Mechanism of formation of aluminum trihydroxide from keggin Al13 polymers

The mechanism of formation of layered aluminum hydroxides from aluminum chloride solution progressively neutralized by caustic soda has been studied by infrared spectroscopy, small-angle X-ray scattering, and 27Al high-resolution solid-state nuclear magnetic resonance. In the starting solution most of the cationic species belong to the so-called Al13 polymer (Keggin structure) made from 12 octahedra surrounding an aluminum tetrahedron. This polymer is stable in solution for hydrolysis ratio R = (OH)(Alt) 2.8, the hydrolysis mechanism is driven faster, leading to the formation of highly polymerized octahedral layers and to the disappearance of tetrahedral aluminum. For R = 3, the short-range order characteristic of bayerite is obtained very rapidly (⋍24 h). Thus crystalline Al trihydroxides are not formed by progressive condensation of flat hexamers made from Al octahedra but by solid-state structural rearrangement, without redissolution steps.