Structure and surface acid property of aluminas chlorinated at high temperature

Structure and surface acid properties of eta- and gamma-aluminas chlorinated at high temperature were characterized using XRD, SEM, IR, XRF and XPS. When the aluminas were treated in Cl-2 and HCl gas, transformation from eta- or gamma-phase to theta- and alpha-phases drastically occurred. Especially, cubic alpha-alumina crystallite particles of 2.0-2.5 mu m were formed by the treatment with HCl (93 kPa) at 1273 K. The transformation was promoted in the presence of Na as bulk impurity. In the IR absorption band of vibration of pyridine-ring on the aluminas chlorinated in chlorine (11 kPa), only four peaks were assigned to pyridine molecules coordinated on Lewis acid sites, and shifted to the high frequency by 5 cm(-1). The intensity of the 19b mode peak after outgassing at 773 K for 30 min showed a maximum value at 973-1123 K, which was beyond two times that of the sample treated in vacuo at the same temperature. Correlation was also established between the intensity of the 19b mode peak and the amount of chlorine on the surface. It was suggested that the chlorine atoms bonding to aluminium cation(2.1 atoms nm(-2)) promoted Lewis acidity strength of the nearest aluminium cations and the binding energies of Na, Si and Ti as bulk impurity corresponded to their oxide forms, of which the total amount did not exceeded the number of lattice defects in gamma-alumina spinel structure.