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
≈ −TMS < −I), whereas the selectivities seem
to follow a trend more related to pore-steric effects (−TMS
< −I < −Me < −F).