Thermodynamic Separation of Hydrogen Isotopes Using Hofmann-Type Metal-Organic Frameworks with High-Density Open Metal Sites.

Hydrogen isotope separation with nanoporous materials is a very challenging yet promising approach. To overcome the limitation of the conventional isotope separation strategy, quantum sieving-based separation using nanoporous materials has been investigated recently. In this study, to see the thermodynamic deuterium separation phenomena attributed to the chemical affinity quantum sieving effect, we examine Hofmann-type metal-organic frameworks (MOFs), Co(pyz)[M(CN) 4 ] (pyz = pyrazine, M = Pd 2+ , Pt 2+ , and Ni 2+ ), which have microporosity (4.0 × 3.9 Å 2 ) and an extraordinarily high density of open metal sites (∼9 mmol/cm 3 ). Owing to the preferential adsorption of D 2 over H 2 at strongly binding open metal sites, the Hofmann-type MOF, Co(pyz)[Pd(CN) 4 ] exhibited a high selectivity ( S D 2 /H 2 ) of 21.7 as well as a large D 2 uptake of 10 mmol/g at 25 K. This is the first study of Hofmann-type MOFs to report high selectivity and capacity, both of which are important parameters for the practical application of porous materials toward isotope separation.