Balancing affinity and confinement: Structural tuning of MOFs for synergistic hydrogen isotope sieving

Efficient separation of hydrogen isotopes, especially H2 from D2, is critical for applications such as heavy-water production and fueling nuclear fusion. Achieving high selectivity at low energy cost remains a formidable challenge due to the isotopes' nearly identical physical properties. Metal-organic frameworks (MOFs) offer a promising low-energy, high-selectivity alternative for H2/D2 separation because their tunable porous structures can exploit subtle quantum effects. Here, we investigate how structural modifications to a prototypical MOF, FJI-Y11, influence its H2/D2 separation performance via quantum-sieving mechanisms. Using a suite of quantum and classical simulations, we show that subtle structural modifications, such as Zn substitution and Cl functionalization, significantly affect quantum sieving performance. In particular, the chloride functionalization synergistically enhances both zero-point-energy-driven adsorption affinity and confinement-driven quantum-exclusion mechanisms, markedly improving the H2/D2 selectivity. Our findings demonstrate that balancing pore size, framework flexibility, and adsorption-site chemistry can optimize hydrogen-isotope separation performance and guide the rational design of MOFs.