Synthesis of nanomorphic zeolites by sodium-surfactant cooperative structure direction and their catalytic applications

Abstract

The work presented in this thesis addresses the synthesis of nanomorphic zeolites with controlled thickness and their catalytic application. Zeolites are a family of crystalline aluminosilicates exhibiting ordered micropores. Due to their micropores at the molecular scale and their acidity, they have been widely used in catalysis and separation. Conventional zeolites synthesized with the use of small organic ammonium ions or alkali metals as structure-directing agents (SDAs) typically have micrometer-sized crystals. The limited diffusion of molecules in the crystals often induces an overload on the crystal surface and fast deactivation. To solve this problem, decreasing the diffusion length has been intensively investigated by the synthesis of zeolite as nanomorphic structures. Among the large number of approaches tried, use of an organic surfactant that is functionalized with a zeolite structure-directing group as SDA is one of the most promising strategies. In this thesis, MFI zeolites were synthesized using a small amount of $C_{18} H_{37}-N^{+}(CH^3)_{2} -C_{6}H_{12} -N^{+} (CH_{3})_{2}-C_{6}H_{13}(Br^{-})_{2}$ surfactant $(C_{18-6-6})$ in the absence and presence of $Na^{+}$. In the presence of $Na^{+}$, 2.5 nm thick MFI nanosheets were generated followed by the progressive growth of the nanosheets with the consumption of amorphous aluminosilicate. Under $Na^{+}$-free conditions, the product consisted of 2.5 nm thick nanosheet and amorphous product. It was clearly elucidated that C18-6-6 acted as a SDA for the 2.5 nm zeolite nanosheets, and $Na^{+}$ was involved in the structure direction of the MFI to grow the nanosheets. The structure direction of $C_{18-6-6}$ and $Na^{+}$ was systematically balanced to control the thickness of the MFI zeolites in the range of 2.5 to 20 nm. In addition, the properties of the zeolites were characterized and compared in the conversion of methanol to hydrocarbons. Thicker crystals exhibited higher hydrocarbon yields and shorter lifetimes. These results are explained by the hydrocarbon-pool mechanism and the diffusion in crystals.