The dehydration of boehmite, γ-AlOOH,to γ-Al2O3

Electron microscopic observations of microstructure, together with studies of the evolution of X-ray and electron diffraction patterns, have been used to provide mechanistic information on the dehydration of boehmite to γ-Al₂O₃. Firing boehmite in air at 400°C produces slow development of a fine, lamellar, porous microstructure, oriented parallel to (001)_γ, the dimensions of which are consistent with the loss of one-quarter of the oxygen atoms of the boehmite lattice. The mechanism proposed for the dehydration is controlled by diffusion in a direction determined by the hydrogen bond chains in the boehmite structure and involves countermigration of the Al cations and protons with crystallographic formation of voids in a coherent cubic close-packed matrix. Final reorganization to give the spinel structure of γ-Al₂O₃ is suggested to involve gradual filling of the tetrahedral cation sites.