Figure 3. Cumulative moles of CO2 formed
normalized by the total iron content (left axis) and per
Fe3+ open-metal site (right axis) with increasing
activation temperature (423 - 523 K). Reaction conditions: 473 K, 14.5
kPa N2O, 1.5 kPa CH4, t = 2 h, activated
at temperature for 12 h in He.
Gagliardi, Snurr, and co-workers have reported density functional theory
(DFT)-derived energy profiles for the decomposition of
N2O over M2+ sites to form M(IV)=O
species;39,40,53 crucially, calculations reported by
Barona and Snurr suggest the potential for M3+coordinatively unsaturated sites situated in trimeric MOF nodes to
participate in methane oxidation with
N2O.54 For example, a
V3+ open-metal site in the
(Ni2+)(V3+)2mixed-metal node was proposed to be oxidized by N2O to
Ni2+(V4+=O)V4+,
with both of the trivalent vanadium centers donating electrons to form
the oxo intermediate. N2O activation barriers over
Fe3+ open-metal sites in
[(Cr2+)(Fe3+)2]
and
[(Mn2+)(Fe3+)2]
mixed-metal nodes (144 and 146 kJ mol-1, respectively)
were found to be comparable to those predicted for
Fe2+ open-metal sites on an
(Fe2+)(Fe3+)2 node
(146 kJ mol-1). The involvement of
Fe3+ sites in methane oxidation may proceed through
the involvement of metal-oxo bond formation steps of such a nature, and
a fuller understanding of these steps may require knowledge of changes
in formal oxidation states over the entire trimer rather than merely one
of three metal atoms constituting the node.