Microporous and mesoporous solid-state materials such as activated carbon, car­bon-based molecular sieves, mesoporous silicas, and zeolites have been demon­strated to have a significant, and in some cases selective, CO2 adsorption capacity. Such materials have advantages for CO2 capture over the amine solvents currently employed in industry as they are endowed with better stabilities and lower energies of regeneration. Zeolites in particular have been widely studied for the purpose of CO2 capture due to their defined and controllable pore size, insensitivity to mois­ture, and high uptake at non-extreme conditions (for example, zeolite 13X has a CO2 uptake of 3.6 mmol. g-1 at 25 °C) [16]. At higher, more industrially-relevant temperatures, these zeolites tend to lose adsorption capacity and also suffer low selectivity for CO2 over other gases (e. g. N2 and H2) as a result of the physisorptive nature of the CO2-adsorbate interaction. To enhance selectivity for CO2, amine — impregnated or amine-modified materials have been explored, which couple the chemisorption approach used in conventional liquid-amine capture with the phys — isorption approach traditionally seen in porous solid materials. This technique has also been employed using a number of porous silicas, such as MCM-41 meso — porous molecular sieves impregnated with polyethylenimine [17] and SBA-15 mesoporous silicas covalently tethered with hyperbranched amines [18]. Despite the increase in CO2 selectivity of such materials achieved using this approach, they often suffer low stabilities over repeated cycles.

For industrial purposes, solid materials with high selectivity and capacity for CO2 uptake, as well as stability to extreme industrial conditions and a low energy for regeneration, are desired. Metal-organic frameworks (MOFs) are a highly promising class of material for this application due to their structural and chemical versatility, arising from different combinations of metal coordination-spheres as well as multidentate bridging ligands with different lengths, shapes, and direc­tionalities of the coordinating groups. While this versatility sometimes comes at the expense of being able to predict structure accurately, the MOF scaffold provides a unique platform upon which to systematically tune the functionalities of known structures to obtain a desirable property [19]. They may be rationally engineered to have a high surface area and porosity, can be post-synthetically modified to allow for increased selectivity for CO2, and can possess excellent stability under indus — trially-relevant conditions [20]. High surface areas and the possibility to possess coordinatively-unsaturated metal sites make MOFs particularly attractive as gas — selective adsorbents. Coordinatively-unsaturated metal centres have been generated in such materials via chelation by post-synthetically modifying bridging ligands or via insertion into open-ligand sites [9]. However, they are most often created through the evacuation of MOFs that have metal-bound solvent molecules. An effective strategy in tuning the selectivity of MOFs for CO2 is the introduction of functional groups into the pores such as amine [21, 22] and sulfone groups (a SO2 group attached to two C atoms) [23], known to specifically interact with CO2 preferentially over other gases of industrial interest.

Topological control is another strategy employed in the design of MOFs for gas separation, however, this approach is frequently serendipitous due to unknown mechanisms of formation of these materials. Despite this, it has been shown that pore size and shape modulation can determine the diffusion dynamics of the molecules to be separated (e. g. metal formates, M(HCO2)2 (M = Mg, Mn, Co, or Ni), have been shown to selectively adsorb CO2 over CH4, suggesting a size — exclusion effect by the small pores) [24]. Generally, attempts to increase pore size through the incorporation of longer ligands results in framework interpenetration. While this is disadvantageous from a gas-storage standpoint, it may be favourable for some guest separations by their kinetic-diameter differences (e. g. CO2 over CH4) [25, 26]. This type of “molecular sieving” approach may also be achieved by taking advantage of the structural flexibility in MOFs. For example, the material Cr (OH)(bdc), where bdc = 1, 4-benzenedicarboxylate and also known as MIL-53(Cr), exhibits a two-step CO2 uptake isotherm compared to a single-step CH4 uptake isotherm, indicative of a specific “gating” effect [27].