Synthesis And Crystallization Of Polymorph A Enriched Zeolite Beta

Zeolite beta was first synthesized by Mobil Oil Corporation in1967using tetraethylammonium ion asthe organic structure-directing agent (OSDAs). It consists of3D interconnecting channels with12-memberedrings. Its structure which was determined by Treacy, Newsam, and Higgins in1988is built of twopolymorphs, polymorph A (P4122or P4322) and polymorph B (C2/c). Zeolite beta is a highly disorderedstructure formed by the random intergrowth of polymorph A and B, in a ratio of45:55. Polymorph A has achiral structure, with a helical channel running along the c axis. The other polymorph B is achiral.Up to now, almost pure polymorph B of zeolite beta has been synthesized. However, chiral polymorphA is still hypothetical. Considering the valuable potential applications of chiral polymorph A, much effort hasbeen devoted to the synthesis of pure polymorph A or polymorph A enriched zeolite beta. However, thesuccess is very limited. Selective crystallization of polymorph A remains as one of the most difficultchallenges in zeolite synthesis.The aim of the thesis is to synthesize pure chiral polymorph A zeolite beta or polymorph A enrichedzeolite beta. In the presence of two new structure-directing agents, i.e. trimethyl-cyclohexyl ammoniumhydroxide or ethyl-dimethyl-cyclohexyl ammonium hydroxide, we successfully synthesized the chiralpolymorph A enriched zeolite beta.Firstly, we investigated the influence of water content of the synthetic system on the percentage ofpolymorph A enrichment in zeolite beta when the tetraethylammonium hydroxide is used as astructure-directing agent. It was found that the normal zeolite beta with a percentage of ca.45%ofpolymorph A was obtained when the water content is high. The polymorph A enriched zeolite beta can onlybe obtained from the synthetic system with an extremely low water content. A further investigation on the crystallization process of polymorph A enriched zeolite beta wasconducted. The results showed that the enrichment of chiral polymorph A occurred in the early stage ofcrystallization and the degree of the enrichment of polymorph A did not change during the crystallization.Our results illustrated that the chiral polymorph A enriched zeolite beta or even its pure form can beobtained by carefully selecting suitable OSDAs and optimizing the synthetic condition. This route may beapplied in the synthesis of one enantiomorph of other chiral zeolites or one polymorph of intergrownmaterial.