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 d_z² 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.