Structure, surface area and morphology of aluminas from thermal decomposition of Al(OH)(CH3COO)2 crystals

Abstract

Crystalline aluminium hydroxiacetate was prepared by reaction between aluminium powder (AL-COA 123) and aqueous solution of acetic acid at 96°C±1°C. The white powder of Al(OH)(CH3COO)2 is constituted by agglomerates of crystalline plates, having size about 10μm. The crystals were fired from 200°C to 1550°C, in oxidizing atmosphere and the products characterized by X-ray diffraction, scanning electron microscopy and surface area measurements by BET-nitrogen method. Transition aluminas are formed from heating at the following temperatures: gamma (300°C); delta (750°C); alpha (1050°C). The aluminas maintain the original morphology of the Al(OH)Ac2 crystal agglomerates, up to 1050°C, when sintering and coalescence of the alpha-alumina crystals start and proceed up to 1550°C. High surface area aluminas are formed in the temperature range of 700°C to 1100°C; the maximum value of 198m2/g is obtained at 900°C, with delta-alumina structure. The formation sequence of transition aluminas is similar to the sequence from well ordered boehmite, but with differences in the transition temperatures and in the development of high surface areas. It is suggested that the causes for these diversities between the two sequences from Al(OH) Ac2 and boehmite are due to the different particle sizes, shapes and textures of the gamma-Al2O3 which acts as precursor for the sequence gamma-to alpha-Al2O3.