Synthesis of Nay-Type Nanozeolites and Their Assembling into Microporous Membranes

In this paper we report on a divalent procedure for the synthesis of FAU zeolites with particle size distribution in the nanometer range and for the concomitantly assembling of a uniform layer of a quasi-solid gel containing nanozeolite precursor species on the support surface. The crystallization has been conducted at room temperature, in absence of organic structure directing agents (SDAs). A highly reactive sodium hydroxide rich hydrogel was used as starting synthesis system, while microsized zeolite crystals, preliminarily coated on the support surface before the synthesis, oriented and promoted the nucleation and crystallization processes. Nanosized FAU crystals with particle size of ca. 56 nm have been obtained after 24 hours. Alongside, a thick gel stratum was formed on the support surface, which contains FAU nanoparticles of 20-30 nm. The gel matrix assembled on the support surface was used as starting material for the zeolite membrane synthesis and it was rapidly converted into a nanocrystalline layer upon hydrothermal treatment at higher temperature. The FAU layer has a thickness of ca. 2 mu m and is constituted by closely packed nanocrystals, whose dimension is still. 20-30 nm. The mass transport properties of the prepared membranes were probed by feeding dry single gases (N-2 and CO2) at ambient temperature, obtaining low permeance and ideal selectivity higher than the corresponding Knudsen value. The results reported in this study indicate that supersaturation as well as the elevate amount of sodium ions are synergistic factors to reach a high degree of conversion of the hydrogel into a nanocrystalline material at near ambient conditions and to induce the assembling of soluble aluminosilicate species on the support surface, thus effectively promoting the heterogeneous nucleation process.