Phase selection controlled by sodium ions in the synthesis of FAU/LTA composite zeolite

Zeolite faujasite (FAU), Linde type A (LTA) and FAU/LTA composite have been synthesized using tetramethylammonium cation (TMA⁺) as template, by adjusting only the concentration of Na⁺ ions in the initial solution (1.00 Al₂O₃ 4.36 SiO₂ : 2.39 (TMA)₂ O : β Na₂ O : 249.00H₂ O). Na⁺ ions alter the phase composition of the product more than TMA⁺ or OH^- ions. When Na₂ O concentration [Na₂ O] increases from 0.024 to 0.168, the product gradually changes from pure FAU to pure LTA via the formation of FAU/LTA composite with increasing LTA fraction. Interestingly, the induction periods of FAU and LTA in the FAU/LTA composite zeolite ([Na₂ O] is 0.072) are both 13 h, quite different from the induction periods of their individual pure phases-45 h for FAU and 4 h for LTA. During the crystallization, the LTA/(FAU + LTA) fraction in the composite zeolite decreases in a nearly linear fashion. Scanning electron microscopy, thermogravimetry and differential thermal analysis indicate some difference between the properties of the FAU/LTA composite zeolite and of the mechanical mixture.