Chemistry of the calcination of 15-crown-5 and 18-crown-6 ethers occluded in faujasite polytype zeolites

Abstract

The decomposition of sodium 18-crown-6 and 15-crown-5 ether complexes, occluded in FAU and EMT zeolite cages, under oxidative and inert atmospheres, has been studied using thermoanalysis (TG–DTA), IR and mass spectroscopy. At ca. 373 K, the Na–crown ether complex is hydrolysed yielding a bridging hydroxy group in the zeolite framework and a neutral [Na–crown ether]⁺ OH^– species. This bridging hydroxy group, with a stretching vibration frequency of 3608 cm^–1, is generated at an unusually low temperature. This heterolytic splitting of water molecules is most pronounced for sodium 18-crown-6 complexes occluded in EMT zeolites. Based on in situ IR spectra of adsorbed degradation products and MS analysis of desorbing products, the decomposition chemistry of the crown ether complexes is established. In an oxidative atmosphere, the crown ethers degrade to aldehydes via an alkenic intermediate, and oxidize further to acetic acid and finally to carbon oxides. In an inert atmosphere, acetaldehyde is formed, desorbing at elevated temperatures.