| Summary: | 博士 === 國立臺灣大學 === 化學研究所 === 91 === Since scientists at Mobil Oil Research and Development synthesized a new family of mesoporous molecular sieves (M41S), M41S has attracted much attention. Especially, MCM-48 with three-dimensional channels seems to be more attractive than MCM-41 with one-dimensional channels for catalytic applications since MCM-48 can better avoid pore blockage during catalytic reactions. Because of the amorphous nature of the pore walls, MCM-41 and MCM-48 have weaker acidity and much less hydrothermal stability than conventional zeolites. This limits their catalytic applications. In this thesis, we strive to improve the hydrothermal stability and acidity via four methods.
The first method is by adding salts (NaCl, NaBr, Na4EDTA etc.) to synthesis gels to facilitate the condensation of silanol groups during the formation of the framework. Significant improvements in the thermal stability (at 900 ºC for 2h) and the hydrothermal stability (in boiling water for 6h) have been achieved. The second one is using a two-step process consisting transformation in acid solution and a mild surface modification to obtain silanes grafted MCM-48 with high coverage. The third one is to synthesize highly ordered ultrastable acidic mesoporous aluminosilicates with cubic structure (MCM-48-S) from cetyltrimethylammonium bromide (CTAB) and pre-formed zeolite Beta seeds. The materials retain its structure after soaking in boiling water for 10 days. From the NH3-TPD-MASS curves of the materials, desorption lasts until a fairly high temperature of 500 ºC indicating a fairly acidic mesoporous aluminosilicates in MCM-48-S. Efforts were made to understand the physical chemistry of surfactant/silicate self-organization. The last one is to reduce the pH value during rehydrothermal process so that a better condensation of the silicate on the wall can be achieved. Thus, well-ordered MCM-48-S (MCM-48-SH) can be obtained. The MCM-48-SH displays better hydrothermal stability and more uniform pore size than MCM-48-S.
Furthermore, the catalytic application of the solid acid catalysts (MCM-41-S and MCM-48-S) was demonstrated with the Friedel-Crafts acylation of aromatics. High conversion (>90%) and high selectivity (100%) were obtained. Mesoporous carbon was obtained by the impregnation of sucrose on MCM-48 or MCM-48-S followed by dehydration with concentrated sulfuric acid. Because of the strong acidity of MCM-48-S the mesoporous carbon could be synthesized without adding sulfuric acid. The synthesized mesoporous carbon has high surface area (∼2000cm2/g) and small pore size (15 Å). The melting/freezing behavior of water in the pore system was also briefly studied.
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