Thermal Evolution of Transitional Aluminas Followed by NMR and IR Spectroscopies

Abstract

Thermal decomposition of boehmite and bayerite has been studied by X-ray diffraction and by NMR and IR spectroscopies. Coordination and site distortion of Al polyhedra have been estimated by 27Al NMR spectroscopy while IR optical properties of transitional aluminas, η-, γ-, δ-, and θ-Al2O3 were derived from IR near-normal specular reflectance technique. In the low-temperature phases, aluminum vacancies are located in tetrahedral positions in γ-Al2O3 while in η-Al2O3 they are distributed at random between tetrahedral and octahedral positions. In addition, a small amount (5%) of pentahedrally coordinated aluminum was found in γ-alumina, which has been tentatively assigned to Al at the external surface of alumina particles. At 800 °C a notable increase in structural ordering was detected in both phases; in the case of η-Al2O3, this change accelerates the continuous transformation into θ-Al2O3 which is totally achieved at 1000 °C. On the contrary, γ-Al2O3, does not show any sign of transformation until 900 °C; at 950 °C pentahedral cations disappear and formation of θ-Al2O3 is detected by IR and NMR techniques. From 900 to 1200 °C, γ-, θ-, and α-Al2O3 coexist in samples obtained from boehmite. Above 1200 °C corundum is the only thermodynamically stable phase in both series of samples.