The role of water in acetaldehyde formation was investigated further by testing the reactivity of ethanol over activated MIL-100(Cr). Stoichiometric conversion of ethanol was observed at 373 K over activated MIL-100(Cr) in quantities consistent with the density of Cr²⁺ sites (Table 2 and Figure S15b). Lower activation temperatures that allow for the concurrent presence of Cr²⁺ and Cr³⁺-OH^-groups enable acetaldehyde formation in amounts that track with Cr²⁺ density, but activation under vacuum at 523 K which creates Cr²⁺ sites far in excess of terminal hydroxyls (Cr³⁺-OH^-) results in a decrease in cumulative acetaldehyde formation (Figure 9). Specifically, cumulative moles of acetaldehyde formed (0.07 mol (mol Cr)^-1) under this activation condition more closely approximate the moles of terminal hydroxides (0.05 mol Cr³⁺-OH^- (mol Cr)^-1) than the moles of reduced metal sites (0.28 mol Cr²⁺ (mol Cr)^-1), suggesting that while methoxy formation is predicated on the availability of reduced metal sites, terminal hydroxides may be necessary for the dehydrogenation of ethanol to acetaldehyde.

Table 2. For MIL-100(Fe) and MIL-100(Cr), the activation condition employed, the corresponding M²⁺ site density, and the total quantity of acetaldehyde formed when ethanol (0.11 kPa) is fed over the activated material at 373 K.