(101d) Investigating Cyclization and Dehydrogenation Routes Toward the Conversion of Aromatic Compounds during Methanol-to-Olefins in MFI Framework Zeolites

Aromatic species cocatalyze the formation of olefins and may subsequently form polyaromatic species that cause catalyst deactivation during methanol-to-olefins (MTO) processes in zeolites. Our density functional theory (DFT) calculations show that aromatics form by the dehydrogenation of cyclic compounds in H-ZSM-5. Cyclization reactions to form precursors to aromatics can occur directly from unsaturated olefins, from the cycloaddition of dienes and alkenes (e.g., Diels-Alder), or by formaldehyde-mediated dehydration reactions via dienic-alcohol intermediates. We contrast these cyclic formation and dehydrogenation pathways by modeling them in both channels and intersections within MFI. Once formed, C₆ cyclization products undergo methyl-mediated dehydrogenation, estimated to occur with activation free energies of 124, 77, and 64 kJ mol^−1 more favorable than their proton-mediated counterparts for cyclohexane, cyclohexene, and cyclohexadiene, respectively. Preliminary data suggest that methyl-substituents in C₆ rings hinder dehydrogenation events when present at the geminal and ortho-position relative to the hydride transfer event (C–H heterolytic cleavage); however, when these are present at the meta- and para-positions, electronic dehydrogenation barriers remain unchanged. This suggests that the dehydrogenation of methylated-cyclization products may also contribute to the formation of toluenes and xylenes during MTO. Furthermore, dehydrogenation barriers trend opposite with degree of alkyl substitution on the hydride acceptor in alkyl-mediated routes, with activation barriers for cyclohexane dehydrogenation of 101, 76, 64, and 57 kJ mol^−1 for methyl-, ethyl-, 2-propyl, and tert-butyl-mediated routes, respectively. Finally, we contrast the bimolecular cycloaddition of dienes and alkenes to the direct cyclization of unsaturated alkenes. Preliminary data suggest that surface-bound mediated cyclization of unsaturated compounds is limited by the formation of their surface-bound species precursors, with activation barriers of 77 and 130 kJ mol^−1 for hexadiene and hexatriene, respectively. Cyclization into cyclohexene and cyclohexadiene from their corresponding precursors then proceeds with estimated barriers of 48 and 98 kJ mol^−1, respectively.