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

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 degrees 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 degrees 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.