The properties of salt-filled sodalites. Part 4. Synthesis and heterogeneous reactions of iodate-enclathrated sodalite Na8[AlSiO4]6(IO3)2−x(OH·H2O)x; 0.7 < x < 1.3

Abstract

Sodalite solid solutions Na8[AlSiO4]6(IO3)2−x(OH·H2O)x; (0.7 < x < 1.3) have been obtained in the system Na2O2SiO2Al2O3NaIO3H2O under hydrothermal conditions and characterized by X-ray powder diffraction, IR and MAS-NMR spectroscopy, as well as by simultaneous thermoanalysis in different gas atmospheres. Syntheses have been performed by the alkaline transformation of kaolinite at a temperature of 473 K in the presence of different amounts of sodium iodate salt. The composition range could only be varied down to x = 0.7, even with high amounts of sodium iodate in the initial mixtures. For the synthesis of a solid solution with a low content of iodate, a certain high concentration of the template salt has been found to be essential for the formation of a homogeneous sodalite solid solution. As well as its synthesis, the thermal properties of basic iodate sodalite have been investigated. A discontinuity of the thermal expansion could be detected as a result of temperature-induced host-guest interactions. Whereas at room temperature and during the initial heating process the aluminosilicate framework of sodalite is partially collapsed, at elevated temperatures (T = 773 K) an onset of a maximal expansion of the unit cell volume was apparent from the high-temperature X-ray powder diffraction results. Further investigations of the thermal behaviour of basic iodate sodalite (Na8[AlSiO4]6 (IO3)2−x(OH·H2O)x; x = 1) in different gas atmospheres gave evidence for intra-cage heterogeneous reactions, resulting in the formation of carbonate, nitrate or sulphite anions inside those sodalite cages, formerly filled with the “hydrated-hydroxide”. During this high temperature process, the decomposition of the iodate to iodide could be observed inside the sodalite cavities, causing high thermal stability in the resulting carbonate/iodide-, nitrate/iodide- or sulphite/iodide-sodalite solid solution.