Synthesis and the Phase Transformation of Mesoporous Silica using CTEABr as the pore-directing agent
Date Issued
2005
Date
2005
Author(s)
Liu, Ming-Chang
DOI
en-US
Abstract
Mesopouors silicas have attracted much attention due to their possible applications as catalyst supports, optical materials, sensors, low-k dielectrics, membranes, and selective adsorbents. The porous structure and stability of the material are very important factors for applications. Therefore, the understanding of the factors which control the mesostructures is one of the goals of this study. Mesoporous silica synthesized using cetyltriethylammonium bromide (CTEABr) as the structure-directing agent and tetraethyl orthosilicate (TEOS) as a silica source is of great interest because various mesostructures can be formed by changing the synthesis condition. In the present study, four types of well-ordered mesophases, 3-D hexagonal P63/mmc, cubic Pm3n, 2-D hexagonal p6mm and cubic Ia3d, have been synthesized in the presence of H2SO4, HCl, and HNO3 acids. By using in-situ XRD techniques, the details of formation mechanism were studied. The anions were found to play important roles on the mesophases formed, and drying also led to phase transformation. In the competition of anion experiments, these three acids were mixed at different ratios in order to compare their abilities in affecting the structure formed, which was found to decrease in the order of NO3-> Cl-> HSO4 . This sequence follows the Hofmeister series. The interfacial anions between the template and silica were found to be exchangeable with the anions in the solution. Sodium salts of HSO4-, H2PO4-, F-, OAc-, Cl-, Br-, NO3-, SCN- were utilized to compare their strength of influence. The phase transformation was found toward the mesostructures of increasing the g-value, where g is the packing parameter of surfactants. The influences of anions follow the Hofmeister series of salting-in. Because the polarity and lifetime of pyrene fluorescence are anion-dependent, the anion exchange equilibrium was confirmed by the fluorescence experiments with pyrene seated in the micelle.
A novel synthesis route to prepare SBA-1 in alkaline condition was developed, and the material was termed NTU-1 to distinguish it from that formed in acid. The formation mechanism was different from that in the acidic condition. The cubic mesophase was not affected by post-treatment with other anions. The materials showed high hydrothermal stability and porosity. The alkaline route facilitated the incorporation of Al, Fe, and Cu into the silica framework. It should be a good candidate as a catalyst.
Subjects
介孔二氧化矽
離子效應
四級銨鹽
Mesoporous silica
ionic effect
Hofmeister series
Type
thesis
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