ARTICLES Study of Type-A Zeolites. Part 1: Mechanism of He and Ne Encapsulation Yacov Finkelstein, Avraham Saig, Albert Danon, and Jacob E. Koresh* Chemistry DiVision, Nuclear Research Center of the NegeV, Beer-SheVa 84190, Israel ReceiVed: March 13, 2003; In Final Form: June 26, 2003 The mechanism of ambient pressure encapsulation of He and Ne in the R and  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  cages of the Sodalite units, whereas the K-exchange form (3A) offers both R and  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  cages. Introduction Controlling the accessibility of crystalline voids to inert gases and selectively entrapping them is an obvious desirable require- ment from both experimental and applied perspectives. Ad- ditional advantage is gained if such confinement is achieved in a reversible, stable fashion and under nondestructive, as ambient as possible, conditions. For that matter, zeolite molecular sieves, 1-3 which are silicoaluminate crystalline frameworks of extremely regular pore structure and uniform small characteristic apertures, may be considered as potential candidates. In particular, most promising is the synthetic type-A class, available in three grades uniquely differing from one another by their molecular sieving properties. Type 4-, 3- and 5-A, respectively relating to the Na and its K- and Ca-exchanged forms, correspondingly admit species with effective kinetic diameters of up to roughly 4, 3, and 5 Å. Structurally, these microporous crystalline structures are considered as rigid sponge construc- tions, capable of imbibing large amounts of molecules that are small enough or of the right shape, to penetrate the intracrys- talline pores, yet excluding molecules having the “wrong” sizes or shapes. The sieving and selectivity properties of zeolites are strongly related to the presence of exchange cations in the framework and their locations therein. They may occupy sites adjacent to apertures located between one void and the next and, according to their charge, number, size, and location, act as sentinels, effectively barring passage of larger molecules while allowing passage of smaller ones. This familiar sieving property is thus very sensitive to the size of the cationic constituents relative to the dimensions of the zeolite windows. Type A zeolite exhibits two classes of regular voids. The smaller cavities ( cages), having the structure of Sodalite units, can be entered only through their eight O 6 -ring faces (i.e., six oxygens ringed in a plane) of ∼ 2.3 Å free diameter. 4-7 The second class (R cages) possess larger cavities comprising six O 8 circumferential windows and share eight O 6 windows of eight joint  cages. Every window of each R cage is partly blocked by a sentinel counterion, 7-9 due to which its effective aperture free dimension, and thus the access to its inner available free volume, varies between 3 and 5 Å, whence the terms 3A, 4A, and 5A. The phenomenon of type A zeolitic encapsulation was studied to some detail in relation to the potential of molecular sieves to serve as storage mediums for inert gases. 6,9-16 Sorption proce- dures were performed in these studies under practical vitrifying conditions, between 300 and 800 °C and 40 to 1000 bar, at which the zeolite irreversibly turns into an amorphous glassy form with an almost complete loss of its specific surface area. 14 We have recently reported on the first observation of a reversible room-temperature encapsulation of He and Ne in the  cages of NaA following a nondestructive sorption procedure taken at atmospheric pressure and around 200 °C. 17 The  cages were shown to act as tunable gates for a selective and reversible admission and efficient entrapment of He and Ne. The present study complements the previous one with explicit detailed measurements of He and Ne in 3A and 5A, providing a broader insight into the overall physical picture of the mechanism ruling the phenomena of encapsulation in zeolites. Principle of the TPD-MS-SMB Method In temperature programmed desorption mass spectrometry (TPD-MS), desorption is generated by programmed heating of * Corresponding author. 9170 J. Phys. Chem. B 2003, 107, 9170-9174 10.1021/jp034644f CCC: $25.00 © 2003 American Chemical Society Published on Web 07/30/2003