Zeolite-catalyzed methanol-to-olefins (MTO) reactions involve the parallel consumption of methanol via olefins- and aromatics-based cycles. Within these complex reaction networks, aromatic species cocatalyze olefin formation and may subsequently form polyaromatic species that cause catalyst deactivation. Binding energy calculations for polycyclic species as big as pyrene suggest that these species may readily form inside MFI zeolite-type catalysts. Therefore, the mechanistic details of aromatics formation during MTO are relevant to 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 MFI, which is frequently used in MTO catalysis. Aromatics form from dehydrogenation of cycloalkane and cycloalkene species derived from cyclization of other MTO intermediates. Cyclic compounds can form via the internal 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 H<sub>2</sub>) or alkyl-assisted dehydrogenation (hydride transfer reactions) to form arenes. We contrast these cyclization 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 relevant to aromatic formation (i.e., contrasting the formation of benzene, toluene, and xylenes) during cyclization 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 are governed by the stability of their carbocation intermediates and the steric hindrances for larger transition state structures. These steric effects, in turn, are influenced by the specific location of large intermediates within MFI. Maximum rate analysis suggests that the rate-determining step in the sequential dehydrogenation of cycloalkanes into aromatics is sensitive to both temperature and pressure of gas-phase products. Entropic gains from desorption of products drive aromatic formation rates to increase with higher temperatures, and this is more relevant for subsequent dehydrogenation steps due to their inverse dependence on gas-phase pressures; as such, aromatic formation rates are inhibited by its products.

Investigating the formation of aromatic compounds during methanol-to-olefins in MFI framework zeolites

ACS Spring 2022

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