Catalytic properties of iron oxide. III. Oxidative dehydrogenation of butenes over iron oxide catalysts.

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Oxidative dehydrogenation of butenes was investigated by use of a flow reactor over three iron oxide catalysts having different structures, i.e., α-Fe2O3, γ-Fe2O3, and Fe3O4, which were prepared in situ by controlled reduction and reoxidation with a pulse method. Under the working conditions of catalytic oxidation, the structures of α- and γ-Fe2O3 remained unchanged, while Fe3O4 was oxidized to γ-Fe2O3, the oxidation state of iron being Fe(III) in all of three iron oxide catalyst. These results are consistent with the kinetics that the reaction was of zeroth order in the partial pressure of oxygen and of nearly first order in the butene partial pressure. γ-Fe2O3, as well as γ-Fe2O3 formed from Fe3O4 under the catalytic conditions, was very active and selective for the reaction, as expected from the results obtained previously with a pulse reactor. It was concluded from the following facts that the oxygen species responsible for this reaction is lattice oxygen of γ-Fe2O3 and that the reaction proceeds by the redox cycles of γ-Fe2O3\rightleftharpoonsγ-Fe2O3−x. i) Reactivity of γ-iron oxide surface in the stoichiometric oxidation of butenes to butadiene did not change, when the surface oxygen was stepwisely removed by CO-pulses from γ-Fe2O3 to γ-Fe2O2.8. ii) Formation of butadiene from a butene pulse was little affected by whether butenes were pulsed with oxygen or not.
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