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

• Published in: Journal of physical chemistry. C Vol. 120; no. 48; pp. 27221 - 27229
• Main Authors: Bhattacharyya, Souryadeep, Pang, Simon H, Dutzer, Michael R, Lively, Ryan P, Walton, Krista S, Sholl, David S, Nair, Sankar
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 08.12.2016
• Subjects: carbon capture; catalysis (heterogeneous); defects; INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; materials and chemistry by design; membrane; synthesis (novel materials); synthesis (scalable processing); synthesis (self-assembly)
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/acs.jpcc.6b09197

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 SO2 exposure in dry, humid, and aqueous environments has been investigated in detail. Combined characterization by several techniques (PXRD, N2 physisorption, EDX, XPS, and FTIR) allowed us to track the structural and compositional properties of ZIF-8 before and after SO2 exposure. ZIF-8 is stable after prolonged exposure in dry SO2 and in humid air without SO2. However, exposure to 10–20 ppm concentrations of SO2 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 SO2 did not lead to bulk degradation. Humid SO2 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 SO2 and humidity is proposed. Attack by sulfuric and sulfurous acid species (generated in humid SO2) 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.