Application of Metal-Organic Frameworks (MOFs) for Capturing CO₂: Advancement and Challenges

    

    

    Figure 5: (a) Crystal structures of mmen-Mg2(dobpdc) (M = Mg or Mn), with portions of the crystal structure displayed on the right side (Green, grey, red, blue and
white spheres represent M, C, O, N and H atoms, respectively; with some H atoms omitted for clarity); CO2 and N2 adsorption isotherms for (b) mmen-Mg2(dobpdc)
and (c) mmen-Mn2(dobpdc) [22,24].
    

Fuel production: Natural gas/Biogas upgrading: Methane is the lightest and the cleanest hydrocarbon fuel with the lowest carbon to hydrogen ratio. Most of the sources of CH₄ such as natural gas and biogas contain significant amount of CO₂ together with other impurities such as H₂S and humidity that could be emitted into the atmosphere [29-31]. As a result, capturing of CO₂ impurities is very crucial for upgrading of natural gas and biogas and for reducing emissions into the atmosphere. Owing to the disadvantages associated with the currently employed amine scrubbing technology [13], the development of alternative cost and energy-efficient processes and materials, such as adsorption-based technology, that could offer great prospects to address this challenge is a necessity. The key in this regard is the choice of appropriate adsorbent with high stability towards humidity and H₂S, with good CO₂ adsorption capacity and selectivity.

Owing to their tunable porous structures, MOFs are considered to be suitable candidates to overcome the challenges in natural gas and biogas sweetening processes. In the same way as that of MOFs for post-combustion CO₂ capture, targeting structures with high density of CO₂ binding sites can help in the assembly of MOFs relevant for natural gas/biogas upgrading. Additionally, MOFs with adjusted pore aperture sizes to distinguish CO₂ and CH₄ based on their different molecular sizes (kinetic diameters of CO₂ and CH₄ are 3.3Å and 3.8Å, respectively), can be suitable targets for the targeted application. The stability of the frameworks towards moisture and H₂S is also additional parameter to be envisaged. Among the different MOFs that showed good performance for CO₂/CH₄ separation, UTSA-49 (Zn(mtz)₂; mtz = 5-methyl-1H-tetrazolate), showed CO₂/ CH₄ selectivity of 33.7 at 298 K for 1:1 mixture of CO₂ and CH₄ [32]. The presence of uncoordinated nitrogen atoms on the structure and two types of small pore openings (2.9Å×3.6 Å and 3.6 Å×4.0 Å), are supposed to play role for preferential adsorption of CO₂ over CH₄. The mixed-metal ZIF, ZIF-204 (Zn₂Cu₃(Im)₁₀; Im = imidazolate), was also explored for its CO₂ capture potential from a ternary gas mixture of CO₂, CH₄ and H₂O with the MOF demonstrating selective adsorption of CO2 from the mixture without being affected by the presence of water [33]. Similarly, two polymorphic MOFs, Qc-5-Cu-a and -β [Cu(quinoline-5-carboxylate)2], that exhibit pore sizes suitable for size selective CO₂/CH₄ separation, were shown to have potential for molecular sieving of CO₂ from CH₄ in a humid environment [34]. Even though it is at its infancy, the development of H₂S stable MOFs that can simultaneously capture CO₂ and H₂S from natural gas and biogas streams is a recent research interest. In this regard, the Gasoc- MOF [35] and the rare earth cluster based fcu-MOFs [36] gave interesting results for selective capture of acid gases from methane with good H₂S/CO₂ selectivity as well. Recently, Ultra-microporous fluorinated MOFs showed an exception with highly tunable/stable removal capabilities for both CO₂ and H₂S [37]. However, there remains a lot to be done in the design and preparation of MOFs suitable for selective removal of CO₂ from CH₄ in the presence of other impurities such as H₂S. To have full practical implementation of MOFs for natural gas/biogas upgrading, further research works that target to overcome hurdles related to MOF stability towards such impurities need to be addressed.

H₂ production: Pre-combustion capture: The other process that requires the capture of CO₂ at large-scale is the pre-combustion C-capture, which mainly involves the capture of CO₂ from hydrogenrich synthesis gas prior to combustion. A typical pre-combustion mixture might contain 15-60% CO₂ mixed with H₂ and other impurities at relatively high pressures. In the quest for highly selective and efficient adsorbent materials for the targeted application, as a substitute for the commonly used energy-intensive liquid amine adsorption, MOFs have attracted intense research interest owing to their tailorable structures [38]. Compared to the CO₂/N₂ and CO₂/ CH₂ separations, the number of studies on CO₂/H₂ separations using MOFs is very limited. UTSA-16 [39], a microporous MOF that has been explored for capture of CO₂ using PSA unit from steam-methane reforming off-gases (76% H₂, 17% CO₂, 3% CH₄ and 4% CO); Cu-BTTri [40,41], H₃[(Cu₄Cl)₃(BTTri)₈] (BTTri³- = 1,3,5-benzenetristriazolate) that displays high CO₂/H₂ separation capability (IAST selectivity up to 860) for 80:20 and 60:40 H₂/CO₂ gas mixtures due to its exposed cation sites (adsorbs 4.5 CO₂ molecules per Cu at saturation); Mg- MOF-74 (dobdc^4- = 1,4-dioxido-2,5-benzenedicarboxylate) [42], another MOF with open metal sites which adsorbs 1.8 CO₂ molecules per Mg at saturation; and Ni-4PyC [43], an ultra-microporous (3.5 and 4.8 Å pores) Ni-(4-pyridylcarboxylate)₂ MOF that exhibits CO₂/ H₂ selectivity of 285 for 20:80 and 230 for 40:60 mixtures at 10 bar and 40°C with facile adsorption-desorption cycles and CO₂ selfdiffusivity (~3×10^-9 m₂/s), are among the well-known MOFs that have been experimentally tested for CO₂/H₂ separation under precombustion CO₂ capture conditions with significant improvements in performances over zeolites and activated carbons. Due to the high pressure of the pre-combustion gas mixture, MOF-based membranes could be targeted for better efficiency besides the necessity to work on the development of MOFs with high CO₂ capacity.

Small-scale emissions/removal

Compared to the traditional CO₂ capture processes from large point sources, little attention has been given to CO₂ emissions from small-scale sources such as buildings, aircrafts, cars, trucks, and other confined air spaces mainly due to their expected smaller contribution to greenhouse gas emissions (about 10% to 15%) and the higher cost of the process than the larger-scale CO₂ capture [44]. However, the CCS from small and mobile source provides a means to adjust/stabilize the atmospheric CO₂ concentrations in confined air spaces and in other places where mitigation efforts fall short of targets [45]. As a result, development of feasible and economical CCS technologies for small and medium-scale CO₂ emissions must receive more attention. In this section, we make a perspective analyses on the application of small-scale CCS for medical applications and highlight some works on the CO₂ capture from confined air spaces.

Medical applications: CO₂ is known for a number of its medical applications. To mention a few, a liquefied CO₂, supplied in cylinders filled to a high pressure, is used as an insufflations gas during bodily investigations and for minimal invasive surgery (laparoscopy, endoscopy, and arthroscopy). Medical CO₂ also provides respiratory stimulation during and after anesthesia. In a solid form (dry ice), it serves as a cryotherapy agent to achieve a temperature of -76 °C, which in turn enables freezing of tissues and removal of wart, moles, and skin tags. For effective utilization of CO₂ for the clinical applications with minimum cost and improved patient care, there is a critical need to develop practical and economical way of selective capturing and, hence storing the gas. This could be achieved using fully recyclable CO₂ sorbents where CO₂ can be effectively extracted and properly used. The CO₂ sorbents to be utilized for this purpose should have highly efficient and selective CO₂ separation potential and high gravimetric CO₂ uptake capacity. To date, there are no welldocumented CO₂ sorbents that have been practically used for medical purposes [46].

Applications in confined spaces: Environments involving occupants in sealed air spaces such as aircrafts, submarines, vehicles, buildings, etc. lead to build-up of CO₂ concentrations over time due to respiration. The humans inside the confined spaces can then be affected by the rising levels of CO₂. In such environment, the safe exposure limit is about 1% CO₂ since higher concentration can lead to adverse side effects such as headaches and lethargy [47]. The high level of CO₂ in such environment is also linked to operational issues due to dry ice formation [48] and hence it must be removed from air prior to liquefaction. As a result, the development of appropriate techniques and materials that can maintain safe levels of CO₂ by efficiently adsorbing such low concentration of CO₂ is essential. Among the different techniques, direct air capture [49] has been implemented for the past a few decades to capture small-scale CO₂ emissions in submarines [1] and spaceships [2]. However, the technique is expensive due to its large energy consumption. Porous materials to be used for this application require suitable binding sites that possess sufficiently strong interactions with CO₂ at low coverage but at the same time being easier for regeneration without the need for a high energy input [2]. In recent years, MOFs have shown very promising potential to tackle such challenge. Among the different examples of MOFs, NbOFFIVE-1-Ni [NiNbOF₅(pyrazine)₂•2H₂O], due to its suitable structural features such as appropriate sized square channels, pore walls with polar functional groups and the lower energy requirement for regeneration, has been shown to be a viable material for this application [50]. Other MOFs, functionalized with polar groups, have also shown attractive features for lowconcentration CO₂ uptake [51,52]. Given the possibility to tune their structures for a particular application, a lot of room is then available for further design of new MOFs with more enhanced performance.

State-of-the-Art in Research, Development, and Innovation for MOF Deployment in CO₂ Capture

Although most of CO₂ capture adsorbents studies at the labscale are based on purely equilibrium thermodynamics where the main separation driving force is the difference in the interaction between CO₂ and the less absorbable products/secondary products, few recent studies demonstrated disruptive advanced concepts that allow the combined maximization of CO₂ uptake and selectivity while intrinsically reducing the level of interaction with adsorbents [20,53]. The benefit of such endeavor is to maximize the reduction of adsorptive cycling’s cost via optimal adsorbent regeneration, primarily linked to MOF structural and functionality features. Such control could have a dramatic effect on the main metrics for the separation while affording mild conditions for cycling. With this regard, it is obvious that CO₂-MOF interactions with physical nature are more preferred to achieve such objective. In this section, we will go over the main works highlighting the use of MOF as chemical adsorbents and the recent works that allowed making dissimilarity between physical adsorption mechanisms as function of the extent of thermodynamics and kinetics involved in the CO₂ capture process. Several types of MOFs have been proposed for CO₂ capture, including (i) MOFs with open metal sites [7,42,54-63]; (ii) MOFs without open metals sites [64-80]; (iii) MOFs with narrow pore size via interpenetration [53,81,82] or shortening the size of the ligands [53,67] and (iv) MOFs decorated with specific functional groups, including (NH₂, OH, etc.) [76,83-87]. From these types of materials, we discuss selected ones that offer the maximized uptake-selectivity performances at the lab-scale. For a more comprehensive account on CO2 adsorbents, we direct the reader to reviews by Sumida et al. 2012 [88] and Sayari et al. 2011 [89].

Chemical adsorption

Chemisorption, which relies on chemical reaction between CO₂ and sorbent, is the most widely employed and well-developed CO₂ capture technique. Among the state-of-the-art chemical adsorption techniques, liquid amine scrubbing is the most popular approach for bulk-scale carbon capture [45]. In an attempt to improve the performance of amine-based sorbents, solid chemisorbents were developed from amine-modified porous materials. Amine-grafted or functionalized MOFs are among those materials that have been explored for CO₂ capture applications [24,90]. Compared to the unmodified porous materials, the amine-functionalized counterparts were found to exhibit significantly improved CO₂ affinity. The regeneration costs are also relatively lower than that of liquid aminebased techniques. However, problems associated with the use of such chemisorbents such as the high temperature requirement for regeneration (even though it is lower than that of liquid chemisorbents), the difficulty in the preparation of the materials, the high cost of the precursors, the poor chemical and thermal stabilities, etc. hamper the further development of such materials for CO₂ capture applications [90].

Physical adsorption

The use of physisorbent materials that selectively bind CO₂ at low concentrations is an alternative to chemical absorption. However, it was reported in different works that physical adsorption is too weak to handle low concentration CO₂ removal in highly or average diluted gas streams [12,91]. Even at lab-scale, the use of physical adsorbents, from the family of porous materials available commercially, for CO₂ capture from diluted gas streams was not recognized as a viable promising solution. In this subsection, we will shed light on this topic to break this old, prolonged misconception. For this purpose, the revealed advanced concepts involving kinetics will be discussed only macroscopically with the link to the structural and functional features of the MOF adsorbents.

CO₂ capture based on adsorption equilibrium thermodynamics: CO₂ adsorption based on equilibrium thermodynamics represents the bulk of the studies for CO₂ capture in wide range of conditions (pressure, temperature, and compositions). The thermodynamic system based on this mechanism is diffusion resistance free, as most the porosity features of the CO₂ separation agents involved are optimal (enough large aperture size and pores diameter) to allow to achieve high self and transport diffusion parameter toward and from the highly homogeneous pore network.

Various examples of MOFs have been reported to be suitable for thermodynamically driven separation of CO₂ from CH₄ and N₂. One of the earlier example that has been explored for such application is the interpenetrated microporous MOF, Zn(BDC)(4,4’-Bipy)_0.5 (MOF- 508) [92]. The MOF can allow diffusion of all the three molecules with little or no restriction since the size of its one-dimensional channels (~4.0 Å x 4.0 Å) (Figure 6) is higher than the kinetic diameters of CO₂, CH₄ and N₂ (3.30 Å, 3.64 Å and 3.80 Å, respectively). The preferential adsorption of CO₂ over N₂ or CH₄ (breakthrough selectivity of 3 and 6 for CO₂/CH₄ and CO₂/N₂, respectively) together with a higher adsorption enthalpy for CO₂ with respect to the other two gases can then prove the enthalpic-driven mechanism [92]. Due to the absence of specific functional groups that can interact with CO₂, the adsorption selectivity is not high. However, this is one of the first examples of MOFs that permitted further practice of pore size reduction and interpenetration as design principles for the targeted application.

Figure 6: Crystal structure of MOF-508 showing its one-dimensional channel (a); Isosteric heat of adsorption for N₂, CH₄, and CO₂ (b); Single-component at 323 K and 1 bar (c) and Multicomponent (1:1:1 mixture of CH₄/N₂/CO₂) at 303 K and 4 bar (d) breakthrough curves for N₂ (square), CH₄ (circle), and CO₂ (triangle) [92].

    

    

    Figure 6: Crystal structure of MOF-508 showing its one-dimensional channel (a); Isosteric heat of adsorption for N2, CH4, and CO2 (b); Single-component at 323 K and 1 bar (c) and Multicomponent (1:1:1 mixture of CH4/N2/CO2) at 303 K and 4 bar (d) breakthrough curves for N2 (square), CH4 (circle), and CO2 (triangle) [92].
    

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The other examples of MOFs that showed potential for enthalpicdriven separation of CO₂ are SIFSIX-2-Cu and the self-interpenetrated analogue SIFSIX-2-Cu-i, which contain square grid layers from the connection of Cu²⁺ cations and 4,4’-dipyridylacetylene linkers and are intercalated by (SiF₆)^2- pillars [53,93,94]. Both materials exhibited CO₂/N₂ separation with a selectivity ranging from 140 to 13.7 (298K, 1 bar, gas mixture CO₂/N₂: 10/90) based on Ideal Adsorbed Solution Theory (IAST). Similar results were obtained for CO₂/CH₄ separation with a selectivity increasing from 5.3 to 33 (298 K, 1 bar, gas mixture CO₂/CH₄: 50/50) upon using SIFSIX-2-Cu-i. The pore dimensions of SIFSIX-2-Cu and SIFSIX-2-Cu-i are 13.5 Å and 5.15 Å, respectively, which are above the molecular sizes of the three molecules. These results and the enhanced CO₂-MOF interactions as reflected in the higher isosteric heat of adsorption evidenced the thermodynamic equilibrium adsorption mechanism. These and other examples [68,95-97] demonstrate the importance of interpenetration and pore size constriction to induce MOF-CO₂ interactions and hence better enthalpy-driven separation performances. MOFs without interpenetration but with reduced pore apertures such as UTSA-16 also gave interesting results in terms of thermodynamic separation of CO₂ from gas streams [98,99].

The other most practiced strategy in the design of MOFs for thermodynamic separation of CO₂ is the incorporation of high charge density (CO₂-philic groups) such as amine functional groups, coordinative unsaturated metal sites, etc. The use of functionalized linkers to assemble MOFs suitable for enthalpic-driven CO₂ separation is exemplified by the isoreticular series of RE-fcu-MOFs, constructed from the assembly of the 12-c RE hexanuclear cluster MBBs, [RE₆(μ₃-OH)₈(O₂C-)₁₂], with a series of ditopic heterofunctional as well as homo-functional linkers of various lengths and functionalities [7]. Despite the triangular pore aperture diameters of the MOFs (5-6 Å) being above the kinetic diameters of CO₂ and N₂, a high CO₂/N₂ adsorption selectivity (ca. 370) was predicted by IAST calculation. These results together with the high value of the calculated isosteric heats of adsorption for CO₂ (58.1 kJ mol^-1 at low loading for Tb-FTZB-fcu-MOF)) evidenced thermodynamic mechanism for the observed selective separation. The high charge density within the frameworks due to the presence of tetrazolate moieties, fluorine groups and coordinative unsaturated sites is responsible for the enhanced MOF-CO₂ interaction. The other very popular MOFs that have been widely investigated for equilibriumbased CO2/CH4, CO₂/N₂ and CO₂/H₂ separations are Mg-MOF-74 and its derivatives [24,42,51,55] made from 2,5-dihydroxybenzene- 1,4-dicarboxylic acid (H₂DOBDC) or elongated form of the linker and magnesium salt sources. The presence of high density of open metal sites and the amenability of the MOF for post-synthetic grafting of functionalized groups that can interact with CO₂ made it to be one of the well-studied MOFs for selective CO₂ capture.

CO₂ Capture based on Kinetics: Although important and critical metric in the selection of adsorbents in general and CO₂ adsorbents in particular, selective kinetics CO₂ adsorption is rarely studied as the main driving force for separating CO₂ from N₂, CH₂ and H₂. At the exception for H₂, CO₂ is expected to diffuse much faster than N₂ and CH₄ into and from any pore network. The desirable cap enhancement on the diffusion properties could be achieved by using MOF with reduced pore aperture size that will potentially enhance the kinetics selectivity toward CO₂. Due to the difficulty in measuring experimentally the diffusion of gases into the pores, only few works reported kinetic gas-based separation. Among the early examples of MOFs, [Cu₂(bza)₄(pyz)]_n (bza = benzoic acid; pyz = pyrazine), which possesses two types of confined 1D pore channels with apertures of 2 Å, has been shown to be promising for possible kinetic separation of CO₂ from N₂ or CH₂ [100,101]. In spite of the fact that it is difficult to extract adsorption kinetics data, gas mobilities were demonstrated using single-crystal membrane of the MOF with high permeabilities for CO₂ as compared to N₂ and CH₄ (1.5×10^-12, 1.1×10^-13 and 5.9×10^-14 mol m m^-2s^-1Pa^-1 for CO₂, N₂ and CH₄, respectively) (Figure 7) [102].

Figure 7: Comparison of gas permeabilities along the channels (red) and vertical to the channels (blue) of the single crystal membrane [102].

    

    

    Figure 7: Comparison of gas permeabilities along the channels (red) and vertical to the channels (blue) of the single crystal membrane [102].
    

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CO₂/CH₄ kinetics-driven separation was also demonstrated using a layered doubly-interpenetrated Cu-MOF, Cu(hfipbb)(H₂hfipbb)_0.5 (H₂hfipbb = 4,4'-(hexafluoroisopropylidene) bis(benzoic acid)) [103]. The narrow window (~3.5×3.5 Å) of the MOF, that are comparable to the molecular dimensions of CO₂ and CH₂, make it a suitable candidate for kinetic separation of the gases [103]. Despite the slight differences in heats of adsorptions for both gases at zero loading (29.7 and 21.4 kJ mol^-1 for CO₂ and CH₄, respectively), high overall CO₂/ CH₄ selectivity of 25 (Figure 8) was observed due to the significantly faster diffusion of CO₂ (2.97×10^-3 s^-1) than CH₄ (1.14×10^-4 s^-1) [103].

Figure 8: Left: crystal structure of Cu(hfipbb)(H₂hfipbb)_0.5; Right: Fractional adsorption uptake of CO₂ (black) and CH₄ (red) at 298K [103].

    

    

    Figure 8: Left: crystal structure of Cu(hfipbb)(H2hfipbb)0.5; Right: Fractional adsorption uptake of CO2 (black) and CH4 (red) at 298K [103].
    

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CO₂ Capture based on cooperative effect between Thermodynamics and Kinetics: The synergy between thermodynamics and kinetics was also found to play role in the selective capture of CO₂ using MOFs. In this regard, one peculiar platform is the M-SIFSIX MOFs [53]. Among the these family of MOFs, the Zn-pyrazine (pyr) MOF, SIFSIX-3-Zn or [Zn(pyr)2(SiF6)] n, with square pore channels of ~3.84 Å, exhibited highly selective CO₂/N₂ (selectivity = 495) and CO₂/CH₄ (selectivity = 109) separation potential as evidenced by low-pressure pure-component CO₂, N₂ and CH₄ adsorption data, column breakthrough studies using CO₂/ N₂:10/90 and CO₂/CH₂:50/50 gas mixtures at 298K and atmospheric pressure, IAST calculations to predict adsorption equilibria for CO₂/CH₄:05/95, CO₂/CH₄:50/50, CO₂/N₂:10/90 and CO2/H2:30/70 mixtures and competitive adsorption kinetic studies of the aforementioned gas mixtures (Figure 9). The contracted pore aperture of the MOF, due to the short ligand employed to assemble it, was asserted to provide sufficient diffusion barriers to differentiate the adsorption rates of CO₂ and N₂. Moreover, the high charge density within the structure favors thermodynamic separation. The adsorption properties of this 1st generation of fluorinated-MOF demonstrated the interplay of both kinetics and thermodynamics for selective capture of CO₂ from gas mixtures.

Figure 9: (a) Crystal structure of SIFSIX-3-Zn; Column breakthrough experiment for CO₂/N₂:10/90 (b), for CO₂/CH₄:50/50 (c) carried out on SIFSIX-2-Cu-i and SIFSIX-3-Zn (298 K, 1 bar); Calculated (using IAST) CO₂ adsorption selectivity for two different CO₂/CH₄ mixtures (d) and for CO2/N2:10:90 (e) on SIFSIX-3-Zn compared to SIFSIX-2-Cu-I, Mg-dobdc and 13X zeolite at 298 K; Kinetics of adsorption of SIFSIX-3-Zn for pure gases and gas mixtures containing various compositions of CO₂ (f) [53].

    

    

    Figure 9: (a) Crystal structure of SIFSIX-3-Zn; Column breakthrough experiment for CO2/N2:10/90 (b), for CO2/CH4:50/50 (c) carried out on SIFSIX-2-Cu-i and
SIFSIX-3-Zn (298 K, 1 bar); Calculated (using IAST) CO2 adsorption selectivity for two different CO2/CH4 mixtures (d) and for CO2/N2:10:90 (e) on SIFSIX-3-Zn
compared to SIFSIX-2-Cu-I, Mg-dobdc and 13X zeolite at 298 K; Kinetics of adsorption of SIFSIX-3-Zn for pure gases and gas mixtures containing various
compositions of CO2 (f) [53].
    

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One major weakness in the 1^st generation of fluorinated-MOFs is the limited stability of the adsorbents in the presence of moisture. To circumvent such problem, the 2^nd generation of hydrolytically stable CO₂ selective fluorinated-MOFs, such as AlFFIVE-1-Ni and NbOFFIVE-1-Ni, 3-periodic pcu-MOFs based on the 2-periodic Ni- (pyrazine)₂ square grid, bridged by (AlOF₅)^2- or (NbOF₅)^2- pillars, were developed (Figure 10) [50,104]. The NbOFFIVE-1-Ni was found to exhibit high gravimetric and volumetric CO₂ uptake with very high selectivity, the uptake being very similar to the capacities obtained for the 1^st generation fluorinated-MOFs.

Figure 10: Crystal structure of NbOFFIVE-1-Ni showing the interaction with CO₂ (Top), CO₂ adsorption isotherm for NbOFFIVE-1-Ni up to 1 bar and 298 K (Bottom left), and breakthrough adsorption for the NbOFFIVE-1-Ni with CO₂/N₂ (1%/99%) mixed-gas at 1 bar and 298 K in both dry and humid conditions (Bottom right) [50].

    

    

    Figure 10: Crystal structure of NbOFFIVE-1-Ni showing the interaction with CO2 (Top), CO2 adsorption isotherm for NbOFFIVE-1-Ni up to 1 bar and 298 K (Bottom
left), and breakthrough adsorption for the NbOFFIVE-1-Ni with CO2/N2 (1%/99%) mixed-gas at 1 bar and 298 K in both dry and humid conditions (Bottom right) [50].
    

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Advanced concept to develop CO₂ adsorbents with high selectivity and capacity: The successful practice of CO₂ capture using MOFs lies in the structural tunability of the adsorbents that paves the way for the design of materials with improved CO₂- framework interactions. The possibility to introduce appropriate pore dimensions and suitable CO₂ binding sites in the frameworks have played very important role in the design and preparation of various CO₂ adsorbent MOFs with remarkable uptake capacities and selective adsorptions of the gas in the presence of other gases/vapors such as N₂, CH₄, H₂O, etc. For example, the fluorinated-MOFs proved the power of favorable CO₂-MOF thermodynamics and kinetics to develop CO₂ selective MOFs. Even though the 2^nd generation fluorinated MOFs showed high CO₂/N₂ separation performance with very high thermoand chemical stability, one common drawback associated with those MOFs is their low CO₂ uptakes necessitating the development of advanced concepts to further enhance the capacity via enhanced packing of the adsorbed molecules. Hence, the 3^rd generation of fluorinated-MOFs were then envisaged from 3,6-di(4-pyridyl)-1,2,4,5-tetrazine (dptz) as the ligand and either SiF₆ ^2- or TiF₆^2- as the pillar [20]. Among that family of MOFs, dptz-CuTiF₆, was found to exhibit the highest CO₂ uptakes at 10% CO₂ and 298 K, with the gravimetric and volumetric uptakes being superior to the first- and second-generation MOFs. The MOF also requires significantly lower energy input for regeneration making it a better replacement for the common aqueous amine scrubbing technique (38 kJmol^-1 vs. 105 kJmol^-1) [20]. The crystal structure of dptz-CuTiF₆ and the evolution of the fluorinated-MOFs as CO₂ adsorbents are displayed in Figure 11.

Figure 11: (a) The evolution of fluorinated-MOFs as CO₂ adsorbent for flue gas treatment, (b) Crystal structure description for dptz-CuTiF₆, (c) Dynamic column breakthrough tests for dptz-CuTiF₆ with CO₂-N₂ (10:90) at 298 or 328 K and 1 bar under both dry and humid conditions, and (d) structure depicting the packing arrangement of CO₂ molecules within the channels [20].

    

    

    Figure 11: (a) The evolution of fluorinated-MOFs as CO2 adsorbent for flue gas treatment, (b) Crystal structure description for dptz-CuTiF6, (c) Dynamic column
breakthrough tests for dptz-CuTiF6 with CO2-N2 (10:90) at 298 or 328 K and 1 bar under both dry and humid conditions, and (d) structure depicting the packing
arrangement of CO2 molecules within the channels [20].
    

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Despite the numerous opportunities the structural features of MOFs provide, there are certain challenges associated with the successful implementation of MOFs for CCS applications, the challenges in the incorporation of some CO₂ binding functional groups during synthesis, selective adsorption of CO₂ in the presence of water and acid gases, and the large-scale application of the adsorbents for the intended target. To fabricate MOFs that possess all the necessary attributes for the desired capacity and selectivity, the development of advanced concepts with respect to design, synthesis, characterization, and applications should remain to be the subject of further research.

Challenges Facing MOFs in the Process of Transition to High Technology Readiness Levels for CO₂ Capture

Compared to many other porous materials, MOFs have shown outstanding CO₂ capture potentials and selective adsorptions in the presence of other gases/vapors. There are, however, several challenges regarding the synthesis and CO₂ capture applications of MOFs that need to be overcome to place this class of adsorbents at high Technology Readiness Levels (TRLs). Among the different concerns that need to be addressed prior to commercialization, the problems associated with MOF stability and scale-up will be assessed here.

Formulation and stability

One of the MOF formulation challenge with respect to CO₂ capture applications is the difficulty to introduce multiple binding sites within the same structure. The presence of several CO₂-philic groups is particularly important to improve the selective adsorption of CO₂ at low partial pressures. The strategy of amine grafting has already shown some promising potential for enhancing CO₂ adsorption [51,105]. There are still lots of rooms to increase the density of grafted molecules for enhanced MOF-CO₂ interactions. Related to this, another challenge while attempting to introduce multiple binding sites is the dramatic decrease in surface areas and pore volumes and hence the resultant CO₂ adsorption capacities. A balance between functionalization and porosity must be maintained while designing and formulating MOFs for the targeted application. Adsorbent regeneration energies need also to be considered while formulating MOFs with multiple CO₂ binding sites. An increase in the regeneration energies is obviously expected upon incorporation of several CO₂-philic groups [106].

As already mentioned, many of the process conditions for CO₂ capture consist of other competing gases/vapors such as moisture/ water and acidic gasses such as SO₂ and H₂S. Different MOFs showed great promise as sorbents for CO₂ capture [19,107] but many MOFs that have been explored for CO₂ capture lack the requisite stability in the presence of these chemicals due to structure collapse and irreversible binding of the sulfur contaminants [36,108-116]. This is usually reflected in the reduction of adsorption uptakes upon cyclic measurements due to the continuous decrease in the materials’ surface areas and/or pore volumes. The design of MOFs to fully alleviate emissions of such corrosive and anthropogenic molecules then remains a fundamental challenge. As a result, it is extremely important to continue work in areas coupled to rational design and controlled crystallization. So far, pre- and post-synthetic MOF synthesis strategies have been considered and a significant improvement has been achieved regarding water and acid stabilities of MOFs [117-120, 125]. But the suits of acid and base stable MOFs for CO₂ capture applications are still underexplored.

The formulation of MOF adsorbents that preferentially binds CO₂ over the other competing molecules is another challenge. Owing to their higher polarity, H₂O or acidic gasses are preferentially adsorbed over CO₂ on the binding sites of MOFs. One option that is implemented to alleviate this challenge is to dehydrate and remove the sulfur contaminants prior to the CO₂ capture step. Although mature technologies are implemented for this purpose, it incurs, however, extra cost to the process and makes it more expensive. This is then one of the research opportunities to be considered when formulating a MOF for CO₂ capture in the presence of water and/or acid gases [37,104,114].

Scale-UP

MOFs, at small scale, have shown to be promising candidates for future development of alternative CO₂ capture technologies. However, they are yet to be transitioned for bulk-scale CO₂ capture applications at industry level. One of the main obstacles that slow down the transition of MOFs to TRLs is the problem associated with the scale-up in the synthesis of MOFs [121]. Pilot-scale production of MOFs and their CO₂ capture performance testing are currently under investigation, but the large-scale production of MOFs is still underexplored. MOFs are usually prepared in a lab-scale by employing solvo(hydro)thermal syntheses approaches. However, transformation of the lab-scale synthetic conditions to large-scale production through either direct mixing of reagents in solvents or solvent-free mechanochemical methods is not always practical [122]. Efforts must be made towards the development of MOF synthetic protocols that are amenable to scale-up. In fact, there are some ongoing MOF scale-up approaches with promising results for further development [123]. For example, a continuous solvent-free solidstate grinding synthesis with a kg per hour-scale MOF production has been achieved by twin-screw extrusion [124]. To position MOFs at high TRLs, the development of synthesis protocols for scaledup production in general and methods amenable for continuous production in particular should then be a subject of further research.

Lack of multidisciplinary research approaches

The other challenge that slows down the generation of CO₂ capturing MOFs at industrial scale and their transition towards TRLs is lack of multidisciplinary research approach. The scientific community with significant mass of expertise (chemists, crystallographers, computational experts, chemical/ process engineers, etc.) should create extensive collaborations. The basis for the development of tailored-MOFs that can satisfy the requirements of CO₂ capture processes for various technologies lies on the understanding of structure-function relationships. Besides designing and preparing the adsorbents at lab-scale, the mechanisms how the materials perform in an actual CO₂ capture process should be elucidated. Consequently, the different experts must work handin- hand to address the structure-property relationships. Large-scale screening of MOF for their adsorption properties is unlikely to be achieved through experimental studies. Therefore, the development of computational modeling tools that can successfully predict the MOFs’ CO₂ capture performance must be further developed. Besides the material design and testing, engineering economics models must be established to evaluate lifecycle, scale-up and capture costs of adsorbents for various processes.

Conclusion

Remarkable advances have been made toward the development MOFs for CO₂ capture. Some of the achievements that have been made in the design and preparation of MOFs for CO₂ capture include the design and synthesis of various tunable MOFs at lab-scale that possess high CO₂ adsorption capacities at low partial pressures (~1- 10 % CO₂ concentration) with suitable CO₂/N₂ selectivity, and in the presence of humidity. In order to achieve the above-mentioned features, the optimal pore structures (size and functional decoration) are key elements for selective adsorption of CO₂ with sufficient selectivity over N₂/CH₄ under humid conditions, which are the requirements for post-combustion CCS process, and could be also reworked for other applications such as natural gas/biogas upgrading or confined space applications. Even though the aforementioned achievements indicate an important step in moving forward, there are still several other issues that need to be addressed including long-term chemical and mechanical stabilities, selective CO₂ capture in the presence of contaminants such as H₂S and SO₂, assembly of MOFs that combine high CO₂ uptake capacity with high selectivity particularly in the presence of humidity and acid gases, large-scale production and deployment of MOFs and assessment of the stability and adsorption properties of MOFs under real industrial conditions. Most of the challenges associated with the applicability of MOFs for CO₂ capture stem from the little research carried out in the area and lack of multidisciplinary approaches. Given the promise of MOF structures for tunability of their chemical structures and their crystalline nature that allows researchers to experimentally probe structure-property relationships or mechanism of CO₂-framework interactions, performing application-oriented research in the area can guarantee to overcome the challenges. Most of the previous research reports in the area are based on evaluation of MOFs’ CO₂ overall uptake capacities under static adsorption tests (i.e., from single component isotherms) rather than on the actual industry relevant conditions (dynamic adsorptions using mixtures of gases/ vapors). Future research must then be centered on testing/assessment of MOFs’ CO₂ capture performance in dynamic settings at elevated technology readiness levels.

Acknowledgement

The authors acknowledge the financial support of Mohammed VI polytechnic University.

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Citation: Assen AH, Adil K and Belmabkhout Y. Application of Metal-Organic Frameworks (MOFs) for Capturing CO₂: Advancement and Challenges. Austin Chem Eng. 2021; 8(1): 1086.

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