Molecular dynamics simulations of cations and hydrofluorocarbons in faujasite-type zeolites.

摘要

We have developed and applied a new force field for the simultaneous movement of cations and hydrofluorocarbons (HFCs) in zeolites, which explicitly distinguishes Si and Al atoms, as well as different types of oxygen in the framework. The aim of the work is to model three interesting phenomena occurring upon adsorption of HFCs in cationic zeolites: the separation of isomers HFC-134 and HFC-134a at low loadings; the preference upon adsorption of the gauche conformer of HFC-134 as opposed to the gas-phase preferred trans conformer; and the cation migration that takes place in Na-Y after the adsorption of HFCs. Energy minimizations and molecular dynamics simulations done with this force field give excellent agreement with experimental data on heats of adsorption, guest-host distances, cation positions, infrared spectra and conformer ratio for different loadings of HFC-134 and HFC-134a in Na-X and Na-Y. Energy minimizations show that Na cations in site I are not at the center of hexagonal prisms, but rather in one of two I sites displaced symmetrically by about 0.6 A along the [111] direction. The force field also accounts partially for the observed cation migration, which is the result of the large host-guest interactions. These interactions are strong enough to draw cations into the supercages despite originating sites (I′ ) been energetically preferred to the arriving sites (III′ ). This migration occurs in a two-step mechanism that involves cations from several sites. In the preferred binding site, HFCs are anchored by a site II cation and a site III′ cation. Many other less energetic binding sites are observed, in which HFCs are anchored by only one cation or by just hydrogen bonds, as recently proposed. The HFC separation capacity of the zeolite is ascribed to the preferential adsorption of HFC-134 compared to HFC-134a, and their competition for binding sites in the zeolite. The main contribution to the heat of adsorption in Na-X and Na-Y is by far the host-guest interaction energy followed by the guest-guest attraction. In Na-Y, there are additional contributions that arise from the cation migration. The most important of those are the decreases in Na-Na electrostatic repulsion and in Na-O electrostatic attraction. This last contribution has a sign opposite to the other contributions. The binding energy for the gauche conformer of HFC-134 is larger than for trans at low loadings, but as loading increases, the difference MD simulations at 300 K show that most cations and adsorbed HFCs are immobile in NaX and NaY on the MD time scale. A small-amplitude cation motion was observed in bare zeolites Na-X and Na-Y. This motion was highly correlated and involved several cations vibrating simultaneously between sites I and I′, II and II′ and III and III′. This motion is quenched upon adsorption of HFCs, becoming less rapid and uncorrelated. Most of the HFCs remain in the adsorbed sites during the MD calculations and only those bound to the framework by just hydrogen bonds can migrate to other supercages.