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 ratio100) can only be prepared in a solid-like precursor with low H2O/SiO2 ratio ~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)2SiF6) as silica source. Pure phase all-silica CHA with size between 0.3–2 μm can be prepared in a broad temperature range (373–433 K) with low TMAdaOH/SiO2 ratio of 0.3. The use of (NH4)2SiF6 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. All-silica CHA crystals prepared in a diluted mother liquor with NH4(SiF6)4 as silica source. [Display omitted] •Synthesis of all-silica CHA in diluted mother liquor for the first time.•Ammonium hexafluorosilicate as silica source led to fast synthesis at low temperature.•Sub-micron all-silica CHA crystals obtained with proper seeding.•(NH4)2SiF6 can be replaced with regular silica source and ammonium fluoride.