Preparation of Magnetically Separable Mesoporous Silica

Mar 3, 2009 - The magnetic microspheres were characterized by transmission electron microscopy, energy-dispersive X-ray analysis (EDAX), powder X-ray...
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Ind. Eng. Chem. Res. 2009, 48, 3441–3445

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Preparation of Magnetically Separable Mesoporous Silica Microspheres with Open Pore Systems in Supercritical Carbon Dioxide Zhimin Chen, Mingchuan Zhuo, Fengfeng Xue, Jiafu Chen, and Qun Xu* College of Materials Science and Engineering, Zhengzhou UniVersity, Zhengzhou 450052, China

Magnetically separable mesoporous silica microspheres with open pore systems have been synthesized in supercritical carbon dioxide (SC-CO2). In the experiments, using the excellent transport properties of supercritical carbon dioxide, an acetone solution containing iron(III) acetylacetonate [Fe(acac)3] was easily infiltrated into the mesopores of the microspheres. After thermal decomposition, the Fe(acac)3 was easily transformed into Fe3O4 nanoparticles. A comparison experiment verified that the carrying effect of Fe(acac)3 using SC-CO2 was better than that observed for the common method. The magnetic microspheres were characterized by transmission electron microscopy, energy-dispersive X-ray analysis (EDAX), powder X-ray diffraction, and nitrogen adsorption-desorption. The magnetically separable property of the microspheres was also tested. 1. Introduction Silica mesoporous materials have been investigated intensively in the past 16 years for their large surface areas, tunable porosities, uniform pore size distributions, and high thermal stabilities.1,2 These materials have application prospects in heterogeneous catalysis, host-guest chemistry, environmental technology, adsorption, and many other fields.3,4 In recent years, a new application of mesoporous silica as a drug vehicle in drug-delivery systems has also been explored,5-8 not only because silica is nontoxic and highly biocompatible, but also because pore walls containing free silanol groups can react with appropriate drug functional groups. However, the use of bulk mesoporous silica materials in many applications, especially in targeted drug-delivery mechanisms as carriers and in the separation of certain substances from multiphase complex systems, suffers from some inherent limitations. Thus, for many of the envisaged applications, small particle sizes are advantageous. However, these materials also cause a separability problem in liquid-phase processes. Magnetic separation provides a convenient method for the removal of magnetizable particles by applying an appropriate magnetic field. If one could combine the advantages of smallsized mesoporous silica microspheres and magnetic particles to fabricate a nanocomposite with a high surface area, welldefined pore size, and magnetic separability, a promising novel targeted drug-delivery carrier, adsorbent, or catalyst support material might be accessible. Attempts toward this strategy have been published before: magnetic nanoparticles of iron oxide, cobalt, or FePt nanoparticles have been incorporated into porous silica materials such as MCM-41, MCM-48, and SBA-15.9-12 However, such an approach often causes two kinds problems: One is long-time consumption because of the many time cycles of impregnation; the other is the clogging of the pore system, which, in turn, can lead to mass-transfer problems. In addition, the occupied pore space restricts further functionalization of the silica inner surface. In recent years, supercritical fluids (SCFs) have been widely used as a viable alternative to conventional liquid solvents for the synthesis and processing of porous materials.13 Among SCFs, supercritical carbon dioxide (SC-CO2) is by far the most * To whom correspondence should be addressed. Tel.: +86-37167767827. Fax: +86-371-67767827. E-mail: [email protected].

frequently used because it is nonflammable, nontoxic, inexpensive, naturally abundant, and environmentally benign and has relatively low critical temperature and pressure (Tc ) 31 °C, Pc ) 73.8 bar),14,15 which allows CO2 to be used safely under laboratory and commercial operating conditions. The chemical potential of SC-CO2 is highly tunable with pressure and temperature and can be used to manipulate physical and chemical properties.16 This tunability and excellent transport properties and interfacial properties are beneficial for processing polymers,15 ordered metal nanoparticle films,17 and mesoporous materials.18-20 Another advantage is that CO2 can be easily and completely removed from products and the porous structure can be obtained without collapse of the structure. In addition, the solvent can be easily recycled from gaseous CO2 after the pressure is diminished. Organic solution-phase decomposition of an iron precursor at high temperature has been widely used in the synthesis of iron oxide nanoparticles in recent years, such as the direct decomposition of FeCup321 or iron(III) acetylacetonate [Fe(acac)3]22,23 and the decomposition of Fe(CO)5 followed by oxidation.24 However, as far as we know, there are few reports on the thermal decomposition of Fe(acac)3 for the preparation of magnetically separable mesoporous silica microspheres with open pore systems. Yang et al. recently reported that Fe(acac)3 can be directly transformed into Fe3O4 nanoparticles through simple heating of Fe(acac)3 powder at 300 °C in air for 1 h.23 In their work, using a soft lithography technique and a direct thermal decomposition method, they were able to fabricate patterned magnetite films at a low reaction temperature and in a quite short time. In this work, we extend this thermal decomposition technique to Fe(acac)3-filled mesoporous silica microspheres to prepare magnetically separable mesoporous silica microspheres with open pore systems. The Fe(acac)3-filled mesoporous silica microspheres was prepared using the iron(III) acetylacetonate [Fe(acac)3] as the precursor with a nanoscale casting process under supercritical conditions. Considering the dissolvability of Fe(acac)3 in pure SC-CO2, acetone was used as the cosolvent for Fe(acac)3. The structure of the magnetic mesoporous silica microspheres was characterized by transmission electron microscopy (TEM), nitrogen adsorption-desorption isotherms, energy-dispersive X-ray analysis (EDAX), and

10.1021/ie801557y CCC: $40.75  2009 American Chemical Society Published on Web 03/03/2009

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powder X-ray diffraction. The magnetically separable effect was also tested with a magnet. 2. Experimental Section 2.1. Materials. Ammonium hydroxide (NH4OH, 28 wt % in water), tetraethyl orthosilicate (TEOS, 28 wt %), cetyltrimethylammonium bromide (CTAB, 99.8 wt %), iron(III) acetylacetonate [Fe(acac)3], acetone, and ethanol were of analytical grade and were used as received. CO2 with a purity of 99.9% was used as received for SC-CO2 treatment. 2.2. Synthesis of Mesoporous Silica Microspheres. For sample A, 16 mL of ammonium hydroxide and 600 mL of deionized water were added to a 1000-mL three-neck flask fitted with a reflux condenser and a mechanical stirrer. This system was placed in a water bath, and the temperature was raised to 50 °C. Then, 0.37 g of CTAB was added to the water with rapid stirring. The solution was allowed to cool to room temperature, and 1.89 mL of TEOS was added. After 2 min, the solution became slightly turbid, indicating that rapid hydrolysis of the silicate was taking place. The reaction was kept for 24 h, and the obtained sample was then centrifuged at 8000 rpm for 15 min and calcined in air at 600 °C for 6 h. For sample B, 290 mL of ethanol, 324 mL of deionized water, and 23.2 mL of ammonium hydroxide were mixed in a 1000mL three-neck flask fitted with a reflux condenser and a mechanical stirrer. Following this, 0.56 g of CTAB was added with rapid stirring at room temperature. After 5 min, 2.78 mL of TEOS was added. The solution became slightly turbid 5 min later, indicating that hydrolysis of the silicate was taking place. The reaction was allowed to proceed for 24 h, and the obtained sample was then centrifuged at 8000 rpm for 15 min and calcined in air at 600 °C for 6 h. 2.3. Synthesis of Magnetically Separable Mesoporous Silica Microspheres with Open Pore Systems. In a typical experiment, 0.2 g of Fe(acac)3 was dissolved in 4 mL of acetone; then, the solution was placed in the bottom of a stainless steel autoclave of 50-mL capacity. Subsequently, 0.2 g of sample A or sample B was placed in a stainless cage fixed in the upper part of the autoclave. The temperature of the autoclave was adjusted to 40 °C, and CO2 was then added to the autoclave with a syringe pump (DB-80, Beijing Satellite Manufacturing Factory) until the desired pressure of 20 MPa was obtained. After 8 h, the pressure was released by venting. After the system had cooled to room temperature, the samples were heated at 300 °C at a 5 °C/min heating rate for 1 h in air, and finally, the magnetically separable mesoporous silica microspheres with open pore systems were obtained. 2.4. Characterization. The morphologies of the samples were characterized by transmission electron microscopy (FEI Tecnai G2 20) at an accelerating voltage of 200 kV. Nitrogen adsorption-desorption isotherms at 77 K were collected on a Quantachrome NOVA 1000e surface area and pore size analyzer. Before these measurements, the samples were heated at 473 K in 10-6 Torr nitrogen for 1 h. The Barret-JoynerHallenda (BJH) method was used to calculate the pore size distributions. Samples for EDAX (JEOL JSM-5600LV) were coated with thin gold film in order to avoid the influence of charge effects during scanning electron microscope operation. Powder X-ray diffraction (XRD) data were collected on a Siemens D-5005 X-ray diffractometer with Cu KR radiation (λ ) 1.5418 Å).

Figure 1. Sketch of preparation of magnetically separable mesoporous silica microspheres with open pore systems in supercritical carbon dioxide.

Figure 2. TEM images of bare mesoporous silica microspheres: (a) sample A, (b) high-magnification image for a detailed view of sample A, (c) sample B, (d) high-magnification image for a detailed view of sample B.

3. Results and Discussion Figure 1 shows the procedure for preparing the magnetically separable mesoporous silica microspheres with open pore systems in SC-CO2. In the first step, mesoporous silica microspheres were placed in a stainless steel cage fixed in the upper part of the autoclave, and CO2 was added to the autoclave at the desired temperature and pressure to form a supercritical fluid state with the mixed solution of acetone and Fe(acac)3. This state was maintained for a certain time to allow Fe(acac)3 to sufficiently diffuse into the pores of the microspheres. In the second step, after the pressure of the system had been released, the CO2 was vented, and the mixture of acetone and Fe(acac)3 was left in the pores. Through the treatment of this stage, the mesoporous silica microspheres turned from white to yellow brown. In the cooling procedure, most of the solvent of acetone was vaporized, and Fe(acac)3 was left in the microspheres. In the third step, the silica mesoporous microspheres containing Fe(acac)3 were placed in the furnace and sintered at 300 °C in air for 1 h; then, the Fe(acac)3 in the microspheres was transformed into magnetic Fe3O4 nanoparticles. Figure 2 shows TEM images of the mesoporous silica microspheres fabricated by the method described in the Experimental Section. The images in parts a and b of Figure 2 are of sample A, whereas those in parts c and d are of sample B. For sample A, the particles were spherical, with an average particle size of approximately 150 nm. Figure 2b is an expanded image of sample A in which a hexagonal array of ordered mesopores is clearly visible. The pore center-to-center distance

Ind. Eng. Chem. Res., Vol. 48, No. 7, 2009 3443 Table 1. Loadings of Fe(acac)3 and Fe3O4 in Sample A and Sample B Silica Microspheres Fabricated with the Assistance of SC-CO2. sample

SiO2 (g)

SiO2@Fe(acac)3 (g)

SiO2@Fe3O4 (g)

Fe(acac)3 (wt %)

Fe3O4 (wt %)

A B

0.2003 0.2001

0.2279 0.2222

0.2066 0.2052

12.24 9.95

3.05 2.49

was approximately 4 nm. For sample B, the particles were larger, with an average size of approximately 440 nm. Figure 2d is a magnified TEM image of sample B. From the image, one can see that, although these microspheres were mesoporous, the size of these pores was very small. The average diameter of the pores was about 2.2 nm. Table 1 shows the conditions in terms of the loadings of Fe(acac)3 and Fe3O4 in silica microspheres obtained from samples A and B with the assistance of SC-CO2. From the table, one can see that Fe(acac)3 was successfully carried into these microspheres, at concentrations of 12.24 and 9.95 wt %, respectively. After the microspheres had been sintered at 300 °C in air for 1 h, the Fe(acac)3 was transformed into Fe3O4. According to the final weights of the products, we calculated the amounts of Fe3O4 in samples A and B as 3.05 and 2.49 wt %, respectively. The weight loss of the samples corresponded approximately to the transformation from Fe(acac)3 to Fe3O4. The presence of ferric oxide also can be verified by EDAX analysis. Figure 3 shows an EDAX spectrum of the magnetic mesoporous silica microspheres derived from sample A, in which one can see Si, O, and Fe signals, thus confirming the presence of Fe in the microspheres. The weak signal of Fe also indicates that the content of ferric oxide is very low. Figure 4 presents TEM images of the magnetically separable mesoporous silica microspheres with open pore systems. Figure 4a,b shows the morphology of the magnetic microspheres derived from sample A, whereas Figure 4c shows the magnetic microspheres fabricated from sample B. In Figure 4a, although the magnetic nanoparticles in the spheres are not clearly visible, one can see that the microspheres in this image are darker than those in Figure 2a. On the microspheres, many faint dots are dispersed on the surface, and this phenomenon is more obvious at the boundary of the microspheres. Figure 4b is a magnified image of Figure 4a in which one can clearly see the dark magnetic nanoparticles in the microspheres. These magnetic nanoparticles were mainly distributed in the pores or coated on the pore walls at the boundary of the microspheres and resulted in a darkening of the boundary of the microspheres. These pores were not fully filled with Fe3O4, and the coating rate decreased from the boundary to the center of the microspheres. From this image, we approximated that, after being filled with magnetic nanoparticles, these mesoporous silica microspheres also contained open pores. This result can be explained as follows: In the process of treating these mesoporous silica microspheres with SC-CO2, the Fe (acac)3, acetone, and CO2 formed a homogeneous supercritical system. Because of the excellent transport properties of SC-CO2, after a certain time, the concentration of Fe(acac)3 in the pores of the microspheres became the same as that throughout the stainless steel autoclave. During the process of releasing the CO2, the pressure of CO2 quickly decreased, and the homogeneous composite of Fe(acac)3, acetone, and CO2 in the center of mesoporous silica microspheres dispersed to the boundary. During the dispersion process, more and more Fe(acac)3 in the microspheres tended to precipitate in the boundary pores. Because the components in the pores were CO2, Fe(acac)3, and acetone in the process of precipitating Fe(acac)3, when the CO2 and acetone had completely vaporized, the

Figure 3. EDAX spectrum of magnetic mesoporous silica microspheres derived from sample A.

Figure 4. TEM images of magnetically separable mesoporous silica microspheres with open pore systems. These images correspond to samples fabricated from (a,b) sample A with the assistance of SC-CO2 (b is a highmagnification image), (c) sample B with the assistance of SC-CO2, (d) sample A with the assistance of sonication.

remaining Fe(acac)3 could not fully occupy the pores. In the later sintering procedure, the Fe(acac)3 coated on the pore walls further shrank and finally formed magnetic nanoparticles. This resulted in only a thin layer of magnetic nanoparticles being coated on the pore walls and the final products having open pore systems. These phenomena are also reflected by sample B in Figure 4c. In this figure, one can see that, near the boundary of the microspheres, more dark dots can be found, and these dots also do not fully fill the pores. To illustrate that our method is more suitable for carrying Fe(acac)3 into mesoporous materials than other general methods, we performed a comparative experiment. For this experiment, 0.2 g of Fe(acac)3 was dissolved in 4 mL of ethanol, and the solution was placed in a glass tube of 50mL capacity. Next, 0.2 g of sample A was added to the glass tube, and the whole system was sonicated for 8 h at 40 °C and a frequency of 40 kHz. The sonication instrument was model KQ-100E manufactured by Kunshan Sonication Apparatus Ltd. The obtained sample was centrifuged at 8000 rpm for 10 min and then heated to 300 °C at a 5 °C/min heating rate for 1 h in air. Figure 4d shows a TEM image of these mesoporous silica microspheres treated with the assistance of sonication. Compared to Figure 4b, one can see that only very small dark dots are

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Figure 5. Photograph of the magnetically separable mesoporous silica microspheres with open pore systems. The bottle at the left shows that these microspheres could be dispersed in ethanol and form an emulsion. The bottle at the right shows that these microspheres could be separated in a magnetic field.

Figure 7. Nitrogen adsorption/desorption isotherms for (a) bare mesoporous silica microspheres of sample B, (b) magnetically separable mesoporous silica microspheres derived from sample B with the assistance of SC-CO2. The insets are the corresponding distributions of the pore radii of the samples. Table 2. Experimental Sorption Data for Bare Sample B (1) and Sample B Filled with Fe3O4 (2) Figure 6. XRD pattern of the Fe3O4 powder derived from treating Fe(acac)3 at 300 °C for 1 h in air.

present in the mesoporous silica microspheres. This indicates that, using sonication, carrying Fe(acac)3 into the pores of the silica microspheres was very difficult. The carrying effect of Fe(acac)3 also can be tested through the magnetic separability properties. In this experiment, 0.05-g samples treated with SC-CO2 or sonication were sonicated and dispersed in 3 mL of ethanol in small bottles, and then a magnet was placed beside the bottles. After 30 min, the sample treated with SC-CO2 presented obvious magnetic separability properties, as its microspheres were attached to the side wall of the bottle. In contrast, for the sample treated with sonication, the microspheres could not be separated by a magnet. This result also indicates that our method is more suitable for carrying Fe(acac)3 into mesoporous materials. Figure 5 shows the emulsion of the magnetically separable microspheres derived from sample B using SC-CO2. In the bottle at the left, one can see that the magnetic sample can be welldispersed in ethanol and form a homogeneous emulsion. The yellow-brown color of the sample indicates that magnetic nanoparticles have formed in the microspheres. The bottle at the right shows that these magnetic microspheres can be separated in a magnetic field. To illustrate that the Fe(acac)3 in the mesoporous silica microspheres has been transformed into Fe3O4, the sintered products should be characterized by powder

sample

surface area (m2/g)

pore volume (cm3/g)

pore diameter (nm)

1 2

1227.89 1133.16

0.6849 0.6212

2.23 2.19

X-ray diffraction analysis. However, the content of ferric oxide is lower than 5 wt %, which cannot be detected by X-ray diffraction analysis. Therefore, we directly treated the Fe(acac)3 powder at 300 °C in air for 1 h. Figure 6 shows an X-ray diffraction pattern of this powder, from which the phases of iron oxide can be identified as either magnetite or maghemite. Distinguishing between Fe3O4 and γ-Fe2O3 is difficult using X-ray diffraction because of peak broadening (due to small crystallite size) and the similarity of these two iron oxide structures (both γ-magnetite and maghemite crystallize with the cubic inverse spinel structure). Nonetheless, it is considered highly likely that the oxide phase in our synthesized microspheres is Fe3O4 for two reasons. On one hand, the lower decomposition temperature and shorter reaction time could result in insufficient oxidation of the Fe(acac)3, which would make the final product Fe3O4. On the other hand, the high-angle powder XRD patterns of our sample matched exactly that of the standard Fe3O4. Furthermore, Yang et al.23 also found that, under these conditions, the final product is Fe3O4. This result can indirectly confirm that the Fe(acac)3 that was carried into the silica microspheres through the SC-CO2 method was transformed into Fe3O4. Because of the presence of Fe3O4

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nanoparticles, these mesoporous silica microspheres had fine magnetism and could be separated from the emulsion by a magnet. To verify that these microspheres have open pore systems and also have the potential for further functionalization at the inner wall, we performed nitrogen adsorption/desorption experiments. Figure 7 shows nitrogen sorption/desorption isotherms for sample B before (Figure 7a) and after (Figure 7b) being filled with magnetic nanoparitcles. The isotherms show type IV adsorption behavior with H1 hysteresis loops, as defined by IUPAC.25 The relatively narrow hysteresis indicates that the mesopores in the microspheres have a narrow size distribution. The isotherms recorded before and after the particles had been filled with Fe3O4 do not show obvious changes, which indicates that the presence of Fe3O4 affects the characteristics of the pores very little. Table 2 lists the nitrogen adsorption data for the two samples.. From this table, one can see that, after the mesopores had been decorated with Fe3O4, the surface areas, pore volumes, and pore diameters of the microspheres decreased only slightly. The small decrease not only confirms that Fe3O4 formed in the microspheres, but also indicates that the content of Fe3O4 in the microspheres was very low and that the magnetic microspheres also had open pore systems after being decorated with Fe3O4. 4. Conclusion Magnetically separable mesoporous silica microspheres with open pore systems have been synthesized in supercritical carbon dioxide. Through the combined use of the SC-CO2 technique and the direct thermal decomposition method, we provide a rapid, simple, convenient, and environmentally benign platform for the synthesis of magnetically separable mesoporous silica microspheres with open pore systems. The large accessible surface areas and pore volumes of the magnetic carrier materials will undoubtedly broaden the potential applications of this type of material in the fields of catalysis, separation, and targeted drug delivery. For example, by embedding functional drugs in the pores of these particles, these microspheres could be used for targeted drug-delivery. Acknowledgment We are grateful for the financial support from the National Natural Science Foundation of China (No. 20804040), the Prominent Research Talents in University of Henan Province, the Prominent Youth Science Foundation of Henan Province (No. 0512001200), and the Open Project of State Key Laboratory of Supramolecular Structure and Materials (SKLSSM200704). Literature Cited (1) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Ordered Mesoporous Molecular Sieves Synthesized by a Liquid-Crystal Template Mechanism. Nature 1992, 359, 710. (2) Chan, Y.; Zimmer, J. P.; Stroh, M.; Steckel, J. S.; Jain, R. K.; Bawendi, M. G. Incorporation of Luminescent Nanocrystals into Monodisperse Core-Shell Silica Microspheres. AdV. Mater. 2004, 16, 2092. (3) Ciesla, U.; Schuth, F. Ordered Mesoporous Materials. Microporous Mesoporous Mater. 1999, 27, 131. (4) Ying, J. Y.; Mehnert, C. P.; Wong, M. S. Synthesis and Applications of Supramolecular-Templated Mesoporous Materials. Angew. Chem., Int. Ed. 1999, 38, 56.

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ReceiVed for reView October 16, 2008 ReVised manuscript receiVed December 25, 2008 Accepted February 04, 2009 IE801557Y