Soft synthesis of FAU nanozeolites and microporous membranes.

Abstract

An alternative strategy to the conventional seeded-assisted synthesis of Faujasite (FAU) nanozeolites and their assembling into thin films on ceramic substrates are reported. The method, which can be easily extended to other zeolite topologies, offers good opportunities for industrial applications.
The molar composition of the precursor gel used for the synthesis of FAU nanozeolites and membranes was conceived to achieve a high degree of supersaturation and trigger a uniform and abundant nucleation. Tubular alumina supports seeded with NaX particles of 2μm have been used to address the crystallization process. The synthesis was carried out in “soft condition” at near ambient temperature (30°C). Pure-phase FAU zeolites having uniform particle size dimensions in the range of 35-56 nm have been produced in high yield. Moreover, a uniform gel layer containing precursor entities has been concomitantly formed on the inner surface of the support after immersion in the synthesis gel. Thin and dense FAU membranes have been prepared through drygel conversion at higher temperatures. SEM images showed that the FAU layers have a thickness of ca. 2 μm and are constituted by closely packed, well intergrown nanocrystals, whose dimension was ca. 20-30 nm. The mass transport properties of the prepared membranes were probed by feeding dry single gases (N₂ and C_O2) at ambient temperature, obtaining low permeance (ca. 3.8·10^-9 molm^-2s^-1Pa^-1 for N₂) and ideal selectivity higher than the corresponding Knudsen value.