Cationic and Anionic Vacancies in the Crystalline Phases of Sol−Gel Magnesia−Alumina Catalysts

Abstract

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.

Influence of calcination temperature on textural and structural properties, reducibility, and catalytic behavior of mesoporous γ-alumina-supported Ni–Mg oxides by one-pot template-free route

“…With further elevating the calcination temperature, the reduction peak temperatures for both dispersed NiO species and aluminate-like species in TPR profiles increased, implying that the interaction between Ni species and support strengthened with the calcination temperature. On the other hand, the incorporation of divalent Mg ions in the tetrahedral or octahedral sites in c-Al 2 O 3 structure resulted in a decrease in the cationic deficiency of c-Al 2 O 3 , thus stabilizing c-Al 2 O 3 -NiAl 2 O 4 nanoparticles and elevating the reduction temperature of Ni 2+ ions in spinel phase [56]. As a whole, the increase in the reduction peak intensity for aluminate-like species with the calcination temperature suggested that more Ni ions diffused into the lattices of c-alumina to form spinel.…”

Comparative Study of Mg/M(III) (M=Al, Ga, In) Layered Double Hydroxides Obtained by Coprecipitation and the Sol–Gel Method

“…In order to modify the textural properties of LDHs and their calcination products, different LDHs have been obtained using the sol-gel method. This includes the hydrolysis of alkoxide precursors and has been used to obtain solids of different nature, including TiO 2 -MgO (18), MgO-Al 2 O 3 (19) and Ga 2 O 3 -Al 2 O 3 (20) systems. This paper reports on the synthesis of Mg/Al, Mg/Ga and Mg/In LDH using the sol-gel method.…”

Nanocomposite Based Multifunctional Coatings

“…For instance the polyol method performed by the hydrolysis of the metal acetate salts in ethylene glycol or di ethylene glycol under heating produces pure acetate intercalated LDH, this in absence of pH and atmosphere control and without an excess of a base (Prevot et al, 2005). Alternatively the sol-gel route in an organic medium yields LDH materials after hydrolysis and condensation polymerization reactions of metal alkoxide precursors (Wang et al, 1999). Of lower relevance for large scale production and from application related aspects, reversed micelles or micro emulsions may be employed to drastically change the LDH platelets morphology into fiber-like particles (Hu & O'Hare, 2005), and a sophisticated method using inverse opals is reported to yield three dimensional ordered macroporous LDH materials, the polystyrene beads based opal array acting as a sacrificial template (Géraud et al, 2006).…”