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 complex with benzene have been determined at 5 K by Rietveld analysis of powder neutron diffraction data in space group Fd3. For NaX, a = 25.0328(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 were located preferentially in sites SI' and SII, both of which are fully 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 located. Our simulations on cation positioning in NaX (Si:Al = 1) predict the preferred occupancy of SIII' sites that are facing AlO4 tetrahedra and support our crystallographic results. Less than half the benzene molecules were found at the six-ring window in the supercage, where they interact with the SII cations. No benzene was found in the 12-ring window, presumably because of the presence of the SIII' sodium cations, but there remains a possibility that undetected benzene is adsorbed near these cations.