Aggregation mechanism of nanoslabs with zeolite MFI-type structure

The transformation of a suspension of MFI nanoslabs into colloidal Silicali te-l at 373 K was monitored with in situ low angle and wide angle X-ray sca ttering (XRS). The low angle region of the diffractograms taken in the cour se of the crystallization could be fitted by 1 to 3 Lorentzian lines, repre senting 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-l zeolite. The m easured X-ray data at low angles were at all times in agreement with the pr esence of entities that are multiples of the nanoslab, measuring 1.3 x 4 x 4 nm. The crystallization was performed in an open vessel with reflux coole r, 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 fined with first-order reaction kinetics for the conversion of the nanoslab volume into intermediates and an autocatalytic conversion of intermediates into large particles. This agg regation mechanism is supported by the interaction potentials of the differ ent faces of the nanoslabs decorated with tetrapropylammonium cations, esti mated using extended DLVO theory. The proposed mechanism can account for th e nature of the intermediates, the preferential growth of Silicalite-1 in t he crystallographic "c" direction, the strain in the colloidal Silicalite-l crystals in the crystallographic "a" direction, and the influence of react ion conditions on the crystallization kinetics.