(344aa) Contrasting Diene Formation Pathways with Methyls and Larger Alkyls during Methanol-to-Olefin Reactions in MFI and CHA Using DFT

Brønsted acid zeolites, such as CHA and MFI, catalyze methanol-to-olefin (MTO) processes, in which a pool of alkene and aromatic species co-catalyze the production of light olefins. Olefins that do not egress as products may undergo hydride transfers to dienes. Dienes can cyclize to form aromatics and eventually polyaromatics, which induce catalyst deactivation by obstructing acid sites. Prior studies have demonstrated the rate of catalyst deactivation decreases with the introduction of formaldehyde scavenging Y₂O₃, implicating that formaldehyde promotes polyaromatic formation. Previous studies suggest hydrogen transfer reactions govern diene formation; however, these reaction pathways have not been extensively studied with DFT. Here, we investigate two potential pathways for diene formation during MTO processes–alkene disproportionation and CH₂O-mediated– in CHA and MFI zeolite frameworks to provide insight into catalyst deactivation. Multiple routes are studied for each mechanisms; however, we find that the sequential route, which involves a hydride transfer between a surface-bound alkyl and donor species (C₄H₈ or CH₃OH for alkene disproportionation and CH₂O-mediated, respectively), is preferred for both mechanisms. Therefore, the ability of C₄H₈ and CH₃OH to participate in hydride transfers determines which pathway dominates. Barriers of both pathways are comparable for C₁–C₃ surface-bound alkyls, suggesting the preferred mechanism is largely governed by the ratio of CH₃OH to C₄H₈ pressures; because of higher CH₃OH pressures at MTO conditions, the CH₂O-mediated pathway is likely preferred. Due to their larger size and therefore enhanced carbocation stability, surface-bound C₄ species react readily with CH₃OH at rates > 10⁷-times faster than C₁ alkyls. Steric interactions hinder hydride transfer between surface-bound C₄ and C₄H₈–making these reactions unfavorable. Overall, we find dienes are likely formed via the reaction of CH₃OH and surface-bound tert-butyl in MFI and CHA—imparting valuable insight into the mechanism of diene formation that can potentially be exploited to mitigate catalyst deactivation.