Combined in situ29Si NMR and small-angle X-ray scattering study of precursors in MFI zeolite formation from silicic acid in TPAOH solutions

Abstract

Silicic acid powder was dissolved and polymerized in a concentrated aqueous tetrapropylammonium (TPA) hydroxide solution at room temperature. Two complementary techniques were employed to follow this process leading to silicalite-1 zeolite upon heating. The formation of small silicates and specific oligomers involved in the assembly of silicalite-1 nanoprecursors was investigated using ²⁹Si NMR. Small-angle X-ray scattering (SAXS) was used to follow processes at a colloidal level. Dissolution and polymerization of silicic acid could then be related to events occurring at both molecular and colloidal scales. The appearance of very well-defined colloidal particles was linked to a specific intermediate already observed in systems using an organic and monomeric silica source. In situ time-resolved ultra-small-angle X-ray scattering (USAXS) using synchrotron radiation showed a linear growth of the average crystal diameter, which was slower than of that encountered in Na⁺ containing synthesis mixtures. Using the results presented here, we propose a mechanism describing the TPA-mediated self-assembly of silicalite-1 from silicic acid powder as silica source. This model is in agreement with rising evidence of a common mechanism involving nanoblock aggregation for organic mediated crystallization of high-silica zeolites.