Mesoporous MCM-41 silica hierarchical structures consisting of a high yield (∼98%) of uniform-sized hollow spheres of diameter 5.0 ± 1.0 μm have been obtained. The nanochannels on the shell are in latitude direction and the inside of the micron spheres has intricate pillar structures producing structures of various topological genus ranks.
Mixed cationic surfactants were used in the synthesis of MCM-41 materials. Synthetic conditions for controlling the nanometer-scale channels size and micrometer-scale morphology in mixed alkyltrimethylammonium halides systems (n \ 8È18 ; X \ Cl or Br) were studied. The spacing (C n TMAX) d 100 of MCM-41 synthesized using the system decreases linearly with increase in the C 18 TMABrÈC 14 TMABr molar fraction. Such linearity was also reproduced in other systems with some deviation at high C 14 TMABr molar fractions of short chain length surfactants (n \ 14). The morphology in mixed system can be C n TMABr changed from particle form to a tubules-within-tubule (TWT) structure by adjusting the average surfactant chain length. It was found that the optimum average chain length for preparing a TWT structure is 15È16 for the mixed system and 16È17 for the mixed system. A high yield TWT morphology also C n TMABr C n TMACl can be prepared by adding the cosolvent acetone to a suitable value of the hydrophobicÈlipophilic balance (HLB). The change in morphology of the products with respect to average chain length, counter ions and cosolvents are explained by the principle of hydrophilicÈhydrophobic (lipophilic) balance (HLB).
We report a “delayed neutralization” process for the preparation of highly‐ordered aluminosilicate MCM‐41 molecular sieves with high thermal and hydrothermal stability, and sharp pore size distribution. However, the structural order and pore size are dependent on the carbon chain length. In the mixture surfactant systems, the pore size of the MCM‐41 materials could be fine‐tuned. The pore size can be extended from 2.5 to 4.5 nm by adding a suitable amount of hydrocarbons. The tubular morphology of the MCM‐41 material of 0.3 to 10 micrometers diameter, where the wall consists of coaxial cylindrical pores of nanometers MCM‐41, can be obtained by careful control of the surfactant‐water content and the rate of condensation of silica. An optimum condition for automatic synthesis of the hierarchical TWT structure has been accomplished. The addition of 1‐alkanols as cosurfactant would not only improve the order of the MCM‐41 hexagonal structure but also promote the formation of micrometer‐sized hierarchical materials, for example: tubules‐within‐tubule and uniform‐sized hollow spheres of diameter 5.0 ± 1.0 μm. However, the inside of the micron spheres has intricate structures possessing various topological genus ranks. The MCM‐41 is a good supporter for Molybdenum oxide catalysts. The rate of deactivation in the catalytic reaction of ethyl‐benzene dehydrogenation to styrene increases in the order: MT < MP < SiO2. The physically mixed samples have higher catalytic activity than impregnated ones.
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