Layered intermediates in zeolite synthesis: Are structures related?

Layered organic-inorganic compounds have been found during the crystallizat ion of zeolites ZSM-48 and beta with hexamethonium and tetraethylammonium c ations, respectively. These materials are obtained from gels with relativel y high concentrations in organic species but are not stable under hydrother mal conditions. While highly crystalline solids are formed after short synt hesis periods, their structure gradually collapses with synthesis time unti l a completely amorphous compound is formed. The stability of these materia ls can be increased by lowering the synthesis temperature, but zeolites sta rt to crystallize only after the complete degradation of the layered struct ures. Even though the synthesis conditions, the gel compositions, and the s tructure 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 d isplay interesting intercalation properties toward long alkylammonium catio ns. Silica layers can be swollen at low temperature without any modificatio n of their crystallinity. Spectroscopic characterization of the materials a s well ais their evolution over the whole crystallization period tends to i ndicate that zeolites are not formed via layered intermediates, as was prev iously claimed. Actually, these solids only provide the synthesis medium wi th silicate and aluminosilicate species necessary for the crystallization o f the zeolite, whose structure is directed by the organic species present i n the solution.