Metal–organic frameworks for carbon dioxide capture


Detailed report on MOFs for CO2 adsorption on the basis of ligands employed, OMSs, and structures. Systematic report on the high- and low-pressure CO2 capture. Report on the mechanism of CO2 capture.

A review on the promising field of MOF-based carbon capture and storage is presented. We discuss here the main features of MOFs applicable for CO2 capture and separation, the linker functionalization role, and the most important CO2-binding sites as also the most efficient and significant technologies, and a systematic report on the high- and low-pressure CO2 capture.

Discussion Points

  • CO2 capture and sequestration continue to play a fundamental role and to be the main strategy to reduce emissions of CO2 in the atmosphere.

  • MOFs with very large surface areas can be successfully employed for CO2 capture, separation, concentration and storage, transport, and conversion.


At the dawn of the industrial revolution, the CO2 concentration was at 280 parts per million (ppm) in the atmosphere. By 1958, it had reached 315 ppm. It exceeded 350 ppm in 1986 and 400 ppm in 2013.1 At July 2019, the concentration reached 408.65 ppm. CO2 levels are now alarming and indicate the need to take immediate action to prevent serious repercussions on climate change.2,3

Although, during the Paris Climate Conference (COP21), 195 countries agreed to prevent a global temperature increase of less than2 °C, however, it is now fairly common knowledge that CO2 emissions are projected to increase by over 40% in 2040 compared with those of 2010, all due to the increase in energy demand.

Clean energy alternatives and new green technologies are constantly being investigated, and in some cases, they are used as important solutions, however, in order to allow a significant reduction in CO2 emissions; both a cultural change and changes in the energy distribution plants and in infrastructures capable of implementing new technologies will be necessary. CO2 capture and sequestration and its conversion continue to play a fundamental role and to be the main strategy to reduce emissions of CO2 in the atmosphere. It is possible to reduce the anthropogenic CO2 emission by the removal of CO2 from fossil fuel-burning power plants, through carbon capture and storage technologies, however always accompanied by further CO2 emissions due to regeneration of adsorbents and limited adsorption capacities.4 In addition, it is also possible to capture and transform CO2 in high value-added chemical compounds or fuels.57

Different methods were employed to capture and/or separate CO2 from the absorption to cryogenic and micro-algal biofixation,8 but to date the better method seems the adsorption due to the ease and cost of the process.9 Several materials and technologies have been also developed that involve the use of metal oxides,10,11 zeolites,12 ionic liquids,13 activated carbons,14 fluorinated solvents,15 membranes,16 molecular sieves,17 metal–organic frameworks (MOFs),18 and covalent organic frameworks,19 the former being one of the most promising.

MOFs2026 are crystalline porous compounds relevant in materials science due to their large variety of structures and properties, widely applied in catalysis, ion exchange, molecular recognition, and gas storage and separation. In the last 15 years, MOFs with very large surface areas were employed for CO2 capture, separation, concentration and storage, transport, and conversion.27 To well evaluate the CO2 adsorption, it is very important to take into account the adsorption capacity and enthalpy, the latter directly impacting on the requirements for material regeneration and desorption of CO2.

In this review, we report the most important and high-performing MOFs in the CO2 capture and separation with the aim to identify the structural and chemical properties requested for the best adsorption capacity and selectivity.

MOFs for CO2 capture: adsorption and capacity

Amine scrubbing is the currently CO2 capture method used in industry, which suffers from the drawbacks of amine corrosive nature and volatility, decomposition, and high energy cost of regeneration. MOF-based CO2 capture could be considered a promising alternative when the MOFs have high CO2 uptake capacity and selectivity, high stability, and production/regeneration reasonable cost. Tailoring MOF structures by the appropriate selection of metal cations (primary building units) or metal clusters (secondary building units) and functionalized organic linkers allows specific affinity toward CO2 molecules obtaining adsorption capacity and selectivity properties not usual for traditional CO2 solid porous adsorbents.2830

To identify an efficient adsorbent material for carbon capture, important benchmark points must be considered. CO2 sorbent needs to have high adsorption capacity, i.e., the amount of adsorbate taken up by the adsorbent per unit mass (gravimetric CO2 uptake mg/g, w/w) or volume (volumetric CO2 uptake - v/v) of the adsorbent.31 Single-component gas adsorption isotherms are usually used to estimate the pure CO2 capture capacity at specific temperature and pressure. In microporous CO2 adsorbents, as MOFs, Type I isotherm is typically observed which is characterized by Langmuir-type adsorption concave to the pressure axis, and the amount of the quantity adsorbed approaches a limiting value (Fig. 1). This limiting uptake depends on the accessible micropore volume and corresponds to the completion of a surface monolayer.

Figure 1.

Type I isotherm for nitrogen and argon adsorption at 77 and 87 K, respectively. At low p/p, a steep uptake is observed due to enhanced adsorbent–adsorptive interactions in narrow micropores. Type I (a) is observed for narrow micropores of width approximately <1 nm and Type I (b) is observed for wider micropores and possibly narrow mesopores (approximately <2.5 nm).31 Adapted from ref. [31 ].

MOFs are considered a competitive system when they demonstrate a CO2 adsorption capacity of 3.0 mmol/g or higher. A crucial parameter is the adsorption heat that indicates the strength of the interaction between the pore surface and CO2 and the energy required to regenerate the sorbent. It is usually expressed as isostatic adsorption heat, Qst, released during adsorption, and calculated from adsorption isotherms collected at different temperatures using virial equation [Eq. (1)].32 The mathematical fitting parameters are then used to calculate Qst [Eq. (2)].

$${\rm In}\,P = {\rm In}\,N + \displaystyle{1 \over T}\sum\limits_{i = 0}^m {a_iN^i} + \sum\limits_{i = 0}^n {b_iN^i} $$
$$Q_{{\rm st}} = {-}R \sum\limits_{i = 0}^m {a_iN^i} $$

where ai and bi are virial coefficients and m and n are the number of virial coefficients to adequately fitting the isotherm, P is the pressure (bar), N is the adsorbed quantity (mmol/g), and R is the gas constant (8.314 J/mol K).33 A high value of Qst means a stronger CO2 interaction and high adsorption capacity, desorption requiring, in this case, a large quantity of energy. Conversely, a weaker interaction results in low adsorption capacity, Qst decreasing, and favoring an easier regeneration. The optimal Qst for CO2 is between 35 and 50 kJ/mol in order to reduce regeneration costs.34 Additionally, fast adsorption/desorption kinetics for CO2 over other gases help to minimize the energy input for regeneration, reducing the cycle time as well as the amount of sorbent needed.

The performances in CO2 capture can be related to the real amount of CO2 adsorbed,35 i.e., the working capacity, estimated from the difference between the CO2 uptake at adsorption and desorption conditions of temperature and pressure of the process (Fig. 2). Large working capacities are highly recommended in an effective adsorbent, especially in pressure-swing adsorption and temperature-swing adsorption separation processes.36,37

Figure 2.

Schematic gas adsorption isotherms for a microporous adsorbent, in the pressure-swing and temperature-swing cycles, working capacity being given by nadsndes.36,37 Adapted from refs. [36,37 ].

As CO2 is the minor component in flue gas, the adsorptive selectivity of CO2 over other gases is a key parameter in screening CCS materials.

The adsorption selectivity of binary gas mixtures is defined by a normalized selectivity factor (S) as given in the following equation:

$$S = \displaystyle{{q_i/q_j} \over {\,p_i/p_j}}$$

where qi and qj represent the quantity of adsorbent components, pi and pj represent their relative partial pressure, and S being derived from single-component adsorption isotherm; therefore, it does not take into consideration the competition of gas molecules for the adsorption sites on the pore surface.38 Nevertheless, it is the best method to compare the CO2 selectivity performance of post-combustion CCS materials. Henry's law selectivity is an alternative method to determine adsorption selectivity based on the calculation of Henry's constants (KH) for the gases from single-component isotherms.39

The ratio between KH of different gas components (m and n) provides selectivity Sm,n. Equation (4) will reflect the real mixture selectivity only at very low pressure and low loadings on the adsorbent.

$$S_ = \displaystyle{{K_{{\rm H}m}} \over {K_{{\rm H}n}}}$$

IAST is the most widespread method to predict selectivity in multicomponent adsorption equilibria from single-component adsorption isotherms of CO2, in which competitive adsorption of gases is considered. IAST is based on three important assumptions: (i) gas components must be considered ideal, (ii) adsorbent surfaces must be homogeneous and have accessible equal area, and (iii) intermolecular interactions have the same strength in the adsorbed phase.40 In the simple case of two-component gas mixtures (i and j), pure component isotherms ni,j(P) for each gas at fixed T should be mathematically fitted by a single-site or dual-site Langmuir–Freundlich, Langmuir, or BET modes to extract the molar fraction of each species in the adsorbed phase by solving the following expression (5):

$$\int_0^{P\cdot y_i/x_i} {\displaystyle{{{\rm Isotherm}\,{\rm fit}\,{\rm for}\,{\rm component}\; i (P)} \over P} } dp = \int_0^{P\cdot y_j/x_j} {\displaystyle{{{\rm Isotherm}\,{\rm fit}\,{\rm for}\,{\rm component\ }\;j (P)} \over P} } dp$$

where xi and yi are the molar fractions in the adsorbent phase and P is the total pressure. Then, at given pressure, the adsorbent amount in the mixture can be determined from the following equation (6):

$$\displaystyle{1 \over n_{{\rm tot}}} = \displaystyle{{x_i} \over {n_i^0 }} + \displaystyle{{x_j} \over {n_j^0 }}$$

where ntot is the total number of adsorbed moles in the mixture, and nj is the amount of the i pure component adsorbent per gram of adsorbent. IAST is the common method employed to evaluate the adsorption of gaseous mixtures to post- and pre-combustion CO2 capture in the MOF.4043

IAST and Henry's law methods can be used to estimate the separation performance of an adsorbent simply from single-gas adsorption isotherms unlike breakthrough experiments where the equipment is needed to perform mixed-gas separation measurement. In the breakthrough experiments, absorbent bed is exposed to a multicomponent gas mixture, and the composition of outgoing gas streams is analyzed by gas chromatography or mass spectrometry. This the most accurate way to evaluate the performance of MOF in CO2 separation for a variety of binary gas mixtures such as CO2/N2, CO2/C2H4, CO2/C2H2, and H2/CO2.44,45

High CO2 selectivity over other components is essential in CO2 capture applications. Selective adsorption can be defined in different ways depending on a separation mechanism, i.e., kinetic or thermodynamic separation. Kinetic separation is based mainly on the size/shape exclusion (molecular sieving), where the size of pores in the framework allows molecular diffusion for specific kinetic diameters. In rigid MIL-9646 and Zn2(cnc)2(dpt),47 the free OH groups inside the pores attract CO2, whereas the small apertures hinder the CH4 adsorption. They showed interesting adsorption selectivity toward CO2 over CH4 based on size/shape exclusion useful in gas purification/separation industrial process frameworks.

In thermodynamic separation, adsorptive selectivity can be attributed to the adsorbate physical properties (physisorptive mechanism) such as polarizability and quadrupole moment. For example, the presence of highly charged groups (polar group) or exposed metal cation sites can enhance the selectivity toward CO2. A breakthrough experiment on Mg-MOF-74 indicated that the interaction between Mg and CO2 provides a complete separation of CO2 over CH4.48 Furthermore, framework interpenetration in the MOF leads to small pore volume and control of the pore size. A double-interpenetrated microporous MOF, Cu(FMA)(4,4′-Bpe)0.5, showed a highly CO2 selective sorption at 195 K.49

Alternatively, chemical interaction between CO2 and functionalized pore surfaces can result in larger selective than physisorptive adsorption. In [In((Me2NH2)NH2BDC)2]⋅DMF⋅H2O, amine functionalization has been effective to improve selectivity for CO2.50 The high selectivity is derived from the combined effect of size/shape exclusion and adsorbate–surface interactions.

High stability is a key parameter in terms of recyclability in CO2-adsorption materials. MOFs can be regenerated by common methods such as PSA,51,52 VSA,53,54 and TSA,55 in which sorbent is exposed to many pressure/temperature swings. Additionally, adsorbents should possess a good mechanical stability to be pulverized and densely packed for maximum volumetric capacity. Chemical stability toward acid gas contaminants (such as SOx, NOx, H2S, and HCl), H2O, and O2 is fundamental in terms of the structural integrity of CO2 sorbent. In general, H2O sensitivity is one of the major weakness of MOFs in an industrial CCS process. The first example of H2O effect, under humidity conditions, has been observed in zinc carboxylate-based MOF-5, the H2O directly interacting with Zn and the hydrolysis reaction taking place upon displacement of the BDC ligand.56 In some cases, the presence of H2O influences the CO2 capacity. In HKUST-1, CO2 uptake increases of 45% at 1 bar in the presence of small amount of H2O.57

Furthermore, metal–ligand bond strength is crucial toward thermodynamic, kinetic, and hydrolytic stabilities of MOFs. According to the Pearson's hard/soft acid/base principle, hard bases as carboxylate-based ligands can form stable MOFs with high-valent metal ions.58 Alternatively, soft azolate ligands can bind soft metal ions thus leading a stronger bond.59,60

Ligand functionalization role and CO2-binding sites

Bridging organic linkers play an essential role in the overall topology of an MOF, connecting the metal or metal cluster yielding crystalline porous structures. A careful choice of the linker based on geometry, length, functionality, and connectivity can help to obtain specific framework properties. Pore sizes, total pore volume, and surface area, in fact, are relatively easy to tune by the choice of the appropriate organic linkers, longer molecules allowing large pores, and a higher level of porosity that affect gas diffusion.61

Also, if the CO2 capture ability could be lower in large pore MOFs, their selectivity could be largely improved by the presence of functional groups on the linkers able to interact with CO2 molecules.62 The presence of polar groups could produce effects comparable to those found in MOFs having open metal sites.63 Moreover, ligand extensions with conjugated aromatic linkers or asymmetric linkers can contribute to tuning the backbone flexibilities of the framework able to respond to external stimuli.64,65 In addition to increase the CO2 uptake, it is necessary to consider groups of different polarity, hydrophilicity, and acidity chosen in order to have the right balance between the increasing of the MOF–CO2 interaction and the possible reduction of the pore volume and the detraction of the surface area.66,67

In this section, we have pointed out below an overview of most relevant ligands, useful as appropriate starting materials for design, synthesis, and production of CO2 capture adsorbents.


One of the most important strategies to improve CO2 capture and separation is the use of organic ligands and linkers containing basic functional groups. The most performing MOFs in CO2 capture employ alkyl amine-functionalized (RNHR*, or RNH2) pores that selectively react with CO268 forming covalent C−N bonds. It is possible both post-synthetic functionalization of MOFs by introducing in theman NH2 group and direct synthesis of MOFs containing NH2-based linkers (Fig. 3).69

Figure 3.

(a) Binding interactions between CO2 and amine functional groups. (b) Amine-functionalized ligand in MOF synthesis. (c) Post-functionalized-based ligand of MOF.69 Adapted from ref. [69 ].

The pores of IRMOF-74-III have been functionalized with CH2NH2, covalently attached to each linker, and the pores of IRMOF-74-III-CH2NH2 are able to absorb CO2 at low pressure.70 The aliphatic amine functionality present in the etb-type structure formed by MgO rods and terphenylene binds CO2, gavea new carbamate species (see section the “Mechanism of CO2 capture”).71 IRMOF-74-III is able to capture CO2 also in the presence of H2O. CO2 adsorption takes place at the alkyamine level, the H2O not accessing to the open Mg site, IRMOF-74-III-CH2NH2 preserving its structure under 65% relative humidity.

Micro–mesoporous amino-decorated BPZ-based MOFs have been investigated as CO2 capture material, CO2/N2 Henry and IAST selectivity being 17 and 14, respectively.72 [Zn(Bmic)(AT)], a MOF containing amino groups, uncoordinated carboxylates, and OMSs exhibits strong CO2 adsorption with high adsorption enthalpy [Fig. 4(a) ].73 The affinity of [Zn2(Atz)2(ox)] to take up CO2 molecules has been investigated, and the MOF has demonstrated to have high CO2 adsorption capacity and high enthalpy of adsorption.75 The investigation of the activated sites through computational studies suggests two active sites: (i) free amine groups via H-bonding with O in CO2; (ii) the N-lone pair with the C of CO2. en-Mg2(dobpdc) prepared via post-modification shows significant CO2 uptake at 0.15 bar comparable to the CO2 partial pressure of a post-combustion flue gas. The adsorption of CO2 onto the free amines of en leads to the formation of a carbamic –NHCOOH moiety, which requires equivalent numbers of CO2 and amine groups [Fig. 4(b) ].14,74,76

Figure 4.

(a) CO2 interactions with −NH2, uncoordinated oxygen atoms, and multiple OMSs in 3D Zn(Bmic)(AT) structure.73 (b) CO2 adsorption in diamine post-functionalized Mg2(dobpdc).74 Adapted from refs. [73 ] and [74 ].

Tethering covalently two primary alkylamines to the linkers of IRMOF-74-III gave species able to uptake CO2 also at low pressure via a chemisorption mechanism, the main product being carbamic acid. In the presence of H2O, the main product is the carbamate suggesting a determinant H2O role in the pores of MOF.77 NH2-MIL-101 has been impregnated with TEPA, which was grafted to coordinatively unsaturated chromium centers. The sterically hindered TEPA produces a diminution of the space inside the MOF and of its calculated surface area, but despite the decreased volume the CO2 adsorption capacity increases, most of the CO2 molecules are absorbed near the inner surfaces of cages.78

Theoretical calculations demonstrated that the amine group plays a fundamental role in increasing the adsorption capacity of an MOF.78 PEI is a polymer with highly concentrated amines. The CO2 absorption process on a PEI@UiO-66 composite (UiO-66 is a very good supporting material with significant alkaline resistance) is greater than that on UiO-66.79 PEI-impregnated MIL-101 shows a dramatically enhanced CO2 adsorption capacity at low pressure. The stability of the polymer toward SOx and NOx was also tested, the adsorption capacity being reduced due to the formation of nitrate and sulfate compounds on the surface.80


MOFs containing carboxylate substituents exhibit significant CO2 adsorption capacities under humid gas streams due to their polarity. A study describing the effect of OMSs and adsorbate polarity on the adsorption of CO2 on Cu-BTC and MIL-101 has been reported, CO2 showing a higher capacity on Cu-BTC with respect to CO and CH4. In addition, it has been reported that CO has a larger Henry constant on MIL-101 at low pressure and CO2 better capacity at high pressure.81 MIL-101(Cr) exhibits a selectivity of CO2 versus N2 of 12.6 at 303 K and demonstrates a great potential for CO2 adsorption with respect to different typology of silica, zeolite, and carbons.82

Isostructural MOFs built from bi-functional ligands carrying both azolyl and chelating carboxylate groups and employing acetate moieties as “modulators” enabled selective adsorption of CO2 and retained about 80% of the capture capacity upon exposure to 75% RH, so they can be considered efficient candidates for humid CO2 capture. The most favorable CO2 adsorption sites have been identified via simulated annealing methods, from which the presence of polarized CO2 molecules located adjacent to the π-electron-rich pore walls can be seen.83 MOFs based on trinuclear and tetranuclear metal–carboxylate–pyrazolate clusters prepared by reacting Co with 4,4′-biphenyldicarboxylic acid and methyl-functionalized Me4BPZ are very porous with both high CO2 loadings and CO2/N2 selectivity. GCMC simulation evidenced two favorable CO2 sorption sites located sequentially near Co3(pz)3 and Co3(CO2)2(pz) motifs of the tetranuclear cluster.84 [Ni(btzip)(H2btzip)]⋅2DMF⋅2H2O, chemically stable to water, acid heterogeneous catalyst featured by intersected 2D channels decorated by free COOH groups and uncoordinated triazolyl N atoms, exhibits not only high CO2 adsorption capacity but also significant selective capture for CO2 over CH4 and CO in the range of 273–333 K.85

The UiO-derived [Zn(H2BTC)(phen)2][H2BTC]⋅H2O exhibits storage capacity for H2, CH4, and CO2, with an unusual stepwise adsorption for liquid CO2 with a sequential filling mechanism on different adsorption sites.86 [Ni8(OH)4(H2O)(BDP-COOH)6] is highly porous and extraordinarily robust in boiling water and in strongly basic aqueous solution. It is characterized by COOH arms into its pore surfaces, which enhance affinity for CO2 molecules (Fig. 5) and improve the CO2/N2 and CO2/CH4 adsorption selectivity, due to the polarity of COOH strongly interacting with CO2 due to their large quadrupole moment.87

Figure 5.

(a) CO2-binding interactions and free carboxylate group. (b) The framework structure of [Ni8(OH)4(H2O)(BDP-COOH)6].87 Adapted from ref. [87 ].

Uncoordinated carboxyl groups have been inserted into the amine-containing MOF MIL-101(Cr)-NH2 through a post-synthetic strategy involving acyl chlorides. Most of the amine groups (>80%) were grafted to carboxyl groups, suggesting post-synthetic modification very effective. The adsorption selectivity of CO2/N2 was enhanced, despite the BET surface areas and total pore volumes were reduced.88

[Zn2(tdc)2dabco] shows a greater CO2 uptake and CO2/N2 selectivity compared with the nonthiophene analog [Zn2(BDC)2dabco]. Preferred binding sites in [Zn2(tdc)2dabco] have been unambiguously determined by in situ single-crystal diffraction studies on its CO2-loaded form. This study unveils the role of the thiophene moieties in the specific CO2 binding via an induced dipole interaction between CO2 and the sulfur center, confirming that an enhanced CO2 capacity in [Zn2(tdc)2dabco] is achieved without the presence of OMSs.89

[Ce2(NDC)3(DMF)2], solvothermally prepared, upon thermal activation revealed a phase transformation at about 300 °C involving the removal of the solvent molecules. [Ce2(NDC)3] is porous toward CO2 at −78 °C, with a surface area of about 200 m/g.89

The incorporation of H2O-repellent functional groups in MOFs increases sorption capability and H2O resistance. Trifluoromethoxy groups have been inserted in species analogous to IRMOF-1, substantial advantages being reported.90 A highly selective adsorption of CO2 has been reported for the green CD-MOF-2 formed by the renewable cyclic oligosaccharide γ-cyclodextrin and RbOH, the reversible carbon fixation occurring through carbonate formation and decomposition at room temperature.91

Nitrogen-donor ligands

MOFs based on N-donor linkers have witnessed an important role in the last decade as prospective materials for CO2 adsorption, and ligands such as Hpz, HIm, Htz, and Httz generally bind strongly to Lewis acid metal centers,92,93 yielding species highly stable to air, humidity, acidic and basic conditions, and potentially practical for the gas storage.9496 CO2 adsorption capacity, CO2Qst, and CO2/N2 selectivity in ZnBPZ-X derivatives have been determined by analyzing single- and mixed-ligand systems, the effect of different substituents on the azole being evidenced. For example, in the case of the BPZNO2 derivatives, the interaction site is between the O-atoms of CO2 and the N-atoms of the NO2 groups (Fig. 6).97100

Figure 6.

The most significant intermolecular interaction in M-BPZNO2 (a). Portion of the crystal structure of (b) Zn-BPZNO2 with tetrahedral MN4 nodes and (c) Cu-BPZNO2 with a square-planar MN4 node.98,99 Adapted from refs [98 ] and [99 ].

The azo functional group (–N=N–) shows chemical properties like Lewis basicity, hydrogen bond accepting and polarity helpful in gas adsorption and separation, and catalysis. The (–N=N–) group increases the Lewis base–acid interaction with CO2 and the response to the external stimuli. PCN-123, for example, adsorbs different amounts of CO2 after UV or heat treatment.101

The introduction of highly polarized heterocyclic functional groups seems a promising approach to improved CO2 adsorption performance. When benzo[c][1,2,5]oxadiazole, having a high dipole moment, is incorporated in ZJNU-41a, a considerable affinity and capacity toward CO2, as a consequence of stronger dipole–quadrupole interactions, was found.102

Heterocyclic azine-based MOFs are successfully applied in CO2 storage and selective captures. The interaction between MOFs containing pyridine, diazine or triazine, and CO2 has been investigated, the most important contribution coming from Lewis acid–base N⋯CO2 electrostatic interactions. The affinity for CO2 decreases when the electron deficiency on nitrogen atoms increases.103

Adenine is a suitable ligand for the presence of uncoordinated free nitrogen sites and for capacity to form robust frameworks and possibility to give interligand π–π interactions. The interaction between adenine and CO2 is stronger with respect to that found for other N-donor-based linkers.104 The Ad role is well recognized in bio-MOF-11, the strong adsorption capacity found being due to the narrow pore dimensions and pyrimidine and amino groups in Ad (Fig. 7).105

Figure 7.

(a) Interactions between CO2 and pyrimidine and amino groups of adenine linkers. (b) Crystal structure of bio-MOF-11 in which cobalt−adeninate−acetate “paddle-wheel” clusters are linked together through the apical coordination of the adeninate N7 atoms to Coions on neighbouring clusters.105 Adapted from ref. [105 ].


The OH ligand can be useful not only to stabilize the initial M-CO2 adducts actingas a strong hydrogen bonding donor, but OH can also facilitate the C–O bond breaking due to local proton sources.106 Dipolar or quadrupolar interactions between OH and CO2 are responsible of both increasing CO2 uptake and isosteric heat of CO2 adsorption.107 The importance of the role played by OH has been explored in a series of OH-functionalized isostructural MOFs showing higher uptakes and high adsorption heats.108

The iso-frameworks {[Cd(BIPA)(IPA)]⋅DMF}n, and {[Cd(BIPA)(HIPA)]⋅DMF}n, having both amino and phenolic hydroxyl groups, exhibit CO2 uptakes of more than 20 wt%, indicating that amino- and phenolic hydroxyl-functionalized groups are beneficial to CO2 adsorption.109

The adsorption sites in (OH)-decorated [M2(OH)2(BPTC)] (M = In, Ga, and Al) show that the captured CO2 interacts with the cis2-OH groups in an end-on mode through moderate to weak H-bond.110

In [Zn2(H2dht)(dht)0.5(azpy)0.5(H2O)]⋅4 H2O, two different types of channels have been found, one calyx-shaped along the [001] direction and another rectangle-shaped along the [101] direction occupied by guest water molecules. The pore surfaces of the dehydrated form are decorated with unsaturated Zn sites and pendant OH groups from H2dht, thereby resulting in a highly polar pore surface. When this MOF was dehydrated, a highly selective adsorption of CO2 over other gases, such as N2, H2, O2, and Ar, at 195 K has been observed.111 In the OH-modified Al-MIL-53-OHx (x means 25, 50, and 75 mol% 2,5-dihydroxyterephthalate incorporated in the linker), a low OH content is in accordance with a positive role in the low-pressure CO2 adsorption capacity of MIL-53, whereas high OH content means reduced capacity as a consequence of poor porosity.108


The introduction of amide groups into MOF can improve the CO2 uptake due to the possibility for CONH groups to form hydrogen bond with CO2, the amides acting not only as H-bond acceptors (via C=O) but also as hydrogen bond donors (via N–H). The amide groups, with low affinity to nonpolar/quadrupolar CH4 and N2, usually not coordinated to the metal centers, influence the MOFs structural features and the adsorption properties, as in the pores of a Cu-based MOF producing larger CO2 uptake, enhancing adsorption heats, and increasing selectivity with respect to analogous MOFs containing alkyne groups.112

Nanosized triangular amide-bridging hexacarboxylate linkers were employed to synthesize iso-reticular rht-type copper-based MOFs that through a marked gate-opening process have high CO2 uptake at room temperature 113 lower only to that reported for MOF-177114 and MOF-205.115 Very recently, the amide-functionalized MOF based on Sc (NJU-Bai49) showed the high value of selective CO2 uptake due to both decorated amide groups and OMSs.116 By using the flexible N,N′,N″-tris(carboxylmethyl)-1,3,5-benzenetricarboxamide, a H2O-stable pillar-layered MOF has been obtained showing a good performance in CO2 uptake in spite of relatively a low BET surface,117 whereas an amide-functionalized pyrimidyl Cu(II) carboxylate compound has been shown to have higher CO2-, lower CH4-, and negligible N2-uptake.118

MFM-188, based on a tetra-amide octacarboxylate linker, showed a high surface area (BET = 2568 mg) and a fully reversible adsorption with a CO2 uptake of 120 cm/g (23.7 wt%) at 298 K and 1 bar.119 {[Sr(BDPO)0.5(H2O)]⋅2H2O}, based on a oxalamide ligand, possesses polar tubular channels decorated with a high density of Lewis acid metal sites. By using GCMC simulations, the interactions between the framework and CO2 have been examined and identified the crucial role of both open metal sites and oxalamide groups in the high CO2 loading (Fig. 8).120

Figure 8.

(a) Interactions between CO2 and BDPO: C–H⋯O hydrogen bond with the phenyl ring, C⋯O and C⋯N interactions with the oxalamide group. (b) Portion of the crystal structure of {[Sr(BDPO)0.5(H2O)⋅2H2O}.120 Adapted from ref. [120 ].


Other functional groups based on heteroatoms can be used to enhance the CO2 adsorption, an increase in CO2 uptake being favored by strongly polarizing groups and extended ligand functionalization.

A NO2-functionalized imidazolate framework and a series of mixed-ligand MOFs containing different substituents on the linkers were investigated, and as expected, the NO2-containing species showed a higher CO2 uptake capacity at low pressures due to strong attraction between polar functional groups and CO2.121 Lower CO2 uptake has been observed in the case of MOFs containing CN and NO2, Cl and NO2, and Br and NO2 groups. A MOF, partially functionalized with the alkyl group, has been shown to adsorb the least amount of CO2.122

The NO2 group increases overall N2 and CO2 uptake approximately four times in a Zn(II)-paddlewheel DMOF-based MOF due to the increased flexibility.123 Also, in IRMOF series and in MTV-MOF-5-EHI, the incorporation of NO2 groups was a common technique for best-performing CO2 adsorption.124,125

The insertion of different functional groups in UiO-66(Zr)–X (X = –Br, –NH2, –NO2, –(CF3)2, –(OH)2, –SO3H, and –CO2H) suggested that, in accordance with the polarity trend of the functional group, SO3H and CO2H groups possess the highest CO2 selectivity, good working capacities, and medium-ranged CO2 adsorption enthalpy. Additionally, the geometry of the UiO-66(Zr)–SO3H and UiO-66(Zr)–CO2H allows direct interaction between the inserted groups and CO2.107,126 In the pillared-bilayer 2D Cu(II) MOF, [Cu(tdc)(bpe)]n⋅2n(H2O)⋅n(MeOH), the Sthiophene atom can interact strongly with electron-deficient C atom in CO2. The hydrophilic/polar pores of the framework interact strongly with the CO2.127

In MOFs with ligands containing halogen-based functional groups, polar C–X bonds and high density of negative charge on halide atoms improve the dipolar (C-X)⋯(CO2) quadrupole interaction.128,129 Organic- (F,130,131 CF3,132,133 and (CF2)n134 ) and inorganic- (SiF6 ,TiF6 BF, PF, AlF5(H2O), NbOF5, and CF3SO3)135140 fluoro-based ligands can give a significant advantage as tuning the pores with fluorine atom generally increases H2O stability and hydrophobicity in the corresponding MOFs.130,134 Strong PO–M bonds in phosphonate-based MOFs yielded a lower solubility and high stability against heat, air, and humidity. CuBDPR, based on phosphonate monoester linkers with methyl tether, is able to capture CO2 with a high Qst of 45 kJ/mol.141 Furthermore, phosphonate groups can be applied as high polar groups in MOFs for strong interaction with quadrupolar guests like CO2.19,142,143

In ether-based MOFs, the flexibility and polarity of –C–O–C– moiety leads to a selective CO2 capture.144 SNU-21H and SNU-21S, containing the flexible H4TCM, show selective and reversible CO2 adsorption over N2 gas at room temperature.145 The functionalization with alkyl ether groups of the rigid [Zn2(BDC)2(dabco)]n tuned the structural flexibility, as well as sorption selectivity toward CO2 over N2 and CH4. The CO2 sorption capacity of ether-functionalized MOFs is very similar, but the N2 sorption capacity is very different, the reason for the high selectivity being due to very polar methoxy-terminated pore surface.146

Mechanism of CO2 capture

Efficient methods for CO2 capture and separation based on MOFs are continuously reported but for their implementation on an industrial scale, and the corresponding mechanisms need to be known. Mechanisms of CO2 capture by MOFs have been investigated by spectroscopic, diffraction measurements and DFT studies. Very recently, also a combination of the ab initio calculations using the DFT/CC method and microcalorimetry has been used to study an adsorption mechanism in CO2 capture by widely described MOFs such as HKUST-1 that showed a rather unexpected dependence of the adsorption enthalpies on the coverage. In the low-coverage regime, CO2 molecules adsorb onto coordinatively unsaturated copper sites, whereas at higher coverage CO2 preferentially occupies sites in the windows of small cages. At even higher coverage, CO2 adsorbs in the center of small cages and in large cages. The lateral interactions between CO2 molecules at rather different sites are well explained by DFT/CC methods.147 A CO2 capture mechanism depends also on the presence of coordinatively unsaturated metal centers. Diffraction studies, for example, have been used to study the interaction between the open Lewis acid sites of Ni- and Mg-MOF-74 and CO2 which is end-on coordinated to the OMSs.148,149 The intermolecular angles between CO2 and Ni or Mg cations suggest a strongly electrostatic and physisorptive interaction.150 A copper-based MOF with a high density of active sites, showing both acid and base stability and high volumetric CO2 uptake at room temperature, demonstrated significant adsorption capacity due to a synergistic effect of multiple active sites.151

The role played by different cations on CO2 adsorption of rho-ZMOF has been theoretically calculated, the Qst increasing with a corresponding increase of the charge-to-diameter cation ratio.152 The strength of the chemical bond between CO2 and the unsaturated metal plays an important role: the relatively high ionic character of the Mg–O bond in Mg-MOF-74 increases the degree of polarization, enhancing CO2 adsorption.128 153 The CO2 adsorption on Mg-MOF-74 has been also studied by DFT calculations using the Crystal Program and VT IR spectroscopy: an angular Mg⋯OCO complex has been observed, and calculations demonstrated that dispersion forces account for about one-half of the adsorption enthalpy.154

The CO2 adsorption isotherms for Mg2(dobdc) were measured, and DFT calculations have been performed on the addition of CO2 molecules into the MOF pores one to one in order to mimic the CO2 adsorption process, the CO2 adsorption energies being reported for both OMSs and the secondary sites, i.e., the organic linkers. It has been found that when CO2 pressure increases, CO2 firstly occupies the OMSs and then interacts with the organic linkers. A continuous charge transfer from the adsorbed CO2 molecules to the frameworks occurs, resulting in an electrostatic environment change. Upon that six CO2 molecules (Fig. 9) are loaded in a single pore, each CO2 symmetrically interacts with the MOF at the same distances and angles.155

Figure 9.

Six CO2 molecules adsorbed in one unit of cell of Mg/DOBDC.155 Adapted from ref. [155 ].

Due to higher affinity of amine solutions for CO2, a number of MOFs based on Lewis basic sites such as NH2 and alkyl- and aryl-amines have been prepared, the amine being tethered to unsaturated OMSs or to organic linkers. For example, in diamine-tagged MOFs, two CO2 adsorption models were identified. The first model is defined “pair” and consists in one amine bonded to the metal site, the other amine absorbing CO2, forming a carbamic acid.156 The second model, defined “chain,” consists in CO2 binding to the metal-bound nitrogen end of the amine, forming in this case a carbamate, the neighboring amine interacting with the carbamate by removing a proton and forming an ammonium species (Fig. 10).

Figure 10.

Schematic representations of (a) carbamic acid pair formation and (b) ammonium carbamate chain formation upon CO2 adsorption.156158 Adapted from refs. [156158 ].

DFT and lattice models employed to study the effect of CO2 adsorption in amine-appended MOFs indicated a step in the adsorption isotherm in accordance with a phase change. It has been reported that an ammonium carbamate species is formed via the insertion of CO2 into the M–N amine bonds, the CO2-binding energies being strongly dependent on the M–N amine-bond strengths.157 Hydrogen bond in the zwitterionic ammonium carbamate species and hydrogen bond between carbamic acid and amine are possible.158 In some cases, a cooperative insertion process into the M–N bond has been hypothesized.157

In the alkylamine-functionalized mmen-Mg2(dobpdc), the CO2 adsorption mechanism has been investigated by quantum chemical calculation.159 The exceptional capacity of mmen-M2(dobpdc) species is likely due to a structural transition yielding an extended chain structure held together by ion pairing between carbamate and ammonium groups.160 It has been found that 2:2 amine:CO2 stoichiometry imparts a higher capacity with respect to 2:1 stoichiometry, likely due to a hydrogen-bonded complex involving two carbamic acid moieties derived from the CO2 adsorption onto secondary amines.159 The use of less-basic arylamines favors CO2 physisorption rather than chemisorption.161 Adenine is a good ligand, as both amino and pyridine nitrogens can interact with CO2,105 but it has been observed that uncoordinated N-donor sites in ligands such as pyridine, azole, and azolate have a small effect on the enhancing of the CO2 capacity likely due to weak coordination between these N-donors and CO2.162

Flexible frameworks allow reversible structural changes to occur as a response to external stimuli such as guest inclusion.163 [Cu(tzc)(dpp)0.5], also defined as elastic single-molecule trap, has been transformed in the activated form with a dramatic change in the conformation of the linking propylene chain of dpp, which shrinks the unit cell volume from 2489 to 2226 Å, and opening the kinetic pore diameter from 2.9 to 4.4 Å. Upon CO2 loading, also if the overall volume is almost identical, changes in the unit cell parameters and in the pore size and shape have been found. The strong interaction between CO2 and this MOF produces selectivity of CO2 over N2.164

MOFs with interpenetrated structures seem very promising in separation and adsorption of small molecules such as CO2; the interpenetration generally leads to a small pore volume and lowers the maximum CO2 uptake at high pressure,61 and also produces new CO2 adsorption sites with high binding affinity as in a Cu-based MOF, doubly interpenetrated, recently reported.165 A MOF containing a cavity designed for the capture of a single CO2 (single-molecule traps), built with four bridging ligands (3,3′-(naphthalene-2,7-diyl)dibenzoic acid) and two Cu clusters, shows a cage where a strong electrostatic interaction with the negative oxygen of CO2 is possible without the formation of chemical bonds.166

The CO2 adsorption-stimulated structural variation found for the flexible ZIF-7 is due to the CO2 migration through the nonuniform porous structure and not to a proactive opening of the ZIF-7 guest-hosting pores.167 Whereas the Qst of CO2 adsorption on MOF-5 decreased upon increasing in surface coverage by CO2 molecules, in accordance with a negligible intermolecular interaction between CO2 molecules and disappearance of favorable adsorption sites.168

In conclusion, a highly selective CO2 capture and a strong binding affinity can be obtained by conjunctive effects as the combination of size-exclusive effects with LBSs and OMSs.

Methods and technologies for CO2 capture

Post-combustion capture

Post-combustion capture (Fig. 11) refers to the separation of CO2 from flue gas derived from combusting fossil fuels (e.g., coal, natural gas, or oil) in air. Flue gas generated from combustion of coal in air is mainly composed of N2 (73–77%) and other components (H2O and O2: 3–7%; SOx, NOx, and CO: 10–1000 ppm).169,170 Steam is released at a total P of about 1 bar to remove impurities (NOx, SOx) and incondensable gases, and then flue gas interacts with CO2 scrubber at temperature between 40 and 60 °C. In post-combustion technology, an important step is the CO2/N2 separation at low P, with CO2 partial P of 0.15 bar, a weak driving force for its separation.60,171 High adsorption capacity and selectivity for CO2 over the other flue gas components at low pressures is highly desirable for post-combustion capture sorbent material. In addition, rapid diffusion of the gas through the adsorbent, minimal energy penalty for its regeneration and long-term stability under the operating conditions are other desirable features.172 Although in recent years significant progress has been made, synthesis of MOFs suitable for post-combustion capture is currently a challenge as their pore surface properties can increase the adsorption CO2 selectivity and capacity in post-combustion technology.55,74,169 To date, post-combustion capture is considered the most feasible technology option applicable in the short time to the existing fossil fuel-consuming power plants.173 The most promising MOFs in adsorption and selectivity under post-combustion conditions have been discussed below in the section “Low pressure CO2 capture.”

Figure 11.

Post-combustion, pre-combustion, oxy-combustion, and direct air capture systems.

Pre-combustion capture

Pre-combustion CO2 capture (Fig. 11) requires fuel decarbonation through gasification or the reforming process. At high T and P, coal is gasified to produce “syngas,” a mixture of H2, CO2, CO, and H2O.174 Steam was then added to syngas in a shift reactor, producing CO2 and H2 at high P and slightly higher T (5–40 bar and 40 °C).63 Due to the differences in kinetic diameters, polarizability, and quadrupole moment of gas molecules, CO2/H2 separation from high pressure-shifted syngas is easier with respect to other CO2/N2 and O2/N2 separations (see Table 1).175,176

Table 1.

Properties of gas molecules involved in CO2 capture process.

A high pressure allows implementations of PSA processes where gas species can be separated from a gas mixture at low T by passing through a reactor containing the sorbent. Then, the pressure is reduced and the gas is released or desorbed.177 The involvement of efficient solid adsorbents for the separation of CO2/H2 could make H2 purification more energetically favorite and workable on a large scale.178 In this contest, MOFs have been investigated as a fixed-bed adsorbent material in CO2/H2 separation at high pressure.48,179 Single-component high-pressure isotherms at room temperature can be used to evaluate MOF performances in PSA pre-combustion capture.114,180

Selected MOFs have been studied in CO2/H2 separation. Adsorption isotherms at 313 K, pressure up to 40 bar, and IAST have been employed to estimate mixed-gas adsorption behavior.180 The rigid MOF-177181 and Be-BTB,182 and the flexible CoBDP183 were selected for their high surface area. Cu-BTTri184 and Mg2(dobdc),185 possessing exposed metal cation sites, showed a promising CO2/H2 selectivity as well as high working capacity due to the presence of strongly adsorbing sites. Interestingly, MOFs with large aromatic surfaces without charges were found to display low CO2/H2 selectivity in the range of 5–10, making them unsuitable sorbent material for pre-combustion CO2 capture.

By harnessing the difference in the kinetic diameters and diffusion properties of CO2 and H2, MOF membranes were employed as molecular sieves in pre-combustion separation. Moreover, high pressure pre-combustion acts as a driving force for membrane separation. MOF-5,186 MIL-47, and MOF-53(Cr) have been tested as separation membranes, the H2 diffusivity resulting higher than that of CO2.178 A molecular sieve membrane with apertures on a single layer of Zn2(bim)4 of 2.9 Å, which works as channels for the small-size H2 (2.89 Å) and also as filters for CO2, increases the CO2/H2 selectivity.187

Oxy-fuel combustion capture

Oxy-fuel combustion (Fig. 11) is a technology to mitigate CO2 emission in which coal or other carbon-based fuel burned in a pure O2 environment. The fuel generated from oxy-combustion, after H2O and impurities removal, is almost completely CO2, making simpler the CO2 capture step. In general, oxy-fuel setup involves a separation unit based on cryogenic methods to remove nitrogen from the air-producing oxygen.188 O2 is then injected into the fuel in a boiler where combustion takes place. Steam is generated and used to power turbines and make electricity. Final flue gas consists mainly of CO2 (55–65%) and H2O (25–35%); water is easily condensed and removed and pure CO2 stem is obtained for transport and storage.189

Significant drawbacks to employ an oxy-fuel combustion method include high temperature operations, large energy costs for O2 separation from the air, and the use of expensive materials for the high combustion temperatures.190 In this O2/N2 separation scenario, microporous solid as MOFs could potentially reduce the energy cost, improving the selectivity in oxygen adsorption from the air. MOFs can facilitate effective separation of O2 from N2 harnessing the different chemical behavior of the two gas molecules. Differently from N2, O2 is able to accept electrons from electron-rich open coordination sites present in MOFs. These redox-active metal sites interact with O2 from the air through a reversible electron transfer.191 Recently, MOFs with a strong reduction ability have showed strong affinity and selectivity toward O2. In Cr3(BTC)2, electron transfer from Cr and O2 leads to the formation of Cr-superoxide adduct. This strong chemical interaction has resulted in O2 adsorption of 11 wt% at 298 K and 2 mbar and an O2/N2 selectivity factor of 19.3.192 1D hexagonal channel framework Fe2(dobdc) at 298 K and 1 bar showed O2 uptake capacity of 10 wt% and just 1.3 wt% for N2 under the same conditions.193 Recyclability is the weakness of these MOFs, as, for example, Fe2(dobdc) cannot be regenerated at temperature above 222 K for irreversible oxidation.

Direct air capture

Over the last few years, nontraditional environmental technologies have received much attention to reduce the global atmospheric CO2 concentration, where CO2 capturing does not occur from the large point sources, but directly from the air.194 Direct air capture (DAC; Fig. 11) is “negative carbon technology” in which sorbent liquid or solid is evaluated to extract CO2 from air under atmospheric condition and generate a concentrated stream of CO2 for sequestration or re-use.195 The theoretical minimum energy input to separate CO2 from a 400 ppm stream has been estimated to be ≈20 kJ/mol, while a more realistic minimum energy estimation is roughly 30 kJ/mol. One of the major advantages of DAC is its ability to suppress CO2 emission from both point sources and distributed sources. In addition, DAC has not specific location, the process can be set up anywhere, and it is not exposed to the high concentrations of contaminants such as SOx, NOx, and mercury present in a flue gas capture process.196

Physiosorbent materials such as Zeolite 13X under DAC conditions exhibit 0.0035 mmol/g of CO2 uptake capacity. This negligible value is due to the absence of strong interaction. MOFs seem to be highly promising. Mg-MOF-74 and SIFSIX-3-Zn at 400 ppm and 298 K show limited uptake lower than 0.2 mmol/g under DAC conditions.74 Subsequently, upon tuning and contracting the pore size of SIFSIX-3-M series, it has been found that SIFSIX-3-Cu displays CO2 uptake of about 1.24 mmol/g higher than that reported for Zn analog. en-Mg2(dobpdc) and mmen-Mg2(dobpdc) have demonstrated excellent CO2 uptake capacities of 2.83 and 2 mmol/g, respectively, at 400 ppm, 298 K, and 1 bar due to the presence of amine acting as strong chemisorption sites.197,198

High-pressure CO2 capture

The most relevant high-pressure adsorption data for selected MOFs are reported in Table 2. Among carbon-capture technologies, pre-combustion capture technology has several advantages. Pre-combustion capture involves the easier separation of CO2 from H2 due to much large disparities in their chemical properties (Table 1). In addition, the separation is carried out at high CO2 concentration under high pressures (5–40 bar); therefore, less energy is required to regenerate the adsorbent material applying a PSA-type regeneration process. In these conditions, solid adsorbents provide higher selectivity for CO2 over H2 using a physiosorptive process. CO2 uptake capacity at high pressure is closely related to the storage ability of adsorbent and is dependent on surface areas and MOF pore volumes that can be implemented with organic linkers characterized by extended length.217

Table 2.

High-pressure CO2 adsorption capacities.

Pre-combustion CO2 capture has been investigated at 50 bar and 313 K180 with rigid MOF-177 and Be-BTB that showed extraordinary adsorption capacity for CO2 and featured, however, lower performances in CO2 selectivity related to the dimension of the saddle-shaped rings.205 Mg2(dobdc) and CuBTTri show the best performance in terms of selectivity. Mg2(dobdc), possessing surfaces coated with exposed metal cations, shows a greater CO2 selectivity (350) at 40 bar with adsorption capacity around 39.7 wt%. In the flexible Co(BDP),both adsorption and selectivity are drastically low.

Highly porous MOFs based on the Zn4O(CO2)6 unit have been investigated for their gas uptake capacities. Particularly, MOF-210 exhibits the highest BET and the Langmuir surface area (6240 and 10,400 m/g) and pore volume (3.60 and 0.89 cc g) with respect to all other MOFs so far reported. In addition, MOF-210 showed the highest CO2 uptake (70.5 wt%) at 298 K and 50 bar.115 An isoreticular PCN-6X series, based on dendritic hexacarboxylate and Cu paddle-wheel cluster ligands, has been studied for applications in gas storage and separation.218 The incorporation of mesocavities with microwindows in the (3,24)-connected network of this series increases the stability of MOFs with higher surface areas. PCN-68 has the highest gravimetric CO2 storage capacity (57.2 wt%) at 35 bar, and its BET surface area is increased from 3000 m/g (PCN-61) to 5109 m/g by extension with a ligand size of 11.2 Å.206

UMCM-1, containing both BDC and BTB, shows mesoporous 1D hexagonal (2.7 nm × 3.2 nm) and microporous channels (1.4 nm × 1.7 nm). The large pores and high BET surface area (4160 m/g) lead to excellent adsorption properties, particularly at high CO2 pressures (24 mmol/g).207

In the 1D-honeycomb frameworks, such as M-MOF-74, CPO-27-M, and M2(dhtp) where M = Mg, Mn, Fe, Co, Ni, Cu, or Zn, the large micropores of about 11–12 Å diameter and the high number of OMSs favor the CO2 adsorption. In Table 2, we reported the maximum adsorption capacity at high pressure for Zn-MOF-74, Ni-MOF-74, and Mg-MOF-74.114,179 The CO2 uptake is higher for the Mg species with respect to the other isostructural M-MOF-74 compounds, and the high affinity found for CO2 is due to the lighter weight of the metal and higher adsorption heat.

HKUST-1, with exposed Cu sites acting as Lewis acid sites, interacts strongly with quadrupolar CO2. The amount of adsorbed CO2 increases with pressure, and the adsorption capacity ranges from 35.9 wt% at 15 bar (298 K) to 42.8 wt% at 30 bar (313 K).212,213,219 It is possible to improve the CO2 capacity in HKUST-1 by alkali metal ions doping. Li ions, through post-synthetic modification, have been introduced in Cu sites available for interaction with lithium naphthalenide after H2O removal, Li@HKUST-1 being obtained. The enhancement of gas adsorption has been obtained by maintaining the Li content at low concentration (0.07 Li/Cu mol/mol).215 The CO2 uptake increases from 295 mg/g for unmodified HKUST-1 to 469 mg/g to Li@ HKUST-1 at 298 K and 18 bar due to the strong affinity of Li toward gas molecules (Fig. 12).220

Figure 12.

Doping HKUST-1 by Li ions: the strong binding energy originates from the non-bonded electrostatic Li-ion and quadrupolar CO2 interaction.220 Adapted from ref. [220 ].

In MIL-100 and MIL-101 containing (Cr3(μ-O)) units and BTC and BDC, respectively, the pores space derives from two cages with diameters of 2.9 and 3.4 nm, connected by windows with diameters of 1.2 and 1.45 nm, respectively.221 These mesoporous MOFs adsorb a large amount of CO2 and CH4 at relatively high pressures (50 bar) at 300 K. The CO2 uptake is approximately 44 wt% for MIL-100 and 63.7 wt% for MIL-101 due to strong interaction with Lewis acid chromium sites (O = C = O⋯Cr). The partial coordination of terephthalate to the metal sites decreases the MIL-101 heat of adsorption at −44 kJ/mol with respect to MIL-100 (Qst −63 kJ/mol).222,223 A mesoporous MOF obtained from the reaction of H3TATB with Tb(NO3)3⋅5H2O shows a zeotype network composed of hierarchical 3.9 and 4.7 nm cages, similar to those of MIL-100 and MIL-101. After activation, Tb16(TATB)16(DMA)24 showed a BET surface area of 1783 m/g with a CO2 adsorption of 18 mmol/g at 45 bar and ambient temperature.211

Cu paddle-wheel SBUs connected by octatopic linkers TDCPTM with a scu topology (NOTT-140) have a 4,8-connected framework constituted by two types of polyhedral cage with diameters of 5.5 and 8.9 Å. The BET surface area is 2620 m/g, and, at 30 bar, the CO2 uptake reaches 20.72 and 19.53 mmol/g at 283 and 293 K, respectively.208 DUT-9, formed by five octahedrally coordinated nickel atoms bridged by two μ-O atoms and six BTB linkers with large pores up to 25 Å wide and high concentration of nickel sites, also demonstrated a high potential for CO2 storage (capacity 61.5 wt%), gas separation, and catalytic application.203

MOF-5, characterized by a large open pore within a simple tetrahedral [Zn4O] cluster bridged by ditopic BDC ligands and a cubic structure, has a high surface area allowing easy diffusion of molecules into the pores.224 The partial substitution in MOF-5 of Zn by Co has been performed during crystallization processes.200 Co-MOF-5 has a higher adsorption capacity for CO2 at high pressure than its Co-free homologs, suggesting high influence of the Co-doping. The gas uptake slightly raises due to the fact that Co is incorporated into unexposed metal sites less accessible to gas molecules, but it is more pronounced for Co-rich MOF, especially at high pressure. The isoreticular Zn4O(FMA)3 characterized by a cubic rigid framework composed of octahedral Zn4O units bridged by FMA dianions (α-Po structure) took up a large amount of CO2 (69 wt%) at 300K and 28 bar.199

A computational model has been developed to prepare NU-100 based on hexacarboxylate ligand and unsaturated Cu sites with an activated BET surface area of approximately 6,143 m/g and a large total pore volume of 2.82 cm/g that enable a high CO2 storage capacity (69.8%) at 40 bar.225

The highest CO2 capacity adsorption at high pressure have been found in series of amide-functional MOFs (Table 2). The incorporation of this functional group in organic linkers enhances the adsorption affinity and selectivity for CO2 due to specific binding and H-bond formation between CO2 and amide groups.226

Cu-TPBTM, built from the flexible C3-symmetric hexacarboxylate ligand with the acylamide group and Cu2(carboxylate)4 paddle-wheel clusters,112 exhibits the same topology as the prototypical rht-type MOF and other isoreticular MOFs such as the PCN-6X series.206,218 Cu-TPBTM and the isostructural analog PCN-61 possess the same pore size, surface area, and number of Cu(II) sites, the unique difference being the substitution of the acetylene moiety in PCN-61 with an amide moiety. Under the same conditions, 20 bar and 298 K, the CO2 adsorption capacity of Cu-TPBTM was 23.53 mmol/g with a corresponding Qst of −26.3 kJ/mol at zero coverage,112 a value higher with respect to those reported for the PCN-6X series206 and due to the large dipole moment of the CONH groups, which facilitate dipole–quadrupole interactions between the acylamide groups and CO2, and NH⋯OCO hydrogen bond formation.112 Moreover, Cu-TPBTM and PCN-61 exhibit different CO2/N2 selectivity due to the positive effect of the amide group, which improve the steepness of the CO2 isotherms.

In NJU-Bai3, an agw-type porous framework high storage and adsorption selectivity CO2/N2 (60.8) and CO2/CH4 (46.6) are combined.209 This MOF characterized by three types of cages, in which decorated amide units are directly exposed to each individual cavity, exhibits a BET surface area of 2690 m/g. The high-pressure CO2 adsorption showed an unsaturation excess of CO2. Moreover, the densely decorated amide groups in the pore surfaces enhanced the strong binding affinity to CO2. The adsorption mechanism investigated at the molecular level by GCMC simulations suggests the amide groups as the main adsorption sites for CO2 molecules,112 especially at the carbonyl side of the amide group, but also indicated that open Cu site clearly favors the CO2 adsorption.225

The water stable pillar-layered porous Cu2(TCMBT) shows a great stability after treatment at room temperature and boiling water. In spite of low BET surface area (808.5 m/g), Cu2(TCMBT) exhibited a large amount of CO2 uptake (5.8 mmol/g) than CH4 (2.5 mmol/g) and N2 (1.0 mmol/g) at 20 bar. The adsorption enthalpy for CO2 was calculated to be 26.7 kJ, and it was mainly attributed to the large dipole moment of the bridging acylamide groups along the small channels that enhance the dipole−quadrupole interactions with CO2, which leads to the selectivity of CO2 over CH4 and N2.216 Two rht-typed MOFs containing nanosized acylamide-bridging hexacarboxylate ligands have been prepared to expand via linker elongation, isoreticular analogous of Cu-BTB and Cu-TATB. These two mesoporous MOFs exhibit a high BET surface area of approximately 3288 Cu-BTB and 3360 Cu-TATB m/g and high CO2 adsorption of 60.9 wt% at 20 bar and 273 K as well as good selectivity of CO2/CH4 (8.6) and CO2/N2 (34.3). The high-pressure isotherm of CO2 adsorption showed a type IV-like profile with stepwise behavior and hysteresis, a typical feature of flexible MOFs with hierarchically mesopores and a gate-opening process. GCMC simulations and first-principle calculations support advantages of acylamide groups for CO2 capture.113

A microporous NbO-type MOF, HNUST-1 built with nanosized rectangular acylamide-bridging tetracarboxylate linkers and paddle-wheel [Cu2(COO)4] SBUs, shows a moderate BET surface area of 1400 m/g, a large CO2 storage capacity of 34.7 wt% at 20 bar and 273 K, as well as good selectivity of CO2/CH4 (7.2) and CO2/N2 (39.8). Also, in this case, the adsorption enthalpy of CO2 was 31.2 kJ/mol as a result of strong dipole–quadrupole interactions between the acylamide groups in HNUST-1 and CO2.214 The oxalamide-functionalized NbO-type MOF HNUST-3, with a high BET surface area of 2412 m/g, which is among the highest surface area of NbO-type MOF series reported to date, shows a CO2 uptake at 20 bar of about 22.47 and 20.23 mmol/g at 273 and 298 K.208 NOTT-125 with a fof topology and the same oxalamide incorporated within the pore walls provide a variety of strong binding sites for CO2.210

The eea-type MFM-136, a rare (3,6)-connected pyrimidyl isophthalate acylamide-decorated MOF, in which all Cu sites have been fully coordinated to carboxylate and pyrimidyl groups, exhibits a CO2 uptake of 12.6 mmol/g at 20 bar and 298 K. Neutron powder diffraction and inelastic neutron spectroscopy have been used to investigate the precise role of free amides as a binding site for CO2, without the competitive binding of the open Cu sites. Surprisingly, the two methods revealed that the strongest binding site for both adsorbent CO2 and CD4 molecules are the phenyl-isophthalate rings. The inelastic neutron spectroscopy indicates retention of vibrational motion of the amide group upon CO2 binding that confirmed an absence of interactions between CO2 molecules and the pendant amide groups in the pore.118

Low-pressure CO2 capture

High adsorption capacity at lower pressure is a highly desirable condition for post-combustion CO2 capture. Selective CO2 capture at low pressure and low concentration (15–16%) requires higher strong chemical affinity toward CO2, and the adsorption is dominated by adsorbent–adsorbate interactions.

The most common strategy to improve CO2 capture is the functionalization of the pore surface of MOF with unsaturated metal centers or grafting polar functional groups, in order to achieve stronger binding interactions between MOF and CO2. However, narrow pores result in higher overlap of the adsorption potentials of opposing pore walls and increases the adsorption strength, leading to more adsorption at lower pressures. For these reason, it would be to keep in an equilibrium surface area, pore size, and adsorption enthalpy to maximize the adsorption capacity. To compare different MOFs performances, we reported the CO2 adsorption capacities at low pressure (1 bar) in Table 3.

Table 3.

Low-pressure CO2 adsorption capacities.

The low-cost UiO-66, easy to synthesize highly stable toward humidity and active catalyst,273 has a fcu-topology framework consisting of a very stable Zr6O4(OH)4(CO2)12 cluster extending in 12 directions to form a cubic closed packed (ccp) structure, which generates tetrahedral and octahedral cages of sizes 8 and 11 Å, respectively. These cages are connected by a triangular window of size 6 Å (Fig. 13).274 At 298 K and low pressures, the introduction of polar functionalities produces a significant increase in adsorption over the nonpolar groups. Specifically, UiO-66−NH2 shows the highest CO2 adsorption loadings (3 mmol/g), followed by nearly identical low-pressure loadings for the NO2 (2.57 mmol/g) and OMe (2.63 mmol/g) variants. The lower CO2 adsorption in UiO-66−NO2 and UiO-66−2,5-(OMe)2 have been attributed to reduced pore volume and surface area induced by the bulkier functional groups. UiO-66 with no specific site and UiO-66−1,4-Naphtyl with nonpolar organic linkers exhibited lower loadings at low pressures (1.8 and 1.53 mmol/g, respectively).272

Figure 13.

(a) UiO-66 (University of Oslo) is obtained by the reaction of [Zr6O4(OH)4] clusters with H2BDC. (b) Octahedral and tetrahedral cages in UiO-66.274 Adapted from ref. [274 ].

The porous UiO-66-ADn (n = 4, 6, 8 and 10) has been synthesized by UiO-66 post-synthetic modification through the substitution of terephthalate linkers with diverse flexible alkanedioic acid. UiO-66-AD6, containing adipic acid pendants, showed the highest CO2 adsorption capacities and CO2 desorption-free energy with respect to UiO-66 and UiO-66-ADn series. The dangling adipic acid groups have an appropriate length to allow a strong and multiple interaction between CO2 and the modified framework that increases the enthalpy loss and mitigate the entropy loss on CO2 adsorption.265

Among all UiO-66-type MOFs, UiO-66-(CH3)2 exhibits the highest CO2 uptake 5.8 mmol/g at 1 bar and 273 K, which represents an enhancement of ~33% compared with the unmodified UiO-66. The enhancement in CO2 adsorption could be ascribed to the open metal Zr sites after thermal activation and the small pore size in the presence of two methyl groups on the linkers. It is likely that the stronger intermolecular interactions of CO2 in smaller pores contribute to the high Qst of UiO-66-(CH3)2.247

H2O stability improves CO2 adsorption. When, in UiO-66, the linkers have been replaced through a metalated-ligand exchange process by incubating UiO-66 for 5 d at 85°C with aqueous solution of BTEC or mellitic acid and alkaline hydroxide (LiOH, NaOH, and KOH), the formed UiO-66-(COOLi)4-EX showed the highest BET surface area with a CO2 adsorption capacity at 273 K of about 3.44 mmol/g, the combined effect of the higher Li polarization ability and of the small pores size increasing the van der Waals and electrostatic interactions toward CO2.269

Another success case of post-synthetic modification is UiO-66(Ti56): in fact, CO2 uptake of UiO-66 has been enhanced by 81% via PSE of Zr with Ti ions. The UiO-66(Zr100) dispersed in a DMF solution of TiCl4(THF)2 gives a heterometallic MOF with 56 wt% Ti atom loading. This higher Ti content in UiO-66 was obtained upon longer exposing time during PSE. The replacement of Zr(IV) (heavy metal) with Ti(IV) (lighter metal) can increase the surface area of MOFs and leads to an increase in CO2 enthalpy and uptake.275 Moreover, the presence of shorter Ti–O bond decreases both octahedral pore size and MOF density, bringing the CO2 uptake capacity from 2.2 of UiO-66(Zr100) to 4 mmol/g of UiO-66(Ti56).262

A ukv-type Cd MOF based on 4,4′-(hexafluoroisopropylidene)diphthalate shows a strong quadrupole–quadrupole interaction with CO2 and reduced pore size due to the presence of highly polar CF3 groups. In addition, Cd–carboxylate chains and Cd sites exposed in the porous surface can generate an electric field interacting with quadrupole CO2 molecules.270

A series of microporous anionic C3N4-type (choline)3[In3(BTC)4]⋅2DMF has been synthesized using different organic and deep eutectic solvents and ionic liquids. Solvents were incorporated in the MOF as extra-framework charge-balancing cations occupying a large portion of the pore space. In (choline)3[In3(BTC)4]⋅2DMF, also if extra-framework choline cations decrease pore size, the OH on the choline increases the CO2 uptake capacity at 1 atm and 273 K (3.2 mmol/g). The pore space partition is an efficient method to tune the pore space and achieve gas molecular adsorption.271 CPM-5, which is characterized by a cage-within-cage structure, where large Archimedean In24 cage (truncated octahedral cage or sodalite cage) encapsulates a small Archimedean In12 cage (tetrahedral cage), presents a pore size partition and open Insites leading to CO2 adsorption of 3.2 mmol/g at 273 K and 1 atm.267

A Ni-trimer-based MOFs, CPM-X (X = a–d), has been obtained upon the insertion of a second ligand into MIL-88 (Fig. 14). The Tpt linker, with a C3 symmetry, has been selected in this symmetry-matching regulated ligand insertion strategy. CMP-33a and CPM-33b have the same backbone structure with the open metal site coordinated by a Tpt ligand, but CPM-33b has two OH groups on the benzene ring. CPM-33b shows moderate isosteric heat and higher CO2 uptake capacity under 1 bar with respect to CPM-33a.234

Figure 14.

Pore space partition through symmetry-matching regulated ligand insertion.234 Adapted from ref. [234 ].

A post-synthetic variable-spacer installation strategy has been used in the robust flexible LIFM-33 to control and tuning the pore surface. The insertion of spacer substituents enhances the CO2 adsorption capacity (3.6 mmol/g) with a high Qst (39.7 kJ/mol) due to the strong NH2–CO2 interactions.268 The synergistic effect of dynamic spacer installation and post-synthetic covalent modification strategies lead to the synthesis of LIMF-92 with a medium amount of azide groups that exhibits the CO2 uptake with the value of 4.3 mmol/g.259

Cation exchange in ionic bio-MOF-1 has been used to systematically modify pore size in a straightforward fashion.104 DMA cations inside the pore of anionic bio-MOF-1 have been replaced by three similar organic cations. TMA@bio-MOF-1, TEA@bio-MOF-1, and TBA-bio-MOF-1 have been obtained by soaking bio-MOF-1 in a DMF solution of TMA, TEA, and TBA, respectively. From the data in Table 3, the higher CO2 adsorption values are for TMA@bio-MOF-1 and TEA@bio-MOF-1.257 The guanidinium derivatives gave a significant increase in CO2 adsorption due to the greater affinity between the Lewis basic cation and CO2.253

When, in [(CH3)NH2]3[(Cu4Cl)3(BTC)8]⋅9DMA, the (CH3)2NH2 cations were replaced by tetraalkylammonium cations such as TMA, TEA, and TPA, the CO2 adsorption isotherms followed a decreasing trend. 276 The small pores limited its CO2 adsorption capacity. These results highlighted the importance of a suitable pore volume for the gas storage.277,278

The impregnation of various metal ions such as Li, Mg, Ca, Co, and Ni in the pores of the anionic SNU-100 significantly enhances uptake capacity, selectivity, and isosteric heat of the CO2 adsorption of the MOF. Among various metal ions, the Co and Ni species showed the higher CO2 uptake, whereas the Ca derivative displays the large increase of the isosteric heat.264

MOFs based on rigid spacers such as BPZ are typically characterized by high thermal and chemical stabilities.277279 In Zn(BPZ), the tetrahedral Zn ions are coordinated by four N atoms of four BPZ, creating a 3D porous network containing 1D channels of square shape. The interactions between the aromatic system and the molecular quadrupole of CO2 allowed a CO2 uptake of 4.4 mmol/g at 273 K and 1 bar.280 At the same adsorption conditions, the MOF decorated by nitro-groups Zn(BPZNO2) showed an enhancement of CO2 uptake of 4.7 mmol/g.99 The affinity for CO2 increased significantly in Zn(BPZNH2), which is the best-performing adsorbent of the group, considering the amount adsorbed per SSA unit.281

In a follow-up study, a series of Zn(II) mixed-ligand MOFs (MIXMOFs) has been synthetized by varying the quantity and type of ligand functionalization. In the isoreticular MIXMOF, the adsorption properties exhibit a trend depending on ligand tag nature and “dilution.” Specifically, the amino-decorated compounds show higher Qst values and CO2/N2 selectivity similar to the nitro-functionalized analogs; in addition, tag “dilution” increased CO2 adsorption selectivity over N2. The simultaneous presence of NH2 and NO2 groups has been found not beneficial for CO2 uptake. Among the compounds studied, Zn(BPZ)x(BPZNH2)1−x had been the best compromise among uptake capacity and CO2/N2 selectivity.254

Higher CO2 adsorption values were reported for the MOFs Fe2(BPEB)3, Ni(BPEB), and α-Zn(BPEB) containing the long and rod-like bis-pyrazolate H2BPEB spacer.229 Uncoordinated N atoms in the pore wall can enhance Lewis base–acid and dipole–quadrupole interactions between CO2 molecules and basic N sites. The tetrazolate group in CPF-6 showed two types of binding modes, a bidentate and a monodentate fashion. The high concentration (67%) of uncoordinated N atoms on aromatic ring allowed CO2 uptake of 4.4 mmol/g at 1 atm and 273 K.162 The high CO2 adsorption of MAF-66 is ascribed to the amino-functional group and to N1 atom uncoordinated on pore surface of triazolate ligand bonded to tetrahedral Zn ion through N2 and N4 atoms.282

The accessibility of uncoordinated nitrogens plays a crucial role on the CO2 adsorption in isostructural copper MOFs, ZJNU-43, ZJNU-44, and ZJNU-45. IAST and simulated breakthrough calculations showed that ZJNU-44a performed better than the other MOFs for CO2 adsorption and selective CO2/CH4 and CO2/N2 separations.252 Noteworthy, also NbO-type ZJNU-40 containing the highly polarized benzothiadiazole unit as a spacer between two isophthalate moieties, OMSs and accessible polar donor sites, exhibited high CO2 uptake capacity as well as adsorption selectivity of CO2 over CH4 (6.6) and N2 (22.9).235

In most of cases, the introduction of N-containing functional groups enhances selective CO2 adsorption ability at low pressures. Cu-BTTri has been also modified by introducing a secondary amine through post-synthetic functionalization, i.e., mmen, grafted onto the exposed Cu sites. At the same adsorption condition, unmodified Cu-BTTri and mmen-CuBTTri adsorb 3.2 and 4.2 mmol/g of CO2, respectively. The mmen-Cu-BTTri drastically enhanced the CO2 adsorption capacity reaching about 9.5 wt% CO2 at 298 K and 0.15 bar and the IAST selectivity of CO2/N2 being 372/298 K. Moreover, Cu-BTTri showed large Qst of −96 kJ/mol at zero coverage with an unexpected easily regeneration using mild temperature.260

dmen-Mg2(dobpdc), which contains a heterodiamine with both primary and tertiary amines, adsorbs 4.34 mmol/g at 323 K and 5 mmol/g at 298 K of CO2 at 1 bar. DFT calculations on the binding energy of each possible Mg–amine pair explain the formation of alkylammonium carbamate at a specific partial pressure (Fig. 10). Additionally, dmen-Mg2(dobpdc) showed an exceptionally high selectivity for CO2 over N2 and high working capacity at low regeneration temperatures helpful in a post-combustion CO2 capture process.255

Amine-functionalized ligands influence the adsorption and catalytic properties of Ti-incorporated MIL-125. The quasi-cubic tetragonal MIL-125 structure induced stronger van der Waals interaction with CO2 allowing an adsorption of 4 mmol/g at 273 K and 1 bar.283 Upon replacement of H2BDC with H2BDCNH2 in the substrate, CO2 adsorption raises until 5 mmol/g (273 K and 1 bar). At 298 K, NH2-MIL-125 shows moderate CO2 adsorption capacity (3 mmol/g) but excellent selectivity over N2 (>27:1). Furthermore, four consecutive CO2 adsorption–desorption cycles over NH2-MIL-125 showed completely reversible adsorbent regeneration at 298 K under helium flow.249 In three-dimensional NH2-functionalized zeolitic tetrazolate framework ZTF-1, the high CO2 capacity (5.6 mmol/g at 273 K and 1 bar) is due to the narrow pores, exposed NH2 functionality, and free tetrazole nitrogen. At zero coverage, Qst is approximately 25.4 kJ/mol and increasing loading, Qst increases.248 Large CO2 uptake can be achieved through a combination of functionalization and appropriate pore size, which, in a synergic fashion, improves the interactions with CO2. In this regard, CAU-1, built from aluminum hydroxide clusters linked by BDC-NH2, demonstrated an excellent CO2 uptake of 7.2 mmol/g at 1 bar and 273 K. The 0.3–0.4 nm pore windows decorated with amino groups not only improve the CO2 adsorption, but also the CO2/N2 (101:1) and CO2/CH4 28:1 selectivity at 273 K, respectively.263

In the rht-type NTU-105 and in Cu-TDPAT, the combination of both OMSs and LBSs produces high and selective CO2 uptake.206,230,232 Cu-TDPAT showed CO2 uptake of 5.9 mmol/g at 298 K and 10.2 mmol/g at 273 K and 1 bar, and the high adsorption enthalpy at zero loading (42.2 kJ/mol) suggested a strong adsorbate–adsorbent interaction owing to high density of LBSs with the small cages.230 The thermally stable Dy(BTC) after removal of the terminal H2O showed high CO2 storage capacity at low pressure due to the presence of the available Lewis acid metal.244

A favorable interaction between adsorbed CO2 and unsaturated Mcenters in {M3}x SBUs [NH2(CH3)2]2[M3(BTA)(BTC)2(H2O)]2 (M = Cd and Zn) provides an additional electric field that not only increases CO2 uptake but also beneficial for CO2 selective adsorption with respect to other gases under room temperature conditions.206,230,244,250 FJI-H14, a MOF with a high density of open Cu(II) sites and LBSs, showed high volumetric CO2 uptake (7.6 mmol/g at 298 K and 12.5 mmol/g at 195 K) and high stability in water and in an acid/base environment without loss of adsorption capacity.284

MAF-X25 and MAF-X27 have been functionalized by monodentate OH yielding MAF-X25ox [MnMn(OH)Cl2(bbta)] and MAF-X27ox [CoCo(OH)Cl2(bbta)]. The OH groups strongly interact with CO2, the high Qst being ascribed to chemisorption interaction (Fig. 15).238

Figure 15.

The framework structure of (a) MAF-X25 and (b) MAF-X25ox and the interactions between M(III)–OH and M(II) with CO2.238 Adapted from ref. [238 ].

Strong interactions with CO2 can be induced also by nonmetallic functional groups present in the SBUs. In the SIFSIX series, in which M ions (e.g., Cu and Zn) are connected by linear bifunctional ligands to form a two-dimensional square grid, SiF6 constitutes a favorable functional group for efficient, reversible adsorption–desorption, therefore relevant to CO2 separation in the context of post-combustion (Fig. 16).246 Specifically, interpenetrated SIFSIX-2-Cu-i exhibited higher volumetric uptake at low CO2 partial pressure (5.41 mmol/g at 1 bar and 298 K). These materials exhibit a CO2 selectivity over N2, H2, and CH4 in the presence of moisture.285

Figure 16.

The structures of (a) SIFSIX-2-Cu and (b) SIFSIX-2-Cu-i with the 4,4′-dipyridylacetylene (dpa) linker.246 Adapted from ref. [246 ].

Another potentially useful adsorbent material for post-combustion CO2 capture is UTSA-16. In the simulated PSA process with a ratio CO2/N2 of 15/85%, the breakthrough time of UTSA-16 resulted lower with respect to other systems. UTSA-16 displayed extraordinary performance for the CO2 capture at ambient conditions (7.1 mmol/g). Neutron diffraction studies revealed that in its diamonded cage of about 4.5 Å in diameter, two pairs of CO2 molecules interact through K-OH2⋯O2C hydrogen-bonding, thereby enforcing the high separation selectivity. More importantly, after 3 days of exposition air, UTSA-16 maintains the same CO2 sorption capacities, resulting no-air sensitive.231

Some MOFs have been shown to have defects as active sites to enhance CO2 capture and its catalytic transformation. Sulfone-functionalized Al-based MOFs synthesized by the reaction of Al(NO3)3⋅9H2O with Sbpdc in DMF upon the addition of trifluoroacetic acid as a modulator to generate a structural defect show significant CO2 capture ability.266 The mixed-ligand strategy has been employed to synthesize rare sulfonate-based 3D Cu(II) MOF possessing primitive cubic topology with twofold interpenetration. The high density and strong polarity of sulfonate groups play an important role in CO2 adsorption helping to overcome the moisture sensitivity of conventional Cu2 paddle-wheel MOFs.241

Interpenetrating MOFs has a considerable potential for CO2 adsorption and separation at low pressure. The interpenetration produces new CO2 adsorption sites with high affinity and thus remarkably enhances CO2 uptake capacity at low pressure more than twice enhancement with respect to noninterpenetrating MOFs.61,165,286 A series of interpenetrated SUMOF-n has been synthetized by connecting Zn4O clusters with rigid dicarboxylate anions. At 1 bar, the CO2 uptake for this SUMOF ranges from 4.26 to 3.60 mmol/g. The trend observed for the uptake has been assigned to the increased electric field gradients in the small pores of interpenetrated frameworks that increase interaction with the quadrupole moment of CO2.258

The CO2 uptake capacity of the doubly interpenetrated SNU-71 is 1.3 times greater than that of noninterpenetrated SNU-70 at 298 K and 1 bar.204 NOTT-202 consists of two interpenetrated frameworks, in which only one has been partially occupied. The CO2 adsorption properties of the desolvated analogous are related to the stepwise filling of pores and structural vacancies generated by this partially interpenetrated 1.75 defect material.228 Finally, the CO2 sorption properties of isostructural NbO-type MOFs have been demonstrated in Cu-based MOFs with and without accessible metal sites. The presence of OMSs in SNU-5 increased significantly the CO2 adsorption capacity due to a stronger interaction between OMSs and CO2. Notably, the formation of OMSs can be controlled precisely by selecting different activation conditions.227

MOF composites for CO2 capture

The combination of MOFs with other suitable materials can improve their CO2 adsorption capabilities generating further pore environments and additional interaction between CO2 and sorbent materials. Moreover, certain physical and chemical properties of compounds utilized in the formation of MOF composites can improve stabilities and mechanical performance of adsorbents. For example, a series of PEI-decorated MOF-101(Cr) has been prepared with different amounts of PEI via a wet impregnation method. At high PEI loading, the BET surface area and pore volume decreased, but the CO2 uptake capacity of the composite was significantly enhanced at low pressure. In PEI@MIL-101-100, CO2 adsorption was over 12 times greater (4.2 mmol/g at 298 K and 0.15 bar) than parent MIL-101 (0.33 mmol/g at 298 K and 0.15 bar), and the CO2/N2 selectivity (in a 15/75 v/v mixture) was 120 at 298 K.287

In the composite PN@MOF-5, the PN polymer has been trapped within the channels of MOF-5 via Bergman cyclization and subsequent radical polymerization (Fig. 17). The incorporation of PN partitioned the channels that increased ultra-micropores and the number of exposed aromatic edges and surfaces allowing CO2 uptake of 3.5 mmol/g at 273 K and 1 bar. More importantly, PN significantly improved moisture stability of MOF-5, PN@MOF-5 exhibiting greater stability under humidity conditions and keeping almost the same value of dynamic CO2 adsorption obtained under dry conditions.288

Figure 17.

Polymerization of DEB in MOFs and formation of PN@MOF-5 composite.288 Adapted from ref. [288 ].

1-Ethyl-3-methylimidazolium thiocyanate has been incorporated in IRMOF-1 and its presence in the structure causes an increment of CO2 adsorption due to the strong chemisorption of the CO2 dissolved in such compounds.289

Several materials have been used as a substrate for the growth of MOFs on the surface or within the pores. In MCGr-X composites, benzoic acid-functionalized graphene has been used as supports to growth of M-MOF-74 (M = Mg, Ni and Co). The resulting composites possess significantly enhancement in gas adsorption, for example MgCGr-10 adsorbs 8.1 mmol/g CO2 at 1 bar and 298 K, and the graphene improves elastic modulus and hardness.290

Mesoporous silica has been used to growth a Mg-MOF-74 composite. SBA-15@Mg-MOF-74 hybrid materials were synthesized via the immobilization of Mg-MOF-74 into mesoporous SBA-15 rods. In this case, the presence of SBA-15 components decreased the CO2 uptake of approximately 45% in total uptake with respect to pristine Mg-MOF-74.291 On the contrary, the SBA-15@HKUST-1 composite prepared by in situ self-assembly of HKUST-1 and SBA-15 (1 wt% of SBA-15 based on the mass of the metal precursor) showed an ordered morphology and hierarchical structure that increased CO2 adsorption capacities of 15.9% with respect to the original at 298 K and 1 bar. In addition, SBA-15@HKUST-1 exhibited an excellent reversibility of CO2 adsorption.292

Graphene oxide (GO) has been used as templates to build HKUST-1. The epoxy groups into the graphene layer acted as a seed site for well-dispersed nanocrystal growth of HKUST-1. GO@HKUST-1 exhibits about a 30% increase in CO2 storage capacity with respect to the no-composite analogous.293 Moreover, in GO@HKUST-1 with 1% of GO loading, the CO2/CH4 adsorption selectivity raises to 14, which was almost twice that of HKUST-1.293 In GO@HKUST-1/urea-modified, due to the presence of amide groups and the highest number of defects/unsaturated copper centers, the CO2 adsorption reached significant values depending on the temperature and pressure. In addition, this composite showed a totally reversible adsorption process without any external thermal treatment.294 HKUST-1 has also been incorporated in carbon nanotubes (CNTs) to give CNT@HKUST-1. Herein, crystals of HKUST-1 are formed by heteronucleation on the carboxylic groups of CNT to support continuous growth, leading an enhancement of pore volume (from 0.73 to 0.87 cm/g). The CO2 adsorption was about twice as high that HKUST-1 under the same conditions.215 In a series of ZIF-8, the optimal hydroxy-functionalized CNTs content improved CO2 adsorption capacity and relative selectivity for CO2/N2. Due to the large surface area, the CNT@ZIF-8 composite (with 3.63 wt % of CNTs) showed slightly higher CO2 and N2 uptake capacities at both low and high pressures.295

MOF-based membranes for CO2 capture

Mixed matrix membranes applicable to CO2 capture during energy regeneration are continuously developed. Targets in terms of selectivity and productivity also on the basis of the existing literature are well identified, and recent efforts have been directed to the production of MOFs based on MMMs. The use of MOFs as potential fillers in the polymeric matrix seems very promising due to the high affinity of the polymer chains for MOFs in comparison to other inorganic fillers as a consequence of their partially organic nature and the presence of easily adjustable cavities in terms of size, shape, and chemical functionalities that can be tuned by choosing the appropriate ligands in the synthesis or by post-synthetic functionalization. MOFs commonly have a higher pore volume and a lower density than zeolites, meaning that their effect on the membrane properties can be larger for a given weight percentage of the filler.

Already, in 2004, a three-dimensional copper(II) biphenyl dicarboxylate-triethylenediamine MOF embedded in PAET has been prepared and applied in gas separation.296 HKUST-1, ZIF-8, and MIL-53(Al) with and without the amino group have been the most studied MOFs, whereas the organic phase investigated has been mainly the low flux glassy polymers (i.e., PSF,297,298 PPEES,299 PVAc,300 Ultem® Matrimid®,186 or PBI)301 and the high flux polymers PDMS and PMPS (rubbery),302 and 6FDA-DAM (glassy).303 For all the membranes tested at high pressures, it was observed that upon MOF addition, the plasticization of the membrane at high CO2 pressures was partially suppressed, maintaining large separation factors over a wider pressure range than that observed for the pure polymer304,305 or even increasing the selectivity at high pressures.306

Membranes formed by ZIF-8 and 6FDA-DAM:DABA, PIM-1, or 6FDA-durene,307 CPO-27 and XLPEO,308 and [Zn2(1,4-BDC)2(dabco)]⋅4DMF⋅0.5H2O and 6FDA-4MPD exhibit selectivity and permeance very close to those required for an attractive membrane-based post-combustion CO2/N2 separation.309

H2/CO2 separation membranes310,311 formed, for example, by 6FDA, polyimides, and ZIFs demonstrated highest performance. Significant results were reported for asymmetric membranes prepared with HKUST-1 and PMDA-ODA30312 and dense membranes containing ZIF-8 and PBI313 for which the commercial interest is reached with MOF loadings of 6 and 30 wt%, respectively. The polyimide 6FDA-mPD crystallites have been mixed with MIL-101(Cr) and HKUST-1. The HKUST-1@6FDA-mPD MMMs demonstrated an enhancement in CO2 permeability as well as in CO2/CH4 selectivity, which were favorite by good dispersion and nonagglomeration of HKUST particles.314

MMMs prepared from Pebax®1657 with the nano-sized ZIF-7 have been successfully deposited as a thin layer (<1 μm) on a porous PAN support. The combination of molecular sieving effect from ZIF-7 filler and the high solubility for CO2 in Pebax®1657 increased both selectivity (CO2/N2 up to 97 and CO2/CH4 up to 30) and permeability (CO2 up to 145 barrier).315

MOF films

Monolithic structures are important platform for scaling up a gas separation process. MOF-74(Ni) and UTSA-16(Co) have been immobilized on cordierite monolith (600 cpsi) and studied for the adsorptive performance in CO2 capture. Different techniques have been employed to optimize loading, thickness, and adsorption characteristics of the MOFs films. The choice of suitable coating procedure depends primarily on the type of the MOF material used. The layer-by-layer technique followed by a secondary growth is a suitable method for MOF-74(Ni) film growth on the monolith walls which give rise to ~52 wt% MOF loading, whereas, for UTSA-16(Co), in situ dip coating was found to be a promising coating method which results in ~55 wt% MOF weight gain. The MOF-coated monoliths displayed relatively moderate CO2 adsorption capacity with fast kinetics.316

Ag-nanoparticles constituting the plasmonic substrate were coated with a HKUST-1 MOF thin film using the layer-by-layer or LPE method. The layer-by-layer method yields excellent control over MOF thickness, which showed a significant influence on a sensor response of pure SF6 and CO2. The HKUST-1/AgNP sample adsorbs larger amounts of CO2 than N2 at 1 bar. Preferential concentration of CO2 within the HKUST-1 pores produces a 14-fold signal enhancement for CO2 sensing.317


In recent years, the research on MOFs for CO2 capture, storage, and transformation has become one of the more important fields in inorganic and organic chemistry, and most of the collected results strongly suggests MOFs as possible adsorbents toward real industrial applications.

In this review, we have shown that MOFs can be powerful and promising candidates for CO2 capture and transformations for their porous structures resulting in very large surface area and for their tunable physicochemical properties, matching the adsorbed CO2. The versatility of these materials is mainly due to the tunability of the organic ligands into the structures, and we have reported the role played by amines, carboxylates, nitrogen-donor ligands, amide, hydroxides, and the effects originated by grafting functional groups of different polarity onto the MOFs surfaces. To provide a better understanding of the role of the ligands, we have discussed the chemical properties useful in CO2 capture through a brief explanation of the best MOFs in terms of adsorption. The ligand functionalization strongly influences the CO2 adsorption properties and separation performances of MOFs as reported in the comprehensive tables referred to high and low pressures. The collected results suggest that the ligand and metal properties influence MOF⋯CO2 interactions, whereas the insertion of functional groups also by PSE could reduce porosity due to the lack of void volume, and that both factors should be considered to have best performances. In CO2 adsorption at low pressure, amine, heterocyclic azole, and oxalamide ligands show higher performance through their Lewis basicity, polarity, and hydrogen bonding. On the other hand, the high surface area and pore volume are decisive at high-pressure CO2 adsorption.

We reviewed here also the four approaches for CO2 capture: post-combustion capture, pre-combustion capture, oxy-fuel combustion, and direct air capture. In the post-combustion application, MOFs need to be chemical and thermal stable to withstand at the high levels of water present in the flue gas stream and the condition required for the regeneration process. In addition, impurity components can affected the long-term stability and separation performance of the MOF materials. Thus, MOF materials with high strength of the metal–ligand bond (e.g., high-valent metal cations strongly binds heterocycle azolate ligands) are promising in this area. At high pressure, high CO2 adsorption and CO2/H2 selectivity are essential features for efficient MOFs in the pre-combustion CO2 capture. MOFs containing exposed redox active metals (Cr, Mn, Fe, Co, and Cu) that reversibly bind O2 via charge–transfer interactions allow oxy-fuel combustion application near ambient temperature with high selectivity and working capacity.

In the section dedicated to the mechanism of CO2 capture, we have tried to give a clear picture of the proposed mechanisms to date reported not only by distinguish chain and pair models in amine-functionalized MOFs but also addressing the role of OMSs, LBSs, and size-exclusive effects. The possibility to have best performances by combining MOFs with other suitable materials or by preparing MOF films and MOF membranes has also been exhaustively discussed.

MOF-based membranes outperform polymeric membranes in terms of permselectivity and separation performance. Prevent defects during the thin membrane formation and control the orientation of MOF crystal size and morphology are the two main challenges to improve the processability of the membranes allowing selective transport for CO2 molecules.

The carbon dioxide capture and storage processes and technologies discussed in this review bring out the relevant aspect of CO2 utilization. The conversion of CO2 feedstock into valuable chemicals, materials, and energy products provides a long-term solution than sequestration, and it represents a link to a circular carbon economy. Metal–organic frameworks are very active catalyst for CO2 conversion through chemical fixation, photocatalysis, and electrocatalysis. CO2 can be transform into different energetic species such as HCOOH, CO, HCHO, CH3OH, and CH4 or cyclic carbamate.


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