NEUTRON-DIFFRACTION AND COMPUTATIONAL STUDY OF ZEOLITE NAX - INFLUENCE OF SIII' CATIONS ON ITS COMPLEX WITH BENZENE

The crystal structure of zeolite NaX (Si:Al = 1.2) and that of its com plex with benzene have been determined at 5 K by Rietveld analysis of powder neutron diffraction data in space group Fd3. For NaX, a = 25.03 28(5) Angstrom, R-wp = 4.25%, and R-p = 3.27%. For NaX + benzene, a = 25.0496(7) Angstrom, R-wp = 4.60%, and R-p = 3.52%. The sodium ions we re located preferentially in sites SI' and SII, both of which are full y occupied. The remaining cations were found at the SIII' site in the 12-ring window. These cations are coordinated by two oxygen atoms, O(1 ) and O(4), in a corner of the 4-ring window where the aluminum is loc ated. Our simulations on cation positioning in NaX (Si:Al = 1) predict the preferred occupancy of SIII' sites that are facing AlO4 tetrahedr a and support our crystallographic results. Less than half the benzene molecules were found at the six-ring window in the supercage, where t hey interact with the SII cations. No benzene was found in the 12-ring window, presumably because of the presence of the SIII' sodium cation s, but there remains a possibility that undetected benzene is adsorbed near these cations.