(510c) Facet-Specific Stability of ZIF-8 in the Presence of Acid Gases

Microporous materials such as metal-organic frameworks (MOFs) and zeolitic imidazolate frameworks (ZIFs) are typically described by their bulk properties, such as crystal structure, internal surface area, and metal-ligand connectivity. However, little consideration is given to the external surfaces of these materials, which are necessarily undercoordinated compared to the bulk structure and become increasingly more significant as the crystal sizes approach the nanoscale. It is currently unclear how these surfaces are terminated and how the specific external termination affects the stability of a given crystal facet and bulk structure as a whole. It is expected that the external surface will play a large role in the crystal stability under exposure of these materials to acid gases such as SO₂.

In this contribution, we examine the stability of the (110) and (100) crystallographic facets of ZIF-8 under mild acidic conditions. We find that though the (110) facet is more thermodynamically stable than (100), it is more susceptible to degradation by acid exposure. We hypothesize that the mechanism of particle degradation follows a shrinking-core model, with surface imidazolates being replaced by hydroxyls, as suggested by XPS and FTIR studies. Computational investigations support this hypothesis and suggest that the activation barrier for insertion of water into the Zn-MeIm bond is higher on the (100) facet than (110) due to steric constraints. Additionally, the barrier for water insertion is lowered to a greater extent in the presence of SO₂ on the (110) facet.