A New Strategy for Synthesizing Zeolites and Zeolite-Like Materials

Citation

Lee, Hyunjoo (2005) A New Strategy for Synthesizing Zeolites and Zeolite-Like Materials. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/N5NX-2T83. https://resolver.caltech.edu/CaltechETD:etd-05122005-144223

Abstract

Zeolite and molecular sieve materials are broadly used as ion-exchangers, adsorbents and catalysts in the chemical industry. Zeolites are typically synthesized by using organic molecules as structure-directing agents (SDA). The SDA should be removed from the pore cavity of the zeolite framework to create microporous void space before the zeolite can be used for further purposes. Porous zeolites have been prepared by calcination, or extraction in very limited cases. However, calcination has several undesired aspects mainly resulted from a high temperature. A combustion-free methodology is developed in this work by using a new concept for the SDAs. An organic molecule that can be easily cleaved into smaller fragments and subsequently recombined into the original molecule by simple treatments can be used as a 'recyclable SDA'. That is, after synthesizing a zeolite using this type of organic molecule as the SDA, the molecule can be fragmented in the pore cavity, and its fragments removed due to their smaller size. The recovered fragments are then recombined into the original SDAs, which can be used for further zeolite syntheses. The cyclic ketal molecule, 8,8-dimethyl-1,4-dioxa-8-azaspiro[4,5]decane, is used here to prove this new methodology. The ketal is fragmented into its corresponding ketone and diol molecules after structure-directing the synthesis of the zeolite, ZSM-5. The fragments are successfully removed by ion-exchange, and the prepared porous ZSM-5 shows equivalent porosity, catalytic activity and shape selectivity as conventional ZSM-5. In some cases, the SDA can be so tightly packed inside the pore cavity that the small reagent molecules required for fragmentation by acid hydrolysis have no access to the pore cavity. Therefore, the original methodology was expanded to provide a solution for this problem by utilizing two different kinds of organic molecules, a SDA and a pore-filling agent (PFA), during the zeolite syntheses. The removal of the PFA by simple extraction generates the necessary space inside the pore cavity for agents necessary for the hydrolysis to transport into the zeolite. Using this methodology, ZSM-5, ZSM-12, VPI-8 and MOR are successfully synthesized with various ketal SDAs whose hydrolysis depends on the hydrophilicity and pore connectivity of the synthesized zeolites.

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