Corundum α-Al2O3 Formation from the Dehydration of Boehmite γ-AlOOH under Hydrothermal Conditions. II. The Reaction Rates and the Mechanism

Abstract

The authors have measured the reaction rates of corundum formation from boehmite under some hydrothermal conditions for the purpose of discussing the reaction mechanism. They have proposed that, in such cases as the crystallization of corundum or boehmite, there exists the hydrated poly ion of the structure intimately related to the lattice of the product phase and that the formation process often becomes the rate-determinant if no seed crystals of the product phase have been added beforehand. The seed crystal supplies the poly ions which accelerate the simple ions to transform themselves into the poly ions. The mechanism has been illustrated schematically and discussed in detail.

Glycothermal synthesis of metal oxides

“…Practical synthesis of α-alumina under hydrothermal conditions has been examined by many researchers. For example, Yamaguchi et al [14] reported that 445 • C and 10 h are required for complete conversion of boehmite into α-alumina even though the reaction was carried out in a 0.1N NaOH solution in the presence of α-alumina seed crystals. On the other hand, under glycothermal conditions, complete conversion into α-alumina was attained at 285 • C for 4 h. The difference between glycothermal and hydrothermal reaction can be attributed to the stability of the intermediate phases, that is, the glycol derivative of boehmite is thermodynamically much less stable than well-crystallized boehmite and therefore the former compound has a larger driving force for conversion into α-alumina.…”