RUB-55, a new hydrous layer silicate with silicate layers known as motives of the sodalite and octadecasil frameworks: Synthesis and crystal structure

摘要

The new hydrous layer silicate, RUB-55, was synthesized at 160 °C by hydrothermal synthesis from a reaction mixture of SiO _2/ tetramethylammonium hydroxide/ H _2O. The idealized chemical composition of as-made RUB-55 is [(CH _3) _4N] _2[Si _(10)O _(18)(OH) _6] · 16H _2O. The XRD powder pattern was indexed in tetragonal symmetry with lattice parameters of a _0 = 8.8? and c _0 = 14.6?. Physicochemical characterization using solid-state NMR spectroscopy, SEM, TG-DTA, and DRIFT spectroscopy confirmed that RUB-55 is a hydrous layer silicate (HLS). The crystal structure was solved from model building. Rietveld structure refinement of the average structure in space group P42 _1m converged to residual values of R _(Bragg) = 0.029 and R _F = 0.032 confirming the structure model. The structure of RUB-55 contains silicate layers with a topology that resembles the (001)-section of the sodalite framework as well as a (001)-section of the octadecasil framework. The layer can be regarded as made up by interconnected open, cup-like cavities. These "half-cages" are occupied by tetramethylammonium (TMA) ions which were used as a structure directing agent (SDA) during synthesis. A sheet of hydrogen bonded water molecules is intercalated between the silicate layers. The structure analysis also clearly revealed the presence of very strong hydrogen bonds between the terminal O atoms (silanol/siloxy groups) confirmed by ~1H MAS NMR and DRIFT spectroscopy. RUB-55 is structurally and chemically related to the layer silicates RUB-15, HUS-1 and β-helix- layered-silicate.