Verified synthesis of pure silica CHA-type zeolite in fluoride media

• Pure silica CHA was synthesized reproducibly within a wide range of water contents.
• TMAdaF/SiO2 significantly influenced the phase selectivity of CHA and STT.
• H2O/SiO2 and calcination temperature influenced the CHA microstructural properties.

Among the increasingly popular zeolites with eight-membered ring channels, CHA-type zeolites are considered to have great potential for various applications such as catalysis and separation. While CHA-type aluminosilicates with Si/Al ratios below 100 can be synthesized in both hydroxide and fluoride media, to date, pure silica CHA-type zeolites can only be prepared in a fluoride medium, with a low H2O/SiO2 of 3. Herein, we investigated the effect of various synthesis conditions on the formation of pure silica CHA-type zeolites. Single phase crystalline pure silica CHA was synthesized within a very narrow range of H2O/SiO2 ratio, and small deviations in the water content from this range led to the formation of STT-type zeolites. High N,N,N-trimethyl-1-adamant ammonium fluoride/SiO2 ratios allowed the formation of pure silica CHA, with no by-products, in the synthesis gel, with a high H2O/SiO2 ratio of 8. Additionally, synthesis conditions, in particular the H2O/SiO2 ratio, significantly influenced the specific surface and pore volume of CHA.

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