Adsorption of chlorofluorocarbons in nanoporous solids;: a combined powder neutron diffraction and computational study of CFCl3 in NaY zeolite

The crystal structure of NaY (Na54Si138Al54O384)+ CFCl3 has been determined at 20 K by Rietveld analysis of powder neutron diffraction data in space group Fd (3) over barm. The sodium cations were located preferentially in sites II, which are fully-occupied and also in sites I, which are half-occupied. A new cation position in the beta-cages was detected around x = 0.125, y = 0.125, z = 0.031. Important cation migration is observed upon adsorption of CFCl3, as shown by the absence of cations in sites I', known to be a favorable cation site in bare NaY, together with the detection of this new site in the beta-cages. Part of extraframework cations remained undetected, that were assumed to be located in the supercages in low symmetry sites. The CFCl3 molecules are found in the 12-ring windows, where they accomodate Cl...O-zeolite and F...O-zeolite van der Waals interactions together with Cl...Na(II) electrostatic interactions. (N,V,T) Monte Carlo simulations were performed in order to elucidate the role of extraframework cations in the adsorption of CFCl3 in NaY. They show that the presence of Na ions in the 12-ring window (sites III') has to be taken into account in order to get adsorption geometries consistent with that obtained from neutron diffraction.