Identification of precursor species in the formation of MFI zeolite in theTPAOH-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 concen trated aqueous solution of tetrapropylammonium hydroxide (TPAOH) at low tem peratures. The composition was characterized by a molar hydrolysis ratio (H 2O/TEOS) of 6 and a molar TPAOH/TEOS ratio of 0.37. The Si-29 NMR spectra a nd the infrared spectra of the samples recorded at different reaction times and temperatures were assigned to a limited number of specific silicate po lyanions containing three and five rings. The structure directing action of tetrapropylammonium cations was evidenced by the formation of silicate pol yanions with a curved hydrophobic SiO2 surface, such as the bicyclic pentam er, pentacyclic octamer, and the tetracyclic undecamer. At room temperature , the polycondensation process leads to the selective formation of a specie s containing 33 Si atoms. It occluded a tetrapropylammonium molecule and ha d the same framework connectivity as in bulk MFI zeolite. This TEOS polycon densation process may be relevant for the first steps of the crystallizatio n of MFI type zeolites.