Synthesis and real structure of RUB-11, a novel high-density silica zeolite based on magadiite layers

Abstract

The discovery of new zeolite framework types plays an important role in producing new porous materials for applications such as adsorption, catalysis, separation, etc. RUB-11, a new all-silica zeolite with high density (2.11 g cm−3), was synthesised at 160 °C from reaction mixtures consisting of SiO2/ethylenediamine/H2O in a xenon atmosphere of 30 bar for a long reaction time (140 d). Physico-chemical characterisation using solid-state NMR spectroscopy, SEM, TG-DSC and ATR-FTIR spectroscopy confirmed that RUB-11 is a framework silicate. The atomic structure was solved by 3D electron diffraction using the fast-automated diffraction tomography method. The structure model of monoclinic symmetry with lattice parameters of a0 = 7.3929(5) Å, b0 = 7.3942(3) Å, c0 = 26.1786(13) Å and β = 98.372(7)° (space group: Pc) was refined against electron diffraction data (dynamical refinement) and powder diffraction data. An additional distance-least-squares refinement confirmed the feasibility of forming a stress-free silica framework of RUB-11 topology. The chemical composition of RUB-11 per unit cell is 30 SiO2. The framework silicate RUB-11 is structurally closely related to layer silicate magadiite and can be regarded as an interlayer expanded zeolite (IEZ) based on magadiite-type layers. Both materials contain topologically identical, dense layers, named [m with combining low line][a with combining low line][g with combining low line] layers. In the case of RUB-11, these layers are interconnected via additional silicon atoms leading to a complete framework with a 2-dimensional pore system consisting of intersecting 8-ring channels. The synthesis route leading to RUB-11 is in contrast to typical IEZs, which are obtained in a two-step process. According to the electron diffraction data and the XRD powder patterns, RUB-11 has a disordered structure. A detailed analysis revealed that two different types of disorder concerning the stacking of layer-like building units (consisting of [m with combining low line][a with combining low line][g with combining low line] layers plus interconnecting silicon atoms) contribute to the real structure of RUB-11. It is surprising that the channel-like pores of RUB-11 are completely empty when separated from the reaction mixture.