Reversible thermal behavior of the layered double hydroxide of Mg with Al: mechanistic studies

The layered double hydroxide (LDH) of Mg with Al decomposes at 450 degreesC to yield a mixed metal oxide. The oxide reconstructs back to the parent LDH either on cooling in air or soaking in water. This reversible thermal behaviour was attributed to the formation of an unstable defect rocksalt phase (comprising Al3+ ions partially substituting for Mg2+ in MgO) on decomposition of the LDH. However a simple oxide mixture of MgO and Al2O3 taken in the 6:1 molar ratio is also found to yield a LDH-like phase when soaked in an aqueous solution of a suitable anion, suggesting that the reconstruction occurs through a simple dissolution-reprecipitation mechanism. The formation of the defect rocksalt phase is not a necessary precondition for reconstruction. However, the kinetics of the reconstruction reaction depends upon the nature of the anion and the thermal history of the oxides. The presence of carbonate ions and unsintered oxides hastens the reconstruction reaction.