3.4. Prevalence of primary versus secondary reactions over tri-iron nodes
The ability of MIL-100(Fe) to convert methane to methanol has been reported by multiple groups. As described above, under identical conditions (0.35-0.70 kPa H2O, 373-473 K), MIL-100(Cr) exhibits a propensity to convert methane to C2 oxygenates through secondary interactions of methanol with methoxy species formed on Cr2+ sites. C2 oxygenate formation, however, appears to not necessarily be precluded on MIL-100(Fe) materials, as demonstrated by the formation of ethanol upon product extraction with methanol at 0.12 kPa and 373 K (Figure 7b), and by the formation of acetaldehyde upon feeding ethanol over the partially-dehydrated material (0.11 kPa ethanol, 373 K - Figure S15a, Table 2). To test whether C2 oxygenate formation could occur over tri-iron clusters upon extraction with water, the water partial pressure during extraction was increased from 0.35 to 1.0 kPa (Figure 11), resulting in the detection of minor amounts of acetaldehyde (fractional molar selectivity = 0.03). Analogous to tri-chromium clusters, increasing inlet water partial pressures can be used to ’force’ secondary reactions between Fe3+-methoxies and methanol, but the water partial pressures and/or flow rates required to access meaningful cumulative acetaldehyde selectivities may be much higher on tri-iron nodes than tri-chromium ones. Accessibility to C2 oxygenate production within lower water partial pressure and flow rate regimes enabled by MIL-100(Cr) likely reflect the greater propensity for Cr3+-OCH3-intermediates to undergo C-C bond formation reactions with gas phase methanol compared to Fe3+-OCH3-intermediates, and point to metal identity being a reliable lever for tuning product selectivity in the partial oxidation of light alkanes over supported poly-metal oxo clusters.