Cr-MIL-101 encapsulated Keggin phosphotungstic acid as active nanomaterial for catalysing the alcoholysis of styrene oxide

Article abstract of DOI:10.1039/c3gc41988f

Mesoporous chromium-based terephthalate metal-organic framework (MIL-101) encapsulated Keggin phosphotungstic acid (HPW) [MIL-101(HPW)] was demonstrated to be an active heterogeneous catalyst for selective catalysis of the ring opening reaction of styrene

Full text of DOI:10.1039/c3gc41988f

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Cr-MIL-101 encapsulated Keggin phosphotungstic
acid as active nanomaterial for catalysing the
alcoholysis of styrene oxide†
Cite this: Green Chem., 2014, 16,
1351
Lik H. Wee,*a Francesca Bonino,b Carlo Lamberti,b Silvia Bordigab and
Johan A. Martensa
Mesoporous chromium-based terephthalate metal–organic framework (MIL-101) encapsulated Keggin
phosphotungstic acid (HPW) [MIL-101(HPW)] was demonstrated to be an active heterogeneous catalyst
for selective catalysis of the ring opening reaction of styrene oxide with methanol, achieving 99% yield of
2-methoxy-2-phenylethanol in 20 minutes at 40 °C. Similar MIL-101 samples prepared using one-pot
microwave synthesis in the absence of HPW or in the presence of hydrofluoric acid (HF) were less active.
The impact of fluoride and HPW polyanion incorporation on the acidity of MIL-101 was investigated by
the in situ infrared spectroscopy technique using CO as a probe molecule. Additional hydroxyl groups
and Lewis acid sites are present in MIL-101(HPW) explaining the observed superior catalytic performance
in styrene oxide methanolysis.
Received 23rd September 2013,
Accepted 21st November 2013
DOI: 10.1039/c3gc41988f
www.rsc.org/greenchem
Chromium terephthalate MIL-101 (Materials of the Institut
Lavoisier no. 101) is a mesoporous MOF reported by Férey and
1. Introduction
Metal–organic frameworks (MOFs) are a rapidly growing family co-workers.2a MIL-101(Cr) was constructed from linkage of 1,4-
of organic–inorganic hybrid materials built from organic benzenedicarboxylate (H2BDC) anions and inorganic trimeric
ligands bridging metal ions.1 By systematic design and fine- Cr building units leading to a three-dimensional cubic struc-
tuning, MOF structures having high accessible porosity and ture of corner-sharing supertetrahedra. MIL-101 has mesosize
pores with dimensions in the range of several angstroms to cages of 2.9 and 3.4 nm, accessible through microporous
nanometres have been achieved.2 These unique and outstand- windows of 1.2 and 1.45 nm, giving rise to a high BET surface
ing features of MOFs have resulted in a vast range of promising area of 5900 m2 g−1. Several studies on MIL-101 synthesis8 and
potential applications such as gas storage,3 separation,4 drug its applications in gas storage,9 separation10 and catalysis7,11a,b
release,5 catalysis6 and others. The engineering of MOFs with have been reported.
strong Lewis or Brønsted acidity via grafting of functional
Polyoxometalates (POMs) are excellent homogeneous acid
groups and encapsulation of porphyrin and nanoparticles catalysts. However, the recovery and reuse of POM catalysts
makes them excellent candidates as heterogeneous catalysts.7 in liquid reaction media is often hampered by their high solu-
In addition, the well-defined pore architecture in MOFs is bility in water and organic solvents.12 To overcome this limit-
ideally suited for size- and shape-selective catalysis. A draw- ation, permanent heterogenisation of these homogeneous
back of MOFs is their limited chemical and hydrothermal catalysts into various host supports such as zeolite,13 silica14
stability. Therefore, the most successful application area for and activated carbon15 has been reported. However, these
MOFs in heterogeneous catalysis is reactions running at low systems tend to have limited POM loading and present some
temperature under liquid phase conditions.
leaching, and the supported POMs show a tendency for
agglomeration.16 MIL-101 is an attractive host matrix for the
encapsulation of POM taking into account the benefit of its (i)
large mesocages for the encapsulation of POM molecules, (ii)
good POM dispersion, (iii) high surface area, (iv) a simple and
efficient one-pot synthesis of the formulation and (v) a rela-
tively high framework stability among MOF materials. The
encapsulation of POM into the pores of MIL-101 has been
achieved via either impregnation2a,17 or the one-pot hydrother-
mal synthesis approach.18,19 These MIL-101(POMs) materials
aCentre for Surface Chemistry and Catalysis, KU Leuven, Kasteelpark Arenberg,
23, B3001, Leuven, Belgium. E-mail: likhong.wee@biw.kuleuven.be;
Fax: +32 (0) 16321498; Tel: +32 (0) 163 76748
bNIS Centre of Excellence and INSTM Reference Center, Department of Chemistry,
University of Turin, Via Quarello 15, 10135, Turin, Italy
†Electronic supplementary information (ESI) available. See DOI:
10.1039/c3gc41988f
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show catalytic activity for Knoevenagel condensation of benz- flow overnight prior to analysis). The 31P MAS NMR spectrum
aldehyde with ethyl cyanoacetate, esterification reaction of was recorded on a Bruker AMX300 spectrometer (7.0 T) using
acetic acid with n-butanol, alkene epoxidation, carbohydrate the single-pulse excitation method. 3503 scans were accumu-
dehydration
to
5-hydroxymethylfurfural
and
Baeyer lated with a recycle delay of 20 s. The pulse length was 2.0 μs.
condensation.17–20
The sample was packed in 4 mm Zirconia rotors, and the spin-
Given the promising catalytic activity, we synthesized ning frequency of the rotor was 6 kHz. A solution of 2 mL
MIL-101 in the absence of HF, and with encapsulated Keggin ortho phosphoric acid (85%, Normapur VWR) in 2 mL water
HPW ions. The active sites of the MIL-101 catalysts were was used as a chemical shift reference. FTIR spectra were col-
characterized in detail using in situ infrared spectroscopy and lected on a Vertex 70 Bruker spectrophotometer equipped with
CO as a probe molecule at low temperature. This work adds to a MCT detector at 2 cm−1 resolution on a thin self-supported
the greening of a chemical process considering that (i) an HF wafer. The sample was activated in a home-made low tempera-
free synthesis method for the preparation of MIL-101(HPW) is ture cell under high vacuum (residual pressure 10−4 mbar) at
achieved; (ii) the selective synthesis of 2-methoxy-2-phenyletha- 200 °C for 2 h. CO gas (peq = 50 torr) was dosed on the sample
nol achieving 99% yield under mild reaction conditions rep- at RT. Successively, the sample in contact with CO is cooled
resents an energy saving and chemical waste minimisation down by means of liquid nitrogen and then, once the tempera-
and (iii) the encapsulation of homogeneous POM into MIL-101 ture equilibrium is reached, it is progressively outgassed.
enables facile catalyst recovery and recycling.
2.3. Catalysis
Ring opening reactions of styrene oxide with methanol were
2. Experimental section
2.1. Synthesis
performed in closed glass vessels inserted into a copper block
equipped with a temperature control thermocouple set at
40 °C. In a standard experiment, the reaction mixture con-
tained 1.25 mmol of styrene oxide (97+%, ACROS) and 10 mL
of methanol (HPLC grade, BDH) and 50 mg MOF catalyst and
the content was magnetically stirred at 600 rpm. The reactions
were performed for 20 min. Aliquots of the reaction mixture
were periodically withdrawn with a microsyringe after 5, 10
and 20 minutes of reaction. The catalyst was removed through
centrifugation prior to gas chromatography analysis (GC,
Chrompack 8760 WCOT fused silica capillary column, 30 m
long). The yields of the product were determined using
nonane as an external standard. The leaching test and evi-
dence of heterogeneity in the catalysis of styrene oxide ring
opening by methanol was determined by removing the solid
catalysts via centrifugation after 5 min of reaction time and
analysis of the reaction mixture was continued under similar
reaction conditions for another 15 min. The solid was separ-
ated by centrifugation, washed with ethanol and dried at 60 °C
for subsequent recycling tests.
MIL-101 samples were prepared according to a previously
reported method by Bromberg et al.20 involving microwave
heating of
a
synthesis mixture containing
4
g
of
Cr(NO3)3·9H2O (99%, Sigma-Aldrich), and 1.66 g of terephtha-
lic acid (H2BDC) (98%, Aldrich) in 50 mL of distilled water to
produce MIL-101 (H2O), or with the addition of 1.44 g of
H3PW12O40·nH2O to produce MIL-101(HPW), or with 0.2 mL of
hydrofluoric acid to produce MIL-101 (HF). The synthesis
mixture was heated to 210 °C in 10 min (microwave heating at
1000 W) and kept at 210 °C for 40 min. The product was
washed with DMF (≥99% (GC), Sigma-Aldrich) via a series of
centrifugation and redispersion steps in an ultrasonic bath.
The product was then dried in an air oven at 60 °C overnight
followed by Soxhlet extraction in ethanol (laboratory type,
Chem-Lab) for 48 h, and washed with ethanol prior to oven
drying. Samples were activated under vacuum overnight at
140 °C. Elemental analysis (wt%) of: MIL-101(H2O): C, 37.00;
Cr, 6.31; MIL-101(HF): C, 40.93, Cr, 6.40 F, 5.60; MIL-101
(HPW): C, 24.84; Cr, 6.40; W, 34.98. Based on the elemental
analysis results and molecular weight of hydrated HPW
(H3PW12O40·nH2O) (W content, 75.65%; MW, 2916 g mol−1),
the content of hydrated HPW was calculated according to the
formula, HPW (μmol g−1) = 1 × 106 (W content in the hybrid
material, %)/(2916 × 75.65%).20 The estimated HPW content in
MIL-101(HPW) material was 158 μmol g−1 of dry powder, or
48 wt%.
3. Results and discussion
3.1. Synthesis and characterization of MOFs
In this study, MIL-101 samples were prepared in the presence
or absence of HF or HPW via microwave heating at 210 °C of a
synthesis mixture having a molar composition of 1Cr(NO3)3·
9H2O : 1H2BDC : 280H2O for 40 min. MIL-101(HF) was first
synthesized by Chang and co-workers9a via microwave heating.
Therefore, the MIL-101(HF) sample is an ideal reference
2.2. Characterization
The synthesized MOFs were characterized using a scanning material for direct comparison with MIL-101(H2O) and
electron microscope (SEM, Philips XL-30 FEG equipped with a MIL-101(HPW) samples for phase purity, porosity, pore
tungsten filament), a powder X-ray diffractometer (XRD, STOE volume, crystal morphology, acidity and catalytic activity.
StadiP diffractometer in high-throughput transmission mode; According to the SEM image (Fig. 1b), MIL-101(H2O) appears
Cu Kα1 radiation), and a N2 physisorption instrument (Micro- like spherical particles with a uniform morphology and par-
meritics Tristar 3000, samples were degassed at 150 °C in N2 ticle size around 100 nm. MIL-101(HF) has a less defined
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Fig. 1 SEM images of (a) MIL-101(HF), (b) MIL-101(H2O) and (c) MIL-101(HPW).
Fig. 2 XRD patterns of (a) MIL-101(HF), (b) MIL-101(H2O) and (c)
MIL-101(HPW).
particle morphology (Fig. 1a). MIL-101(HPW) appears like an
aggregate of nanocrystallites (Fig. 1c). XRD patterns (Fig. 2) are
characteristic of MIL-101.2a
The permanent porosity of MIL-101(H2O) and MIL-101
(HPW) was verified by N2 physisorption analysis (Fig. 3a). N2
adsorption isotherms show strong uptake at a low relative
pressure which is the typical characteristic of a microporous
material.21 MIL-101(H2O) and MIL-101(HF) samples have a
similar BET (Brunauer–Emmett–Teller) surface area and a pore
volume of 2995 m2 g−1 and 1.31 cm3 g−1, respectively. This BET
surface area is lower than that for micron-sized MIL-101 reported
in the literature (5900 m2 g−1) but similar to the reported nano-
sized MIL-101 prepared in the presence of HF.16 According to the
BJH (Barrett–Joyner–Halenda) method based on nitrogen adsorp-
tion isotherms, the average pore diameter of MIL-101(HF),
MIL-101(H2O) and MIL-101(HPW) was estimated to be in the
range of 3.5–3.8 nm. The physiochemical properties of MIL-101
samples are also presented in Table S1 (see ESI†). It should be
mentioned that MIL-101 with a BET surface area greater than
4500 m2 g−1 is very difficult to obtain due to the presence of
Fig. 3 (a) N2 adsorption isotherms, (b) IR spectra collected after out-
gassing at 200 °C and (c) 31P solid-state MAS NMR spectrum. MIL-101
(HF) (red), MIL-101(H2O) (blue) and (b) MIL-101(HPW) (black).
trapped recrystallised H2BDC synthesis residues within the pores
of MIL-101.16 The presence of free dicarboxylic acid impurities in
MIL-101 samples was evidenced by IR spectra which showed an
absorption band at 1728 cm−1 (Fig. S1, ESI†).11h Although many
attempts have been made to increase the purity of MIL-101
through various purification steps, the BET surface area of
MIL-101 is limited to the range of 2700–4300 m2 g−1 16
Two
.
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distinctive secondary uptakes around P/Po of 0.1 and 0.2 are epoxide to β-alkoxyalcohol, which is a valuable synthetic inter-
attributed to characteristic micropore window effects.2a The mediate for 1,2-diol and 1,2-diol mono-ether organic syn-
shallow hysteresis loops are likely due to capillary condensation theses, is highly desirable.26 Current processes often suffer
of nitrogen in interstitial void volumes between packed nano- from long reaction times, drastic reaction conditions, unsatis-
particles revealed by the SEM images (Fig. 1).22
factory conversion and poor regioselectivity.27
The BET surface area and pore volume of the MIL-101
The catalytic activity of MIL-101, MIL-101(HF) and MIL-101
(HPW) sample was found to be lower compared to MIL-101 (HPW) in the ring opening reaction of styrene oxide with
parent material, which is to be expected from the weight of the methanol performed under mild reaction temperature (40 °C) is
heavy heteropolyacid molecules and the pore space they reported in Fig. 4. The MIL-101(HPW) catalyst gave an excellent con-
occupy. Nevertheless, considerable BET surface area and pore version (99.8%) and regioselectivity (≥99%) to 2-methoxy-2-phenyl-
volume of 2124 m2 g−1 and 0.96 cm3 g−1, respectively, were pre- ethanol in 20 min of reaction time, higher than MIL-101(HF)
served. The incorporation of Keggin HPW in MIL-101 was con- (80% conversion) and MIL-101(H2O) (22% conversion)
firmed by IR spectroscopy based on the characteristic bands at (Fig. 4a). Reaction performed without catalyst under these
822, 903 ν(W–O–W), 983 ν(WvO) and 1084 cm−1 ν(P–O) reaction conditions gave less than 1% conversion. The hetero-
(Fig. 3b, black spectrum).22,23 The integrity of the Keggin poly- geneity of the MIL-101(HPW) catalyst was verified by removing
oxometal anion within the pores of MIL-101 was further sup- the solids from the reaction mixture after 5 min of reaction
ported by 31P solid-state MAS NMR which revealed a single time, and heating the filtrate for 15 more minutes. After fil-
signal at −15.4 ppm (Fig. 3c). Both the IR and 31P solid-state tration, a completely clear filtrate was obtained (Fig. 4b,
MAS NMR results were in good agreement with the previously insert). No further conversion of styrene oxide was noted,
reported results.2a The Keggin polyanion has a relatively large which confirmed that the reaction catalyzed by MIL-101(HPW)
particle size of ca. 1.3 nm diameter and 2.25 nm3 in volu- is truly heterogeneous (Fig. 4b). According to ICP analysis, the
me.2a,11a Five Keggin ions take up a 10.1 nm3 volume, which is detected concentration of W in the supernatant corresponded
approximately 50% of the total volume of a large cage to less than 0.001 wt% of the nominal amount of HPW in the
(20.6 nm3) as reported by Férey and co-workers.2a,11a Elemental catalyst. No Cr was detected. The MIL-101(HPW) catalyst could
analysis of MIL-101(HPW) revealed that HPW represented ca. be reused and recycled up to 3 times without significant loss
48% of weight indicating that each cage is loaded with five of catalytic activity and selectivity (Fig. 4c). The crystallinity of
Keggin ions, confirming the encapsulation of the Keggin HPW MIL-101(HPW) was preserved after catalytic testing as evi-
ions in these large cages of MIL-101. Although it is more likely denced by XRD (Fig. 4d). This is another illustration of the
that only the large cages can host the HPW,2a it has been stabilization of MIL-101 by incorporation of HPW reported
recently claimed by Hatton and co-workers that HPW can earlier.2d,28 However, a gradual loss of surface area and pore
reside both in the large and the small cages of MIL-101.20
volume was noted for the recovered MIL-101(HPW) (cycle 3)
In Fig. 3b, all MIL-101 spectroscopic fingerprints in the sample (BET surface area = 1417 cm2 g−1 and pore volume =
1200–700 cm−1 range are present. The highest frequency range 0.57 cm3 g−1) (Fig. S2, ESI†). This could be the reason for the
(3850–2500 cm−1) shown in the inset shows again many simi- observed loss of catalytic activity after the recycling test
larities between the MIL-101(H2O) and MIL-101(HPW) (Fig. 4c). A similar observation has also been reported by Khol-
samples. These IR spectra were recorded on samples activated deeva and co-workers for the MIL-101(HPW) catalysed
at 200 °C to remove all residual solvent and physisorbed water.
In this spectral range a lot of bands are due to combi-
nation and overtone modes. Nevertheless, some fingerprints
are clearly distinguishable: the bands at 3070 cm−1 and
3592 cm−1 due to ν(C–H) aromatic linker stretching vibration
and to ν(O–H) stretching mode, respectively. The presence of
an OH group is to compensate for the negative default charge
per trimer of chromium octahedra. It should be mentioned
that similar weak bands were observed by Vimont et al. in the
case of Cr-MIL-10024 and Al-MIL-10025 upon treatment in
vacuo at moderate temperatures. MIL-101(HPW) shows an
additional feature at 3665 cm−1, probably due to the presence
of some new hydroxyl groups.
3.2. Ring opening reactions of styrene oxide with methanol
Ring opening reaction of styrene oxide with methanol was
selected as an acid catalyzed model reaction to compare the
catalytic activities of the three types of MIL-101. This reaction
Fig. 4 (a) Ring opening reaction of styrene oxide with methanol cata-
lyzed by MIL-101(H2O), MIL-101(HF), and MIL-101(HPW), (b) leaching
also is of practical interest, and especially a heterogeneous
test of MIL-101(HPW), (c) catalyst recycling tests for MIL-101(HPW) and
catalyst for converting this readily available, inexpensive (d) XRD patterns of MIL-101(HPW) before and after three catalytic runs.
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cyclohexene oxidation reaction.11g The stability of MIL-101 proposed: (i) some defects in encapsulated polyanion forming
(HPW) after 10 catalytic cycles was confirmed by XRD (Fig. S3, new Lewis sites or (ii) Cr3+ sites with different acidity because
ESI†). The 9th reuse MIL-101(HPW) catalyst still gave 30% con- of the presence of HPW.31 The use of pyridine or ammonia as
version and high selectivity (>90%). The lower residual activity a probe molecule to monitor the acidity and in particular
is partly due to a loss of the catalyst material in the catalyst Brønsted acid of MIL-101(HPW) was not possible due to the
+
recovery operations and partly due to pore blockage by trapped complete overlapping of NH4 or PyH+ vibrational modes with
reaction products. Many catalysts have been evaluated for this carboxylate vibrational modes of MIL-101.32
reaction, such as MOFs (e.g. MIL-10029a and HKUST-129b), zeo-
The CO probing study revealed that the presence of the flu-
lites,29c mesoporous aluminosilicate,29d zirconium(IV)-grafted oride ion has no significant Lewis acidity enhancement effect
hexagonal MCM-41,29e silica supported copper oxide29f and in MIL-101(HF). Interestingly, in styrene oxide methanolysis
polymer supported iron complexes.29g The catalytic perform- MIL-101(HF) was more active than MIL-101(H2O) (Fig. 4a).
ance of MIL-101(HPW) in ring opening of styrene oxide with According to the chemical composition of MIL-101(HF), the
methanol appears to be exceptional.
mole ratio of F/Cr is 0.37, which agrees well with the theore-
tical value of 0.33 for the idealized structure.2a It has been pre-
viously reported that the fluoride ion has a significant
influence on the catalytic performance of some solid catalysts
such as zeolites33 and alumina.34 In MIL-100(Fe), that the pres-
ence of fluoride acting as the electron-withdrawing group sig-
nificantly strengthens the Lewis acidity in the neighboring
unsaturated metal sites has been reported by Serre and co-
workers.35 Another example is the enhancement of Lewis
acidity in UiO-66 by introducing electron withdrawing groups
such as F, CI, Br, and NO2 on the terephthalate linker.36
Although in our studies, no significant Lewis acidity enhance-
ment was noted for MIL-101(HF) according to the recorded IR
spectra, an additional Lewis acid site was noted for MIL-101
(HPW).
3.3. Infrared spectroscopic analysis
The catalytic activity of MIL-101 catalysts in ring opening of styrene
oxide is in the increasing order: MIL-101(H2O) < MIL-101(HF)
< MIL-101(HPW). The intriguing difference in catalytic per-
formance of the three different MIL-101 samples prompted
further investigation of the catalytic sites by in situ IR spec-
troscopy using CO as a probe molecule known to be a very sen-
sitive probe19 (see Fig. 5). In all the three samples, already at
room temperature, a quite intense and sharp band is always
present at 2194 cm−1 (light green spectra) showing the pres-
ence of strong Cr3+ exposed Lewis sites. By lowering the temp-
erature with liquid nitrogen, the band increases in intensity
and shifts to 2197 cm−1 (dark green spectra).24,30 In parallel at
lower frequency, bands due to CO physisorbed (liquid-like)
species arise (main feature centered at 2135 cm−1). Besides
these features in common with the other two samples,
MIL-101(HPW) shows a broadening of the band centered at
2197 cm−1. In particular, when at low temperature CO coverage
is decreased, a further component centered at 2202 cm−1
clearly appears. The assignment is not straightforward, but we
can hypothesize the presence of other Lewis sites, different
from the Cr3+ environment present in MIL-101(H2O) and
MIL-101(HF) samples. Two possible new species can be
4. Conclusions
The catalytic activity of MIL-101 and modified versions with
fluoride and Keggin phosphotungstate incorporated during
synthesis in styrene oxide methanolysis was investigated. The
acid sites were probed using in situ FTIR using CO as a probe
molecule. The occluded Keggin polyoxyanions in the pore
structure of MIL-101 generate additional hydroxyl groups and
Lewis acid sites inferred to be responsible for efficient metha-
nolysis of styrene oxide giving excellent yield of 2-methoxy-2-
phenylethanol (99%) in a short reaction time at 40 °C. The
MIL-101(HPW) catalyst is stable and can be reused without sig-
nificant loss of activity and selectivity. This is another example
of stabilization and catalytic activity enhancement of a MOF
through incorporation of heteropolyanions. The presence of
HF or HPW in the synthesis mixture results in the formation
of MIL-101 nanocrystallites with less-defined morphology
whereas uniform nanospheres of MIL-101 were obtained in
the absence of these additives. These uniform nanocrystals of
MIL-101 though less useful in catalysis could be useful as seed
layers for membrane/thin film preparation.
Fig. 5 FTIR spectra of (a) MIL-101(H2O), (b) MIL-101(HF) and (c) MIL-101
(HPW) recorded after outgassing at 200 °C for 2 hours (blue, red, black
color respectively), after adsorption of CO at room temperature (light
green) and low temperature (dark green); black curves show the effect
of outgassing in vacuo at low temperature.
Acknowledgements
The authors gratefully acknowledge financial support from the
EU FP7 project (CP-IP-228604-2), the Flemish Government
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