Microporosity of the amorphous aluminosilicate precursors of zeolites: The case of the gels of synthesis of mordenite

The amorphous aluminosilicates formed at the beginning of the hydrothermal synthesis of mordenite present a zeolite-like microporosity. The disorder of the aluminosilicate framework is witnessed by the absence of structural bands in IR spectra, the width of the 29Si MAS-NMR signals and the broad X-ray diffraction patterns. The microporosity of the hydrothermal gels can be easily overlooked as it completely collapses under evaporative drying. Aerogels formed by CO2 supercritical drying retains the structure of the parent gel and allows the microporosity to be detected and characterised.The role of CO2 in the stabilisation of the porosity is not limited to the suppression of the capillary forces in the drying process. The extraction by liquid CO2 of a fraction of the Na+ cations which compensate the charge of deprotonated silanols brings about the condensation of surface silanols, with a corresponding increase of the strength of the material.