Synthesis and Phase Relationships between Smectite and Low-silica Zeolites

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Low-silica zeolite samples and smectite were hydrothermally synthesized from Si-Al hydrous oxides. The synthesized zeolite phases changed to structures having lower Si/Al ratio with increases in the amount of NaOH in the slurry, and time of hydrothermal treatment. When tetraethyl ammonium hydroxide (TEAOH) was added instead of NaOH, ammonium smectite was produced without zeolite. SEM and TEM images suggested that crystallization occurred at the solid-liquid interface. Upon addition of NaOH (Na/Si<0.2) to the smectite, zeolite nucleation was observed on the surface of the clay film. The zeolite phase changes depended on the dissolution of the solid phase and the resultant change in composition of the liquid phase. The observed phase relationships agreed with the calculated formation free energies and solubilities. Thus, low-silica zeolite formation under hydrothermal conditions was strongly affected by the composition and concentration of the solution. The phase relationships based on solubility curves, can be used to control the synthesis of metastable aluminous silicates and can be applied to the design of some industrial materials such as zeolite membranes and molecular sieves using hydrophilic low-silica zeolites and smectites.

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