Systematic Experimental Study on Quantum Sieving of Hydrogen Isotopes in Metal‐Amide‐Imidazolate Frameworks with narrow 1‐D Channels

• Published in: Chemphyschem 2019-05, Vol.20 (10), p.1311-1315
• Main Authors: Mondal, Suvendu Sekhar, Kreuzer, Alex, Behrens, Karsten, Schütz, Gisela, Holdt, Hans‐Jürgen, Hirscher, Michael
• Format: Article
• Language: English
• Subjects: Apertures; Channels; gas adsorption; Hydrogen isotopes; Isotope separation; Materials selection; metal-organic frameworks; quantum sieving; Selectivity; Thermal desorption spectroscopy; Time dependence
• ISSN: 1439-4235; 1439-7641
• DOI: 10.1002/cphc.201900183

Quantum sieving of hydrogen isotopes is experimentally studied in isostructural hexagonal metal‐organic frameworks having 1‐D channels, named IFP‐1, −3, −4 and −7. Inside the channels, different molecules or atoms restrict the channel diameter periodically with apertures larger (4.2 Å for IFP‐1, 3.1 Å for IFP‐3) and smaller (2.1 Å for IFP‐7, 1.7 Å for IFP‐4) than the kinetic diameter of hydrogen isotopes. From a geometrical point of view, no gas should penetrate into IFP‐7 and IFP‐4, but due to the thermally induced flexibility, so‐called gate‐opening effect of the apertures, penetration becomes possible with increasing temperature. Thermal desorption spectroscopy (TDS) measurements with pure H2 or D2 have been applied to study isotope adsorption. Further TDS experiments after exposure to an equimolar H2/D2 mixture allow to determine directly the selectivity of isotope separation by quantum sieving. IFP‐7 shows a very low selectivity not higher than S=2. The selectivity of the materials with the smallest pore aperture IFP‐4 has a constant value of S≈2 for different exposure times and pressures, which can be explained by the 1‐D channel structure. Due to the relatively small cavities between the apertures of IFP‐4 and IFP‐7, molecules in the channels cannot pass each other, which leads to a single‐file filling. Therefore, no time dependence is observed, since the quantum sieving effect occurs only at the outermost pore aperture, resulting in a low separation selectivity. Go in single file: Experiments on quantum sieving of hydrogen isotopes in frameworks having 1‐D channels restricted by small apertures show a constant selectivity with exposure time and pressure. The relatively small cavities between the apertures do not allow the passing of adsorbed molecules, leading to a single‐file filling of the channels. Therefore, quantum sieving occurs only at the outermost pore aperture.