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.