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.