POM@IL-MOFs-inclusion of POMs in ionic liquid modified MOFs to produce recyclable oxidation catalysts

Article abstract of DOI:10.1039/c6cy02662a

In this study, we present for the first time the immobilization of phosphotungstic acid (H₃PW₁₂O₄₀, HPW) into the nanocages of a dual amino-functionalized ionic liquid (DAIL)-modified MIL-101(Cr) framework under mild condi

Full text of DOI:10.1039/c6cy02662a

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POM@IL-MOFs – inclusion of POMs in ionic liquid
modified MOFs to produce recyclable oxidation
catalysts†
Cite this: DOI: 10.1039/c6cy02662a
Sara Abednatanzi,ab Karen Leus,a Parviz Gohari Derakhshandeh,bc Fady Nahra,c
Katrien De Keukeleere,c Kristof Van Hecke,c Isabel Van Driessche,c Alireza Abbasi,b
a
Steven P. Nolancd and Pascal Van Der Voort
*
In this study, we present for the first time the immobilization of phosphotungstic acid (H3PW12O40, HPW)
into the nanocages of a dual amino-functionalized ionic liquid (DAIL)-modified MIL-101IJCr) framework un-
der mild conditions. The obtained PW/DAIL/MIL-101IJCr) was evaluated as a catalyst in the oxidation of ben-
zyl alcohol using TBHP in chloroform. In order to obtain insights into the role of the DAIL during catalysis,
a DAIL-free catalyst (HPW/MIL-101IJCr)) was also prepared. Both catalysts were fully characterized using
several techniques. In comparison with the HPW/MIL-101IJCr) material, PW/DAIL/MIL-101IJCr) showed an
enhanced catalytic performance in the selective oxidation of benzyl alcohol. This was mainly attributed to
the ability of the DAIL groups to form hydrogen bonds, thus enhancing the accessibility of TBHP. Further-
more, the immobilization of the DAIL groups onto MIL-101IJCr) resulted in increased thermal stability of the
obtained PW/DAIL/MIL-101IJCr) which showed stability up to 400 °C. Moreover, the PW/DAIL/MIL-101IJCr)
catalyst exhibited good recyclability and selectivity. The catalyst could be reused for at least five cycles with
no significant leaching of the tungsten species or loss of crystallinity and activity.
Received 22nd December 2016,
Accepted 6th February 2017
DOI: 10.1039/c6cy02662a
rsc.li/catalysis
actions, owing to their redox and acid properties.7–10 More-
over, with this type of chemistry, incorporation of organic
1 Introduction
Selective oxidation of primary alcohols to aldehydes is one of
the most sought-after transformations since aldehydes are
highly important targets in both academia and industry.1–4
From an industrial point of view, one of the significant chal-
lenges in such oxidation is to avoid the over-oxidation of alde-
hydes into acids, as selectivity targeting the sole production
of aldehydes would impact processes where aldehydes act as
intermediates in the synthesis of pharmaceutical and agro-
chemical compounds.5,6
units into POM anions has received considerable interest as
an efficient approach to develop POM-derived materials.11
Within this context, functionalized organic ionic liquid (IL)
cations have been demonstrated to be among the most effec-
tive modifiers for POMs by increasing the catalytic activity of
the latter in selective oxidation reactions.12–15
Despite these interesting features of POM-based ILs and
recent advances in the design and synthesis of these highly
active catalysts, clean separation and recycling still pose a
considerable challenge.16–18 Therefore, for practical applica-
tions, it is of paramount importance to immobilize the cata-
lytically active POM-based ILs onto appropriate solid supports
which combine the attractive features of POM-based ILs with
the benefits of heterogeneous catalysis.19
Metal–organic frameworks (MOFs) constitute an emerging
class of porous materials due to their thermal and chemical
stabilities and large surface areas; they possess regular and
accessible pores that make them outstanding candidates to
act as a support for various active centres.20–22 Many attempts
have been made to insert catalytically active molecules into
the nanocages of MOFs via a post-synthetic strategy.23–27
Within this context, the mesoporous chromium terephthal-
ate, denoted as MIL-101IJCr), has been the most examined
host material for the immobilization of catalytic species.28–32
Polyoxometalates (POMs), a class of structurally diverse
anionic transition metal oxide clusters, have already been
shown to be promising catalysts for a number of oxidation re-
a Department of Inorganic and Physical Chemistry, Center for Ordered Materials,
Organometallics and Catalysis (COMOC), Ghent University, Krijgslaan 281-S3,
9000 Ghent, Belgium. E-mail: Pascal.VanDerVoort@UGent.be
b School of Chemistry, College of Science, University of Tehran, P.O. Box 14155-
6455, Tehran, Iran
c Department of Inorganic and Physical Chemistry, Ghent University, Krijgslaan
281-S3, 9000 Ghent, Belgium
d Chemistry Department, College of Science, King Saud University, PO Box 2455,
Riyadh, 11451, Saudi Arabia
† Electronic supplementary information (ESI) available: Characterization of
HPW/MIL-101IJCr), textural properties and elemental analysis of PW/DAIL/MIL-
101IJCr) materials, characterization of the recycled PW/DAIL/MIL-101IJCr). See
DOI: 10.1039/c6cy02662a
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The large surface area with accessible mesoporous cages and
high thermal and chemical stabilities make this MOF an at-
tractive solid support.33,34
The surface area and pore volume of the samples were de-
termined by nitrogen adsorption/desorption measurements
at liquid nitrogen temperature using a Belsorp-mini II gas an-
alyzer. Before the adsorption experiments, the samples were
degassed under vacuum at 90 °C for 24 h. Thermogravimetric
analysis (TGA) was performed using a NET-ZSCH STA 409 PC/
PGTG instrument. The samples were heated from 30 to 800
°C in air at a constant rate of 10 °C min−1. An ultra-fast GC
equipped with a flame ionization detector (FID) and a 5%
diphenyl/95% polydimethylsiloxane column with a 10 m
length and a 0.10 mm internal diameter was used to monitor
the conversion of the products during the catalytic tests.
Helium was used as the carrier gas and the flow rate was
programmed to be 0.8 mL min−1. The reaction products were
identified using a TRACE GC × GC (Thermo, Interscience)
coupled to a TEMPUS TOFMS detector (Thermo, Inter-
science). The first column consists of a dimethyl polysiloxane
package and has a length of 50 m with an internal diameter
of 0.25 mm, whereas the second column has a length of 2 m
with an internal diameter of 0.15 mm. The package of the lat-
ter is 50% phenyl polysilphenylene-siloxane. Helium was
used as the carrier gas with a constant flow (1.8 mL min−1).
1H NMR spectra are recorded in D2O on a Bruker 300
MHz AVANCE spectrometer with chemical shifts (δ) expressed
in ppm relative to a tetramethylsilane standard. Elemental
analyses were performed using an ICP-OES Optima 8000 (in-
ductively coupled plasma optical emission spectroscopy)
atomic emission spectrometer. X-ray fluorescence (XRF) mea-
surements were carried out by means of a NEX CG from
Rigaku using a Mo X-ray source. The nitrogen content of the
modified materials was determined using a Thermo Flash
200 elemental analyzer with V2O5 as a catalyst. Transmission
electron microscopy (TEM) and high angular annular dark
field scanning transmission electron microscopy (HAADF
STEM) images were obtained using a JEOL JEM-2200FS trans-
mission electron microscope with a post-sample Cs corrector
and an accelerating voltage of 200 kV. For the sample prepa-
ration, the powder was dispersed in methanol and treated ul-
trasonically for 2 minutes. A copper support TEM grid (200
mesh) is dipped in the dispersion and air-dried before
measurement.
Although there are some reports about the preparation of
POM-based IL hybrid materials using mesoporous silica and
magnetic nanoparticles,35–40 to the best of our knowledge,
there is only one report on the utilization of ILs using MOFs
as a solid support for the immobilization of POMs.41 In the
latter case, a heteropolyanion-based IL was encapsulated
within the framework of MIL-100. The prepared catalyst
exhibited an enhanced catalytic performance in the esterifica-
tion of oleic acid in comparison with the encapsulated
heteropolyanion within the framework. The presence of a sul-
fonic acid group-functionalized IL led to an increased num-
ber of acid sites, which resulted in a higher catalytic activity
of the prepared material.41
It is interesting to note that immobilization of POMs onto
MOFs using a post-synthetic strategy is challenging, as most
POM-loaded MOFs reported so far lack sufficient thermal
and chemical stabilities to prevent leaching of the active
components.30 As a consequence, there is a strong need to
develop new POM–MOF stable catalysts. For the first time, we
report on the utilization of
a supported dual amino-
functionalized ionic liquid (DAIL) in designing a new and ef-
ficient POM-based oxidation catalyst using MIL-101IJCr) as a
support. For this purpose, MIL-101IJCr) was synthesized in wa-
ter under HF-free conditions. The coordinatively unsaturated
chromium sites (CUS) present in the MIL-101IJCr) structure
are used as anchoring points for the immobilization of the IL
through a post-synthetic strategy. In addition, the supported
dual amino-functionalized ionic liquid was used for the im-
mobilization of the Keggin-type H3PW12O40 (HPW) inside the
nanocages of MIL-101IJCr) through anion exchange. The HPW
can easily diffuse into the pores of MIL-101IJCr) and react with
the DAIL groups to form a POM-based IL. The remaining free
amino groups in the DAIL play an important role in enhanc-
ing the accessibility of TBHP through the formation of hydro-
gen bonds which results in an increased catalytic perfor-
mance. The catalytic activity of the prepared PW/DAIL/MIL-
101IJCr) material was examined in the selective oxidation of
benzyl alcohol.
2.2. Catalyst preparation
2 Experimental
2.2.1. Synthesis of the dual amino-functionalized ionic
liquid 1,3-diIJ2′-aminoethyl)-2-methylimidazolium bromide
(DAIL). The dual amino-functionalized ionic liquid was syn-
thesized according to a previously reported procedure42 and
is depicted in Scheme 1. The successful synthesis of the ma-
2.1. Catalyst characterization
All reagents were purchased from Sigma-Aldrich or TCI Eu-
rope and used without further purification.
Diffuse reflectance infrared Fourier transform spectro-
scopy (DRIFTS) measurements were obtained using a Thermo
Nicolet 6700 FT-IR spectrometer equipped with a nitrogen-
cooled MCT-A (mercury–cadmium–tellurium) detector and a
KBr beam splitter. Powder X-ray diffraction (PXRD) patterns
were recorded on a Thermo Scientific ARL X’Tra diffractome-
ter, operated at 40 kV and 40 mA using Cu-Kα radiation (λ =
1.5406 Å).
1
terial was confirmed by H NMR spectroscopy.
2.2.2. Preparation of the supported DAIL (DAIL/MIL-
101IJCr)). The MIL-101IJCr) sample was prepared under HF-
free conditions according to a slightly modified procedure de-
scribed by Edler et al.43 Typically, 0.6645 g of terephthalic
acid was mixed with 1.6084 g of CrIJNO3)3·9H2O and 20 mL of
distilled water. The mixture was transferred into a Teflon-
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3 Results and discussion
3.1. Preparation and structural information of immobilized
PW onto the supported DAIL (PW/DAIL/MIL-101IJCr))
MIL-101IJCr), a three-dimensional chromiumIJIII) terephthalate
MOF, possesses regular mesoporous cages of 29 and 34 Å,
which are accessible through windows of ca. 12 and 16 Å.33
Interestingly, the existence of coordinatively unsaturated
chromium sites (CUSs) in the framework provides opportuni-
ties for the incorporation of active species. In this regard, the
treatment of the activated MIL-101IJCr) with the prepared
DAIL afforded DAIL/MIL-101IJCr), as there is strong coordina-
tion between the metal centres and the amino groups of the
supported DAIL. In the following, the immobilization of PW
onto the supported DAIL was performed through anion ex-
change of the corresponding cluster and the DAIL. The PW
cluster, with dimensions of about 12 Å, has the appropriate
size to diffuse through the larger windows of the framework.
A schematic illustration for the preparation of the catalyst is
outlined in Scheme 2.
Scheme 1 Synthetic route to DAIL. (a) Et3N, CH2Cl2, room temp., 24 h;
(b) 2-methylimidazole and NaH (60%), DMF, reflux,
8 h; (c) HCl,
CH3OH, room temp., 24 h; (d) NaOH, H2O, room temp., 3 h.
lined autoclave, sealed, and then heated for 2 h to 210 °C at
which it was held for 8 h. After cooling to room temperature,
the MIL-101IJCr) was filtered, washed with acetone, and
stirred in dimethylformamide for 24 h to remove any
unreacted terephthalic acid. Thereafter, the MIL-101IJCr) was
filtered and dried under vacuum at 90 °C overnight. The
dehydrated form of MIL-101IJCr) was obtained by heating the
pristine material at 150 °C under vacuum for 24 h to generate
open metal centers. The immobilization of the DAIL was
achieved by the addition of MIL-101IJCr) (150 mg) to a solu-
tion of DAIL (0.3, 0.6, 0.9, 1.5 mmol g−1) in 2.5 mL ethanol.
The mixture was stirred at room temperature for 24 h. The
resulting solid was collected by filtration and washed several
times with ethanol. Then, the resulting solid was dried under
vacuum overnight.
2.2.3. Preparation of immobilized PW onto the supported
DAIL (PW/DAIL/MIL-101IJCr)). The PW/DAIL/MIL-101IJCr) sam-
ple was prepared by adding DAIL/MIL-101IJCr) (150 mg) to a
solution of HPW in acetonitrile (0.23 mmol g−1, 2.5 ml). The
mixture was stirred at ambient temperature for 24 h. After-
wards, the catalyst was collected by filtration, washed several
times with acetonitrile (3 × 20 ml) and dried under vacuum
for 24 h.
3.2. Characterization of immobilized PW onto the supported
DAIL (PW/DAIL/MIL-101IJCr))
3.2.1. X-ray diffraction, nitrogen adsorption and
thermogravimetric analysis. In Fig. 1, the PXRD patterns of
MIL-101IJCr) and the post-modified materials are presented.
The pattern of the pristine MIL-101IJCr) corresponds well with
the simulated one, indicating the phase purity of MIL-101IJCr)
under the applied synthesis conditions. However, the ob-
served broadening in the PXRD patterns of the as-synthesized
MIL-101IJCr) (MIL-101IJCr) before modification) is due to the
fact that the resulting product consisted of crystals having
various sizes, as supported by TEM images. Moreover, the
PXRD patterns revealed a shift for all peaks to higher 2θ an-
gles. This behavior might be due to the shrinkage of the
framework depending on the removal of guest molecules
2.2.4. Preparation of immobilized HPW onto MIL-101IJCr)
(HPW/MIL-101IJCr)). MIL-101IJCr) (150 mg) was suspended in
a solution of HPW in acetonitrile (0.227 mmol g−1, 2.5 ml)
and the mixture was stirred at ambient temperature for 24 h.
The solid was collected by filtration and washed several times
with acetonitrile (3 × 20 ml).
2.3. General procedure for the oxidation of benzyl alcohol
During a typical catalytic test, PW/DAIL/MIL-101IJCr) (6 μmol
based on W), tert-butyl hydroperoxide (TBHP, 5-6 M in dec-
ane, 4.5 mmol), dodecane as an internal standard (0.5
mmol), benzyl alcohol (1 mmol) and chloroform (1 ml) were
added into a Schlenk tube. The tube was sealed and then
heated to 100 °C for 6 h. The PW/DAIL/MIL-101IJCr) material
was activated under vacuum at 90 °C prior to catalysis. After
each catalytic run, the catalyst was recovered by filtration,
washed with chloroform and used directly in the subsequent
runs. Samples were withdrawn after 6 h reaction, and after
cooling and dilution with the solvent, were analyzed using an
ultra-fast gas chromatograph (Thermo, Interscience)
equipped with a flame ionization detector (FID).
Scheme 2 Schematic illustration for the preparation of PW/DAIL/MIL-
101IJCr).
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Fig. 2 Nitrogen sorption isotherms of MIL-101IJCr) and the modified
materials.
and pore volume (2704 m2 g−1 and 1.35 cm3 g−1, respectively)
compared to the DAIL/MIL-101IJCr) sample, which is a clear
indication of the successful immobilization of PW inside the
framework. The textural properties and elemental analysis of
all the samples are listed in Table S1.†
Fig. 1 XRD patterns of MIL-101IJCr) and the modified materials.
Thermal analysis was performed to monitor the decompo-
sition profiles of the pristine MIL-101IJCr), DAIL/MIL-101IJCr)
and PW/DAIL/MIL-101IJCr) catalyst. The obtained results are
depicted in Fig. 3. The presence of the DAIL and PW mole-
cules in MIL-101IJCr) was also confirmed by means of TGA. In
the TGA profile of the pristine MIL-101IJCr), three distinct
weight loss steps can be observed: the first weight loss (be-
tween 25–150 °C) corresponds to the loss of guest water and
organic solvent molecules. With the further increase in tem-
perature, the removal of chemically bonded water occurs. In
the third step (between 325–470 °C), the weight loss of ap-
proximately 57% is attributed to the elimination of OH
groups and the framework decomposition. The TGA curves
for the DAIL/MIL-101IJCr) and PW/DAIL/MIL-101IJCr) materials
are similar to that of the pristine MIL-101IJCr), albeit the ini-
tial weight loss up to 150 °C is less pronounced since the
inside the pores after drying under vacuum at 90 °C. A simi-
lar broadening to the parent MIL-101(Cr) was observed for
the isoreticular NH2-MIL-101(Cr).28 The PXRD pattern of the
functionalized MIL-101IJCr) with the DAIL groups is in good
accordance with that of the parent MIL-101IJCr), which im-
plies that the crystal structure remained intact after the intro-
duction of the DAIL groups. In the PXRD pattern of the PW/
DAIL/MIL-101IJCr) catalyst, one can see that all the main dif-
fraction peaks are present, however, some variations in the
relative intensity of the diffraction peaks are observed. Simi-
lar observations were reported previously for the modified
POM-MILs due to the interaction of the clusters with the
MIL-101IJCr) or changes in the symmetry of the clusters
within the nanocages of the framework.44
Nitrogen
adsorption/desorption
experiments
were
performed to evaluate the porosity of the pristine and modi-
fied samples. Fig. 2 shows the N2 adsorption/desorption iso-
therms of MIL-101IJCr) and the modified materials. The
pristine MIL-101IJCr) exhibits secondary uptakes at about p/p0
= 0.1 and p/p0 = 0.2, which indicates the presence of two
nanoporous windows.45 The Langmuir surface area and total
pore volume for the prepared MIL-101IJCr) are 3735 m2 g−1
and 1.48 cm3 g−1, respectively. Compared with the pristine
MIL-101IJCr), the DAIL-modified sample shows a decrease in
both the Langmuir surface area and pore volume (3049
m2 g−1 and 1.41 cm3 g−1, respectively) owing to the insertion
of the DAIL groups into the nanocages of the framework.
However, the modified sample still possesses a large surface
area to provide sufficient space for the introduction of PW
POM. As expected, the PW/DAILIJ0.3)/MIL-101IJCr) material
shows an additional decrease in the Langmuir surface area
Fig. 3 TGA curves of MIL-101IJCr) and the modified materials.
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cages of MIL-101IJCr) were occupied by the DAIL and PW. The
TGA curve of DAIL/MIL-101IJCr) indicates a weight loss at
250–380 °C, which is probably due to the decomposition of
the DAIL inside the pores. In contrast to the latter material,
PW/DAIL/MIL-101IJCr) shows good thermal stability at these
temperatures.
The enhanced thermal stability of PW/DAIL/MIL-101IJCr)
compared to DAIL/MIL-101IJCr) may be due to the additional
interactions between DAIL and PW molecules. After 600 °C, a
shoulder can be seen in the TGA curve, which is ascribed to
the decomposition of the PW POM.
3.2.2. Spectroscopic analysis. DRIFT measurements were
performed to gain insight into the structural features of the
pristine and functionalized samples. The DRIFT spectra of all
the samples are presented in Fig. 4. In the DRIFT spectrum
of the DAIL, the characteristic peaks of the imidazole frag-
ment appear at 1590 and 1668 cm−1 which can be assigned to
the vibrations of the CC and CN groups.46 The bands at
1111, 1160 and 2953 cm−1 can be ascribed to the C–N, C–C
and C–H stretching vibrations of the alkyl chain.46 These vi-
brations can also be seen in the DRIFT spectrum of the DAIL/
MIL-101IJCr) sample, which clearly confirms the successful at-
tachment of the DAIL to the free metal sites. As shown in
Fig. 4, the characteristic bands at approximately 1080, 979
and 900 cm−1, which appeared in the DRIFT spectrum of the
supported PW/DAIL/MIL-101IJCr), can be assigned to the vi-
brations of P–O, WO and W–O–W in the POM, respec-
tively.44 These DRIFT results demonstrate that PW molecules
have been successfully deposited onto DAIL/MIL-101IJCr).
3.2.3. Transmission electron microscopy measurements.
In order to investigate the morphology and composition of
the PW/DAIL/MIL-101IJCr) material, TEM, STEM and elemen-
tal mapping were conducted. Fig. 5a shows a dark-field STEM
micrograph of the modified MIL-101IJCr) compound demon-
strating a typical truncated octahedral morphology. More-
over, the material shows perfect TEM images of the pore dis-
tribution within the crystals (Fig. 5c and d). These
observations confirm that the morphology of the support was
preserved after the modification process. Interesting features
could be observed from both TEM and elemental mapping
images. The TEM images in Fig. 5c and d show places with
darker contrast that could be assigned to the presence of
polyoxometalate molecules inside the MIL-101IJCr) cavities
and not on the external surface. Elemental mapping (Fig. 6)
further revealed the elements, including W, P and Cr, from
both polyoxometalates and MIL-101IJCr). In addition, this im-
age displays a very nice and homogeneous distribution of
polyoxometalates in the MIL-101IJCr) material.
3.3. Catalytic tests
The catalytic activity of the obtained PW/DAIL/MIL-101IJCr)
material was evaluated in the selective oxidation of benzyl al-
cohol to benzaldehyde using TBHP as the oxidant. The sur-
face area, as well as the distribution of the active sites within
the porous support, plays an important role in the catalytic re-
action. In this case, MIL-101IJCr) as the solid support provides
a high surface area even after the immobilization of the PW
molecules, which further allows the facile diffusion of the re-
actants within the framework. In order to obtain insights into
the function of the DAIL moiety, the DAIL-free catalyst (HPW/
MIL-101IJCr)) was synthesized through the impregnation of
MIL-101IJCr) with HPW POM using the original reaction
Fig. 5 (a) Dark-field STEM image, (b) bright-field STEM image, (c) and
(d) TEM images of PW/DAIL/MIL-101IJCr).
Fig. 4 DRIFT spectra of MIL-101IJCr) and the modified materials.
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ucts is of great importance.5,6 These over-oxidation reactions
occur due to the large activation energy required to bring
about the first step in the oxidation reaction.47 In all the cases
examined in this study, complete selectivity towards the for-
mation of benzaldehyde is obtained (>99%), demonstrating
that the POM-based catalyst exhibits excellent product selec-
tivity under the reaction conditions. PW/DAIL/MIL-101IJCr) ex-
hibits a higher catalytic performance in the oxidation of ben-
zyl alcohol to benzaldehyde in comparison with HPW/MIL-
101IJCr) (Table 1, entry 5). Further tests were carried out to de-
termine the effect of DAIL groups on the catalytic properties
of the PW/DAIL/MIL-101IJCr) catalyst. Therefore, various
amounts of DAIL were loaded onto the material. By increasing
the loading of DAIL from 0.3 mmol g−1 to 0.9 mmol g−1, the
conversion increased (Table 1, entries 1–3). Based on these
data, it can be concluded that the presence of DAIL groups
within the framework of MIL-101IJCr) has a positive effect on
the catalytic efficiency. The hydrophilic amino-functionalized
IL moiety in the framework, especially the remaining free
amino groups anchored on the imidazolium, plays a crucial
role in achieving excellent catalytic activity by enhancing the
accessibility of TBHP. Within this context, Yang et al. reported
the efficient epoxidation of cis-cyclooctene over a vanadium
catalyst using TBHP as the oxidant in the presence of 1-ethyl-
3-methylimidazolium bisIJtrifluoromethylsulfonyl)imide as a
co-solvent.48 The superior catalytic activity of this IL-involved
catalytic system was attributed to the hydrogen bonding be-
tween the IL and the by-product t-butanol, which prevents the
coordination of t-butanol with the vanadium centre and hence
improves the catalytic efficiency. So, the combination of ILs
and the catalytically active centres in these catalysts can pro-
mote better catalytic efficiency by enhancing the accessibility
of oxidants through hydrogen bonding. Additional DAIL
groups loaded into the framework resulted in a reduced cata-
lytic activity which may be due to diffusion limitations
Fig. 6 EDX elemental mapping images for the PW/DAIL/MIL-101IJCr)
material.
conditions of the PW/DAIL/MIL-101IJCr) catalyst. HPW/MIL-
101IJCr) was fully characterized (see Fig. S1–S4†). The W load-
ing was determined by means of XRF measurements, indicat-
ing a loading of 0.18 mmol g−1 and 0.23 mmol g−1 for the
HPW/MIL-101IJCr) and the PW/DAIL/MIL-101IJCr) materials, re-
spectively. The slightly higher metal loading in the PW/DAIL/
MIL-101IJCr) catalyst may be due to less significant PW
leaching into the liquid environment, which arises from the
additional interactions between DAIL and PW molecules.
In a typical catalytic test, a catalyst loading of 6 μmol W
was used and the substrate/oxidant ratio was 1 : 4.5 (mmol).
The results of the catalytic performance of the PW/DAIL/MIL-
101IJCr) material are given in Table 1.
As mentioned above, the selective oxidation of alcohols to
aldehydes and ketones without forming over-oxidized prod-
Table 1 Catalytic performance of various catalysts in the oxidation of benzyl alcohol
Entry
Catalyst
DAIL loading (mmol g−1
)
Oxidant/solvent
Conversiona (%)
TONb
1
2
3
4
5
6
7
8
PW/DAIL/MIL-101IJCr)
PW/DAIL/MIL-101IJCr)
PW/DAIL/MIL-101IJCr)
PW/DAIL/MIL-101IJCr)
HPW/MIL-101IJCr)
PW/DAIL/MIL-101IJCr)
PW/DAIL/MIL-101IJCr)
PW/DAIL/MIL-101IJCr)
PW/DAIL/MIL-101IJCr)
—
0.3
0.6
0.9
1.5
—
0.9
0.9
0.9
0.9
—
—
0.9
—
—
0.9
0.9
TBHP/CHCl3
TBHP/CHCl3
TBHP/CHCl3
TBHP/CHCl3
TBHP/CHCl3
TBHP/CH2Cl2
TBHP/CH3CN
H2O2/CH3CN
H2O2/CHCl3
TBHP/CHCl3
TBHP/CHCl3
TBHP/CHCl3
TBHP/CHCl3
TBHP/CHCl3
TBHP/CHCl3
TBHP/CHCl3
78
85
95
58
70
81
65
19
9
22
43
47
59
25
71
56
1560
1700
1900
1160
1400
1620
1300
380
180
—
—
—
—
—
1420
1120
9
10
11
12
13
14
15
16
MIL-101IJCr)
DAIL/MIL-101IJCr)
HPW
DAILc
SiW/DAIL/MIL-101IJCr)
PMo/DAIL/MIL-101IJCr)
Benzyl alcohol (1 mmol), oxidant (4.5 mmol), solvent (1 ml), dodecane (0.5 mmol), catalyst (6 μmol based on W or Mo, 0.5 μmol based on PW,
a
b
SiW or PMo), time (6 h), 100 °C. GC yield based on the starting substrate. Calculated as mmol of the product formed per mmol of
polyoxometalate (PW, SiW or PMo) in the catalyst, selectivity towards the formation of benzaldehyde >99% (determined by GC, entries 1–16).
c
0.23 mmol DAIL was used as the catalyst.
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(Table 1, entry 4). Further tests were carried out to determine
the influence of the solvent on the catalytic properties of PW/
DAIL/MIL-101IJCr). Higher reactivity of the substrate was
obtained in chloroform compared with dichloromethane and
acetonitrile. In addition, the PW/DAIL/MIL-101IJCr) catalyst
also exhibited higher catalytic activity when TBHP was used
as the oxidant. A very low conversion is observed in the ab-
sence of the catalyst (22%, Table 1, entry 10).
cases, in most reports no turnover number (TON) or turnover
frequency (TOF) values were stated. The superiority of our
catalysts lies in obtaining high selectivity towards benzalde-
hyde with an excellent TON. Moreover, higher thermal stabil-
ity is obtained compared to those prepared by the immobili-
zation of POM-based ILs onto mesoporous silica,40,51,52
which may be attributed to the high stability of MIL-101IJCr)
as the solid support.
To clarify the catalytic role of the prepared PW/DAIL/MIL-
101IJCr) catalyst, the oxidation reaction was also carried out
using the unmodified MIL-101IJCr) and DAIL/MIL-101IJCr) as
catalysts. MIL-101IJCr) with unsaturated metal centers at the
nodes showed 43% conversion of the benzyl alcohol to benz-
aldehyde. When DAIL/MIL-101IJCr) was applied as the catalyst,
the conversion of benzyl alcohol is slightly higher than that
of pristine MIL-101IJCr) (Table 1, entry 12). Moreover, the cata-
lytic performances of the HPW and DAIL were evaluated in
the selective oxidation of benzyl alcohol to benzaldehyde,
and the results are summarized in Table 1. It is found that
the various catalysts of PW/DAIL/MIL-101IJCr) were much
more active than the Keggin-type HPW (Table 1, entries 1–3
cf. 13), which proves the proficiency of DAIL/MIL-101IJCr) in
loading the PW anions.
The comparative data for the selective oxidation of benzyl
alcohol over the present catalyst and reported POM-based cat-
alysts are given in Table 2. Patel et al. reported the selective
aerobic oxidation of benzyl alcohol to benzaldehyde catalyzed
by PMo11Co.49 28–33% conversion of benzyl alcohol was
obtained with 92–93% selectivity for benzaldehyde using
TBHP as an initiator. Although a very high TON (3628–4281)
was achieved, the catalyst gives a relatively low conversion
and requires long reaction times (24 h). Later, Wang et al. de-
veloped a new recyclable amino-functionalized bipyridine–
heteropolyacid ionic hybrid, and 100% conversion was ob-
served with 93% selectivity for benzaldehyde.50 Alternatively,
attempts have been made to synthesize supported POM-
based IL materials as well. This is another efficient way to
fine-tune the surface properties of the support and transfer
the ILs' properties to the material. Mesoporous silica and
magnetic nanoparticles have been applied as the solid sup-
ports, and the results are shown in Table 2.37,40,51,52 Although
high catalytic activity and selectivity were achieved in all the
Catalytic oxidation of benzyl alcohol was carried out in the
presence of different POMs. Within this context, H4SiW12O40
and H3PMo12O40 were also immobilized onto DAIL/MIL-
101IJCr) to give SiW/DAIL/MIL-101IJCr) and PMo/DAIL/MIL-
101IJCr), respectively. The catalytic results in Table 1 clearly
show that PW/DAIL/MIL-101IJCr) is the most active catalyst in
terms of benzyl alcohol conversion (Table 1, entry 3 cf. 15–
16). PMo/DAIL/MIL-101IJCr) showed a much lower catalytic
performance in the oxidation of benzyl alcohol. This differ-
ence in reactivity might be due to the different acidic
strengths of the prepared catalysts and the oxidation poten-
tials of their metal centers.
With the aim of examining the catalytic potential of PW/
DAIL/MIL-101IJCr) towards other alcohols, various aromatic
electron-donating and electron-withdrawing alcohols were ex-
amined. The obtained results are listed in Table 3. For aro-
matic alcohols, the nature of substituents (electron-withdraw-
ing or electron-donating) has a significant effect on aldehyde
formation. For example,
a high conversion (78%) of
4-methoxybenzyl alcohol into its corresponding aldehyde was
observed after 4 h, while for 4-chlorobenzyl alcohol only 45%
of its corresponding aldehyde was produced after 6 h.
3.4. Stability and reusability tests
In order to investigate the heterogeneous nature of the PW/
DAIL/MIL-101IJCr) catalyst, a hot filtration test was conducted
to analyse the existence of the active PW species in solution.
Hence, the reaction mixture was filtered after 4 h reaction,
and the filtrate was left to react for a further 8 h. The benzyl
alcohol conversion reached 80% after 8 h, suggesting that
the oxidation reaction does not proceed after the removal of
the catalyst (Fig. 7). Additionally, to evaluate the recyclability
and reusability of the PW/DAIL/MIL-101IJCr) catalyst, we
Table 2 Comparison of the catalytic activities for the oxidation of benzyl alcohol with reported catalysts
Entry
1
Catalyst
PMo11
Reaction conditions
Conversion
Selectivity
Ref.
49
M
Alcohol (100 mmol), catalyst (25 mg), O2,
28–33
92–93
(M = Co, Mn, Ni)
TBHP (0.2%), 90 °C, 24 h.
2
3
4
5
6
7
8
DPyAMIJH)-PW
Alcohol (10 mmol), catalyst (0.1 g), H2O2 (6 mmol), 90 °C, 0.5 h.
Alcohol (2 mmol), catalyst (1 mmol), 2 atm O2, 80 °C, 5 h.
Alcohol (1 mmol), catalyst (100 mg), air (3.4 atm), 100 °C, 12 h.
Alcohol (10 mmol), catalyst (0.1 g), H2O2 (30 mmol), 100 °C, 6 h.
Alcohol (5 mmol), catalyst (0.1 g), H2O2 (15 mmol), 100 °C, 6 h.
Alcohol (5 mmol), catalyst (0.1 g), H2O2 (15 mmol), 100 °C, 6 h.
Alcohol (1 mmol), catalyst (6 μmol based on W),
TBHP (4.5 mmol), 100 °C, 6 h.
100
98
93.2
—
—
50
37
51
52
40
40
PV2Mo10O40/IL/SMNP
PV2Mo10O40/IL/SBA15
PW-NH2-IL-SBA-15
HPW-IL-PMO-15/5m
HPW-IL-SBA-15/5m
PW/DAIL/MIL-101IJCr)
98a
92
91
88
93
95
>99
>99
>99
This work
a
Based on the GC yield.
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Table 3 Catalytic performance of PW/DAIL/MIL-101IJCr) in the oxidation of various alcohols
Entry
1
Catalysta
Substrate
Time (h)
4
Conversionb (%)
100
TONc
2000
PW/DAIL/MIL-101IJCr)
2
3
PW/DAIL/MIL-101IJCr)
PW/DAIL/MIL-101IJCr)
4
6
78
59
1560
1180
4
PW/DAIL/MIL-101IJCr)
6
45
900
Reaction conditions: aromatic alcohol (1 mmol), TBHP (4.5 mmol), CHCl3 (1 ml), dodecane (0.5 mmol), catalyst (6 μmol based on W, 0.5 μmol
a
b
c
based on PW), 100 °C. 0.9 mmol DAIL was used for the preparation of the catalyst. GC yield based on the starting substrate. Calculated as
mmol of the product formed per mmol of polyoxometalate (PW) in the catalyst, selectivity towards the formation of benzaldehyde >99%
(determined by GC, entries 1–4).
performed the oxidation reaction of benzyl alcohol under the
optimized reaction conditions. After reaction completion, the
mixture was filtered, and the recycled catalyst was washed
with chloroform and acetone. The results presented in Fig. 8
indicate that the catalyst can be recycled and reused up to
five times with no significant decrease in catalytic activity.
From this, one can conclude that there is little leaching or
deactivation of the active components, owing to the strong
interaction between the PW and the DAIL/MIL-101IJCr) sup-
port. In addition, the filtrate obtained after the recovery of
the catalyst was characterized by utilizing the ICP-OES tech-
nique to verify if tungsten leaching occurred. The results
showed no significant tungsten content in the filtrate during
five successive runs (0.00021 wt%).
Fig. 8 Recyclability of the PW/DAIL/MIL-101IJCr) catalyst.
Further evidence of the high stability of the catalyst was
confirmed by DRIFT and PXRD analyses. The results from
the DRIFT characterization of the recycled PW/DAIL/MIL-
101IJCr) catalyst (see the ESI,† Fig. S5) show all the character-
istic peaks of the DAIL and PW molecules, suggesting that
the main structure of the catalyst is preserved. The similarity
of the PXRD patterns of PW/DAIL/MIL-101IJCr) before catalysis
and after each run clearly suggests the high stability of the
structure under reaction conditions (see the ESI,† Fig. S6).
4 Conclusions
In summary, we have designed and synthesized a new,
environmentally-friendly POM-based catalyst using dual
amino-functionalized basic ionic liquid (DAIL)-modified MIL-
101IJCr) as a solid support. The good thermal stability and
also the presence of two primary amine groups, in which the
free amino group is able to form hydrogen bonds, make the
DAIL a good candidate to be immobilized onto MIL-101IJCr).
This proved to be a successful and efficient approach to in-
crease the catalytic activity of the resulting catalyst. The cata-
lyst enables the oxidation of benzyl alcohol with complete se-
lectivity towards benzaldehyde. The PW/DAIL/MIL-101IJCr)
catalyst can be recovered and reused at least five times with
no significant loss in activity. Moreover, the combination of
Fig. 7 Hot filtration test for the PW/DAIL/MIL-101IJCr) catalyst.
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MIL-101IJCr), DAIL and HPW resulted in a heterogeneous cat-
alyst with enhanced thermal stability and Langmuir surface
area compared to those prepared by the immobilization of
POM-based ILs onto mesoporous silica.
This work is an important step towards the hetero-
genization of ionic liquids to design heterogeneous POM-
based ionic liquids.
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Acknowledgements
Sara Abednatanzi gratefully acknowledges financial support
from the University of Tehran. Karen Leus acknowledges fi-
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