Preparation of uniform magnetic recoverable catalyst microspheres with hierarchically mesoporous structure by using porous polymer microsphere template
© Ren et al.; licensee Springer. 2014
Received: 16 December 2013
Accepted: 21 March 2014
Published: 4 April 2014
Merging nanoparticles with different functions into a single microsphere can exhibit profound impact on various applications. However, retaining the unique properties of each component after integration has proven to be a significant challenge. Our previous research demonstrated a facile method to incorporate magnetic nanoparticles into porous silica microspheres. Here, we report the fabrication of porous silica microspheres embedded with magnetic and gold nanoparticles as magnetic recoverable catalysts. The as-prepared multifunctional composite microspheres exhibit excellent magnetic and catalytic properties and a well-defined structure such as uniform size, high surface area, and large pore volume. As a result, the very little composite microspheres show high performance in catalytic reduction of 4-nitrophenol, special convenient magnetic separability, long life, and good reusability. The unique nanostructure makes the microspheres a novel stable and highly efficient catalyst system for various catalytic industry processes.
PACS: 61.46.-w; 75.75.-c; 81.07.-b
KeywordsMagnetic nanoparticles Gold nanoparticles Mesoporous silica microspheres Hierarchical pores Catalysts
Owing to their higher catalytic activity, better selectivity, and longer stability than Pd and Pt catalysts, the catalysis of gold nanoparticles (Au NPs) in liquid-phase reactions has become the subject of increasing interest in recent years[1–15]. It has been proven that smaller Au NPs show higher catalytic activity as they have much greater surface to volume ratio[16–18]. However, small Au NPs easily aggregate to minimize their surface area, resulting in a remarkable reduction in their catalytic activity[19, 20]. Immobilizing Au NPs onto solid supports to form composite catalysts is regarded as a practical strategy to solve this problem[21–26]. For liquid-phase reactions, the catalysts need to be separated easily from the mixture for recycling. Among various kinds of supports with different nanostructures, porous magnetic composite nanomaterials have aroused considerable attention since they could satisfy two requirements simultaneously: high surface area and facile recycle[22–24, 27–31]. The high surface area comes from the hierarchically porous structure which provides enough exposure of the composite catalysts to the reactants. The facile recyclability results from the magnetic nature of the composite catalysts, which enables fast separation of the solid catalysts from the reaction mixture by applying an external magnet.
Several strategies have been developed to immobilize Au NPs onto/into the magnetic composite supports[27–35]. Generally, Au NPs are pre-synthesized and then incorporated into the modified supports. Ge et al. reported the synthesis of a nanostructured hierarchical composite composed of a central magnetite/silica composite core and many small satellite silica spheres. Au NPs were immobilized on the silica satellites through gold-amine complexation. The obtained supported gold catalysts showed fast magnetic separation ability and high catalytic activity for 4-nitrophenol reduction. Deng et al. deposited Au NPs onto modified Fe3O4@SiO2 microspheres followed by a surfactant-assembly sol-gel process and synthesized multifunctional Fe3O4@SiO2-Au@mSiO2 microspheres with well-defined core-shell nanostructures, confined catalytic Au NPs, and accessible ordered mesopore channels. However, most of these methods are tedious and time-consuming. Recently, Zheng et al. successfully developed an approach to in situ load Au NPs on Fe3O4@SiO2 magnetic spheres. After the Fe3O4@SiO2 magnetic nanoparticles were firstly prepared, AuCl4- was introduced to the surface and then reduced by Sn2+ species that were linked to the surface of the Fe3O4@SiO2 precursor. The synthesis step and the reaction cost were remarkably decreased. Despite of these researches, in situ fabrication is limited[25, 36–39], and it is still a challenge to develop an efficient and facile method to immobilize Au NPs in solid magnetic supports without compromising the catalytic activity.
The silica precursor tetraethylorthosilane (TEOS) was purchased from Alfa Aesar (Beijing, China). The template polymer microspheres are a polymer of glycidyl methacrylate (GMA) cross-linked with ethylene glycol dimethacrylate (EGDMA) supplied by Nano-Micro Technology Company (Jiangsu, China). Ferric chloride hexahydrate (FeCl3 · 6H2O), sodium oleate, trimethylamine (TMA) hydrochloride, sodium hydroxide, ammonium hydroxide (28% aqueous solution), and ethanol were purchased from Shanghai Chemical Reagent Corp. (Shanghai, China). Hexanes, chloroform, sodium borohydride (NaBH4), 4-nitrophenol (4-NP), and 1-octadecene were purchased from Alfa Aesar. Anhydrous alcohol and chloroauric acid tetrahydrate (HAuCl4 · 4H2O) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China) Water was purified by distillation followed by deionization using ion exchange resins. Other chemicals were of analytical grade and used without any further purification.
Synthesis of magnetic γ-Fe2O3 nanoparticles
Monodisperse magnetic γ-Fe2O3 nanoparticles were synthesized through the thermal decomposition of organometallic precursors with modifications. Typically, 10 g of ferric chloride hexahydrate and 35 g of sodium oleate were dissolved in a mixture of 90 ml of ethanol, 70 ml of water, and 130 ml of hexane. The mixed solution was heated to 70°C for 4 h. The resulting ferric oleate was washed four times with 50 ml of distilled water and dried at 50°C. Then, 36 g of the iron-oleate complex synthesized as described above and 5.7 g of oleic acid were dissolved in 200 g of 1-octadecene at room temperature. The reaction mixture was heated to 320°C with a constant heating rate of 3.3°C/min and then kept at 320°C for 30 min. When the reaction temperature reached 320°C, the initial transparent solution became turbid and brownish black. The resulting solution containing the nanoparticles was then cooled to room temperature, and 500 ml of ethanol was added to the solution to precipitate the nanoparticles, which were subsequently separated by centrifugation. The weight of dry oleate-capped magnetic nanoparticles was 8.2 g.
Preparation of magnetic polymer composite microspheres doped with γ-Fe2O3 nanoparticles
Magnetic nanoparticles (0.2 g) were added into 50 ml of toluene. After ultrasonic treatment in water bath for 1 h, a homogeneous yellow solution was obtained. Another 100 ml toluene containing 2 g of P(GMA-EGDMA) microspheres was prepared. Under stirring, the magnetic nanoparticle solution was added into the polymer microsphere solution. After 2 h, magnetic nanoparticle-embedded porous polymer microspheres were filtrated and washed repeatedly with toluene and ethanol. The brown magnetic polymer composite microspheres were dried at 50°C under vacuum.
Surface modification of magnetic polymer composite spheres
Brown composite spheres (2 g) were dispersed in 250-ml mixture of ethanol and water (volume ratio = 2:1). Then, 2 g of trimethylamine hydrochloride and 1 g of sodium hydroxide were added to the mixture solution. After the resulting mixture was stirred in water bath at 50°C for 24 h, the resulting TMA-treated magnetic P(GMA-EGDMA) composite microspheres were filtrated and washed repeatedly with distilled water. The brown functionalized magnetic polymer composite microspheres were dried at 50°C under vacuum.
Functionalized magnetic polymer composite microspheres adsorbed with gold precursors
TMA-treated magnetic P(GMA-EGDMA) composite microspheres (1.0 g) were added to a 100-ml round-bottomed flask, and then 50 ml deionized water and 5 ml aqueous HAuCl4 · 4H2O (1.0 wt%) were subsequently added at room temperature with mechanical stirring. After 4 h, the reddish brown precipitate was recovered by a magnet and washed with water for five times. The precipitate for further characterization was dried at 60°C for 6 h.
Preparation of mesoporous silica microspheres embedded with γ-Fe2O3 and Au nanoparticles
In a 250-ml three-necked, round-bottomed flask equipped with a mechanical stirrer, 80 ml of ethanol and 20 g of water were placed. With vigorous stirring in the flask, 0.5 g of magnetic P(GMA-EGDMA)-N+/AuCl4- composite microspheres and 2 ml of ammonia hydroxide were introduced over a period of 0.5 h. A 10% TEOS solution (in ethanol) of 30 ml was then added dropwise into the mixture in 1.5 h. The sol-gel transformation of TEOS to silica in the pore of the composite polymer microspheres was carried out at 30°C for 24 h. The brown γ-Fe2O3/polymer/gold/silica microspheres obtained were washed repeatedly with ethanol and distilled water before being dried at 50°C overnight. The dried microspheres were calcined at 600°C for 10 h (ramp rate of 10°C/min) under air. After calcination, yellow hierarchically porous silica microspheres embedded with γ-Fe2O3 and Au nanoparticles were obtained.
Catalytic reduction of 4-NP
The reduction of 4-NP by NaBH4 was chosen as a model reaction for investigating the catalytic performance of the porous SiO2/Au/γ-Fe2O3 composite microspheres. Typically, aqueous solution of 4-NP (5 mM, 1 ml) was mixed with fresh aqueous solution of NaBH4 (0.4 M, 5 ml). Two milliliters of aqueous suspension of the SiO2/Au/γ-Fe2O3 composite microspheres (1.0 mg) was rapidly added. Subsequently, 2 ml aqueous suspension at a given interval was sampled and filtered through 0.45-μm membrane filters. The UV-visible absorption spectra of the filtrates were recorded at room temperature.
The morphology and structure of the porous SiO2/Au/γ-Fe2O3 composite microspheres were studied using a field emission scanning electron microscope (FESEM; Hitachi S4800, Chiyoda-ku, Japan) and a transmission electron microscope (TEM; FEI Tecnai G2, Hillsboro, OR, USA). The particle hydrodynamic size was measured by using a Beckman Coulter Counter laser size analyzer (Multisizer 3, Fullerton, CA, USA). The thermogravimetric analysis was conducted on a DuPont TGA 2050 (Wilmington, DE, USA), with a temperature ramp of 10°C/min. The magnetization curve was measured at room temperature under a varying magnetic field with a vibrating sample magnetometer (ISOM, UPM, Madrid, Spain). N2 adsorption and desorption isotherms were measured at 77 K on a Micromeritics TriStar II 3020 (Norcross, GA, USA). The X-ray diffraction (XRD) pattern of the prepared powder sample was collected using a Rigaku D/Max-2200PC X-ray diffractometer with Cu target (40 kV, 40 mA, Shibuya-ku, Japan). The γ-Fe2O3 content in the silica microspheres was determined by atomic absorption spectroscopy (AAS; PerkinElmer 3110, Waltham, MA, USA) of an extract from the sample obtained with dilute HCl (1:1) and HF (1:1) at 80°C for 6 h. UV absorbance spectra were measured using a NanoDrop 2000 spectrophotometer (Thermo Fisher Scientific, Waltham, MA, USA).
Results and discussion
Characterization of γ-Fe2O3/Au/mSiO2 microspheres
Application of the porous γ-Fe2O3/Au/SiO2 microspheres for catalytic reduction of 4-NP
A small quantity (1.0 mg) of the γ-Fe2O3/Au/SiO2 microspheres is added and the adsorption peak at 400 nm significantly decreases as the reaction proceeds, revealing the reduction of 4-NP to form 4-AP. Figure 6B shows the UV-vis spectra as a function of reaction time for a typical reduction process. The full reduction of 4-NP by NaBH4 is completed within approximately 13 min, and the bright yellow solution gradually becomes colorless. Linear relationships between ln(Ct/C0) and reaction time are obtained in the reduction catalyzed by the γ-Fe2O3/Au/mSiO2 microspheres (Figure 6C), which well matches the first-order reaction kinetics. The rate constant κ is calculated to be 0.4/min.
The reduction reaction occurs via relaying electrons from the donor BH4- to the acceptor 4-NP after the adsorption of both onto the catalyst surface. The hydrogen atom, which is formed from the hydride, after electron transfer to the Au NPs attacks 4-NP molecules to reduce them. For comparison, the catalytic ability of the equal amount of γ-Fe2O3/mSiO2 is also studied. Without Au catalyst, the reduction reaction does not proceed, as evidenced by a nonvarying absorption spectrum.
To investigate the reusability of the γ-Fe2O3/Au/mSiO2 microspheres, we use a magnet to separate the catalysts from the solution and then rinse it with deionized water. Then, the microspheres are dispersed into deionized water for the next cycle of catalysis. As shown in Figure 6D, the γ-Fe2O3/Au/mSiO2 microspheres could be successfully recycled and reused for at least ten times within 10 min. The microstructures are well retained after the repeating catalytic processes.
In summary, by employing a functionalized magnetic polymer microsphere template, we have successfully synthesized monodisperse, hierarchically mesoporous γ-Fe2O3/Au/mSiO2 microspheres with high surface area. Quaternary ammonium in the surface of the microspheres serves not only as a reducing agent but also as a protecting ligand, which makes the adsorption of gold nanoparticles simple and convenient. Gold nanoparticles are reduced in situ and incorporated into the matrix of porous microspheres. The resulting multicomponent microspheres have high magnetization and can be conveniently separated from the reaction solution using external magnetic fields. They exhibit excellent catalytic performance and high reusability for the reduction of 4-NP in the presence of NaBH4. This functional microsphere holds great promise as a novel gold-based catalyst system for various catalytic applications. Additionally, the approach for the fabrication of γ-Fe2O3/Au/SiO2 microspheres can be extended to synthesize other multicomponent nanostructures for advanced applications in chemical/biosensor, environmental detection, and electromagnetic devices.
- Au NPs:
energy-dispersive X-ray spectroscopy
ethylene glycol dimethacrylate
scanning electron microscope
transmission electron microscopy
This work was financially supported by China Postdoctoral Science Foundation 2012 M510250 and the Shenzhen Strategic Emerging Industries Project (JCYJ201206141509581, JCYJ20130329181034621, JCYJ20120614151035045, CXZZ20130322142615483). This work is financially supported by grants from the National Basic Research Program of China (2010CB923303 to J. Z.). J. Z. thanks the National Natural Science Foundation of China (91013009) for the support.
- Hashmi ASK, Hutchings GJ: Gold catalysis. Angew Chem Int Edit 2006, 45: 7896–7936. 10.1002/anie.200602454View ArticleGoogle Scholar
- Haruta M, Kobayashi T, Sano H, Yamada N: Novel gold catalysts for the oxidation of carbon-monoxide at a temperature far below 0-degrees-C. Chem Lett 1987, 2: 405–408.View ArticleGoogle Scholar
- Haruta M, Yamada N, Kobayashi T, Iijima S: Gold catalysts prepared by coprecipitation for low-temperature oxidation of hydrogen and of carbon-monoxide. J Catal 1989, 115: 301–309. 10.1016/0021-9517(89)90034-1View ArticleGoogle Scholar
- Yoon B, Wai CM: Microemulsion-templated synthesis of carbon nanotube-supported Pd and Rh nanoparticles for catalytic applications. J Am Chem Soc 2005, 127: 17174–17175. 10.1021/ja055530fView ArticleGoogle Scholar
- Ko S, Jang J: A highly efficient palladium nanocatalyst anchored on a magnetically functionalized polymer-nanotube support. Angew Chem Int Edit 2006, 45: 7564–7567. 10.1002/anie.200602456View ArticleGoogle Scholar
- Ge JP, Huynh T, Hu YX, Yin YD: Hierarchical magnetite/silica nanoassemblies as magnetically recoverable catalyst-supports. Nano Lett 2008, 8: 931–934. 10.1021/nl080020fView ArticleGoogle Scholar
- Deng YH, Cai Y, Sun ZK, Liu J, Liu C, Wei J, Li W, Liu C, Wang Y, Zhao DY: Multifunctional mesoporous composite microspheres with well-designed nanostructure: a highly integrated catalyst system. J Am Chem Soc 2010, 132: 8466–8473. 10.1021/ja1025744View ArticleGoogle Scholar
- Zheng JM, Dong YL, Wang WF, Ma YH, Hu J, Chen XJ, Chen XG: In situ loading of gold nanoparticles on Fe3O4@SiO2 magnetic nanocomposites and their high catalytic activity. Nanoscale 2013, 5: 4894–4901. 10.1039/c3nr01075aView ArticleGoogle Scholar
- Zhang ZY, Shao CL, Zou P, Zhang P, Zhang MY, Mu JB, Guo ZC, Li XH, Wang CH, Liu YC: In situ assembly of well-dispersed gold nanoparticles on electrospun silica nanotubes for catalytic reduction of 4-nitrophenol. Chem Commun 2011, 47: 3906–3908. 10.1039/c0cc05693fView ArticleGoogle Scholar
- Liu B, Zhang W, Feng HL, Yang XL: Rattle-type microspheres as a support of tiny gold nanoparticles for highly efficient catalysis. Chem Commun 2011, 47: 11727–11729. 10.1039/c1cc13717dView ArticleGoogle Scholar
- Boyen HG, Kastle G, Weigl F, Koslowski B, Dietrich C, Ziemann P, Spatz JP, Riethmuller S, Hartmann C, Moller M, Schmid G, Garnier MG, Oelhafen P: Oxidation-resistant gold-55 clusters. Science 2002, 297: 1533–1536. 10.1126/science.1076248View ArticleGoogle Scholar
- Shi F, Zhang QH, Ma YB, He YD, Deng YQ: From CO oxidation to CO2 activation: an unexpected catalytic activity of polymer-supported nanogold. J Am Chem Soc 2005, 127: 4182–4183. 10.1021/ja042207oView ArticleGoogle Scholar
- Hashmi ASK: Gold-catalyzed organic reactions. Chem Rev 2007, 107: 3180–3211. 10.1021/cr000436xView ArticleGoogle Scholar
- Deng YH, Wang CC, Shen XZ, Yang WL, An L, Gao H, Fu SK: Preparation, characterization, and application of multistimuli-responsive microspheres with fluorescence-labeled magnetic cores and thermoresponsive shells. Chem Eur J 2005, 11: 6006–6013. 10.1002/chem.200500605View ArticleGoogle Scholar
- Stratakis M, Garcia H: Catalysis by supported gold nanoparticles: beyond aerobic oxidative processes. Chem Rev 2012, 112: 4469–4506. 10.1021/cr3000785View ArticleGoogle Scholar
- Ma Z, Dai S: Design of novel structured gold nanocatalysts. ACS Catal 2011, 1: 805–818. 10.1021/cs200100wView ArticleGoogle Scholar
- Min BK, Friend CM: Heterogeneous gold-based catalysis for green chemistry: low-temperature CO oxidation and propene oxidation. Chem Rev 2007, 107: 2709–2724. 10.1021/cr050954dView ArticleGoogle Scholar
- Daniel MC, Astruc D: Gold nanoparticles: assembly, supramolecular chemistry, quantum-size-related properties, and applications toward biology, catalysis, and nanotechnology. Chem Rev 2004, 104: 293–346. 10.1021/cr030698+View ArticleGoogle Scholar
- Zhao MQ, Sun L, Crooks RM: Preparation of Cu nanoclusters within dendrimer templates. J Am Chem Soc 1998, 120: 4877–4878. 10.1021/ja980438nView ArticleGoogle Scholar
- Zhu CZ, Han L, Hu P, Dong SJ: In situ loading of well-dispersed gold nanoparticles on two-dimensional graphene oxide/SiO2 composite nanosheets and their catalytic properties. Nanoscale 2012, 4: 1641–1646. 10.1039/c2nr11625aView ArticleGoogle Scholar
- Budroni G, Corma A: Gold–organic–inorganic high-surface-area materials as precursors of highly active catalysts. Angew Chem Int Edit 2006, 45: 3328–3331. 10.1002/anie.200600552View ArticleGoogle Scholar
- Lin FH, Doong RA: Bifunctional Au-Fe3O4 heterostructures for magnetically recyclable catalysis of nitrophenol reduction. J Phys Chem C 2011, 115: 6591–6598. 10.1021/jp110956kView ArticleGoogle Scholar
- Shylesh S, Schunemann V, Thiel WR: Magnetically separable nanocatalysts: bridges between homogeneous and heterogeneous catalysis. Angew Chem Int Edit 2010, 49: 3428–3459. 10.1002/anie.200905684View ArticleGoogle Scholar
- Ma Z, Dai S: Development of novel supported gold catalysts: a materials perspective. Nano Res 2011, 4: 3–32. 10.1007/s12274-010-0025-5View ArticleGoogle Scholar
- Wang S, Zhao QF, Wei HM, Wang JQ, Cho MY, Cho HS, Terasaki O, Wan Y: Aggregation-free gold nanoparticles in ordered mesoporous carbons: toward highly active and stable heterogeneous catalysts. J Am Chem Soc 2013, 135: 11849–11860. 10.1021/ja403822dView ArticleGoogle Scholar
- Valden M, Lai X, Goodman DW: Onset of catalytic activity of gold clusters on titania with the appearance of nonmetallic properties. Science 1998, 281: 1647–1650.View ArticleGoogle Scholar
- Leung KCF, Xuan SH, Zhu XM, Wang DW, Chak CP, Lee SF, Ho WKW, Chung BCT: Gold and iron oxide hybrid nanocomposite materials. Chem Soc Rev 2012, 41: 1911–1928. 10.1039/c1cs15213kView ArticleGoogle Scholar
- Zhu YH, Shen JH, Zhou KF, Chen C, Yang XL, Li CZ: Multifunctional magnetic composite microspheres with in situ growth Au nanoparticles: a highly efficient catalyst system. J Phys Chem C 2011, 115: 1614–1619. 10.1021/jp109276qView ArticleGoogle Scholar
- Wang Y, He J, Chen JW, Ren LB, Jiang BW, Zhao J: Synthesis of monodisperse, hierarchically mesoporous, silica microspheres embedded with magnetic nanoparticles. ACS Appl Mater Interfaces 2012, 4: 2735–2742. 10.1021/am300373yView ArticleGoogle Scholar
- Shokouhimehr M, Piao YZ, Kim J, Jang YJ, Hyeon T: A magnetically recyclable nanocomposite catalyst for olefin epoxidation. Angew Chem Int Edit 2007, 46: 7039–7043. 10.1002/anie.200702386View ArticleGoogle Scholar
- Stevens PD, Li GF, Fan JD, Yen M, Gao Y: Recycling of homogeneous Pd catalysts using superparamagnetic nanoparticles as novel soluble supports for Suzuki, Heck, and Sonogashira cross-coupling reactions. Chem Commun 2005, 35: 4435–4437.View ArticleGoogle Scholar
- Du XY, He J, Zhu J, Sun LJ, An SS: Ag-deposited silica-coated Fe3O4 magnetic nanoparticles catalyzed reduction of p-nitrophenol. Appl Surf Sci 2012, 258: 2717–2723. 10.1016/j.apsusc.2011.10.122View ArticleGoogle Scholar
- Graf C, Dembski S, Hofmann A, Ruhl E: A general method for the controlled embedding of nanoparticles in silica colloids. Langmuir 2006, 22: 5604–5610. 10.1021/la060136wView ArticleGoogle Scholar
- Shin KS, Choi JY, Park CS, Jang HJ, Kim K: Facile synthesis and catalytic application of silver-deposited magnetic nanoparticles. Catal Lett 2009, 133: 1–7. 10.1007/s10562-009-0124-7View ArticleGoogle Scholar
- Yi DK, Lee SS, Ying JY: Synthesis and applications of magnetic nanocomposite catalysts. Chem Mater 2006, 18: 2459–2461. 10.1021/cm052885pView ArticleGoogle Scholar
- Wang X, Liu DP, Song SY, Zhang HJ: Pt@CeO2 multicore@shell self-assembled nanospheres: clean synthesis, structure optimization, and catalytic applications. J Am Chem Soc 2013, 135: 15864–15872. 10.1021/ja4069134View ArticleGoogle Scholar
- Yin HF, Wang C, Zhu HG, Overbury SH, Sun SH, Dai S: Colloidal deposition synthesis of supported gold nanocatalysts based on Au-Fe3O4 dumbbell nanoparticles. Chem Commun 2008, 36: 4357–4359.View ArticleGoogle Scholar
- Zhang J, Liu XH, Guo XZ, Wu SH, Wang SR: A general approach to fabricate diverse noble-metal (Au, Pt, Ag, Pt/Au)/Fe2O3 hybrid nanomaterials. Chem Eur J 2010, 16: 8108–8116. 10.1002/chem.201000096View ArticleGoogle Scholar
- Wang Y, Shen YH, Xie AJ, Li SK, Wang XF, Cai Y: A simple method to construct bifunctional Fe3O4/Au hybrid nanostructures and tune their optical properties in the near-infrared region. J Phys Chem C 2010, 114: 4297–4301. 10.1021/jp9099804View ArticleGoogle Scholar
- Biffis A, Minati L: Efficient aerobic oxidation of alcohols in water catalysed by microgel-stabilised metal nanoclusters. J Catal 2005, 236: 405–409. 10.1016/j.jcat.2005.10.012View ArticleGoogle Scholar
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