The role of adsorption strength and quantum confinement on the isotopologue selectivity of H₂ complexation

Heavy hydrogen isotopes are an essential resource with multiple applications in energy production, medicine and science. Unfortunately, the relative scarcity of heavy hydrogen in comparison to the much more abundant light hydrogen makes the enrichment of the heavy isotopes extremely challenging. State-of-the-art separation methods based on chemical exchange remain energy-consuming processes. In this thesis, density functional and ab initio methods are used to make several contributions to the investigation of one alternative approach to isotope separation: the isotopologue-selective adsorption of hydrogen at undercoordinated metal sites based on the different zero-point energy of adsorption of the isotopologues.

Because the zero-point energy is proportional to the curvature of the potential energy surface, which in turn correlates with the adsorption energy, stronger adsorption should in principle also lead to higher selectivity. Therefore, in the first part, the prerequisites for a high adsorption energy are investigated. It is concluded that main contributors are the chemical nature of the metal site (presence of d orbitals) and very acute ligand-metal-ligand angles, whereas the nature of the ligands seems to play a minor role.

The second part focuses on the selectivity-enhancing effect of confinement introduced by a pore around the adsorption site. It is found that confinement does not play a significant role in Cu(I)-MFU-4l. Although some confinement may be introduced via appropriate modifications of the linker, it is found that this comes at the cost of a sharp loss in H₂ affinity. Studying a more strongly binding model, it is nevertheless concluded that confinement can indeed significantly enhance H₂ isotopologue selectivity at very strong adsorption sites.

B₁₂X₁₁⁻ (X = H, F, Cl, Br, I, CN), a system showing very strong H₂ adsorption and a high D₂ /H₂ selectivity is investigated in the third part.

In the fourth part, the CoRE MOF database is screened for potential H₂-affine sites based on geometric criteria. Some promising adsorption site motifs are identified, studied with preliminary DFT calculations and their deeper investigation in future works is envisioned.

To conclude, this thesis advances the field of isotopologue-selective H₂ adsorption by identifying both general structural prerequisites for high selectivity as well as specific model systems and structural motifs worthy of further study.