Heterogenous events in the crystallization of zeolites

While for many years zeolite crystallization has been assumed to proceed ma inly 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 p articipation of the amorphous raw phase in the formation of the zeolite nuc lei has been clearly established. Moreover, in some cases the zeolite cryst als grow by reorganization of the hydrogel through solid-solid transformati ons. In zeolite synthesis starting from clear solutions, the crystallizatio n mechanism has been recently studied by in situ light scattering technique s, showing the formation in the first stages of an X-ray amorphous gelatino us phase, consisting of particles with sizes below 10 nm. These nanoparticl es seem to be involved directly in the crystallization, as their aggregatio n and subsequent densification have been proposed to lead to the formation of the zeolite crystals. The crystallization of zeolites through heterogene ous versus homogeneous pathways is favoured at high solid concentrations or when the solubility of the silicate species is low, as occurs during zeoli te synthesis through the fluoride route.