Thermal crystallization of ion-exchanged zeolite A

The zeolite 4A (Na12Al12Si12O48.27H2O) was subjected to cationic exchange with an aqueous solution of Li+ 0.5 M at 70 °C. After three exchanges for 24 h, about 100% of Na+ was removed obtaining the Li-exchanged form (Li)-A. Despite this quite complete exchange, some undesired and occluded NaAlO2 was detected in the typical structural cages of precursor. The partial exchange of Li with equivalent amounts of Ca or Mg, allowed the formation of (Li–Ca)- or (Li–Mg)-exchanged zeolitic forms characterized by different Ca or Mg contents, respectively. These various zeolitic precursors have been thermally treated at increasing temperatures up to 1170 °C. The transformations into other crystalline phases occur immediately after the formation of intermediate amorphous phases originating from the thermal collapse of zeolitic precursors. From the (Li)-A zeolite-based precursor, a small amount of nepheline (NaAlSiO4) and b-eucryptite (LiAlSiO4) crystallize, while during the thermal transformation of Li–Ca-containing precursors, anorthite [(Ca,Na)Al2Si2O8] and b-eucryptite-like phase are formed. Stuffed derivatives of quartz relatively richer in SiO2 crystallize from (Li–Mg)-containing precursors. The Mg content of the zeolitic precursor affects the type of secondary crystallized phases and consequently the composition of the corresponding stuffed derivative of quartz.