Identification of precursor species in the formation of MFI zeolite in the TPAOH-TEOS-H2O system

Si-29 liquid NMR and in situ infrared spectroscopy was used to investigate the polycondensation process of tetraethyl orthosilicate (TEOS) in a concentrated aqueous solution of tetrapropylammonium hydroxide (TPAOH) at low temperatures. The composition was characterized by a molar hydrolysis ratio (H2O/TEOS) of 6 and a molar TPAOH/TEOS ratio of 0.37. The Si-29 NMR spectra and the infrared spectra of the samples recorded at different reaction times and temperatures were assigned to a limited number of specific silicate polyanions containing three and five rings. The structure directing action of tetrapropylammonium cations was evidenced by the formation of silicate polyanions with a curved hydrophobic SiO2 surface, such as the bicyclic pentamer, pentacyclic octamer, and the tetracyclic undecamer. At room temperature, the polycondensation process leads to the selective formation of a species containing 33 Si atoms. It occluded a tetrapropylammonium molecule and had the same framework connectivity as in bulk MFI zeolite. This TEOS polycondensation process may be relevant for the first steps of the crystallization of MFI type zeolites.