4. Conclusions
Tri-iron and tri-chromium nodes on MIL-100 frameworks effectuate the
conversion of methane to methanol and acetaldehyde at low temperatures
and sub-ambient reactant pressures. In-situ titrations with NO over
MIL-100(Fe), combined with the equivalence between methoxy densities and
Cr2+ open-metal site concentrations in MIL-100(Cr)
across a range of thermal activation conditions suggest that the
formation of both products occurs solely over M2+sites. CO2 transient formation rates that are
insensitive to Fe3+-methoxy coverages, water
titrations over surfaces pre-titrated with NO, and cumulative
CO2 yields per M3+ site that remain
invariant as a function of activation temperature all point to the
involvement of M3+ sites in deep oxidation. Methoxy
intermediates play a key role in both methanol and acetaldehyde
formation, with acetaldehyde formation in the absence of co-fed methanol
occurring through secondary reactions of methanol molecules formed as a
result of primary methoxy-water reactions. Acetaldehyde formation does
not occur when purely methanol is fed over a methoxy-covered surface,
suggesting that the presence of water is necessary for producing
acetaldehyde, not ethanol, as the C2 oxygenate. Relative rates of
propagation of water and methanol concentration fronts determine the
ratio of cumulative methoxy-water and methoxy-methanol encounters, and
hence acetaldehyde selectivity; we show that these encounters can be
’forced’ in either direction by varying inlet water partial pressures
and/or flow rates. Although tri-chromium and tri-iron nodes both enable
C-C bond formation steps, tri-chromium ones do so at significantly lower
water extraction pressures, evidencing the potential for tuning C2
oxygenate selectivity merely by varying the identity of the metal atom
constituting the oxo-bridged trimer. Elucidation of the role of metal
identity and oxidation state in C-H bond scission and C-C bond formation
steps is enabled, first and foremost, by the high level of definition,
uniformity, and thermal stability of MIL-100 nodes. We assert, in this
vein, that the study and use of MOF materials toward catalytic
applications more generally could play a meaningful role in overcoming
persistent limitations in the development of structure-property
relationships that result at least in part directly from intractable
levels of heterogeneity in (in)active site speciation.