Layered intermediates in zeolite synthesis: Are structures related?

Layered organic-inorganic compounds have been found during the crystallization of zeolites ZSM-48 and beta with hexamethonium and tetraethylammonium cations, respectively. These materials are obtained from gels with relatively high concentrations in organic species but are not stable under hydrothermal conditions. While highly crystalline solids are formed after short synthesis periods, their structure gradually collapses with synthesis time until a completely amorphous compound is formed. The stability of these materials can be increased by lowering the synthesis temperature, but zeolites start to crystallize only after the complete degradation of the layered structures. Even though the synthesis conditions, the gel compositions, and the structure of the final zeolite are different, the two layered materials seem to possess very similar structures, the main difference resulting from the nature of the interlayer organic molecules. These intermediate compounds display interesting intercalation properties toward long alkylammonium cations. Silica layers can be swollen at low temperature without any modification of their crystallinity. Spectroscopic characterization of the materials as well ais their evolution over the whole crystallization period tends to indicate that zeolites are not formed via layered intermediates, as was previously claimed. Actually, these solids only provide the synthesis medium with silicate and aluminosilicate species necessary for the crystallization of the zeolite, whose structure is directed by the organic species present in the solution.