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 Ni(2+) in the regular sites in periclase and spinel structure in the Fe-Ni/Mg(Al)O. Contrarily, Co species exists as a mixture of Co(3+)/Co(2+) 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 Fe(3+) active sites by forming Fe(3+)-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 pi-adsorbed aromatic ring. This suggests that ethylbenzene was strongly adsorbed on the Fe(3+) acid sites via pi-bonding and the dehydrogenation was initiated by alpha-H(+) abstraction from ethyl group on Mg(2+)-O(2-) basic sites, followed by C-O-Mg bond formation. The alpha-H(+) abstraction by O(2-)(-Mg(2+)) was likely followed by beta-H abstraction, leading to the formations of styrene and H(2). Such catalytic mechanism by the Fe(3+) acid-O(2-)(-Mg(2+)) base couple and the Fe(3+)/Fe(2+) reduction-oxidation cycle was further assisted by Co(3+)/Co(2+), leading to a good catalytic activity for the dehydrogenation of ethylbenzene. (C) 2011 Elsevier B.V. All rights reserved.