Inelastic neutron spectroscopy to probe selective H2/D2 separation in zeolites

Separating hydrogen and deuterium is among the most formidable separation challenges. There is a growing demand for deuterium, due to its use for isotope specific experiments and labelling studies in chemistry, biology, materials science, nuclear fusion, and medicine. Current technologies to separate hydrogen and deuterium, either require incredibly low temperatures (~30 K), have low separation efficiencies, or low uptake capacities. An alternative method is using porous materials as Quantum Sieves, as D2 adsorbs more readily within micropores than H2, at cooler conditions. Chabazite zeolites (CHA) are highly effective at separating the two species due to the small pore windows (8 membered rings, 3.8 Å) within the crystalline framework. Purely siliceous CHA has poor D2/H2 selectivity. But replacing framework Si species with Al, leads to cations binding to the zeolite. These cations improve D2 selectivity in zeolites, specifically in CHA species where a “trapdoor mechanism” is suggested, due to extra-framework cations sitting in the centre of the CHA 8 membered ring. Our recent work on CHA with a potassium counterions (K-CHA) suggests that at ~159 K, K+ moves to one side, allowing a D2 molecule into the framework, acting as a “trapdoor”, which does not occur with H2. Below this temperature we see no uptake as the ions block access to the pores. Therefore, this provides a unique opportunity to exploit this system for D2/H2 uptake at comparatively high temperatures.