Seeded synthesis of all-silica CHA zeolites in diluted mother liquor

Owing to their valuable structural characteristics, CHA-type zeolites have great potential for applications in catalysis, adsorption and membrane separation. Aluminosilicate CHA with Si/Al ratio<100 can be prepared easily with few limitations. However, all-silica and high silica CHA zeolites (Si/Al ratio >100) can only be prepared in a solid-like precursor with low H2O/SiO2 ratio similar to 5, which creates difficulties for the synthesis of CHA powders and membranes. Herein, we systematically studied the effect of various parameters on the synthesis of all-silica CHA in a diluted mother liquor with H2O/SiO2 ratio of 30. The key is the use of ammonium hexafluorosilicate ((NH4)(2)SiF6) as silica source. Pure phase all-silica CHA with size between 0.3-2 mu m can be prepared in a broad temperature range (373-433 K) with low TMAdaOH/SiO2 ratio of 0.3. The use of (NH4)(2)SiF6 can be replaced with regular silica sources and ammonium fluoride. The successful synthesis of all-silica CHA zeolite in diluted mother liquor not only simplifies its synthesis, but also facilitates its applications as sorbents and membranes.

Automated summary

The synthesis of all-silica CHA zeolite in diluted mother liquor with a H2O/SiO2 ratio of 30 was successfully achieved by using ammonium hexafluorosilicate as the silica source. This method allows for the preparation of pure phase all-silica CHA with a size between 0.3-2 µm in a broad temperature range (373-433 K) with a low TMAdaOH/SiO2 ratio of 0.3. This simplified synthesis method not only facilitates the production of CHA zeolites, but also enables their applications as sorbents and membranes.