Molecular Dynamics Studies of Hydrofluorocarbons in Faujasite-type Zeolites: Modeling Guest-Induced Cation Migration in Dry Zeolites

We have developed and applied a new force field for simultaneously modeling the dynamics of hydrofluorocarbons (HFCs) and exchangeable Na cations in faujasite-type zeolites. Our aim is to account for (i) the zeolite’s capacity of separating HFC isomers, (ii) the experimentally observed unusual cation migration in Na-Y from the â-cages into the supercages upon the adsorption of HFCs, and (iii) the abnormal trans/ gauche ratio in these systems. Energy minimizations and molecular dynamics simulations performed with this force field give excellent agreement with experimental data on heats of adsorption, guest-host distances, infrared spectra, and conformer ratios for different coverages of HFC-134 (CF2H-CF2H) and HFC-134a (CH2F-CF3) in Na-X (Si:Al ) 1.2) and Na-Y (Si:Al ) 2.4). The force field also accounts partially for the observed cation migration at intermediate loadings and fully at high loadings. The extent of migration is found to be influenced by the competition among Na-O, Na-F, and Na-Na interactions. The Na-O interaction disfavors migration because Na(I′) sites are found to be energetically more favorable than Na(III′) sites; the Na-F attraction obviously favors migration; and surprisingly, the Na-Na repulsion also favors migration because moving Na cations into supercages leads to better cation dispersion. This migration occurs in a two-step mechanism that involves first a concerted two-cation jump, S I′ f SII′/S II f (SIII or S III′), followed by a S II′ f SII jump, leading to a net process S I′ f (SIII or S III′). The preferred binding site in both Na-X and Na-Y involves HFCs anchored by both site II and site III′ cations. The loading dependence of the heat of HFC adsorption in zeolite Na-X is predicted to be different from that in Na-Y, because of the energetics of cation migration in Na-Y. HFC-134 is generally more strongly bound to both zeolites, because of its ability to make close Na-F and O-H contacts with the zeolites. The binding energy for the gauche conformer of HFC-134 is larger than that for trans at low loadings, but as loading increases, the difference decreases. The highly correlated small-amplitude motion predicted for cations in bare faujasites is quenched upon adsorption of HFCs. Most of the HFCs are too strongly bound to exhibit diffusive behavior during our molecular dynamics simulations.