Crystallization pathway from a highly viscous colloidal suspension to ultra-small FAU zeolite nanocrystals

Abstract

The crystallization kinetics of template free ultra-small FAU-type zeolite (X) in highly alkaline viscous precursor suspensions is investigated. We focus on understanding the crystallization pathway from a viscous amorphous precursor suspension to nanosized FAU crystals with high phase purity and yield. The crystallization develops in a four-step process: (1) preparation of a clear and stable precursor suspension by a controlled mixing of silica and alumina colloidal suspensions at low temperature (4 °C), (2) aging the colloidal suspension at room temperature, (3) partial evaporation of the water of the colloidal suspension to produce a highly viscous suspension with amorphous nanoparticles with a monomodal particle size distribution, (4) hydrothermal treatment at low temperature (50 °C) to transform the amorphous nanoparticles into FAU type nanocrystals with the same particle size while preventing agglomeration and sedimentation. The properties (size and morphology) of the amorphous particles depend on the mixing procedure (temperature and speed of mixing) and alkalinity of the colloidal suspension. High alkalinity of the suspension leads to the formation of ultra-small discrete nanoparticles with a monomodal particle size distribution. During the aging step (2), secondary building units characteristic of the FAU framework structure are detected (Raman and IR spectroscopies) in the amorphous aluminosilicate particles. During the evaporation of the water of the colloidal suspension (3), prior to hydrothermal synthesis, double-six membered rings (D6R), the secondary building units of the FAU structure, were detected by in situ²⁹Si NMR spectroscopy. Formation of the very first micropores in the nanosized particles was further confirmed by HP ¹²⁹Xe NMR spectroscopic measurements.