The Theoretical Investigation of Oxidative Dehydrogenation of Ethane to Ethene over Fe-ZSM-5: A QM/MM Study

The complete detailed reaction mechanism for the oxidative dehydrogenation of ethane over Fe-ZSM-5 zeolite has been systematically investigated by means of the ONIOM(MP2/6-31G(d,p):UFF)//ONIOM(B3LYP/6-31G(d,p):UFF) scheme. Two types of reaction mechanisms for the oxidative dehydrogenation of ethane have been suggested: stepwise and concerted. The concerted mechanism, the concurrent abstraction of two hydrogen atoms from ethane was found to be unattainable. Two routes of the stepwise pathway were proposed. The reaction at the straight channel takes place via the alkoxide intermediate, while the key intermediate of the reaction occurring at the sinusoidal channel is an “ethyl radical” one. The activation energies of the reaction observed at the straight channel are 12.4 and 54.9 kcal/mol, which is quantitatively higher than those at the sinusoidal channel (10.3 and 4.8 kcal/mol). The stepwise reaction taking place via the radical intermediate has been proved to be a dominant step in generating the ethene molecule.