Two new members of the Silica-X family of materials: RUB-5, a silica zeolite with a very high framework density and RUB-6, a hydrous layer silicate

Abstract

The new zeolite RUB-5 and the new phyllo silicate RUB-6 were synthesized at temperatures between 130 °C and 200 °C from reaction mixtures consisting of SiO2/LiOH/B(OH)3/OA/H2O or SiO2/KOH/OA/H2O (OA = organic additive). Physico-chemical characterization using solid-state NMR spectroscopy, SEM, TG-DTA, and ATR-FTIR spectroscopy confirmed that RUB-5 is a framework silicate while RUB-6 is a layer silicate. The XRD powder patterns were indexed in monoclinic symmetry (space group: C2) with lattice parameters of a0 = 10.2699 (4) Å, b0 = 10.6556 (4) Å, c0 = 18.1551 (7) Å and β = 106.35 (1)° (RUB-5), and a0 = 10.1100 (43) Å, b0 = 10.6956 (51) Å, c0 = 20.5448 (44) Å and β = 105.79 (1)° (RUB-6). The chemical compositions of RUB-5 and RUB-6 are [Si42O84] (with a few silanol defects) and [Si40O76(OH)8] • C6H14N2, respectively. According to the XRD powder patterns, RUB-5 and RUB-6 have a disordered structure and belong to the Silica-X family of materials. The materials are structurally closely related and contain topologically identical, dense layer-like building units, named γ-layer. In the case of RUB-5 these building units are interconnected via additional Si atoms leading to a 2-dimensional pore system consisting of intersecting 8-ring channels. RUB-5 is a pure silica zeolite of very high density (2.20 g/cm3) which shows a negative thermal expansion between 20 °C and 470 °C. In the case of RUB-6, the layers are intercalated by organic cations which generate weak ionic bonds between cation and silicate layer. Formally, RUB-6 may be transformed into RUB-5 via a condensation reaction involving the γ-layers ([Si40O76(OH)8]) of RUB-6 and additional Si(OH)4 units from the reaction mixture to form the framework of RUB-5 ([Si42O84]) and water. In this sense, RUB-5 is the interlayer expanded zeolite based on the silicate layers of RUB-6.