Effect of Calcination on the Structure and Texture of Gamma-Alumina Prepared by Thermolysis of Aluminium Sulphate

Abstract

Thermal decomposition of aluminium sulphate at 1173 K produces mesoporous γ-alumina with a BET surface area of 134 m² g⁻¹. This active alumina was subsequently calcined in air at temperatures ranging from 1173 to 1523 K. The microcrystalline structure, surface area and porous texture of the resulting materials were analysed by X-ray diffraction and nitrogen adsorption measurements. The results suggest that γ-alumina is converted into the α-modification in the sequence: γ-alumina → δ-alumina → θ-alumina → α-alumina. All oxides calcined within the temperature range 1173–1473 K were found to be mesoporous. The most frequent pore radius increased slowly, from 10.5 to 13.5 nm, as the temperature was raised from 1173 to 1423 K. However, calcination at 1473 K resulted in a sharp increase of pore radius (up to 18.5 nm) with simultaneous reduction of the BET surface area and total pore volume by over 75%. The sample fired at 1523 K showed a BET surface area smaller than 5 m² g⁻¹.