(509ba) Role of Metal Speciation on Product Selectivity in the Partial Oxidation of Methane over Trimetal Oxo Clusters in Metal-Organic Framework MIL-100

Metal-organic framework MIL-100 has garnered attention as a material for the partial oxidation of light alkanes [1] due to its well-defined structure and endowment with metal nodes uniform in nuclearity and oxidation state. Thermal activation of MIL-100 leads to the formation of a M(III)₂M(II)O mixed-valence node featuring coordinatively unsaturated, open-metal sites. Partial oxidation of CH₄ with N₂O over Fe-containing MIL-100 leads to the selective formation of surface methoxy intermediates in a quantity of which is equivalent to the density of reduced Fe(II) open-metal sites across a range of thermal activation conditions. Exposure to water vapor is essential to the generation of the final CH₃OH product into the gas phase. In-situ titration of Fe(II) sites under reaction conditions indicated their sole contribution to the formation of CH₃OH. Minor quantities of CO₂ formed during reaction, however, were unaffected by NO feeds. Co-titration with increasing quantities of H₂O resulted in a linear decrease in the CO₂ yield. Complete suppression of product formation at a quantity of 0.62 mol H₂O (mol Fe)^-1 adsorbed, suggesting the contributions of Fe(III) sites which exist in a concentration of 0.66 mol (mol Fe)^-1 and remain untitrated by NO under the tested conditions. Evaluation of the Cr-analogue of MIL-100 provided that acetaldehyde could be formed selectively when water was fed after the reaction of CH₄ and N₂O, indicating the increased propensity for methanol to undergo secondary reaction during the water product extraction step. The results presented in this work demonstrate the ability to identify relationships between metal speciation and product selectivity in the partial oxidation of CH₄ over MIL-100 materials, emphasizing the value of evaluating materials with well-defined active moieties for elucidating fundamental structure-function relationships.

References: [1] Hall, J.N., Bollini, P., Chem. Eur. J. 26 (2020) 16639.