC
H2C 1
.O
OCH3
HC
HC
H2Cδ
+
O
OCH3
HC δ+
δ+
δ+
δ+
Coniferaldehyde type structuresThis structure contains an α -keto
group. Notice that a free phenolic
hydroxyl groups is not needed!
O
OCH3
C O
C
HC "
. 1
O
OCH3
C O
C
HC δ/
δ/
( Or: r or l: 1 ( OH: Or: or Ol
Notes
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Important Issues!!!!
•When learning about alkaline pulpingmechanisms, remember to ask yourselves
these questions!»Which reactant are we concerned with:OH- orHS-?
»Does the lignin structure have a free phenolichydroxyl group or is it etherified?
»Which linkage are you hoping to cleave?»Is there anα-carbonyl or benzyl hydroxyl?
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Reactions of α-O-4 LinkagePhenolic and Etherified
•In kraft pulping, α-O-4linkages do not reactwith HS-
•Reaction with OH-»Phenolic Units:α -O-4 arevery rapidly cleaved byalkali. This is the fastest ofthe lignin degradationreactions. (Will occur at lowtemperatures)
»Etherified Units:α -O-4linkages are stable (noreaction).
»Please work out reactionmechanism.
OH
O
CH3O
C OH
+.,
+.,
O
CH3O
CH
O
+.,
O
CH3O
C OH
OH+.,
%o ea-tion
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Reactions of ()(* Linages:
+ree Phenolic !"dro$"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
(