The significant effects of the alkali-metal cations on ZSM-5 zeolite synthesis: From mechanism to morphology

Abstract The effects of the addition of Na+ and K+ on the synthesis of ZSM-5 were investigated with multi-techniques. Results show that, Na+ and K+ had remarkable acceleration effect on the crystallization of ZSM-5. However, the Na+ additive produced aggregates of very small crystals, the crystallization curve and SEM images showed the gel underwent ‘sudden’ crystallization. Whereas, the K+ additive produced large coffin-like crystals (∼30 μm), the gels showed ‘little by little’ crystallization. In addition, the liquid phase of Na+ adding gel had weaker solubility for aluminosilicate, while in solid phase an obvious IR absorption band around 580 cm−1 attributed to aluminosilicate species of five-membered structures was observed. During crystallization, the band shifted gradually from 580 to 550 cm−1, meanwhile the SiO2/Al2O3 molar ratio of solid phase kept close to the initial gel. Contrarily, the liquid phase of K+ adding gel had stronger solubility for aluminosilicate. And its solid phase did not have IR absorption band around 580 cm−1. In K+ adding system SiO2/Al2O3 molar ratio of solid phase was always much lower than that of initial gel during crystallization. These results suggested that Na+ adding gel crystallized by solid hydrogel transformation mechanism, while K+ adding gel by solution-mediated transport mechanism. The acceleration effect of Na+ and K+ on crystallization of ZSM-5 was achieved by different ways, the former played the structure directing role and the later enhanced the aluminosilicate dissolution. This difference was attributed to the structure forming effect of Na+ and the structure breaking effect of K+.