Polyoxometalates Hosted in Layered Double Hydroxides: Highly

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Polyoxometalates Hosted in Layered Double Hydroxides: Highly Enhanced Catalytic Activity and Selectivity in Sulfoxidation of Sulfides Kai Liu, Zhixiao Yao, and Yu-Fei Song* State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, P. R. China

Downloaded by UNIV OF LETHBRIDGE on September 14, 2015 | http://pubs.acs.org Publication Date (Web): September 11, 2015 | doi: 10.1021/acs.iecr.5b02298

S Supporting Information *

ABSTRACT: Layered double hydroxides (LDHs) are a class of layered materials with tunable interlayer galleries. In this paper, a series of Mg3Al−P2W17X (X = MnIII, FeIII, ZnII, CoII, CuII, NiII) have been prepared successfully by intercalating the polyoxometalate (POM) of [α2-P2W17O61(X·OH2)]n− ([P2W17X]n−, n = 7 or 8) into a Mg3Al−suberic precursor. Catalytic tests for H2O2-based sulfoxidation of various sulfides demonstrate that Mg3Al−P2W17Zn exhibits better catalytic efficiency and selectivity than pure POM or LDH precursors under mild conditions. The uniform and well-ordered dispersion of POM in the confined gallery of LDHs and the multiple interactions between POMs and LDHs contribute to the excellent catalytic performance. Moreover, Mg3Al−P2W17Zn is stable and can be easily separated from the reaction system. The recycled Mg3Al− P2W17Zn maintains both the intact structures of the POM anion and LDHs. The scaled-up experiment provides further support for its potential use for industrial applications. properties, acidities, solubilities, etc., can be finely tuned by adjusting the metal ions and counterions, which endow POMs to be superior catalysts for oxidative and acid catalysis.23−31 In the last decades, various kinds of POMs have been developed for the H2O2- and O2-based green oxidations of alcohols, alkenes, and sufides.32−37 However, there are two main drawbacks impeding their potential application in industrial production. First, high solubility in solution makes them difficult to separate from the mixture of the reaction. Many efforts have been made in order to solve this problem. The incorporation of homogeneous POMs into inorganic supporting materials, such as SiO2,38,39 Al2O3,40,41 layered rare-earth hydroxides,42 and some molecular sieves,43,44 has been widely studied. Although these hybrids can be separated and recovered for recycling, they have shown disadvantages such as low activity, catalyst leaching, or less stability during the reaction. Second, some POM-based catalysts show less selectivity in catalytic reactions because of their strong oxidizing property and strong acidity. Therefore, it is highly desirable to develop an efficient and selective POM-based heterogeneous catalyst. Layered double hydroxides (LDHs) are a class of layered materials with tunable interlayer galleries.45 The general molecular formula of LDHs is [M2+1−xM3+x(OH)2](An−)x/n· mH2O, where M2+ and M3+ are di- and trivalent metal cations and An− is a counteranion (denoted as MII(1−x)/xMIII-A). LDHs can provide “flexible” confining space that can be adjusted by changing the size and arrangement of guest molecules. The flexible interlayer space can not only fit small-sized moieties but also accommodate bulky catalytic sites46,47 that have difficulty entering the rigid pores. Thus, LDHs are ideal supports for bulky POM catalysts.

1. INTRODUCTION The selective oxidation of sulfides is an important process for the preparation of sulfoxides or sulfones because these products are versatile intermediates and/or bioactive compounds in organic synthesis and asymmetric catalysis.1−3 Previously, various strong oxidants such as m-chloroperbenzoic acid, oxone, NaIO4, NaClO, and KMnO4 were used for the oxidation of sulfides.4−8 However, these systems not only corrode the equipment but also generate large amounts of wastes during the reaction process. As an alternative, oxidation of sulfides with H2O2 is one of the most attractive processes because it is inexpensive, ecofriendly, readily available, and an easy operation.9,10 To date, a number of catalysts have been applied for the oxidation of sulfides with H2O2 as the oxidant such as HAuCl4·4H2O,11 SiO2−H2SO4,12 nanocrystallite TiO2,13 TaCl5,14 Si-V10-2,15 and some molybdenum- and tungsten-containing catalysts including MoO2Cl2,16 PPh 4 [MO(O 2 ) 2 L], 1 7 Na 2 WO 4 /C 6 H 5 PO 3 H 2 /[CH 3 (nC8H17)3N]HSO4,18 WO3/MCM-48,19 and WO42−/silica− NH3+20 (see Table S1). For example, Wang et al. reported a novel ionic liquid-based polyoxometalate (POM)21 catalytic system that shows exclusive oxidation of sulfoxide at room temperature with a very short time. It is noted that some of these catalytic systems suffer from low activity and selectivity, less thermostability, difficult recycling, etc. Some of them are impossible to use for further industrial application because of the economy/environmental consideration and/or potential leaching problems. As a result, there is enough space for the development of highly efficient heterogeneous catalysts for the selective oxidation of sulfides with H2O2 as the oxidant, which can be easily recovered and recycled many times. POMs are a large class of anionic metal oxides of vanadium, molybdenum, tungsten, niobium, etc., in their highest oxidation states, and they are thermally and oxidatively stable compared with commonly used organic ligands.22 The chemical properties of POMs such as the redox potential, electron-transfer © XXXX American Chemical Society

Received: June 24, 2015 Revised: August 19, 2015 Accepted: September 4, 2015

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DOI: 10.1021/acs.iecr.5b02298 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research


Scheme 1. Schematic Representation of the Synthesis of [α2-P2W17O61(X·OH2)]n− Intercalated into LDHs

°C/min from 32 to 800 °C. XPS measurements were performed with monochromatized Al Kα exciting X-radiation (PHI Quantera SXM). Inductively coupled plasma emission spectroscopy (ICP-ES; Shimadzu ICPS-7500) was used to measure the concentration of tungsten in the catalysts. Gas chromatography (GC) analyses were performed with an Agilent 7820A GC system using a 30 m 5% phenylmethyl silicone capillary column with an i.d. of 0.32 mm and 0.25 mm coating (HP-5). 2.4. Oxidation of Sulfides. In a typical experiment, 1 mmol of thioanisole, 102 μL of aqueous H2O2 (30 wt %), 2.5 μmol of Mg3Al− CO3, 2.5 μmol of Mg3Al−suberic or Mg3Al−P2W17Zn as the catalyst, and 200 μL of ethanol were added to a 20 mL glass bottle, and the reaction mixture was kept for vigorous stirring at room temperature. After 6 h, the reaction was effectively quenched. The reaction mixture was diluted with 1 mL of water and 6 mL of diethyl ether, then stirred for 5 min, and filtered using a 0.22 μm microfilter. The filtrate was analyzed by GC and identified by 1H NMR to determine the conversion and selectivity. After being washed with acetone, the heterogeneous catalyst of Mg3Al−P2W17Zn was recovered by centrifugation, dried under vacuum, and recycled for the next run.

Herein, six POM-intercalated LDH catalysts of Mg3Al− P2W17X have been prepared by intercalating a series of POM clusters of [α2-P2W17O61(X·OH2)]n− (for X = MnIII and FeIII, denoted as [P2W17X]7−; for X = ZnII, CoII, CuII, and NiII, denoted as [P 2 W 17 X] 8− ) into the Mg 3 Al−suberic support. The application of the designed catalysts of Mg3Al−P2W17X for the sulfoxidation of various sulfides with H2O2 as the oxidant suggests that Mg3Al−P2W17Zn exhibits the highest activity and selectivity, coupled with convenient recovery and reuse. The recovered catalyst of Mg3Al−P2W17Zn remains its structure, which has been confirmed by Fourier transforn infrared (FT-IR), X-ray diffraction (XRD), 31P NMR, and X-ray photoelectron spectroscopy (XPS) characterization.

2. EXPERIMENTAL SECTION 2.1. Materials. Analytically pure Mg(NO3)2·6H2O, Al(NO3)3· 9H2O, suberic acid, hexamethylenetetramine (99%), KHCO3, Zn(NO3)2·6H2O, MnCl2·4H2O, Co(NO3)·6H2O, KCl, K2S2O8, aqueous H2O2 (30 wt %), methanol (99%), ethanol (99%), diethyl ether (99%), and various sulfides were all purchased from Alfa Aesar. 2.2. Catalyst Preparation. Preparation of POMs. K8[α2P2W17O61(Zn·OH2)]·13H2O and other POMs were synthesized according to the procedures reported previously.48 The LDH support Mg3Al−CO3 was prepared according to literature methods.49 Preparation of an LDH−POM Catalyst. Taking Mg3Al−P2W17Zn as an example, the detailed procedure is as follows: the slurry of Mg3AlCO3 was obtained by dispersing 1 g of Mg3Al−CO3 in 90 mL of methanol under N2 for 30 min. A solution of suberic acid (5.81 mol, 1.0 g) in 15 mL of methanol was added dropwise to the slurry of Mg3Al− CO3 under N2 with rapid stirring for 5 h. After that, the reaction mixture was filtered and the precipitate was washed with methanol (note: the precipitate is Mg3Al−suberic). Then, the slurry of Mg3Al−suberic was prepared by dispersing Mg3Al−suberic in 120 mL of decarbonated H2O. A solution of 6 g of K8−P2W17O61(Zn·OH2)]·13H2O in 62 mL of decarbonated H2O was added dropwise to the above-mentioned Mg3Al−suberic slurry under N2 with rapid stirring. The mixture was stirred for 5 h at 100 °C. Finally, a Mg3Al−P2W17Zn catalyst was obtained as a white solid after being washed with the boiled and decarbonated water three times and dried at 60 °C overnight under vacuum. 2.3. Catalyst Characterization. Powder XRD patterns were recorded on a Rigaku XRD-6000 diffractometer under the following conditions: 40 kV, 30 mA, Cu Kα radiation (λ = 0.154 nm). FT-IR spectra were recorded on a Bruker Vector 22 IR spectrometer using KBr pellets. The solid-state NMR experiments were carried out at 121.0 MHz for 31P on a Bruker Avance 300 M solid-state spectrometer equipped with a commercial 5 mm magic-angle-spinning (MAS) NMR probe. The N2 adsorption−desorption isotherms were measured using a Quantachrome Autosorb-1 system at the liquid-N2 temperature. Scanning electron microscopy (SEM) images and energy-dispersive X-ray (EDX) analytical data were obtained using a Zeiss Supra 55 scanning electron microscope equipped with an EDX detector. Transmission electron microscopy (TEM) micrographs were recorded using a Hitachi H-800 instrument. Thermogravimetric and differential thermal analyses (TG-DTA) were performed on a TGA/DSC 1/1100 SF analyzer from Mettler Toledo in flowing N2 with a heating rate of 10

3. RESULTS AND DISCUSSION Scheme 1 shows the synthetic procedure for the preparation of the designed heterogeneous catalyst. In step 1, suberic acid was

Figure 1. XRD patterns of (a) Mg3Al−CO3 and Mg3Al−suberic and (b) Mg3Al−P2W17X (X = MnIII, FeIII, CoII, NiII, CuII, ZnII). B


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Figure 4. (a) TG-DTA of Mg3Al−P2W17Zn. (b) Adsorption− desorption isotherms of Mg3Al−CO3, Mg3Al−suberic, and Mg3Al− P2W17Zn.

Figure 2. (a) FT-IR spectra of Mg3Al−CO3, Mg3Al−suberic, Mg3Al− P2W17Zn, and K−P2W17Zn. (b) 31P CP/MAS NMR spectrum of Mg3Al−P2W17Zn.

resultant Mg3Al−P2W17X (X = MnIII, FeIII, ZnII, CoII, CuII, NiII) has been demonstrated by using various spectroscopic methods. 3.1. XRD Analysis. Powder XRD patterns of Mg3Al−CO3, Mg3Al−suberic, and Mg3Al−P2W17Zn in Figure 1 show characteristic reflections of Mg3Al LDHs. The corresponding

used to intercalate into Mg3Al−CO3 to widen the gallery, which was followed by ion exchange of the Dawson-type POM anions of [α2-P2W17O61(X·OH2)]n− with the presupported Mg3Al− suberic in step 2. The detailed structural characterization of the

Figure 3. Survey XPS spectra of (A) the Mg3Al−P2W17Zn sample, (B) Mg 1s, (C) Al 2p, (D) P 2p, (E) W 4f, and (F) Zn 2p. C


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Industrial & Engineering Chemistry Research Table 1. Physiochemical Properties of Mg3Al−CO3, Mg3Al− Suberic, and M3Al−P2W17Zn material


Mg3Al−CO3 Mg3Al−suberic Mg3Al− P2W17Zn

surface area (m2/g)

pore volume (cm3/g)

average pore diameter (nm)

14 10 18

0.052 0.027 0.048

79.46 45.83 68.17

Scheme 2. Model Reaction of Sulfoxidation of Thioanisole

Table 2. Sulfoxidation of Thioanisole in Ethanol with H2O2 by Different Catalystsa

basal spacing of d(003) has been summarized in Table S2. The gallery height value of 1.25 nm is obtained by subtracting the thickness of the host layer (0.48 nm)45 from the value of the d(003) spacing of Mg3Al−P2W17Zn. The gallery height is in good accordance with the estimated diameter of the Dawson anion along the short (C2) axis (1.23 nm). Therefore, the orientation of the intercalated [α2-P2W17O61(Zn·OH2)]7− (denoted as [P2W17Zn]7−) has the C2 axis orthogonal to the LDH layers in order to optimize the hydrogen-bonding interactions between the LDH layer hydroxyl groups and the apical oxygen atoms on the POM frameworks.50 Similar intercalation and orientation of other POM anions of [α2P2W17O61(X·OH2)]n− (X = MnIII, FeIII, CoII, CuII, NiII) can be found, as shown in Figure 1b. 3.2. FT-IR and Solid-State NMR Analysis. The FT-IR spectrum of the Mg3Al−suberic (Figure 2a) precursor shows a strong peak at 1558 cm−1, which is assigned as the ν3 vibration of the −COO− anions in the interlayer galleries. The peak at 1558 cm−1 disappears in the FT-IR spectrum of the corresponding Mg3Al−P2W17Zn sample, indicating complete exchange of the [P2W17Zn]8− anions and suberic anions. The FT-IR spectrum of K8[α-2-P2W17O61(Zn·OH2)]·13H2O (denoted as K−P2W17Zn) in Figure 2a exhibits peaks at 950, 914, and 782 cm−1, which can be attributed to the vibrations of W−Ot, W−Oe−W, and W− Oc−W (t, terminal; c, corner sharing; e, edge sharing), respectively. These W−O stretching bands can be clearly observed in the FT-IR spectrum of Mg3Al−P2W17Zn (Figure 1c) with slight shifts to 949, 919, and 798 cm−1, respectively. The slight shift of these peaks is due to the presence of hydrogenbonding interactions between the hydroxyl groups in the layers

entry

catalyst

yieldb (%)

selectivityb (%)

TONc

1 2 3 4 5 6 7 8 9 10 11 13 14 15

Mg3Al−P2W17Zn Mg3Al−CO3 Mg3Al−suberic K−P2W17Zn Mg3Al−CO3 + K−P2W17Zn Mg3Al−suberic + K−P2W17Zn Mg(OH)2 Al(OH)3 none Mg3Al−P2W17Mn Mg3Al−P2W17Fe Mg3Al−P2W17Cu Mg3Al−P2W17Co Mg3Al−P2W17Ni

96 37 45 69 70 72 32 33 33 56 64 70 70 68

>99.9 >99.9 >99.9 87 93 95 83 82 82 >99.9 >99.9 >99.9 >99.9 >99.9

384 148 180 240 260 274 106 108 108 224 256 280 280 272

a

Reaction conditions: 1 mmol of substrate, 1 mmol of (30% aqueous) H2O2, 2.5 μmol of catalyst, 200 μL of ethanol, room temperature, 6 h. b Yields were determined by GC analysis. cTON = moles of product/ moles of catalyst used.

of LDHs and the oxygen atoms of POMs.51 Meanwhile, the peaks at 1088 and 1016 cm−1 are ascribed to the bonds of P−O, which are the same in the FT-IR spectrum of Mg3Al−P2W17Zn and K−P2W17Zn. FT-IR spectra of Mg3Al−P2W17X (X = MnIII, FeIII, CoII, NiII, CuII) can be found in the Supporting Information. Furthermore, two sets of signals centered at −12.9 and −5.7 ppm in the 31P cross polarization (CP)/MAS NMR spectrum of Mg3Al−P2W17Zn imply that [α2-P2W17O61(Zn·OH2)]8− anions

Figure 5. (A and B) SEM images of Mg3Al−P2W17Zn. (C) High-resolution TEM image of Mg3Al−P2W17Zn. (D) EDX of Mg3Al−P2W17Zn. D


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second weight loss of 7.11% at 245−460 °C is due to collapse of the layered structure. The third weight loss of 2.88% at 460− 1000 °C corresponds to decomposition of the POM anions.54 The Mg3Al−P2W17Zn sample in Figure 4b shows a type I adsorption isotherm at lower relative pressure (P/P0 < 0.1) and a H3 type N2 hysteresis loop at higher relative pressure (P/P0 > 0.5) according to BDDT (Brunauer−Deming−Deming−Teller) classification,55 indicating the presence of both interlayer micropores and interparticle mesopores. The Brunauer− Emmett−Teller surface area of Mg3Al−P2W17Zn (18 m2/g) is higher than that of Mg3Al−suberic (10 m2/g; see Table1). The experimental W/Zn atomic ratio of Mg3Al−P2W17Zn (2.17/0.13 = 16.69) by ICP measurement is in good agreement with the theoretical one (W/Zn ratio = 17/1 = 17) of the transition-metal-substituted Dawson POM cluster of K− P2W17Zn. On the basis of TG-DTA and elemental analysis, the elemental composition of the product can be expressed as Mg0.75Al0.25(OH)2[P2W17O61(Zn·OH2)]0.031·1.38H2O (Table S2). 3.5. SEM, TEM, and EDX Analysis. SEM (Figure 5A,B) and TEM (Figure 5C) images of Mg3Al−P2W17Zn show slightly irregular hexagonal platelike LDH crystallites. The plate sizes of Mg3Al−P2W17Zn are in the range of 1−2 μm. EDX of Mg3Al− P2W17Zn (Figure 5D) demonstrates that all of the elements of LDHs and the POM anions exist in the sample, which implies that the POM anions have been intercalated into the interlayer of LDHs. 3.6. Effect of Different Catalysts on Sulfoxidation of Thioanisole. To explore the effect of different catalysts for the sulfoxidation reaction, thioanisole and ethanol are selected as the substrate and solvent, respectively, for the model reaction (Scheme 2). Initially, thioanisole is used as a model substrate to check the feasibility of oxidation using 30% H2O2 as the oxidant. As shown in Table 2, the reaction proceeds well in ethanol in the presence of a catalytic amount of Mg3Al−P2W17Zn at room temperature. An excellent yield of sulfoxide (96%) can be obtained with exclusive selectivity (Table 1, entry 1). However, only 33% yield of sulfoxidation is obtained in the absence of catalyst (entry 9). The reaction proceeds sluggishly by using Mg3Al−CO3, Mg3Al−suberic, and K−P2W17Zn or the simple physical mixture of them (entries 2−6). To prove that the catalytic activity is not from the impurities, Mg(OH)2 and Al(OH)3 (entries 7 and 8) are applied, and they exhibit 32% and 33% yield, respectively. Contrast experiments (entries 10−15) are carried out by utilizing different transition-metal-substituted Dawson clusters such as Mg3Al−P2W17X (X = MnIII, FeIII, CoII, NiII, CuII). The results indicate that Mg3Al−P2W17Zn is the most efficient catalyst for sulfoxidation. The catalytic yield of Mg3Al− P2W17Zn is higher than that of K−P2W17Zn. It was reported that oxidation of organosulfur compounds occurred through the electrophilic attack of an activated peroxo complex to sulfide.56 Thus, the catalytic activity for such sulfoxidation is supposed to increase with an increase of the electron deficiency in the catalytic active center. Because of the positive charge of the brucite-like layers, the electronic density of the [P2W17Zn]8− anions hosted in the LDHs is less than that of the corresponding K−P2W17Zn, which can account for the difference of the catalytic activity. The above experimental results strongly suggest that the cooperative and confined effect of Mg3Al−P2W17Zn plays a significant role for such considerable enhancement of sulfoxide yields. 3.7. Effect of Different Solvents on Sulfoxidation of Thioanisole. The study of the solvent effect on the sulfoxidation reaction (Figure 6) shows that yields of 75%, 59%, 81%, 96%,

Figure 6. Effect of solvents on sulfoxidation of thioanisole by Mg3Al− P2W17Zn. Reaction conditions: 1 mmol of substrate, 1 mmol of (30% aqueous) H2O2, 2.5 μmol of catalyst, 200 μL of solvent, room temperature, 6 h.

Figure 7. Sulfoxidation of thioanisole by Mg3Al−P2W17Zn and ln(Ct /C0) as a function of the reaction time. Reaction conditions: 1 mmol of substrate, 1 mmol of (30% aqueous) H2O2, 2.5 μmol of catalyst, 200 μL of ethanol, room temperature, 6 h.

exist in the LDHs (Figure 2b). The broad peak at −12.9 ppm in the 31P NMR spectrum of M3Al−P2W17Zn (Figure 2a) can be assigned to the phosphorus atom distal to the zinc center, which is in accordance with the pure POM K−P2W17Zn (−12.8 ppm), while the appearance of a new peak at −5.7 ppm in the spectrum of M3Al−P2W17Zn, which cannot be observed in the spectrum of K−P2W17Zn, may be attributed to the hydrogen-bonding and electrostatic interactions between the LDH layers and the guest POM anions.52 3.3. XPS Analysis. Survey XPS analysis of Mg3Al−P2W17Zn shows that the W 4f spectrum (Figure 3F) can be deconvoluted into doublets, which consist of W 4f7/2 at 35.3 eV and W 4f5/2 at 37.4 eV, respectively. The doublets are ascribed to the tungsten in the W−O bond configuration that typically can be observed for W6+.53 This result is in good accordance with the oxidation state of the tungsten element of Mg3Al−P2W17Zn. 3.4. TG-DTA, N2 Adsorption−Desorption, and ICP Mass Spectrometry Analysis. TG-DTA has been performed on Mg3Al−P2W17Zn. As shown in Figure 4, three weight-loss stages can be observed with an increase of the temperature from 25 to1000 °C. The first weight loss of 9.52% in the range of 30 to 245 °C is attributed to the removal of water molecules absorbed on the surface and interlayer space of Mg3Al−P2W17Zn. The E


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Table 3. Sulfoxidation of Various Sulfides to Sulfoxides Catalyzed by Mg3Al−P2W17Zn in Ethanola

Reaction conditions: 1 mmol of substrate, 1 mmol of (30% aqueous) H2O2, 2.5 μmol of Mg3Al−P2W17Zn, 200 μL of solvent, room temperature, 6 h. bYields were determined by GC analysis. cTON = molar of product/molar of catalyst used. a

65%, 6%, 71%, and 45% can be achieved in acetone, 1-butanol, acetonitrile, ethanol, water, chloromethane, i-propanol, and none, respectively. Such a difference can be attributed to the

solvation effect and different interactions between thioanisole and the solvents. As a result, the optimized solvent for such a sulfoxidation reaction is ethanol. F


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Powder XRD patterns of Mg3Al−P2W17Zn recycled 3, 7, and 10 times (Figure S6B) show the characteristic reflections of the layered structure, and the corresponding basal spacing d(003) of the recycled Mg3Al−P2W17Zn is 1.73 nm. FT-IR spectra of recycled Mg3Al−P2W17Zn exhibit characteristic peaks at 1088, 949, 919, and 798 cm−1 (Figure S6C). The 31P CP/MAS NMR and XPS spectra of M3Al−P2W17Zn can confirm that the structure of the reused catalyst remains unchanged during the catalytic reaction. ICP results show that the composition of recycled Mg3Al−P2W17Zn is nearly the same as that of the fresh one (Table S3). To confirm that the catalyst of Mg3Al−P2W17Zn is a truly heterogeneous catalyst, the sulfoxidation reaction of thioanisole was carried out using Mg3Al−P2W17Zn as the catalyst and H2O2 as the oxidant in ethanol at room temperature. When the yield reached about 80%, the solid catalyst was filtered by simple filtration. The reaction mixture was allowed to proceed with the filtrate under the same experimental conditions as those above. It was found that no additional sulfoxide of thioanisole can be obtained (Figure 8, red line). While the solid catalyst Mg3Al− P2W17Zn was added back to the reaction mixture, the catalytic reaction can continue as before. This experimental result suggests that Mg3Al−P2W17Zn is a heterogeneous catalyst. The leaching test was carried out after the catalyst of Mg3Al− P2W17Zn was reused 5 and 10 times, the reaction mixture was filtered, and ICP measurement of the filtrate showed that no detectable tungsten and zinc can be found, indicating that almost no leaching of the catalyst can be observed. These results clearly reflect the advantage of applying LDHs as carriers for the active POM anions because of the presence of multiple interactions between the host LDHs and guest POM anions. 3.12. Scaled-Up Experiments. The scaled-up experiments by expansion to 100 times the original experimental conditions suggest that the reaction proceeds smoothly in ethanol in the presence of 250 μmol of Mg3Al−P2W17Zn at room temperature (reaction conditions: 100 mmol of substrate, 100 mmol of (30% aqueous) H2O2, 250 μmol of Mg3Al−P2W17Zn, 20 mL of ethanol, room temperature, 6 h). An excellent yield of sulfoxide of 94% can be obtained with 99% selectivity of the corresponding sulfoxide. This result demonstrates the great potential of Mg3Al− P2W17Zn for further industrial applications. Detailed experiments are under investigation.


Figure 8. Effect of the removal of Mg3Al−P2W17Zn in 3 h from the reaction mixture on sulfoxidation of thioanisole.

3.8. Kinetic Study of Sulfoxidation of Thioanisole. To obtain the kinetic parameters for sulfoxidation of thioanisole by Mg3Al−P2W17Zn, experiments have been performed with H2O2 as the oxidant in ethanol at room temperature. The percentage of conversion and ln(Ct/C0) are plotted against the reaction time in Figure 7, in which C0 and Ct are the initial thioanisole concentration and the concentration at time t, respectively. The linear fit of the data reveals that the catalytic reaction exhibits pseudo-first-order kinetics for sulfoxidation (R2 = 0.9943). The rate constant k of the sulfoxidation reaction can be determined to be 0.5364 h−1. Sulfoxidation of thioanisole can be complete in about 6 h. Thus, the catalyst exhibits high catalytic efficiency for sulfoxidation of thioanisole, and the catalytic reaction obeys pseudo-first-order kinetics with 100% selectivity for sulfoxide of thioanisole.

−dCt = kCt dt ln

C0 = kt Ct

(1)

(2)

3.9. Sulfoxidation of Different Sulfides. To test the generality and selectivity of the as-prepared catalyst of Mg3Al− P2W17Zn, a series of thioethers have been subjected to the optimized reaction conditions (Table 3). The results demonstrate that dialkyl thioethers and alkylaryl thioethers almost give excellent yields and selectivity. However, diphenyl thioether shows a relatively slow reaction, which might be due to the low electron density on the sulfur atom of the substrate (Table 3, entry 14). Interestingly, the phenylallyl thioether only gives the desired sulfoxide in 54% yield along with an unknown byproduct (Table 3, entry 13). 3.10. Influence of the Amount of H2O2 on the Selectivity of Sulfoxidation. In a subsequent catalytic reaction, we gradually increase the amount of the oxidant H2O2 in order to see whether it is possible to obtain the sulfoxide and sulfone under the experimental conditions. It is worth noting that the catalyst of Mg3Al−P2W17Zn shows a remarkable selectivity toward sulfoxides with an excess amount of H2O2 (2.0−5.0 equiv to the substrates). 3.11. Catalyst Recycling and the Leaching Experiment. To have a better understanding of the heterogeneous catalyst of Mg3Al−P2W17Zn, it is recycled by simple filtration after the catalytic reaction each time and reused for sulfoxidation of thioanisole in the next run. As shown in Figure S6, it can be noted that the catalytic results remain almost unchanged after 10 times.

4. CONCLUSIONS In summary, we have synthesized and fully characterized a number of Mg3Al−P2W17X (X = Mn, Fe, Co, Ni, Zn) compounds successfully. Intercalation of [P2W17Zn]8− anions into the positively charged LDH interlayers leads to the electron deficiency of [P2W17Zn]8− anions in contrast to pure K− P2W17Zn. It has been demonstrated that Mg3Al−P2W17Zn shows great reactivity and selectivity for sulfoxidation of various sulfides under mild conditions. The 2D confinement effect of LDHs leads to the uniform and well-ordered dispersion of [P2W17Zn]8− anions. The multiple host−guest interactions endow the heterogeneous catalyst of Mg3Al−P2W17Zn with excellent recycling performance. The leaching experiment suggests that there is no detectable tungsten in the filtrate. It is noted that Mg3Al−P2W17Zn exhibits excellent selectivity for sulfoxide even with an increase of the H2O2 amount to 2−5 times that of the substrate. The scaled-up experiment shows the great potential of Mg3Al−P2W17Zn for further application. G


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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.5b02298. FT-IR spectra of Mg3Al−suberic, Mg3Al− P2W17(Mn,Fe,Co,Ni,Cu), and K− P2W17(Mn,Fe,Co,Ni,Cu) (Figures S1−S5), oxidation of thioanisole to sulfoxides by different catalysts with H2O2 as the oxidant (Table S1), catalyst recycling experiments and characterization of the reused catalyst of Mg3Al−P2W17Zn (Figure S6), composition of Mg3Al−suberic and Mg3Al− P2W17Zn (Table S2), and ICP measurements of the composition of the catalyst (Mg3Al−P2W17Zn) (Table S3) (PDF)

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AUTHOR INFORMATION


Corresponding Author

*Tel/Fax: +86 10 64431832. E-mail: [email protected] or [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Basic Research Program (Grant 2014CB932104), National Science Foundation of China (Grants 21222104 and U1407127), Fundamental Research Funds for the Central Universities (Grants RC1302 and YS1406), and Beijing Engineering Centre for Hierarchical Catalysts.

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Polyoxometalates Hosted in Layered Double Hydroxides: Highly

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