(152bn) Selective CO2 Capture from Mixtures Using Zeolitic Imidazolate Framework Sorbents: A Gcmc and Molecular Dynamics Study

Zeolitic Imidazolate Frameworks (ZIFs) are a subclass of porous Metal-Organic Frameworks (MOFs) composed of metal ions linked together by organic imidazole-based ligands forming three-dimensional structures with channels or cavities. Depending upon the metal- centers and the organic ligands, variable properties such as pore size, surface area, and affinity to guest molecules by specific functionality make them unique materials for different applications. To understand their potential for CO₂ capture and separation, we investigated pure component CO₂ and N₂ gas adsorption as well as the selective CO₂ adsorption from binary flue gas-like mixtures of two different compositions in defect-free and rigid ZIF-8 (Zinc with 2-methyl Imidazolate linker) and ZIF-90 (Zinc with 2-Carboxyaldehyde Imidazolate linker) using Grand Canonical Monte Carlo (GCMC) simulations. Our simulations showed that both the materials have good potential for CO₂ separation with good selectivity from binary CO₂/N₂ mixtures, with 10 times better selectivity in the case of ZIF-90 than ZIF-8. This is due to the stronger interaction of CO₂ with sites containing (-CHO) in 2-Carboxyaldehyde Imidazolate than (-CH₃) in the 2-methyl Imidazolate rings. Further, we performed simulations using defective ZIF-8 structures as sorbent for CO₂ capture as these defects are very common in a humid condition. The water-induced defects lowered the performance of the sorbent, which indicates that during the adsorption cycle, water vapor present in the flue gas may lead to the formation of defective sites, ultimately reducing the capacity of the adsorbent, which is undesirable. Further, Molecular Dynamics (MD) simulations are being carried out to investigate the effects of rotation and structural flexibility [2] of organic imidazole ligands on adsorption capacity. Our work provides molecular-level insights to understand the capture of CO₂ and its selectivity using ZIFs as adsorbents.

REFERENCES:

[1] C. Han, R. J. Verploegh, and D. S. Sholl, “Assessing the Impact of Point Defects on Molecular Diffusion in ZIF-8 Using Molecular Simulations,” Journal of Physical Chemistry Letters, vol. 9, no. 14, pp. 4037–4044, Jul. 2018, doi: 10.1021/ACS.JPCLETT.8B01749/ASSET/IMAGES/LARGE/JZ-2018-01749M_0004.JPEG.

[2] D. Fairen-Jimenez, S. A. Moggach, M. T. Wharmby, P. A. Wright, S. Parsons, and T. D€ Uren, “Opening the Gate: Framework Flexibility in ZIF-8 Explored by Experiments and Simulations,” J. Am. Chem. Soc, vol. 133, pp. 8900–8902, 2011, doi: 10.1021/ja202154j.