Figure 6. (a) Fractional selectivity for oxygenated products
formed during product extraction (473 K, 0.35 kPa H2O,
1.54 x 10-6 mol s-1) and the
quantity of methane reacted (per total mol M) for MIL-100(Fe) and
MIL-100(Cr) activated at 523 K under He flow and in vacuum,
respectively. (Reaction conditions: 473 K, 2.9 kPa N2O,
1.5 kPa CH4, 2 h). (b) Cr2+ open-metal
site densities estimated from IR spectroscopy measurements at various
activation conditions and the corresponding cumulative moles of
CH4 reacted normalized by the Cr2+open-metal site density (Reaction conditions: 423 K, 14.5 kPa
N2O, 1.5 kPa CH4, 2 h).
We hypothesize that secondary reactions of methoxy intermediates with
gas phase methanol are responsible for acetaldehyde formation. The
presence of these secondary reactions prevent the use of
D2O for evidencing the prevalence of methoxy
intermediates over MIL-100(Cr) upon exposure to methane and
N2O. Instead, the prevalence of these secondary
reactions can be verified through extraction with 0.12 kPa methanol at
373 K which leads to the formation, exclusively, of ethanol in both
MIL-100(Fe) and MIL-100(Cr) (Figure 7), with the moles of ethanol formed
approximating to the M2+ site density for both
materials (Table S6, SI). This result is consistent with the same
methoxy-covered surface being prevalent in both MIL-100(M) variants
following oxidation of CH4, and product distributions
detected upon exposure to water vapor being dependent on the differing
propensities of Fe and Cr-methoxy intermediates to undergo C-C bond
formation steps. It also appears that whereas carbon-carbon bond
formation is not predicated on the presence of water vapor, it seems to
be necessary for the formation of acetaldehyde (as opposed to ethanol)
in MIL-100(Cr), as indicated by the observation of methanol and
acetaldehyde as products under aqueous extraction conditions (Figure
6a). Both water and methanol appear to be necessary for
acetaldehyde formation, as evident in experiments involving extraction
using equimolar feeds at 373 K. As a reference condition, a temperature
of 373 K and a water partial pressure of 0.12 kPa were chosen for the
reason that significant water coverages are attained (0.42 mol
H2O mol Cr-1) in the absence of
measurable amounts of methanol or acetaldehyde formation (Figure 8a and
Table 1). Co-feeding equimolar mixtures of methanol and water (0.12 kPa
each) under these conditions leads to the formation of 0.93 moles
acetaldehyde per mole methane reacted (or equivalently, per mol methoxy
formed), evidencing near-complete coupling of methoxies with gas phase
methanol. These results suggest that although methoxies can react with
methanol in the absence of significant partial pressures of water, the
presence of water is necessary for the production of acetaldehyde,
rather than ethanol, as the C2 product, and point to water playing a
role in acetaldehyde formation over and above (and may be even
independent of) their involvement in primary reaction steps that lead to
the formation of gas-phase methanol.