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Magnetic Resonance Investigations of Adsorbate Dynamics in Metal-Organic Frameworks

  • Author(s): Chen, Joseph Jung-Wen
  • Advisor(s): Reimer, Jeffrey A
  • et al.
Abstract

Metal-organic frameworks (MOFs), a new class of porous materials, are crystalline networks of metal clusters or ions connected by organic linkers through coordination bonds. These frameworks exhibit a rich diversity of structures, chemistries, and topologies, as evidenced by the explosion of new MOF structures in the last decade. However, the nearly infinite number of possible network connectivities and framework compositions, as well as the significant impact of minor changes in reaction conditions on the structure obtained, impedes optimization. Although high-throughput synthesis can greatly accelerate the discovery of new materials, the speed of subsequent characterization, such as gas adsorption measurements, limits the rate of optimization. In response to this challenge, we describe the development of a high-throughput nuclear magnetic resonance (NMR) porosity screening tool that uses NMR relaxation times of adsorbed molecules to estimate porosity. The diffusion and exchange processes uncovered during the development of this NMR screening tool highlight the lack of a molecular understanding of how adsorbed molecules, or guests, move in metal-organic frameworks. Thus, we investigate adsorbate dynamics in detail using NMR relaxation and diffusion experiments. Our relaxation experiments, combined with molecular simulations, establish the presence of a new condensation phenomenon occurring in a model adsorbate-MOF system. Our diffusion studies explore the interplay between the adsorbate-adsorbent interaction energy and this new condensation phenomenon on adsorbate diffusion in MOFs. These fundamental investigations, as well as our more practical efforts in developing a porosity screening tool, provide detailed insight into molecular dynamics in confined systems, and this knowledge possesses broad implications for applications in separations and catalysis.

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