(129g) Effect of Acid Gas Interactions on the Structure and Porosity of ZIF-8

Metal organic frameworks (MOFs) are a class of nanoporous materials that have received considerable attention for use in adsorptive and membrane separations. Zeolitic Imidazolate Frameworks (ZIFs) are well known amongst MOFs for their structural stability with respect to temperature and humidity. ZIFs can be synthesized utilizing different organic linker molecules and synthesis conditions, leading to a set of diverse materials with tunable surface areas, pore sizes, surface functionalities and possible applications. In many practical separation processes such as CO₂ capture from post combustion flue gas, landfill gas separation, natural gas sweetening and others, adsorbents are exposed to a variety of molecular species including acid gases that can have a detrimental effect on performance parameters such as selectivity or adsorption capacity, or can lead to irreversible structural changes in the adsorbent that may restrict its prolonged use. However, there is little knowledge of the effects of such acid gas exposure on the adsorption and separation properties of ZIFs.

In this study, the stability of a model ZIF material (ZIF-8) under SO₂ exposure in dry, humid, and aqueous environments was investigated in detail. To determine structural and textural changes, pXRD and N₂ physisorption were conducted before and after exposure. Furthermore, techniques such as EDX, XPS and FTIR were used to study the mechanistic aspects of these changes. ZIF-8 was found to be stable after prolonged exposure in both dry and aqueous SO₂ environments and also in the presence of humid air without SO₂. However interactions with ppm concentrations of SO₂ in the presence of high relative humidity led to its irreversible structural degradation over time as evinced by substantial losses in crystallinity, BET surface areas and pore volumes. EDX analysis showed a significant presence of S even after reactivation of the exposed ZIF-8. FTIR studies revealed the presence of S-O vibrations and the protonation of the nitrogen in the imidazole ring, suggesting cleavage of the metal-linker bond. A mechanism of ZIF-8 degradation combining the synergistic effects of SO₂ and humidity is proposed, which advances the present understanding of the stability of ZIF materials towards acid gas exposure.