(510a) The Degradation Mechanism of MOFs during Acid Gas Exposure: An Experimental and Computational Study

Developing sorbents for the adsorption of acid gases has garnered much research attention in the field of porous materials. Impregnated activated carbons have been widely explored for acid gas separation applications; however, metal-organic frameworks (MOFs) have emerged as a promising class of materials for acid gas separations. MOFs are characterized by metal clusters and organic linkers, large surface areas, and tunable chemical properties that make them excellent candidates for these gas adsorption applications.[1] Several MOFs using titanium for the metal source have shown promising adsorption properties for CO₂, water, and H₂S.[2] However, MOFs often degrade and are unable to retain their performance in the presence of SO₂, NO₂ or other acid gases that are commonly present in industrial applications of interest.[3-5] In this study, MIL-125 and MIL-125-NH₂ were investigated with SO₂ exposure in dry, humid, and aqueous environments using in situ IR experiments and timed, controlled exposures. MIL-125 was found to be unstable in both humid and aqueous environments while the amine functionalized MIL-125-NH₂ was stable under all conditions for extended time periods, showing no change in textural properties or visual degradation as observed through SEM. Both materials were found to be stable under pure water and pure dry SO_2, thereby suggesting the formation of sulfurous acid and the associated bisulfite ion is likely a key step in the degradation mechanism. In situ IR experiments confirmed the presence of sulfite species supporting the hypothesis that the presence of the bisulfite ion likely leads to the degradation of the MIL-125 structure. Computational investigation of several potential reaction mechanisms in the MIL-125 framework indicated the reaction involving the bisulfite ion is favored over reaction with pure water or pure SO₂. DFT simulations support the observation that MIL-125-NH₂ is stable in humid and aqueous conditions as all reactions are less favorable with the functionalized framework compared to the unfunctionalized framework. This study advances the fundamental understanding of MOF degradation mechanisms during acid gas exposure.

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[3] S.G. Han, Y.G. Huang, T. Watanabe, S. Nair, K.S. Walton, D.S. Sholl, J.C. Meredith, Micro. Meso. Mater., 173 (2013) 86-91.

[4] C. Petit, B. Levasseur, B. Mendoza, T.J. Bandosz, Micro. Meso. Mater., 154 (2012) 107-112.

[5] W.P. Mounfield III, U. Tumuluri, Y. Jiao, M. Li, S. Dai, Z. Wu, K.S. Walton, Micro. Meso. Mater., 227 (2016) 65-75.