Effect of synthesis conditions on the properties of nanocrystalline faujasites

Abstract

The effect of crystallization parameters on the properties of nanocrystalline zeolites Y has been studied to optimize the synthesis conditions. Zeolites Y are prepared by the three-stage hydrothermal crystallization with a gradual rise in synthesis temperature from room temperature to 60°C using the Na₂O/SiO₂ ratio in the reaction mixture and the number and duration of individual synthesis stages as variable synthesis parameters. It has been shown that an increase in the Na₂O/SiO₂ molar ratio in the reaction mixture above 9.6 leads to a change in the crystallization selectivity and causes formation of zeolite X with a crystal size of 200 nm as a crystalline product. An increase in the duration of the synthesis stage at 60°C is accompanied by increase in the crystal size and entails formation of an analcime impurity phase. A decrease in the duration of reaction mixture aging stages at 25 and 38°C leads to an increase in the zeolite Y nanocrystal sizes and a decrease in the crystalline product yield. It has been found that a pure zeolite Y phase with a nanocrystal size of 320–350 nm and a SiO₂/Al₂O₃ ratio of 3.6–4.2 is formed during the three-stage synthesis from a (7.2–9.6)Na₂O · Al₂O₃ · 14.4 SiO₂ · 290 H₂O reaction mixture with a yield of 0.60–0.88 g/g of reaction mixture.