STRUCTURAL EVIDENCE FOR PI-COMPLEXES IN CATALYTICALLY ACTIVE Y-ZEOLITES WITH ORTHO-XYLENE, META-XYLENE, AND PARA-XYLENE

The structures of Yb,Na-Y zeolites containing sorbed perdeuterated xylenes have been studied for two different xylene converages at 5 K by powder neutron diffraction. The space group is Fd3mBAR. The ytterbium ions are located in the sodalite cage on position I' or in the hexagonal prism on position I. The sodium ions are all located on position II' in the six-ring window shared by the sodalite and the supercages. The molecules are located in the supercage, the plane of the aromatic ring being perpendicular to the <111> axis with short contacts to the Na+ cations on II'. The para and ortho isomers are statistically disordered with respect to the 6-fold axis of the ring, and are equally distributed over these orientations, whereas m-xylene molecules occupy only three of these orientations due to interactions of the methyl groups with the framework oxygens. A second crystallographic site was found occupied by the para and ortho isomers at low coverages. It is the less favored one: the molecules are at longer distances from the Na+ cations and at no distance shorter than 0.32 nm from the framework. Even at high coverages this second site is less occupied. Not all the adsorbed molecules, however, can be located in the high-coverage samples. The powder diffraction pattern appears to indicate a liquidlike phase of xylenes within the cavities.