A theoretical investigation into thiophenic derivative cracking mechanism over acidic and cation-exchanged beta zeolites

A two-layer ONIOM study has been carried out to investigate the unimolecular and bimolecular cracking mechanisms of thiophene catalyzed by acidic and metal cation-exchanged (Li+, Na+, K+, Cu+, and Ag+) beta zeolites. The unimolecular cracking reactions of 3,4-dimethylthiophene (3,4-DMT) and benzothiophene (BT) catalyzed by acidic beta zeolite are also calculated. It was found that the rate-determining steps in these two mechanisms are both C-S bond cracking. Furthermore, the activation energy of C-S bond cracking in bimolecular mechanism is lower than that in unimolecular mechanism, indicating that the bimolecular cracking process is more favorable. In addition, although the activation energies of C-S bond cleavage of 3,4-DMT and BT over H-beta zeolite are relatively lower than thiophene, the practical process would be difficult to occur due to still high energy barriers. A close analysis of activation energies over cation-exchanged zeolites suggests that the metal cations do not reduce and contrarily increase their difficulty of thiophene cracking reactions. (C) 2015 Elsevier B.V. All rights reserved.