J Inorg Organomet Polym DOI 10.1007/s10904-013-9919-5
M-hexaferrite–APTES/Pd(0) Magnetically Recyclable Nano Catalysts (MRCs) M. Demirelli • E. Karaoglu A. Baykal • H. Sozeri
•
Received: 19 June 2013 / Accepted: 20 July 2013 Ó Springer Science+Business Media New York 2013
Abstract Magnetically recovable BaFe12O19–APTES– Pd(0) and SrFe12O19–APTES–Pd(0) catalysts were easily synthesized by immobilizing Pd nanoparticles on the surface of magnetic hexaferrite–NH2 microspheres. It was found that the combination of BaFe12O19, SrFe12O19 and 3-aminopropyltriethoxysilane (APTES) could give rise to structurally stable catalytic sites. Furthermore, BaFe12O19– APTES–Pd(0) and SrFe12O19–APTES–Pd(0) magnetically recyclable nano catalysts (MRCs) can be recovered by magnet and reused for nine runs for hydrogenation of 4-nitroaniline and dinitribenzene without significant loss in its catalytic activity which shows the indicative of a potential applications of these catalyst in industry. Keywords Nanostructures Chemical synthesis Infrared spectroscopy X-ray diffraction Catalytic properties
M. Demirelli (&) E. Karaoglu A. Baykal Department of Chemistry, Faculty of Arts and Sciences, Fatih University, 34500 B.Cekmece-Istanbul, Turkey e-mail:
[email protected] M. Demirelli Department of Chemistry, Faculty of Arts and Sciences, Yıldız University, Davutpas¸ a Campus, D Block No:1010, Esenler-Istanbul, Turkey E. Karaoglu Departments of Basic Medical Sciences, Medical Biochemistry, Faculty of Medicine, Sakarya University, Korucuk, Sakarya, Turkey H. Sozeri TUBITAK-UME, National Metrology Institute, PO Box 54, 41470 Gebze, Kocaeli, Turkey
1 Introduction Recently, magnetic nanoparticles (MNPs) have attracted much attention in physics, biomedicine, biotechnology, material science and catalysis, because of the large ratio of surface area to volume, paramagnetic behavior and low toxicity [1–3]. The magnetic property of these nanoparticles allow a convenient way of removing and recycling MNPs supported catalysts in heterogenous reactions by applying an appropriate magnetic field. As compared with the filtration and centrifugation methods, this kind of magnetic separation is not time-consuming and prevents the loss of solid catalyst in the process. Due to the absence of these problems with magnetically recyclable nano catalysts (MRCs), magnetic separation a green process and it avoids use of extra chemicals and additional filtration or centrifugation step during the separation process. Moreover, they are accessible from inexpensive materials and can be easily tuned by structural appropriate surface modification [4–9]. Moreover it enhances products purity, optimizes operational costs and MRCs shows high dispersion and reactivity with a high degree of chemical stability [1, 10, 11]. To our best of knowledge, this is the first study in which hexaferrite nanoparticles as core shell were used as a catalyst for the hydrogenation of various organic compounds. There are very few studies for the usage of hexaferrites for catalyst. In one of this study, hexaferrite was as catalyst for the clean combustion of methane [12, 13]. In this study, we reported a convenient procedure for synthesizing of a magnetically recyclable MFe12O19– APTES–Pd(0) (M = Ba, Sr) MRCs for the hydrogenation reactions of 4-nitroaniline, 1, 3 dinitrobenzene respectively.
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2 Experimental 2.1 Chemicals All chemicals Sr(NO3)24H2O, Ba(NO3)24H2O, Fe(NO3)3 9H2O, PdCl2, NaOH, APTES (3-aminopropyl-triethoxysilane), NaBH4 (sodium borohydride), 4-nitroaniline and 1,3 dinitrobenzene were obtained from Merck and were used without further purification.
obtained separately. The obtained MFe12O19 NPs were dispersed in ethanol/water (volume ratio 1:1) solution by sonication for 30 min, and then 6 ml APTES (99 %) was added to the mixture. After mechanical stirring at 40 °C for 8 h, the suspended MFe12O19 NPs were separated magnetically. The settled product was re-dispersed in ethanol by sonication and then was isolated with magnetic decantation for three times. The precipitated product (SrFe12O19–APTES and BaFe12O19–APTES nanocomposites) were dried at room temperature under vacuum (Scheme 1).
2.2 Instrumentation X-ray powder diffraction (XRD) analysis was conducted on a Rigaku SmartLab Diffractometer operated at 40 kV and 35 mA using Cu Ka radiation. High resolution transmission electron microscopy (HR-TEM) analysis was performed using a JEOL JEM 2100 microscope. A drop of diluted sample in alcohol was dripped on a TEM grid. Fourier transform infrared (FT-IR) spectra were recorded in transmission mode with a Perkin Elmer BX FT-IR infrared spectrometer. The powder samples were ground with KBr and compressed into a pellet. FT-IR spectra in the range 4000-400 cm-1 were recorded in order to investigate the nature of the chemical bonds formed. VSM measurements were performed by using a Vibrating sample magnetometer (LDJ Electronics Inc., Model 9600). The magnetization measurements were carried out in an external field up to 15 kOe at room temperature. UV–Vis measurement was done using a Shimadzu UV– Vis 2600. ICP analysis was done by using the Perkin Elmer Optima 4300DV model. 2.3 Procedure 2.3.1 Preparation of the magnetic MFe12O19–APTES (M:Ba, Sr) nanocomposites
3 Results and Discussion 3.1 XRD analysis Phase investigation of the crystallized products was performed by XRD and their powder diffraction patterns were presented in Fig. 1a, b respectively. The XRD patterns
Fe(NO3)3·9H2O
stirring at 50 °C Citric acid NaOH Sr(NO3)2 or Ba(NO3)2
+
hexaferrite
200 °C
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Synthesized M-hexaferrite–APTES powder was added to 30 mL of 0.01 g PdCl2 aqueous solution and stirred for 1 day at 35 °C. After producing M-hexaferrite–APTES– Pd(?2), Pd(?2) ions were reduced to Pd(0) by 30 mL of 0.1 M NaBH4 aqueous solution which resulted in the formation of a black powder (M-hexaferrite–APTES–Pd(0)) (Scheme 2). The solid catalysts were separated by external magnet and washed several times with ethanol.
calcined at 1100 °C
For the synthesis of MFe12O19–APTES–Pd(0) (M = Ba, Sr) MRCs, citrate sol–gel combustion method was applied. For this purpose, stoichiometric amounts of Fe(NO3)39H2O, Ba(NO3)24H2O and Sr(NO3)24H2O were dissolved in a minimum amount of de-ionized H2O by stirring at 50 °C with Fe/M ratio (of 11.5. Citric acid was then added to the mixture solution of M2? and Fe3? to chelate these ions. The molar ratios of citric acid to metal ions used were 1:1. Ammonia was added to adjust the pH value to 7. The clear solution was slowly evaporated at 80 °C under constant stirring, forming a viscous gel. By increasing the temperature up to 200 °C, the gel precursors were combusted to form brown loose powders. Finally, the obtained powder was calcined at 1100 °C for 1 h. MFe12O19 NPs were thus
2.3.2 Preparation of the magnetically recyclable MFe12O19–APTES–Pd(0) (M = Ba, Sr) (MRCs)
6 ml APTES o
stirring at 40 C f or 8 h
Scheme 1 Preparation steps for fabricating APTES-functionalized MFe12O19 NPs (M = Ba, Sr)
J Inorg Organomet Polym Scheme 2 Schematic representation for the synthesis of MFe12O19–APTES–Pd(0) (M = Ba, Sr)
O O Si
Pd(Cl)2
hexaferrite
hexaferrite
O
1hr stirring (NaBH4) 24 h 35 oC
NH2
O Si O
NH2
Pd(0)
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APTES
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% Transmission (a.u.)
Intensity (arb. units)
107 114
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D= 29+2 nm Pd
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1014 219
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exp fit
Pd
008 106
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006
20
Fig. 2 FT-IR spectra of MFe12O19–APTES–Pd(0) MRCs a M = Sr and b M = Ba
D= 26±3 nm
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Intensity (arb. units)
(b)
70
2 Theta (deg.) Fig. 1 XRD powder pattern of MFe12O19–APTES–Pd(0) MRCs a M = Sr and b M = Ba with line profile fitting
indicate that both products (SrFe12O19 and BaFe12O19) are M-type hexaferrites and the diffraction peaks are broadened owing to very small crystallite size. All of the observed diffraction peaks in Fig 1a, b are indexed by the cubic structure of BaFe12O19 (JCPDS no. 84-0757) and SrFe12O19 (JCPDS no. 84-1531) revealing a high phase purity of hexaferrite respectively. The average crystallite size of both products were calculated by using line profile fitting [14] and found as 29 ± 2 nm for SrFe12O19 and 26 ± 3 nm for BaFe12O19 for observed 19 peaks with the following miller indices: (006), (110), (008), (107), (114), (200), (201), (203), (116), (205), (206), (209), (217), (304), (2011), (218), (220), (1114), (2014). The following miller indices (111), (200),
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(a)
(b)
Relative Frequency %
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DTEM =26.4 nm σ=1.6
20
10
0 20
30
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Diameter (nm) Fig. 3 a The TEM micrographs of SrFe12O19–APTES–Pd(0) MRC with different magnifications; and b size distribution histogram obtained from the TEM micrographs
(220). ICDD card no: 46-1043 showed the presence of Pd(0) NPs in both SrFe12O19–APTES–Pd(0) and BaFe12O19– APTES–Pd(0) MRCs [15, 16]. According to the ICP results, Pd content in the products were determined as 12 and 15 % by weight for SrFe12O19–APTES–Pd(0) and BaFe12O19– APTES–Pd(0) MRCs respectively. 3.2 FT-IR Analysis The FT-IR spectra of MFe12O19–APTES–Pd(0) (M = Ba, Sr) MRCs were presented in Fig. 2a, b respectively. As prepared powder presents characteristic peaks that are exhibited by the SrFe12O19 and BaFe12O19 powders:
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characteristic absorption bands for MFe12O19 at around 450 and 590 cm-1 (corresponding to vibrations of the tetrahedral and octahedral sites for MFe12O19) [17, 18]. The adsorption of APTES onto the surface of magnetite particles was confirmed by the bands at 1110, 1050 and 990 cm-1 assigned to the SiO–H and Si–O–Si groups [19]. The silica network is adsorbed on the magnetite surface by M–O–Si bond (584 cm-1 which overlaps with the M–O vibration of CoFe2O4 NPs. The two broad bands at 3417 and 1625 cm-1 can be ascribed to the N–H stretching vibration and NH2 bending mode of free NH2 group, respectively [20, 21]. The Si–O–Si stretching was verified at 1150–1100 cm-1 for MFe12O19–APTES–Pd(0) catalyst
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histogram presented in Fig. 3a was obtained by counting a total of 150 nanoparticles from several micrographs and average particle size of SrFe12O19–APTES–Pd(0) MRC was estimated as 26.4 ± 1.6 nm. Size estimated from TEM micrographs agrees well with the crystallite size estimated from XRD line profile fitting. 3.3 Magnetization Measurements
Fig. 4 The TEM micrographs of BaFe12O19–APTES–Pd(0) MRC
30
(a) (b)
20
M, emu/g
10 0 -10 -20
Magnetic characterization of MFe12O19–APTES–Pd(0) (M = Ba, Sr) MRCs have been done by measuring M–H hysteresis curve at room temperature in the external field range of ±15 kOe. Figure 5 reveals that saturation magnetization values (Ms) of these nanocomposites (*24–28 emu/g) is considerably lower than previously reported value of 78.5 emu/g [25], probably due to the adsorption of surfactant molecules to the surface of both SrM and BaM nanoparticles. Coercive field, on the other hand, is nearly the same with high purity bulk SrM particles reported as 3500 Oe in Ref. [25]. It is well known that coercive field (Hc) depends on the particle size in such a way that as Hc increases particle size decreases. This means that we have synthesized fine BaM and SrM nanoparticles which are smaller than the single domain limit (i.e., \1 lm). It has been reported that the squareness ratio (Mr/Ms) at or above 0.5 indicates that the material is in single magnetic domain [26] and below 0.5 can be attributed to the formation of multidomain structure. In the present study, the squareness ratio (Mr/Ms) is found as 0.56, indicating that all the samples have single magnetic domain structure and magnetization reversal mechanism is due to the coherent rotation [27].
-30 -15000
-10000
-5000
0
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15000
3.4 Catalytic Activity and UV Measurements
H, Oe
Fig. 5 Room temperature M–H hysteresis curve of MFe12O19– APTES–Pd(0) (M = Ba, Sr) MRCs a M = Ba and b Sr
[22]. The surface of MFe12O19–APTES–Pd(0) catalyst has been successfully functionalized with amino groups in the synthetic process, in agreement with the earlier reports [23, 24]. Schematic representation of this confirmation was presented in Scheme 2. 3.2.1 TEM analysis Morphology of SrFe12O19–APTES–Pd(0) MRC along with their size distribution histogram and BaFe12O19–APTES– Pd(0) MRCs were investigated with TEM and were presented in Figs. 3 and 4 respectively. The size distribution
Due to the very slow reaction rate of the chemical reduction of nitro group with sodium borohydride (NaBH4), the use of a catalyst is necessary [24]. For catalytic activity and UV measurements 4-nitroaniline and 1,3-dinitrobenzene was selected respectively. Catalytic activity of magnetically recyclable MFe12O19–APTES– Pd(0) MRCs were done on the reduction of each compound by NaBH4. In a typical run, 0.2 mL magnetically recyclable MFe12O19–APTES–Pd(0) MRCs aqueous solution of 0.1 mg/mL was added to a mixture of 1.4 ml 4-nitroaniline and 1,3-dinitrobenzene, 1.4 mL NaBH4 (30 mM) respectively. The absorption spectra of the reaction solution were recorded at different times after the addition of the catalyst. The easy magnetic separation of both MRCs after hydrogenation of 1,3-dinitrobenzene were shown in Fig. 6a, b respectively.
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J Inorg Organomet Polym Fig. 6 Catalytic reaction of MFe12O19–APTES–Pd(0) (M = Ba, Sr) MRCs a M = Sr, b Ba with 1,3-Dinitrobenzene
3.4.1 Catalytic Activity MFe12O19–APTES–Pd(0) MRCs for hydrogenation of 1,3-dinitrobenzene and 4nitroaniline The UV–Vis spectra of hydrogenation reaction of 1,3dinitrobenzene with MFe12O19–APTES–Pd(0) MRCs were given in Fig. 7d, f. At the same time, the UV–Vis spectra of hydrogenation of 1,3-dinitrobenzene without catalyst were also depicted in Fig. 7b. The characteristic UV–Vis peak of m-DNB appears at around 240 nm [28]. After the reduction reaction, this characteristic peak disappeared and new peak was observed at around 280 nm which indicated the formation of benzenediamine. It was observed that there was a slow decrease of absorbance during the chemical reaction with Pd(0) in comparison to MRC. As it is seen in Fig. 7d and f after addition of MFe12O19– APTES–Pd(0), the characteristic band of 1,3-dinitrobenzene disappear immediately and new band appear at around 280 nm comes due to benzenediamine. These results showed that the catalytic activity of both catalyst were same (Fig. 6d, f). The UV–Vis spectra of hydrogenation reaction of 4-nitroaniline with MFe12O19–APTES–Pd(0) MRCs were given in Fig. 7c, e. At the same time, the UV–Vis spectra of hydrogenation of 1,3-dinitrobenzene without catalyst were also depicted in Fig. 7a. For 4-nitroaniline, the characteristic band arises at 380 nm due to -NO2 group. It was recognized that during the reaction the characteristic band of -NO2 group disappeared and new band observed
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at around 240 nm due to the formation of 4-phenylenediamine as new product [29]. The comparison of catalytic activity of Pd(0) and MFe12O19–APTES–Pd(0) MRCs were depicted in Fig. 7a, f respectively. As it was shown in Fig. 7a, the disappearance of this peak (-NO2) occurred within 20 and 1 min after the addition of Pd(0) and MFe12O19–APTES–Pd(0) MRCs respectively. These results revealed that the catalytic activity of MFe12O19–APTES–Pd(0) MRCs is much more better than that of Pd(0) catalyst itself. Figure 7 represents the absorption spectra of 4-nitroaniline, NaBH4 and Pd(0) mixed aqueous solution after 1,2,5 and 20 min; and MFe12O19–APTES–Pd(0) MRCs mixed aqueous solution after 1 min of 1st, 3rd, 5th, 7th, 9th run. To ensure the recyclable property of MFe12O19–APTES–Pd(0) MRCs, both catalysts were used nine times subsequently without any significant lose of its catalytic activity. As it was shown in Scheme 1; Pd particles sit on NH2 comes from APTES group. This modification prevent the agglomerations of bulk Pd particles and increase the surface area of catalyst. Because of that well dispersed Pd particles play very efficient role during the reduction reaction [8, 30].
4 Conclusion In this study, the synthesis, characterizations and catalytic activity MFe12O19–APTES–Pd(0) (M = Ba, Sr) MRCs
J Inorg Organomet Polym
(a)
(b)
1min 2min 5min 20 min
Absorbance
4-nitroaniline, NaBH 4 and Pd(0)
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4-Nitroaniline, NaBH 4 and SrFe 12 O 19 -APTES -Pd(0)
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Fig. 7 Absorption spectra of a 4-nitroaniline with NaBH4 ? Pd(0) for 1st, 2nd, 5th, 20th min, b 1,3-dinitrobenzene with NaBH4 ? Pd(0) for 1st,2nd,5th,20th min, c 4-nitroaniline with NaBH4 ? BaFe12O19– APTES–Pd(0) MRC, d 1,3-dinitrobenzene with NaBH4 ? BaFe12O19–
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APTES–Pd(0) MRC, e 4-nitroaniline with NaBH4 ? SrFe12O19–APTES–Pd(0) MRC, f 1,3-dinitrobenzene with NaBH4 ? SrFe12O19– APTES–Pd(0) MRC mixed aqueous solution after 3 min of 1st, 3rd, 5th, 7th, 9th run
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(M = Sr, Ba) as highly effective catalysts for reduction reactions for 4-nitroaniline and 1,3-dinitrobenzene in liquid phase. Products (SrFe12O19 and BaFe12O19 NPs) have exhibited an average crystallite size of size of 26 ± 3 nm and 29 ± 2 respectively. Super paramagnetic character with a high saturation magnetization makes these catalysts an attractive material that was possible to recover from solution using magnetic decantation. It is noteworthy fabricated MFe12O19–APTES–Pd(0) (M = Ba, Sr) MRCs could be easily separated using a magnet and had good activity until nine times recycling in reduction reactions of 4-nitroaniline and 1,3-dinitrobenzene in liquid phase. The application of this catalytic system to some other catalytic reactions (Suzuki, Heck reaction etc.) will be progressed as further study. Acknowledgments This work is supported by Fatih University under BAP Grant No P50021104-B.
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