Interactions of SO2-Containing Acid Gases with ZIF-8: Structural Changes and Mechanistic Investigations

Zeolitic imidazolate frameworks (ZIFs) are a set of nanoporous metal–organic frameworks (MOFs) with tunable porosity and functionality. Among MOFs, they also show relatively good stability with respect to temperature and humidity. These characteristics lead to their possible applications in separation processes. In many practical separation processes, adsorbents are exposed to a variety of molecular species including acid gases. However, there is little knowledge of the effects of such acid gas exposure on the adsorption and separation properties of ZIFs. Here, the stability of a model ZIF material (ZIF-8) under SO 2 exposure in dry, humid, and aqueous environments has been investigated in detail. Combined characterization by several techniques (PXRD, N 2 physisorption, EDX, XPS, and FTIR) allowed us to track the structural and compositional properties of ZIF-8 before and after SO 2 exposure. ZIF-8 is stable after prolonged exposure in dry SO 2 and in humid air without SO 2 . However, exposure to 10–20 ppm concentrations of SO 2 in the presence of high relative humidity led to its irreversible structural degradation over time as evidenced by substantial losses in crystallinity and textural properties. Exposure to similar concentrations of aqueous SO 2 did not lead to bulk degradation. Humid SO 2 exposed ZIF-8 showed a significant presence of sulfur (S) even after reactivation, with vibrational characteristics corresponding to (bi)sulfite and (bi)sulfate groups. A mechanism of ZIF-8 degradation combining the synergistic effects of SO 2 and humidity is proposed. Attack by sulfuric and sulfurous acid species (generated in humid SO 2 ) leads to protonation of nitrogen in the imidazole ring, resulting in cleavage of metal–linker (Zn–N) bonds. Our detailed experimental findings serve as a starting point for developing a generalized mechanism of acid gas interactions with ZIF materials.