Sandwich type polyoxometalates encapsulated into the mesoporous material: Synthesis, characterization and catalytic application in the selective oxidation of sulfides

Article abstract of DOI:10.1039/c8ra03659d

The A-type sandwich polyoxometalates of [(HOSn^IVOH)₃(PW₉O₃₄)₂]^12- (P₂W₁₈Sn₃) and [(OCe^IVO)₃(PW₉O₃₄)₂]^12- (P₂W₁₈Ce₃) were immobilized for the first time into the porous metal-organic framework MIL-101(Cr). FT-IR, powder X-ray diffraction, SEM-EDX, ICP analysis, N₂ adsorption and thermogravimetric analysis collectively confirmed immobilization and good distribution of polyoxometalates into cages of MIL-101(Cr). The catalytic activities of the homogeneous P₂W₁₈Sn₃ and P₂W₁₈Ce₃ and the corresponding heterogeneous catalysts were examined in the oxidation of sulfides to sulfones with H₂O₂ as the oxidant at room temperature. The effects of different dosages of polyoxometalates, type of solvent, reaction time, amount of catalyst and oxidant in this catalytic system were investigated. The new P₂W₁₈Sn₃@MIL-101 and P₂W₁₈Ce₃@MIL-101 nanocomposites exhibited good recyclability and reusability in at least five consecutive reaction cycles without significant loss of activity or selectivity.

Full text of DOI:10.1039/c8ra03659d

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Sandwich type polyoxometalates encapsulated into
the mesoporous material: synthesis,
Cite this: RSC Adv., 2018, 8, 28249
characterization and catalytic application in the
selective oxidation of sulfides
*
Elham Naseri and Roushan Khoshnavazi
The
A-type
sandwich
polyoxometalates
of
[(HOSnIVOH)3(PW9O34)2]12ꢀ
(P2W18Sn3)
and
[(OCeIVO)3(PW9O34)2]12ꢀ (P2W18Ce3) were immobilized for the first time into the porous metal–organic
framework MIL-101(Cr). FT-IR, powder X-ray diffraction, SEM-EDX, ICP analysis, N2 adsorption and
thermogravimetric analysis collectively confirmed immobilization and good distribution of
polyoxometalates into cages of MIL-101(Cr). The catalytic activities of the homogeneous P2W18Sn3 and
P2W18Ce3 and the corresponding heterogeneous catalysts were examined in the oxidation of sulfides to
sulfones with H2O2 as the oxidant at room temperature. The effects of different dosages of
polyoxometalates, type of solvent, reaction time, amount of catalyst and oxidant in this catalytic system
were investigated. The new P2W18Sn3@MIL-101 and P2W18Ce3@MIL-101 nanocomposites exhibited good
recyclability and reusability in at least five consecutive reaction cycles without significant loss of activity
or selectivity.
Received 28th April 2018
Accepted 30th July 2018
DOI: 10.1039/c8ra03659d
rsc.li/rsc-advances
decomposition of guest-free MOFs.16–20 MIL-101 family of
materials (MIL: Materials of the Institute Lavoisier) are a very
1. Introduction
stable group of MOFs, result from the three-dimensional cova-
lent connection of inorganic clusters and organic linkers. Its
open-pore structure with the pores (ꢁ3.5 nm) and pore windows
(ꢁ1.5 nm) are large enough to give access to voluminous reac-
tant molecules diffusing into the pores. These properties and
also able to be functionalized, accessible cages and very high
specic surface area, make it an excellent candidate to support
catalytic species.21–25
Polyoxometalates (POMs) as a class of metal cluster complexes
comprising transition metal oxide anions attract attention in
light of their signicant potential in medicine, structural
chemistry, analytical chemistry, surface science, electrochem-
istry and photochemistry.1–3 Their utility for catalysis applica-
tions has been limited, notably due to their tendency toward low
surface area (typically # 10 m2 gꢀ1) and porosity (lower than 0.1
cm3 gꢀ1) of bulk POMs which hinder the accessibility of the
active sites, together with a high solubility in polar solvents
making it inconvenient for recovery and reutilize.4–6 In this
sense, the heterogenization or immobilization of POMs onto
various solid supports via dative,7 covalent,8 or electrostatic9–11
binding, such as silica, activated carbon, magnetic nano-
particles and titanium dioxide is highly desired to prepare new
and robust heterogeneous catalysts, capable of being easily
separated from a reaction mixture and recycled.12–14 Recently,
metal–organic frameworks (MOFs) materials were also used for
immobilization of POMs.15 Because of their structural features,
MOFs relative to other porous matrices play an important role
in the development of different catalysts, including those for
enantioselective chiral reactions, asymmetric epoxidation of
alkenes and allyl alcohols, oxidation of alcohols and in
a synthesis of porous carbon materials through thermal
´
In 2005, Ferey et al. reported incorporation of the lacunary
7ꢀ
polytungstate of PW11O39
within the large cage of MIL-
101(Cr).26 Following work by Ferey et al., numerous POM@MIL-
101 composite materials containing various catalytically active
polyoxometalates has been widely studied, and the resulting
composites have been applied to H2O2-based alkene epoxida-
tion, knoevenagel condensation, aldehyde–alcohol reactions,
oxidative desulfurization, aerobic so on.27–30 Recently, a range of
hybrid materials based on various Keggin polyoxometalates
building blocks and copper organic frameworks with pyrazine
derivatives have been discussed by Haddadi et al. and used as
a heterogeneous catalyst for the selective oxidation of suldes to
corresponding sulfoxides and sulfones using H2O2.31 A-type
sandwich polyoxometalate of K11H[(HOSnIVOH)3(PW9O34)2]$
20H2O was immobilized for the rst onto Nd-doped TiO2
nanoparticles as support material and these new nano-
composites used for the oxidation of different suldes and
alcohols.32 Herein we report the synthesis and application of
´
Department of Chemistry, University of Kurdistan, PO. Box 66135-416, Sanandaj,
Iran. E-mail: r.khoshnavazi@uok.ac.ir; Fax: +98 87336224133; Tel: +98 87336224133
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Scheme 1 The synthesis pathway for preparation of nanocomposites.
new POM@MIL-101 composites materials containing A-type (CH3CN, Merck) and hydrogen peroxide (H2O2, Merck 35%)
sandwich polyoxometalates in selective oxidation of suldes. were also used as received. The A-type sandwich poly-
To the best of our knowledge, this is the rst report about the oxometalates of
K
11H[(HOSnIVOH)3(PW9O34)2]$20H2O and
immobilization of various potassium salts of [(HOSnIVOH)3(XW9- K9(NH4)H2[(OCeIVO)3(PW9O34)2]$20H2O were prepared accord-
O34)2]12ꢀ (X ¼ P and Si), [(OCe)3(XW9O34)2]12ꢀ (X¼ P) and sodium ing to the previously described procedures.33,34
salts [WM3(H2O)2(XW9O34)2]12ꢀ (X ¼ M ¼ Zn and Co) into a MIL-
101 metal–organic framework resulting in a series of POM@MOF
composite materials in selective oxidation of suldes. The struc-
ture of P2W18Sn3 and P2W18Ce3 consists of two A-type PW9O34
anions linked by three tin(IV) or cerium(IV) cations leading to A-type
sandwich POMs (Scheme 1).
2.2. Characterization methods
The synthesized MOF and nanocomposites were characterized
using; Infrared absorption spectra were recorded on a Bruker
Vector 22 infrared spectrometer using KBr pellet method.
Thermogravimetric analysis (TGA) was carried out under N2
ow while gradually increasing the temperature with a rate of
10 ꢂC minꢀ1, using a STA PT-1000 LINSEIS. The morphology of
nanocomposites was revealed by a scanning electron micro-
scope (MIRA3 FEG-SEM). The elements in the nanocomposite
2. Experimental
2.1. Materials
All chemicals were used as received from different commercial samples were probed with energy-dispersive X-ray (EDX) spec-
sources. All the reagents used in the preparation of the troscopy accessory to the MIRA3 FEG-SEM scanning electron
composite materials, namely analytically pure chromium(III) microscopy. Nitrogen adsorption–desorption isotherms used to
nitrate nonahydrate (Cr(NO3)3$9H2O, Aldrich), benzene-1,4- obtain the MIL-101(Cr) and POM@MIL 101(Cr) specic surface
dicarboxylic acid (C6H4(CO2H)2, Aldrich), uorhydric acid (HF, areas and pore volumes, calculated by the Brunauer–Emmett–
Merck) and DMF (N,N-dimethylformamide, Merck) were Teller (BET) method. All as-prepared samples were degassed at
purchased and used without further purication. The chem- 100 ꢂC in vacuum for overnight prior to adsorption measure-
ꢂ
icals used for desulfurization experiments, namely diphenyl ments. The experiments took place at ꢀ196 C under variable
sulde, methyl phenyl sulde, dimethyl sulde, 2-(methylthio) relative pressure. The equipment was a Belsorp mini II
ethanol, methyl 3-(methylthio)propionate, 2-furfuryl methyl adsorption analyzer (Bel-Japan) at ꢀ196 ꢂC. Powder X-ray
sulde, allyl sulde, dibutyl sulde (Fluka, 95%), acetonitrile diffraction (XRD) analyses were collected at ambient
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temperature by a X'PertPro Panalytical, Holland diffractometer support MIL-101 according to the preparation procedure of
˚
using a CuKa radiation (l ¼ 1.5418 A). The accelerating voltage P2W18Sn3@MIL-101 composite unless, P2W18Sn3 was replaced
and applied currents were 40 kV and 30 mA, respectively. The C by P2W18Ce3.
microanalyses was carried out with CHNS–O Elemental
Analyzer Vario EL III, ELEMENTARY, Hanau-Germany while the
amount of W and Cr were measured by inductively coupled
A mixture of bulk polyoxometalates (P2W18Sn3 or P2W18Ce3) or
plasma mass spectrometry (ICP-MS).
2.4. General test for the oxidation
composites (P2W18Sn3@MIL-101 or P2W18Ce3@MIL-101) (50
mg) as catalyst, 35% H2O2 aqueous solution (5.85 mmol) and
2.3. Preparation of the materials
solvent (2.5 mL) were placed in a 25 mL glass bottle. Aer 5 min,
the substrate (1 mmol) was added under stirring. The reaction
time was counted aer the addition of sulde, and then the
reaction mixture was stirred at the experiment temperatures for
the appropriate time. The sample was collected from the
mixture at time intervals and then the progress of the reaction
was followed by TLC (eluent: n-hexane/EtOAc, 3 : 1) and stopped
when a complete conversion of the substrate was observed. The
catalyst was ltered off at the end of reactions, washed several
times with ethyl acetate followed by ethanol (4 ꢃ 5 mL), heated
2.3.1. Solid support MIL-101(Cr). The porous metal–
organic framework (MOF) material MIL-101(Cr) was prepared
hydrothermally, using a modied procedure of the method
described by Ferey et al.26 Benzene-1,4-dicarboxylic acid
´
(H2BDC) (0.332 g, 2 mmol) was added in small portions to an
aqueous solution of Cr(NO3)3$9H2O (0.8 g, 2 mmol in 10 mL of
deionized water) and stirred at room temperature to obtain an
homogeneous suspension for 20 min, then uorhydric acid (0.1
mL) was added to the above suspension and stirred for 30 min.
The dark blue-colored suspension was placed in a Teon-lined
autoclave (125 mL,ꢂPaar model 4748), for 9 h at a heating
temperature of 220 C without stirring. Aer slowly cooling to
ambient temperature (inside the oven), the green powder was
collected by repeated centrifugation and thorough washing with
deionized water. Since the product was a mixture of MIL-101
powder and few amounts of needle-like crystals of recrystal-
lized H2BDC. It was washed overnight with DMF under reux
against the removal of recrystallized H2BDC. Aerwards, to
remove the DMF solvent, the resulting solids were washed with
deionized water several times. The product was dried in an air
oven at 70 ꢂC overnight followed by soxhlet extraction in ethanol
for 24 h. Samples were activated under vacuum 3 days at 70 ꢂC
with a high yield 83.8% based on chromium.
ꢂ
in an oven at 70 C overnight and then reused using the same
reaction conditions. The starting material and product are
insoluble in water and it was used just as an environment for
stirring. Therefore, the reaction mixture was transferred to
a separating funnel and the product was extracted with CH2Cl2
(3 ꢃ 5 mL). Aer evaporation of organic layer, the crude prod-
ucts were recrystallized from hot ethanol and the pure products
were obtained in 94–98% yield. Stability test of the P2W18-
Sn3@MIL-101 and P2W18Ce3@MIL-101 catalysts were carried
out running six consecutive experiments.
3. Results and discussion
3.1. Synthesis
2.3.2. P2W18Sn3@MIL-101 composite. The composite The MIL-101(Cr) is built up from a corner-sharing of so-called
material P2W18Sn3@MIL-101 was prepared through the immo- super tetrahedron (hereaer noted ST), which is formed by
bilization of the potassium salt of P2W18Sn3 in the porous solid rigid chromium(III) octahedral trimers (the vertices of the ST)
support MIL-101 using a modied procedure of the method and terephthalate anions (the edges of the ST) (see Scheme 1).
described by Balula et al.35 In the “impregnation” method,
The connection between of the corners of the ST building
0.0733 mmol of dry MIL-101 synthesized in an autoclave as blocks ensures a 3D cubic zeotype structure with two types of
˚
described above was added to an aqueous solution of mesoporous cages (B ¼ 29 and 34 A) (extended MTN topology),
˚
a moderate amount of P2W18Sn3 (0.25 mM, 0.5 mM, 1 mM, accessible through microporous windows (B ¼ 12 and 16 A).
˚
˚
2 mM, 4 mM; 15 mL) and the mixture was stirred at room The large cages possess both 12 A pentagonal and 16 A hexag-
˚
temperature for 24 h. The solid separated by centrifugation and onal windows with internal free diameters of 34 A. Therefore,
then washed several times thoroughly with deionized water and these cavities are sufficiently spaced to accommodate large
dried in an air oven at 60 ꢂC overnight followed by activated guest molecules (such as the successful encapsulation of the
ꢂ
under vacuum 3 days at 70 C. Elemental analysis (w/w %): C, sandwich-type anions, P2W18Sn3 or P2W18Ce3 (ꢁ10.4 ꢃ 10.4 ꢃ
3
˚
33.5; Cr, 8.18; W, 21.52. Based on the elemental analysis results 15.2 A )), into cages of MIL-101(Cr). The MIL-101(Cr) has been
and molecular weight of
K
11H[(HOSnIVOH)3(PW9O34)2]$ employed as a useful and versatile solid support for preparation
20H2O, W content, 57.98%; MW, 5708, we estimated the of heterogeneous catalysts because of its open-pore structure
P2W18Sn3 content of the P2W18Sn3@MIL-101 materials to be with large and accessible cages.26 The incorporation of POMs in
approximately 65 mmol gꢀ1 of dry powder, or 37 w/w %. The the mesoporous MIL-101(Cr) has been carried out by the
content of P2W18Sn3 was calculated according to the formula, anionic exchange between the counter-ions of the MIL-101(Cr)
mmol gꢀ1 ¼ 1 ꢃ 106 (W content in the P2W18Sn3@MIL-101, (nitrate ions coming from the Cr(NO3)3 precursor) and the
%)/(MW of P2W18Sn3 ꢃ W content in the P2W18Sn3%).28
negatively charged P2W18Sn3 or P2W18Ce3.36 For rst time, MIL-
2.3.3. P2W18Ce3@MIL-101 composite. The composite 101(Cr) with Cr3F(H2O)2O[(O2C)–C6H4–(CO2)]3$nH2O formula
material P2W18Ce3@MIL-101 was prepared through the immo- was synthesized by the HF–Cr(NO3)3–H2BDC–H2O system.26 The
bilization of the potassium salt of P2W18Ce3 in the porous solid composites of the sandwich-type POM of P2W18Sn3 and
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Fig. 1 FT-IR spectra of (A) P2W18Sn3@MIL-101 composite (a) the precursors of MIL 101 (b) and P2W18Sn3 (c) and (B) P2W18Ce3@MIL-101
composite (a) the precursors of MIL 101 (b) and P2W18Ce3 (c).
P2W18Ce3 with MIL-101 were prepared by impregnation of the due to the departure of chemically bonded water and organic
synthesized MIL-101 in an autoclave as described above in solvent (EtOH/DMF) molecules.30 The third step occurs in the
aqueous solution of P2W18Sn3 or P2W18Ce3.
range 272–800 ꢂC with loss of about 52.71% relates to the
departure of OH/F groups and the decomposition of the
framework.
3.2. Characterization of the material
The residual solid is Cr2O3 for MIL-101. Although, the TGA
prole of the composite P2W18Sn3@MIL-101 in the rst event,
3.2.1. FT-IR spectroscopy. The FT-IR spectra of composite
materials and those of the precursor compounds of poly-
oxometalates and the MIL-101 were compared in Fig. 1. The FT-
IR spectra of composites (Fig. 1A(a) and B(a)) exhibit the char-
acteristic bands of both the MIL-101 support (Fig. 1A(b) and
B(b)) and sandwich polyoxometalates P2W18Sn3 or P2W18Ce3
(Fig. 1A(c) and B(c)). The new bands at 1093, 956, 891, 792 cmꢀ1
or 1062, 1018, 948, 784 cmꢀ1 (shown with dashed lines)
ꢂ
occurs in the range 30–310 C (2.27%), may be assigned to the
removal of water molecules present render in the structure
electrostatically neutral or the guest molecules inside the pores.
A larger loss of ca. 36.91% is due to relates to the departure of
OH/F groups and the decomposition of the framework. The
minor weight loss of 2.97% in the temperature range 600–
800 ꢂC in TGA prole of the composite P2W18Sn3@MIL-101 can
attribute to the loss of oxygen atoms from the residual metal
oxides resulted from decomposition of P2W18Sn3. The residual
mixture solid is Cr2O3–WO3–P2O5–SnO2–K2O (55.80%).40 Also,
the total weight loss of 42.15% for P2W18Sn3@MIL-101 is lower
observed in the spectra of P2W18Sn3@MIL-101 and P2W18
-
Ce3@MIL-101 compared to the that of MIL-101 were attributed
to the P–O and W–O vibrations of the sandwich poly-
oxometalates P2W18Sn3 (ref. 32) or P2W18Ce3,37 respectively. The
bands in MIL-101, P2W18Sn3@MIL-101 and P2W18Ce3@MIL-101
at 1400 cmꢀ1 and 1552 cmꢀ1 are correspond to (O–C–O)
symmetric vibrations implying the presence of dicarboxylate
within the framework, broad and strong bands at around
3430 cmꢀ1 and 1620 cmꢀ1 also conrms the presence of
adsorbed water molecules stretching and bending vibrations or
the guest molecules inside the pores and the other weak bands
in the spectral region of 600–1600 cmꢀ1 are attributed to
benzene, including the stretching vibration C]C groups
(1514 cmꢀ1), in plane and out–of plane bending modes of COO
groups (400–700 cmꢀ1), and the out–of plane deformation
vibrations of benzene ring C–H groups at 1164, 1020, 889, and
748 cmꢀ1
.
38,39
3.2.2. Thermogravimetric analysis (TGA). The TGA curves
for the neat MIL-101 (a) and the composites of P2W18Ce3@MIL-
101 (b) and P2W18Sn3@MIL-101 (c) under an inert atmosphere
at a constant rate of 10 ꢂC minꢀ1 are shown in Fig. 2. A thermal
gravimetric analysis study on the pristine MIL-101 showed
continuous weight loss in the range of 30–800 ꢂC. Two main
weight-loss steps were observed below 400 ꢂC: the rst (5.83%),
relates to the removal of guest water molecules in the small
Fig. 2 TGA curves of MIL-101 (a), P2W18Ce3@MIL-101 (b) and P2W18
-
cages, occurs in the range 30–120 ꢂC; a larger loss of ca. 6.51% is Sn3@MIL-101 (c).
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Fig. 3 SEM images of MIL-101 (a) and P2W18Ce3@MIL-101 (b).
than that for the parent MIL-101 (65.05%), as would be expected
from the presence additional residual from partial decomposi-
tion of P2W18Sn3 in the cages of composite. The TGA curve of
the P2W18Ce3@MIL-101 composite shows a two-stage weight
3.2.4. N2 adsorption. Chromium terephthalates contained
signicant amounts of free guest molecules (solvent or other
chemicals used during the synthesis for example; terephthalic
acid or DMF) within the pores. To evacuate the free molecules
from the MOF with compromising its structural integrity and
hence porosity, two effective activation steps were performed.
The exchange of the high-boiling point solvent, (e.g., DMF), used
for purication, by a lower boiling point solvent (e.g., EtOH) fol-
lowed by simple heat and vacuum treatment.42 DMF molecules
were completely removed from the MIL-101 aer solvent
ꢂ
loss below 600 C, such as the P2W18Sn3@MIL-101 composite,
and indicates a lower weight loss than MIL-101. From TGA
curves can be recognized that polyoxotungstate anions improve
the thermal stability of the neat MIL-101.41
3.2.3. SEM and EDX. The SEM image of the MIL-101 was
displayed frequently octahedral with some hexagonal MOF rods
(Fig. 3a). As depicted in Fig. 3b, the SEM image the P2W18-
Ce3@MIL-101 composite material appears like an aggregation
state of nano crystallites with the similar morphology of the
solid support MIL-101, pointing to the preservation of structure
the solid support aer the incorporation of P2W18Ce. EDX
ꢂ
exchange for one day and activation at 70 C for 3 days under
vacuum. Aer post-treatment, the pore textural properties
enhanced for the as-synthesized MIL-101. Nitrogen adsorption
isotherms for P2W18Sn3, MIL-101 (activated with ethanol), P2-
W18Sn3@MIL-101 and P2W18Ce3@MIL-101 are shown in Fig. 5 at
boiling temperatureꢂ(77 K) aer evacuating guest molecules from
the samples at 100 C for overnight. As noted above, the major
disadvantages of POMs as catalyst are low surface areas as well as
POMs in the bulk phase display no characteristic porosity which
limit their utility in many catalytic reactions.
analyses of MIL-101 and nanocomposites materials of P2W18
-
Sn3@MIL-101 and P2W18Ce3@MIL-101 demonstrate that all of
the elements of MIL-101 and the polyoxometalate anions in the
samples which it conrms the presence of the polyoxotungstate
anions in POMs@MIL-101 (Fig. 4).
Fig. 4 EDX spectra of (a) MIL-101 and the composite of P2W18Sn3@MIL-101 (b) and P2W18Ce3@MIL-101 (c).
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Fig. 5 N2 adsorption–desorption isotherms at 77 K of (A) P2W18Sn3 (a), the composite P2W18Sn3@MIL 101 (b) and MIL 101 (c). (B) The composite
P2W18Ce3@MIL 101. Inset shows BJH pore-size distribution plots of the samples.
To overcome these problems, it is proposed that increasing the results of the Barrett–Joyner–Halenda (BJH) analysis, mes-
the surface area can be achieved by the deposition of sandwich oporous structure are present in both nanocomposites, espe-
POM into porous solid supports with high surface area. The cially in P2W18Sn3@MIL-101 catalyst, in agreement with its
results of the BET analysis show that the specic surface area of hysteresis loop at relative pressure (P/P0) between 0.4 and 1.0.45
P2W18Sn3 is much lower compared to composite (Fig. 5A(a)).
The results of the N2 adsorption–desorption isotherms and pore
Nitrogen adsorption isotherms show that the specic surface size distributions of the synthesized nanocomposites show that
area of the dehydrated MIL-101 decrease from 946.91 m2 gꢀ1 to the type of polyoxometalate loaded in the cavities of the MIL-101
573.08 m2 gꢀ1 for P2W18Sn3@MIL-101. Importantly, compared affects the volume and size distribution of the pores in the
with MIL-101, P2W18Sn3@MIL-101 shows a smaller pore volume material.
(1.312 cm3 gꢀ1 to 0.491 cm3 gꢀ1), which may help to interpret
3.2.5. Powder X-ray diffraction. Fig. 6 shows the XRD
the lower specic surface area for composite. Combined with N2 patterns of MIL-101, P2W18Sn3@MIL-101, P2W18Ce3@MIL-101
sorption isotherms analysis and the pore size distribution (PSD) and sandwich polyoxometalates P2W18M3 (M ¼ Sn and Ce) in
results, it can be concluded that the doping of the MIL-101 the 2q range of 5–50ꢂ. XRD patterns of the obtained MIL-101
porous structure with POM ions can change morphological metal–organic framework was analyzed referring to the simu-
characteristics (surface area, pore volume and pore size lated XRD patterns of the MIL-101 single crystal. The simulated
distributions).43,44
XRD patterns of MIL-101 (Fig. 6) exhibited ve strong peaks at
Adsorption isotherms of P2W18Sn3@MIL-101 (Fig. 5A(b)), as
well as MIL-101 (Fig. 5A(c)), reveal typical type-I behavior with
remarkable H4 hysteresis loop which is coincident with the
mesoporous structures. This hysteresis is usually characteristic
of solids consisting of aggregates of particles forming slit-
shaped pores (plates or edged particles like cubes), with
uniform or non-uniform size and/or shape. These results are in
good agreement with the results of the pore size distribution
(calculated by BJH method based on the adsorption branch) of
P2W18Sn3, the MIL-101 and P2W18Sn3@MIL-101 materials.
From the distribution curves, the samples have a broad pore-
size distribution in the range of 1.2–30 nm (Fig. 5 inset).
The N2 adsorption isotherm and the BJH pore size distri-
bution curve of P2W18Ce3@MIL-101 catalyst shown in Fig. 5B.
Importantly, compared with the MIL-101, P2W18Ce3@MIL-101
catalyst shows the surface area and total pore volume is
reduced to 690.14 m2 gꢀ1 and 0.376 cm3 gꢀ1, in agreement with
the presence of heavy polyoxometalate moieties. According to
Fig. 6 Powder X-ray diffraction patterns of simulated and prepared
MIL-101 (a) P2W18Sn3@MIL-101 (b) and P2W18Ce3@MIL-101 (c) sand-
wich polyoxometalates P2W18M3 (d).
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2q ¼ 5.2ꢂ, 5.6ꢂ, 5.9ꢂ, 8.4ꢂ, 9.1ꢂ corresponding to the 511, 440, reaction times, inconvenient reaction conditions, environmen-
351, 822, 911 reection, respectively.46,47 As seen in Fig. 6a, the tally unfavorable catalysts (poor recovery of expensive metal
all of the diffraction peaks corresponding to the obtained MIL- catalysts), low yields and the formation of toxic wastes.49,50
101 are in good agreement with the standard sample. It can see Various catalyst systems of organocatalysts, acid catalysts,
the diffraction peaks of the P2W18Sn3@MIL-101 (Fig. 6b) enzymes, metal catalysts and organic–inorganic hybrid solid
include both characteristic peaks of MIL-101 and P2W18Sn3 materials have been used for this reaction with H2O2 as an
polyoxotungstate (Fig. 6d), revealing the existence of P2W18Sn3 oxygen source. Recently, the use of polyoxometalates was
and their basically intact the crystalline characters of the parent promising not only due to bifunctional redox acid catalysts
metal–organic framework. Powder X-ray diffraction pattern of properties but also due to their unusual properties such as high
P2W18Sn3@MIL-101 shows a less-ordered structures pattern charges, low cost and low environmental impact.51–53
which implying that the occupation of pore channels of MIL-101
by polyoxometalate and change in electronic environment 5.85 mmol of H2O2 as an oxidant, employing 1 mmol (0.17 mL)
around Cr atoms (Fig. 6b).48
diphenyl sulde and 50 mg of catalyst in CH3CN (2.5 mL) at
The reaction initially performed in the presence of
The X-ray diffraction pattern of the P2W18Ce3@MIL-101 room temperature. In order to nd the optimum reaction
nanocomposite is in good agreement with P2W18Sn3@MIL- conditions, the effect of different reaction parameters such as
101, thus conrming both polyoxotungstates have the same the amount of catalyst, the stoichiometry of H2O2 respect to the
diffraction pattern (Fig. 6c).
catalyst and substrate, solvent, reaction time, different dosages
of POM into MIL-101 and the catalytic activities of the different
catalysts on the selectivity and conversion of the oxidation
reaction were studied. No product was obtained in the absence
of any catalyst (Table 1, entry 1). At rst, the catalytic perfor-
mance was evaluated for the selective oxidation of suldes
using MIL-101 support and the POMs (P2W18Sn3 and P2W18Ce3)
as homogeneous catalysts in different reaction conditions
(Table 1, entries 2–6).
The P2W18Sn3 as a homogeneous catalyst in the presence of
hydrogen peroxide in H2O or CH3CN as solvent unsuitable
conversion and selectivity was found even in prolonged reaction
time (Table 1 entries 3 and 4). The P2W18Ce3 as a homogeneous
catalyst in the presence of hydrogen peroxide, although in H2O
do not show suitable activity but, it catalyzes selectively the
oxidation of suldes to sulfone in about 180 min in EtOH (Table
1 entries 5 and 6). Although the conversion and the selectivity of
3.3. Catalytic activity
Organosulfur compounds, such as sulfoxides and sulfones, are
useful synthetic intermediates for the construction of various
pharmaceutically and biologically active compounds. They are
widely
utilized
as
anti-bacterial,
anti-fungal,
anti-
atherosclerotic, anti-ulcer, vasodilators and cardiotonic agents
and as well as activation of enzymes. Different synthetic
methods for the controlled oxidation of conventional suldes
have been previously reported for fundamental transformation.
Traditionally, these transformations take place with stoichio-
metric amounts of electrophilic reagents, such as peracids,
dioxiranes, hypochlorites, periodates and highly toxic oxo metal
oxidants (NaIO4, MnO2, CrO3, SeO2, PhIO, NH4MnO4 and so on)
but, many of these procedures are accompanied by particular
disadvantages such as toxic and corrosive oxidants, long
Table 1 Optimization of the reaction conditions with respect to the effect of the different catalystsc and solvent on the oxidation of diphenyl
sulfide
Entry
Catalyst (mg)
Solvent
Time (min)
Sulfoxide (%)a
Sulfone (%)a
1b
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
Catalyst free
MIL-101 (50)
CH3CN
CH3CN
CH3CN
H2O
H2O
EtOH
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
H2O
EtOH
H2O/PEG
DMF
1440
1440
1440
1440
1440
180
120
35
210
260
320
210
220
220
220
60
Trace
40
35
Trace
Trace
—
—
—
—
—
—
—
—
—
30
—
—
—
20
10
Trace
Trace
98
98
98
98
98
98
98
98
98
30
98
P2W18Sn3 (50)
P2W18Sn3 (50)
P2W18Ce3 (50)
P2W18Ce3 (50)
Catalyst (1) (50)
Catalyst (2) (50)
Catalyst (3) (50)
Catalyst (4) (50)
Catalyst (5) (50)
Catalyst (1) (50)
Catalyst (1) (50)
Catalyst (1) (50)
Catalyst (1) (50)
Catalyst (2) (50)
Catalyst (2) (50)
EtOH
H2O
900
92
a
b
Isolated yield. Reaction conditions: diphenyl sulde (1 mmol), 35% H2O2 (5.85 mmol), solvent (2.5 mL),catalyst (50 mg), r.t.
c
[(HOSnIVOH)3(PW9O34)2]12ꢀ @MIL-101 (1), [(HOCeIVOH)3(PW9O34)2]12ꢀ @MIL-101 (2) [(HOSnIVOH)3(SiW9O34)2]14ꢀ @MIL-101 (3),
[WCo3(H2O)2(CoW9O34)2]12ꢀ @MIL-101 (4) [WZn3(H2O)2(ZnW9O34)2]12ꢀ @MIL-101 (5).
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P2W18Ce3 in EtOH were good, but there are two problems about at room temperature in CH3CN (Table 1, entries 7–11). The
its separation and recovery. We found that like some of POMs, results in Table 1 clearly show that P2W18Sn3@MIL-101 and
the structure of P2W18Ce3 in the presence of hydrogen peroxide P2W18Ce3@MIL-101 showed the highest selectivity for
was converted to a mixture of peroxopolytungstophosphate oxidizing of diphenyl sulde in short reaction time with 98%
species.54,55 As shown in Table 1, the oxidation of substrate was yield of the corresponding sulfone. Increasing catalysts over
carried out using the heterogeneous catalysts. Catalytic oxida- 50 mg and also H2O2 more over than 1 mL doesn't have an
tion of suldes was carried out in the presence of the MIL-101 appreciable effect on the reaction times. The role of different
composites of various sandwich-type polyoxometalates.
solvents for the oxidation of diphenyl sulde was examined.
Within this context, [(HOSnIVOH)3(PW9O34)2]12ꢀ@MIL- As shown in Table 1; in presence P2W18Sn3@MIL-101 when
101, [(OCeIVO)3(PW9O34)2]12ꢀ@MIL-101, [(HOSnIVOH)3(- we used acetonitrile as solvent, the desired product (sulfone)
SiW9O34)2]14ꢀ@MIL-101, [WCo3(H2O)2(CoW9O34)2]12ꢀ@MIL- can be obtained in a shorter time (Table 1, entry 7). By using
101 and [WZn3(H2O)2(ZnW9O34)2]12ꢀ@MIL-101 composites H2O, EtOH or H2O/PEG as solvent similar catalytic conver-
were prepared and their catalytic activities investigated for sion and selectivity obtained but at prolonged reaction time
the oxidation of diphenyl sulde by using H2O2 as an oxidant (Table 1, entries 12–14). DMF as solvent, the mixture of
Table 2 Selective oxidation of various sulfides to sulfones using H2O2 catalyzed by P2W18Sn3@MIL-101
Sulfonea,b
Sulfonea,c
Isolated yield
(%)
Entry
1
Substrate
Time (min)
Isolated yield (%)
Time (min)
150
210
98
80
98
98
2
98
120
3
4
10
15
97
95
5
5
98
96
5
15
97
5
97
6
7
200
240
94
94
20
30
98
95
8
15
96
5
97
a
b
All the products are known and were characterized by FT-IR and by melting point comparison with those of authentic samples. Reaction
c
conditions: sulde (1 mmol), 35% H2O2 (5.85 mmol), catalyst (50 mg), in H2O, r.t. Reaction conditions: sulde (1 mmol), 35% H2O2 (5.85
mmol), catalyst (50 mg), in CH3CN, r.t.
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sulfoxide and sulfone is obtained (Table 1, entry 15). Gener-
ally, the solvent type is chosen based on the reaction kinetic
and catalyst structure, so choosing the suitable solvents
according to the chemical structure, molecular design and its
physical and chemical properties is very important in reac-
tion systems. With regard to the porous catalyst in our study,
it was not possible to use solvent-free conditions because
reactants have to interact with catalytic active species located
in the cavities. It is necessary to mention, the reaction in
acetonitrile showed a good reactivity (excellent conversion
and selectivity), in our this work. On the other hand, in protic
solvents such as water, it is possible that Lewis acid sites
(CrIII) in the structure of the nanocomposite interact with
protons of solvent and thus the cavities are blocked, so it is
difficult to insertion–extraction for reactants.
Fig. 7 The reusability of P2W18Sn3@MIL-101 (gray) and P2W18Ce3@-
MIL-101 (orange) catalysts in the oxidation of sulfides.
The inuence of temperature in the diphenyl sulde
oxidation is illustrated with keeping H2O2 and substrate
Table 3 Selective oxidation of various sulfides to sulfones using H2O2 catalyzed by P2W18Ce3@MIL-101
Sulfonea,b
Entry
1
Substrate
Time (min)
Isolated yield (%)
30
98
2
3
4
60
5
98
97
96
10
5
20
97
6
7
30
70
96
94
8
15
96
a
b
All the products are known and were characterized by FT-IR and by melting point comparison with those of authentic samples. Reaction
conditions: sulde (1 mmol), 35% H2O2 (5.85 mmol), catalyst (50 mg), in EtOH, r.t.
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Fig. 8 The FT-IR spectra of (A) P2W18Sn3@MIL-101 composite (a) and P2W18Sn3@MIL-101 after six consecutive runs (b) and (B) P2W18Ce3@MIL-
101 composite (a) and P2W18Ce3@MIL-101 after six consecutive runs (b).
amount constant. We found similar results at 25 to 50 ꢂC. The as aromatic and aliphatic suldes. For example, 2-methylthio
reaction was completed selectively in 60 ꢂC at shorter reaction ethanol contains the hydroxyl group was transformed to the
time. Increasing temperature to 85 ꢂC conversion was corresponding sulfone compound in high conversion and
completed while the selectivity decreased. P2W18Ce3@MIL-101 selectivity without dehydrogenation of the hydroxyl group.
the best catalytic activity observed in CH3CN and EtOH as
solvent (Table 1, entries 8 and 16), while in H2O the same
catalytic activity observed only at very prolonged time (Table 1,
entry 17).
3.4. Stability of the catalysts
In our nal experimental work, stability tests (recovery and
reusability) of the new P2W18Sn3@MIL-101 and P2W18Ce3@-
MIL-101 catalysts were carried out running six consecutive
experiments in the oxidation of diphenyl sulde at a constant
time.
At the end of each reaction, the catalyst was recovered by
simple centrifuge and was washed with ethyl acetate and
ethanol (4 ꢃ 3 mL), dried under vacuum then used again as the
catalyst. The results show the reactions were completed in run
of 1–5 at average time of 210 min (in H2O) and 60 min (in EtOH)
for P2W18Sn3@MIL-101 and P2W18Ce3@MIL-101, respectively.
At the same reaction times the 6th runs were completed up to 80
and 90% with P2W18Sn3@MIL-101 and P2W18Ce3@MIL-101
respectively (Fig. 7).
To investigate the structural stability of catalyst aer oxida-
tive reactions, IR spectra fresh and reused nanocomposites were
recorded aer six catalytic cycles (Fig. 8). The IR spectra of fresh
catalyst are identical to those of reused catalyst, conrming the
integrity of the support material and also the presence of the
sandwich polyoxotungstate anions in the composite material
aer the catalytic performance.
The used polyoxometalates have similar structure and the
difference among them is the metal ions in the belt of the used
sandwich type polyoxometalates. Therefore, the difference in
their catalytic activity can be attributed to type of metal ions.
The nanocomposites of P2W18Sn3@MIL-101 and P2W18Ce3@-
MIL-101 show the best catalytic activity which it can be
attributed to the SnIV or CeIV ions in their structures. A
comparison between the latters, show that the best results
obtained with P2W18Ce3@MIL-101. Two aligned reasons can
be proposed for preference of P2W18Ce3@MIL-101 composite.
First, particular ability of cerium in store/release oxygen as an
oxygen storage via facile reciprocal transformation of CeIV and
CeIII ions under oxidizing and reducing conditions respec-
tively. Due to this feature, activated oxygen species produced
from H2O2, may be stored on the composite, which in turn,
may be responsible for oxidation of sulde.56–58 Second, stan-
dard electrode potential for reduction of CeIV to CeIII in the
P2W18Ce3@MIL-101 is very more positive than that SnIV to SnII
in the P2W18Sn3@MIL-101. As an interesting result, in contrast
to homogenous condition, the P2W18Ce3 structure was pro-
tected in the P2W18Ce3@MIL-101 composite in the presence of
hydrogen peroxide.
4. Conclusions
To investigate the applicability of this procedure, we carried
out oxidation of different types of suldes under the distinc-
tive reaction conditions and the expected products afforded at
high yields in CH3CN and H2O solvents for P2W18Sn3@MIL-
101 (Table 2) and for P2W18Ce3@MIL-101 in EtOH (Table 3).
As a result from this study, all used suldes were oxidized to
Organic–inorganic frameworks based on different sandwich-
type polyoxometalates using commercially and readily avail-
able materials were synthesized and their catalytic activities
studied in the oxidation of different suldes with an environ-
mentally benign oxidant in different solvents. The nano-
composites have been achieved by impregnation of the
synthesized MIL-101 in aqueous solution of the sandwich-type
polyoxometalates. This is important point that by increasing
concentration of polyoxometalates in aqueous solution, their
loading into cages of MIL-101 is not signicantly altered. Even
the corresponding sulfone and also noteworthy that P2W18
-
Ce3@MIL-101 composite shows better catalytic activity. In
other word, substrates have high selectively to oxidation of
sulfur singly even with the presence of functional groups such
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aer these relatively large anions signing into the MIL-101,
there is still enough space to enter the reactants and get out
of the product. The POMs insertion into porous MIL-101 solid
was investigated using several methods: XRPD, FT-IR, SEM-
8 R. Villanneau, A. Marzouk, Y. Wang, A. B. Djamaa, G. Laugel,
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Conflicts of interest
The authors declare that they have no competing interests.
´
24 S. H. Jhung, J. H. Lee, J. W. Yoon, C. Serre, G. Ferey and
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Acknowledgements
The authors would like to thank the University of Kurdistan
Research Council for their support of this work.
´
26 G. Ferey, C. Mellot-Draznieks, C. Serre, F. Millange,
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