Energetics and structures of fluoro- and chlorofluorocarbons in zeolites: Force field development and Monte Carlo simulations

Canonical Monte Carlo simulations on the adsorption of a series of fluoro-, chlorofluoro-, and hydrofluorocarbons (CF(4), CF(3)Cl, CF(2)Cl(2), CFCl(3), CHF(3)) in siliceous Y and NaY zeolites have been performed and are compared with available calorimetric data on the same host-guest systems. A new force field for fluorocarbon-type molecules in zeolites has been developed, and our (N,V,T) simulations predict adsorption heats with good accuracy. Further insights into the key features of host-guest interactions are gleaned from the relative contributions of the short-range and electrostatic interactions to the total adsorption heats and the analysis of host-guest pair functions. In siliceous Y, host-guest interactions are driven primarily by F ... O(zeolite) and Cl ... O(zeolite) van der Waals interactions, and H ... O(zeolite) hydrogen bonding in the case of hydrogen-containing fluorocarbons. When the fluorocarbon is adsorbed in a cation-containing zeolite, such as NaY, additional F ... Na(zeolite) electrostatic interactions with Na cations of the supercage are clearly revealed and control the orientation of the sorbate molecules within the supercages. In addition, (N,V,ir) simulations have enabled us to compare the behavior of CHF(3) with that of CHCl(3). Tnt: heats of adsorption at zero loading are very similar, but the relative contributions of the short-range and long-range interactions are inverted between the two systems. with the electrostatic term dominating in the case of the fluorocarbon.