Effect of AI³⁺ introduction into hydrothermally prepared ZnFe₂O₄

The effect of aluminum introduction into the zinc ferrite ZnFe₂-_xAl_xO₄ spinel structure was studied in the concentration range of 0.0 ≤ x ≤ 1.0. Spinel ferrites were obtained by hydrothermal method at 140°C in the whole range of Al concentration (x). The resulting powders were calcined at 550 and 750°C, and their physicochemical and catalytic properties were compared with those prepared by coprecipitation method [J.A. Toledo, P. Bosch, M.A. Valenzuela, A. Montoya, N. Nava, J. Mol. Catal. 125 (1997) 53]. As in coprecipitated samples, isomorphic substitution of Fe³⁺ by Al³⁺ into the octahedral sites was observed. The aluminum introduction gives rise to a lattice distortion caused by the introduction of a metal with smaller atomic radius. This lattice distortion facilitates a charge transfer from Fe³⁺ to O^2-, which increases the basicity of the oxygen atoms in the Fe-O-Al bonds, increasing its proton affinity. Therefore, it favors the acid-base dissociation that takes place in the C-H bond during the abstraction of the hydrogen atom in the oxidative dehydrogenation (OXD) process. After calcination at 550°C, a maximum in the intrinsic activity and butadiene yield was obtained for an Al concentration of 0.2 ≤ x ≤ 0.5, whereas in coprecipitated catalysts, a higher aluminum content was necessary (0.75 ≤ x ≤ 1.0). In the hydrothermally treated samples calcined at 750°C, the promoter effect of aluminum was not evident. Indeed, the pure ZnFe₂O₄ showed the maximum activity to butadiene. However, the activity of the structural surface sites increased by a factor about 2 as the calcining temperature increased from 550 to 750°C.