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  and traces CO2 concentration . 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).
 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.
 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