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

The A-type sandwich polyoxometalates of [(HOSnIVOH)3(PW9O34)2]12− (P2W18Sn3) and [(OCeIVO)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.


Introduction
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, electrochemistry and photochemistry. [1][2][3] Their utility for catalysis applications has been limited, notably due to their tendency toward low surface area (typically # 10 m 2 g À1 ) and porosity (lower than 0.1 cm 3 g À1 ) 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][5][6] In this sense, the heterogenization or immobilization of POMs onto various solid supports via dative, 7 covalent, 8 or electrostatic [9][10][11] binding, such as silica, activated carbon, magnetic nanoparticles 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][13][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 decomposition of guest-free MOFs. [16][17][18][19][20] MIL-101 family of materials (MIL: Materials of the Institute Lavoisier) are a very stable group of MOFs, result from the three-dimensional covalent 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 reactant 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][22][23][24][25] In 2005, Férey et al. reported incorporation of the lacunary polytungstate of PW 11

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 N 2 ow while gradually increasing the temperature with a rate of 10 C min À1 , using a STA PT-1000 LINSEIS. The morphology of nanocomposites was revealed by a scanning electron microscope (MIRA3 FEG-SEM). The elements in the nanocomposite samples were probed with energy-dispersive X-ray (EDX) spectroscopy accessory to the MIRA3 FEG-SEM scanning electron microscopy. Nitrogen adsorption-desorption isotherms used to obtain the MIL-101(Cr) and POM@MIL 101(Cr) specic surface areas and pore volumes, calculated by the Brunauer-Emmett-Teller (BET) method. All as-prepared samples were degassed at 100 C in vacuum for overnight prior to adsorption measurements. The experiments took place at À196 C under variable relative pressure. The equipment was a Belsorp mini II adsorption analyzer (Bel-Japan) at À196 C. Powder X-ray diffraction (XRD) analyses were collected at ambient temperature by a X'PertPro Panalytical, Holland diffractometer using a CuKa radiation (l ¼ 1.5418Å). The accelerating voltage and applied currents were 40 kV and 30 mA, respectively. The C 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 plasma mass spectrometry (ICP-MS).

Preparation of the materials
2.3.1. Solid support MIL-101(Cr). The porous metalorganic framework (MOF) material MIL-101(Cr) was prepared hydrothermally, using a modied procedure of the method described by Férey et al. 26 Benzene-1,4-dicarboxylic acid (H 2 BDC) (0.332 g, 2 mmol) was added in small portions to an aqueous solution of Cr(NO 3 ) 3 $9H 2 O (0.8 g, 2 mmol in 10 mL of deionized water) and stirred at room temperature to obtain an homogeneous suspension for 20 min, then uorhydric acid (0.1 mL) was added to the above suspension and stirred for 30 min. The dark blue-colored suspension was placed in a Teon-lined autoclave (125 mL, Paar model 4748), for 9 h at a heating temperature of 220 C without stirring. Aer slowly cooling to ambient temperature (inside the oven), the green powder was collected by repeated centrifugation and thorough washing with deionized water. Since the product was a mixture of MIL-101 powder and few amounts of needle-like crystals of recrystallized H 2 BDC. It was washed overnight with DMF under reux against the removal of recrystallized H 2 BDC. Aerwards, to remove the DMF solvent, the resulting solids were washed with deionized water several times. The product was dried in an air oven at 70 C overnight followed by soxhlet extraction in ethanol for 24 h. Samples were activated under vacuum 3 days at 70 C with a high yield 83.8% based on chromium.
2.3.2. P 2 W 18 Sn 3 @MIL-101 composite. The composite material P 2 W 18 Sn 3 @MIL-101 was prepared through the immobilization of the potassium salt of P 2 W 18 Sn 3 in the porous solid support MIL-101 using a modied procedure of the method described by Balula et al. 35 In the "impregnation" method, 0.0733 mmol of dry MIL-101 synthesized in an autoclave as described above was added to an aqueous solution of a moderate amount of P 2 W 18 Sn 3 (0.25 mM, 0.5 mM, 1 mM, 2 mM, 4 mM; 15 mL) and the mixture was stirred at room temperature for 24 h. The solid separated by centrifugation and then washed several times thoroughly with deionized water and dried in an air oven at 60 C overnight followed by activated under vacuum 3 days at 70 C. Elemental analysis (w/w %): C, 33.5; Cr, 8.18; W, 21.52. Based on the elemental analysis results and molecular weight of K 11 H[(HOSn IV OH) 3 (PW 9 O 34 ) 2 ]$ 20H 2 O, W content, 57.98%; MW, 5708, we estimated the P 2 W 18 Sn 3 content of the P 2 W 18 Sn 3 @MIL-101 materials to be approximately 65 mmol g À1 of dry powder, or 37 w/w %. The content of P 2 W 18 Sn 3 was calculated according to the formula, mmol g À1 ¼ 1 Â 10 6 (W content in the P 2 W 18 Sn 3 @MIL-101, %)/(MW of P 2 W 18 Sn 3 Â W content in the P 2 W 18 Sn 3 %). 28 2.3.3. P 2 W 18 Ce 3 @MIL-101 composite. The composite material P 2 W 18 Ce 3 @MIL-101 was prepared through the immobilization of the potassium salt of P 2 W 18 Ce 3 in the porous solid support MIL-101 according to the preparation procedure of P 2 W 18 Sn 3 @MIL-101 composite unless, P 2 W 18 Sn 3 was replaced by P 2 W 18 Ce 3 .

General test for the oxidation
A mixture of bulk polyoxometalates (P 2 W 18 Sn 3 or P 2 W 18 Ce 3 ) or composites (P 2 W 18 Sn 3 @MIL-101 or P 2 W 18 Ce 3 @MIL-101) (50 mg) as catalyst, 35% H 2 O 2 aqueous solution (5.85 mmol) and 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 Â 5 mL), heated 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 CH 2 Cl 2 (3 Â 5 mL). Aer evaporation of organic layer, the crude products were recrystallized from hot ethanol and the pure products were obtained in 94-98% yield. Stability test of the P 2 W 18 -Sn 3 @MIL-101 and P 2 W 18 Ce 3 @MIL-101 catalysts were carried out running six consecutive experiments.

Synthesis
The MIL-101(Cr) is built up from a corner-sharing of so-called super tetrahedron (hereaer noted ST), which is formed by rigid chromium(III) octahedral trimers (the vertices of the ST) and terephthalate anions (the edges of the ST) (see Scheme 1).
The connection between of the corners of the ST building blocks ensures a 3D cubic zeotype structure with two types of mesoporous cages (B ¼ 29 and 34Å) (extended MTN topology), accessible through microporous windows (B ¼ 12 and 16Å). The large cages possess both 12Å pentagonal and 16Å hexagonal windows with internal free diameters of 34Å. Therefore, these cavities are sufficiently spaced to accommodate large guest molecules (such as the successful encapsulation of the sandwich-type anions, P 2 W 18 Sn 3 or P 2 W 18 Ce 3 ($10.4 Â 10.4 Â 15.2Å 3 )), into cages of MIL-101(Cr). The MIL-101(Cr) has been employed as a useful and versatile solid support for preparation of heterogeneous catalysts because of its open-pore structure with large and accessible cages. 26 The incorporation of POMs in the mesoporous MIL-101(Cr) has been carried out by the anionic exchange between the counter-ions of the MIL-101(Cr) (nitrate ions coming from the Cr(NO 3 ) 3 precursor) and the negatively charged P 2 W 18 Sn 3 or P 2 W 18 Ce 3 . 36 26 The composites of the sandwich-type POM of P 2 W 18 Sn 3 and P 2 W 18 Ce 3 with MIL-101 were prepared by impregnation of the synthesized MIL-101 in an autoclave as described above in aqueous solution of P 2 W 18 Sn 3 or P 2 W 18 Ce 3 .

Characterization of the material
3.2.1. FT-IR spectroscopy. The FT-IR spectra of composite materials and those of the precursor compounds of polyoxometalates and the MIL-101 were compared in Fig. 1. The FT-IR spectra of composites ( Fig. 1A(a) and B(a)) exhibit the characteristic bands of both the MIL-101 support ( Fig. 1A(b) and B(b)) and sandwich polyoxometalates P 2 W 18 Sn 3 or P 2 W 18 Ce 3 (Fig. 1A(c) and B(c)). The new bands at 1093, 956, 891, 792 cm À1 or 1062, 1018, 948, 784 cm À1 (shown with dashed lines) observed in the spectra of P 2 W 18 Sn 3 @MIL-101 and P 2 W 18 -Ce 3 @MIL-101 compared to the that of MIL-101 were attributed to the P-O and W-O vibrations of the sandwich polyoxometalates P 2 W 18 Sn 3 (ref. 32) or P 2 W 18 Ce 3 , 37 respectively. The bands in MIL-101, P 2 W 18 Sn 3 @MIL-101 and P 2 W 18 Ce 3 @MIL-101 at 1400 cm À1 and 1552 cm À1 are correspond to (O-C-O) symmetric vibrations implying the presence of dicarboxylate within the framework, broad and strong bands at around 3430 cm À1 and 1620 cm À1 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 cm À1 are attributed to benzene, including the stretching vibration C]C groups (1514 cm À1 ), in plane and out-of plane bending modes of COO groups (400-700 cm À1 ), and the out-of plane deformation vibrations of benzene ring C-H groups at 1164, 1020, 889, and 748 cm À1 . 38, 39 3.2.2. Thermogravimetric analysis (TGA). The TGA curves for the neat MIL-101 (a) and the composites of P 2 W 18 Ce 3 @MIL-101 (b) and P 2 W 18 Sn 3 @MIL-101 (c) under an inert atmosphere at a constant rate of 10 C min À1 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 C. Two main weight-loss steps were observed below 400 C: the rst (5.83%), relates to the removal of guest water molecules in the small cages, occurs in the range 30-120 C; a larger loss of ca. 6.51% is due to the departure of chemically bonded water and organic solvent (EtOH/DMF) molecules. 30 The third step occurs in the range 272-800 C with loss of about 52.71% relates to the departure of OH/F groups and the decomposition of the framework.
The residual solid is Cr 2 O 3 for MIL-101. Although, the TGA prole of the composite P 2 W 18 Sn 3 @MIL-101 in the rst event, 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 C in TGA prole of the composite P 2 W 18 Sn 3 @MIL-101 can attribute to the loss of oxygen atoms from the residual metal oxides resulted from decomposition of P 2 W 18 Sn 3 . The residual mixture solid is Cr 2 O 3 -WO 3 -P 2 O 5 -SnO 2 -K 2 O (55.80%). 40 Also, the total weight loss of 42.15% for P 2 W 18 Sn 3 @MIL-101 is lower  than that for the parent MIL-101 (65.05%), as would be expected from the presence additional residual from partial decomposition of P 2 W 18 Sn 3 in the cages of composite. The TGA curve of the P 2 W 18 Ce 3 @MIL-101 composite shows a two-stage weight loss below 600 C, such as the P 2 W 18 Sn 3 @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 P 2 W 18 -Ce 3 @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 P 2 W 18 Ce. EDX analyses of MIL-101 and nanocomposites materials of P 2 W 18 -Sn 3 @MIL-101 and P 2 W 18 Ce 3 @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).

N 2 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) followed by simple heat and vacuum treatment. 42 DMF molecules were completely removed from the MIL-101 aer solvent 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 P 2 W 18 Sn 3 , MIL-101 (activated with ethanol), P 2 -W 18 Sn 3 @MIL-101 and P 2 W 18 Ce 3 @MIL-101 are shown in Fig. 5 at boiling temperature (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. Fig. 4 EDX spectra of (a) MIL-101 and the composite of P 2 W 18 Sn 3 @MIL-101 (b) and P 2 W 18 Ce 3 @MIL-101 (c). To overcome these problems, it is proposed that increasing the surface area can be achieved by the deposition of sandwich POM into porous solid supports with high surface area. The results of the BET analysis show that the specic surface area of P 2 W 18 Sn 3 is much lower compared to composite (Fig. 5A(a)).
Nitrogen adsorption isotherms show that the specic surface area of the dehydrated MIL-101 decrease from 946.91 m 2 g À1 to 573.08 m 2 g À1 for P 2 W 18 Sn 3 @MIL-101. Importantly, compared with MIL-101, P 2 W 18 Sn 3 @MIL-101 shows a smaller pore volume (1.312 cm 3 g À1 to 0.491 cm 3 g À1 ), which may help to interpret the lower specic surface area for composite. Combined with N 2 sorption isotherms analysis and the pore size distribution (PSD) results, it can be concluded that the doping of the MIL-101 porous structure with POM ions can change morphological characteristics (surface area, pore volume and pore size distributions). 43,44 Adsorption isotherms of P 2 W 18 Sn 3 @MIL-101 ( Fig. 5A(b)), as well as MIL-101 (Fig. 5A(c)), reveal typical type-I behavior with remarkable H 4 hysteresis loop which is coincident with the mesoporous structures. This hysteresis is usually characteristic of solids consisting of aggregates of particles forming slitshaped 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 P 2 W 18 Sn 3, the MIL-101 and P 2 W 18 Sn 3 @MIL-101 materials. From the distribution curves, the samples have a broad poresize distribution in the range of 1.2-30 nm (Fig. 5 inset).
The N 2 adsorption isotherm and the BJH pore size distribution curve of P 2 W 18 Ce 3 @MIL-101 catalyst shown in Fig. 5B. Importantly, compared with the MIL-101, P 2 W 18 Ce 3 @MIL-101 catalyst shows the surface area and total pore volume is reduced to 690.14 m 2 g À1 and 0.376 cm 3 g À1 , in agreement with the presence of heavy polyoxometalate moieties. According to the results of the Barrett-Joyner-Halenda (BJH) analysis, mesoporous structure are present in both nanocomposites, especially in P 2 W 18 Sn 3 @MIL-101 catalyst, in agreement with its hysteresis loop at relative pressure (P/P 0 ) between 0.4 and 1.0. 45 The results of the N 2 adsorption-desorption isotherms and pore size distributions of the synthesized nanocomposites show that the type of polyoxometalate loaded in the cavities of the MIL-101 affects the volume and size distribution of the pores in the material.
3.2.5. Powder X-ray diffraction. Fig. 6 shows the XRD patterns of MIL-101, P 2 W 18 Sn 3 @MIL-101, P 2 W 18 Ce 3 @MIL-101 and sandwich polyoxometalates P 2 W 18 M 3 (M ¼ Sn and Ce) in the 2q range of 5-50 . XRD patterns of the obtained MIL-101 metal-organic framework was analyzed referring to the simulated XRD patterns of the MIL-101 single crystal. The simulated XRD patterns of MIL-101 (Fig. 6) exhibited ve strong peaks at Fig. 6 Powder X-ray diffraction patterns of simulated and prepared MIL-101 (a) P 2 W 18 Sn 3 @MIL-101 (b) and P 2 W 18 Ce 3 @MIL-101 (c) sandwich polyoxometalates P 2 W 18 M 3 (d). 2q ¼ 5. 2 , 5.6 , 5.9 , 8.4 , 9.1 corresponding to the 511, 440, 351, 822, 911 reection, respectively. 46,47 As seen in Fig. 6a, the all of the diffraction peaks corresponding to the obtained MIL-101 are in good agreement with the standard sample. It can see the diffraction peaks of the P 2 W 18 Sn 3 @MIL-101 (Fig. 6b) include both characteristic peaks of MIL-101 and P 2 W 18 Sn 3 polyoxotungstate (Fig. 6d), revealing the existence of P 2 W 18 Sn 3 and their basically intact the crystalline characters of the parent metal-organic framework. Powder X-ray diffraction pattern of P 2 W 18 Sn 3 @MIL-101 shows a less-ordered structures pattern which implying that the occupation of pore channels of MIL-101 by polyoxometalate and change in electronic environment around Cr atoms (Fig. 6b). 48 The X-ray diffraction pattern of the P 2 W 18 Ce 3 @MIL-101 nanocomposite is in good agreement with P 2 W 18 Sn 3 @MIL-101, thus conrming both polyoxotungstates have the same diffraction pattern (Fig. 6c).

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, antiatherosclerotic, 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 stoichiometric amounts of electrophilic reagents, such as peracids, dioxiranes, hypochlorites, periodates and highly toxic oxo metal oxidants (NaIO 4 , MnO 2 , CrO 3 , SeO 2 , PhIO, NH 4 MnO 4 and so on) but, many of these procedures are accompanied by particular disadvantages such as toxic and corrosive oxidants, long reaction times, inconvenient reaction conditions, environmentally unfavorable catalysts (poor recovery of expensive metal catalysts), low yields and the formation of toxic wastes. 49,50 Various catalyst systems of organocatalysts, acid catalysts, enzymes, metal catalysts and organic-inorganic hybrid solid materials have been used for this reaction with H 2 O 2 as an oxygen source. Recently, the use of polyoxometalates was promising not only due to bifunctional redox acid catalysts properties but also due to their unusual properties such as high charges, low cost and low environmental impact. [51][52][53] The reaction initially performed in the presence of 5.85 mmol of H 2 O 2 as an oxidant, employing 1 mmol (0.17 mL) diphenyl sulde and 50 mg of catalyst in CH 3 CN (2.5 mL) at room temperature. In order to nd the optimum reaction conditions, the effect of different reaction parameters such as the amount of catalyst, the stoichiometry of H 2 O 2 respect to the 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 performance was evaluated for the selective oxidation of suldes using MIL-101 support and the POMs (P 2 W 18 Sn 3 and P 2 W 18 Ce 3 ) as homogeneous catalysts in different reaction conditions (Table 1, entries 2-6).
The P 2 W 18 Sn 3 as a homogeneous catalyst in the presence of hydrogen peroxide in H 2 O or CH 3 CN as solvent unsuitable conversion and selectivity was found even in prolonged reaction time (Table 1 entries 3 and 4). The P 2 W 18 Ce 3 as a homogeneous catalyst in the presence of hydrogen peroxide, although in H 2 O 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  [12][13][14]. DMF as solvent, the mixture of sulfoxide and sulfone is obtained (Table 1, entry 15). Generally, 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 reaction 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 (Cr III ) 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. The inuence of temperature in the diphenyl sulde oxidation is illustrated with keeping H 2 O 2 and substrate Fig. 7 The reusability of P 2 W 18 Sn 3 @MIL-101 (gray) and P 2 W 18 Ce 3 @-MIL-101 (orange) catalysts in the oxidation of sulfides. 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 P 2 W 18 Sn 3 @MIL-101 and P 2 W 18 Ce 3 @-MIL-101 show the best catalytic activity which it can be attributed to the Sn IV or Ce IV ions in their structures. A comparison between the latters, show that the best results obtained with P 2 W 18 Ce 3 @MIL-101. Two aligned reasons can be proposed for preference of P 2 W 18 Ce 3 @MIL-101 composite. First, particular ability of cerium in store/release oxygen as an oxygen storage via facile reciprocal transformation of Ce IV and Ce III ions under oxidizing and reducing conditions respectively. Due to this feature, activated oxygen species produced from H 2 O 2 , may be stored on the composite, which in turn, may be responsible for oxidation of sulde. [56][57][58] Second, standard electrode potential for reduction of Ce IV to Ce III in the P 2 W 18 Ce 3 @MIL-101 is very more positive than that Sn IV to Sn II in the P 2 W 18 Sn 3 @MIL-101. As an interesting result, in contrast to homogenous condition, the P 2 W 18 Ce 3 structure was protected in the P 2 W 18 Ce 3 @MIL-101 composite in the presence of hydrogen peroxide.
To investigate the applicability of this procedure, we carried out oxidation of different types of suldes under the distinctive reaction conditions and the expected products afforded at high yields in CH 3 CN and H 2 O solvents for P 2 W 18 Sn 3 @MIL-101 (Table 2) and for P 2 W 18 Ce 3 @MIL-101 in EtOH (Table 3). As a result from this study, all used suldes were oxidized to the corresponding sulfone and also noteworthy that P 2 W 18 -Ce 3 @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 as aromatic and aliphatic suldes. For example, 2-methylthio ethanol contains the hydroxyl group was transformed to the corresponding sulfone compound in high conversion and selectivity without dehydrogenation of the hydroxyl group.

Stability of the catalysts
In our nal experimental work, stability tests (recovery and reusability) of the new P 2 W 18 Sn 3 @MIL-101 and P 2 W 18 Ce 3 @-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 Â 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 H 2 O) and 60 min (in EtOH) for P 2 W 18 Sn 3 @MIL-101 and P 2 W 18 Ce 3 @MIL-101, respectively. At the same reaction times the 6 th runs were completed up to 80 and 90% with P 2 W 18 Sn 3 @MIL-101 and P 2 W 18 Ce 3 @MIL-101 respectively (Fig. 7).
To investigate the structural stability of catalyst aer oxidative 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.

Conclusions
Organic-inorganic frameworks based on different sandwichtype polyoxometalates using commercially and readily available materials were synthesized and their catalytic activities studied in the oxidation of different suldes with an environmentally benign oxidant in different solvents. The nanocomposites 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 Fig. 8 The FT-IR spectra of (A) P 2 W 18 Sn 3 @MIL-101 composite (a) and P 2 W 18 Sn 3 @MIL-101 after six consecutive runs (b) and (B) P 2 W 18 Ce 3 @MIL-101 composite (a) and P 2 W 18 Ce 3 @MIL-101 after six consecutive runs (b).