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Typical Composition of Wood
Cellulose (41-53%)Hemicellulose (25-41%)Lignin (16-33%)Extractives (2-5%)Inorganics (<1%)Nitrogen compounds (traces)
Compositions of Several North American Pulpwood Species
Species Cellulose, % Hemicelluloses, % Lignin, % Balsam Fir 44 27 29 Eastern Hemlock 42 26 33 Eastern White Cedar 44 25 31 White Spruce 44 29 27 Jack Pine 41 30 29 Tamarack 43 28 29 “Typical” Softwood 43 28 29 Beech 42 36 22 Trembling Aspen 53 31 16 White Elm 49 27 24 Red Maple 41 35 24 White Birch 41 40 19 Yellow Birch 40 39 21 “Typical” Hardwood 46 34 20
Cellulose (1)
One sugarLinearLongReducing and nonreducing endsPartly crystalline
Hemicelluloses
More than one sugarBranched or substitutedShortReducing and nonreducing endsGenerally not crystalline
Xylan (1)
Primary hemicellulose in hardwoodsLinear chain backbone of xylose unitsSubstituents are 4-O-methylglucuronic acid, acetate groups and, in some, arabinoseSubstituents prevent crystallization, but may be removed during pulping and bleaching
Xylan (2)
O
OO
O
OOH
O
O
OH
HOOCH3CO
HO
O
OOH
OOOH
O
1
4
1
1 4
4
1
4-β−D-Xylp-1
β
α
4-β−D-Xylp-1 4-β−D-Xylp-1 4-β−D-Xylp-1
2
1
3
α-L-Araf
O
OH
HOH2C
1α
3
OH
4
1OHOH
4-O-Me-α−D-Glc pA2
4-β−D-Xyp-15
1
HOβ
Glucomannan (1)
Chief hemicellulose component of softwoodsStraight chain backbone of mannose units and a smaller number of glucose unitsMore vulnerable than xylan to chemical degradation
Glucomannan (2)
O
O
HOH2C
O
OH
O
RO
O
O
OH
HOH2C
HO
O
O
HOH2C
OROHOH2C
ROO
Ac
6
1
4
1
1 4
4
1
4-β−D-Glcp-1
β
α
4-β−D-Manp-1 4-β−D-Manp-1 4-β−D-Manp-16
1α-D-Galp
2,3
4
O
HO RO
HO
OR OR
1
β
LigninNative lignin is a polymer of coniferyl alcohol, sinapyl alcohol, and p-coumaryl alcohol
A heterogeneous, branched and cross-linked polymer with phenylpropane units linked by C-C and C-O bondsCross-linked with polysaccharide chains
R1
OH
R2
HO
12
3
4
5
6
αβ
γ
Coniferyl alcohol/guaiacyl: R1=OMe, R2=HSinapyl alcohol/ syringyl: R1=R2=OMepCoumaryl alcohol: R1=R2=H
Beta-5, Beta-1 and Beta-Beta Linkages
SW: 9-12%
HW: 6%
SW: 7%
HW: 7%
SW: 2%
HW: 3%
Lignin Reactions During Kraft PulpingFragmentation
Alpha- and beta- aryl ether cleavage in free phenolic unitsEnol ether formationAlpha- and beta- aryl ether cleavage in etherified unitsC-C bond cleavage
CondensationBonding to carbohydratesOther reactions
Residual Lignin Structure
Phenolic hydroxyl groupsBeta-aryl ethersC-C bondsCondensation productsFunctional group analysisLinkages to carbohydrates
Phenolic Hydroxyl Groups in Dissolved and Residual Lignins
2.0
2.2
2.4
2.6
2.8
3.0
3.2
1015202530Unbleached kappa number
Phen
olic
hyd
roxy
l con
tent
(m
mol
/g) Kraft lignins
Residual lignins
Phenolic hydroxyl content of various kraft (from black liquor) and residual lignins versus unbleached kappa number
Froass, Peter M.; Ragauskas, Arthur J.; Jiang, Jian E. NMR studies. Part 3. Analysis of lignins from modern kraft pulping technologies. Holzforschung (1998), 52(4), 385-390.
Froass, Peter M.; Ragauskas, Arthur J.; Jiang, Jian-er. Nuclear Magnetic Resonance Studies. 4. Analysis of Residual Lignin after Kraft Pulping. Industrial & Engineering Chemistry Research (1998), 37(8), 3388-3394
Beta-Aryl Ethers in Residual and Dissolved Lignins
0.10
0.12
0.14
0.16
0.18
0.20
0.22
1015202530Unbleached kappa number
Cγ
in β
-O-4
(per
aro
mat
ic u
nit) Residual lignins
Kraft lignins
Content of Cγ in β-O-4 structures for various kraft (from black liquor) and residual lignin versus unbleached kappa number
Functional Groups in Lignin Wood/Pulp
Froass, Peter M.; Ragauskas, Arthur J.; Jiang, Jian-er. Chemical structure of residual lignin from kraft pulp. Journal of Wood Chemistry and Technology (1996), 16(4), 347-365.
Hexenuronic Acid Formation
O
O
CO2H
H3CO
OHHO
Xylan
O
HO2C
OHHO
Oxylan
MeGlcA HexA
Kraft pulping
HH
+ CH3O-
