Structure of strontium ion-exchanged ETS-4 microporous molecular sieves

The structure of strontium ion-exchanged ETS-4 titanosilicate has been refined from X-ray powder diffraction data and compared to the structure of sodium ETS-4. The framework of ETS-4 is highly faulted in two directions and can be described as an intergrowth of four polymorphs. Despite the faulting, both materials have open 8-ring channels in the b direction. Faulting probabilities in the a and c directions close to 50% allow the structures to be modeled using a superposition of the possible polymorphs for the purposes of Rietveld refinement. While the sodium ions in Na-ETS-4 are found to be distributed over two different cation sites, the ions in Sr-ETS-4 are found close to the same positions with the strontium ions selectively occupying the cation site coordinated to the chain-bridging titanium leaving unexchanged sodium ions in the 6-ring cation site. The chain-bridging titaniums in Sr-ETS-4 were found to be five-coordinated in square-pyramidal polyhedra, as indicated by an occupancy of the apical oxygen of 1.03 oxygens per unit cell and a Ti-O bond distance of 1.75 +/- 0.04 Angstrom to the apical oxygen. The ideal formula for Sr-ETS-4 was determined to be NaSr4Si12Ti5O38(OH). 12H(2)O with lattice constants a = 23.1962(12) Angstrom, b = 7.23810(33) Angstrom, c = 6.96517(31) Angstrom, alpha = beta = gamma = 90 degrees in the Cmmm space group. Site ordering of the cations and the presence of five-coordinated titanium may help understanding the recently reported methane/nitrogen gas separation properties of this new molecular sieve.