3.1. Active site requirements for methanol and
CO2 formation
We previously reported the oxidation of methane to methanol and
CO2 at low temperatures (423 – 483 K) and sub-ambient
pressures over tri-iron oxo clusters hosted by MIL-100(Fe)
nodes.35 Co-feeding N2O and methane at
473 K over MIL-100(Fe) activated at 523 K resulted in transient
dinitrogen formation rates that decrease asymptotically toward a
constant value and CO2 formation
rates that appear to be invariant in time (Figure 1). Extraction with
water vapor (0.35 kPa, 473 K) subsequent to exposure to methane and
N2O resulted in the formation of methanol, the
cumulative yield of which asymptotically approaches a maximum value with
reaction time (Figure 1). Reported N2 formation rates
are consistent with those expected based on the combination of methane
oxidation to methanol, CO2 and water based on the
reaction stoichiometry (N2O + CH4 →
N2 + CH3OH; 4N2O +
CH4 → 4N2 + CO2 +
2H2O), as reflected by the close correspondence between
measured cumulative nitrogen yields and those estimated from reaction
stoichiometry (Figure 1). The maximum cumulative methanol yield measured
corresponds to the theoretical maximum density of Fe2+sites in MIL-100 (0.33 mol Fe2+/mol total Fe).
Furthermore, Fe2+ site densities estimated from FTIR
signatures of either terminal hydroxyl ligands or NO coordinated to
Fe2+ sites created upon hydroxyl removal serve as
proxies for cumulative methanol formation, lending credence to the
possible sole involvement of Fe2+ sites in methanol
formation.35