The reconstruction of layered double hydroxides with the hydrotalcite-like structure from material previously calcined at increasing temperatures has been studied. Both the calcination temperature and the time the samples spent equilibrating with a controlled water vapour pressure have been considered. The crystallinity of the samples and the crystalline phases formed have been assessed by powder X-ray diffraction, while the local environment of the Al ions has been probed by ²⁷ A1 MAS NMR. It has been found that reconstruction is complete when the sample has been calcined at or below 550 °C, after 24 h rehydration, while equilibration for 3 days is adequate to reconstruct samples previously calcined at 750 °C. Only partial reconstruction is observed after calcination at 1000 °C. However, the NMR results indicate that, in all cases, some of the Al ions remain in tetrahedral sites.

Layered double hydroxides (LDHs) with the hydrotalcite-like structure have been prepared containing different amounts of Ni²⁺, Cu²⁺ and Al³⁺ in the brucite-like layers by a coprecipitation method. The samples have been characterized by elemental chemical analysis, powder X-ray diffraction, differential thermal analysis and thermogravimetric analysis, FT-IR and UV–VIS diffuse reflectance spectroscopies, temperature-programmed reduction, and specific surface area and porosity assessment by N₂ adsorption at −196 °C. The nature of the phase obtained, crystallinity of the HT-like phase and thermal behaviour of these materials were influenced by M^II/Al atomic composition and concentration of bivalent metal ion. Calcination at 500 °C leads to mostly amorphous solids containing NiO, and also CuO (tenorite) for large Cu²⁺ contents (Cu/Ni molar ratio equal to 4). When the calcination temperature is increased to 850 °C, crystallization of well defined phases (NiO, CuO and NiAl₂O₄, depending on the relative amounts of Ni, Cu and Al) takes place. Reduction of Cu²⁺ species in these samples takes place at a lower temperature than that for Ni²⁺, while the temperature of reduction of nickel is increased with an increase in aluminium and copper content of the samples.