The selective conversion of methane to oxygenated fuels is an attractive strategy for obtaining value-added chemicals from abundant natural gas. To reduce complexity, direct low temperature oxidative routes are highly desired. The activation of C-H bonds is kinetically accessible, as in enzymatic systems such as methane monooxygenases.1 Among promising inorganic analogs are copper exchanged ZSM-5 (Cu-ZSM-5) and mordenite (Cu-MOR)2 where CuII-O-CuII species selectively oxidize methane to methanol. Methane is oxidized into a surface bound methoxy group and is then hydrolyzed into methanol with water. In Cu-ZSM-5, the CuII-O-CuII core abstracts hydrogen from methane to form a methyl radical, which then rebounds to form a methoxy species bound to a copper center.3 It is not clear if the methyl radical 1) rebounds onto the same reactive center to form a metal-methoxy species, 2) travels to another metal site, or 3) readsorbs elsewhere. In this work, the conversion of methane to acetic acid is demonstrated in Cu-MOR using a tandem oxidation/carbonylation reaction sequence. The carbonylation reaction serves as a site-specific probe to locate the methoxy groups within the zeolite,4 thereby providing important mechanistic insight for methane oxidation. After the oxidation/carbonylation sequence, Cu-MOR with Bronsted acidity generates more acetic acid. We hypothesize that a fraction of the methoxy species are bound to the Cu2+ species are labile and capable of diffusing and adsorbing onto acid sites in the 8MR pockets of MOR as carbonylation active methoxy species. A novel copper site was observed when Bronsted acidity was in Cu-MOR.

Finding new methane oxidation (NEMO) sites in metal-exchanged zeolites: Insights from the tandem oxidation and carbonylation of methane to acetic acid in Cu-MOR

251st National Meeting (2016)

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