Industrial Crops and Products 23 (2006) 180–193
Comparative study of organosolv lignins from wheat straw
Feng Xu a , Jin-Xia Sun b,c , RunCang Sun c,a,∗ , Paul Fowler c , Mark S. Baird d
b
a State Key Laboratory of Pulp & Paper Engineering, South China University of Technology, Guangzhou 510641, China
College of Forestry, The North-Western University of Agricultural and Forest Sciences and Technology, Yangling 712100, China
c The BioComposites Centre, University of Wales, Bangor LL57 2UW, UK
d Department of Chemistry, University of Wales, Bangor LL57 2UW, UK
Received 10 March 2004; accepted 19 May 2005
Abstract
Dewaxed wheat straw was treated with acetic acid–H2 O (65/35, v/v), acetic acid–H2 O (80/20, v/v), acetic acid–H2 O (90/10,
v/v), formic acid–acetic acid–H2 O (20/60/20, v/v/v), formic acid–acetic acid–H2 O (30/60/10, v/v/v), methanol–H2 O (60/40,
v/v) and ethanol–H2 O (60/40, v/v) using 0.1% HCl as a catalyst at 85 ◦ C for 4 h, in which 78.2, 80.0, 88.2, 89.4, 94.1, 23.5
and 37.4% of the original lignin, and 42.4, 58.7, 70.0, 65.1, 76.5, 14.2 and 22.2% of the original hemicelluloses was released,
respectively. Lignins obtained were characterized by their content of hemicelluloses, composition of phenolic acids and aldehydes,
molecular weight, thermal stability and by UV, Fourier transform infrared (FT-IR), 1 H and 13 C nuclear magnetic resonance (NMR)
spectroscopy. The results showed that aqueous organic acid was more effective than aqueous organic alcohol for extensive
delignification and selective fractionation of cellulose, lignin and hemicelluloses from the straw. In particular, the addition of
formic acid gave a significant effect on the dissolution of lignin. All the acid-insoluble lignin fractions contained small amounts
of contaminated hemicelluloses as shown by their content of neutral sugars, 0.9–4.3%, and had weight-average molecular
weight between 3960 and 4340 g mol−1 . An increase in concentration of acetic acid or formic acid in organosolv resulted in an
increment in release of guaiacyl units and in lignin condensation. However, the lignin preparations released during the treatment
with aqueous organic alcohol without organic acid contained almost equal amounts of non-condensed guaiacyl and syringyl
units with fewer p-hydroxyphenyl units. The -O-4 ether bonds together with -, -5 and 5-5′ carbon–carbon linkages were
identified to be present in lignin substructures.
© 2005 Elsevier B.V. All rights reserved.
Keywords: Wheat straw; Lignin; Organic acids; Phenolics; FT-IR; 13 C NMR
1. Introduction
∗
Corresponding author. Tel.: +44 1248 370588;
fax: +44 1248 370594.
E-mail address: bcs00a@bangor.ac.uk (R. Sun).
0926-6690/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.indcrop.2005.05.008
Large amounts of lignin are produced every year
by the pulping processes as a by-product of wood and
non-wood delignification. However, the availability of
F. Xu et al. / Industrial Crops and Products 23 (2006) 180–193
lignin is not as high as could be expected from this fact
(Oliet et al., 2001). The commercial pulping processes
(kraft and sulphite technologies) lead to high quality
pulps, but fractions such as lignin and hemicelluloses
(accounting for 50–55% of the dry weight of wood and
straw) are employed in low added value applications
such as producing process energy (Vila et al., 2003).
Organosolv pulping, based on the utilization of organic
solvents as delignification agents, provides an interesting alternative to the current commercial technologies,
since they lead to a solid phase enriched in cellulose
and to liquors containing hemicellulose-degradation
products and lignin-degradation products free from
sulphur.
In organosolv pulping, a mixture of organic solvent
and water is used as cooking liquor. The solvent primarily acts on the promotion of vegetal tissue impregnation
and the solubilization of the lignin fragments so produced (Balogh et al., 1992; Gilarranz et al., 2000).
In non-catalyzed pulping (autocatalyzed), the cooking liquor becomes acidified due to the acetic acid
released from the wood. However, in catalyzed pulping
the liquor can be acidic, neutral or alkaline depending on the nature of the additives employed (Aziz and
Sarkanen, 1989). During organosolv acid delignification, the Acetosolv process (based on the utilization of
HCl-catalyzed acetic acid media) and Formacell process (formic acid-catalyzed media) have proved to be
promising process to achieve complete utilization of
lignocellulosics without impact to environment. Both
processes have ability to cause extensive removal of
both lignin and hemicelluloses under mild conditions,
with no significant cellulose degradation. By either of
these processes (Nimz and Casten, 1986; Sano et al.,
1990; Pan and Sano, 2000), wood and non-wood can
be simply fractionated to pulp, lignin and monosaccharides or hemicellulosic-degradation products, which
makes it easy to utilize them for more valuable products. The pulp can be used for either paper or cellulose derivatives, such as carboxymethylcellulose, cellophane, viscose or cellulose acetate. The lignin can
be converted to valuable products, such as carbon fibre
(Uraki et al., 1995), activated carbon fibres (Uraki et al.,
1997) and adhesives (Pan and Sano, 1998). From the
monosaccharides or hemicellulosic-degradation products, sugars or other chemicals, sweetening materials,
food additives, fuel and polymers can be obtained
(Fengel and Wegener, 1984).
181
Lignin is an extremely complex three-dimensional
polymer (typically found in vascular plants)
formed by dehydrogenative polymerization of phydroxycinnamyl, coniferyl and sinapyl alcohols.
These three lignin precursors (‘monolignols’) give
rise to the so-called p-hydroxyphenyl (H), guaiacyl
(G) and syringyl (S) phenylpropanoid units, which
show different abundances in lignins from different
groups of vascular plants, as well as in different plant
tissues and cell-wall layers. During polymerization
of the above p-hydroxycinnamyl alcohols involved,
the formation of aryl ether (involving C4 ) interunit
linkages is strongly favoured. In addition, a small
proportion of lignin units remains as phenolic, being
linked only by C–C bonds, such as -5, -1, -5,
- and ␣- linkages. Although this phenolic moiety
represents a low (and variable) fraction of the total
lignin, it can strongly affect the reactivity of the
polymer (Camarero et al., 1999). During organosolv
acid delignification, lignin is dissolved essentially by
acid-catalyzed cleavage of such bonds as ␣-aryl ether
and arylglycerol--aryl ether in the lignin macromolecule (Sarkanen, 1990). However, the cleavage of
-aryl ether bonds occurs at lower extent (Goyal et al.,
1992). The cleavage of ether bonds gives rise to new
phenolic hydroxyl groups in lignin, which affect some
industrial uses of lignins and lignocellulosic materials,
since they increase lignin solubility (favouring its
alkaline extraction during paper or pulp manufacture),
and modifies the reactivity of technical lignins to be
used as raw material for manufacture of lignin-based
adhesives and other applications (Camarero et al.,
1999).
The aim of this work was to study the influence of
various organosolvs on physico-chemical properties of
lignins dissolved from wheat straw at atmospheric pressure. Two favourable organic acids were chosen to be
used, formic acid and acetic acid. In comparison, aqueous methanol and ethanol were also used as cooking
solvents. In order to get more information on chemical
structures and relationship between physical properties and chemical structures, the lignins were chemically characterized in this paper by their fractional
yield, sugar composition, content of phenolic acids and
aldehydes, and molecular weights. Other techniques
such as Fourier transform infrared (FT-IR) and 1 H and
13 C nuclear magnetic resonance (NMR) spectroscopy
were also used to investigate the changes occurring
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F. Xu et al. / Industrial Crops and Products 23 (2006) 180–193
in lignin structure during the organosolv treatment
processes.
Wheat straw (Variety Riband) was kindly supplied
by B Lloyd Co., Llangefni. The composition (%, w/w)
of the straw is cellulose 39.0%, hemicelluloses 38.7%,
lignin 17.0%, ash 1.8% and wax 1.9% on a dry weight
basis. The deviations of these contents from their
respective means were all less than 8%. After being
dried at 60 ◦ C in an oven for 16 h, the straw was ground
to pass through a 0.7 mm screen and then extracted with
toluene–ethanol (2/1, v/v) in a Soxhlet extractor for 6 h.
The dewaxed straw was further dried at 60 ◦ C for 12 h.
All chemicals used were of analytical or reagent grade.
The hemicelluloses (hemicellulosic degradation products) were isolated from the concentrated hydrolysates
by precipitation with three volumes of 95% ethanol
(22 ◦ C, 12 h). The solubilized lignins were obtained
from the corresponding supernatants by reprecipitation after evaporation of all the organic solvents. The
acid-insoluble lignin fractions were then washed with
acidified water and freeze-dried. Note that the treatments with acetic acid–H2 O (65/35, v/v) was for lignin
preparation L1 , acetic acid–H2 O (80/20, v/v) for L2 ,
acetic acid–H2 O (90/10, v/v) for L3 , formic acid–acetic
acid–H2 O (20/60/20, v/v/v) for L4 , formic acid–acetic
acid–H2 O (30/60/10, v/v/v) for L5 , methanol–H2 O
(60/40, v/v) for L6 and ethanol–H2 O (60/40, v/v) for
L7 , respectively. The scheme for organosolv treatment
and the isolation of lignin is illustrated in Fig. 1. All
experiments were performed at least in duplicate. Yield
of lignin is given on a dry weight basis related to the
starting material.
2.2. Organosolv treatment
2.3. Characterization of the lignin preparations
Organosolv treatment was carried out in a 500 ml
glass reactor at atmospheric pressure. The extractive
free powder (10.0 g) was treated under seven different organosolv systems (Table 1) using 0.1% HCl as a
catalyst at 85 ◦ C for 4 h with a liquor to solid ratio of
20:1 (ml/g) under stirring, respectively. After the reactor was loaded with wheat straw and the cooking liquor,
it was heated to the operating temperature, which was
then maintained throughout the experiment. After treatment, the residue enriching in cellulose was filtrated
on a nylon cloth, then washed with hot acid–water
mixture and finally, washed with hot distilled water.
The neutral sugar composition of the contaminated hemicelluloses in lignins was determined as their
alditol-acetate derivatives by gas chromatography (GC)
after hydrolysis with 2 M trifluoroacetic acid for 2 h at
120 ◦ C (Blakeney et al., 1983). The chemical composition of phenolics liberated from alkaline nitrobenzene oxidation of the lignins (175 ◦ C, 2.5 h) was
determined on a Hichrom H5ODS HPLC column of
dimensions 250 mm × 4.6 mm (Phenomenex Co., Beijing). The identification of the individual compounds
was detected at 280 nm by computer comparison of
the retention times and peak areas with the authentic
2. Materials and methods
2.1. Materials
Table 1
Extraction conditions of organosolv-soluble lignins
Experiment no.
Organosolv system
Catalyst
Temperature (◦ C)
Liquor to solid
ratio (ml/g)
Reaction time (h)
W1
W2
W3
W4
Acetic acid–H2 O (65/35, v/v)
Acetic acid–H2 O (80/20, v/v)
Acetic acid–H2 O (90/10, v/v)
Formic acid–acetic acid–H2 O
(20/60/20, v/v/v)
Formic acid–acetic acid–H2 O
(30/60/10, v/v/v)
Methanol–H2 O (60/40, v/v)
Ethanol–H2 O (60/40, v/v)
0.1% HCl
0.1% HCl
0.1% HCl
0.1% HCl
85
85
85
85
20:1
20:1
20:1
20:1
4
4
4
4
0.1% HCl
85
20:1
4
0.1% HCl
0.1% HCl
85
85
20:1
20:1
4
4
W5
W6
W7
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F. Xu et al. / Industrial Crops and Products 23 (2006) 180–193
300 spectrometer at 300 and 74.5 MHz, respectively.
1 H NMR spectrum was recorded at 25 ◦ C from 25 mg
of sample dissolved in 1.0 ml DMSO-d6 . For each sample, 16 scans were collected. The 13 C NMR spectrum
was recorded at 25 ◦ C from 250 mg of sample dissolved
in 1.0 ml DMSO-d6 after 30,000 scans. A 70◦ pulse flipping angle, a 10 s pulse width and a 15 s delay time
between scans were used.
3. Results and discussion
3.1. Fractional yield and purity of lignin
In comparison to other organosolv processes under
neutral and alkaline conditions, the recovery of Acetosolv lignin in this study does not need a previous step
of reduction, and consequently the further neutralization of liquors is also avoided. The acid-insoluble lignin
fractions were therefore obtained from the supernatants
by reprecipitation in water solution after evaporation
of all the organic solvents. Table 2 gives the fractional
yields of the lignins dissolved in various organosolvs.
Obviously, the results indicated that the higher yield
of lignin was obtained when the treatment was performed using organic acid as a solvent between lignin
preparations L1 and L5 ; in this case, the yield of lignin
was 13.3–16.0%, corresponding to 78.2–94.1% on total
lignin of straw. Interestingly, an increase in acetic acid
concentration from 65 to 80% and to 90% resulted in an
increment in lignin yield from 13.3% (in L1 ) to 13.6%
(in L2 ) and to 15.0% (in L3 ), respectively. The current
results were consistent with the studies on acetic acid
Fig. 1. Scheme for isolation of acid-insoluble lignin preparations
obtained by treatment of dewaxed wheat straw with organosolv under
acidic conditions.
phenolics. Methods of UV spectra recording, measurement of the molecular-average weights and thermal
analysis of lignin samples have been described in previous papers (Sun et al., 1999, 2003). All nitrobenzene oxidation results represent the mean of at least
triplicate samples and each oxidation mixture was
chromatographed twice. Other experiments were performed in duplicate. The standard errors or deviations
were observed to be lower than 6.5% except for the
variation among the triplicate nitrobenzene oxidation
(7.0–16.8%).
FT-IR spectra were obtained on an FT-IR spectrophotometer (Nicolet 750) using a KBr disc containing 1% finely ground samples. The solution-state 1 H
and 13 C NMR spectra were obtained on a Bruker MSLTable 2
The yield (% dry matter) of the lignin preparations
Lignin fraction
Lignin preparationa
L1
L2
L3
L4
L5
L6
L7
Total solubilized lignins
Acid-insoluble ligninsb
Acid-soluble ligninsc
Lignin associated in solubilized hemicelluloses
13.3
9.0
3.8
0.5
13.6
9.1
3.7
0.8
15.0
11.6
2.4
1.0
15.2
11.7
2.5
1.0
16.0
12.5
2.0
1.5
4.0
3.0
0.8
0.2
6.4
4.8
1.4
0.2
a L , L , L , L , L , L and L represent the lignin preparations obtained by treatment of the dewaxed wheat straw with acetic acid–H O
1
2
3
4
5
6
7
2
(65/35, v/v), acetic acid–H2 O (80/20, v/v), acetic acid–H2 O (90/10, v/v), formic acid–acetic acid–H2 O (20/60/20, v/v/v), formic acid–acetic
acid–H2 O (30/60/10, v/v/v), methanol–H2 O (60/40, v/v) and ethanol–H2 O (60/40, v/v) and 0.1% HCl as a catalyst at 85 ◦ C for 4 h, respectively.
b Represent the lignin fractions obtained by precipitation of the supernatant solution at pH 1.5 after isolation of the solubilized hemicelluloses.
c Represent the lignin fractions which are still solubilized in the pH 1.5 supernatant after precipitation of the acid-insoluble lignin fractions
and obtained by difference (total solubilized lignin–acid-insoluble lignin–lignin associated in the solubilized hemicelluloses).
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F. Xu et al. / Industrial Crops and Products 23 (2006) 180–193
pulping of wheat straw under atmospheric pressure by
Pan and Sano (1999). The authors stated that acetic acid
had been proved to be an effective organosolv for both
extensive delignification of wood or non-wood and
selective fractionation of cellulose, lignin and hemicelluloses. Similar results have been reported by Nimz and
Casten (1986) and Davis et al. (1986) during the delignification of lignocellulosics using acetic acid–water
mixture.
More importantly, as the data shown in Table 2,
the highest yield of lignin was obtained when formic
acid was added into acetic acid. Treatment with formic
acid–acetic acid–H2 O (20/60/20 and 30/60/10, v/v/v)
led to a release of 88.2% (in L4 ) and 94.1% (in L5 )
of the original lignin from the straw, respectively. This
indicated that the addition of formic acid showed a significant effect on the dissolution of lignin. In this case,
the yield of lignin increased by 1.9 and 2.7% when
formic acid increased from 0 to 20% and to 30%. Based
on atmospheric acetic acid pulping of wood, Nimz and
Schoene, 1993 concluded that when 5–10% of formic
acid was added into acetic acid, the pulps could be
obtained with improved qualities and kappa number
below 5. In other words, these processes achieved a
‘fractionation’ of the lignocellulosic raw materials into
separate streams containing hemicellulose-degradation
products, lignin-degradation products and cellulose, all
of them being utilizable for different end-product applications.
It should be noted that the yield of lignin obtained
by using methanol–H2 O (60/40, v/v) and ethanol–H2 O
(60/40, v/v) together with 0.1% HCl as a catalyst at
85 ◦ C for 4 h, was less than half the yield of organic
acid isolated lignin, 4.0% in L6 and 6.4% in L7 . This
confirmed again that organic acid, e.g. formic acid and
acetic acid, is more powerful solvent for delignification
than aqueous alcohol, such as methanol/ethanol–water
mixtures. The fairly low lignin yields of L6 and L7 can
partially be explained by the relatively low hydroxonium ion concentration in aqueous alcohol system,
since lignin dissolution is expected to be proceeded
by the acid-catalyzed cleavage of ␣-aryl and -aryl
ether linkages in the lignin macromolecule and the
rate of delignification is highly pH-dependent. This
effective role of organic acid at high concentration is
probably due to rapid hydrolysis of hemicelluloses,
resulting in increased porosity and accessibility of the
solvent to lignin, since significant amounts of hemi-
celluloses (42.4, 58.7, 70.0, 65.1 and 76.5% of the
original hemicelluloses obtained by treatment with
acetic acid–H2 O (65/35, v/v), acetic acid–H2 O (80/20,
v/v), acetic acid–H2 O (90/10, v/v), formic acid–acetic
acid–H2 O (20/60/20, v/v/v) and formic acid–acetic
acid–H2 O (30/60/10, v/v/v), respectively, data not
shown) were released or degraded during the treatments using organic acids, while only small quantities
of them (14.2 and 22.2% of the original hemicelluloses,
data not shown) dissolved during the treatments with
aqueous methanol and ethanol, respectively. Furthermore, as expected, the acid-insoluble lignin fraction in
Table 2 was the major fraction, comprising 67.7–78.1%
of the total solubilized lignins, while the lignin fraction associated in the solubilized hemicelluloses, was
accounted only 3.1–9.4% of the released lignins. This
result suggested that treatment with various organosolvs under the acidic conditions given substantially
cleaved the ether bonds between lignin and hemicelluloses from the cell walls of wheat straw.
In this study, UV–vis absorption measurements of
the seven acid-insoluble lignin fractions were performed using a dioxane–water mixture, which verify the purity of lignins at λ = 250–380 nm. Spectra
of acid-insoluble lignin fractions L2 (spectrum L2 ),
L3 (spectrum L3 ), L4 (spectrum L4 ) and L6 (spectrum L6 ) solubilized during the treatment with acetic
acid–H2 O (80/20, v/v), acetic acid–H2 O (90/10, v/v),
formic acid–acetic acid–H2 O (20/60/20, v/v/v) and
methanol–H2 O (60/40, v/v), respectively, are illustrated in Fig. 2. The maximum absorption at 280 nm
originates from non-conjugated phenolic groups in the
lignin. The presence of a second characteristic region
of lignin absorption around 318 nm can be assigned
to the presence of both ferulic and p-coumaric acids
(Scalbert et al., 1986). Interestingly, as shown in the
spectra, a highest absorption coefficient occurred in L6
fraction, suggesting that the most pure lignin preparation can be obtained when aqueous alcohol was used as
an organosolv. On the other hand, the lowest absorption
coefficient of L4 fraction, released during the treatment
with formic acid–acetic acid–H2 O (20/60/20, v/v/v),
was undoubtedly due to the highest amounts of bound
hemicelluloses and other non-lignin materials. It can be
seen that the purity of the lignin decreased in the order:
aqueous alcohol-soluble lignin, acetic acid–watersoluble lignin, formic acid–acetic acid–water-soluble
lignin.
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F. Xu et al. / Industrial Crops and Products 23 (2006) 180–193
Fig. 2. UV spectra of acid-insoluble lignin preparations isolated with acetic acid–H2 O (80/20, v/v, sample L2 , spectrum L2 ), acetic acid–H2 O
(90/10, v/v, sample L3 , spectrum L3 ), formic acid–acetic acid–H2 O (20/60/20, v/v/v, sample L4 , spectrum L4 ) and methanol–H2 O (60/40, v/v,
sample L6 , spectrum L6 ) combining with 0.1% HCl as a catalyst at 85 ◦ C for 4 h from dewaxed wheat straw.
Organosolv treatment under the acidic conditions used resulted in intensive hydrolysis of lignin–
hemicellulose bonds and, accordingly, in low hemicellulose content in the isolated lignins. Table 3 shows
the content of sugars determined after acid hydrolysis of acid-insoluble lignin fractions. Even under mild
organic alcohol reaction conditions, the total hemicellulose content never exceeded 5%. The relatively
high amount of xylose and arabinose suggested that the
main hemicelluloses bonded to lignin were arabinoxylans. Glucose and galactose could also be found in all
lignin samples. Trace amount of mannose was present
in some cases. No clear effect of treatment conditions
on the amount of hemicelluloses in lignin preparations was observed except for the lignin preparation
L4 , which had a noticeable amount of hemicelluloses
as determined by the sugar content, 4.3%. This high-
est content of hemicelluloses in L4 preparation implied
that there were more chemical linkages between lignin
and polysaccharides in L4 than in other lignin preparations, which were stable to aqueous formic acid–acetic
acid hydrolysis. p-Coumaric and ferulic acids present
in wheat straw lignin might be favourable to this kind
of linkages. In particular, ferulic acid might form crosslinks between lignin by ether bonds through its phenolic oxygen in the cell walls of wheat straw and hemicelluloses by simultaneous esterification of their carboxyl
group to the C-5 position of arabinose substituents
of arabinoglucuronoxylans (Himmelsbach and Barton,
1980). The presence of linkages between hemicelluloses and lignin makes it difficult to prepare wheat straw
lignin free from polysaccharides. Similar results were
reported for purification of wheat straw lignins by BenGhedalia and Yosef (1994).
Table 3
The content of neutral sugars (% lignin sample, w/w) in the acid-insoluble lignin preparations
Neutral sugars/uronic acids
Lignin preparationa
L1
L2
L3
L4
L5
L6
L7
Arabinose
Xylose
Mannose
Galactose
Glucose
0.6
1.0
Trb
0.2
0.2
0.4
0.8
Tr
0.2
0.4
0.4
0.8
Tr
0.3
0.4
0.9
2.1
Tr
0.6
0.7
0.6
1.1
Tr
0.4
0.6
0.3
0.3
Tr
0.1
0.2
0.3
0.4
Tr
0.1
0.2
Total
2.0
1.8
1.9
4.3
2.8
0.9
1.0
a
Corresponding to the lignin preparations in Table 2.
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F. Xu et al. / Industrial Crops and Products 23 (2006) 180–193
increment in release of guaiacyl units. This means that
the fraction with more guaiacyl units were much easier
to extract than that with more syringyl units during the
preparation of organosolv lignins with relatively high
concentration of acetic acid or formic acid, or that guaiacyl units were less condensed or cross-linked than
syringyl units in the cell walls of wheat straw. On the
other hand, these data also suggested that syringyl units
were condensed to a great extent during the treatment
with the concentration (90%) of aqueous acetic acid
and formic acid–acetic acid–water mixture than other
units. The total yield of oxidation products decreased
from 38.69% (L2 ) to 24.68% (L3 ) with an increase in
acetic acid concentration from 80 to 90% and reduced
from 23.89% (L4 ) to 17.97% (L5 ) when the formic
acid increased from 20 to 30% in the treating mixture of formic acid–acetic acid–water, implying that
condensation degree of the lignins increased as the
concentration of acetic acid or formic acid raised. In
other words, the lignin preparations extracted with a
relatively high concentration of acetic acid or formic
acid were highly condensed and difficult to oxidize
while the lignin preparation dissolved during the treatment with 60% aqueous methanol (L6 ) was least condensed and easiest to oxidize, due to its highest yield
of nitrobenzene oxidation products (50.68%). These
different degrees of lignin condensation demonstrated
3.2. Composition of phenolic acids and aldehydes
The results of alkaline nitrobenzene oxidation of
seven organosolv lignin preparations are listed in
Table 4. As can be seen, all the lignin preparations gave
vanillin and syringaldehyde as main products accompanied with their corresponding aromatic acids in minor
quantity, suggesting that the organosolv lignins mainly
resulted from guaiacyl–syringyl units. The presence
of fewer p-hydroxybenaldehyde and p-hydroxybenzoic
acid was considered most probably to the indicative of
non-condensed p-hydroxyphenyl units, indicating the
incorporation of p-hydroxycinnamoyl alcohol in wheat
straw organosolv lignin. The occurrence of significant
amounts of non-condensed guaiacyl and syringyl units
with relatively fewer p-hydroxyphenyl units revealed
that the seven organosolv lignins could be justified
as GSH-lignin such as grass type lignin. The relative molar ratios of G (the relative total moles of
vanillin, vanillic acid and acetovanillin) to S (the relative total moles of syringaldehyde, syringic acid and
acetosyringone) and to H (the relative total moles of
p-hydroxybenzaldehyde and p-hydroxybenzoic acid)
were found to be 4:3:1 in L1 and L2 , 7:4:1 in L3 ,
4:4:1 in L4 , 5:4:1 in L5 , 4:4:1 in L6 and 6:6:1 in L7 .
Apparently, an increase in acetic acid or formic acid
concentration in a mixture of organosolv resulted in an
Table 4
The composition (% lignin sample, w/w) of phenolic acids and aldehydes from nitrobenzene oxidation of the acid-insoluble lignin preparations
Phenolic acids and aldehydes
Lignin preparationa
L1
L2
L3
L4
L5
L6
L7
p-Hydroxybenzoic acid
p-Hydroxybenzaldehyde
Vanillic acid
Vanillin
Syringic acid
Syringaldehyde
Acetovanillin
Acetosyringone
p-Coumaric acid
Ferulic acid
Cinnamic acid
1.49
1.68
1.36
12.86
0.63
11.38
0.53
1.00
0.38
0.47
0.030
1.50
2.36
2.81
15.89
1.27
11.37
0.54
1.88
0.40
0.64
0.026
0.25
1.03
0.68
9.68
0.32
9.65
0.38
0.69
0.35
0.33
0.041
0.77
1.34
1.02
9.20
0.89
8.93
0.36
0.72
0.24
0.30
0.12
0.57
0.78
0.72
7.22
0.46
7.17
0.23
0.40
0.18
0.16
0.078
1.34
2.72
2.52
17.18
0.82
21.46
1.65
1.60
0.72
0.58
0.092
0.40
1.58
2.02
11.78
1.22
14.26
0.80
0.76
0.56
0.28
0.080
Total
31.81
38.69
24.68
23.89
17.97
50.68
33.74
4:3:1
4:3:1
7:4:1
4:4:1
5:4:1
4:4:1
6:6:1
Molar ratio
a
(G:S:H)b
Corresponding to the lignin fractions in Table 2.
G represents the sum of total moles of vanillin, vanillic acid and acetovanillin; S represents the sum of total moles of syringaldehyde, syringic
acid and acetosyringone; H represents the sum of total moles of p-hydroxybenzaldehyde and p-hydroxybenzoic acid.
b
187
F. Xu et al. / Industrial Crops and Products 23 (2006) 180–193
the different behaviour of lignins during the various
organosolv treatments. In addition, the low yields of
L3 , L4 and L5 could also be explained either as the
removal of p-coumaric and ferulic acids ester and/or
ether-linked to protolignin, which exist in a remarkable amount in gramineae lignin (Sun et al., 2000), or
as the occurrence of condensation between the units of
the lignin during the concentration (90%) of aqueous
acetic acid and formic acid–acetic acid–water mixture
in the atmospheric organic acid treatment with 0.1%
HCl as a catalyst.
3.3. Molecular weight
Weight-average (M̄w ) and number-average (M̄n )
molecular weights and polydispersity (M̄w /M̄n ) of the
seven acid-insoluble lignin preparations from wheat
straw are summarized in Table 5. Clearly, the seven
lignin fractions showed no significant difference in
their molecular-average weights, which ranged M̄w
from 3960 to 4340 g mol−1 . These data indicated that
the organosolv under the conditions given had no substantial influences on the M̄w of the lignins from wheat
straw. In other words, aqueous organic acids and aqueous alcohols appeared to have equal effect on the degradation of lignin into small segments by cleavage of
␣-aryl and -aryl ether linkages when 0.1% HCl was
used as a catalyst. In addition, the seven lignin fractions
also gave a fairly analogous polydispersity, ranging
between 1.56 and 1.71.
3.4. FT-IR spectra
Among the analysis techniques described in the literature, FT-IR spectroscopy shows interesting characteristics such as high sensitivity and selectivity, high
signal-to-noise ratio, accuracy, data handling facility, mechanical simplicity and short time and small
Fig. 3. FT-IR spectra of acid-insoluble lignin preparations isolated
with acetic acid–H2 O (80/20, v/v, sample L2 , spectrum 1), acetic
acid–H2 O (90/10, v/v, sample L3 , spectrum 2) and formic acid–acetic
acid–H2 O (20/60/20, v/v/v, sample L4 , spectrum 3) combining with
0.1% HCl as a catalyst at 85 ◦ C for 4 h from dewaxed wheat straw.
amount of sample required for the analysis (Hortling
et al., 1997). In addition, the spectrum of a lignin
sample gives an overall view of its chemical structure
(Gilarranz et al., 2001). Fig. 3 shows FT-IR spectra of
acid-insoluble lignin preparations isolated with acetic
acid–H2 O (80/20, v/v, sample L2 , spectrum 1), acetic
acid–H2 O (90/10, v/v, sample L3 , spectrum 2) and
formic acid–acetic acid–H2 O (20/60/20, v/v/v, sample L4 , spectrum 3) combining with 0.1% HCl as a
catalyst at 85 ◦ C for 4 h from dewaxed wheat straw.
The spectral profiles and the relative intensities of
the bands were rather similar in three spectra, which
confirmed that the ‘core’ of lignin structure did not
change significantly during the aqueous organic acid
treatment.
Table 6 gives the assignments of FT-IR absorption bands of the lignins, aromatic skeleton vibrations
occur at 1606, 1507 and 1434 cm−1 , in which the
aromatic semicircle vibration (a vibration involving
Table 5
Weight-average (M̄w ) and number-average (M̄n ) molecular weights and polydispersity (M̄w /M̄n ) of the acid-insoluble lignin preparations
Lignin preparationa
M̄w
M̄n
M̄w /M̄n
a
L1
L2
L3
L4
L5
L6
L7
4130
2530
1.63
4330
2760
1.57
3960
2330
1.70
4140
2650
1.56
4170
2660
1.57
4340
2630
1.65
4280
2500
1.71
Corresponding to the lignin fractions in Table 2.
188
F. Xu et al. / Industrial Crops and Products 23 (2006) 180–193
Table 6
Assignments of FT-IR absorption bands (cm−1 )
Absorption bands
Assignment
3429
2945
OH stretching
CH stretching of methyl, methylene
or methane group
C O stretch in unconjugated ketone
and carboxyl group
C O stretch in conjugated ketone
Aromatic skeletal vibrations
Aromatic skeletal vibrations
Aromatic methyl group vibrations
Aromatic skeletal vibrations
Aliphatic C–H stretch in CH3
Syringyl ring breathing with C–O
stretching
Aromatic C–O stretching
C–O stretch in ester groups
Aromatic C–H in-plane deformation
for syringyl type
Aromatic C–H in-plane deformation
for guaiacyl type
Aromatic C–H out of plane bending
1732, 1726
1660, 1653
1606
1507
1460
1434
1374
1328
1242
1165
1135
1043
855, 844
Fig. 4. FT-IR spectra of acid-insoluble lignin preparations isolated
with formic acid–acetic acid–H2 O (30/60/10, v/v/v, sample L5 , spectrum 1), methanol–H2 O (60/40, v/v, sample L6 , spectrum 2) and
ethanol–H2 O (60/40, v/v, sample L7 , spectrum 3) combining with
0.1% HCl as a catalyst at 85 ◦ C for 4 h from dewaxed wheat straw.
both carbon–carbon stretching and a change of the
H–C–C bond angle) is assigned at 1507 cm−1 (Colthup
et al., 1990). The band at 1732 cm−1 is originated from
the carbonyl and unconjugated ketone and carboxyl
Fig. 5. 1 H NMR spectrum of lignin preparation L4 obtained by treatment with formic acid–acetic acid–H2 O (20/60/20, v/v/v) combining with
0.1% HCl as a catalyst at 85 ◦ C for 4 h from dewaxed wheat straw.
F. Xu et al. / Industrial Crops and Products 23 (2006) 180–193
group stretching, while the small band at 1653 cm−1 is
attributed to conjugated carbonyl stretching in organosolv lignins. Moreover, aliphatic C–H stretch in CH3
and syringyl ring breathing with C–O stretching are
clearly seen at 1374 and 1328 cm−1 , respectively. The
bands at 1135 and 1043 cm−1 are indicative of the aromatic C–H in-plan deformation for syringyl type and
guaiacyl type, respectively. Aromatic C–H out of bending exhibits at 844 cm−1 (Collier et al., 1997; Faix,
1991).
Fig. 4 illustrates FT-IR spectra of acid-insoluble
lignin preparations isolated with formic acid–acetic
acid–H2 O (30/60/10, v/v/v, sample L5 , spectrum 1),
methanol–H2 O (60/40, v/v, sample L6 , spectrum 2)
and ethanol–H2 O (60/40, v/v, sample L7 , spectrum 3)
combining with 0.1% HCl as a catalyst. As can be
seen, the changes of the carbonyl absorption region
189
at 1726 and 1660 cm−1 might enable the evaluation
of the effects of organic acid and organic alcohol
treatment. Obviously, a remarkable increase of carboxyl absorption observed in spectrum 1 at 1726 cm−1
revealed that a noticeable oxidation of the lignin structure did occur during the organic acid treatment. On
the other hand, a much stronger peak at 1660 cm−1 in
spectra 2 and 3 demonstrated that the lignin preparations obtained by treatment with organic alcohols contained higher amounts of conjugated carbonyl groups
than those of the lignins isolated with organic acids.
Furthermore, the intensity of band at 1374 cm−1 for
aliphatic C–H stretch in CH3 in spectrum 1 was much
lower than that of the lignins in spectra 2 and 3,
indicating that the oxidation occurred mainly at ␥position of lignin side chains during the organic acid
treatment.
Fig. 6. 13 C NMR spectrum of lignin preparation L4 obtained by treatment with formic acid–acetic acid–H2 O (20/60/20, v/v/v) combining with
0.1% HCl as a catalyst at 85 ◦ C for 4 h from dewaxed wheat straw.
190
F. Xu et al. / Industrial Crops and Products 23 (2006) 180–193
3.5. 1 H and 13 C NMR spectra
Analysis of the 1 H NMR signal intensity in the range
of 8.0–6.2 ppm provides an indirect method of monitoring the level of substitution on the aromatic ring
of lignin. The 1 H NMR spectrum of lignin preparation L4 obtained by treatment with formic acid–acetic
acid–H2 O (20/60/20, v/v/v) is shown in Fig. 5. The integrals of signals between 6.2 and 6.8 ppm are attributed
to aromatic protons in syringylpropane and guaiacylpropane structures (Faix et al., 1992), indicating the
presence of similar relative contents of syringyl and
guaiacyl units in the lignin. The small signals around
7.3–7.4 ppm are assigned to the aromatic protons in
positions 2 and 6, in structures containing a C␣ O
group, to aromatic protons in positions 2 and 6 of phydroxyphenyl units conjugated with a double bond,
to the proton in C␣ C structures and to aromatic
protons in p-coumaric and ferulic acids, confirming
the presence of p-hydroxyphenyl units, C␣ O groups,
and p-coumaric and ferulic acids in the lignin fraction (Seca et al., 2000). The H in -O-4 structures
exhibits two weak signals between 5.0 and 4.8 (data
not shown). Methoxyl protons (–OCH3 ) give a sharp
signal at 3.7 ppm. The signal at 3.3 ppm is due to protons in water in DMSO. Two intense signals around
2.5 ppm are indicative of protons in DMSO. Protons in
aliphatic groups such as in the side chains of lignin or
in acetyl groups in xylans produce signals between 2.0
and 0.8 ppm.
In order to gain a more complete understanding
of the structures in the isolated lignins, a qualitative
13 C NMR spectrum of lignin preparation L , isolated
4
with formic acid–acetic acid–H2 O (20/60/20, v/v/v),
was investigated (Fig. 6), and their chemical shifts (δ,
ppm), intensity and assignment list in Table 7. Most
of the observed signals have been previously assigned
in straw and wood lignin spectra (Nimz et al., 1981;
Scalbert et al., 1986). It should be noted that the spectra of the lignin sample do show some signals at 174.5,
102.0, 97.6, 92.4, 82.6, 76.6, 74.6, 73.1, 70.1 and 69.7,
65.6 and 63.0 ppm attributed to hemicellulose carbons.
This suggested the partial cleavage of linkages between
hemicelluloses and lignin during the treatment with
formic acid–acetic acid–water mixture under the condition used.
Table 7
Carbon chemical shifts (δ, ppm), intensity and assignment of the lignin preparation L4 in 13 C NMR spectrum
δ (ppm)
Intensitya
Assignmentsb
δ (ppm)
Intensity
Assignments
181.8
174.5
171.9
170.3
166.5
159.8
157.3
152.5
149.3
147.8
145.2
134.9
130.2
127.9
125.5
123.1
119.2
115.7
111.1
103.9
w
w
s
m
w
w
w
s
m
s
m
m
s
w
w
w
m
s
m
s
C O in dienone or quinone
C-6 in 4-O-MeGlcA
–COOH, aliphate
–C O in acetyl group
C-␥, PC ester
C-4, PC ester
C-4, H
C-3/C-5, S
C-3, G etherified
C-4, G etherified
C-4, G non-etherified
C-1, S etherified; C-1, G etherified
C-2/C-6, PC ester
C-2/C-6, H
C-1, PC ester
C-6, FE ester
C-6, G
C-3/C-5, PC ester
C-2, G
C-2/C-6, S
102.1
97.6
92.4
84.8
82.6
79.5
76.6
74.6
73.1
72.2
70.1
69.7
65.6
63.0
59.7
59.3
56.0
33–24
22.0
13.9
s
m
w
w
w
w
m
w
w
w
m
m
w
w
m
m
vs
w
s
m
C-1, Xyl internal unit
C-1, Xyl reducing end unit, -anomer
C-1, Xyl reducing end unit, ␣-anomer
C- in -O-4
C-4, 4-O-MeGlcA
C-3 in arabinfuranose
C-3, Xyl non-reducing end unit
C-3, Xyl internal unit
C-2, Xyl internal unit
C-␣ in -O-4
C-4, Xyl non-reducing end unit
C-4, Xyl non-reducing end unit
C-5, Xyl non-reducing end unit
C-5, Xyl internal unit, C-␥ in -5
C-␥ in -O-4
C-␥ in -O-4
OCH3 in S and G units
␣-, -Methylene groups
CH3 in acetyl group
␥-Methyl in n-propyl side chains
a
Abbreviations: s, strong; m, mean, w, weak; vs, very strong.
Abbreviations: G, guaiacyl unit; S, syringyl unit; H, p-hydroxylphenyl unit; PC, p-coumaric acid; FE, ferulic acid; Xyl, xylose; 4-O-MeGlcA,
4-O-methylglucuronic acid.
b
F. Xu et al. / Industrial Crops and Products 23 (2006) 180–193
Signal at 181.8 ppm could be assigned to carbonyl groups in dienone or quinone substructures
(Zhang and Gellerstedt, 1999). The carbonyl resonances from uronic acids and esters may contribute to signal at 174.5 ppm, which corresponds
to C-6 in methyl urinates (Himmelsbach and Bar-
191
ton, 1980). A strong signal at 171.9 ppm arises
from –COOH groups in aliphatic acids. The acetyl
groups in xylans give a peak at 170.3 ppm. This
phenomenon confirmed again that hemicelluloses are
strongly associated with lignin in the cell walls of wheat
straw.
Fig. 7. The thermograms of the acid-insoluble lignin preparations: (a) L2 , isolated with acetic acid–H2 O (80/20, v/v) and (b) L7 , isolated with
ethanol–H2 O (60/40, v/v) combining with 0.1% HCl as a catalyst at 85 ◦ C for 4 h from dewaxed wheat straw.
192
F. Xu et al. / Industrial Crops and Products 23 (2006) 180–193
One of most important features of lignin can be seen
in the aromatic region (103.9–168.0 ppm). The syringyl
(S) residues were identified by signals at 152.5 and
152.2, 147.8 and 147.4, 134.9 and 134.7, and 103.9
and 103.5 ppm. Guaiacyl (G) residues were verified
by signals at 149.3, 147.8 and 147.4, 145.2, 134.9
and 134.7, 119.2 and 119.0, 114.9, and 111.1 ppm.
The p-hydroxyphenyl (H) residues give a small signal
at 127.9 ppm. These signals confirmed that the lignin
preparation could be justified as GSH grass lignin. The
signals at 166.5 and 166.3, 159.8, 130.2 and 130.0,
125.5 and 124.9, and 115.7 ppm are attributed to esterified p-coumaric acid. Etherified ferulic acids exhibit
signals at 168.0 ppm (data not shown). Esterified ferulic acid was detected with a signal at 123.1 ppm. These
observations indicated that p-coumaric acid is linked to
lignin by ester bonds, while the ferulic acid is linked to
lignin by both ether and ester bonds.
The most abundant substructure in lignin is an O-4 structure. Side chain carbons: C-␣, C- and C-␥
in -O-4 can be seen at 72.2, 84.8 and 59.7–59.3 ppm,
respectively. In addition, other common carbon–carbon
linkages such as -, -5 and 5-5′ type structures,
are also important substructures in lignin. The presence of - substructures can be seen from the C-␥
signal at 70.1 ppm, although C-␣ and C- signals are
overlapped with other signals. Side chain carbons of
C-␥ in -5 substructures can be observed at 63.0 ppm.
Signal at 125.5 ppm could also relate to 5-5′ substructures. These observations indicated that the lignin
preparation is mainly composed of -O-4 ether bonds
together with small amounts of -, -5 and 5-5′
carbon–carbon linkages. The strong signal at 56.0 ppm
corresponds to the OCH3 group in syringyl and guaiacyl units. The signals representing the ␥-methyl, ␣
and -methylene groups in n-propyl side chains occur
at 13.9 and 18.4–33.6 ppm, respectively. In addition,
CH3 in acetyl group give a strong signal at 22.0 ppm,
indicating that the lignin fraction contained noticeable
amounts of acetyl groups, coming from the residual
solvent.
lignin samples began to decompose at 190 (L2 ) 186 ◦ C
(L7 ). When weight loss reached 50%, the temperature
increased to 467 ◦ C (L2 ) and 486 ◦ C (L7 ). It is clear that
after about 200 ◦ C the thermal degradation of the lignin
took place rapidly. This almost same thermal stability
of lignins corresponded to their equal values of molecular weights. The DSC curve of the lignin preparations
showed a wide exothermic peak ranged between 200
and 600 ◦ C and centred at 476 ◦ C (L2 ) and 380 ◦ C (L3 ),
due to exothermic reactions of the lignin.
4. Conclusion
The treatment of wheat straw with aqueous organic
acids is very well suitable for separation of the principal chemical components. This is practically true for
isolation of lignins. It was found that aqueous organic
acid was much effective for delignification of wheat
straw than aqueous organic alcohol under the conditions used. FT-IR spectra indicated that the lignins
dissolved during the treatment with aqueous organic
acid, had more unconjugated and fewer conjugated carbonyl groups than those of the lignins isolated with
aqueous organic alcohol. In addition, a remarkable
increase of carboxyl absorption observed in the spectra of organic acid lignins suggested that a noticeable
oxidation of the lignin structure did occur during the
organic acid treatment. Furthermore, the treatment with
organic acid, particularly acetic acid, also resulted in
noticeable amounts of acetyl groups in the lignin preparations. 1 H and 13 C NMR spectra revealed that the
lignin preparation comprised guaiacyl, syringyl and a
small amount of p-hydroxyphenyl units. p-Coumaric
acid is linked to lignin by ester bonds, while the ferulic acid is linked to lignin by both ether and ester
bonds. The results also showed that the lignin preparation is mainly composed of -O-4 ether bonds together
with small amounts of -, -5 and 5-5′ carbon–carbon
linkages.
Acknowledgements
3.6. Thermal stability
Fig. 7 illustrates the thermograms of the acidinsoluble lignin preparations L2 (Fig. 7a) isolated with
acetic acid–H2 O (80/20, v/v) and L7 (Fig. 7b) isolated
with ethanol–H2 O (60/40, v/v). As can be seen, the two
The authors are grateful for the financial support of
this research from National Natural Science Foundation of China (Nos. 30271061 and 30430550), Guangdong Natural Science Foundation (No. 013034) and
Ministry of Education China for a major project.
F. Xu et al. / Industrial Crops and Products 23 (2006) 180–193
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