25908
J. Phys. Chem. B 2006, 110, 25908-25915
Engineered Complex Emulsion System: Toward Modulating the Pore Length and Morphological Architecture of Mesoporous Silicas He Zhang,† Junming Sun,† Ding Ma,† Gisela Weinberg,‡ Dang Sheng Su,‡ and Xinhe Bao*† State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, P. R. China, and Department of Inorganic Chemistry, Fritz-Haber Institute of the Max Planck Society, Berlin D-14195, Germany ReceiVed: September 5, 2006; In Final Form: October 18, 2006
In the complex alkane/P123/TEOS/H2O emulsion system, an emulsion engineering method to modulate pore length and morphological architecture of mesoporous materials has been built. With fine tuning of the synthetic parameters (e.g., the composition of the synthetic mixtures, temperature, stirring, etc.), a series of chemically significant mesostructures (i.e., short-pore SBA-15 materials) with tunable pore length and morphological architecture have been successfully constructed. The effects of alkane solubilizates on pore length and particle morphology are discussed. The resulting short-pore materials would have potential applications in the fields of adsorption/separation of biomolecules and inclusion chemistry of guest species, etc.
1. Introduction During the past few decades, supermolecular self-assembly has initiated a new era, in which the controlled bottom-up construction of various functional porous materials has become the interest of many researchers. As a promising support, the construction of ordered mesoporous silicas is always one of the research focuses in this newly developed field.1 Almost along with the discovery of M41s,2 studies to control the morphology/ texture of these obtained mesoporous materials have begun, aiming at getting good performance in the specific applications. Due to the existence of complicated interactions among the different species in the synthetic mixtures, a slight change in synthetic parameters, such as temperature, shearing, components, additives, etc., might alter the self-assembly behavior significantly. Not surprisingly, mesoporous materials with diverse morphologies/textures resulted.3-13 The construction of chemically significant mesostructures has long been one of the hottest issues in the supermolecular selfassembly age.6 Particularly, mesoporous materials with short and open channels are extensively investigated because they are more favorable for the mass transfer. For example, in the adsorption experiments of biomolecules, due to the long, curved mesostructures and densely packed secondary morphologies of traditional SBA-15,14 the diffusion of large biomolecules (e.g., lysozyme) within their channels is suppressed, and thus, the adsorption equilibrium was often reached on a time scale of more than 24 h. If SBA-15 with straight pore channels and separated rodlike morphologies was used, significant improvement in the diffusion of lysozyme molecules was observed, and adsorption equilibrium time was shortened to a few hours.15 More recently, by using nanoscale large-pore mesoporous silicas, the time scale for lysozyme adsorption equilibrium has been further decreased to a few minutes.16 It was also demonstrated that more accessible mesopores will result in a significant increase in the amount of lysozyme adsorption. All these results * Towhomcorrespondenceshouldbeaddressed.E-mail:
[email protected]. Fax: 86-411-84694447. † Chinese Academy of Sciences. ‡ Fritz-Haber Institute of the Max Planck Society.
indicate that the design and synthesis of chemically significant morphological architectures are very important for the practical applications of mesoporous materials. To get short and open porous structures, decreasing the size of the particles was the most used method, by changing quenching procedure, dilution of the reaction solutions, spray drying, as well as a secondary surfactant mediated process, etc.17-20 These methods were mainly focused on the synthesis of the cationic surfactants directed MCM-41 nanoparticles. On the other hand, controlling the length of surfactant micelles21-23 could also result in the short-pore materials; however, few reports were successful until now.24 Since 1998, the nonionic surfactant-templated mesoporous SBA-15 silicas have attracted more and more attention because of their larger surface areas, tunable pore size, thicker pore walls, and therefore higher hydrothermal stability.14 However, as mentioned above, the long end to end and side by side densely packed morphologies of the traditional SBA-15 silicas are not suitable for many practical applications, especially where a fast mass transfer is required. Although various strategies have been developed for the modulation of their morphological architectures, limited cases about the construction of chemically significant structures20 (e.g., nanosized particles) were reported. Even for these successful cases, the sacrifice of ordering degree of mesostructures is often observed. In our previous studies, large amounts of decane were used to modulate the length of P123 micelles; as a result, unusual SBA-15 materials with parallel channels running along the short axis have been constructed.24 Our recent studies show that the interaction between different alkanes and P123 micelles are not the same.25 By strictly controlling the initial reaction temperature, alkanes with shorter chain length (e.g., hexane) could be used to construct highly ordered SBA-15.25 On the basis of this knowledge, herein, by finely tuning the synthetic parameters (e.g., the composition of the synthetic mixtures, temperature, stirring, etc.) of the alkane (from pentane to hexadecane)/P123/ water/TEOS complex emulsion systems, a series of well-ordered chemically significant mesostructures with tunable pore length and morphological architectures, from uniform submicrometersized SBA-15 columns to short-pore SBA-15 hexagonal slices
10.1021/jp065760w CCC: $33.50 © 2006 American Chemical Society Published on Web 11/30/2006
Mesoporous Silicas
J. Phys. Chem. B, Vol. 110, No. 51, 2006 25909
TABLE 1: Parameters of SBA-15 Materials Prepared by Using Different Alkanes alkane
initial reaction temp (K)
D100 spacing (nm)
unit cell (a0, nm)
pore diameter (nm)
surface area (m2/g)
none hexadecane dodecane decane nonane octane heptane hexane pentane
315 ( 2 316 ( 2 313 ( 2 308 ( 2 300 ( 2 298 ( 2 293 ( 2 288 ( 2 285 ( 2
10.1 11.6 11.6 11.6 12.3 13.0 13.4 14.2 14.6
11.7 13.4 13.4 13.4 14.2 15.0 15.5 16.4 16.8
9.7 12.0 12.0 12.0 13.4
659 539 600 560 637
15.0 15.7
620 614
to mesoporous free-standing films and monoliths with shortpore SBA-15 units, have been successfully constructed. At the same time, the effects of alkanes on pore length and particle morphology are discussed, and possible mechanisms for the formation of these short-pore materials are proposed. Significantly, most of these short-pore mesoporous silicas have shown superiority against traditional SBA-15 in the fields of biomolecular adsorption, separation, and inclusion of guest species.16,26 2. Experimental Methods 2.1. Synthesis. As a typical synthesis procedure, 2.4 g of EO20PO70EO20 (P123) was dissolved in 84 mL HCl solution (1.20 M), followed by the addition of 0.027 g of NH4F. The mixture was then stirred at a given temperature (from 285 to 315 K) until the solution became clear. Different alkanes (from pentane to hexadecane) and TEOS were premixed and then introduced into the solution under mechanical stirring (300360 rpm) (final P123/HCl/NH4F/H2O/TEOS/alkane molar ratios ) 1/261/1.8/11278/x/y; x ) 48-110, y ) 0-755). The above mixture was stirred at the given temperature (Table 1) for 20 h, and then transferred into an autoclave for further reaction at 373 K for 48 h. The products were collected by filtration, dried in air, and calcined at 813 K for 5 h to remove the templates. 2.2. Characterization. SEM was done on the Hitachi S4800 field-emisson scanning electron microscope. The TEM images were obtained with a Philips CM 200 transmission electron microscope equipped with a CCD camera. XRD patterns were collected on a Rigaku D/MAX 2400 diffractometer equipped with a Cu KR X-ray source operating at 40 kV and 50 mA. The N2 adsorption-desorption isotherms were recorded on an ASAP 2000 instrument. 3. Results 3.1. Effect of Alkane Solubilizates on the Pore Length of SBA-15. Various alkanes from pentane to hexadecane (final P123/HCl/NH4F/H2O/TEOS/alkane molar ratio ) 1/261/1.8/ 11278/60/235) have been used in the synthesis. Figure 1 shows SAXD patterns of the samples prepared with different alkanes. It is clear that all the samples show four well-resolved peaks that can be indexed as (100), (110), (200), (210) diffractions associated with a 2-D hexagonal symmetry (p6mm), characteristic of highly ordered SBA-15 silicas. In addition, shift of the (100) diffraction to a low angle with the decrease of alkane chain length indicates the increase of SBA-15 unit cell.25 More physical parameters of the obtained SBA-15 materials can be found in Table 1. Low magnified SEM images (Figure S1) show that the morphological architectures of the obtained SBA-15 are different depending on the alkane solubilizates used. Most importantly, HRSEM images at higher magnification demonstrate that the pore length of the obtained SBA-15 could be modulated by using different alkanes (Figure 2). Without alkane, the obtained materials have a bundle morphology, which consists of long
ropelike particles (above 1 µm) with parallel channels running along the long morphological axis (Figure 2a). When dodecane is used, the long ropelike morphology is maintained, except that the particles turn thinner (ca. 1 µm, Figure 2b). When decane is used, however, the morphology changes into uniform columnlike particles, with a pore length of ca. 260 ( 50 nm (Figure 2c). When the alkane chain length is further decreased (n-C9, Figure S1e), the morphology of the obtained SBA-15 changes back into long rodlike particles again (ca. 700-900 nm long). Surprisingly, the pores of the obtained SBA-15 are running along the short morphological axis with a pore length of only ca. 150 ( 50 nm (Figure 2d). Upon decreasing the alkane chain length to n-C8 (octane), the structure with parallel channels running along the short axis is maintained, but the pore length increases to ca. 210 ( 50 nm (Figure 2e). When heptane is used, the morphology of the obtained materials changes significantly, and micrometer-scale single or twin bunches were obtained. Moreover, the TEM image (inset in Figure S1g) shows that the micrometer-sized bunches are in fact composed of fused shortpore SBA-15 particles (
