Study of Type-A Zeolites. Part 2: Effect of Dehydration on the Effective Aperture Dimension

Abstract

A new insight into the role played by water molecules in the crystalline framework of type-A zeolites is demonstrated. The effect of dehydration on the effective free aperture dimension, D f , is studied by utilizing temperature-programmed decapsulation of He and Ne. The interplay between the various known mechanisms governing D f is directly sensed by the decapsulation behavior of differently sized inert atoms. The results are qualitatively interpreted using pure dimensional considerations, revealing the occurrence of a strict relation between the content of water molecules and the redistribution of zeolitic cations in determining D f . Water content is shown to have a strong effect on the blocking state of the O 8 and O 6 zeolitic windows, which in turn, governs gas accessibility to the a and β cages. It is proposed that D f is regulated via a subtle balance between two coupled principal mechanisms. One involves direct lattice adjustments, where two opposite sub-mechanisms seem to play competitive roles. Removal of water molecules by simultaneous heating and pumping enlarges D f , while hydroxyl groups elimination from within the zeolitic channels results in their partial collapse, thus reducing D f . The second mechanism involves the regulation of aperture blocking via relocations of the counterions initiated by increasing vacancies due to removal of water molecules. While dehydration continuously contracts the O 6 apertures, a two-stage effect is observed for the wider O 8 windows. Upon dehydration at large water contents, O 8 apertures are reduced; then, beyond a critical extent of dehydration, the net effect is reversed, widening the O 8 windows.