Molecular Dynamics simulations have been carried out in NaX and NaY Faujasite systems to deepen understanding of the cation rearrangement during the CO2 adsorption process suggested by our recent diffusivity measurements. This study is a major contribution since the rearrangement of the cations in Faujasite, the most promising adsorbent for CO2 storage, can represent a significant breakthrough in understanding the adsorption and diffusion processes at the mircroscopic scale. For NaY, it has been shown that at low and intermediate loadings, SII cations can migrate toward the center of the supercage due to strong interactions with the adsorbates, followed by a hopping of SI'cation from the sodalite cage into the supercage to fill the vacant SII site. The SI cations are only displaced at a higher loading, leading to cation de-trapping out of the double six rings into the vacant SI' sites. For NaX, the SIII' cations which occupy the most accessible adsorption sites move significantly upon coordination to the carbon dioxide molecules. The SI' and SII cations remain consistently located in their initial sites whatever the loading. Indeed, the most probable migration mechanism involves SIII' cation displacements into nearby vacant SIII' sites.

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Molecular dynamics simulations have been carried out to address the question of cation migration upon adsorption of methanol in NaY and NaX faujasite systems as a function of the loading. For NaY, it has been shown that, at low and intermediate loadings, SII cations can migrate toward the center of the supercage due to strong interactions with the adsorbates, followed by a hopping of SI' from the sodalite cage into the supercage to fill the vacant SII site. A SI' cation can also migrate across the double six ring and takes a SI' vacant position. SI cations mainly remain trapped in their initial sites whatever the loading. At high loading, only limited motions are observed for SII cations due to steric effects induced by the presence of adsorbates within the supercage. For NaX, the SIII' cations which occupy the most accessible adsorption sites are significantly moving upon coordination to the methanol molecules; the extent of this mobility exhibits a maximum for 48 methanol molecules per unit cell before decreasing at higher loadings due to steric hindrance. In addition, the SI' and SII cations remain almost trapped in their initial sites whatever the loading. Indeed, the most probable migration mechanism involves SIII' cation displacements into nearby SIII' sites.

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This paper describes a computational study of the mixed metal fluorides LiCaAlF 6 and LiSrAlF 6 , which have potential technological applications when doped with a range of elements, especially those from the rare earth series. Potentials are derived to represent the structure and properties of the undoped materials, then defect properties are calculated, and finally solution energies for rare earth elements are calculated, enabling preferred dopant sites and charge compensation mechanisms to be predicted.

Molecular dynamics simulations were performed in order to obtain a detailed understanding of the self-diffusion mechanisms of methanol in the zeolite NaY system. We derived a new force-field term to describe the interactions between the methanol molecules and the extraframework cations. From the simulations, we show that diffusive behavior in the high-temperature range consists of a combination of both short- and long-range motions at low and intermediate loadings. This type of motion is characterized by an activation energy that decreases as the loading increases. At low loadings, we also observe short-range diffusive behavior based on a surface-mediated mechanism. The short-range behavior corresponds to motion only on the length scale of an FAU supercage, whereas the long-range behavior involves intercage diffusion. For the saturation loading corresponding to 96 methanol molecules per unit cell, only short-range motions within the same supercage predominate. Finally, the preferential arrangement of the adsorbate molecules around the extraframework cations are examined and compared with those previously deduced from experimental data.

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