2D-to-disguised 3D materials with built-in acid sites: H+-[Al]-RUB-18

Abstract

Al³⁺ heteroions have been incorporated into the silicate layers of Na-RUB-18 in three different molar ratios Si/Al_ICP (22, 40 and 42) through a post-synthesis procedure under hydrothermal conditions. Systematic characterization (XRD, ²⁹Si NMR, ²⁷Al NMR and SEM) showed that these Na-[Al]-RUB-18 layered aluminosilicates are free from impurities, and the aluminum did not affect the crystallinity of the material, and also did not change its morphology. ²⁷Al NMR showed that aluminum was incorporated in the tetrahedral position and the layered sample with molar ratio Si/Al_ICP = 22 has also displayed octahedral coordination. These layered aluminosilicates which consist of lamella stacking in the c direction had their interlayer space expanded by ion exchange with CTA⁺. The swollen samples were pillared with TEOS, as the SiO₂ pillar precursor, and octylamine, as the hydrolyzing agent and co-surfactant. Pillared materials presented type IV isotherms with a BET surface area in the range of 223–323 m² g⁻¹ and mesoporous with 2–4 nm, while their layered precursors Na-[Al]-RUB-18 showed type II isotherms with a BET surface area in the range of 7–12 m² g⁻¹. Additionally, the acidity present in the layers of these pillared solids was proved by in situ ethanol dehydration monitored by FTIR.