Sintering of pseudo-boehmite and γ-Al₂O₃

Abstract

Sintering of pseudo-boehmite, acicularγ-Al₂O₃ produced by dehydration of pseudo-boehmite, andγ-Al₂O₃ ex alum was investigated. The sintering process was studied by X-ray diffraction, transmission electron microscopy with selected area electron diffraction and BET surface area measurements. The solid state reaction toα-Al₂O₃ causes a steep drop of the surface area to less than 10 m²g⁻¹. The acicular pseudo-boehmite andγ-Al₂O₃ supports exhibit an intermediate state where the acicular particles assume a rod-like shape and the surface area falls from about 300 to 100 m²g⁻¹. It was established that reaction to α-Al₂O₃ and, hence, sintering proceeds via a nucleation and growth mechanism. The rate-limiting step is nucleation of α-Al₂O₃. Consequently, the contacts between the elementary alumina particles dominate the sinter process. The contact between the acicular elementary particles of pseudoboehmite andγ-Al₂O₃ studied leads to the reaction to α-Al₂O₃ to be almost complete after keeping samples for 145 h at 1050 °C. Decomposition of alum produces very small particles showing negligible mutual contacts. Consequently an elevated thermal stability is exhibited. Treatment of the alumina ex alum with water and drying results in a xerogel in which contact between elementary particles is much more intimate. Accordingly, treatment at 1050 °C causes a sharp drop in surface area.