Thermal behaviour of alumina precursor obtained by the aluminium sulphate–urea reaction in boiling aqueous solution

An aqueous solution containing 0.1 M Al 3+ and excess urea was boiled for 4 h to obtain an alumina precursor precipitate. The precursor precipitate was separated into several portions and each portion was heated at a different temperature for 4 h between 100–1100 °C. Thermally treated samples were investigated by X-ray diffraction (XRD) analysis, differential thermal analysis (DTA), thermogravimetric analysis (TGA) and scanning electron microscopy (SEM) techniques. The particle size distributions (PSD) and nitrogen adsorption–desorption isotherms at 77 K of the same samples were also determined. The XRD data showed that the previously amorphous precursor was transformed respectively into δ-Al2O3 at 850 °C and α-Al2O3 at 1100 °C. The TGA data were used to calculate the activation energies for the dehydration and dehydroxylation of the precursor according to the Coats–Redfern equation and the results were discussed. From the SEM and PSD data, it was determined that most of the precursor particles were agglomerated. The adsorption data were used to calculate the specific surface areas (A) and the desorption data were used to calculate the specific micropore–mesopore volumes (V). At 100 °C, the A and V values were respectively, 15 m2 g−1 and 0.030 cm3 g−1. They increased by increasing temperature and reached their maximum values of 117 m2 g−1 and 0.350 cm3 g−1 respectively at 900 °C and then decreased promptly at 1000 °C.