The mechanism of ambient pressure encapsulation of He and Ne in the R and β crystalline cages of type-A zeolites is demonstrated. Reversible and highly selective gas admission and entrapment are readily achieved at characteristic temperatures occurring between 77 and 570 K. The permeability of the zeolitic windows is governed by an interplay between the critical diameter of the encapsulate and the effective apertures dimension, which is shown to be strongly dependent on temperature. The blocking state of the zeolitic apertures is determined by a simultaneous thermal activation of both cation mobility and structural dilation/constriction of crystalline windows. Encapsulation in NaA (4A) principally occurs in the β cages of the Sodalite units, whereas the K-exchange form (3A) offers both R and β encapsulations. The effective free aperture dimension of the Ca exchange form (5A) is found to be too large to allow a practical gas enfoldment in either class of cavities, even at 77 K, where only poor encapsulation is observed. The counterion location vs size dependence, known only from crystallographic data, is sensed here for the first time by an encapsulation process, via the manifestation of different aperture occupancy states. While the blocking extent of the wider O 8 windows of the R cages is consistent with the size of exchangeable cations, a reverse correlation is evident for the narrower O 6 windows of the β cages.

The He͑Ne͒/NaA-zeolite system was studied using temperature programmed desorption mass spectrometry ͑TPD-MS͒ with a supersonic molecular-beam inlet. Controllable, stable, and reversible entrapment of He and Ne by the ␤ cages of NaA zeolite was experimentally achieved at ambient pressure and around 200°C. Decapsulation of either He or Ne from NaA is shown to be of a doublet character, indicating on the occurrence of effectively two classes of ␤ cavities: completely blocked cages, never previously observed, and partly blocked ones. The encapsulation of Ne and He in NaA is associated with the coupling of two reversible mechanisms governing the effective free aperture dimension, i.e., apertures thermal dilation and activated ion mobility. Characteristic admission temperatures between 130°C and 200°C, show highly selective sieving effect between He and Ne, suggesting its potential utilization for gas separation via a temperature swing practice and for a possible experimental realization of quantum sieving.

The adsorption-desorption pattern of He and Ne from amorphous carbon molecular sieve fibers (CMSF) was found to be governed by an encapsulation mechanism. He and Ne undergo reversible and efficient entrapment by the micropores of CMSF. Selective adjustment of pore openings to meet the critical dimensions of He and Ne which allow their admission is achieved via two principal steps: (i) the well-known pore widening by means of an irreversible removal of surface oxide groups upon evacuation at elevated temperatures and (ii) regulation of the effective pore opening via reversible thermal dilation/contraction. The occurrence of a markedly high activation energy, which previously could not be solely attributed to pure adsorption, is now understood as that needed for overcoming the energetic barrier imposed by the thermally constricted pores. The present results imply that the encapsulation phenomenon can considerably affect dead volume measurements in porous materials.

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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 O8 and O6 zeolitic windows, which in turn, governs gas accessibility to the α 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 O6 apertures, a two-stage effect is observed for the wider O8 windows. Upon dehydration at large water contents, O8 apertures are reduced; then, beyond a critical extent of dehydration, the net effect is reversed, widening the O8 windows.