Open-Framework Copper Titanosilicates
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Open-Framework Copper Titanosilicates Xiqu Wang, Lumei Liu, Lingbao Wang and Allan. J. Jacobson Department of Chemistry and Center for Materials Chemistry, University of Houston, Houston, TX 77204-5003 ABSTRACT Four open-framework copper titanosilicates have been synthesized by hydrothermal techniques. The crystal structure of Phase 1 determined from single crystal X-ray data is closely similar to that reported for the titanosilicate ETS-4 with ca. one third TiO5 pyramids randomly replaced by CuO4 squares. Phase 2 also has the ETS-4 structure but all the TiO5 pyramids are replaced by CuO4 squares which are not randomly disordered. Phase 3 with the composition Na8CuTi3Si16O43 has the same structure as the mineral narsarsukite, a titanosilicate closely related to the microporous titanosilicate ETS-10. Narsarsukite and ETS-10 have the same straight single chains of TiO6 octahedra which are partially substituted by CuO4 squares in 3. Phase 4 with the composition K4CuTiSi8O21 contains CuO4 squares and TiO5 tetragonal pyramids that crosslink a new type of silicate double layer to form an open framework. INTRODUCTION Microporous transition metal silicates may have important uses in diverse areas such as catalysis, ion-exchange and gas separations as recently demonstrated by the titanosilicate ETS-4. [1-3] We have been interested in metal silicate frameworks built by cross-linking 1- or 2dimensional silicate units with transition metal coordination polyhedra. By linking various silicate layers with [VO5] pyramids we obtained a number of novel microporous frameworks. [4] Using [CuO4] squares or [UO6] distorted octahedra to replace the [VO5] pyramids led to more open-framework structures. [5,6] In all these structures, only one transition metal was incorporated in the frameworks. Adding a second transition metal to the framework provides an additional route to tune the structure and properties of the materials. Therefore, we have been investigating synthesis of silicates incorporating more than one transition metal into the framework. As first results, four new copper titanosilicates (NaxH10-x)[Ti4(TiO)0.7Cu0.3Si12O38] (H2O)y (1), (Na9H)[Ti4CuSi12O38](H2O)6 (2), Na8CuTi3Si16O43 (3), and K4CuTiSi8O21 (4) are reported here. EXPERIMENTAL Synthesis For the synthesis of 1, 0.9 g silica gel (Sigma S-4883) was mixed with 0.6 g NaOH, 0.1 g Cu and 0.04 g Ti metal foil. The mixture was put in a Teflon tube with an open end. The tube was subsequently placed in a sealed Parr autoclave. The autoclave was filled with water to ~ 30% inner volume and the water was kept outside the Teflon tube. After sealing in air, the autoclave was heated at 245 ˚C for three days and then cooled to room temperature in air within four h. The product was vacuum-filtered, washed with water and dried in air. Compound 2 was synthesized in a similar way. The initial mixture consists of 0.9 g silica gel, 0.9 g NaOH, 0.1 g Cu and 0.2 g
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Ti metal foil, 0.5 ml 0.8 M Ti2(SO4)3 solution and 1 ml water. Compound 3 was similarly synthesized from a mixt
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