Hydrogen isotope separation in metal-organic frameworks: Kinetic or chemical affinity quantum-sieving?

Recently we reported hydrogen isotope separation by quantum sieving in metal-organic framework MFU-4, a framework exhibiting gates of about the same size as the molecular radii of D₂ and H₂. Due to its smaller effective particle size, D₂ penetrates preferentially through the framework, resulting in remarkable selectivity. Surprisingly, MFU-4l, a material of very similar composition, but with substantially larger gate openings, shows appreciable hydrogen isotopologue selectivity. This selectivity occurs at low temperature and is smaller compared to earlier reported CPO-27, a framework exhibiting open metal sites. We show that this is caused by different adsorption enthalpies which are the result of quantum effects. It turns out that two independent hydrogen isotope separation mechanisms have been reported for MOFs: while kinetic quantum sieving works at cryogenic temperatures for materials with small pores, different adsorption energies allow chemical affinity quantum sieving. This effect is maximized by strong adsorption centers, which allow high selectivity at high temperatures (100 K and above), and is more appropriate for the rational design of isotope separation membranes.