Heterogenous events in the crystallization of zeolites

Abstract

While for many years zeolite crystallization has been assumed to proceed mainly through solution-mediated transformations, recent works point towards the presence of heterogeneous events in both nucleation and crystal growth steps. When the zeolite synthesis starts from a solid-containing gel, the participation of the amorphous raw phase in the formation of the zeolite nuclei has been clearly established. Moreover, in some cases the zeolite crystals grow by reorganization of the hydrogel through solid–solid transformations. In zeolite synthesis starting from clear solutions, the crystallization mechanism has been recently studied by in situ light scattering techniques, showing the formation in the first stages of an X-ray amorphous gelatinous phase, consisting of particles with sizes below 10 nm. These nanoparticles seem to be involved directly in the crystallization, as their aggregation and subsequent densification have been proposed to lead to the formation of the zeolite crystals. The crystallization of zeolites through heterogeneous versus homogeneous pathways is favoured at high solid concentrations or when the solubility of the silicate species is low, as occurs during zeolite synthesis through the fluoride route.