https://doi.org/10.1002/cplu.202000072
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The introduction of designated uncoordinated functional groups (−NH2, (−COOH or (−OH) in the organic linkers is a typical method to build a MOF with higher affinity between the crystalline framework (host) and the gas (guest).49, 50 Using ligands with specific functional groups, it is possible to increase the MOF uptake performances, this is a consequence of higher adsorption enthalpy. MOF surfaces can be modified either by choosing appropriate linker precursors for the synthesis or through post-synthetic modification of an already synthesized MOF compound. Consequently, a broad variety of methods is available to introduce the desired functionality in the material backbone. Amino groups (both alkyl and aromatic) are the most employed functionalities to increase the MOF affinity to CO2, thanks to their high polarity and nucleophilicity. For example, a moderate CO2 uptake increase has been demonstrated by simply replacing terephthalic acid by 2-aminoterephthalate in the CAU-1 MOF.51
Similarly, very good results in terms of both selectivity and capacity towards CO2 have been achieved using bio-MOF-11, a cobalt based MOF which includes adeninate in its structure. The nucleobase adenine contains an aromatic amine, a pyrimidine and a diazole, this exceptional variety of amino groups allows a strong affinity for the acidic CO2.52 Several examples of amino functionalization of MOFs are reported in the literature, especially tetrazole and triazole derivatives have shown interesting results.53, 54
The role of the amine group in the CO2 adsorption has been widely discussed. Considering the immense variety of the MOFs, a general behavior cannot be proclaimed. An interesting example is that of the NH2-MIL-53(Al) MOF, as its adsorption properties derive from a fragile interaction of weak dispersion forces determining the flexibility of the MOF material.55 The role of the basic amine moiety on the adsorption properties is only indirect. The lack of strong chemical interactions with CO2 is crucial for the general activity of the sorbent, since full regeneration can be completed rapidly at 80 °C.
A completely different strategy to design a MOF suitable for carbon capture consists in increasing the hydrophobicity of the material. Although the affinity to highly polarizable molecules such as CO2 will not benefit from this methodology, the exclusion of water from the pores can increase the overall uptake performance. Considering the ubiquitous humidity of the atmosphere, the design of a MOF for DAC must include a strategy to reduce the water affinity of the framework.
Methodologies to increase the framework hydrophobicity, such as polydimethylsiloxane (PDMS) coating,56 introduction of long alkyl substituents57 and surface pyrolysis58 have been already successfully tested to selected MOFs. However, these approaches are associated with consistent decrease of the materials’ pore volumes and/or specific surface areas and can be applied only to limited classes of MOF materials.
On the other hand, some promising results have been also achieved. The group of Prof. Yaghi prepared hydrophobic zeolitic imidazolate frameworks (ZIFs) with the chabazite (CHA) topology by including two different imidazolate linkers. ZIF-300 prepared using 2-methylimidazolate and 5-(6)-bromobenzimidazolate turned out to be the most effective material for the dynamic separation of CO2 from N2. The experiments were carried out under dry as well as humid conditions and no loss of activity was registered over three subsequent cycles.59 Ding et al. carried out an in-situ polymerization of aromatic acetylenes to obtain polynaphthylene inside the channels of MOF-5. Compared with pristine MOF-5, the resultant material (PN@MOF-5) exhibited a 2-fold CO2 capacity, 23-times higher CO2/N2 selectivity and significantly improved moisture stability.60 Prof. Farha and coworkers developed a solvent-assisted ligand incorporation (SALI) to functionalize the zirconium MOF NU-1000 with perfluoroalkane carboxylates of various chain lengths (C1–C9) (Figure 3). CO2 adsorption studies indicated that perfluoroalkane-functionalized zirconium-nodes act as the primary CO2 binding sites and the Qst constantly increase with the chain length.61 More recently D’Amato et al. prepared two materials with UiO-66 and MIL-140 topologies using Cerium as metal and 2,3,5,6-tetrafluoroterephthalate as linker. These materials outperform their Zr-based analogs. The calculated CO2/N2 selectivity resulted to be among the highest ever reported for metal-organic frameworks (>1900).62
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