The structure and phase transformations in fumed transition alumina by X-ray diffraction and solid-state NMR

Powder X-ray diffraction, 27Al solid-state NMR and TEM were used to study the structure and phase transformations in a series of transition alumina materials with BET surface areas ranging from 220 to 30 m2/g produced by gas-to-particle flame method. During the short-lived synthesis, highly disordered γ-alumina progressively transforms into a mixture of δ and θ-alumina. 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 AlO 4 sites pertaining to the δ phase alone. We propose here based on data acquired on high-quality δ and θ-alumina that the bulk structures of both phases must be ordered.