Unusual Pathway of Crystallization of Zeolite ZSM-5 in a Heterogeneous System: Phenomenology and Starting Considerations

Abstract

Crystallization of zeolite ZSM-5 from a diluted heterogeneous system (12.5Na2O–Al2O3–8TPABr–60SiO2–4000H2O) 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.