Brønsted-acid catalyzed alkylation reactions are ubiquitous in many industrially relevant processes such as alcohol dehydration, alkene oligomerization, and methanol-to-hydrocarbon (MTH) reactions. During these reactions, zeolite surfaces, alkenes, and arenes can be methylated by a combination of methanol (CH3OH) and dimethyl ether (CH3OCH3). The methylation of alkenes and arenes can occur through two distinct pathways: a sequential mechanism in which the methylation agent first reacts with the zeolite surface to form a surface methoxy species preceding the methylation of a guest species; or a concerted mechanism in which the methylation agent directly reacts with the arene. During MTH, alkenes grow through repeated methylation reactions to a size capable of cracking to form olefins. Alternatively, alkenes can undergo hydride transfer and cyclization reactions to form arenes. These arenes are methylated during MTH to form one of thirteen C6–C12 methylbenzene rings which co-catalyze the formation of light alkene products via isomerization and dealkylation reactions. Arene methylation is also important in the formation of toluene from benzene, toluene disproportionation to xylenes, and other transalkylation reactions. Despite the ubiquity of arene methylation reactions, few studies investigate methylation mechanisms across all possible arene species. Here, we utilize fully periodic DFT to investigate the methylation of all methylbenzene species via the concerted and sequential mechanisms with CH3OH and CH3OCH3. Our investigations suggest that both methylation mechanisms can occur, with the dominant mechanism depending on the number and position of methyl-substituents on the arene and the methylation agent. At MTH conditions, methylation barriers do not significantly increase as the number of methyl-substituents on the arene increases; however, the formation of higher methylated arenes is thermodynamically favorable, suggesting that C10+ species are likely to be the predominant arenes at MTH conditions. Knowledge of the dominant methylbenzene species during MTH reactions can guide future studies investigating alkene forming reactions and pathways. Furthermore, understanding these methylation routes and methylation products is a preliminary step in improving catalyst design to prevent the formation of large polyaromatic species that lead to deactivation while also maintaining desired activity during MTH reactions.

Interconversion of aromatic co-catalysts during MTO in H-MFI zeolites: Reacting methanol and dimethyl ether with C6-C12 methylbenzenes

2019 Spring National Meeting

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