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Superstable magnetic nanoreactors with high efficiency for Suzuki-coupling reactions†
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Weihe Zhang,a Xuecheng Chen,*b,c Tao Tang*b and Ewa Mijowskac Magnetic nanoreactors based on Pd nanoparticles supported on magnetic carbon nanotubes that are coated by a mesoporous silica layer, (Fe3O4-CNT-Pd)@m-SiO2, have been selectively synthesized by facile steps. The prepared system exhibits high catalytic efficiency in Suzuki-coupling reactions because Received 26th July 2014, Accepted 22nd August 2014 DOI: 10.1039/c4nr04245j www.rsc.org/nanoscale
of the Pd nanoparticles supported on carbon nanotubes (CNT-Pd). Most importantly, induced magnetic properties make the separation of the catalyst easy from the reaction media, which can be re-used several times with significant activity. Moreover, the existence of a mesoporous silica layer makes the Pd catalyst very stable and does not allow the Pd nanoparticles to get detached from the CNT supports. The above-mentioned observations make the obtained catalyst a great candidate for future applications.
To design novel catalysts with high activity, selectivity and resistance to deactivation is always the long-time goal for the research scientists.1 In regards to chemical reactions, all of the catalytic reactions occur in reactors, which accommodate the catalysts and reactants, and allow the chemicals to be transported in and out of the reactor. For conventional catalysis, the design of a reactor is totally unrelated to catalyst and support. However, recently, some scientists investigated this area and showed that some catalysts have uniform internal structures that resemble a reactor on the nanoscale. For example, mesoporous silica allows the reactants to get into its porous channels, where the catalyst is located. Thus, the mesopores within the silica can be considered as nanotubular reactors.2 Multifunctional nanoreactors are also realized using hollow spheres, in particular mesoporous silica hollow spheres. Recently, Li et al. have encapsulated several chiral catalysts into mesoporous silica SBA-16 nanocages, the resulting catalytic system shows remarkable activity for asymmetric synthesis.3 Yamada et al. reported TiO2 inside hollow silica spheres for photoreactions.4 The yolk/shell structured nanoreactors with movable cores and porous shells were also successfully prepared and showed good performance.5
a Division of Chemical Biology and Medicinal Chemistry, Eshelman School of Pharmacy, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, USA b State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Science, Remin Road 5625, Changchun, 130022, China c Institute of Chemical and Environment Engineering, West Pomeranian University of Technology, Poland. E-mail: [email protected], [email protected] † Electronic supplementary information (ESI) available. For ESI see DOI: 10.1039/c4nr04245j
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Noble metals have been widely used in catalysts because of their super catalytic performances in many areas.6 Numerous industrially important catalytic reactions, including CO oxidation,7 partial oxidation,8 cracking9 of hydrocarbons and combustion10 reactions, are carried out at temperature above 300 °C. For these catalytic reactions, restricting the catalysts in isolated nanoreactors can effectively protect them against aggregation. For example, Somorjai et al. designed a Pt@CoO composite for the hydrogenation of ethylene.11 Song et al. fabricated a Au@SiO2 yolk/shell structure for the catalytic reduction of p-nitrophenol.12 Yin et al. designed multilayer core/shell structures which resemble nanoreactors, where Pd nanoparticles were protected by a silica coating.13 Palladium based catalysts have primarily showed industrial applications in both heterogeneous and homogeneous catalytic reactions.14 Pd nanoparticles dispersed in water-in-oil microemulsion have also been reported to show important catalytic activity for the hydrogenation of olefins.15 It is found that ligand-stabilized nanosized Pd catalysts are advantageous for the hydrogenation of olefin over homogeneous and heterogeneous catalysts.16 Moreover, Pd nanoparticles supported on silica17 and polymer18 have also been successfully used in selective hydrogenation reactions. Pd nanoparticle systems stabilized by ionic liquids,19 dendrimers, polymers, or fluorous ligands20 have also been used as active catalysts.21,22 Conventionally, heterogeneous catalysis is favored over homogeneous catalysis because of its simplicity in recovery and regeneration.23–25 Magnetic separation renders the recovery of catalysts from a liquid reaction system much more easily than the traditional separation procedures, such as filtration and centrifugation. Thus, design and synthesis of a multifunctional nanoreactor system would open new opportunities for their applications in catalysis.
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In this study, we designed a magnetic nanoreactor that is composed of mesoporous silica, carbon nanotubes (CNT), as support, and Pd nanoparticles, residing inside the mesoporous silica shell. The existence of the mesoporous silica layer not only allows mass transportation but also isolates the catalytically active Pd nanoparticles and prevents the leaching of the Pd nanoparticles from CNT supports during the catalytic reactions. Therefore, the Pd nanoparticles are all fully accessible to external environments through perpendicular channels. The native magnetic properties of (Fe3O4-CNT-Pd)@m-SiO2 enables the catalyst to be easily separated from the reaction very quickly. All these features enable the proposed system to be very active in catalysis. To test it in practical applications, Suzuki-coupling reactions were carried out to explore the catalytic activities of (Fe3O4-CNT-Pd)@m-SiO2. The results showed that the obtained (Fe3O4-CNT-Pd)@m-SiO2 is very stable, it is highly efficient in diverse Suzuki-coupling reactions and it can be easily separated after the process. From the viewpoint of fundamental research and practical application, this multifunctional catalyst system provides a powerful platform for designing novel nanoreactors for catalysis.
Experimental section Deposition of Fe3O4 particles on carbon nanotubes (Fe3O4-CNT) Pristine CNTs were first dispersed in concentrated nitric acid at 130 °C with constant stirring for 6 h. Then, the solution was diluted with distilled water and rinsed several times until the pH value reached neutral. The resulting CNTs were separated from the solution by filtration and dried in vacuum at 60 °C for further use. In our experiments, the formation of magnetic Fe3O4 particles on CNTs was carried out by hydrothermal reactions.26 Typically, 0.65 g of FeCl3, 1.8 g of sodium acetate (NaAC) and 0.5 g of PEG were dissolved into 20 mL ethylene glycol. The prepared solution was orange. Then, 100 mg of functionalized CNTs were dispersed in the solution by ultrasonication for 1 h. The mixture was transferred to the autoclave and allowed to react for 12 h and the suspension was then cooled to room temperature. The reaction temperature was set to 200 °C. The final products were rinsed with ethanol several times and dried at 100 °C for 24 h (CNT-Fe3O4). Deposition of Pd nanoparticles on Fe3O4-CNT ((Fe3O4-CNT-Pd) In a typical preparation procedure, the Fe3O4-CNT nanocomposites obtained from the abovementioned steps were suspended in 60 mL of 0.05 M aqueous SDS solution and sonicated for 20 min. Subsequently, 60 mg of palladium acetate was added to the Fe3O4-CNT/SDS dispersion and the reaction mixture was refluxed for 6 h.27 After cooling down to room temperature, the reaction mixture was filtered through a 0.2 μm PTFE membrane filter, extensively washed with water and ethanol to remove the excess of surfactant and dried in a vacuum (Fe3O4CNT-Pd).
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Coating of mesoporous silica layer on Fe3O4-CNT-Pd (Fe3O4-CNT-Pd@m-SiO2) In a typical synthesis, the above obtained dry sample (Fe3O4CNT-Pd) and CTAB (0.3 g) were dispersed into 9 mL deionized water, and the mixtures were sonicated for 1 hour. Next, 24 mL of anhydrous ethanol was introduced to the mixture and further sonicated for 0.5 h to form a stable dispersion. Subsequently, 0.6 mL NH3·H2O was added into the as-prepared Fe3O4-CNT-Pd dispersion. Then, a TEOS solution (0.3 mL TEOS in 12 mL ethanol) was dropped in with mechanical stirring, and the reaction mixture was stirred for next 12 h. Finally, the mixture was centrifuged and washed with ethanol. The process results in the formation of a uniform and mesoporous silica coating on Fe3O4-CNT-Pd. The surfactants were removed by refluxing in acetone at 80 °C for 48 h to obtain mesoporous silica coated Fe3O4-CNT-Pd. Characterizations X-ray diffraction (XRD) study was conducted on a Philips diffractometer using Cu Kα radiation. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HR-TEM) were performed on the FEI Tecnai F30 transmission electron microscope with a field emission gun operating at 200 kV to examine the dimensions and structural details. The N2 adsorption/desorption isotherms were obtained at liquid nitrogen temperature (77 K) using a Micromeritics ASAP 2010 M instrument, and the specific surface area was calculated by the Brunauer–Emmett–Teller (BET) method. The pore size distribution was determined using the Barrett–Joyner–Halenda (BJH) method. The magnetic properties of the samples were measured by a vibrating sample magnetometer (VSM, Nanjing University HH-15) operating at room temperature (300 K).
Results and discussion The sword/sheath structured (Fe3O4-CNT-Pd)@m-SiO2 was prepared in four steps (Scheme 1): (1) first deposition of Fe3O4 nanoparticles on the carbon nanotubes surface (Fe3O4-CNT), (2) subsequent supporting Pd nanoparticles on Fe3O4-CNT (Fe3O4-CNT-Pd), (3) coating of Fe3O4-CNT-Pd by mesoporous silica layer, generating the as-synthesized (Fe3O4-CNT-Pd)@ m-SiO2-CTAB mesostructures, (4) removal of the CTAB templates by refluxing in acetone. After these steps (Fe3O4CNT-Pd)@m-SiO2 catalyst was obtained. The high stability derived from the shell structure suggested that the (Fe3O4CNT-Pd)@m-SiO2 is an excellent nanoreactor for catalytic reactions or surface chemical processes. In order to visualize the microstructure of the sample after each step of the catalyst preparation, TEM was used. Fig. 1a represents the TEM image of the functionalized carbon nanotubes after refluxing with HNO3. The diameter of CNT is in the range of 20–40 nm. CNTs beaded with magnetic Fe3O4 particles are shown in Fig. 1b and c. Through hydrothermal reaction, Fe3O4 particles of ca. 200 nm are successfully supported
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Fig. 2 a) Wide-angle XRD pattern of (Fe3O4-CNT-Pd)@m-SiO2. (b) Small-angle XRD pattern of (Fe3O4-CNT-Pd)@m-SiO2.
Procedures for the preparation of (Fe3O4-CNT-Pd)@m-SiO2
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Scheme 1 catalyst.
Fig. 1 Representative TEM images of (a) pristine CNTs, (b–c) Fe3O4CNT, (d–f ) Fe3O4-CNT-Pd and (g–i) (Fe3O4-CNT-Pd)@m-SiO2 nanocomposite.
on CNT. As shown in Fig. 1d–f, Pd nanoparticles are successfully supported on carbon nanotubes by the refluxing process with Pd(AC)2 as the precursor. The diameter of Pd nanoparticles is in the range of 2–4 nm. The large black particles are magnetic Fe3O4 beads (as indicated by white arrows). In Fig. 1g–i, mesoporous silica layers coated on Fe3O4-CNT-Pd are shown. The thickness of mesoporous silica layer is around 50 nm (Fig. 1i). It should be noted that all of the ordered nanochannels in silica layer are perpendicular to the CNT. The Pd nanoparticles can be accessible to the external environment through these parallel nanochannels. The pore size of these nanochannels is ca. 2 nm, which will prohibit big molecules to contact with Pd nanoparticles. In Fig. 1i, the black dots indicated by the white arrows represent Pd nanoparticles supported on CNTs. CNT is also pointed out by the black arrow.
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The existence of mesoporous silica layer allows only small sized molecules to come in contact with Pd nanoparticles, thus inhibited large molecules pass through. Additionally, the silica layer coating on CNT-Pd should provide protection to stop the Pd nanoparticles from getting detached from CNT supports, thus improving the stability of (Fe3O4-CNT-Pd)@ m-SiO2. The crystallinity and phase composition of the catalyst was investigated by XRD. As shown in Fig. 2a, all the peaks (30.3°, 35.6°, 43.3°, 57.1° and 63.2°) are assigned to Fe3O4. There is also a diffraction peak at 2θ = 26° attributed to the CNTs. The XRD pattern of (Fe3O4-CNT-Pd)@m-SiO2 contains a series of characteristic diffraction peaks (40.1°, 46.6° and 68°) which are indexed to the Pd(0) indicating the presence of Pd(0) in mesoporous silica nanotubes. According to the Debye-Scherrer equation, the diameter of Pd(0) nanoparticles is calculated to be ∼3 nm, which is in agreement with TEM observation. There is also a strong peak at 2.65° ascribed to ordered mesoporous silica (Fig. 2b). The XRD results are also in agreement with TEM observations from Fig. 1(g–i). The porosity of (Fe3O4-CNT-Pd)@m-SiO2) was confirmed by N2 physisorption method. The N2 adsorption/desorption isotherm of (Fe3O4-CNT-Pd)@m-SiO2 (Fig. 3a) revealed that these nanotubes were mesoporous, which can be identified by the increase of the adsorption amount in the relative pressure (P/P0) range of 0.2–0.3. The pore size distribution curve calculated from the adsorption branch of the isotherms (Fig. 3b) exhibited a maximum at 2.3 and 4.3 nm, and the Brunauer– Emmett–Teller (BET) surface area of (Fe3O4-CNT-Pd)@m-SiO2 was calculated to be 440 m2 g−1, indicating the highly mesoporous nature of the silica shell. The isolation and reuse of the catalyst are the crucial requirements for any practical applications in terms of cost and environmental protection. These are the greatest merits of our catalyst. In this study, it is clearly seen that for the sample without coating with mesoporous silica layer (Fe3O4-CNT-Pd), Pd nanoparticles were leaching from the CNT support after the catalytic reaction, changing the solution color to brown (Fig. 4a). However, for the catalysts of (Fe3O4-CNT-Pd)@ m-SiO2, the solution was still transparent after chemical reactions, indicating that no Pd nanoparticles were detached from CNT supports (Fig. 4b). More importantly, catalyst (Fe3O4CNT-Pd)@m-SiO2 can be easily recovered by an external magnet as shown in Fig. 4c. Magnetic response of (Fe3O4CNT-Pd)@m-SiO2 was also investigated by a magnetometer at
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Fig. 3 (a) N2 sorption/desorption isotherm and (b) pore-size distributions of the (Fe3O4-CNT-Pd)@m-SiO2 catalyst.
Fig. 4 Photographs of catalyst (a) Fe3O4-CNT-Pd and (b) (Fe3O4CNT-Pd)@m-SiO2 after Suzuki-coupling reactions. (c) Magnetic separation of the catalysts (Fe3O4-CNT-Pd)@m-SiO2 from the reaction medium. (d) Field-dependent magnetization curves of (Fe3O4-CNT-Pd)@m-SiO2.
300 K in the applied magnetic field ranging from −10 000 to 10 000 Oe. As illustrated in Fig. 4d, the isothermal magnetization of (Fe3O4-CNT-Pd)@m-SiO2 indicates superparamagnetism. The saturated magnetization value of (Fe3O4-CNT-Pd)@ m-SiO2 reached 8.94 emu g−1. As a result, the catalyst in their homogeneous dispersion shows fast movement to the applied magnetic field and quickly re-disperses with a slight shake,
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once the magnetic field is removed. This suggests that the catalyst possesses excellent magnetic response and dispersibility. The catalytic activities of catalysts were evaluated by catalyzing Suzuki-coupling reactions as listed in Table 1. As summarized in Table 1 (entry 3a–e), when Fe3O4-CNT-Pd was used as catalyst, the coupling reactions of phenyl boronic acid with substituted aryl bromides was completed in short time (10 min) with good yields at 150 °C. The reactivity is almost similar to the homogeneous catalyst Pd(PPh3)4 (entry 3c). However, after reactions, lots of Pd nanoparticles were leached from CNT supports, leading to the deterioration of catalytic activity (as shown in Fig. 4a). In order to fully use the Pd nanoparticles and maintain the catalytic activity, a mesoporous silica layer was coated on Fe3O4-CNT-Pd catalyst. After coating with silica, no Pd nanoparticles were detached from CNT supports when Suzuki-coupling reactions were finished. As shown in Table 1 (entry 3f–i), the catalyst (Fe3O4-CNT-Pd)@m-SiO2 successfully catalyzed the Suzuki-coupling reactions. Various aromatic bromides 1 reacted with phenylboronic acid or substituted phenylboronic acids produced the coupling product 3 with excellent yields. The yields for this reaction were produced at the same level when moderate steric R1 and/or R4 were introduced in the bromides (entry 3c, 3f and 3g). When we compare Table 1 (entry 3c), the product yields are almost the same, the performance of (Fe3O4-CNT-Pd(0))@m-SiO2 catalyst even behaves better than Fe3O4-CNT-Pd(0). However, no reaction occurred when R1 and R4 were substituted as steric isopropyl groups (entry 3l). As compared with the general catalyst (Pd(PPh3)4), the prepared catalyst ((Fe3O4-CNT-Pd(0))@ m-SiO2) has better efficacy (entry 3k vs. 3j). We proposed that the high specific surface area of mesoporous silica layer adsorbs more reactant molecules to increase the reactant concentrations within the silica channels, leading to higher production yields (93%). The effects of solvent on the product yields were also investigated, as shown in entry 3j and 3l. Higher yields were obtained by using DMF–H2O (93%) than dioxane–H2O (67%) as a solvent because of its better solubility for K2CO3. It is significant noting that the electronic nature of the substituents on aryl bromides had no obvious effect on the catalysis, and no by-products such as homo-coupling products from aryl bromides or phenylboronic acid had been detected. After completion of the coupling reactions, (Fe3O4-CNT-Pd)@m-SiO2 could be readily separated and simply recovered from the reaction mixture by magnet, which maintained its catalytic activity and selectivity after five cycles for the coupling reaction between 2-bromotoluene and 4-methoxyphenylboronic acid as illustrated in Table 2.
Conclusions We have demonstrated that magnetic Pd nanoreactors with mesoporous silica coating have been successfully fabricated facilely by reduction of palladium acetate to Pd nanoparticles
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Table 1
Suzuki cross-coupling by using Fe3O4-CNT-Pd(0)) or (Fe3O4-CNT-Pd(0))@m-SiO2 as catalyst
Entry
Compd #
R1
R2
R3
R4
R5
X
Catalyst
Solvent
Reaction time
Yielda (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
3a 3b 3c 3d 3e 3c 3c 3f 3g 3h 3i 3j 3k 3l 3l
H H H H H H H CH3 Ph CH3 CH3 CH3 CH3 CH(CH3)2 CH3
Br H H CO2H H H H H H H H H H H H
H H H H CHO H H H H H H H H H H
H H H H H H H H H CH3 CH3 CH3 CH3 CH(CH3)2 CH3
F F H H H H H OCH3 OCH3 H CHO OCH3 OCH3 OCH3 OCH3
I Br Br Br Cl Br Br Br Br Br Br Br Br Br Br
Fe3O4-CNT-Pd Fe3O4-CNT-Pd Fe3O4-CNT-Pd Fe3O4-CNT-Pd Fe3O4-CNT-Pd Pd(PPh3)4 (Fe3O4-CNT-Pd)@m-SiO2 (Fe3O4-CNT-Pd)@m-SiO2 (Fe3O4-CNT-Pd)@m-SiO2 (Fe3O4-CNT-Pd)@m-SiO2 (Fe3O4-CNT-Pd)@m-SiO2 (Fe3O4-CNT-Pd)@m-SiO2 Pd(PPh3)4 (Fe3O4-CNT-Pd)@m-SiO2 (Fe3O4-CNT-Pd)@m-SiO2
Dioxane–H2O Dioxane–H2O Dioxane–H2O Dioxane–H2O Dioxane–H2O Dioxane–H2O DMF–H2O DMF–H2O DMF–H2O DMF–H2O DMF–H2O DMF–H2O DMF–H2O DMF–H2O Dioxane–H2O
15 min 15 min 15 min 15 min 15 min 15 min 10 min 10 min 10 min 10 min 10 min 10 min 10 min 10 min 10 min
89 86 89 78 83 91 90 94 86 82 80 93 85 —b 67
a
Separated yield. b No desired product was separated.
Table 2
The cyclability of (Fe3O4-CNT-Pd(0))@m-SiO2 catalyst
Notes and references
Entry Substrates
Yielda Cycles (%)
1 2 3 4 5
2-Bromotoluene + 4-methoxyphenylboronic acid 2-Bromotoluene + 4-methoxyphenylboronic acid 2-Bromotoluene + 4-methoxyphenylboronic acid 2-Bromotoluene + 4-methoxyphenylboronic acid 2-Bromotoluene + 4-methoxyphenylboronic acid
1 2 3 4 5
a
Average yield based on two independent experiments.
93.2 93 86.3 82.6 89.6
on the surface of magnetic CNTs. The resultant (Fe3O4CNT-Pd)@m-SiO2 showed excellent catalytic activity for the Suzuki-coupling reactions, which are attributed to high specific surface area of the Pd nanoparticles and the limited size of mesopores on the coated silica layer. The existence of the mesoporous layer not only ensures the special catalytic reactions to be performed only in the nanochannels but also protects Pd nanoparticles from being removed from the CNT support, allowing the catalyst to be used for many cycles without the deterioration of its performances. Moreover, the recovery of the catalyst is simple and efficient using outer magnet. This work provides a new idea for designing nanoreactors, in particular the ideal heterogeneous catalysts for reactions like Suzuki-coupling reactions.
Acknowledgements This work was supported by the National Natural Science Foundation of China (the project no. 51303170) and National Science Centre, Poland within the project no. UMO-2011/03/B/ ST5/03239.
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Superstable magnetic nanoreactors with high efficiency for Suzuki-coupling reactions†
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Weihe Zhang,a Xuecheng Chen,*b,c Tao Tang*b and Ewa Mijowskac Magnetic nanoreactors based on Pd nanoparticles supported on magnetic carbon nanotubes that are coated by a mesoporous silica layer, (Fe3O4-CNT-Pd)@m-SiO2, have been selectively synthesized by facile steps. The prepared system exhibits high catalytic efficiency in Suzuki-coupling reactions because Received 26th July 2014, Accepted 22nd August 2014 DOI: 10.1039/c4nr04245j www.rsc.org/nanoscale
of the Pd nanoparticles supported on carbon nanotubes (CNT-Pd). Most importantly, induced magnetic properties make the separation of the catalyst easy from the reaction media, which can be re-used several times with significant activity. Moreover, the existence of a mesoporous silica layer makes the Pd catalyst very stable and does not allow the Pd nanoparticles to get detached from the CNT supports. The above-mentioned observations make the obtained catalyst a great candidate for future applications.
To design novel catalysts with high activity, selectivity and resistance to deactivation is always the long-time goal for the research scientists.1 In regards to chemical reactions, all of the catalytic reactions occur in reactors, which accommodate the catalysts and reactants, and allow the chemicals to be transported in and out of the reactor. For conventional catalysis, the design of a reactor is totally unrelated to catalyst and support. However, recently, some scientists investigated this area and showed that some catalysts have uniform internal structures that resemble a reactor on the nanoscale. For example, mesoporous silica allows the reactants to get into its porous channels, where the catalyst is located. Thus, the mesopores within the silica can be considered as nanotubular reactors.2 Multifunctional nanoreactors are also realized using hollow spheres, in particular mesoporous silica hollow spheres. Recently, Li et al. have encapsulated several chiral catalysts into mesoporous silica SBA-16 nanocages, the resulting catalytic system shows remarkable activity for asymmetric synthesis.3 Yamada et al. reported TiO2 inside hollow silica spheres for photoreactions.4 The yolk/shell structured nanoreactors with movable cores and porous shells were also successfully prepared and showed good performance.5
a Division of Chemical Biology and Medicinal Chemistry, Eshelman School of Pharmacy, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, USA b State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Science, Remin Road 5625, Changchun, 130022, China c Institute of Chemical and Environment Engineering, West Pomeranian University of Technology, Poland. E-mail: [email protected], [email protected] † Electronic supplementary information (ESI) available. For ESI see DOI: 10.1039/c4nr04245j
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Noble metals have been widely used in catalysts because of their super catalytic performances in many areas.6 Numerous industrially important catalytic reactions, including CO oxidation,7 partial oxidation,8 cracking9 of hydrocarbons and combustion10 reactions, are carried out at temperature above 300 °C. For these catalytic reactions, restricting the catalysts in isolated nanoreactors can effectively protect them against aggregation. For example, Somorjai et al. designed a Pt@CoO composite for the hydrogenation of ethylene.11 Song et al. fabricated a Au@SiO2 yolk/shell structure for the catalytic reduction of p-nitrophenol.12 Yin et al. designed multilayer core/shell structures which resemble nanoreactors, where Pd nanoparticles were protected by a silica coating.13 Palladium based catalysts have primarily showed industrial applications in both heterogeneous and homogeneous catalytic reactions.14 Pd nanoparticles dispersed in water-in-oil microemulsion have also been reported to show important catalytic activity for the hydrogenation of olefins.15 It is found that ligand-stabilized nanosized Pd catalysts are advantageous for the hydrogenation of olefin over homogeneous and heterogeneous catalysts.16 Moreover, Pd nanoparticles supported on silica17 and polymer18 have also been successfully used in selective hydrogenation reactions. Pd nanoparticle systems stabilized by ionic liquids,19 dendrimers, polymers, or fluorous ligands20 have also been used as active catalysts.21,22 Conventionally, heterogeneous catalysis is favored over homogeneous catalysis because of its simplicity in recovery and regeneration.23–25 Magnetic separation renders the recovery of catalysts from a liquid reaction system much more easily than the traditional separation procedures, such as filtration and centrifugation. Thus, design and synthesis of a multifunctional nanoreactor system would open new opportunities for their applications in catalysis.
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In this study, we designed a magnetic nanoreactor that is composed of mesoporous silica, carbon nanotubes (CNT), as support, and Pd nanoparticles, residing inside the mesoporous silica shell. The existence of the mesoporous silica layer not only allows mass transportation but also isolates the catalytically active Pd nanoparticles and prevents the leaching of the Pd nanoparticles from CNT supports during the catalytic reactions. Therefore, the Pd nanoparticles are all fully accessible to external environments through perpendicular channels. The native magnetic properties of (Fe3O4-CNT-Pd)@m-SiO2 enables the catalyst to be easily separated from the reaction very quickly. All these features enable the proposed system to be very active in catalysis. To test it in practical applications, Suzuki-coupling reactions were carried out to explore the catalytic activities of (Fe3O4-CNT-Pd)@m-SiO2. The results showed that the obtained (Fe3O4-CNT-Pd)@m-SiO2 is very stable, it is highly efficient in diverse Suzuki-coupling reactions and it can be easily separated after the process. From the viewpoint of fundamental research and practical application, this multifunctional catalyst system provides a powerful platform for designing novel nanoreactors for catalysis.
Experimental section Deposition of Fe3O4 particles on carbon nanotubes (Fe3O4-CNT) Pristine CNTs were first dispersed in concentrated nitric acid at 130 °C with constant stirring for 6 h. Then, the solution was diluted with distilled water and rinsed several times until the pH value reached neutral. The resulting CNTs were separated from the solution by filtration and dried in vacuum at 60 °C for further use. In our experiments, the formation of magnetic Fe3O4 particles on CNTs was carried out by hydrothermal reactions.26 Typically, 0.65 g of FeCl3, 1.8 g of sodium acetate (NaAC) and 0.5 g of PEG were dissolved into 20 mL ethylene glycol. The prepared solution was orange. Then, 100 mg of functionalized CNTs were dispersed in the solution by ultrasonication for 1 h. The mixture was transferred to the autoclave and allowed to react for 12 h and the suspension was then cooled to room temperature. The reaction temperature was set to 200 °C. The final products were rinsed with ethanol several times and dried at 100 °C for 24 h (CNT-Fe3O4). Deposition of Pd nanoparticles on Fe3O4-CNT ((Fe3O4-CNT-Pd) In a typical preparation procedure, the Fe3O4-CNT nanocomposites obtained from the abovementioned steps were suspended in 60 mL of 0.05 M aqueous SDS solution and sonicated for 20 min. Subsequently, 60 mg of palladium acetate was added to the Fe3O4-CNT/SDS dispersion and the reaction mixture was refluxed for 6 h.27 After cooling down to room temperature, the reaction mixture was filtered through a 0.2 μm PTFE membrane filter, extensively washed with water and ethanol to remove the excess of surfactant and dried in a vacuum (Fe3O4CNT-Pd).
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Coating of mesoporous silica layer on Fe3O4-CNT-Pd (Fe3O4-CNT-Pd@m-SiO2) In a typical synthesis, the above obtained dry sample (Fe3O4CNT-Pd) and CTAB (0.3 g) were dispersed into 9 mL deionized water, and the mixtures were sonicated for 1 hour. Next, 24 mL of anhydrous ethanol was introduced to the mixture and further sonicated for 0.5 h to form a stable dispersion. Subsequently, 0.6 mL NH3·H2O was added into the as-prepared Fe3O4-CNT-Pd dispersion. Then, a TEOS solution (0.3 mL TEOS in 12 mL ethanol) was dropped in with mechanical stirring, and the reaction mixture was stirred for next 12 h. Finally, the mixture was centrifuged and washed with ethanol. The process results in the formation of a uniform and mesoporous silica coating on Fe3O4-CNT-Pd. The surfactants were removed by refluxing in acetone at 80 °C for 48 h to obtain mesoporous silica coated Fe3O4-CNT-Pd. Characterizations X-ray diffraction (XRD) study was conducted on a Philips diffractometer using Cu Kα radiation. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HR-TEM) were performed on the FEI Tecnai F30 transmission electron microscope with a field emission gun operating at 200 kV to examine the dimensions and structural details. The N2 adsorption/desorption isotherms were obtained at liquid nitrogen temperature (77 K) using a Micromeritics ASAP 2010 M instrument, and the specific surface area was calculated by the Brunauer–Emmett–Teller (BET) method. The pore size distribution was determined using the Barrett–Joyner–Halenda (BJH) method. The magnetic properties of the samples were measured by a vibrating sample magnetometer (VSM, Nanjing University HH-15) operating at room temperature (300 K).
Results and discussion The sword/sheath structured (Fe3O4-CNT-Pd)@m-SiO2 was prepared in four steps (Scheme 1): (1) first deposition of Fe3O4 nanoparticles on the carbon nanotubes surface (Fe3O4-CNT), (2) subsequent supporting Pd nanoparticles on Fe3O4-CNT (Fe3O4-CNT-Pd), (3) coating of Fe3O4-CNT-Pd by mesoporous silica layer, generating the as-synthesized (Fe3O4-CNT-Pd)@ m-SiO2-CTAB mesostructures, (4) removal of the CTAB templates by refluxing in acetone. After these steps (Fe3O4CNT-Pd)@m-SiO2 catalyst was obtained. The high stability derived from the shell structure suggested that the (Fe3O4CNT-Pd)@m-SiO2 is an excellent nanoreactor for catalytic reactions or surface chemical processes. In order to visualize the microstructure of the sample after each step of the catalyst preparation, TEM was used. Fig. 1a represents the TEM image of the functionalized carbon nanotubes after refluxing with HNO3. The diameter of CNT is in the range of 20–40 nm. CNTs beaded with magnetic Fe3O4 particles are shown in Fig. 1b and c. Through hydrothermal reaction, Fe3O4 particles of ca. 200 nm are successfully supported
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Fig. 2 a) Wide-angle XRD pattern of (Fe3O4-CNT-Pd)@m-SiO2. (b) Small-angle XRD pattern of (Fe3O4-CNT-Pd)@m-SiO2.
Procedures for the preparation of (Fe3O4-CNT-Pd)@m-SiO2
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Scheme 1 catalyst.
Fig. 1 Representative TEM images of (a) pristine CNTs, (b–c) Fe3O4CNT, (d–f ) Fe3O4-CNT-Pd and (g–i) (Fe3O4-CNT-Pd)@m-SiO2 nanocomposite.
on CNT. As shown in Fig. 1d–f, Pd nanoparticles are successfully supported on carbon nanotubes by the refluxing process with Pd(AC)2 as the precursor. The diameter of Pd nanoparticles is in the range of 2–4 nm. The large black particles are magnetic Fe3O4 beads (as indicated by white arrows). In Fig. 1g–i, mesoporous silica layers coated on Fe3O4-CNT-Pd are shown. The thickness of mesoporous silica layer is around 50 nm (Fig. 1i). It should be noted that all of the ordered nanochannels in silica layer are perpendicular to the CNT. The Pd nanoparticles can be accessible to the external environment through these parallel nanochannels. The pore size of these nanochannels is ca. 2 nm, which will prohibit big molecules to contact with Pd nanoparticles. In Fig. 1i, the black dots indicated by the white arrows represent Pd nanoparticles supported on CNTs. CNT is also pointed out by the black arrow.
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The existence of mesoporous silica layer allows only small sized molecules to come in contact with Pd nanoparticles, thus inhibited large molecules pass through. Additionally, the silica layer coating on CNT-Pd should provide protection to stop the Pd nanoparticles from getting detached from CNT supports, thus improving the stability of (Fe3O4-CNT-Pd)@ m-SiO2. The crystallinity and phase composition of the catalyst was investigated by XRD. As shown in Fig. 2a, all the peaks (30.3°, 35.6°, 43.3°, 57.1° and 63.2°) are assigned to Fe3O4. There is also a diffraction peak at 2θ = 26° attributed to the CNTs. The XRD pattern of (Fe3O4-CNT-Pd)@m-SiO2 contains a series of characteristic diffraction peaks (40.1°, 46.6° and 68°) which are indexed to the Pd(0) indicating the presence of Pd(0) in mesoporous silica nanotubes. According to the Debye-Scherrer equation, the diameter of Pd(0) nanoparticles is calculated to be ∼3 nm, which is in agreement with TEM observation. There is also a strong peak at 2.65° ascribed to ordered mesoporous silica (Fig. 2b). The XRD results are also in agreement with TEM observations from Fig. 1(g–i). The porosity of (Fe3O4-CNT-Pd)@m-SiO2) was confirmed by N2 physisorption method. The N2 adsorption/desorption isotherm of (Fe3O4-CNT-Pd)@m-SiO2 (Fig. 3a) revealed that these nanotubes were mesoporous, which can be identified by the increase of the adsorption amount in the relative pressure (P/P0) range of 0.2–0.3. The pore size distribution curve calculated from the adsorption branch of the isotherms (Fig. 3b) exhibited a maximum at 2.3 and 4.3 nm, and the Brunauer– Emmett–Teller (BET) surface area of (Fe3O4-CNT-Pd)@m-SiO2 was calculated to be 440 m2 g−1, indicating the highly mesoporous nature of the silica shell. The isolation and reuse of the catalyst are the crucial requirements for any practical applications in terms of cost and environmental protection. These are the greatest merits of our catalyst. In this study, it is clearly seen that for the sample without coating with mesoporous silica layer (Fe3O4-CNT-Pd), Pd nanoparticles were leaching from the CNT support after the catalytic reaction, changing the solution color to brown (Fig. 4a). However, for the catalysts of (Fe3O4-CNT-Pd)@ m-SiO2, the solution was still transparent after chemical reactions, indicating that no Pd nanoparticles were detached from CNT supports (Fig. 4b). More importantly, catalyst (Fe3O4CNT-Pd)@m-SiO2 can be easily recovered by an external magnet as shown in Fig. 4c. Magnetic response of (Fe3O4CNT-Pd)@m-SiO2 was also investigated by a magnetometer at
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Fig. 3 (a) N2 sorption/desorption isotherm and (b) pore-size distributions of the (Fe3O4-CNT-Pd)@m-SiO2 catalyst.
Fig. 4 Photographs of catalyst (a) Fe3O4-CNT-Pd and (b) (Fe3O4CNT-Pd)@m-SiO2 after Suzuki-coupling reactions. (c) Magnetic separation of the catalysts (Fe3O4-CNT-Pd)@m-SiO2 from the reaction medium. (d) Field-dependent magnetization curves of (Fe3O4-CNT-Pd)@m-SiO2.
300 K in the applied magnetic field ranging from −10 000 to 10 000 Oe. As illustrated in Fig. 4d, the isothermal magnetization of (Fe3O4-CNT-Pd)@m-SiO2 indicates superparamagnetism. The saturated magnetization value of (Fe3O4-CNT-Pd)@ m-SiO2 reached 8.94 emu g−1. As a result, the catalyst in their homogeneous dispersion shows fast movement to the applied magnetic field and quickly re-disperses with a slight shake,
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once the magnetic field is removed. This suggests that the catalyst possesses excellent magnetic response and dispersibility. The catalytic activities of catalysts were evaluated by catalyzing Suzuki-coupling reactions as listed in Table 1. As summarized in Table 1 (entry 3a–e), when Fe3O4-CNT-Pd was used as catalyst, the coupling reactions of phenyl boronic acid with substituted aryl bromides was completed in short time (10 min) with good yields at 150 °C. The reactivity is almost similar to the homogeneous catalyst Pd(PPh3)4 (entry 3c). However, after reactions, lots of Pd nanoparticles were leached from CNT supports, leading to the deterioration of catalytic activity (as shown in Fig. 4a). In order to fully use the Pd nanoparticles and maintain the catalytic activity, a mesoporous silica layer was coated on Fe3O4-CNT-Pd catalyst. After coating with silica, no Pd nanoparticles were detached from CNT supports when Suzuki-coupling reactions were finished. As shown in Table 1 (entry 3f–i), the catalyst (Fe3O4-CNT-Pd)@m-SiO2 successfully catalyzed the Suzuki-coupling reactions. Various aromatic bromides 1 reacted with phenylboronic acid or substituted phenylboronic acids produced the coupling product 3 with excellent yields. The yields for this reaction were produced at the same level when moderate steric R1 and/or R4 were introduced in the bromides (entry 3c, 3f and 3g). When we compare Table 1 (entry 3c), the product yields are almost the same, the performance of (Fe3O4-CNT-Pd(0))@m-SiO2 catalyst even behaves better than Fe3O4-CNT-Pd(0). However, no reaction occurred when R1 and R4 were substituted as steric isopropyl groups (entry 3l). As compared with the general catalyst (Pd(PPh3)4), the prepared catalyst ((Fe3O4-CNT-Pd(0))@ m-SiO2) has better efficacy (entry 3k vs. 3j). We proposed that the high specific surface area of mesoporous silica layer adsorbs more reactant molecules to increase the reactant concentrations within the silica channels, leading to higher production yields (93%). The effects of solvent on the product yields were also investigated, as shown in entry 3j and 3l. Higher yields were obtained by using DMF–H2O (93%) than dioxane–H2O (67%) as a solvent because of its better solubility for K2CO3. It is significant noting that the electronic nature of the substituents on aryl bromides had no obvious effect on the catalysis, and no by-products such as homo-coupling products from aryl bromides or phenylboronic acid had been detected. After completion of the coupling reactions, (Fe3O4-CNT-Pd)@m-SiO2 could be readily separated and simply recovered from the reaction mixture by magnet, which maintained its catalytic activity and selectivity after five cycles for the coupling reaction between 2-bromotoluene and 4-methoxyphenylboronic acid as illustrated in Table 2.
Conclusions We have demonstrated that magnetic Pd nanoreactors with mesoporous silica coating have been successfully fabricated facilely by reduction of palladium acetate to Pd nanoparticles
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Table 1
Suzuki cross-coupling by using Fe3O4-CNT-Pd(0)) or (Fe3O4-CNT-Pd(0))@m-SiO2 as catalyst
Entry
Compd #
R1
R2
R3
R4
R5
X
Catalyst
Solvent
Reaction time
Yielda (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
3a 3b 3c 3d 3e 3c 3c 3f 3g 3h 3i 3j 3k 3l 3l
H H H H H H H CH3 Ph CH3 CH3 CH3 CH3 CH(CH3)2 CH3
Br H H CO2H H H H H H H H H H H H
H H H H CHO H H H H H H H H H H
H H H H H H H H H CH3 CH3 CH3 CH3 CH(CH3)2 CH3
F F H H H H H OCH3 OCH3 H CHO OCH3 OCH3 OCH3 OCH3
I Br Br Br Cl Br Br Br Br Br Br Br Br Br Br
Fe3O4-CNT-Pd Fe3O4-CNT-Pd Fe3O4-CNT-Pd Fe3O4-CNT-Pd Fe3O4-CNT-Pd Pd(PPh3)4 (Fe3O4-CNT-Pd)@m-SiO2 (Fe3O4-CNT-Pd)@m-SiO2 (Fe3O4-CNT-Pd)@m-SiO2 (Fe3O4-CNT-Pd)@m-SiO2 (Fe3O4-CNT-Pd)@m-SiO2 (Fe3O4-CNT-Pd)@m-SiO2 Pd(PPh3)4 (Fe3O4-CNT-Pd)@m-SiO2 (Fe3O4-CNT-Pd)@m-SiO2
Dioxane–H2O Dioxane–H2O Dioxane–H2O Dioxane–H2O Dioxane–H2O Dioxane–H2O DMF–H2O DMF–H2O DMF–H2O DMF–H2O DMF–H2O DMF–H2O DMF–H2O DMF–H2O Dioxane–H2O
15 min 15 min 15 min 15 min 15 min 15 min 10 min 10 min 10 min 10 min 10 min 10 min 10 min 10 min 10 min
89 86 89 78 83 91 90 94 86 82 80 93 85 —b 67
a
Separated yield. b No desired product was separated.
Table 2
The cyclability of (Fe3O4-CNT-Pd(0))@m-SiO2 catalyst
Notes and references
Entry Substrates
Yielda Cycles (%)
1 2 3 4 5
2-Bromotoluene + 4-methoxyphenylboronic acid 2-Bromotoluene + 4-methoxyphenylboronic acid 2-Bromotoluene + 4-methoxyphenylboronic acid 2-Bromotoluene + 4-methoxyphenylboronic acid 2-Bromotoluene + 4-methoxyphenylboronic acid
1 2 3 4 5
a
Average yield based on two independent experiments.
93.2 93 86.3 82.6 89.6
on the surface of magnetic CNTs. The resultant (Fe3O4CNT-Pd)@m-SiO2 showed excellent catalytic activity for the Suzuki-coupling reactions, which are attributed to high specific surface area of the Pd nanoparticles and the limited size of mesopores on the coated silica layer. The existence of the mesoporous layer not only ensures the special catalytic reactions to be performed only in the nanochannels but also protects Pd nanoparticles from being removed from the CNT support, allowing the catalyst to be used for many cycles without the deterioration of its performances. Moreover, the recovery of the catalyst is simple and efficient using outer magnet. This work provides a new idea for designing nanoreactors, in particular the ideal heterogeneous catalysts for reactions like Suzuki-coupling reactions.
Acknowledgements This work was supported by the National Natural Science Foundation of China (the project no. 51303170) and National Science Centre, Poland within the project no. UMO-2011/03/B/ ST5/03239.
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