Efficient confinement of ionic liquids in MIL-100(Fe) frameworks by the "impregnation-reaction-encapsulation" strategy for biodiesel production

Article abstract of DOI:10.1039/c6ra00579a

A new, simple and effective strategy to confine dicationic acid ionic liquids (DAILs, 1,4-bis[3-(propyl-3-sulfonate) imidazolium] butane hydrogen sulfate) in the cages of MIL-100(Fe) frameworks was constructed. The target catalyst, defined as MIL-100(Fe)@DAILs, was characterized by XRD, FTIR, SEM, TEM, EA, TGA and N₂ adsorption-desorption. Meanwhile, the catalytic activity of the MIL-100(Fe)@DAILs catalyst was evaluated by the esterification reaction with oleic acid and methanol. The results indicated that the DAILs had been effectively encapsulated within the cages of the MIL-100(Fe) frameworks. Moreover, the influence of reaction time, reaction temperature, molar ratio of methanol to oleic acid and catalyst dosage on the conversion of oleic acid was studied by univariate analysis. The conversion of oleic acid decreased from 93.5% to 86.0% when the catalyst was reused five times, which indicated that the target catalyst possessed higher catalytic activity and superior catalytic activity. Finally, an esterification mechanism catalyzed by this novel catalyst was illustrated.

Full text of DOI:10.1039/c6ra00579a

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Efficient Confinement of Ionic Liquids in the MILꢀ100(Fe) Frameworks by the
“ImpregnationꢀReactionꢀEncapsulation” Strategy for Biodiesel Production
Mingjuan Hana, Zheng Gua, Chong Chenb, Zuowang Wub, Yigen Queb, Qiang Wanga, Hui Wanb*,
Guofeng Guanb*
a College of Chemistry and Molecular Engineering, Nanjing Tech University,
b College of Chemical Engineering, Nanjing Tech University, Nanjing 210009, China
* Corresponding author: Eꢀmail address: wanhui@njtech.edu.cn; guangf@njtech.edu.cn
ABSTRACT:
A new, simple and effective strategy to confine dicationic acid ionic liquids (DAILs,
1,4ꢀbis[3ꢀ(propylꢀ3ꢀsulfonate) imidazolium] butane hydrogen sulfate) in the cages of
MILꢀ100(Fe) framework was constructed. The target catalyst, defined as
MILꢀ100(Fe)@DAILs, was characterized by XRD, FTIR, SEM, TEM, EA, TGA and
N2 adsorptionꢀdesorption. Meanwhile, the catalytic activity of MILꢀ100(Fe)@DAILs
catalyst was evaluated by the esterification reaction with oleic acid and methanol. The
results indicated that the DAILs had been effectively encapsulated within the cages of
MILꢀ100(Fe) framework. Moreover, the influence of reaction time, reaction
temperature, molar ratio of methanol to oleic acid and catalyst dosage on the
conversion of oleic acid had been studied by univariate analysis. And the conversion
of oleic acid decreased from 93.5% to 86.0% when the catalyst was reused five times,
which indicated that the target catalyst possessed higher catalytic activity and superior
catalytic activity. Finally, an esterification mechanism catalyzed by this novel catalyst
was illustrated.
Keywords: Biodiesel; Ionic liquids; Metalꢀorganic frameworks; Encapsulation
1. Introduction
Biodiesel had emerged as one of the most promising energy sources to replace current
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petroleumꢀbased diesel due to its sustainable, biodegradable and nonꢀtoxic1, 2. And it
could be easily produced through baseꢀ or acidꢀcatalyzed esterification and
transesterification reactions3, particularly highly reactive homogeneous Brønsted acid
had been proved to be effective catalysts in esterification4ꢀ6. Unfortunately,
homogeneous base or acid catalysts suffered from serious contamination and
corrosion problems that required the implementation of good separation and
purification steps, and their high costs and unendurable viscosity restricted their
industrial applications7. Therefore, these homogeneous acidꢀ or baseꢀcatalyzed
systems were not an excellent choice for commercial application.
Recently, ionic liquids (ILs) as the new environmentallyꢀfriendly liquid acidic
catalysts, had received extensive attention due to their special properties such as
negligible volatility, remarkable solubility, no corrosion, nonꢀflammable and thermal
stability and especially showed the excellent catalytic activity in the synthesis of
biodiesel8, 9, but their drawbacks, such as limited solubility with organic compounds
(especially polar molecules) not only caused catalysis loss but also resulted in
purification difficulty, and the high viscosity of the ILs limited their industrial
applications10. Attempts had been made to solve these problems by immobilizing
acidic ILs onto silica gel11, ordered mesoporous silica12, polystyreneꢀbased polymers13
or poly divinylbenzene (PDVB)14 and magnetic nanoparticles (MNP)15, 16
.
Mealꢀorganic frameworks (MOFs), an emerging class of hybrid inorganicꢀorganic
porous materials, had been receiving considerable attention for their potential
application in gas storage, molecular separation, heterogeneous catalysts, and catalyst
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supports17ꢀ20. MOFs as heterogeneous catalysts or catalyst supports offered the main
advantages of having the large surface area together with a large number of active
sites, high porosity, regular and accessible pores, and crystalline open structures21.
Therefore, different approaches had been employed for the introduction of strong
Brønsted acidity in MOFs22, these included mainly postꢀsynthetic and encapsulation
routes, like controlled frameworks destruction to increase the number and acidity of
CUS23, encapsulation of heteropolyacids24, and direct synthesis of free sulfonic acid
functionalized MOFs25. Otherwise, up to date, there were few investigations focusing
on introducing the strong Brønsted acidity in MOFs matrix except for Luo and his
coꢀworkers26 and our previous study27. In the above investigations, Brønsted acidic
ionic liquid confined inside wellꢀdefined MILꢀ101 nanocages by organic electronꢀrich
Nꢀheterocyclic compound as a chemical bridge26 and the anion exchange strategy27.
Considering that iron (III) trimesate MOFs (MILꢀ100(Fe), MIL standing for Material
of Institut Lavoisier) were effectively assembled from iron(III) trimers and trimesic
acid linkers, and had two kinds of mesocages (cage diameters are 25 and 29 Å)
accessible through microporous windows (5.8 and 8.6 Å)28. It meant iron (III)
trimesate MOFs possess the regular and accessible pore size, and crystalline open
structures. Therefore, if we introduced some small molecular acidic ILs to pass
through the windows and stayed in the cages of MOFs, then made them react in the
cages to become larger molecular acidic ILs. Efficient encapsulation could be realized
when the larger ILs couldn’t escape from the cages of MOFs because of their enough
large bodies.
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Herein, in this work, we proposed a new, simple and effective strategy to confine acid
ionic liquids in porous MILꢀ100(Fe) frameworks. First, we chose a type of ionic
liquids were easily diffused into the pores of MILꢀ100(Fe) frameworks, and then we
attempted to excite them to grow up so that they could not escape from the cages of
MILꢀ100(Fe) matrix. Then, the ideal MILꢀ100(Fe)ꢀsupported acid catalysts for their
efficient catalytic activities were performed to check the validity of encapsulation by
the esterification reactions. Meanwhile, the characterization of the target catalyst
(defined as MILꢀ100(Fe)@DAILs) was studied by XRD, FTIR, SEM, TEM,EA, TGA
and N2 adsorptionꢀdesorption.
2. Experimental
2.1. Materials and instrumentation
Benzeneꢀ1,3,5ꢀtricarboxylic acid, ferric chloride hexahydrate (FeCl3·6H2O) were
purchased from SigmaꢀAldrich. Imidazole (99 %), 1,3ꢀpropane sultone (99 %),
diethyl ether, methanol, ethanol, oleic acid and other chemicals were obtained from
Aladdin and used without further purification.
The powder XRD experiments were done with the Smart Lab diffractometer (Rigaku,
Japan) using Niꢀfiltered CuKα radiation (λ=0.154 nm). The materials were in
advance degassed for 4 h at 150 °C, then their Nitrogen adsorptionꢀdesorption
isotherms were measured using the ASAP 2020 system (Micromeritics, USA) at
ꢀ196 °C. The FTIR spectra were recorded by using a Nicolet 6700 FTIR spectrometer
(Thermo Fisher Scientific, USA). Scanning electron microscopy (SEM) was
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performed on the Sꢀ4800 fieldꢀemission microscope (Hitachi, Japan). Transmission
electron microscopy (TEM) was conducted with the JEMꢀ2100 highꢀresolution
transmission electron microscope (JEOL, Japan). Thermogravimetric analysis (TGA)
was carried out in the temperature range of 25ꢀ600 °C (heating rate: 10 °Cminꢀ1)
under N2 flow. Elemental analyses (EA) were conducted with Elementar Vario EL III
system to determine the C, H, and N content of the prepared catalysts.
2.2. Synthesis of MILꢀ100(Fe)
MILꢀ100(Fe) was hydrothermally synthesized in the absence of HF as previously
reported24. FeCl3·6H2O (2.70 g) and benzeneꢀ1,3,5ꢀtricarboxylic acid (1.39 g) were
dissolved in deionized water (50 mL). The mixture was then transferred to a
Teflonꢀlined autoclave, and was heated at 130 °C for 72 h. The orange solid was
recovered through filtration and washed several times with methanol. After the solid
product had been obtained, guest molecular exchange was carried out with
dichloromethane at room temperature for 3 days. Finally, the material was evacuated
under vacuum at 150 °C for 12 h.
2.3. Synthesis of DAILs
AILs [SO3Hꢀ(CH2)3ꢀHIM][HSO4] were synthesized as described in our previous
work29. DAILs [SO3Hꢀ(CH2)3ꢀIM]2C4[HSO4]2 were prepared as following steps. First,
AILs (1.2 g) was added into methanol (20 mL) and stirred for 30 min. Second,
1,4ꢀDibromobutane was added dropwise at 0 °C and stirred at 25 °C for 12 h, and
then the mixture was stirred at 60 °C for another 12 h. Finally, DAILs was achieved
by washing with diethyl ether and drying under vacuum at 80 °C for 12 h.
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2.4. Synthesis of MILꢀ100(Fe)@DAILs
The “impregnationꢀreactionꢀencapsulation” process of encapsulating DAILs
[SO3Hꢀ(CH2)3ꢀIM]2C4[HSO4]2 into the framework of MILꢀ100(Fe) was briefly as
follows. First, AILs was added into the solution of methanol(20 mL) with
MILꢀ100(Fe)(1.0 g) and stirred for 30 min. Second, 1,4ꢀDibromobutane was added
dropwise at 0 °C and stirred at 25 °C for 12 h, and then the mixture was stirred at
60 °C for another 12 h. Finally, the orange solid was collected through centrifugation
and washed with methanol. The product was performed using hot methanol at 40 °C
for 12 h and evacuated under vacuum at 80 °C for 12 h. In order to optimize the
amount of DAILs species occluded in the cavities of MILꢀ100(Fe), the samples 1ꢀ4
with the different adding amount of AILs were shown in Table 1 and sample 2 was
chosen to be the target catalyst.
MILꢀ100(Fe)/AILs was prepared through direct impregnation of MILꢀ100(Fe) with
AILs. The solid was performed using hot methanol at 40 °C for 12 h and evacuated
under vacuum at 80 °C for 12 h.
2.5. Catalytic reaction
The catalytic activity of MILꢀ100(Fe)@DAILs was evaluated by using the
esterification of oleic acid with methanol. In a typical reaction, oleic acid (10 mmol)
and methanol (100 mmol) were mixed in a 50 mL round bottom flask equipped with a
magnetic stirrer and a water condenser system, then added an appropriate amount of
the catalyst. The reaction was kept at 67 °C for 4 h. After the reaction, the catalyst was
separated from the reaction mixture by centrifugation, washed with copious amounts
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of methanol, performed using hot methanol at 40 °C for 12 h and evacuated under
vacuum at 80 °C for 12 h. A gas chromatograph equipped with a flame ionization
detector (SPꢀ6890; SEꢀ54 capillary column: 30 m×0.25 mm×0.3 mm) was employed
to analyze the conversion of oleic acid.
3. Results and discussion
3.1. Characterization of the catalysts
The XRD patterns of MILꢀ100(Fe), MILꢀ100(Fe)@DAILs and the recovered
MILꢀ100(Fe)@DAILs in Fig. 1 showed that the diffraction peaks of MILꢀ100(Fe)
were consistent with its standard XRD data reported in the literature30. The slight
decrease in relative intensities could be found at 2ꢀ5°, which indicated that the
encapsulation of ILs in MILꢀ100(Fe) matrix had changed its crystal structure24.
Compared with MILꢀ100(Fe)@DAILs (Fig. 1b), the diffraction peaks of recovered
MILꢀ100(Fe)@DAILs(Fig. 1c) were almost unchanged. The result showed that the
crystal structure of target catalyst was stable after the catalytic reaction.
As shown in Fig. 2, the FTIR spectra of AILs, DAILs, MILꢀ100(Fe) and
MILꢀ100(Fe)@DAILs were recorded in the spectral range 400ꢀ4000 cmꢀ1. The peak
of DAIL at 3443 cmꢀ1 was related to OꢀH vibrations of physical adsorbed water in Fig.
2b, and the peak of AILs at this area was related to ν(N－H) stretching vibration Fig.
2a. The characteristic peaks of MILꢀ100(Fe) at 1714, 1382, and 1600ꢀ1446 cmꢀ1 could
be observed in Figs. 2c, 2d and 2e. The peaks were attributed to ν(C=O)stretching
vibration, ν(C－O) stretching vibration and ν(C=C) vibrations of aromatic groups in
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MILꢀ100(Fe) matrix, respectively. The peaks (at 1580, 1217, 1035 and 3443 cmꢀ1 in
Fig. 2b) of DAILs were also displayed in Figs. 2d and 2e, which were related to
ν(C=N) stretching vibrations in imidazole ring, ν(S=O) asymmetric, symmetric
stretching vibrations of sulfonic acid group and OꢀH vibrations of physical adsorbed
water, respectively. This indicated DAILs were successfully encapsulated in
MILꢀ100(Fe) matrix. To further confirm above discussion, the optimized geometric
structures of the AILs, DAILs and Oleic acid were optimized by a semiꢀempirical
PM3 method and using Gaussian09 program package (revision B.01; Gaussian, Inc.,
Wallingford CT, 2010), as shown in Fig. 3. First, AILs (L: 1.05 nm; W: 0.6 nm,
shown in Figs. 3a) could pass through the bigger hexagonal windows (~0.86 nm,
shown in Figs. 3d) of big mesoporous cages, therefore, AILs easily diffused into the
microcages of MOFs. Then, according to the optimized geometric structures of
DAILs (L: 1.93 nm; W: 0.9 nm, shown in Fig. 3b), an efficient encapsulation could be
realized since the MILꢀ100(Fe) had the big mesocages with diameters of 2.9 nm and
accessible microporous windows of 0.86 nm. It indicated that DAILs could stay in the
big mesocages and could not escape from the windows of 0.86 nm due to their enough
large bodies. Finally, the results showed that the optimized geometric structures of the
oleic acid were close to 2.35 nm (length) and 0.35 nm (width) (shown in Fig.3c), and
it indicated that the carboxyl terminal of oleic acid molecules could smoothly insert
through the windows of 0.6 nm or 0.86 nm and react with active components. In
conclusion, both FTIR spectra and semiꢀempirical PM3 method proved DAILs were
successfully confined in the frameworks of MILꢀ100(Fe) by the
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“impregnationꢀreactionꢀencapsulation” strategy.
The MILꢀ100(Fe) kept the same wellꢀdefined octahedral shape before and after the
encapsulation of DAILs, as investigated by SEM and TEM (Fig. 4). The average
particles size of MILꢀ100(Fe) and MILꢀ100(Fe)@DAILs was about 500 nm, only the
external surface of MILꢀ100(Fe)@DAILs tended to become rough. In comparison to
the pristine MILꢀ100(Fe), the crystal morphology of the recovered catalyst had
changed slightly and the average particles size was still approximately 500 nm, which
suggested that catalytic reaction would have little effect on the carrier structure.
In order to further investigate the target catalysts, the N2 adsorptionꢀdesorption
isotherms of MILꢀ100(Fe) and MILꢀ100(Fe)@DAILs were exhibited in Fig. 5. As
same as the pristine MILꢀ100(Fe), the N2 isotherm of MILꢀ100(Fe)@DAILs exhibited
a typeꢀI/IV mixed type isotherm. The surface area and total pore volume decreased
from 1183.7 to 170.2 m2gꢀ1 and from 0.72 to 0.20 cm3gꢀ1 for MILꢀ100(Fe) and
MILꢀ100(Fe)@DAILs, respectively. The decreasing BET surface area and pore
volume could be attributed to the presence of DAILs inside the MILꢀ100(Fe)
nanocages. Moreover, the open cavities and high surface areas were retained, which
benefited the free diffusion of the reactants or products. The elemental analysis (C
26.25, H 3.24, N 4.97) of MILꢀ100(Fe)@DAILs showed that DAILs were effectively
encapsulated into MILꢀ100(Fe), and the loading amount was 0.8875 mmolgꢀ1 by the
calculation of the nitrogen content. In conclusion, all results confirmed the effective
encapsulation of DAILs within the cages of MILꢀ100(Fe).
TGA curves of MILꢀ100(Fe) and MILꢀ100(Fe)@DAILs were shown in Fig. 6. At the
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temperature below 250 °C, the weight loss of MILꢀ100(Fe) (almost 30%) and
MILꢀ100(Fe)@DAILs (<10%) were resulted from the adsorbed water on the inner
and outer surfaces of the frameworks of MILꢀ100(Fe). Due to the occupation of
DAILs inside the MOFs pore, MILꢀ100(Fe)@DAILs showed less weight loss than the
pristine MILꢀ100(Fe). There was a rapid weight loss of MILꢀ100(Fe)@DAILs in the
temperature range of 250ꢀ350 °C, which was attributed to decomposition of DAILs.
When the temperature increased above 350 °C, MILꢀ100(Fe) begun to degrade with
the decomposition of benzeneꢀ1,3,5ꢀtricarboxylic acid and the collapse of the
MILꢀ100(Fe) framework.
3.2. Catalytic activity
3.2.1. Conversion of oleic acid catalyzed by samples 1ꢀ4
In order to optimize the amount of DAILs species occluded in the cavities of
MILꢀ100(Fe), the loading amount, surface area, total pore volume and conversion of
the samples were characterized and shown in Table 1. Table 1 showed that the loading
amount calculated by the elemental analysis was increased from 0.6786 to 0.9678
mmolgꢀ1 when the AILs were added from 0.8 g to 2.0 g, correspondingly, the surface
area and total pore volume decreased from 232.1 to 118.0 m2gꢀ1 and from 0.26 to 0.17
cm3gꢀ1, respectively. Meanwhile, the conversion of oleic acid was increased to the
maximum value (conversion: 89.5%), then decreased to 87.8% when the AILs were
continuously added. The main reason for this was because the cavities of MILꢀ100(Fe)
were occluded by the more DAILs and couldn’t give enough room to facilitate the
access of oleic acid molecules.
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3.2.2. Univariate analysis
The catalytic activity of MILꢀ100(Fe)@DAILs was evaluated by the esterification
reaction with oleic acid and methanol. Univariate analysis15 was conducted to study
the influence of reaction time, reaction temperature, molar ratio of methanol to oleic
acid and catalyst dosage on the conversion of biodiesel, as shown in Fig. 7.
Fig. 7a showed that the conversion of oleic acid was first increased and then slightly
decreased with increasing the reaction time, the maximum value was 92.6% when the
time was 5 h. At the beginning of the reaction, the catalyst could not contact with
reactants adequately, so the conversion rate was not high, but as the time increased,
the conversion rate of oleic acid increased due to enough contact. The effect of
temperature on the conversion of oleic acid was shown in Fig. 7b. It showed that the
conversion of oleic acid was increased with the increment of reaction temperature (up
to 67 °C), and then slightly decreased, which was attributed to the change of catalytic
activity of MILꢀ100(Fe)@DAILs with temperature. Considering both energy
conservation and environmental protection, the most suitable reaction temperature
was 67 °C. In Fig. 7c, it was observed that the conversion of oleic acid increased with
increasing the molar ratio of methanol to oleic acid (up to 8:1) and then kept constant,
when reaction time and temperature were kept at 5 h and 67 °C, respectively. The
reasons for this were: more dosage of methanol benefited to the dispersion of the
MILꢀ100(Fe)@DAILs catalyst and increased the chance of collision between
reactants and catalyst from the molecular level. The influence of catalyst dosage on
the conversion of oleic acid was studied at 5 h, 67 °C and molar ratio of methanol to
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oleic acid 8:1, as displayed in Fig. 7d. When adding more catalysts, the conversion of
oleic acid was higher because of the adequate number of active sites.
Based on the above analysis, the optimized experimental conditions were obtained:
reaction time 5 h, reaction temperature 67 °C, molar ratio of methanol to oleic acid
8:1 and catalyst dosage 15 wt%, and then the conversion of oleic acid reached 93.5%.
3.3. Catalytic activities of different catalysts
To test the catalytic activity of target catalyst (MILꢀ100(Fe)@DAILs), the catalytic
activities of different catalysts (some from references31ꢀ34 and some from our work)
for the esterification of oleic acid were compared with each other and shown in Table
2. It should be noted that reaction conditions performed by us were reaction time 5 h,
reaction temperature 67 °C, molar ratio of methanol to oleic acid 8:1 and catalyst
dosage 15 wt%. As shown in Table 2, first, the esterification reaction seemed to hardly
perform without a catalyst (entry 1). Second, entries 2ꢀ3 showed that homogeneous
catalysts (AILs and DAILs) possessed high catalytic activities due to their typical
properties of Brønsted acid. Third, the pristine MILꢀ100(Fe) also showed catalytic
performance (shown in entry 4) due to the Lewis acid sites which provided by the
unsaturated coordinated metal centers (FeIII). Fourth, the excellent catalytic activities
from heterogeneous catalysts (MILꢀ100(Fe)/AILs and MILꢀ100(Fe) @DAILs) could
be obtained because of the typical property of Brønsted acid from ILs, and the Lewis
acid sites from the pristine MILꢀ100(Fe)). The heterogeneous catalysts were our target
catalysts due to their easy separation although the catalytic activities of homogeneous
ionic liquids were high. Finally, in order to further prove the excellent catalytic
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activities of target catalysts, we employed some catalysts from references (entries
7ꢀ10) to compare with our target catalysts. All the results showed that the target
catalysts MILꢀ100(Fe)/AILs and MILꢀ100(Fe)@DAILs could be the promising and
excellent catalysts for esterification reaction.
3. 3. Reusability of the catalyst
It was necessary to test the reusability of catalysts, therefore, the esterification
reactions catalyzed by heterogeneous catalysts (MILꢀ100(Fe)/AILs and MILꢀ100(Fe)
@DAILs) were carried out for six times under the optimized experimental condition,
shown in Fig. 8. After each reaction, the catalysts were recovered by centrifugation,
dried under vacuum and reused in the subsequent reaction. From Fig. 8, it showed that
the conversion of oleic acid decreased from 93.5% to 86.0% for
MILꢀ100(Fe)@DAILs catalyst, otherwise, sharply decreased from 90.2% to 22.3%
for MILꢀ100(Fe)/AILs catalyst, when the catalyst was reused five times. Therefore,
this result indicated the MILꢀ100(Fe)@DAILs catalyst possessed the higher
reusability than that of MILꢀ100(Fe)/AILs catalyst and could be used in industry. The
slight decrease of conversion for MILꢀ100(Fe)@DAILs catalyst might be due to mass
loss of blockage of MOFs channels and it was natural phenomenon. It was obvious
that conversion of MILꢀ100(Fe)/AILs were sharply decreased from 90.2% to 22.3%,
which could be attributed to major leaching of ILs from the channels of MOFs. All in
all, the results confirmed that the DAILs were effectively encapsulated into the MOF
cages, and the target catalyst MILꢀ100(Fe)@DAILs possessed superior catalytic
activity.
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3. 4. Mechanism of esterification
The classical esterification mechanism had two key steps, one was the protonation of
oleic acid and the other was the nucleophilic attack of the protonated acid by the
shortꢀchain alcohol11. Therefore, the esterification mechanism of oleic acid catalyzed
by the target catalyst MILꢀ100(Fe)@DAILs could be displayed in Fig.9 and the whole
process was divided into five steps. First, DAILs encapsulated within the framework
of MILꢀ100(Fe) would support the proton, and oleic acid was protonated to generate a
carbocation (product 2). Second, the nucleophilic attack of the methanol molecule to
the carbocation generated a middle product (product 3). Third, the protonated
hydroxyl in product 4 had been formed when H+ of product 3 transferred. Fourth, one
molecule water took off the backbone of product 4. Finally, methyl oleate and catalyst
were recovered when the bond of hydrogen and oxygen of product 5 had been broken.
4. Conclusion
DAILs were effectively encapsulated within the framework of MILꢀ100(Fe) by an
“impregnationꢀreactionꢀencapsulation” strategy, and confirmed by corresponding
techniques. The catalytic activity of MILꢀ100(Fe)@DAILs was evaluated by the
esterification reaction with oleic acid and methanol. The optimized experimental
condition was obtained: reaction time 5 h, reaction temperature 67 °C, molar ratio of
methanol to oleic acid 8:1 and catalyst dosage 15 wt%, and then the highest
conversion of oleic acid reached 93.5%. Moreover, the conversion of oleic acid
decreased from 93.5% to 86.0% when the catalyst was reused six times. The results
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showed that the catalyst possessed superior catalytic activity and could be easily
recovered without obvious leaching of the active component.
Acknowledgments
We gratefully acknowledge the National Natural Science Foundation of China
(21476110, 21003074, 21176121 and 21373112), the Key Project for University
Natural Science Foundation of Jiangsu Province (14KJA530001) and the Natural
Science Foundation of Jiangsu Province (BK20131407, BK20151531), the
Foundation from State Key Laboratory of Materialsꢀoriented Chemical Engineering,
Nanjing Tech University(ZK201305), the Research Innovation Training Project for
Graduate Students of Jiangsu Province(KYZZ_0227), and the Practice and Innovation
Training Project for College Students of Jiangsu Province(201410291041Z,
201310291036Z) for financial support.
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(c)
(b)
(a)
5
10
15
20
25
30
2 Theta(degree)
Fig.1. XRD patterns of (a) MILꢀ100(Fe), (b) MILꢀ100(Fe)@DAILs, and (c) recovered
MILꢀ100(Fe)@DAILs.
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(e)
(d)
(c)
1714
1580
1382
1217
(b)
(a)
3443
1035
4000 3500 3000 2500 2000 1500 1000
500
Wavenumbers (cm-1)
Fig. 2. FTIR spectra of (a) AILs, (b) DAILs, (c) MILꢀ100(Fe), (d)MILꢀ100(Fe)@DAILs and (e)
recovered MILꢀ100(Fe)@DAILs.
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Fig 3. Optimized geometric structures of the AILs (a), DAILs (b) and oleic acid (c) were fully
optimized by a semiꢀempirical PM3 method and using Gaussian09 program package (revision
B.01; Gaussian, Inc., Wallingford CT, 2010.); Note: bigger hexagonal windows (d) for the
MILꢀ100(Fe) was deduced from previous report28, big mesoporous cages and smaller pentagonal
opening (e)of small mesoporous cages.
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(d)
(a)
(b)
(c)
(e)
(f)
Fig. 4. SEM images (a, b, c) and TEM images (d, e, f): MILꢀ100(Fe)(a, d);
MILꢀ100(Fe)@DAILs(b, e); recovered MILꢀ100(Fe)@DAILs(c, f).
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(a)
(b)
0.0
0.2
0.4
0.6
0.8
1.0
Relative presure
(p/p0)
Fig. 5. N2 adsorptionꢀdesorption isotherms of (a) MILꢀ100(Fe) and (b) MILꢀ100(Fe)@DAILs.
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100
90
80
70
60
50
40
30
20
(b)
(a)
100
200
300
400
500
600
Temperature (OC)
Fig. 6. TGA curves of (a) MILꢀ100(Fe) and (b) MILꢀ100(Fe)@DAILs.
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100
95
90
85
80
75
70
65
60
100
95
90
85
80
75
70
65
60
(a)
(b)
0
1
2
3
4
5
6
40
50
60
70
80
90
100
Reaction Time (h)
Reaction Temperature (OC)
100
95
90
85
80
75
70
65
60
100
95
90
85
80
75
70
65
60
(d)
(c)
2
4
6
8
10
12
0
2
4
6
8
10 12 14 16 18 20
Molar Ratio of Methanol to Oleic Acid
Catalyst Dosage (%)
Fig. 7. Optimization of esterification of oleic acid catalyzed by MILꢀ100(Fe)@DAILs: reaction
time (a), (77 °C, 10:1, 15 wt%); reaction temperature (b), (5 h, 10:1, 15 wt%); methanol/oleic acid
molar ratio (c), (5 h, 67 °C, 15 wt%); catalyst dosage (d), (5 h, 67 °C, 8:1).
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100
80
60
40
20
0
1
2
3
4
5
6
Recycling times
Fig. 8. Recycling of MILꢀ100(Fe)@DAILs (blue) and MILꢀ100(Fe)/AILs (red) for the
esterification of oleic acid with methanol under the experimental conditions.
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Fig.
9.
Mechanism
of
esterification
catalyzed
by
MILꢀ100(Fe)@DAILs,
R=CH3(CH2)7CH=CH(CH2)7, R’=CH3.
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Table 1. The loading amount, surface area, total pore volume and conversion of oleic acid with the
different samples.
mMILꢀ100(Fe) mAILs Loading amount Surface area Total pore volume Conversion
Sample
/g
/g
/mmolgꢀ1
0.6786
0.8875
0.9250
0.9678
/m2gꢀ1
232.1
170.2
146.4
118.0
/cm3gꢀ1
0.26
/%
1
2
3
4
1.0 g
1.0 g
1.0 g
1.0 g
0.8 g
1.2 g
1.6 g
2.0 g
83.6
89.5
88.2
87.8
0.20
0.19
0.17
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Table 2. Catalytic activities of different catalysts for the esterification of oleic acid.
Entry
Catalyst
Without catalyst
AILs
Alcohol
methanol
methanol
methanol
methanol
methanol
methanol
ethanol
Alcohol/Acid Conversion/%
Reference
1
2
8:1
8:1
8:1
8:1
8:1
8:1
9:1
22:1
9:1
10:1
4.4
This work
This work
This work
This work
This work
This work
31
94.5
94.2
12.8
90.2
93.5
93.0
83.4
80.6
84.5
3
DAILs
4
MILꢀ100(Fe)
5
MILꢀ100(Fe)/AILs
MILꢀ100(Fe)@DAILs
Exchange resin
[BMIM][FeCl4][a]
[BMIM][HSO4]
MPEGꢀ350ꢀILs[b]
6
7
8
methanol
methanol
methanol
32
9
33
10
34
[a] BMIM=1ꢀButylꢀ3ꢀmethylimidazolium,
[b] MPEGꢀ350=Polyethylene glycol monomethylether (polymerization degree=350);
ILs=[SO3Hꢀ(CH2)4ꢀHIM][HSO4].
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99x98mm (300 x 300 DPI)