On the dehydration enthalpies of some beta-aluminas

Abstract

The energetics of the dehydration reaction of single crystal (Na, Na 50%-Li, Li) and ceramic Na beta-aluminas have been studied. From the dependence of the dehydration enthalpy values on the water content it has been deduced that lattice water can be bound in two different ways i.e. by ion-dipole interactions with conducting cations and by hydrogen bonds with spinel block oxygens. As expected, the first one gives binding energies depending on the nature of the monovalent cation (ΔH=84.5 and 59.4 kJ/mol H2O for Li and Na beta-alumina respectively). In contrast, the enthalpy change associated with the second one is identical for the three beta-aluminas (ΔH=15.1 kJ/mol H2O). The ceramic Na beta-alumina undergoes a surface reaction too (with CO2) leading to the formation of carbonates and bicarbonates.