The Theoretical Investigation of Oxidative Dehydrogenation of Ethane to Ethene over Fe-ZSM-5: a QM/MM Study
K. Bobuatong, J. Sirijaraensre, P. Khongpracha, P. Pantu, J. Limtrakul
Laboratory for Computational and Applied Chemistry, Physical Chemistry Division, TH
Keywords: ethene, Fe-ZSM-5 zeolite, ONIOM, oxidative dehydrogenation, ethane
Abstract: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, based on the “single site of the Fe model”, was found to be unattainable. However, the oxidative dehydrogenation of ethane via the stepwise mechanism was seen to take place. Subsequently, the shape selectivity in zeolite was systemically examined in this latter reaction. The reaction pathways were investigated at different topologies of the pores of MFI zeolite: sinusoidal and straight channels, respectively. These results nicely demonstrate that the shape selectivity for the stability of intermediate complexes in oxidative dehydrogenation of ethane is highly influenced by the channel structure in this type of molecular-sieving material. 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 12.43 and 54.94 kcal/mol for the first and second H-abstraction steps, respectively. These energy barrier values are quantitatively higher than those at the sinusoidal channel (10.33 and 4.84 for the first and second H-abstractions, respectively). The stepwise reaction taking place via the radical intermediate has been proved to be a dominant step in generating the ethene molecule. However, this process competes against the alkoxide formation, which is a key intermediate in obstructing the ethene production.