Phase separation in hydrated LTA zeolite

Abstract The coexistence of two distinct phases, [Li 0.08 (NH 4 ) 0.92 ]- and [Li 0.32 (NH 4 ) 0.68 ]–A, was observed for a mixed crystal of [Li x (NH 4 ) 1− x ]–A of LTA (Linde type A) zeolite in the range of 0.08  x x 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 [Li x (NH 4 ) 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 [Li x (NH 4 ) 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.