Immobilization of a thiol-functionalized ionic liquid onto HKUST-1 through thiol compounds as the chemical bridge

Article abstract of DOI:10.1039/c6ra03317b

A novel heterogeneous catalyst [HVIm-(CH₂)₃SO₃H]HSO₄@HKUST-1 (IL@HKUST-1), with both Lewis and Br?nsted acid sites, was developed for the esterification of oleic acid with short-chain alcohols. HKUST-1 was chemically modified with ethanedithiol, and the vinyl-containing ionic liquid was then grafted onto the carrier through thiol groups. The catalyst IL@HKUST-1 was characterized by XRD, N₂ adsorption-desorption, FT-IR, SEM, TG, elemental analysis, and ICP. The results proved that HKUST-1 had typical microporous structure, and the thiol groups were incorporated into the channels of the carrier. Through the reaction of vinyl and thiol, the ionic liquid was successfully immobilized onto SH-HKUST-1 by chemical covalent bonds. The catalyst was applied in the esterification of oleic acid with ethanol, and the optimal conditions were determined as follows: molar ratio of ethanol to oleic acid 12 : 1, catalyst amount 15 wt% (based on oleic acid), reaction time 4 h, and reaction temperature 90 °C. Under the conditions, the conversion of oleic acid was 92.1%. After 5 times of recycling, there was no significant decrease in conversion, showing a certain stability and good reusability of the catalyst. The catalyst also exhibited high catalytic activity in esterification of oleic acid with other short-chain alcohols.

Full text of DOI:10.1039/c6ra03317b

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Page 1 of 43
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Immobilization of a thiol-functionalized ionic liquid onto HKUST-1
through thiol compounds as the chemical bridge
Chong Chen, Zuowang Wu, Yigen Que, Bingxue Li, Qirui Guo, Zhong Li, Lei Wang,
Hui Wan*, and Guofeng Guan*
State Key Laboratory of MaterialsꢀOriented Chemical Engineering, College of
Chemical Engineering, Jiangsu National Synergetic Innovation Center for Advanced
Materials, Nanjing Tech University, Nanjing 210009, P. R. China
*Corresponding
author,
Telephone:
+86ꢀ25ꢀ8358
7198.
Eꢀmail:
wanhui@njtech.edu.cn; guangf@njtech.edu.cn
Abstract:
A
novel heterogeneous catalyst [HVImꢀ(CH2)3SO3H]HSO4@HKUSTꢀ1
(IL@HKUSTꢀ1), with both Lewis and Brønsted acid sites, was developed for the
esterification of oleic acid with shortꢀchain alcohols. HKUSTꢀ1 was chemically
modified with ethanedithiol, and the vinylꢀcontaining ionic liquid was then grafted
onto the carrier through thiol groups. The catalyst IL@HKUSTꢀ1 was characterized
by XRD, N2 adsorptionꢀdesorption, FTꢀIR, SEM, TG, elemental analysis, and ICP.
The results proved that HKUSTꢀ1 had typical microporous structure, and the thiol
groups were incorporated into the channels of the carrier. Through the reaction of
vinyl and thiol, the ionic liquid was successfully immobilized onto SHꢀHKUSTꢀ1 by
chemical covalent bond. The catalyst was applied in the esterification of oleic acid
with ethanol, and the optimal conditions were determined as follows: molar ratio of
ethanol to oleic acid 12:1, catalyst amount 15 wt% (based on oleic acid), reaction time
1

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o
4 h, and reaction temperature 90 C. Under the conditions, the conversion of oleic
acid was 92.1%. After 5 times of recycling, there was no significant decrease in
conversion, showing a certain stability and good reusability of the catalyst. The
catalyst also exhibited high catalytic activity in esterification of oleic acid with other
shortꢀchain alcohols.
1. Introduction
Biodiesel is well recognized as one of the most promising substitutes for the
traditional fossilꢀbased fuels.1 Due to its excellent properties2, such as
biodegradability, renewability, low viscosity and pollution emissions, biodiesel can be
applied as diesel engine fuels without further modifications. Compared with regular
diesel, the biodiesel shows better engine performance and is environmentally
beneficial.3 Biodiesel can be obtained by the esterification of longꢀchain fatty acids
with shortꢀchain alcohols to mono alkyl esters in the presence of a catalyst.4, 5 To
improve its production, an efficient and green catalyst is urgently needed, attracting
more and more attention in this field.
Functionalized acidic ionic liquids play an important role in acid catalysis fields.6
As a class of novel environmental friendly catalysts, they combine the advantages of
solid acids and liquid acids, meanwhile have the characteristics of low volatility,
adjustable acidity, high acidity density, and good fluidity.7ꢀ10 However, functionalized
acidic ionic liquids still have some serious defects such as high viscosity and difficult
to recycle.11ꢀ13 An effective solution to these problems is the immobilization of ionic
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liquid onto solid material with high surface area and certain mechanical strength,14ꢀ19
such as SiO2, ZrO2, SBAꢀ15, and MCMꢀ41. In this way, the performance of ionic
liquid can be improved and its applications will be broadened. Meanwhile, the
introduction of functionalized acidic ionic liquid can endow the solid with diversity
and variability. H. Li and coꢀworkers20 designed a grapheneꢀanalogue hexagonal
boron nitride (GꢀhꢀBN) supported with tungstenꢀbased ionic liquid heterogeneous
catalyst based on postꢀsynthetic modification strategy. The prepared catalyst presented
excellent activity in oxidation of dibenzothiophene, which was superior to
homogeneous catalysts of ionic liquids themselves.
In recent years, metal organic frameworks (MOFs) have been the focus of
academic research due to their excellent properties.21, 22 As new functional materials,
MOFs have been applied very widely in many fields such as gas storage, adsorption
separation, drug delivery, and heterogeneous catalysis.23ꢀ26 And as more and more
MOFs with the advantages of large surface area, high porosity, stable structure,
adjustable pore size, and available functionalization are synthesized,27ꢀ29 they have
attracted increasing attention and shown great potential application in catalytic field.
During the past decade, the study of using MOFs as carriers to support active
components has been reported.30ꢀ33 L H Wee et al.34 prepared the Cu3(BTC)2 (BTC =
benzene tricarboxylic acid) incorporating heteropolyacid nanomaterial and evaluated
its catalytic activity in esterification of acetic acid with 1ꢀpropanol. Y Pan and
coꢀworkers35 reported the oneꢀstep synthesis of methyl isobutyl ketone over palladium
nanoparticles deposited on a chromium terephthalate MILꢀ101 (MIL = material from
3

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institute Lavoisier) catalyst. These studies strongly indicate that MOFs are a class of
promising carrier materials. However, there are only a handful of reports describing
the immobilization of ionic liquid onto MOFs.
Postꢀsynthetic modification (PSM) refers to the chemical modification of MOFs
after the formation of crystals, while the basic frameworks remain unchanged.36, 37
This is an effective way to construct interactions between MOFs and the functional
guest molecules. Y K Hwang et al.38 selected ethylenediamine and diethylenetriamine
as grafting agents to produce the amineꢀgrafted MILꢀ101 and both of them exhibited
high activities in the Knoevenagel condensation. F Ke et al.39 described
thiolꢀfunctionalization of MOFs by a facile coordinationꢀbased postꢀsynthetic strategy
and the samples showed high adsorption capacity for Hg2+ from water. In these cases,
the functionalization mainly relies on the coordinatively unsaturated metal centers
(UMCs) of MOFs and the active species are grafted through coordinate covalent
bond.40ꢀ42 With the similar method, the immobilization of functionalized ionic liquid
onto MOFs can also be realized. Q Luo and coꢀworkers43 developed a functionalized
MILꢀ101 material for acetalization, in which a Brønsted acidic quaternary ammonium
salt ionic liquid was confined inside wellꢀdefined nanocages. This was the first
successful attempt to combine the advantages of ionic liquid with the desirable
microenvironment in MOFs. All these works and some other studies44ꢀ48 prove that
the introduction of ionic liquid into MOFs is attainable and hopeful in catalytic
applications.
Herein, a novel biꢀfunctional catalyst IL@HKUSTꢀ1 (HKUST = Hong Kong
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university of science and technology) is synthesized by a PSM strategy and applied in
the esterification reaction. HKUSTꢀ1, also known as Cu3(BTC)2 or MOFꢀ199, is
selected as the carrier. Considering the strong coordination ability of metal centers
(Cu2+), ethanedithiol is used as the grafting agent to introduce thiol groups. By the
reaction of vinyl and thiol, the ionic liquid [HVImꢀ(CH2)3SO3H]HSO4 can then be
immobilized onto HKUSTꢀ1 to achieve the catalyst. Although the pristine carrier
exists a certain amount of Lewis acid sites, the acidity is far from enough to meet the
requirement of the acid catalytic system. The introduction of ionic liquid greatly
improves the Brønsted acidity of the material, thus making it an efficient catalyst for
the esterification of oleic acid with ethanol. The structure and property of the prepared
IL@HKUSTꢀ1 are systematically characterized by XRD, N2 adsorptionꢀdesorption,
FTꢀIR, SEM, TG, elemental analysis, and ICP. At the same time, the optimal reaction
conditions are obtained and the reusability of the catalyst is also evaluated.
2. Experimental
2.1. Materials
All chemicals were commercially available and used without further purification.
Copper nitrate trihydrate (Cu(NO3)2·3H2O), benzeneꢀ1,3,5ꢀtricarboxylic acid
(H3BTC), 1,2ꢀethanedithiol, 2,2ꢀazoꢀbisꢀisobutyronitrile (AIBN), and oleic acid were
of analytical reagent grade and purchased from SigmaꢀAldrich (Shanghai) Trading
Co., Ltd. 1ꢀVinylimidazole (99%) was purchased from Ninghai Kaile Chemical Co.,
Ltd. 1,3ꢀPropane sultone (99%) was purchased from Wuhan Fengfan Surface
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Engineering Co., Ltd. Sulfuric acid, anhydrous ethanol, and ethyl acetate were
purchased from Wuxi City Yasheng Chemical Co., Ltd. Diethyl ether was purchased
from Sinopharm Chemical Reagent Beijing Co., Ltd. Toluene (99.5%) was purchased
from Shanghai Lingfeng Chemical Reagent Co., Ltd. All other chemicals used in
synthesis were of analytical grade.
2.2. Methods
The powder Xꢀray diffraction (XRD) experiments were carried out at the
Smartlab diffractometer (RIGAKU, Japan) equipped with Niꢀfiltered Cu Kα radiation
(λ=0.15418 nm). The Xꢀray tube was operated at 40 kV and 40 mA. The scanning
angle was from 0.6o to 30o with scanꢀsteps of 0.02o per second. To calculate the
BrunauerꢀEmmettꢀTeller (BET) surface area, N2 adsorptionꢀdesorption measurements
were performed on the Micromeritics ASAP 2020 system model instrument at ꢀ196 oC.
o
The samples were outgassed under vacuum at 200 C for 8 h before measurement.
Fourier transform infrared (FTꢀIR) spectra were recorded on the Nicoletꢀ6700
spectrometer (Thermo Fisher Scientific, U.S.) using anhydrous KBr (Nicolet, United
States) as dispersing agent. The spectra were collected after accumulation of 128
scans with a resolution of 5 cmꢀ1. Scanning electron microscopy (SEM) images were
obtained with the S4800 fieldꢀemission scanning electron microscope (HITACHI,
Japan). Before the test the samples were dispersed with ultrasonic treatment to
remove the aggregated grains. The thermal behavior was examined by the STA 409PC
thermo gravimetric (TG) analyzer. The temperature ranged from 30 oC to 600 oC with
a heating rate of 10 oC/min under N2 atmosphere. Elemental analyses were performed
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on Elementar Vario EL III system to determine the loading amount of ionic liquid in
the prepared catalysts. The amount of Cu in the catalyst was analyzed by inductively
coupled plasma (ICP, Optima 7000 DV, PerkinꢀElmer, USA) after dissolving the
sample in hydrofluoric acid.
2.3. Catalyst preparation
The acidic ionic liquid [HVImꢀ(CH2)3SO3H]HSO4 was synthesized referring to
our previous work (Scheme 1).49 Specifically, 9.4 g of 1ꢀvinylimidazole was added
into a 100 mL round bottom flask, adding certain amount of ethyl acetate as solvent.
1,3ꢀpropane sultone (12.2 g) was then slowly dropped into the solution under vigorous
stirring condition at 0 oC and the mixture was continuously agitated for another 24 h.
After suction filtration, washing with ethyl acetate, and vacuum drying, the white
precipitates were collected. Then the formed products were dissolved in a small
amount of deionized water, and equal molar sulfuric acid was added dropwise in an
ice bath. The mixture was slowly heated up to 60 oC and reacted for 12 h with stirring.
After the reaction finished, the target ionic liquid was obtained by reduced pressure
distillation, washing with diethyl ether, and vacuum drying.
The thiolꢀfunctionalized HKUSTꢀ1 (SHꢀHKUSTꢀ1) was synthesized by a
convenient postꢀsynthetic modification method.39 First, 2.174 g of Cu(NO3)2·3H2O
was dissolved in 30 mL deionized water, while 1.05 g of benzeneꢀ1,3,5ꢀtricarboxylic
acid was dissolved in 30 mL ethanol. The two solutions were mixed thoroughly and
stirred for 30 min. Then the mixture was transferred to a 100 mL Teflon autoclave
o
liner and heated for 12 h at 120 C. After cooling down to the room temperature, the
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blue powder was obtained by filtering, washing with deionized water and ethanol
o
alternately, and drying under vacuum at 60 C for 12 h. Second, the synthesized
HKUSTꢀ1 sample was chemically modified with 1,2ꢀethanedithiol. The solid powder
was activated at 150 oC for 12 h before use to remove the water molecules. Typically,
0.25 g of ethanedithiol was dissolved in 100 mL of anhydrous toluene, and 1.0 g of
HKUSTꢀ1 was added into the solution gradually. The mixture was stirred at room
temperature for 24 h, and then the suspension was centrifuged. The dark green solid
was washed with ethanol for 5 times, and dried overnight under vacuum at room
temperature.
The preparation process of IL@HKUSTꢀ1 was illustrated in Scheme 2. In a
typical process, 3.0 g of the vinylꢀcontaining ionic liquid was dissolved in 50 mL of
anhydrous ethanol, and 1.0 g of thiolꢀfunctionalized HKUSTꢀ1 was subsequently
o
dispersed to the mixture with stirring. After the temperature was increased to 80 C,
0.03 g of AIBN was added under the protection of nitrogen and the reaction was
lasted for 30 h. The product was filtered and washed with ethanol repeatedly to
remove the excess ionic liquid. The target catalyst IL@HKUSTꢀ1 was obtained after
drying in vacuum at 60 oC for 12 h.
2.4. Catalytic Activity Measurement
Taking the esterification of oleic acid with ethanol as a probe reaction, the
catalytic activity of IL@HKUSTꢀ1 was investigated. In a typical procedure,
appropriate amounts of oleic acid and ethanol were mixed in a 50 mL round bottomed
flask equipped with magnetic stirring and condensate water, and IL@HKUSTꢀ1 was
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added as the catalyst. Then the esterification was allowed to proceed at a certain
temperature for a period of time. For this study, the molar ratios of ethanol to oleic
acid were varied from 8:1 to 16:1, and the catalyst amounts (based on oleic acid) were
set from 5 wt% to 25 wt% (the total mass of the catalyst). The reaction times ranging
o
from 2 h to 6 h were investigated. The reaction temperatures changing from 50 C to
o
130 C were also discussed. After reaction, the upper liquid was obtained by
centrifugation for further analysis and the remaining catalyst was recovered by
washing with diethyl ether and drying under vacuum. The recycling experiment was
carried out as the same procedure to study the reusability of the catalyst.
The conversion of oleic acid was calculated according to the literature with
modification.50 Briefly, 1.0 g of reaction liquid was accurately weighed and dissolved
in 20 mL of ethanol, adding a few drops of phenolphthalein as indicator. The mixture
was then titrated with 0.02 mol/L NaOH ethanol solution. The acidity index of the
sample could be calculated based on the volume of NaOH solution consumed by the
following equation:
A = (Vt ꢀ Vo) Mc / m
(1)
where A is the acidity index of the sample, mg/g; Vt is the final reading of the burette,
mL; Vo is the initial reading of the burette, mL; M is the molecular weight of NaOH,
g/mol; c is the concentration of NaOH solution, mol/L; m is the weight of the sample
used for titration, g.
According to the equation below, the conversion of oleic acid could be obtained.
Y = (Ao ꢀ At) / Ao × 100%
(2)
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where Y represents the conversion of oleic acid, %; Ao represents the initial acidity
index of the sample before reaction, mg/g; At represents the final acidity index of the
sample after reaction, mg/g.
3. Results and discussion
3.1. Catalyst characterization
The XRD patterns of HKUSTꢀ1, SHꢀHKUSTꢀ1, IL@HKUSTꢀ1, and the
recovered catalyst were illustrated in Figure 1. It could be seen that the diffraction
peak positions of all samples were primarily the same. The main characteristic
diffraction peaks of HKUSTꢀ1 crystals appeared at 2θ=6.66 o, 9.50 o, 11.68 o, 13.44 o,
o
o
19.12 , and 25.96 , which were aligned with the data reported in literature.51 After
the chemical modification with ethanedithiol (Figure 1(b)), the intensities of
diffraction peaks of SHꢀHKUSTꢀ1 were accordingly declined, indicating the decrease
in the crystallinity degree of the sample. This was because a small amount of
HKUSTꢀ1 crystallites were unstable and decomposed in the functionalization process.
As shown in Figure 1(c), the introduction of vinylꢀcontaining ionic liquid did not
cause further damage to the frameworks and the MOF structure remained unchanged
during immobilization. The pattern of the recovered catalyst was akin to that of the
fresh one, proving a good stability of the prepared catalyst in esterification reactions.
The N2 adsorptionꢀdesorption isotherms of HKUSTꢀ1, SHꢀHKUSTꢀ1, and
IL@HKUSTꢀ1 were shown in Figure 2. The curves of the bare HKUSTꢀ1 were typical
IVꢀtype isotherms with an H4ꢀtype hysteresis loop, which was characteristic of
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microporous materials with regular channel structure. Compared with HKUSTꢀ1, the
surface area and pore volume of the thiolꢀfunctionalized sample were both declined.
The surface area dropped from 1395.3 m2/g to 816.0 m2/g, while the pore volume
decreased from 0.56 cm3/g to 0.40 cm3/g. The main reason for this phenomenon was
that the pores of the carrier were partially occupied by ethanedithiol after modification,
inferring that the thiol functional groups existed inside the channels of the framework
rather than outside the surfaces. For IL@HKUSTꢀ1, the surface area and pore volume
further reduced to 386.9 m2/g and 0.23 cm3/g respectively due to the partial blocking
of the pores. It demonstrated that the ionic liquid was successfully immobilized onto
HKUSTꢀ1 by the reaction of vinyl and thiol. As observed in Figure 2(b) and (c), the
hysteresis loops of the two samples had a tendency to translate into H3ꢀtype, which
could be ascribed to the slight decomposition of the framework.
The FTꢀIR spectra (A) and partial enlargement (B) of [HVImꢀ(CH2)3SO3H]HSO4,
1,2ꢀethanedithiol, HKUSTꢀ1, SHꢀHKUSTꢀ1, IL@HKUSTꢀ1, and recovered
IL@HKUSTꢀ1 were presented in Figure 3. For vinylꢀcontaining ionic liquid, the
bands around 1643 cmꢀ1, 1565 cmꢀ1, and 3143 cmꢀ1 could be attributed to the C=C,
C=N, and CꢀH stretching vibrations of the imidazole ring, respectively. The band
corresponding to the endꢀgroup C=C shear vibration was observed at 1417 cmꢀ1, while
the bands at 1176 cmꢀ1 and 1050 cmꢀ1 were assigned to the S=O asymmetric and
symmetric stretching vibrations of ꢀSO3H group.52 In Figure 3(c), the characteristic
peaks of bare HKUSTꢀ1 were observed around 1646 cmꢀ1 and 1375 cmꢀ1, which were
related to the C=O asymmetric and symmetric stretching vibrations of carboxylic
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ligands. The characteristic vibrational band of the C=C group at 1449 cmꢀ1 was also
observed corresponding to the aromatic group. The thiolꢀfunctionalized sample
showed two new peaks at 2590 cmꢀ1 and 695 cmꢀ1, indicating the presence of SꢀH and
CꢀS vibrations. Meanwhile, the wavenumbers of the aliphatic CꢀH stretching
vibrations around 2800ꢀ3000 cmꢀ1 were slightly shifted to larger values in comparison
with the standard spectrum of ethanedithiol (Figure 3(b)), which demonstrated that the
thiol groups had been successfully grafted onto the unsaturated metal sites (Cu2+) in
the HKUSTꢀ1 framework through SꢀCu coordination bond.39 It could be found in
Figure 3(d) and (e) that the characteristic adsorption peaks of SꢀH and endꢀgroup C=C
totally disappeared after the introduction of ionic liquid.49 Furthermore, the main
peaks of HKUSTꢀ1 as well as [HVImꢀ(CH2)3SO3H]HSO4 occurred in curve (e). These
results also confirmed that the ionic liquid was grafted onto SHꢀHKUSTꢀ1 by
chemical covalent bond. There was no obvious difference in characteristic peaks
between recovered IL@HKUSTꢀ1 and the fresh one, ensuring a considerable
reusability of the catalyst. The results obtained were well accordant with the analysis
of XRD.
To investigate the surface morphologies and crystalloid structures of the samples,
the SEM characterization was conducted and the correlative images were given in
Figure 4. The original HKUSTꢀ1 crystals had regular octahedron geometry with
smooth surfaces, and the average particle size was about 15 ꢁm. After the
thiolꢀfunctionalization, the surfaces of SHꢀHKUSTꢀ1 became rougher and part of the
crystals ruptured. The main reason was that the HKUSTꢀ1 framework was partially
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decomposed during the process. In comparison, the dispersion of the catalyst tended
to get worse after the immobilization of ionic liquid, whereas the structure was
conserved. It was worth noticing that the crystal grains of IL@HKUSTꢀ1 did not
massively aggregated, indicating that most of the vinylꢀcontaining ionic liquid
diffused into the pores of the carrier and reacted with thiol groups rather than
selfꢀpolymerized on the outer surfaces. The results agreed well with those of XRD
and N2 adsorptionꢀdesorption. The morphology of the recovered catalyst was virtually
unchanged, which revealed that the stability of the catalyst was improved owing to the
introduction of ionic liquid.
The TG curves of HKUSTꢀ1, SHꢀHKUSTꢀ1, and IL@HKUSTꢀ1 were illustrated
in Figure 5. The bare HKUSTꢀ1 exhibited two weight loss stages corresponding to the
o
departure of the adsorbed/coordinated water inside the pores (30ꢀ200 C) and the
collapse of HKUSTꢀ1 framework (200ꢀ350 oC). For the thiolꢀfunctionalized sample, a
o
o
new stage between 100 C and 220 C was discovered, the weight loss was related to
the decomposition of ethanedithiol. The TG curve of IL@HKUSTꢀ1 could also be
o
divided into three stages. The weight loss within 200 C was due to the loss of
adsorbed water as well as the coordinated water molecules. In the second step
o
(200ꢀ300 C), the weight loss was attributed to the decomposition of ionic liquid. On
the final stage, the trimesic acid disintegrated and the framework collapsed, as
o
o
indicated by the weight loss between 300 C and 400 C. It was obvious that the
decomposition temperature of IL@HKUSTꢀ1 was the highest among all the samples,
suggesting that the interaction between ionic liquid and the carrier could enhance the
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thermal stability of the catalyst to some extent.
The data of elemental analysis revealed that the loading amount of ionic liquid in
the fresh catalyst was calculated to be 0.97 mmol/g. While in the catalyst reused for 5
times, the corresponding value decreased to 0.93 mmol/g. The main reason was that a
small amount of crystals were decomposed during reaction due to the poor chemical
stability of HKUSTꢀ1,34 which might cause the loss of ionic liquid inside the grains.
The results corresponded well with those of SEM.
3.2. Catalytic activity of IL@HKUST-1
To investigate the catalytic activity of IL@HKUSTꢀ1, the esterification of oleic
acid with ethanol was selected as the probe reaction. Under the standard conditions
(oleic acid 5 mmol, molar ratio of ethanol to oleic acid 12:1, catalyst amount 15 wt%
(based on oleic acid), reaction time 4 h, reaction temperature 90 oC), the esterification
reactions were carried out with different catalysts and the corresponding results were
shown in Table 1.
It was clearly that the pure ionic liquid, as a homogeneous catalyst, showed high
catalytic activity for the esterification of oleic acid. By contrast, only 4.8% conversion
was obtained without catalyst. The activity of HKUSTꢀ1 was slightly enhanced due to
the mild Lewis acidity of the unsaturated coordinated metal center (Cu2+), whereas the
conversion of oleic acid was still below 10%. Besides, the introduction of thiol
functional groups (SHꢀHKUSTꢀ1) had very little impact on the reaction. After the
immobilization of ionic liquid, the conversion of oleic acid reached 92.1%, much
higher than that of the carrier. The possible reason was that the ionic liquid was the
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main active component, and the acidity of the catalyst was greatly improved.
Moreover, the newly joined Brønsted acid sites worked together with Lewis acid sites
to promote the synergistic effect, which was more conducive to the reaction.52, 53
In comparison with the results reported in literature,54ꢀ60 the catalytic activity of
IL@HKUSTꢀ1 was roughly equal to that of the same kind of catalysts. It indicated
that the introduction of acidic ionic liquid into MOFs was feasible and effective.
3.3. Effect of reaction conditions
For esterification of oleic acid with ethanol, the main factors influenced the
conversion of oleic acid were molar ratio of ethanol to oleic acid, amount of catalyst,
reaction time, and reaction temperature. In order to obtain the optimum reaction
conditions, the singleꢀfactor experiments were carried out.
Effect of molar ratio. To investigate the effect of molar ratio on conversion of
oleic acid, the dosage of ethanol was increased whereas other conditions were
invariant. As shown in Figure 6, the conversion increased rapidly with increasing
molar ratio of ethanol to oleic acid from 8:1 to 12:1. However, further increase of
molar ratio led to slight decrease of the conversion. This was due to the reversibility
of esterification reaction, in other words, excess ethanol was unfavorable because the
active sites of catalyst and oleic acid were both diluted.59 Therefore, we could
conclude that the appropriate molar ratio was 12:1, and the corresponding conversion
was 92.1%.
Effect of catalyst amount. The relationship between catalyst amount and the
conversion of oleic acid was given in Figure 7. It was seen that the conversion
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increased in the initial stage (from 5 wt% to 15 wt%) and slowly declined in the latter
part (from 15 wt% to 25 wt%). Within a certain scope, the increase of catalyst amount
could provide more active centers, and thus the conversion rate was improved. Instead,
excessive amount of catalyst might lead to worse dispersion and partial coverage of
the active sites, thereby avoiding the contact between the reactants and catalyst.54 The
optimal catalyst amount was chosen as 15 wt%.
Effect of reaction time. Figure 8 showed the effect of reaction time on
conversion of oleic acid. At first, the conversion obtained rapid increase with the
reaction time extending. After reacting for about 4 h, the esterification basically
achieved equilibrium and the conversion of oleic acid reached 92.1% at this point.
Further prolongation of time did not show significant promoting effect on the
conversion. Taking into account of energy consumption at the same time, the
appropriate reaction time was selected as 4 h.
Effect of reaction temperature. The last but equally important factor we
discussed here was reaction temperature. As shown in Figure 9, the conversion of
oleic acid increased from 62.8% to 92.1% as the reaction temperature was raised up
o
o
o
from 50 C to 90 C. When the temperature was over 90 C, the conversion almost
remained unchanged. The molecule collision was more likely to happen at relatively
high temperatures, therefore the reaction rate was accelerated with temperature
ascending. However, too high temperature was unnecessary because the decrease of
reactant concentration caused by the volatilization of ethanol was not conducive to the
reaction.59 For this reason, the suitable reaction temperature was set as 90 oC.
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Based on the analysis above, the optimal reaction conditions were listed as
follows: molar ratio of ethanol to oleic acid 12:1, catalyst amount 15 wt% (based on
oleic acid), reaction time 4 h, and reaction temperature 90 oC. Under these conditions,
the conversion of oleic acid could reach 92.1%, showing high catalytic activity of the
prepared catalyst.
Effect of different alcohols. The influence of different alcohols (methanol,
ethanol, nꢀpropanol, and nꢀbutanol) as the reactants were also studied in the presence
of IL@HKUSTꢀ1 catalyst. The esterification reactions were conducted under the same
conditions and the results were illustrated in Figure 10.
The experimental results indicated that the prepared catalyst was active in a
series of esterification reactions and all of the conversions were above 90%. The
slight diversity between conversions could be attributed to the different solubility of
reactants. It could be concluded that IL@HKUSTꢀ1 was an efficient heterogeneous
catalyst for esterification of oleic acid with shortꢀchain alcohols.
3.4. Reusability of IL@HKUST-1 and hot filtration test
To investigate the reusability of IL@HKUSTꢀ1, the esterification of oleic acid
with ethanol was repeated for several times under the optimal reaction conditions. It
was observed in Figure 11 that the conversion of oleic acid decreased from 92.1% to
86.3% (dropped by 5.8%) after 5 times of recycling. The hot filtration test was carried
out to clarify the reasons for deterioration in activity. After reacting for 0.5 h, the
esterification was stopped and the catalyst was removed from the reaction mixture.
The liquid phase was analyzed and allowed to react for another 4.5 h. A slight
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increase in conversion (Figure 12) was found in the subsequent process, which might
be ascribed to the leaching of active component (ionic liquid) inside the channels. To
further assess the leaching phenomenon, the ICP technique was performed to examine
the variation of Cu2+ ions. The Cu2+ content in the catalyst decreased by 3.12% after 5
times recycle, indicating that the HKUSTꢀ1 framework was slightly decomposed
during the process. It also suggested that the decomposition of the framework was the
main reason for the loss of active component. There was only a minor decrease in
conversion rate, revealing that the catalyst could still remain stable under reaction
conditions and the catalyst had good reusability within a certain scope.
3.5. Catalytic mechanism of esterification over IL@HKUST-1 catalyst
In this experiment, the entity relationship (ER) model was applied to illustrate
the possible reaction mechanism of esterification catalyzed by IL@HKUSTꢀ1. As
shown in Figure 13, the whole process could be divided into five steps. At first, the
carbonyl group of carboxylic acid was protonated by the immobilized ionic liquid
catalyst, which was then attacked by the nucleophilic O atom of alcohol. In the third
stage, the proton from oxonium ion transferred and the activated complex 4 was
formed. After loss of water and subsequent deprotonation, the product ester was
obtained and the catalyst IL@HKUSTꢀ1 was regenerated.
4. Conclusions
In this study, the acidic ionic liquid [HVImꢀ(CH2)3SO3H]HSO4 was synthesized
and successfully grafted onto the thiolꢀfunctionalized HKUSTꢀ1 by chemical covalent
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bond. The ethanedithiol bridged the gap between ionic liquid and HKUSTꢀ1, thus the
vinylꢀcontaining ionic liquid could be immobilized onto the carrier through thiol
groups. Furthermore, the introduction of ionic liquid slightly enhanced the thermal
stability of framework and greatly improved the acidity of the catalyst. The
biꢀfunctional IL@HKUSTꢀ1 catalyst exhibited good activity for esterification of oleic
acid with ethanol, and a satisfactory conversion of 92.1% was obtained under optimal
reaction conditions. The catalyst could be easily recovered and reused for at least 5
times without obvious decrease in activity. However, the chemical stability of
HKUSTꢀ1 in acidic medium is still not so satisfactory. Thus, it is a crucial problem to
prevent the decomposition of framework and it is worth investigating in the further. In
conclusion, this work provides a new thinking for the combination between ionic
liquid and MOF, which is very important for designing novel immobilized ionic liquid
catalysts.
Acknowledgements
We gratefully acknowledge the National Natural Science Foundation of China
(No. 21176121, No. 21476110), the National Natural Science Foundation of Jiangsu
Province of China (No. BK20151531), the Key Project for University Natural Science
Foundation of Jiangsu Province (No. 14KJA530001), the Foundation from State Key
Laboratory of MaterialsꢀOriented Chemical Engineering, Nanjing Tech University
(No. ZK201305), and the Practice and Innovation Training Project for College
Students of Jiangsu Province (No. 201410291041Z).
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Captions for Tables and Figures
Table 1. Activities of various catalysts for the esterification of oleic acid with short
alcohols.
Scheme 1. Preparation of the ionic liquid [HVImꢀ(CH2)3SO3H]HSO4.
Scheme 2. Preparation of the immobilized ionic liquid IL@HKUSTꢀ1.
Figure 1. The XRD patterns of HKUSTꢀ1 (a), SHꢀHKUSTꢀ1 (b), IL@HKUSTꢀ1 (c),
and recovered IL@HKUSTꢀ1 (d).
Figure 2. The N2 adsorptionꢀdesorption isotherms of HKUSTꢀ1 (a), SHꢀHKUSTꢀ1 (b),
and IL@HKUSTꢀ1 (c).
Figure 3. The FTꢀIR spectra (A) and partial enlargement (B) of
[HVImꢀ(CH2)3SO3H]HSO4 (a), 1,2ꢀethanedithiol (b), HKUSTꢀ1 (c), SHꢀHKUSTꢀ1 (d),
IL@HKUSTꢀ1 (e), and recovered IL@HKUSTꢀ1 (f).
Figure 4. The SEM images of HKUSTꢀ1 (a1, a2), SHꢀHKUSTꢀ1 (b1, b2),
IL@HKUSTꢀ1 (c1, c2), and recovered IL@HKUSTꢀ1 (d1, d2).
Figure 5. The thermogravimetric curves of HKUSTꢀ1 (a), SHꢀHKUSTꢀ1 (b), and
IL@HKUSTꢀ1 (c).
Figure 6. Effect of molar ratio on conversion of oleic acid. Catalyst amount = 15 wt%,
t = 4 h, T = 90 oC.
Figure 7. Effect of catalyst amount on conversion of oleic acid. Molar ratio = 12:1, t
= 4 h, T = 90 oC.
Figure 8. Effect of reaction time on conversion of oleic acid. Molar ratio = 12:1,
catalyst amount = 15 wt%, T = 90 oC.
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Figure 9. Effect of reaction temperature on conversion of oleic acid. Molar ratio =
12:1, catalyst amount = 15 wt%, t = 4 h.
Figure 10. Esterification of oleic acid with different alcohols catalyzed by
IL@HKUSTꢀ1. Molar ratio = 12:1, catalyst amount = 15 wt%, t = 4 h, T = 90 oC.
Figure 11. The recycling of IL@HKUSTꢀ1 catalyst for esterification of oleic acid
with ethanol. Molar ratio = 12:1, catalyst amount = 15 wt%, t = 4 h, T = 90 oC.
Figure 12. The hot filtration test for esterification with IL@HKUSTꢀ1 catalyst.
Figure 13. Catalytic mechanism of esterification over IL@HKUSTꢀ1 catalyst.
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Table 1. Activities of various catalysts for the esterification of oleic acid with short alcohols.
Entry
Catalyst
without catalyst
[HVImꢀ(CH2)3SO3H]HSO4
HKUSTꢀ1
Alcohol Alcohol/Acid Conversion (%)
Ref.
this work
this work
this work
this work
this work
54
1
2
3
4
5
6
7
8
ethanol
ethanol
ethanol
ethanol
ethanol
ethanol
ethanol
ethanol
12:1
12:1
12:1
12:1
12:1
12:1
6:1
4.8
94.6
8.2
SHꢀHKUSTꢀ1
IL@HKUSTꢀ1
H2SO4
8.7
92.1
90.1
88.0
93.0
H3PW/ZrO2
55
sulfonated cation exchange
resin
9:1
56
9
[BMIM][FeCl4]
[BMIM][HSO4]
IL/FeꢀSBAꢀ15
MPEGꢀ350ꢀILs
methanol
methanol
methanol
methanol
22:1
9:1
83.4
80.6
87.7
84.5
57
58
59
60
10
11
12
6:1
10:1
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Scheme 1. Preparation of the ionic liquid [HVImꢀ(CH2)3SO3H]HSO4.
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Scheme 2. Preparation of the immobilized ionic liquid IL@HKUSTꢀ1.
29