Structure and phase changes of alumina produced by flame hydrolysis

Fumed alumina from the combustion of AlCl₃ produced nano particles with specific areas from 30 to 220 m² g^-1 (BET) which were characterized by powder X-ray diffraction, ²⁷Al solid-state NMR and transmission electron microscopy. During the short-lived synthesis, highly disordered γ-alumina progressively transforms into a mixture of δ and θ-alumina. For the γ-alumina particles, only for particles with the highest specific area a significant amount of five-coordinated Al can be found which is only partially located in the particle surface. Water can be bound reversibly and increases the coordination number of aluminium atoms in the particle surface. For the well-crystallized mixture of δ and θ-alumina, high resolution and high S/N powder XRD pattern features a large number of superstructure reflections along with the commonly observed diffuse reflections. ²⁷Al MQMAS NMR provides a total of 8 crystallographic sites with an unusually high resolution in the tetrahedral region, with 4 distinct AlO₄ sites pertaining to the δ phase alone. The results suggest that the δ-alumina phase produced in this process can be described as an ordered structure.