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-Al 2 O 3, and brucite, hydrotalcite, and glushinskite into periclase. In magnesium-rich samples, magnesium ions were incorporated into the γ-Al 2 O 3 lattice, expanding its unit cell. Sample dehydroxylation produced oxygen vacancies in boehmite, brucite, and θ-Al 2 O 3 . Periclase and γ-Al 2 O 3, 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.