Slight channel difference influences the reaction pathway of methanol-to-olefins conversion over acidic H-ZSM-22 and H-ZSM-12 zeolites
The methanol to olefins (MTO) process, in which low-value carbon-rich feedstocks are converted to value-added petrochemical products, is one of the most prominent alternatives for the production of light olefins. In order to reveal the confinement effect of zeolites on the catalytic reactions, the MTO mechanisms and reactivity over two unidimensional zeolites (H-ZSM-12 and H-ZSM-22) with a channel difference of only 0.3 Å have been systematically explored by DFT calculations in this work. The calculated activation barriers and reaction energies demonstrated that the 0.3 Å channel difference between H-ZSM-12 and H-ZSM-22 zeolites results in a dramatic discrepancy in their transition state selectivity associated with the aromatic-based hydrocarbon pool (HCP) mechanism. For the larger H-ZSM-12 zeolite, the formation of pentamethybenzenium cation was favored, which would be the active HCP species in the MTO reaction. For the H-ZSM-22 zeolite with a 0.3 Å smaller pore structure, the traditional methylation at the C–H sites of polymethylbenzenes occurred exclusively. When the alternative olefin-based cycle is followed for the MTO reaction, both of the zeolites are active catalysts for the formations of butene and propene. A comparison of the activation barriers for the olefin-based cycle revealed that the larger H-ZSM-12 possesses a higher catalytic activity than the H-ZSM-22 zeolite. Our theoretical results demonstrate that both the aromatic-based cycle and the olefin-based cycle can proceed during the MTO reaction over H-ZSM-12, with the latter cycle being predominant.