Heating Properties of Boemite (AlO(OH)) Gel Prepared by Alkoxy-Method

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Abstract
The chemical properties of boemite gel prepared by alkoxy-method were studied by means of differential thermal analysis, thermogravimetric analysis and X-ray diffraction. 1) When boemite gel was heated in air at an atmospheric and reduced pressures, it was converted into a-Al2O3 via the following steps. boemite gel→amorphous alumina→γ-A12O3→θ-Al2O3→a-A12O3 2) Boemite gel, in the case of sample weight 1 g, was easily converted into amorphous alumina gel by grinding in air, but this amorphous alumina gel did not change in spite of grinding for 250 hrs. On the other hand, the ground amorphous alumina gel was directly converted into α-Al2O3 at 990°C by heating. 3) In the conversion to α-Al2O3 through θ-Al2O3 from γ-Al2O3, the nucleation process may be the rate-determining step. The apparent activation energy for this process was about 115 kcal/mol. 4) Reactions of γ-Al2O3 and α-Al2O3 with ZnO were fitted to Dander's equation. At 850°C, the reaction rate constant of ZnO-γ⋅Al2O3 system was found to be about eighty six times as large as that of ZnO-a⋅Al2O3 system. The activation energies of ZnAl2O4 formation in ZnO-γ⋅Al2O3 system and ZnO-α⋅Al2O3 system were about 51 kcal/mol and 88 kcal/mol, respectively.

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