Deactivation of Zeolites and Zeotypes in Methanol-to-Hydrocarbons Catalysis: Mechanisms and Circumvention

Abstract

Conspectus Solid catalysts deployed in industrial processes often undergo deactivation, requiring frequent replacement or regeneration to recover the loss in activity. Regeneration occurs under conditions distinct from, and typically more harsh than, the catalysis, placing strict requirements on physicochemical material properties that divert catalyst optimization toward addressing regenerability over high activity and selectivity. Deactivation arises from mechanical, structural, or chemical modifications to active sites, promoters, and their surrounding matrices, and the prevailing mechanism for deactivation varies with the reaction, the catalyst, and the reaction conditions. Methanol-to-hydrocarbons processes utilize zeolites and zeotypescrystalline, microporous oxides widely deployed as catalysts in the refining and petrochemical industriesas solid acid catalysts. Deposition and growth of highly unsaturated carbonaceous residues within the micropores congest molecular transport and block active sites, resulting in deactivation. In this Account, we describe studies probing the underlying mechanisms of deactivation in methanol-to-hydrocarbons catalysis and discuss examples of leveraging the acquired mechanistic insights to mitigate deactivation and prolong catalyst lifetime. These fundamental principles governing carbon deposition within zeolites and zeotypes provide opportunity to broaden versatility of processes for C1 valorization and to relax constraints imposed by hydrothermal catalyst stability considerations to achieve more active and more selective catalysis. Methanol-to-hydrocarbons catalysis occurs via a chain carrier mechanism. A zeolite/zeotype cavity hosts an unsaturated hydrocarbon guest to together constitute the supramolecular chain carrier that engages in a complex network of reactions for chain carrier propagation. Productive propagation reactions include olefin methylation, aromatic methylation, and aromatic dealkylation. Methanol undergoes unproductive dehydrogenation to formaldehyde via methanol disproportionation and olefin transfer hydrogenation. Subsequent alkylation reactions between formaldehyde and active olefinic/aromatic cocatalysts instigate cascades for dehydrocyclization, resulting in the formation of inactive polycyclic aromatic hydrocarbons and termination of the chain carrier. Addition of a distinct catalytic function that selectively decomposes formaldehyde mitigates chain carrier termination without disrupting the high selectivity to ethylene and propylene in methanol-to-hydrocarbons catalysis on small-pore zeolites and zeotypes. The efficacy of this bifunctional strategy to prolong catalyst lifetime increases with increasing proximity between the active sites for formaldehyde decomposition and the H+ sites of the zeolite/zeotype. Coprocessing sacrifical hydrogen donors mitigates chain carrier termination by intercepting, via saturation, intermediates along dehydrocyclization cascades. This strategy increases in efficacy with increasing concentration of the hydrogen donor a...

Mechanisms of Alkene and Diene Hydrogenation Reactions in H-MFI and H-CHA Zeolite Frameworks During MTO

“…41 Overall barriers (relative to the Z-H state) of concerted and sequential hydrogenation are less than 10 kJ mol −1 different for C2H4, 41 indicating that the two mechanisms are likely competitive. Additionally, overall potential energy barriers of CH2O hydrogenation are facile (60 kJ mol −1 ) and are over 100 kJ mol −1 lower than those of ethene, indicating that hydrogenation of CH2O, 41 a diene precursor, 8,24,42 may contribute to the experimentally observed increases in catalyst lifetime.…”

“…Zeolite catalysts, however, are susceptible to deactivation via the formation of large, polyaromatic species thus limiting their efficiency and requiring the use of recirculating fluidized bed reactors in industrial applications. [8][9][10][11][12][13] Two complementary cycles form C2-C4 alkenes in methanol-to-olefins (MTO) processes. [14][15][16][17][18][19] Olefins methylate and grow to a size capable of cracking into C3-C5 compounds in the olefin cycle.…”

Contrasting Arene, Alkene, Diene, and Formaldehyde Hydrogenationin H-ZSM-5, H-SSZ-13, and H-SAPO-34 Zeolite Frameworks during MTO

“…Additionally, overall potential energy barriers of CH2O hydrogenation are facile (60 kJ mol −1 ) and are over 100 kJ mol −1 lower than those of ethene. Hydrogenation of CH2O, 42 a diene precursor, 8,26,44 may contribute to the experimentally observed increases in catalyst lifetime. Scheme 1.…”

“…Zeolite catalysts, however, are susceptible to deactivation via the formation of large, polyaromatic species thus limiting their efficiency and requiring the use of recirculating fluidized bed reactors in industrial applications. [8][9][10][11][12][13] Two complementary co-catalyzed cycles form products from methanol during MTO. [14][15][16][17][18][19] Alkenes methylate and grow to a size capable of cracking into C3-C5 compounds in the olefin cycle.…”

Disentangling Reaction Processes of Zeolites within Single‐Oriented Channels

“…However, the detrimental formation of coke limits catalyst activity and lifetime, serving as a major roadblock in the application of the zeolite materials in any high-demand catalytic processes, such as the methanol-to-hydrocarbons (MTH) process. [7][8][9][10][11][12] ZSM-5 coking in the MTH process has long been studied, [13] and it is known that the deactivation is caused by the coking of aromatic species, and that the deactivation heavily depends on the location of the coke. [13,[19][20][21][22][23][24] To fully elucidate the key descriptor for zeolite deactivation, it is of utmost importance to study the reaction behaviors at the level of single-oriented zeolite-channels.…”