Application of pyridinium ionic liquid as a recyclable catalyst for acid-catalyzed transesterification of jatropha oil

Article abstract of DOI:10.1007/s10562-010-0409-x

A series of pyridinium ionic liquids were synthesized and characterized. Acid-catalyzed transesterifications of Jatropha oil were carried out with these ionic liquids under mild reaction conditions. [BSPy]CF₃SO₃ showed the best catal

Full text of DOI:10.1007/s10562-010-0409-x

Catal Lett (2010) 139:151–156
DOI 10.1007/s10562-010-0409-x
Application of Pyridinium Ionic Liquid as a Recyclable Catalyst
for Acid-Catalyzed Transesterification of Jatropha Oil
•
•
•
Kai-Xin Li Li Chen Zong-Cheng Yan
Hong-Lin Wang
Received: 10 May 2010 / Accepted: 9 July 2010 / Published online: 4 August 2010
Ó Springer Science+Business Media, LLC 2010
Abstract A series of pyridinium ionic liquids were syn-
thesized and characterized. Acid-catalyzed transesterifica-
tions of Jatropha oil were carried out with these ionic
liquids under mild reaction conditions. [BSPy]CF3SO3
showed the best catalytic activity and biphasic behavior in
the proposed process. The products could simply be sepa-
rated from the catalyst phase. The catalyst exhibited con-
stant activity for seven successive cycles after being
recycled.
great interest due to its environmental friendliness [5–7].
But some alcohols such as methanol deactivate the lipase to
some extent and the enzyme stability is poor. Moreover,
glycerol,which is a byproduct in the transesterification,
easily adsorbs on the surface of the lipase and inhibits the
enzyme activity. Although solid acid catalysts are widely
used in these organic reactions because of their regenera-
tive capacity, as well as the ability to eliminate corrosion
while solving environmental and toxicity problems [8],
some disadvantages such as low activity, easy deactivation
and high mass-transfer resistance are the major limits for
further application. Because of the detrimental effects of
these catalysts, great efforts have been directed towards the
benign development of environmentally friendly catalysts
in the transesterification of vegetable oil.
Keywords Pyridinium ionic liquid Á Transesterification Á
Esterification Á Recyclable catalyst Á Jatropha oil
1 Introduction
The task specific ionic liquids (TSILs), which are
environmentally friendly solvents and catalysts, have
gained the attention of scholars from various fields due to
their adjustable physical and chemical properties. They are
non-flammable, exhibit negligible vapor pressure, as well
as high catalytic acitivity and offer the potential for recy-
clability [9–11]. Since Cole etal. [12] designed a series of
The transesterification of vegetable oil is one of the major
industrial processes for the production of biodiesel. The
fatty acid methyl ester (FAME), which is the main com-
ponent of biodiesel fuel, is prepared through the transe-
sterification of triglycerides (TGs) and the esterification of
free fatty acids (FFAs) in the present of acid or base cat-
alysts [1–4]. However, these catalysts cannot be readily
recovered nor reused. Moreover, they have other disad-
vantages such as equipment corrosion, more byproducts, a
tedious workup procedure, and difficult separation from the
products. These disadvantages result in a series of envi-
ronmental problems and reaction inefficiency. The use of
lipases as biocatalysts for biodiesel production has been of
¨
water-stable Bronsted acidic ionic liquids in 2002, many
organic reactions involving esterification [13], nitration
[14], and acetylation [15], have been performed with these
task-specific ionic liquids, resulting in excellent yields and
selectivity. All of these studies offer us the possibility of
designing suitable catalysts for the appointed reaction.
Aiming to create an environmentally friendly procedure
for the synthesis of biodiesel from Jatropha oil and an easy
separation of catalyst from the product,we are especially
interested in the application of pyridinium ionic liquids in
the acid-catalyzed transesterificaiton and esterificaiton of
Jatropha oil with high level FFA. The pyridinium ionic
liquids are highly active [16], water-stable, nonvolatile,
K.-X. Li Á L. Chen Á Z.-C. Yan (&) Á H.-L. Wang
School of Chemistry and Chemical Engineering, South China
University of Technology, Guangzhou 510640,
People’s Republic of China
e-mail: zcyan@scut.edu.cn; 120557927@qq.com
123

152
K.-X. Li et al.
instrument. The quantitative analysis of the product was
directly carried out by an Agilent 1100 HPLC using an
ultraviolet detector and was determined by inner standard
method. A Hypersil ODS (C18) column (4.6 mm 9 5 lm 9
250 mm) was used and the mobile phase was chromato-
graphic grade acetonitrile at a flow rate of 1.0 mL/min. The
column temperature was 40 °C and the detection wave-
length was 210 nm. The biodiesel samples were diluted with
acetone (HPLC grade)
N
N
SO3H CF3SO3
SO3H HSO4
[BSPy] [CF3SO3]
[BSPy] [HSO4]
N
N
H2PO4
SO3H
p-CH3(C6H4)SO3
SO3H
[BSPy][ p-TSA]
[BSPy] [H2PO4]
3
3
PMo12O40
PW12O40
N
N
SO3H
SO3H
3
3
2.2 Preparation of TSILs with Pyridinium Cations
[BSPy]3PW12O40
[BSPy]3PMo12O40
The TSILs with pyridinium cations were prepared in a
procedure similar to the previous reported paper [17, 18].
The synthesis of [BSPy] was carried out under the fol-
lowing condition: pyridine was dissolved by stirring it with
1,4-butane sultone at 40 °C for 24 h. Then, the zwitterions
([BSPy]) formed as white solid. They were washed by
ether three times to remove residual material and dried in a
vacuum at 80 °C for 10 h. When the corresponding acid
was liquid, equimolar of [BSPy] and acid solutions were
mixed and stirred for 8 h at 80 °C. Then the combined
solution was dried in a vacuum at 80 °C to remove water.
Finally, the produced TSILs were washed repeatedly with
diethyl ether to remove left over material and dried in a
vacuum again.
3
SiW12O40
N
SO3H
3
[BSPy]3SiW12O40
Scheme 1 Structures of pyridinium ionic liquids prepared and used
in this paper
noncorrosive and immiscible with many organic solvents,
which may further facilitate the reaction and make the
separation of the product from the catalyst convenient.
Moreover, a discussion of pyridinium ionic liquids as cat-
alysts for the acid-catalyzed transesterification of Jatropha
oil with high level FFA is still absent from the literature.
In this paper, several acidic ionic liquids with pyridini-
um cations (as shown in Scheme 1) were prepared and used
in the process for the transesterification of jatropha oil.
These ionic liquids were characterized by infrared (IR)
spectroscopy, nuclear magnetic resonance (NMR) and
electrospray ionization mass spectrometry (ESI–MS). The
temperature, reaction time, yields in the process and dis-
tribution of esters in the biphasic system were studied.
Finally, the recycling performance of the pyridinium ionic
liquid is examined.
When the corresponding acid was solid, such as p-tol-
uenesulfonic acid hydrate (p-TSAÁH2O) or phosphotungstic
acid (H3PW12O40), the reaction was carried out with proper
proportion of [BSPy] and an aqueous solution of acid, and
then the mixture was stirred at 80 °C for 10 h, resulting in
the formation of objective products. The resulting liquids
were purified and dried in a similar procedure as proposed
above. Finally, the TSILs were evaluated by IR measure-
ment, NMR spectroscopy and ESI–MS. The spectral data
was agreed with their structures showed in Scheme 1.
The spectral data for [BSPy] [CF3SO3]:FT-IR(KBr):
3072, 2949, 1637, 1493, 1339, 1170, 1030 cm-1. 1H NMR
(400 MHz, D2O, TMS): d 1.68 (m, 2H), 2.05 (m, 2H), 2.88
(t, 2H, J = 7.6 Hz), 4.57 (t, 2H, J = 7.2 Hz), 4.71 (s, 2H),
7.99 (t, 2H, J = 6.8 Hz), 8.47 (t, 1H, J = 7.2 Hz), 8.76 (d,
2H, J = 6.4 Hz).13C NMR (400 MHz, D2O): d 20.5, 27.8,
49.5, 60.7, 128.6, 145.1, 145.7. Positive-ion electro-
spray ionization mass spectrometry (ESI–MS) m/z: 216.5
(BSPy?). Negative-ion electrospray ionization mass spec-
trometry (ESI–MS) m/z: 149.6 (CF3SO3-).
2 Methods
2.1 Chemicals and Instruments
Pyridine (99%), 1,4-butane sultone (99%), trifluoromethane
sulfonic acid (99%), phosphotungstic acid (AR),phospho-
molybdic acid (AR) and other chemicals (AR) were
commercially available and were used without further
purification unless otherwise stated. NMR spectra were
recorded on a BRUKER AV400 spectrometer in D2O and
were calibrated with tetramethylsilane (TMS) as the internal
standard. IR measurements were performed on a Nico-
let.510P FT-IR absorption spectrometer using KBr windows
suitable for fourier transform infrared (FTIR) transmittance
technology to form a liquid film. ESI–MS spectra were
obtained on BRUKER Corporation Esquire HCT PLUS
The spectral data for [BSPy][HSO4]: FT-IR(KBr):3072,
2951,1638,1492,1173,1036 cm-1
.
1H NMR (400 MHz,
D2O, TMS):d 1.62 (m, 2H), 2.03 (m, 2H), 2.80 (t, 2H,
J = 15.2 Hz), 4.50 (t, 2H, J = 14.8 Hz), 4.70 (s, 2H), 7.90
(t, 2H, J = 14.4 Hz), 8.39 (t, 1H, J = 1.2 Hz), 8.69 (d, 2H,
J = 6.4 Hz).13C NMR (400 MHz, D2O): d 23.8, 32.4,
52.8, 64.2, 131.4, 146.8, 148.6. Positive-ion electrospray
123

Application of Pyridinium Ionic Liquid
153
2.4 Recycling Experiment
ionization mass spectrometry (ESI–MS) m/z: 216.4
(BSPy?). Negative-ion electrospray ionization mass spec-
trometry (ESI–MS) m/z: 96.9 (HSO4-).
The ionic liquid phase was simply separated from the
product by decantation and washed with ethyl acetate. The
excess methanol and water were removed from the ionic
liquid phase by atmospheric distillation at 100 °C. The
crude glycerin was removed from the ionic liquid phase by
vacuum distillation at a temperature range of 180–210 °C.
Finally, the ionic liquid was directly reused in subsequent
runs.
The spectral data for [BSPy]3 PW12O40: FT-IR(KBr):
3070, 2948, 1636, 1491, 1170, 1035, 1080, 983, 893,
1
800 cm-1. H NMR (400 MHz, D2O, TMS):d 1.61 (m,
2H), 2.12 (m, 2H), 2.89 (t, 2H, J = 15.2 Hz), 4.51 (t, 2H,
J = 14.8 Hz), 4.73 (s, 2H), 7.95 (t, 2H, J = 14.4 Hz), 8.41
(t, 1H, J = 1.2 Hz), 8.70 (d, 2H, J = 6.4 Hz).13C NMR
(400 MHz, D2O): d 21.8, 29.4, 51.2, 60.2, 128.4, 141.8,
145.6. Positive-ion electrospray ionization mass spec-
trometry (ESI–MS) m/z: 216.5 (BSPy?). Negative-ion
electrospray ionization mass spectrometry (ESI–MS) m/z:
959.08 (PW12O403-).
3 Results and Discussion
We first studied the effect of ionic liquids with different
anions on the acid-catalyzed transesterification of Jatropha
oil. The results are summarized in Table 1. It can be seen
that little product could be detected when the reaction was
heated at 100 °C for 8 h in the absence of ionic liquid
(Table 1, entry 1), which indicated that the catalysts should
be absolutely necessary for this reaction. [BSPy][CF3SO3]
shows the best catalytic performance among all the ionic
liquids and the optimized reaction conditions went to entry
4 in Table 1. Other catalysts (entries 11–16) are shown to
be less active under the optimized conditions.
The spectral data for [BSPy]3PMo12O40: FT-IR(KBr):
3073, 2947, 1632, 1493, 1171, 1032, 1041, 974,
917,789 cm-1. 1H NMR (400 MHz, D2O, TMS):d 1.58 (m,
2H), 2.01 (m, 2H), 2.78 (t, 2H, J = 15.2 Hz), 4.51 (t, 2H,
J = 14.8 Hz), 4.71 (s, 2H), 7.92 (t, 2H, J = 14.4 Hz), 8.38
(t, 1H, J = 1.2 Hz), 8.72 (d, 2H, J = 6.4 Hz).13C NMR
(400 MHz, D2O): d 25.6, 33.2, 54.3, 62.2, 130.4, 145.8,
146.6. Positive-ion electrospray ionization mass spec-
trometry (ESI–MS) m/z: 216.4 (BSPy?). Negative-ion
electrospray ionization mass spectrometry (ESI–MS) m/z:
607.9 (PMo12O403-).
The anion of an ionic liquid has a significant effect on its
¨
catalytic activity [17, 19], which is related to the Bronsted
2.3 Preparation of Biodiesel from Jatropha Oil
acidic strength of anion [20]. As an organically stronger
acid, trifluoromethane sulfonic acid could ionize a proton
rapidly because of the negative induction effect of fluorine
atoms between the–CF3 group and the–SO3H group [21].
The proton combined with carbonyl group to form a car-
bocation intermediate, which reacted with methanol through
nucleophilic substitution reaction and finally formed one
molecular of diglyceride, FAME and a produced proton to
catalyze the following reaction (Scheme 3). Therefore, as a
The reaction was carried out in a 100 mL cylindrical
stainless steel reactor, equipped with a thermostat,
mechanical stirring and a sampling outlet. Jatropha oil,
methanol and an ionic liquid catalyst with different molar
ratios were quantitatively introduced into the reactor, suc-
cessively. The reaction was allowed to proceed for 1–8 h
with vigorous stirring and heating at the desired tempera-
ture. After the reaction, the mixture was placed for a few
hours for the formation of two phases (Scheme 2). The
upper phase mainly consisted of the produced methyl
esters, and the lower phase contained the ionic liquid cat-
alyst, water, excess methanol and a portion of the produced
ester.
¨
stronger Bronsted-acidity ionic liquid, [BSPy][CF3SO3]
facilitates the acid-catalyzed transesterification.
On the other hand, the different partial immiscibility of
obtained esters with the TSILs might also affect TSILs
catalytic activity. The TSILs are miscible with water but
partially immiscible with esters, which finally results in a
biphasic system. Most of hydrophilic products remained in
TSILs to form a heavy phase and most of the produced esters
formed the light phase. As shown in Table 2, the distribution
of esters between the two phases is variable in the different
TSILs under the optimized condition. [BSPy][CF3SO3]
presents the best immiscibility with the produced esters,
which facilitates the shifting of the esterification and
transesterification equilibrium to the product side.
The sample of the upper layer was purified by filtration,
water washing and distillation at 100 °C to remove the
hydrophilic residue. The TG conversion and FAME yield
were determined by HPLC analysis.
CH2OH
CHOH
CH2OH
R'COOCH3
R''COOCH3
R'''COOCH3
H2C OCOR'
HC OCOR''
TSILs
+
+ 3CH3OH
Since results were most favorable for [BSPy][CF3SO3],
different parameters including temperature, reaction time,
FAME yield and TG conversion were carefully examined
H2C OCOR'''
Scheme 2 Transesterification of Jatropha oil catalyzed by TSILs
123

154
K.-X. Li et al.
Table 1 Effect of ionic liquids
with different anions on the
transesterification reaction
Entry TSIL
Temperature (°C) Molar ratio
of oil/alcohol/TSIL
Time (h) FAME
TG
yielda (%) conversion (%)
1
–
100
1:10
8
8
6
5
8
5
5
6
8
6
8
6
5
5
5
5
5
1.5
70.5
85.6
92.0
90.3
90.8
91.5
86.2
88.3
83.3
88.0
88.6
30.3
75.5
80.1
80.3
79.5
3.2
90.2
95.1
99.1
96.5
99.2
99.3
99.1
97.5
99.0
96.9
99.8
71.2
90.6
91.4
91.3
90.8
2
[BSPy][CF3SO3] 100
[BSPy][CF3SO3] 100
[BSPy][CF3SO3] 100
[BSPy][CF3SO3] 100
[BSPy][CF3SO3] 100
[BSPy][CF3SO3] 100
1:10:0.05
1:10:0.10
1:10:0.12
1:10:0.12
1:10:0.15
1:10:0.18
1:10:0.12
1:10:0.12
1:10:0.12
1:10:0.12
1:10:0.12
1:10:0.12
1:10:0.12
1:10:0.12
1:10:0.12
1:10:0.12
3
4
5
6
7
8
[BSPy][CF3SO3]
[BSPy][CF3SO3]
[BSPy][CF3SO3]
[BSPy][CF3SO3]
[BSPy][HSO4]
90
90
9
10
11
12
13
14
15
16
17
80
80
100
100
100
[BSPy][H2PO4]
[BSPy][p-TSA]
[BSPy]3PW12O40 120
[BSPy]3PMo12O40 120
[BSPy]3SiW12O40 120
a
Isolated yield and conversion
were determined by HPLC
using internal standard method
H+
Scheme 3 Acid-catalyzed
reaction mechanism for the
transesterification of
triglycerides
H
H
O
H
O
O
+
O
O
O
H+
O R3
O R3
O
+
O
O
OH
O
R1
R2
OH
R2
O R3
O
O
R4
R4
O
O
R1
R2
R2
O
O
R4
O
O
R4
+
O
O
O
O
R1
R3
R1,R2,R3:carbon chain of the fatty acids
R4: alkyl group of the alcohol
can be noticed that the FAME yield increases with higher
temperature and varies smoothly with enough reaction time
(entries 4, 5, 9, 11). The use of [BSPy][CF3SO3] as a
catalyst in the acid-catalyzed transesterification reduced the
temperature compared with the previous work [16, 22, 23].
Table 2 Distribution of esters in the biphasic system after reaction
Entry TSIL
Fraction of produced esters Total estersa
Light phase Heavy phase
1
2
3
4
[BSPy][CF3SO3] 97.9
[BSPy][HSO4] 96.8
2.1
3.2
92.0
83.6
75.5
30.3
-
The higher acidity of the anion CF3SO3 should probably
be responsible for its higher catalytic activity in this
reaction. These results also indicated that increasing tem-
perature is favorable to the improvement of catalyst
activity and reaction rate, which is in agreement with the
previous work [16].
[BSPy][p-TSA] 69.8
[BSPy][H2PO4] 50.6
30.2
49.4
After the esterification and transesterification of Jatropha oil with
methanol, the reaction systems were biphasic. The produced esters in
the light phase were isolated by decantation, and the esters dissolved
in the heavy phase were extracted with ethyl acetate. Reaction con-
ditions:molar ratio of oil/alcohol/catalyst 1:10:0.12, temperature
100 °C, time 5 h
It can also be found from entry 4, 5 that the TGs con-
version was as high as 99.1% and slightly decreased as the
FAME yield reached about 92.0%. Similar trend has been
found from entries 8, 9 and entries 10, 11. As all of the
reactions are reversible, some monoglycerides (MGs) and
diglycerides (DGs) would form at the end of the reaction,
and isomerization may occur, which results in the formation
of some isomerized products. We had examined trace
a
Isolated yields were determined by HPLC
(Table 1, entries 2–11). With a rise of temperature, the
FAME yield and TG conversion reached a maximum of
92.0 and 99.1% at 100 °C for 5 h, respectively (entry 4). It
123

Application of Pyridinium Ionic Liquid
155
amounts of isomerized products by GC–MS during our
experiment, which is in agreement with our supposition. In
this work, the decrease in TG conversion with reaction time
is supposed to be due to the inverse transesterification of
MGs, DGs. And the existence of the isomerized products
may reduce the FAME selectivity in TG transesterification
slightly.
contrast, the experiment without removing glycerin showed
poorer FAME yield.
Therefore, we can conclude that the catalytic activity of
the ionic liquid was almost not changed after seven
repeated times, the decrease in FAME yield may mainly
result from the accumulative amount of glycerin in the
seventh recycling experiment and the gradual lost of cat-
alyst in the phase separation and decantation. The good
reusability of [BSPy][CF3SO3] may be due to its molec-
ular structure. The alkane sulfonic acid group is the con-
stituent of the active sites, and it is covalently tethered to
the cation of ionic liquid, which is not readily lost.
Moreover, the electrostatic force between the cation and
anion in ionic liquid is stronger than Vander Waals force.
Therefore, the [BSPy][CF3SO3] presents more excellent
stability and reusability than traditional molecular cata-
lysts. We may conclude that it is a very promising recy-
clable catalyst for the acid-catalyzed transesterification of
Jatropha oil.
To investigate the reusability of [BSPy][CF3SO3] as an
environmental friendly catalyst for the acid-catalyzed
transesterification of Jatropha oils, we performed a series of
recycling experiments. After the first reaction, water and
excess methanol were easily removed by atmospheric
distillation, glycerin was removed by vacuum distillation.
The experimental results with and without removing
glycerin were given in Fig 1. It was found that almost the
same yields were obtained between the two experiments in
the six successive trials. The FAME yield was not obvi-
ously affected by the recycling system without removing
glycerin even in the sixth time, which may be due to the
high thermodynamic constant of the reaction and the little
amount of glycerin. Since the vacuum distillation to
remove glycerin is a difficult separating process, it may be
neglected in the six successive recycling experiments.
We also found that the yield decreased slightly without
removing glycerin in the seventh run. In order to examine
the reason of the decrease in the FAME yield, gas chro-
matography (GC) analysis for determining the free glycerin
in the ionic liquid phase after six runs was performed using
EN 14105 standard. After six cycling experiments, the
ionic liquid phase was distillated under high vacuum
pressure for a few hours and then analyzed by GC detec-
tion. No free glycerin was detected within the detection
limits. And the ionic liquid [BSPy][CF3SO3] still present
excellent yield in the seventh cycling experiment. In
4 Conclusion
In summary, some pyridinium ionic liquids were synthe-
sized and [BSPy][CF3SO3] presents the best catalytic
activity and biphasic behavior among these ionic liquids in
the acid-catalyzed transesterificaiton of Jatropha oil. The
product was easily separated from the catalyst with high
conversion and good yield. Furthermore, the ionic liquid
can be conveniently separated from the products and
reused at least seven times without obvious change in the
catalytic activity. Thus, the effect of the transesterification
on the environment will be significantly reduced and the
ionic liquid may be the suitable recyclable catalyst for the
acid-catalyzed transesterification of Jatropha oil.
Acknowledgment This work was financially supported by the Fun-
damental Research Funds for the Central Universities, 20092M0226.
before vacuum distillation
after vacuum distillation
80
60
40
20
0
References
1. Igor MN, Abdelhamid S (2008) Appl Catal A 339:45
2. Mohamad IAW, Ali OAS (2002) Bioresour Technol 85:253
3. Freedman B, Butterfield RO, Pryde EH (1986) J Am Oil Chem
Soc 63:1375
4. Galen SJ, Mohanprasad DA, Eric DJ, Pratik MJ, Michael GJ
(2004) Appl Catal A 257(2):213
5. Wu H, Zong MH, Lou WY (2004) Chin J Catal 25:903
6. Xu YY, Du W, Liu DH, Zeng J (2003) Biotechnol Lett 25:1239
7. Du W, Xu YY, Liu DH, Zeng J (2004) J Mol Catal B 30:125
8. Li Y, Zhang XD, Sun L, Zhang J, Xu HP (2010) Appl Energy
87:156
1
2
3
4
5
6
7
9. Kenneth SR (1997) J Chem Technol Biotechnol 68:351
10. Thomas W (1999) Chem Rev 99:2071
11. Wilkes JS (2002) Green Chem 4:73
Run
Fig. 1 Reusability of the ionic liquid [BSPy][CF3SO3]
123

156
K.-X. Li et al.
12. Cole AC, Jensen JL, Ntai I, Tran KLT, Weaver KJ, Forbes DC,
Davis JH (2002) J Am Chem Soc 124:5962
13. Gu YL, Shi F, Deng YQ (2004) J Mol Catal A 212:71
14. Qiao K, Hagiwara H, Yokoyama C (2006) J Mol Catal A 246:65
15. Wang WJ, Cheng WP, Shao LL, Yang JG (2008) Catal Lett
121:77
16. Wu Q, Chen H, Han MH, Wang DZ, Wang JF (2007) Ind Eng
Chem Res 46:7958
17. Xing HB, Wang T, Zhou ZH, Dai YY (2007) J Mol Catal A
264:55
19. Pralhad AG, Gigi G, Jagannath D (2008) J Mol Catal A 279:183
20. Zhao YW, Long JX, Deng FG, Liu XF, Li Z, Xia CQ, Peng JJ
(2009) Catal Commun 10:733
21. Bhardwaj RC, Enayetullah MA, Bockris JO’M (1990) J Elect-
rochem Soc 137:2072
22. Han MH, Yi WL, Wu Q, Liu Y, Hong YC, Wang DC (2009)
Bioresour Technol 100:2308
23. Kaewta S, Edgar L, Kanokwan N, James GG Jr (2009) Ind Eng
Chem. Res 48:2810
18. Leng Y, Wang J, Zhu DR, Ren XQ, Ge HQ, Shen L (2009)
Angew Chem Int Ed 48:170
123