Ethylbenzene dehydrogenation over binary FeO_x-MeO _y/Mg(Al)O catalysts derived from hydrotalcites

A series of FeO_x-MeO_y/Mg(Al)O catalysts were prepared from hydrotalcite-like compounds as precursors and were tested in the ethylbenzene dehydrogenation to styrene in He atmosphere at 550 °C. The hydrotalcite-like precursors of the metal compositions of Mg₃Fe _0.25Me_0.25Al_0.5 (Me = Cu, Zn, Cr, Mn, Fe, Co and Ni) were coprecipitated from the nitrates of metal components and calcined to mixed oxides at 550 °C. After the calcination, the mixed oxides showed high surface area of 150-200 m² g_cat^-1, and were mainly composed of (MgMe)(Fe³⁺Al)O periclase in the bulk, whereas the surface was enriched by (MgMe)(Fe³⁺Al)₂O ₄ pinel. Among the Me species tested, Co²⁺ was the most effective, followed by Ni²⁺. Co²⁺ addition increased the activity of original FeO_x/Mg(Al)O catalyst, whereas Ni²⁺ increased the activity at the beginning of reaction, but deactivated the catalyst during the reaction. The other metals formed isolated MeO_x species in the catalyst, resulting in a decrease in the activity compared to the original FeO_x/Mg(Al)O catalyst. The active Fe species exists as metastable Fe³⁺ on the FeO_x/Mg(Al)O catalyst. By the addition of Co²⁺, the reduction-oxidation between Fe³⁺ and Fe²⁺ was facilitated and, moreover, the active Fe³⁺ species was stabilized. It is likely that the dehydrogenation proceeds on the active Fe³⁺ species via its reduction-oxidation assisted by Co ²⁺.