Origins of Acid-Gas Stability Behavior in Zeolitic Imidazolate Frameworks: The Unique High Stability of ZIF-71

Zeolitic
imidazolate frameworks (ZIFs) are promising materials
for industrial process separations, but recent literature reports
have highlighted their vulnerability to acid gases (e.g., SO2, CO2, NO2, H2S), often present
in practical applications. While previous work has documented the
widely varying stability behavior of many ZIFs under varying (humid
and dry) acid gas environments, efforts to explain or correlate these
experimental observations via empirical descriptors have not succeeded.
A key observation is that ZIF-71 (RHO topology) is an extraordinarily
stable ZIF material, retaining both structure and porosity under prolonged
humid SO2 exposure whereas many other well-known ZIFs with
different linkers and topologies (such as ZIF-8) were shown to degrade.
Through a combination of hybrid quantum mechanics/molecular mechanics
(QM/MM) based methods and statistical mechanical models, we successfully
explain this important experimental observation via atomistic investigations
of the reaction mechanism. Our holistic approach reveals an ∼9
times lower average defect formation rate in ZIF-71 RHO compared to
ZIF-8 SOD, leading to the conclusion that the observed experimental
stability of this material rises from kinetic effects. Moreover, our
analysis reveals that differing stability of the two materials is
determined by the distributions of acid gas molecules,
which is difficult to capture using empirical descriptors. Our results
suggest wider applicability of the present approach, toward identifying
tuned functional groups and topologies that move the acid gas distributions
away from more reactive sites and thus allow enhanced kinetic stability.