Synergistic effect of paired Cu(II) open metal sites for enhanced high-temperature hydrogen isotope separation.

The selective separation of hydrogen isotopes under mild cryogenic conditions remains a formidable challenge due to their nearly identical physicochemical properties. Here, we report a dual strategy of pore topology design and paired Cu(II) open metal sites (OMS) synergistic engineering to amplify chemical affinity quantum sieving (CAQS). Among three tailored Cu(II)-MOFs, Cu-ATC exhibited exceptional performance, achieving a D2/H2 selectivity of 20 at 50 K (10 mbar) and 1.8 in breakthrough experiments at 77 K, demonstrating excellent H2/D2 separation performance. The ultramicroporous topology of Cu-ATC fixes a Cu···Cu distance of 5.98 Å within one-dimensional channels, while Jahn-Teller distortion induces axial elongation at each Cu(II) center, thereby enhancing the accessibility of the dz2 orbitals for interaction with hydrogen isotope molecules. This structural combination creates two closely spaced OMSs that enhance differential interactions with H2 and D2, thereby driving isotope separation via CAQS. The distinct binding strength is evidenced by in situ DRIFTS (v(H-H)/ v(D-D) red-shift of 203 cm-1/ 147 cm-1) and by DFT calculations showing stronger adsorption of H2 ( - 9.7 kJ mol-1) and D2 ( - 13.0 kJ mol-1). These microscopic differences account for the observed D2/H2 selectivity, highlighting the potential of paired OMSs engineering for CAQS-based isotope separation under mild cryogenic conditions.