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