Lattice-driven Gating in a Cu-based Zeolitic Imidazolate Framework for Efficient High-temperature Hydrogen Isotope Separation

For the separation of hydrogen isotopes (H/D), traditional kinetic quantum sieving (KQS) takes advantage of the diffusion barriers created by the flexibility of organic linkers and the breathing frameworks in porous solids. While the phenomena have been observed typically below 77 K, in this study, we present that a copper-based zeolite imidazolate framework (Cu-ZIF-gis) can show KQS above 120 K. Since Cu-ZIF-gis has narrow channels with ca. 2.4 Å in aperture, the small pore size itself acts as a diffusion barrier. This barrier changes with temperatures, leading to pore contraction or expansion through lattice-driven gating (LDG). The H adsorption isotherms measured at 40 - 150 K reflect the temperature sensitivity of the pore properties. Quasi-elastic neutron scattering (QENS) experiments indicate a notable difference in the molecular mobility of H and D, even at temperatures exceeding 150 K. Temperature-variation powder X-ray diffraction measurements at 20 - 300 K show a small but gradual increase in the unit cell volume, indicating that LDG gives rise to the KQS at temperatures above 120 K. These findings can be applied to develop sustainable isotope separation technologies using existing LNG cryogenic infrastructure.