Probing Structural Transformations and Degradation Mechanisms by Direct Observation in SIFSIX-3-Ni for Direct Air...

Despite global efforts to reduce carbon dioxide (CO₂) emissions, continued industrialization threatens to exacerbate climate change. This work investigates methods to capture CO₂, with a focus on the SIFSIX-3-Ni metal-organic framework (MOF) as a direct air capture (DAC) sorbent. SIFSIX-3-Ni exhibits promising CO₂ adsorption properties but suffers from degradation processes under accelerated aging, which are akin to column regeneration conditions. Herein, we have grown the largest SIFSIX-3-Ni single crystals to date, facilitating single crystal X-ray diffraction analyses that enabled direct observation of the H₂O and CO₂ dynamics through adsorption and desorption. In addition, a novel space group (I4/mcm) for the SIFSIX-3-Ni is identified, which provided insights into structural transitions within the framework and elucidated water's role in degrading CO₂ uptake performance as the material ages. In situ X-ray scattering methods revealed long-range and local structural transformations associated with CO₂ adsorption in the framework pores as well as a temperature-dependent desorption mechanism. Pair distribution function analysis revealed a partial decomposition to form nonporous single-layer nanosheets of edge-sharing nickel oxide octahedra upon aging. The formation of these nanosheets is irreversible and reduces the amount of active material for the CO₂ sorption. These findings provide crucial insights for the development of efficient and stable DAC sorbents, effectively reducing greenhouse gases, and suggest avenues for enhancing MOF stability under practical DAC conditions.