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Abstract Copper‐exchanged zeolites are useful for stepwise conversion of methane to methanol at moderate temperatures. This process also generates some over‐oxidation products like CO and CO2. However, mechanistic pathways for methane over‐oxidation by copper‐oxo active sites in these zeolites have not been previously described. Adequate understanding of methane over‐oxidation is useful for developing systems with higher methanol yields and selectivities. Here, we use density functional theory (DFT) to examine methane over‐oxidation by [Cu3O3]2+active sites in zeolite mordenite MOR. The methyl group formed after activation of a methane C−H bond can be stabilized at a μ‐oxo atom of the active site. This μ‐(O−CH3) intermediate can undergo sequential hydrogen atom abstractions till eventual formation of a copper‐monocarbonyl species. Adsorbed formaldehyde, water and formates are also formed during this process. The overall mechanistic path is exothermic, and all intermediate steps are facile at 200 °C. Release of CO from the copper‐monocarbonyl costs only 3.4 kcal/mol. Thus, for high methanol selectivities, the methyl group from the first hydrogen atom abstraction stepmust bestabilizedawayfrom copper‐oxo active sites. Indeed, it must be quickly trapped at an unreactive site (short diffusion lengths) while avoiding copper‐oxo species (large paths between active sites). This stabilization of the methyl group away from the active sites is central to the high methanol selectivities obtained with stepwise methane‐to‐methanol conversion.
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Mechanistic role of water in HSSZ-13 catalyzed methanol-to-olefins conversion
Co-feeding water leads to a simultaneous attenuation of chain initiation and chain termination rates in HSSZ-13 catalyzed methanol-to-olefins (MTO) conversion. Density functional theory calculations and transient stoichiometric experiments support the plausibility of formaldehyde hydrolysis occurring over zeolitic Brønsted acid sites at MTO-relevant temperatures. A monotonic decrease in MTO chain initiation and termination rates, and a concurrent monotonic increase in total turnovers as a function of water co-feed partial pressure are consistent with the occurrence and mechanistic relevance of formaldehyde hydrolysis effected by co-fed water. Initiation/termination rates and total turnovers normalized by their corresponding values in the absence of water co-feeds at the same temperature show the expected trends as a function of reaction temperature, assuming equilibrium between formaldehyde and methanediol. These results underscore the implications of formaldehyde hydrolysis chemistry when assessing the mechanistic role of water in methanol-to-olefins conversion specifically, and deactivation mechanisms in zeolite-catalyzed hydrocarbon conversion processes more generally.
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- Award ID(s):
- 1701534
- PAR ID:
- 10113795
- Date Published:
- Journal Name:
- Catalysis Science & Technology
- Volume:
- 9
- Issue:
- 16
- ISSN:
- 2044-4753
- Page Range / eLocation ID:
- 4374 to 4383
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/C9CY01015G
