Heterogenization of ionic liquid based on mesoporous material as magnetically recyclable catalyst for biodiesel production

Article abstract of DOI:10.1016/j.molcata.2014.12.002

To develop green and reusable catalysts for the biodiesel preparation, a novel magnetically separable nanocatalyst was fabricated by covalent bonding of 3-sulfopropyl-1-(3-propyltrimethoxysilane) imidazolium hydrogen sulfate ([SO₃H-PIM-TMSP]HSO₄) on mesoporous silica modified Fe₃O₄ nanoparticle. The catalyst possessed uniform core-shell structure with magnetic response core and Br?nsted ionic liquid functionalized mesoporous silica shell. The multifunctional catalyst also exhibited appealing physicochemical features like high specific surface area, mesoporous channel, magnetic response, and fine catalytic activity in esterification of oleic acid with alcohol. Under mind conditions, the conversion of oleic acid was up to 93.5%. The catalyst exhibited recyclable property based on magnetism and the separation of the catalyst would be simply using external magnetic field without obvious mass loss, allowing a clean biodiesel product.

Full text of DOI:10.1016/j.molcata.2014.12.002

Journal of Molecular Catalysis A: Chemical 398 (2015) 127–132
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Heterogenization of ionic liquid based on mesoporous material as
magnetically recyclable catalyst for biodiesel production
Hui Wan, Zuowang Wu, Wen Chen, Guofeng Guan∗, Yuan Cai, Chong Chen, Zhong Li,
Xiaoqin Liu
State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering, Nanjing Tech University,
Nanjing 210009, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 23 October 2014
Received in revised form 2 December 2014
Accepted 3 December 2014
Available online 8 December 2014
To develop green and reusable catalysts for the biodiesel preparation, a novel magnetically separable
nanocatalyst was fabricated by covalent bonding of 3-sulfopropyl-1-(3-propyltrimethoxysilane) imida-
zolium hydrogen sulfate ([SO3H-PIM-TMSP]HSO4) on mesoporous silica modified Fe3O4 nanoparticle. The
catalyst possessed uniform core-shell structure with magnetic response core and Brønsted ionic liquid
functionalized mesoporous silica shell. The multifunctional catalyst also exhibited appealing physic-
ochemical features like high specific surface area, mesoporous channel, magnetic response, and fine
catalytic activity in esterification of oleic acid with alcohol. Under mind conditions, the conversion of
oleic acid was up to 93.5%. The catalyst exhibited recyclable property based on magnetism and the sepa-
ration of the catalyst would be simply using external magnetic field without obvious mass loss, allowing
a clean biodiesel product.
Keywords:
Ionic liquid
Magnetic
Heterogeneous catalyst
Biodiesel
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
many organic solvents, can afford higher yields and selectivities
with potential application in replacing conventionally homoge-
Biodiesel has been a modern and technological area due
to the relevance to environmental benefits and the increasing
in the petroleum price. As an alternative diesel fuel derived
from vegetable oils, animal fats, and microalgae etc., biodiesel is
biodegradable, renewable, and nontoxic, having low emission pro-
files [1,2]. Generally, both of the transesterification of triglycerides
(TGs) with alkaline catalysts and esterification of free fatty acids
(FFAs) with acidic catalysts can be applied to prepare biodiesel.
But alkaline catalysts require anhydrous conditions and feed stocks
with low levels of FFAs to avoid saponification [3]. In that situation,
many approaches involving acid catalysis have been developed [4].
Specially, heterogeneous acid catalysts have played the important
role for green and recyclable catalysts [5,6].
Recently, ionic liquid has become a new eco-benign approach
toward modern chemistry, having found applications in a wide
range of areas, including catalysis, synthesis, analysis, and gas
absorption, etc., [7], especially those functionalized with Lewis
and/or Brønsted acid sites [8,9]. The ionic liquid, with the properties
of flexibility, non-volatility, non-corrosivity, and immiscibility with
neous/heterogeneous acid catalysts [10]. It has been reported that
esterification of oleic acid can be catalyzed by functionalized ionic
liquid with satisfied yield [11,12]. However, several drawbacks
like unsuitable viscosity, tedious separation procedures, and large
dosage in catalysis, hamper their widespread applications. As sep-
aration and recycle of catalysts are critically required to reduce
the cost of catalysts as well as to avoid the generation of waste,
it requires the heterogenization of ionic liquid [13]. Based on the
facts that the chemical and structural versatility of ionic liquid
make them potential guest molecule candidates for the post-
functionalization of solid materials [14], supported ionic liquid (SIL)
is prepared by immobilizing IL on solid material [15,16]. The sup-
ported ionic liquid has better performance in many fields, with the
merits of both homogeneous ionic liquid and heterogeneous solid
materials [17,18].
To develop efficient and recyclable catalysts, nano-magnetite
has been developed as a new support with the properties of easy
synthesis and functionalization, facile separation by magnetic force
[19,20]. At the same time, ordered mesoporous materials have
been widely used in adsorption, separation, catalysis, etc., due to
the unique properties and functionalities [21]. Thus the introduc-
tion of magnetic nanoparticles in mesoporous materials realizes
the combination of ionic liquid heterogenization with techniques
∗
Corresponding author. Tel.: +86 25 8358 7198.
E-mail address: guangf@njtech.edu.cn (G. Guan).
http://dx.doi.org/10.1016/j.molcata.2014.12.002
1381-1169/© 2014 Elsevier B.V. All rights reserved.

128
H. Wan et al. / Journal of Molecular Catalysis A: Chemical 398 (2015) 127–132
Fig. 1. The schematic representation of the synthetic route of FSS–IL and the application in esterification of oleic acid for biodiesel production.
for magnetic separation [22,23]. Magnetite-supported ionic liquid
catalysts are an important and growing arena in heterogeneous
catalysis, having been successfully applied in numerous important
reactions [24,25].
[SO3H-PIM-TMSP][HSO4] was prepared referring to the
previous published article [26]: imidazole (0.05 mol) was dis-
solved in ethanol and equimolar sodium ethylate was added
afterwards. The reaction was carried out at 348 K for 12 h. 3-
Chloropropyltrimethoxysilane (0.05 mol) was added dropwise and
the mixture was stirred for another 12 h in N2 atmosphere. After
the generated sodium chloride was removed, 1,3-propane sultone
(0.05 mol) was added and stirred at 323 K for 12 h. Sulfuric acid
(0.05 mol) was added, reacting at the same temperature for 6 h.
[SO3H-PIM-TMSP][HSO4] was achieved by washing with diethyl
ether and drying under vacuum.
The magnetic Fe3O4 microspheres were synthesized by a
solvothermal reaction [27]. By the hydrolysis and condensation
of tetraethyl orthosilicate (TEOS) in the mixture of water, ethanol
and ammonia, individual Fe3O4 microsphere was coated with uni-
form silica layer, generating Fe3O4@SiO2 microsphere. With TEOS
as silica source and cetyltrimethylammonium bromide (CTAB)
as structure-directing agent, a CTAB/silica composite layer was
formed on the surface of Fe3O4@SiO2 microsphere. Magnetic FSS
mesoporous microspheres were gained after CTAB was removed
[28].
In this work, we design and prepare a magnetic mesoporous
material Fe3O4@SiO2@mSiO2 (FSS) supported ionic liquid [SO3H-
PIM-TMSP]HSO4 (IL) catalyst, defined as FSS–IL, for biodiesel
production. Magnetic mesoporous support FSS is fabricated with
super magnetic Fe3O4 nanoparticle as core and silica layers as
shells. The inner silica layer of FSS acts as the surface protector and
the outer mesoporous silica shell works as the support, which ben-
efits the diffusion of guest molecules and improves mass transfer.
The catalyst is systematically characterized and applied in het-
erogeneous catalytic esterification of oleic acid, allowing a clean
biodiesel product and easy catalyst removal from the reaction mix-
ture under magnetic force.
2. Material and methods
2.1. Material and instruments
Ethylene glycol, ethanol, imidazole, sulfuric acid, anhydrous
FeCl3, toluene, 1,3-propane sultone, cetyltrimethylammonium bro-
mide (CTAB), sodium acetate anhydrous, sodium citrate, ethyl
silicate (TEOS), ␥-chloropropyltrimethoxysilane (CPMS), oleic acid
(85%), diethyl ether, ammonium hydroxide were commercially
available and were used without further purification unless oth-
erwise stated.
The catalyst FSS–IL was prepared as following. IL (1.5 g) and FSS
(1.0 g) was dissolved in dry toluene (50 ml). The mixture was heated
to reflux for 24 h in N2 atmosphere. FSS–IL catalyst was obtained
after washing with diethyl ether and drying in vacuum at 323 K for
8 h. The total synthetic route of FSS–IL was represented in Fig. 1.
2.3. Catalytic activity measurement
Scanning electron microscopy (SEM) images were gained by
a S4800 Field-emission scanning electron Microscope (HITACHI,
Japan). Transmission electron microscopy (TEM) images were
taken with a JEM-2100 (HR) (JEOL, Japan), and the samples were
daubed onto carbon film supported on copper grids for analy-
sis. X-ray diffraction (XRD) patterns were recorded on a Smartlab
diffractometer (RIGAKU, Japan). Fourier-transform infrared (FT-IR)
spectra were measured by a Nicolet-6700 spectrometer (Thermo
Fisher Scientific, United States) using anhydrous KBr as dispersing
agent. N2 adsorption–desorption measurements were carried out
on a Micromeritics ASAP 2020 system model instrument. The spe-
cific surface area was calculated by Brunauer–Emmett–Teller (BET)
algorithm. The mesoporous pore size distribution was obtained
through the Barrett–Joyner–Halenda (BJH) theory. The magnetic
hysteresis loops were measured by a lakeshore 7407 vibrating sam-
ple magnetometer (China Trend Limited, China).
Biodiesel production from oleic acid with alcohol was carried
out with FSS–IL as catalyst. In a typical run, oleic acid (10 mmol),
alcohol (60 mmol), and catalyst (0.2 g) were added in a flask (50 ml)
and kept under 373 K for 4 h. When the reaction was finished, a
magnet was put at the bottom of the reaction flask and the cat-
alyst would be totally adsorbed to the magnet. The upper layer
was dumped out for analysis and the yield of biodiesel was cal-
culated based on oleic acid referring to previous publication [11].
The left catalyst was washed with acetone for three times, setting
in vacuum at 323 K for 6 h for recycling experiment. The recovered
catalyst was also weighted by electronic balance (d = 0.0001 g) for
calculating catalyst mass loss data.
3. Results and discussion
3.1. Catalyst characterization
2.2. Catalyst preparation
Brønsted
trimethoxysilylpropyl)
ionic
liquid
imidazolium
1-(propyl-3-sulfonate)-3-(3-
hydrogen sulfate
According to the SEM/TEM images, the FSS particle (Fig. 2a)
exhibited good monodispersity and uniform spherical morphology.

H. Wan et al. / Journal of Molecular Catalysis A: Chemical 398 (2015) 127–132
129
Fig. 2. Representative SEM images (a, c, e) and TEM images (b, d, f) of FSS (a, b), FSS–IL(c, d), and recovered FSS–IL(e, f).
calculated to be 547 m2/g, 175 m2/g, and 168 m2/g, respectively.
Based on the uniformly interpenetrating mesopore and large sur-
face area, FSS could interact with guest IL not only at the external
surface, but also through the whole internal pore system. The
decrease of FSS–IL surface area was caused due to the immobi-
lization of ionic liquid when compared with FSS. Recovered FSS–IL
showed similar curve when compared with fresh FSS–IL, and the
slight surface area decrease might be caused by the adsorption of
organic molecules and the drop of the order degree of structure.
Successful immobilization of IL on FSS could also be inferred
from the results of FT-IR in Fig. 5. Band around 580 cm−1 were
A core-shell structure with Fe3O4 core and two distinct silica layers
was clearly shown in the TEM images (Fig. 2b,d,f). The inner shell
was fabricated as non-porous silica with a thickness of ∼40 nm,
with the purpose of protecting Fe3O4 core from decay in some harsh
reaction circumstances. The mesoporous channels in the outer shell
provided the main area for immobilizing IL and favored the access
of guest molecules, serving as nanoreactors. The morphology and
structure of recovered FSS–IL catalyst (Fig. 2e and f) had no obvious
change when compared with the fresh (Fig. 2c and d), indicating the
good stability and high corrosion resistance against acidic reaction
environment.
The wide-angel XRD pattern of magnetic Fe3O4 microsphere
(Fig. 3) exhibited six distinct peaks, which matched well with
previous report on crystalline magnetic microspheres [29]. The
extra-large shoulder centered at around 2theta = 23◦ was attributed
to the silica component. The XRD results indicated that the mag-
netic core Fe3O4 was well retained in the silica matrix, consistent
with the TEM results (Fig. 2).
assigned to the Fe O stretching. The Si
O Si stretching modes
of the silica shell could be observed as a strong peak around
1088 cm−1. The IR curve of IL showed typical bands at 1168 and
1054 cm−1 (S O asymmetric and symmetric stretching vibrations),
2940, 2864 and 1458 cm−1 (alkyl chain stretching and deformation
vibrations), 1564 cm−1 (C N and C C vibrations of the imidazole
ring) [30]. In the spectrum of FSS–IL, the characteristic peaks were
The N2 adsorption–desorption isotherms of FSS showed char-
acteristic type IV curve (Fig. 4), indicating a mesoporous structure.
The BET surface areas of FSS, FSS–IL, and recovered FSS–IL were
FSS
311
220
FSS-IL
440
400
551
422
FSS-IL
FSS
Recovered FSS-IL
Fe O
3
4
0.0
0.2
0.4
0.6
0.8
1.0
20
30
40
50
60
70
Relative Pressure
(
p/p
)
o
2 theta (degree)
Fig. 3. Wide-angle XRD patterns of Fe3O4, FSS, and FSS–IL.
Fig. 4. N2 adsorption–desorption isotherms of FSS, FSS–IL, and recovered FSS–IL.

130
H. Wan et al. / Journal of Molecular Catalysis A: Chemical 398 (2015) 127–132
Fe O
3
4
580
FSS
IL
1088
1564
1458
2864
2940
1168 1054
FSS-IL
4000 3500 3000 2500 2000 1500
1000
500
-1
Wavenumbers(cm )
Fig. 6. Magnetic hysteresis loops of FSS and FSS–IL. Inset was the schematic repre-
sentation of the separation of FSS–IL.
Fig. 5. FT-IR spectra of Fe3O4, FSS, IL, and FSS–IL.
at the same wavenumbers, proving that IL were successfully sup-
ported on FSS.
3.2. FSS–IL for biodiesel production
Magnetic properties were tested and the saturation magneti-
zation values were 20.6 emu/g and 13.2 emu/g for FSS and FSS–IL,
respectively (Fig. 6). The decrease of saturation magnetization value
was caused by the layer of supported ionic liquid. No hysteresis loop
and remanence were detected for both of the samples. The super-
paramagentic property afforded the easy separation of FSS–IL after
reaction by simply using a magnet (Fig. 6 inset).
FSS–IL was applied in the esterification of oleic acid with ethanol
and univariate analysis was applied to research the influence of
reaction temperature, time, alcohol/acid molar ratio, and catalyst
amount on the yield of biodiesel.
A high value of 93.5% was achieved when temperature reached
383 K (Fig. 7a), which might be due to the increase of the reaction
rate and decrease of the mass transfer limitation between reactants
100
100
b
a
90
90
80
70
60
50
40
80
70
60
50
40
320
340
360
380
400
420
0
1
2
3
4
5
Temperature (K)
Time (h)
100
100
d
c
90
80
70
60
50
40
30
90
80
70
60
50
40
2
4
6
8
10
12
14
0
2
4
6
8
10
Catalyst Amount (%)
Alcohol/ free fatty acids molar ratio
Fig. 7. Optimization of FSS–IL-catalyzed esterification of oleic acid: reaction temperature (a), (4 h, 6:1, 10.6 wt%); reaction time (b), (383 K, 6:1, 10.6 wt%); alcohol/oleic acid
molar ratio (c), (4 h, 383 K, 10.6 wt%); catalyst amount (d), (4 h, 383 K, 6:1).

H. Wan et al. / Journal of Molecular Catalysis A: Chemical 398 (2015) 127–132
131
Table 1
Performances of various catalysts for esterification of oleic with short alcoholsa Oleic Acid + Alcohol ꢀ Biodiesel + Water.
Entry
Catalyst
Alcohol
Catalysis type
Catalyst separation
Separation time (min)
Catalyst mass loss (wt%)b
Yield (%)
1
2
3
4
5
6
7
8
9
Blank
H2SO4
FSS
Ethanol
Ethanol
Ethanol
Ethanol
Methanol
Ethanol
Ethanol
n-Propanol
n-Butanol
–
–
–
>10
<1
>30
<1
<1
<1
<1
<1
–
100
1.2
15.6
1.5
1.3
2.6
1.8
1.6
5.2
90.1
6.4
94.2
89.6
93.5
87.4
92.0
91.5
Homogeneous
Heterogeneous
Homogeneous
Heterogeneous
Heterogeneous
Heterogeneous
Heterogeneous
Heterogeneous
Chemical reaction + skimming
Magnetic field
Distillation + skimming
Magnetic field
Magnetic field
Magnetic field
Magnetic field
Magnetic field
IL
FSS–IL
FSS–IL
FSS–ILc
FSS–IL
FSS–IL
a
Reaction conditions: nalcohol:noleicacid = 6:1; temperature, 383 K; time, 4 h; catalyst amount (based on oleic acid) 10.6 wt%.
Catalyst mass loss was calculated based on the first run except Entry 7.
The 6th run.
b
c
and catalyst. As esterification was reversible, the equilibrium was
arrived around 4 h (Fig. 7b). As excess alcohol would favor the for-
ward reaction [31], experiments with different alcohol/oleic acid
molar ratios were carried out and the results was shown in Fig. 7c.
As the molar ratio increased, a fine biodiesel yield was achieved at
6:1. The effect of catalyst amount on the yield of biodiesel was stud-
ied (Fig. 7d). The maximum yield was achieved in the presence of
10.6 wt% FSS–IL (based on oleic acid) when enough catalytic active
sites were available [32].
Based on the above data, suitable conditions were obtained as
follows: reaction temperature, 383 K; reaction time, 4 h; ethanol
to oleic acid molar ratio, 6:1; and catalyst amount, 10.6 wt%. Under
these conditions, the conversion of oleic acid was 93.5%. The esteri-
fication could be carried out under mind conditions and the catalyst
could be isolated conveniently and totally in magnetic field, which
allowed for a high biodiesel yield and purity.
Fig. 8. Reusability of FSS–IL.
yields from 89.6% to 93.5% were obtained under the above reaction
conditions. The slight difference in carbon chains have no signifi-
cant effect on the yield. Esterification of methanol gave relatively
lower yield compared with other alcohols due to its lower boiling
point. As ethanol also could come from biological resource, it would
be the best choice for studies and industrial applications.
3.3. Catalytic activity assessment
Table 1 summarized the performances of various catalysts for
esterification of oleic with short alcohols. Without any catalyst,
oleic acid allowed a low conversion due to the autocatalysis itself
(Entry 1). When sulfuric acid, IL, and FSS–IL were applied as cat-
alysts, high yields of biodiesel could be obtained. However, when
taking catalyst separation, catalyst separation time, and catalyst
mass loss into consideration, FSS–IL showed the best catalytic per-
formance due to its easy separation, short separation time, and
trace amount of mass loss (Entry 2,4,6).
3.4. Catalyst reusability
As recyclability was an important factor for any catalyst, a
six-cycle experiment was carried out to check the catalyst per-
formance. FSS–IL was easily separated from the reaction mixture
using magnetic field by taking advantage of its high magnetization
and no obvious catalyst mass loss was observed. The recovered
FSS–IL was washed with acetone for three times. After dried in
To investigate the universality of FSS–IL, several most common
alcohols were applied as raw materials (Entry 5,6,8,9). Satisfactory
Fig. 9. The mechanism of esterification catalyzed by FSS–IL catalyst.

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H. Wan et al. / Journal of Molecular Catalysis A: Chemical 398 (2015) 127–132
vacuum, FSS–IL was directly applied in esterification of oleic acid
with ethanol (reaction conditions: time, 4 h; temperature, 383 K;
molar ratio, 6:1; and catalyst amount, 10.6 wt%) and the results
were shown in Fig. 8. Notably, the catalyst could maintain good
catalytic activity even after being reused for six runs, with only a
slight decrease from 93.5% to 87.4%. The decrease in catalytic per-
formance might be probably attributed to minor leaching of acidic
protons or the adsorption of reactant molecules on the active sites
[33]. The catalytic activity no longer decreased after five cycles,
indicating the good catalytic property and reusability of FSS–IL.
In order to clarify the reaction process, esterification mechanism
catalyzed by FSS–IL catalyst was illustrated in Fig. 9. The process
had two key steps, one was the protonation of oleic acid and the
other was the nucleophilic attack of the protonated acid by the alco-
hol. As displayed in Fig. 9, ionic liquid part [SO3H-PIM-TMSP]HSO4
supported the proton, and oleic acid was protonated to generate a
carbocation. After the nucleophilic attack of the alcohol molecule to
the carbocation, a middle product was generated. When the bond
of hydrogen and oxygen of product 5 was broken, biodiesel was
formed and catalyst FSS–IL was regenerated.
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