Helium separation from gas mixtures using a temperature-regulated trapdoor zeolite

Alternative methods of recovering helium from natural gas such as adsorption processes are often more cost-effective than cryogenic distillation approaches for small-scale reserves. However, conventional adsorbents tend to selectively adsorb bulk components like nitrogen and methane rather than the minor helium component, increasing overall gas production costs. Here, we show that a trapdoor potassium chabazite, K-CHA can selectively capture helium from a helium‐nitrogen mixture. Isobaric measurements of helium adsorption capacities on K-CHA were found to exhibit a distinctive bell shape indicative of a trapdoor guest admission rather than a monotonic increase of adsorption capacity with decreasing temperature. The threshold temperature for helium admission on K-CHA was determined to be 100.6 K from these measurements, which is lower than that for nitrogen (254 K). Helium trapped inside the zeolite cavities was observed to escape slowly over an extended period even when the temperature remained well below the threshold, which is an observation not previously reported for trapdoor zeolites. The time constant of the exponential associated with this helium release was 5.5 h at 77 K when the K-CHA was used to encapsulate 1.8 mol/kg with approximately 4 % of the total encapsulated helium released during the first hour. Nevertheless, it was possible to use the trapdoor mechanism within encapsulation cycles lasting 1 h to enrich helium from an initial 4.5 mol% to 80 mol% with a helium recovery rate of 60 %, achieving a helium‑nitrogen separation factor of 89. Our results highlight a potential new process with selectivity for inert, minor components in gas mixtures.