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

The topotactic transformation of boehmite (AlOOH) taking place under calcin ation and leading to gamma -alumina (gamma -Al2O3) has been investigated th eoretically 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 characteri stics appear. The proposed mechanism demonstrates the existence of an equil ibrium structure for gamma -alumina containing 25-31% of tetrahedral alumin um sites in agreement with nuclear magnetic resonance (NMR) results. Temper ature effects on the thermodynamic stability of the different structures in volved in the mechanism has been investigated. Theoretically simulated X-ra y diffraction (XRD) patterns confirm the validity of our model structures. According to this mechanism, a coherent skeleton of gamma -alumina inherite d from the original AlOOH network is constructed. This skeleton can be rega rded as a primary matrix providing an insight into structural properties of gamma -alumina. Finally, the structures involved in this process may const itute a basis for further investigation on the series of metastable alumina phases appearing before the final conversion of AlOOH to alpha -alumina (a lpha -Al2O3).