LOCATION OF META-XYLENE IN BAX ZEOLITE AND MODEL FOR THE FILLING OF THE SUPERCAGES

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 f or the filling of the supercages. For up to two molecules per supercag e, strong interactions are observed between the aromatic ring and the Ba-2+ cation (d = 2.44(1) angstrom), 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 Ba-2+ catio ns than at low coverage (d = 2.75(1) angstrom), the interactions betwe en the methyl groups and the framework are weakened, and a second crys tallographic site for the xylene is observed. This site has lower occu pancy and apparently is the less favoured one since the aromatic ring is 2.90(3) angstrom from the Ba-2+ cation. From these results, we demo nstrate that the crystallographic arrangement of the sorbate is strong ly dependent on intermolecular interactions. When coverage increases, different molecular orientations are adopted in order to maximize meth yl-methyl distances and thereby minimize the intermolecular repulsion.