Transcript Lignin Reactions

Agenda
• Lignin Structure - Linkages
• Lignin Reactions
»
»
»
»
Cleavage of b-O-4 linkages
Cleavage of a-O-4 Linkages
Non Cleavable Linkages
Condensation Reactions
• Kinetics of Lignin Removal
1
The Goal of Lignin Reactions
in Kraft Pulping
H2COH
CH2
CH2
H2COH
CH
CH
OH
OCH3
6
16
H3CO
OH
H COH
HC
CH2OH
OH C CH CH2OH
O
H3CO
OCH3
H3CO
5
7
HC
HC
HC
H2C
O
HC
4
OH
HOH2C
O
OCH3
HOCH
3
HC
H2C
O
O
O
H3CO
HC
27
OHC CH CH2OH
O
CH2OH
H3CO
HC
HC
H3CO
1
O
C O
CH
CH2
O
OH
O
CH2OH
CH H3CO
CH
24
HC
HCOH
Kraft Pulping
CH
H2COH
HOCH
O
HC
CHO
12
O
23
OCH3
O
17
O
9
O
HOCH
CH
H2COH
HC
H2COH
H3CO
HC
22
OCH3
O
21
H3CO
HO
19
HC
O
CH
H2COH
CH2
HC
H COH
OCH3
O
20
Soluble
Fragments
CHO
CH
CH
18
11
H3CO
O
H2 COH
CH2OH
10
H2 COH
H2COH
HC
CH
H3CO
13
CH2
H3CO
CH
CH
O
14
CH
C O
CH
CH2OH
OCH3
CH
O
OH
HCOH
HCOH
H2C O
CH
CH
HC O
25
H3CO
2
O
O
OCH3
CH2OH
8
CH
26
CH
H C O Carbohydrate
15
O
CH2OH
28
HO
H2 COH
CH
H3CO
CH3
CH3O
The goal in kraft pulping
is to take large insoluble
lignin and turn it into small
alkali soluble fragments.
OCH3
OH
OH
2
Lignin Reactions:
Linkage Frequencies
Linkage
b-O-4
Softwood
%
50
Hardwood
%
60
a-O-4
2-8
7
b-5
9-12
6
5-5
4-0-5
10-11
5
4
7
b-1
7
7
b-b
2
3
C
C
C
C
O
C
C
C
O
O
b-O-4
a-O-4
C
O
C
C
O
C
O
b-1
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
O
O
O
O
C
O
O
b-b
5-5
O
O
4-O-5
b-5
Notes
3
Lignin Nomenclature

C
b
C
a
C
Side Chain
1
6
2
3
5
4
OCH3
OH
Phenylpropane Unit
C9
Met hoxyl Group
Phenolic Hydroxyl
}
Common Names
4
Reactions of a-O-4 Linkage
Phenolic and Etherified
• In kraft pulping, a-O-4
linkages do not react with
HS-.
• Reaction with OH» Etherified Units: a-O-4
linkages are stable
» Phenolic Units: a-O-4 are
very rapidly cleaved by
alkali. This is the fastest of
the lignin degradation
reactions. Characteristic
reaction in initial
delignificaton
R
HC
R
HC
O
(-)
(-)
O
OH
CH3O
O
CH3O
(-)
O
R
HC
O
OH
(-)
No Reaction
CH3O
OR
5
Kraft Reactions of b-O-4
Linkage
• This ether linkage accounts for 5060% of the linkages in lignin
» Unlike most carbon-carbon
bonds, this linkage can be
cleaved under kraft pulping
conditions.
• Major variables which affect whether
linkage can be cleaved:
» Free phenolic versus etherified
phenolic hydroxyl.
» Presence of HS-.
CH2OH
HC
CH
HOH2C
HC
HC
OCH3
O
OR
OCH3
OH
b-O-4 Linkage
6
Reactions of b-O-4 Linkages:
Free Phenolic Hydroxyl/Benzyl Hydroxyl
• Reaction with OH» The ether linkage is not
cleaved; a vinyl ether
structures is formed.
» Vinyl ether linkages are
difficult to cleave.
HC
HC
O
O
HC
HC O R
-
OH
O-
OCH3
O-
OCH3
Vinyl Ether Linkage
• Reaction with HS» HS- is a very strong
nucleophile which cleaves the
b-O-4 linkage.
» Reaction is very rapid even at
lower temperatures.
CH3O
CH3O
H2COH
CH3O
H2COH
HC
H2COH
O
HC
HC O R
-
O-
OCH3
CH3O
HC
+ -O
OH
HS
O-
OCH3
7
Reactions of b-O-4 Linkages:
Etherified Phenolic Hydroxyl/Benzyl hydroxyl
• Reaction with OH» The b-O-4 linkage is cleaved but
only very slowly.
» Cleavage of the b-O-4 linkage
generates a free phenolic
hydroxyl. Which can undergo
rapid cleavage in the presence of
HS-
• Reaction with HS» HS- will only react with groups
containing a free phenolic
hydroxyl so therefore there is not
cleavage.
CH3O
H2COH
HC
H2COH
O
HC
HC O R
-
+ -O
OH
OCH3
O
OCH3
O R
R
CH3O
H2COH
HC
CH3O
HC
O
HC O R
-
OH
OCH3
O
HS-
No Reaction
R
8
Reactions of b-O-4 Linkages:
Alpha Carbonyl Group
• These reactions occur whether
there is a free or etherified
phenolic hydroxyl group.
• Reaction with OH-.
» No reaction
• Reaction with OH-/HS» Rapid cleavage of linkage.
CH2OH
HC
CH
HOH2C
OCH3
HC
HC
O
O
OCH3
OR
9
Non Cleavable Lignin
Linkages/Condensation
•
b-1, b-5, b-b, 4-0-5, 5-5 linkages do not cleave!
» There are reactions which will occur with these linkages which
consume NaOH and Na2S:
- Formation of stilbenes
- Addition of HS-
• Condensation Reactions
» Under alkaline conditions, there are reactions that will build
molecular weight through bond formation. Potential source of
residual lignin.
OCH3
HC O H
HC
(-)
HC
OH
+
O
OCH3
OCH3
OCH3
O
OH
10
Review of the Kraft Pulping
Reactivities of Lignin Linkages
Free Phenolic OH
Etherified Phenolic OH
Linkage
OH-
HS-
OH-
HS-
b-O-4
No
Yes
Yes
No/Yes*
a-O-4
Yes
No
No
No
b-5, b-b, b1, 5-5, 4-O-5
No
No
No
No
* No if a hydroxyl, yes if a carbonyl
11
Lignin Removal during Kraft
Pulping
200
80
150
60
100
40
Lignin
20
50
Temperature (C)
Lignin Yield (%)
100
Temperature
0
0
0
50
100
150
200
250
Time (minutes)
12
Lignin Removal:
Effect of Effective Alkali
200
100
150
80
60
100
40
15.8% EA
50
25% EA
20
Temperature (C)
Lignin Yield (%)
120
Temperature
0
0
0
50
100
150
200
250
300
Time (minutes)
Increase in EA charge
results in faster pulping
13
Kraft Pulping:
Reaction Phases of Lignin Removal
•Delignification is divided into three phases: Initial, Bulk, Residual
•Each phase has different pulping characteristics and kinetics that
are governed by the lignin chemistry
14
Kraft Pulping:
Effective Alkali (g/l NaOH)
Reaction Phases of Lignin Removal
60
70°C
70°C
137°C
50
40
30
Initial Phase
170° C
20
Bulk Phase
Residual Phase
10
0
0
5
10
15
20
25
30
Yield of Lignin (%)
15