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.