Abstract
Various synthesis approaches of gel or colloidal solution system have been applied to prepare LTA zeolite with Si/Al ratio >1 to improve their thermal stability for applications in catalysis and separation process. While a cooperative templating of the mixed templates as structure-directing agents was typical for the synthesis from the gel system, the templating process of colloidal system may be varied depending on their synthesis composition systems. Using SANS technique, we probe nucleation of Si-rich LTA (Si/Al ratio > 1) nanocrystals from a colloidal solution synthesis system comprising of Ludox AS-40, Aluminium isopropoxide, TMAOH, NaOH and water. The SANS measurements at contrast match condition were also performed to contrast out of silica for examining the scattering signatures of the species other than the silica. The study brings new insights to the formation of industrially important Si-rich LTA nanocrystals of sizes ca. 50–100 nm. The relative change in the nucleation behaviour of the typical LTA (Si/Al ratio = 1) as observed by the SANS was also discussed. Our findings showed a dual heterogeneous nucleation of the LTA from the colloidal solution by forming -Si-O-Al- (aluminosilicate) nanomaterials in the solution as well as on the solid surface. The Si-rich LTA formation was predominantly originated from the nucleation in the liquid phase whereas the heterogeneous nucleation of the typical LTA mainly occurred on the exterior solid surface of the silica precursor. Thus, the different types of nanoscale structures nucleated from the colloidal syntheses should lead into the formation of LTA nanocrystals of different properties.
A dual heterogeneous nucleation of the LTA nanocrystals as probed by SANS. [Display omitted]
•New insights to the nucleation of industrially important Si-rich LTA nanocrystals.•A dual heterogeneous nucleation process of colloidal syntheses as probed by SANS.•LTA of different properties depending upon the nucleation process.•Si-rich LTA formation predominantly from the nucleation in the solution system.