CFacile biodiesel synthesis from esterification of free fatty acids catalyzed by SO3H-Functionalized Ionic Liquid

Article abstract of DOI:10.14233/ajchem.2013.12930

The esterifications of free fatty acids with methanol were efficiently accomplished over a SO3H-functionalized ionic liquid, 1-(4-sulphonic acid)butyl-3-methylimidazolium hydrogen sulphate ([BSMim]HSO4), which are commonly considered as both one of the typical synthetic routes and the representative pretreatment process for biodiesel production. Particularly, the maximum yield of methyl oleate was obtained in 99.9 %. Methyl oleate could be isolated through simple decantation from the biphasic system, which is attributed to the immiscibility of [BSMim]HSO4 with hydrophobic ester and the good stability and operability of ionic liquid were demonstrated by the six times reuses without dramatic decrease in methyl oleate yield.

Full text of DOI:10.14233/ajchem.2013.12930

Asian Journal of Chemistry; Vol. 25, No. 1 (2013), 240-244
http://dx.doi.org/10.14233/ajchem.2013.12930
Facile Biodiesel Synthesis from Esterification of Free Fatty
Acids Catalyzed by SO3H-Functionalized Ionic Liquid
1
1
1,*
1
1,2
SHUAI ZHOU , LU LIU , BO WANG , FENG XU and RUNCANG SUN
1Institute of Biomass Chemistry andTechnology, College of Materials Science andTechnology, Beijing Forestry University, 100083 Beijing, P.R. China
2Key State Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou 510640, P.R. China
*Corresponding author: Fax: +86 10 62336592; Tel: +86 10 62336592; E-mail: mzlwb@bjfu.edu.cn
(Received: 24 October 2011;
Accepted: 20 July 2012)
AJC-11855
The esterifications of free fatty acids with methanol were efficiently accomplished over a SO3H-functionalized ionic liquid, 1-(4-sulphonic
acid)butyl-3-methylimidazolium hydrogen sulphate ([BSMim]HSO4), which are commonly considered as both one of the typical synthetic
routes and the representative pretreatment process for biodiesel production. Particularly, the maximum yield of methyl oleate was obtained
in 99.9 %. Methyl oleate could be isolated through simple decantation from the biphasic system, which is attributed to the immiscibility
of [BSMim]HSO4 with hydrophobic ester and the good stability and operability of ionic liquid were demonstrated by the six times reuses
without dramatic decrease in methyl oleate yield.
Key Words: Biodiesel, SO3H-Functionalized ionic liquid, Low-cost feedstocks, Oleic acid, Esterification.
alternative technologies such as using heterogeneous catalysts9,
INTRODUCTION
enzyme10 and membrane11 have been investigated. However,
the relatively high cost or complexity and instability of catalytic
Biodiesel, a clean-burning diesel fuel with much lower
toxic air emissions and biodegradable features1-3, usually
system unquestionably restrains their wide applications and
produced from non-toxic and biologically renewable resources,
large-scale production.
has been paid much attention all over the world and considered
In pace with the intensive study for task-specific ionic
as the best candidate for the substitution of petroleum products
in the future4. Along with the rapid reduction of arable land
tendentiously emerged as special acidic catalysts to perfect
liquids, a great range of acid-functionalized ionic liquids
and improve the organic synthesis12, especially including
and exceeding shortage of resources in the world, effective
biodiesel synthesis process13. Neto et al.14 has developed a
recycling and reusing low-cost feedstocks for biodiesel produc-
tion, such as non-edible and waste frying oils, have gradually
biphasic catalytic system that tin complexes is immobilized in
become one of the significant approaches to reducing the cost
1-n-butyl-3-methylimidazolium tetrachloro-indate ([BMim]InCl4)
of biodiesel production, which is the most vital theme of
ionic liquid. However, the insuperable defects in this system
such as the metal componet leaching and non-reusability of
biodiesel science5. However, the high content of free fatty
acids in the non-edible oils usually leads to serious processing
catalyst usually impair this methodology. Although the
problems, which require multiple chemical steps or alternate
approaches to produce biodiesel and consequently increase
biodiesel synthesis from soybean oil has been proceeded using
[Et3NH]Cl-AlCl3 as catalyst reported byYang et al.15. It is well
the production cost, even reduce the biodiesel yield below the
known that the chloroaluminate ionic liquids could not be
applied to the large-scale production just due to its sensitivity
standards6. Therefore, an esterification pretreatment of the free
fatty acids to alkyl esters in the presence of an acid catalyst
to air and water. Nevertheless, the research and application of
could efficiently eliminate the problems. In this regard, the
various SO3H-functionalized ionic liquids with strong Brønsted
traditionally used acid catalysts such as sulphuric acid, organic
acidicity have received much attention due to their potential
acids and solid acids etc., are usually associated with one of
applications in replacing conventionally catalytic systems16,17
.
the following disadvantages such as high temperature, long
reaction time, high cost and moisture sensitivity of the catalyst,
tedious isolation procedures, equipment corrosion and environ-
mental problems7,8. In order to abate these deficiencies, several
In the research of biodiesel synthesis, SO3H-functionalized
ionic liquids have revealed the peculiar characteristics that they
could serve as both acid catalyst and reaction media, leading
to simplify the product isolation and achieve the catalysts'

Vol. 25, No. 1 (2013)
Facile Biodiesel Synthesis from Esterification of Free Fatty Acids 241
recovering and recycling. In a particular case, the biodiesel
preparation from waste oils catalyzed by pyridine-based
sulphonic acid-functionalized ionic liquids has been reported,
however the relatively lower catalytic activities have to bring
the high-acid value waste oils. A special procedure:
[BSMim]HSO4 (2.0 g, 6.3 mmol) was charged into a 50 mL
round-bottom flask followed by the addition of oleic acid
(10 mL, 31.6 mmol) and MeOH (2.6 mL, 63.2 mmol) under
magnetic stirring. Then the mixture was heated to 80 ºC and
kept stirring for desired duration time. The biphasic system
formed when the stirrer stopped. The upper phase containing
methyl oleate and unreacted oleic acid was isolated by simple
decantation. And the ionic liquid phase retaining methanol is
always the lower phase, which can be recovered after simple
treatment under vacuum and used directly for the next run.
on the harsh reaction conditions18,19
.
During the course of the preparation of high value
materials from biomass, we found that the imidazolium-based
SO3H-functionalized ionic liquids have been proved to be the
excellent acidic catalytic media and applied to many kinds of
organic reactions due to their inherent strong acidity and
thermal stability20. This prompted us to investigate the esterifi-
cation of free fatty acids as the model pretreatment process of
biodiesel synthesis catalyzed by 1-(4-sulphonic acid)butyl-3-
methylimidazolium hydrogen sulphate ([BSMim]HSO4) ionic
liquid for the purpose of exploiting the low-cost feedstocks
and oleic acid and methanol were designated as the model
substrates for the esterification reaction as shown in Scheme-I.
1
The identification of samples was detected from H NMR
spectra, which were recorded on a MERCURY-PLUS 400
NMR spectrometer.
RESULTS AND DISCUSSION
Activity comparison of different acidic catalysts for
esterification of oleic acid: Initially, an activity comparison
study was carried out with several catalysts and ionic liquids
which are similar with chemical structure, in the esterification
of oleic acid and the results are summarized in Table-1. It can
be seen that, H2SO4 as the common inorganic acid catalyst
gives rise to a good ester yield (96.3 %), which is much better
than that from the organic acid catalysts such as p-toluene-
sulphonic acid (p-TSA, 38.2 %) and sulfamic acid (NH2SO3H,
68.1 %), respectively under the same reaction conditions.
However, the toxicity, corrosivity and contamination of these
catalysts forced people to use the more benign alternatives.
Herein, the sulphonic acid functionalized ionic liquids were
chosen as the friendly catalyst for the esterification of oleic
acid and methanol. And [BSMim]HSO4 ionic liquid shows
better reactivity than p-TSA and NH2SO3H. Methyl oleate yield
could come up to 92.0 %, which is undoubtedly better than
that over H2SO4 from the viewpoint of environmental pro-
tection, although there is a little decrease in ester yield
comparatively. While the ionic liquids with the same cations
[BSMim]H2PO4 and [BSMim]OAc, just only provide the
ester yields in 33.5 and 17.2 %, respectively, even if the reaction
time is extended twice. At the same time, the negligible ester
yield (3.4 %) was achieved when reaction was carried out only
using [BMim]Cl ionic liquid as catalyst. This demonstrated
that the butyl sulphonic acid group attached to the cation has
relatively little influence on the esterification of oleic acid.
On the contrary, the anions of ionic liquids decisively predo-
minate their own catalytic performance depending on the
corresponding acid strength of anions22.
EXPERIMENTAL
Preparation of [BSMim]HSO4 ionic liquid: The SO3H-
functionalized ionic liquid [BSMim]HSO4 was synthesized
according to the literature method21. The detailed preparation
procedure was as follows: To 150 mL toluene in 500 mL three-
necked round-bottom flask equipped with mechanical stirrer,
1,4-butanesulfone (51.1 mL, 0.5 mol) was added followed by
an equal-mole N-methylimidazole (39.6 mL, 0.5 mol) trans-
ferring at room temperature and the mixture was kept stirring
for overnight to form white precipitate. Then the white solid
was filtered out, washed with diethyl ether thrice (30 mL ×
3 mL) to remove non-ionic residues and dried at 80 ºC under
vaccum for overnight to get the zwitterion, 4-(1-methylimi-
1
dazolium-3-yl)butane-1-sulfonate (yield 97.1 %). H NMR
(400 MHz, D2O): δ 1.670 (m, 2H), 1.977 (m, 2H), 2.878 (m,
2H), 3.851 (s, 3H, J = 3.0 Hz), 4.203 (m, 2H), 7.414 (m, 1H),
7.475 (m, 1H), 8.710 (s, 1H).
Nextly, the white zwitterion was charged into 100 mL
H2O in 500 mL round-bottom flask and a stoichiometric
amount of concentrated sulphuric acid was added dropwise at
room temperature under vigorously magnetic stirring. Then,
the reaction mixture was gradually heated up to 90 ºC and
kept stirring for 8 h followed by removing water under vacuum
at 90 ºC, giving [BSMim]HSO4 ionic liquid (yield 98.0 %).
1H NMR (400 MHz, DMSO): δ 1.586 (m, J = 7.32 Hz, 2H),
1.905 (m, J = 7.44 Hz, 2H), 2.624 (t, J = 7.56 Hz, 2H), 3.873
(s, 3H), 4.204 (t, J = 7.04 Hz, 2H), 7.724 (m, J = 1.64 Hz,
1H), 7.786 (m, J = 1.64 Hz, 1H), 9.177 (s, 1H), 9.294 (bs,
2H).
Additionally, it is well known that current biodiesel pro-
duction in industry through alkali-catalyzed transesterification
using KOH, NaOH or K2O as catalysts, usually suffered from
the saponification, which entirely contributes to the decrease
in the biodiesel yield and catalyst efficiency23. Based on this,
Esterification of oleic acid and methanol catalyzed by
[BSMim]HSO4: The esterification of oleic acid with methanol
catalyzed by [BSMim]HSO4 was investigated as the model
pretreating reaction for the biodiesel synthesis derived from
H
H
H
H
ROH, R = short chain alkyl
( )4
C
H3C (CH2)7
C
OR
C
H3C (CH2)7
C
OH
(CH2)7 C
(CH2)7 C
N
N+
HSO4
SO3H
O
O
-
Scheme-I: Model biodiesel synthesis over esterification of oleic acid and methanol catalyzed by [BSMim]HSO4

242 Zhou et al.
Asian J. Chem.
TABLE-1
COMPARISON OF ESTERIFICATION OF OLEIC ACID AND
MeOH IN THE PRESENCE OF DIFFERENT CATALYSTSa
100
80
60
40
20
0
Catalyst
H2SO4
Cat. (mol %)b Time (h)
Yield of ester (%)c
40
40
6
6
96.3
68.1
38.2
3.4
NH2SO3H
p-TSA
40
6
[BMim]Cl
40
24
6
[BMim]ClH2SO4
[BMim]HSO4
[BSMim]HSO4
[BSMim]H2PO4
[BSMim]Ac
[BMim]OH
40/40
40
97.5
84.8
92.0
33.5
17.2
15.7
6
40
6
40
16
16
16
40
0
5
10
15
20
100
b
aOleic acid (2 mL), methanol (0.5) mL, 80 ºC; Based on oleic acid;
cTested by 1H NMR analysis, but yields by weight were also evaluated
and quantitative like the 1H NMR analysis data.
Reaction time (h)
reaction time/h
Fig. 2. Effect of reaction time on the esterification of oleic acid with MeOH
catalyzed by [BSMim]HSO4 (oleic acid: MeOH = 1:2; 40 mol % of
[BSMim]HSO4; 80 ºC)
we have expected the basic ionic liquid [BMim]OH (1-butyl-
3-methylimidazolium hydroxide)24 would eradicate the saponi-
fication problem. However, it seems that the elementary explo-
ration is not successful (entry 7), the activity of [BMim]OH
has been destroyed on account of the interaction between ionic
liquid and fatty acid.
12 h could be optimal for the esterification (97.5 %) of oleic
acid with methanol catalyzed by [BSMim]HSO4 ionic liquid.
Fig. 3 revealed that as the proportion of ionic liquid increasing,
a significant rise in the initial rate was observed when the ionic
liquid amount was less than 15 mol % (based on substance).
However, further improving the proportion of ionic liquid to
40 mol % based on oleic acid, the increased rate of product
yield started to decrease slightly. For a rise in content of ionic
liquid from 15-40 mol %, the yield of methyl oleate just only
increased about 1.7 % under the same reaction conditions.
Moreover, it looks like that too much ionic liquid has a detri-
mental effect on the ester yield (100 mol % of [BSMim]HSO4
based on oleic acid, 93.3 % of ester yield), which is probably
due to the relatively poor dispersion of ionic liquid in the
reaction mixture. As far as the reusability of ionic liquid is
concerned, 20 mol % proportion of ionic liquid was chosen as
the best option.
Reaction conditions optimization: We set out to exami-
ning the effect of reaction temperature, time, catalyst amount,
molar ratio of reactants on the esterification of oleic acid and
methanol in the presence of [BSMim]HSO4. As shown in
Fig. 1, 62.5 % of ester yield was achieved after 6 h at room
temperature, indicating the inherent efficiency of the acidic
ionic liquid. For a 20 ºC rise in temperature, the yield of methyl
oleate could increase about 10 % during the same reaction
time. Consequently, when the reaction was performed under
reflux (80 ºC in oil bath), the yield of methyl oleate could
reach the maximum (92 %).
100
90
80
70
60
100
95
90
85
20
40
60
80
0
50
Ionic liquid (%)
100
Temperature (ºC)
temperature/
℃
ionic liquid percent/%
Fig. 1. Effect of reaction temperature on the esterification of oleic acid
with MeOH catalyzed by [BSMim]HSO4 (oleic acid: MeOH = 1:2;
40 mol % of ionic liquid; 6 h)
Fig. 3. Effect of ionic liquid amount on the esterification of oleic acid with
MeOH catalyzed by [BSMim]HSO4 (80 ºC; 12 h; oleic acid: MeOH
= 1:2)
As the data shown in Fig. 2, the reaction proceeded fast at
the beginning and 85.2 % of ester yield was obtained after 4 h
at 80 ºC. When reaction time was prolonged to 8 h, 94.6 % of
methyl oleate was achieved. This result was comparable to
the yield data (95 %) using [NMP]CH3SO3 ionic liquid as acidic
catalyst25. Subsequently, the yield of methyl oleate increased
slowly and finally the reaction nearly reached to completion
(99.9 %) at 18 h. Taking the energy consumption into account,
Undoubtedly, an excess of alcohol is necessary for the
esterification of oleic acid. However, a cost-efficient molar
ratio of MeOH to oleic acid should be favorable for the prac-
tical application of the catalytic system. Fig. 4 showed that
the yield of methyl oleate increased slightly from 97.5-99.5 %
with the molar ratio of methanol to oleic acid increasing from
2:1 to 10:1. Thus, it seems that too much excess of methanol

Vol. 25, No. 1 (2013)
100
Facile Biodiesel Synthesis from Esterification of Free Fatty Acids 243
to producing biodiesel, increase in the production cost and
depression of biodiesel quality below the standards6. Encour-
aged by the promising results in hand, we have attempted to
perform the esterification of several other fatty acids e.g., (lauric
acid 98.3 %, myristic acid 97.4 %, palmitic acid 95.7 % and
stearic acid 95.6 %) with methanol under the above-mentioned
optimized conditions. Evidently, these data revealed that the
alkyl chain length of fatty acid has negligible little effect on
the esterification of long-chain fatty acids.Accordingly, it can
be said that all of the free fatty acids involved in the waste
frying oils could be converted into esters in the presence of
[BSMim]HSO4, no matter what kind of fatty acids they are,
which is very valuable for the effective recycling and reusing
of low-cost feedstocks.
99
98
97
96
2
6
10 14
MeOH:fatty acid
18
MeOH:Fatty acid
Fig. 4. Effect of molar ratio of MeOH to oleic acid on the eaterification of
oleic acid with MeOH catalyzed by [BSMim]HSO4 (20 mol % of
[BSMim]HSO4; 80 ºC; 12 h)
Conclusion
In summary, esterification of fatty acids and methanol has
been performed smoothly using [BSMim]HSO4 ionic liquid
(20 mol %) as catalyst and methyl oleate could be obtained
with 96.8 % of yield at 80 ºC after 12 h, which is commonly
deemed as both one of the typical synthetic routes and the
representative pretreatment process for biodiesel production.
Furthermore, the reactivity of sulphonic acid functionalized
ionic liquids for the esterification of oleic acid was probably
predominated by the anions paired to imidazolium cation.
Particularly, the product separation was easily accomplished
through the decantation of the biphasic system attributed to
the immiscibility of the acidic ionic liquid with organic ester.
Moreover, [BSMim]HSO4 could be reused several times just
only to need simple vacuum treatment. This method is far
superior to others performed using toxic acidic catalysts such
as H2SO4, because [BSMim]HSO4 ionic liquid has combined
the advangtages of friendly benign acid catalyst with an
excellent product separation methodology, as well as the cost
efficiency and green aspect, which are in line with the demand
of sustainable chemistry.
is not always good for the production of biodiesel, probably
due to the dilution effect of [BSMim]HSO4 catalyst by methanol.
The similar phenomenon has been displayed by Zhang et al.25.
Therefore, the molar ratio of 2:1 was believed to be optimal in
the [BSMim]HSO4 catalytic system with regard to the produc-
tion cost. It is worth mentioning that increasing the amount of
methanol did not influence the biphasic system formation after
reaction and the product ester still could be separated easily
by decantation.
Reusability of [BSMim]HSO4 in esterfication of oleic
acid: In this study, [BSMim]HSO4 ionic liquid was separated
from the organic phase after first run and treated under vacuum
before it was directly used in the next run. From the results
given in Fig. 5, there is no dramatic decrease in ester yield
after [BSMim]HSO4 was reused six times in the esterification
of oleic acid, which gives evidence of the thermal stability
and feasibility of [BSMim]HSO4 ionic liquid for industrial
production of biodiesel.
100
90
ACKNOWLEDGEMENTS
Financial supports from the Fundamental Research Funds
for the Central Universities (YX2011-36, TD2011-1, TD2010-1),
the Forestry UniversityYoung Scientist Fund (BLX2009003),
National Natural Science Foundation of China (31170556),
Research Fund for the Doctoral Program of Higher Education
of China (20100014120007), China Postdoctoral Science
Foundation (20110490303), Major State Basic Research
Development Program of China (973 Program, No. 2010CB
732204) and China Ministry of Education (No. 111 project)
are gratefully acknowledged.
80
70
60
50
1
2
3
4
5
6
Run
Run
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Fig. 5. Reuse of [BSMim]HSO4 in the esterification of oleic acid with
MeOH (40 mol % [BSMim]HSO4; oleic acid: MeOH = 1:2; 80 ºC;
12 h)
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