Synthesis of faujasite zeolites with crown-ether templates

Abstract

The function of crown ethers in the synthesis of faujasite zeolites has been studied. Hexagonal faujasite (EMT) was prepared from 18-crown-6 ether and not from 15-crown-5 ether as the organic template under hydrothermal conditions, whereas cubic faujasite (FAU) was obtained from a reaction gel using either 18-crown-6 ether or 15-crown-5 ether. During the early stages of synthesis, 18-crown-6 ether and 15-crown-5 ether were partially occluded in the aluminosilicate hydrogel and EMT or FAU nuclei, but 18-crown-6 ether was only adsorbed on the surface of the hydrogel for FAU crystals. Thermal analysis, Raman spectroscopy and composition analysis measurements indicate that sodium cations play a more important role than 18-crown-6 ether in directing the formation of FAU nuclei and balancing the charge on the framework of FAU when 18-crown-6 ether is used at the organic template. The stacking arrangement and Al content of the synthesized faujasite zeolites were found to depend on the SiO₂ : Al₂O₃ and Na₂O : crown ether molar ratios of the reaction mixtures. The structure-directing role of the crown ether in the formation of faujasite zeolites was also studied by theoretical calculations.