Monte Carlo calculations of Xe arrangement and energetics in the NaA alpha cage

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Abstract
The canonical ensemble Monte Carlo technique is used to calculate density distributions and internal energies of Xe trapped in the alpha cage of zeolite NaA. This cage is modeled as an assembly of ions positioned in accord with crystallographic data for the dehydrated crystal. We explore the effects that cage chemistry, adsorbate loading, and temperature have on the structure and energy of the adsorbed phase. In order to examine the effect of slight changes in the detailed cage structure, we compare current results with those of an earlier study in which a simplified cage structure was used. Intense Xe density maxima are established at discrete adsorption sites, the locations of which change in a discontinuous way as loading increases. The arrangement of these sites depends on the number and position of the charge balancing cations. The sites are arranged at the vertices of a cuboctahedron for the alpha cage of the zeolite NaA, but they make an octahedron for a dealuminated cage (Si/Al = 2·0). When the number of Xe atoms exceeds the number of sites available in the original arrangement, a new polyhedron is formed. The arrangement of the adsorption sites affects the degree of site-site interaction, which must be accounted for in adsorption theories. Trends in the potential energy with loading can be correlated to changes in the Xe density arrangement. The maximum occupancy for which adsorption is energetically favoured in this study is much greater than that found previously for a cage idealized as a truncated cuboctahedron; this observation shows that the isotherm is extremely sensitive even to slight zeolite structural changes.

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