Crystallization and assembling of FAU nanozeolites on porous ceramic supports for zeolite membrane synthesis

Abstract

In this study, a novel route for preparing nanocrystalline porous FAU layers s stably supported on porous α-Al2O3 tubular substrates is reported. A highly reactive sodium hydroxide rich hydrogel was used to promote the nucleation process on the support surface. After 24 h of treatment at near ambient condition, a heavily nucleated quasi-solid gel layer containing nanozeolite precursor species was first formed on the support surface. This matrix was used as starting material for the synthesis of nanozeolite-based membranes. The support coated with the extensively nucleated gel phase was isolated and cured at higher temperature in different conditions. A rapid and diffuse crystallization was observed by treating the gel matrix with a synthesis solution of lower alkalinity, leading to the formation of a dense nanocrystalline FAU layer firmly adhered to the substrate surface. The FAU layers (Si/Al ratio of ca. 2) have a thickness of ca. 2 μm and are constituted by closely packed, well intergrown nanocrystals (crystallite dimension of ca. 20–30 nm). The results of this study indicate that supersaturation as well as the elevate amount of sodium ions synergistically contribute to induce the assembling of soluble aluminosilicate species on the support surface and to effectively promote the heterogeneous nucleation process.