Metal-organic framework (MOF) nodes offer an opportunity for investigating structure-catalytic property relationships over well-defined, uniform, isolated active site moieties for key hydrocarbon conversions. Elucidating structure-catalytic property relationships using this class of materials is, however, first and foremost reliant on an accurate characterization of both the density and speciation of open-metal sites that effectuate catalytic turnovers. In this talk I will present a new method for counting active sites that exploits a near-universal characteristic of MOF materials- the irreversible coordination of a labile ligand in the metal’s first coordination sphere. We demonstrate the use of this technique for MIL-100- a material that has attracted attention for its use as a catalyst for ethene oligomerization1 and the partial oxidation of light alkanes.2 We assert that while water adsorption onto open-metal sites is irreversible at room temperature, sorption elsewhere onto the MOF material is reversible. We exploit this fundamental characteristic of MOF materials to show that capacities obtained from difference isotherms (original – repeat) can serve as a marker for open-metal site density. Comparison with infrared spectroscopy measurements tracking dehydration and dehydroxylation events as a function of activation temperature provide confirmation of site counts estimated using water adsorption. Methods for characterizing the oxidation state of open-metal sites were also developed through the use of three distinct probe molecules (NO, CO, and ethylene) that help evidence and quantify M2+ formation. We envision that rigorously characterized site counts and oxidation states elucidated as part of this study can be used to interpret light alkane activation data over metal-oxo sites on MOF nodes, and more broadly to interpret catalytic data on MOF-based model catalyst systems for a variety of heterogeneously catalyzed reactions.

Assessing open-metal site density and speciation in metal-organic framework catalysts for light alkane conversion

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.