Metal-organic frameworks are a class of crystalline materials with extended porous structures that have the potential to overcome existing limitations of biomimetic catalysts due to their well-defined structures and endowment with metal nodes uniform in nuclearity and oxidation state. Specifically, MIL-100 (MIL = Materials of Institut Lavoisier) is a framework which has attracted recent attention as a catalyst for the partial oxidation of light alkanes. The inorganic metal node of this framework consists of three octahedrally coordinated metal atoms sharing an oxygen vertex (M3O). Coordinatively unsaturated or open-metal site densities can be controlled by thermal activation temperature, leading to the formation of a M3+2M2+O mixed-valence node in the fully activated material. Results of the oxidation of CH4 with N2O over Fe-containing MIL-100 revealed the selective formation of surface-bound methoxy intermediates in a quantity equivalent to the maximum theoretical density of Fe2+ sites in the material (0.33 mol (mol Fe)-1). Exposure to water vapor after reaction led to the evolution of CH3OH into the gas phase. Fe2+ site densities enumerated through NO equilibrium adsorption measurements approximated the CH3OH yield as a function of activation temperature and selective in-situ titration of Fe2+ sites with NO confirmed their sole contribution to CH3OH formation. Minor quantities of CO2 formed during reaction were unaffected by NO co-feeds, however, increasing amounts of H2O added to the reactant flow led to a linear decrease in the CO2 yield. CO2 formation was fully suppressed at a quantity of 0.62 mol H2O (mol Fe)-1 adsorbed, suggesting the contributions of Fe3+ sites which exist in a concentration of 0.66 mol (mol Fe)-1 and remain untitrated by NO under the tested conditions. Extension to the Cr-analogue of MIL-100 revealed the selective formation of acetaldehyde when water is fed after the reaction of CH4 and N2O, demonstrating the increased propensity for secondary reaction between gas-phase methanol and surface-bound methoxys in this material. Furthermore, this study demonstrates the capability to identify clear relationships between metal speciation and product selectivity in MIL-100 materials for the partial oxidation of CH4, underlying the utility of exploring classes of materials hosting sites in well-defined coordination environments for the purposes of relating active site structure and catalytic function.

Role of metal identity and oxidation state in the partial oxidation of methane over trimeric nodes in a metal-organic framework

ACS Spring Meeting 2021

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