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PSE 406 Lecture 11 1
Wood Chemistry
Wood ChemistryPSE 406/Chem E 470
Lecture 11
Lignin Structure
PSE 406 Lecture 11 2
Wood Chemistry Class Agenda Functional groups
» Methoxyl, phenolic hydroxyl, aliphatic hydroxyl, carbonyl
Lignin structures Lignin – carbohydrate complexes Lignin analytical procedures Lignin trivial facts Appendix
PSE 406 Lecture 11 3
Wood Chemistry
Lignin Functional GroupsPhenolic Hydroxyl
15-30 free phenolic hydroxyl/100C9: Softwood
10-15 free phenolic hydroxyl/100C9: Hardwood
Reactivity» Units containing free phenolic
hydroxyl groups much more susceptible to cleavage reactions - hydrolysis of and aryl ether linkages
» Structures much more reactive towards modification reactions
OH
OCH3
R
O
OCH3
R
CH CHOHOH2C
Free PhenolicHydroxyl
Etherified PhenolicHydroxyl
PSE 406 Lecture 11 4
Wood Chemistry
Lignin Functional GroupsMethoxyl
~0.95/C9 in softwoods ~1.5/C9 in hardwoods Generally resistant to acid and
alkali HS cleaves to form thiols,
mercaptans (Kraft mill odor)OR
OCH3CH3O
Syringyl (both)
Guaiacyl (one)
PSE 406 Lecture 11 5
Wood Chemistry
Lignin Functional GroupsAliphatic Hydroxyl
Majority of aliphatic hydroxyl groups are primary: on carbon» Site relatively non-reactive» In some species, carbon oxygen linked
through ester linkage to -coumaric acid, etc
Benzyl alcohols» Debated amount: 16-40/100 C9 in spruce» Play dominant role in delignification
reactions
OH
OCH3
CH
CH2OH
HC O
OH Benzyl Alcohol
Primary Alcohol
OH
OCH3
CH
CH
COOH
-Coumaric Acid
PSE 406 Lecture 11 6
Wood Chemistry
Lignin Functional GroupsCarbonyl Groups
Total carbonyl groups 20/100C9 in spruce» 1/2 Conjugated Structures
– Coniferaldehyde and -keto structures
– Play important role in delignifcation reactions
» 1/2 Non-conjugated Structures– Glyceraldehyde from -1 coupling
Larger amount in certain hardwoods and grasses due to esters.
OH
OCH3
C OHC O
CH2OH
Carbonyl(-keto group)
OH
OCH3
CHCH
CHO
Coniferaldehyde
Conjugated Carbonyl Groups
Non-Conjugated Carbonyl Groups
O
OCH3
R
CH CHOHOH2C GlyceraldehydeType Group
PSE 406 Lecture 11 7
Wood Chemistry
Lignin StructureSakakibara
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
Text
PSE 406 Lecture 11 8
Wood Chemistry
Lignin-Carbohydrate Complex
All purified holocellulose materials contain a certain amount of lignin
All purified lignin fractions contain a certain amount of monosaccharides
LCCs have been enzymatically prepared from lignin and monosaccharide model compounds
Significant work studying isolated LCCs» No definitive information on exact covalent bonding patterns» Generally accepted bonding patterns
PSE 406 Lecture 11 9
Wood Chemistry
Lignin-Carbohydrate Complex
Proposed Linkages
OH
C
OCH3
O CH
CHO
CH2OH
O
O
OH
OHCH3O
Xyl-Xyl-Xyl
O
Benzyl Ester Linkage
O
OH
OH
OH
C
OCH3
O CH2
O
H
CHO
CH2OH
Xyl-Xyl-Xyl
Benzyl Ether Linkage
OCH2OH
OH HOOMan-Glu-Man
O
CHO
CH2OH
C
OCH3
H
Phenolic Glycosidic Linkage
PSE 406 Lecture 11 10
Wood Chemistry
Lignin-Carbohydrate Complex
General Information
Mw of isolated LCCs 60015,000 LCC linkage stability
» Esters: alkali labile, acid labile» Ethers: selectively alkali labile, mildly acid labile» Glycosides: mildly alkali labile, acid labile
Formation during pulping processes possible LCCs: residual lignin and bleaching
» Removal of that last little bit of lignin» Enzyme assisted bleaching
PSE 406 Lecture 11 11
Wood Chemistry
Lignin StructureAnalytical Procedures
All analysis require model compound studies!
Linkages » Enzymatic dehydrogenation (test tube studies)» Degradation studies (see appendix)» NMR
Functional groups» Wet Chemistry techniques» Spectroscopy
PSE 406 Lecture 11 12
Wood Chemistry Lignin Trivial Facts I
Solubility» Native lignins: limited/no solubility in all solvents
without modification
Molecular Weight» Average Mw for softwood ~20,000, lower for
hardwoods» Polydispersity ~ 2.5-3.0» Mw measured for lignosulfonates as high as
1,000,000
PSE 406 Lecture 11 13
Wood Chemistry Lignin Trivial Facts II
Elemental Composition MWL» Spruce C9H7.92O2.40(OCH3)0.92
C9H8.83O2.37(OCH3)0.96
» Beech C9H8.50O2.86(OCH3)1.43
C9H7.93O2.95(OCH3)1.46
UV Absorption» Strong adsorption at 205 and 280nm» Carbohydrates do not adsorb at 280nm
Compression Wood (Softwoods)» High % lignin (~40%), high % of -hydroxy units (to 70%)
Tension Wood (Hardwoods)» Reduced lignin content
PSE 406 Lecture 11 14
Wood Chemistry
Lecture 11
Appendix
PSE 406 Lecture 11 15
Wood Chemistry
Lignin Structure Elucidation StudiesNitrobenzene Oxidation
OH
OCH3
COHH
HC
CH2OH
6
7
H
HC CH
O
O CH
CHO
H2C
H3C
8
OHC CH CH2OH
CH2OH
O
O
C
H3C9
10
O
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
O
CH
OCH
CH2O
C
OCH3
O
NO2
CHO
OCH3
OH
Vanillin (25% yield, softwoods)175° C /NaOH
CHO
OCH3
OHOCH3
Syringaldehyde (hardwoods)
Text
PSE 406 Lecture 11 16
Wood Chemistry
Lignin Structure Elucidation StudiesPermanganate Oxidation
OH
OCH3
COHH
HC
CH2OH
6
7
H
HC CH
O
O CH
CHO
H2C
H3C
8
OHC CH CH2OH
CH2OH
O
O
C
H3C9
10
O
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
O
CH
OCH
CH2O
C
OCH3
O
1.
KMnO4 2.
H2O2 Oxidation
Oxidation
Alklylation
3.
4. Methylation
OCH2CH3
OCH3
CO2CH3
OCH2CH3
OCH3
CO2CH3
CH3O2C
OCH2CH3
OCH3
CO2CH3
H3COCH3CH2O
CH3O2C
Text
PSE 406 Lecture 11 17
Wood Chemistry
Lignin Structure Elucidation StudiesAcidolysis
OH
OCH3
COHH
HC
CH2OH
6
7
H
HC CH
O
O CH
CHO
H2C
H3C
8
OHC CH CH2OH
CH2OH
O
O
C
H3C9
10
O
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
O
CH
OCH
CH2O
C
OCH3
O
Dioxane
HCl
OH
OCH3
COHH
C O
CH3
Hibbert's Ketones
Text
PSE 406 Lecture 11 18
Wood Chemistry Lignin Structure: Adler
PSE 406 Lecture 11 19
Wood Chemistry
Lignin Structure: Freudenberg
PSE 406 Lecture 11 20
Wood Chemistry Lignin Structure: Nimz
PSE 406 Lecture 11 21
Wood Chemistry Lignin Structure: Forss
