Temperature regulated Br?nsted acidic ionic liquid-catalyze esterification of oleic acid for biodiesel application

Article abstract of DOI:10.1016/j.molstruc.2016.10.049

By combining phosphotungstic acid (PW) and SO₃H- functioned zwitterion, heteropoly anion-based Br?nsted acidic ionic liquids (HPA-ILs) were successfully obtained. Scanning electron microscopy and energy dispersive X-ray spectroscopy were provid

Full text of DOI:10.1016/j.molstruc.2016.10.049

Accepted Manuscript
Temperature regulated Brønsted acidic ionic liquid‐catalyze esterification of oleic acid
for biodiesel application
Ezzat Rafiee, Fakhrosadat Mirnezami
PII:
S0022-2860(16)31090-0
10.1016/j.molstruc.2016.10.049
MOLSTR 23050
DOI:
Reference:
To appear in: Journal of Molecular Structure
Received Date: 15 June 2016
Revised Date: 27 September 2016
Accepted Date: 14 October 2016
Please cite this article as: E. Rafiee, F. Mirnezami, Temperature regulated Brønsted acidic ionic liquid‐
catalyze esterification of oleic acid for biodiesel application, Journal of Molecular Structure (2016), doi:
10.1016/j.molstruc.2016.10.049.
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Temperature regulated Brønsted acidic ionic liquid‐catalyze esterification of oleic acid
for biodiesel application
Ezzat Rafiee , a, b, Fakhrosadat Mirnezami aa
a Faculty of Chemistry, Razi University, Kermanshah, 67149, Iran
b Institute of Nano Science and Nano Technology, Razi University, Kermanshah, 67149,
Iran
*Corresponding author. Tel./fax: +98-833-427-4559; e-mail: ezzat_rafiee@yahoo.com;
e.rafiei@razi.ac.ir
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Abstract
By combining phosphotungstic acid (PW) and SO3H- functioned zwitterion, heteropoly anion-
based Brønsted acidic ionic liquids (HPA-ILs) were successfully obtained. Scanning electron
microscopy and energy dispersive X-ray spectroscopy were provided the morphology and
composition of the prepared material. Catalytic performance and reusability of the catalysts
were evaluated through an esterification reaction between oleic acid and methanol for
production of biodiesel. Relationship between catalytic activities and acidity of the catalysts
have been discussed by potentiometric titration. The results showed that HPA-ILs had good
activity and reusability. HPA-ILs dissolved in the reaction mixture during the reaction process
and could be precipitated and separated from products at lower temperature.
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Keywords: Brønsted acidic ionic liquid, Polyoxometalate, Biodiesel, Thermolegulated
catalyst, Esterification.
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1. Introduction
Fatty acid methyl esters (FAME), biodiesel, a suitable substitute for petroleum-derived
diesel, is a renewable and non-polluting fuel that consists of methyl or ethyl esters resulting
from either the transesterification of triglycerides or the esterification of free fatty acids (FFA)
[1]. Recently, biodiesel have attained great importance, and research has intensified for the use
of plant/tree oil and animal fat as a source of fatty acids (FA) [2]. In many cases, the use of
fatty acids as feedstock for biodiesel production is economically attractive as they are the
byproducts of food processing plants [3]. Biodiesel contains no aromatics which are believed to
contribute to higher particulate and nitrogen oxides emissions [4]. The contents of water and
free fatty acids in the raw oil need to be very low in order to prevent saponification, which
induces depletion of catalysts thus reducing their catalytic activities [5, 6]. In recent years,
many investigations on catalysts for the esterification step have demonstrated the use of solid
superacids, [7–9] polyoxometalates, [10, 11] or cation‐exchange resins with strong acidity [12,
13] as catalysts. These catalysts can displace sulfuric acid and thereby solve process problems
such as equipment corrosion and environmental pollution. However, the preparation of these
catalysts is relatively complicated, they are difficult to recycle, and the production cost of the
catalysts is high. Therefore, it is necessary to develop an environmentally‐friendly, efficient,
and novel catalyst for the synthesis of biodiesel by esterification.
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In response to the development of more environmental benign esterification process,
ionic liquids (ILs) seem to be a good option. ILs, defined as molten salts, consist of only
cations and anions, and usually appear as liquids under 100 °C. ILs are considered as
replacements for volatile organic solvents due to negligible vapor pressure, [14] which
eliminate problems associated with volatility and flammability. Furthermore, cations and
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anions of ILs can be designed to bind a series of groups with specific properties. Diversified
ILs containing various cations and anions have been synthesized over the past few decades
[15]. Recently, a new Brønsted acidic IL emerged, which bears an alkyl sulfonic acid group in
cations [16]. Cole et al. firstly synthesized Brønsted acidic ILs that bear an alkyl sulfonic acid
group in imidazolium cations or triphenylphosphine cations [17]. ILs were used as Brønsted
acidic catalysts for the esterifications and transesterification [18-20] and alkylation reaction
[21]. Indeed, heteropoly anions (HPA) have been used as counter anions for ILs and some
HPA-IL compounds have been prepared and employed as inorganic–organic hybrid catalysts
[22-30]. Encouraged by those results, in this paper, we prepared three Brønsted acidic ILs and
used them in the production of biodiesel. The aim of this work is to find the most active
catalyst for this reaction and to analyze the influences of experimental conditions on the
reaction. Specially, the catalytic activity for esterification and the reuse performance of
Brønsted acidic IL were examined. These catalysts were dissolved in the reaction mixture at 65
°C and thus, it can homogeneously catalyze the reaction. It was precipitated quantitatively at
the end of the reaction upon cooling to lower temperature and thus, it can recover
heterogeneously.
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2. Experimental
2. 1. General remarks
All organic materials and H3PW12O40 (PW) were purchased commercially from Fluka Chemical
Corp. and Merck & Co., Inc. and were used without further purification. FTIR spectra were
recorded with KBr pellets using a WQF-510 FTIR Rayleigh. The feedstock and products were
analyzed by the gas chromatographic-mass spectroscopy (GC-MS) using an Agilent GC-6890
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N/MS-5973 with a HP-5 capillary column (30 m length, 0.32 mm i.d., 0.25 mm film
thickness). Scanning electron microscopy (SEM) has been performed using an LEO1450 VP
microscope with scanning range from 0 to 30 keV. Electron dispersive X-ray spectroscopy
(EDX) measurements were made with oxford company model 7353 attached to SEM. The
potential variation was measured with a Hanna 302 pH meter using a double junction electrode.
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2. 2. Preparation of PW-based sulfonated Brønsted acidic IL catalysts
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Isoquinoline (Q) (0.01 mol) and 1,4-butane sultone (Bs) (0.01 mol) were mixed at 50
°C. After 3 h, 5 mL toluene was added and stirred for 48 h under nitrogen atmosphere. A white
solid zwitterion, QBs, was formed; it was filtered off and washed with ether for three times,
then dried in a vacuum. The resulting aqueous solution of QBs (0.03 mol) was added to the
aqueous solution of PW (0.01 mol), followed by the stirring of the mixture at room temperature
for 24 h. Water was removed in vacuum to give the final product as solid (QBs-PW). PhPyBs-
PW and PyBs-PW were prepared based on the same method with using corresponding starting
materials (Scheme 1) according to our previously published paper [31, 32].
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Scheme 1. Structures of prepared HPA–ILs.
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2. 3. Acidity Measurement
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The acidity of samples was measured using potentiometric titration. For the
potentiometric titration, 0.05 g of solid was suspended in acetonitrile (90 mL) and stirred for 15
min. The suspension was titrated with 0.05 N solution of n-butylamine in acetonitrile. The
potential variation was measured with a Hanna 302 pH meter using a double junction electrode.
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2. 4. General procedure for the production of biodiesel
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The esterification of oleic acid with methanol (4, 5 and 6 wt.% methanol to oleic acid)
in the presence of various amounts of the HPA–IL as catalyst (1, 3, 5 and 7 wt.% catalyst to
oleic acid) was poured in a 25 mL flask equipped with a reflux condenser. The reaction
mixture was refluxed for 2 h at 65 °C. Completion of the reaction was indicated by TLC (n-
heptane/ethyl acetate 6:1). After the completion of the reaction, the mixture of reaction was
cooled to 5 °C. The catalyst precipitate as solid at the bottom layer and could be easily
separated by decantation. The unreacted alcohol was recovered by rotary evaporator. A mixture
of n-heptane:water (1:1, v/v) was added to become biphasic system. The upper phase is mainly
containing the desired biodiesel, n-heptane and un-reacted FFAs, and the bottom phase
containing a mixture of possibly HPA-IL, water and trace amounts of ester. The non-polar
phase was analyzed using GC-MS for characterization of the products.
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3. Results and discussion
The synthetic approach used to assemble the zwitterionic precursors to acidic IL.
Characterization data of PyBs-PW, QBs-PW and 4-PhPyBs-PW were previously reported [31,
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32]. First reaction of the neutral nucleophiles such as pyridine (Py), Q and 4-phenyl-pyridine
(PhPy) with Bs, produces the requisite zwitterion. In the second step, the zwitterion is mixed
with strong Brønsted acid, PW, to obtain PW-based sulfonated Brønsted acidic IL. The yields
of the Brønsted acidic ILs are excellent, as no byproduct is produced in the Brønsted acidic IL
synthetic procedure. The fresh PyBs-PW, QBs-PW and 4-PhPyBs-PW are respectively creamy,
orang and golden solids at room temperature.
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Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS)
were also used to characterize the morphology and composition of the hybrid material. Fig. 1 is
the SEM images of amorphous QBs-PW, which is made up of irregular block structures with a
size of several micrometers. The corresponding EDS spectrum (Fig. 2) indicates the presence
of C, O, W, S and P elements. W percentage of 56.31% in the hybrid catalyst was suggested by
EDX quantitative analysis (Fig. 2). The amount of determined W is near to stoichiometric
calculated value, 60.01% and thus, the EDX suggested the presence of PW in the catalyst.
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Fig. 1. SEM images of QBs-PW
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Fig. 2. EDS spectrum of QBs-PW
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Esterification is one type of classical acid-promoted reaction and is usually used to
check an acidic catalytic activity. The methanolysis of FFA is a typical reversible acid-
catalyzed reaction that produces ester and water as by-product (Scheme 2). In the present case,
the conversion can be increased by increasing the concentration of either alcohol or FFA. In
practicality, for economic reasons, in order to obtain maximum conversion, the reactant that is
usually less expensive is taken in excess. In the present study corresponding alcohol was taken
in excess amount. The catalytic performances of HPA–ILs were investigated in the
esterification of oleic acid with methanol (Scheme 2). Three catalysts with different cations
were investigated to choose the best catalyst in point of reusability in parallel with catalytic
reactivity of methanolysis of oleic acid. The experiments were carried out at 65 °C in the
presence of 3.0 wt.% of the catalyst and molar ratio of methanol: oleic acid 5:1 after 2 h (Table
1). QBs-PW, PhPyBs-PW presented the best catalytic performances in terms of conversion of
oleic acid. Typically, for synthesis of hybrid materials, first, N-heterocyclic compounds are
reacted with Bs to obtain a solid zwitterion. This zwitterion is then protonated with a strong
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Brønsted acid such as HPAs to obtain the desired material. In fact, ion exchanging was
happened, this happening can explain how the HPAs actives cations. According to the obtained
results in literature [33, 34], the super acidity of hybrid materials was responsible for high
catalytic activity. In their sample, propane sultone (Ps) acid sites in organic cations worked as
the active centers for esterification. The catalytic activity of sample increased with increasing
the amount of cation. This verifies explicitly a very high activity of the acid site provided by Ps
in organic cations of HPA salts. From the results reported herein, it can be assumed that the
cations are responsible for the high activity in esterification. At the end of the reaction, the
catalyst was recovered by cooling the reaction mixture to 5 °C, dried in vacuum and weighed.
The loss of 12 and 29 wt.% of the PhPyBs-PW and QBs-PW (compared with the amount of the
catalyst in the first run) was found. But, PyBs-PW was soluble and did not appear at the bottom
of the reactor as solid. Therefore this catalyst could not collected and weighted. So among three
catalysts, QBs-PW was found to be the most promising in recycling (71 wt.% of the catalyst
was precipitated at the end of the reaction) and gave the highest yield (90% conversion).
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CH3OH
OH
OH
+
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6
6
OH
O
H+
+
HOCH3
OCH3
OH
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6
O
+
OH
H2O
H+
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Scheme 2. Schematic illustration for acid-catalyzed esterification of oleic acid with methanol.
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Table 1
Results of esterification of different catalysts.
Entries
Catalyst
QBs-PW
Conversion (%)
Recyclability of the catalyst
(wt.%)
1
2
3
90
80
56
71
PhPyBs-PW
4-PhPyBs-PW
-
88
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Reaction conditions: Catalyst loading, 3.0 wt.% to oleic acid; methanol: oleic acid,
5:1; at 65 °C after 2h.
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The typical GC-MS chromatogram of oleic acid sample after the esterification reaction
in the presence of 3.0 wt.% QBs-PW as catalyst and molar ratio of methanol: oleic acid, 5:1 at
65 °C after 2 h, is shown in Fig. 3. The composition of the sample was oleic acid (82.54 wt.%),
myristic acid (3.54 wt.%), palmitic acid (6.17 wt.%), palmitoleic acid (5.83 wt.%) and stearic
acid (1.92 wt.%). The catalytic activity was expressed as yield of conversion of oleic acid. Also
the effect of various reaction parameters, such as, type of the catalyst, molar ratio of methanol
to oleic acid, temperature, content of water and amount of the catalyst were investigated in the
esterification of oleic acid with methanol using QBs-PW as the best catalyst (Table 2).
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Fig. 3. Typical GC-MS chromatogram of oleic acid sample after the esterification reaction.
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Acidity measurements of the catalysts by means of potentiometric titration with n-
butylamine were carried out to determine if there is any relationship between acidic properties
and catalytic reactivity (Fig. 4). This method enables determining the distribution and total
number of acid sites and their acid strength. The initial electrode potential (Ei) demonstrates the
maximum strength of the acid sites and the value from which the plateau is reached (mmole
amine/g solid, n) indicates the total number of acid sites that are present in the titrated solids.
The acidic strength of the acid sites can be classified according to the following ranges: Ei>100
mV (very strong acid sites), 0<Ei<100 mV (strong acid sites), -100<Ei<0 mV (weak acid sites)
and Ei<-100 mV (very weak acid sites) [35]. As shown in Fig. 4, all kinds of POM based
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catalysts presented very strong acid sites, Ei> 100 mV. As a result, the higher activity of QBs-
PW is related to higher acidic strength of this catalyst (Ei= 180 mV). The number of acid sites
in HPA-ILs are approximately the same. The catalytic activities were in the following
sequence: QBs-PW>PhBs-PW>4-PhBs-PW (Fig. 4). Thus, as shown in Table 1, it seems that
QBs-PW can be chosen as the best catalyst in esterification.
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-50
-150
-250
0
0.5
1
1.5
mmole amin/g catalyst
4-PhBs-PW PhBs-PW
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QBs-PW
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Fig. 4. Potentiometric titration curves of different POM-ILs (QBs-PW, 4-PhBs-PW,
PhBs-PW).
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Fig. 5 shows the reaction time effect on production of biodiesel within the range of 10-120
min. The catalyst was efficient in the first hour of the reaction with the yield reaching up to
60%. After that there was no considerable change in the yield, therefore the reaction time of 2 h
was identified as the optimum reaction time.
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80
60
40
20
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0
50
100
150
Time (min)
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Fig. 5. Effect of the reaction time on biodiesel production (QBs-PW loading, 3.0 wt.% to
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oleic acid; methanol: oleic acid, 5:1; at 65 °C).
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Temperature is one of the vital factors which influence the reaction and yield of
esterification. The effect of reaction temperature on the esterification of oleic acid with
methanol in the presence of QBs-PW was investigated in the temperature range of 30-75 °C for
2 h (Fig. 6). The other variables were fixed as: catalyst loading 3.0 wt.%, methanol:oil ratio 5:1
and time 2 h. The biodiesel yield increased as the temperature increasing, which suggests that
increasing the reaction temperature will increase solubility and mixing of the reactants and
hybrid catalyst. The highest methyl ester yield was achieved at almost 65 °C. Therefore, in the
present study 65 °C was decided as the optimum reaction temperature.
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The model reaction was studied in the absence of the catalyst, only 5% of the
corresponding product was obtained after 7 h (Table 2, entry 1). Effect of the amount of the
catalyst on the esterification of oleic acid was investigated and the results are listed in Table 2
(entries 2-5). As a general trend, the increase in the amount of the catalyst up to 3.0 wt.%
caused improvement in the methyl oleate yield with QBs-PW as catalyst. However, note that
beyond a certain limit (which seems to be 3.0 wt.%) an increase in the amount of the catalyst
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did not cause a remarkable change in the reaction conversion. As clearly shown in Table 2,
almost identical methyl oleate yields were achieved when the amount of QBs-PW used was
either 7.0 wt.%.
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Temparature (°C)
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Fig. 6. Effect of the temparature on biodiesel production (QBs-PW loading, 3.0 wt.% to
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oleic acid; methanol: oleic acid, 5:1; for 2 h).
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Oleic acid conversion can be increased by introducing an excess amount of methanol to
shift the equilibrium in the forward direction. In order to test the effect of molar ratio of
methanol to oleic acid, esterification reaction was carried out by varying mole ratio of
methanol to oleic acid (4:1 to 6:1). It is observed that the oleic acid conversion increases with
increase in the methanol to oleic acid ratio and reaches a maximum when molar ratio is 5:1.
With further increase in molar ratio, decrease in conversion was observed. Further increase of
this mole ratio did not increase the conversion probably because oleic acid and HPA-IL were
too diluted with excess methanol. Hence 5:1 molar ratio of methanol to oleic acid was
optimized for further studies.
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Table 2
Effect of different operating conditions on the esterification of oleic acid with methanol using
QBs-PW as catalyst at 65 °C after 2 h.
Entry
Catalyst loading
(wt.%)
Methanol: Oleic acid Water content
Yield
(%)a
5
(w:w)
5:1
5:1
5:1
5:1
5:1
4:1
6:1
5:1
5:1
5:1
5:1
(wt.%)
1
-
-
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1.0
3.0
5.0
7.0
3.0
3.0
3.0
3.0
3.0
3.0
31
3
-
85
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-
82
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80
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30
7
-
73
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2
82
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10
13
15
80
69
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30
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a by GC-MS
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The inhibitory effect of water on the ester formation is known. It is reasonable to state
that the decline in the ester yields could also be caused by a smaller number of available
protons as the proportion of coordinating water increases in the reaction medium. So water-
tolerant catalysts needed to be explored. To investigate the effect of water on catalytic activity,
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the experiment was carried out by adding some excess water (2.0 to 15.0 wt.%) to the mixture
of reaction under the same conditions (methanol: oleic acid, 5:1; catalyst 3.0 wt.%; at 65 °C; 2
h). Up to 13.0 wt.% of water to oleic acid the catalytic activity was fairly stable. The present
catalyst gave only 30% conversion at 15 wt.% water content. Therefore, the QBs-PW is a
relatively water-tolerant catalyst up to 13.0 wt.%.
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Activity of the reused catalyst was investigated through the esterification of oleic acid
with methanol in the presence of QBs-PW as catalyst (Fig. 7). Recovery of the catalyst was
very convenient. After each run, the reaction mixture was cooled, catalyst appeared at the
bottom of the reactor as solid and then could be recovered by decantation (or filtration). There
was no significant decrease in yield after four subsequent runs. After four cycles, the catalyst
was weighed, loss of 15 wt.% of the catalyst (compared with the amount of the catalyst at the
first run) was found. These results indicated that the catalyst showed no significant change in
its activity and only a trace declining in the yield was observed.
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80
60
40
20
0
Run 1
Run 2
Run 3
Run 4
Yield of Product
wt.% of recovered catalyst
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Fig. 7. Reusability of the catalyst (QBs-PW loading, 3.0 wt.% to oleic acid; methanol: oleic
acid, 5:1; at 65 °C after 2 h).
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The FTIR spectra are quite useful to identify structural and bonding changes in the
Keggin unit present in HPA-ILs. FTIR spectra of PW, QBs and QBs-PW are illustrated in Fig.
8a to c respectively. It can be seen that in wavenumber region of 800–1100 cm−1, PW gave four
featured peaks 1080 (P–Oa), 984 (W=Od), 889 (W–Ob–W) (corner-sharing), and 806 cm−1 (W–
Oc–W) (edge-sharing)) assigned to the Keggin structure. Two bands around 3500 and 1610
cm−1 were assigned to the modes of O–H and H2O, respectively [36]. On the other hand, S=O
stretching vibrations at 1037 and 1170 cm−1 were detected to verify the presence of sulfonic
group in catalysts. The FTIR spectrum for the recycled QBs-PW sample is shown in Fig. 8.
The four characteristic peaks of the Keggin anion were observed at 1080, 984, 889, and 806
cm−1. It was shown that the Keggin structure of the PW is well retained after the reusing (Fig.
8d). Also, characteristic peaks at 1037 and 1176 cm−1 (S=O) for the cation indicated good
structural stability of the reused catalyst.
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Potentiometric titration of reused catalysts (QBs-PW and its reused) was investigated to
understand what happens to the surface acidity of the catalysts after recovery. As shown in Fig.
9, Ei of QBs-PW comes down to about 165 mV in reused compared to fresh QBs-PW (Ei =180
mV in fresh QBs-PW). However, there were no significant differences in ‘‘n’’values between
fresh and reused catalysts. Fresh and reused catalysts presented very strong acid sites, Ei> 100
mV.
4. Conclusion
In this study, we report in detail the catalytic behavior of the SO3H- functioned HPA-
ILs in esterification, taking the esterification of oleic acid with methanol for production of
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biodiesel. Results of SEM indicate morphology of surface catalyst, results of EDX provide an
evidence of existence of the Keggin structure unit in catalyst. Acidity of the catalyst were
investigated by potentiometric titration method. It was found that QBs-PW acted as
homogeneous catalyst at the reaction temperature, and upon the cooling down of the reacted
mixture, it resumed the solid state that could be easily recovered and reused without significant
change in the catalytic activity in four-runs test.
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Fig. 8. FTIR spectra of (a) PW, (b) QBs, (c) QBs-PW and (d) recycled QBs-PW catalyst.
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250
150
50
-50
-150
-250
0
0.5
1
1.5
2
2.5
3
3.5
4
me amin/ g catalyst
QBs-PW
Reused
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2
3
Fig. 9. Potentiometric titration curves of QBs-PW and its reused.
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Highlights:
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SO3H-functionalized polyoxometalate-based ionic liquid was synthesized and characterized.
Catalytic activity of these ionic liquids were investigated in preparation of biodiesel.
Reusability and weight loosing of these catalysts were studied.