Theoretical study of the dehydration process of boehmite to γ-alumina

The topotactic transformation of boehmite (AlOOH) taking place under calcination and leading to gamma -alumina (gamma -Al2O3) has been investigated theoretically and a step-by-step mechanism for the structural transformation is proposed. This mechanism reveals two major steps. First, the structural collapse of boehmite occurs, after hydrogen transfers and water extraction. Then, through an aluminum migration process, the gamma -alumina characteristics appear. The proposed mechanism demonstrates the existence of an equilibrium structure for gamma -alumina containing 25-31% of tetrahedral aluminum sites in agreement with nuclear magnetic resonance (NMR) results. Temperature effects on the thermodynamic stability of the different structures involved in the mechanism has been investigated. Theoretically simulated X-ray diffraction (XRD) patterns confirm the validity of our model structures. According to this mechanism, a coherent skeleton of gamma -alumina inherited from the original AlOOH network is constructed. This skeleton can be regarded as a primary matrix providing an insight into structural properties of gamma -alumina. Finally, the structures involved in this process may constitute a basis for further investigation on the series of metastable alumina phases appearing before the final conversion of AlOOH to alpha -alumina (alpha -Al2O3).