Direct Observation of Xe and Kr Adsorption in a Xe-Selective Microporous Metal–Organic Framework

The cryogenic separation of noble
gases is energy-intensive and
expensive, especially when low concentrations are involved. Metal–organic
frameworks (MOFs) containing polarizing groups within their pore spaces
are predicted to be efficient Xe/Kr solid-state adsorbents, but no
experimental insights into the nature of the Xe–network interaction
are available to date. Here we report a new microporous MOF (designated
SBMOF-2) that is selective toward Xe over Kr under ambient conditions,
with a Xe/Kr selectivity of about 10 and a Xe capacity of 27.07 wt
% at 298 K. Single-crystal diffraction results show that the Xe selectivity
may be attributed to the specific geometry of the pores, forming cages
built with phenyl rings and enriched with polar −OH groups,
both of which serve as strong adsorption sites for polarizable Xe
gas. The Xe/Kr separation in SBMOF-2 was investigated with experimental
and computational breakthrough methods. These experiments showed that
Kr broke through the column first, followed by Xe, which confirmed
that SBMOF-2 has a real practical potential for separating Xe from
Kr. Calculations showed that the capacity and adsorption selectivity
of SBMOF-2 are comparable to those of the best-performing unmodified
MOFs such as NiMOF-74 or Co formate.