Ind. Eng. Chem. Res. 2005, 44, 8707-8714
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Emulsion-Templated Hierarchically Porous Silica Beads Using Silica Nanoparticles as Building Blocks Haifei Zhang and Andrew I. Cooper* Donnan and Robert Robinson Laboratories, Department of Chemistry, University of Liverpool, Crown Street, Liverpool L69 3BX, United Kingdom
Uniform silica beads were produced using emulsion-templated polymer beads as templates and silica nanoparticles as building blocks. Hybrid silica/alumina and silica/titania beads were also prepared by immersing silica-polymer composite beads or silica beads in the corresponding precursor solutions. Inorganic beads were produced by calcining the polymer-inorganic composites at 520 °C in air. The resulting materials exhibited a hierarchical mesoporousmacroporous structure, with high pore volumes and high surface areas. The mechanical stability of the beads could be enhanced by sintering at 1450 °C under argon or by using sodium silicate as a precursor, but with a concomitant reduction in the total surface area of the materials. Introduction Emulsion-templating has been used widely for the preparation of macroporous polymers1,2 and inorganic materials.3 For example, as pioneered by Sherrington and co-workers, macroporous open-cell polymers have been prepared by polymerizing the continuous phase of high internal phase emulsions (HIPEs) where the internal phase volume is greater than 74.05% v/v.1 These materials are usually prepared in the form of monoliths. More recently, we have developed an oil-inwater-in-oil (O/W/O) sedimentation polymerization process to prepare monodisperse emulsion-templated polymer beads4 based on Ruckenstein’s previous work.5 These beads exhibit highly interconnected pores and high intrusion volumes. The average diameter of the beads was around 2 mm with standard deviations in diameter as low as 2.6%. As a result of their size and uniformity, the beads are suitable for a range of potential applications. Highly porous emulsion-templated materials have found applications as separation media,6 supports for heterogeneous catalysis,7 polymersupported reagents,8 and tissue-engineering scaffolds.9 Porous inorganic materials tend to have enhanced chemical and thermal stability and are, therefore, useful in applications such as heterogeneous catalysis. Our O/W/O sedimentation polymerization method was extended to prepare uniform emulsion-templated silica beads.10 An O/W HIPE including tetraethyl orthosilicate (TEOS) in the continuous phase was prepared and then injected into a hot oil sedimentation medium. Polymersilica composite beads were generated, which were then calcined to produce uniform, hierarchically porous silica beads with a surface area of 421.9 m2 g-1 and an intrusion volume (macropore volume) of 5.81 cm3 g-1.10 Porous inorganic materials have also been generated by two-step “immersing methods”. In this approach, a porous organic material (often a synthetic polymer) is immersed in an inorganic precursor solution and a composite material is obtained by gelation of the precursors. Porous inorganic materials are then obtained * To whom correspondence should be addressed. Tel.: 0044 151 7943548. Fax: 0044 151 7943588. E-mail: aicooper@ liv.ac.uk.
by calcining the organic phase. For example, titania and titania-zirconia network structures were fabricated using polymer gel templating11 and meso-macroporous inorganic monoliths were produced from polymer foams.12 Recently, we have developed a “two-step” method to produce hierarchically porous uniform silica and metal oxide beads (e.g., alumina, titania, and zirconia) from emulsion-templated polymer beads.13 First, uniform porous poly(acrylamide) (PAM) beads were produced by O/W/O sedimentation polymerization.4 These PAM beads were then soaked in various inorganic precursor solutions to produce polymer-inorganic composites. Metal oxide beads with high surface areas and emulsiontemplated macropore structures were obtained after calcining the PAM phase.13 In addition to using reactive molecular inorganic precursor such as TEOS, nanoparticles have been used as building blocks for the preparation of porous inorganic materials. For example, starch gel and silicate nanoparticles were used to produce macroporous zeolite materials.14 Similarly, synthetic copolymer gels were soaked in colloidal dispersions of Fe3O4 or TiO2 nanoparticles to prepare ceramic monoliths.15 Gold microspheres16 and porous gold monoliths17 were also prepared using a colloidal gold sol or gold paint, respectively. Recently, we have produced emulsion-templated gold beads using PAM beads as templates and gold nanoparticles as building blocks.18 We have found that PAM beads can adsorb gold nanoparticles from their solutions irreversibly and decolorize the sols. Substantial mass gains of gold on the PAM scaffolds could be achieved in this way, and emulsion-templated porous gold beads were prepared by calcining the polymer phase. Surprisingly, the emulsion-templated structure is completely retained, despite the high mass loss that occurs upon calcination.18 In this paper, we report the synthesis and characterization of hierarchically porous silica beads and silicatealumina materials using PAM beads as templates and silica nanoparticles (or related inorganic precursors) as building blocks. One disadvantage is that the resulting porous inorganic beads are generally found to be quite weak due to the high levels of macroporosity. We have therefore developed a sintering process and alternative precursors such as sodium silicate to produce materials
10.1021/ie0502988 CCC: $30.25 © 2005 American Chemical Society Published on Web 05/25/2005
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Ind. Eng. Chem. Res., Vol. 44, No. 23, 2005
Table 1. Characterization Data for Polymer-Inorganic Composite Beads
entry
preparation conditions (v/v)f
mass gain (%)
1a 2a,b 3a 4a 5c 6c 7d 8e 9
methanol + HS-30 (3:5) methanol + HS-30 (3:5) 35% PEG-b-PPG-b-PEG + HS-30 (1:1) Na2SiO3 solution 35% PEG-b-PPG-b-PEG + Ti(OiPr)4 (1:1) 18% PEG-b-PPG-b-PEG + Al(O-s-Bu)3 (2:1) 18% PEG-b-PPG-b-PEG + Al(O-s-Bu)3 (2:1) 18% PEG-b-PPG-b-PEG + Al(O-s-Bu)3 (2:1) air-dried silica colloids
110.4 164.2 (88.1) 110.1 290.7 30.0 45.0 13.4 20.0 -
soaking time (h)
skeletal density (kg cm-3)g
surface area (m2 g-1)h
pore size (nm)i
2.7 (21.5 + 5.5) 2 22.5 21.5 21 18 18 -
1.80 1.88 1.61 1.76 1.75 1.72 -
69.4 110.2 25.7 2.1 48.9 71.2 22.7 195.4
7 8, 20 7 3.5, 5 6 150 µm) from the total intrusion. Skeletal densities were measured using a Micromeritics Helium AccuPyc 1330 pycnometer. Experimental Results and Discussion Polymer Beads. Uniform, highly porous PAM beads were prepared by O/W/O sedimentation polymerization as templates to produce the inorganic beads.4 A highly interconnected, emulsion-templated porous structure was observed for the PAM beads by SEM, and it was confirmed that the macropores were open to the bead surface. The structures were also investigated by confocal laser microscopy (CLM) as used previously to characterize emulsion-templated PAM19 and silica13 by inclusion of a fluorescent dye in the materials. Colloidal gold can be used as a labeling reagent for CLM analysis: for example, a 3D confocal reflection image of a rat thymocyte could be observed by labeling antiCD43 with a 40 nm gold-conjugated secondary antibody.20 Gold particles were also injected directly into EMT-6 tumors in order to investigate dose enhancement and radiosensitization.21 We have shown that PAM beads can adsorb gold nanoparticles irreversibly from solution,18 and this was used here to “stain” the PAM scaffold material prior to analysis. PAM beads were soaked in a 60-nm gold nanoparticle sol22 and then transferred into an isorefractive solvent, benzyl alcohol.19 Figure 1a shows an image obtained for a single emulsion-templated PAM bead: the black color can be attributed to the gold that adheres to the highly porous PAM surface. Using CLM, the internal pores in the
Ind. Eng. Chem. Res., Vol. 44, No. 23, 2005 8709
Figure 1. Images obtained using confocal laser microscopy (CLM): (a) gold nanoparticle adsorption on the polymer leads to the observation of the porous surface structure and (b) solventswollen internal pore structure of a PAM bead.
material can also be observed directly in a “solventswollen” state (Figure 1b), which is not possible using conventional SEM under high vacuum. Figure 1b is a reasonable representation of the O/W emulsion structure immediately prior to gelation. The black cycles arise from the deposition of gold nanoparticles on the pore walls. The fact that these circles overlap reflects the concentrated nature of the parent O/W emulsion (nominally 80% v/v oil).4 Composite Polymer-Inorganic Beads. A commercially available silica colloid (LUDOX HS-30) was used as the silicon source to prepare PAM-silica composite beads. The size of the silica nanoparticles in the sol was about 15 nm, as characterized by transition electron microscopy (TEM). The colloidal silica suspension was too viscous to allow full penetration into the PAM pore structure during immersion and the sol was therefore diluted with methanol in order to lower the viscosity. The beads were recovered after soaking by filtration and then allowed to dry at room temperature in air. It was found, however, that the composite beads shrank significantly during drying, which led to loss of the spherical bead shape and collapse of the emulsiontemplated pore structure. This was avoided by washing
the beads with acetone after soaking. This served two purposes: first, the addition of acetone destabilized the aqueous silica colloids very effectively, thus precipitating silica onto the polymer scaffolds, rigidifying the structure, and making it less prone to deformation during drying. Second, we have shown previously that exchanging the aqueous phase for a water-miscible organic solvent prior to drying can prevent pore collapse in composite beads prepared by O/W/O sedimentation polymerization.4,13 Table 1 shows the characterization data for various polymer-inorganic composite beads produced by this immersion route. Coating the PAM structure with silica colloids (Table 1, entries 1 and 2) leads to mass gains in the range 100-160%. These organic-inorganic composite materials exhibit much higher surface areas (70110 m2 g-1) than the starting PAM beads, the latter of which had a surface area of
