Preferential adsorption of CO2 on cobalt ferrite sites and its role in oxidative dehydrogenation of ethylbenzene

Abstract

The role and the preferred form of CO2 adsorption in the oxidative dehydrogenation of ethylbenzene to styrene on a CoFe2O4–MCM-41 surface were investigated. It was observed from XRD, Mossbauer and magnetic results that the cobalt ferrite was maintained when the reaction was performed in the presence of CO2 and it was reduced in tests carried out in the absence of CO2, showing that CO2 has the role of regenerating the lattice oxygen and iron species of spinel, consuming H2 through the reverse water–gas-shift reaction. Thermogravimetric analyses showed that the reaction in the presence of CO2 deposited less coke compared to the test in the absence of CO2, confirming the role of CO2 in oxidation of deposited carbon through gasification of coke and the Boudouard reaction. The proposed mechanism shows that adsorption of CO2 occurs preferentially at the O2−–Fe3+–O2− octahedral sites as bicarbonate, according to TPD-CO2 and FTIR-CO2 results, where C from carbon dioxide interacts with lattice oxygen (O2−) and the oxygen from CO2 interacts with the acid site (Fe3+). The formation of a bridged bicarbonate structure on the ferrite surface occurs through a proton transfer and an intermolecular process involving the coadsorption of hydroxyl and/or carbonate groups.