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

Abstract

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 ‚-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 )i n 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