Strong Cu(I)–He interaction at open metal sites enables isotope-selective helium adsorption

The rare isotope helium-3 (3He) is in very short supply on Earth and highly desired for various applications, including cryogenics, neutron detection and potentially as fuel for nuclear fusion reactors. The main terrestrial resources are tritium decay and the very small fraction in helium-4 dominated supplies obtained from natural gas. This calls for the development of efficient techniques for efficient separation of 3He from isotope mixtures of He gas. In this study, we examine the circumstances that enable the selective adsorption of helium isotopes at open metal sites that commonly occur in metal–organic frameworks (MOFs) and zeolites. To this end, we first focus on small and computationally tractable model complexes of the transition metal Cu(I). We investigate the various aspects of bonding between the metal site and the adsorbed He using couple-cluster methods. In several of our model systems, we observe a substantial difference in adsorption zero-point energy between 3He and 4He, ranging from 0.2 kJ·mol⁻¹ for bare Cu+ to 0.7 kJ·mol⁻¹ for the CuOH complex. For the latter, we predict a remarkable isotope selectivity of adsorption at the technologically relevant temperatures with separation factors of up to 47.2 at 20 K (liquid hydrogen, LH2) and 2.5 at 77 K (liquid nitrogen, LN2). This demonstrates the potential of adsorptive techniques for efficient helium isotope separation at highly attractive open metal sites with a suitable ligand environment. Encouraged by these findings, we expand our investigation to a series of realistic materials of varying complexity, ranging from crown ethers and porphyrins to zeolites and MOFs, as potential hosts for active Cu(I) sites. While all realistic systems exhibit more modest selectivity compared to the simpler model complexes, substantial selectivity approaching a separation factor of 4 is observed at LH2 temperature. Our results emphasize the crucial role of the ligand environment in enhancing selectivity. Among the studied systems, Cu(I) anchored at tetrahedral sites in zeolites and in m502_p3 MOF emerge as particularly promising candidates.