Polymethylbenzene or Alkene Cycle? Theoretical Study on Their Contribution to the Process of Methanol to Olefins over H-ZSM-5 Zeolite

Polymethylbenzene (polyMB) and alkene cycles are considered as two main routes forming light olefins in the process of methanol to olefins (MTO); however, the contribution that each cycle makes to MTO is still unclear. In this work, density functional theory considering dispersive interactions (DFT-D) was used to elucidate the catalytic roles that the polyMB and the alkene cycles may play in forming ethene and propene from methanol in MTO over H-ZSM-5. The results demonstrated that ethene and propene can be produced in nearly the same probability via the polyMB cycle, as they have a very close free energy height as well as a similar free energy barrier for the rate-determining steps. Via the alkene cycle, however, propene is the dominant product, because the methylation and cracking steps to get propene have a much lower free energy barrier in comparison with those to form ethene. As a result, ethene is predominantly formed via the polyMB cycle, whereas propene is produced via both the polyMB and the alkene cycles. The contribution of the alkene cycle is probably larger than that of the polyMB cycle, resulting in a high fraction of propene in the MTO products. Meanwhile, both cycles are interdependent in MTO, as the aromatic species generated by aromatization via the alkene cycle can also serve as new active centers for the polyMB cycle, and vice versa. Moreover, the catalytic activity of H-ZSM-5 zeolite is directly related to its acid strength; weaker acid sites are unfavorable for the polyMB cycle and then enhance relatively the contribution of the alkene cycle to forming light olefins. These results can well interpret the recent experimental observations, and the theoretical insights shown in this work may improve our understanding of the MTO mechanism, which are conducive to developing better MTO catalysts and reaction processes.