Crystallization mechanism of all-silica zeolite beta in fluoride medium

Abstract

The crystallization mechanism of pure silica zeolite beta in a fluoride medium has been investigated through the characterization of samples obtained at different synthesis times. The crystallization takes place in the presence of tetraethyl ammonium and fluoride ions at neutral pH, following a non-conventional mechanism based on the reorganization of an amorphous gel phase. The solid yield, referenced to the silica weight, remains almost constant and close to 100% during the whole crystallization process. Likewise, the amount of TEA + and F − ions present in the solid is not significantly changed as its crystallinity varies from 0% to 100%. The amorphous material initially observed is formed by a gel phase consisting of non-isolated primary units around 10–30 nm size distributed throughout the gel phase. The first crystals detected in the system are very large, with sizes around 7 μm. These crystals growth directly from the amorphous solid phase through a process of aggregation and densification of the primary units. The crystals finally observed in highly crystalline samples have sizes around 14 μm. This crystallization mechanism based on solid–solid transformations is probably favored by the low solubility of the silica species in the fluoride medium under neutral pH.