Seed-assisted crystallization of high-silica cubic and hexagonal faujasite polymorphs in the presence of the tetraethylammonium (TEA+) cation

Abstract

Silica-rich Y (FAU topology) and EMC-2 (EMT topology) zeolites have been obtained in the presence of the tetraethylammonium (TEA⁺) cation as a structure-directing molecule and the corresponding protonic zeolites as seeds. In the presence of USY seeds, Y zeolites were fully crystallized after a few hours at 130 °C but they were unstable and rapidly converted into chabazite, a zeolite with CHA topology. Their stability could nonetheless be greatly improved by decreasing the temperature to 110 °C without significantly modifying the morphology of the crystals and the composition of the framework. Framework SiO₂/Al₂O₃ ratios up to 13 could be obtained, which is the highest value ever reported for Y zeolites synthesized with TEA⁺ cations as the sole organic molecule. The use of H-EMC-2 as seeds generally led to FAU–EMT intergrowths in which the relative proportion of EMT depended on the composition of the seeds. However, a pure EMT zeolite could be obtained under specific conditions. With a framework SiO₂/Al₂O₃ ratio of 11.2, it constitutes the first example of Si-rich EMT obtained in the absence of crown ether from conventional silica and alumina sources.