Methane as a surrogate for krypton for adsorption capacity evaluation

Krypton is scarce, costly, and periodically experiences supply shortages, motivating the search for alternative probe molecules. Methane, which shares many physicochemical properties with krypton—such as kinetic diameter, polarizability, boiling point, and a lack of quadrupole moment—presents itself as a promising and more accessible surrogate. This study addresses whether methane can be used as a substitute for krypton for adsorption studies. Here, we demonstrate that across several representative adsorbents—including activated carbon, a zeolite, and flexible as well as rigid metal-organic frameworks—the adsorption-desorption isotherms of methane and krypton are remarkably similar in both shape and magnitude over a wide range of conditions. Comparative analysis reveals that differences in maximum adsorption capacities and affinity parameters between methane and krypton are minor, generally within experimental uncertainty, and that the qualitative adsorption mechanisms—including phase transitions in flexible MOFs—are consistently captured by both probes. To the best of our knowledge at the exception of H 2 isotopes, we show for the first time that same adsorption capacity data can be obtained with two different adsorbates.