Study of phase transformations during calcination of aluminum hydroxides by selected area electron diffraction

Abstract

Bayerite, fl-AI(OH)3, and boehmite, 7-A1OOH, both with crystallite sizes of some tenths of a micron, were used for this investigation.The transformation of bayerite to T-alumina and of boehmite to 7-and &alumina showed a strict pseudomorphosis.7-and 7-alumina have spinel lattices that differ in disorder: T-alumina has a strong one-dimensional disorder of the cubic close-packed stacking; for 7-alumina the oxygen sublattice is fairly well ordered, whereas the tetrahedral A1 lattice is strongly disordered.The crystallographic texture of 7-and 7-alumina could be related to their microporous texture and this in its turn to the length of the OH bonds in the hydroxides.The formation of the well ordered super spinel structure ~-A120 3 is suppressed by obstructing the diffusion of the A1 ions either by the surface of numerous pores (T-alumina from bayerite or 7-alumina from gelatinous boehmite) or by incorporating large ions such as Na ions (y-alumina from crystalline boehmite).In these cases the disordered spinel type aluminas are transformed at a high temperature to 0-and a-Al~O 3 by complete recrystallization to small particles which still have some preferred orientation.