Insights into the Stability of Zeolitic Imidazolate Frameworks in Humid Acidic Environments from First-Principles Calculations

Understanding
the degradation mechanisms of zeolitic imidazolate
frameworks (ZIFs) is crucial to improve their chemical stability and
realize their potential industrial applications. Here, we conduct
a comprehensive study utilizing dispersion-corrected density functional
theory calculations to investigate the chemical stability of bulk
ZIFs and their external surfaces under conditions of acid-gas exposure.
We examine the influence of steric factors such as topology and ligand
functionalization on the relative chemical stability of prototypical
ZIFs (ZIF-2 and ZIF-8), including their hypothetical polymorphs. We
find that defect formation is more thermodynamically and kinetically
favorable at ZIF external surfaces versus the bulk, and that both
topology and ligand functionalization impact defect formation. In
addition, we provide a detailed mechanism for the reaction of ZIFs
with sulfurous and sulfuric acids, of which the latter serves as a
catalyst in potential degradation reactions of ZIFs. We also provide
information about the adsorption strength of a range of acid gases
to defective ZIF structures, which can inform potential strategies
to regenerate ZIFs and/or achieve defect engineering in these materials.