Spectroscopic definition of the copper active sites in mordenite: selective methane oxidation.
Two distinct [Cu-O-Cu](2+) sites with methane monooxygenase activity are identified in the zeolite Cu-MOR, emphasizing that this Cu-O-Cu active site geometry, having a ∠Cu-O-Cu ∼140°, is particularly formed and stabilized in zeolite topologies. Whereas in ZSM-5 a similar [Cu-O-Cu](2+) active site is located in the intersection of the two 10 membered rings, Cu-MOR provides two distinct local structures, situated in the 8 membered ring windows of the side pockets. Despite their structural similarity, as ascertained by electronic absorption and resonance Raman spectroscopy, the two Cu-O-Cu active sites in Cu-MOR clearly show different kinetic behaviors in selective methane oxidation. This difference in reactivity is too large to be ascribed to subtle differences in the ground states of the Cu-O-Cu sites, indicating the zeolite lattice tunes their reactivity through second-sphere effects. The MOR lattice is therefore functionally analogous to the active site pocket of a metalloenzyme, demonstrating that both the active site and its framework environment contribute to and direct reactivity in transition metal ion-zeolites.
†Department of Chemistry, Stanford University, Stanford, California 94305, United States.|‡Center for Surface Chemistry and Catalysis, KU Leuven, Kasteelpark Arenberg 23, 3000 Leuven, Belgium.
Pieter
P
Vanelderen
†Department of Chemistry, Stanford University, Stanford, California 94305, United States.
Benjamin E R
BE
Snyder
†Department of Chemistry, Stanford University, Stanford, California 94305, United States.
Ming-Li
ML
Tsai
†Department of Chemistry, Stanford University, Stanford, California 94305, United States.
Ryan G
RG
Hadt
‡Center for Surface Chemistry and Catalysis, KU Leuven, Kasteelpark Arenberg 23, 3000 Leuven, Belgium.
Julie
J
Vancauwenbergh
‡Center for Surface Chemistry and Catalysis, KU Leuven, Kasteelpark Arenberg 23, 3000 Leuven, Belgium.
Olivier
O
Coussens
‡Center for Surface Chemistry and Catalysis, KU Leuven, Kasteelpark Arenberg 23, 3000 Leuven, Belgium.
Robert A
RA
Schoonheydt
‡Center for Surface Chemistry and Catalysis, KU Leuven, Kasteelpark Arenberg 23, 3000 Leuven, Belgium.
Bert F
BF
Sels
†Department of Chemistry, Stanford University, Stanford, California 94305, United States.
Edward I
EI
Solomon