Large‐Scale Design of Cubic Ia3d Mesoporous Silica Monoliths with High Order, Controlled Pores, and Hydrothermal Stability (original) (raw)
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Advanced Materials, 2003
Mesoporous silica with large pores varying widely in size and with three-dimensional (3D) architectures are potential candidates for numerous applications. [1±5] Enlarging the pore size of mesoporous silica materials is an area that is actively researched. However, considerable attention has also been devoted to synthetic strategies for tailoring mesoporous silica dimensions in the 20±500 range through the use of various surfactant molecules, auxiliary chemicals, and synthetic conditions. [6±15] Furthermore, 3D mesoporous structures have been prepared through the phase transition of cylindrical phase assemblies by adding auxiliary molecules at ambient synthesis conditions. [7] The mesoporous silicates produced had an enlarged pore size; however, a loss of long-range order over the array was often observed, as revealed by the less well-resolved X-ray diffraction spectra and the broadening of the high-intensity peaks. In general, powdery mesoporous silica in micrometer-sized particles (1±2 lm) has been obtained; the formation of large, uniform monoliths was limited. [14±19] Furthermore, the utilization of a dilute surfactant in the synthesis of the mesoporous silica severely limited the ability to predict the mesophase topologies, because the primary phase of an amphiphile is affected by the interactions between the surfactant assemblies and the inorganic precursors. [5±7] Sol±gel synthesis of silica in a bulk lyotropic liquid-crystalline phase allows the inorganic precursors to precipitate in the synthesis solution during the solidification (i.e. rational precipitation), thereby allowing fabrication of large monoliths of the desired size and shape. [20±22] The use of a high template concentration (> 30 %) preserves the pre-existence of liquidcrystalline phases prior to solidification of the silica network, and thus allows a high degree of control over the amphiphilic phase domains and morphological organization of the mesopores. However, even in these syntheses, the pore size is limited to a maximum of 40 by the type and composition of the amphiphiles. [20±23] COMMUNICATIONS
A facile way to synthesize mesoporous silica with Ia3d cubic symmetry
Materials Letters, 2008
A facile synthesis route of the mesoporous silica with Ia3d cubic symmetry is reported for the first time, in which the MCM-48 type materials can be prepared at room temperature, instead of high or low temperature as usual, by using cetylpyridinium chloride (CPyCl) as template, tetraethyl orthosilicate (TEOS) as a silica source and HNO 3 as the acid catalyst in the presence of phenol. The resulting samples exhibit a larger surface area than their analogues synthesized under alkaline conditions, which will be beneficial for their potential applications in adsorption and catalysis. Moreover, the influences of HNO 3 /TEOS molar composition and the amount of phenol on the formation of Ia3d cubic mesostructure are examined and the actual function of these is described in terms of g value.
Surfactant-Templated Synthesis of Ordered Silicas with Closed Cylindrical Mesopores
Chemistry of Materials, 2012
Ordered mesoporous silicas with 2-dimensional hexagonal arrays of closed cylindrical pores were synthesized via templating with block copolymer surfactant followed by calcination at appropriately high temperatures. Precursors to closed-pore silicas, including SBA-15 silicas and organosilicas, were selected based on the existence of narrow passages to the mesopores. The increase in calcination temperature to 800−950°C led to a dramatic decrease in nitrogen uptake by the materials, indicating the loss of accessible mesopores, whereas small-angle X-ray scattering (SAXS) indicated no major structural changes other than the framework shrinkage. Since SAXS patterns for ordered mesoporous materials are related to periodic arrays of mesopores, the existence of closed mesopores was evident, as additionally confirmed by TEM. The formation of closed-pore silicas was demonstrated for ultralarge-pore SBA-15 and large-pore phenylene-bridged periodic mesoporous organosilicas. The increase in the amount of tetraethyl orthosilicate in standard SBA-15 synthesis also allowed us to observe the thermally induced pore closing. It is hypothesized that the presence of porous plugs in the cylindrical mesopores and/or caps at their ends was responsible for the propensity to the pore closing at sufficiently high temperatures. The observed behavior is likely to be relevant to a variety of silicas and organosilicas with cylindrical mesopores.
Triblock Copolymer Syntheses of Mesoporous Silica with Periodic 50 to 300 Angstrom Pores
Science, 1998
Use of amphiphilic triblock copolymers to direct the organization of polymerizing silica species has resulted in the preparation of well-ordered hexagonal mesoporous silica structures (SBA-15) with uniform pore sizes up to approximately 300 angstroms. The SBA-15 materials are synthesized in acidic media to produce highly ordered, two-dimensional hexagonal (space group p 6 mm ) silica-block copolymer mesophases. Calcination at 500°C gives porous structures with unusually large interlattice d spacings of 74.5 to 320 angstroms between the (100) planes, pore sizes from 46 to 300 angstroms, pore volume fractions up to 0.85, and silica wall thicknesses of 31 to 64 angstroms. SBA-15 can be readily prepared over a wide range of uniform pore sizes and pore wall thicknesses at low temperature (35° to 80°C), using a variety of poly(alkylene oxide) triblock copolymers and by the addition of cosolvent organic molecules. The block copolymer species can be recovered for reuse by solvent extraction...
Angewandte Chemie, 2003
The hydrothermal stability of mesoporous materials is currently of great interest because of this requirement for potential applications. A number of successful examples of mesoporous materials with good hydrothermal stability were reported recently, [7] for example, an ordered hexagonal SBA-15 with thicker pore walls, vesicle-like MSU-G materials with a high SiO 4 cross-linking, disordered KIT-1, and stable mesoporous aluminosilicates from a grafting route and from a preformed solution of "zeolite seeds". [7] Notably, these mesostructured materials are prepared at room temperature or relatively low temperatures (80-150 8C). This is quite different from the higher temperatures (150-220 8C) used for the syntheses of many microporous zeolites or phosphates because the surfactant molecules are not able to direct the mesoporous structure formation due to the unfavorable conditions for micelle formation at the higher temperatures. In some cases, the large-chain surfactants will even decompose at temperatures greater than 150 8C. As with silica-based materials, a critical factor in increasing hydrothermal stability is to have more silica condensation on the pore walls, but low synthetic temperatures result in imperfectly condensed mesoporous walls with large amounts of terminal hydroxyl groups that make the mesostructure unstable, especially under hydrothermal or steam conditions. It can be expected that the level of silica condensation will be enhanced by increasing the crystallization temperature. As suggested above, the strategy of using higher crystallization temperature for the synthesis of mesoporous materials may require special surfactants that can be used as template at high temperature. Fluorocarbon surfactants are a kind of stable surfactant, which are widely used at high temperatures (> 200 8C). However, due to the rigidity and strong hydrophobicity of the fluorocarbon chains, fluorocarbon surfactants are not suitable as templates for the preparation of well-ordered mesoporous mateials. We demonstrate herein that when a fluorocarbon surfactant
Formation Mechanism of Anionic Surfactant-Templated Mesoporous Silica
Chemistry of Materials
The synthesis mechanism of anionic surfactant-templated mesoporous silica (AMS) is described. A family of highly ordered mesoporous silica structures have been synthesized via an approach based on the self-assembly of anionic surfactants and inorganic precursors by using aminopropylsiloxane or quaternized aminopropylsiloxane as the co-structure-directing agent (CSDA), which is a different route from previous pathways. Mesophases with differing surface curvatures, varying from cage type (tetragonal P4 2 /mnm; cubic Pm3 hn with modulations; cubic Fd3 hm) to cylindrical (two-dimensional hexagonal p6mm), bicontinuous (cubic Ia3 hd and Pn3 hm), and lamellar have been obtained by controlling the charge density of the micelle surfaces by varying the degree of ionization of the carboxylate surfactants. Changing the degree of ionization of the surfactant results in changes of the surfactant packing parameter g, which leads to different mesostructures. Furthermore, variation of the charge density of positively charged amino groups of the CSDA also gives rise to different values of g. Mesoporous silicas, functionalized with amino and quaternary ammonium groups and with the various structures given above, have been obtained by extraction of the surfactant. This report leads to a deeper understanding of the interactions between the surfactant anions and the CSDA and provides a feasible and facile approach to the mesophase design of AMS materials.
Large-Pore Ethylene-Bridged Periodic Mesoporous Organosilicas with Face-Centered Cubic Structure
2010
Low-temperature (∼15°C) Pluronic-F127-templated synthesis of periodic mesoporous organosilicas (PMOs) with ethylene (-CH 2 -CH 2 -) framework groups and a face-centered cubic structure of spherical mesopores (Fm3m symmetry) was greatly enhanced through the use of judiciously chosen swelling agents and the optimization of synthesis conditions. The resulting materials were characterized by using small-angle X-ray scattering (SAXS), nitrogen adsorption, transmission electron microscopy (TEM), and thermogravimetry. While it was confirmed that 1,3,5-trimethylbenzene is a facile swelling agent for F127-templated ethylene-bridged PMOs with cubic Fm3m structure and our optimization of the synthesis afforded hitherto unreported unit-cell size and pore size, it was also shown that swelling agents predicted to have a higher extent of solubilization in Pluronics provide vast new opportunities. In particular, xylene was found to afford highly ordered PMOs with large unit-cell size and pore diameter, and a wide range of moderately or weakly ordered organosilicas with very large unit-cell parameters (up to ∼50 nm) and pore diameters (up to ∼18 nm). In the case of xylene, the pore size and unit-cell size were tunable by adjusting the amount of inorganic salt (KCl) in the synthesis mixture. The use of toluene allowed us to obtain unprecedented well-ordered PMOs with large unit-cell size (∼42 nm), very large mesopores (nominal diameter 16-17 nm; capillary condensation pressure up to ∼0.87 p/p 0 ), and enhanced primary mesopore volume, and also afforded large-pore (nominal diameter ∼15 nm) PMO in the absence of an inorganic salt. The use of benzene also afforded large-pore PMO under salt-free conditions. Ethylene-bridged PMO was successfully converted to closed-pore ordered mesoporous silica at a temperature as low as 400°C. The identification of new swelling agents for large-pore ethylenebridged PMO with spherical mesopores is likely to be extendable on PMOs of other framework compositions and on other related materials.
Versatile approach to synthesis of 2-D hexagonal ultra-large-pore periodic mesoporous organosilicas
Journal of Materials Chemistry, 2010
Periodic mesoporous organosilicas (PMOs) with methylene (-CH 2 -), ethylene (-CH 2 CH 2 -), ethenylene (-CH]CH-) and phenylene (-C 6 H 4 -) framework groups were synthesized with 2-dimensional hexagonal structures of very large cylindrical mesopores. A combination of a commercially available triblock copolymer Pluronic P123 (EO 20 PO 70 EO 20 ) with a judiciously chosen micelle swelling agent (cyclohexane and 1,3,5-triisopropylbenzene) was used as a micellar template, and the initial step of the synthesis was performed at temperature between 10 and 18 C, followed by a hydrothermal treatment at 100-150 C. The PMOs were characterized using small-angle X-ray scattering (SAXS), nitrogen adsorption, transmission electron microscopy, and solid-state 29 Si NMR. For all PMO compositions, the formation of 2-D hexagonal structures with (100) interplanar spacing, d 100 , up to 21-26 nm was achieved, which is at least seven nanometres larger than d 100 reported earlier for any PMO. The nominal (BJH) pore diameters up to 20-27 nm were achieved for the considered compositions of PMOs with 2-D hexagonal ordering, while even larger pore sizes were sometimes attained for disordered or weakly ordered structures. The mesopores exhibited constrictions or narrow entrances that were widened by increasing the hydrothermal treatment temperature. The pore diameter tended to increase as an initial synthesis temperature decreased, allowing for the pore size adjustment, but the useful temperature range depended on the bridging group. The present work suggests that the low-temperature micelle-templated synthesis with judicious selected swelling agents is a general pathway to ultra-large-pore 2-D hexagonal PMOs with both aliphatic and aromatic bridging groups.