Green Chemistry PAPER
Cite this: Green Chem., 2013, 15, 2913
Tungstate based poly(ionic liquid) entrapped magnetic nanoparticles: a robust oxidation catalyst† Ali Pourjavadi,*a Seyed Hassan Hosseini,a Firouz Matloubi Moghaddam,b Behzad Koushki Foroushanib and Craig Bennettc A novel magnetically recoverable oxidation catalyst was prepared in which magnetic nanoparticles were
Received 4th July 2013, Accepted 7th August 2013
entrapped in a tungstate functionalized poly(ionic liquid) matrix. Using H2O2 as an oxidant, a wide range of substrates including alcohols, sulfides and olefins were selectively oxidized with excellent yields. The resulting catalyst was characterized by FTIR, TGA, SEM, TEM, XRD, XRF, CHN, vibrating sample magneto-
DOI: 10.1039/c3gc41307a
meter (VSM) and atomic adsorption spectroscopy (AAS). The catalyst can be readily recovered and reused
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at least ten times under the described reaction conditions without significant loss of reactivity.
Introduction Over the past several decades, oxidation reactions based on H2O2 have received a lot of attention because of their environmental and economic benefits.1–3 Several transition metals such as manganese,4 molybdenum,5 rhenium6 and tungsten7,8 have been used as homogenous catalysts in oxidation systems based on aqueous H2O2. Among them, tungsten based catalysts are the most interesting owing to their high efficiency and selectivity in the oxidation reactions. Direct oxidation of cyclohexene to adipic acid, introduced by Noyori, is a previous example of using a tungsten based catalyst in oxidation reactions.9 Noyori’s halide-free method was developed for various substrates such as olefins, alcohols, and sulfides. The oxidation reaction in this method is catalyzed by sodium tungstate, however, a phase transfer catalyst is also required. Several catalytic systems based on tungstate and various phase transfer catalysts have been developed for oxidation reactions but most of them are homogenous and encountered many problems related to catalyst separation and catalyst reusability.10–14 To solve the catalyst separation and recyclability problems, a promising route is the heterogenizing of homogenous catalysts. The catalytically active species are immobilized onto the solid supports and utilized in oxidation reactions. Many support materials have been used for immobilization of tungstate such as polymers,15,16,44,45 silica particles,17–19,41
a Polymer Research Laboratory, Department of Chemistry, Sharif University of Technology, Tehran, Iran. E-mail:
[email protected]; Tel: +(982)166165311 b Laboratory of Organic Synthesis and Natural Products, Department of Chemistry, Sharif University of Technology, Tehran, Iran c Department of Physics, Acadia University, Wolfville, Nova Scotia, Canada † Electronic supplementary information (ESI) available. See DOI: 10.1039/c3gc41307a
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mesoporous silica20–23 and magnetic nanoparticles.15,24,25 However, these supported catalysts have many disadvantages such as high tungstate leaching, low activity due to heterogeneity, low catalyst loading, decomposition of H2O2 because of basic sites on the support surface and low penetration of organic substrates and H2O2 into the catalyst surface arising from the hydrophilicity of the catalyst surface. In recent years, ionic liquids (ILs) have received a lot of attention due to their unique properties. They are safe, nonflammable and have negligible vapor pressure with various polarities.26,27 They can dissolve in various organic solvents and are also good solvents for other materials. Moreover, they are tunable and can be tailored by various functionalizations and modifications. Recently, catalytic aspects of ILs have attracted more attention28,29 but the homogenous phase of these catalytic systems, which lowers the recyclability of expensive ILs, limits their widespread use in organic transformations.30,31 To overcome the recyclability problems, the concept of supported ionic liquid catalysts has emerged.32,33 This concept combines the advantages of ionic liquids with various inorganic supports such as silica particles,34,35 mesoporous silica36,37 and magnetic nanoparticles.38–40 With regard to the oxidation reactions, several supported IL catalysts have been developed for various oxidation processes.41–43 Moreover, poly(ionic liquids) have recently received a lot of attention in catalyst preparation for oxidation reactions.16,44,45 However, these systems suffer from some drawbacks such as low catalyst loading and difficult catalyst separation. Among the support materials, magnetic nanoparticles with their magnetic properties can facilitate the catalyst separation without the need for any filtration process. Herein, we report the synthesis and characterization of a novel oxidation catalyst based on supported tungstate based
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Paper ionic liquids. In this system, magnetic nanoparticles are entrapped and covalently attached to the multi-layered crosslinked poly(ionic liquid) which bears tungstate anions as counteranions. The hydrophobic surface and the multi-layered nature of the resulting catalyst improve the catalytic activity for various oxidation reactions. Moreover, catalyst separation is very easy after reaction due to their magnetic properties.
Experimental Reagents and analysis Ferric chloride hexahydrate (FeCl3·6H2O), ferrous chloride tetrahydrate (FeCl2·4H2O), ammonia (30%), 3-methacryloxypropyltrimethoxy-silane (MPS, 98%) and 1-bromododecane were obtained from Merck. 1-Vinylimidazole was obtained from Aldrich and was distilled before use. 1,4-Dibromobutane was obtained from Aldrich, while 2,2′-azobisisobutyronitrile (AIBN, Kanto, 97%) was recrystallized from ethanol. FTIR spectra of samples were taken using an ABB Bomem MB-100 FTIR spectrophotometer. Thermogravimetric analysis (TGA) was performed under a nitrogen atmosphere using a TGA Q 50 thermogravimetric analyzer. The morphology of the catalyst was observed using a Philips XL30 scanning electron microscope (SEM) and transmission electron microscopy (TEM) images were taken using a Philips CM30 electron microscope.
Synthesis of vinyl functionalized magnetic nanoparticles 6.80 g of FeCl3·6H2O and 2.50 g of FeCl2·4H2O were dissolved in 300 mL deionized water under nitrogen at room temperature. 70 mL of ammonia solution was added dropwise into the mixture with vigorous stirring. After the color of solution turned black, the magnetite precipitates were magnetically separated and washed several times with deionized water. 1 g of dried Fe3O4 nanoparticles was suspended in 100 mL of a 4/1 ethanol–water mixture by ultrasonication and the pH of solution was adjusted to 10 using ammonia solution. Then, 10 mL of tetraethoxysilane (TEOS) was added dropwise into the magnetite solution and the mixture was stirred under nitrogen atmosphere at 60 °C. The stirring was continued for another 3 h, and then the silica coated nanoparticles were magnetically separated and washed three times with deionized water and two times with ethanol. The final dark brown silica coated Fe3O4 (denoted as MNP@SiO2) was dried at 50 °C under vacuum for 24 h. 1 g of MNP@SiO2 was added to dry ethanol and then 2 mL of ammonium solution was added to the flask. An excess amount (10 mmol) of the 3-(trimethoxysilyl)propylmethacrylate (MPS) solution was then added dropwise and the mixture was stirred at 60 °C for 48 h. The MPS coated magnetic nanoparticles (denoted as MNP@MPS) were magnetically separated and washed several times with methanol and dried under vacuum at 50 °C.
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Green Chemistry Synthesis of IL monomer and cross-linker Typically, 1-vinylimidazole (2.82 g, 30 mmol) and 1-bromododecane (7.44 g, 30 mmol) were stirred in 10 mL methanol at 60 °C for 20 h. The reaction mixture was then cooled to room temperature and added to 250 mL of diethyl ether. The resulting translucent solution was placed in a refrigerator for 5 h. Solid products were separated by decantation of supernatant and washed three times with diethyl ether and dried under vacuum at 50 °C. 3-n-Decyl-1-vinylimidazolium bromide46 (sludgy at room temperature). 1H NMR (DMSO-d6, δ, ppm): 9.53 (1H, s), 8.20 (1H, s), 7.93 (1H, s), 7.29 (1H, dd), 5.96 (1H, dd), 5.41 (1H, dd), 4.18 (2H, t), 1.81 (2H, m), 1.22 (14H, m), 0.85 (3H, t). The same procedure was used for the synthesis of 1,4-butanediyl-3,3′-bis-L-vinylimidazolium dibromide (BVD) except that the molar ratio of 1-vinylimidazole to 1,4-dibromobutane was 2 : 1. 1,4-Butanediyl-3,3′-bis-L-vinylimidazolium dibromide46 1 (white solid, mp = 150 °C). H NMR (DMSO-d6, δ, ppm): 9.59 (2H, s), 8.22 (2H, s), 7.95 (2H, s), 7.32 (2H, dd), 5.98 (2H, dd), 5.44 (2H, dd), 4.27 (4H, s), 1.85 (4H, s). Synthesis of the catalyst MNP@MPS (0.1 g), IL monomer (2 g) and BVD (0.3 g) were loaded into a 100 mL round bottom flask and 40 mL methanol was added. The mixture was sonicated for 20 min and then deoxygenated under argon for another 20 min. Afterwards, AIBN was added to the mixture and the flask was equipped with a condenser and placed in an oil bath at 70 °C. After 18 h, the solid products were magnetically separated and washed three times with methanol and dried under vacuum at 50 °C. The resulting powdered materials were subjected to anion exchange reaction. 0.5 g of powdered magnetic PIL was added to 50 mL water and an excess amount of Na2WO4·H2O (1.5 g) was added to the solution. The mixture was vigorously stirred (1200 rpm) for 3 days at room temperature. The solid products (MNP@PILW) were then magnetically separated, washed five times with water (5 × 100 mL) and twice with methanol (2 × 20 mL) and dried under vacuum at 50 °C. Oxidation reactions catalyzed by MNP@PILW Substrate (1 mmol), solvent (5 mL) and 30% H2O2 (3 mmol) were loaded into a 25 mL round bottom flask and then MNP@PILW (10 mg) was added to the mixture. The reactions were vigorously stirred at certain temperature for a defined time. After the reaction, methanol was added and the catalyst was magnetically separated, washed with methanol and dried for another run. The product mixtures were extracted by ethyl acetate and analyzed by gas chromatography (GC).
Results and discussion Fabrication of the catalyst The MNPs were synthesized by co-precipitation of iron(II) and iron(III) in alkali solution based on the previously reported
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Fig. 3 Fig. 1
TGA curves of MNP@MPS (a), MNP@PILBr (b) and MNP@PILW (c).
Preparation of MNP@PILW.
method.39 The surface of MNPs was functionalized by MPS to ensure that MNPs covalently bonded to the copolymer. Moreover, active vinyl groups on the surface of MNPs cause that polymerization to occur more easily on the surface of MNPs. IL monomer 3-n-dodecyl-L-vinylimidazolium bromide and cross-linker 1,4-butanediyl-3,3′-bis-L-vinylimidazolium dibromide (BVD) were prepared by quaternization of L-vinylimidazole. An IL monomer with long alkyl chains is chosen since the resulting polymer would be hydrophobic and behave as a phase transfer catalyst in oxidation reactions. MNP entrapped cross-linked poly(ionic liquid) was prepared by free radical initiation polymerization of IL monomer in the presence of MNP@MPS (Fig. 1). Copolymerization was initiated by AIBN and cross-linked insoluble copolymers were precipitated from the solution, while MNPs were entrapped and covalently attached to the copolymer. Such an approach for the preparation of heterogeneous catalysts improves the
Fig. 2 FTIR spectra of silica coated MNP@SiO2, MNP@MPS, MNP@PILBr, MNP@PILW and Na2WO4·H2O.
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loading level of the active catalyst part and a lower amount of catalyst could be used in reactions. This is certainly useful for large-scale applications to prevent using large amounts of solvent. Moreover, the magnetic properties of MNP@PILW facilitate separation of catalyst at the end of the reaction.
Characterization of the catalyst The FTIR spectra of silica coated MNP and MNP@MPS are shown in Fig. 2. Both spectra show stretching vibration of Fe–O at 640 cm−1 and Si–O at 1035 cm−1. In addition, MNP@MPS shows stretching vibration of a carbonyl group at 1713 cm−1 and CvC at 1461 cm−1 which demonstrates successful attachment of MPS on MNP. In the FTIR spectrum of MNP@PILBr, characteristic peaks of imidazolium rings are observed at 1565 cm−1 and 1640 cm−1, attributed to CvC and CvN. The strong peak at 2923 cm−1 is attributed to C–H of alkyl chains. Similar peaks were observed in the FTIR spectrum of MNP@PILW and a new peak also appeared at 822 cm−1 which is attributed to a WvO bond (the same peak for Na2WO4 is observed in Fig. 2). These results confirm that MNPs were
Fig. 4
The XRD pattern of MNP@PILW.
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entrapped into the copolymers and bromide anions were successfully exchanged with WO4 anions. Fig. 3 shows the thermal gravimetric analysis (TGA) of (a) MNP@MPS, (b) MNP@PILBr and (c) MNP@PILW. The weight loss for all samples on heating to 150 °C was attributed completely to the loss of adsorbed water molecules. From
Fig. 5
Magnetization of MNP and MNP@PILW.
Fig. 6
TEM and SEM images of MNP@PILW.
Table 1
the weight losses in the TGA curve of MNP@MPS, the loading amount of MPS was calculated as 0.72 mmol g−1. From the TGA curve of MNP@PILBr and MNP@PILW it was seen that the copolymer content in the catalyst is about 50 wt%. It is notable that the loading amount of the monomer and the cross-linker cannot be calculated individually by TGA due to the identical nature of the monomer and the cross-linker. Fig. 4 shows the XRD pattern of MNP@PILW with characteristic peaks and relative intensity, which completely match with the standard Fe3O4 sample (red lines). The magnetization curves of MNP and MNP@PILW show small coercivities, which indicate the superparamagnetic nature of both materials (Fig. 5). The saturation magnetization of MNP@PILW is smaller than that of bare MNP due to entrapment of MNPs into the nonmagnetic materials; however, the magnetization is still large enough for separation of the catalyst. Fig. 6 shows the TEM and SEM images of MNP@PILW. As may be seen in the TEM image, the size of MNP particles is around 10 nm and they are dispersed into the polymeric matrix. The loading amount of Br in MNP@PILBr was measured by the standard method (titration by AgNO3, according to the Moher method) and it was found that the loading amount of Br in MNP@PILBr was 1.32 mmol g−1. The loading amount of tungstate ions in MNP@PILW was calculated by atomic absorption spectroscopy (AAS) using the standard samples. It was found that the loading amount of WO4 ions in MNP@PILW was 0.61 mmol g−1. The XRF analysis of MNP@PILW confirms the results of AAS and the amount of W was found to be 11.36 wt% which is equal to a loading amount of 0.62 mmol g−1 of WO4 ion. Also, XRF analysis shows that no significant amount of Na ions (
