Crystal Structure of a Helix Layered Silicate Containing Tetramethylammonium Ions in Sodalite-Like Cages

• Published in: Chemistry of materials Vol. 13; no. 4; pp. 1286 - 1295
• Main Authors: Ikeda, T, Akiyama, Y, Izumi, F, Kiyozumi, Y, Mizukami, F, Kodaira, T
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 16.04.2001
• Subjects: Chemistry; Colloidal state and disperse state; Condensed matter: structure, mechanical and thermal properties; Exact sciences and technology; General and physical chemistry; Inorganic compounds; Physics; Porous materials; Salts; Structure of solids and liquids; crystallography; Structure of specific crystalline solids; Inorganic compounds; Organic cation; Metastable phases; Porous materials; XRD; Silicates; Experimental study; Charge distribution; Quaternary ammonium compound; Microporosity; Lamellar structure; Open framework; Hydrogen bonds; Polysilicates; Helical structure; Hydroxosilicates; NMR spectra; Electron density; Sodium compounds; Hydrates; Crystal structure
• ISSN: 0897-4756; 1520-5002
• DOI: 10.1021/cm000736v

The crystal structure of a helix layered silicate (HLS), [(CH3)4N]2Na2[Si10O20(OH)4]·5.53H2O, with tetramethylammonium (TMA) ions as templates was determined by ab initio structure analysis with X-ray powder diffraction data. The HLS is orthorhombic with space group Amm2 and lattice parameters of a = 22.8641(2) Å, b = 12.5388(2) Å, and c = 12.4648(2) Å. A Q3−Q4−Q3 silicon network with an amount-of-substance ratio of Si/O = 1:2.4 exhibits cup-shaped cage topology with four- and six-membered rings. Such a cage is similar to a sodalite cage divided into two pieces. 29Si MAS NMR showed the silicate layer to have Q3- and Q4-types of Si atoms with a Q3/Q4 ratio of ca. 4.0, which is consistent with our structural model. The framework is distorted, with Si−O bond lengths varying widely. TMA ions are incorporated into the cup-shaped cage. Na+ ions and H2O molecules are also located between two silicate layers, with interlayer distances varying alternately. Disordering of TMA ions, Na+ ions, and H2O molecules was clearly visualized in electron-density maps obtained by combining a maximum-entropy method and whole-pattern fitting. H2O molecules surround Na+ ions and form hydrogen bonds with O atoms in silanol groups in the silicate layers, by which the lattice instability due to the distorted framework is compensated. The present compound is regarded as a metastable phase, as would be expected for precursors to new types of microporous materials with silicate frameworks such as the sodalite cage.