Probing Structural Transformations and Degradation Mechanisms by Direct Observation in SIFSIX-3-Ni for Direct Air Capture
Michael L. Barsoum, J. Hofmann, Haomiao Xie, Zhihengyu Chen, Simon M. Vornholt, Roberto dos Reis, Nicholas Burns, S. Kycia, Karena W. Chapman, Vinayak P. Dravid, Omar K. Farha
Abstract
Despite global efforts to reduce carbon dioxide (CO 2 ) emissions, continued industrialization threatens to exacerbate climate change. This work investigates methods to capture CO 2, with a focus on the SIFSIX-3-Ni metal–organic framework (MOF) as a direct air capture (DAC) sorbent. SIFSIX-3-Ni exhibits promising CO 2 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 H 2 O and CO 2 dynamics through adsorption and desorption. In addition, a novel space group ( I 4/ mcm ) for the SIFSIX-3-Ni is identified, which provided insights into structural transitions within the framework and elucidated water’s role in degrading CO 2 uptake performance as the material ages. In situ X-ray scattering methods revealed long-range and local structural transformations associated with CO 2 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 CO 2 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.