Steric and Electronic Effects on the Interaction of Xe and Kr with Functionalized Zirconia Metal-Organic Frameworks

The separation of xenon and krypton from their mixtures has been an enduring and complex venture due to their similar sizes and unreactive nature. Metal-organic frameworks (MOFs) have shown the potential to complete these challenging separations by utilizing pressure-swing adsorption (PSA) as a sustainable alternative to current cryogenic distillation techniques. To rationally design materials to better realize this goal, two main approaches have emerged: pore-size optimized and polarizability-based separations. To ascertain the efficacy of these strategies, we designed a series of UiO-type MOFs with terphenyl linkers that systematically varied their steric and electronic properties, including -Me, -F, -TMS, and -I functionalities, to assess their interactions with xenon and krypton. The prepared MOFs are all isoreticular and have similar pore size distributions, allowing us to directly evaluate the effects imposed by the functional groups. We found that the xenon uptake could be increased with greater polarizability of the functional group (-F < -Me approximate to -TMS < -I), whereas the selectivities seem to follow a trend more related to pore-steric effects (-TMS < -I < -Me < -F).