(616f) Investigating the Formation of Aromatic Compounds during Methanol-to-Olefins in MFI Framework Zeolites

Zeolite-catalyzed methanol-to-olefins (MTO) reactions involve the parallel consumption of methanol via olefins- and aromatics-based autocatalytic cycles. These complex reaction networks include the methylation and cracking of olefins, the methylation and dealkylation of arenes, and hydride transfers and cyclization reactions between the two cycles. Aromatic species cocatalyze the formation of olefins and may subsequently form polyaromatic species that cause catalyst deactivation. Therefore, the mechanistic details of aromatics formation during MTO processes are critical to understanding both the relative propagations of the aromatic and olefin cycles, and catalyst deactivation. We aim to clarify aromatic-formation mechanisms using density functional theory (DFT) calculations in the MFI framework to gain insights into the behavior of H-ZSM-5 catalysts. Aromatic formation broadly takes place by the formation of cyclic compounds, followed by their dehydrogenation. The formation of cyclic compounds can occur via the cyclization of dienes and trienes or by bimolecular routes (e.g., Diels-Alder) involving alkenes and dienes. Once formed, cyclic compounds can undergo direct dehydrogenation (forming H₂) and alkyl-assisted dehydrogenation (hydride transfer reactions) to form arenes. We contrast these cyclic formation and dehydrogenation pathways modeled in both channels and intersections within the MFI framework. Furthermore, we assess the effects of chain length and substitution on the rates and mechanisms associated with aromatic formation (i.e., contrasting the formation of benzene and toluene, among others) in cyclic formation and dehydrogenation pathways, as well as the effect of alkyl substitution in the alkyl-assisted routes. Preliminary data suggest that dehydrogenation occurs more favorably via alkyl-assisted routes, which themselves are governed by the stability of their carbocation intermediates and the steric hindrances which occur for larger transition state structures (i.e., the 10-C atom complex formed by hydride transfer between cyclohexene and t-butyl). These steric effects, in turn, are influenced by the specific location (i.e., channels or intersections) within the MFI framework.