Molecules 2014, 19(1), 699-712; doi:10.3390/molecules19010699

Article
Magnetically Separable and Recyclable Fe3O4-Supported Ag Nanocatalysts for Reduction of Nitro Compounds and Selective Hydration of Nitriles to Amides in Water
Hyunje Woo 1, Kyoungho Lee 1, Sungkyun Park 2 and Kang Hyun Park 1,*
1
Department of Chemistry and Chemistry Institute for Functional Materials, Pusan National University, Busan 609-735, Korea; E-Mails: hjwoo@chemistry.or.kr (H.W.); kyoungho@chemistry.or.kr (K.L.)
2
Department of Physics, Pusan National University, Busan 609-735, Korea; E-Mail: psk@pusan.ac.kr
*
Author to whom correspondence should be addressed; E-Mail: chemistry@pusan.ac.kr; Tel.: +82-51-510-2238; Fax: +82-51-980-5200.
Received: 28 November 2013; in revised form: 27 December 2013 / Accepted: 30 December 2013 /
Published: 7 January 2014

Abstract

: As hybrid nanostructures have become more important in many fields of chemistry, Ag nanoparticles (NPs) are being increasingly immobilized onto Fe3O4 microspheres in situ. Structural characterization reveals that the Ag NPs are uniformly immobilized in the Fe3O4 microsphere-based supports. Moreover, Ag NPs are more stable in the hybrid structure than in the naked state and show high catalytic activity for the reduction of nitro compounds and hydration of nitriles to amides in water. The Fe3O4 microspheres were recycled several times using an external magnet.
Keywords:
silver; Fe3O4 support; reduction; hydration; magnetic separation

1. Introduction

In recent years, numerous attempts have been made toward designing and synthesizing hybrid nanostructures, which combine or even improve the physical and chemical properties of the constituent parts [1]. Many studies have discussed the syntheses of these multicomponent nanostructures with increased functionality [2,3]. The presence of multicomponent functions combined with the enhanced chemical and physical properties make hybrid nanostructures suitable for research fields pertaining to the study of magnetic, plasmonic, and semiconducting properties [4]. Metal NPs immobilized onto metal oxide supports can be used as catalysts in organic reactions, wherein they show higher catalytic activity than the naked metal NPs because of electron transfer across the interface [5,6,7,8].

Among the existing metal oxide supports, Fe3O4 has gained significance owing to its nanostructure and cutting-edge technological applications, including its usage as a magnetic storage medium [9], in biosensors [10], as well as medical applications such as targeted drug delivery [11,12]. Furthermore, Fe3O4 microspheres have been used as supports for the immobilization of metal NPs in nanocatalysis because of the ease of their recyclability using external magnets after the reactions [13].

Recently, many researchers have described the preparation of Ag NPs and studied their various applications in highly active surface-enhanced Raman scattering substrates [14], antibacterial coatings [15], electrochemical and biosensors [16], and as efficient catalysts for organic reactions [17]. A significant number of studies on the catalytic reactions of Ag NPs have been conducted, such as alcohol dehydrogenation, oxidation of phenylsilanes, reduction of aromatic compounds, and Diels-Alder cycloadditions [17,18,19,20]. However, during a reaction, the highly active surface atoms destabilize the NPs. To overcome this problem, metal NPs have been immobilized onto supports to increase the stability of the catalysts [21,22].

4-Nitrophenol (Scheme 1) is considered to be one of the most refractory pollutants in wastewaters generated by industrial sources such as companies that manufacture explosives and dyes [23,24]. Due to the importance of 4-aminophenol, there have been many reports about applications of Ag nanoparticles on various supports as catalysts for the reduction of 4-nitrophenol [25,26,27]. Among them, the use of water as a solvent under mild conditions has attracted much attention in the environmental field. Also, the hydration of nitriles to yield the corresponding amides is of tremendous significance to researchers in academia and industry alike because the resulting amides have numerous applications in synthetic organic and pharmaceutical chemistry. Among various heterogeneous nanocatalysts, supported Ag NPs have been used for this reaction as catalyst [28,29]. In our earlier work, Ag NPs were also used as catalysts for the hydration of nitriles to amides [30].

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Scheme 1. Reduction of a nitro compound and hydration of a nitrile to an amide catalyzed by Ag/Fe3O4 catalyst.

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Scheme 1. Reduction of a nitro compound and hydration of a nitrile to an amide catalyzed by Ag/Fe3O4 catalyst.
Molecules 19 00699 g005 1024

In this paper, Ag NP, immobilized onto Fe3O4 microspheres were synthesized and tested as a catalyst in reduction of nitro compounds and nitrile hydration reactions in water (Scheme 1). The monodisperse Ag NPs were easily immobilized without any pretreatment such as attaching functional groups onto the Fe3O4 microspheres. This catalyst showed increased Ag NP stability and could be easily recycled using an external magnet after the completion of the reaction.

2. Results and Discussion

2.1. Catalyst Preparation and Characterization

The Fe3O4 microspheres were synthesized using the solvothermal method [31]. The partial reduction of FeCl3 was done at 200 °C using ethylene glycol as a solvent, sodium acetate as a reducing agent, and Na3Cit as an electrostatic stabilizer. No pretreatment procedures such as attaching functional groups and coating polymers or carbon onto Fe3O4, were required prior to the immobilization of Ag NPs onto the Fe3O4 microspheres. In fact, Ag NPs were easily immobilized on the surface of Fe3O4 in situ because of the interaction between the Fe atoms and carboxyl groups of Na3Cit with Ag. Figure 1 illustrates the morphology of the Ag/Fe3O4 catalyst. Figure 1a shows a scanning electron microscopy (SEM) image of spherical Fe3O4 microspheres. The immobilized Ag NPs on Fe3O4 microspheres were observed in the transmission electron microscopy (TEM) images (Figure 1b–d). The X-ray diffractometer (XRD) pattern of the Ag/Fe3O4 and reused Ag/Fe3O4 microspheres corresponded to a cubic spinel structure of Fe3O4 (JCPDS No. 19-0629) and face-centered cubic of Ag (JCPDS No. 04-07831) (Figure 2a).

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Figure 1. (a) SEM image of Fe3O4 microspheres; (b,c) TEM images of Ag/Fe3O4; and (d) Immobilized Ag NPs onto Fe3O4 microspheres.

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Figure 1. (a) SEM image of Fe3O4 microspheres; (b,c) TEM images of Ag/Fe3O4; and (d) Immobilized Ag NPs onto Fe3O4 microspheres.
Molecules 19 00699 g001 1024

In particular, the crystalline pattern was assigned to the (220), (311), (422) and (511) reflections of Fe3O4 and (111), (200) and (311) reflections of Ag. The superconducting quantum interference device (SQUID) results in Figure 2b shows the magnetic curves as a function of the applied field at 300 K. The saturation magnetization value of Ag/Fe3O4 was 58.7 emu∙g−1, which was similar to that of the Fe3O4 microspheres (56.9 emu∙g−1). The small decrease of the magnetization value of Ag/Fe3O4 microspheres compared to that of Fe3O4 microspheres can be attributed to the slight increase of mass due to the immobilized Ag nanoparticles on the surface of Fe3O4 microspheres [32]. After hydration, the saturated magnetization value was decreased to 13.7 emu∙g−1. Moreover, both the remanence (Mr) and coercivity (Hc) of Fe3O4 microspheres were close to zero, indicating superparamagnetism. The average diameters of Fe3O4 microspheres and Ag NPs were 150 nm and 12 nm, respectively, as determined from the TEM images (Figure 2c). The elemental compositions of Ag/Fe3O4 catalysts were obtained using energy-dispersive X-ray spectroscopy (EDS), showing Ag content (atomic: 6.15% and weight: 17 wt%) (Figure 2d).

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Figure 2. (a) XRD pattern of Ag/Fe3O4 and reused Ag/Fe3O4; (b) SQUID data; (c) Size distributions of Ag NPs and Fe3O4 microspheres; and (d) EDS spectrum of Ag/Fe3O4.

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Figure 2. (a) XRD pattern of Ag/Fe3O4 and reused Ag/Fe3O4; (b) SQUID data; (c) Size distributions of Ag NPs and Fe3O4 microspheres; and (d) EDS spectrum of Ag/Fe3O4.
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2.2. Reaction Tests

2.2.1. Reduction of Nitro Compounds

As shown in Figure 3a, the UV/Vis spectrum of the reaction mixture was monitored with time during the catalytic reduction of 4-nitrophenol. Specifically, the absorption of 4-nitrophenol at 400 nm decreases rapidly with a concomitant increase in the peak at 300 nm, which is attributed to the reduction product, 4-aminophenol. The control experiments, where only Fe3O4 microspheres were used as the catalyst, showed no reaction (entry 1, Table 1). In the absence of NaBH4, Ag/Fe3O4 catalyst showed no catalytic activity (entry 2, Table 1). As expected, increasing the number of equivalents of NaBH4 increased the catalytic activity (entries 3–5, Table 1). As shown in Figure 3b, the reaction rate constant k is compared under different temperatures at 2.0 mol% of Ag/Fe3O4 and 50 equiv. of NaBH4. The highest catalytic efficiency (0.924 min−1) was obtained at 35 °C.

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Figure 3. (a) Time-dependent UV/vis absorption spectra for the reduction of 4-nitrophenol over hybrid Ag/Fe3O4 catalyst in aqueous media at 303 K (NaBH4: 50 equiv.); and (b) Plot of ln(Ct/C0) versus time for the reduction of 4-nitrophenol over hybrid Ag/Fe3O4 catalysts under different temperatures at 2.0 mol% of catalyst and 50 equiv. of NaBH4.

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Figure 3. (a) Time-dependent UV/vis absorption spectra for the reduction of 4-nitrophenol over hybrid Ag/Fe3O4 catalyst in aqueous media at 303 K (NaBH4: 50 equiv.); and (b) Plot of ln(Ct/C0) versus time for the reduction of 4-nitrophenol over hybrid Ag/Fe3O4 catalysts under different temperatures at 2.0 mol% of catalyst and 50 equiv. of NaBH4.
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Table 1. Optimization of reaction conditions.

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Table 1. Optimization of reaction conditions.
Molecules 19 00699 i001
EntryCatalyst (mol%)Temp. (°C)NaBH4 (equiv.)TimeTOF (h−1)
1Fe3O4 (2.0)253004 hNo reaction
2Ag/Fe3O4 (2.0)2504 hNo reaction
3Ag/Fe3O4 (2.0)255020 min150
4Ag/Fe3O4 (2.0)252003 min1000
5Ag/Fe3O4 (2.0)253004 min 10 s1059
6Ag/Fe3O4 (2.0)253002 min 5 s1440

Reaction conditions: 10 mL of 7.50 * 10−4 M 4-nitrophenol, 0.026 mg of Ag/Fe3O4 [Ag base (17 wt%): 2.0 mol%], 1.0 mL of 2.22 M NaBH4 (300 equiv. to the substrate).

We then sought to optimize reaction temperature during the reduction (entries 5 and 6, Table 1). The reduction was finished in 2 min 5 s where Ag/Fe3O4 was used as the catalyst (2.0 mol% of catalyst, 300 equiv. of NaBH4 per equiv. substrate). Ag/Fe3O4 catalyst exhibited superior catalytic activity to previous reported Ag/halloysite nanocomposites and Ag nanoshell-coated cationic polystyrene beads in the comparison with turnover frequency (TOF) value [33,34].

We also confirmed the catalytic activity of hybrid Ag/Fe3O4 for the reduction of other nitroarene analogues (Table 2). As shown in Table 2, we found that our Ag/Fe3O4 catalyst promoted high reactivities and excellent yields for a series of model nitrophenols and aniline compounds, regardless of the types and positions of the substituents.

Table 2. Reduction of various nitroarenes using hybrid Ag/Fe3O4 catalyst.

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Table 2. Reduction of various nitroarenes using hybrid Ag/Fe3O4 catalyst.
EntrySubstrateProductTimeTOF (h−1)
1 Molecules 19 00699 i002 Molecules 19 00699 i0032 min 5 s1440
2 Molecules 19 00699 i004 Molecules 19 00699 i0051 min 40 s1800
3 Molecules 19 00699 i006 Molecules 19 00699 i0076 min 50 s439
4 Molecules 19 00699 i008 Molecules 19 00699 i0096 min500
5 Molecules 19 00699 i010 Molecules 19 00699 i0115 min 40 s529
6 Molecules 19 00699 i012 Molecules 19 00699 i01318 min 20 s164

Reaction conditions: 10 mL of 7.50 * 10−4 M 4-nitrophenol, 0.026 mg of Ag/Fe3O4 [Ag base (17 wt%): 2.0 mol%], 1.0 mL of 2.22 M NaBH4 (300 equiv. to the substrate) at 308 K.

Interestingly, when the reductions of 4-, 3-, and 2-nitrophenols were catalyzed by Ag/Fe3O4, the 4- and 3-nitrophenol reductions showed better activities than that of 2-nitrophenol because of a steric effect (entries 1–3, Table 2). Remarkably, the TOF in Entry 2 is 1,800 h−1, which is calculated using the moles of nitroarene consumed per mole of the hybrid Ag/Fe3O4 catalyst for a reaction time of 1 h under the present reaction condition. Interestingly, m-nitro compounds exhibited better conversion efficiencies than other nitro compounds because of a resonance effect (entries 4–6, Table 2).

2.2.2. Hydration of Nitriles to Amides

The reaction of benzonitrile in water was chosen as the test model. A possible mechanism of nitrile hydration by Ag catalyst is shown by Satsuma’s group [28]. Almost no reaction occurred in the absence of a catalyst (entry 1, Table 3). Generally, time, temperature, and quantity of catalyst are important considerations for increasing rate of the conversion. A 2% conversion was obtained at 100 °C within 2 h of the reaction in the presence of 1.0 mol% catalyst (entry 2, Table 3). When the temperature was increased to 150 °C, 25% conversion was achieved within the same reaction time, i.e., 2 h (entry 3, Table 3). As mentioned in entry 4, the conversion increased slightly within 6 h of the reaction. Finally, the optimum reaction conditions were found to be as follows: nitrile (0.1 mL, 1.0 mmol) with Ag/Fe3O4 (19.0 mg, 3.0 mol%) in H2O (3.0 mL) in a stainless steel reactor (entry 5, Table 3). Under these conditions, Ag/Fe3O4 catalyst showed better catalytic activity than a previously reported SiO2-supported Ag nanocatalyst [28]. 27% conversion was obtained with a reaction time of 6 h at 120 °C (entry 6, Table 3).

Table 3. Optimization of reaction conditions.

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Table 3. Optimization of reaction conditions.
Molecules 19 00699 i014
EntryCatalyst (mol%)Temp. (°C)Time (h)Conversion a (%)
1Blank15068
2110022
31150225
41150636
531506>99
63120627
72150685
83150131
9Recovered from #51506>99
10Recovered from #91506>99
11Recovered from #101506>99

a Determined by using gas chromatography-mass spectrometery (GC-MS) spectroscopy. Reaction conditions: nitriles (1.0 mmol), Ag/Fe3O4 catalyst (3.0 mol%), H2O (3.0 mL).

Interestingly, the conversion decreased only slightly when 2.0 mol% Ag/Fe3O4 was used (entry 7, Table 3). As expected, lowering the reaction time effectively decreased the conversion (entry 8, Table 3). Among the three factors of reaction time, temperature, and quantity of catalyst, we found that the reaction temperature was the most important because the conversion decreased drastically at 120 °C (entries 5 and 6, Table 3). Remarkably, the Ag/Fe3O4 catalysts were easily separated using an external magnet (Scheme 2) after the completion of the reaction, and reused three times under the same reaction conditions without any loss of catalytic activity (entries 9–11, Table 3). As shown in Figure 4, the structure of the Ag/Fe3O4 microspheres remained unchanged after the reaction ended, thereby demonstrating catalyst recyclability.

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Scheme 2. Magnetic separation and recycling of the Ag/Fe3O4 catalyst.

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Scheme 2. Magnetic separation and recycling of the Ag/Fe3O4 catalyst.
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The optimized reaction conditions determined for the Ag/Fe3O4 catalyst system were applied to various other substituents (Table 4).

Table 4. Hydration of various nitriles catalyzed by Ag/Fe3O4 catalyst.

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Table 4. Hydration of various nitriles catalyzed by Ag/Fe3O4 catalyst.
EntrySubstrateProductYield a (%)
1 Molecules 19 00699 i015 Molecules 19 00699 i016>99
2 Molecules 19 00699 i017 Molecules 19 00699 i01896
3 Molecules 19 00699 i019 Molecules 19 00699 i02060
4 Molecules 19 00699 i021 Molecules 19 00699 i02240
5 Molecules 19 00699 i024 Molecules 19 00699 i02498
6 Molecules 19 00699 i025 Molecules 19 00699 i02686
7 Molecules 19 00699 i027 Molecules 19 00699 i02813
8H3C—CH Molecules 19 00699 i029>99
9 Molecules 19 00699 i030 Molecules 19 00699 i03187

a Determined by using GC-MS Spectroscopy. Reaction conditions: nitriles (1.0 mmol), Ag/Fe3O4 catalyst (3.0 mol%), H2O (3.0 mL), 150 °C, 6 h.

We confirmed that this reaction could be extended to a wide variety of nitriles. The reaction rates were not influenced substantially by the electronic effects of the substituents on the aromatic rings of nitriles. Both, 4-chlorobenzonitrile and 4-bromobenzonitrile showed high conversions of more than 95% (entries 1 and 2, Table 4). The nitro and amino groups on the nitrile were less active than the halide substituents (entries 3 and 4, Table 4). When the hydration of o-, m-, and p-tolunitriles was catalyzed by Ag/Fe3O4, steric effects of ortho-substituted nitriles on the reaction rates were observed (entries 5–7, Table 4). The hydration of aliphatic nitriles such as acetonitrile and acrylonitrile was also accomplished with high conversions (entries 8 and 9, Table 4)

3. Experimental

3.1. General Remarks

The morphology of each sample was characterized by TEM (FEI, Tecnai F30 Super-Twin) located at the National Nanofab Center (Daejeon, South Korea) by placing a few drops of the corresponding colloidal solution on carbon-coated copper grids (200 mesh, F/C coated, Ted Pella Inc., Redding, CA, USA). The SEM images were taken using a SEM (VEGA3, TESCAN, Busan, South Korea). Magnetization data were taken using a SQUID (MPMS-7, Quantum Design, Busan, South Korea). The elemental compositions of the hybrid catalysts were obtained using EDS (550i, IXRF Systems, Inc., Busan, South Korea), while the XRD patterns were recorded by a Rigaku RINT 2200 HK diffractometer (Rigaku Corporation, Tokyo, Japan). Mass spectra were obtained on a Shimadzu GC/MS, QP-2010 SE (EI) (Shimadzu Co., Kyoto, Japan). Reagents were purchased from Aldrich Chemical Co., TCI and Strem Chemical Co. and used as received without further purification. The concentration of 4-nitrophenol was determined at a wavelength of 400 nm using a SINCO S-3150 spectrophotometer (SINCO, Daejeon, Korea).

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Figure 4. TEM images of Ag/Fe3O4, (a) before and (b) after the third cycles.

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Figure 4. TEM images of Ag/Fe3O4, (a) before and (b) after the third cycles.
Molecules 19 00699 g004 1024

3.2. Synthesis of Fe3O4 Microspheres

Magnetite particles were synthesized using a solvothermal method [31]. The details were as follows: FeCl3•6H2O (0.36 g, 1.3 mmol) and trisodium citrate (Na3Cit, 0.072 g, 0.24 mmol) were dissolved in ethylene glycol/ethanol (36 mL/4 mL) solution; then, sodium acetate (0.48 g, 5.9 mmol) was added under vigorous stirring for 10 min. The resulting mixture was then transferred to a Teflon-lined stainless-steel autoclave (with a capacity of 50 mL) for heating at 200 °C for 10 h. Then, the autoclave was carefully taken out and allowed to cool to room temperature. The as-made black products were thoroughly washed with ethanol three times, and they were then vacuum-dried.

3.3. Immobilization of Ag NPs onto Fe3O4 Microspheres

Ag NPs were immobilized onto Fe3O4 microspheres according to modified procedure of Yang et al. [31]. Fe3O4 microspheres were incubated in 0.2 M NaOH aqueous solution to ionize the carboxyl groups. The residual NaOH was removed by washing with deionized water through centrifugation. Then, the ionized Fe3O4 microspheres were dispersed in the solution (30 mL, 0.1 M) of AgNO3 during 1 h under sonication. After this, the microspheres were harvested with the aid of the magnet and washed with deionized water three times. Then, the microspheres were redispersed in 20 mL of deionized water, and 50 mM NaBH4 aqueous solution (1 mL) was added dropwise under ice water bath cooling during 5 min with vigorous stirring. The final product was purified through washing with water three times and dried under vacuum.

3.4. A Typical Procedure for Reduction of Nitroarenes

As a representative example, 7.50 × 10−4 M 4-nitrophenol solution and 0.026 mg of Ag/Fe3O4 [Ag base (17 wt%): 2.0 mol%] were mixed and sonicated for 30 s at room temperature. Then, 1.0 mL of 2.22 M NaBH4 (300 equiv. to per equiv. of substrate) solution was added to the mixture. The reaction progress was monitored by UV/vis spectrometer.

3.5. A Typical Procedure for the Hydration of Nitriles

Ag/Fe3O4 catalyst (3.0 mol%), water (3.0 mL), and the corresponding nitrile (1.0 mmol) were introduced into a stainless steel reactor. After the reaction, the catalysts were separated from the solution by external magnet. The reaction products were analyzed by mass spectra on GC-MS. (Figures S1–S10).

3.5.1. GC-MS Data

Benzamide (Table 3, entry 5). To a stainless steel reactor equipped with magnetic stirrer were added benzonitrile 0.1 mL (1.0 mmol), Ag/Fe3O4 19.0 mg (0.03 mmol) and H2O (3 mL). The mixture was heated at 150 °C for 6 h. After cooling to room temperature, the solution was extracted with ethylacetae (20 mL). MS (EI) m/z: 28(100), 32(34), 51(30), 77(74), 105(75), 121(60).

4-Chlorobenzamide (Table 4, entry 1). 4-Chlorobenzonitrile (138 mg, 1.0 mmol) was hydrolysed as above. MS (EI) m/z: 75(33), 111(52), 139(100), 155(51).

4-Bromobenzamide (Table 4, entry 2). 4-Bromobenzonitrile (182 mg, 1.0 mmol) was hydrolysed as above. MS (EI) m/z: 28(27), 50(48), 139(100), 155(51), 183(100), 199(51).

4-nitrobenzamide (Table 4, entry 3). 4-nitrobenzonitrile (148 mg, 1.0 mmol) was hydrolysed as above. MS (EI) m/z: 28(100), 118(28), 150(78), 166(55).

4-aminobenzamide (Table 4, entry 4). 4-aminobenzonitrile (118 mg, 1.0 mmol) was hydrolysed as above. MS (EI) m/z: 28(100), 32(33), 65(29), 92(31), 120(75), 136(55).

4-Methylbenzamide (Table 4, entry 5). 4-Methylbenzonitrile (117 mg, 1.0 mmol) was hydrolysed as above. MS (EI) m/z: 28(85), 32(28), 65(25), 91(72), 119(100), 135(62).

3-methylbenzamide (Table 4, entry 6). 3-methylbenzonitrile (117 mg, 1.0 mmol) was hydrolysed as above. MS (EI) m/z: 65(26), 91(83), 119(100), 135(71).

2-methylbenzamide (Table 4, entry 7). 2-methylbenzonitrile (117 mg, 1.0 mmol) was hydrolysed as above. MS (EI) m/z: 65(33), 91(100), 119(87), 135(84 ).

Acetamide (Table 4, entry 8). acetonitrile (0.052 mL, 1.0 mmol) was hydrolysed as above. MS (EI) m/z: 28(100), 32(32), 44(77), 59(89).

Acrylamide (Table 4, entry 9). acrylonitrile (0.066 mL, 1.0 mmol) was hydrolysed as above. MS (EI) m/z: 27(75), 28(73), 32(17), 43(31), 74(83), 55(64), 71(100).

4. Conclusions

Ag NPs were immobilized onto Fe3O4 microspheres in situ by substituting sodium cations with Ag ions. The Ag NPs were uniformly immobilized in the Fe3O4 support while preserving their particle size and crystallinity, as well as the structural integrity of the Fe3O4 support. The Ag/Fe3O4 catalyzed the reduction of nitro compounds and hydration of nitriles to amides in water with high conversion. Furthermore, the Ag/Fe3O4 catalyst was readily separated using an external magnet and could be reused at least three times with benzonitrile under the optimized reaction conditions without any loss of catalytic activity. The magnetic separability eliminated the requirement of catalyst filtration after the completion of the reaction.

Supplementary Materials

Supplementary materials can be accessed at: http://www.mdpi.com/1420-3049/19/1/699/s1.

Acknowledgments

This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (No. 2013R1A1A1A05006634) and Ministry of Education (MOE) and National Research Foundation of Korea (NRF) through the Human Resource Training Project for Regional Innovation (No. 2012H1B8A2026225).

Conflicts of Interest

The authors declare no conflict of interest.

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  • Sample Availability: Samples of the compounds are available from the authors.
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