(677c) Mechanisms of Diene and Formaldehyde Formation and Their Selective Elimination By H2 to Elongate Catalyst Lifetimes during Methanol-to-Olefins Reactions
AIChE Annual Meeting
2020
2020 Virtual AIChE Annual Meeting
Catalysis and Reaction Engineering Division
Fundamentals of Catalysis and Surface Science III: Solvent Effects in Microporous Materials
Thursday, November 19, 2020 - 8:30am to 8:45am
Brønsted acid containing zeolite frameworks, such as MFI and CHA, effectively convert methanol to C2âC4 alkenes via methanol-to-olefin (MTO) processes. During MTO, C2âC4 alkenes are formed from a hydrocarbon pool consisting primarily of olefins and aromatics, which isomerize and crack to form the product alkenes. Zeolite catalysts rapidly deactivate via the formation of polyaromatic species, likely formed by reaction of aromatics with dienes, which block access to the acid site and prevent diffusion through the zeolite pores. Previous work has suggested that CH2O contributes to catalyst deactivation and co-feeding formaldehyde scavenging species, Y2O3, increases catalyst lifetimes. Furthermore, co-feeding high pressure H2 elongates catalyst lifetimes by ~2-fold. Understanding the mechanisms of diene formation and catalyst lifetime elongation by H2 are critical to mitigate catalyst deactivation. Here, we investigate diene formation via alkene disproportionation and CH2O-mediated routes and show that the critical transition states involve a hydride transfer between a surface-bound alkyl and butene, via alkene disproportionation, or CH3OH, via CH2O-routes. Both routes demonstrate similar barriers, suggesting that the relative rates of diene formation from CH2O-mediated and alkene disproportionation is governed by CH3OH:C4H8 ratios, which are typically large during MTO reactions, favoring CH2O-mediated routes. Intrinsic diene formation rates are 7 orders of magnitude larger when CH3OH reacts with a surface-bound tert-butyl alkyl, formed from isobutene, rather than a surface methyl, formed from dimethyl ether or methanol. This large rate ratio indicates that minute amounts of branched alkene products can dramatically increase the rate of CH2O, and thus diene formation. Once formed, butadiene and CH2O can be hydrogenated by H2 with barriers 10â20 kJ molâ1 lower than hydrogenation of C2âC4 alkenes and benzene in CHA and MFI frameworks. Therefore, we suggest that H2 elongates catalyst lifetimes by eliminating dienes and diene precursors (CH2O) to prevent formation of polyaromatic species.