Green Chemistry

PAPER

Cite this: Green Chem., 2013, 15, 2913

Received 4th July 2013,Accepted 7th August 2013

DOI: 10.1039/c3gc41307a

www.rsc.org/greenchem

Tungstate based poly(ionic liquid) entrapped magneticnanoparticles: 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

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-

meter (VSM) and atomic adsorption spectroscopy (AAS). The catalyst can be readily recovered and reused

at least ten times under the described reaction conditions without significant loss of reactivity.

Introduction

Over the past several decades, oxidation reactions based onH2O2 have received a lot of attention because of their environ-mental and economic benefits.1–3 Several transition metalssuch as manganese,4 molybdenum,5 rhenium6 and tungsten7,8

have been used as homogenous catalysts in oxidation systemsbased on aqueous H2O2. Among them, tungsten based catalystsare the most interesting owing to their high efficiency andselectivity in the oxidation reactions. Direct oxidation of cyclo-hexene to adipic acid, introduced by Noyori, is a previousexample of using a tungsten based catalyst in oxidation reac-tions.9 Noyori’s halide-free method was developed for varioussubstrates such as olefins, alcohols, and sulfides. The oxi-dation reaction in this method is catalyzed by sodium tung-state, however, a phase transfer catalyst is also required. Severalcatalytic systems based on tungstate and various phase transfercatalysts have been developed for oxidation reactions but mostof them are homogenous and encountered many problemsrelated to catalyst separation and catalyst reusability.10–14

To solve the catalyst separation and recyclability problems,a promising route is the heterogenizing of homogenous cata-lysts. The catalytically active species are immobilized ontothe solid supports and utilized in oxidation reactions. Manysupport materials have been used for immobilization oftungstate such as polymers,15,16,44,45 silica particles,17–19,41

mesoporous silica20–23 and magnetic nanoparticles.15,24,25

However, these supported catalysts have many disadvantagessuch as high tungstate leaching, low activity due to hetero-geneity, low catalyst loading, decomposition of H2O2 becauseof basic sites on the support surface and low penetration oforganic substrates and H2O2 into the catalyst surface arisingfrom the hydrophilicity of the catalyst surface.

In recent years, ionic liquids (ILs) have received a lot ofattention due to their unique properties. They are safe, non-flammable and have negligible vapor pressure with variouspolarities.26,27 They can dissolve in various organic solventsand are also good solvents for other materials. Moreover,they are tunable and can be tailored by various functionali-zations and modifications. Recently, catalytic aspects ofILs have attracted more attention28,29 but the homogenousphase of these catalytic systems, which lowers the recyclabilityof expensive ILs, limits their widespread use in organictransformations.30,31

To overcome the recyclability problems, the concept of sup-ported ionic liquid catalysts has emerged.32,33 This conceptcombines the advantages of ionic liquids with various in-organic supports such as silica particles,34,35 mesoporoussilica36,37 and magnetic nanoparticles.38–40 With regard to theoxidation reactions, several supported IL catalysts have beendeveloped for various oxidation processes.41–43 Moreover, poly-(ionic liquids) have recently received a lot of attention in cata-lyst preparation for oxidation reactions.16,44,45 However, thesesystems suffer from some drawbacks such as low catalystloading and difficult catalyst separation. Among the supportmaterials, magnetic nanoparticles with their magnetic pro-perties can facilitate the catalyst separation without the needfor any filtration process.

Herein, we report the synthesis and characterization of anovel oxidation catalyst based on supported tungstate based

†Electronic supplementary information (ESI) available. See DOI:10.1039/c3gc41307a

aPolymer Research Laboratory, Department of Chemistry, Sharif University of

Technology, Tehran, Iran. E-mail: purjavad@sharif.edu; Tel: +(982)166165311bLaboratory of Organic Synthesis and Natural Products, Department of Chemistry,

Sharif University of Technology, Tehran, IrancDepartment of Physics, Acadia University, Wolfville, Nova Scotia, Canada

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ionic liquids. In this system, magnetic nanoparticles areentrapped and covalently attached to the multi-layered cross-linked poly(ionic liquid) which bears tungstate anions as coun-teranions. The hydrophobic surface and the multi-layerednature of the resulting catalyst improve the catalytic activity forvarious oxidation reactions. Moreover, catalyst separation isvery easy after reaction due to their magnetic properties.

ExperimentalReagents and analysis

Ferric chloride hexahydrate (FeCl3·6H2O), ferrous chloride tetra-hydrate (FeCl2·4H2O), ammonia (30%), 3-methacryloxypropyl-trimethoxy-silane (MPS, 98%) and 1-bromododecane wereobtained from Merck. 1-Vinylimidazole was obtained fromAldrich and was distilled before use. 1,4-Dibromobutane wasobtained from Aldrich, while 2,2′-azobisisobutyronitrile (AIBN,Kanto, 97%) was recrystallized from ethanol.

FTIR spectra of samples were taken using an ABB BomemMB-100 FTIR spectrophotometer. Thermogravimetric analysis(TGA) was performed under a nitrogen atmosphere using aTGA Q 50 thermogravimetric analyzer. The morphology of thecatalyst was observed using a Philips XL30 scanning electronmicroscope (SEM) and transmission electron microscopy(TEM) images were taken using a Philips CM30 electronmicroscope.

Synthesis of vinyl functionalized magnetic nanoparticles

6.80 g of FeCl3·6H2O and 2.50 g of FeCl2·4H2O were dissolvedin 300 mL deionized water under nitrogen at room tempera-ture. 70 mL of ammonia solution was added dropwise into themixture with vigorous stirring. After the color of solutionturned black, the magnetite precipitates were magnetically sep-arated and washed several times with deionized water.

1 g of dried Fe3O4 nanoparticles was suspended in 100 mLof a 4/1 ethanol–water mixture by ultrasonication and the pHof solution was adjusted to 10 using ammonia solution. Then,10 mL of tetraethoxysilane (TEOS) was added dropwise intothe magnetite solution and the mixture was stirred undernitrogen atmosphere at 60 °C. The stirring was continued foranother 3 h, and then the silica coated nanoparticles weremagnetically separated and washed three times with deionizedwater and two times with ethanol. The final dark brown silicacoated Fe3O4 (denoted as MNP@SiO2) was dried at 50 °Cunder vacuum for 24 h.

1 g of MNP@SiO2 was added to dry ethanol and then 2 mLof ammonium solution was added to the flask. An excessamount (10 mmol) of the 3-(trimethoxysilyl)propylmethacry-late (MPS) solution was then added dropwise and the mixturewas stirred at 60 °C for 48 h. The MPS coated magnetic nano-particles (denoted as MNP@MPS) were magnetically separatedand washed several times with methanol and dried undervacuum at 50 °C.

Synthesis of IL monomer and cross-linker

Typically, 1-vinylimidazole (2.82 g, 30 mmol) and 1-bromo-dodecane (7.44 g, 30 mmol) were stirred in 10 mL methanol at60 °C for 20 h. The reaction mixture was then cooled to roomtemperature and added to 250 mL of diethyl ether. The result-ing translucent solution was placed in a refrigerator for 5 h.Solid products were separated by decantation of supernatantand washed three times with diethyl ether and dried undervacuum at 50 °C.

3-n-Decyl-1-vinylimidazolium bromide46 (sludgy at roomtemperature). 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-butane-diyl-3,3′-bis-L-vinylimidazolium dibromide (BVD) except thatthe molar ratio of 1-vinylimidazole to 1,4-dibromobutane was2 : 1.

1,4-Butanediyl-3,3′-bis-L-vinylimidazolium dibromide46

(white solid, mp = 150 °C). 1H 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) wereloaded into a 100 mL round bottom flask and 40 mL methanolwas added. The mixture was sonicated for 20 min and thendeoxygenated under argon for another 20 min. Afterwards,AIBN was added to the mixture and the flask was equippedwith a condenser and placed in an oil bath at 70 °C. After 18 h,the solid products were magnetically separated and washedthree times with methanol and dried under vacuum at 50 °C.

The resulting powdered materials were subjected to anionexchange reaction. 0.5 g of powdered magnetic PIL was addedto 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 fivetimes 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 thenMNP@PILW (10 mg) was added to the mixture. The reactionswere vigorously stirred at certain temperature for a definedtime. After the reaction, methanol was added and the catalystwas magnetically separated, washed with methanol and driedfor another run. The product mixtures were extracted by ethylacetate and analyzed by gas chromatography (GC).

Results and discussionFabrication of the catalyst

The MNPs were synthesized by co-precipitation of iron(II) andiron(III) in alkali solution based on the previously reported

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method.39 The surface of MNPs was functionalized by MPS toensure that MNPs covalently bonded to the copolymer. More-over, active vinyl groups on the surface of MNPs cause thatpolymerization to occur more easily on the surface of MNPs.

IL monomer 3-n-dodecyl-L-vinylimidazolium bromide andcross-linker 1,4-butanediyl-3,3′-bis-L-vinylimidazolium dibro-mide (BVD) were prepared by quaternization of L-vinylimida-zole. An IL monomer with long alkyl chains is chosen sincethe resulting polymer would be hydrophobic and behave as aphase transfer catalyst in oxidation reactions.

MNP entrapped cross-linked poly(ionic liquid) was preparedby free radical initiation polymerization of IL monomer in thepresence of MNP@MPS (Fig. 1). Copolymerization wasinitiated by AIBN and cross-linked insoluble copolymers wereprecipitated from the solution, while MNPs were entrappedand covalently attached to the copolymer. Such an approachfor the preparation of heterogeneous catalysts improves the

loading level of the active catalyst part and a lower amount ofcatalyst could be used in reactions. This is certainly useful forlarge-scale applications to prevent using large amounts ofsolvent. Moreover, the magnetic properties of MNP@PILWfacilitate separation of catalyst at the end of the reaction.

Characterization of the catalyst

The FTIR spectra of silica coated MNP and MNP@MPS areshown in Fig. 2. Both spectra show stretching vibration of Fe–Oat 640 cm−1 and Si–O at 1035 cm−1. In addition, MNP@MPSshows stretching vibration of a carbonyl group at 1713 cm−1

and CvC at 1461 cm−1 which demonstrates successful attach-ment of MPS on MNP. In the FTIR spectrum of MNP@PILBr,characteristic peaks of imidazolium rings are observed at1565 cm−1 and 1640 cm−1, attributed to CvC and CvN. Thestrong peak at 2923 cm−1 is attributed to C–H of alkyl chains.

Similar peaks were observed in the FTIR spectrum ofMNP@PILW and a new peak also appeared at 822 cm−1 whichis attributed to a WvO bond (the same peak for Na2WO4 isobserved in Fig. 2). These results confirm that MNPs were

Fig. 1 Preparation of MNP@PILW.

Fig. 3 TGA curves of MNP@MPS (a), MNP@PILBr (b) and MNP@PILW (c).

Fig. 2 FTIR spectra of silica coated MNP@SiO2, MNP@MPS, MNP@PILBr,MNP@PILW and Na2WO4·H2O. Fig. 4 The XRD pattern of MNP@PILW.

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entrapped into the copolymers and bromide anions weresuccessfully exchanged with WO4 anions.

Fig. 3 shows the thermal gravimetric analysis (TGA) of(a) MNP@MPS, (b) MNP@PILBr and (c) MNP@PILW. Theweight loss for all samples on heating to 150 °C was attributedcompletely to the loss of adsorbed water molecules. From

the weight losses in the TGA curve of MNP@MPS, the loadingamount of MPS was calculated as 0.72 mmol g−1. From theTGA curve of MNP@PILBr and MNP@PILW it was seen thatthe copolymer content in the catalyst is about 50 wt%. It isnotable that the loading amount of the monomer and thecross-linker cannot be calculated individually by TGA due tothe identical nature of the monomer and the cross-linker.

Fig. 4 shows the XRD pattern of MNP@PILW with characteri-stic peaks and relative intensity, which completely match withthe standard Fe3O4 sample (red lines).

The magnetization curves of MNP and MNP@PILW showsmall coercivities, which indicate the superparamagneticnature of both materials (Fig. 5). The saturation magnetizationof MNP@PILW is smaller than that of bare MNP due to entrap-ment of MNPs into the nonmagnetic materials; however, themagnetization is still large enough for separation of thecatalyst.

Fig. 6 shows the TEM and SEM images of MNP@PILW. Asmay be seen in the TEM image, the size of MNP particles isaround 10 nm and they are dispersed into the polymericmatrix.

The loading amount of Br in MNP@PILBr was measured bythe standard method (titration by AgNO3, according to theMoher method) and it was found that the loading amount ofBr in MNP@PILBr was 1.32 mmol g−1. The loading amount oftungstate ions in MNP@PILW was calculated by atomicabsorption spectroscopy (AAS) using the standard samples. Itwas found that the loading amount of WO4 ions inMNP@PILW was 0.61 mmol g−1. The XRF analysis ofMNP@PILW confirms the results of AAS and the amount of Wwas found to be 11.36 wt% which is equal to a loading amountof 0.62 mmol g−1 of WO4 ion. Also, XRF analysis shows that nosignificant amount of Na ions (<0.004 wt%) remained inMNP@PILW. From these results, we can conclude that ionexchange was performed with a 2 : 1 ratio of imidazoliumcations and WO4 anions with the yield of ion exchange process

Fig. 5 Magnetization of MNP and MNP@PILW.

Fig. 6 TEM and SEM images of MNP@PILW.

Table 1 Control experiment of oxidation of benzyl alcohola

Entry Catalyst Cat. loading (mg) 30% H2O2 (mmol) Solvent T (°C) Time (h) Conv.b (%) Yieldc (%)

1 None — 3 Solvent free 90 2 — Trace2 Na2WO4 30 10 Solvent free 90 3 39 223 MNP@PILBr 50 6 Solvent free 90 3 — Trace4 MNP@PILW 30 6 Solvent free r.t 5 38 375 MNP@PILW 30 4 Solvent free 90 1.5 95 956 MNP@PILW 30 1 Solvent free 90 2 36 357 MNP@PILW 15 4 Solvent free 90 2 74 738 MNP@PILW 15 4 Water 100 3 51 509 MNP@PILW 30 4 Water 100 3 75 6610 MNP@PILW 15 4 CH3CN 90 3 64 6311 MNP@PILW 30 4 CH3CN 90 3 93 8412 MNP@PILW 30 1 CH3CN 90 3 66 6113 MNP@PILW 15 4 n-Hexane 70 3 41 4114 MNP@PILW 30 4 n-Hexane 70 3 81 8015 MNP@PILW 15 4 CH3OH–CH2Cl2 60 3 23 2316 MNP@PILW 30 4 CH3OH–CH2Cl2 60 3 61 59

a Reaction condition: benzyl alcohol (1 mmol), solvent (5 mL). b Conversions were calculated based on initial mmol of benzyl alcohol. c Yieldswere determined by GC.

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Table 2 Oxidation reactions catalyzed by MNP@PILW

Entry Substrates Products Conditiona Time (h) Conv.b (%) Yieldc (%)

1 A 1.5 96 95

2 A 2 92 92

3 A 2 95 93

4 A 2.5 85 85

5 A 2.5 85 84

6 A 3 79 75

7 A 3.5 80 78

8 A 1 89 89

9 A 3 93 90

10 B 2.5 82 82

11 B 1.5 96 96

12 B 1 99 99

13 B 1.5 88 88

14 B 2 97 96

15 B 2.5 98 98

16 C 4 88 86

17 C 2 99 99

18 C 2 99 99

19 C 4 99 98

a Reaction conditions: A, alcohols (1 mmol), 30% H2O2 (4 mmol), MNP@PILW (30 mg), 90 °C; B, sulfides (1 mmol), 30% H2O2 (3 mmol),MNP@PILW (15 mg), 23 °C; C, alkenes (1 mmol), 30% H2O2 (3 mmol), MNP@PILW (15 mg), CH3CN (5 mL), 70 °C. bConversions calculated basedon substrates. c Yields were determined by GC.

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calculated as about 94%. This indicates that polymer chains ofthe inner layers did not exchange Br ions with WO4 as well asthe outer layers.

Catalytic performance of MNP@PILW

The catalytic performance of MNP@PILW was first investigatedin the oxidation of benzyl alcohol (Table 1). As seen in Table 1,no significant amount of aldehyde was produced when H2O2

was used as an oxidant. Using Na2WO4 as a non-supportedcatalyst gave only 39% conversion and 22% yield in solventfree condition. Also, MNP@PILBr was inactive and no alde-hyde was obtained. Using 30 mg of MNP@PILW at room temp-erature produced a 37% yield in 5 h. However, increasing thereaction temperature to 90 °C improved the oxidation yield upto 95%. Also, it was found that the selectivity of reaction underthese conditions was 97%. Using equimolar of H2O2 as anoxidant under solvent free conditions with 30 mg of the cata-lyst at 90 °C gave only 35% yield. This could be due to difficultmixing of the substrate under solvent free conditions ordecomposition of H2O2 at high temperature. To solve themixing problem, the same reaction was performed in CH3CNand 61% yield was obtained. These results show that 4 mmolH2O2 is the optimum amount under these conditions. Theyield of oxidation was dramatically reduced under the sameconditions when catalyst loading was reduced to 15 mg. Theidentical reactions were performed in acetonitrile and a1/1 mixture of methanol–dichloromethane, but the yield of oxi-dation was higher under solvent free conditions than whenusing solvents.

All of these experiments demonstrate the key role ofMNP@PILW in the oxidation of benzyl alcohol. The diversityof the oxidation reaction catalyzed by MNP@PILW wasextended and the oxidation of various alcohols, sulfides andalkenes was investigated (Table 2). Various benzyl alcoholscontaining electron withdrawing and electron donor groupswere readily oxidized to corresponding benzaldehydes. The oxi-dation of two kinds of secondary alcohols was also investigatedand good conversions were obtained. In addition, several sul-fides were subjected to oxidation reaction catalyzed byMNP@PILW. First, the oxidation reaction of diphenyl sulfidewas tested and a good conversion was obtained under con-dition B (MNP@PILW (15 mg), H2O2 (3 mmol), at room temp-erature). It was found that diphenyl sulfide was selectivelyoxidized to diphenyl sulfoxide (99% selectivity). Several sul-fides with different molecular structures were oxidized to thecorresponding sulfoxides. Encouraged by these results, severalcyclic and linear alkenes were subjected to oxidation reaction.Under condition C (alkenes (1 mmol), 30% H2O2 (3 mmol),MNP@PILW (15 mg), CH3CN (5 mL), 70 °C) a good conversionwas obtained for epoxidation of alkenes.

A possible reaction mechanism is the reaction of WO4

anion with 4 mmol H2O2 to produce W(O2)4, as previouslyreported.47 Then oxidation will occur and W(O2)4 will reactwith 1 mmol alcohol to produce W(O2)3O and correspondingaldehyde. Then W(O2)3O will react with another H2O2 andproduce W(O2)4 again and the oxidation cycle will repeat.

The W(O2)4 or W(O2)3O anions at the catalyst separationprocess when the catalyst is drying, will lose O2 and produceWO4 anions again.

47

Recycling and leaching experiments

Along with the high activity of MNP@PILW in oxidation reac-tions, another useful advantage of the catalyst is simple separ-ation. At the end of a reaction, the catalyst could be easilyseparated from the solution mixture using an external magnet(Fig. S6†). The reusability of MNP@PILW for the oxidation ofbenzyl alcohol was also investigated. As shown in Fig. 7 thecatalyst was reused 10 times and no significant loss of activitywas observed.

In order to investigate catalyst leaching in the oxidation ofbenzyl alcohol as a model reaction, the reaction was stopped athalf the reaction time (45 min) and the catalyst was completelyseparated from solution. The rest of the reaction mixture(without catalyst) was allowed to stir for another period of halfthe reaction time. As seen in Fig. 8, no benzaldehyde was pro-duced after catalyst separation. After separation of the catalyst,the reaction mixture was analyzed by AAS and no tungsten was

Fig. 7 Recycling experiment.

Fig. 8 Leaching experiment. Reaction condition: benzyl alcohol (1 mmol), 30%H2O2 (4 mmol), MNP@PILW (30 mg), 90 °C. Blue line: reaction with MNP@PILWand red line: reaction without a catalyst.

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detected. These results show that MNP@PILW is truly hetero-geneous and catalyst leaching is negligible under these con-ditions. Moreover, there is no significant change in the TEMimage and the FTIR spectrum of the recycled catalyst (Fig. S3and S5†), demonstrate that the catalyst is stable even after10 times use.

Conclusion

In conclusion, we have prepared a novel magnetically recover-able catalyst based on poly(ionic liquid). The resulted catalystwas proven to be an effective and robust catalyst for selectiveoxidation of various organic substrates. The catalyst could berecovered using an external magnet and reused several timeswithout losing activity. From the viewpoint of green chemistry,the present catalyst and protocol was proven to be useful andhas potential for use in large scale applications.

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