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Selective Hydrogen Isotope Separation via Breathing Transition in MIL-53(Al) Jin Yeong Kim, Linda Zhang, Rafael Balderas-Xicohtencatl, Jaewoo Park, Michael Hirscher, Hoi Ri Moon, and Hyunchul Oh J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 27 Nov 2017 Downloaded from http://pubs.acs.org on November 28, 2017
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Journal of the American Chemical Society
Selective Hydrogen Isotope Separation via Breathing Transition in MIL-53(Al) Jin Yeong Kim,† Linda Zhang,‡ Rafael Balderas-Xicohténcatl,‡ Jaewoo Park,§ Michael Hirscher,*,‡ Hoi Ri Moon*,† and Hyunchul Oh*,§ †
Department of Chemistry, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea Max Planck Institute for Intelligent Systems, Heisenbergstr. 3, 70569 Stuttgart, Germany § Department of Energy Engineering, Gyeongnam National University of Science and Technology (GNTECH), Jinju 52725, Republic of Korea Supporting Information Placeholder ‡
ABSTRACT: Breathing of MIL-53(Al), a flexible metal– organic framework (MOF), leads to dynamic changes as narrow pore (np) transition to large pore (lp). During the flexible and reversible transition, the pore apertures are continuously adjusted, thus providing the tremendous opportunity to separate mixtures of similar-sized- and similar-shaped molecules that require precise pore tuning. Herein, for the first time, we report a strategy for effectively separating hydrogen isotopes through the dynamic pore change during the breathing of MIL-53(Al), representative of flexible MOF. The experiment shows that the selectivity for D2 over H2 is strongly related to the state of the pore structure of MIL-53(Al). The highest selectivity (SD2/H2 = 13.6 at 40 K) was obtained by optimizing the exposure temperature, pressure, and time to systematically tune the pore state of MIL-53(Al).
Structural flexibility is a unique property of some metal– organic frameworks (MOFs) that clearly distinguishes them from other inorganic porous materials such as zeolite and porous silica.1-3 This interesting property is the result of the detectable changes in local or overall structural properties in response to external stimuli (e.g., heat, pressure, and presence of guest molecules).4-6 The smart response enables many possible applications in various fields for flexible MOFs.7-9 In particular, upon exposure of a MOF to specific adsorbates, large changes in the unit cell parameters are manifested as expansion and contraction, an effect called breathing, and this effect has attracted the attention of many researchers in the field of separation and sorption.10,11 For example, MIL-53 family, the most studied flexible MOFs, has demonstrated selective separation of specific molecules such as CO2 over that of N2 or CH4, o-xylene over xylene isomers, and acidic gases over CH4.12-15 In these cases, the efficient separation can be attributed to the chemical affinity between the inner surface of MIL-53 and the adsorbates, as well as the size selectivity of adsorbates through the aperture of narrow pore (np) or large pore (lp). This observation initiated many theoretical and experimental investigations to gain a deeper understanding of the breathing mechanism between the np and lp phases of the MIL-53 system.1624 However, these studies overlooked the dynamic pore changes, which ought to occur during the np-to-lp phase transition. Even though there is not enough direct structural information on this transitional state, it has been generally agreed that molecular adsorption on the pore surfaces triggers breathing along the onedimensional (1D) channel, resulting in dynamic changes in the
pore diameter during the transitional state between two structural phases. This phenomenon of dynamic change in pore aperture can be applied to the separation of gas mixtures containing molecules of similar size and shape, which means it would entail fine poretuning, as required for the separation of hydrogen isotopes. Hydrogen isotope separation through the use of porous materials was previously studied based on a quantum sieving (QS) effect, in which D2, which has a shorter de Broglie wavelength than H2, can diffuse faster through the confined pore at a cryogenic temperature.25-30 Chen et al. suggested the sorption of H2 and D2 within
