Effect of AI3+ introduction into hydrothermally prepared ZnFe2O4 Original Research Article

The effect of aluminum introduction into the zinc ferrite ZnFe2−xAlxO4 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 O2−, 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.