THE HYDROSODALITE SYSTEM NA6+X[SIALO4]6(OH)X.NH2O - FORMATION, PHASE-COMPOSITION, AND DEHYDRATION AND REHYDRATION STUDIED BY H-1, NA-23, AND SI-29 MAS-NMR SPECTROSCOPY IN TANDEM WITH THERMAL-ANALYSIS, X-RAY-DIFFRACTION, AND IR SPECTROSCOPY

Hydrosodalites of general composition Na6+x(SiAlO4)6(OH)x.nH2O have been prepared from kaolinite and aqueous NaOH solution by hydrothermal synthesis under various conditions and subsequent NaOH extraction with water and/or thermal dehydration. Detailed characterization of the products by Si-29, Na-23, and H-1 MAS-NMR spectroscopy in combination with thermal analysis, powder X-ray diffraction, and infrared spectroscopy verifies the composition of the following five distinct hydrosodalite phases: Na8[SiAlO4]6(OH)2.2H2O, Na8[SiAlO4]6(OH)2 (''basic hydrosodalites''), Na6[SiAlO4]6.8H2O, Na6[AlSiO4]6.4H2O, and Na6[SiAlO4]6H (''nonbasic hydrosodalites''). Hydrosodalite phases of other compositions such as ''superbasic sodalites'' Na10[SiAlO4]6(OH)4 and basic sodalite hydrates Na8[SiAlO4]6(OH)2.nH2O with n > 2 proposed by several authors could not be identified. The primary product of hydrothermal synthesis is always the basic hydrosodalite Na8[SiAlO4]6(OH)2.2H2O. However, the latter is partially transformed into nonbasic Na6[SiAlO4[6.8H2O sodalite due to intracage NaOH/H2O exchange during the washing of the crude product with water, generally applied to remove NaOH impurities from the outer surface of the crystallites. The extent of the exchange depends on the washing conditions and is very effective for microcrystalline powders but strongly reduced for large single crystals of 0.5-1 mm in size. Thus the basic hydrosodalites of composition Na8[SiAlO4]6(OH)2.nH2O With n > 2 described previously are in fact mixtures of basic and nonbasic hydrosodalites. In contrast to the highly hydrophilic anhydrous nonbasic sodalite Na6[SiAlO4]6, the dehydrated basic sodalite Na8[SiAlO4]6(OH)2 is hydrophobic and cannot be rehydrated. During slow rehydration of the former an intermediate phase of composition Na6[SiAlO4]6.4H2O has been positively identified. This study clearly demonstrates the power of combined application of multinuclear MAS-NMR spectroscopy, X-ray diffraction, IR spectroscopy, and thermal analysis for providing detailed information on the specific cage fillings and host/guest chemistry in sodalites.