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

Abstract

The crystal structure of NaY (Na₅₄Si₁₃₈Al₅₄O₃₈₄) + CFCl₃ has been determined at 20 K by Rietveld analysis of powder neutron diffraction data in space group Fdm. 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 β-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 β-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.