Biomacromolecules 2005, 6, 2815-2821
2815
Hydrogen Bonding in Lignin: A Fourier Transform Infrared
Model Compound Study
Satoshi Kubo and John F. Kadla*
Biomaterials Chemistry, Department of Wood Science, The University of British Columbia,
Vancouver, BC V6T 1Z4, Canada
Received April 21, 2005; Revised Manuscript Received May 27, 2005
Hydrogen bonding plays an important role in the thermal and mechanical properties of biopolymers. To
investigate hydrogen bond formation in lignin, an abundant natural polymer found in plants, Fourier transform
infrared (FTIR) analysis of various lignin model compounds was performed. Four monomeric model
compounds and one dimeric model compound were studied under various conditions. FTIR analysis revealed
aliphatic hydroxyl groups form stronger hydrogen bonds than phenolic hydroxyl groups. Further, the dimeric
biphenyl-type structure formed significantly stronger intermolecular hydrogen bonds as compared to the
other monomeric model compounds. Results from the model compound studies were used to explain the
observed complex hydrogen-bonding system present in both softwood and hardwood technical lignins.
Together with chemical analysis, we discuss the difference in hydrogen bonding between hardwood and
softwood lignin and the observed differences in the glass transition temperature.
Introduction
Lignin is a natural polymer found in wood and is one of
the most abundant biomacromolecules, second only to
cellulose in natural abundance. A complex three-dimensional
network polymer, lignin serves as a continuous matrix
component in plant cell walls, providing mechanical strength
and structural support. Produced by dehydrogenative polymerization of phenylpropane units, native lignins possess
a large array of interunit linkages. Depending on the wood
species, e.g., softwood (conifers) vs hardwood (angiosperms),
the types and amounts of interunit linkages and phenylpropane units present will vary.1,2 Numerous spectroscopic and
degradative techniques have been developed to characterize
the structural features and functional groups present in
lignin.3 Chemically, softwood lignins consist largely of
guaiacylpropane units, while hardwood lignins are made up
of both guaiacyl- and syringylpropane units. Both lignins
contain predominately glycerol-aryl ether linkages, but there
are several types of C-C bonds which likely serve as crosslinks between relatively short, linear chains of phenylpropane
units. Softwood lignins contain fewer glycerol-aryl ether
linkages and subsequently a more complex macromolecular
network structure.4
There is no method to isolate lignin from plants in the
native form; chemical and physical modifications are unavoidable during lignin isolation. Therefore, the chemical
structure and thermal behavior of lignin is strongly dependent
on the isolation method used. Isolated lignin displays both
thermoplastic and thermosetting behavior.5 The thermal
properties of lignin are usually explained in association with
its chemical structure, i.e., molecular weight,6 degree of
condensation,7 and chemical modification during the prepa* To whom correspondence should be addressed. Phone: (604) 8575254. Fax: (604) 822-9104. E-mail: [email protected].
ration process.8,9 However, noncovalent interactions, such
as hydrogen bonding, will also affect the thermal properties
of lignin; strong interactions will reduce the thermal molecular motion of the lignin molecules.
A powerful tool to detect specific intermolecular interactions in polymers is Fourier transform infrared (FTIR)
spectroscopy. Observed changes in hydroxyl, carbonyl, and
ether vibrations provide direct evidence of specific interactions between components.10,11 The magnitude of the shift
in wavenumber (∆ν) arising due to the composition and
chemical structure yields a measure of the average strength
of the intermolecular interactions.12 However, due to the
complex structure and the amorphous nature of the lignin
macromolecule, it is difficult to study such interactions.
Therefore, in this study, lignin model compounds are used
to probe the complex hydrogen-bonding system in lignin.
FTIR spectroscopy is utilized to examine the hydrogen bonds
formed between the model compounds, and the results
obtained are compared to those from lignin.
Materials and Methods
Materials. Hardwood (HKL) and softwood (SKL) Kraft
lignins were commercially produced and obtained from
Westvaco Corp. The lignin was repeatedly washed with
dilute HCl to exchange sodium counterions and then vacuumdried over P2O5. Both lignins were recovered as a fine
powder after drying and were stored over desiccant prior to
analysis. Five lignin model compounds, four monomeric,
3-(3,4-dimethoxyphenyl)-1-propanol (I), 1-(3,4-dimethoxyphenyl)ethanol (II), 2-methoxyphenol (III), and 2,6dimethoxyphenol (IV), and one dimeric, 3,3′-dimethoxy-5,5′dimethyl-[1,1′-biphenyl]-2,2′-diol (V), were used in this
study. The model compounds are shown in Figure 1 and
represent γ-OH (I), R-OH (II), phenolic guaiacyl (III),
phenolic syringyl (IV), and 5,5′-biphenol (V) substructures
10.1021/bm050288q CCC: $30.25
© 2005 American Chemical Society
Published on Web 06/30/2005
2816
Biomacromolecules, Vol. 6, No. 5, 2005
Figure 1. Chemical structure of lignin model compounds.
found in lignin.1-4 Model compounds I, III, and IV, 3,4dimethoxyacetophenone, and 2-methoxy-4-hydroxyphenol
were purchased from Aldrich Chemicals and recrystallized
from benzene/petroleum ether (6:4) prior to use. For all
compounds purity was checked by GLC. All compounds
were stored over desiccant.
1-(3,4-Dimethoxyphenyl)ethanol (II) was prepared by
reacting 2 equiv of NaBH4 (0.42 g, 11.0 mmol) with 3,4dimethoxyacetophenone (1.0 g, 5.5 mmol) in 3:1 EtOH/H2O
(50 mL) and heating the reaction mixture under reflux for 3
h. The reaction mixture was then cooled, neutralized by
bubbling CO2 through the supernatant solution, and extracted
with 1,2-dichloroethane (3 × 50 mL). A quantitative conversion of the acetophenone was obtained. MS: m/z (rel intens)
182 (M+, 59), 167 (87), 153 (47), 139 (100), 124 (32), 108
(21), 93 (50), 77 (21), 65 (25), 51 (11), 43 (41). 1H NMR:
δ (ppm) 1.48 (d, 3H), 3.90 (d, 6H), 4.83 (q, 1H), 6.84 (q,
1H), 6.86 (q, 1H), 6.93 (d, 1H).
3,3′-Dimethoxy-5,5′-dimethyl-[1,1′-biphenyl]-2,2′-diol (V)
was prepared by reacting a solution of 0.25 g (2.0 mmol) of
III in a mixture of 5.0 mL of EtOH and 1.0 mL of 10%
aqueous NaOH at 5 °C with a solution of 0.82 g (2.5 mmol)
of K3Fe(CN)6 in 5 mL of water followed by 5 mL of EtOH
and 1.0 mL of 10% aqueous NaOH to facilitate stirring. After
2.0 h, the reaction mixture was diluted with 50 mL of
saturated NH4Cl, adjusted to pH 5.5, and extracted with
EtOAc, and the dried (MgSO4) extract evaporated in vacuo
to leave 0.21 g of crude product. Recrystallization from
benzene-petroleum ether afforded colorless crystals. Tm )
125 °C (DSC). 1H NMR: δ (ppm) 2.30 (s, 6H), 3.84 (s, 6H),
6.04 (s, D2O-exchangeable), 6.68 (s, 2H), 6.72 (s, 2H).
Lignin Characterization. The methoxyl content of the
lignin samples was determined according to the modified
procedure of Viebock and Schwappach.13 Aliphatic and
aromatic hydroxyl contents were determined using 1H NMR.
The NMR spectra were recorded on a Bruker AVANCE 300
MHz spectrometer at 300 K using CDCl3 as the solvent.
Chemical shifts were referenced to TMS (0.0 ppm). The
quantitative 1H NMR spectrum of acetylated lignins was
recorded at a lignin concentration of ∼2% in CDCl3, a 90°
pulse width, and a 1.3 s acquisition time. A relatively high
relaxation delay of 7 s was applied to ensure complete
relaxation of aldehyde protons. A total of 128 scans were
collected. Quantification was obtained from the integration
Kubo and Kadla
ratios of aliphatic and aromatic acetoxy protons of acetylated
lignin preparations relative to the internal standard pnitrobenzaldehyde.
The lignin preparations were acetylated by dissolving 200
mg of the lignin in 10 mL of pyridine/acetic anhydride (1:1,
v/v) and reacted for 48 h at room temperature. The solution
was poured over crushed ice and filtered. The resulting
precipitate was then washed with cold water/HCl, dried, and
subjected to a second acetylation treatment.
The average molecular mass and molecular mass distribution of the lignin samples were determined by GPC (Waters
Associates, UV and RI detectors) using styragel columns at
50 °C and DMF/LiCl (0.1 mol) as the eluting solvent. The
GPC system was calibrated by using standard polystyrene
samples with molecular weights ranging between 580 and
1800K. The UV detector was employed at wavenumbers of
280 and 270 nm for the lignin and polystyrene standards,
respectively. The injection volume was 100 µL, and the lignin
concentration was 1 mg mL-1 DMF/LiCl (0.1 mol).
DSC analysis was performed using a TA Instruments
model Q1000 with a scan rate of 20 °C/min over the
temperature range of -90 to +200 °C. The measurements
were made using 4.5 mg samples under a nitrogen atmosphere. The glass transition temperature (Tg) was recorded
as the midpoint temperature of the heat capacity transition
of the second heating run. Samples were run in duplicate,
are reported as the average of the two runs, and were within
experimental error of each other ((1.0 °C).
FTIR analysis of the model compounds in the solution
state was performed in CCl4 (dried over 4 Å activated
molecular sieves before use). An adjustable liquid cell with
ZnSe windows and 1.0 and 0.015 mm Teflon spacers were
used to obtain the two experimental concentrations of 0.01
and 1 mol (as a OH group) L-1, respectively. FTIR spectra
of solid samples were collected without any dilution.
Individual model compounds and mixtures were spread
between the ZnSe plates and heated under a nitrogen
atmosphere. A total of 16-32 scans were acquired at a
spectral resolution of between 2.0 and 4.0 cm-1 on a PerkinElmer 16PG FTIR spectrometer using a dry nitrogen purge.
For the Kraft lignin samples FTIR spectra were collected as
KBr pellets at a lignin concentration of 1 wt %. To ensure
sufficient spectral quality (signal-to-noise, baseline) for
subsequent deconvolution, 256 scans were acquired at a
spectral resolution of 4.0 cm-1.14 Care was taken to ensure
all samples remained dry during sample preparation and
FTIR analysis.
The recorded spectra were analyzed using Peak Fit
software (SPSS Inc., Chicago, IL). Deconvolution was
performed using the Lorentzian or Gaussian peak shape and
a full width at half-maximum (fwhm; cm-1) of 20-40
cm-1scompared to ensure good resolution of peaks without
overfitting. Prior to deconvolution the second-derivative
spectra were analyzed to determine the number of hidden
peaks. All data were analyzed and compared on the basis of
peak areas. Selection of peaks and calculations of peak areas
as a measure of spectral intensity were performed by
maximum likelihood peak fitting, and all data were fit with
r2 values of greater than 0.995.
Biomacromolecules, Vol. 6, No. 5, 2005 2817
Hydrogen Bonding in Lignin
Figure 2. DSC analysis of HKL and SKL (second heating run at a
scan rate of 20 °C/min using 4.5 mg samples under a nitrogen
atmosphere).
Table 1. Chemical Properties of Lignin
functional group concn (mmol g-1)
hydroxyl groups
molecular weight
lignin
aliphatic
phenolic
methoxyl
S G-1
Mw
dispersity
HKL
SKL
4.1
5.6
4.3
3.8
5.9
4.2
1.2
28000
36000
2.8
2.3
Results and Discussion
Lignin Characterization. Thermomechanical analysis of
isolated lignins has shown that the thermal transition temperature of softwood lignin is higher than that of hardwood
lignin.7 DSC analysis (Figure 2) of the HKL and SKL shows
that the glass transition temperature (Tg) of the HKL (93
°C) is lower than that of the SKL (119 °C).
Lignin contains various types of functional groups depending on the wood species and isolation procedure. Some
isolated lignin’s prepared under acid conditions have relatively large amounts of carbonyl groups.15 In the case of Kraft
lignin, sulfur is introduced into the original lignin structure.16
However, compared with those of hydroxyl and methoxyl
groups, the amount of carbonyl or sulfur groups in Kraft
lignin is relatively low.17 To study the effect of hydrogen
bond formation on the thermal properties of lignin, functional
group analysis of the HKL and SKL were performed. The
properties of the HKL and SKL are listed in Table 1. The
total hydroxyl group content of HKL is ∼10% lower than
that of SKL. However, the phenolic hydroxyl group content
of HKL is higher than that of SKL, but SKL contains a larger
amount of aliphatic hydroxyl groups. As expected, methoxyl
group contents are higher in HKL compared to SKL.3
Softwood lignin, often referred to as guaiacyl lignin is
primarily comprised of coniferyl alcohol units, which make
up more than 95% of the structural units in the lignin, with
the remainder consisting mainly of p-coumaryl alcohol-type
units. Hardwood lignin is composed of coniferyl alcoholand sinapyl alcohol-derived units in varying proportions
(commonly referred to as guaiacyl-syringyl lignin). Figure
3 shows the FTIR spectra of softwood and hardwood Kraft
lignin. The peak positions of the major IR bands and their
relative absorbance (the intensity of the highest absorbance
peak normalized to unity) are summarized in Table 2.
Infrared spectroscopy has been proven to be a highly
effective means of investigating specific interactions within
and between molecules.18,19 FTIR can be used to qualitatively
and quantitatively study the mechanism of intermolecular
Figure 3. FTIR analysis of hardwood (HKL) and softwood (SKL).
interaction through hydrogen bonding. Spectral differences
between the hardwood and softwood Kraft lignin are
observed in the fingerprint region (1800 and 900 cm-1).20
In the SKL, the bands at 1513 cm-1 (aromatic skeletal
vibration) and 1269 cm-1 (guaiacyl ring breathing with
carbonyl stretching) dominate, whereas the bands at 1514,
1215, and 1116 cm-1 dominate in the HKL (Figure 3). The
intensity of bands at 1603 cm-1 (aromatic skeletal vibration
breathing with CdO stretching) and 1461 cm-1 (C-H
methyl and methylene deformation) is lower than that of the
1513 cm-1 band in softwood lignin. The intensity of the 1113
cm-1 band (C-H in-plane deformation of the syringyl unit)
is much higher than that of the 1030 cm-1 band (C-H
deformation in guaiacyl with C-O deformation in the
primary alcohol) in hardwood lignin, whereas the intensity
of the band at 1030 cm-1 is equal to or greater than that of
the 1113 cm-1 band in softwood lignin. In softwood lignin,
the 1269 cm-1 band is more intense than the 1214 cm-1 band
and has no syringyl absorption at 1327 cm-1, whereas the
opposite is true for hardwood lignins, that is, a weak 1269
cm-1 band, a strong band at 1215 cm-1, and a syringyl
absorption at around 1327 cm-1. The presence of a syringyl
unit in hardwood lignin is also evident from the higher
intensity of the band at 1462 cm-1.
In addition to the fingerprint region, apparent differences
exist between the SKL and HKL band envelopes associated
with the hydroxyl stretching region (νOH ≈ 3700-3000
cm-1). A clear difference in the band envelope shape can
be seen in Figure 3. The SKL hydroxyl stretching region is
broader, indicating a more complex hydrogen-bonding
system. In both lignins a broad band center and a shoulder
are observed. The broad band center at ∼3350 cm-1 (SKL)
and ∼3420 cm-1 (HKL) and the shoulder at ∼3500 cm-1
can be assigned to the average stretching of intermolecular
hydrogen bonding.9
Figure 4 shows the deconvoluted hydroxyl stretching
region of the SKL and HKL. From analysis of the secondderivative spectra eight peaks were detected for both lignins.
Application of Gaussian peak shape assumptions was chosen
as it yielded fewer bands, 7-8, as compared to Lorentzian
assumptions, which yielded 12-14, potentially overfitting
the data. Increasing the fwhm values from 20 to 30 cm-1
2818
Biomacromolecules, Vol. 6, No. 5, 2005
Kubo and Kadla
Figure 4. Deconvoluted FTIR spectra of the νOH region of Kraft lignin.
Table 2. Summary of the IR Bands Observed in Hardwood (HKL) and Softwood (SKL) Kraft Lignin14
hardwood
no.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
softwood
band position
(cm-1)
absorbance
3421
2937
2840
1682
1603
1514
1462
1425
1327
1269
1215
1151
1116
50
36
21
19
60
97
75
56
55
65
95
54
100
1033
36
band position
(cm-1)
absorbance
assignment
3349
2934
2840
1704
1594
1513
1463
1427
42
41
29
23
46
98
63
54
1269
1214
1150
100
83
61
1081
1031
52
47
O-H stretching
C-H stretching
C-H stretching
CdO stretching (unconjugated)
aromatic skeletal vibration + CdO stretching
aromatic skeletal vibration
C-H deformation (methyl and methylene)
C-H in-plane deformation with aromatic ring stretching
C-O of the syringyl ring
C-O of the guaiacyl ring
C-C + C-O stretch
aromatic C-H in-plane deformation in the guaiacyl ring
aromatic C-H deformation in the syringyl ring
C-O deformations of secondary alcohols and aliphatic ethers
aromatic C-H in-plane deformation (G > S)
Table 3. Summary of the FTIR (νOH Region) Band Centers (cm-1) for the Lignin Model Compounds Measured in Carbon Tetrachloride
L-1
0.01 mol
1 mol L-1
I
II
III
IV
V
phenol
3639
3510 (d)
3406 (m)
3616
broad (d)
3422 (m)
3558
3556 (i)
shoulder
3554
3555 (i)
shoulder
3552
3611
3610 (free)
3356 (m)
did not affect the number of bands; however, at fwhm values
of 35 and 40 cm-1 the number of bands dropped dramatically
to only three to four. The Gaussian (fwhm of 25 cm-1) assumptions gave bands for the lignins which correspond well
with those of lignin model compounds (discussed below).
The number of bands and the band centers of the HKL
and SKL do not vary significantly. However, a change in
the relative intensity of several bands is evident. This is
particularly apparent for the bands at ∼3612 and ∼3192
cm-1. In the SKL the band at ∼3192 cm-1 is more than
double the intensity of that in the HKL, whereas the band at
∼3612 cm-1 is substantially lower in intensity. To further
understand the complex hydrogen-bonding system present
in both the SKL and HKL, FTIR analysis of various lignin
model compounds representing the various hydroxyl groups
present in lignin was performed.
FTIR Analysis of Lignin Model Compounds: Solution
State. In diluted solution the extent of intermolecular
interaction between compounds should be substantially
reduced if not eliminated. Depending on the molecular
structure of the hydroxylated compound, the hydroxyl
stretching band should be sharp and appear at a relatively
high wavenumber, ∼3590-3650 cm-1. As illustrated in
Figure 5A and listed in Table 3, all of the model compounds
display only one narrow hydroxyl stretching (νOH) band at
high wavenumber in dilute dry CCl4 (0.01 mol L-1). The
νOH bands of III, IV, and V appeared at a lower wavenumber
as compared to those of I and II. Included in Figure 5A
(broken line) is the νOH band of phenol. It can be seen that
the νOH band appears at a wavenumber comparable to that
of the benzylic hydroxyl group, 3611 cm-1. It appears that
methoxylation of the phenol leads to a shift in the wavenumber of the phenolic hydroxyl group. This can be the result
of a change in the bond dissociation energy of the phenolic
hydroxyl group due to the addition of the methoxyl group
in an ortho position or as a result of intramolecular hydrogen
bonding between the hydrogen atom of the hydroxyl group
and the oxygen atom of the adjacent methoxyl substituent.
Hydrogen Bonding in Lignin
Biomacromolecules, Vol. 6, No. 5, 2005 2819
Figure 5. FTIR spectra of the νOH region of lignin model compounds
measured by the liquid film method. The Roman numerals refer to
the compounds in Figure 1.
The pKa value of phenol (∼10.0) is only slightly lower than
that of methoxylated phenol (∼10.2).21 Further, all of the
phenolic compounds in this study, regardless of the substitution pattern, show a single νOH band at ∼3555 cm-1 that is
slightly broader than that of phenol. This combined with the
fact that an intramolecular hydrogen bond would exist as a
five-membered ring supports the conclusion that intramolecular hydrogen bonds are formed in these phenolic model
compounds even in diluted solutions.
With increasing concentration the νOH band will become
broader as a result of the formation of a variety of
intermolecular hydrogen bonds. At 1 mol L-1, the aliphatic
hydroxyl group compounds I and II show a dramatic change
in the νOH band envelope, two new broad band centers at
∼3510 and ∼3422-3406 cm-1 with a trace of the free
hydroxyl groups (Figure 5B). By contrast, the main νOH band
for the phenolic model compounds III and IV remains at
the same position as that in diluted conditions, despite the
fact that for phenol a large broad multimer band is observed
at ∼3356 cm-1. These results suggest that the intramolecular
hydrogen bonding between the phenolic hydroxyl and
methoxyl groups hinders the formation of intermolecular
hydrogen bonds and that aliphatic hydroxyl groups in lignin
are more likely to form intermolecular hydrogen bonds than
phenolic hydroxyl groups.
FTIR Analysis of Lignin Model Compounds: Solid
State. To represent the hydrogen bonding in lignin more
accurately, FTIR spectra of the model compounds (neat) were
collected (Figure 6 and Table 4). It can be seen that the band
profiles of the aliphatic model compounds I and II are very
similar to those measured for the 1.0 mol L-1 solutions (I
and II in Figure 5B). One noticeable difference is the absence
of the free νOH band (>3600 cm-1). Similarly, the band
centers assigned to dimers and multimers (∼3506, 3421, and
3350 cm-1) are slightly shifted to the lower wavenumber
Figure 6. FTIR spectra of the νOH region of lignin model compounds
(A) and binary mixtures (B) measured in the solid phase.
region, ∆νOH ≈ 20 cm-1. The νOH band center of the
multimer for I appears at a lower wavenumber position than
that of II, and the band profile is broader than that of II.
This indicates that stronger and a wider variety of hydrogen
bonds are formed in I. The νOH band for the monomethoxysubstituted phenol III is broader than that observed in
solution. The dimer complex band center appears at roughly
the same wavenumber position as that of the aliphatic model
compounds I and II (Table 4). However, the multimer band
is located approximately 28-54 cm-1 higher at 3444 cm-1.
The dimethoxy-substituted phenol IV shows two sharp bands
(dimer and multimer) at 3490 and 3458 cm-1, respectively.
Multimer formation in IV will be sterically restricted due to
the bulky methoxyl groups that flank the phenolic hydroxyl
group. However, the lower wavenumber position of the dimer
complex might indicate that IV has a potential to form
stronger intermolecular hydrogen bonding than III. Further,
these observations indicate that aliphatic hydroxyl groups
in lignin have the potential to form stronger intermolecular
hydrogen bonds than phenolic hydroxyl groups. As shown
in Table 1, SKL contains a higher amount of aliphatic
hydroxyl groups than HKL. Therefore, hydrogen bonding
between aliphatic hydroxyl groups will be greater in SKL
than HKL and may be a contributing factor to the decreased
thermal segmental motion and higher Tg of the SKL.
Owing to the poor solubility in CCl4 of the lignin model
dimer V, FTIR analysis could not be conducted at high
concentration (1.0 mol L-1). Therefore, analysis of the
intermolecular hydrogen bonding was examined in the solid
state. Figure 6A includes the FTIR spectra of the νOH region
2820
Biomacromolecules, Vol. 6, No. 5, 2005
Kubo and Kadla
Table 4. Summary of the FTIR (νOH Region) Band Centers (cm-1) of the Lignin Model Compound Mixtures
I
II
III
IV
V
phenol
I
II
III
IV
3500 (d)
3390 (m)
3494 (d)
3406 (m)
3491 (d)
shoulder (m)
3505 (d)
shoulder (m)
3498 (d)
3238 (m)
3484 (d)
3416 (m)
shoulder (d)
3444 (m)
3499 (d)
shoulder (m)
3484 (d)
3242 (m)
3508 (d)
3444 (m)
3500 (d)
3454 (m)
3515 (d)
3191 (m)
3445 (d)
shoulder (m)
3448 (d)
shoulder (m)
shoulder (m)
3407 (m)
3490 (d)
3458 (m)
3502 (d)
3446 (m)
3165 (m)
shoulder (m)
3444 (m)
V
phenol
3219 (m)
3200 (m)
3226 (m)
Table 5. Assignment of the FTIR (νOH Region) Bands of the Lignin Model Compound Blends
wavenumber at
band center (cm-1)
>3600 (3639-3616)
∼3550 (3558-3552)
∼3500 (3515-3484)
∼3450 (3458-3444)
∼3400 (3416-3390)
∼3240 (3242-3238)
3219
∼<3200 (3191-3165)
hydrogen bond
free hydroxyl groups in an alcoholic group
intramolecular hydrogen bond in a phenolic group
dimeric formation of an intermolecular hydrogen bond
multiple formation of an intermolecular hydrogen bond between
phenolic groups and their combinations with alcoholic groups
multiple formation of an intermolecular hydrogen bond between
alcoholic groups
multiple formation of an intermolecular hydrogen bond between
biphenols and their combinations with alcoholic groups
multiple formation of an intermolecular hydrogen bond in
biphenols
multiple formation of an intermolecular hydrogen bond between
biphenol and other phenolic groups
of V. A broad band envelope is observed at a significantly
lower wavenumber region than that of any of the other model
compounds. As describe above, a single sharp band was
observed at ∼3552 cm-1 and was assigned to intramolecular
hydrogen bonding on the basis of the wavemunber position
relative to those of the other phenolic model compounds
(Figure 5A). These results indicate that the biphenyl-type
phenolic hydroxyl groups form stronger hydrogen bonds than
the other hydroxyl groups examined in this study. Although
the proportion of such moieties in lignin is difficult to
accurately determine, softwood lignins contain a substantial
amount of these units.4 Therefore, these units can be expected
to be a substantial contributor to the poor thermal mobility7
and low thermal processability of SKL.22
It is clear from Figure 4 that the νOH region of both the
HKL and SKL is quite complex, involving extensive
hydrogen bonding. To further interpret the deconvolution
data, FTIR analysis was performed on mixtures of the various
model compounds. The νOH band profiles of the various
mixed samples (a total of 10 combinations) are shown in
Figure 6B and can be broadly classified into three groups,
those involving (i) only aliphatic hydroxyl groups, (ii)
aliphatic and phenolic hydroxyl groups, and (iii) biphenol
groups. The νOH band of the mixture of I and II is very
similar to that of the spectra obtained from pure I or II
(Figure 6A). Broad band centers at 3494 and 3406 cm-1
appear between those observed for I and II, 3500/3484 and
3390/3416 cm-1 (Table 4), respectively. For the aliphatic-
phenolic mixtures (I + III, I + IV, II + III, and II + IV)
the band envelopes are less broad than that observed for the
I + II mixture, consistent with the restricted hydrogen
bonding of the phenolic compounds. The broad band centers
or shoulders representing dimers appeared at almost the same
wavenumber position as for the I + II mixture. However,
the shoulder or broad band center associated with multimer
formation appears at a higher wavenumber position than in
the I + II mixture. The intermolecular hydrogen bonds
formed between the aliphatic and phenolic hydroxyl groups
are weaker than those formed between aliphatic hydroxyl
groups. As expected, the band envelope of the phenolic
model mixture III + IV is less broad and there is no
significant difference in band position as compared to the
other mixtures in this group. This indicates that the hydrogen
bond strengths between phenolic hydroxyl groups and with
aliphatic hydroxyl groups are essentially the same, although
the distribution (relative intensity) of hydrogen bonding
differs slightly between compounds.
The mixtures involving biphenol V (I + V, II + V, III
+ V, IV + V) show broad band centers at two different
wavenumber regions (Figure 6B). A weak band center is
observed at high wavenumber, consistent with the dimeric
region assigned to the other model monomers (see Table 4).
The exception is the IV + V blend in which the higher
wavenumber band is quite strong, clearly showing two
distinct band centers at 3502 and 3446 cm-1. The lower
wavenumber band position varies depending on the mono-
Hydrogen Bonding in Lignin
meric model compound, and the band profile is distinctly
different from that of V (Figure 6A). The broad band center
of V is at ∼3219 cm-1 and is shifted to lower and higher
wavenumber regions by blending with phenolic, δνOH ) -54
and -19 cm-1 for IV + V and III + V, respectively, and
aliphatic hydroxyl, δνOH ) 19 and 23 cm-1 for I + V and
II + V, respectively, model compounds. The band center
for biphenol + phenol blends appears at 3200 cm-1, δνOH
) -19 (data not shown). Therefore, it seems that the second
methoxyl group plays a key roll in the formation of multiple
“strong” hydrogen bonds with V, despite the observed effect
with other monomeric aliphatic hydroxyl and phenolic
compounds. Further, stronger hydrogen bonds form in the
phenolic + biphenol blends than the aliphatic hydroxyl +
biphenol blends, which is in contrast to the results obtained
for the phenolic + aliphatic hydroxyl and aliphatic +
aliphatic hydroxyl blends.
Conclusions
Hydrogen bond formation in lignin was examined using
lignin model compounds. In dilute solution non-hydrogenbonded free hydroxyl groups were only observed in the
model compounds possessing aliphatic hydroxyl groups. The
compounds with phenolic hydroxyl groups show intramolecular hydrogen bonding with the adjacent methoxyl group.
Deconvolution of the νOH band profile of the kraft lignins
shows the presence of these bands (3558-3652 cm-1), the
extent of which is greater in the HKL than the SKL. The
formation of dimeric intermolecular hydrogen-bonded complexes (3515-3484 cm-1) was observed for all of the lignin
model monomeric compounds and binary mixtures. Further,
both the HKL and SKL show a predominate peak at ∼3610
cm-1. Similarly, the HKL and SKL show prominent peaks
over the range of 3450-3330 cm-1. This is consistent with
the model compound data and represents strong multiple
intermolecular hydrogen bonding, wherein the aliphatic
hydroxyl groups appear to form stronger and more extensive
multiple-hydrogen-bonding complexes as compared to the
phenolic groups. In fact, the relative intensity of these bands
in the SKL is greater than that of the bands in the HKL,
consistent with the higher aliphatic hydroxyl group content
of the softwood lignin.
Compared to that of the monomeric model compounds,
the νOH region of the biphenol model compound is quite
different, showing a single broad band at low wavenumber
indicating the existence of strong intermolecular hydrogen
bonding. Binary mixtures with the other model compounds
show two broad bands, classified into three groups representing intermolecular hydrogen bonding between biphenyl and
aliphatic hydroxyl group compounds (3242-3238 cm-1),
between biphenol compounds (∼3219 cm-1), and between
biphenol and phenolic hydroxyl group compounds (31913165 cm-1), respectively. Both the SKL and HKL show
Biomacromolecules, Vol. 6, No. 5, 2005 2821
deconvoluted peaks within these regions; as expected the
SKL shows a larger amount of these peaks. As lignins have
highly complex three-dimensional network structures, unrestricted intermolecular hydrogen bond formation will likely
not occur. This may account for some of the differences
observed between the model compounds and the two lignins,
although fairly good agreement was obtained. It is clear that
hydrogen bonding, in particular intermolecular hydrogen
bonding between biphenol and phenolic moieties, will have
a significant contribution to the thermal mobility of lignin,
and contribute to the higher Tg observed for softwood lignins
as compared to hardwood lignins.
Acknowledgment. Financial support from the USDA
(Award Number 2001-52104-11224), the Canada Research
Chair Program, and the Canadian Foundation for Innovation
is gratefully acknowledged.
Supporting Information Available. Tables of enthalpies
of hydrogen bond formation. This material is available free
of charge via the Internet at http://pubs.acs.org.
References and Notes
(1) Sarkanen, K. V. In Lignins: occurrence, formation, structure and
reactions; Ludwig, C. H., Ed.; Wiley-Interscience: New York, 1971;
pp 95-163.
(2) Ralph, J.; Hatfield, R. D.; Piquemal, J.; Yahiaoui, N.; Pean, M.;
Lapierre, C.; Boudet, A. M. Proc. Natl. Acad. Sci. U.S.A. 1998, 95,
12803-12808.
(3) Lin, S. Y.; Dence, C. W. Methods in lignin chemistry; SpringerVerlag: Berlin, New York, 1992.
(4) Capanema, E. A.; Balakshin, M. Y.; Kadla, J. F. J. Agric. Food Chem.
2004, 52, 1850-1860.
(5) Uraki, Y.; Kubo, S.; Nigo, N.; Sano, Y.; Sasaya, T. Holzforschung
1995, 49, 343-350.
(6) Hatakeyama, H.; Iwashita, K.; Meshitsuka, G.; Nakano, J. Mokuzai
Gakkaishi 1975, 21, 618-623.
(7) Kubo, S.; Uraki, Y.; Sano, Y. Holzforschung 1996, 50, 144-150.
(8) Glasser, W. G. In Lignin: Historical, Biological, and Materials
PerspectiVes; American Chemical Society: Washington, DC, 2000;
Vol. 742, pp 216-238.
(9) Kadla, J. F.; Kubo, S. Macromolecules 2003, 36, 7803-7811.
(10) Kuo, S. W.; Chang, F. C. Macromolecules 2001, 34, 4089-4097.
(11) Wang, J.; Cheung, M. K.; Mi, Y. L. Polymer 2002, 43, 1357-1364.
(12) Purcell, K. F.; Drago, R. S. J. Am. Chem. Soc. 1967, 89, 28742880.
(13) Chen, C.-L. In Methods in lignin chemistry; Dence, C. W., Ed.;
Springer-Verlag: Berlin, New York, 1992; pp 465-472.
(14) Faix, O. In Methods in lignin chemistry; Dence, C. W., Ed.; SpringerVerlag: Berlin, New York, 1992; pp 83-109.
(15) Kubo, S.; Kadla, J. F. Macromolecules 2004, 37, 6904-6911.
(16) Gierer, J.; Smedman, L. A. Acta Chem. Scand. 1965, 19, 11031112.
(17) Kadla, J. F.; Chang, H.-M.; Jameel, H. Holzforschung 1999, 53, 277284.
(18) Joesten, M. D.; Drago, R. S. J. Am. Chem. Soc. 1962, 84, 38173821.
(19) Drago, R. S.; Vogel, G. C. J. Am. Chem. Soc. 1992, 114, 95279532.
(20) Faix, O.; Beinhoff, O. J. Wood Chem. Technol. 1988, 8, 505-522.
(21) Citra, M. J. Chemosphere 1998, 38, 191-206.
(22) Kadla, J. F.; Kubo, S.; Venditti, R. A.; Gilbert, R. D.; Compere, A.
L.; Griffith, W. Carbon 2002, 40, 2913-2920.
BM050288Q