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.