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 which become increasingly 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 provide experimental evidence that supports selective degradation in the surface region of ZIF-8 without affecting the bulk properties of the microporous material. Bulk properties of the crystals were probed via x-ray diffraction, cryogenic nitrogen physisorption, and pulsed-field gradient nuclear magnetic resonance experiments to probe the intracrystalline diffusion properties. The external surface of the ZIF-8 crystals was probed via scanning electron microscopy, x-ray photoelectron spectroscopy, and macroscopic diffusion measurements. These results, paired with vibrational spectroscopy studies, suggest selective removal of 2-methylimidazolate linkers from the surface region of the crystals, and replacement by hydroxyl species. Furthermore, these investigations provide evidence that the (110) facet of ZIF-8 is selectively degraded, whereas the (100) facet appears to be more resistant to aqueous SO2 attack.

Effect of acid gas exposure on the external surfaces of ZIF-8

251st National Meeting (2016)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.