chemistry of biomass lecture 2. agenda l cellulose l hemicelluloses l lignin they are all polymers
TRANSCRIPT
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Chemistry of biomass
Lecture 2
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Agenda
Cellulose
Hemicellulose
s
Lignin
They are all
POLYMERS
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Major carbohydrates (Fisher projection)
CHO
OHH
HHO
HHO
OHH
CH2OH
D-Galactose
CHO
OHH
HHO
OHH
OHH
CH2OH
D-Glucose
CHO
HHO
HHO
OHH
OHH
CH2OH
D-Mannose
OHH
HHO
OHH
CH2OH
CHO
D-Xylose
OHH
HHO
HHO
CH2OH
CHO
L-Arabinose
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Major carbohydrates (Haworth)
Hexoses
O OH
CH2OH
OH
HO
OH
-D-Galactopyranose
O OH
CH2OH
OH
OHHO
-D-Glucopyranose
O OH
CH2OH
OHHO
HO
-D-Mannopyranose
PentosesO OH
OH
OH
HO
-L-Arabinopyranose
O OH
OH
OH
HO
-D-Xylopyranose
O
OH
OH
HOH2C OH
-L-Arabinofuranose
HO
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Important monosaccharide projections
CHO
OHH
HHO
OHH
OHH
CH2OH
O
H
HO
H
HO
H
HO
OHHH
OH
Haworth Chair ConfigurationNotes
D-glucose
Fisher
α-D-glucopyranose
O
HO OH
OH
OH
CH2OH
α-D-glucopyranose
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Cellulose
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Cellulose: the basics
Linear polymer made up of -D glucopyranose units linked with glycosidic bonds.
Repeating unit = glucose (cellobiose) Glucopyranose units in chair form - most
thermodynamically stable. Only 1% or less in other forms.
O
O O
O
O
O
O
O
CH2OH
OHHO
OHHO
CH2OHOH
CH2OH
HO
OH
CH2OH
HO
O
Cellulose
Cellobiose Unit
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Cellulose: DP
Degree of Polymerization of Cellulose
DP = molecular weight of cellulose
molecular weight of one glucose unit
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Degree of polymerization
Material Degree of polymerizationCotton (unopened) 15,300
Aspen (Hardwood) 10,300
Jack Pine (Softwood) 7900
Bacteria 5000
Sulfite pulp, bleached 1255
Kraft pulp 975
Rayon 305
Notes
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Is cellulose like spaghetti?
In the woody cell wall, exactly what is the cellulose doing?» Is cellulose like uncooked
spaghetti? i.e. random orientation of rigid cellulose chains.
» Is cellulose like cooked spaghetti? i.e random orientation of flexible cellulose chains
» Or is cellulose like those clumps of spaghetti you get when you don’t stir the spaghetti when cooking?
O
O O
O
O
O
O
O
CH2OH
OHHO
OHHO
CH2OHOH
CH2OH
HO
OH
CH2OH
HO
O
Cellulose
Cellobiose Unit
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Amorphous cellulose
A portion of the cellulose in the cell wall can be though of as flexible spaghetti. This is amorphous cellulose.
Every different cellulose preparation has different percentages of amorphous and crystalline cellulose (see next slide).
These 2 forms of cellulose have different properties and reactivities.
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Cellulose: physical properties
Sorptive Properties» Crystalline cellulose does not dissolve in most
solvents– Molecular length– Inter molecular bonding
» Amorphous regions have large number of hydrogen bonding sites available
– Cellulose can absorb large amounts of water– Fully hydrated cellulose very flexible– Dry cellulose inflexible and brittle
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Cellulosecrystalline versus amorphous*
Cellulose Crystallinity (%)
Cotton Linters 71.3
Bleached Kraft (spruce) 68
Bleached Sulfite (spruce) 67.8
Bleached Kraft (birch) 65.1
Bleached Kraft (bamboo) 59.9
Rayon 45
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How is the cell wall put together?
Cell wall is assembled by gluing together a bunch of very small fibers called macrofibril
The glue holding the macrofibrils together is lignin
Macrofibrils are made up of microfibrils which in turn are made up of cellulose and hemicellulose polymers» The glue holding all this together is lignin
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Representation of cell wall components
Cellulose (elementary fibril)
Hemicelluloses
Lignin
Notes
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Cellulose in cell walls (1)
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Types of cellulose
Cellulose I: Native cellulose (cellulose as found in nature.
Cellulose II: Native cellulose which has been soaked in alkali or regenerated cellulose. Large structural changes have occurred in the molecule
Cellulose III or IV: Forms of cellulose which have been treated with various reagents
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Cellulose I unit cells
a
b
Notes
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Cellulose I bonding
a
b
Notes Hydrogen Bonds
Hydrogen Bonds
v.d. Waals
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Bond strength comparison
Linkage Compound EnergykJ/mol
v.d. Waals H2O 0.155
O-H....O H2O 15
O-H....O Cell-OH 28
O-H 460
C-C 347
Notes
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Hemicellulose
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Hemicellulose-general information
Cell wall supporting components 27-30% of wood
» ~27% softwoods» ~30% hardwoods» ~30% agricultural biomass
Short branched polymers» 50-300 DP
In wood they are not crystalline» Very accessible to chemicals» Very reactive
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Cellulose/hemicellulose comparison
Hemicellulose FragmentHemicellulose Fragment
Folded Cellulose FragmentFolded Cellulose Fragment
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Hemicellulose classifications
Softwood Hemicelluloses» Galactoglucomannan (Mannans)-main» Arabinoglucuronoxylan (Xylans)» Arabinogalactan» Pectins
Hardwood Hemicelluloses» Glucuronxylan (Xylans)-main » Glucomannan
Grasses» Arabinoxylan-main
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Softwood Xylans
4--D-Xly-14--D-Xly-14--D-Xly-14--D-Xly4--D-Xly
4-O-Me--D-Glc
-L-Araf
O
O
HO
O
OO
HO
OH
OO
O
OH
OO
HO
OH
O
O
H3CO
HO
OH
COOHH
OH
HO
O
HO
1
31
4
2
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Hardwood Xylans
4 - - D - Xly-14 - - D - Xly-14 - - D - Xly-14 - - D - Xly
R
R = Acetyl
4 - O - Me - - D - Glc
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Arabinogalactan
3--D-Gal-1 3--D-Gal-1 3--D-Gal-1 3--D-Gal-1 3--D-Gal-16
-D-Gal
1
-D-Gal
1
6-D-Gal
1
-D-Gal
1
6-D-Gal
1
-D-Gal
1
6
6 6 6 6
1
R
R = galactopyranose or L-Arabinofuranose or D-glucopyranosyluronic acid
-L-Araf
1
-L-Ara
1
3
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Starch in plants
Starch serves as an energy reserve in plants.
» High concentrations of starch are found in seeds, bulbs, and tubers.
» Starch can be as high as 70-80% of certain tubers and seeds.
Wood contains minor amounts of starch in the form of granules in living parenchyma cells.
» Typical amounts: 0.2-0.6% of total wood
» Sapwood >3%
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Chemical composition of starch
Plants contain two types of starch, linear (helix) amyloses and branched amylopectins.
The amounts of each of these starch types present is plant dependent.» Typical amounts are 25% amylose, 75%
amylopectin » Mutant species can have from 50-90%
amylose
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Lignin
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What is holding all these fibers, vessels together in the
biomass? Lignin
» Three dimensional polymer» No sugars in it» Nature’s glue – very similar to phenolic resin
used in plywood. Holds cellulose and hemicelluloses together
» Second most plentiful natural material» Must be removed or weakened to separate
fibers; turn wood to pulp» Dark in nature – especially after reacting with
alkali – must be de-colored or removed to bleach pulp
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Lignin for chemists
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
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Lignin has been described as 3 dimensional chicken wire. Picture taken from Katy’s chicken page.
Lignin for non-chemists
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Lignin biosynthesisNomenclature
OH
OCH3
C
C
C
Methoxyl Group
Phenolic Hydroxyl
1
2
3
45
6
Phenylpropane UnitC9 } Common Names
Side Chain
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Lignin nomenclature
Once incorporated into lignin, the ring structures of the precursors are given these names.
OH
R
-Hydroxyphenyl
OH
R
OCH3
Guaiacyl Syringyl
OH
R
OCH3CH3O
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Lignin structureMethoxyl content
Lignin Source Grass Softwood Hardwood
-Coumaryl Alcohol
(No Methoxyl)
10-25% 0.5-3.5% Trace
Coniferyl Alcohol
(1 methoxyl)
25-50% 90-95% 25-50%
Sinapyl Alcohol
(2 Methoxyls)
25-50% 0-1% 50-75%
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Common lignin linkages
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
The linkages shown on the right are those formed in dehydrogenation polymers and also found in wood.
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Extractives
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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 is protection
Extractives can cause serious problems for processing Extractives are responsible for the characteristic color and
odor of biomass
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Tree extractives (1)
Besides the big three wood compounds, trees contain other compounds that serve a variety of functions including:» Protection (from insects, animals, and rot).» Attractants (flowers, fruits)» Food storage
The amount of extractives in wood can range from 1-20% (species, position in the tree, season, geographical location)
More in heartwood
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Tree extractives (2)
Extractives add significant properties to wood:» Color» Odor» Density
These compounds are typically present in very limited amounts but still affect the wood properties greatly.
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The fragrance of a tree
Each tree has a unique fragrance.» Some have strong fragrances than others like Cedar.» Some have only light odor.
The aroma is due to volatile compounds produced by the tree (the odor chemicals become gases easily).
These chemicals can be isolated and sold.
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Flavonoids
Serve many roles in plants:» Protection» Coloration» Other unique roles. (Western Hemlock)
O
OH
OH
OH
OH
HO
Catechin
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FlavonoidsMedicinal uses
Higher flavonoid content in diet reduces chance of:» Heart Disease
– Strengthen capillaries– Dilates blood vessels
» Stroke» Cancer: all types reduced
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Alkaloids These nitrogen containing
compounds are found in a variety of different plants.
» Located in the leaves, fruits, and bark.
You are all aware of the alkaloids shown on this page you probably have never seen their structures. These are typically found in small amounts in plants but are worth large sums of cash.
NO
O
N
N
N
N
O
O
N
O
O
Nicotine Caffeine
Cocaine
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What is the chemical makeup of wood?
0
10
20
30
40
50
60
%
Douglas Fir Redwood Yellow Pine Balsam Fir
Cellulose*Hemicellulose*Lignin*Extractives
* Data for Cellulose, Hemicellulose & Lignin on extractive free wood basis