Hydrothermal Synthesis of Layered Double Hydroxides (LDHs) from Mixed MgO and Al2O3:  LDH Formation Mechanism

The formation of MgAl layered double hydroxide (LDH) from physically mixed MgO and Al2O3 oxides upon hydrothermal treatment has been extensively investigated, and a formation mechanism has been proposed. We observed that the formation of LDH from the oxide mixture occurs upon heating at 110 °C. In general, LDH is the major component while the minor phases are mainly determined by the initial pH of the oxide suspension as well as the MgO/Al2O3 ratio. The neutrality in the initial suspension results in a minor Mg(OH)2 as the impure phase, while the alkalinity in the suspension keeps some MgO unreacted throughout the whole hydrothermal treatment. We suggest that MgO and Al2O3 be hydrated into Mg(OH)2 and Al(OH)3, respectively, in the initial stage for all samples. We further suggest that in the neutral condition Mg(OH)2 be quickly dissociated to Mg2+ and OH- which then deposit on the surface of Al(OH)3/Al2O3 to form a MgAl pre-LDH material. Al(OH)4-, ionized from Al(OH)3 in the basic solution, deposits on the surface of Mg(OH)2/MgO to result in a similar MgAl pre-LDH material. Such a pre-LDH material is then well crystallized upon continuous heating via the diffusion of metal ions in the solid lattice. Such a dissociation−deposition−diffusion mechanism via two pathways has been supported by the phase composition, morphological features of crystallites, and [Mg]/[Al] ratios on the crystallite surface, and presumably applied to the general formation of LDHs with various synthetic methods, such as coprecipitation, homogeneous preparation, and reconstruction.