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Preparation of porous silica spheres with self-dispersing properties
G Model PARTIC-499; No. of Pages 5
ARTICLE IN PRESS Particuology xxx (2013) xxx–xxx
Contents lists available at SciVerse ScienceDirect
Particuology journal homepage: www.elsevier.com/locate/partic
Preparation of porous silica spheres with self-dispersing properties Yujiao Li, Bo Zou, Xiaofeng Wang ∗ , Zichen Wang College of Chemistry, Jilin University, Changchun 130012, China
a r t i c l e
i n f o
Article history: Received 3 October 2012 Received in revised form 13 December 2012 Accepted 7 January 2013 Keywords: Porous material Silica sphere Emulsion Self-dispersal
a b s t r a c t A sol–gel procedure in a water/oil emulsion was introduced for the synthesis of porous silica spheres. Tetraethoxysilane was used as the silica source. The specific surface area and total pore volume of the product reached 772.3 m2 /g and 0.663 cm3 /g, respectively. The electrolyte washing process conferred a surface charge to the product, which displayed self-dispersal properties in water. The porous spheres have potential applications in the fields of drug delivery, controlled release capsules, indoor air pollutant scavengers, and hydrogen storage agents. The oil phase, which accounts for over 80% of the chemical cost of the procedure, could largely be recycled by filtering, standing, and layering. The whole procedure is suitable for application as an industrial process. © 2013 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.
1. Introduction Silica is one of the most popular cost-effective carriers and matrixes. As observed in the literature, porous silica spheres are extensively used for drug delivery and controlled release. When ethyl ether is used as the cosolvent, organic molecules can be readily encapsulated in situ into silica nanocapsules; the encapsulated molecules may then be released in a controlled way into aqueous media (Chen, He, Tang, & Yan, 2008). Chen, Ding, Wang, and Shao (2004) have used CaCO3 templates to form porous hollow silica nanoparticles as drug vehicles and find that such carriers can markedly delay the release of cefradine. A typical sustainedrelease pattern of Brilliant Blue F has also been exhibited without any observable burst effect (Li, Wen, Shao, & Chen, 2004). Modified porous silica spheres are often designed to possess various functional properties. Porous magnetic silica spheres decorated with Co2+ and Ni2+ are designed to capture the histidinetagged green fluorescent protein (Benelmekki et al., 2012), while MCM-41 type amine-functionalized mesoporous silica nanoparticles are designed for the selective sequestration of carboxylic acids from biomass fermentation (Kim, Huang, Sawatdeenarunat, Sung, & Lin, 2011). Thiol-functionalized silica spheres may be utilized as adsorbents in the presence of water vapor due to the stable hydrophobic properties of their porous surfaces (Kosuge, Murakami, Kikukawa, & Takemori, 2003). Porous silica spheres containing a copper oxide catalyst can catalyze the oxidation of carbon monoxide to carbon dioxide (van der Grift et al., 1990). A Pd-loaded
∗ Corresponding author. Tel.: +86 431 85155358; fax: +86 431 85155358. E-mail address: [email protected] (X. Wang).
mesoporous hollow silica sphere catalyst exhibits high catalytic activity and good recyclability in a liquid phenol hydrogenation under mild reaction conditions (Yang, Liao, Zeng, & Liang, 2011). Other utilizations of porous silica spheres include indoor air pollutant scavengers (Delaney et al., 2010), hydrogen storage agents (Du et al., 2009), and HPLC packing particles (Kirkland, 1992). As a result, the synthesis and utilization of porous silica has long been a focus of research. The most commonly used preparation methods are template, emulsion and self-assembly synthesis. Polystyrene–methyl acrylic acid latex can be used as a template for the preparation of porous hollow silica spheres (Ge et al., 2009). Mesoporous silica spheres with a hexagonal structure can be prepared using low concentrations of hexadecyl trimethyl ammonium bromide (CTAB) and propanol as templating agents (Das, Parida, & Mishra, 2007; Han, Hou, Xu, & Li, 2004). Hollow silica nanospheres with diameters ranging from 43 to 70 nm may be prepared by removing Fe3 O4 templates with hydrochloric acid from silica-coated Fe3 O4 composites (Liu, Wang, Yin, Shen, & Jiang, 2012). Supercritical CO2 can be an effective medium for the synthesis of spherical silica structures with different types of porosity using cationic templates under basic conditions (Chatterjee et al., 2010). Hollow silica SBA-16 spheres with cubic-ordered mesoporous shells are synthesized via an emulsiontemplate method using Pluronic F127 (purity ≥98.0%, ACS reagent, Fluka) as a structure-directing agent (Ballem, Johansson, Córdoba, & Odén, 2011). Hierarchical mesoporous silica hollow spheres with monodisperse microcapsules and uniform shells are also prepared via the emulsion-template method using a triblock copolymer as a template (Wang, Miao, Li, & Deng, 2010). Silica microcapsules (hollow spheres) are readily prepared using an interfacial reaction with a water/oil/water (W/O/W) emulsion system (Fujiwara,
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Shiokawa, Tanaka, & Nakahara, 2004), and stable hollow silica microspheres are synthesized using a sol-gel method in a nonionic water/oil (W/O) emulsion (Li, Sha, Dong, & Wang, 2002). Low-molecular-weight poly (ethylene glycol)-grafted silica particles are prepared in a W/O emulsion (Oh, Ki, Chang, & Oh, 2005). Thiol-functionalized porous silica spheres with a 70-m mean diameter are synthesized using 1-alkylamine templates through a S+ X− I+ assembly pathway (Jiang, Ding, et al., 2005). Macroporous/mesoporous silica materials with a well-ordered two-dimensional hexagonal mesostructure have been synthesized through a confinement self-assembly approach in the skeletal scaffolds of commercial polyurethane foams (Xue, Wang, Tu, & Zhao, 2009). A novel combination of stabilized condensation and dynamic self-assembly has been successfully utilized in the fabrication of porous silica nanocapsules and silica nanospheres (Chen et al., 2008). In this paper, silica spheres with a bimodal pore structure were synthesized using a sol–gel procedure in a W/O emulsion. The electrolyte washing process conferred a surface charge to the product, causing it to display self-dispersal properties. The preparation methods are very amenable to industrialization. The recycling of the oil phase could effectively reduce manufacturing costs while avoiding excessive waste generation. 2. Materials and methods 2.1. Materials Cyclohexane (99.5 wt%), polyethylene glycol-10000 (PEG10000), sorbitan monooleate (Span-80), ammonium chloride (99.0 wt%), sulphuric acid (95–98 wt%), tetraethoxysilane (TEOS, 28.0 wt%) and absolute ethanol (99.7 wt%) were commercially available. All chemicals were analytical grade and were used without further purification. Distilled water was used throughout the process. 2.2. Preparations Porous silica spheres were obtained by a sol–gel procedure in a water/oil emulsion. A typical case may be described using the water phase (WP) and the oil phase (OP). 2.2.1. Water phase First, 1 g of PEG-10000 was added to a two-necked flask, followed by the addition of 7 mL of ethanol and 8 mL of 1 wt% H2 SO4 . After approximately 20 min, under mechanical stirring and heating via a water-bath (70–80 ◦ C), the PEG dissolved completely. At this point, 4 mL of TEOS was added to the flask, which was then stirred until a certain viscosity rise in the water phase could be observed (2 h or longer). 2.2.2. Oil phase Using a Fluko FA25 emulsifying machine set to 10,000 rpm, 1 g Span-80 and 80 mL cyclohexane were mixed in a 250 mL beaker while heating in a water-bath at 70–80 ◦ C. Under vigorous emulsifying, the WP was slowly poured into the OP, and the beaker was sealed immediately with sealing film. Within 10 min, a white solid appeared at the liquid level along the beaker wall. The solution was stirred until the reaction came to completion, and the solution was filtered while hot. The filtrate was collected; after standing and layering, the oil phase could be reused simply without any other purification. The obtained solid product was first washed with 70–80 ◦ C ∼2 wt% NH4 Cl solution. Afterwards, it was washed with 70–80 ◦ C ethanol, followed by washing with 70–80 ◦ C water. The product was then dried at 50 ◦ C. At last, the
obtained powder was calcinated at 400 ◦ C for 2 h to remove the PEG. 2.2.3. Characterization Transmission electron micrograph (TEM) images were obtained using an FEI Tecnai G2 F20 S-twin D573 transmission electron microscope working at 200 kV. Scanning electron micrograph (SEM) images were acquired using a field-emission-scanning electron microscope (JEOL JSM-6700F, 10 kV). The porous properties of the products were characterized by nitrogen adsorption isotherms. Nitrogen adsorption–desorption measurements were performed at 77 K using a Micromeritics ASAP 2010. The BET surface area was calculated from the N2 adsorption isotherms using the Brunauer–Emmett–Teller (BET) equation. The total pore volumes were calculated from the amount of adsorbed N2 at P/P0 = 0.996. The small angle X-ray diffractogram was examined using a Rigaku D/max-2550 Powder X-ray diffractometer, examining Cu K␣ radiation at a scanning rate of 0.2◦ /min with 2Â values ranging from 0.7◦ to 6.0◦ . 3. Results and discussion 3.1. Predicted formation mechanism The schematic drawing of the suggested formation mechanism is shown in Fig. 1. TEOS first hydrolyze into small silica cores very quickly. The cores then gradually grow larger and form particles. The particles simultaneously start to aggregate (Fig. 1(a)). When the water phase (WP) is added to the oil phase (OP), a water/oil (W/O) emulsion is formed (Fig. 1(b)). Span-80 acts as a structure-directing agent. The sol–gel procedure continues in the water droplets until porous silica spheres are formed. Both PEG-10000 and the aggregation process can contribute to the formation of pores in the silica spheres. In this procedure, the PEG had two main functions: speeding up the viscosity increase and forming the pores. 3.2. Affects of the experiment conditions 3.2.1. Hydrolysis time of TEOS To obtain a good spherical morphology, the control of the water phase viscosity when added to the oil phase is important (Jiang, Yang, et al., 2005; Jiang et al., 2004). As the hydrolysis time of the TEOS increases, the viscosity slowly increases until the silica gel is formed. The temperature is kept at 70 ◦ C, with other reaction conditions kept identical to the previous procedure. Fig. 2 shows the morphologies of silica samples prepared at different hydrolysis times. When the viscosity is quite low, only small and irregular silica (Fig. 2(a)) is obtained. Higher viscosity results in a more perfect spherical morphology. However, if the water phase is already gelatinized, silica fragments (Fig. 2(d)) are obtained. According to the experimental results, the viscosity at 10–15 min before the gelatinization is optimal. 3.2.2. Reaction temperature The emulsification temperature also significantly affects the structure and morphology of the porous silica spheres. Generally speaking, samples prepared at higher temperatures have a more regular spherical morphology (Fig. 3), while both their surface area and pore size are smaller (Fig. 4). During the hydrolysis process of TEOS, the growth of silica cores competes with particle aggregation. When the reaction temperature is higher, the relative speed of particle aggregation is faster than that of core growth. As a result, smaller cores aggregate into more regular spheres (Fig. 3(b)), while the pore size is smaller. On the contrary, if the reaction
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Fig. 1. The schematic drawing of the suggested formation mechanism of porous silica spheres.
Fig. 2. Silica samples of different TEOS hydrolyzing times (a) 2 h, (b) 3 h, (c) 4 h, and (d) 4.5 h.
temperature is lower, larger silica cores aggregate (Fig. 3(a)) and a larger pore size is obtained.
3.2.3. Stirring rate The particle size can range from the sub-micron level to several tens of micrometers simply by adjusting the stirring rate. However, if the rate is too low (i.e., lower than 600 rpm), a stable emulsion cannot be formed. In addition, the particle size has a lower limit due to the preparation method.
3.3. Porous properties of the products The peak between 1◦ and 2◦ in the small angle X-ray diffractogram (Fig. 5) indicates that mesostructures may exist in the product. The pore size distribution calculated using the density functional theory (DFT) method confirms the existence of meso- and micro-pores, thus validating a bimodal pore structure for the products (Fig. 4(a)). According to the BDDT (Brunauer–Deming–Deming–Teller) classification, the isotherm in this work (Fig. 4(b)) is classified as types I–IV (Tompsett, Krogh,
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Fig. 3. SEM images of porous silica spheres prepared at (a) 70 ◦ C and (b) 80 ◦ C (bar length: 2 m); inset: TEM images.
Griffin, & Conner, 2005; Wang et al., 2011). The amount of adsorbed N2 increases rapidly when the relative pressure is lower than 0.1 P/P0 , indicating that the sample has well-developed micro-pores. The hysteresis loop at a relative pressure of approximately 0.6 proves the existence of meso-pores. The spheres prepared at lower temperatures have larger pore volumes and pore sizes (Fig. 4(a)). The specific surface area and total pore volume calculated from the nitrogen isotherms are 772.3 m2 /g and 0.663 cm3 /g, respectively. Meanwhile, the pore size of the sample seems to decrease as the temperature increases (Fig. 4(a)). The BET surface area and total pore volume are 685.3 m2 /g and 0.578 cm3 /g, respectively.
Fig. 4. (a) DFT method pore size distribution and (b) N2 adsorption isotherm of porous silica spheres prepared at 70 ◦ C and 80 ◦ C.
sample (Fig. 6(b)). In this case, the powder sinks directly to the bottom without diffusing in solution. The mechanism of the self-dispersal properties observed can be explained as follows. The electrolytic NH4 Cl solution washing treatment can confer a charge to the surfaces of the porous spheres (Abendroth, 1970, 1972). The complex charge forces among the treated silica spheres, the hydrogen and the hydroxyl ions act as
3.4. Self-dispersal properties of the products The NH4 Cl solution washing treatment confers a self-dispersing property to the product in water. The dispersal process is observed using a control test. Several tens of milligrams of treated silica powder is added to a test tube containing 90 vol% of distilled water (Bala et al., 2005; Wang et al., 2007). Then, without stirring or shaking, the dispersal process is observed. Photographs taken at different time points during the process are shown in Fig. 6(a). The silica powder initially floats on the surface. After a few seconds, the powder starts to sink and simultaneously disperses throughout the solution. After approximately 5 min, the powder is thoroughly dispersed. The suspension can be stable for approximately 20 min. However, this phenomenon is not observed when testing the untreated silica
Fig. 5. Small angle X-ray diffractogram of porous silica spheres prepared at 70 ◦ C.
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Fig. 6. The self-dispersal process of (a) NH4 Cl aq. treated and (b) blank silica in water over time (min): (I) 0, (II) 1, (III) 3, (IV) 5, (V) 10, (VI) 15, and (VII) 20.
counterweights to gravity. Therefore, the powders can remain suspended for a period of time rather than directly sinking to the bottom of the solution. 4. Conclusions A water/oil emulsion system was designed for the preparation of porous silica spheres. When adding the water phase to the oil phase, the water phase viscosity significantly affected the morphologies obtained. The samples prepared at lower temperatures displayed comparatively good porous properties, while the samples prepared at higher temperatures had better spherical morphologies. For the lower-temperature synthesized porous silica, the surface area determined using the BET method reached 772.3 m2 /g, with a total pore volume of 0.663 cm3 /g. The NH4 Cl solution washing process added a surface charge to the product, causing it to display self-dispersal properties in water. The oil phase may be readily recycled to reduce the chemical costs. The obtained product has extensive fields of utilization, and the entire procedure in this work is suitable for industrialization. Acknowledgements This work was supported by the Key Project of the National Eleventh Five-Year Research Program of China (2008BAE66B00) and by the Scientific and Technological Planning Project of Jilin Province (200, 75, 009). References Abendroth, R. P. (1970). Behavior of a pyrogenic silica in simple electrolytes. Journal of Colloid and Interface Science, 34, 591–596. Abendroth, R. P. (1972). Surface charge development of porous silica in aqueous solution. Journal of Physical Chemistry, 76, 2547–2549. Bala, H., Fu, W., Zhao, J., Ding, X., Jiang, Y., Yu, K., et al. (2005). Preparation of BaSO4 nanoparticles with self-dispersing properties. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 252, 129–134. Ballem, M. A., Johansson, E. M., Córdoba, J. M., & Odén, M. (2011). Synthesis of hollow silica spheres SBA-16 with large-pore diameter. Materials Letters, 65, 1066–1068.
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Benelmekki, M., Xuriguera, E., Caparros, C., Rodríguez-Carmona, E., Mendoza, R., Corchero, J. L., et al. (2012). Design and characterization of Ni2+ and Co2+ decorated Porous Magnetic Silica spheres synthesized by hydrothermal-assisted modified-Stöber method for His-tagged proteins separation. Journal of Colloid and Interface Science, 365, 156–162. Chatterjee, M., Chatterjee, A., Ikushima, Y., Kawanami, H., Ishizaka, T., Sato, M., et al. (2010). Preparation of silica sphere with porous structure in supercritical carbon dioxide. Journal of Colloid and Interface Science, 348, 57–64. Chen, J.-F., Ding, H.-M., Wang, J.-X., & Shao, L. (2004). Preparation and characterization of porous hollow silica nanoparticles for drug delivery application. Biomaterials, 25, 723–727. Chen, H., He, J., Tang, H., & Yan, C. (2008). Porous silica nanocapsules and nanospheres: Dynamic self-assembly synthesis and application in controlled release. Chemistry of Materials, 20, 5894–5900. Oh, C., Ki, C. D., Chang, J. Y., & Oh, S.-G. (2005). Preparation of PEG-grafted silica particles using emulsion method. Materials Letters, 59, 929–933. Das, D. P., Parida, K. M., & Mishra, B. K. (2007). A study on the structural properties of mesoporous silica spheres. Materials Letters, 61, 3942–3945. Delaney, P., Healy, R. M., Hanrahan, J. P., Gibson, L. T., Wenger, J. C., Morris, M. A., et al. (2010). Porous silica spheres as indoor air pollutant scavengers. Journal of Environmental Monitoring, 12, 2244–2251. Du, L., Liao, S., Liu, Q., Yang, X., Song, H., Fu, Z., et al. (2009). Porous grape-like spherical silica with hydrogen storage capability, synthesized using neutral dual surfactants as templates. International Journal of Hydrogen Energy, 34, 3810–3815. Fujiwara, M., Shiokawa, K., Tanaka, Y., & Nakahara, Y. (2004). Preparation and formation mechanism of silica microcapsules (hollow sphere) by water/oil/water interfacial reaction. Chemistry of Materials, 16, 5420–5426. Ge, C., Zhang, D., Wang, A., Yin, H., Ren, M., Liu, Y., et al. (2009). Synthesis of porous hollow silica spheres using polystyrene–methyl acrylic acid latex template at different temperatures. Journal of Physics and Chemistry of Solids, 70, 1432–1437. Han, S., Hou, W., Xu, J., & Li, Z. (2004). Synthesis of hollow spherical silica with MCM-41 mesoporous structure. Colloid & Polymer Science, 282, 1286–1291. Jiang, Y., Ding, X., Zhao, J., Hari, B., Zhao, X., Tian, Y., et al. (2005). A facile route to synthesis of hollow SiO2 /Al2 O3 spheres with uniform mesopores in the shell wall. Materials Letters, 59, 2893–2897. Jiang, Y., Yang, S., Ding, X., Guo, Y., Bala, H., Zhao, J., et al. (2005). Synthesis and catalytic activity of stable hollow ZrO2 –SiO2 spheres with mesopores in the shell wall. Journal of Materials Chemistry, 15, 2041–2046. Jiang, Y., Zhao, J., Bala, H., Xu, H., Tao, N., Ding, X., et al. (2004). Synthesis of stable hollow spheres of Si/Al composite oxide with controlled pore size in the shell wall. Materials Letters, 58, 2401–2405. Kim, S.-H., Huang, Y., Sawatdeenarunat, C., Sung, S., & Lin, V. S.-Y. (2011). Selective sequestration of carboxylic acids from biomass fermentation by surfacefunctionalized mesoporous silica nanoparticles. Journal of Materials Chemistry, 21, 12103–12109. Kirkland, J. J. (1992). Superficially porous silica microspheres for the fast highperformance liquid chromatography of macromolecules. Analytical Chemistry, 64, 1239–1245. Kosuge, K., Murakami, T., Kikukawa, N., & Takemori, M. (2003). Direct synthesis of porous pure and thiol-functional silica spheres through the S+ X− I+ assembly pathway. Chemistry of Materials, 15, 3184–3189. Li, W., Sha, X., Dong, W., & Wang, Z. (2002). Synthesis of stable hollow silica microspheres with mesoporous shell in nonionic W/O emulsion. Chemical Communications, 20, 2434–2435. Li, Z.-Z., Wen, L.-X., Shao, L., & Chen, J.-F. (2004). Fabrication of porous hollow silica nanoparticles and their applications in drug release control. Journal of Controlled Release, 98, 245–254. Liu, C., Wang, A., Yin, H., Shen, Y., & Jiang, T. (2012). Preparation of nanosized hollow silica spheres from Na2 SiO3 using Fe3 O4 nanoparticles as templates. Particuology, 10, 352–358. Tompsett, G. A., Krogh, L., Griffin, D. W., & Conner, W. C. (2005). Hysteresis and scanning behavior of mesoporous molecular sieves. Langmuir, 21, 8214–8225. van der Grift, C. J. G., Boon, A. Q. M., van Veldhuizen, A. J. W., Trommar, H. G. J., Geus, J. W., Quinson, J. F., et al. (1990). Preparation and characterization of porous silica spheres containing a copper(oxide) catalyst. Applied Catalysis, 65, 225–239. Wang, L., Guo, Y., Zou, B., Rong, C., Ma, X., Qu, Y., et al. (2011). High surface area porous carbons prepared from hydrochars by phosphoric acid activation. Bioresource Technology, 102, 1947–1950. Wang, C., Liu, Y., Bala, H., Pan, Y., Zhao, J., Zhao, X., et al. (2007). Facile preparation of CaCO3 nanoparticles with self-dispersing properties in the presence of dodecyl dimethyl betaine. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 297, 179–182. Wang, X., Miao, X., Li, Z., & Deng, W. (2010). Fabrication of mesoporous silica hollow spheres using triblock copolymer PEG–PPG–PEG as template. Journal of NonCrystalline Solids, 356, 898–905. Xue, C., Wang, J., Tu, B., & Zhao, D. (2009). Hierarchically porous silica with ordered mesostructure from confinement self-assembly in skeleton scaffolds. Chemistry of Materials, 22, 494–503. Yang, X., Liao, S., Zeng, J., & Liang, Z. (2011). A mesoporous hollow silica sphere (MHSS): Synthesis through a facile emulsion approach and application of support for high performance Pd/MHSS catalyst for phenol hydrogenation. Applied Surface Science, 257, 4472–4477.
Please cite this article in press as: Li, Y., et al. Preparation of porous silica spheres with self-dispersing properties. Particuology (2013), http://dx.doi.org/10.1016/j.partic.2013.01.002
ARTICLE IN PRESS Particuology xxx (2013) xxx–xxx
Contents lists available at SciVerse ScienceDirect
Particuology journal homepage: www.elsevier.com/locate/partic
Preparation of porous silica spheres with self-dispersing properties Yujiao Li, Bo Zou, Xiaofeng Wang ∗ , Zichen Wang College of Chemistry, Jilin University, Changchun 130012, China
a r t i c l e
i n f o
Article history: Received 3 October 2012 Received in revised form 13 December 2012 Accepted 7 January 2013 Keywords: Porous material Silica sphere Emulsion Self-dispersal
a b s t r a c t A sol–gel procedure in a water/oil emulsion was introduced for the synthesis of porous silica spheres. Tetraethoxysilane was used as the silica source. The specific surface area and total pore volume of the product reached 772.3 m2 /g and 0.663 cm3 /g, respectively. The electrolyte washing process conferred a surface charge to the product, which displayed self-dispersal properties in water. The porous spheres have potential applications in the fields of drug delivery, controlled release capsules, indoor air pollutant scavengers, and hydrogen storage agents. The oil phase, which accounts for over 80% of the chemical cost of the procedure, could largely be recycled by filtering, standing, and layering. The whole procedure is suitable for application as an industrial process. © 2013 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.
1. Introduction Silica is one of the most popular cost-effective carriers and matrixes. As observed in the literature, porous silica spheres are extensively used for drug delivery and controlled release. When ethyl ether is used as the cosolvent, organic molecules can be readily encapsulated in situ into silica nanocapsules; the encapsulated molecules may then be released in a controlled way into aqueous media (Chen, He, Tang, & Yan, 2008). Chen, Ding, Wang, and Shao (2004) have used CaCO3 templates to form porous hollow silica nanoparticles as drug vehicles and find that such carriers can markedly delay the release of cefradine. A typical sustainedrelease pattern of Brilliant Blue F has also been exhibited without any observable burst effect (Li, Wen, Shao, & Chen, 2004). Modified porous silica spheres are often designed to possess various functional properties. Porous magnetic silica spheres decorated with Co2+ and Ni2+ are designed to capture the histidinetagged green fluorescent protein (Benelmekki et al., 2012), while MCM-41 type amine-functionalized mesoporous silica nanoparticles are designed for the selective sequestration of carboxylic acids from biomass fermentation (Kim, Huang, Sawatdeenarunat, Sung, & Lin, 2011). Thiol-functionalized silica spheres may be utilized as adsorbents in the presence of water vapor due to the stable hydrophobic properties of their porous surfaces (Kosuge, Murakami, Kikukawa, & Takemori, 2003). Porous silica spheres containing a copper oxide catalyst can catalyze the oxidation of carbon monoxide to carbon dioxide (van der Grift et al., 1990). A Pd-loaded
∗ Corresponding author. Tel.: +86 431 85155358; fax: +86 431 85155358. E-mail address: [email protected] (X. Wang).
mesoporous hollow silica sphere catalyst exhibits high catalytic activity and good recyclability in a liquid phenol hydrogenation under mild reaction conditions (Yang, Liao, Zeng, & Liang, 2011). Other utilizations of porous silica spheres include indoor air pollutant scavengers (Delaney et al., 2010), hydrogen storage agents (Du et al., 2009), and HPLC packing particles (Kirkland, 1992). As a result, the synthesis and utilization of porous silica has long been a focus of research. The most commonly used preparation methods are template, emulsion and self-assembly synthesis. Polystyrene–methyl acrylic acid latex can be used as a template for the preparation of porous hollow silica spheres (Ge et al., 2009). Mesoporous silica spheres with a hexagonal structure can be prepared using low concentrations of hexadecyl trimethyl ammonium bromide (CTAB) and propanol as templating agents (Das, Parida, & Mishra, 2007; Han, Hou, Xu, & Li, 2004). Hollow silica nanospheres with diameters ranging from 43 to 70 nm may be prepared by removing Fe3 O4 templates with hydrochloric acid from silica-coated Fe3 O4 composites (Liu, Wang, Yin, Shen, & Jiang, 2012). Supercritical CO2 can be an effective medium for the synthesis of spherical silica structures with different types of porosity using cationic templates under basic conditions (Chatterjee et al., 2010). Hollow silica SBA-16 spheres with cubic-ordered mesoporous shells are synthesized via an emulsiontemplate method using Pluronic F127 (purity ≥98.0%, ACS reagent, Fluka) as a structure-directing agent (Ballem, Johansson, Córdoba, & Odén, 2011). Hierarchical mesoporous silica hollow spheres with monodisperse microcapsules and uniform shells are also prepared via the emulsion-template method using a triblock copolymer as a template (Wang, Miao, Li, & Deng, 2010). Silica microcapsules (hollow spheres) are readily prepared using an interfacial reaction with a water/oil/water (W/O/W) emulsion system (Fujiwara,
1674-2001/$ – see front matter © 2013 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.partic.2013.01.002
Please cite this article in press as: Li, Y., et al. Preparation of porous silica spheres with self-dispersing properties. Particuology (2013), http://dx.doi.org/10.1016/j.partic.2013.01.002
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Shiokawa, Tanaka, & Nakahara, 2004), and stable hollow silica microspheres are synthesized using a sol-gel method in a nonionic water/oil (W/O) emulsion (Li, Sha, Dong, & Wang, 2002). Low-molecular-weight poly (ethylene glycol)-grafted silica particles are prepared in a W/O emulsion (Oh, Ki, Chang, & Oh, 2005). Thiol-functionalized porous silica spheres with a 70-m mean diameter are synthesized using 1-alkylamine templates through a S+ X− I+ assembly pathway (Jiang, Ding, et al., 2005). Macroporous/mesoporous silica materials with a well-ordered two-dimensional hexagonal mesostructure have been synthesized through a confinement self-assembly approach in the skeletal scaffolds of commercial polyurethane foams (Xue, Wang, Tu, & Zhao, 2009). A novel combination of stabilized condensation and dynamic self-assembly has been successfully utilized in the fabrication of porous silica nanocapsules and silica nanospheres (Chen et al., 2008). In this paper, silica spheres with a bimodal pore structure were synthesized using a sol–gel procedure in a W/O emulsion. The electrolyte washing process conferred a surface charge to the product, causing it to display self-dispersal properties. The preparation methods are very amenable to industrialization. The recycling of the oil phase could effectively reduce manufacturing costs while avoiding excessive waste generation. 2. Materials and methods 2.1. Materials Cyclohexane (99.5 wt%), polyethylene glycol-10000 (PEG10000), sorbitan monooleate (Span-80), ammonium chloride (99.0 wt%), sulphuric acid (95–98 wt%), tetraethoxysilane (TEOS, 28.0 wt%) and absolute ethanol (99.7 wt%) were commercially available. All chemicals were analytical grade and were used without further purification. Distilled water was used throughout the process. 2.2. Preparations Porous silica spheres were obtained by a sol–gel procedure in a water/oil emulsion. A typical case may be described using the water phase (WP) and the oil phase (OP). 2.2.1. Water phase First, 1 g of PEG-10000 was added to a two-necked flask, followed by the addition of 7 mL of ethanol and 8 mL of 1 wt% H2 SO4 . After approximately 20 min, under mechanical stirring and heating via a water-bath (70–80 ◦ C), the PEG dissolved completely. At this point, 4 mL of TEOS was added to the flask, which was then stirred until a certain viscosity rise in the water phase could be observed (2 h or longer). 2.2.2. Oil phase Using a Fluko FA25 emulsifying machine set to 10,000 rpm, 1 g Span-80 and 80 mL cyclohexane were mixed in a 250 mL beaker while heating in a water-bath at 70–80 ◦ C. Under vigorous emulsifying, the WP was slowly poured into the OP, and the beaker was sealed immediately with sealing film. Within 10 min, a white solid appeared at the liquid level along the beaker wall. The solution was stirred until the reaction came to completion, and the solution was filtered while hot. The filtrate was collected; after standing and layering, the oil phase could be reused simply without any other purification. The obtained solid product was first washed with 70–80 ◦ C ∼2 wt% NH4 Cl solution. Afterwards, it was washed with 70–80 ◦ C ethanol, followed by washing with 70–80 ◦ C water. The product was then dried at 50 ◦ C. At last, the
obtained powder was calcinated at 400 ◦ C for 2 h to remove the PEG. 2.2.3. Characterization Transmission electron micrograph (TEM) images were obtained using an FEI Tecnai G2 F20 S-twin D573 transmission electron microscope working at 200 kV. Scanning electron micrograph (SEM) images were acquired using a field-emission-scanning electron microscope (JEOL JSM-6700F, 10 kV). The porous properties of the products were characterized by nitrogen adsorption isotherms. Nitrogen adsorption–desorption measurements were performed at 77 K using a Micromeritics ASAP 2010. The BET surface area was calculated from the N2 adsorption isotherms using the Brunauer–Emmett–Teller (BET) equation. The total pore volumes were calculated from the amount of adsorbed N2 at P/P0 = 0.996. The small angle X-ray diffractogram was examined using a Rigaku D/max-2550 Powder X-ray diffractometer, examining Cu K␣ radiation at a scanning rate of 0.2◦ /min with 2Â values ranging from 0.7◦ to 6.0◦ . 3. Results and discussion 3.1. Predicted formation mechanism The schematic drawing of the suggested formation mechanism is shown in Fig. 1. TEOS first hydrolyze into small silica cores very quickly. The cores then gradually grow larger and form particles. The particles simultaneously start to aggregate (Fig. 1(a)). When the water phase (WP) is added to the oil phase (OP), a water/oil (W/O) emulsion is formed (Fig. 1(b)). Span-80 acts as a structure-directing agent. The sol–gel procedure continues in the water droplets until porous silica spheres are formed. Both PEG-10000 and the aggregation process can contribute to the formation of pores in the silica spheres. In this procedure, the PEG had two main functions: speeding up the viscosity increase and forming the pores. 3.2. Affects of the experiment conditions 3.2.1. Hydrolysis time of TEOS To obtain a good spherical morphology, the control of the water phase viscosity when added to the oil phase is important (Jiang, Yang, et al., 2005; Jiang et al., 2004). As the hydrolysis time of the TEOS increases, the viscosity slowly increases until the silica gel is formed. The temperature is kept at 70 ◦ C, with other reaction conditions kept identical to the previous procedure. Fig. 2 shows the morphologies of silica samples prepared at different hydrolysis times. When the viscosity is quite low, only small and irregular silica (Fig. 2(a)) is obtained. Higher viscosity results in a more perfect spherical morphology. However, if the water phase is already gelatinized, silica fragments (Fig. 2(d)) are obtained. According to the experimental results, the viscosity at 10–15 min before the gelatinization is optimal. 3.2.2. Reaction temperature The emulsification temperature also significantly affects the structure and morphology of the porous silica spheres. Generally speaking, samples prepared at higher temperatures have a more regular spherical morphology (Fig. 3), while both their surface area and pore size are smaller (Fig. 4). During the hydrolysis process of TEOS, the growth of silica cores competes with particle aggregation. When the reaction temperature is higher, the relative speed of particle aggregation is faster than that of core growth. As a result, smaller cores aggregate into more regular spheres (Fig. 3(b)), while the pore size is smaller. On the contrary, if the reaction
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Fig. 1. The schematic drawing of the suggested formation mechanism of porous silica spheres.
Fig. 2. Silica samples of different TEOS hydrolyzing times (a) 2 h, (b) 3 h, (c) 4 h, and (d) 4.5 h.
temperature is lower, larger silica cores aggregate (Fig. 3(a)) and a larger pore size is obtained.
3.2.3. Stirring rate The particle size can range from the sub-micron level to several tens of micrometers simply by adjusting the stirring rate. However, if the rate is too low (i.e., lower than 600 rpm), a stable emulsion cannot be formed. In addition, the particle size has a lower limit due to the preparation method.
3.3. Porous properties of the products The peak between 1◦ and 2◦ in the small angle X-ray diffractogram (Fig. 5) indicates that mesostructures may exist in the product. The pore size distribution calculated using the density functional theory (DFT) method confirms the existence of meso- and micro-pores, thus validating a bimodal pore structure for the products (Fig. 4(a)). According to the BDDT (Brunauer–Deming–Deming–Teller) classification, the isotherm in this work (Fig. 4(b)) is classified as types I–IV (Tompsett, Krogh,
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Fig. 3. SEM images of porous silica spheres prepared at (a) 70 ◦ C and (b) 80 ◦ C (bar length: 2 m); inset: TEM images.
Griffin, & Conner, 2005; Wang et al., 2011). The amount of adsorbed N2 increases rapidly when the relative pressure is lower than 0.1 P/P0 , indicating that the sample has well-developed micro-pores. The hysteresis loop at a relative pressure of approximately 0.6 proves the existence of meso-pores. The spheres prepared at lower temperatures have larger pore volumes and pore sizes (Fig. 4(a)). The specific surface area and total pore volume calculated from the nitrogen isotherms are 772.3 m2 /g and 0.663 cm3 /g, respectively. Meanwhile, the pore size of the sample seems to decrease as the temperature increases (Fig. 4(a)). The BET surface area and total pore volume are 685.3 m2 /g and 0.578 cm3 /g, respectively.
Fig. 4. (a) DFT method pore size distribution and (b) N2 adsorption isotherm of porous silica spheres prepared at 70 ◦ C and 80 ◦ C.
sample (Fig. 6(b)). In this case, the powder sinks directly to the bottom without diffusing in solution. The mechanism of the self-dispersal properties observed can be explained as follows. The electrolytic NH4 Cl solution washing treatment can confer a charge to the surfaces of the porous spheres (Abendroth, 1970, 1972). The complex charge forces among the treated silica spheres, the hydrogen and the hydroxyl ions act as
3.4. Self-dispersal properties of the products The NH4 Cl solution washing treatment confers a self-dispersing property to the product in water. The dispersal process is observed using a control test. Several tens of milligrams of treated silica powder is added to a test tube containing 90 vol% of distilled water (Bala et al., 2005; Wang et al., 2007). Then, without stirring or shaking, the dispersal process is observed. Photographs taken at different time points during the process are shown in Fig. 6(a). The silica powder initially floats on the surface. After a few seconds, the powder starts to sink and simultaneously disperses throughout the solution. After approximately 5 min, the powder is thoroughly dispersed. The suspension can be stable for approximately 20 min. However, this phenomenon is not observed when testing the untreated silica
Fig. 5. Small angle X-ray diffractogram of porous silica spheres prepared at 70 ◦ C.
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Fig. 6. The self-dispersal process of (a) NH4 Cl aq. treated and (b) blank silica in water over time (min): (I) 0, (II) 1, (III) 3, (IV) 5, (V) 10, (VI) 15, and (VII) 20.
counterweights to gravity. Therefore, the powders can remain suspended for a period of time rather than directly sinking to the bottom of the solution. 4. Conclusions A water/oil emulsion system was designed for the preparation of porous silica spheres. When adding the water phase to the oil phase, the water phase viscosity significantly affected the morphologies obtained. The samples prepared at lower temperatures displayed comparatively good porous properties, while the samples prepared at higher temperatures had better spherical morphologies. For the lower-temperature synthesized porous silica, the surface area determined using the BET method reached 772.3 m2 /g, with a total pore volume of 0.663 cm3 /g. The NH4 Cl solution washing process added a surface charge to the product, causing it to display self-dispersal properties in water. The oil phase may be readily recycled to reduce the chemical costs. The obtained product has extensive fields of utilization, and the entire procedure in this work is suitable for industrialization. Acknowledgements This work was supported by the Key Project of the National Eleventh Five-Year Research Program of China (2008BAE66B00) and by the Scientific and Technological Planning Project of Jilin Province (200, 75, 009). References Abendroth, R. P. (1970). Behavior of a pyrogenic silica in simple electrolytes. Journal of Colloid and Interface Science, 34, 591–596. Abendroth, R. P. (1972). Surface charge development of porous silica in aqueous solution. Journal of Physical Chemistry, 76, 2547–2549. Bala, H., Fu, W., Zhao, J., Ding, X., Jiang, Y., Yu, K., et al. (2005). Preparation of BaSO4 nanoparticles with self-dispersing properties. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 252, 129–134. Ballem, M. A., Johansson, E. M., Córdoba, J. M., & Odén, M. (2011). Synthesis of hollow silica spheres SBA-16 with large-pore diameter. Materials Letters, 65, 1066–1068.
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Benelmekki, M., Xuriguera, E., Caparros, C., Rodríguez-Carmona, E., Mendoza, R., Corchero, J. L., et al. (2012). Design and characterization of Ni2+ and Co2+ decorated Porous Magnetic Silica spheres synthesized by hydrothermal-assisted modified-Stöber method for His-tagged proteins separation. Journal of Colloid and Interface Science, 365, 156–162. Chatterjee, M., Chatterjee, A., Ikushima, Y., Kawanami, H., Ishizaka, T., Sato, M., et al. (2010). Preparation of silica sphere with porous structure in supercritical carbon dioxide. Journal of Colloid and Interface Science, 348, 57–64. Chen, J.-F., Ding, H.-M., Wang, J.-X., & Shao, L. (2004). Preparation and characterization of porous hollow silica nanoparticles for drug delivery application. Biomaterials, 25, 723–727. Chen, H., He, J., Tang, H., & Yan, C. (2008). Porous silica nanocapsules and nanospheres: Dynamic self-assembly synthesis and application in controlled release. Chemistry of Materials, 20, 5894–5900. Oh, C., Ki, C. D., Chang, J. Y., & Oh, S.-G. (2005). Preparation of PEG-grafted silica particles using emulsion method. Materials Letters, 59, 929–933. Das, D. P., Parida, K. M., & Mishra, B. K. (2007). A study on the structural properties of mesoporous silica spheres. Materials Letters, 61, 3942–3945. Delaney, P., Healy, R. M., Hanrahan, J. P., Gibson, L. T., Wenger, J. C., Morris, M. A., et al. (2010). Porous silica spheres as indoor air pollutant scavengers. Journal of Environmental Monitoring, 12, 2244–2251. Du, L., Liao, S., Liu, Q., Yang, X., Song, H., Fu, Z., et al. (2009). Porous grape-like spherical silica with hydrogen storage capability, synthesized using neutral dual surfactants as templates. International Journal of Hydrogen Energy, 34, 3810–3815. Fujiwara, M., Shiokawa, K., Tanaka, Y., & Nakahara, Y. (2004). Preparation and formation mechanism of silica microcapsules (hollow sphere) by water/oil/water interfacial reaction. Chemistry of Materials, 16, 5420–5426. Ge, C., Zhang, D., Wang, A., Yin, H., Ren, M., Liu, Y., et al. (2009). Synthesis of porous hollow silica spheres using polystyrene–methyl acrylic acid latex template at different temperatures. Journal of Physics and Chemistry of Solids, 70, 1432–1437. Han, S., Hou, W., Xu, J., & Li, Z. (2004). Synthesis of hollow spherical silica with MCM-41 mesoporous structure. Colloid & Polymer Science, 282, 1286–1291. Jiang, Y., Ding, X., Zhao, J., Hari, B., Zhao, X., Tian, Y., et al. (2005). A facile route to synthesis of hollow SiO2 /Al2 O3 spheres with uniform mesopores in the shell wall. Materials Letters, 59, 2893–2897. Jiang, Y., Yang, S., Ding, X., Guo, Y., Bala, H., Zhao, J., et al. (2005). Synthesis and catalytic activity of stable hollow ZrO2 –SiO2 spheres with mesopores in the shell wall. Journal of Materials Chemistry, 15, 2041–2046. Jiang, Y., Zhao, J., Bala, H., Xu, H., Tao, N., Ding, X., et al. (2004). Synthesis of stable hollow spheres of Si/Al composite oxide with controlled pore size in the shell wall. Materials Letters, 58, 2401–2405. Kim, S.-H., Huang, Y., Sawatdeenarunat, C., Sung, S., & Lin, V. S.-Y. (2011). Selective sequestration of carboxylic acids from biomass fermentation by surfacefunctionalized mesoporous silica nanoparticles. Journal of Materials Chemistry, 21, 12103–12109. Kirkland, J. J. (1992). Superficially porous silica microspheres for the fast highperformance liquid chromatography of macromolecules. Analytical Chemistry, 64, 1239–1245. Kosuge, K., Murakami, T., Kikukawa, N., & Takemori, M. (2003). Direct synthesis of porous pure and thiol-functional silica spheres through the S+ X− I+ assembly pathway. Chemistry of Materials, 15, 3184–3189. Li, W., Sha, X., Dong, W., & Wang, Z. (2002). Synthesis of stable hollow silica microspheres with mesoporous shell in nonionic W/O emulsion. Chemical Communications, 20, 2434–2435. Li, Z.-Z., Wen, L.-X., Shao, L., & Chen, J.-F. (2004). Fabrication of porous hollow silica nanoparticles and their applications in drug release control. Journal of Controlled Release, 98, 245–254. Liu, C., Wang, A., Yin, H., Shen, Y., & Jiang, T. (2012). Preparation of nanosized hollow silica spheres from Na2 SiO3 using Fe3 O4 nanoparticles as templates. Particuology, 10, 352–358. Tompsett, G. A., Krogh, L., Griffin, D. W., & Conner, W. C. (2005). Hysteresis and scanning behavior of mesoporous molecular sieves. Langmuir, 21, 8214–8225. van der Grift, C. J. G., Boon, A. Q. M., van Veldhuizen, A. J. W., Trommar, H. G. J., Geus, J. W., Quinson, J. F., et al. (1990). Preparation and characterization of porous silica spheres containing a copper(oxide) catalyst. Applied Catalysis, 65, 225–239. Wang, L., Guo, Y., Zou, B., Rong, C., Ma, X., Qu, Y., et al. (2011). High surface area porous carbons prepared from hydrochars by phosphoric acid activation. Bioresource Technology, 102, 1947–1950. Wang, C., Liu, Y., Bala, H., Pan, Y., Zhao, J., Zhao, X., et al. (2007). Facile preparation of CaCO3 nanoparticles with self-dispersing properties in the presence of dodecyl dimethyl betaine. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 297, 179–182. Wang, X., Miao, X., Li, Z., & Deng, W. (2010). Fabrication of mesoporous silica hollow spheres using triblock copolymer PEG–PPG–PEG as template. Journal of NonCrystalline Solids, 356, 898–905. Xue, C., Wang, J., Tu, B., & Zhao, D. (2009). Hierarchically porous silica with ordered mesostructure from confinement self-assembly in skeleton scaffolds. Chemistry of Materials, 22, 494–503. Yang, X., Liao, S., Zeng, J., & Liang, Z. (2011). A mesoporous hollow silica sphere (MHSS): Synthesis through a facile emulsion approach and application of support for high performance Pd/MHSS catalyst for phenol hydrogenation. Applied Surface Science, 257, 4472–4477.
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