Post-combustion carbon capture and conversion using advanced materials of Zn-based metal-organic frameworks: A mini review

Abstract

Developing environmentalyl friendly and energy-efficient CO₂ adsorbents for post-combustion capture is a critical step toward achieving toward carbon neutrality. While aqueous amines and metal oxides have play pivotal roles in CO₂ capture, their application is limited by issues such as secondary pollution and high energy consumption. In contrast, Zn-based metal-organic frameworks (Zn-based MOFs) have emerged as a green alternative, offering low toxicity reduced regeneration temperatures, and high efficiency in both CO₂ adsorption and catalytic conversion into valuable fuels and chemicals. This mini review begins with a general introduction to MOFs in CO₂ capture and conversion, followed by an overview of early studies on Zn-based MOFs for CO₂ capture. It then summarizes recent research advancements in Zn-based MOFs for integrated CO₂ capture and conversion. Finally, it discusses key challenges and future research directions for post-combustion CO₂ capture and conversion using Zn-based MOFs.

Graphical abstract

Keywords

Zn-based metal-organic frameworks (Zn-based MOFs) / post-combustion capture / CO₂ reduction reaction (CO₂RR)

1 Introduction

As of May 2024, 151 countries have pledged their respective carbon neutrality targets, with 120 of them having formalized their commitments through laws or policy measures. Although zero-carbon technologies such as renewable energy generation, electric vehicles, and green hydrogen, continue to advance, a remarkable gap remains between current zero-carbon capacity and the scale required to achieve climate goals. Furthermore, global CO₂ emissions are still increasing [1]. Carbon capture, utilization, and storage (CCUS) is recognized as prospective solution for carbon neutrality and principal pathway for a green future [2,3].

The main strategies for CO₂ capture include oxy-fuel combustion, pre-combustion capture, and post-combustion capture. Among the various CO₂ emissions sources, industrial processes contribute the most to global CO₂ emissions. Capturing CO₂ directly from flue gas—without requiring significant alterations to existing infrastructure—could significantly reduce associated costs, making post-combustion capture a particularly attractive option [4].

To date, aqueous amines and metal oxides have been the most widely used materials for CO₂ adsorption. However, aqueous amines commonly operate below 100 °C and suffer from issues such as corrosiveness and secondary pollution [5] while metal oxides often exhibit poor cycling performance and entail additional energy input for CO₂ release [6].

Metal-organic frameworks (MOFs), a class of porous crystalline materials composed of metal ions or metal clusters coordinated with organic linkers through coordination bonds, offer a promising alternative [7]. Thanks to their high surface areas, tunable structures, and abundant functional sites, MOFs have been extensively investigated for CO₂ capture [8].

While many reviews have addressed MOF-based CO₂ capture from flue gas [4,8–11], few have focused specifically on Zn-based MOFs for post-combustion capture and conversion. Moreover, recent advances by Long and coworkers in high-temperature CO₂ capture represent a transformative leap forward [12], yet these breakthroughs have not been comprehensively covered in the existing literature.

To addresses these gaps, this review summarizes the characteristics, stability, and synthesis methods of MOFs for CO₂ capture and conversion. It traces the development of Zn-based MOFs for post-combustion CO₂ capture, analyzes their current status in electrochemical CO₂ conversion, and discusses key challenges and future directions. By integrating insights across Zn-based MOFs, post-combustion capture, and CO₂ electrochemical conversion, this review aims to provide a valuable reference for advancing efficient CCUS technologies (Fig.1).

2 General introduction to MOFs in CO₂ capture and conversion

This section presents a comprehensive overview of MOFs in CO₂ capture and conversion, focusing on the fundamental characteristics that govern their adsorption performance, the stability challenges these materials they encounter under practical operating conditions, and the diverse synthesis strategies employed to produce both pristine MOFs and their derived nanostructures. This discussion establishes a foundational understanding of how MOFs design and processing critically influence their effectiveness in carbon capture and conversion applications.

2.1 Characteristics of MOFs with excellent CO₂ capture capabilities

MOFs employed for CO₂ capture can be broadly classified into two categories: those utilizing physical adsorption sites and those relying on chemical adsorption sites.

Physical adsorption involves mechanisms primarily governed by electrostatic interactions, along with other weak forces such as hydrogen bonding and van der Waals forces. The nature of CO₂ interaction with MOFs during this process is largely determined by factors including the characteristics of the binding sites, pore dimensions, and the flexibility of the framework [13].

MOFs typically exhibit exceptionally strong physical adsorption capabilities toward CO₂ under the following conditions:

Open metal sites: In MOFs with open metal sites, unsaturated metal ions with high charge density function as Lewis acids, forming strong electrostatic bonds with the O^δ- atoms in CO₂, thereby improving adsorption performance [14].

Incorporation of inorganic fluorometallate anions: MOFs incorporating inorganic fluorometallate anions constitute a unique class of reticular frameworks, in which metal cations (e.g., Zn²⁺) are coordinated and charge-balanced by fluorometallate anions. The elevated charge density of the oxygen and fluorine atoms in these anions promotes strong CO₂ binding [15].

Ultramicropores: MOFs with ultramicroporous structures can significantly enhance CO₂ uptake. This is because weak hydrogen bonds and van der Waals interactions play an important role.

The intrinsic flexibility of MOFs impacts their adsorption behavior. Dynamic frameworks often exhibit S-shaped isotherms during both CO₂ uptake and release, leading to increased working capacity and reduced desorption pressure required for effective CO₂ capture. However, it only works under specific concentration ranges [16].

Chemical adsorption refers to an adsorption mechanism governed by chemical bonds. Studies have shown that the introduction of strong Lewis base functional groups as chemical adsorption sites on pore surfaces can significantly improve CO₂ capture. For instance, aliphatic amines exhibit high reactivity toward CO₂, resulting in strong and selective adsorption [17].

MOFs that perform poorly in CO₂ capture often suffer from the absence of critical features such as open metal sites, Lewis base functionalities, or poorly designed pore structures. Additionally, insufficient structural stability under operating conditions further limits their practical application.

2.2 Stability of MOFs

The durability of MOFs is crucial for their long-term application in demanding CO₂ capture conditions. Flue gas generally contains 73%–77% N₂, 15%–16% CO₂, 5%–7% H₂O, 3%–4% O₂, and trace amounts of SO_x, NO_x, CO, HCl, and Hg vapor, creating a highly corrosive and complex environment for capture materials. Under such conditions, maintaining the structural integrity of MOFs, including their stability in humid air, thermal stability, and chemical stability, is a major concern for practical deployment [9].

A major limitation of MOFs for post-combustion CO₂ capture is their poor performance under humid conditions, where strong interactions between water molecules and metal nodes lead to the breaking of coordination bonds and the collapse of the framework. Additionally, H₂O can interact with acidic gases in the air, posing challenges to the chemical stability of MOFs. Section 3.1 will explore in detail the resistance of MOFs to interference from H₂O and other acid gases.

The high temperature of the flue gas, typically exceeding 200 °C, pose another challenge by accelerating the thermal degradation of MOFs. This degradation involves the cleavage of node-linker bonds, which subsequently leads to the combustion of the linker when MOFs are subjected to high temperatures. In general, the thermal stability of a MOF depends on both the bond strength between its metal nodes and organic linkers, as well as the connectivity (i.e., the number of linkers attached to each node); stronger bonds and higher connectivity generally result in improved thermal stability. When high temperature and high humidity coexist, the damage to MOFs can be significantly amplified. Besides, high temperatures cause CO₂ to preferentially desorb rather than adsorb [18]. Section 3.2 will delve further into CO₂ capture under high-temperature conditions.

Chemical stability refers to a MOF’s ability to retain its structure under harsh aqueous conditions, including acidic and basic conditions. Acidic or basic substance can significantly impact both the adsorption and catalytic performance of MOFs. Therefore, for CCUS applications, MOFs must be chemically stable [18].

In addition to chemical, thermal, and humidity stability, mechanical stability is also an important consideration in the design of MOFs for CO₂ capture, particularly in applications such as CO₂ sensing and gas filtration. Mechanical properties are critical for MOFs used in CO₂ sensing and filtration. The fundamental framework of a MOF determines key physical attributes such as elastic moduli, compressibility, and rigidity. Although mechanical stability is not a central focus of this review, because this review does not address CO₂ sensing and filtration, it remains a relevant aspect of MOF design [19].

2.3 Synthesis methods of MOFs and MOF-derived nanostructures

Various MOF synthesis techniques have been developed to accommodate factors such as cost, time efficiency, the nature of metal nodes and organic linkers, solvent properties, and other experimental variables. These techniques include solvothermal, sonochemical, mechanochemical, microwave-assisted, electrochemical, and slow evaporation methods [20,21].

The solvothermal method is the most commonly employed technique for MOF synthesis. In this technique, insoluble metal salts and organic linkers are heated in aqueous or alternative solvents (e.g., ethanol, DMF) at elevated temperatures over several days. For instance, when DMF is used as the solvent, its thermal decomposition produces an amine base that dehydrogenates the organic linker, thereby promoting MOF formation. This approach has been successfully commercialized by companies such as BASF for the mass production of MOFs [22].

Among alternative approaches, one popular approach is sonochemical synthesis, which employs ultrasonic irradiation to expedite MOF formation, drastically shortens crystallization times compared to conventional techniques, while yielding high-quality products. Another notable approach is mechanochemical synthesis, a solvent-free method relying on grinding the reactants to enhance their contact area, thereby improving reactivity and reducing reaction times. This method not only shortens reaction time but also produce materials with relatively larger surface areas.

Less commonly used techniques include microwave-assisted synthesis, which employs microwave energy for rapid, uniform heating, enabling the production of nanoscale MOF crystals within an hour. This method accelerates MOF synthesis due to its highly efficient heating process. Electrochemical synthesis, on the other hand, involves the anodic dissolution of metal electrodes to generate metal ions in situ, which then react with organic linkers and electrolytes to form MOFs. This technique eliminates the need for added metal salts and supports continuous MOF production. Conversely, the slow evaporation method, one of the simplest techniques due to its room-temperature operation and lack of external energy requirements, suffers from a significant drawback—long synthesis times, often from several hours to days [21].

In addition to direct synthesis, MOF-derived materials obtained by using MOFs as precursors, represent an emerging class of functional materials that retain certain advantages of the parent MOFs while exhibiting new properties. For instance, MOFs can be transformed into structures where individual atoms are evenly dispersed; these frameworks lower the surface energy of the isolated atoms and keep them stable and accessible to reactants. MOF-derived nanostructures can also be used for carbon capture. There are several established methods for preparing MOF-derived nanostructures. The most common method involves heating MOF precursors in an inert atmosphere (such as nitrogen or argon) at elevated temperatures. This thermal treatment decomposes the organic ligands, leading to the formation of MOF-derived carbon and oxide [23]. By carefully controlling parameters such as temperature, heating rate, and atmosphere, the porosity, surface area, and chemical composition of the resulting nanostructures can be finely tuned.

After pyrolysis, MOF-derived carbons can be further activated using chemical agents like KOH or H₃PO₄. This treatment creates additional porosity and increases the surface area, enhancing the adsorption properties crucial for CO₂ capture. Besides, selective etching using acid or base solutions can remove specific components from the MOF-derived structure, thereby exposing active sites or generating further porosity. Additionally, post-synthetic modifications, such as heteroatom doping (e.g., nitrogen or sulfur), can further tailor the chemical and catalytic properties of these nanostructures [24].

An alternative strategy is template-assisted synthesis, in which a sacrificial soft or hard template is introduced during MOF synthesis. Upon subsequent removal via chemical etching or calcination, a well-defined porous structure remains. This method allows for precise control over the morphology and pore architecture of the resulting nanostructured materials [25]. Finally, hydrothermal or solvothermal post-treatments can be applied after thermal conversion to further modify the morphology and crystallinity of the derived nanostructures, offering an additional means of optimizing the surface chemistry and structure of MOF-derived nanomaterials for CO₂ capture and conversion applications [25].

3 Zn-based MOFs for post-combustion capture

In 2005, Rosi et al. [26] first reported Zn₂(dobdc) (dobdc^4- = 2,5-dioxido-1,4-benzenedicarboxylate), also known as MOF-74, in which the metal ions constitute linear, infinite-rod secondary building units, and the organic linkers connect these units to form a hexagonal, one-dimensional pore structure. Upon removal of solvent molecules (usually H₂O or N,N-dimethylformamide) from the pores, open metal sites are generated that show strong coordination interactions with CO₂ molecules. Therefore, MOF-74 possesses high selectivity for CO₂ under low pressure conditions [27].

Based on this framework, Mcdonald et al. [28] synthesized isostructural analogs of MOF-74 using longer ligand, resulting in M₂(dobpdc) (M = Zn, Mg; dobpdc^4- = 4,4′-dioxidobiphenyl-3,3′-dicarboxylate) which feature enlarged pores. The CO₂ adsorption capacity of these frameworks increased significantly after introducing N, N’-dimethylethylenediamine (mmen) into the pores [29]. In this configuration, mmen are bound to M(II) through one amine group, leaving the other amine group exposed on the surface of the framework. CO₂ can then reversibly insert into the metal-amine bond under reasonable pressure, resulting in extremely high selectivity [29].

Additionally, other Zn-based MOFs have also been developed for CO₂ capture. For instance, Liao et al. [30] utilized H₂btm (bis(5-methyl-1H-1,2,4-triazol-3-yl) methane) to synthesize Zn₂(btm)₂, designated as MAF-23. This structure exploits two pairs of nitrogen atoms on the triazole rings to chelate CO₂, thereby enhancing both the adsorption heat and selectivity of CO₂/N₂.

Previous studies on Zn-based MOFs have primarily focused on evaluating the adsorption capacity of CO₂ and the adsorption selectivity of CO₂/N₂ for Zn-based MOFs at low pressures. However, significant challenges remain in the industrial application of these findings. The primary obstacle lies in the complex composition of flue gas emitted from industrial plants, which in addition to N₂, includes various acidic gases and water vapor (H₂O). The structural and chemical stability of MOFs in the presence of these impurities needs further investigation and validation.

Another critical limitation is the temperature of flue gas, which typically exceeds 200 °C, far above the optimal operating temperature range for most reported MOFs (Fig.2). Consequently, research in the past five years has increasingly focused on addressing these two important issues: improving MOF stability under humid and chemically aggressive environments, and enhancing their performance at elevated temperatures.

3.1 Anti-interference

The anti-interference performance of MOFs is a critical factor that plays a vital role in CO₂ capture in harsh environments [31]. In most cases, acidic gases and water vapor (H₂O) in flue gases are common sources of interference that can significantly affect MOF performance [32]. In this context, Dunstan et al. [33] synthesized Zn₂(1,2,4-triazolate)₂(oxalate) with ultramicropores, denoted as CALF-20, using a solvothermal method. CALF-20 shows a CO₂ adsorption capacity of 4.07 mmol/g at 1.2 bar and 293 K, and a CO₂/N₂ selectivity of 230 for a 10:90 CO₂/N₂ mixture. Moreover, CALF-20 exhibits preferential CO₂ physisorption and suppresses H₂O adsorption by CO₂ at relative humidity (RH) below 40%, due to the fact that CO₂ has a strong binding site in the center of CALF-20, which prevents the formation of hydrogen-bonded network at high RHs, a key factor for H₂O uptake. Furthermore, CALF-20 shows extraordinary chemical stability, with only a 1.3% loss in CO₂ capacity after 6 days in real flue gas. Notably, the raw materials (oxalic acid and triazole) and solvents (H₂O and < 25 wt% methanol) used in the synthesis of CALF-20 are low-cost bulk chemicals. With a solid content of > 35%, a yield of > 90% and an acceptable reaction time, CALF-20 achieves a space-time yield of 550 kg/m³ per day, significantly higher than zeolite (50–150 kg/m³). The two points contribute to promise for applications of CALF-20 in industrial activities [33].

Following the development of CALF-20, several improvement strategies have been implemented. Recognizing that H₂O disrupts CO₂ capture at RH levels above 40%, Wang et al. [34] introduced methyl groups as hydrophobic moieties, resulting in the creation of CALF-20M-w and CALF-20M-e (Fig.3(a)). These modified materials show CO₂ adsorption capacities ranging from approximately 1.81 to 2.13 mmol/g at 0.15 bar and room temperature, with negligible N₂ adsorption. Both materials retain approximately 20% of their initial CO₂ capture efficiency at 70% RH, where CALF-20 is ineffective. Grand canonical Monte Carlo simulations confirm that the methyl groups in the pores block the formation of the hydrogen-bonded network, which enhances selectivity of CO₂ at high RHs [34].

Moreover, Feng et al. [35] synthesized spherical CALF-20@poly(acrylate) composite hydrophobic materials using an in situ crystallization method. This composite material exhibits a CO₂ adsorption capacity of 2.15 mmol/g at 313 K and 0.1 bar. At 10% CO₂, 90% N₂ and 90% RH, its capacity decreases by less than 3%. Gopalsamy et al. [36] fabricated a series of linker-substituted ultramicroporous CALF-20, also referred to as SquCALF-20 through silico design. Compared to CALF-20, SquCALF-20 adsorbs H₂O at lower RH but maintains a CO₂ capacity of approximately 4 mmol/g at 0.2 bar and room temperature when RH is beneath 20%. It also demonstrates a high CO₂/N₂ selectivity of 500 under a 15% CO₂, 85% N₂ mixture, representing a significant improvement in performance over the pristine CALF-20.

Apart from these targeted improvements in CALF-20’s ability to handle impurities [34–36] and the studies on its interaction with CALF-20 and H₂O [34,37–39], other investigations concerning the broader anti-interference potential of MOFs are also in progress. For example, Boyd et al. [40] screened 325000 hypothetical MOFs for CO₂ adsorption capacity and CO₂/N₂ selectivity, identifying 8325 MOFs with CO₂ adsorption capacity exceeding 2 mmol/g and CO₂/N₂ selectivity over 50 (Fig.3(b)). Through comparative analysis, three structures were found which can provide ideal binding sites for CO₂: Two parallel aromatic rings with interatomic spacings of approximately 7 Å (A1), metal-oxygen-metal bridges (A2), and unsaturated metal sites (A3). A secondary screening of these three types of structures for affinity to H₂O (Fig.3(c)) revealed that MOFs with the A1 structure have lower Henry coefficients, demonstrating superior hydrophobicity (Fig.3(d)), while MOFs with A2 and A3 structures are relatively hydrophilic [40].

Younas et al. [9] provided a comprehensive review of strategies to enhance the stability of MOFs in harsh environments, identifying the strength of node-linker bounds, the number of linkers per node, and hydrophobicity of surface as key factors^[41] affecting the stability of MOFs.

Additionally, Shi et al. [41] proposed a strategy that attaches amino groups to triazolyl linkers in MOFs, improving their chemical stability under aqueous, acidic, and alkaline conditions (Fig.3(e)). ZnF(daTZ), synthesized using this approach, shows a CO₂ adsorption capacity of 1.8 mmol/cm³ at 298 K and 0.15 bar, with negligible decrease in CO₂ adsorption capacity after 50 adsorption–desorption cycles under simulated flue gas conditions. Besides, ZnF(daTZ) presents a thermodynamic adsorption selectivity beyond 150 and a kinetic adsorption selectivity of 50.

3.2 High-temperature operation

Although several MOFs with stability in flue gas have been identified, their application has largely been limited to low-temperature conditions [42–44]. In contrast, Mg-based MOFs demonstrate superior performance at elevated temperatures. Zhu et al. [45] reported that diamine-appended Mg₂(olz) frameworks (olz^4-=(E)-5,5′-(diazene-1,2-diyl)bis(2-oxidobenzoate)) exhibit CO₂ adsorption steps ranging from approximately 35  to 135 °C, depending on the specific diamine used. Notably, the ee₂–Mg₂(olz) variant (ee₂=N,N-diethylethylenediamine) shows effective CO₂ uptake at 40 °C under simulated coal flue gas conditions, with desorption occurring at 85 °C, resulting in high working capacities and low regeneration energy requirements . Additionally, tetraamine-functionalized Mg₂(dobpdc) materials—particularly the Mg₂(dobpdc)(3-4-3) variant (3-4-3=spermine)—demonstrate a cooperative CO₂ adsorption mechanism. This initial adsorption step occurs near 150 °C, one of the highest reported for amine-appended materials [46]. However, while the initial adsorption occurs near 150 °C, these materials can effectively capture CO₂ from flue gas at operating temperatures around 100 °C (Fig.4(a)).

For Zn-based MOFs, most CO₂ adsorption experiments are conducted at room temperature. Although some materials show strong thermal stability (capable of withstanding temperatures above 150 °C), their operating temperature for CO₂ adsorption remains below 50 °C. For instance, CALF-20 retains its structure and adsorption properties even after repeated regeneration cycles at 150 °C. However, industrial-scale tests have shown that CALF-20 maintains robust CO₂ capture performance in simulated flue gas conditions in temperatures ranges of 45–50 °C [33]. To date, there have been no studies exploring the use of MOFs at temperatures above 150 °C [47].

Recently, Rohde and coworkers [12] reported a ZnH-MFU-4l (MFU-4l = Zn₅Cl₄(btdd)₃; H₂btdd= bis(1H-1,2,3-triazolo[4,5-b],[4′,5′-i])dibenzo [1,4]dioxin), in which ZnH-MFU-4l can selectively and reversibly capture CO₂ above 200 °C (Fig.4(b)–4(c)). This study resolves the challenges associated with low-temperature operation and high energy consumption of adsorbents via their high activation energy, making a crucial breakthrough in post-combustion CO₂ capture.

ZnCl-MFU-4l was alkylated using ZnEt₂ to produce ZnEt-MFU-4l, followed by protonolysis to obtain Zn(O₂CH)-MFU-4l. Thereafter, the intermediate was heated in an inert atmosphere to yield ZnH-MFU-4l. The transformation from ZnH-MFU-4l to Zn(O₂CH)-MFU-4l was confirmed during CO₂ adsorption. In the isothermal adsorption test, ZnH-MFU-4l demonstrated unprecedented properties: its CO₂ adsorption capacity increased with growing temperature. At 150 °C, the capacity was nearly threefold that at 25 °C (3.27 mmol/g) at 1 bar, which approximates the theoretical value of 3.3 mmol/g (Fig.4(d)). Thermodynamic calculations suggest that activation energies for adsorption are 95 kJ/mol at 25 °C and 124 kJ/mol at 275 °C (Fig.4 (e)). According to the Eyring equation, the rate constant for CO₂ insertion at 275 °C was five orders of magnitude higher than that at 25 °C, implying that high-temperature facilitates CO₂ in overcoming substantial energy barrier. This remarkable temperature-triggered mechanism clarifies why the operating temperature of this material can surpass 200 °C [48].

Despite the high-temperature operability, there are still two main challenges in capturing CO₂ directly from industrial exhaust gases: the low partial pressure of CO₂, and the interference from other gases. To evaluate the practicability of ZnH-MFU-4l, experiments were conducted at 300 °C under low-pressure conditions (200 mbar of pure CO₂ for adsorption, and 20 mbar of CO₂ for desorption). The material shows extraordinary performance: its CO₂ adsorption capacity in the first cycle was 1.24 mmol/g. After 508 cycles over 150 h, the capacity decreased by less than 4%. Moreover, the material could be regenerated in N₂ at the same temperature, indicating that no extra energy required for the adsorption–desorption cycle. Besides, the effect of H₂O and SO₂ on CO₂ adsorption was also examined. After 10 cycles, although the material maintained high selectivity for CO₂, its adsorption capacity was reduced, highlighting the need for further enhancement of its long-term stability in the presence of impurities.

4 Zn-based MOFs for electrochemical CO₂ conversion

The aforementioned studies provide carbon source for resourceful utilization of CO₂. As a frontier in mitigating climate change, electrochemical CO₂ conversion technology drives the CO₂ reduction reaction (CO₂RR) using external electrical energy, offering a crucial pathway for producing value-added products [49]. In the field of electrochemistry, in addition to being used as catalysts for hydrogen evolution reaction (HER) [50,51] and oxygen evolution reaction (OER) [52,53], certain Zn-based materials and MOFs have been recognized as effective catalysts for electrocatalytic CO₂ conversion in recent studies.

When Zn-based MOFs are used as catalysts for CO₂RR, the reaction typically initiates with the adsorption of CO₂ onto the catalytic sites. Afterward, CO₂ is converted to the *COOH intermediate by accepting one electron, which then progressively transforms into *CO. If the binding energy is sufficient, *CO will further gain six electrons to generate CH₄; otherwise, it will desorb from the surface of MOFs (Fig.5) [54]. Guided by this mechanism, Han et al. demonstrated the application of Zn₃(btc)₂ (H₃btc = 1,3,5-benzenetricarboxylic acid) deposited on carbon paper as cathodes, using imidazolium ionic liquids as the electrolytes, for CO₂RR. The results highlight that the selectivity for CH₄ can reach as high as 80% at a current density of 3 mA/cm² and an overpotential of 0.25 V. During electrolysis, Zn₃(btc)₂ adsorbs imidazolium cations and transfers electrons to CO₂, reducing CO₂ to CO after a series of reactions. Since the adsorption capacity of CO is higher than that of CH₄, CO can further be reduced by accepting 6 electrons to generate CH₄ [55].

Another noteworthy study was conducted by Kang et al. [55], who synthesized Zn(mIM)₂ (mIM = 2-methylimidazolate), also known as ZIF-8, using various zinc sources. The ZIF-8 prepared with ZnSO₄ has excellent activity for CO₂RR, achieving a Faradaic efficiency of 65% at −1.8 V versus the saturated calomel electrode. The study showed that the discrete zinc nodes contribute to the high CO₂ selectivity, as they facilitate anion exchange to maintain charge balance when the oxidation state of the Zn nodes varies. Additionally, NaCl was found be an effective electrolyte due to its ease of anion exchange [56,57].

5 Conclusions and perspectives

Aqueous amines and metal oxides have long been established as reliable adsorbents for post-combustion CO₂ capture due to their proven industrial performance. However, recent advancements in Zn-based MOFs present promising alternatives with unique advantages. These Zn-based MOFs offer high CO₂ selectivity, enabling effective separation even in complex flue gas mixtures, and are designed with environmental sustainability in mind. Their excellent recyclability ensures high capture efficiency over numerous cycles, reducing both waste and operational costs. Additionally, their low regeneration energy requirements address a major limitation of traditional adsorbents. As research progresses, Zn-based MOFs are poised to play a crucial role in developing sustainable and cost-effective carbon capture technologies, thereby contributing significantly to global greenhouse gas reduction efforts.

Despite these advantages, several challenges remain before Zn-based MOFs can replace aqueous amines and metal oxides on a large scale. First, ZnH-MFU-4l is currently the only Zn-based MOFs that functions at temperatures as high as 150 °C, but it suffers from limited anti-interference capabilities. While considerable progress has been made in improving anti-interference properties of other Zn-based MOFs, their operating temperatures remain low. Developing Zn-based MOFs capable of directly capturing CO₂ from untreated flue gas is a critical avenue for future research.

Secondly, the synthesis costs of Zn-based MOFs are generally high, making the large-scale production of these materials a key challenge. Advancing materials such as CALF-20 to be more scalable and cost-effective is crucial for broader adoption.

Thirdly, to close the carbon cycle, CO₂ capture must be integrated with CO₂ conversion technologies. While there has been significant progress in CO₂ capture, Zn-based MOFs in CO₂RR remain relatively underexplored compared to Cu-based MOFs, etc. [58–61], which deserves further development.

Finally, integrating machine learning algorithms and molecular simulations into the design process could accelerate the discovery of optimal pore architectures and active sites, helping to overcome current limitations related to operating temperature and anti-interference properties.

In addition to Zn-based MOFs, other CO₂ capture and conversion pathways are also worth exploring [62–66], particularly high-temperature solid oxide electrolysis cells (SOECs). SOECs enable CO₂ conversion using renewable electricity and residual heat, providing a promising route to reduce the carbon footprint. Besides, SOECs show outstanding flexibility and a wide range of applications. For instance, SOECs can achieve co-electrolysis of H₂O and CO₂ for creating syngas (H₂/CO). This multi-gas co-electrolysis process is extremely compatible with downstream fuel and chemical synthesis methods, such as Fischer-Tropsch synthesis, simplifying the manufacturing process [67–69].

Recent studies have also demonstrated significant improvements in the durability and efficiency of SOEC systems, which further enhance their economic viability for large-scale deployment [70]. Given these advancements, sustained research and multidisciplinary collaboration are critical to overcoming technological challenges and accelerating the transition to carbon neutrality [71–75].

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