(465h) Effect of Topology and Structural Tunability on Selective Capture of CO2 Using Zeolitic Imidazolate Frameworks

A typical post-combustion flue gas predominantly consists of N₂, CO₂, H₂O and O₂,¹ with the concentration of CO₂ being 15% and N₂ as the major component. Presence of moisture and acid gas impurity, it has become a daunting task to selectively separate CO₂ from the mixture. Commercial CO₂ capture technology, which involves amine-based solvents or alkaline solutions, suffers from high energy penalty. Hence, it is essential to develop cost-effective and energy-efficient CO₂ capture technologies. In this regard, the discovery of metal-organic frameworks (MOFs) has opened up new possibilities as the adsorption-based CO₂ capture from the multi-component gas mixture is increasingly considered a promising technology.

MOFs possess high surface area, large pore volume, excellent structural flexibility and tremendous tuneable functionality. However, they often suffer largely from a lack of chemical stability and high synthesis cost. Among the MOF family of materials, the zeolitic imidazolate frameworks (ZIFs), possessing similar topologies to zeolites, are considered to possess high thermal and good chemical stability and can be synthesized with ease in comparison to many other MOFs. For our study, we have selected a few ZIF-class of materials, e.g., ZIF-7 and ZIF-11, with zinc as a metal centre and benzimidazole as a linker. ZIF-7 and ZIF-11 use the same precursors but different synthesis methods, resulting in materials with varied topology, viz., SOD and RHO, respectively. Consequently, we use these as exemplary materials to investigate the effect of topology on CO₂ adsorption in detail. Furthermore, we consider an exceptionally stable material, i.e., ZIF-71 with RHO topology and Cl-substituted imidazole linker (4,5-dichloroimidazole), for comparison. All the three materials employed in this study are stable in humidity and that ZIF-71 shows outstanding chemical stability even with underexposure of humid CO₂ and/or SO₂.²

Preliminary CO₂ adsorption studies on ZIF-7 and ZIF-71 indicate that the former can show significant potential for CO₂ capture with an uptake capacity at 300 K and 1.2 bar is 9 wt% (~ 2 mmol/g) while for the latter it is 3 wt% (~ 0.7 mmol/g). Under similar experimental conditions, ZIF-7 not only has a 3 times higher uptake than the corresponding ZIF-71 but also possesses an interesting gate-opening phenomena triggered by CO₂ molecule leading to an S-shape adsorption isotherm under atmospheric pressure. This structural flexibility could further improve the selectivity of CO₂/N₂ which is essential for separation processes. On the other hand, since ZIF-71 is not affected by the presence of impurities, this material could prove to be a better candidate for CO₂ capture from gas streams containing impurities. In this context, our future direction would be to conduct dynamic breakthrough experiments under a 15% CO₂/N₂ mixture using a packed bed of ZIF-7, ZIF-11 and ZIF-71. The outcome of this experiment would enable us to correlate the effect of functional group, topology and structural flexibility in the relation to the gas mixture separation. Furthermore, we can also hypothesize that ZIF-7 would probably have a longer CO₂ retention time owing to its CO₂ selectivity, while the presence of benzene ring and -Cl group as functionality in ZIF-11 and ZIF-71 will tend to interact differently with CO₂ molecule causing distinctive breakthrough time.

References

1. Sumida, K., Rogow, D.L., Mason, J.A., McDonald, T.M., Bloch, E.D., Herm, Z.R., Bae, T.-H. and Long, J.R., Carbon Dioxide Capture in Metal-Organic Frameworks. Chem. Rev., 112, 724-781 (2012).
2. Bhattacharyya, S., Han, R., Kim, W.-G., Chiang, Y., Jayachandrababu, K.C., Hungerford, J.T., Dutzer, M.R., Ma, C., Walton, K.S., Sholl, D.S. and Nair, S., Acid Gas Stability of Zeolitic Imidazolate Frameworks: Generalized Kinetic and Thermodynamic Characteristics. Chem. Mater., 30, 4089-4101 (2018).