Applied Catalysis A: General 198 (2000) 235–245 Effect of AI 3+ introduction into hydrothermally prepared ZnFe 2 O 4 J.A. Toledo a,∗ , M.A. Valenzuela b , P. Bosch c , H. Armendáriz a , A. Montoya a , N. Nava a , A. Vázquez a a Instituto Mexicano del Petróleo, Coordinación de Simulación Molecular, Lázaro Cárdenas #152, A.P. 14-805, 07730, México, D.F., México b Instituto Politécnico Nacional, ESIQIE, UPALM-Zacatenco, 07738 México, D.F., México c Depto. Qu´ ımica, Universidad Autónoma Metropolitana-Iztapalapa, Av. Michoacán y la Pur´ ısima, A.P. 55-532, 09340 México, D.F., México Received 25 May 1999; received in revised form 12 November 1999; accepted 12 November 1999 Abstract The effect of aluminum introduction into the zinc ferrite ZnFe 2−x Al x O 4 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 Fe 3+ by Al 3+ 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 Fe 3+ 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 ZnFe 2 O 4 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. ©2000 Elsevier Science B.V. All rights reserved. Keywords: Spinel; Ferrites; Mixed oxides; Aluminum zinc ferrite; Mössbauer spectra; Oxidative dehydrogenation; Hydrothermal method 1. Introduction Ferrites are well known materials, often used as high frequency circuit, pigments or heterogeneous catalysts [1–4]. They present, indeed, high magnetic saturation moment, high electric resistance and catalyze selec- tive oxidation reactions. They have shown adequate ∗ Corresponding author. E-mail address: jtoledo@www.imp.mx (J.A. Toledo) activity and selectivity in the oxidative dehydrogena- tion (OXD) of alkanes and alkenes [5]. A spinel structure [6], as ferrite structure, presents two types of interstitial sites: tetrahedral and octahe- dral; these sites are surrounded by four and six oxygen atoms, respectively. The formula of the spinel oxides is AB 2 O 4 where the A 2+ ions occupy the tetrahedral and B 3+ ions the octahedral positions. In fact, most of the ferrite properties depend on the ion distribu- tion in the tetrahedral or octahedral sites in the spinel structure. It has been reported that electronic proper- 0926-860X/00/$ – see front matter ©2000 Elsevier Science B.V. All rights reserved. PII:S0926-860X(99)00514-1