Location of meta-xylene in BaX zeolite and model for the filling of the supercages

Abstract

The structure of BaX zeolite containing sorbed deuterated meta-xylene has been studied for two different coverages at 11 K by powder neutron diffraction. From the two refinements, we suggest a molecular model for the filling of the supercages. For up to two molecules per supercage, strong interactions are observed between the aromatic ring and the Ba²⁺ cation (d=2.44(1) Å), and between the methyl groups and the framework oxygens. When coverage is increased above two molecules per supercage, a significant molecular rearrangement is observed. The aromatic molecules reorient at a larger distance from the Ba²⁺ cations than at low coverage (d=2.75(1) Å), the interactions between the methyl groups and the framework are weakened, and a second crystallographic site for the xylene is observed. This site has lower occupancy and apparently is the less favoured one since the aromatic ring is 2.90(3) Å from the Ba²⁺ cation. From these results, we demonstrate that the crystallographic arrangement of the sorbate is strongly dependent on intermolecular interactions. When coverage increases, different molecular orientations are adopted in order to maximize methyl-methyl distances and thereby minimize the intermolecular repulsion.