Elementary hydrogen has three isotopes: protium, deuterium, and tritium. These isotopes play a critical role in hydrogen fuel production, nuclear fusion, and the development of advanced pharmaceuticals.
However, it’s very difficult to isolate these isotopes at room temperature. This is because they have similar sizes and shapes. In addition, each of them has one proton and one electron, leading to similar chemical and thermodynamic properties. The methods currently used for separating hydrogen isotopes are resource-intensive as they require extreme conditions to work.
Knut Asmis is a chemistry professor at Leipzig University. He said, “It has been known for almost 15 years that porous metal-organic frameworks can, in principle, be used to purify and separate hydrogen isotopes. However, this has only been possible at very low temperatures, around minus 200 degrees Celsius—conditions that are very costly to implement on an industrial scale.”
Asmis and his colleagues recently published a report that provides important insights into how hydrogen isotopes can be isolated at room temperature and at a low cost. Their research reveals that the secret is water ligand.
When porous metal ions such as Cu+ interact with hydrogen under extreme conditions, they perform selectively absorption of the different isotopes. The study authors note that “Adsorption is a process by which atoms, ions, or molecules from a gas or liquid adhere to a solid, often porous, surface”.
During their study, the researchers discovered that when water molecules are used as ligands with the copper ion, the metal becomes better at attracting and holding hydrogen molecules. In addition, the resulting copper-water complex is better at distinguishing the energy difference in the bonds between H2 (regular hydrogen) and D2 (heavy hydrogen) compared to bare copper.
The authors continue that “Combining experimental and computational methods, we demonstrate a high isotopologue selectivity in dihydrogen binding to Cu+(H2O), which results from a large difference in the adsorption zero-point energies (2.8 kJ mol−1 between D2 and H2, including an anharmonic contribution of 0.4 kJ mol−1).”
Unlike bare copper ions, the copper water complex doesn’t need large amounts of energy to achieve hydrogen isotope isolation. This suggests that it might lead to more efficient, less resource-intensive, and highly cost-effective ways of obtaining hydrogen isotopes.
The research shows that porous metal complexes with water ligands are promising candidates for hydrogen isotope isolation. It also suggests that metal-water complexes might be used to study chemical reactions that take place at particular sites in large systems.
The authors added that “These systems are ideal model complexes for gas-phase studies of the chemistry at individual active sites as they occur in framework materials.”
Asmis and his team also carried out spectroscopy and complex quantum calculations to understand the interaction between the hydrogen isotopes in detail. Their findings may reveal more practical ways of selective absorption of isotopes.
Thomas Heine is one of the study authors and an expert in theoretical chemistry at Technische Universität Dresden. He said, “For the first time, we have been able to show the influence of the individual atoms of the framework compounds on adsorption. We can now optimize them in a targeted manner in order to obtain materials with high selectivity at room temperature.”