Phase separation in hydrated LTA zeolite

The coexistence of two distinct phases, [Li0.08(NH4)0.92]- and [Li0.32(NH4)0.68]-A, was observed for a mixed crystal of [Lix(NH4)1−x]-A of LTA (Linde type A) zeolite in the range of 0.08 < x < 0.32 at 295 K, where A denotes the aluminosilicate framework of LTA containing water molecules. In contrast, for x < 0.08 and x > 0.32, a uniform monophase was observed for the mixed crystal. The coexistence of the separated phases was deduced from ion-exchange isotherm measurements at 295 K and thermodynamic analyses. Spinodal and binodal curves were obtained. The monophasic and biphasic states were confirmed by powder X-ray diffraction. The excess Gibbs energy and the excess volume of [Lix(NH4)1−x]-A were unsymmetrical with respect to x = 1/2. The Li ions favored specific ion-exchange sites. The preferential occupation of Li ions in the six-membered oxygen ring of [Lix(NH4)1−x]-A was proved by Li-isotope measurements, and was demonstrated by ab initio molecular orbital calculations. The separation of two LTA-phases due to ordered Li-occupation within the six-membered oxygen rings is discussed.