Targeted modifications to the metal unit of MOF for CO2 capture

The heat of adsorption (Qst) of a guest molecule is the fundamental physical characteristic, which drives the physisorption of CO2 to a MOF compound. Considering the high polarizability and quadrupole moment of carbon dioxide compared to other gases being much more abundant in the atmosphere such as O2 and especially N2, the high electrostatic charge of an ion or a coordinative unsaturated metal provides the ideal requisite to increase the Qst of CO2. In other terms, a strong and selective interaction between the guest molecule and the host structure will be established.

Among the most studied MOFs for CO2 adsorption is the MOF-74 family.34 This material is based on a bivalent metal such as Mg(II), Mn(II), Fe(II), Co(II), Zn(II), Ni(II) and 2,5-dihydroxyterephthalate. Mg-MOF-74 (Figure 1) exhibits the highest ever reported CO2 adsorption capacity (27.5 wt%) under standard conditions (298 K and 1 bar).35 The high Qst value for Mg-MOF-74 (47 kJ mol−1) indicates that the structure of this material is quite effective to host CO2 molecules. Indeed, considering the structure of MOF-74 the metal sites are only five-coordinated and the sixth coordination site is available to bind carbon dioxide.34

UTSA-16 [K(H2O)2Co3(cit)(Hcit)] (H4cit=citric acid)36 has also emerged as a potential material for carbon dioxide capture, due to its high density of open metal sites in conjunction with a mild affinity for water. The coordinated water molecules play a fundamental role in CO2 uptake within UTSA-16. Indeed, removing the water from UTSA-16 at elevated temperature, originates a non-porous material.37 Additionally, the lower CO2 interaction energy of UTSA-16 makes it a competitive material for CO2 capture and separation at technical level.38

In the zirconium based MOF compound UiO-66 coordinative unsaturated sites (cus) have been systematically tuned aiming to increase the Qst and increase the CO2 adsorption capacity.39 Although a remarkable influence of the defects on the adsorption process was observed, an increase in the CO2 uptake performance proportional to the number of defects has not been achieved. The cus in UiO-66 are introduced by removing the terephthalic acid linker from the network causing a reduced hydrophobicity of the MOF, which is a limitation to further increase the CO2 adsorption capacity. A substantial uptake increase (up to 48 %) was achieved grafting various nitrogen-containing monocarboxylates at defective sites of UiO-66.40 Post-synthetic defect exchange (PSDE) appears to be a promising approach to enhance the CO2 uptake performance of zirconium based MOFs.

Defective sites can also be used to introduce metal substituted (Li+, Na+, K+) 1,2,4,5-benzenetetracarboxylic acid, performing a metalated-ligand-exchange (MLE) in UiO-66. Following this procedure, it is possible to tune pore sizes (for van der Waals interactions) and to insert polar metal sites (for electrostatic interactions) at the same time, obtaining superior CO2 separation efficiency. Indeed, a 51 % CO2 working capacity increase was registered after MLE.41

The release of CO2 from cus is a process that can be carried out with a low energy expense, applying both pressure or temperature swing adsorption (PSA or TSA) methodology.4244 In PSA CO2 is adsorbed at a given pressure and released by reducing it, while in TSA the gas is released upon a heating of the sorbent material.

The above described examples show that MOF design and modification are powerful strategies to realize a material appropriate for DAC. Nevertheless, a major challenge to face when utilizing MOFs with such structural properties is the competitive adsorption of water in the cus. For example, experimental results show that Mg-MOF-74 when exposed to moisture has a much lower capacity for CO2.45 Therefore, an innovative strategy has to be developed to mitigate this major drawback. For example, increasing the hydrophobicity of a MOF can reduce the water sensitivity. Zinc-based MOF-5 that was thermally annealed below its framework decomposition temperature showed a high CO2 uptake together with increased moisture/air stability.46

The group of Prof. Eddaoudi developed another strategy to prepare MOF materials presenting outstanding CO2 uptake and selectivity even at low CO2 partial pressure. In this case the strong interactions between the CO2 molecules and the secondary building unit of the MOF are responsible for the excellent performances achieved. The MSiF6(pyrazine)2 ⋅ 2H2O (known as SIFSIX-3-M where M=Ni, Cu and Zn) material, thanks to the highly electrostatic inorganic anion (SiF62−) and the narrow binding sites, contribute to establish the interactions between the framework and a polarizable gas such as CO2.47 Nevertheless, although the SIFSIX-3-M MOFs do not present cus, they show reduced CO2 uptake capacity after extended exposure to water vapor. (NbOF5)2− was tested as an alternative inorganic anion to replace SiF62− due to its related distinct qualities: (i) because of the larger size of Nb5+ the Nb−F (1.899 Å) bond is longer than the Si−F (1.681 Å), suggesting the reasonable convergence of the fluorine groups inside the channel; (ii) the greater nucleophile character of the (NbOF5)2− is predicted to stabilize the obtained MOF against water. Up to date, the reported NiNbOF5(pyrazine)2 ⋅ 2H2O (NbOFFIVE-1-Ni) (Figure 2) is the best physical-adsorbent material for carbon dioxide capture from atmospheric and confined spaces. In detail, the NbOFFIVE-1-Ni shows the highest uptake for CO2 at 400 ppm and relatively low regeneration energy.48

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