Aluminas Rehydration and Dehydration

In this review paper, results on the A12O3-water interactions will be critically examined. The synthesis of alumina precursors rich in structural defects can be operated in an aqueous solution containing ligands with high affinity for A13+aq or by restricted hydrolysis of Al-alkoxides. Calcination of the gels yields transition aluminas containing four (IV), five (V) and six (VI) fold coordinated aluminum. The pore-size distribution, the Alv content and the degree of crystallinity are controlled by the degree of condensation of the oligomer species in the gel. In a dehydrated alumina two kinds of AlIV with isotropic chemical shifts at ∼73 and 58 ppm, respectively, are present. The distorted AlIV (chemical shift at 58 ppm) and Alv are surface species. Upon water chemisorption the distorted AlIV line disappears while the Alvline is reinforced. As bulk rehydration progresses the Alv line intensity decreases. It is only partially restored upon recalcination. Thus, hydration-rehydration cycles cure the solid from its defects and increase crystallinity. The onset of rehydration is the chemisorption of water on the surface Lewis sites just as Lewis sites result from a thorough dehydration.