Tin-derivative sandwich-type polyoxometalates supported on Nd-doped TiO2 nanoparticles as efficient and reusable catalysts in the oxidation of sulfides and alcohols

New nanocomposites, including different loading levels of sandwich-type polyoxometalates [(HOSnIVOH)3(XW9O34)2]n− (X = As (1), P (2) n = 12 and Si (3) n = 14) on Nd-doped TiO2 nanoparticles were prepared by a simple impregnation method. The nanocomposites were characterized by X-ray diffraction (XRD), scanning electron microscopy, transmission electron microscopy, Fourier transform infrared (FTIR), and energy-dispersive X-ray spectroscopy. Compounds 1–3 were successfully loaded on Nd-doped crystallized anatase-phase TiO2 nanoparticles of 20–25 nm. Catalytic activities of nanocomposites were examined by carrying out the oxidation of sulfides and alcohols with H2O2. Simple synthesis method, reusability, and low amounts of the heterogeneous catalysts with a slight excess of H2O2 and mild reaction conditions make these oxidation reactions an environmentally benign chemical process.


Introduction
Polyoxometalates (POMs) are composed of negatively charged inorganic metaloxygen-building blocks and cations. These compounds can self-assemble into 3-D structures with special electronic structures at multilevel length scales [1,2]. The diversity in composition and structure of POMs make them attractive materials in many fields, including their use as catalysts in oxidation and acid-assisted reactions [3]. POMs exhibit catalytic activity in the oxidation of olefins to epoxides, oxidative dehydrogenation of alkanes or isomerization reactions, which has attracted attention in both homogeneous and heterogeneous catalysis [4][5][6][7]. Despite some advantages, there are some drawbacks concerning catalysis with POMs-based systems; they are nonporous solids with a small surface area (<10 m 2 /g) and have high solubility in polar solvents. Therefore, in a homogeneous catalytic reaction, the isolation of the products and reuse of the catalyst become difficult. To overcome these limitations, POMs have been supported on inert or weakly acidic materials with high surface area [8][9][10]. In recent years, numerous studies have focused on catalytic properties of Keggin and Dawson-type polyoxometalates. Sandwich-type polyoxometalates in which three or four metal ions are sandwiched between two Keggin or Dawson moieties, a well-known class of polyoxometalates, have been less studied. The catalytic properties of these compounds may be improved due to synergistic effect of metal ions and polyoxometalates. Many structural species of such polyoxometalates are known but attention has focused more on the transition-metal-substituted sandwich-type polyoxometalates. Neumann and coworkers used the sandwich-type transition metal-substituted polyoxometalate K 10 [(WZnRh III 2 )(ZnW 9 O 34 ) 2 ] for the epoxidation of alkenes [11]. Sandwich-type polyoxometalates of Na x [(WZnM II 2 (ZnW 9 O 34 ) 2 ] (M = Ru, Mn, Zn, Pd, Pt, Co, Fe, Rh) [12] were used as an effective catalyst for the selective oxidation of alcohols by aqueous hydrogen peroxide in a biphasic medium [13]. These and other transition-metal polyoxometalates have also been used for oxidative transformations with iodosobenzene [14], N-oxides [15], periodate [16], ozone [17], nitrous oxide [18], sulfoxides [19], molecular oxygen [20], and hydrogen peroxide [21,22] [25] have been reported. Sandwich-type polyoxometalates containing main group metal ions were synthesized first by Pope in 1996. In this research, we investigate the catalytic properties of tin-containing sandwich-type polyoxometalates, [(HOSn IV OH) 3 (XW 9 O 34 ) 2 ] n− . To the best of our knowledge, there is no report on the catalytic properties of these sandwich-type polyoxometalates. Tin-containing sandwich-type polyoxometalates are more hydrolytically stable than transition metal-substituted sandwich-type polyoxometalates in aqueous solution [26][27][28] and thus are more suitable for oxidation reactions. To overcome limitations of surface area, separation and recycling and, of course, to improve efficiency, sandwich-type polyoxometalates were supported on Nd-doped TiO 2 nanoparticles. A series of effective nanocomposites including different loading levels of sandwich-type polyoxometalates of [(HOSn IV OH) 3 (X-W 9 O 34 ) 2 ] n− (X = As (1), P (2) n = 12 and Si (3) n = 14) (10-30%) on Nd-doped TiO 2 nanoparticles were prepared by simple impregnation method and were used as heterogeneous catalysts. The catalytic activities of the tin-containing sandwich-type polyoxometalates and the prepared nanocomposites were examined in reactions such as oxidation of alcohols and sulfides with H 2 O 2 .

Chemical and apparatus
All reagents were commercially obtained and used without purification. Neodymium chloride hexahydrate (NdCl 3 ·6H 2 O), titanium(IV) isopropoxide (TTIP) and absolute ethanol were obtained from Merck. Potassium salts of [(HOSn IV OH) 3 (XW 9 O 34 ) 2 ] n− (X = As (1), P (2) n = 12 and Si (3) n = 14) were prepared according to the literature [26,27]. The crystal structure of the synthesized nanocomposites were characterized by an X'PertPro Panalytical, Holland diffractometer in 40 kV and 30 mA with CuKα radiation (λ = 1.5418 Å). Infrared spectra were recorded on a Bruker Vector 22 FTIR using KBr pellets. The morphology of nanocomposites was revealed by a scanning electron microscope (FESEM-TESCAN MIRA3) and a transmission electron microscope (TEM, Zeiss -EM10C -80KV). TEM images of the samples were prepared by dropping the ethanol dispersion on a lacey carbon-coated grid Cu Mesh 300. The elements in the nanocomposite were probed by energy-dispersive X-ray (EDX) spectroscopy accessory to the FESEM-TESCAN MIRA3 (SEM).

Preparation of Nd-TiO 2 nano-sized
Nd-TiO 2 nano-sized was synthesized using the reported procedure [29]. About 10 mL of Ti{OCH(CH 3 ) 2 } 4 was added to 30 mL of EtOH at room temperature. Concentrated HCl was added to the mixture to obtain pH 1.5, and the solution was marked as A. NdCl 3 ·6H 2 O (0.2 mmol, 0.07 g), the lanthanide precursor, was dissolved into the solution containing H 2 O (2 mL) and EtOH (4 mL) and was marked as B. Then, solution B was added dropwise into the A solution in approximately 10 min. The resulting acidic mixture was stirred constantly for 12 h until the sol was obtained. The sol was maintained for 24 h until gel formation. The gel was dried at 373 K for 4 h and then calcinated at 823 K for 3 h. The lanthanide doping in the support is 1.0% [29].

2.4.
General procedure for oxidation of sulfides to sulfoxides 2a (20%, 50 mg) was added to a mixture of sulfide (1 mmol) and 30% H 2 O 2 (6-8 mmol) in CH 3 CN (3 mL) and the mixture was stirred at room temperature for the time specified (table 1). Progress was monitored by TLC (EtOAc/n-hexane, 4/10). After completion of the reaction, the catalyst was separated from the product by centrifuge. The product was extracted with CH 2 Cl 2 (3 × 5 mL) and the combined organic solutions washed with brine (10 mL) and dried over anhydrous Na 2 SO 4 . The solvent was removed under reduced pressure to give the corresponding pure sulfoxide in most cases.

General procedure for the oxidation of sulfides to sulfones
Typically, 2a (20%, 50 mg) was added to a mixture of sulfide (1 mmol) and 30% H 2 O 2 (6-20 mmol) in H 2 O (1.5 mL) and the mixture was stirred at room temperature for the time specified (table 2). Progress was monitored by TLC (EtOAc/n-hexane, 4/10). After completion of the reaction, the catalyst was separated from the product by centrifuge. The product was extracted with CH 2 Cl 2 (3 × 5 mL) and the combined organic solutions washed with brine (10 mL) and dried over anhydrous Na 2 SO 4 . The solvent was removed under reduced pressure to give the corresponding pure sulfone in most cases.

General procedure for oxidation of benzylic alcohols
To a mixture of alcohol (1 mmol) and POMs-Nd-TiO 2 (40 mg) in H 2 O (1 mL), 30% H 2 O 2 (6-8 mmol) was added and the mixture was stirred at 80°C for the time specified (table 3). Progress was monitored by TLC (EtOAc/n-hexane, 4/10). After completion of the reaction, the mixture was cooled to room temperature and the catalyst was separated from the product by centrifuge. The product was extracted with CH 2 Cl 2 (3 × 5 mL) and the combined organic extractions washed with brine (10 mL) and dried over anhydrous Na 2 SO 4 . The solvent was removed under reduced pressure to give the corresponding pure carbonyl compound in most cases.

Preparation and characterization of POM-Nd-TiO 2 nanocomposites
Various synthetic techniques have been developed to produce TiO 2 in different forms [30][31][32][33][34][35][36][37]. The sol-gel method with metal alkoxides as starting materials is widely used to obtain titanium oxide nanoparticles [38]. The sol-gel technique and impregnation method were used for the preparation of POM-Nd-TiO 2 nanocomposites. In this research, sandwich-type polyoxometalates of [(HOSn IV OH) 3 (XW 9 O 34 ) 2 ] n− (X = As (1), P (2) n = 12 and Si (3) n = 14) as potassium salt are placed by different loadings (10-30%) onto the Nd-TiO 2 nanoparticle by impregnation. Desired polyoxometalates are prepared from the oxidation of [(Sn II ) 3 (XW 9 O 34 ) 2 ] n− (X = As, P, Si) by bromine [27]. The tin(IV) derivatives consist of two A-type XW 9 O 34 n− anions linked by three tin(IV) cations into an assembly of virtual D 3h symmetry (figure 1). Each Sn has two terminal OH − ligands due to high charge on the Sn IV [39][40][41]. Consistent with the elemental analysis, acid/base titrations show that 1, 2 and 3 have one acidic proton. The surface of Nd-TiO 2 particles is positively charged in acidic  media, whereas the POMs are negatively charged, thus facilitating the adsorption of the POM species on the surface of TiO 2 particles by simple Coulombic interactions. The interaction of acidic hydrogen of 1, 2, and 3 may occur by an exchange reaction leading to the formation of water and the replacement of the surface hydroxyl anion by a polyoxometalate anion [42,43]. Furthermore, Sn ions in the structures may have interaction with the surface of TiO 2 nanoparticles via their external OH groups. Therefore, by simple impregnation process, sandwich-type polyoxometalates strongly interact via oxygen atoms, through acidic hydrogen and also external OH groups of Sn ions with a surface of titania nanoparticles.  polyoxometalate on the Nd-TiO 2 . As a result, some characteristic peaks of POM unit in 2a composites and other composites are overlapped in this area (see figure 2). FTIR spectra of other samples are given in the Supporting Information (figures S1 and S2). Attenuated total reflectance Fourier Transform IR spectroscopy (ATR FTIR) has been used for surface analysis of the nanocomposites. As expected, the appearance of POMs component on the surface of the prepared nanocomposites must be greater than that expected from the mass ratio. The ATR FTIR spectrum of 2a shows strong peaks corresponding to 2, evidence for loading the desired POM. The ATR FTIR spectrum of 2a is given in the Supporting Information ( figure S3). Figure 3 shows the X-ray diffraction (XRD) testing results. XRD patterns of Nd-TiO 2 and pure TiO 2 are similar; both of them crystallized in  [44]. However, no characteristic peak of neodymium oxide is found, which implies that neodymium oxide content is very small and highly dispersed [45]. From figure 3 it can be seen that all the nanocomposites with 20% loading levels also have an anatase phase, and only anatase phase is present and no separate polyoxometalate-related phase is observed. These features probably result in the high dispersity of POM on Nd-TiO 2 nanoparticles [46]. The particle diameters calculated from the Debye-Scherrer formula are 20-50 nm.

SEM and TEM images.
The SEM images 1a (20%), 2a (20%), and 3a (20%) reveal that particles are regular spheres and less than 30 nm in diameter (figure 4), which is consistent with the XRD results. The TEM images of 1a (20%) and 2a (20%) are presented in figure 5, which reveal that most of the particles have a spherical shape. The average size of these nanoparticles is 20-30 nm which show a close agreement with the SEM images and the values calculated by XRD analysis.

EDX spectra.
The composition of the nanocomposites was characterized by energy dispersive X-ray analysis (EDX). The EDX spectra confirm the expected elemental composition of nanocomposites. Figure 6 shows the EDX spectrum of 2a (20%) which confirms its elemental composition (Nd, Ti, P, W, Sn and K). The EDX spectra of other samples are given in the Supporting Information (figures S4 and S5).

Catalytic application of POMs-Nd-TiO 2 in the oxidation of sulfides to sulfoxides and sulfones
The chemoselective preparation of sulfoxides and sulfones is extremely important in organic chemistry. Organosulfur compounds, such as sulfoxides and sulfones, are valuable intermediates for the synthesis of fine chemicals and biologically active compounds [47].   Oxidation of sulfides is the most straightforward method for the synthesis of these valuable materials. Concerning green oxidants, hydrogen peroxide is one of the most powerful candidates next to molecular oxygen because it is inexpensive, readily available, has high atom efficiency, and water is expected to be the only by-product generated from the reaction [48]. Oxidation of sulfides with H 2 O 2 is slow; hence, extensive studies have been undertaken to develop new catalysts for this reaction [49][50][51][52][53]. Although these protocols represent considerable progress, there is still a demand for new chemoselective, recoverable and reusable catalysts for the chemoselective oxidation of sulfides to sulfoxide and sulfone using green oxidants. We report the chemoselective oxidation of sulfides to sulfoxides or sulfones with H 2 O 2 in the presence of POM-Nd-TiO 2 catalyst. First, in order to find the best catalytic system for the selective oxidation of sulfide, the influence of various catalysts on the oxidation of methyl phenyl sulfide as a model compound using hydrogen peroxide in CH 3 CN at room temperature (table 1) was evaluated. As shown in table 1, the reaction was incomplete in the absence of any catalyst and in the presence of catalytic amount of free TiO 2 nanoparticles, Nd-TiO 2 and POMs even after prolonged reaction time (table 1, entries 1-6). All synthesized POM-Nd-TiO 2 (1a-3a) catalyzed the oxidation of methyl phenyl sulfide to methyl phenyl sulfoxide or sulfone in short reaction times in CH 3 CN at room temperature. Catalyst 1a produced only methyl phenyl sulfoxide while 2a or 3a as catalyst led to the corresponding methyl phenyl sulfone under the same reaction conditions (table 1, entries 7-9). To optimize the loading of POMs on Nd-TiO 2 nanoparticles, different loadings of POMs were prepared and used in the oxidation of methyl phenyl sulfide under the same reaction conditions. 20% loading of POMs (50 mg) was found to be ideal for the complete conversion of methyl phenyl sulfide to methyl phenyl sulfoxide or sulfone. When 10% loading of POMs is used, the reactions are completed in the presence of 100 mg of the catalysts (table 1, entries 10-12). The reaction was incomplete in the presence of 40% Figure 6. The EDX spectrum of 2a (20%).
loading even after prolonged reaction time (table 1, entry 13). One explanation for reducing the reaction rate is that excess loading (40% and more) probably act as Nd-TiO 2 deactivators. As shown in table 1, with 1a the reaction is completed in lower time (table 1, entry 10) than nanocomposite containing of 1 loaded on naked TiO 2 nanoparticle, possibly due to synergic effect between TiO 2 and Nd(III) (table 1, entry 14) [54]. When water was used as a solvent instead of CH 3 CN, only methyl phenyl sulfoxide was obtained in the presence of catalytic amount of all of three catalysts 1a-3a (table 1, entries 15-17). Therefore, the oxidation of methyl phenyl sulfide to sulfoxide or sulfone in the presence of 2a or 3a can be controlled by changing the solvent. To develop the scope of the oxidation of sulfides further, H 2 O 2 (6-8 mmol) in the presence of 20% loading of 2a (50 mg) in H 2 O or CH 3 CN at room temperature was selected for chemoselective oxidation of sulfides to sulfoxides or sulfones. Various sulfides were subjected to oxidation under the optimized reaction conditions and the expected products were obtained in short times and at high yields (table 2). To show the chemoselectivity of this method, the sulfide containing oxidation-prone functional group (2-thio ethanol) was subjected to oxidation; the OH group remained intact during the conversion of sulfide to sulfoxide or sulfone (table 2, entry 4).

Catalyst recovery and reuse.
For practical purposes, recyclability of the catalyst is highly desirable. To investigate this issue, the reusability of 2a was examined for oxidation of methyl phenyl sulfide as a model substrate. For each of the repeated reactions, the catalyst was easily separated from the product by centrifuge, washed with H 2 O and dichloromethane to remove the residual product, dried at 100°C for 2 h and reused in a subsequent reaction. The catalyst was reused six and 10 times in the oxidation of methyl phenyl sulfide to methyl phenyl sulfoxide and methyl phenyl sulfone, respectively, without a detectable catalyst leaching or a significant loss of its activity (figures 7a and 7b). The FTIR spectrum did not show significant structural changes for catalysts after 10 consecutive runs ( figure S6).

Catalytic application of POMs-Nd-TiO 2 in the oxidation of benzylic alcohols
The selective oxidation of alcohols to their corresponding carbonyl compounds is a challenging reaction in organic chemistry [55]. Traditionally, the oxidation of alcohols has been achieved with stoichiometric inorganic oxidants; the disadvantages of these oxidants are that they are expensive, produce large quantities of waste and cause serious environmental problems [56]. Consequently, the use of green and inexpensive oxidants such as oxygen and hydrogen peroxide in effective catalytic systems has attracted attention because of their economic and environmental benefits [57][58][59]. Owing to the success of POM-Nd-TiO 2 for carrying out oxidation of sulfides, we studied the possibility of applying this new catalyst for the oxidation of alcohols. First, the catalytic activity of various catalysts including Nd-TiO 2 , 1, 2, 3, 1a, 2a, and 3a was investigated in the oxidation of benzyl alcohol with 30% hydrogen peroxide as a model reaction in water at 80°C ( . The results showed that benzylic alcohols were more reactive than aliphatic substrates and were transformed to the corresponding carbonyl compound with good to high yields. Among benzylic alcohols, electron-rich alcohols are more reactive than electron-poor ones (table 4, entry 2). These results prompted us to explore chemoselectivity of this heterogeneous catalytic system in a binary mixture of benzyl alcohol (as a model for primary benzylic alcohol) and 2-phenyl ethanol (as a model for primary alcohol) in the presence of 2a. The benzyl alcohol was completely converted to benzaldehyde, while 0% conversion was observed for 2-phenyl ethanol to the corresponding carbonyl compounds (scheme 1). This reveals that this method can be applied for the chemoselective oxidation of benzylic alcohols in the presence of primary aliphatic alcohols. We have found that 1a and 2a in the oxidation of alcohols are more active than 3a, maybe due to higher charge As(V) and P(V) in the structure of 1a and 2a compared to Si(IV) in the structure of 3a.
4.2.1. Catalyst recovery and reuse. The reusability of the heterogeneous catalyst, 2a, was examined using benzyl alcohol as a model substrate. After the first use of catalyst in oxidation of benzyl alcohol to give benzaldehyde, the catalyst was easily separated from the product by centrifuge, washed with H 2 O and dichloromethane to remove the residual product, dried at 100°C for 2 h and reused for subsequent experiments under similar reaction conditions. As shown in figure 8, the catalyst was reused at least five times without significant loss of activity.

Reactions mechanism.
Catalytic oxidation of various organic substrates with hydrogen peroxide as oxidant may be performed via homolytic or heterolytic cleavage of  [59]. We ran the oxidation of alcohols and sulfides catalyzed by 1a, 2a, or 3a and hydrogen peroxide as oxidant in the presence of 2,2′-azobis(isobutyronitrile) as radical scavenger. It has been observed that addition of 2,2′-azobis(isobutyronitrile) has no significant effect on the yield of the product or reaction times. This result confirmed that with 1a, 2a, or 3a the preferred reaction path is heterolytic cleavage of the O-O bond.

Conclusion
We have developed noncovalently immobilized sandwich-type polyoxometalates [(HOSn I-V OH) 3 (XW 9 O 34 ) 2 ] n− (X = As (1), P (2) n = 12 and Si (3) n = 14) using Nd-TiO 2 nanoparticles as support. The synthesized catalysts were confirmed by XRD, FTIR, SEM, TEM, and EDX. These catalysts were used for the oxidation of sulfides and benzylic alcohols  efficiently in the presence of H 2 O 2 (30%) as oxidant. These procedures offer several major advantages,which are as follows: (1) the use of a commercially available, cheap and green oxidant; (2) control over the degree of oxidation it offers sulfoxides or sulfones; (3) excellent chemoselectivity for the oxidation of benzylic alcohols; (4) the heterogeneous catalysts can be easily separated and reused several times; (5) the method conforms to several of the guiding principles of green chemistry.

Disclosure statement
No potential conflict of interest was reported by the authors.