Structure and properties of ITQ-8: a hydrous layer silicate with microporous silicate layers

Abstract

ITQ-8 is a new hydrous layer silicate (HLS) with a chemical composition of [C₄H₈(C₇H₁₃N)₂]₈ [Si₆₄O₁₂₈(OH)₁₆]·48H₂O per unit cell. The synthesis of ITQ-8 was first described in 2002 by Díaz-Cabañas et al., the structure of this material, however, remained unsolved at that time. Physico-chemical characterization using solid-state NMR spectroscopy, SEM, TG-DTA, and FTIR spectroscopy confirmed that ITQ-8 is a layer silicate. The XRD powder pattern was indexed in the monoclinic system with lattice parameters of a₀ = 35.5168(5) Å, b₀ = 13.3989(2) Å, c₀ = 16.0351(2) Å, β = 106.74(2)°. The crystal structure was solved by simulated annealing. Rietveld refinement of the structure in space group C2/c converged to residual values of R_Bragg = 0.023, R_F = 0.022 and chi² = 2.3 confirming the structure model. The structure of ITQ-8 contains silicate layers with a topology that resembles a (11–1) section of the framework of zeolite levyne. So far, this layer topology is unique among layer silicates. The layer can be regarded as made up of 4-, 6-, double-six and 8-rings which are interconnected to form cup-like “half-cages”. Unlike other HLSs, which possess impermeable silicate layers, ITQ-8 contains 8-rings pores with a free diameter of 3.5 Å × 3.4 Å and can be regarded as a “small-pore layer silicate”. In the crystal structure, the organic cations, 1,4-diquiniclidiniumbutane, used as structure directing agents during synthesis are intercalated between the silicate layers. Clusters (bands) of water molecules which are hydrogen bonded to each other and to the terminal Si–OH/Si–O⁻ groups are located between the organic cations and interconnect the silicate layers. ITQ-8 is a very interesting material as precursor for the synthesis of microporous framework silicates by topotactic condensation or interlayer expansion reactions leading to 3D micro-pore systems which may be useful in applications as e.g. catalysts, catalyst supports and adsorbents of for separation.