(513en) Characterization of M2+ Active Sites in MIL-100 Metal-Organic Framework Catalysts for Light Alkane Oxidation

Metal-organic frameworks (MOFs) are a class of porous materials that offer an opportunity to study crystalline structures featuring well-defined, monodisperse metal centers for pointed applications in heterogeneous catalysis and selective gas sorption. 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.^1,2 Prior to application, open-metal sites must first be created through thermal activation which removes terminally coordinated species such as water molecules and anions.³ Reduced M²⁺ atoms created through anion removal have been proposed as the active site for N₂O decomposition to form highly-active M⁴⁺=O sites for alkane oxidation.^1,2 Understanding of the necessary conditions for M²⁺ site formation in MIL-100 is limited and in many cases the maximum theoretical concentration of M²⁺ sites (0.33 mol (mol Fe)^-1) is unattainable. Herein we show the exploitation of exclusively hydroxyl anions in accessing the maximum density of Cr²⁺ sites in MIL-100(Cr) when activated at 523 K in vacuum (P < 6.7 x 10^‑5 bar). This result is verified through a combination of infrared spectroscopic characterization and probe molecule adsorption with NO, CO, and ethene. The results obtained emphasize that anion identity, resulting from choice of synthesis precursors, is a key feature in accessing high concentrations of reduced metal sites and that dehydroxylation events leading to Cr²⁺ formation are a strong function of activation temperature, consistent with observed trends in adsorption. Furthermore, we demonstrate the utility of M²⁺ sites in MIL-100 for the oxidation of light alkanes in which the concentration of active sites is controlled through thermal pretreatment temperature.

[1] J. Vitillo, et al. ACS Catal. 9 (2019) 2870.[2] M. C. Simons, et al. J. Am. Chem. Soc. 141 (2019) 18142.[3] J. Yoon, et al. Angew. Chem. Int. Ed. 49 (2010) 5949.