Time-resolved in situ X-ray diffraction study of the liquid-phase reconstruction of Mg–Al–carbonate hydrotalcite-like compounds

Abstract

The brucite-like Mg²⁺/Al³⁺ layered double hydroxide (LDH) was synthesized by using the coprecipitation method and calcined at a variety of temperatures. The starting LDH and the calcination products (mixtures of binary oxides and spinels) were characterized by X-ray diffraction, IR spectroscopies and thermogravimetric analysis. These techniques allowed us to confirm that the double hydroxide possesses a brucite-like structure in which cations are octahedrally arranged. At moderate temperatures, the calcination products have a MgO-like structure where Al ions are dissolved in the lattice to form a solid solution. Calcination at high temperatures (up to 1000 °C) yields a mixture of well-crystallized phases corresponding to MgO and MgAl₂O₄ (spinel). The reconstruction of layered double hydroxides from material calcined at 400 °C has been studied in situ by time resolved energy dispersive X-ray diffraction (EDXRD). Kinetic data for the reaction have been determined and modelled using the Avrami–Erofe'ev nucleation–growth model, consistent with the process taking place by dissolution of the starting material and crystallisation of the LDH from the solution.