Si 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 (H 2 O/TEOS) of 6 and a molar TPAOH/TEOS ratio of 0.37. The 29 Si 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 SiO 2 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.

The silica species contained in an aged clear suspension, which upon heating gives rise to the crystallization of Silicalite-1, were extracted with 80% efficiency using a sequence of acidification, salting out, phase transfer into organic solvent, and freeze-drying methods. This silica powder was characterized by X-ray scattering, transmission electron microscopy, atomic force microscopy, and 29Si magic angle spinning nuclear magnetic resonance. These techniques gave evidence for the presence of a very specific morphology, corresponding to slab shaped particles, with dimensions of 1.3 × 4.0 × 4.0 nm. The nanoslabs have the MFI structure with nine channel intersections per particle, each containing a TPA cation. The identity of the extracted nanoslabs with the species in suspension is evidenced with in situ and ex situ X-ray scattering.

The formation of silicate particles upon gradual addition of TEOS to concentrated TPAOH and TBAOH solutions, and upon dilution with water and aging, was studied with in situ X-ray scattering (XRS) and gel permeation chromatography (GPC). The samples for the GPC analyses were obtained by extraction of the particles from solution via a sequence of acidification with HCl, salting out with NaCl, and phase transfer into tetrahydrofuran. XRS and GPC reveal the presence of populations of particles with discrete sizes and number molecular weights, respectively, growing through aggregation. The entities forming in TPAOH are identified on the basis of their size and number molecular weight relationships with species previously identified in these suspensions by 29Si NMR and other independent techniques. A mathematical expression for the X-ray scattering function is derived for particles with slab shape. When dimensions of 1.3 × 4.0 × 4.0 nm, corresponding to the nanoslab with MFI structure, or integer multiples of them are assumed, the derived function resembles the measured intensity pattern in shape and position. The observed scattering at 6.6° 2θ at very early stages is assigned to a trimer of tetracyclic undecamer. All larger particles including the nanoslab observed by XRS and GPC are multiples of this trimer. The evolution of the system in TBAOH also obeys a stacking sequence of particles of the same size, number and molecular weights, although the kinetics are totally different. Up to the formation of nanoblocks, TBAOH and TPAOH affect the reaction mixture in a very similar way, indicating that the first molecular steps in the early stages of formation of MFI and MEL zeolite structures in these systems are very similar. The nanoslabs representing a specific fragment of MFI or MEL structures have channel intersections which are at least to two sides open and contain TPA or TBA molecules, respectively. The surface of these Silicalite-1 and -2 fragments is decorated on the ac and bc planes with alkyl groups sticking out of the surface. In nanoslabs with TBA, unfavorable template−template interaction suppresses aggregation along a and b. The final product is a “double” nanoslab, probably connected along the later c direction of the MEL structure in which there is only a very small repulsion of the butyl chains. TPA favors particle aggregation to give larger particles measuring up to 15.6 × 8 × 8 nm, even at room temperature.

The transformation of a suspension of MFI nanoslabs into colloidal Silicalite-1 at 373 K was monitored with in situ low angle and wide angle X-ray scattering (XRS). The low angle region of the diffractograms taken in the course of the crystallization could be fitted by 1 to 3 Lorentzian lines, representing 1 to 3 populations of particles with different sizes corresponding to nanoslabs, intermediates, and large particles. The large particles gave rise to the Bragg diffraction characteristic of Silicalite-1 zeolite. The measured X-ray data at low angles were at all times in agreement with the presence of entities that are multiples of the nanoslab, measuring 1.3 × 4 × 4 nm. The crystallization was performed in an open vessel with reflux cooler, and in a closed container. In both conditions, a consecutive conversion pattern of nanoblocks into intermediates into large particles was observed. The evolution of volume populations can be fitted with first-order reaction kinetics for the conversion of the nanoslab volume into intermediates and an autocatalytic conversion of intermediates into large particles. This aggregation mechanism is supported by the interaction potentials of the different faces of the nanoslabs decorated with tetrapropylammonium cations, estimated using extended DLVO theory. The proposed mechanism can account for the nature of the intermediates, the preferential growth of Silicalite-1 in the crystallographic "c" direction, the strain in the colloidal Silicalite-1 crystals in the crystallographic "a" direction, and the influence of reaction conditions on the crystallization kinetics.