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Industrial Crops and Products 26 (2007) 116124
Analytical methods for determining functionalgroups in various technical lignins
Nour-Eddine El Mansouri 1, Joan Salvado
Rovira i Virgili University, Department of Chemical Engineering, Avinguda dels Pasos Catalans 26,
43007 Tarragona (Catalunya), Spain
Received 8 March 2006; accepted 7 February 2007
Abstract
In this paper we compare various analytical methods for determining functional groupsin technical lignins of five differentorigins:
kraft, sulfite, soda/anthraquinone, organosolv and ethanol process lignins. These lignins were characterized in terms of methoxyl,
phenolic and aliphatic hydroxyl, carbonyl, carboxyl and sulfonate groups. The analytical methods used were: gas chromatography,
aminolysis, UV-spectroscopy, 1Hand 13C NMR spectroscopy, the oximating method, FTIRspectroscopy, acid number determination,
and non-aqueous and aqueous potentiometry.
The statistical comparison of the various analytical methods for hydroxyl groups (phenolic and aliphatic) shows that the results
obtained are not fully comparable. Aminolysis and non-aqueous potentiometry are assumed to be the most reliable for phenolic
hydroxyl. We observed the same trend for the methods for carbonyl groups and selected the oximating method as reliable for
determining total carbonyl. The results for the methods used for carboxylic groups showed correspondence at a significance level of
0.05. We selected aqueous and non-aqueous titration as being reliable for the lignins studied. We also compare all the commercial
lignins in terms of functional groups.
Finally, by completely characterizing the functional groups of various technical lignins, we have established the most complete
representative expanded formula C9for each lignin under study.
2007 Elsevier B.V. All rights reserved.
Keywords: Characterization; Technical lignins; Analytical methods; Functional groups; Expanded molecular formula C9
1. Introduction
With the exception of cellulose, no other renewable
natural resource is more abundant than lignin. Lignin is a
highly-branched, three dimensional polymer with a wide
variety of functional groups providing active centers
for chemical and biological interactions. In wood, the
Corresponding author. Tel.: +34 977 559 641;
fax: +34 977 558 544.
E-mail addresses: [email protected](N.-E. El Mansouri),
[email protected] (J. Salvado).1 Tel.: +34 977 558 656; fax: +34 977 558 544.
lignin content generally ranges from 19 to 35% (Dence
and Lin, 1992).It is extracted by several pulping tech-
niques and ethanol production process as a by-product
available inexpensively in large quantities. Technical
lignins are divided into two categories (Gosselink et
al., 2004b). The first one comprises sulfur-containing
commercial lignins, including lignosulfonates and kraft
lignins, which are produced in huge quantities. The
second one compriseslignins without sulfurin their com-
position, such as organosolv, soda/anthraquinone lignin
and lignin from the ethanol process production.
The potential of lignins is clearly not valued because
almost all are burned to generate energy and recover
0926-6690/$ see front matter 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.indcrop.2007.02.006
mailto:[email protected]:[email protected]://localhost/var/www/apps/conversion/tmp/scratch_4/dx.doi.org/10.1016/j.indcrop.2007.02.006http://localhost/var/www/apps/conversion/tmp/scratch_4/dx.doi.org/10.1016/j.indcrop.2007.02.006mailto:[email protected]:[email protected] -
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N.-E. El Mansouri, J. Salvad o / Industrial Crops and Products 26 (2007) 116124 117
chemicals. Only a limited quantity has been used for
applications such as biomaterials, fuels, biocides and
biostabilisers, animal feed, health products and crops
cultivations (Lora and Glasser, 2002).However, indus-
trial applications are only possible if lignins added value
is enhanced, which is only possible if industrial and
scientific research can be intensified to find better appli-cations. Current research faces several problems that
could be avoided. These problems are the low purity, het-
erogeneity, odour, colour of lignin-based products and
the absence of reliable analytical methods (Gosselink
et al., 2004a). Thus, the availability of the analytical
methods for chemical and physical properties adopted
by both suppliers and users can allow any laboratory
to reproduce the results and analyze any various exist-
ing types of lignins. Using these methods lignin can be
properly characterized and its behavior with regard to
several potential uses can be determined (Gosselink etal., 2004c).
Several studies have established new methods or
compared existing methods to characterize lignins
(Gosselink et al., 2004a; Milne et al., 1992; Faix et al.,
1998).Much interest has focused on functional groups
analyses. The main chemical functional groups in lignin
are the hydroxyl, methoxyl, carbonyl and carboxylic
groups. The proportion of these groups depends on the
genetic origin and isolation processes applied. Func-
tional group analysis can be used to determine the lignin
structure. However, the increasing interest in using ana-lytical methods to determine the functional groups is
mainly due to the following reasons: (i) the appearance
of new technical lignin generated from new and more
environmentally friendly cellulose-production methods.
To understand the reaction mechanisms during delig-
nification and to predict and develop different uses for
byproducts of the pulping process, we therefore need to
study their functional properties; (ii) lignin is currently
of interest to the specialist in various fields of science
and industry searching for new practical applications.
Functional group analysis is therefore an indispensable
research method. The only way to achieve these aims isto compare the various analytical methods.
In this paper, we review the main analytical methods
in the field of lignin chemistry, especially for functional
groups analysis, and select 11 analytical methods. We
selected five technical lignins for the structural charac-
terization, focusing on different functional groups, with
these analytical methods. These characterization per-
mit a critical comparison between these methods, and
choose the most adequate in each case, and the compar-
ison between these lignins in term of functional groups.
Finally, we established the most representative formula
C9, which contains the important information about the
structure of each lignin.
2. Materials and methods
2.1. Raw materials
Kraft lignin (KL) and lignosulfonate (LS) derived
from softwood were purchased from Ligno-Tech Iber-
ica. Soda/anthraquinone lignin (SAL) from a mixture of
long fiber plants, was supplied by CELESA Celulosa de
Levante S.A. of Tortosa, Catalonia, Spain. Organosolv
lignin (ORS) obtained fromMiscanthus sinensis, was of
the formasolv lignin type, which was supplied by the
University of Santiago de Compostela (Galicia-Spain).
Ethanol process lignin (EPL) was supplied by CIEMAT
(Centro de Investigacion Energeticas, Medioambientales
y Tecnologicas) of Madrid, Spain, from Populus woodpretreated by steam explosion and the simultaneous sac-
charification and fermentation process (SSF).
These lignins were purified and analyzed for chem-
ical composition in a previous study (El Mansouri and
Salvado, 2006).The characteristics of these lignins are:
total lignin content of over 94% (except lignosulfonate)
and a sugar content of close to 2% (except ethanol
process lignin). All lignins were air-dried at room tem-
perature to equilibrium moisture content and stored in
plastic bottles for characterization. The technical lignins
were analyzed in this study by the following methods.
2.2. Analytical methods
2.2.1. Elemental analysis
Carbon, hydrogen, sulfur and nitrogen contents were
determined using a Perkin Elmer 640-C Analyzer. After
correction for ash content, the percentage of oxygen was
calculated by difference.
2.2.2. FTIR spectroscopy for unacetylated lignins
The FTIR spectra of the unacetylated lignin samples
embedded in KBr disk were obtained with a BRUKERspectrometer using a resolution of 4 cm1 and 32 co
addition scans in a frequency range of 4004600 cm1.
The spectra were analyzed by Nicolet software to com-
pare the absorbance corresponding to each functional
group. The absorption bands were assigned as suggested
byFaix (1992).
2.2.3. Methoxyl groups
Methoxyl group was determined as suggested by
Vazquez et al. (1997). The lignin (0.15 g) was treated
with refluxing concentrated sulfuric acid (10 ml) for
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118 N.-E. El Mansouri, J. Salvado / Industrial Crops and Products 26 (2007) 116124
Fig. 1. Types of phenolic structure determined in different lignin samples.
10 min. The reaction mixture was cooled, 70 ml of dis-
tilled water was added, and the methanol produced in the
reaction was distilled off under vacuum and quantified
by gas chromatography.
2.2.4. Acetylation
A weighted amount of each lignin except lignosul-fonate was acetylated for 48 h with a mixture of purified
pyridine-acetic anhydride (1:1, v/v). Methanol was used
to quench the remaining acetic anhydride. Finally, a flow
of nitrogen was applied to evaporate the solvents and the
samples were dried under vacuum (Chum et al., 1985).
2.2.5. Hydroxyl groups: aliphatic and phenolic
Phenolic hydroxyl groups were determined by
three wet chemical methods (aminolysis, ultraviolet-
spectroscopy and non-aqueous potentiometry) and two
spectroscopy methods (
1
H NMR and
13
C NMR). Thetwo spectroscopy methods enabled aliphatic hydroxyl
quantification. These methods are described below.
2.2.5.1. Aminolysis. The procedure described by Lai
was used to determine free phenolic hydroxyl groups
in lignin (Lai, 1992). The acetylated lignin, dis-
solved in 1.0ml of dioxane containing 5 mg of
1-methylnaphtalene, was treated with 1.0 ml of dioxane-
pyrrolidine (1:1, v/v) solution, which initiated the
aminolysis reaction. After the addition of pyrrolidine,
samples were taken from the reaction mixture at different
times (total reaction time was approximately 120 min)and analyzed by gas chromatography. The amount of 1-
acetylpyrrolidine formed (equivalent to the amount of
hydroxyl groups) was recorded as a function of time.
The content of phenolic hydroxyl groups was calculated
by extrapolation of the curve at zero time.
2.2.5.2. Phenolic hydroxyl groups by ultraviolet-
spectroscopy ( method). The content of various
phenolic units in lignin samples was determined by
UV spectroscopy as described by Zakis (1994). This
method is based on the difference in absorption at 300
and 360 nm between phenolic units in neutral and alka-
line solutions. The content of ionizing phenol hydroxyl
groups can be quantitatively evaluated by comparing the
values of substances studied at certain wavelengths
to the values ofof the respective model compounds
(I, II, III, IV types) (Fig. 1).
2.2.5.3. Proton nuclear magnetic resonance spec-
troscopy (1H NMR). We used proton nuclear magnetic
resonance to analyze all acetylated technical lignins
under study.1H NMR spectra of 10 mg acetylated lignin
samples dissolved in 0.5 ml of CDCl3 were recorded
on a VARIAN GEMINI 300 Hz apparatus using tetram-
ethylsilane as internal standard under the same condition
as those described byLundquist (1992).Proton signals
were integrated from the baseline and referred to the
integrated signal of the methoxyl protons for proton
quantification of aliphatic and phenolic hydroxyl.
2.2.5.4. Carbon nuclear magnetic resonance spec-
troscopy 13C NMR. 13C nuclear magnetic resonance
is the most suitable method for determining benzylic
alcohol groups in lignins. For all acetylated lignins, the13C NMR spectra were recorded in acetone-d6 under
the same conditions as those described by Robert and
Brunow (1984). The quantitative estimation of differ-
ent hydroxyl groups (located at 170.8 and 170 ppm
of primary and secondary aliphatic hydroxyl groups,
respectively, and 168.9 ppm for the phenolic hydroxyl
group) were achieved by expanding ten times, beforeintegration, the signal areas corresponding to each func-
tional group and combining these results with those of
elemental analysis and methoxyl groups.
2.2.6. Carbonyl groups
Carbonyl groups for all lignins were determined by
two wet chemical methods: the Modified Oximating
method and differential UV-spectroscopy. The Modified
Oximating method was described byFaix et al. (1998)
that present a correction technique, which is necessary
for lignins containing carboxyl groups. Differential UV-
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N.-E. El Mansouri, J. Salvad o / Industrial Crops and Products 26 (2007) 116124 119
Fig. 2. Types of carbonyl structures determined in various lignins.
spectroscopy was developed by Alder and Marton in
1966 and reported by Zakis (1994). It involves differ-ential absorption measurements that take place when
carbonyl groups are reduced at the benzylic alcohol
corresponding with sodium borohydride. This method
determines some carbonyl lignin structures such as alde-
hydes and ketones structures described inFig. 2.
2.2.7. Carboxyl groups
We analyzed carboxyl groups using three methods:
acid number determination and aqueous and non-
aqueous titration methods. These methods are described
below.
2.2.7.1. Acid number determination. Carboxylic
groups were determined as described by Gosselink et
al. (2004a).The pH of 100 ml of 95% ethanol in water
was adjusted to 9.0 using 0.1 mol/l sodium hydroxide in
water. After adding 1 g of dried lignin, the mixture was
stirred for 4 h and subsequently titrated back to pH 9.0
with 0.1 mol/l sodium hydroxide solution.
2.2.7.2. Aqueous titration method. This method was
used by Gosselink et al. (2004a). A weight of lignin
sample (1 g) was suspended in 100 ml of alkaline aque-ous solution. After stirring for 3 h, the pH was adjusted
to 12 with sodium hydroxide. After stirring again, the
solution was potentiometrically titrated with 0.1 mol/l
aqueous hydrochloride acid.
2.2.7.3. Non-aqueous potentiometry method. This pro-
cedure, reported by Dence, involves a non-aqueous
potentiometric titration of lignin with tetra-n-butyl-
ammonium hydroxide in the presence of an internal
standard, which isp-hydroxybenzoic acid (Dence, 1992;
Gosselink et al., 2004a). The advantage of this method is
that it determines not only the carboxyl groups in lignin
butit concurrentlydetermines the weakly acidic phenolichydroxyl groups. When combined with an ion-exchange
treatment, the aforementioned titrimetric procedure was
also used to determine the strongly acidic groups (sul-
fonates groups) in lignosulfonate.
2.2.8. Sulfonate groups
Sulfonate groups were determined by non-aqueous
potentiometry, as described above (Dence, 1992).
2.2.9. Expanded C9 formulae
The expanded formulae C9 contain complete infor-mation about the lignin structure. They are obtained
by combining the results from elementary analysis and
functional groups analysis.
2.2.10. Statistical analysis
We compared the methods for determining the func-
tional groups in lignins by applying paired two-sided
t-tests at a 95% confidence level for mean values and
combining the two methods. The results are presented
as averages and their standard deviation.
3. Results and discussions
3.1. Structural characterization with FTIR
spectroscopy
The IR absorption spectra of the five technical lignins
studied were recorded in the 4004000 cm1 region (see
Fig. 3).These spectra show that there were clear differ-
ences between these lignins. The band at 3400 cm1,
which is attributed to OH groups in lignins, had a lower
absorption intensity for KL and SAL than for ORS, EPL
andLS. This is attributed to thehigh oxidation anddegra-
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120 N.-E. El Mansouri, J. Salvado / Industrial Crops and Products 26 (2007) 116124
Fig. 3. FTIR spectra of unacetylated lignin samples.
dation power of soda during the two pulping processes.
The 30002800 cm1 region of the C H stretch in the
methyl and methylene groups was present in different
quantities. These bands, which were mainly attributed
to methoxyl groups, were substantially higher for SAL,
EPL and ORS and presented relatively lower absorbance
bands for KL and LS. The carbonyl stretching vibra-
tion at 1720 cm1 appeared in the IR spectra of KL,SAL and ORS but was absent in the spectra of EPL and
LS. At 1600 and 1500 cm1, aromatic skeletal vibration
bands were observed for all lignins. Between 1300 and
1000 cm1, the bands and peak ratios were very differ-
ent due to various vibrations modes such as C O, C H
and C O. The distinct band appearing at 620 cm1 in
the spectra of LS was assigned to the sulphonic groups
(S O stretching vibration) formed from the reaction of
sodium sulphite with the secondary OH of the aliphatic
side chain of lignin. FTIR spectroscopy showed that the
lignins studied were clearly structurally different. Thestructural differences between other lignins analyzed
by FTIR spectroscopy were reported by Carmen et al.
(2004). This will be analyzed in further detail in this
study.
3.2. Hydroxyl groups: phenolic and aliphatic
hydroxyl
The phenolic hydroxyl groups of all lignin sampleswere determined by several methods: aminolysis, UV-
spectroscopy, 1H NMR, 13C NMR and non-aqueous
potentiometric titration (Table 1). Aliphatic hydroxyl
groups were determined by 1H NMR and 13C NMR
spectroscopy (Table 2). The amounts of the various
phenolic structures present in lignin as determined by
UV-spectroscopy are shown inTable 3.
Comparison of the methods used for phenolic
hydroxyl quantification by statistical analysis (paired
t-test) as listed in Table 4 shows that aminolysis/13C
NMR, UV-spectroscopy/13
C NMR and non-aqueous-potentiometry/1H NMR show a poor correspondence
Table 1
Phenolic hydroxyl content in various technical lignins determined by different methods (%, w/w)
Aminolysis Non-aqueous potentiometry 1H NMRa 13C NMR UV-spectroscopya
KL 4.60 (0.04) 4.54 (0.15) 4.10 4.99 4.50 (0.32)
SAL 4.90 (0.07) 5.10 (0.23) 4.50 5.31 4.40 (0.30)
ORS 2.80 (0.10) 3.56 (0.12) 3.33 3.23 2.66 (0.32)
EPL 2.55 (0.08) 2.92 (0.18) 2.65 2.70 2.30 (0.36)
LS NA 2.55 (0.31) NA NA 2.00 (0.16)
NA: Not acetylated; ( ) standard deviation.a
Data fromEl Mansouri and Salvado (2006).
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N.-E. El Mansouri, J. Salvad o / Industrial Crops and Products 26 (2007) 116124 121
Table 2
Aliphatic hydroxyl content of various technical lignins determined by
NMR spectroscopy methods (%, w/w)
1H NMR 13C NMR
KL 10.09 9.80
SAL 3.10 2.45
ORS 3.50 3.20EPL 4.73 4.55
at a significance level of 0.05. In contrast, the other
paired analyses show a correspondence at a significance
level of 0.05. This variability in results is attributed to
an incomplete acetylation in the case of methods based
on lignin acetylation, such as 1H NMR, 13C NMR and
aminolysis. This incomplete acetylation was confirmed
by Gosselink et al. for sulphur-free lignin and model
compounds that may be attributed to steric hindranceby the methoxyl groups present in lignins (Gosselink et
al., 2004a).Moreover, NMR-spectroscopy is character-
ized by an overlapping signal that lowers the accuracy
of these techniques. Also, UV-spectroscopy determines
only some phenolic structures, so the phenolic groups
might be underestimated. For non-aqueous potentiome-
try it is difficult to observe the inflection point with some
lignins.
From theresults obtained,we cansee that the methods
used are not fully comparable. The standard deviations
for each lignin analysis lead us to assume that aminolysisand non-aqueous potentiometry are the most reliable for
the determination of phenolic hydroxyl. These results
are in agreement with those ofMilne et al. (1992)and
Gosselink et al. (2004a).The two selected methods pro-
vide quantitative data on the frequency with which the
phenolic OH occurs in lignin, but they do not reveal the
structural environment in which it occurs. This informa-
tion about the lignin structure can be obtained by the
spectral techniques. The UV spectroscopy is an easy
method to quickly estimate some phenolic hydroxyl
structures.
The 1H NMR showed a poor correspondence inthe results with 13C NMR at a significance level of
0.05 for aliphatic hydroxyl determination (p-value is
0.04 < 0.05). This is attributed to the overlapping signals
that can easily introduce significance errors and to the
well-known incomplete acetylation of lignin with NMR
spectroscopy. A similar discrepancy was observed by
Gosselinket al. (2004a) whenestimating the ratio of phe-
nolic/aliphatic hydroxyl by methods such as 1H NMR
and 13C NMR spectroscopy. 1H NMRand 13C NMR are
therefore not comparable for aliphatic hydroxyl quan-
tification.These results show the phenolic hydroxyl contents
were highest for kraft and soda/anthraquinone lignins,
high for organosolv lignin and relatively low for ethanol
process lignin and lignosulfonate.The aliphatic hydroxyl
content was highest for the kraft lignin and relatively low
for the other samples.
3.3. Carbonyl groups
Table 5 shows the quantitative determination ofcarbonyl groups by differential UV-spectroscopy and
modified oximating method with and without the cor-
rection technique.Table 6shows the amount of different
carbonyl structures as determined by differential UV-
spectroscopy.
Table 3
Relative abundance of different phenolic structures in lignins determined by UV-spectroscopy (%, w/w)
KL LS SAL ORS EPL
Non-conjugated phenolic
structures (I + III)
[OH]I 2.63 1.34 2.74 0.89 1.43
[OH]III 0.49 0.48 0.57 0.44 0.68Conjugated phenolic
structures (II + IV)
[OH]II 1.30 0.14 1.10 1.31 0.14
[OH]IV 0.08 0.03 0.02 0.02 0.05
Table 4
Comparison of methods for the determination of phenolic hydroxyl content by paired t-test (two-sidedp-values)
Method Aminolysis UV-spectroscopy 1H NMR 13C NMR Non-aqueous potentiometry
Aminolysis 0.07 0.79 0.01 0.16
UV-spectroscopy 0.48 0.013 0.051H NMR 0.20 0.0213C NMR 0.88
Non-aqueous potentiometry
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122 N.-E. El Mansouri, J. Salvado / Industrial Crops and Products 26 (2007) 116124
Table 5
Content of carbonyl groupsin samples from various analytical methods
(%, w/w)
Lignin types Oximating method UV-spectroscopy
Without correction With correction
KL 3.13 (0.05) 2.91 (0.05) 2.35 (0.32)SAL 2.62 (0.10) 2.13 (0.10) 1.94 (0.25)
LS 5.30 (0.10) 4.50 (0.10) 4.70 (0.27)
ORS 4.05 (0.10) 3.94 (0.09) 2.90 (0.19)
EPL 6.48 (0.11) 5.73 (0.11) 5.20 (0.23)
( ) Standard deviation.
Table 7 indicate that the correspondence between
the results for the carbonyl groups determined by
oximating method without correction and for the
oximating method with correction and differential UV-spectroscopy method were poor at a significance level
of 0.05. These differences in the results are attributed
to a correction method introduced in order to sub-
tract CO from carboxylic origin in the oximating
method and to the existence of other forms of carbonyl
groups underestimated by differential UV-spectroscopy
for example quinone forms, which exist in highly oxi-
dized lignins such as those in this study. The results
from UV-spectroscopy and the oximating method with
the correction technique corresponded at a 0.05 signifi-
cance level. These results show that the methods are not
completely comparable. From the standard deviationsof each lignin analysis, we concluded that the oximating
method with the correction technique is reliable for total
carbonyl quantification, which was confirmed byFaix et
al. (1998).Differential UV-spectroscopy enables some
carbonyls, such as aldehydes and ketones structures, to
be determined.
Ethanol process lignin and lignosulfonate showed
higher contents of carbonyl groups than other lignins.
Values for kraft and organosolv lignins were within
the range found by Faix et al. (1998) when analyz-
ing alcell-organosolv from yellow poplar (4.40%) and
kraft indultin AT (3.32%). The higher carbonyl con-
tents of technical lignins than of ball milled enzymelignin 2.2% are plausible because technical lignins
underwent oxidation during the treatment process (Faix
et al., 1998).
3.4. Carboxyl groups
Table 8lists the carboxyl content for lignins deter-
mined by acid number and aqueous and non-aqueous
titration methods, as described above. Statistical com-
parison of these methods shows that there were
no significant differences at a 95% confidence level
(Table 9).However, the carboxylic contents of lignins
were different for the three titration methods. These
differences were due to the solubility of the lignins in
the selected solvents. The same trend was observed by
Gosselink when analyzing soda lignins with the same
methods (Gosselink et al., 2004a).The accessibility of
the carboxylic groups is therefore higher when DMF is
used as solvent for non-aqueous titration and when the
agitation time is longer in the alkaline medium for aque-
oustitration. From the standarddeviations foreach lignin
analysis, we concluded that non-aqueous titration andaqueous titration, in this order, provide reliable results
for the determination of carboxyl groups. The acid num-
ber method cannot be used for lignosulfonate because
this lignin is insoluble in 95% ethanol. With this method
the solubility of the other lignins is also poor, which is
reflected in the low values for the carboxylic groups.
Table 6
Relative abundance of some aldehydes and ketones types in samples obtained by differential UV-spectroscopy (%, w/w)
KL LS SAL ORS EPL
Coniferyl aldehydestructures (I + II)
[CO]I 0.38 0.98 0.31 1.03 1.50[CO]II 1.09 1.80 0.56 1.14 1.53
Ketones structures
(III+IV)
[CO]III 0.51 0.90 0.73 0.66 1.28
[CO]IV 0.37 1.02 0.34 0.07 0.89
Table 7
Comparison of methods for the determination of carbonyl content by paired t-test (two-sidedp-values)
Method Oximating without correction Oximating with correction UV-spectroscopy
Oximating without correction 0.03 0.01
Oximating with correction 0.11
UV-spectroscopy
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N.-E. El Mansouri, J. Salvad o / Industrial Crops and Products 26 (2007) 116124 123
Table 8
Contents of carboxylic and sulfonate obtained by the analytical methods (%, w/w)
Lignin type Non-aqueous potent Acid number Aqueous titration Sulfonate
KL 7.06 (0.15) 5.97 (0.50) 7.10 (0.31)
SAL 6.91 (0.22) 5.42 (0.56) 6.90 (0.37)
ORS 3.15 (0.20) 2.79 (0.60) 2.86 (0.45)
EPL 2.02 (0.27) 1.82 (0.69) 2.17 (0.41) LS 4.63 (0.21) a 4.30 (0.42) 12.23 (0.39)
( ) Standard deviation.a Sample not completely dissolved.
Table 9
Comparison of methods for the determination of carboxyl content by
pairedt-test (two-sidedp-values)
Methods Non-aqueous
potent
Acid
number
Aqueous
titration
Non-aqueous potent 0.08 0.40
Acid number 0.10Aqueous titration
The contents of carboxylic groups for kraft and
soda/anthraquinone were higher than for the other
lignins. This indicates that the two lignins were highly
degraded during the kraft and soda/anthraquinone pulp-
ing. Ethanol process lignin seemed to be less degraded.
3.5. Sulfonate groups
Table 8shows the sulfonate group contents of lig-nosulfonate. These groups ensure ready water solubility
in the presence of a suitable counter ion (Na, Ca, Mg,
NH4, etc.). These results are in agreement with those
in the literature. The results from non-aqueous potentio-
metric titration and elementary analysis show that not
all the sulfur content in lignosulfonate is in the form of
sulfonate.
3.6. Expanded molecular formulae
Table 10lists the expanded molecular formulae for
the various technical lignins under study. The expanded
Table 11
Elemental composition of different lignins studied (El Mansouri and
Salvado, 2006)
%C %H %N %S %O
KL 65.00 5.41 0.05 1.25 28.24
SAL 65.00 6.12 0.17 0.00 28.64
LS 44.84 5.15 0.02 5.85 44.14ORS 63.51 5.55 0.02 0.00 30.92
EPL 58.34 6.01 1.26 0.00 34.40
C9 formulae were obtained from elemental analysis
(Table 11)and functional groups analysis, which pro-
vides a number for each functional group per expanded
formula C9. Each expanded formula C9summarizes all
the information about the structure of these technical
lignins.
4. Conclusions
We have conducted a comparative study of the dif-
ferent analytical methods for the functional groups in
various technical lignins. Statistical comparison shows
that the methods used for phenolic OH are not fully
equivalent. Each method has some disadvantage or
other: incomplete acetylation with techniques based on
acetylation, an overlapping signal in nuclear magnetic
resonance, the difficulty of showing the inflection point
in non-aqueous titration, and the underestimation of phe-
nolic hydroxyl content with UV-spectroscopy. Despite
Table 10
Expanded molecular formulae for the technical lignins studied
Lignins Expanded formulae C9
KL C9H6,010O0,269N0,006S0,065(OCH3)0,597(OHAr)0,425(OH
Al)1,046(OCO)0,183(OOHCOOH)0,277SAL C9H6,825O0,560N0,020S0,065(OCH3)1,166(OH
Ar)0,493(OHAl)0,338(OCO)0,141(OOHCOOH)0,286
LS C9H10,360O2,880N0,003S0,070(OCH3)0,730(OHAr)0,260(OCO)0,354(OOHCOOH)0,227(HSO3)0,330
ORS C9H6,705O1,205N0,002(OCH3)0,971(OHAr)0,396(OH
Al)0,380(OCO)0,260(OOHCOOH)0,130EPL C9H9,036O2,270N0,166(OCH3)0,646(OH
Ar)0,289(OHAl)0,515(OCO)0,378(OOHCOOH)0,083
OCH3: Methoxyl groups; OHAr: aromatic phenolic hydroxyl; OHAl: aliphatic phenolic hydroxyl; OCO: carbonyl groups; OOHCOOH: carboxyl
groups; HSO3: sulfonate groups.
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124 N.-E. El Mansouri, J. Salvado / Industrial Crops and Products 26 (2007) 116124
these contradictory results, we selected aminolysis and
non-aqueous titration with TnBAH as reliable methods.
Non-aqueous titration with TnBAH can be used with all
technical lignins and can determine not only phenolic
OH but also carboxylic and sulfonate groups. Also, the
methods used for the aliphatic hydroxyl groups are not
comparable because the spectral technique is based onacetylation, which is incomplete for lignin, and because
an overlapping signal affects reliability. The methods for
quantifying carbonyl are also not comparable because
one determines total carbonyl content and the other deter-
mines only some carbonyl structures. The oximating
method is reliable for determining total carbonyl groups.
The methods used to determine carboxylic groups are
comparable and we selected non-aqueous titration and
aqueous titration as reliable methods for the technical
lignins in this study.
By analyzing the various lignin functional groups,we determined their structural characteristics. Several
analytical methods showed that the highest content of
phenolic hydroxyl were in kraft and soda/anthraquinone
lignins and that there was a high content in organosolv
lignin but a relatively low content in ethanol process
lignin and lignosulfonate. Kraft lignin had the highest
content of aliphatic hydroxyl: the other lignin samples
had low contents. Lignosulfonate and ethanol process
lignin had the highest contents of carbonyl groups than
the other lignins. Carboxyl groups analysis also showed
that Kraft lignin and soda/anthraquinone were morehighly degraded than the other lignins under study. In
conclusion, the technical lignins analyzed in this study
have different functional group contents.
By combining elementary analysis and functional
groups analysis, we can represent the expanded formu-
lae C9, which contains all the information about the
structural environment of the lignins.
Acknowledgements
The authors would like to thank Ligno-Tech Iberica,S.A., Santiago de Compostela University, the Cen-
tro de Investigaciones energeticas, medioambientales
y tecnologicas (CIEMAT) and Celulosa de Levante,
S.A. (CELESA) for supplying the lignins. We would
also like to express our sincere appreciation to the
Rovira i Virgili University for their award of a schol-
arship, the Spanish Ministry of Science and Technology
for providing finance under project number ENE2004-
07624-C03-03, and the autonomous government of
Catalonia also for providing finance under project num-
ber 2005SGR00580.
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