CO2 and Air Capture Using Physical Adsorbents at Moderate Temperatures. Are Metal-Organic Frameworks Becoming the New Benchmark Materials?

CO₂ capture using physical adsorbents such as activated carbon, zeolites, and metal-organic frameworks (MOFs) have attracted a lot of attention because of the technical capability of adsorption technology using such separation agents to retrofit the energy demanding liquid amine scrubbing process. Nevertheless, this will be conceivable only if the separation agents (adsorbents) in question fulfil many unambiguous attributes in terms of porosity, CO₂ affinity, kinetics, energetics, stability, throughout the right capture mechanism, in addition to the adsorbents cost.

Our work over a long period of time, using different MOFs showed that one of the key factors for design of new materials, which is directly and indirectly related to other attributes such as porosity, kinetics…etc, is the uniformity of suitable adsorption sites over a wide range of CO₂ adsorption loading (Figure 1). Therefore uniform and enough strong CO₂ interaction (energy) distribution is one of the strict requirements to ensure maintaining high selectivity over a broad range of CO₂ adsorption loading [1, 2]. This uniform high charge density in addition to narrow pore size led to unveil, for the first time, a combined mechanism involving optimal thermodynamics (energetics) and kinetics for CO₂ capture at intermediate, low [1] and traces CO₂ concentration [2]. This unique combination of high charge density and optimal pore size allowed to push the boundaries of CO₂ energetics to the upper limit of physical reversible adsorption (45-52 kJ/mol) combined with highly favourable kinetics to CO₂, owing to the small pore size (3.84 Å) as in the case of SiF₆-Zn-pyrazine-MOF and the SiF₆-Cu-pyrazine-MOF (3.5 Å).

References

[1] P. Nugent*, Y. Belmabkhout*, S. D. Burd, A. J. Cairns, R. Luebke, K. Forrest, T. Pham, S. Ma, B. Space, L. Wojtas, M. Eddaoudi and M. J. Zaworotko, Nature, 2013, 495, 80-84.

[2] O. Shekhah^*, Y. Belmabkhout^*, Z. Chen, V. Guillerm, A. Cairns, K. Adil, and M. Eddaoudi. MOF crystal chemistry: The road to made-to-order materials for low concentration CO₂ removal and air capture. 2014, Nature Communication, DOI: 10.1038/ncomms5228