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Pd nanoparticles in hollow magnetic mesoporous spheres: High activity, and magnetic recyclability
Journal of Molecular Catalysis A: Chemical 367 (2013) 46–51
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Catalysis A: Chemical journal homepage: www.elsevier.com/locate/molcata
Pd nanoparticles in hollow magnetic mesoporous spheres: High activity, and magnetic recyclability Jian Sun, Zhengping Dong, Xun Sun, Ping Li, Fengwei Zhang, Wuquan Hu, Haidong Yang, Haibo Wang, Rong Li ∗ The Key Laboratory of Nonferrous Metal Chemistry and Resources Utilization of Gansu Province, College of Chemistry and Chemical Engineering Lanzhou University, Lanzhou 730000, China
a r t i c l e
i n f o
Article history: Received 2 February 2012 Received in revised form 20 October 2012 Accepted 22 October 2012 Available online 30 October 2012 Keywords: Suzuki coupling reaction Palladium Hollow silica sphere Mesoporous Magnetic properties
a b s t r a c t The nanoreactor of hollow magnetic mesoporous silica spheres (Pd/HMMS), with Pd and Fe3 O4 nanoparticles embedded in the mesoporous silica shell, were successfully prepared by using the colloidal carbon spheres of glucose, Pd and Fe3 O4 heteroaggregates as the hard template together with a coating of tetraethoxysilane (TEOS) and cetyltrimethylammonium bromide (CTAB) mixture. The synthesized Pd/HMMS shows excellent catalytic activity in the Suzuki cross-coupling reaction of iodobenzene with phenylboronic acid with over 99% yield in 3 min and can be recycled multiple times without any significant loss in catalytic activity. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Hollow mesoporous silica spheres (HMS) have attracted considerable attention because their nanopore channels with large surface area and pore volume, high porosity, ordered and tunable size. They can be used as nanoreactors for applications in adsorption, separation, catalysis [1–9], and so forth. Compared to the conventional mesoporous silica materials such as MCM41 [10–14], SBA-15 [15–18], FSM-16 [19] and MCF [20], HMS exhibited good catalyst loading properties for confined cooperative catalysis, as they could prevent aggregation of metallic nanoparticles and promote the mass diffusion and transport of reactants. As is well known, magnetic nanoparticles have been widely applied in heterogeneous catalyst systems [21–25]. Generally, magnetic microspheres consisting of an iron oxide core and functional shell have gained much attention due to their unique separable feature. Therefore, to enhance their performance in practical applications, many research efforts have recently been devoted to HMMS to combine the high surface area and magnetic properties. Meanwhile, considerable efforts focused on the adsorption or drug delivery application
∗ Corresponding author. Tel.: +86 0931 891 3597; fax: +86 0931 891 2582. E-mail address: [email protected] (R. Li). 1381-1169/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molcata.2012.10.023
[26–31], and rare work succeeded in designing HMMS with metal nanoparticles and well-defined nanostructures for advanced catalysis [32–35]. In this paper, we developed a facile route to synthesize HMMS nanoreactors with Pd/Fe3 O4 nanoparticles resided inside the mesoporous spheres by using the colloidal carbon spheres as the templates. The fabrication procedure involved four steps as shown in Fig. 1. The first step involved the preparation of the colloidal carbon spheres with attached Fe3 O4 nanoparticles. In the next step, deposited highly dispersed Pd nanoparticles onto the surface of carbon nanospheres with Fe3 O4 nanoparticles. Then, the organosilicate incorporated silica shells were deposited on the colloidal carbon spheres through the simultaneous sol–gel polymerization of tetraethoxysilane (TEOS) and cetyltrimethylammonium bromide (CTAB). Finally, the HMMS nanoreactors were obtained after the calcination to remove the carbon templates and the organic groups of CTAB. The designed Pd/HMMS nanoreactors possess large magnetization, highly open and ordered mesopores, and stably confined but exposed Pd nanoparticles. The multicomponent nanostructured materials showed excellent activity for Suzuki cross-coupling reaction and could be easily recycled multiple times without visible decrease in the catalytic performance. Furthermore, the method is also extended to introduce other noble metal nanoparticles including Au, Ag, Pt, and their bimetallic nanoparticles inside the HMMS to prepare other new catalysts.
J. Sun et al. / Journal of Molecular Catalysis A: Chemical 367 (2013) 46–51
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Fig. 1. Scheme of the synthetic procedure for the preparation of Pd/hollow magnetic mesoporous spheres.
2. Experimental 2.1. Materials and reagents Glucose, ethanol, sodium formate, tetraethoxysilane (TEOS), ammonia solution (28 wt%), SnCl2 ·2H2 O, PdCl2 , K2 CO3 , FeCl3 ·6H2 O (99%), FeCl2 ·4H2 O (99%), were purchased from Tianjing Chemical Reagent Co. Iodobenzene, phenylboronic acid and cetyltrimethylammonium bromide (CTAB) were bought from Alfa Aesar. All chemicals were of analytical grade and were used as received without further purification.
environment for adsorbing metal ion-nanoparticles by electrostatic attraction in acidic solution. 0.6 g Fe3 O4 nanoparicles and 1.0 g negatively charged carbon nanospheres were dispersed in 80 mL and 60 mL hydrochloric acid solution (pH ≈ 2.3) with ultrasonic for 20 min, respectively. Then the latter suspension was added dropwise into the former under vigorous stirring at room temperature. After 6 h, Fe3 O4 nanoparticles attaching on the surface of carbon nanospheres, were separated from the solution by an external magnet and washed several times with distilled water until the pH value of the solution was close to 7. Finally, the products were oven-dried at 50 ◦ C for further use.
2.2. Preparation of catalysts 2.2.1. Synthesis of carbon nanospheres [36] Glucose (6.0 g) was dissolved in 40 mL distilled water to form a clear solution and then transferred into a 100 mL Teflon-sealed autoclave. The autoclave was maintained at 180 ◦ C for 4.5 h. The products were separated by centrifugation, followed by washing three times with distilled water and ethanol. Finally, the products were oven-dried at 50 ◦ C for the following work. 2.2.2. Synthesis of iron oxide nanoparticles (Fe3 O4 ) Magnetic nanoparticles were prepared using simple chemical co-precipitation. Typically, 4.8 g of FeCl3 ·6H2 O and 2.0 g of FeCl2 ·4H2 O were dissolved in 40 mL of distilled water in a threenecked bottom (100 mL). The obtained transparent solution was degassed with nitrogen for 1 h. After being rapidly magnetic stirred for 30 min, the transparent solution was heated to 90 ◦ C. Thereafter, 12 mL of ammonia hydroxide was added to the reaction mixture in 0.1 min time using a syringe, and the mixture was stirred for 2 h at 90 ◦ C. After cooling to room temperature, the obtained magnetic dispersion was subjected to magnetic separation with a magnet, and dried at 60 ◦ C for 6 h. 2.2.3. Preparation of carbon nanospheres with attached Fe3 O4 nanoparticles (Fe3 O4 /C) [31] These hydroxyl carbon nanospheres possess functional groups ( OH) on the surface, which offer an important chemical
2.2.4. Loading Pd nanoparticles onto the carbon nanosphere with Fe3 O4 nanoparticles (Pd/Fe3 O4 /C) [37] 200 mg Fe3 O4 /C was dispersed in 50 mL distilled water and stirred for 20 min as part A. 0.2 g SnCl2 was dissolved in 40 mL 0.02 M HCl solution as part B. Parts A and B were mixed together under stirring for 30 min. The suspension was separated from the solution by an external magnet, washed several times with distilled water and dispersed in 80 mL distilled water. Then 50 mL 10 mg PdCl2 was added into it. 10 min later, 20 mL of 0.15 M sodium formate solution was added, following stirring for 8 h. The suspension was separated from the solution by an external magnet, washed with distilled water and ethanol for several times and dried at 50 ◦ C for 12 h.
2.2.5. Production of Pd/Fe3 O4 @mesoporous SiO2 nanoreactor (Pd/HMMS) The Pd/Fe3 O4 /C composite obtained from last step was first dispersed in the solution containing 80 mL H2 O, 60 mL ethanol, 0.3 g CTAB and 1.2 mL NH3 ·H2 O with ultrasonic for 20 min. The 300 L TEOS was added into the mixture, and vigorously stirred for 8 h. The suspension was separated from the solution by an external magnet, washed several times with distilled water and ethanol, respectively, then oven-dried at 50 ◦ C. Then the product was calcined at 400 ◦ C in air atmosphere for 7 h to remove carbon sphere, CTAB template and other organic species. The amount of Pd in
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J. Sun et al. / Journal of Molecular Catalysis A: Chemical 367 (2013) 46–51
Fig. 2. XRD patterns of (a) HMMS and (b) Pd/HMMS.
the Pd/HMMS nanoreactor was found to be 2.0 wt% based on ICP analysis. 2.3. Measurement of catalytic performance For Suzuki cross-coupling reaction, 20 mg Pd/HMMS nanoreactor catalyst, 0.5 mmol iodobenzene, 1 mmol phenylboronic acid, 1 mmol K2 CO3 were added to 4 mL ethanol under magnetic stirring. The reaction was carried out at reflux (ca. 80 ◦ C) for definite time. Then, the catalyst was collected by an external magnet, and the reaction system was analyzed by gas chromatography. For recycling, the recovered catalyst was washed with ethanol and distilled water several times, respectively, and then dried under vacuum at 60 ◦ C. 2.4. Characterization of catalysts Powder X-ray diffraction (XRD) patterns were obtained on a Rigaku D/max-2400 diffractometer using Cu-K␣ radiation as the Xray source in the 2Â range of 10–80◦ . The conversion was estimated by GC (P.E. AutoSystem XL) or GC–MS (Agilent 6890N/5973N). Pd content of the catalyst was measured by inductively coupled plasma (ICP) on IRIS Advantage analyzer. The morphology of the catalyst was observed by a Tecnai G2 F30 transmission electron microscopy and samples were obtained by placing a drop of a colloidal solution onto a copper grid and evaporating the solvent in air at room temperature. X-ray photoelectron spectroscopy (XPS) was recorded on a PHI-5702 instrument and the C1s line at 297.8 eV was used as the binding energy reference. Magnetic measurement of Pd/Fe3 O4 @mesoporous SiO2 nanoreactor was investigated with a Quantum Design vibrating sample magnetometer (VSM) at room temperature in an applied magnetic field sweeping from −15 to 15 kOe.
that the Pd particles have been coated by hollow mesoporous SiO2 . Transmission electron microscopy (TEM) experiments have been carried out to observe a possible structure of the metallic Pd particles (Fig. 3). Apparently, Fe3 O4 and Pd nanoparticles are located at the inside pore mouths of the mesopores. This is consistent with the XRD patterns (Fig. 2). The morphologies and structures of the products at different synthetic steps were observed by TEM. Fig. 3a shows the TEM image of carbon nanospheres, in which well-dispersed particles with average diameter about 200 nm could be observed clearly. These carbon nanospheres possess functional groups ( OH, C O) on the surface, which offer an important chemical environment for adsorbing metal ion-nanoparticles by electrostatic attraction in acidic solution. Fig. 3b is a TEM image of the Fe3 O4 /C composite. The Fe3 O4 particles with an average size of about 9 nm are uniformly distributed on the surfaces of the carbon nanospheres. Using TEOS as the silica source and CTAB as a soft template, a thin layer of mesoporous silica is coated onto the Pd/Fe3 O4 /C spheres. Then, the Pd/HMMS nanoreactors were obtained after the calcination to remove the carbon templates and the organic groups of CTAB. As shown in Fig. 3c and d, the TEM images show that Pd, Fe3 O4 particles were evenly coated with a layer of mesoporous shells about 40 nm thick, while the Fe3 O4 particles remained initial their cluster morphologies in the Pd/HMMS. To further probe the components of the mesoporous shell, we used EDX measurement to analyze the Pd/HMMS after calcination at 400 ◦ C. The EDX spectra confirmed that the nanoreactor consisted of Fe, Si, Sn, Pd and O elements (Cu and C came from the TEM copper grid of the sample holder). EDX analysis (Fig. 3e) further confirms the existence of Pd. According to the ICP analysis, the mass percent of Sn for the composites was 0.9 wt%, and the presence of Sn was not significantly affected the catalytic activity of the nanoreactor [8]. Fig. 4 shows the XPS spectrum of the synthesized Pd/HMMS nanoreactor. Peaks corresponding to oxygen, silicon, carbon, palladium, tin and iron are observed. However, typical peaks of Fe, Sn and Pd elements are not obviously found. Fig. 6 shows the XPS spectrum of the elemental survey scan of Pd/Fe3 O4 /C. Meanwhile, typical peaks of Fe, Sn and Pd elements are clearly found. This indicates that Sn, Fe3 O4 and Pd particles have been coated by mesoporous SiO2 , since the analysis depth of XPS is only several nanometers. In Fig. 4, both the peaks of Si 2s (153.5 eV) and Si 2p (102.6 eV) are quite clear and this means that SiO2 is the main component of the outer surface. To ascertain the oxidation state of the Pd, X-ray photoelectron spectroscopy (XPS) studies were carried out. In Fig. 5, the binding energy of Pd exhibits two strong peaks centered at 342.6 and 337.2 eV, which are assigned to Pd 3d3/2 and Pd 3d5/2 , respectively. On the basis of above analysis, it can be concluded that the XPS data further identified the hollow mesoporous structure of the synthesized Pd/HMMS (Fig. 6). Magnetization curves revealed the superparamagnetic behavior of the magnetic nanoreactor, the hysteresis loop of Pd/HMMS is shown in Fig. 7. The magnetization saturation (Ms ) of the mesoporous spheres reaches 21.0 emu g−1 , which suggests that Fe3 O4 particles become encapsulated in hollow mesoporous spheres. This is consistent with the TEM patterns (Fig. 3). The catalyst can be efficiently separated easily from the solution with the help of an external magnetic force.
3. Results and discussion 3.2. Suzuki reaction catalyzed by Pd/HMMS nanoreactor 3.1. Characterization of catalysts The wide angle XRD patterns of the samples are presented in Fig. 2. An almost identical XRD pattern was obtained for Pd/HMMS comparing with HMMS (Fig. 2b), indicating the crystalline structure of the support was well maintained in the Pd samples. No obviously peaks due to metallic Pd were detected, owing to the fact
The catalytic activity of Pd/HMMS nanoreactor was tested in the Suzuki reaction of aryl halides, namely the reactivity of aryl iodides and aryl bromides with phenylboronic acid in ethanol solvent using K2 CO3 as a base. The reaction results are summarized in Table 1. A range of activities were observed from 25% to >99% yields depending on the nature of aryl bromo and iodo derivatives. Iodobenzene
J. Sun et al. / Journal of Molecular Catalysis A: Chemical 367 (2013) 46–51
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Fig. 3. (a) TEM of carbon nanospheres, (b) TEM of Fe3 O4 /C nanospheres, (c) TEM of final composite nanoreactor (Pd/HMMS) in low magnification, (d) TEM of final composite nanoreactor in high magnification, (e) energy-dispersive X-ray absorption spectroscopy (EDX) of the Pd/HMMS nanoreactor.
Fig. 4. XPS spectrum of the elemental survey scan of Pd/Fe3 O4 @mesoporous SiO2 . Fig. 5. XPS spectrum of the Pd/HMMS showing Pd 3d5/2 and Pd 3d3/2 binding energies.
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J. Sun et al. / Journal of Molecular Catalysis A: Chemical 367 (2013) 46–51 Table 2 Recyclability of catalysta . Entry
Run
1 2 3 4 5
Fresh First Second Third Fourth
Yieldb (%) >99 >99 99 99 99
a Reaction conditions: iodobenzene (0.5 mmol), phenylboronic acid (1 mmol), K2 CO3 (2 mmol), ethanol (4 mL), 80 ◦ C, catalyst (20 mg, with Pd loading 2.0 wt%). b Yield based on GC analysis.
can be easily recovered in a facile manner from the reaction solution by using a permanent magnet. Furthermore, the catalyst revealed a remarkable activity and was reused up to four consecutive cycles without any significant loss in catalytic activity (Table 2). 4. Conclusions
Fig. 6. XPS spectrum of the elemental survey scan of Pd/Fe3 O4 /C.
In summary, we have developed a facile method for fabrication of uniform Pd/HMMS nanoreactor with desired hollow core/mesoporous shell structures, high catalytic activity and magnetic recyclability properties. The present method is versatile, and many transition noble metals and their bimetallic nanoparticles can be resided inside the HMMS via the sol–gel process. The nanoreactor shows superior catalytic activity in the Suzuki cross-coupling reaction of iodobenzene with phenylboronic acid with over 99% yield in 3 min and can be recycled multiple times without any significant loss in catalytic activity. Acknowledgment This work was supported by the Fundamental Research Funds for the Central Universities (lzujbky-2012-68). References
Fig. 7. Room temperature magnetization curves of Pd/HMMS.
derivatives were found to give a good yield in 5 min reaction time (Table 1, entries 2 and 3). It was observed that with the increase in reaction time the conversion of bromobenzene increases (Table 1, entry 4). Bromobenzene derivatives were found to give a moderate yield in 30 min reaction time (Table 1, entries 5–7). The catalyst Table 1 Suzuki reactions catalyzed by the Pd/HMMS.a
. Entry
X
R
Time/min
1 2 3 4
I I I Br
H 2-NH2 4-Me H
5 6 7
Br Br Br
4-COMe 4-NH2 4-Me
3 5 5 3 30 30 30 30
Yieldb (%) >99 86 66 25 46 57 84 49
a Reaction conditions: iodobenzene (0.5 mmol), phenylboronic acid (1 mmol), K2 CO3 (2 mmol), ethanol (4 mL), 80 ◦ C, catalyst (20 mg, with Pd loading 2.0 wt%). b Determined by GC or GC–MS.
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Contents lists available at SciVerse ScienceDirect
Journal of Molecular Catalysis A: Chemical journal homepage: www.elsevier.com/locate/molcata
Pd nanoparticles in hollow magnetic mesoporous spheres: High activity, and magnetic recyclability Jian Sun, Zhengping Dong, Xun Sun, Ping Li, Fengwei Zhang, Wuquan Hu, Haidong Yang, Haibo Wang, Rong Li ∗ The Key Laboratory of Nonferrous Metal Chemistry and Resources Utilization of Gansu Province, College of Chemistry and Chemical Engineering Lanzhou University, Lanzhou 730000, China
a r t i c l e
i n f o
Article history: Received 2 February 2012 Received in revised form 20 October 2012 Accepted 22 October 2012 Available online 30 October 2012 Keywords: Suzuki coupling reaction Palladium Hollow silica sphere Mesoporous Magnetic properties
a b s t r a c t The nanoreactor of hollow magnetic mesoporous silica spheres (Pd/HMMS), with Pd and Fe3 O4 nanoparticles embedded in the mesoporous silica shell, were successfully prepared by using the colloidal carbon spheres of glucose, Pd and Fe3 O4 heteroaggregates as the hard template together with a coating of tetraethoxysilane (TEOS) and cetyltrimethylammonium bromide (CTAB) mixture. The synthesized Pd/HMMS shows excellent catalytic activity in the Suzuki cross-coupling reaction of iodobenzene with phenylboronic acid with over 99% yield in 3 min and can be recycled multiple times without any significant loss in catalytic activity. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Hollow mesoporous silica spheres (HMS) have attracted considerable attention because their nanopore channels with large surface area and pore volume, high porosity, ordered and tunable size. They can be used as nanoreactors for applications in adsorption, separation, catalysis [1–9], and so forth. Compared to the conventional mesoporous silica materials such as MCM41 [10–14], SBA-15 [15–18], FSM-16 [19] and MCF [20], HMS exhibited good catalyst loading properties for confined cooperative catalysis, as they could prevent aggregation of metallic nanoparticles and promote the mass diffusion and transport of reactants. As is well known, magnetic nanoparticles have been widely applied in heterogeneous catalyst systems [21–25]. Generally, magnetic microspheres consisting of an iron oxide core and functional shell have gained much attention due to their unique separable feature. Therefore, to enhance their performance in practical applications, many research efforts have recently been devoted to HMMS to combine the high surface area and magnetic properties. Meanwhile, considerable efforts focused on the adsorption or drug delivery application
∗ Corresponding author. Tel.: +86 0931 891 3597; fax: +86 0931 891 2582. E-mail address: [email protected] (R. Li). 1381-1169/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molcata.2012.10.023
[26–31], and rare work succeeded in designing HMMS with metal nanoparticles and well-defined nanostructures for advanced catalysis [32–35]. In this paper, we developed a facile route to synthesize HMMS nanoreactors with Pd/Fe3 O4 nanoparticles resided inside the mesoporous spheres by using the colloidal carbon spheres as the templates. The fabrication procedure involved four steps as shown in Fig. 1. The first step involved the preparation of the colloidal carbon spheres with attached Fe3 O4 nanoparticles. In the next step, deposited highly dispersed Pd nanoparticles onto the surface of carbon nanospheres with Fe3 O4 nanoparticles. Then, the organosilicate incorporated silica shells were deposited on the colloidal carbon spheres through the simultaneous sol–gel polymerization of tetraethoxysilane (TEOS) and cetyltrimethylammonium bromide (CTAB). Finally, the HMMS nanoreactors were obtained after the calcination to remove the carbon templates and the organic groups of CTAB. The designed Pd/HMMS nanoreactors possess large magnetization, highly open and ordered mesopores, and stably confined but exposed Pd nanoparticles. The multicomponent nanostructured materials showed excellent activity for Suzuki cross-coupling reaction and could be easily recycled multiple times without visible decrease in the catalytic performance. Furthermore, the method is also extended to introduce other noble metal nanoparticles including Au, Ag, Pt, and their bimetallic nanoparticles inside the HMMS to prepare other new catalysts.
J. Sun et al. / Journal of Molecular Catalysis A: Chemical 367 (2013) 46–51
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Fig. 1. Scheme of the synthetic procedure for the preparation of Pd/hollow magnetic mesoporous spheres.
2. Experimental 2.1. Materials and reagents Glucose, ethanol, sodium formate, tetraethoxysilane (TEOS), ammonia solution (28 wt%), SnCl2 ·2H2 O, PdCl2 , K2 CO3 , FeCl3 ·6H2 O (99%), FeCl2 ·4H2 O (99%), were purchased from Tianjing Chemical Reagent Co. Iodobenzene, phenylboronic acid and cetyltrimethylammonium bromide (CTAB) were bought from Alfa Aesar. All chemicals were of analytical grade and were used as received without further purification.
environment for adsorbing metal ion-nanoparticles by electrostatic attraction in acidic solution. 0.6 g Fe3 O4 nanoparicles and 1.0 g negatively charged carbon nanospheres were dispersed in 80 mL and 60 mL hydrochloric acid solution (pH ≈ 2.3) with ultrasonic for 20 min, respectively. Then the latter suspension was added dropwise into the former under vigorous stirring at room temperature. After 6 h, Fe3 O4 nanoparticles attaching on the surface of carbon nanospheres, were separated from the solution by an external magnet and washed several times with distilled water until the pH value of the solution was close to 7. Finally, the products were oven-dried at 50 ◦ C for further use.
2.2. Preparation of catalysts 2.2.1. Synthesis of carbon nanospheres [36] Glucose (6.0 g) was dissolved in 40 mL distilled water to form a clear solution and then transferred into a 100 mL Teflon-sealed autoclave. The autoclave was maintained at 180 ◦ C for 4.5 h. The products were separated by centrifugation, followed by washing three times with distilled water and ethanol. Finally, the products were oven-dried at 50 ◦ C for the following work. 2.2.2. Synthesis of iron oxide nanoparticles (Fe3 O4 ) Magnetic nanoparticles were prepared using simple chemical co-precipitation. Typically, 4.8 g of FeCl3 ·6H2 O and 2.0 g of FeCl2 ·4H2 O were dissolved in 40 mL of distilled water in a threenecked bottom (100 mL). The obtained transparent solution was degassed with nitrogen for 1 h. After being rapidly magnetic stirred for 30 min, the transparent solution was heated to 90 ◦ C. Thereafter, 12 mL of ammonia hydroxide was added to the reaction mixture in 0.1 min time using a syringe, and the mixture was stirred for 2 h at 90 ◦ C. After cooling to room temperature, the obtained magnetic dispersion was subjected to magnetic separation with a magnet, and dried at 60 ◦ C for 6 h. 2.2.3. Preparation of carbon nanospheres with attached Fe3 O4 nanoparticles (Fe3 O4 /C) [31] These hydroxyl carbon nanospheres possess functional groups ( OH) on the surface, which offer an important chemical
2.2.4. Loading Pd nanoparticles onto the carbon nanosphere with Fe3 O4 nanoparticles (Pd/Fe3 O4 /C) [37] 200 mg Fe3 O4 /C was dispersed in 50 mL distilled water and stirred for 20 min as part A. 0.2 g SnCl2 was dissolved in 40 mL 0.02 M HCl solution as part B. Parts A and B were mixed together under stirring for 30 min. The suspension was separated from the solution by an external magnet, washed several times with distilled water and dispersed in 80 mL distilled water. Then 50 mL 10 mg PdCl2 was added into it. 10 min later, 20 mL of 0.15 M sodium formate solution was added, following stirring for 8 h. The suspension was separated from the solution by an external magnet, washed with distilled water and ethanol for several times and dried at 50 ◦ C for 12 h.
2.2.5. Production of Pd/Fe3 O4 @mesoporous SiO2 nanoreactor (Pd/HMMS) The Pd/Fe3 O4 /C composite obtained from last step was first dispersed in the solution containing 80 mL H2 O, 60 mL ethanol, 0.3 g CTAB and 1.2 mL NH3 ·H2 O with ultrasonic for 20 min. The 300 L TEOS was added into the mixture, and vigorously stirred for 8 h. The suspension was separated from the solution by an external magnet, washed several times with distilled water and ethanol, respectively, then oven-dried at 50 ◦ C. Then the product was calcined at 400 ◦ C in air atmosphere for 7 h to remove carbon sphere, CTAB template and other organic species. The amount of Pd in
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J. Sun et al. / Journal of Molecular Catalysis A: Chemical 367 (2013) 46–51
Fig. 2. XRD patterns of (a) HMMS and (b) Pd/HMMS.
the Pd/HMMS nanoreactor was found to be 2.0 wt% based on ICP analysis. 2.3. Measurement of catalytic performance For Suzuki cross-coupling reaction, 20 mg Pd/HMMS nanoreactor catalyst, 0.5 mmol iodobenzene, 1 mmol phenylboronic acid, 1 mmol K2 CO3 were added to 4 mL ethanol under magnetic stirring. The reaction was carried out at reflux (ca. 80 ◦ C) for definite time. Then, the catalyst was collected by an external magnet, and the reaction system was analyzed by gas chromatography. For recycling, the recovered catalyst was washed with ethanol and distilled water several times, respectively, and then dried under vacuum at 60 ◦ C. 2.4. Characterization of catalysts Powder X-ray diffraction (XRD) patterns were obtained on a Rigaku D/max-2400 diffractometer using Cu-K␣ radiation as the Xray source in the 2Â range of 10–80◦ . The conversion was estimated by GC (P.E. AutoSystem XL) or GC–MS (Agilent 6890N/5973N). Pd content of the catalyst was measured by inductively coupled plasma (ICP) on IRIS Advantage analyzer. The morphology of the catalyst was observed by a Tecnai G2 F30 transmission electron microscopy and samples were obtained by placing a drop of a colloidal solution onto a copper grid and evaporating the solvent in air at room temperature. X-ray photoelectron spectroscopy (XPS) was recorded on a PHI-5702 instrument and the C1s line at 297.8 eV was used as the binding energy reference. Magnetic measurement of Pd/Fe3 O4 @mesoporous SiO2 nanoreactor was investigated with a Quantum Design vibrating sample magnetometer (VSM) at room temperature in an applied magnetic field sweeping from −15 to 15 kOe.
that the Pd particles have been coated by hollow mesoporous SiO2 . Transmission electron microscopy (TEM) experiments have been carried out to observe a possible structure of the metallic Pd particles (Fig. 3). Apparently, Fe3 O4 and Pd nanoparticles are located at the inside pore mouths of the mesopores. This is consistent with the XRD patterns (Fig. 2). The morphologies and structures of the products at different synthetic steps were observed by TEM. Fig. 3a shows the TEM image of carbon nanospheres, in which well-dispersed particles with average diameter about 200 nm could be observed clearly. These carbon nanospheres possess functional groups ( OH, C O) on the surface, which offer an important chemical environment for adsorbing metal ion-nanoparticles by electrostatic attraction in acidic solution. Fig. 3b is a TEM image of the Fe3 O4 /C composite. The Fe3 O4 particles with an average size of about 9 nm are uniformly distributed on the surfaces of the carbon nanospheres. Using TEOS as the silica source and CTAB as a soft template, a thin layer of mesoporous silica is coated onto the Pd/Fe3 O4 /C spheres. Then, the Pd/HMMS nanoreactors were obtained after the calcination to remove the carbon templates and the organic groups of CTAB. As shown in Fig. 3c and d, the TEM images show that Pd, Fe3 O4 particles were evenly coated with a layer of mesoporous shells about 40 nm thick, while the Fe3 O4 particles remained initial their cluster morphologies in the Pd/HMMS. To further probe the components of the mesoporous shell, we used EDX measurement to analyze the Pd/HMMS after calcination at 400 ◦ C. The EDX spectra confirmed that the nanoreactor consisted of Fe, Si, Sn, Pd and O elements (Cu and C came from the TEM copper grid of the sample holder). EDX analysis (Fig. 3e) further confirms the existence of Pd. According to the ICP analysis, the mass percent of Sn for the composites was 0.9 wt%, and the presence of Sn was not significantly affected the catalytic activity of the nanoreactor [8]. Fig. 4 shows the XPS spectrum of the synthesized Pd/HMMS nanoreactor. Peaks corresponding to oxygen, silicon, carbon, palladium, tin and iron are observed. However, typical peaks of Fe, Sn and Pd elements are not obviously found. Fig. 6 shows the XPS spectrum of the elemental survey scan of Pd/Fe3 O4 /C. Meanwhile, typical peaks of Fe, Sn and Pd elements are clearly found. This indicates that Sn, Fe3 O4 and Pd particles have been coated by mesoporous SiO2 , since the analysis depth of XPS is only several nanometers. In Fig. 4, both the peaks of Si 2s (153.5 eV) and Si 2p (102.6 eV) are quite clear and this means that SiO2 is the main component of the outer surface. To ascertain the oxidation state of the Pd, X-ray photoelectron spectroscopy (XPS) studies were carried out. In Fig. 5, the binding energy of Pd exhibits two strong peaks centered at 342.6 and 337.2 eV, which are assigned to Pd 3d3/2 and Pd 3d5/2 , respectively. On the basis of above analysis, it can be concluded that the XPS data further identified the hollow mesoporous structure of the synthesized Pd/HMMS (Fig. 6). Magnetization curves revealed the superparamagnetic behavior of the magnetic nanoreactor, the hysteresis loop of Pd/HMMS is shown in Fig. 7. The magnetization saturation (Ms ) of the mesoporous spheres reaches 21.0 emu g−1 , which suggests that Fe3 O4 particles become encapsulated in hollow mesoporous spheres. This is consistent with the TEM patterns (Fig. 3). The catalyst can be efficiently separated easily from the solution with the help of an external magnetic force.
3. Results and discussion 3.2. Suzuki reaction catalyzed by Pd/HMMS nanoreactor 3.1. Characterization of catalysts The wide angle XRD patterns of the samples are presented in Fig. 2. An almost identical XRD pattern was obtained for Pd/HMMS comparing with HMMS (Fig. 2b), indicating the crystalline structure of the support was well maintained in the Pd samples. No obviously peaks due to metallic Pd were detected, owing to the fact
The catalytic activity of Pd/HMMS nanoreactor was tested in the Suzuki reaction of aryl halides, namely the reactivity of aryl iodides and aryl bromides with phenylboronic acid in ethanol solvent using K2 CO3 as a base. The reaction results are summarized in Table 1. A range of activities were observed from 25% to >99% yields depending on the nature of aryl bromo and iodo derivatives. Iodobenzene
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Fig. 3. (a) TEM of carbon nanospheres, (b) TEM of Fe3 O4 /C nanospheres, (c) TEM of final composite nanoreactor (Pd/HMMS) in low magnification, (d) TEM of final composite nanoreactor in high magnification, (e) energy-dispersive X-ray absorption spectroscopy (EDX) of the Pd/HMMS nanoreactor.
Fig. 4. XPS spectrum of the elemental survey scan of Pd/Fe3 O4 @mesoporous SiO2 . Fig. 5. XPS spectrum of the Pd/HMMS showing Pd 3d5/2 and Pd 3d3/2 binding energies.
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J. Sun et al. / Journal of Molecular Catalysis A: Chemical 367 (2013) 46–51 Table 2 Recyclability of catalysta . Entry
Run
1 2 3 4 5
Fresh First Second Third Fourth
Yieldb (%) >99 >99 99 99 99
a Reaction conditions: iodobenzene (0.5 mmol), phenylboronic acid (1 mmol), K2 CO3 (2 mmol), ethanol (4 mL), 80 ◦ C, catalyst (20 mg, with Pd loading 2.0 wt%). b Yield based on GC analysis.
can be easily recovered in a facile manner from the reaction solution by using a permanent magnet. Furthermore, the catalyst revealed a remarkable activity and was reused up to four consecutive cycles without any significant loss in catalytic activity (Table 2). 4. Conclusions
Fig. 6. XPS spectrum of the elemental survey scan of Pd/Fe3 O4 /C.
In summary, we have developed a facile method for fabrication of uniform Pd/HMMS nanoreactor with desired hollow core/mesoporous shell structures, high catalytic activity and magnetic recyclability properties. The present method is versatile, and many transition noble metals and their bimetallic nanoparticles can be resided inside the HMMS via the sol–gel process. The nanoreactor shows superior catalytic activity in the Suzuki cross-coupling reaction of iodobenzene with phenylboronic acid with over 99% yield in 3 min and can be recycled multiple times without any significant loss in catalytic activity. Acknowledgment This work was supported by the Fundamental Research Funds for the Central Universities (lzujbky-2012-68). References
Fig. 7. Room temperature magnetization curves of Pd/HMMS.
derivatives were found to give a good yield in 5 min reaction time (Table 1, entries 2 and 3). It was observed that with the increase in reaction time the conversion of bromobenzene increases (Table 1, entry 4). Bromobenzene derivatives were found to give a moderate yield in 30 min reaction time (Table 1, entries 5–7). The catalyst Table 1 Suzuki reactions catalyzed by the Pd/HMMS.a
. Entry
X
R
Time/min
1 2 3 4
I I I Br
H 2-NH2 4-Me H
5 6 7
Br Br Br
4-COMe 4-NH2 4-Me
3 5 5 3 30 30 30 30
Yieldb (%) >99 86 66 25 46 57 84 49
a Reaction conditions: iodobenzene (0.5 mmol), phenylboronic acid (1 mmol), K2 CO3 (2 mmol), ethanol (4 mL), 80 ◦ C, catalyst (20 mg, with Pd loading 2.0 wt%). b Determined by GC or GC–MS.
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