Journal
JOURNAL OF THE AMERICAN CHEMICAL SOCIETY
Volume 137, Issue 5, Pages 1825-1832Publisher
AMER CHEMICAL SOC
DOI: 10.1021/ja5106927
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Funding
- Massachusetts Institute of Technology Energy Initiative's Seed Fund
- NSF [CHE-9808061, DBI-9729592]
- National Institutes of Health [EB-002026]
- Natural Sciences and Engineering Research Council of Canada
- Government of Canada
- Netherlands Organization for Scientific Research (NWO)
- MIT-Japan Hayashi Seed Grant Project
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The selective low temperature oxidation of methane is an attractive yet challenging pathway to convert abundant natural gas into value added chemicals. Copper-exchanged ZSM-5 and mordenite (MOR) zeolites have received attention due to their ability to oxidize methane into methanol using molecular oxygen. In this work, the conversion of methane into acetic acid is demonstrated using Cu-MOR by coupling oxidation with carbonylation reactions. The carbonylation reaction, known to occur predominantly in the 8-membered ring (8MR) pockets of MOR, is used as a site-specific probe to gain insight into important mechanistic differences existing between Cu-MOR and Cu-ZSM-5 during methane oxidation. For the tandem reaction sequence, Cu-MOR generated drastically higher amounts of acetic acid when compared to Cu-ZSM-5 (22 vs 4 mu mol/g). Preferential titration with sodium showed a direct correlation between the number of acid sites in the 8MR pockets in MOR and acetic acid yield, indicating that methoxy species present in the MOR side pockets undergo carbonylation. Coupled spectroscopic and reactivity measurements were used to identify the genesis of the oxidation sites and to validate the migration of methoxy species from the oxidation site to the carbonylation site. Our results indicate that the Cu-II-O-Cu-II sites previously associated with methane oxidation in both Cu-MOR and Cu-ZSM-5 are oxidation active but carbonylation inactive. In turn, combined UVvis and EPR spectroscopic studies showed that a novel Cu2+ site is formed at Cu/Al <0.2 in MOR. These sites oxidize methane and promote the migration of the product to a Bronsted acid site in the 8MR to undergo carbonylation.
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