Synthesis Mechanism of Cationic Surfactant Templating Mesoporous Silica under an Acidic Synthesis Process (original) (raw)
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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.
Journal of Colloid and Interface Science, 2007
Mixed surfactant systems have the potential to impart controlled combinations of functionality and pore structure to mesoporous metal oxides. Here, we combine a functional glucopyranoside surfactant with a cationic surfactant that readily forms liquid crystalline mesophases. The phase diagram for the ternary system CTAB/H 2 O/n-Octyl-β-D-glucopyranoside (C 8 G 1) at 50 °C is measured using polarized optical microscopy. At this temperature, the binary C 8 G 1 /H 2 O system forms disordered micellar solutions up to 72 wt% C 8 G 1 , and there is no hexagonal phase. With the addition of CTAB, we identify a large area of hexagonal phase, as well as cubic, lamellar and solid surfactant phases. The ternary phase diagram is used to predict the synthesis of thick mesoporous silica films via a direct liquid crystal templating technique. By changing the relative concentration of mixed surfactants as well as inorganic precursor species, surfactant/silica mesostructured thick films can be synthesized with variable glucopyranoside content, and with 2D hexagonal, cubic and lamellar structures. The domains over which different mesophases are prepared correspond well with those of the ternary phase diagram if the hydrophilic inorganic species is assumed to act as an equivalent volume of water.
Well-ordered mesoporous silica prepared by cationic fluorinated surfactant templating
Microporous and Mesoporous Materials, 2004
We describe the synthesis and characterization of ordered mesoporous silica using the template 1H,1H,2H,2H-perfluorooctylpyridinium chloride. This surfactant forms several lyotropic mesophases at room temperature, including hexagonal close packed cylinders, an isotropic liquid crystal, and a lamellar phase. Using this surfactant, mesoporous silica is synthesized by room temperature precipitation and surfactant extraction. Both the surfactant and product material are thoroughly characterized. From nitrogen adsorption, the product has a specific surface area of 982 m 2 /g and a pore diameter of 2.6 nm. X-ray diffraction and transmission electron microscopy confirm 2-dimensional close-packed hexagonal long-range ordering. This is the first example of cationic fluorinated surfactant templating of an ordered porous ceramic material.
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.
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
2020
In this paper, a facile method for preparing sub-micron spherical mesoporous silica by the sol-gel process and cationic surfactant cetyltrimethylammonium bromide (CTAB) as a soft template was reported. Moreover, the effect of surfactant concentration on the specific surface area and the total pore volume was investigated. The specific surface area, pore characteristic, morphology, chemical composition, and structure of mesoporous silica were studied using various methods. The N2 adsorption test showed that increasing the CTAB concentration from 4.6 mM to 7.2 mM increases the specific surface area from 416.48 to 564.07 m2g-1. However, the maximum pore volume was obtained at 5.9 mM CTAB. The spherical shape of the powders was confirmed by field emission scanning electron microscopy. Besides, X-ray diffraction, fourier transform infrared spectra, and energy dispersive spectrometry analysis indicated that the synthesized samples are SiO2, with an amorphous structure. Based on the struct...
Nanoscale Research Letters, 2013
Acidic interfacial growth can provide a number of industrially important mesoporous silica morphologies including fibers, spheres, and other rich shapes. Studying the reaction chemistry under quiescent (no mixing) conditions is important for understanding and for the production of the desired shapes. The focus of this work is to understand the effect of a number of previously untested conditions: acid type (HCl, HNO 3 , and H 2 SO 4 ), acid content, silica precursor type (TBOS and TEOS), and surfactant type (CTAB, Tween 20, and Tween 80) on the shape and structure of products formed under quiescent two-phase interfacial configuration. Results show that the quiescent growth is typically slow due to the absence of mixing. The whole process of product formation and pore structuring becomes limited by the slow interfacial diffusion of silica source. TBOS-CTAB-HCl was the typical combination to produce fibers with high order in the interfacial region. The use of other acids (HNO 3 and H 2 SO 4 ), a less hydrophobic silica source (TEOS), and/or a neutral surfactant (Tweens) facilitate diffusion and homogenous supply of silica source into the bulk phase and give spheres and gyroids with low mesoporous order. The results suggest two distinct regions for silica growth (interfacial region and bulk region) in which the rate of solvent evaporation and local concentration affect the speed and dimension of growth. A combined mechanism for the interfacial bulk growth of mesoporous silica under quiescent conditions is proposed.
Journal of Physical Chemistry C, 2016
We investigate the mechanism responsible for the formation of mesoporous silica formed with the so-called co-structure directing agent (CSDA) route. The synthesis relies on the interaction between silica source (tetraethylorthosilicate), cationic surfactant (C18H37N + (CH3)2(CH2)3N + (CH3)3Br2) and CSDA (carboxyethylsilanetriol), that results in a material functionalized with carboxylic groups. Depending on the concentration of HCl in the synthesis, the structure is defined by Fm3 ̅ m (at high pH) and by Fd3 ̅ m (at low pH), with a gradual transition in the intermediate pH range. Here we aim at finding the origin for the structural change triggered by pH and investigate the effects of the hydrolysis of the silica source on the overall kinetics of the synthesis. A fast process results in Fm3 ̅ m, regardless of pH, and a slow process results in Fd3 ̅ m. The hydrolysis step is the important structural control parameter. We studied the cross-linking of silica and CSDA using 29 Si NMR. The cross-linking is similar for the two structures, and possibly Fd3 ̅ m structure contains slightly more CSDA. 13 C PT ssNMR was used to investigate the surfactant mobility/rigidity during the synthesis. The rigidity of the Fm3 ̅ m is established much faster than the Fd3 ̅ m.