Correlation between the magnetism of non-stoichiometric zinc ferrites and their catalytic activity for oxidative dehydrogenation of 1-butene

doi:10.1016/S0926-860X(02)00212-0

Applied Catalysis A: General

Volume 234, Issues 1–2, 8 August 2002, Pages 137-144

https://doi.org/10.1016/S0926-860X(02)00212-0 Get rights and content

Abstract

Non-stoichiometric zinc ferrites, which were used as catalysts for the oxidative dehydrogentation of 1-butene, were synthesized by using co-precipitation and hydrothermal methods. Their atom local order was analyzed with Mössbauer spectroscopy and the refinement of the crystalline structures; their macroscopic magnetization was measured by the vibrating sample method. Their magnetic and catalytic properties depended on the synthesis conditions; the analysis of the local atom distribution shows that these properties were related to the occupation of the tetrahedral sites with iron ions. This occupation was larger when the ferrites were prepared under hydrothermal conditions than by co-precipitation. The parallelism between the macroscopic magnetization of the ferrites and their capacity of transforming 1-butene into butadiene, CO₂ and 2-butene, by an oxidative dehydrogenation reaction, suggests that the “freezing” of the magnetic moments in the octahedral sites could cause this catalytic behavior.

Introduction

Oxidative dehydrogenation of hydrocarbons is commonly catalyzed with a transition metal oxide [1]; for example hematite, which is very active for transforming butene [2], [3], [4] and ethylbenzene [5]; its selectivity for converting butene into butadiene, however, is low [6]. To increase its selectivity for this reaction Zn (Mg, Ni or Co) oxide is added to it, giving rise to a ferrite characterized by having the spinel crystalline structure [7].

For 1-butene oxidative dehydrogenation, the catalytic activity of a ferrite strongly depends on the cation distribution in lattice [7], [8]. For instance, the MgFe₂O₄ and the γ-Fe₂O₃ ferrites, which have an inverted spinel structure (magnesium and iron can occupy octahedral and tetrahedral positions), have similar catalytic activity but higher than the one of stoichiometric ZnFe₂O₄ ferrite, whose crystalline structure is non-inverted. These results suggest that the distribution of iron in the tetrahedral and octahedral sites of a ferrite could determine this catalytic activity. Therefore, it is necessary to study in detail the atomic distribution in ZnFe₂O₄ ferrite.

The spinel-type crystalline structure has the formula AB₂O₄; where A corresponds to eight equivalent sites with a tetrahedral symmetry produced by oxygen anions and B to 16 sites with an octahedral symmetry also produced by oxygen ions, both kind of sites are occupied by cations. In the stoichiometric ZnFe₂O₄ ferrite, tetrahedral sites are occupied by Zn²⁺ ions, while the octahedral ones by Fe³⁺ ions [9].

Fe³⁺ ions have a magnetic moment because of the partially filled 3d orbitals; when the sites with octahedral symmetry are not occupied by another magnetic ion, Fe³⁺ ions interact magnetically between each other producing an anti-ferromagnetic order with a Néel temperature of 10 K [9], [10]. Recently, however, it is reported that nanocrystalline ZnFe₂O₄ shows partial inversion producing ferrimagnetic order even at room temperature [11], [12], [13], [14], [15]. This ferrimagnetism, however, can also be explained by assuming that the zinc ferrite is non-stoichiometric, as reported by Hochepied and co-workers [16], [17], who used a novel synthesis method and showed that cation and anion distribution determined the magnetic behavior. The cation distribution is responsible of the number of vacancies and the non-stoichiometry, altering bulk and surface properties of the corresponding system, which is of special interest for catalysis [18].

In the present work, non-stoichiometric zinc ferrite was synthesized by using two methods: co-precipitation and crystallization under hydrothermal conditions. With both methods it was possible to change the iron distribution in the tetrahedral sites of the ferrite. Samples were characterized with X-ray powder diffraction, Mössbauer spectroscopy, vibrating sample magnetometry; eventually they were tested for oxidative dehydrogenation of 1-butene.

Section snippets

Co-precipitation

The co-precipitation was carried out from an aqueous solution of zinc and iron nitrates (Aldrich, 99.9%) in stoichiometric amounts (Fe/Zn atomic ratio=2) and an aqueous solution of NH₄OH (Baker) (50 vol.%). The precipitation was done at 50 °C at a pH of 7.5, producing a gel, which was washed with distilled water, filtered and dried at 120 °C overnight in air. The dried powder was annealed in airflow for 6 h at 550 or 750 °C.

Hydrothermal

The synthesis under hydrothermal conditions was carried out in a 500 ml

Results and discussion

Calcined samples contained only two crystalline phases: ZnFe₂O₄ (franklinite) and α-Fe₂O₃ (hematite) (Fig. 1, Fig. 2). ZnFe₂O₄ has the spinel structure, which was refined with a cubic unit cell having the symmetry described by the space group Fd3m and the initial atom positions given in [23]. α-Fe₂O₃ has the corundum structure, which was refined with a hexagonal unit cell described by the space group R-3c and the initial atom positions given in reference [24].

The elemental analysis of the

Conclusions

The magnetic and catalytic properties of non-stoichiometric zinc ferrites depend on their synthesis conditions. The analysis of the local atom distribution via Mössbauer spectroscopy and X-ray diffraction shows that these properties were related to the occupation of the tetrahedral sites with iron ions. This occupation was larger when the ferrites were prepared under hydrothermal conditions than by co-precipitation. The parallelism between the macroscopic magnetization of the ferrites and their

Acknowledgements

We would like to thank Mr. A. Morales and Mr. M. Aguilar for technical assistance. This work was financially supported by IMP Project D.01234.

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Correlation between the magnetism of non-stoichiometric zinc ferrites and their catalytic activity for oxidative dehydrogenation of 1-butene

Abstract

Introduction

Section snippets

Co-precipitation

Hydrothermal

Results and discussion

Conclusions

Acknowledgements

J. Catal.

J. Catal.

J. Catal.

Appl. Catal.

J. Magn. Magn. Mater.

J. Phys. Chem. Sol.

J. Magn. Magn. Mater.

J. Solid State Chem.

Appl. Catal. A

Solid State Commun.

J. Catal.

Ind. Eng. Chem. Prod. Res. Dev.

J. Phys. Chem.

J. Phys. Chem.

J. Phys. Chem.

Phys. Rev. B