Thermal decomposition, gas phase hydration and liquid phase reconstruction in the system Mg/Al hydrotalcite/mixed oxide: A comparative study

The thermal decomposition of a synthetic Mg/Al hydrotalcite and the subsequent hydration of the calcined products in the presence of water vapor at 30 °C were studied by in situ XRD. A comparative study between the gas phase hydration and reconstruction of calcined hydrotalcite in Na2CO3 solution at 25 and 70 °C was investigated. TG coupled to a mass spectrometer was used to study the thermal behaviour of synthetic hydrotalcite. SEM images were taken to show the layering and size of crystals. Thermal decomposition of hydrotalcite led to the formation of a dehydrated intermediate phase at 200 °C, followed by crystalline MgO formation at 450 °C. Hydration of calcined hydrotalcite in H2O/N2 resulted in the formation of broad patterns of the meixnerite phase after 2 h. This phase retained its structure even after 52 h exposure to water vapor. Reconstruction of the calcined products in carbonated water at 25 and 70 °C resulted in intensified patterns of the hydrotalcite phase. Due to the different structural and compositional properties of the layered products obtained by different chemical reactions of the mixed oxide phase a clarification of nomenclature for all phase transforming steps is proposed.

Research highlights
► Thermal decomposition of hydrotalcite-like materials into mixed oxides.
► Mixed oxide treatment with carbonate solution recovers the hydrotalcite-like structure.
► Mixed oxide treatment in CO2-free water/N2 leads to a meixnerite-type structure.
► Systematic nomenclature for the different transformations is proposed.