(376a) Characterization of Open-Metal Site Density and Speciation in Mixed-Valence Trimetallic Nodes of Metal-Organic Framework MIL-100

The metal-organic framework (MOF) MIL-100 has attracted significant recent attention as an adsorbent and catalyst in key applications including the partial oxidation of light alkanes [1], in part due to its well-defined structure with monodisperse metal centers. Thermal activation protocols leading to the removal of terminal ligands (e.g. H₂O, OH^-) in MIL-100 lead to the creation of coordinatively unsaturated open-metal sites [2], the quantification and characterization of which is essential to evaluating their use for future applications. A wide range of techniques have been developed for analyzing open-metal sites in MOFs; however, a method involving mild adsorption conditions, benign substrates, and providing the ability to directly measure the density of open-metal sites is still lacking. We hypothesized that water adsorption onto open-metal sites is irreversible at room temperature, whereas that elsewhere on the MOF is reversible. Comparison of volumetric measurements of irreversible water adsorption on MIL-100 activated at several temperatures with values estimated from infrared spectroscopy measurements evidence the validity and accuracy of this method. In-situ spectroscopic characterization in the presence of NO as a probe molecule revealed the increasing presence of reduced M(II) metal sites upon dehydroxylation, dissimilar from M(III) metal sites created upon dehydration. Site densities and oxidation states were found to be consistent with trends in the adsorption of carbon monoxide and ethene, both of which exhibit more favorable Ï€-backbonding interactions with M(II) over M(III) sites, and were used to interpret the rates of low-temperature methane activation. The combination of characterization methods used herein are applicable to MOF materials more generally, and could enable the rigorous assessment of open-metal site density and speciation in a range of adsorption and catalysis applications.

References: [1] Hall, J.N., Bollini, P., Chem. Eur. J. 26 (2020) 16639. [2] J. Yoon, et al. Angew. Chem. Int. Ed. 49 (2010) 5949.