posted on 2021-08-31, 14:41authored byBrandon
R. Barnett, Hayden A. Evans, Gregory M. Su, Henry Z. H. Jiang, Romit Chakraborty, Didier Banyeretse, Tyler J. Hartman, Madison B. Martinez, Benjamin A. Trump, Jacob D. Tarver, Matthew N. Dods, Lena M. Funke, Jonas Börgel, Jeffrey A. Reimer, Walter S. Drisdell, Katherine E. Hurst, Thomas Gennett, Stephen A. FitzGerald, Craig M. Brown, Martin Head-Gordon, Jeffrey R. Long
Coordinatively unsaturated metal
sites within certain zeolites
and metal–organic frameworks can strongly adsorb a wide array
of substrates. While many classical examples involve electron-poor
metal cations that interact with adsorbates largely through physical
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 nondissociative chemisorption of H2 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 situ powder neutron diffraction experiments enable crystallographic characterization
of this intermediate, the first time that this has been accomplished
for any material. Evidence 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.