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Review

CO2-Selective Capture from Light Hydrocarbon Mixtures by Metal-Organic Frameworks: A Review

College of Environmental Science and Engineering, State Key Laboratory of Pollution Control and Resource Reuse, Tongji University, Siping Rd 1239, Shanghai 200092, China
*
Author to whom correspondence should be addressed.
Clean Technol. 2023, 5(1), 1-24; https://doi.org/10.3390/cleantechnol5010001
Submission received: 18 October 2022 / Revised: 12 December 2022 / Accepted: 19 December 2022 / Published: 20 December 2022
(This article belongs to the Special Issue Application of Porous Materials in CO2 Capture)

Abstract

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CO2 represents a typical impurity in light hydrocarbon feedstocks, which affects the quality of subsequent chemical products. Owing to their highly similar nature, industrial separation requires large amounts of energy. Adsorptive gas separation based on porous materials is considered an efficient alternative, as it can offer faster kinetics, higher selectivity, long-term stability and more energy-efficient regeneration. For the adsorption separation method, preferential CO2 capture from gas mixtures in one step is more energy-efficient for direct purification than light hydrocarbons, saving about 40% energy by eliminating energy-intensive post-regeneration processes such as countercurrent vacuum blowdown. Therefore, CO2-selective adsorbents are more sought-after than light hydrocarbon-selective adsorbents. Metal-organic frameworks (MOFs) have been demonstrated as outstanding physisorbents for CO2 capture due to their configurable channels for CO2 recognition, structural flexibility and large specific surface area. Many highly selective CO2 adsorption behaviors of MOFs have been reportedly achieved by precise modulation of pore size, pore chemistry or structural flexibility. In this review, we discuss the emerging development of MOFs for CO2-selective capture from different light hydrocarbon mixtures. The challenges of CO2 recognition and the strategies employed to achieve CO2 selectivity over light hydrocarbon mixtures by MOFs are summarized. In addition, the current challenges and prospects in the field of MOFs for CO2 capture are discussed and elaborated.

1. Introduction

Carbon dioxide (CO2) is a critical cause of global warming, which is a widespread public concern [1,2]. Global annual CO2 emissions increased by approximately 130% between 1970 and 2020 and are expected to increase to 48–55 Gt/year by 2050 without intervention [3,4]. Therefore, carbon capture, utilization and storage have been considered key strategies by policymakers and oil companies, of which CO2 capture is the core issue [5,6,7]. In the chemical industry, CO2 is often formed as an impurity during the production of light hydrocarbons. For instance, natural gas or biogas (CH4) usually contains 5–70% carbon dioxide, which needs to be reduced to less than 2% for pipeline transportation and 50 ppm for liquefied natural gas [8,9]. Acetylene (C2H2) is typically produced by partial combustion of CH4 or hydrocarbon cracking, which generally results in several gaseous impurities such as CH4 and CO2 [10]. The presence of CO2 in light hydrocarbons affects the quality of subsequent chemical products, and it is challenging to trap CO2 from their mixture due to their highly similar physical properties (Table 1) [11]. Absorption with aqueous amine and cryogenic distillation are traditionally used for CO2 capture and light hydrocarbon purification, but these methods suffer from large amounts of waste streams and high regeneration energy consumption [12,13,14]. Comparatively, adsorptive gas separation based on porous adsorbents in hydrocarbon purification is considered more energy-efficient, as it can offer faster kinetics, higher selectivity and more energy-efficient regeneration [5,15,16,17,18]. The adsorption separation process is performed by introducing a gas mixture into a column filled with porous materials, adsorbing the strongly adsorbed components and obtaining the outlet weakly adsorbed components; the strongly adsorbed components are subsequently recovered in the desorption process. However, the desorption process involves energy-intensive steps such as countercurrent evacuation [19,20,21]. It is estimated to cost approximately 40% more energy to harvest high-purity products through the desorption process than to collect the weakly adsorbed gas product directly at the outlet [22,23]. Therefore, porous materials for CO2-selective adsorption are more sought-after than hydrocarbon-selective materials, as hydrocarbons are the target products in practical applications. Direct CO2 capture from light hydrocarbon mixtures is more energy-efficient and effective.
The development of porous materials has explosively grown in the last few decades, the most promising of which are metal-organic frameworks (MOFs), also referred to as porous coordination polymers (PCPs). MOFs assembled from organic linkers and metal cluster based secondary building units (SBUs) generally possess high surface areas and easily controlled pore sizes and environments, with infinite possibilities. MOFs have been demonstrated as outstanding physisorbents for CO2 capture due to their highly configurable channels for CO2 molecular recognition (Figure 1) [24,26,27,28,29,30,31,32]. Therefore, a detailed update on the current status of MOFs with CO2 selectivity for CO2/light hydrocarbon separation is urgently needed and would provide insight into their future development. Through this review, we aim to discuss the advancement of MOFs for the selective capture of CO2 from different light hydrocarbon mixtures (CO2/CH4, CO2/C2H2, CO2/C2H4, etc.) in one-step purification of light hydrocarbons. As a departure from other reviews, we focus on evaluating MOFs with CO2 selectivity. Herein, we describe the challenges of CO2-selective recognition and summarize the strategies adopted to achieve CO2 selectivity. Furthermore, we discuss the current challenges and prospects in the field of MOFs for CO2 capture.

2. CO2-Selective Capture from CO2/CH4 Mixture

Natural gas, the main component of which is CH4, is a clean energy source and a feedstock for bulk chemicals, for which the CO2 concentration is required to be less than 2% for pipeline transportation and 50 ppm for liquefied natural gas [8,9]. Therefore, the separation of the CO2/CH4 mixture is an essential process for the industrial utilization of natural gas. However, CO2 and CH4 exhibit close polarizability (CH4, 26.0 × 10−25 cm3; CO2, 29.11 × 10−25 cm3) and kinetic diameters (CH4, 3.76 Å; CO2, 3.30 Å). By exploiting the slight differences in molecular size and polarizability, there are two solutions available using MOFs to trap CO2 from CO2/CH4: (1) modulating the pore size/shape of MOFs for kinetic separation or molecular sieving and (2) constructing functional sites with enhanced CO2 interactions, such as open metal sites (OMSs), quadrupole interaction sites, hydrogen bonding sites, Lewis basic sites, van der Waals interaction sites, etc. (Table 2).
Bastin et al. presented the first example of the removal of CO2 from a binary CO2/CH4 mixture by MOFs [33,34]. MOF-508 showed CO2 uptake of 1.78 mmol/g and moderate selectivity in the range of 3−6 at 303 K, 1 bar. The very low adsorption enthalpy of 14.9 kJ/mol for CO2 and 5.1 kJ/mol for CH4 suggested that the CO2 interaction with MOF-508 probably originated from quadrupole interaction and/or van der Waals interaction.
Due to the slight differences in molecular size between CO2 and CH4, some rigid MOF materials with molecular sieving effects achieved CO2 capture from CO2/CH4 with high selectivity by reducing the aperture of MOFs so that only smaller CO2 could diffuse into the pores, whereas larger CH4 was completely excluded.
UTSA-280 is a typical rigid MOF with a molecular sieving effect [49,55]. The cross-sectional area of its 1D channel is approximately 14.4 Å2, which is smaller than that of CH4 (minimum: 15.1 Å2) but larger than that of CO2 (minimum: 10.7 Å2). The CO2 uptake capacity of UTSA-280 was 3.0 mmol/g at 1 atm, 298 K and the density of CO2 in the pore channel was up to 733 g/L, which was comparable to the liquid CO2 density of ~1.1 kg/L (236 K, 11.1 bar), indicating that the CO2 molecules in UTSA-280 were tightly packed. However, the relatively high isosteric heat of adsorption (Qst) of 42.9 kJ/mol was not favorable for material regeneration.
Another example of molecular sieving is Cu-F-pymo with dual functionality, exhibiting extremely high selectivity for CO2 over CH4 [52,56]. In Cu-F-pymo, Cu(II) atoms are coordinated to four N atoms from four different ligands to form a 1D pore channel with a pore diameter of ~3.3 Å. The IAST selectivity was calculated to be ultra-high, at 107 for CO2/CH4 (v/v, 50/50) mixtures at 298 K and 1 bar. Grand canonical Monte Carlo (GCMC) simulation indicated that the binding affinity for CO2 molecules was further enhanced by the pore channels functionalized with oxygen moieties through electrostatic and hydrogen bonding interactions. Furthermore, it could be easily synthesized in aqueous solution under ambient conditions. However, Cu-F-pymo showed a relatively low uptake capacity of 3.09 mmol/cm3 (1.61 mmol/g) at 298 K and 1 bar due to the compact pore volume.
A higher CO2 uptake achieved by molecular sieving adsorbent was achieved using SIFSIX-14-Cu-i [43]. SIFSIX-type MOFs are a typical kind of microporous MOF designed according to the isoreticular principle. These MOFs with 1D pores are composed of 2D metal–dipyridyl layers and SiF62−-type anion pillars in pcu topology. The replacement of organic ligands with different molecule lengths and/or framework interpenetrations provides extensive adjustment of the pore diameter, and the presence of F atoms in the channel tends to provide high affinity for CO2 [57]. The pore cavity of SIFSIX-14-Cu-i was embellished with a high density of SiF62− anions and thus showed high CO2 capacity (4.71 mmol/g) under ambient conditions.
Qc-5-Cu demonstrated a strategy of crystal engineering to tune pore size in MOFs (Figure 2) [42]. HQc (quinoline-5-carboxylic acid) and five kinds of metal ions were used to synthesize five different MOFs, Qc-5-M-dia (dia indicates a twofold and 3D diamondoid network) and Qc-5-Cu-sql-α (sql indicates a 2D square lattice network). Qc-5-Cu-sql-β was the desolvated phase of Qc-5-Cu-sql-α, with a contract cell volume of 1004.1 ± 0.1 to 908.0 ± 0.1 Å3 and a pore size of 3.8 to 3.3 Å. Such regulated channels of Qc-5-Cu-sql-β fit the kinetic diameter of CO2 and could exclude methane molecules, leading to high selectivity (≈3300) for CO2. Qc-5-Cu-sql-β also exhibited higher Qst (36 kJ/mol) than its supramolecular isomer, Qc-5-Cu-dia (34 kJ/mol), and Qc-5-Ni-dia (32 kJ/mol). Molecular simulations of CO2 adsorption revealed that the interaction sites were the H atoms in the Qc ligands, which could attract the negatively charged CO2 oxygen atoms. In the modeled CO2-loaded structure, the distance between the O atoms and interaction sites was <2.5 Å in Qc-5-Cu-sql-β but >3.0 Å in Qc-5-Ni-dia, which explains the difference in their adsorption behavior and Qst. However, the CO2 capacity of the former was only 2.16 mmol/g at 293 K, 1 bar, which is negative for practical applications.
Jiang et al. found a new strategy to precisely tune the multinuclear clusters of MOFs via symmetry-upgrading isoreticular transformation and obtained MOFs with high CO2 selectivity [44]. As a precursor, Cu(IN)2 (termed FZU, IN = isonicotinic acid) with binuclear clusters was used for isoreticular transformation towards NJU-Bai34 and NJU-Bai35 with clusters of higher symmetry (Figure 3). The binuclear cluster of FZU offered certain structural flexibility, and the uncoordinated O atoms made it possible to incorporate additional metal ions and upgrade the symmetry of the inorganic clusters without changing connectivity. With an increase in the concentration of Cu2+ ions and the addition of water, acetonitrile and acetic acid, NJU-Bai34 was obtained from [Cu4(μ3-O)2(COO)4N4O2]. All Cu atoms in NJU-Bai34 were five-coordinated, and the remaining coordination sites were saturated by CH3COO. However, the coordination of CH3COO requires ((CH3)2NH2)+ generated by DMF hydrolysis to neutralize the entire complex. Therefore, part of the pore channel is blocked by CH3COO and the counterions. To avoid this phenomenon, the reaction was conducted at 120 °C using DMF/H2O as a solvent. The Cl atoms in copper chloride started to coordinate and shrunk the Cu clusters more than NJU-Bai34. After the symmetry of clusters was upgraded, the channels sizes of NJU-Bai35 were 3.6 × 3.6 Å2 (along the a axis), 3.4 × 3.4 Å2 (along the b axis) and 3.6 × 3.6 Å2 (along the c axis) and perfectly fit the CO2 molecules, which resulted in a higher CO2 uptake capacity of 7.20 wt% than NJU-Bai34 and FZU at 298 K, 0.15 bar. Furthermore, NJU-Bai35 exhibited a high CO2/CH4 selectivity of 11.6 for an equimolar mixture at 298 K, 1 bar, mainly due to the molecular sieve effect, with a pore size of 3.6 × 3.6 Å2. As a result, breakthrough curves further demonstrated that NJU-Bai35 has high potential for natural gas purification.
Despite the extremely high selectivity, the strategy of molecular sieving typically suffers from low capacity limited by the compact pore volume, which is not favorable for MOF applications. The discrepancy in polarizability and acidity inspired some researchers to construct functional sites such as OMSs and Lewis basic sites with enhanced CO2 interactions.
The open magnesium sites in Mg-MOF-74 were discovered by Bao et al. with high affinity for CO2 [36]. At 298 K and 1 atm, the uptake of CO2 was as high as 8.61 mol/kg (37.8 wt.%), which is significantly higher than that of the general amine-treated adsorbents. Comparatively, the uptake of CH4 was only 1.05 mol/kg (1.7 wt.%), and the Henry’s Law selectivity for CO2/CH4 (50/50) was 8 at 298 K and 1 atm. Intense guest–framework interaction also induced an extremely high Qst of 73 kJ/mol, which was not conducive to the regeneration of materials.
Amine functionalization is considered a feasible strategy to improve the CO2 capture capacity of MOFs. PEI-incorporated amine-MIL-101(Cr) represented a polyethyleneimine-decorated MOF adsorbent [41]. After loading PEI, the structure of amine-MIL-101(Cr) was not degraded in scanning electron microscopy images; however, the pore size and polarity were affected as the PEI loading increased. Taking advantage of the porous characteristics of MIL-101(Cr) and Lewis basic –NH2 groups, PEI-incorporated amine-MIL-101(Cr) showed a high CO2 capacity of 3.6 mmol/g and an IAST selectivity of 931 for equimolar CO2/CH4 mixtures under 100 kPa, 298 K.
In(aip)2 is a 2D stacked MOF composed of In clusters and 5-aminoisophthalic acid with abundant amine groups (–NH2) in the channels, serving as hydrogen bonding sites and Bronsted basic sites to improve the interaction with CO2 [48]. The Horvath–Kawazoe (HK) model determined the experimental average pore diameter of In(aip)2 to be approximately 3.57 Å, which is between the molecular sizes of CH4 and CO2. A synergetic size-sieving effect and –NH2 interaction sites resulted in an extremely high IAST selectivity of 1808 for CO2/CH4 (50/50, v/v) but limited uptake capacity of 1.27 mmol/g for CO2 under ambient conditions.
Uncoordinated N donors as active sites could also enhance the CO2 adsorption affinity and have been introduced into MOFs. Based on the MOF SYSU platform, two isoreticular MOFs, [Cu(L2)·DMF]∞ (NJU-Bai7) and [Cu(L3)·DMF·H2O] (NJU-Bai8), were created by altering the coordination sites of the ligands and adjusting the pore size while introducing uncoordinated nitrogen atoms to the inner surface (Figure 4) [40]. The porosity and specific surface area of the three MOFs remained almost unchanged after channel narrowing and polarization. However, at low pressure (0.15 bar), NJU-Bai7 and NJU-Bai8 could adsorb 8.0 wt% and 5.4 wt% of CO2 at 298 K, respectively—higher than the 3.6 wt% for SYSU. Both NJU-Bai7 and NJU-Bai8 showed much higher Qst for CO2 (40.5 and 37.7 kJ/mol, respectively) and separation ratios at 273 K (14.1 and 40.8, respectively) compared with the MOF SYSU platform, which may be attributed to the narrow channels, favoring the interaction with CO2. In contrast, NJU-Bai8 showed stronger CO2 recognition and higher CO2/CH4 selectivity than NJU-Bai7 because of unsaturated N atoms, which made it a better choice for trapping CO2 from the CO2/CH4 mixture.
IRHs-(1–3) are three isoreticular lanthanide MOFs from cyamelurate linkers characterized by a large number of accessible N-donor sites [47]. Due to the abundant uncoordinated N on the hydrophilic pore walls, the interaction with CO2 was enhanced, and CH4 with nonpolar covalent bonds was excluded, facilitating the adsorption of CO2 over CH4. Through GCMC simulations, researchers found that no CH4 was distributed around the metal center as a result of the existence of water molecules. The uptake of CO2 and CH4 in IRH-3 was 2.7 and 0.07 mmol/g, respectively, and the IAST selectivity for equimolar CO2/CH4 reached 27 at 298 K and 1 bar.
3-amino-1,2,4-triazole (Hatz) was used to produce a zeolite-like MOF, [Zn(atz)2] (MAF-66), at room temperature with -NH2 groups and uncoordinated triazolate nitrogen atoms in the channels [37]. MAF-66 exhibited a high CO2 uptake capacity of 4.41 mol/kg, and the IAST selectivity for equimolar CO2/CH4 was 5.8 at 298 K and 1 bar. The Qst of MAF-66 at zero coverage was 26.0 kJ/mol, which is significantly lower than those of PCPs based on OMSs. A low Qst is beneficial to the adsorption–desorption cycle of the material.
UiO-66(N10%-Zr) was designed through the in situ functionalization of node chemistry of UiO-66 with 2-aminobenzimidazole (2-AMI) by a novel microwave-assisted strategy [50]. The grafted 2-AMI on the nodes provided the N-containing five-membered heterocycle as an electron donor. Through electrostatic interactions, –NH2 could act as a binding site for Lewis acidic CO2 and produce a specific recognition effect. The stability and hydrophobicity would also be enhanced because the benzene ring in 2-AMI modified the frameworks, making them inert. The node-engineered UiO-66(N10%-Zr) showed excellent separation performance for CO2/CH4 through a synergetic equilibrium–kinetic effect. The CO2 uptake was 2.1 mol/kg, and the IAST selectivity was up to 326 for a 50/50 CO2/CH4 mixture at 298 K, 1 atm—both higher than those of UiO-66. The Qst of UiO-66(N10%-Zr) was 35.7 kJ/mol, which is also higher than the Qst of UiO-66 (22.0 kJ/mol), indicating that node-engineering contributed to the stronger affinity for CO2.
The low-cost and water-stable SIFSIX-1-Cu [Cu(bpy)2(SiF6)] exhibited an extremely high capacity of 5.2 mmol/g for CO2 at 298 K, 1 atm, and a high selectivity of 10.5 for an equimolar CO2/CH4 mixture [38]. Furthermore, the Qst value for CO2 was as low as 27 kJ/mol. SIFSIX-2-Cu and SIFSIX-2-Cu-i were both synthesized from the reaction of 4,4′-dipyridylacetylene (dpa) and CuSiF6. SIFSIX-2-Cu-i was isostructural to SIFSIX-2-Cu but with double interpenetrated nets [39]. After interpenetrating, the pore size formed by SIFSIX-2-Cu-i was smaller than that of SIFSIX-2-Cu (13.05 Å), which was 5.15 Å. Similarly, SIFSIX-3-Zn was synthesized from the shorter pyrazine and thus resulted in smaller pore size of 3.84 Å (Figure 5). Owing to its optimal pore size, the electropositive carbon atoms of CO2 molecules could closely interact with the fluorine atoms of SiF62− and provided a benchmark IAST selectivity of 231 for a 50/50 CO2/CH4 mixture. This research demonstrates the successful control of pore size according to the isoreticular principle. Together with the favorable electrostatic interactions provided by the inorganic anion array, the SIFSIX series MOFs are capable of a synergetic equilibrium–kinetic effect, allowing for the separation of CO2 mixtures, with the potential for high selectivity, recoverability and moisture stability.
For the mixture system of CO2 and CH4, CO2 has a smaller molecular size and is more acidic and can therefore be easily separated by the strategy of molecular sieving and construction of strong CO2 binding sites. It should be noted that although ultra-high selectivity can be achieved using a molecular sieving mechanism, the narrowed pore channels often tend to result in low adsorption capacity. Therefore, future studies should investigate the construction of pore structures with large cavities connected by narrow pore channels, potentially maintaining a large adsorption capacity while sieving at a narrow entrance with high selectivity.

3. CO2-Selective Capture from CO2/C2H2 Mixture

C2H2 is an important fuel for the welding industry and an essential feedstock used to fabricate petrochemical products. Among the gas mixtures of acetylene, the separation of C2H2/CO2 is the most important and challenging due to the ultra-high similarity of these compounds in molecular size, shape and boiling points (Table 1). Moreover, most of reported MOF materials interact more strongly with C2H2 than CO2, as the hydrogens and π-electrons on C2H2 are highly polarizable and acidic and usually interact with acid–base interaction sites. Such C2H2 selectivity is called “normal selectivity” and suffers from two energy-intensive steps of adsorption and desorption to obtain pure C2H2 gas [58,59,60,61,62,63,64,65,66]. CO2 capture from C2H2/CO2 in one step is more energy-efficient for purification of C2H2, saving approximately 40% energy by eliminating other complex post-regeneration operations [22]. Therefore, ideal physisorbents should preferentially capture the trace CO2 from the gas mixture rather than C2H2 (“inverse selectivity”) to further reduce the energy consumption of C2H2/CO2 separation [25,67,68].
In the past few years, various benchmark MOFs have been demonstrated to exhibit “inverse selectivity” for CO2/C2H2 separation, as shown in Table 3. Owing to their very close kinetic diameters, it is difficult to separate CO2/C2H2 with the molecular sieve mechanism. Generally, the following strategies are utilized: (1) exploiting the structural flexibility or phase transition induced by different gas molecules, (2) optimizing pore chemistry through different electrostatic interactions or thermodynamic affinity and (3) constructing kinetic diffusivity differences through precise pore geometry.
It has been reported that some flexible MOFs can achieve efficient separation of highly challenging gas mixtures by mechanisms such as guest-induced “gate opening”. The structural flexibility and/or phase transition of flexible MOFs leads to an abrupt increment in uptake amount at a critical pressure, i.e., a “step” adsorption isotherm [83,84,85,86,87].
[Co(HLdc)] is a dynamic bifunctional MOF with inverse CO2/C2H2 selectivity [69]. The framework flexibility of [Co(HLdc)] is provided by the dangling carboxylic groups and electron-donating nitrogen centers on the channel surface, showing gated adsorption for CO2 instead of C2H2. At 195 K, symmetric CO2 molecules with a permanent quadrupole moment can interact with the MOF frameworks and lead to the rotation of pyridyl rings, opening the “gate”. However, C2H2 could not trigger a similar “gate” rotation because the adsorption temperature (195 K) was higher than its boiling point (189 K), and therefore, more C2H2 uptake was required. The CO2 uptake capacity was 10.69 mol/kg at 195 K, 1 bar, and the CO2/C2H2 uptake ratio was 1.7.
Another flexible MOF with inverse CO2/C2H2 selectivity is [Mn(bdc)(dpe)] [70]. Its small zero-dimensional pores showed gated adsorption for C2H2 instead of CO2 due to their opposite quadrupole moments, resulting in different CH−π and π–π interactions. The adsorption isotherm of C2H2 showed a gated behavior at a gate-opening pressure (Pgo) of 1.45 kPa at 195 K, whereas CO2 did not. The CO2 uptake amount was 2.17 mmol/g at 273 K, 100 kPa, and the IAST selectivity for CO2/C2H2 (v/v, 50/50) gas mixture varied from 8.8 to 13, indicating the excellent potential for CO2 capture from a CO2/C2H2 mixture. In situ X-ray diffraction (XRD) and density functional theory (DFT) calculations illustrate that due to the opposite quadrupole moment, the electrostatic potential of the pore leads to repulsion with C2H2 in an end-on orientation, requiring more structural transformation energy to adapt to C2H2 with a side-on orientation (Figure 6).
The opposite quadrupole moments of CO2 (−13.4 × 10−40 C m2) and C2H2 (+20.5 × 10−40 C m2) have enlightened researchers to adjust the pore environment with inverse electrostatic potential (ESP) or precise steric arrangement of interaction sites on the pore surface, which is more favorable for CO2 adsorption and leads to inverse CO2/C2H2 separation [74].
Cd[Fe(CN)5NO] (Cd-NP) is an ultra-microporous MOF constructed by 3.2 Å pore channels connecting ellipsoidal cavities, which are analogous to the molecular shapes of CO2 and C2H2 [75]. The ESP of the pore surface mapped by DFT calculations shows a positive potential (α) around the Cd center, a positive potential (β) on the N atom of nitrosyl and a negative potential (γ) near the N atom of cyanide. Therefore, the ESP of the pore surface was complementary to CO2 rather than C2H2. Cd-NP provided a high CO2 capacity of 2.59 mol/kg and an IAST selectivity of 85 for an equimolar CO2/C2H2 gas mixture at 298 K, 1 atm. The calculated Qst for CO2 at near-zero coverage was 27.7 kJ/mol, which supported the viability of regeneration under mild conditions.
Zhang et al. proposed that metal nodes in a highly oxidized state can attract electrons from the ligands, generating a pore surface with more polarities that preferentially recognize CO2 molecules [76]. Therefore, they synthesized CeIV-MIL-140-4F from tetrafluoro terephthalate and CeIV with unoccupied 4f orbitals. CeIV-MIL-140-4F provided an optimal pore environment to specifically trap CO2 via strong host–guest interactions, whereas the isostructural ZrIV-MIL-140-4F exhibited normal C2H2 selectivity over CO2. The ESP distributions illustrate that there are two different electron regions at the edge of the channel of the two MIL-140-4F MOFs (electron-rich region I and electron-poor region II). (Figure 7). Compared with ZrIV-MIL-140-4F, the electron cloud density of CeIV-MIL-140-4F was higher at site I, and the potential was more positive at site II. In situ PXRD and in situ FTIR spectra indicate that CO2 was perfectly located on site I of CeIV-MIL-140-4F and bound by four fluorine atoms with strong F···C=O interactions. In contrast, DFT calculations indicate that C2H2 was located on site II of CeIV-MIL-140-4F with weaker ≡C−H···F interactions. The different binding configurations of CO2 and C2H2 in CeIV-MIL-14-4F resulted in a superior CO2 capacity of 110.3 cm3/cm3 (2.24 mmol/g) and an excellent separation selectivity of ~9.5 for 50/50 CO2/C2H2 mixtures at 298 K, 100 kPa. Furthermore, the Qst for CO2 and C2H2 on CeIV-MIL-140-4F was 39.5 and 27.4 kJ/mol, respectively, showing stronger affinity to CO2 than C2H2.
Cu-F-pymo was constructed by Shi et al. from 5-fluoropyrimidin-2-olate for inverse CO2 selectivity [77]. They found that residual solvent molecules under different conditions could provide sites for gas molecules to realize high separation performance. GCMC simulation and DFT were used to calculate ESP, revealing that CO2 and C2H2 molecules were preferentially adsorbed in different pores but the C2H2-preferential pore A was occupied by residual water molecules. CO2 could still be collected with three molecules per unit cell. At 1 atm, 298 K, the partly dehydrated Cu-F-pymo exhibited a CO2 adsorption capacity of 1.19 mmol/g and benchmark CO2/C2H2 selectivity of over 105 for the equimolar CO2/C2H2 mixture.
[Zn(atz)(BDC-Cl4)0.5]n is a 3D pillared-layer ultra-microporous MOF constructed with electronegative Cl atoms embedded in the pore surface for inverse CO2/C2H2 separation [78]. [Zn(atz)(BDC-Cl4)0.5]n showed an adsorption selectivity of 2.4 for a CO2/C2H2 (50/50) mixture at 285 K and 100 kPa and a Qst of 32.7 kJ/mol for CO2. The CO2-selectivity resulted from a match between the pore surface adorned with Cl atoms and the quadrupole moment of CO2. However, [Zn(atz)(BDC-Cl4)0.5]n could only adsorb CO2 with a capacity of 34.6 cm3/cm3 (0.94 mol/kg) at 285 K, 1 bar.
Cai et al. proposed a photoinduced electron-transfer (PIET) strategy to modulate the electrostatic gradient of MOFs for selective adsorption of CO2 over C2H2 [79]. Researchers found that the intrinsic electric field gradients of zwitterions were favorable for selective CO2 adsorption and that the radical π moieties generated by photoelectron transfer had strong affinity for the π systems of CO2. During the PIET process, electrons were transferred from the oxygen-containing carboxylic acid ligand to the zwitterion, which would not change the electric field gradient of the material and prevent a decrease in CO2 adsorption. However, after the loss of one electron, the O atom could not form H-C≡C-H···O hydrogen bonds with C2H2 easily, reducing the adsorption of C2H2 (Figure 8). Based on this strategy, the authors synthesized PMOF-1, which achieved a CO2 uptake of 47.5 cm3/g before irradiation and 53.3 cm3/g after irradiation for 1 h at 273 K and 100 kPa. The uptake ratio was also enhanced from 5.0 to 7.1. Notably, the IAST selectivity for an equimolar CO2/C2H2 mixture reached 694 at 273 K, 1 bar, which was superior to any other porous material reported for inverse CO2 adsorption over C2H2.
Pore chemistry can also be optimized through a precise steric arrangement of interaction sites for favorable CO2 adsorption on the pore surface. Gu et al. demonstrated the strategy of “opposite action” in MOFs, realizing the CO2 selective adsorption and separation of CO2/C2H2 under ambient temperature and pressure [74]. They synthesized two isostructural MOFs, PCP-NH2-bdc and PCP-NH2-ipa, from meso-α,β-di(4-pyridyl) glycol) (dpg), terephthalic acid (bdc) and isophthalic acid (ipa). Owing to the different electronic structures of CO2 and C2H2, they tended to exhibit different binding orientations when adsorbed in microporous channels. Therefore, the precise encoding of interaction sites (-NH2) parallel to the CO2 binding site in the channel could enhance the CO2–framework interaction without significantly changing the adsorption orientation of the CO2 molecules. On the contrary, the additional interaction sites could impact the adsorption orientation of C2H2 and combine with the spatial restriction of the 1D confined channel to inhibit C2H2 adsorption at other sites, ultimately achieving high CO2/C2H2 selectivity of more than 4.4 (Figure 9). PCP-NH2-bdc and PCP-NH2-ipa exhibited remarkably high CO2 uptake of 3.03 and 3.21 mmol/g, respectively. The Qst at zero coverage of CO2 for PCP-NH2-bdc and PCP-NH2-ipa was reduced to 34.57 and 36.6 kJ/mol, respectively, indicating reduced energy consumption for regeneration and recycling.
SIFSIX-type MOFs were reported to provide high affinity for CO2 via extensive adjustable pore size and electron-rich F atoms on the channel surface [57]. For instance, in SIFSIX-3-Ni, each CO2 molecule was located near four F atoms of four SiF62− anions with a short distance of ~2.75 Å between C and F [71]. SIFSIX-3-Ni exhibited a high CO2/C2H2 capacity of 2.5/2.0 mmol/g at 298 K, 0.1 bar, and a selectivity of 7.69 for CO2/C2H2 (2/1; v/v) at 298 K, 1.0 bar. However, with a comparable pore environment, the analogous TIFSIX-2-Cu-i could only provide normal C2H2 selectivity over CO2. The inverse “yin-yang” behavior in two analogous MOFs was explained by the different geometry of host–guest binding sites.
Thulium(iii)-based MOFs embellished with -OH groups and H2O [Tm2(OH-bdc)] with partly and fully dehydrated phases were termed 1a and 1a′, respectively [73]. With its optimized pore size and pore surface modified with –OH groups that could interact reversibly with CO2 through hydrogen bonding, 1a showed a high CO2 capacity of 5.83 mol/kg and an IAST selectivity of 17.5 for CO2/C2H2 (1/2, v/v) at 100 kPa, 298 K. The Qst of CO2 on 1a was 45.2 kJ/mol, which is significantly higher than the Qst of C2H2 (17.8 kJ/mol), showing its strong affinity for CO2. The completely dehydrated phase 1a′ with both hydroxyl groups and open metal sites also provided a CO2 capacity of 6.21 mmol/g but lower selectivity of only 1.65 under the same conditions.
Recently, a new kinetic sieving strategy for C2H2/CO2 separation was demonstrated by ZU-610a with sulfonic anion pillars [81]. ZU-610a was obtained by heat treatment of ZU-610. The pore size was reduced from 3.8 Å and 4.7 Å to 3.2 Å and 4.1 Å after heat treatment, but the topology remained unchanged. Interestingly, after shrinking the pore size, the C2H2-selectivity of ZU-610 was reversed to the CO2 selectivity of ZU-610a, due to the separation process, which transformed thermodynamic control into kinetic control. Kinetic adsorption studies revealed that CO2 achieved equilibrium after ~10 min on Zu-610 and Zu-610a. However, C2H2 did not reach equilibrium until 70 min (Figure 10). The absorption of CO2 on Zu-610a was reduced slightly (1.51 mmol/g) due to pore shrinking, but the capacity of C2H2 fell drastically from 3.22 to 0.12 mmol/g at 100 kPa, 298 K. Profiting from kinetic sieving, ZU-610a showed a selectivity of up to 207 for an equimolar CO2/C2H2 mixture, which is higher than that of many benchmark MOFs. Moreover, comparing the Qst of CO2 with other MOFs, ZU-610a showed the lowest Qst of 27.3 kJ/mol, indicating its extremely low regeneration energy. Kinetic studies of MOFs for CO2/C2H2 separation have been less reported, and this research offered recommendations for the design of MOFs for separation systems with comparable molecular dimensions.
Overall, C2H2 is the most similar substance to CO2 in terms of physical properties. The challenge of directly capturing CO2 instead of C2H2 from CO2/C2H2 is to avoid stronger electrostatic interactions between C2H2 and the adsorbent. The separation by exploiting the flexibility of MOFs is fascinating but less predictable, making it difficult to develop design principles. Optimizing the pore chemistry through different electrostatic interactions or thermodynamic affinity will be more opportune.

4. CO2-Selective Capture from Other Binary Light Hydrocarbon Mixtures

High-purity ethylene (C2H4) and ethane (C2H6) are important raw materials to produce plastics, rubber and other industrial chemicals but always contain contaminants such as CO2, which is corrosive to gas pipelines and can affect the conversion of products. In addition, C2H4 and C2H6 can form maximum pressure azeotrope with CO2, which is a barrier to CO2 removal by distillation [88,89]. Compared to C2H2, however, the kinetic diameter and polarizability of C2H4 and C2H6 differ more from CO2; therefore, many MOFs could exploit these differences for highly efficient separation.
Horike et al. reported an early study of a dense coordination framework [Zn(5NO2-ip)(dpe)] in 2012 [90]. [Zn(5NO2-ip)(dpe)] showed the inherent structural flexibility and CO2 preference over C2H4 and C2H6 under ambient conditions in single-component and mixed gas-flow experiments. For C2H6, the observed adsorption amount could be neglected (less than 10 cm3/g at 273 K, 8 bar). For C2H4, the adsorption isotherm represented a gate-opening behavior. At 298 K, the pressure at which adsorption begins was 720 kPa, whereas the total uptake was only 32 mL/g at 800 kPa. For CO2, however, the adsorption capacity reached 31 mL/g at 298 K and 101 kPa with a type I isotherm. Breakthrough curves were measured for a mixture of C2H4/CO2 (80/20, v/v) at 298 K, with outlet C2H4 purity of almost 100%, exhibiting the significant separation property of CO2 over C2H4.
In a subsequent study, a family of rare earth MOFs (named RE-PCP) were designed with various flexibility from an acrylamide-modified ligand for CO2/C2H6 and CO2/C2H4 separation [88]. At 195 K, Y-PCP and Ho-PCP showed fully reversible type I isotherms for CO2 and C2H4 with higher CO2 capacity than C2H4. However, the adsorption of CO2 in La-PCP showed a “gate opening” phenomenon, and at around 80 kPa, the second uptake occurred. In contrast, even at pressures up to 100 kPa, there was almost no absorption for C2H4 or C2H6 in La-PCP. The total CO2 uptake by La-PCP was approximately 34 mL/g at 273 K, 150 kPa, whereas the adsorption of C2H4 and C2H6 was almost absent before 150 kPa, indicating that La-PCP is a potential candidate for CO2 capture from CO2/C2H4 and CO2/C2H6 mixtures.
Qc-5-Cu has been demonstrated to be efficient in CO2/C2H4 separation, with high thermal and water stability [91]. The pore diameter of Qc-5-Cu was estimated to be approximately 3.3 Å, which is similar to the molecular dimensions of CO2 (3.3 Å) but smaller than those of C2H4 (4.16 Å). Thus, Qc-5-Cu could molecularly sieve CO2 from C2H4 to achieve a high selectivity of 39.95 for a CO2/C2H4 (v/v, 1:99) mixture. The saturation capacity of CO2 also reached 2.48 mmol/g. The Qst value for CO2 and C2H4 was calculated to be 36.2 kJ/mol and 23.1 kJ/mol, respectively, indicating the higher CO2 adsorption intensity. However, these results are considerably lower than those of MOFs with OMSs, leading to lower regeneration energy costs.
[Co(pysa)(H2O)]n (YAU-7) was synthesized from H2pysa (2-(pyrid-4′-yl)-benzimidazole-5-carboxylic acid) with superior separation performance for CO2, C2H2 and C2H4 [92]. Among these four gases, YAU-7 was found to be most favorable for the adsorption of CO2. Importantly, YAU-7 had a high selectivity of 36.8, 80.7 and 31.6 for CO2/CH4, CO2/C2H2 and CO2/C2H4 (v/v, 50/50) mixtures at 298 K and 100 kPa, respectively. However, its CO2 capacity was only 1.53 mmol/g under 298 K, 100 kPa.
The SU-101(M) series MOFs, which comprise Bi, In, Ga and Al, have a small pore size of approximately 6.8 Å and an abundance of carbonyl oxygen atoms in the channel, which could act as Lewis basic sites to bind acidic CO2 [82]. Each metal center in SU-101(M) was coordinated to one coordinated H2O molecule. Compared to the fully activated phase SU-101(M)-a, which did not contain coordinated H2O, the presence of coordinated H2O in SU-101(M) strengthened the interaction between the C atom in CO2 and carbonyl O with more electrons, whereas it weakened interactions with both C2H2 and C2H4. Therefore, SU-101(M) showed a high CO2 capacity of 40.2~53.7 mL/g (1.79~2.4 mmol/g) and a high selectivity of 5.5~15.5. The Qst values of CO2 were also obviously larger than those of C2H2 and C2H4.
The kinetic diameters and polarizabilities of C2H4 and C2H6 differ more from those of CO2 compared to C2H2, which is why many MOFs can efficiently separate CO2 based on these differences. Future research should focus on further increasing the adsorption capacity of CO2 and reducing the energy consumption for regeneration. Moreover, the efficient capture of CO2 from multiple C1-C2 mixtures is also very attractive.
In addition to C1 and C2 components, gaseous light hydrocarbons also include C3 and C4 components, such as propylene (C3H6), propane (C3H8), butene (C4H8), etc. Their physical properties differ more significantly from those of CO2, which are easier to separate. Therefore, CO2 capture from C3 and C4 mixtures has not attracted much attention from researchers and is outside the scope of this review.

5. CO2-Selective Capture from Multicomponent Light Hydrocarbon Mixtures

Porous materials capable of recognizing CO2 with high selectivity and specificity from a more complex system have always been a goal pursued by researchers; therefore, attention needs to be paid to the development of MOFs that capture CO2 from multiple components.
Some MOFs with CO2/C2H2 inverse selectivity mentioned in Section 2 can also exhibit a higher capacity for CO2 than some other light hydrocarbons. For instance, Cd-NP was reported to selectively adsorb CO2 over C2H2 through optimization of the ESP of the pore surface; moreover, with a narrow pore size of 3.2 Å, it did not adsorb larger molecules at all, such as C2H4 and C2H6 [75]. The presence of coordinated H2O in SU-101(M) strengthened the CO2 adsorption but reduced both C2H2 and C2H4 adsorption. Taking SU-101(Al) as an example, the equimolar CO2/C2H2 and CO2/C2H4 selectivities were 15.5 and 8.3 at 298 K 100 kPa, respectively, showing the potential of SU-101(Al) for CO2 capture from C2H2 and C2H4.
[Zn(odip)0.5(bpe)0.5] is a flexible MOF with a gate-opening effect for efficient purification of C1-C2 hydrocarbons and CO2 capture [54]. At 298 K, a gate-opening behavior for CO2 was clearly observed. The adsorption of CO2 first rapidly reached 55.0 mL/g at 20.7 kPa and then suddenly increased to 118.7 mL/g (5.30 mmol/g) until 100 kPa. For C1 and C2 hydrocarbons, no clear gate-adsorption behaviors occurred, but lower adsorption was observed at all temperatures. Owing to the gate-opening behavior for CO2, the selectivities for equimolar mixtures of [Zn(odip)0.5(bpe)0.5] were 376.0 for CO2/CH4, 13.2 for CO2/C2H2, 26.2 for CO2/C2H4 and 27.9 for CO2/C2H6 at 298 K, 100 kPa.
However, practical multicomponent breakthrough experiments to demonstrate separation performance are lacking in these studies. Qazvini et al. provided MUF-16-M (M = Co, Mn, Ni)-type MOFs from 5-aminoisophthalic acid (H2aip) [51]. The pore chemistry of MUF-16-M was precisely designed with complementary size (3.6 × 7.6 Å) and electrostatic potential to CO2. Single-crystal XRD and DFT calculations indicated that the O atoms of CO2 could form N-H···O and C-H···O hydrogen bonds with the -NH2 and phenyl groups, whereas the electropositive C atom contacted the O atom from the two uncoordinated carboxyl groups. The adsorption of N2O with similar size and electrostatic distribution on MUF-16 confirmed this binding mode. The uptake capacities of MUF-16 were 47.78 mL/g for CO2, 1.20 mL/g for CH4, 3.99 mL/g for C2H2, 3.17 mL/g for C2H4, 3.06 mL/g for C2H6, 5.35 mL/g for C3H6 and 4.82 mL/g for C3H8. For equimolar CO2/CH4 and CO2/C2H2 mixtures, the selectivities were as high as 6690 and 510 at 293 K and 1 atm, respectively. MUF-16 successfully captured CO2 from CO2/CH4/C2H6/C3H8 (15/80/4/1) mixtures at 1.1 bar in a breakthrough experiment with steep elution profiles (Figure 11). On account of these features, MUF-16 is a promising physical adsorbent for direct capture of CO2 from multicomponent light hydrocarbon mixtures.
The study of CO2 capture from multiple hydrocarbon mixtures is still at the beginning stage, and it is expected to be a hot research topic, as it is very difficult but urgently needed. Finely tuning the pore chemistry or developing flexible MOFs with gate-opening behavior for CO2 represent very promising strategies. We expect more valuable research to explore how to specifically recognize CO2 and extend it to other CO2 mixture systems.

6. Outlook

Capturing CO2 from various gas mixtures is a necessary prerequisite for subsequent CO2 reduction, conversion, utilization or storage technologies. From this perspective, the issue of CO2 capture has been recognized as one of the major challenges of the 21st century. With further understanding of the pore chemistry, structural flexibility and host–guest interaction of MOFs, additional progress has been made in the field of CO2 capture from light hydrocarbon mixtures. In situ characterization and theoretical computational studies have revealed the mechanisms of selective CO2 recognition behavior at the molecular level, providing an important impetus for the development of MOFs.
Although MOF chemistry has recently made considerable progress, there are several aspects of practical applications that remain challenging and deserve further exploration in the future:
(1)
Elimination of the trade-off between CO2 uptake capacity and selectivity. This problem is more evident in CO2 capture from CO2/C2H2 mixtures, as they are extremely close in nature. To develop optimal MOFs combining high CO2 selectivity and high capacity, in the future, researchers should focus on (a) designing flexible MOFs that can transform structures to accommodate target gas to substantially improve selectivity; (b) hyperfine control of pore size, shape and environment through reticular chemistry or crystal engineering strategies; and (c) developing MOF composites or grading combination strategies to compensate for the shortcomings of individual pristine MOFs.
(2)
CO2-selective capture at trace concentrations. The CO2/hydrocarbon ratios used for experiments in most studies are 1/1, 1/2 or 1/9, whereas the initial concentration of CO2 in real hydrocarbon mixtures is typically much lower (<5%). It is extremely challenging to maintain superior selective adsorption for CO2 at such low concentrations. In this regard, flexible MOFs with gate-opening effects for CO2 at low pressure are highly promising.
(3)
CO2 selective adsorption from multicomponent hydrocarbon mixtures. Porous materials capable of recognizing CO2 in more complex systems with high selectivity and specificity are extremely challenging but sought-after for material design. A grading combination of multiple MOFs or integration of pores with multiple properties into one MOF platform may be effective approaches.
(4)
Combination of desirable separation performance with broader performance for practical applications, such as high thermal/water/mechanical stability and low regeneration energy. Compared to conventional porous materials, MOF materials have more suitable ultra-microporous-level channels for CO2 molecules, high designability and the potential to capture CO2 from more complex systems. However, many MOFs are not as stable as porous carbon materials in terms of moisture stability and thermal stability. The introduction of open metal sites should be avoided, owing to their typically high activation temperature, regeneration energy consumption and poor water stability. An increasing number of researchers are employing strategies such as the construction of robust coordination geometries to build stable MOF materials.
(5)
Further reduction in economic and energy costs (from precursors, solvents, synthesis temperatures, activation conditions, etc.). Unaffordable raw materials and severe synthesis or activation conditions have become one of the most challenging issues in scaling MOFs up for practical applications.
(6)
Large-scale synthesis and industrialization. Taking technology from the laboratory to industrialization is always a challenge. A good adsorbent material should be synthesized on a large scale with high purity; however, this is rarely achieved in MOF studies. Making use of non-toxic metal ions, low-cost sustainable organic linkers and solvent-recoverable or solvent-free reactions may be beneficial in industrialization.
(7)
Introducing the design strategy of MOFs into other porous materials and other gas separation applications. For instance, in reticular chemistry, COFs and HOFs feature organic frameworks similar to that of MOFs and can therefore be developed by introducing design strategies for MOFs. On the other hand, air is a complex gas mixture with a CO2 concentration of only 0.04%. If MOFs for CO2 capture from light hydrocarbons can be extended to direct CO2 capture from air, considerable ecological and economic benefits can be realized.
Overall, it should be acknowledged that MOFs have shown tremendous potential for applications in the direct CO2-selective capture and purification of light hydrocarbons, owing to their high designability. Their implementation on a large scale offers the potential for significant environmental and economic benefits. Although there are still some challenging problems, we have confidence that with the persistent efforts of researchers working in this popular field, additional progress will be made in developing CO2-selective MOFs for light hydrocarbon purification.

Author Contributions

Conceptualization, Y.G. and H.H.; writing—original draft preparation, H.H.; writing—review and editing, Y.G., L.W., X.Z. and H.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Key Research and Development Program of China (2022YFE0110500), the National Natural Science Foundation of China (21906120), the Shanghai Pujiang Program (NO. 21PJ1412600) and the Fundamental Research Funds for the Central Universities of China (22120220153).

Institutional Review Board Statement

Not applicable.

Data Availability Statement

Not applicable.

Acknowledgments

This work was supported by the National Key Research and Development Program of China (2022YFE0110500), the National Natural Science Foundation of China (21906120), the Shanghai Pujiang Program (NO.21PJ1412600) and the Fundamental Research Funds for the Central Universities of China (22120220153).

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Schematic diagram of the process and typical strategies of CO2-selective capture from light hydrocarbon mixtures by metal-organic frameworks.
Figure 1. Schematic diagram of the process and typical strategies of CO2-selective capture from light hydrocarbon mixtures by metal-organic frameworks.
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Figure 2. (a) Pore diameter tuning by supramolecular isomerism. (b) CO2 (circle) and CH4 (star) sorption isotherm for Qc-5-Cu-dia (black) and Qc-5-Cu-sql-β (red) at 293 K. (c) The binding sites in the CO2-loaded Qc-5-Cu-dia (left) and Qc-5-Cu-sql-β (right). Color code: C (gray), H (white), Cu (brown), N (blue), O (red). Reproduced with permission from Chen et al., Angewandte Chemie International Edition; published by John Wiley and Sons, 2016 [42].
Figure 2. (a) Pore diameter tuning by supramolecular isomerism. (b) CO2 (circle) and CH4 (star) sorption isotherm for Qc-5-Cu-dia (black) and Qc-5-Cu-sql-β (red) at 293 K. (c) The binding sites in the CO2-loaded Qc-5-Cu-dia (left) and Qc-5-Cu-sql-β (right). Color code: C (gray), H (white), Cu (brown), N (blue), O (red). Reproduced with permission from Chen et al., Angewandte Chemie International Edition; published by John Wiley and Sons, 2016 [42].
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Figure 3. (a) Isoreticular transformation by symmetry-upgrading Cu clusters. Cu clusters and structures of FZU (1, 4), NJU-Bai34 (2, 5) and NJU-Bai35 (3, 6). (b) CO2 adsorption isotherms measured at 298 K. (c) Breakthrough curves of NJU-Bai35 at 298 K. Reproduced with permission from Jiang et al., Journal of the American Chemical Society; published by American Chemical Society, 2018 [44].
Figure 3. (a) Isoreticular transformation by symmetry-upgrading Cu clusters. Cu clusters and structures of FZU (1, 4), NJU-Bai34 (2, 5) and NJU-Bai35 (3, 6). (b) CO2 adsorption isotherms measured at 298 K. (c) Breakthrough curves of NJU-Bai35 at 298 K. Reproduced with permission from Jiang et al., Journal of the American Chemical Society; published by American Chemical Society, 2018 [44].
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Figure 4. (a) Structure of SYSU (left), NJU-Bai7 (middle) and NJU-Bai8 (right). (b) N2 sorption isotherms for SYSU, NJU-Bai7 and NJU-Bai8 at 77 K. (c) CO2 sorption isotherms and CO2 adsorption enthalpy (insert) for SYSU, NJU-Bai7 and NJU-Bai8 at 298 K. Reproduced with permission from Du et al., Journal of the American Chemical Society; published by American Chemical Society, 2013 [40].
Figure 4. (a) Structure of SYSU (left), NJU-Bai7 (middle) and NJU-Bai8 (right). (b) N2 sorption isotherms for SYSU, NJU-Bai7 and NJU-Bai8 at 77 K. (c) CO2 sorption isotherms and CO2 adsorption enthalpy (insert) for SYSU, NJU-Bai7 and NJU-Bai8 at 298 K. Reproduced with permission from Du et al., Journal of the American Chemical Society; published by American Chemical Society, 2013 [40].
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Figure 5. (ac) Schematic diagram of the frameworks of SIFSIX-2-Cu (a), SIFSIX-2-Cu-i (b) and SIFSIX-3-Zn (c). Color code: Si (yellow), C (gray), F (light blue), N (blue), H (white). For clarity, the green structure indicates the interpenetrated network. (d) Structure diagram of interactions between the CO2 molecules and SiF62−. Color code: C (gray), F (green), N (blue), Si (yellow), O (red), H (white), Zn (purple). (e) Breakthrough curves for a CO2/CH4 (50/50) mixture for SIFSIX-2-Cu-i and SIFSIX-3-Zn at 298 K, 1 atm. Reproduced with permission from Nugent et al., Nature; published by Springer Nature, 2013 [39].
Figure 5. (ac) Schematic diagram of the frameworks of SIFSIX-2-Cu (a), SIFSIX-2-Cu-i (b) and SIFSIX-3-Zn (c). Color code: Si (yellow), C (gray), F (light blue), N (blue), H (white). For clarity, the green structure indicates the interpenetrated network. (d) Structure diagram of interactions between the CO2 molecules and SiF62−. Color code: C (gray), F (green), N (blue), Si (yellow), O (red), H (white), Zn (purple). (e) Breakthrough curves for a CO2/CH4 (50/50) mixture for SIFSIX-2-Cu-i and SIFSIX-3-Zn at 298 K, 1 atm. Reproduced with permission from Nugent et al., Nature; published by Springer Nature, 2013 [39].
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Figure 6. (a,b) Potential adsorption mechanism of [Mn(bdc)(dpe)] for CO2 (a) and C2H2 (b) at 195 K. (c,d) Adsorption isotherms of [Mn(bdc)(dpe)] for CO2 and C2H2 at 195 (c) and 273 K (d). Reproduced with permission from Foo et al., Journal of the American Chemical Society; published by American Chemical Society, 2016 [70].
Figure 6. (a,b) Potential adsorption mechanism of [Mn(bdc)(dpe)] for CO2 (a) and C2H2 (b) at 195 K. (c,d) Adsorption isotherms of [Mn(bdc)(dpe)] for CO2 and C2H2 at 195 (c) and 273 K (d). Reproduced with permission from Foo et al., Journal of the American Chemical Society; published by American Chemical Society, 2016 [70].
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Figure 7. (a,b) The Hirshfeld surface with de (ESP) in CeIV-MIL-140-4F (a) and ZrIV-MIL-140-4F (b) (gradient color from red to blue indicates changes in electron density from high to low). (c,d) Structure of CO2-loaded CeIV-MIL-140-4F and the interaction between CO2 and F atoms. (e,f) The binding sites of C2H2 in CeIV-MIL-140-4F (e) and ZrIV-MIL-140-4F (f). Reproduced with permission from Zhang et al., Angewandte Chemie International Edition; published by John Wiley and Sons, 2021 [76].
Figure 7. (a,b) The Hirshfeld surface with de (ESP) in CeIV-MIL-140-4F (a) and ZrIV-MIL-140-4F (b) (gradient color from red to blue indicates changes in electron density from high to low). (c,d) Structure of CO2-loaded CeIV-MIL-140-4F and the interaction between CO2 and F atoms. (e,f) The binding sites of C2H2 in CeIV-MIL-140-4F (e) and ZrIV-MIL-140-4F (f). Reproduced with permission from Zhang et al., Angewandte Chemie International Edition; published by John Wiley and Sons, 2021 [76].
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Figure 8. (a) PIET strategy for improving the uptake ratio of CO2/C2H2. (b,c) Adsorption isotherms of C2H2 and CO2 before (b) and after (c) irradiation at 273 K. Reproduced with permission from Cai et al., Angewandte Chemie International Edition; published by John Wiley and Sons, 2021 [79].
Figure 8. (a) PIET strategy for improving the uptake ratio of CO2/C2H2. (b,c) Adsorption isotherms of C2H2 and CO2 before (b) and after (c) irradiation at 273 K. Reproduced with permission from Cai et al., Angewandte Chemie International Edition; published by John Wiley and Sons, 2021 [79].
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Figure 9. Schematic diagram of the “opposite action” strategy to improve CO2 selectivity. (a) Different orientations of CO2 and C2H2 into the channel. (b) Precise steric arrangement of interaction sites providing an enhanced CO2–framework interactions and inhibiting C2H2 adsorption. Reproduced with permission from Gu et al., Angewandte Chemie International Edition; published by John Wiley and Sons, 2021 [74].
Figure 9. Schematic diagram of the “opposite action” strategy to improve CO2 selectivity. (a) Different orientations of CO2 and C2H2 into the channel. (b) Precise steric arrangement of interaction sites providing an enhanced CO2–framework interactions and inhibiting C2H2 adsorption. Reproduced with permission from Gu et al., Angewandte Chemie International Edition; published by John Wiley and Sons, 2021 [74].
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Figure 10. (a,b) The structures and rhombic channel of ZU-610 (a) and ZU-610a (b). (c,d) The 1D channel of ZU-610 (c) and ZU-610a (d). (e) Sorption isotherms of C2H2 and CO2. (f) Gas adsorption of C2H2 and CO2 versus time at 298 K, 1 bar. Reproduced with permission from Cui et al., Angewandte Chemie International Edition; published by John Wiley and Sons, 2022 [81].
Figure 10. (a,b) The structures and rhombic channel of ZU-610 (a) and ZU-610a (b). (c,d) The 1D channel of ZU-610 (c) and ZU-610a (d). (e) Sorption isotherms of C2H2 and CO2. (f) Gas adsorption of C2H2 and CO2 versus time at 298 K, 1 bar. Reproduced with permission from Cui et al., Angewandte Chemie International Edition; published by John Wiley and Sons, 2022 [81].
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Figure 11. (a) The adsorption/desorption isotherms for CO2 at 293 K. (b) IAST selectivities of MUF-16 for different mixtures (50/50) at 293 K. (c,d) The adsorption sites (c) and arrangement (d) of adsorbed CO2 molecules in MUF-16(Mn). Color codes: Mn, lilac; N, blue; O, red; C, gray; H, light pink or white; pore surface, orange. (e,f) Breakthrough experiment curves for CO2/C2 (50/50) (e) and CO2/CH4/C2H6/C3H8 (15/80/4/1) (f) at 293 K and 1.1 bar for MUF-16. Reproduced with permission from Qazvini et al., Nature Communications; published by Springer Nature, 2021 [51].
Figure 11. (a) The adsorption/desorption isotherms for CO2 at 293 K. (b) IAST selectivities of MUF-16 for different mixtures (50/50) at 293 K. (c,d) The adsorption sites (c) and arrangement (d) of adsorbed CO2 molecules in MUF-16(Mn). Color codes: Mn, lilac; N, blue; O, red; C, gray; H, light pink or white; pore surface, orange. (e,f) Breakthrough experiment curves for CO2/C2 (50/50) (e) and CO2/CH4/C2H6/C3H8 (15/80/4/1) (f) at 293 K and 1.1 bar for MUF-16. Reproduced with permission from Qazvini et al., Nature Communications; published by Springer Nature, 2021 [51].
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Table 1. Main physicochemical properties of CO2 and common light hydrocarbons [24,25].
Table 1. Main physicochemical properties of CO2 and common light hydrocarbons [24,25].
PropertyCO2CH4C2H2C2H4C2H6
Kinetic diameter (Å)3.303.763.304.164.44
Boiling point (K)194.7111.6189.3169.4184.5
Polarizability
(×10−25 cm3)
29.1126.033.3–39.342.5244.3–44.7
Quadrupole moment
(×1026 esu cm2)
−4.3007.501.50.65
Table 2. Representative MOFs for CO2-selective capture from CO2/CH4 mixtures *.
Table 2. Representative MOFs for CO2-selective capture from CO2/CH4 mixtures *.
MOFFunctional SiteCO2 Capacity (mmol/g)CO2/CH4
Selectivity
Qst for CO2 (kJ/mol)Ref.
MOF-508bquadrupole interactions1.78 a3–6 a14.9[33,34]
ZIF-78Dipole–quadrupole interactions2.3210.6 b——[35]
Mg-MOF-74open metal sites8.61873[36]
MAF-66uncoordinated N atoms4.415.826[37]
SIFSIX-1-CuSiF62−5.210.527[38]
SIFSIX-2-Cu-iSiF62−5.43331.9[39]
SIFSIX-3-ZnSiF62−2.5423145[39]
SYSUnarrow channels3.114.728.2[40]
NJU-Bai7narrow channels2.9114.1 c40.5[40]
NJU-Bai8uncoordinated N atoms2.5740.8 c37.7[40]
PEI-incorporated amine-MIL-101(Cr)amine groups3.6931——[41]
Qc-5-Cu-sql-βmolecular sieving2.16 d330036[42]
SIFSIX-14-Cu-imolecular sieving4.7146.7 e37.7[43]
NJU-Bai35molecular sieving3.12511.633.37[44]
dptz-CuTiF6TiF62−4.52——33.3[28]
dptz-CuSiF6SiF62−4.04——38.2[28]
TIFSIX-3-NiTiF62−2.21315850.0[45]
NbOFFIVE-1-NiNbOF52−2.30836654.0[45]
TIFSIX-2-Cu-iTiF62−4.2291635.8[45]
ZU-66molecular sieving4.5613635[46]
IRH-3uncoordinated N atoms2.727——[47]
In(aip)2molecular sieving and –NH2 groups1.27180834.3[48]
UTSA-280molecular sieving3.00molecular sieving42.9[49]
UiO-66(N10%-Zr)uncoordinated N atoms and kinetic effect2.132635.7[50]
MUF-16N-H···O and C-H··O2.13 d6690 d32.3[51]
MUF-16 (Mn)N-H···O and C-H··O2.25 d470 d36.6[51]
MUF-16 (Ni)N-H···O and C-H··O2.13 d1220 d37.3[51]
Cu-F-pymomolecular sieving1.61 f>10729.1[52]
[Cu33-OH)(PCA)3]open metal sites2.9315.931.5[53]
[Zn(odip)0.5(bpe)0.5]gate opening5.3376.042.3[54]
* Unless otherwise specified, the capacity data were all recorded at 1 bar, 298 K; CO2/CH4 selectivity is the ideal adsorbed solution theory (IAST) selectivity for 50/50 (v/v) CO2/CH4 mixtures at 1 bar, 298 K; Qst for CO2 is the value at zero coverage; “——” indicates that data were not found; a at 303 K; b Henry’s Law selectivity; c separation ratios at 273 K; d at 293 K; e calculated by the uptake ratio of CO2/CH4 at 1 bar; f calculated by (volumetric uptake)/(crystal density).
Table 3. Representative MOFs for CO2-selective capture from CO2/C2H2 mixture *.
Table 3. Representative MOFs for CO2-selective capture from CO2/C2H2 mixture *.
MOFFunctional SiteCO2 Capacity (mmol/g)CO2/C2H2
Selectivity
Qst for CO2 (kJ/mol)Ref.
Co(HLdc)gate opening10.69 a1.7 a,b——[69]
[Mn(bdc)(dpe)]gate opening2.17 c8.8 c29.5[70]
SIFSIX-3-NiSiF62−2.77.69 d50.9[71]
CD-MOF-1uncoordinated primary hydroxyl groups2.876.6 d41.0[72]
CD-MOF-2uncoordinated primary hydroxyl groups2.6516 d67.2[72]
[Tm2(OH-bdc)] (1a)OH groups5.8317.5 d45.2[73]
[Tm2(OH-bdc)] (1a′)OH groups6.211.65 d32.7[73]
PCP-NH2-bdcamino group3.034.434.57[74]
PCP-NH2-ipaamino group3.216.436.6[74]
Cd-NPelectrostatic potential2.598527.7[75]
CeIV-MIL-140-4Felectrostatic potential2.249.539.5[76]
Cu-F-pymoelectrostatic potential1.1910528.8[77]
[Zn(atz)(BDC-Cl4)0.5]nelectrostatic potential0.94 f,g2.4 f32.7[78]
PMOF-1(irra)electrostatic potential2.38 c694 c——[79]
MUF-16electrostatic potential2.13 e510 e32.3[51]
MUF-16(Mn)electrostatic potential2.25 e31 e36.6[51]
MUF-16(Ni)electrostatic potential2.13 e46 e37.3[51]
en-MOFamine groups4.8——71.2[80]
nmen-MOFamine groups4.55——62.3[80]
een-MOFamine groups4.9——68.8[80]
ZU-610akinetic sieving1.5120727.3[81]
SU-101(Bi)carbonyl oxygen atoms2.45.530.5[82]
SU-101(Al)carbonyl oxygen atoms2.3715.531.3[82]
SU-101(In)carbonyl oxygen atoms2.466.228.3[82]
SU-101(Ga)carbonyl oxygen atoms1.7911.127.7[82]
[Zn(odip)0.5(bpe)0.5]Gate Opening5.313.242.3[54]
* Unless otherwise specified, the capacity data were all recorded at 1 bar, 298 K; CO2/C2H2 selectivity is IAST selectivity for 50/50 (v/v) CO2/C2H2 mixtures at 1 bar, 298 K; Qst for CO2 is the value at zero coverage; “——” indicates that data were not found; a at 195 K; b calculated by uptake ratio; c at 273 K; d selectivity for CO2/C2H2 (1/2, v/v); e at 293 K; f at 285 K; g calculated by (volumetric uptake)/(crystal density).
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Huang, H.; Wang, L.; Zhang, X.; Zhao, H.; Gu, Y. CO2-Selective Capture from Light Hydrocarbon Mixtures by Metal-Organic Frameworks: A Review. Clean Technol. 2023, 5, 1-24. https://doi.org/10.3390/cleantechnol5010001

AMA Style

Huang H, Wang L, Zhang X, Zhao H, Gu Y. CO2-Selective Capture from Light Hydrocarbon Mixtures by Metal-Organic Frameworks: A Review. Clean Technologies. 2023; 5(1):1-24. https://doi.org/10.3390/cleantechnol5010001

Chicago/Turabian Style

Huang, Hengcong, Luyao Wang, Xiaoyu Zhang, Hongshuo Zhao, and Yifan Gu. 2023. "CO2-Selective Capture from Light Hydrocarbon Mixtures by Metal-Organic Frameworks: A Review" Clean Technologies 5, no. 1: 1-24. https://doi.org/10.3390/cleantechnol5010001

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