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Observation of an Intermediate to H2 Binding in a Metal–organic Framework
preprintsubmitted on 09.11.2020, 22:20 and posted on 10.11.2020, 12:42 by Brandon Barnett, hayden evans, Gregory M. Su, Henry Z. H. Jiang, Romit Chakraborty, Didier Banyeretse, Tyler Hartman, Madison Martinez, Benjamin A. Trump, Jacob Tarver, Matthew Dods, Walter S. Drisdell, Katherine Hurst, Thomas Gennett, Stephen FitzGerald, Craig M. Brown, Martin Head-Gordon, Jeffrey R. Long
Coordinatively-unsaturated metal sites within certain zeolites and metal–organic frameworks can strongly adsorb various molecules. While many classical examples involve electron-poor metal cations that interact with adsorbates largely through electrostatic interactions, unsaturated electron-rich metal centers housed within porous frameworks can often chemisorb guests amenable to redox activity or covalent bond formation. Despite the promise that materials bearing such sites hold in addressing myriad challenges in gas separations and storage, very few studies have directly interrogated mechanisms of chemisorption at open metal sites within porous frameworks. Here, we show that H2chemisorption at the trigonal pyramidal Cu+sites in the metal–organic framework CuI‑MFU-4l occurs via the intermediacy of a metastable physisorbed precursor species. In situpowder neutron diffraction experiments enable crystallographic characterization of this intermediate, the first time that this has been accomplished for any material. Support for a precursor intermediate is also afforded from temperature-programmed desorption and density functional theory calculations. The activation barrier separating the precursor species from the chemisorbed state is shown to correlate with a change in the Cu+coordination environment that enhances π-backbonding with H2. Ultimately, these findings demonstrate that adsorption at framework metal sites does not always follow a concerted pathway and underscore the importance of probing kinetics in the design of next-generation adsorbents.
Hydrogen Materials Advanced Research Consortum (HyMARC), U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office, Contract DE-AC02-05CH11231
Center for Gas Separations
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