Aluminas rehydration and dehydration
- Univ. of Wisconsin, Milwaukee, WI (United States)
In this review paper, results on the Al{sub 2}O{sub 3}-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 Al{sub aq}{sup 3+} 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 Al{sup V} 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 Al{sup IV} with isotropic chemical shifts at {approximately} 73 and 58 ppm, respectively, are present. The distorted Al{sup IV} (chemical shift at 58 ppm) and Al{sup V} are surface species. Upon water chemisorption the distorted Al{sup IV} line disappears while the Al{sup V} line is reinforced. As bulk rehydration progresses the Al{sup V} 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.
- Sponsoring Organization:
- USDOE, Washington, DC (United States)
- OSTI ID:
- 500983
- Report Number(s):
- CONF-960401--; ISBN 1-55899-335-5
- Country of Publication:
- United States
- Language:
- English
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