Monovacant polyoxometalates incorporated into MIL-101(Cr): Novel heterogeneous catalysts for liquid phase oxidation

Article abstract of DOI:10.1016/j.apcata.2012.12.039

Two novel hybrid composite materials, PW₁₁@MIL-101 and SiW ₁₁@MIL-101, were prepared by the inclusion of the potassium salts of the monovacant polyoxotungstates, [PW₁₁O₃₉]^7- (PW₁₁) and [SiW₁₁O₃₉]^8- (SiW ₁₁), into the porous Metal-Organic Framework MIL-101(Cr). Materials were characterized by a myriad of solid-state methods such as powder X-ray diffraction (XRD), vibrational (FT-IR and FT-Raman) and ³¹P solid-state NMR spectroscopies, elemental analysis, scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX), and textural analysis confirming the incorporation of the POMs into MIL-101(Cr). PW ₁₁@MIL-101 and SiW₁₁@MIL-101 revealed to be active, selective and recyclable catalysts for the oxidation of cis-cyclooctene, geraniol and R-(+)-limonene using the H₂O₂ as oxidant. Only one product was obtained from the epoxidation of cis-cyclooctene and geraniol: 1,2-epoxycylooctane and 2,3-epoxygeraniol, respectively. In the oxidation of R-(+)-limonene the main products were limonene-1,2-epoxide and limonene-1,2-diol, however the diepoxide was also formed. Both composite materials, PW₁₁@MIL-101 and SiW₁₁@MIL-101, are recyclable for, at least, three consecutive cycles without significant loss of activity. The stability of the composites after the catalytic cycles was confirmed by several techniques. Remarkably, the MOF framework was found to play an important role in the stability of the PW₁₁ in the presence of H ₂O₂.

Full text of DOI:10.1016/j.apcata.2012.12.039

Applied Catalysis A: General 453 (2013) 316–326
Contents lists available at SciVerse ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Monovacant polyoxometalates incorporated into MIL-101(Cr): novel
heterogeneous catalysts for liquid phase oxidation
Carlos M. Granadeiroa, André D.S. Barbosaa, Patrícia Silvab, Filipe A. Almeida Pazb,
Vipin K. Sainic, João Piresc, Baltazar de Castroa, Salete S. Balulaa,∗, Luís Cunha-Silvaa,∗
a REQUIMTE & Department of Chemistry and Biochemistry, Faculty of Sciences, University of Porto, 4169-007 Porto, Portugal
b Department of Chemistry, CICECO, University of Aveiro, 3810-193 Aveiro, Portugal
c Departamento de Química e Bioquímica, CQB, Faculdade de Ciências, Universidade de Lisboa, Campo Grande C8, 1749-016 Lisboa, Portugal
a r t i c l e i n f o
a b s t r a c t
Article history:
Two novel hybrid composite materials, PW11@MIL-101 and SiW11@MIL-101, were prepared by the inclu-
7− 8−
(PW11) and [SiW11O39]
Received 20 September 2012
Received in revised form
24 November 2012
Accepted 17 December 2012
Available online 4 January 2013
sion of the potassium salts of the monovacant polyoxotungstates, [PW11O39
]
(SiW11), into the porous Metal-Organic Framework MIL-101(Cr). Materials were characterized by a myr-
iad of solid-state methods such as powder X-ray diffraction (XRD), vibrational (FT-IR and FT-Raman) and
31P solid-state NMR spectroscopies, elemental analysis, scanning electron microscopy (SEM) and energy-
dispersive X-ray spectroscopy (EDX), and textural analysis confirming the incorporation of the POMs into
MIL-101(Cr). PW11@MIL-101 and SiW11@MIL-101 revealed to be active, selective and recyclable catalysts
for the oxidation of cis-cyclooctene, geraniol and R-(+)-limonene using the H2O2 as oxidant. Only one
product was obtained from the epoxidation of cis-cyclooctene and geraniol: 1,2-epoxycylooctane and
2,3-epoxygeraniol, respectively. In the oxidation of R-(+)-limonene the main products were limonene-
1,2-epoxide and limonene-1,2-diol, however the diepoxide was also formed. Both composite materials,
PW11@MIL-101 and SiW11@MIL-101, are recyclable for, at least, three consecutive cycles without signif-
icant loss of activity. The stability of the composites after the catalytic cycles was confirmed by several
techniques. Remarkably, the MOF framework was found to play an important role in the stability of the
PW11 in the presence of H2O2.
Keywords:
Polyoxometalates
Metal organic-framework
Hybrid materials
Heterogeneous catalysis
Oxidation
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
cage. Furthermore, the large cavities of certain MOFs can promote
a concentration effect between the different components from the
Metal-Organic Frameworks (MOFs), also known as coordination
polymers, are nowadays on top of research in materials chemistry.
In the last decade an extraordinary progress on the investiga-
tion and development of MOFs has been noticed, yielding novel
functional materials with remarkable potential for gas storage, sep-
aration [1–5] and catalysis [6–10]. The success of these hybrid
framework-based materials comes from the diversity of metal or
metallic clusters that are interconnected with various functional-
ized organic linkers, often leading to tri-dimensional (3D) porous
structures with large, regular and accessible cages and tunnels.
According with their porous morphology, their cavities may act as
nano-reactors because they can accommodate catalytically active
molecules with adequate shape and size to enter the pores. The
possibility of the inclusion depends on the correlation between
their size and the accessible dimensions of the windows of each
catalytic reaction (catalysts, substrate and others), which may be
required to improve yields and reduce reaction time. Heteroge-
neous catalysis using MOFs is, however, a relatively recent subject
that needs to be explored. Recently, the 3D porous MOF material
MIL-101(Cr) has shown to have important capacities for catalyst
engineering [11–15]. This hybrid material originally prepared by
Férey et al. is one of the most promising porous MOF discovered
to date [16–18]. In fact, MIL-101(Cr) allows the introduction of
nanoparticles, large molecular inorganic species and drugs within
its cages [16,19,20].
The opportunity of incorporating Keggin-type polyoxometa-
lates (POMs) in porous MOFs arises as an attractive pathway
to exploit the catalytic activity of these species and opens the
opportunity to create new catalytic systems that transform cheap
natural compounds into valuable products. Keggin-type POMs are
economical and environmentally attractive catalysts for oxida-
tion reactions, both in laboratorial and industrial processes [21].
During the last decades, POMs have been studied as effective
homogeneous catalysts in the oxidation of several hydrocarbons
with H2O2 [21–28]. More recently much work is focussed in the
∗
Corresponding authors. Tel.: +351 220402576; fax: +351 220402659.
E-mail addresses: sbalula@fc.up.pt (S.S. Balula), l.cunha.silva@fc.up.pt
(L. Cunha-Silva).
0926-860X/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2012.12.039

C.M. Granadeiro et al. / Applied Catalysis A: General 453 (2013) 316–326
317
immobilization of POMs due to the necessity of recovering and
recycling these active catalytic compounds. Different methodolo-
gies have been studied using various solid supports to immobilize
POMs via dative, covalent or electrostatic binding [29–36]. The
immobilization of POMs into nanoporous MIL-101(Cr) was per-
formed only few years ago and the first studies using these
composites as heterogeneous catalysts were only performed very
recently. To date, Kholdeeva et al. have been using POMs@MIL-101
Powder X-ray diffraction analyses were collected at ambient
temperature on a X’Pert MPD Philips diffractometer, equipped
with an X’Celerator detector and a flat-plate sample holder in a
Bragg-Brentano para-focusing optics configuration (45 kV, 40 mA).
Intensity data were collected by the step-counting method (step
0.04◦), in continuous mode and, in the ca. 3.5 ≤ 2ꢀ ≤ 50◦ range.
Solid-state 31P HPDEC NMR spectra were recorded on a 9.4 T
Bruker Avance 400 spectrometer (HPDEC stands for high-power
decoupled).
Scanning electron microscopy (SEM) images were performed
in a high resolution scanning electron microscope Hitachi SU-70
instrument working at 4 kV. The energy-dispersive X-ray spec-
troscopy (EDX) studies and SEM mapping images were recorded in
the same microscope working at 15 kV and using a Bruker QUAN-
TAX 400 EDS microanalysis system. Samples were analyzed as
powders and prepared by deposition on aluminium sample holders
followed by carbon coating using a Emitech K950X carbon evapo-
rator.
GC–MS analyses were performed using a Hewlett Packard 5890
chromatograph equipped with a Mass Selective Detector MSD
series II using helium as the carrier gas (35 cms−1); GC-FID was
performed using a Varian CP-3380 chromatographer to monitor the
homogeneous reactions and heterogeneous reactions. The hydro-
gen was the carrier gas (55 cms−1) and fused silica Supelco capillary
columns SPB-5(30 m × 0.25 mm i.d.; 25 ␮m film thickness) were
used.
as catalysts for oxidative reactions, using titanium, cobalt mono-
3−
substituted phosphotungstates and the Keggin anion [PW12O40
]
as active catalyst centres, with O2 and H2O2 being used as oxidants
[13,31,37,38]. Kapteijn et al. reported the use of POMs@MIL-101,
using the H3[PW12O40] in reactions of condensation, esterification,
oxidation and hydrogenation [39,40]. Furthermore, the inclusion
of H3[PW12O40] in other 3D porous MOF (Cu-btc) to prepare
active heterogeneous catalysts has been also recently investigated
[41–44].
In contrast with the monosubstituted POMs, the catalytic activ-
ity of monovacant POMs is not extensively explored. Furthermore,
the immobilization studies performed with monovacant POMs
are particularly scarce and only a couple of studies describe
them as catalysts for oxidation reactions [34–36,45–47]. The
present investigation reports the notable catalytic activity of the
monovacant phosphotungstate ([PW11O39
]
7−) and silicotungstate
([SiW11O39
]
8−) anions incorporated into the MIL-101(Cr) cavities
for the oxidations of alkenes and allylic alcohols using H2O2 as
oxidant. This is one of the most attractive oxidant because it is
fairly inexpensive and easily handled. The robustness and the recy-
clability of the composites were confirmed and the presence of
monovacant POMs seemed to promote the stability of these hybrid
materials under oxidative environment.
2.2. Synthesis and preparation of materials
Monovacant polyoxotungstates. The potassium and tetra-
7−
butylammonium (TBA) salts of monovacant [PW11O39
]
and
[SiW11O39
]
8−: K7[PW11O39]·10H2O (KPW11), K8[SiW11O39]·13H2O
(KSiW11), TBA4H3[PW11O39] (PW11), TBA4H4[SiW11O39] (SiW11),
were prepared using reported procedures [28,48–50]. All these
polyoxotungstates were characterized by elemental and thermal
analysis, FT-IR and FT-Raman spectroscopy, NMR spectroscopy and
powder XRD, confirming the synthesis of the desired compounds.
Solid support MIL-101 (Cr). The porous metal-organic frame-
work (MOF) material MIL-101(Cr) was prepared by an adaptation
of the method described by Férey et al. [16]. A mixture contain-
ing chromium(III) nitrate (2 mmol), benzene-1,4-dicarboxylic acid
(2 mmol) and hydrofluoric acid (100 ␮L) in 10 mL of H2O was stirred
at room temperature to obtain an homogeneous suspension, trans-
ferred into an autoclave and heated at 493 K for 9 h in an electric
oven. After a slowly cooling process (inside the oven), the material
was isolated by filtration and purified through a double DMF treat-
ment and a double ethanol treatment. Selected FT-IR (cm−1): 3440,
2933, 1671, 1625, 1508, 1405, 1385, 1017, 748, 663, 590; selected
FT-Raman (cm−1): 3077, 2929, 1613, 1496, 1459, 1146, 1044, 872,
812, 632.
2. Experimental
2.1. Materials and methods
All reagents used in the preparation of the support mate-
rial, namely chromium(III) nitrate nonahydrate [Cr(NO3)3·9H2O,
Aldrich, 99%], benzene-1,4-dicarboxylic acid (C8H6O2, Aldrich, 98%)
and hydrofluoric acid (HF, Aldrich, 40–45%) were used as received.
Geraniol (C10H18O, Aldrich, 98%), cis-cyclooctene (C8H14, Aldrich,
95%) and R-(+)-limonene (C10H16, Aldrich, 97%) were purchased
from commercial suppliers and used without further purification,
as well as acetonitrile (CH3CN, Panreac, 99%), hydrogen peroxide
(H2O2, Riedel de-Häen or Aldrich, 30%), ceric sulphate [Ce(SO4)2,
Aldrich, 99%].
Elemental analysis for Cr, P, Si and W were performed by
ICP spectrometry (University of Santiago de Compostela, CACTUS-
LUGO).
Electronic absorption spectra were recorded on Varian cary 50
Bio spectrophotometer.
Composite materials XW11@MIL-101. The composite materials
were prepared through the immobilization of the potassium salts
of monovacant polyoxotungstates (KPW11 and KSiW11) in the
porous solid support MIL-101(Cr) using a modified procedure of
the method described by Maksimchuk et al. [37]. Briefly, aqueous
solutions of KPW11 and KSiW11 (10 mM; 50 mL) were added to the
MIL-101(Cr) (0.5 g) and stirred at room temperature for 24 h. The
solid was filtrated, washed thoroughly with water and dried in a
dessicator over silica gel.
PW11@MIL-101. Anal. found (%): Cr, 10.2; W, 12.9; P, 1.20; load-
ing of PW11: 0.066 mmol per 1 g. Selected FT-IR (cm−1): 3466, 2925,
1623, 1556, 1508, 1403, 1043, 1018, 952, 886, 830, 814, 748, 673,
663. Selected FT-Raman (cm−1): 3073, 2933, 1613, 1493, 1457,
1145, 1028, 1001, 974, 871, 811, 785, 632.
Infrared absorption spectra were obtained on a Jasco 460 Plus
spectrometer, using KBr pellets. FT-Raman spectra were recorded
on a RFS-100 Bruker FT-spectrometer, equipped with a Nd:YAG
laser with excitation wavelength of 1064 nm, with laser power set
to 350 mW.
Nitrogen (Air Liquide, 99.999%) physisorptionexperimentswere
made at −196 ◦C using a volumetric apparatus (NOVA 2200e, sur-
face area and pore size analyzer). For every experiment, about
20 mg of sample was degassed for 2.5 h at 120 ◦C at low pressure
(<0.133 Pa). The pore size distribution (PSD) curves were obtained
by the BdB-FHH method. Micropore volumes V , were obtained
␮
from t-plots, using an appropriate non-porous reference material.
The total pore volume Vtotal, was estimated by the amount adsorbed
at p/p0 ≈ 0.95.
SiW11@MIL-101. Anal. found (%): Cr, 10.2; W, 13.3; Si, 1.34; load-
ing of SiW11: 0.073 mmol per 1 g. Selected FT-IR (cm−1): 3429, 2925,

318
C.M. Granadeiro et al. / Applied Catalysis A: General 453 (2013) 316–326
1711, 1682, 1621, 1554, 1398, 1021, 910, 811, 746, 553. Selected FT-
Raman (cm−1): 3080, 2924, 1613, 1493, 1458, 1145, 1028, 976, 959,
872, 811, 785, 632.
PW11@MIL-101-ac
PW11@MIL-101
2.3. Catalytic studies
The oxidation reactions of cis-cyclooctene (i), geraniol (iii) and
R-(+)-limonene (v) (Scheme 1) were carried out in acetonitrile
(MeCN), using a borosilicate 10 mL reaction vessel, with addition
of H2O2 (30 wt.%), in the presence of the monovacant polyoxo-
metalates (PW11 and SiW11) as homogeneous catalysts and the
respective composite materials (PW11@MIL-101 and SiW11@MIL-
101) as heterogeneous catalysts. In the oxidation of geraniol the
reaction was performed at room temperature and protected from
light. The oxidative reactions of cis-cyclooctene and R-(+)-limonene
were carried out at 75 ◦C. In a typical experiment, the substrate
(1 mmol) and the catalyst (3 ␮mol), placed in the reaction vessel,
were dissolved in MeCN (1.5 ml) and stirred. The H2O2 was added
to the reaction mixture, 500 ␮L (4.5 mmol) for the oxidation of
geraniol and R-(+)-limonene and 105 ␮L (1 mmol) for the oxida-
tion of cis-cyclooctene. The reactions were followed by GC analysis
and stopped when a complete conversion of the substrate was
observed or when the product yields remained constant after two
successive GC analyses. At regular intervals, an aliquot was taken
directly from the reaction mixture with a microsyringe (approxi-
mately 10 ␮L), diluted in MeCN, centrifuged and injected into the
GC or GC–MS equipment for analysis of starting materials and prod-
ucts. Each experiment was reproduced at least twice. The maximum
error obtained between repeated experiments for the conversion
data of each substrate was never higher than 5% for the oxida-
tion of cis-cyclooctene, 9% for the oxidation of geraniol and 8% for
the oxidation of R-(+)-limonene. The reaction products reported
were identified as described elsewhere [51,52]. In the presence of
monovacant composites (42 mg of PW11@MIL-101 and 46 mg of
SiW11@MIL-101), the heterogeneous catalysts were centrifuged at
the end of reactions, washed with MeCN several times to remove
the remaining substrate, reaction products and oxidant. The recov-
ered catalyst was dried at room temperature overnight and reused
in a new reaction under identical experimental conditions, with
readjustment of all quantities, without changing the molar ratios
and reaction concentrations. After the consecutive reaction cycles,
the monovacant composites were analyzed by different techniques
as well as by tungsten analysis. Blank reactions were performed for
all substrates, confirming that no oxidation products are obtained
unless the catalyst and H2O2 are present. Even in the presence of
the support MIL-101(Cr) substrates conversion was not higher than
10% after 24 h of reaction.
MIL-101
PW11
1800 1600 1400 1200 1000
800
600
400
Wavenumber (cm-1)
Fig. 1. FT-IR spectra of PW11, the solid support MIL-101(Cr), and the composite
material before (PW11@MIL-101) and after catalysis (PW11@MIL-101-ac), shown
between 1800 and 400 cm−1. (For interpretation of the references to colour in this
figure legend, the reader is referred to the web version of this article.)
PW11 and SiW11 in support material MIL-101(Cr) was monitored
by UV/Vis spectroscopy. As depicted in Fig. S1 (see the Suppor-
ting Information), the decrease of the absorbance of the band at
ca. 250 nm (assigned to the charge transition from the bridging
O-atoms to W-atoms) for their respective solutions after 24 h of
reaction relatively to the correspondent initial polyoxotungstate
solutions, supports the reduction of the amount of POMs. This
reduction strongly suggests the incorporation of the POMs (PW11
and SiW11) in the MIL-101(Cr) framework, leading to the compos-
ite materials. Indeed, the utilization of a myriad of characterization
methods, such as powder XRD, vibrational (FT-IR and FT-Raman)
and 31P HPDEC NMR spectroscopies, elemental analysis (ICP),
scanning electron microscopy (SEM) and energy-dispersive X-ray
spectroscopy (EDX) and textural analysis (N2 adsorption isotherms
and pore size distribution), further support the preparation of
PW11@MIL-101 and SiW11@MIL-101.
The presence of the polyoxotungstates in the MIL-101(Cr)
framework was initially established by FT-IR and FT-Raman spec-
troscopic studies of the composite materials in comparison with the
isolated compounds. The FT-IR spectra of both composite materi-
als exhibit the typical bands of the MOF as well as some of the
vibrational modes of the POMs (Fig. 1 and Fig. S2-left of the Sup-
porting Information for the PW11 and SiW11 based materials,
respectively). Regarding the PW11@MIL-101 material, the infrared
spectrum shows bands located at 1043, 952 and 814 cm−1 assigned
to the P − O, W = O and W − O − W stretching modes of the phospho-
tungstate, respectively (Fig. 1) [54].
2.4. Titration of hydrogen peroxide
At the end of each heterogeneous catalytic reaction, an aliquot
was taken from the reaction mixture for the determination of
hydrogen peroxide in solution. The sample without solid catalyst
was accurately weighed, quickly dissolved in diluted sulphuric acid
and the peroxides titrated against 0.1 N ceric sulphate, using ferroin
as indicator [53].
Similarly, the spectrum of the composite SiW11@MIL-101
exhibits bands at approximately 1021, 910 and 811 cm−1 that
correspond to Si − O, W = O and W − O − W stretches of the
silicotungstate, respectively (see Fig. S2-left of the Suppor-
ting Information) [54]. The FT-Raman spectra of the composite
materials further support the inclusion of the polyoxotungstates
into the MIL-101(Cr) cavities. Besides the characteristic bands of
MIL-101(Cr), the spectra also exhibit new small bands located at
approximately 1028 and 975 cm−1 assigned to M − O and W = O
stretching modes, respectively (see Fig. 2 for PW11@MIL-101 and
3. Results and discussion
3.1. Composite materials preparation and characterization
The preparation of the composite materials (PW11@MIL-101
and SiW11@MIL-101) was carried out by immobilization through
impregnation of potassium salts of the monovacant polyox-
otungstates (KPW11 and KSiW11, respectively) in the porous
MIL-101(Cr) in aqueous solution. The prospective inclusion of the

C.M. Granadeiro et al. / Applied Catalysis A: General 453 (2013) 316–326
319
Scheme 1. Chemical structure of the substrates investigated and their respective oxidation products.
Fig. S2-right of the Supporting Information for SiW11@MIL-101)
[54].
The formation of the PW11@MIL-101 composite and the
structural stability of the PW11 after inclusion in the MIL-
101(Cr) material are also supported by 31P solid-state NMR
spectroscopy. The 31P HPDEC MAS NMR spectra of the PW11
and composite material PW11@MIL-101 are compared in
Fig. 3. Indeed, the single peak observed at −10.70 ppm in the
7−
PW11 spectrum, unequivocally attributed to the [PW11O39]
anion as confirmed by various literature reports [16,50,55,56], is
PW12@MIL-101
PW11@MIL-101-ac
PW11@MIL-101-ac
PW11@MIL-101
PW11@MIL-101
MIL-101
PW11
PW11
30
20
10
0
-10 -20 -30 -40 -50 -60
(ppm)
1800
1600
1400
1200
1000
800
600
400
Wavenumber (cm-1)
Fig. 3. 31P HPDEC MAS spectra of the PW11, and the PW11 based composite material
before (PW11@MIL-101) and after catalysis (PW11@MIL-101-ac), and the composite
PW12@MIL-101, shown in the chemical shift range of −65 to 35 ppm. (For interpre-
tation of the references to colour in this figure legend, the reader is referred to the
web version of this article.)
Fig. 2. FT-Raman spectra of PW11, the solid support MIL-101(Cr), and the composite
material before (PW11@MIL-101) and after catalysis (PW11@MIL-101-ac), depicted
in the wavenumber range of 1800–400 cm−1. (For interpretation of the references to
colour in this figure legend, the reader is referred to the web version of this article.)

320
C.M. Granadeiro et al. / Applied Catalysis A: General 453 (2013) 316–326
Table 1
Textural properties of MIL-101(Cr), composite materials and their respective sam-
ples after catalysis (ac).
PW11@MIL-101-ac
ABET (m2/g)
V
␮
(cm3/g)
Vtotal (cm3/g)
MIL-101(Cr)
2846
1720
1324
1795
1611
1.17
0.74
0.56
0.76
0.68
1.30
0.80
0.63
0.87
0.80
PW11@MIL-101
PW11@MIL-101-ac
SiW11@MIL-101
SiW11@MIL-101-ac
PW11@MIL-101
slight type IV features [58]. This result is in line with the type
of porosity described for MIL-101(Cr) [59] and total adsorbed
amounts also agree with data previously published in the liter-
ature [16]. Materials show polymodal pore size distribution (see
Fig. S6 of the Supporting Information). The textural properties of
these materials, in particular the BET surface area, microporous
MIL-101
volume (V ) and total pore volume V
are listed in Table 1. The
␮
total
incorporation of PW11 and SiW11 into the MIL-101(Cr) framework
is apparent here, as it leads to reduction of 40% and 37% in sur-
face area, respectively (Table 1). Likewise, it also leads to the 38%
and 33% reduction in total pore volume, respectively. Elemental
analysis results indicate a POM loading of 20% and 21% for the
PW11@MIL-101 and SiW11@MIL-101 composites, respectively. As
previously described by Férey et al. we have assumed that the
POMs, due to their size, could only be present in the large cavities
of MIL-101(Cr). We have estimated the presence of approximately
4 monovacant Keggin-type units per cavity in both composites.
Pore Size Distribution (PSD) gets altered upon incorporation of
POMs (see Fig. S6 of the Supporting Information). In MIL-101(Cr)
the majority of pores are of 2.4 and 3.0 nm sizes and both sizes are in
comparable amounts. But after incorporation of POMs, either PW11
or SiW11, the PSD band for small pores is shifted to a broader PSD
between 2.4 and 2.7 nm, on the other hand, the PSD band for larger
pores get shifted around 3.3 nm. These observations indicate that
smaller pore (with pore window 1.2 nm) [59] could have difficulty
in sufficient loading of POMs units (size ∼1.0 nm) that is compa-
rable to pore window [60]. It also suggests that larger pores with
eventually larger pore window (1.6 nm) have received most of the
POMs loading selectively. Therefore, the broadening of smaller PSD
band is due to the loading of some larger pores with POM units that
has caused narrowing of pore diameter, from 3.3 nm to 2.7 nm. It is
believed that with successive loading of larger pores, some of the
nearest larger pores could get distorted due to strong electrostatic
interactions in the loaded pores. It is probable that such a distor-
tion may have altered the pore dimensions of these few pores, from
3.0 nm to 3.3 nm.
3
5
7
9
11
13
15
17
19
21
23
2 (o)
Fig. 4. Powder XRD patterns of the solid support MIL-101(Cr), and the PW11-based
composite material before (PW11@MIL-101) and after catalysis (PW11@MIL-101-ac).
also found in the PW11@MIL-101 spectra (ı = −10.65 ppm). This
indicates that the structure of the monovacant PW11 is retained
after its inclusion in MIL-101 cages as previously verified for other
composite materials of this monovacant POM [35,45,57].
The powder XRD patterns of the composite materials,
PW11@MIL-101 and SiW11@MIL-101, indicates that the crys-
talline structure of the porous MIL-101(Cr) is maintained after
the inclusion of the polyoxotungstates, as their main diffrac-
tion peaks remained practically unaffected (see Fig.
4 and
Fig. S3 of the Supporting Information, for the PW11 and SiW11
based materials, respectively). Furthermore, the crystalline phases
(packing arrangement) of PW11 and SiW11 are totally modified in
the course of their insertion into the solid support, since diffrac-
tion peaks of these polyoxotungstates are not noticeable in the
diffraction patterns of the respective composite material. This piece
of evidence confirms that both the PW11 and SiW11 complexes
present in the materials are in non-crystalline form, presumably as
consequence of their inclusion and dispersion in the solid support
cavities.
The maintenance of the crystalline structure of the solid sup-
port after the incorporation of PW11 or SiW11 was also confirmed
by electron microscopy (SEM and EDX). As depicted in Fig. 5,
the SEM images of PW11@MIL-101 (Fig. 5b) and SiW11@MIL-101
(Fig. 5d) composite materials reveal cubic micro-crystals with iden-
tical morphology (size and shape) of those of the solid support
MIL-101(Cr) (Fig. 5a), pointing to the preservation of the structure
of the MIL-101(Cr) in the composite materials. EDX spectra (not
shown) reveal the presence of P and W, or Si and W, in the respec-
tive materials PW11@MIL-101 and SiW11@MIL-101. Furthermore,
the EDX elemental mapping images show a uniform distribution
of P and W (from PW11), and Cr and O (from MIL-101(Cr)–see Fig.
S4 of the Supporting Information) confirming a homogenous dis-
persion of the PW11 in the MIL-101(Cr) framework, most probably
placed inside the cavities of the porous support material (Fig. 6a).
Identical results were observed in the EDX mapping analysis of the
other composite material, SiW11@MIL-101 (see Fig. S5 of the Sup-
porting Information).
3.2. Stability of monovacant POMs in the presence of H2O2
The study of the stability of monovacant TBAPW11 and
TBASiW11 in presence of excess of H2O2 and MeCN/H2O mix-
tures was performed by electronic absorption spectroscopy and
31P NMR (for TBAPW11). Keggin-type POMs show a characteristic
UV band attributed to the charge transfer band from the oxygen
to the tungsten. UV–vis and 31P NMR studies were performed for
the ratios [H2O2]/[XW11] of 300 (20 ␮mol of XW11 and 6 mmol
H2O2) and 1500 (20 ␮mol of XW11 and 30 mmol H2O2) mixtures
at room temperature. The [H2O2]/[XW11] = 300 corresponds to the
excess of oxidant to the catalyst used in this work for the oxida-
tion of cis-cyclooctene and the [H2O2]/[XW11] = 1500 is the ratio
used for the oxidation of geraniol and R-(+)-limonene. The study
by UV–vis spectroscopy suggests that the monovacant TBASiW11
seems to be stable or at least no appreciable decomposition was
noticed for [H2O2]/[SiW11] = 300 and 1500 since no significant vari-
ation of UV spectra (not shown) was observed (␭max = 258 nm).
Contrasting, it was noticed that TBAPW11 was not stable in the
The nitrogen adsorption–desorption isotherms of MIL-101(Cr),
and POMs incorporated MIL-101(Cr) samples are presented in
Fig. 7. The shape of these samples is predominantly type I, with

C.M. Granadeiro et al. / Applied Catalysis A: General 453 (2013) 316–326
321
Fig. 5. SEM images of (a) MIL-101(Cr), (b) PW11@MIL-101, (c) PW11@MIL-101-ac, (d) SiW11@MIL-101 and (e) SiW11@MIL-101-ac (ac = after catalysis).
Fig. 6. EDX elemental mapping images for the composite material PW11@MIL-101 and PW11@MIL-101-ac (ac = after catalysis).

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C.M. Granadeiro et al. / Applied Catalysis A: General 453 (2013) 316–326
35
30
25
20
15
10
5
MIL-101
SiW11@MIL-101
PW11@MIL-101
SiW11@MIL-101-ac
PW11@MIL-101-ac
Fig. 8. Conversion obtained for the oxidation of cis-cyclooctene (ratio [H2O2]/[cis-
cyclooctene] = 1, T = 75 ◦C), geraniol (ratio [H2O2]/[geraniol] = 4.5, room tempera-
ture) and R-(+)-limonene (ratio [H2O2]/[R-(+)-limonene] = 4.5, T = 75 ◦C) using the
PW11 (blue), SiW11 (green) and the composite materials PW11@MIL-101 (orange)
and SiW11@MIL-101 (dark red) as catalysts. The entire bars correspond to the con-
version after 6 h of reaction, while the darker part of each bar represents only the
conversion at 30 min. (For interpretation of the references to colour in this figure
legend, the reader is referred to the web version of this article.)
3.3. Catalytic studies
0
0.2
0.4
0.6
0.8
1
The oxidation of cis-cyclooctene (i), geraniol (iii) and R-(+)-
limonene (v) (Scheme 1) was carried out in homogeneous phase
using TBA salts of the monovacant polyoxotungstates (TBAPW11
and TBASiW11). In the heterogeneous phase the composite materi-
als PW11@MIL-101 and SiW11@MIL-101 were tested as catalysts, in
the presence of H2O2 and MeCN as solvent. The catalytic activity of
the support MIL-101(Cr) was investigated for the oxidation of the
three selected substrates and the conversion was negligible, even
after 24 h of reaction. Additionally, in the absence of catalyst using
similar conditions to that of the catalytic reactions no conversion
was verified.
The three distinct substrates were chosen to evaluate the cat-
alytic efficiency of the homogeneous monovacant POMs, as well
as their performance and robustness when incorporated in the
MIL-101(Cr) cavities, for the oxidation reactions in a system com-
bining H2O2-MeCN as oxidant-solvent. Considerable differences
were found for the conversion of the substrates and the distribu-
tion of oxidation products for the homogeneous and heterogeneous
systems. Fig. 8 shows the conversion data for the oxidation of the
selected substrates utilizing the homogeneous monovacant POMs
(PW11 and SiW11) and their composite materials (PW11@MIL-101
and SiW11@MIL-101) as catalysts.
Cis-cyclooctene. The oxidation of cis-cyclooctene (i, Scheme 1)
was studied using equivalent amounts of oxidant and sub-
strate. In the presence of all catalysts only 1,2-epoxycyclooctane
(ii, Scheme 1) was formed. Interestingly, the heterogeneous
catalysts PW11@MIL-101 and SiW11@MIL-101 revealed to be
more active than the corresponding homogeneous monovacant
polyanions (PW11 and SiW11). In fact, with the heterogeneous
catalyst PW11@MIL-101 the conversion of cis-cyclooctene into 1,2-
epoxycyclooctane was practically complete after 10 min of reaction
(yield of 98% and TOF of 1922 mol molPOM−1 h−1). Using an equiv-
alent amount of homogeneous catalyst PW11 the conversion of
cis-cyclooctene after 10 min was basically inexistent, however after
5 h of reaction the yield of 1,2-epoxycyclooctane attain 96%.
The phosphotungstate catalysts PW11 and PW11@MIL-101
seemed to be more active than the analogues silicotungstates. This
behaviour has been observed previously for other oxidative reac-
tions and may be consequence of the weaker interactions between
H2O2 and silicotungstates than with phosphotungstates [23]. In
the presence of SiW11@MIL-101 the almost complete epoxidation
was only achieved after 6 h (93% of yield) and in the presence of
p/p0
Fig. 7. The nitrogen adsorption–desorption isotherms of the studied materi-
als at −196 ◦C: MIL-101(Cr) (green), PW11@MIL-101 (blue), PW11@MIL-101-ac
(red), SiW11@MIL-101 (brown) and SiW11@MIL-101-ac (black) (ac = after cataly-
sis); unfilled and filled symbols represent the adsorption and desorption processes,
respectively. (For interpretation of the references to colour in this figure legend, the
reader is referred to the web version of this article.)
presence of H2O2, in both ratios [H2O2]/[PW11] of 300 and 1500. The
UV–vis spectra were collected at different times and was observed
the decrease of the absorbance for the band with ␭max = 255 nm
(see Fig. S7 of the Supporting Information). The decomposition of
PW11 was faster in the presence of higher excess of H2O2. Identical
study was also performed by 31P NMR for the same [H2O2]/[PW11
]
ratios (see Fig. S8 of the Supporting Information). A single peak
attributed to PW11 was observed at −11.8 ppm until 30 min after
the addition of H2O2 (see Supporting Information, Fig. S8a and
S8b). However, after 1 h the PW11 was completely decomposed
and other two peaks attributed to the products of decomposition
were found at 0.8 and 3.2 ppm (see Fig. S8c of the Supporting Infor-
mation). On the other hand, a similar experiment was performed
with H2O replacing H2O2 under identical condition (in particular
[H2O2]/[PW11] = 1500), however the decomposition of PW11 was
not observed.
The decomposition of the Keggin anion PW12 in several
peroxo-complexes in aqueous solution with H2O2 is well known
since the beginning of the 90 decade [61,62]. Some years later,
Griffith et al. reported the stability of PW11 in the presence
of H2O2 in aqueous solution. In this case also the monova-
cant POM split up to
a variety of peroxo-complexes, such
as [(PO4)(WO(O2)2)2]3−, [(PO4)(WO(O2)2)2(WO(O2)2(H2O)]3− and
[(PO3(OH))(WO(O2)2)2]2−, some previously observed from PW12
decomposition [63]. In the same study the peroxo-complexes
where characterized by 31P NMR, revealing chemical shift near
3.0 ppm. More recently, it was described that the PW11 decomposes
into Venturello complex [(PO4)(WO(O2)2)4]3− in the presence of
H2O2 under a biphasic catalytic system, and the peak characteris-
tic of this complex appears at 0.6 ppm [64]. In fact, this Venturello
complex is probably one of the products of PW11 decomposition we
observed in the presence of H2O2 excess and MeCN solution (see
Fig. S8c of the Supporting Information).

C.M. Granadeiro et al. / Applied Catalysis A: General 453 (2013) 316–326
323
Table 2
Data from the oxidation of geraniol after 1 h of reaction, for different catalysts
investigateda.
Catalyst
Conversion
(%)
TOFb
Selectivityc (%)
(mol molcat−1 h−1
)
No catalyst
PW11
PW11@MIL-101
SiW11
≈0
–
–
87
791
179
1000
875
100
100
100
100
30 (91)d
98
89
SiW11@MIL-101
a
Reaction conditions: 1 mmol of geraniol, 4.5 mmol H2O2, 3 ␮mol of POM or 42
or 46 mg of composite materials that contain 3 ␮mol of POM, room temperature,
light protection.
b
TOF = (moles of geraniol consumed)/[(moles of POM) × 0.08 h].
c
Based on the amount of consumed substrate, corresponding to 2,3-
epoxygeraniol.
d
Conversion data obtained after 6 h of reaction.
lower than its homogeneous PW11 [36]. However, when the
catalytic performance of the two different composites PW11@MIL-
101 and PW11@aptesSBA-15 is compared it is noticed that the
PW11 immobilized into MIL-101(Cr) is more active than the same
immobilized onto aptesSBA-15. After 4 h of reaction the yield
of 2,3-epoxygeraniol was 81% when the reaction was catalyzed
by PW11@MIL-101, instead of 50% obtained in the presence of
PW11@aptesSBA-15 [36]. The catalytic results obtained with the
monovacant MIL-101(Cr) composites were also compared with
other heterogeneous catalytic systems for the oxidation of geran-
iol using H2O2 as oxidant [67–71]. Higher conversion data were
obtained in shorter reaction time and equivalent or even higher
selectivity was achieved in the presence of PW11@MIL-101 and
SiW11@MIL-101 catalysts.
R-(+)-limonene. The oxidation of R-(+)-limonene (v, Scheme 1)
was also investigated in the presence of selected catalysts. The
kinetic profile of PW11 and PW11@MIL-101 is very similar and
the oxidation of the substrate was almost complete after 3 h
(Fig. 10). The turnover frequency obtained in the presence of the
homogeneous PW11 and the heterogeneous PW11@MIL-101 was
869 and 828 mol molPOM−1h−1, respectively. Identical amounts of
limonene-1,2-epoxide (vi, Scheme 1) and limonene-1,2-diol (viii,
Scheme 1) were obtained in the oxidation of R-(+)-limonene in the
presence of the PW11 homogeneous and respective heterogeneous
catalysts. However, small amounts of diepoxide (vii, Scheme 1)
were also formed. The SiW11@MIL-101 composite material also
showed considerable activity in the oxidation of this monoter-
pene, since the conversion achieved after 3 h of reaction was
Fig. 9. Conversion data obtained for the oxidation of geraniol (ratio
[H2O2]/[geraniol] = 4.5, room temperature) using the PW11 (blue), SiW11 (green)
in homogeneous phase, and the composite materials PW11@MIL-101 (red) and
SiW11@MIL-101 (dark red) in heterogeneous phase.
the homogeneous SiW11 catalyst only 53% of conversion of cis-
cyclooctene was attained. These results show that the inclusion
of the active centres PW11 and SiW11 seems to improve the cat-
alytic performance of these catalysts. This phenomenon may be
caused by concentration effects promoted by adsorption of reac-
tants in the cages of MIL-101(Cr). In fact, the mechanism proposed
in the literature for alkenes epoxidation involves the formation of
a hydroperoxo species (POM–OOH) as active intermediate, formed
by the interaction of POM and H2O2 [65]. The epoxide is then
formed by transferring an oxygen atom from the cleaved O–O bond
of the POM–OOH species. The effect of the porous cage where cata-
lysts, oxidant and substrate are confined probably promotes a faster
formation of the hydroperoxide active species and consequently
the higher production of epoxide in shorter reaction time, when
compared with the homogeneous system.
Geraniol. The oxidation of geraniol (iii, Scheme 1) can occur in
different possible groups of this allylic alcohol, such as in the two
double bonds, in the allylic carbon centres and in the carbon of
the CH2OH group. The oxidation of this substrate was performed
at room temperature with light protection and using an excess of
oxidant H2O2 relatively to the substrate. In the presence of both the
homogeneous and heterogeneous catalysts only 2,3-epoxygeraniol
(iv, Scheme 1) was obtained as product, demonstrating the high
chemio-selectivity nature of the studied systems. Fig. 9 depicts
the kinetic profile for the oxidation of geraniol catalyzed by the
homogeneous and heterogeneous monovacant catalysts. The sili-
cotungstates SiW11 and SiW11@MIL-101 showed to be more active
than the respective phosphotungstates PW11 and PW11@MIL-101.
The kinetic profile of SiW11 and SiW11@MIL-101 is similar and
the conversion of geraniol into 2,3-epoxydegeraniol was almost
total after 1 h of reaction. The catalytic activity of PW11@MIL-101
is considerably lower than the corresponding homogeneous cata-
lyst PW11. The turnover frequency calculated after 5 min of reaction
for PW11@MIL-101 is lower than the homogeneous PW11 and the
silicotungstates SiW11 and SiW11@MIL-101 (Table 2). The oxida-
tion of this allylic alcohol involves coordination of the alcohol to
the POM and the formation of a peroxo group adjacent to this site
[66]. Thus, the essential coordination of the substrate to the cata-
lyst occurred differently for PW11 and SiW11, and this fact may be
the main reason for the difference of activity observed.
100
80
60
40
20
0
PW11
PW11
MIL@PW11
PW11@MIL-101
SiW11
SiW11
MIL@SiW11
SiW11@MIL-101
0
1
2
3
4
5
6
Time (hour)
In a recent investigation, our group studied the catalytic
performance of the monovacant PW11 immobilized onto func-
tionalized SBA-15 support (PW11@aptesSBA-15) for the oxidation
of geraniol. The catalytic activity of the material also was much
Fig. 10. Conversion attained for the oxidation of R-(+)-limonene (ratio [H2O2]/[R-
(+)-limonene] = 4.5, T = 75 ◦C) using PW11 (blue), SiW11 (green) as homogeneous
catalysts and the composite materials PW11@MIL-101 (red) and SiW11@MIL-101
(dark red) as heterogeneous catalysts.

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C.M. Granadeiro et al. / Applied Catalysis A: General 453 (2013) 316–326
active species (PW11 or SiW11) of the catalyst should occur during
the first reaction cycle.
To investigate the level of heterogeneity of the heterogeneous
catalysts, leaching tests were performed to analyze the presence
of active catalytic species in solution. Thus, the solid catalyst was
separated from the reaction mixture by careful filtration after
certain time and the reaction was further continued with the
remaining filtrate. This experiment was performed for the oxida-
tion of cis-cyclooctene using SiW11@MIL-101, and for the oxidation
of geraniol using PW11@MIL-101. In the leaching test of SiW11
based material for the oxidation of cis-cyclooctene, the solid cat-
alyst was removed after 10 min of reaction. After this point the
conversion of the substrate into its epoxide practically stopped,
and even after 24 h of reaction the conversion only increased 15%.
The results of the leaching analysis of PW11@MIL-101 for geran-
iol oxidation were similar. The catalyst was removed after 1 h of
reaction, and the conversion practically remained unaltered (even
after 24 h the conversion only increased 17%). These results indicate
that the materials are in fact heterogeneous and the loss of active
monovacant species to the solutions is not significant.
To evaluate the possible loss of the POM anions from the com-
posite materials to the solution, the W analysis of the recovered
materials after the three consecutive reaction cycles was per-
formed. From the initial loading of SiW11 in SiW11@MIL-101 only
10% was leached after three consecutive cycles (0.065 mmol of
SiW11 was found). For PW11@MIL-101, the leaching of PW11 species
was higher, and 17% of the initial loading was lost for the solu-
tion during the reaction cycles (0.048 mmol of PW11 was found in
PW11@MIL-101).
Fig. 11. Conversion data obtained for three consecutive cycles for the oxida-
tion of cis-cyclooctene (ratio [H2O2]/[cis-cyclooctene] = 1, T = 75 ◦C) and geraniol
(ratio [H2O2]/[geraniol] = 4.5, at room temperature) using the composite materi-
als PW11@MIL-101 and SiW11@MIL-101: (a) PW11@MIL-101/cis-cyclooctene after
30 min, (b) PW11@MIL-101/geraniol after 6 h, (c) SiW11@MIL-101/cis-cyclooctene
after 6 h and (d) SiW11@MIL-101/geraniol after 30 min.
approximately complete (Fig. 10). On the other hand, the homoge-
neous SiW11 showed to be slightly less active than its composite
SiW11@MIL-101. As described previously, this fact can be asso-
ciated with the confinement effect produced by the MIL-101(Cr)
cavities.
The literature reports only two examples of heterogeneous cat-
alysts based in Keggin type POMs evaluated in the oxidation of
limonene. One corresponds to a silicotungstate immobilized onto
surface-modified SiO2, which yield 78% of limonene-8,9-epoxide
after 24 h of reaction [30]. The other example, was reported by
our group, and corresponds to the monovacant PW11 immobilized
onto aptesSBA-15 [36]. In this case, the activity was lower than the
corresponding homogeneous PW11 and was necessary 24 h of reac-
tion to approximate of the complete conversion. Therefore, we can
conclude that the heterogeneous catalyst PW11@MIL-101 is more
active than the related PW11@aptesSBA-15 material. Superior con-
version data for shorter reaction time is also found comparing the
catalytic performance of the monovacant MIL-101(Cr) composites
with other silica or titanium heterogeneous catalysts using H2O2
as oxidant [72–74]. However, some of these studies present higher
selectivity data for the epoxide of limonene [72,74].
3.5. Catalyst materials stability and H2O2 efficiency
The structural stability of composite materials PW11@MIL-101
and SiW11@MIL-101 used as heterogeneous catalysts was investi-
gated by several methods. In practice, after the three catalytic cycles
the materials PW11@MIL-101-ac and SiW11@MIL-101-ac (where
ac stands for after catalysis) were recovered and investigated by
powder XRD, FT-IR and FT-Raman spectroscopies, 31P HPDEC NMR
spectroscopy (only for the PW11 based material), SEM, EDX and
textural analysis.
The powder XRD patterns of the two heterogeneous cata-
lysts performed in samples used in catalytic reaction are identical
to the respective diffractogram before catalytic use, confirming
the integrity of the crystalline structure of the solid support
MIL-101(Cr) after various catalytic reaction cycles (see Fig. 4 for
PW11@MIL-101 and Fig. S3 of the Supporting Information for
SiW11@MIL-101). The maintenance of the structure was further
recognized by electronic microscopy, SEM and EDX. As depicted in
Fig. 5, the morphology of the PW11@MIL-101-ac and SiW11@MIL-
101-ac is similar to that of the respective samples before catalysis.
Additionally, the EDX spectra (not shown) and the EDX mapping
confirm the existence of P and W, or Si and W in the composite
materials after catalysis, and consequently points to the preser-
vation of polyoxotungstates in the composite materials even after
several subsequent catalytic cycles (see Fig. 6 and Fig. S5 of the Sup-
porting Information for the PW11 and SiW11 based materials,
respectively).
The similarity of catalytic activity of homogeneous and het-
erogeneous promoted by the MIL-101(Cr) support should be
associated with the morphology of its large cages, probably allow-
ing some mobility to the POM inside the cavities.
3.4. Recyclability and leaching
The recyclability of the solid catalysts was studied for the oxida-
tion reactions of cis-cyclooctene and geraniol. The catalysts were
recovered from the reaction mixture by centrifugation, washed
several times with MeCN and dried at room temperature to be
used in a fresh reaction under identical experimental conditions
(using the same ratio of catalyst: substrate: oxidant: solvent).
Fig. 11 shows the reusability of the composites for the oxidation
of both substrates with hydrogen peroxide. In general, the activity
of PW11@MIL-101 and SiW11@MIL-101 in the oxidation of cis-
cyclooctene and geraniol slightly decreases from the first to the
second reaction cycle during the first hours of reaction. However
after 24 h of reaction the conversion data is the same in all reaction
cycles corresponding to the complete conversion of substrate. From
the second to the third reaction cycle the activity of the catalysts
is retained. This behaviour may indicate that some leaching of the
The retention of the polyoxotungstates in the MIL-101(Cr)
framework after catalysis was also verified by FT-IR and FT-Raman
spectroscopic studies. The comparison with the vibrational spectra
of the as-synthesized materials (Figs. 1 and 2 for the PW11 based
material, and Fig. S2 of the Supporting Information for the SiW11
based material) doesn’t reveal significant changes on the typical
bands of the MOF as well as in the observable vibrational modes of
the POMs.

C.M. Granadeiro et al. / Applied Catalysis A: General 453 (2013) 316–326
325
The nitrogen adsorption–desorption isotherms of PW11@MIL-
101-ac and SiW11@MIL-101-ac are shown in Fig. 7. After catalysis
the total pore volume of PW11@MIL-101 gets reduced by ca. 21%,
however this reduction is only ca. 8% for SiW11@MIL-101. Com-
MIL-101(Cr) showed to have a relevant influence in the PW11
stability in the presence of H2O2. In the perspective view of achiev-
ing novel high performance catalytic systems, the composites
PW11@MIL-101 and SiW11@MIL-101 were tested for the oxidation
of cis-cyclooctene, geraniol and R-(+)-limonene, using H2O2 as oxi-
dant. These composites showed to be active, robust and selective
heterogeneous catalysts. PW11@MIL-101 proved to be more effi-
cient catalyst than the homogeneous PW11 for the epoxidation of
cis-cyclooctene into 1,2-epoxycyclooctane (complete conversion
after the first 10 min of reaction). Similar activity between homo-
geneous monovacant species and the composites were found for
geraniol and R-(+)-limonene oxidation. From the first reaction a
complete conversion into 2,3-epoxygeraniol was achieved after 1 h
of reaction in the presence of SiW11@MIL-101. From the R-(+)-
limonene oxidation, complete conversion was enhanced after 3 h
in the presence of PW11@MIL-101. The integrity and robustness of
both composites materials, PW11@MIL-101 and SiW11@MIL-101,
were confirmed after catalytic uses by several techniques. Follow-
ing the interesting results of the present work, we are currently
investigating the use of other porous MOFs and distinct POMs to
prepare novel catalysts for oxidation reactions.
paring the relative decrease in Vtotal and V after catalysis, it
␮
shows that these decreases are primarily due to decrease in V , it
␮
points towards possibility of blocking in some of the loaded pores
during catalysis. The PSD again gets changed, after catalysis, and
returns to its previous stage, see Fig. S6 of the Supporting Informa-
tion. It indicates that the loaded pores of PW11@MIL-101-ac and
SiW11@MIL-101-ac might be partially stuffed/exhausted and are
therefore inaccessible for N2 adsorption. As a result, the PSD in this
case is evolved mainly from non-loaded pores that remained as
such after loading procedure.
The 31P HPDEC MAS NMR spectrum of the composite
PW11@MIL-101 was performed after catalytic use (Fig. 3). Two
peaks are observed at −15.72 and −13.51 ppm. To investigate the
species corresponding to these peaks, the 31P HPDEC MAS NMR
spectrum of the composite PW12@MIL-101 was performed and a
single peak was found at -15.92 ppm (Fig. 3). Thus, one of the two
peaks observed for the spectrum of PW11@MIL-101 may indicate
a small decomposition of PW11 into PW12 inside of MIL-101(Cr)
cage. The other peak may be attributed to PW11 with a satura-
tion in its lacunary region with other species (such as the oxidant
and/or solvent), originating a shift from -10.65 to -13.51 ppm, as
observed before in the literature for PW11 immobilized into sil-
ica nanoparticles [35,57]. However, the transformation of PW11
into peroxo compounds when excess of H2O2 is present was not
observed when the composite PW11@MIL-101 was exposed to the
same excess of H2O2. These results suggest that the MIL-101(Cr)
can confer some stability to the PW11 structure without changing
its catalytic activity.
The combination of the evidences from the wide range of
characterization methods used to characterize the materials after
catalytic use (after three reaction cycles), PW11@MIL-101-ac and
SiW11@MIL-101-ac, strongly suggests that the materials investi-
gated are reasonable robust and stable heterogeneous catalysts.
The efficiency of usage of H2O2 was determined for the oxidation
of different studied substrates. For the oxidation of cis-cyclooctene
and geraniol the efficiency of oxidant was similar in the presence of
PW11@MIL-101 and SiW11@MIL-101 catalysts. For both substrates
100% of selectivity for the corresponding epoxide was obtained and
an efficiency of H2O2 near 100% and 95% was found for the oxidation
of cis-cyclooctene and geraniol, respectively. The efficiency of H2O2
decreases for the oxidation of R-(+)-limonene when three different
products (epoxide, diol and diepoxide) were obtained. In this case,
the efficiency of H2O2 was higher in the presence of PW11@MIL-101
(43%) than in the presence of SiW11@MIL-101 (60%), even though
the product spectrum was similar.
Acknowledgments
The authors acknowledge the Fundac¸ ão para a Ciência e a
Tecnologia (FCT, MEC, Portugal) for their general financial sup-
port through the strategic projects Pest-C/EQB/LA0006/2011 (to
Associated Laboratory REQUIMTE), PEst-OE/QUI/UI0612/2011
(to CQB) and PEst-C/CTM/LA0011/2011 (to the Associated Lab-
oratory CICECO), the R&D projects PTDC/CTM/100357/2008
and
PTDC/EQU-EQU/121677/2010,
and
the
fellowships
SFRH/BPD/73191/2010 (to CG) SFRH/BPD/77216/2011 (to VKS)
and SFRH/BD/46601/2008 (to PS).
Appendix A. Supplementary data
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/j.apcata.
2012.12.039.
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