Designing Adsorptive Gating via Linker Side-Chain Functionalization in a Honeycomb-MOF

Metal–organic frameworks (MOFs) combine high guest-accessible
porosity with high chemical versatility, which predestines tailor-making
porous materials to overcome challenges in efficient next-generation
gas separation processes. Adsorptive gating is an interesting material
feature that enhances sorption selectivity, while its rational design
is still beyond our knowledge. Herein, we report on a model system
to explore the key factors of induced gating controlled by the implementation
of conformationally flexible side chains at the linkers of a honeycomb-like,
structurally rigid MOF, namely, Zn2(2,5-difunctionalized-1,4-benzendicarboxylate)2(4,4′-bipyridine). The sorption and separation properties
of C2H2 and CO2 were selected as
the study case, and ideal adsorbed solution theory (IAST) selectivities,
isosteric enthalpies of adsorption, Henry constants, breakthrough
experiments, and simulations of dynamic properties were correlated
with the linker functionalization. Two dominating factors that determine
gating properties are identified: polarizability and sorption site
accessibility. These factors are accessible through linker functionalization,
and we show that adsorption strength differences can be influenced
linearly within 1 order of magnitude, while breakthrough experiments
show a selectivity increase toward C2H2, ranging
from 4.1 to 10.9. The results suggest transfer to other MOFs toward
extrinsic gating for sophisticated modulation of their sorption selectivity.