STUDIA UNIVERSITATIS BABEŞ-BOLYAI, PHYSICA, SPECIAL ISSUE, 2003
FT- IR SPECTROSCOPY FOR LIGNINS CHARACTERIZATION
1
Georgeta Cazacu, 2Carmen- Mihaela Popescu, 3V. I. Popa,
2
Gh.Singurel, 1Cornelia Vasile
“Petru Poni” Institute of Macromolecular Chemistry
,Aleea Gr. Ghica Vodă 41 A, 6600-Iaşi, Romania
2
“Al. I.Cuza”University, Optics-Spectroscopy Depart.,
Bd. Carol I, 11, 6600-Iaşi, Romania
3
“Gh. Asachi” Technichal University, Iasi, Romania
1
ABSTRACT
FT-IR spectroscopy was applied to obtain new structural information and
characterization of various native lignins and chemically modified
lignins. The relative content of different functional groups was
appreciated by normalized intensities and deconvolution of the bands.
INTRODUCTION
Lignins characterisation is a very difficult task, because of its diversity in respect
both with provenience and method of separation. The elaboration of well-defined
analytical methods is very important for its introduction as raw material in industry.
The heterogeneity of lignin is caused by variation in polymer composition, size,
cross linking, functional groups, linkage type between the phenylpropane monomers
(p- hydroxy phenyl, guayacil and siringil units). The major chemical functional
groups in lignin include: hydroxyl, methoxyl, carbonyl and carboxyl. [1,2]
Taking into account these general information about structure of lignin is very
evident that FT-IR spectroscopy can be a very useful tool for characterisation of
different native lignins or chemically-modified lignins.
EXPERIMENTAL
Materials
The studied lignin samples received from ATO- Netherlands and GranitSwitzerland were classified in two groups: lignin from woody plants and lignin from
annual plants (see column 1, Table 1). The methods of delignification were also
different. Several samples were chemically modified by oxidation or nitration (NL)
reaction using a mixture of nitric acid and acetic acid.
Method
The IR spectra were recorded for solid samples in KBr tablets using a FT-IR
Bomem MB-104 spectrometer (Canada) with a resolution of 4 cm-1. The
concentration of the sample in the tablets was constant of 2.5 mg samples/ 482 mg
KBr. Processing of the spectra was done by means of Grams/ 32 program. The
normalised intensity was evaluated for main bands characteristic to lignin structure,
such as: 3450 cm-1(OH), 2920 cm-1(asCH2 ,guaiacyl- siringyl), 2840 cm-1(sCH2
POPESCU CARMEN MIHAELA
,guaiacyl- siringyl), 1730 cm-1(C=O non conjugated ketone and aromatic ester),
1658 cm-1(C=O aryl ketone p-substituted, guaiacyl), 1510 cm-1 and 1425 cm-1(C=C
aromatic ring ,guaiacyl- siringyl), 1325 cm-1(s siringyl ring),1275 cm-1(s guaiacyl
ring, asC-O-C),1220 cm-1(s siringyl ring),1030 cm-1 (C-H guaiacyl aromatic ring and
C-OH primary alcohol), and 860 cm-1 (=CH aromatic ring ,guaiacyl- siringyl).[3]
Shoulders and complex bands were deconvoluted for a good assessment.
RESULTS AND DISCUSSION
The spectral data of the studied lignins are given in table 1.
It can be remarked that the strongest hydrogen bonds are present in lignin from Pine
and Curan lignin and also in lignosulfonates whose 3450 cm-1 band is very strong.
The bands at 1275 cm-1and 1220 cm-1 (1330 cm-1) (Fig.1a,b,c) assigned to guaiacyl,
respectively siringyl ring increase/decrease in the series studied of lignin from
woody plants.
a.
b.
c.
Figure.1. Variation of the normalised intensities of the bands at 1275cm-1(a), 1220 cm-1(b)
and 1330 cm-1(c)
The lignin from annual plants have much complex IR spectra, the bonds at 2840 cm1
and 1325 cm-1 are stronger than those of woody plants (see Table 1). The oxidation
leads to the destruction of aromatic ring because of introduction of carbonyl and
carboxyl groups. The bands at 1275 cm-1(s guaiacyl ring) (Fig.2a), 1510 cm-1 (C=C
aromatic ring ,guaiacyl- siringyl)(Fig.2b)and 1700cm-1 (C=O non conjugated ketone)
(Fig.2c) decrease in the series studied of lignin from flax.
The presence of the -NO2 and -ONO2 groups was possible to be evidenced only
often deconvolution (Fig.3b-d), they appearing in spectrum as very weak peaks (880
cm-1), or shoulders (1277 cm-1, 1544 cm-1), or overlapped with other bonds (1614
cm-1)(Fig. 3a).
Therefore FT-IR spectroscopy can be successfully used in the characterisation of
complex molecules as lignin, having in view that its structure depends on the raw
material, extraction/ separation procedure and chemical modification.
FT- IR SPECTROSCOPY FOR LIGNINS CHARACTERIZATION
Lignins from woody plants
Hardwood
organosolv
Aspen wood/
SE
Softwood,
kraft (1)
Softwood,
kraft (2)
Softwood SF
HpH
Softwood SF
LpH
Softwood
L.Sf. (white)
Softwood
L.Sf.
Pine, kraft
Curan wood
Alcell/ wood
3.08
m
m
m
__
m
s
m
w
vw
m
w
vvw
3.17
s
m
m
__
__
m
m
w
vw
m
w
__
3.25
vs
m
__
__
w
m
v w
w
w
w
vvw
3.25
vs
__
__
vw
m
m
w
w
w
w
vvw
3.59
m
__
__
vvw
m
vw
w
w
w
w
__
3.25
m
m
__
__
w
s
w
vv
w
vv
w
vv
w
__
m
m
m
vvw
3.34
m
__
vw
m
m
vw
m
s
vvw
__
__
vw
m
w
m
v
w
__
m
3.59
vv
s
m
m
s
s
__
3.77
3.68
3.25
m
m
m
m
w
m
m
w
m
__
vvw
__
w
w
m
m
w
s
vw
vvw
m
vw
__
w
m
w
vw
m
w
m
m
w
m
vw
vvw
vw
Flax SF (1)
3.34
s
vs
s
__
m
m
vvw
m
m
m
vvw
Flax SF(2)
3.51
s
vs
s
__
m
m
w
m
m
m
vw
Flax
oxidized
PF 3035/
Flax
Straw
BPD/ Straw
Sisal
3.42
s
vs
m
m
m
m
vw
w
m
w
vvw
3.42
s
s
s
__
m
m
vw
w
m
w
vvw
2.99
3.08
3.17
vs
s
s
m
m
m
w
m
m
m
m
w
m
__
__
m
m
m
w
w
w
v
w
v
w
v
w
vv
w
vw
vv
w
m
m
w
m
m
w
__
__
vvw
Sisal/ Soda
pulping
PF 3074/
Hemp
Hemp
3.25
s
s
m
m
s
m
w
w
m
m
vvw
3.42
s
s
m
m
m
vw
vw
w
w
m
vv
w
vv
w
w
w
w
vvw
3.42
vs
s
m
w
m
w
vw
vw
vw
vw
vvw
Jute
2.99
s
s
m
m
m
m
w
vw
m
m
vvw
Abaca
2.65
vs
m
m
__
__
s
m
v
w
v
w
w
__
s
w
w
__
__
__
__
__
__
__
__
vs
s
s
m
__
__
m
m
__
m
__
m
__
__
__
vw
__
__
__
__
Lignins from annual fibre crops
Nitrolignins
NL A
NL B
NL C
NL D
2.75
3.15
3.71
4.24
m
m
m
m
vw
m
w
m
vvw
w
vvw
m
__
s
__
m
vs
s
vs
__
vs - very strong, s - strong, m - medium, w - weak, vw - very weak, NL A-fraction
precipitated with dioxane, NL B-fraction precipitated with ethyl ether/ dichlorethane , NL
C-fraction precipitated with water and NL D- residue
guaiacil - siringil
[…]
1030cm-1
 C-H guayacil
aromatic ring and
-1
C-OH
primary
866cm
=CH alcohol
aromatic ring
 C=C aromatic
ring
guayacil - siringil
1325cm-1
s siringil ring and 
C-O
1275cm-1
s guayacil ring
as C-O-C aromatic
1220cm
ester -1
s siringil ring
 C=C aromatic
ring
guayacil
- siringil
-1
1425cm
 C=O aryl ketone
p-substituted
guayacil-1
1510cm
1730cm-1
 C=O non
conjugated ketone
and aromatic
1658cm-1ester
2840cm-1
s CH2 guayacil siringil
2920cm-1
as CH2 guayacil siringil
Names
EH
(the energy of
hydrogen bond)
3450 cm-1  OH
Table 1.
FT-IR results for lignins of various origins and for chemically modified lignins
POPESCU CARMEN MIHAELA
a.
b.
c.
Figure.2. Variation of the normalised intensity of the bands at 1275cm-1(a), 1510 cm-1(b) and
1700 cm-1(c)
a.
b.
c.
d.
Figure.3. FT-IR spectra of nitrolignin(a) and deconvolutions for 1900-1450 cm-1(b), 14501100 cm-1(c) and 910-840 cm-1(d) regions of the spectra
References
1 .R.J . A.Go s se li n k, A. Ab äc her li, H .S e mk e, R. Mal h erb e, P .K ä up er a nd
J .E.G. va n D a m , Characterisation of sulfur-free lignins frm alkaline pulping of annual
fibre crops, Fifth International Forum of ILI, Bordeaux France, September 7, 2000
2. A. Ab äc h er l i , New lignins from agricultural plants, Idem.
3.A.B e r mel lo , M.d el V a lle, U.Or ea a nd L. R. C a rb allo , Characterisation by IR
spectromerty of lignins of three eucaliptus species,Intern. J. of Polym. Mat., 51: 557-566p,
2002