Cationic and anionic vacancies in the crystalline phases of Sol-Gel magnesia-alumina catalysts

Magnesium-alumina mixed oxide catalysts at Mg:Al atomic ratios of 3:1, 2:1.8, and 1:2.8 were synthesized by using the sol-gel technique. Dehydroxylation and phase transformations were studied with thermogravimetry. Crystalline structures were measured with X-ray powder diffraction. Position and concentration of anionic and cationic vacancies were obtained by refining crystalline structures with the Rietveld technique. When samples were annealed below 400°C, boehmite, brucite, hydrotalcite, and glushinskite were formed. When they calcined at 600°C, boehmite was transformed into γ-Al₂O₃, and brucite, hydrotalcite, and glushinskite into periclase. In magnesium-rich samples, magnesium ions were incorporated into the γ-Al₂O₃ lattice, expanding its unit cell. Sample dehydroxylation produced oxygen vacancies in boehmite, brucite, and θ-Al₂O₃. Periclase and γ-Al₂O₃, however, had cationic vacancies in concentrations that depended on calcining temperature. Different models are proposed for explaining the formation mechanisms of the anionic and cationic vacancies.