(344aa) Contrasting Diene Formation Pathways with Methyls and Larger Alkyls during Methanol-to-Olefin Reactions in MFI and CHA Using DFT
AIChE Annual Meeting
2020
2020 Virtual AIChE Annual Meeting
Catalysis and Reaction Engineering Division
Poster Session: Advances in Zeolite Science and Technology
Friday, November 20, 2020 - 8:00am to 9:00am
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 processesâalkene disproportionation and CH2O-mediatedâ 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 C1âC3 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 C4H8âmaking these reactions unfavorable. Overall, we find dienes are likely formed via the reaction of CH3OH and surface-bound tert-butyl in MFI and CHAâimparting valuable insight into the mechanism of diene formation that can potentially be exploited to mitigate catalyst deactivation.