Controllable and SDA-free synthesis of sub-micrometer sized zeolite ZSM-5. Part 2: Influence of sodium ions and ageing of the reaction mixture on the chemical composition, crystallinity and particulate properties of the products

In this work, the influences of excess amount of sodium ions and the way/duration of ageing of the reaction mixture (hydrogel) on structural, particulate, morphological and chemical properties of the crystalline end products obtained by hydrothermal treatment (heating at 483 K for 2 h) of the TPA-free reaction mixture: 1.0Al₂O₃/100SiO₂/xNa₂O/4000H₂O/yNa₂SO₄ (0.4 ? (x + y) ? 100) seeded by silicalite-1 nanocrystals (260 nm, 4 wt.% of silica in gel mixture), was investigated by different characterization methods such as, powder X-ray diffraction (XRD), scanning-electron microscopy (SEM), particle size distribution (PSD) measuring by laser light scattering (LLS) and X-ray fluorescence (XRF). The obtained results showed that addition of sodium sulfate in low-alkaline reaction mixture enhances the aggregation of the particles of colloidal silica and formation of gel by the action of sulfate oxy-anions while in high-alkaline reaction mixtures the condensation process takes place on the surface of the crystalline end products. Excess amount of sodium ions do not increase the crystallization rate thus showing that the rate-determining factor is concentration of “free” low-molecular weight silicate species, determined by the alkalinity of system. On the other hand, addition of sodium sulfate considerably reduces the formation of crystal aggregates, by combined chemical and electrical interactions. Ageing of the reaction mixture (hydrogel) mainly influences the particle size distribution of the crystalline end products, which is explained by the change in the relative rates of crystal growth and crystal aggregation with the time of ageing. However, hydrogel ageing does not affect the size and number of crystals in the crystalline end product, showing that the growth precursor particles form during hydrothermal treatment of the reaction mixture and not during its room-temperature ageing.