Stable and recyclable MIL-101(Cr)–Ionic liquid based hybrid nanomaterials as heterogeneous catalyst

Article abstract of DOI:10.1016/j.molliq.2017.04.034

Br?nsted acidic ionic liquid, N-methyl-2-pyrrolidonium methyl sulfonate ([NMP]⁺?CH₃SO₃^?) immobilized on MIL-101(Cr) was fabricated by simple impregnation method with a good combination of MIL-101(Cr) and IL species. The worthiness of IL/MIL-101(Cr), as a Br?nsted acid catalyst, has been examined for the esterification of acetic acid with amyl alcohol and Friedel–Crafts acylation of anisole. Our findings demonstrated that IL/MIL-101(Cr) catalyst exhibited distinct catalytic activity with respect to the other catalysts towards the esterification reaction and Friedel–Crafts acylation of anisole. The Br?nsted acidic catalysts loaded on MIL-101(Cr) as a new category of porous materials are probably auspicious heterogeneous catalysts for acid-catalyzed to replace the use of traditional homogeneous catalysts. Furthermore, the catalyst can be easily removed from the reactions mixtures and reuse for posterior reactions, more than six times without any considerable decay in catalytic performance.

Full text of DOI:10.1016/j.molliq.2017.04.034

Accepted Manuscript
Stable and recyclable MIL-101(Cr)–Ionic liquid based hybrid
nanomaterials as heterogeneous catalyst
Hassan M.A. Hassan, Mohamed A. Betiha, Shaimaa K.
Mohamed, E.A. El-Sharkawy, Emad A. Ahmed
PII:
S0167-7322(17)30389-6
doi: 10.1016/j.molliq.2017.04.034
MOLLIQ 7187
DOI:
Reference:
To appear in:
Journal of Molecular Liquids
Received date:
Revised date:
Accepted date:
27 January 2017
9 April 2017
11 April 2017
Please cite this article as: Hassan M.A. Hassan, Mohamed A. Betiha, Shaimaa K.
Mohamed, E.A. El-Sharkawy, Emad A. Ahmed , Stable and recyclable
MIL-101(Cr)–Ionic liquid based hybrid nanomaterials as heterogeneous catalyst. The
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check if appropriate. Molliq(2017), doi: 10.1016/j.molliq.2017.04.034
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ACCEPTED MANUSCRIPT
Stable and Recyclable MIL-101(Cr)–Ionic Liquid Based Hybrid
Nanomaterials as Heterogeneous Catalyst
a*
b*
a
Hassan M. A. Hassan , Mohamed A. Betiha , Shaimaa K. Mohamed , E. A. El-
a
a
Sharkawy , Emad A. Ahmed
a
Department of Chemistry, Faculty of Science, Suez University, Suez, Egypt
Egyptian Petroleum Research Institute, Cairo 11727, Nasr city-Cairo
b
1

ACCEPTED MANUSCRIPT
Abstract
Brønsted acidic ionic liquid, N-methyl-2-pyrrolidonium methyl sulphonate
+
-
(
[NMP] CH SO ) immobilized on MIL-101(Cr) was fabricated by simple impregnation
3 3
method with a good combination of MIL-101(Cr) and IL species. The worthiness of
IL/MIL-101(Cr), as a Brønsted acid catalyst, has been examined for the esterification of
acetic acid with amyl alcohol and Friedel–Crafts acylation of anisole. Our findings
demonstrated that IL/MIL-101(Cr) catalyst exhibited distinct catalytic activity with
respect to the other catalysts toward the esterification reaction and Friedel–Crafts
acylation of anisole. The Brønsted acidic catalysts loaded on MIL-101(Cr) as a new
category of porous materials are probably auspicious heterogeneous catalysts for acid-
catalyzed to replace the use of traditional homogeneous catalysts. Furthermore, the
catalyst can be easily removed from the reactions mixtures and reuse for posterior
reactions, more than six times without any considerable decay in catalytic performance.
Keywords: Solid acid catalyst; MIL-101, Ionic liquid, esterification, Friedel–Crafts,
acylation, acidity.
*
Corresponding author
E-mail: hassan.hassan@suezuniv.edu.eg
mohamed_betiha@hotmail.com
Fax: (+20 62)3707068
2

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1
. Introduction
For assorted applications, such as adsorption, membrane science, and catalysis,
acid catalysts are abundantly under-exploited in the chemical industry [1]. Mineral acids
as homogeneous catalysts usually have high catalytic activity, however, they have many
disadvantages as side reactions, corrosion and huge amounts of harmful wastes, which
can cause environmental problems [2]. Recently, the reusable rigid acidic catalysts
became much significant due to their prospect to substitute harmful homogeneous acidic
catalysts in industrial operations [3, 4]. Acidic heterogeneous catalyzing agents can be
effectively isolated from the reaction blend by filtration or centrifugation and don't
require any special treatments, consequently giving an ecologically favorable process
and decreasing the expense of handling [5]. Another favorability of the heterogeneous
catalyst is the reusability [6, 7]. The field of metal–organic frameworks (MOFs), a new
category of porous materials self-assembled from metal ions or clusters and polytopic
rigid organic linkers, has provided a good candidate for catalytic applications owing to
(i) effective framework in comparison with other micro- and mesoporous materials. (ii)
well-defined channels with a tunable pore size that able to accommodate various species
with different sizes and shape. (iii) MOF backbone can be incorporated with multiple
metals and/or organic linkers (multivariate (MTV) functionalization), leading to
possible functional sites of unique distribution within the pores while the original
structure is retained [8-12].
Recently, several strategies for acid sites functionalizing MOFs have been introduced,
including post-synthesis and metal encapsulation to tuned framework through
coordination bonds between metal center and Lewis or base guests [13], post framework
sulphonation, amination, N-substituted aminosulphonic acids and encapsulation of
heteropolyacids [14-16]. In spite of these promising approaches, the development of
general and facile methods to introduce more sites within MOFs remains a challenge.
Today, an ionic liquid (ILs) as environmental-friendly acidic catalysts have received
wide attention because of their unique characteristics likes considerable solubility,
negligible volatility, non-flammable, non-corrosive and remarkable thermal stability.
Ionic liquids (ILs) are defined as salt with a melting point lower than the boiling point
of water. Typically, ILs are composed of organic onium cations and inorganic anions.
Additionally, the solubility of ionic liquids in various reaction media can be controlled
3

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and easily fine-tuned by changing of both cations and anions [17-24]. Ionic liquids (ILs)
have emerged as alternative solvents for various organic syntheses and attracted
increasing attention for various catalytic applications. However, there are some
drawbacks in the catalytic field such as limited solubility in organic compounds
(especially polar molecules), causing not only the loss of catalytic efficiency but also
resulted in the difficulty of purification due to the high viscosity of the reaction media.
Among attempts have been made to overcome these problems is immobilization the ILs
on solid porous materials such as crosslinked polystyrene-divinylbenzene[24],
mesoporous materials [7] and Fe O4 nanoparticles[25]. Indeed, utilization of
3
immobilized acidic ionic liquids (ILs) on robust backings as heterogeneous catalysts
display many advantages other than the use of acidic ILs directly. Such immobilization
may lead to boost the available effective sites on the catalyst surface and reduces the
used amount of ILs. The ionic liquid layer on the support surface acts as a homogeneous
medium for the catalyzed processes however the catalyst macroscopically seems solid,
consequently it is easily separated from the medium [2].
Despite the fact that there are abundant future prospects to apply MOFs as solid
supports for ILs, ILs/MOFs composites showed considerable properties owing to the
nanosizing of ILs than the bulk ionic liquid that showed the usual melting or freezing
characteristics [26]. Thus, it is very interesting and significant to prepare and explore for
the first time the application of chemically confinement ILs-MOFs for catalysis. As a
prevalent Acidic catalyzed model reaction, Esterification is a standout amongst the most
connected reactions in organic chemistry, with the hugeness in the synthesis of
fragrances, polymers, and paints [27]. As well, Friedel–Crafts acylation of aromatics is
a critical process, which is typically utilized as a part of fine synthetic, pharmaceutical
commercial projects polymers, and paints, etc. [28].
Herein, we demonstrate how to chemically incorporate highly Brønsted acidic IL, N-
methyl-2-pyrrolidonium methyl sulphonate, into porous MIL-101(Cr) to avoiding
leaching of ILs through grafting the IL into the residual terephthalic species coordinated
to CUS (coordinatively unsaturated metal sites) in MIL-101(Cr) by simple impregnation
method under ambient conditions as a significant heterogeneous and economically
acidic catalyst for esterification and Friedel–Crafts acylation reactions.
4

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2
2
. Experimental
.1. Chemicals
All the used chemicals in this work were of analytical grade and used as received
without further purification. Chromium (III) nitrate nonahydrate (Cr(NO ) ·9H O,
3
3
2
9
9%), hydrofluoric acid (HF, 48%), N-methyl-2-pyrrolidone (C H NO, 99.5%), 1-
5 9
pentanol (CH (CH ) OH, 99%), acetic acid, (CH CO H, 99%), anisole (CH OC H ,
3
2 4
3
2
3
6
5
9
9.7%), , Acetic anhydride (CH CO) O, 99.5%) and toluene (C H CH , 99.5%) were
3 2 6 5 3
taken up from Sigma–Aldrich Chemicals. Methane sulfonic acid (CH SO H, 99%),
3
3
ammonium fluoride (NH F, 98%), terephthalic acid (TPA, 98%), ethanol (CH CH OH,
4
3
2
9
2
2
9%), and ethyl acetate (CH COOC H , 99%) were purchased from ACROS Organics.
3 2 5
.2. Synthesis
.2.1. Synthesis of MIL-101(Cr)
Hydrothermal synthesis of MIL-101(Cr) from Cr(NO ) .9H O, TPA and deionized
3
3
2
water was in accordance with a described procedure [29]. Typically, a Cr(NO ) ·9H O
3
3
2
(
6.0 g), terephthalic acid (3.76 g) and HF (0.75 ml) were dissolved in 75 ml of H O in
2
Teflon beaker, then introduced into 150-ml Teflon-line autoclave. The mixture was then
heated in an oven at 220 °C for 8 h. By the end of the reaction, the temperature lowered
to the ambient temperature, and the developed green-colored powder was obtained by
filtration to remove the excess recrystallized terephthalic acid. The MIL-101(Cr) was
o
first activated by suspending into 300 ml water with stirring for 5 h at 70 C and then
o
separated using a fine pore fritted glass filter followed by drying at 150 C on standing
overnight to take out powdered MIL-101(Cr) [30]. The second activation for the same
sample was carried out by soaking the sample in 150 ml 30 mM NH F and stirred for 10
4
o
h at 60 C. Finally, the solid product was separated by filtration and washed with hot
water and then vacuum dried at 80 °C for 12h.
2
.2.2. Synthesis of N-methyl-2-pyrrolidonium methyl sulfate (IL)
The
Brønsted
acidic
IL
(N-methyl-2-pyrrolidonium
methyl
sulfonate,
+
(
[NMP] CH SO ]) was prepared (Scheme 1) as described elsewhere [31] with slight
3
3
modification. Typically, 9.9 g of N-methyl-2-pyrrolidone (0.1 mol) was stirred with (15
mL) ethyl acetate in a 50 mL round flask. After that, 9.6 g of methane sulphonic acid
(0.1 mol) in 15 mL ethyl acetate was added dropwise within 30 min in an ice bath. The
5

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reaction continued for extra 4 h at ambient temperature. Finally, ethyl acetate was taken
o
away under diminished pressure at 80 C till the initial volume added.
2
.2.3. Synthesis of IL/ MIL- 101(Cr) catalysts
The impregnation reaction encapsulation technique was used to encapsulate N-methyl-
-pyrrolidonium methyl sulfonate IL onto MIL-101(Cr) framework. In a typical
2
synthesis, 2.0 g of IL was diluted with 10 times volume of ethyl acetate was added to 1
g of MIL-101(Cr) suspended in 10 mL ethyl acetate under stirring for 4 h at
4
5°C. Then, the solvent was removed under vacuum at 80 °C and stirring of 150 rpm.
The obtained gel was extracted in soxhlet extractor using ethanol/toluene (1:1) as co-
solvent, and the weight of the obtained solid was determined to explore the maximum
capacity of MIL-101(Cr). To maximize the ratio of IL occluded in MIL-101(Cr) pores,
different loading of IL behind maximum capacity are prepared and donated as X%
MIL-101(Cr) where X (5, 10, 15, 20) indicates the percent ratio of the ionic liquid on
MIL-101(Cr). The elemental composition based C, N, H % only for MIL-101(Cr): C,
4
3.00; H, 2.26%, while after grafting the IL become C, 42.73; H, 2.46; N, 0.32% (5%
IL/MIL-101(Cr)), C, 42.44; H, 2.67; N, 0.65% (10% IL/MIL-101(Cr)), C, 42.17; H,
2
1
1
2
.87; N, 0.98% (15% IL/MIL-101(Cr)) and C, 41.96; H, 3.02; N, 1.23% (20% IL/MIL-
01(Cr)). Consequently, the real grafting percent after Soxhlet extraction is of 4.6, 9.2
3.7, and 17.2 % for 5, 10, 15 and 20% IL/MIL-101(Cr), respectively.
.3. Characterization
X-ray diffraction patterns were recorded by means of X'PERT X-ray diffractometer,
equipped with Cu-kα radiation (40 kV, 40 mA) (λ = 1.5406 Å). The diffractogram is in
o
o
the 2 range of 4 to 20 , with a scanning speed of 2 in 2/min. For the FT-IR spectra, a
KBr disk containing the sample was prepared and scanned from 4000 to 500 cm-1 using
(Shimadzu FT-IR, Japan). The nitrogen physisorption isotherms and the specific surface
area were measured at -196°C using the Quantachrome Autosorb iQ MP gas
sorption analyzer. Prior to the analysis, the samples were degassed at 150 oC for 12 h.
Transmission electron microscopy (TEM) images were taken using a JEOL JEM-2010
electron microscope operating at 200 KV. The surface electronic states of the prepared
catalysts were characterized by XPS (X-ray photoelectron spectroscopy) on ESCALAB
2
50 spectrometer. The acidity measurement of the samples was investigated using
potentiometric titration [32, 33].
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2
.4. Catalytic performance
.4.1. Esterification reaction:
2
The esterification reaction was carried out using a certain percent of catalyst (1, 3, 5 and
0%). Typical, a mixture of acetic acid and amyl alcohol (1:1) were stirred and heated
1
to achieve the reaction temperature (30, 60, 80 and 100 °C). The activated catalyst was
then added, and the mixture was stirred for 1 h. The reaction mixture was then collected
and analyzed using GC (Varian Technologies, model 3800 GC, capillary column DHA-
1
00).
2
.4.2. Friedel–Crafts acylation:
Friedel–Crafts acylation of anisole with acetic anhydride was carried out by mixing (20
mmol) anisole, (5 mmol) acetic anhydride followed by magnetically stirred and heated
o
to achieve the reaction temperature (80-120 C). Then activated catalyst (5 wt. %) was
added and the mixture stirred for 1h at the required temperature. After the reaction
completion, the catalyst was separated from the reaction temperature by centrifugation.
Finally, the reaction mixture was analyzed using GC–MS (HP GCD system equipped
with EID). The conversion was calculated depending on the gas chromatographic
analysis of the remaining acetic anhydride.
3
. Results and Discussion
3
.1. XRD analysis of IL/ MIL-101(Cr) catalyst
Powder X-Ray diffraction (PXRD) analysis was performed to investigate the structural
benignity of the parent MIL-101(Cr) after loading with IL species. As shown in Fig.1,
the PXRD pattern of MIL-101(Cr) sample perfectly agreed with that previously cited
PXRD pattern [11, 34], confirming the successful preparation of pure MIL-101(Cr). It is
also worth mentioning that the PXRD patterns of IL/MIL-101(Cr) are closely similar to
that of the parent MIL-101(Cr) although the intensity of the diffraction peak becomes
slightly broader and weaker, indicating the crystal structure of the MIL-101(Cr)
remained unchanged after the loading of the IL species.
3
.2. FT-IR and XPS characterization of IL/ MIL-101(Cr) catalyst
FT-IR studies were performed to check whether the MIL-101(Cr) structures were
unaltered after loading with IL species. FT-IR spectra of the pure MIL-101(Cr), pure IL
species and 20%IL/MIL-101(Cr) samples are presented in Fig. 2a. Inspection of such
7

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FTIR spectra revealed that the main spectral features of MIL-101(Cr) could be
obviously observed even the loading with 20% IL. The representative diagnostic bands
−
1
correspond to the framework -(O-C-O)- groups around 1550 and 1423 cm as reported
−1
by Ferey et al. [29] are observed. Moreover, bands at 1017 and 749 cm assigned to
δ(C-H) and γ(C-H) vibrations of the aromatic rings, respectively. The weak bands in the
−
1
region of 700–400 cm are assigned to both In-plane and out-of-plane bending modes
of the COO-groups [34, 35]. On the other hand, IL species shows five characteristic
-1
bands at 1658, 1200, 1000-750, 1056 and 780 cm which are due to C=O, S=O, S-O,
C-N, and C-H in-plane and out of plane stretching vibration, respectively, of IL species.
-1
-1
It is clear that the band at 1658 cm is shifted to lower wavelength by 25 cm ,
indicating an interaction between C=O of IL and CUS of Cr(III) of MIL-101(Cr).
These features of the IL confirm the loading and stability of the IL species within the
MIL-101(Cr) framework. In addition, the MIL-101(Cr) framework was unaltered by the
loading of IL, and further confirmation of this was obtained from (PXRD)
measurements.
Therefore it was significant to recognize which form of IL was obtained within MIL-
1
01(Cr) by XPS technique. The XPS spectrum of Cr2p for 20%IL/MIL-101(Cr)
material is shown Fig. 2b. Two peaks at 577.4 and 587.2 eV due to Cr2p1/2 and
Cr2p3/2 signals, however, the value of these signal appeared in high value [35],
3
+
suggesting the flow of electron density towards the Cr atom. The XPS spectrum of
C1 displayed three peaks at 284.6, 286.7 and 288.4 eV, corresponding to phenyl, C-N-
S
C, and C=O signals, respectively. Fig. 2c, the N1 peak of nitrogen atom showed two
S
peaks at 399.9 and 401.7 eV corresponding to protonated amine and amine enol form
+
(
C=N ), indicating interaction of IL through oxygen atom with CUS of MIL-101(Cr),
which is in good agreement with FTIR results.
.3 Surface area and porosity of IL/MIL-101(Cr) catalyst
3
The consequences of the sorption measurements of the MIL-101(Cr) and IL/MIL-
01(Cr) samples with different loading percentages on MIL-101(Cr) are depicted in
1
Fig.3. Literature values of Cr-MIL-101 compounds are given for comparison[36]. The
obtained SBET and pore size distribution values of the prepared Cr-MIL-101 fall within
the range of values of previously reported literature. The relatively low surface area may
8

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be due to the presence of small amount of impurities. MIL-101(Cr) exhibited Type I
isotherm with secondary uptake at P/P ∼ 0.1 and P/P ∼ 0.2, indicating the presence of
0
0
microporosity according to IUPAC classification of adsorption isotherms [37]. In all
isotherms, two-characteristic curve stage is observed, resulting from the N -filling of
2
two different size cages [38]. The major adsorption of N in the adsorption isotherms
2
occurred at low relative pressure (i.e., P/P ≈ 0.08). It was reported that the MIL-
0
1
01(Cr) had two types of microporous cages; the smaller one possessed pentagonal
windows with a diameter of 1.2 nm while the larger cage contained pentagonal–
hexagonal windows with a diameter in the range of 1.45–1.60 nm. Hence, a secondary
adsorption step in the range of 0.1 < P/P < 0.2 arose by these two types of microporous
0
windows [29]. The IL/MIL-101(Cr) also showed this secondary adsorption near P/P ≈
0
0
.12. The adsorption of nitrogen in the low-pressure regime by IL/MIL-101(Cr) was
more than that of the virgin one. This suggested that the embedding of the IL species
within the MIL-101(Cr) pores up to 20 wt % provided additional micropores in its
framework. Moreover, the IL/MIL-101(Cr) samples with different IL contents showed
the same isotherm shape further confirming that the original pores structure of MIL-
1
01(Cr) was very well unchanged after IL loading. The summarized textural parameters
of the synthesized materials including the virgin MIL-101(Cr) are cited in Table 1. The
surface area and total pore volume decreased with increasing loading percentage of the
IL species, which may indicate the partial pore clogging of the porous structure of MIL-
1
01(Cr) framework by the ionic liquid species.
3
.4. TEM characterization of IL/MIL-101(Cr) catalyst
Fig. 4 displays typical TEM images of pure MIL-101(Cr) and IL/MIL-101. Based on
the TEM observation, the pure MIL-101(Cr) crystal exhibits cubic shape. As also
evident from Fig.4, the crystal structure of the support material remained unaltered after
loading of the IL species into the surface, which is consistent with PXRD patterns
(Fig.1).
3
.5 Surface acidity measurements
Potentiometric titration investigated the surface acidity of the bare MIL-101(Cr) as well
as various loaded IL/MIL-101(Cr) samples with n-butylamine. The relative strength and
the total number of acid sites present in the solids can be estimated adopting this
9

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technique. The value of meq amine/g solid, where the plateau is reached, represents the
total number of acid centers. While the initial electrode potential (E ) demonstrates the
i
maximum acid sites strength. Accordingly, the acid strength of these sites may be
ranked according to the following scale: E > 100 mV (very strong sites), 0 < E < 100
i
i
mV (strong sites), −100 < E < 0 mV (weak sites) and E < −100 mV (very weak sites)
i
i
[
39, 40]. Fig.5. depicts the titration curves of the prepared catalysts, where the total
number of acid sites/g catalyst and the initial electrode potential (E ) was summarized in
i
Table 1. It was observed, the pure MIL-101(Cr) presented strong acid sites, Ei = 27
mV, while loading of IL species has a significant increase in both surface acidity and
acid strength. With further increase of IL contents up to 20 wt% IL, an increase in
surface acidity and acid strength (Ei = 469 mV) was observed. The observed increase in
the acidity as due to the increase in IL species content from 5 to 20% is quite expected,
simply, because IL did presumably act as the acid site constituents of the examined
systems. Thus, such increase in the IL content has actually reflected the increase in the
concentration of the acidic centers.
3
.6 Catalytic activity of IL/MIL-101(Cr) in esterification reaction
In several South American countries, biofuel ethanol is established from sugar cane
fermentation [41]. There are considerable by-products generated from this process,
among them the so-called fusel oil. The fusel oil is a complex mixture consists mainly
of isoamyl alcohol (>60 wt./wt.%) and water and has received increasing awareness for
its conversion to more valuable products [42, 43]. The amyl acetate is widely used in
the food, pharmaceutical, perfumery industry and as a green solvent. The catalytic
performance was investigated towards the formation of amyl acetate from acetic acid
and amyl alcohol using IL/MIL-101(Cr) as the catalyst in a batch-type process, which
proceeds as described in the following equation:
1
0

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Influence of various operating condition such as IL content, reaction time, catalyst
amount and reaction temperature were studied for optimization the reaction conditions
by the utilization 20% IL/MIL-101(Cr) as a model catalyst.
3
.6.1 Effect of the Ionic Liquid (IL) content
The change in the yield percent of amyl acetate as a function of IL content was
graphically represented in Fig.6a. The reaction was carried out over 1% catalyst amount
o
at 100 C using acetic acid to amyl alcohol 1:1. A control experiment was performed to
earn more insight about the esterification mechanism. The results of the yield percent of
the esterification reaction between acetic acid and amyl alcohol in the presence of pure
MIL-101(Cr) showed almost no activity in the esterification, which is likely owing to
the absence of Brönsted acid sites. While MIL-101(Cr) loaded with different amounts of
IL species showed significantly increased in the yield percentage of amyl acetate, attain
the maximum activity of 75% yield for 20% IL/MIL-101(Cr). Recently, A.E.R.S.Khder
et al., [11] investigated the esterification reaction between acetic acid and n-butanol
o
over 0.05 g HPW/MIL-101(Cr) catalyst at 120 C for 1h. The results revealed that the
sample 70 wt% HPW/MIL-101(Cr) exhibited 92.3% conversion with 100% selectivity.
Also, Juan-Alcaniz et al., [41] showed that the esterification over HPW loaded MIL-101
o
at 110 C exhibited 60% conversion for 5h. Moreover, sulfonic acid functionalized
MIL-101 showed an excellent yield of butyl acetate (99.5%) after 2 h [44]. These results
demonstrate that our catalysts possess incentive catalytic performance under the
reaction conditions in comparison to the others. The reaction in amyl alcohol
esterification proceeded more efficient, and this may be attributed to the more alteration
of pores hydrophobicity caused by the N-methyl-2-pyrrolidonium methyl sulfonate that
enhances the diffusibility of amyl alcohol within the MIL-101(Cr) pores to reach the
catalytic acid centers within pores, and revealed that the catalytic conversion occurred
inside the MIL-101(Cr) pores.
3
.6.2 Effect of the reaction time
The influence of the reaction time on the amyl acetate yield percentage was studied over
o
1
% of 20% IL/MIL-101(Cr) at 100 C using acetic acid to amyl alcohol 1:1. The results
obtained are shown in Fig.6b. A gradual increase in the yield percentage of amyl acetate
with increase the reaction time can be observed. The yield percentage of amyl acetate
1
1

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was observed as (66, 71, 72, 75.05 and 75%) after (1, 2, 3, 4 and 5h) respectively.
Therefore, no clear effect on the yield percentage of amyl acetate was observed on
further increase in the reaction time more than 4 h. This suggests that 4 h is the
optimum reaction time to be sufficient to attain the equilibrium.
3
.6.3 Effect of the catalyst amount
In order to investigate the effect of catalyst amount on the amyl acetate yield, a set of
esterification experiments with various weight percent (1, 3, 5 and 10% based on the
weight of both acetic acid and amyl alcohol) of 20% IL/MIL-101(Cr) catalyst were
o
conducted at 80 C for 4h using acetic acid to amyl alcohol 1:1. The yield percentage of
amyl acetate as a function of catalyst amount (%) are shown in Fig.6c. on the basis of
the results pertaining to the assessment of the relative contribution of catalyst amount on
the catalytic performance, one is inclined to forward that, the yield percentage increased
with an increase in the catalyst amount up to 5%. Such increase would indicate that the
number of Brønsted acid sites was increased. An additional increase in the catalyst
amount up to 10% no significant changes in the yield percent were observed. Therefore,
5
% is the optimum IL/MIL-101(Cr) amount for the esterification of acetic acid to amyl
acetate under the current reaction conditions.
3
.6.4 Effect of reaction temperature
The influence of reaction temperature on the yield percent of amyl acetate was studied
o
by varying the reaction temperature between 30-100 C as shown in Fig.6d, using 1%
catalyst (20% IL/MIL-101(Cr)) conducted for 4h using acetic acid to amyl alcohol 1:1.
Inspection of Fig.6d showed that, increase the esterification temperature from 30 to 80
o
C resulted in a significant increase in the yield percentage of amyl acetate from 50 to
7
2%. However, the esterification reaction remains almost unaltered with further
o
increase the temperature up to 100 C. At higher temperature, the viscosities of the
reaction mixture was decreased result in an increase in the reaction rate as more energy
is being supplied for the reaction to occur. Thus, the yield of the ester product is
o
enhanced. Additionally, the boiling point of N-amyl alcohol is 137 to 139 C.
Temperature higher than this will burn the alcohol and will result in much lower yield
o
[
43]. Consequently, we have considered 80 C as the proper reaction temperature for
amyl acetate formation. Table 2, presents a brief comparison of some catalytic activity
1
2

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data collected from different published papers [11, 31, 41, 45, 46] of various catalysts
for esterification reaction using different catalysts taking into consideration the reaction
conditions. Summing up, the reported results within the frame of present work do reflect
that porous MOFs, supported with ionic liquids as (IL/MIL-101) offers a new, exciting
opportunities for the heterogeneous esterification reaction.
3
.6.5 Reaction mechanism
Although a detailed reaction mechanism of homogeneously catalyzed liquid phase
esterification is well known [47], there still no focusing on the heterogeneously
catalyzed gas phase reaction. Chu et al. [48] studied the esterification reaction of n-
butanol with acetic acid by dodecatungstosilicic acid immobilized on activated carbon
as acidic catalyst. Accordingly, the results showed that the esterification mechanism
takes place through the protonation of the alcohol as an intermediate. However, most of
the articles are focusing on the protonation of the carboxylic acid as the reaction
intermediate. Therefore, based on those reports, the esterification mechanism proceed
via the protonation of acetic acid by a proton donated from Brønsted acid sites,
IL/MIL-101(Cr) surface, forming protonated carboxylic acid intermediate, as shown in
Schemes 2. This protonated intermediate then interacts with amyl alcohol generating
the corresponding ester, amyl acetate, and water.
3
.6.6 Catalyst recycling and hot filtration test
The most distinct features of heterogeneous catalysts over homogeneous catalysts is the
ability to separating the catalyst from the reaction mixture and reuse it for subsequent
reactions until the catalyst is sufficiently deactivated. The stability of the catalyst, 20%
o
IL/MIL-101 was conducted at 80 C utilizing 5% of catalyst amount and reactant molar
ratio 1:1. After each esterification run, the reaction products were separated followed by
detection of the yield % of amyl acetate by means of GC. The catalyst was dried and
directly transferred to the fresh reaction mixture for the next runs. The results revealed
that 20% IL/MIL-101 catalyst exhibited outstanding reusability with slight activity loss
as compared with the first run (Fig.7a). The slight decrease in the catalytic activity may
due to the minor catalyst loss during the separation process along with the possibility of
+
3-
partial deactivation of ionic liquid species ([NMP] CH3SO ) [49]. We examined the
leaching behavior of the used catalyst after six repetitive catalytic runs by TEM as
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3

ACCEPTED MANUSCRIPT
shown in Fig.7b. As demonstrated in Fig.7b, the IL leaching little impact on the
reduction of catalyst activity during the recycling process. Other fascinating features of
the original and recovered IL/MIL-101 catalysts include the stability of the catalyst
since identical XRD patterns before and after catalysis (not shown). These results
confirm that the crystal structure of MIL-101 did not change after the ionic liquid
loading as the diffraction peaks of the MIL-101 are retained for all the IL-loaded
samples. Therefore, this catalyst exhibited significant performance for esterification
reaction, and this gives the catalyst promising in industrial scale.
In order to emphasize the heterogeneous nature of esterification reaction using IL/MIL-
1
01(Cr) as a catalyst and that the measured overall activity was not owing to leached IL
species, the hot catalyst filtration test was carried out (Fig.7c). An experiment under the
optimum condition was performed then, the reaction was stopped after 60 min of
reaction, and the catalyst was removed from the reaction mixture by filtration, and the
filtrate was allowed to continue the reaction under the same condition. The results
revealed that no significant increase in the yield % after removal of 20% IL/MIL-
1
01(Cr) catalyst, definitely indicating that the active sites were not leached out from
MIL-101(Cr) into the solution and the observed catalysis is truly heterogeneous on the
catalyst surface.
3
.7 Catalytic activity of IL/MIL-101(Cr) in Friedel–Crafts acylation
3
.7.1 Effect of the Ionic Liquid (IL) content
The change in the conversion % of acetic anhydride as a function of IL content was
o
observed in Fig.8a. The reaction was performed over 5% catalyst amount at 100 C
using anisole: acetic anhydride 4:1 molar ratio for 1 h [50]. It could be observed that
pure MIL-101 did not show up any activity for the acylation of anisole with acetic
anhydride. However, the loading of MIL-101 with IL results in an increase in the
conversion % as a function of IL content passing a maximum at IL content of 20 Wt %.
The loading MIL-101(Cr) with 5 % IL brought about increase in % conversion of acetic
anhydride to 45%. While an increase of percentage conversion of about 79% was
observed upon increasing the IL content up maximum limit 20 % wt.
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3
.7.2 Effect of reaction temperature
The influence of reaction temperature on the conversion % of acetic anhydride was
o
studied by varying the reaction temperature from 30 to 120 C as shown in Fig.8b,
using 5% catalyst (20% IL/MIL-101(Cr)) conducted for 1h using anisole: acetic
anhydride 4:1 molar ratio. Fig.8b demonstrates that the reaction temperature had a
significant influence on the acylation of anisole. The conversion % remarkably
o
increased with increasing the reaction temperature within the range 30-100 C.
o
However, a further increase in the reaction temperature up to 120 C led to a marginal
o
increase in the conversion %. Consequently, we have considered 100 C as the optimum
reaction temperature for Friedel–Crafts acylation of anisole. Table 3, represents a
comparison of some catalytic activity data collected from different articles [50-54] of
various catalysts for Friedel–Crafts acylation with this work taking into consideration
the reaction conditions. Summing up, the reported results within the frame of present
work do reflect that porous MOFs, supported with ionic liquids as (IL/MIL-101(Cr))
offers a new, exciting opportunities for heterogeneous Friedel–Crafts acylation.
3
.7.3 Effect of different substrate on Friedel–Crafts acylation
Fig.8c, displays the impact of the different substrate contain electron-donating group as
anisole and the electron-withdrawing group as benzaldehyde on the Friedel–Crafts
acylation. It was observed that anisole acylation with acetic anhydride was performed
more rapidly than benzaldehyde. These could be attributed to the resonance effect of the
methoxy group far outweighs its polar effect. Hence, the carbocation intermediate (and
the transition state) derived from the electrophilic substitution of anisole is more stable
relative to starting materials than the carbocation (and transition state) derived from the
electrophilic substitution of benzaldehyde. In other words, the methoxy group activates
the benzene ring toward ortho and para substitution.
4
. Conclusion
Economically acidic IL-supported on MIL-101(Cr) catalyst was successively
synthesized by simple impregnation method. IL/MIL-101(Cr) exhibited distinct
catalytic performance and reusability with respect to the other catalysts towards the
esterification of acetic acid with amyl alcohol and Friedel–Crafts acylation of anisole
this give the catalyst promising in industrial sectors. Additionally, from this work, it can
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be suggested that the metal–organic frameworks (MOFs) as new categories of porous
materials supported with the ionic liquid, open a new avenue for designing and
developing the next generation acidic catalyst and offers a new, exciting opportunities
for heterogeneous acid-catalyzed reaction.
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Scheme 1. Schematic representation of synthesis of IL/MIL-101(Cr) catalyst
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3

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20% IL/MIL-101
15% IL/MIL-101
10% IL/MIL-101
5% IL/MIL-101
MIL-101
4
6
8
10
12
14
16
18
20


Fig. 1. XRD patterns of bare MIL-101(Cr) and IL/MIL-101(Cr) catalysts with
-20 wt. % IL species
5
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4

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2
0% IL/ MIL-101(Cr)
MIL-101(Cr)
IL
2000
1800
1600
1400
1200
1000
800
600
400
-1
wavenumber cm
Fig.2(a) FTIR spectra of bare MIL-101(Cr), IL species and 20% IL/ MIL-
01(Cr)
1
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N1
3
97
398
399
400
401
402
403
C1S
2
82
284
286
288
290
292
294
Cr2p1/2
Cr2p3/2
5
70
575
580
Binding energy (eV)
585
590
Fig.2(b) XPS spectra of 20% IL/MIL 101(Cr).
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6

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1000
MIL-101
5
1
1
2
% IL/MIL-101
0% IL/MIL-101
5% IL/MIL-101
0% IL/@MIL-101
800
600
400
200
0
0.0
0.2
0.4
0.6
0.8
1.0
o
P/P
o
Fig.3. Nitrogen adsorption-desorption isotherms at -196 C for bare MIL-101(Cr)
and IL/MIL-101(Cr) catalysts with 5-20 wt.% IL species (full ads., empty des.)
(A)
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Fig.4. TEM images of (a) pure MIL-101(Cr) (b) 10% IL/MIL-101(Cr)
(
c) 20% IL/MIL-101(Cr)
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500
450
400
350
300
250
200
150
100
MIL-101(Cr)
5% IL/MIL-101(Cr)
10% IL/MIL-101(Cr)
15% IL/MIL-101(Cr)
20% IL/MIL-101(Cr)
50
0
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
V (ml) n-butylamine added
Fig. 5. Potentiometric titration curves of n-butyl amine in acetonitrile for bare MIL-
01(Cr) and IL/ MIL-101(Cr) catalysts with 5-20 wt. % IL species
1
80
75
70
65
(b)
(
a)
2
9