Toward separation of hydrogen isotopologues by exploiting zero-point energy difference at strongly attractive adsorption site models

Recent reports on hydrogen isotope separation in metal-organic frameworks are rationalized by the presence of undercoordinated metal sites. In this work, we screen undercoordinated divalent metals for their performance as hydrogen separating agents in common chemical environments (as free dications, as part of M₂(HCOO)₄ paddlewheels and in metal porphyrins). We calculate heat of adsorption and selectivity using the Langmuir model and the density-functional-based potential energy surface. Our calculations show a strong metal ion dependence of the adsorption energy in paddlewheels, but rather small impact of metal choice in stable porphyrins. D₂-over-H₂ selectivities reach top values of 12 for CO²⁺ paddlewheels, while only oxidatively unstable porphyrins incorporating Cr²⁺, V²⁺, and Ti²⁺ show high selectivities.