Crystallization of zeolite ZSM-5 from a diluted heterogeneous system (12.5Na 2 O−Al 2 O 3 −8TPABr−60SiO 2 − 4000H 2 O) was investigated by various experimental methods such as X-ray diffraction (XRD), electron diffraction (ED), infrared spectroscopy (FTIR), X-ray fluorescence (XRF), scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), thermo-gravimetric analysis (TGA), particle size analysis (PSA), pH measurement, inductive coupling plasma (ICP) emission spectrometry, and dynamic light scattering (DLS). The crystallization process is characterized by a very long "induction period" (95% of the entire reaction time) and very fast transformation (5% of the entire reaction time) of amorphous to crystalline phase (zeolite ZSM-5) at the end of the crystallization process. Analysis of the obtained results has shown that the crystallization process takes place by a chain of processes: (i) formation of "primary" amorphous aluminosilicate precursor (gel) at room temperature, (ii) formation of "secondary" amorphous aluminosilicate precursor ("worm-like" particles, WLPs) at increased temperature (170 °C), (iii) formation of "tertiary" amorphous aluminosilicate precursor (condensed aggregates, CAs) by aggregation of the WLPs and densification (condensation) of aggregates, and (iv) formation of nuclei and their growth in the matrixes of CAs; these processes result in the formation of fully crystalline zeolite ZSM-5 in the form of polycrystalline aggregates.

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A critical analysis was carried out for the purpose of understanding the role of subcolloidal (nanosized) (alumino)silicate precursor species in the early stage of crystallization of zeolites in heterogeneous systems (hydrogels). The formation and evolution of these subcolloidal species in both the solid and the liquid phases were investigated by various experimental methods such a scanning electron microscopy (SEM, FE-SEM), transmission electron microscopy, atomic force microscopy, particle size analysis, pH measurement, atomic absorption spectroscopy, and dynamic light scattering, after careful separation of intermediates from reaction mixture by two-step centrifugation treatment. The results revealed that a chain of processes (i) the formation of low-molecular-weight (LMW) silicate species, by dissolution of Al-enriched amorphous silica, and their aggregation into about 3 nm sized primary precursor species (PPSs), (ii) the formation of larger (∼3 to ∼15 nm sized) silicate precursor species (LSPSs) by a rapid aggregation/coalescence of PPSs, (iii) the formation of "gel" (primary amorphous precursor) by a random aggregation of LSPSs at room temperature, and (iv) the formation of the worm-like particles (secondary amorphous precursor) occurred in the solid phase during heating of the reaction mixture (hydrogel) from room temperature to 170 °C. It is interesting that almost the same processes occur in the liquid phase but with decreased rate according to the relative low concentration of LMW silicate species. With the above described findings, it is highly expected that the manipulation of crystallization pathway through controlling the formation/evolution of precursor species in the initial stage of the process can be achieved.

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