%0 Journal Article
%A Narsimhan, Karthik
%A Michaelis, Vladimir
K.
%A Mathies, Guinevere
%A Gunther, William R.
%A Griffin, Robert G.
%A Román-Leshkov, Yuriy
%D 2015
%T Methane
to Acetic Acid over Cu-Exchanged Zeolites:
Mechanistic Insights from a Site-Specific Carbonylation Reaction
%U https://acs.figshare.com/articles/journal_contribution/Methane_to_Acetic_Acid_over_Cu_Exchanged_Zeolites_Mechanistic_Insights_from_a_Site_Specific_Carbonylation_Reaction/2197111
%R 10.1021/ja5106927.s001
%2 https://acs.figshare.com/ndownloader/files/3831442
%K Br ønsted acid site
%K 8 MR pockets
%K acetic acid
%K methane oxidation
%K sites oxidize methane
%K carbonylation
%K MOR side pockets
%K UV
%K ZSM
%K EPR spectroscopic studies
%K methoxy species
%K tandem reaction sequence
%X 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 μmol/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 CuII–O–CuII sites previously associated with methane oxidation in both
Cu-MOR and Cu-ZSM-5 are oxidation active but carbonylation inactive.
In turn, combined UV–vis 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 Brønsted acid site in the 8MR to undergo carbonylation.
%I ACS Publications