3.3. Manipulating methoxy reactivity by controlling metal identity
Implicit in the 1:1 correspondence between Fe2+ site densities and the number of moles of monodeuteromethanol is the lack of secondary reactions between methoxy intermediates and gas phase methanol formed in primary reactions. Over and above this correspondence between active site densities and methanol yield, the absence of C2 oxygenates in the product stream and the fact that all the methane converted can be accounted for (within error) as methanol formed suggests that C2 intermediates, if formed over the MIL-100(Fe) surface under the conditions reported here thus far, may account for a minor fraction of converted methane. Extraction of intermediates formed under identical conditions by methane and N2O over MIL-100(Cr), however, provides (in addition to the formation of methanol) significant yields of acetaldehyde (fractional molar selectivity = 0.81, Figure 6a).C3CHmeImplicit RecationRe Analogous to the case of tri-iron clusters, moles of methane converted over MIL-100(Cr) track with M2+ site densities (Figure 6b) estimated using infrared peak areas corresponding to the hydroxyl anion (Figure S12), pointing to their sole involvement in C-H activation. Cumulative moles of methane reacted per mole Cr3+ do not remain invariant in activation temperature but instead increase with the latter (Figure S13, SI) due to the higher Cr2+/Cr3+ site ratios and the associated lower inactive open-metal site fractions made available through thermal activation at higher temperatures. M2+sites therefore participate in the activation of methane over both the iron and chromium variants of MIL-100(M). Although Cr-NO interactions are significantly stronger over Cr2+ sites compared to Cr3+ sites, as shown using NO adsorption measurements at 303 K reported in our previous work,45 in contrast with MIL-100(Fe), insignificant NO adsorption occurs onto Cr2+ open-metal sites at reaction temperatures (423 – 473 K), as indicated by NO adsorption breakthrough and IR spectroscopy measurements (Section S2.14, SI). We therefore rely solely on the correspondence between product yields and Cr2+open-metal site densities to suggest their involvement in methoxy formation, unlike the case of MIL-100(Fe) in which in-situ titrations with NO provided additional corroborating evidence of the same.