Structural Resolution and Mechanistic Insight into Hydrogen Adsorption in Flexible ZIF-7

Ryan Klein, Sarah Shulda, Philip Parilla, Pierre Le Magueres, Rachelle Richardson, William Morris, Craig Brown, C. McGuirk

Research output: Contribution to journalArticlepeer-review

20 Scopus Citations

Abstract

Flexible metal-organic frameworks offer a route towards high useable hydrogen storage capacities with minimal swings in pressure and temperature via step-shaped adsorption and desorption profiles. Yet, the understanding of hydrogen-induced flexibility in candidate storage materials remains incomplete. Here, we investigate the hydrogen storage properties of a quintessential flexible metal-organic framework, ZIF-7. We use high-pressure isothermal hydrogen adsorption measurements to identify the pressure-temperature conditions of the hydrogen-induced structural transition in ZIF-7. The material displays narrow hysteresis and has a shallow adsorption slope between 100 K and 125 K. To gain mechanistic insight into the cause of the phase transition correlating with stepped adsorption and desorption, we conduct powder neutron diffraction measurements of the D2 gas-dosed structures at conditions across the phase change. Rietveld refinements of the powder neutron diffraction patterns yield the structures of activated ZIF-7 and of the gas-dosed material in the dense and open phases. The structure of the activated phase of ZIF-7 is corroborated by the structure of the activated phase of the Cd congener, CdIF-13, which we report here for the first time based on single crystal X-ray diffraction measurements. Subsequent Rietveld refinements of the powder patterns for the gas-dosed structure reveal that the primary D2 adsorption sites in the dense phase form D2-arene interactions between adjacent ligands in a sandwich-like adsorption motif. These sites are prevalent in both the dense and the open structure for ZIF-7, and we hypothesize that they play an important role in templating the structure of the open phase. We discuss the implications of our findings for future approaches to rationally tune step-shaped adsorption in ZIF-7, its congeners, and flexible porous adsorbents in general. Lastly, important to the application of flexible frameworks, we show that pelletization of ZIF-7 produces minimal variation in performance.

Original languageAmerican English
Pages (from-to)15620-15631
Number of pages12
JournalChemical Science
Volume12
Issue number47
DOIs
StatePublished - 21 Dec 2021

Bibliographical note

Publisher Copyright:
© The Royal Society of Chemistry.

NREL Publication Number

  • NREL/JA-5900-80644

Keywords

  • flexible
  • hydrogen
  • hydrogen storage
  • metal-organic framework
  • powder neutron diffraction

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