Construction of an Anion-Pillared MOF Database and the Screening of MOFs Suitable for Xe/Kr Separation

The separation of xenon/krypton (Xe/Kr)
mixtures is a challenging
process. Many porous materials allow the adsorption of both Xe and
Kr but only with low selectivity. Anion-pillared metal–organic
frameworks (MOFs), featuring the anion groups as structural pillars,
show potential in gas separations, but only a limited number of them
have been synthesized. Here, we describe a collection of 936 anion-pillared
MOFs based on 22 experimentally available structures. We performed
density functional theory (DFT) optimization and then assigned density-derived
electrostatic and chemical (DDEC) charges for each MOF to make them
well suited to molecular simulations. The structural properties of
the MOFs vary more strongly with the choice of the organic ligand
than with other aspects like fluorine groups and metal centers. We
then screened the entire collection of MOFs in the context of Xe/Kr
separation at room temperature. Compared with previously reported
MOFs, the interpenetrated MOF SIFSIX-6-Cd-i is predicted to perform
better for Xe/Kr separations, with a good balance between working
capacity (1.62 mmol/g) and separation selectivity (16.4) at 298 K
and 100 kPa. We also found that the heterogeneity of fluorine groups
within a MOF can help to enhance Xe working capacity without reducing
the Xe/Kr selectivity, suggesting that synthesis of anion-pillared
MOFs with mixed fluorine groups may lead to improved Xe/Kr separation
performance.