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.