Catalytic mechanism of the dehydrogenation of ethylbenzene over Fe-Co/Mg(Al)O derived from hydrotalcites

Catalytic mechanism of ethylbenzene dehydrogenation over Fe-Co/Mg(Al)O derived from hydrotalcites has been studied based on the XAFS and XPS catalyst characterization and the FTIR measurements of adsorbed species. Fe-Co/Mg(Al)O showed synergy, whereas Fe-Ni/Mg(Al)O showed no synergy, in the dehydrogenation of ethylbenzene. Ni species were stably incorporated as Ni²⁺ in the regular sites in periclase and spinel structure in the Fe-Ni/Mg(Al)O. Contrarily, Co species exists as a mixture of Co³⁺/Co²⁺ in the Fe-Co/Mg(Al)O and was partially isolated from the regular sites in the structures with increasing the Co content. Co addition enhanced Lewis acidity of Fe³⁺ active sites by forming Fe³⁺-O-Co ^3+/2+(1/1) bond, resulting in an increase in the activity. FTIR of ethylbenzene adsorbed on the Fe-Co/Mg(Al)O clearly showed formations of C-O bond and π-adsorbed aromatic ring. This suggests that ethylbenzene was strongly adsorbed on the Fe³⁺ acid sites via π-bonding and the dehydrogenation was initiated by α-H⁺ abstraction from ethyl group on Mg²⁺-O^2- basic sites, followed by C-O-Mg bond formation. The α-H⁺ abstraction by O^2-(-Mg ²⁺) was likely followed by β-H abstraction, leading to the formations of styrene and H₂. Such catalytic mechanism by the Fe ³⁺ acid-O^2-(-Mg²⁺) base couple and the Fe ³⁺/Fe²⁺ reduction-oxidation cycle was further assisted by Co³⁺/Co²⁺, leading to a good catalytic activity for the dehydrogenation of ethylbenzene.