Multifunctional catalyst based on sandwich-type polyoxotungstate and MIL-101 for liquid phase oxidations

Article abstract of DOI:10.1016/j.cattod.2012.12.003

A sandwich-type polyoxometalate {(TBA)₇H₃[Co ₄(H₂O)₂(PW₉O₃₄) ₂]} was immobilized for the first time into the three-dimensional porous metal-organic framework, MIL-101. The composite material Co ₄(PW₉)₂@MIL-101 was prepared and the incorporation of the Co₄(PW₉)₂ was confirmed using a myriad of solid state methods (FT-IR, FT-Raman, powder XRD, SEM-EDX and ICP analysis). Co₄(PW₉)₂@MIL-101 reveals to be an active, selective and recyclable catalyst for the oxidation of different substrates: geraniol, R-(+)-limonene, styrene and cyclooctane, using H ₂O₂ as eco-sustainable oxidant. The stability and robustness of the heterogeneous catalyst was confirmed by different techniques. The MIL-101 framework demonstrates to be a suitable and efficient support to accommodate catalytically active sandwich-type polyoxotungstates.

Full text of DOI:10.1016/j.cattod.2012.12.003

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Catalysis Today
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Multifunctional catalyst based on sandwich-type polyoxotungstate and MIL-101
for liquid phase oxidations
Salete S. Balulaa,∗, Carlos M. Granadeiroa, André D.S. Barbosaa, Isabel C.M.S. Santosb, Luís Cunha-Silvaa,∗
a REQUIMTE & Departamento de Química e Bioquímica, Faculdade de Ciências, Universidade do Porto, 4169-007 Porto, Portugal
b Departamento de Química, QOPNA, Universidade de Aveiro, 3810-193 Aveiro, Portugal
a r t i c l e i n f o
a b s t r a c t
Article history:
A sandwich-type polyoxometalate {(TBA)7H3[Co4(H2O)2(PW9O34)2]} was immobilized for the first
time into the three-dimensional porous metal-organic framework, MIL-101. The composite material
Co4(PW9)2@MIL-101 was prepared and the incorporation of the Co4(PW9)2 was confirmed using a myriad
of solid state methods (FT-IR, FT-Raman, powder XRD, SEM-EDX and ICP analysis). Co4(PW9)2@MIL-101
reveals to be an active, selective and recyclable catalyst for the oxidation of different substrates: geran-
iol, R-(+)-limonene, styrene and cyclooctane, using H2O2 as eco-sustainable oxidant. The stability and
robustness of the heterogeneous catalyst was confirmed by different techniques. The MIL-101 framework
demonstrates to be a suitable and efficient support to accommodate catalytically active sandwich-type
polyoxotungstates.
Received 3 September 2012
Received in revised form 4 December 2012
Accepted 5 December 2012
Available online xxx
Keywords:
Sandwich-type polyoxometalate
Oxidation
Hydrogen peroxide
Metal-organic framework
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
studied using a variety of solid supports to immobilize POMs via
dative, covalent or electrostatic binding [28–35]. Tri-dimensional
Polyoxometalates (POMs) with sandwich structure, B-␣-
[M4(H2O)2(PW9O34)2]10−, M = transition metal, are characterized
by the clustering of transition-metal (M) cations, which make
them interesting in different fields such as catalysis and elec-
trocatalysis, magnetochemistry and medicine [1–3]. These anions
(3D) metal-organic frameworks (MOFs) are potential materials
for the immobilization of POMs, since some of these compounds
have porous structures with large, regular and accessible cages
and tunnels, which can act as reactors since they can accommo-
date catalytically active molecules with adequate shape and size
to enter the pores. However, heterogeneous catalysis using MOFs
is a relatively recent subject that needs to be further explored.
Recently, the 3D porous MOF material MIL-101 has shown to
have important capacities for catalyst production [36–39]. How-
ever, MIL-101 was only used a few years ago and the first studies
using these composites as heterogeneous catalysts were per-
formed very recently. To the best of our knowledge, Kholdeeva and
co-workers [31,40–42] were the pioneers in the use POMs@MIL-
101 as catalysts for oxidative reactions with O2 and H2O2 as
oxidants.
are frequently obtained in aqueous solution by reaction of appro-
9−
priate amounts of the tri-lacunary Keggin anion B-␣-[PW9O34
]
and transition metal ions [4,5]. The molecular structure of this
type of anions was presented for the first time by Weakley
et al. [6] for the Co analog. Untill our first work with these
types of POMs [7], only a small number of papers reported their
potential application as catalysts for the oxidation of hydrocar-
bons, in particular, the iron and manganese sandwich complexes
[8–11]. More recently, these types of complexes have been
used for water oxidation, mainly the cobalt sandwich complex
[12–16].
POMs have been widely studied as effective homogeneous cat-
alysts in the oxidation of several hydrocarbons [17–27]. In the
last years considerable effort has been focused in the immobi-
lization of POMs due to the necessity of recovering and recycling
these active catalytic compounds. Different approaches have been
Keggin-type POMs are extensively studied in homogeneous and
heterogeneous catalysis for hydrocarbons oxidation, but for the
sandwich-type POMs, with the formula [M4(H2O)2(PW9O34)2]10−
,
namely with M = Co(II), only few reports can be found in the lit-
erature [7,43,44]. The immobilization of this catalytically active
polyoxotungstates was performed for the first time by Yun and
Pinnavaia [45] in 1996 using LDH as support material; how-
ever its catalytic performance was not investigated. Recently,
[Co4(H2O)2(PW9O34)2]10− was immobilized onto amine-modified
SBA-15 and this composite demonstrated to be catalytically active
for oxidation of some monoterpenes [46].
∗
Corresponding author. 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).
0920-5861/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.cattod.2012.12.003
Please cite this article in press as: S.S. Balula, et al., Multifunctional catalyst based on sandwich-type polyoxotungstate and MIL-101 for liquid
phase oxidations, Catal. Today (2013), http://dx.doi.org/10.1016/j.cattod.2012.12.003

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Following our recent work in the preparation and study of
composite materials based in POMs@MIL-101 as potential hetero-
geneous catalysts [47–49], we report a novel heterogeneous cata-
lyst prepared by the immobilization of [Co4(H2O)2(PW9O34)2]10−
into a metal-organic framework containing large cavities (MIL-
101). The multifunctional catalytic capacity of this composite
was evaluated for the oxidation of different hydrocarbons (geran-
iol, R-(+)-limonene, styrene and cyclooctane), using H2O2 as
the oxygen donor as a consequence of its benefits (inexpen-
sive, environmentally clean and easily handled). The stability
and the recyclability of the composite were also carefully
investigated.
2. Experimental
2.1. Materials and methods
All the reagents used in the preparation of the support mate-
rial {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%)}, as well as the geraniol
(Aldrich), R-(+)-limonene (Aldrich), styrene (Aldrich), cyclooctane
(Aldrich), acetonitrile (Panreac), H2O2 (Aldrich, 30%), were used as
received without further purification.
Elemental analysis for W, Co and P was performed by ICP spec-
trometry (University of Santiago de Compostela, CACTUS-LUGO).
Infrared absorption spectra were obtained on a Mattson 7000
FT-IR 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 and the
laser power set to 200 mW. Scanning electron microscopy (SEM)
and energy-dispersive X-ray spectroscopy (EDX) studies were per-
formed at “Centro de Materiais da Universidade do Porto” (CEMUP,
Porto, Portugal), using a high resolution environmental scanning
electron microscope (FEI Quanta 400 FEG ESEM) equipped with an
energy-dispersive X-ray spectrometer (EDAX Genesis X4M). The
samples were analyzed as powders coated with a thin gold film.
Powder X-ray diffraction (XRD) 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, in the range ca. 3.5◦ ≤ 2ꢀ ≤ 50◦.
GC–MS analysis was performed using a Hewlett Packard 5890
chromatograph equipped with a mass selective detector (MSD)
series II using helium as the carrier gas (35 cm−1); GC-FID was
carried out in a Varian Star 3400CX chromatograph to monitor
homogeneous reactions and a Varian CP-3380 to follow the het-
erogeneous reactions. In both experiments, the hydrogen was the
carrier gas (55 cm−1) and fused silica Supelco capillary columns
SPB-5 (30 m × 0.25 mm i.d.; 25 ␮m film thickness) were used. For
cyclooctane experiments the GC-FID analyses were performed
on a Varian 3900 chromatograph using helium as the carrier gas
(35 cm−1) and Supelco capillary columns SPB-5.
Scheme 1.
2.2.2. Solid support MIL-101
The porous metal-organic framework (MOF) material MIL-101
was prepared by an adaptation of the original method described
by Férey et al. [50]. A mixture containing chromium(III) nitrate
(2 mmol), benzene-1,4-dicarboxylic acid (2 mmol) and hydroflu-
oric acid (100 ␮L) in 10 mL of H2O was stirred at room temperature
to obtain an homogeneous suspension, transferred to an autoclave
and heated at 493 K for 9 h in an electric oven. After a slowly cool-
ing process (inside the oven), the material was isolated by filtration
and purified through a double DMF treatment, followed by 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.
2.2.3. Composite material Co4(PW9)2@MIL-101
The composite material Co4(PW9)2@MIL-101 was prepared
through the immobilization of the TBA salt of Co4(PW9)2 in the
porous solid support MIL-101 using a modified procedure of the
method described by Maksimchuk et al. [41]. Briefly, an acetonitrile
solution of Co4(PW9)2 (10 mM; 50 mL) was added to the MIL-
101 (0.5 g) and stirred at room temperature for 24 h. The solid
was filtrated, washed thoroughly with acetonitrile and dried in a
dessicator over silica gel. Co4(PW9)2@MIL-101. Anal. found (%): W,
38. Loading of Co4(PW9)2: 0.11 mmol per 1 g. Selected FT-Raman
(cm−1): 3078, 1611, 1460, 1144, 972, 872, 632; selected FT-IR
(cm−1): 3436, 1618, 1509, 1393, 1156, 1058, 1018, 958, 888, 826,
747, 660, 583.
2.2. Preparation of catalysts
2.3. Oxidation reactions
2.2.1. Sandwich-type polyoxotungstate
The sandwich-type polyoxotungstate [(TBA)7H3[Co4(H2O)2
The oxidation reactions of geraniol (i, Scheme 1), R-(+)-limonene
(iii, Scheme 1), styrene (vii, Scheme 1) and cyclooctane (xi,
Scheme 1) were carried out in acetonitrile, under air (atmo-
spheric pressure) in a closed borosilicate 10 mL reaction vessel
(PW9O34)2] (Co4(PW9)2, TBA = (C4H9)4N)] was prepared by
a
known procedure, and characterized by elemental and thermal
analysis, FT-IR and FT-Raman spectroscopy, electronic absorption
spectroscopy and powder XRD, confirming the synthesis of the
desired compound.
equipped with
a magnetic stirrer and immersed in a ther-
mostated oil bath. The oxidant used was H2O2 (30 wt.%) and the
Please cite this article in press as: S.S. Balula, et al., Multifunctional catalyst based on sandwich-type polyoxotungstate and MIL-101 for liquid
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composite Co4(PW9)2@MIL-101 was used as heterogeneous cat-
alyst. The sandwich-type Co4(PW9)2 was tested as homogeneous
catalyst for the oxidation of styrene. In the oxidation of geraniol
the reaction was performed at room temperature and protected
from light, and the oxidative reactions of R-(+)-limonene, styrene
and cyclooctane were performed at 80 ◦C. In a typical experiment,
the substrate (1 mmol) and the catalyst (3 ␮mol for R-(+)-limonene
and styrene and 1.5 ␮mol for cyclooctane), placed in the vessel,
were dissolved in acetonitrile (1.5 mL), stirred, and the hydrogen
peroxide was added to the reaction mixture: 500 ␮L (4.5 mmol) for
the oxidation of geraniol, R-(+)-limonene and styrene and 205 ␮L
(2 mmol) for the oxidation of cyclooctane. The reactions were fol-
lowed 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. An aliquot was taken
directly from the reaction mixture with a microsyringe at regular
intervals, diluted in acetonitrile and injected into the GC or GC–MS
equipment for analysis of starting materials and products. The reac-
tion products reported here were identified by GC–MS analysis.
The same procedure was performed for the heterogeneous reac-
tions using the supported polyoxotungstate Co4(PW9)2@MIL-101.
In this case, the heterogeneous catalysts were filtered off at the
end of reactions, washed several times with acetonitrile, dried in
air at room temperature overnight and then reused using the same
reaction conditions.
Co (PW ) @MIL-101-ac
4 9 2
Co
(
PW
)
@MIL-101
MIL-101
4
9 2
Co (PW )
4 9 2
1800 1600 1400 1200 1000 800 600 400
Wavenumber (cm-1)
Fig. 1. FT-Raman spectra of Co4(PW9)2, MIL-101, Co4(PW9)2@MIL-101 and
Co4(PW9)2@MIL-101-ac in the wavenumber region between 400 and 1800 cm−1
(ac stands for after catalysis).
3. Results and discussion
3.1. Catalysts preparation and characterization
small structural arrangements in the MIL-101 due to the inclusion
of Co4(PW9)2 in their pores. Additionally, the crystalline structures
of the sandwich polyoxotungstate Co4(PW9)2 are completely mod-
ified in the course of their inclusion in the solid support, since the
diffraction pattern of the composite material does not reveal any
diffraction peak of the POM. This piece of evidence confirms that the
Co4(PW9)2 present in the composite material are in non-crystalline
The composite material Co4(PW9)2@MIL-101 was prepared by
the reaction of TBA salt of the sandwich-type polyoxotungstate
Co4(PW9)2 with the 3D MOF MIL-101 in acetonitrile solution. The
incorporation of the Co4(PW9)2 in the MIL-101 framework to orig-
inate the composite material was obtained after 24 h of reaction.
Afterwards, the solid was recovered by centrifugation and thor-
oughly washed with acetonitrile. Several characterization methods
were employed, such as powder XRD, vibrational (FT-IR and
FT-Raman) spectroscopy, elemental analysis (ICP), scanning elec-
tron microscopy (SEM) and energy-dispersive X-ray spectroscopy
(EDX), to assure the immobilization of Co4(PW9)2@MIL-101.
The existence of the sandwich polyoxotungstates Co4(PW9)2 in
the MIL-101 framework was primarily recognized by FT-Raman
and FT-IR spectra of the composite material in comparison with
that of the isolated compounds (Figs. 1 and 2). The FT-Raman
spectrum of the composite material reveals the typical bands of
the porous support material MIL-101 and some vibrational modes
of the Co4(PW9)2. In particular, the band located at 982 cm−1
assigned to W O symmetrical stretching mode is clearly evident
in the spectrum of the composite material Co4(PW9)2@MIL-
101 (Fig. 1) [29,49]. Furthermore, the FT-IR spectrum of the
Co4(PW9)2@MIL-101 material also supports the incorporation of
the polyoxotungstate in the MOF framework. Besides the character-
istic bands of the MIL-101, the spectrum of the composite material
exhibits bands located at 956, 888 and 826 cm−1 assigned to the
terminal ꢁas(W Ot), corner-sharing ꢁas(W Ob) and edge-sharing
ꢁas(W Oc), respectively [51,52].
Co (PW ) @MIL-101-ac
4 9 2
Co
(
PW
)
@MIL-101
MIL-101
4
9 2
Co (PW )
4 9 2
The comparison of the powder XRD patterns of the poly-
oxotungstate Co4(PW9)2, the porous support MIL-101 and the
composite material Co4(PW9)2@MIL-101 indicates that the main
features of crystalline structure of the porous MIL-101 are practi-
cally maintained after the incorporation of the polyoxotungstates,
as their main diffraction peaks remained nearly unaffected (Fig. 3).
The increase of the intensity of the peaks around 5◦ (2ꢀ) observable
in the difractogram of the composite material may be related with
1800 1600 1400 1200 1000 800 600 400
Wavenumber (cm-1)
Fig. 2. FT-IR spectra of Co4(PW9)2, MIL-101, Co4(PW9)2@MIL-101 and
Co4(PW9)2@MIL-101-ac in the wavenumber region between 400 and 1800 cm−1
(ac stands for after catalysis).
Please cite this article in press as: S.S. Balula, et al., Multifunctional catalyst based on sandwich-type polyoxotungstate and MIL-101 for liquid
phase oxidations, Catal. Today (2013), http://dx.doi.org/10.1016/j.cattod.2012.12.003

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Co (PW ) ]@MIL-101-ac
4 9 2
Co4(PW9)2]@MIL-101
MIL-101
Fig. 5. Conversion data obtained after 24 h of oxidation of various substrates
with H2O2 as oxidant, acetonitrile as solvent and using the Co4(PW9)2 (blue),
Co4(PW9)2@MIL-101 (red) and Co4(PW9)2@aptesSBA-15 [49] (green) as catalysts.
(For interpretation of the references to color in this figure legend, the reader is
referred to the web version of the article.)
Co4(PW9)2
4
7
10
13
16
19
22
25
2θ
(o)
heterogeneous and the corresponding homogeneous catalyst (TBA
salt of Co4(PW9)2) was also compared. For all catalytic studies H2O2
was used as oxidant and acetonitrile as solvent. The catalytic activ-
ity of the support MIL-101 was investigated for the oxidation of
all substrates and the conversion was negligible, even after 24 h of
reaction. In the absence of catalyst using similar conditions to that
of the catalytic reactions no conversion was verified.
Fig. 3. Powder X-ray diffraction patterns of Co4(PW9)2, MIL-101, Co4(PW9)2@MIL-
101 and Co4(PW9)2@MIL-101-ac in the 2ꢀ angle range from 4 to 25◦ (ac stands for
after catalysis).
form, apparently as a consequence of their encapsulation in the
cavities of the porous MIL-101.
The composite Co4(PW9)2@MIL-101 showed to have a versa-
tile activity for the oxidation of different families of substrates
such as allylic alcohols (geraniol, R-(+)-limonene), alkenylben-
zene (styrene) and alkane (cyclooctane). The oxidation of geraniol,
R-(+)-limonene and cyclooctane catalyzed by the homogeneous
Co4(PW9)2 was recently published [7,46]. Furthermore, the immo-
bilization of this compound recently performed by our research
group using amine-functionalized SBA-15 (Co4(PW9)2@aptesSBA-
15) and its catalytic performance was also investigated for
geraniol and R-(+)-limonene oxidation using the same experi-
mental conditions [46]. Fig. 5 presents the conversion data of
the selected substrates obtained by the homogeneous Co4(PW9)2
and by both heterogeneous catalysts (Co4(PW9)2@MIL-101 and
Co4(PW9)2@aptesSBA-15). After 24 h of reaction, a practically com-
plete conversion of all studied substrates was attained in the
presence of Co4(PW9)2 and Co4(PW9)2@MIL-101. This heteroge-
neous catalyst achieves near 100% of conversion of all studied
substrates, except for cyclooctane, which may be due to the lower
catalyst amount used for this reaction. In opposite, the com-
posite Co4(PW9)2@aptesSBA-15 previously studied shows to be
less active under the same catalytic conditions. The kinetic pro-
file detail between these two heterogeneous catalysts is depicted
The preservation of the main characteristics of crystalline struc-
ture of the solid support MIL-101 after the incorporation of
Co4(PW9)2 was also confirmed by electron microscopy (SEM and
EDX). As depicted in Fig. 4, the SEM images of Co4(PW9)2@MIL-
101 composite materials reveal cubic micro-crystals with identical
morphology (size and shape) of those of the solid support
MIL-101, pointing to the maintenance of the structure of the
MIL-101 in the composite material. Furthermore, EDX spectrum
of Co4(PW9)2@MIL-101 (Fig. S1 in electronic supplementary
information, ESI) reveals the presence of Co, P and W elements and
thus unequivocally confirms the presence of the polyoxotungstate
in the composite material.
3.2. Catalytic studies
The oxidation of substrates (geraniol, R-(+)-limonene, styrene
and cyclooctane) was carried out using the heterogeneous cata-
lyst formed by the encapsulation of tetrabutylammonium (TBA)
salt of the sandwich-type polyoxotungstate Co4(PW9)2 into the
three-dimensional structure of MIL-101 originating the com-
posite Co4(PW9)2@MIL-101. The catalytic performance of the
Fig. 4. SEM images of MIL-101 (left), Co4(PW9)2@MIL-101 (center) and Co4(PW9)2@MIL-101-ac (right; ac stands for after catalysis).
Please cite this article in press as: S.S. Balula, et al., Multifunctional catalyst based on sandwich-type polyoxotungstate and MIL-101 for liquid
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100
100
80
60
40
20
0
80
60
Co4(PW9)2@MIL-101
Co4(PW9)2 in CH3CN
Co4(PW9)2 in BMIPF6
40
20
0
Co4(PW9)2
Co4(PW9)2@MIL-101
0
4
8
12
16
20
24
0
4
8
12
Time (h)
16
20
24
Time (h)
Fig. 6. Kinetic profile for the oxidation of geraniol using Co4(PW9)2 as catalyst [49]
and CH3CN as solvent (red), Co4(PW9)2 as catalyst and BMIPF6 as solvent (green) and
Co4(PW9)2@MIL-101 as catalyst and CH3CN as solvent (blue). (For interpretation of
the references to color in this figure legend, the reader is referred to the web version
of the article.)
Fig. 7. Kinetic profile for the oxidation of styrene using Co4(PW9)2 (red) and
Co4(PW9)2@MIL-101 (blue) as catalysts, CH3CN as solvent and H2O2 as oxidant. (For
interpretation of the references to color in this figure legend, the reader is referred
to the web version of the article.)
the first 6 h of reaction, when 98% was achieved for the homoge-
neous and 90% for the Co4(PW9)2@MIL-101 (Fig. S2 in ESI). On the
other hand, the selectivity was similar between these catalytic sys-
tems and practically equal amounts of 1,2-epoxide and 1,2-diol
were obtained; however, small amounts of diepoxide were also
found.
in Fig. 6 and Fig. S2 in ESI where it can be observed that the
Co4(PW9)2@MIL-101 is a catalyst more active for both allylic alco-
hols than the Co4(PW9)2@aptesSBA-15 even from the first minutes
of reaction. The superior activity observed for Co4(PW9)2@MIL-
101 can be due to some mobility that the polyoxotungstates could
have inside the MIL-101 large cavities. In opposite, in the compos-
ite Co4(PW9)2@aptesSBA-15 the polyoxotungstates should present
some interaction (electrostatic or covalent) with the support that
can involve the active center of the catalyst (probably the two
cobalt metals with the terminal water ligand) decreasing its cat-
alytic activity.
The oxidation of styrene (vii, Scheme 1) catalyzed by Co4(PW9)2
and Co4(PW9)2@MIL-101 yielded benzaldehyde (viii) as the main
product and the hydroxy ketone (ix) as secondary product. In the
presence of the heterogeneous Co4(PW9)2@MIL-101 benzoic acid
(x) is also formed (Table S1 in ESI). Fig. 7 depicts the kinetic profiles
for homogeneous and heterogeneous systems. The heterogeneous
catalyst showed to be more active than the homogeneous. But this
difference of activity is more pronounced after 4 h when the reac-
tion practically stopped when catalyzed by Co4(PW9)2 (64 and 70%
of conversion are obtained after 4 and 24 h, respectively; while
for Co4(PW9)2@MIL-101 85% was achieved after 4 h). This can be
due to the faster degradation of oxidant in the homogeneous cat-
alytic system what causes the absence of oxidant for the evolution
of the reaction. In fact, the mechanism of styrene oxidation into
benzaldehyde with H2O2 catalyzed by polyoxotungstates is well
described in the literature, occurring initially in the formation of
hydroperoxo or bridging peroxo species by the interaction of the
polyoxotungstates with H2O2. Afterwards, styrene is bonded with
one of the metal-peroxo bonds to produce a peroxometallocycle
and styrene oxide is formed [54–57]. A further nucleophilic attack
of H2O2 on the styrene oxide originates benzaldehyde [54,56]. Few
studies had been performed using Keggin-type polyoxotungstates
as catalysts and the green oxidant H2O2 for the oxidation of styrene,
however better catalytic results were found in our work when we
increased the substrate/oxidant ratio to 1:4.5, and the volume of
solvent was decreased to 1.5 mL and a lower amount of catalyst
was used [24,54,57,58].
The oxidation of cyclooctane (xi) was performed for the first
time using a MOF based catalyst. The Co4(PW9)2 had been
already tested before as homogeneous catalyst for the oxida-
tion of this substrate with H2O2 [7]. The comparison of kinetic
performance between the homogeneous Co4(PW9)2 and the het-
erogeneous Co4(PW9)2@MIL-101 catalysts are presented in Table 1.
The conversion data after 24 h of reaction are lower in the
presence of Co4(PW9)2@MIL-101, but on the other hand, higher
selectivity is achieved using the heterogeneous system. Three
different products were obtained from the oxidation of this
alkane when catalyzed by Co4(PW9)2@MIL-101: cyclooctanone
(xii) was always the main product, while cyclooctanol (xiii) and
cyclooctyl hydroperoxide (xiv) were formed in low amounts. The
The selectivity obtained with Co4(PW9)2@MIL-101 and
Co4(PW9)2@aptesSBA-15 was similar for geraniol oxidation,
but was slightly lower for R-(+)-limonene oxidation when the
first catalyst was used. In fact, geraniol (i) is an allylic alcohol that
offers several possible places of oxidative attack, namely at the
two double bonds, at the allylic carbon centers and at the carbon
of the CH2OH group. The oxidation of this substrate with H2O2
at room temperature produces only the 2,3-epoxygeraniol (ii in
Scheme 1) in the presence of the homogeneous Co4(PW9)2 and the
heterogeneous Co4(PW9)2@MIL-101 and Co4(PW9)2@aptesSBA-
15 catalysts, indicating the high chemo-selectivity nature of the
reactions. Comparing the kinetic profile of the homogeneous
Co4(PW9)2 and heterogeneous Co4(PW9)2@MIL-101 both in ace-
tonitrile (Fig. 6), it is possible to observe a complete conversion
of geraniol after 4 h of reaction for both catalysts. In fact, some
difference of activity between the homogeneous and heteroge-
neous system is only observed during the first 4 h when higher
conversion data are obtained using the homogeneous Co4(PW9)2
in acetonitrile. When this solvent was replaced by an ionic liquid
(1-butyl-3-methylimidazolium hexafluorophosphate, BMIPF6) the
activity decreased considerably although near full conversion was
observed after 24 h (Fig. 6). The lower activity observed using
BMIPF6 as solvent may be caused by the higher viscosity of this
ionic solvent and lower diffusion of the reaction components. The
literature refers that the mechanism for the epoxidation of geraniol
requires the coordination of the alcohol to the polyoxotungstate
and the formation of a peroxo group adjacent to this site [53].
The oxidation of R-(+)-limonene (iii, Scheme 1) using
Co4(PW9)2@MIL-101 as catalyst was less selective and three prod-
ucts were obtained: 1,2-epoxide (iv), diepoxide (v) and 1,2-diol
(vi). During the first 30 min of reaction the 1,2-epoxide is the only
product formed but after this time the 1,2-diol is already the main
product. The conversion data in the presence of Co4(PW9)2@MIL-
101 are slightly lower than the corresponding homogeneous during
Please cite this article in press as: S.S. Balula, et al., Multifunctional catalyst based on sandwich-type polyoxotungstate and MIL-101 for liquid
phase oxidations, Catal. Today (2013), http://dx.doi.org/10.1016/j.cattod.2012.12.003

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6
S.S. Balula et al. / Catalysis Today xxx (2013) xxx–xxx
Table 1
To investigate the stability of catalyst after reaction cycles,
the Co4(PW9)2@MIL-101 was recovered after three catalytic
cycles from R-(+)-limonene oxidation (sample identified as
Co4(PW9)2@MIL-101-ac, where ac stands for after catalysis) and
characterized by different techniques. The powder XRD patterns of
the Co4(PW9)2@MIL-101-ac is identical to the diffractogram before
catalysis use, confirming the integrity of the crystalline structure
of the solid support MIL-101 (Fig. 3). The robustness of the support
material was also investigated by SEM. Fig. 4 reveals that the par-
ticles’ morphology of the heterogeneous catalyst before and after
catalytic is identical. Furthermore, EDX spectrum (Fig. S1 in ESI)
confirms the existence of Co and W elements after catalysis and
thus pointing to the presence of the polyoxotungstate in the com-
posite material after their catalytic utilization. On the other hand,
the identification of the polyoxotungstate Co4(PW9)2 inside the
MIL-101 structure after catalysis was performed by FT-IR (Fig. 2)
and FT-Raman spectroscopy (Fig. 1). In fact, the comparison of the
spectra before and after catalytic use does not reveal significant
changes on the typical bands of the MOF as well as in the observ-
able vibrational modes of the Co4(PW9)2. In conclusion, the results
obtained from these different techniques indicate that the support
material is robust and stable after oxidative reactions and also no
noticeable decomposition of active phosphotungstate occurred.
Data for the oxidation of cyclooctanea in the presence of homogeneous [7] and
heterogeneous catalysts.
Catalyst
Time (h)
Conv. (%)
Selectb (%)
xiii
xii
xiv
6
24
6
24
6
24
40
100
55
57
61
84
61
63
56
87
67
76
25
33
16
13
18
10
14
4
28
0
5
14
Co4(PW9)2
Co4(PW9)2@MIL-101
1st cycle
Co4(PW9)2@MIL-101
2nd cycle
a
Reaction conditions: 1 mmol of cyclooctane, 2 mmol of H2O2, 1.5 ␮mol of
Co4(PW9)2 or amount of composite containing 1.5 ␮mol of Co4(PW9)2, 1.5 mL CH3CN
and 80 ◦C.
b
Based on the amount of consumed substrate.
selectivity for the main product is considerable increased from
the 6 to 24 h of reaction using Co4(PW9)2@MIL-101 (Table 1).
Recently, the oxidation of cyclooctane was studied using cobalt
Keggin-type polyoxotungstates immobilized onto MCM-41, how-
ever much lower catalytic activity was found [59]. Considerations
about the mechanism of cyclooctane oxidation were already
reported in the literature, where different concurrent reactions
could take place, mainly hydroperoxidation and hydroxylation of
the substrate and possible dismutation of H2O2 [21,60].
4. Conclusions
The recyclability of Co4(PW9)2@MIL-101 was performed for the
oxidation of different substrates, using different experimental con-
ditions, in order to test their stability and activity in consecutive
reaction cycles. The solid catalyst was recovered at the end of each
reaction by simple filtration followed by washing with acetoni-
trile, dried at room temperature and then reused in a fresh reaction
under identical experimental conditions. Fig. 8 shows the reusabil-
ity of Co4(PW9)2@MIL-101 for three consecutive reaction cycles
after 6 h of reaction. It can be verified that the activity of this cat-
alyst is practically maintained during the consecutive cycles. For
geraniol oxidation the conversion decreases 15% from the first to
the second cycle; however, the same is retained from the second
to the third cycle. In opposite, for R-(+)-limonene and cyclooctane
oxidation a slight increase of conversion was noted from the first to
the second catalytic cycle. The changes in the selectivity between
consecutive cycles is not relevant. To evaluate the possible loss of
polyoxotungstate from the heterogeneous catalyst to the solution,
the W analysis of the recovered materials after the three consecu-
tive reaction cycles was performed. Before catalytic use 0.11 mmol
of Co4(PW9)2 was present per gram of composite and after reaction
cycles 0.091 mmol were found. This indicates that from the initial
loading only 17% was leached after consecutive cycles.
A novel hybrid organic–inorganic heterogeneous catalyst was
prepared. This composite represents the first immobilization per-
formed with a sandwich-type polyoxometalate (Co4(PW9)2) inside
of porous 3D framework of MIL-101. The multifunctionality of
Co4(PW9)2@MIL-101 was demonstrated by its high active per-
formance for the oxidation of various hydrocarbons: geraniol,
R-(+)-limonene, styrene and cyclooctane, in the presence of H2O2.
In general, the activity of the cobalt sandwich polyoxotungstate
catalyst for reaction times less than 24 h increased in the follow-
ing order: geraniol > limonene > styrene > cyclooctane. For 24 h of
reaction, the conversion was practically total, with exception of
cyclooctane oxidation. For this last substrate, the near reaction
completion was only achieved for the second reaction cycle. From
geraniol oxidation, 1,2-epoxygeraniol was the only product formed
with 100% of conversion after 4 h of reaction. For the oxidation
of R-(+)-limonene, 1,2-epoxide is the first product obtained but
after a while the 1,2-diol is the main product and diepoxide is
also produced. Benzaldehyde was the main product obtained from
styrene oxidation and hydroxy ketone and benzoic acid were the
by-products. This reaction was also investigated with the homo-
geneous catalyst (TBA salt of Co4(PW9)2) and higher conversion
data were obtained using the heterogeneous system. The compar-
ison between homogeneous and heterogeneous systems was also
analyzed for the other substrates. The incorporation of Co4(PW9)2
into MIL-101 structure seems not to affect strongly its activity
and at the same time makes possible the recyclability of the cat-
alyst without significant loss of activity for at least three reaction
cycles. The integrity and robustness of the Co4(PW9)2@MIL-101
was confirmed after catalytic uses. Taking into account the inter-
esting results of the present work, other distinct MOFs frameworks
and active sandwich-type polyoxometalates will be prepared and
investigated for oxidation of valuable substrates.
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-C/QUI/UI0062/2011 (to
Fig. 8. Conversion data obtained for three consecutive cycles for the oxidation of
geraniol, R-(+)-limonene and cyclooctane catalyzed by Co4(PW9)2@MIL-101, after
6 h of reaction, using H2O2 as oxidant and CH3CN as solvent.
Please cite this article in press as: S.S. Balula, et al., Multifunctional catalyst based on sandwich-type polyoxotungstate and MIL-101 for liquid
phase oxidations, Catal. Today (2013), http://dx.doi.org/10.1016/j.cattod.2012.12.003

GModel
CATTOD-8313; No. of Pages7
ARTICLE IN PRESS
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7
Research Unit QOPNA), the R&D projects PTDC/CTM/100357/2008
and PTDC/EQUEQU/121677/2010, and the fellowship
SFRH/BPD/73191/2010 (to CG).
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A.M.V. Cavaleiro, Catalysis Communications 12 (2011) 459.
[30] J. Kasai, Y. Nakagawa, S. Uchida, K. Yamaguchi, N. Mizuno, Chemistry – A Euro-
pean Journal 12 (2006) 4176.
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.cattod.
2012.12.003.
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Please cite this article in press as: S.S. Balula, et al., Multifunctional catalyst based on sandwich-type polyoxotungstate and MIL-101 for liquid
phase oxidations, Catal. Today (2013), http://dx.doi.org/10.1016/j.cattod.2012.12.003