Lignin complexity: fundamental
and applied issues
Göran Gellerstedt
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Content
• The lignin structure in wood
• Lignin chemistry in pulping
• Technical lignins
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Content
• The lignin structure in wood
• Lignin chemistry in pulping
• Technical lignins
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Milled Wood Lignin
Spruce: C9H8.62O2.48(OCH3)0.94 Phenolic OH: 20-30%
Birch: C9H8.59O2.86(OCH3)1.52 Phenolic OH:
Linkage
Dimer structure
type
Percent of total linkages
Softwood
Hardwood
-O-4'
Arylglycerol--aryl ether
50
60
-O-4'
Noncyclic benzyl aryl ether
2-8
7
-5'
Phenylcoumaran
9-12
6
5-5'
Biphenyl
10-11
5
4-O-5'
Diaryl ether
4
7
-1'
1,2-Diaryl propane
7
7
'
Pinoresinol/lignan type
2
3
Ref., Adler, 1977
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Monomer yield on thioacidolysis
(theoretical: ~4700-5500 mmol/g)
Sample
Yield of the main monomer(s),
Content of phenolic OH,
mmol/g Klason lignin
Number per 100 C9-units
Spruce wood
1332
n.a.
1682 (31%)
10-13
Spruce MWL
986
20
Spruce TMP (preswollen)
1498
14
Birch wood (preswollen)
672 (G) + 2318 (S) = 2990 (63%)
7.6
403 (G) + 809 (S) = 1212
n.a.
866 (G) + 1942 (S) = 2808 (58%)
10
609 (G) + 863 (S) = 1472
n.a.
Spruce wood (preswollen)
Birch MWL
Aspen wood (preswollen)
Aspen MWL
HO
R
HO
O
R
Lignin O
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H3CO
OCH 3
Lignin
Mechano-chemical cleavage of -O-4
structures in milling
H 3CO
CH2 OH
CH
O
CHOH
CH2 OH
CH
CHOH
L
L
M. E.
OCH 3
L
O
+

OCH3
O
OCH3
O
L
-H
+H
CH2OH
CH2
C O
L
OCH3
OCH3
L
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O
OH
SEC of thioacidolysis products from spruce, eucalyptus
and birch wood
Absorbance
1.2
1
Monomers
Dimers
0.8
Trimers
0.6
Spruce
0.4
Oligomers
Eucalyptus/Birch
0.2
0
20
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25
30
35
40
Time, min
Dissolution of wood/pulp fibres by the use of enzyme
•Endoglucanase
(Novozyme 476)
•Action of urea
- Breaks down the crystallinity of the cellulose by
forming hydrogen bonds between the microfibrils
-
Dissolves any material containing > ~50% lignin
-
Removes enzyme contamination from the fibres
•Action of alkaline borate solution
- Dissolves all remaining components
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Types of LCC isolated from spruce
wood meal
Type of Lignin-Carbohydrate Complex, LCC
Lignin yield, %
GalactoGlucoMannan - Lignin
8
Glucan - Lignin
4
GlucoMannan - Lignin
48
Xylan - Lignin
40
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SEC of acetylated thioacidolysis products from spruce LCCs
Dimer
Monomer
Xylan-rich LCC
(40% lignin on wood)
Response
Glucomannan-rich LCC
(48% lignin on wood)
Wood
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Suggested lignin structures in spruce wood
CHO
OH
OH
HO
Lignin
Lignin
O
O
O
OH
HO
HO
OH
OH
H3CO HO
O
MeO
O
OCH 3
HO
OCH 3
OH
OCH 3
O
OH
H3CO
O
OMe
O
OCH 3
O
H3CO
HO
O
O
O
MeO
OCH 3
OH
O
HO
OMe
O
Glucomannan
OCH 3
Xy lan
HO
O
H3CO
O
MeO
CH 2OH
OH
HO
OH
OH
O
O
OH
OCH 3
OH
OMe
O
HO
HO
OCH 3
OH
O
O
HO
OCH 3
H3CO
Linear xylan-lignin
MeO
OH
O
OH
HO
OH
O
O
OH
OCH 3
O
H3CO
H3CO
O
OCH 3
OCH 3
HO
OCH 3
OH
O
O
HO
HO
OH
OH
O
OH
OH
H3CO
O
O
OH
H3CO
Branched glucomannan-lignin
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OH
Lignin
H3CO
HO
OCH 3
Lignin
HO
OH
OCH 3
S/G ratios in hardwoods
Wood species
S/G-ratio
Method
Reference
Birch
3.8
Thioacidolysis
Gellerstedt et al, 2007
Birch
3.7
Nitrobenzene
Chen, 1992
E. globulus
5.3
Thioacidolysis
Gellerstedt et al, 2007
E.globulus
4.8
Pyrolysis
Gutierrez et al, 2007
E. grandis
3.6
Pyrolysis
Gutierrez et al, 2007
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G-units/S-units in white birch wood
Morphological Differentiation
Guaiacyl/Syringyl
Fibre, S2-layer
12 : 88
Vessel, S2-layer
88 : 12
Ray parenchyma, S-layer
49 : 51
Middle lamella (fibre-fibre)
91 : 9
Middle lamella (fibre-vessel)
80 : 20
Middle lamella (fibre-ray)
Middle lamella (ray-ray)
100 : 0
88 : 12
Ref. Saka and Goring, 1988
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The lignin structure in hardwoods …
contains a high proportion of S-units
which results in a high percentage of
linear lignin – unevenly distributed
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MS-identification of lignin fragment from
E. globulus lignin
OCH 3
O
O
HO H3CO
HO
O
H CO
OCH 33
O
H3CO
HO
OCH 3
OCH 3
OH
OCH 3
OH
OH OH
H3CO
H3CO
HO
O
O
OCH 3
OH OH
Evtuguin et al, 2003
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Lignin in annual plants
Origin
Lignin content
H:G:S
Flax
2.9 (+ 1.6)
57:33:11 (pyrolysis)
Sisal
10.8 (+ 3.0)
1:20:79 (pyrolysis)
Wheat straw
16.0
5:49:46 (thioacidolysis)
Rice straw
6.1
15:45:40 (thioacidolysis)
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Content
• The lignin structure in wood
• Lignin chemistry in pulping
• Technical lignins
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Dissolution of lignin and carbohydrates in
kraft pulping
Residual lignin;
removed by bleaching
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Degree of delignification for different
wood species
Pulp type
Kappa No
Lignin
Delign.
kappa
degree
Pine
28.0
24.6
94.0
Birch
16.5
4.0
98.2
E. globulus
15.9
5.7
97.5
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Kraft pulping of birch and E. globulus
respectively to similar kappa numbers
E. globulus
Birch
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-O-4 structures in wood and pulp based on
thioacidolysis
(birch and eucalyptus)
Degradation product,
mmol/g of lignin
3000
2500
2000
G
S
1500
1000
500
0
Birch B pulp
Klason lignin, %: 16.6
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0.6
Euc
18.3
E pulp
0.9
Size exclusion chromatography (SEC)
of lignin degradation products
(no ”residual lignin” present in wood)
Methodology
•Thioacidolysis of wood/pulp
•Acetylation
•SEC in tetrahydrofuran
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Suggested mode of formation of radical
coupling products in kraft pulping
S
S
S
S
S
S
Lignin
S
S
Lignin
R
H3CO
S S
S
S
S
S
O
H3CO
S S
Lignin
R
OCH 3
O
HO
O
OCH 3

Lignin
OCH 3
H3CO
Lignin
R
R
O
O
S
S
S
S
S
S
R
S
S
H3CO
H
O
H
OCH 3
O
OCH3
H3CO
O
H
O
Low reactivity due to H-bonding
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Principles in the steam explosion process
(Conditions: ~190-240 oC, 1-5 min)
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Chemical composition before and after steam
explosion
100
Lignin
80
Extractives
60
(Ara)-xyl
40
(Gal)-Gluman
20
Spruce samples
Birch samples
Glucan
0
Wood
SO2SE
OneSE
TwoSE
Wood
SO2SE
OneSE
Substantial removal of hemicelluloses and extractives:
SO2SE > TwoSE > OneSE
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Lignin isolation yield (hardwoods)
120
100
80
Residual
60
Extractable,
NaOH
40
20
Birch samples
Aspen samples
0
SO2SE
OneSE
SO2SE
OneSE
SO2SE > OneSE
(missing lignin from aspen  highly soluble lignin)
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SEC of acetylated lignin from steam
exploded aspen wood
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Degradability by thioacidolysis/SEC analysis
Spruce
Condensation  less degradability
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Degradability by thioacidolysis/SEC
analysis, SE aspen
monomers
SO2SE
SE
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Steam explosion chemistry
OH
HO
HO
O
H3CO
High temperature
Lignin
O
Lignin
Stabilisation
OCH 3
OCH 3
Lignin
O
O
Lignin
HO
O
HO
O
OCH 3
H3CO
OH
Lignin
OCH 3
Lignin
O
HO
O
Hydrolysis, H
+
Condensation
O
Lignin
H3CO
OCH 3
OCH 3
O
Lignin
OCH 3
O
Lignin
Acidolysis
OH
O
OCH 3
O
Lignin
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Lignin
Content
• The lignin structure in wood
• Lignin chemistry in pulping
• Technical lignins
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Biomass tree showing the main chemical outlets
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Ref. Rintekno oy,
1984
Highest-value lignin uses to show greatest
future rise (W. Glasser)
As structure of lignin yields
to advances in analytical
techniques, new markets
are projected in adhesives,
foams, films, coatings and
plastics
Ref: C&EN 1984
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The Biorefinery Concept
• Production of large volumes of ethanol will be
necessary in a short term
• New separation process(es) for lignocellulosics
required
• New chemistry based on carbohydrates will be
developed
• Lignin for fuel – and for chemicals
• On a longer term, gasification of biomass to
syngas (biodiesel) will be developed
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Indicative targets for the share of
biofuel in the EU
• 2005: 2% (not achieved)
• 2010: 5.75% (will probably not be achieved)
------------------• 2007: New energy policy document setting a
minimum requirement at 10% by 2020
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From biomass to liquid fuels
• Biodiesel from oils and fat;
rapeseed etc – esterification with methanol
• Biochemical pathways to ethanol;
1) Sugar beet etc – sugar-fermentation
2) Starch crops – hydrolysis-sugar-fermentation
3) Lignocellulosics – separation-hydrolysis-sugarfermentation; lignin as byproduct
• Thermochemical pathways to biofuels;
1) lignocellulosics – pyrolysis-bio oil-biofuels
2) lignocellulosics – gasification-methanol/FT-fuels
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Feedstock sources
• Forestry waste (forest residue, bark, wood
chips, thinnings)
• Agricultural residues (straw, stover, bagasse)
• Energy crops (poplar, willow, switch grass)
• Municipal waste (paper, packaging,..)
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Biomass composition
Structure
Softwood
Hardwood
(Picea abies)
(Betula verrucosa)
Cellulose
42
42
38
Hemicellulose (C6-sugars)
19
4
1
Hemicellulose (C5-sugars)
7
26
24
Lignin
27
23
24
Extractives
2
3
3
Other components
3
2
10
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Wheat straw
The ideal separation of biomass
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… and the reality
•
•
•
•
•
Kraft and soda pulping
Sulfite pulping
Acid hydrolysis
Steam explosion
Organosolv pulping
At present, none of these processes results
in an efficient and cheap separation
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Elemental analysis
Sample
carbon
hydrogen
oxygen
sulfur
Kraft lignin, pine
64.3
6.0
27.9
1.8
Kraft lignin, birch
63.5
6.1
28.0
2.4
Kraft lignin, E. globulus
56.1
5.7
35.4
2.8
Soda lignin, bagasse
61.8
6.0
32.2
0
Steam explosion, beech
57.6
6.0
36.4
0
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Substance Groups in Kraft Black Liquors
(kg/ton of pulp)
Fraction
Pine
Birch
Lignin
490
330
Hydroxycarboxylic acids
320
230
Acetic acid
50
120
Misc. products
200
170
Ref: Sjöström 1993
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Principle for manufacturing of lignin from kraft
black liquor
Black liquor
Evaporation
Acid:
CO2 or H2SO4
Precipitation
pH = 9
Filtration,
Washing
Lignin
Flash drying
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Filtrate,
wash water
Solvent fractionation of softwood kraft lignin
Fraction
Yield
Mn
Mw
Mw/Mn
CH2Cl2
9
4.5 x 102
6.2 x 102
1.4
n-propanol
22
9.0 x 102
1.3 x 103
1.4
Methanol
26
1.7 x 103
2.9 x 103
1.7
CH3OH/CH2Cl2
28
3.8 x 103
8.2 x 104
22
Undissolved
14
5.8 x 103
1.8 x 105
31
Unfractionated
100
1.4 x 103
3.9 x 104
28
Ref: Kringstad et al
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Lignin fractionation
•Material: Industrial black liquor of softwood (pine/spruce), birch
and eucalypt respectively
•Fractionation: Ultra-filtration, 5 kD and 15 kD to remove high
molecular particles / carbohydrates
Permeate
Retentate
•Lignin isolation: Precipitation with CO2 (pH 9),
Acid washing with H2SO4 (pH 2.3),
Drying
•Purification: Cation-exchange to remove traces of Me+
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SEC of kraft lignins before/after
fractionation
eucalypt
0
1
2
3
4
log M (relative polystyrene)
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5
SWL
EL
SP5
EP5
SR5
ER5
dw/d log M
dw/d log M
softwood
6
0
1
2
3
4
log M (relative polystyrene)
5
6
SEC-data from fractionated (5 kDa)
kraft lignins
Sample/
SW
SW
SW
Euc.
Euc.
Euc.
polymer data
lignin
Mw
5600
1800
6100
2300
1300
3400
Mn
900
450
900
530
440
660
Polydispersity
6.2
3.9
6.8
4.4
3.0
5.1
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permeate retentate
lignin
permeate retentate
Thermal analysis of purified kraft
lignins
Lignin sample/
thermal data
SW
lignin
SW
SW
Euc.
Euc.
Euc.
permeate retentate lignin permeate retentate
Tg, oC
148
130
157
133
119
142
Ts, oC
-
181
-
-
182
-
Td, oC
267
260
261
264
260
248
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Even a small lignin withdrawal can be interesting …
650,000 tonnes
of pulp
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Lignin withdrawal of 10%
yields 33,000 tonnes
… converted to
16,000 tonnes of CF
…to support 160,000 cars
with CF-composite
(~40% replacement)
Conclusions
• All native lignins are heterogeneous biopolymers
linked to polysaccharides
• Alkaline or acidic processes result in both lignin
degradation and re-polymerisation
• The up-grading of technical lignins require
purification steps
• Several options exist for an increased lignin use
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