Oxidative dehydrogenation of 1-butene over Zn-Al ferrites

The aluminum introduction into the zinc ferrite ZnFe_2-(x)Al(x)O₄ in the range 0.0 ≤ χ ≤ 1.0 was studied. These ferrites were calcined at two temperatures, 550°C and 750°C. X-ray diffraction (XRD) patterns showed that they have a spinel structure with Al³⁺ replacing Fe³⁺ in octahedral sites. The crystallite size decreased as the aluminum introduction increased. At high calcination temperature, sintering and segregation processes occur. Linear correlations were found between XRD and Mossbauer parameters. Mossbauer spectra showed that the symmetry of the electron distribution decreased around the nucleus of Fe³⁺, as the aluminum content increased. Such asymmetrical distribution of the electron density may be associated to the spatial accommodation of aluminum in the neighboring octahedral sites. Oxygen atoms turn out to be more basic due to the charge transfer from Fe³⁺ to O^2- in the Fe-O bond. Hence the acid-base type dissociation of the C-H bond for the n-butenes activation is favored. The highest yield of butadiene was obtained for the samples treated at 750°C whereas for the samples calcined at 550°C, double bond isomerization of 1-butene occurs in competition with the oxidative dehydrogenation.