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 degrees C, boehmite, brucite, hydrotalcite, and glushinskite were formed. When they calcined at 600 degrees C, boehmite was transformed into gamma-Al2O3, and brucite, hydrotalcite, and glushinskite into periclase. In magnesium-rich samples, magnesium ions were incorporated into the gamma-Al2O3 lattice, expanding its unit cell. Sample dehydroxylation produced oxygen vacancies in boehmite, brucite, and theta-Al2O3. Periclase and gamma-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.