Research Progress on CO₂ Capture and Catalytic Conversion of Metal-Organic Frameworks Materials

Abstract

The increase in CO₂ emissions has been identified as a core driving factor in the intensification of the greenhouse effect. In order to achieve the dual-carbon vision, research on CO₂ capture and its catalytic conversion is receiving growing attention. Due to the high chemical stability of CO₂ itself, traditional separation technologies find it difficult to capture it onto catalysts. Currently, using hydrocarbons as carriers for catalytic reactions is the most common and efficient method. In recent years, metal-organic frameworks (MOFs) have shown their irreplaceable importance in CO₂ capture and catalytic conversion due to their unique adjustable and controllable pore structures and multiple active sites. This study integrates various classification criteria of MOFs, proposes a cooperative mechanism between metal doping and functional groups, and also reveals the CO₂ capture and catalytic conversion processes. In addition, we have conducted an in-depth discussion on the future development of continuous-flow microreactor technology and provided performance and property relationship diagrams for multiple MOF series, offering valuable reference material for future research in related fields.

1. Introduction

The global population is growing exponentially, while modern industries are also developing rapidly. As a result, the reliance on fossil fuel combustion is quite high. With the annual average consumption of fossil fuels continuously rising, this has led to consistently high levels of greenhouse gas emissions, such as CO₂ and methane (see Figure 1) [1,2]. According to data from the Global Carbon Project (GCP), since the 1960s, global fossil fuel CO₂ emissions have steadily increased, with projections suggesting that by 2024, emissions will reach 37.4 billion tons (see Figure 2) [3,4]. Although emissions have experienced significant fluctuations during certain historical phases, such as the oil crisis of the 1970s, the global economic downturn in the early 2000s, and the large-scale COVID-19 pandemic, this trend has remained persistent. The large-scale emission of greenhouse gases has contributed to the intensification of global warming, which has further triggered an increase in extreme weather events. It is worth mentioning that global warming has led to a series of negative impacts, including but not limited to glacier melt, droughts, floods, tsunamis, increased pests and diseases, declining soil quality, as well as changes in crop growth cycles and planting regions [4,5]. It is important to emphasize that although methane has 25 times the greenhouse effect of CO₂ per kilogram, its overall emission in the atmosphere is relatively small due to its primary sources being livestock and landfills. Additionally, its presence in the atmosphere is relatively short, around 10–12 years, while CO₂ can remain in the atmosphere for centuries. Therefore, CO₂ is considered the leading cause of the greenhouse effect. According to the data released by the International Energy Agency (IEA), there has been a significant increase in the concentration of CO₂ in the atmosphere. The concentration of CO₂ has risen from approximately 280 parts per million (ppm) before the industrial revolution to over 420 ppm currently, which is a 52% increase compared to pre-industrial levels. In order to better address the various issues caused by CO₂ emissions, there is an urgent need to develop efficient CO₂ capture and catalytic conversion technologies.

In recent years, MOFs have garnered significant attention in the field of CO₂ capture and catalytic conversion, due to their distinctive structures and exceptional properties [6]. The specific surface area of certain MOFs can reach thousands of square meters per gram, providing a substantial surface area for the adsorption and storage of various substances. In the field of gas storage, these materials exhibit particularly promising adsorption capabilities for CO₂ and other gases, a property crucial in addressing the pressing issue of global warming. Concurrently, metal-organic framework (MOF) materials have demonstrated promise in the capture and conversion of CO₂ at moderate temperatures (typically 523–773 K), highlighting their potential for practical applications. The structural parameters of MOFs, such as their pore size and shape, can be meticulously tailored through the judicious selection of metal ions and organic ligands. This allows the design and synthesis of MOFs with customized pore structures, ensuring their effectiveness in adsorbing and separating substances of various molecular sizes and shapes. The presence of abundant functional groups on the surfaces of MOFs has led to their remarkable performance in catalysis and sensing. In the context of catalysis, MOFs are used as catalysts or catalyst supports, thereby enhancing selectivity and efficiency in chemical reactions. For instance, MOFs can provide numerous adsorption sites, facilitating the adsorption of CO₂. Their adaptable pore configuration can be customized through the strategic design of organic ligands to achieve selective adsorption of CO₂. These materials can achieve high efficiency in capturing CO₂ and converting it into valuable products. In addition, some MOFs have potential catalytic activity sites, with their metal ions and organic ligands serving as the active centers for the catalytic conversion of CO₂ into methanol, formic acid, carbonate, and other valuable products.

2. Metal-Organic Framework Materials (MOFs)

MOFs are self-assembled from metal ions or metal clusters and organic ligands through ligand bonding, forming crystalline materials with a regular pore structure, a highly ordered porous structure, and an ultra-high specific surface area. The unique structure of MOFs gives them the advantages of both inorganic and organic materials, and they exhibit many excellent properties. The unique structure of MOFs enables them to combine the advantages of both inorganic and organic materials. They have many excellent properties and great potential for application in various fields.

2.1. Classification of MOFs

MOFs have a variety of structural characteristics, which has led to the emergence of multiple classification methods. By adopting various classification strategies to study MOFs, we can systematically explore the interrelationship between their structure and function, thereby providing solid theoretical support for efficient material design. This paper classifies MOFs from multiple perspectives.

2.1.1. Categorized by Ligand Type

A popular series of MOFs, classified according to the ligand situation, encompasses approximately six categories, which are further divided into six series: IRMOFs structure series, ZIF structure series, MIL structure series, UIO structure series, HKUST series, and PCN structure series. The molecular structure diagrams of these materials are shown in Figure 3.

Among the popular MOFs mentioned above, the IRMOF series is a microporous crystal structure constructed in the form of an octahedron using inorganic groups and aromatic carboxylic acid ligands. The structural unit (Zn₄O) is formed through self-assembly with organic ligands and is most studied for its large specific surface area, regular pore structure, and large pore volume.

The ZIF series is characterized by a zeolite-like structure, representing a class of compounds formed with a nitrogen-containing heterocyclic ring as a ligand through the binding reaction of Zn or Co with an imidazole ligand. In comparison to other types, the coordination of nitrogen-containing heterocyclic ligands with metal ions confers enhanced flexibility and adjustability. Additionally, these compounds exhibit distinctive properties, such as high porosity and favorable chemical stability.

The MIL series, also known as Levashield skeleton materials, is composed of carboxylates and trivalent cations. The existing MILs include MIL-53, MIL-100, and MIL-101. These materials possess a large specific surface area, a flexible skeleton, and a high degree of stability. The pore size of MIL-101(Cr) can reach up to about 3.4 nm, and its specific surface area is as high as 4100 m²·g⁻¹, providing typical examples of its macromolecular catalytic applications in specific contexts. The activity and stability of MIL-101(Cr) in specific catalytic reactions provide typical examples of its macromolecular catalytic applications. The “breathing effect” of the MIL-53 series (with metal centers such as Al, Cr, etc.) and its property of reversible structural transformation under different solvents, temperatures, or pressure environments are discussed. The high thermal stability of MIL series materials (partial temperature resistance > 573 K) [7].

The UIO structure series, developed at the University of Oslo, is based on zirconium (Zr) clusters and is characterized by its ultra-high chemical stability, with excellent resistance to strong acids and radiation.

The HKUST series, developed by a team from the Hong Kong University of Science and Technology, has found widespread application in catalysis and sensing technologies. These materials are distinguished by their open metal sites and high catalytic activity. Representative materials include HKUST-1 and HKUST-101, which have been utilized for methane storage or electrocatalytic CO₂ reduction. The synthesis and crystal structure of HKUST-1, which is hypothesized to possess open Cu²⁺ sites, have been shown to interact with guest molecules (e.g., CO₂, CH₄, etc.) to enhance gas adsorption [8,9]. HKUST-1 (or its isostructural material MOF-199) was derivatized as a Cu-based electrocatalyst for the electrochemical reduction of CO₂, and the relationship between the structure of the material, the preparation conditions, and the catalytic activity was explored [10].

PCN (Porous Coordination Network) is a class of high-performance MOFs developed by Professor Hong-Cai Zhou’s team at Texas A&M University. The fundamental design objective was to construct framework structures with high porosity. Strong stability and functional tunability can be achieved by precisely regulating the combination of metal nodes and organic ligands. PCN is a series of materials that have attracted much attention in the fields of gas adsorption, catalysis, and drug delivery due to their diverse topologies and excellent performance. A variety of nanoscale MOFs (including some from the PCN series) have been utilized in bioimaging and radiotherapy applications, demonstrating their good biocompatibility and high drug/contrast agent loading capacity [11].

The common MOF materials, along with their corresponding characteristics and limitations, are shown in

Table 1. Characteristics of common MOFs.

Series	Metal Nodes	Organic Ligand	Characteristic	Limitation
IRMOF Series	Zn2+ (e.g., Zn4O clusters)	Carboxylic acid ligands (e.g., terephthalic acid, BDC)	1. High specific surface area: IRMOF-1 (MOF-5) has a specific surface area of 3000 m2·g−1 2. Adjustable pore size: extend the pore by extending the ligand length (e.g., IRMOF-16 pore size up to 2.9 nm) 3. Applications: hydrogen storage, CO2 adsorption	Poor water stability
ZIF Series	Zn2+or Co2+	Imidazole ligands (e.g., 2-methylimidazole)	1. Zeolite topology: mimics the pore structure of natural zeolites (e.g., SOD, RHO topology) 2. High chemical stability: ZIF-8 is stable in boiling water and resistant to acid and alkali. 3. Selective adsorption: ZIF-7 is >100% selective for CO2/CH4. 4. Applications: gas separation, catalytic carriers	Easily restricted pore size
MIL Series	Fe3+, Cr3+, Al3+, etc.	Carboxylic acid ligands (e.g., homotrimellitic acid, BTC)	1. Variety of advantages: e.g., MIL-101 (Cr) 2. Ultra large pores: pore size up to 3.4 nm, specific surface area 4100 m2·g−1 3. Applications: macromolecular adsorption (e.g., drug delivery), catalysis MIL-53(Al) 4. Breathing effect: orifices open and close dynamically with pressure/temperature 1. Applications: CO2 capture, water adsorption. 2. Advantage: high thermal stability (some MIL materials withstand temperatures > 573 K)	Synthesis of high environmental pollution, adsorbent regeneration difficulty
UiO Series	Zr⁶ clusters (e.g., UiO-66, UiO-67)	Carboxylic acid ligands (e.g., terephthalic acid)	1. Ultra-high stability: resistant to water, acid (pH = 1–11), high temperature (773 K) 2. Functionalization potential: Enhancement of CO2 adsorption by post-synthetic modification with the introduction of amino groups (UiO-66-NH2) 3. Applications: nuclear waste treatment, catalysis (e.g., photocatalytic degradation of pollutants)	Limited resistance to high temperatures and susceptibility to UV rays
HKUST Series	Cu2+ (formation of Cu2+ clusters)	Benzene Tricarboxylic Acid (BTC)	1. Open metal sites: Cu2+ is exposed to the pore and enhances gas adsorption (e.g., CO2, CH4). 2. High porosity: Specific surface area ~1500 m2·g−1 3. Applications: gas storage, electrocatalytic CO2 reduction.	Poor gas transfer efficiency due to poor water stability
PCN Series	Combining highly stable metal clusters (e.g., Zr6, Fe3O) with functionalized ligands	-	Applications: photo-/electrocatalysis, biomedical imaging.	Insufficient hardness, limited chemical resistance, limited UV resistance
MOF-74 Series	Mg2+, Zn2+, Ni2+, etc.	Dihydroxyterephthalic acid (DHTA)	1. One-dimensional pores: pore size ~1.4 nm, surface rich in hydroxyl groups. 2. High CO2 adsorption: Mg-MOF-74 adsorbs up to 8.6 mmol·g−1 at 298 K (under low pressure). 3. Applications: carbon capture, natural gas purification	Prone to hygroscopic decomposition, poor chemical stability, harsh synthesis conditions, application limitations
ZJU Series	Rare earth metals (e.g., Y3+)	Polycarboxylic acid ligands	1. Luminous properties: Rare earth metals impart fluorescent properties for use in sensors. 2. High stability: water and heat resistant. 3. Applications: environmental monitoring, optoelectronic devices.	Low heat resistance and susceptibility to corrosion, Poor process performance
Bio-MOF Series	(e.g., Ca2+, Fe3+)	Natural ligands (e.g., amino acids, nucleotides)	1. Low toxicity: suitable for drug delivery (e.g., drug-carrying Bio-MOF-1). 2. Responsive degradation: release of the drug in response to a specific pH or enzyme. 3. Applications: cancer therapy, antimicrobial materials.	Less stable and more difficult to develop synthetically

.

There are several additional series being developed as the research progresses. For example, the Bio-MOF series, particularly Bio-MOF-1, which is zinc-centered, exhibited relatively low toxicity in cellular viability assays. For instance, cell viability was maintained above 80–90% in certain studies, with the tested concentrations ranging from 50–100 mg·L⁻¹. The pores of Bio-MOF-1 are capable of interacting with biomolecules, such as nucleotides. This interaction makes Bio-MOF-1 a potential drug carrier material. Some MOFs containing nucleotide ligands (including Bio-MOF-1) are biocompatible and maintain high cell viability in assays measuring cell toxicity at concentrations up to several hundred micrograms per milliliter. In vitro tests have shown low toxicity to mammalian cells (e.g., HeLa, MCF-7, etc.) [12,13,14]. Furthermore, Bio-MOF doped with silver ions or silver nanoparticles has been shown to achieve a 99% kill rate for both Gram-negative and Gram-positive bacteria within 24 h. The antibacterial effect is attributed to the material’s ability to act gradually. The antimicrobial effect is attributed to the slow release of metal ions (or silver nanoparticles) from the material and the porous structure of the MOF itself [15].

2.1.2. Classified According to the Cambridge Structural Database (CSD)

The University of Cambridge, in conjunction with information management techniques such as databases, has developed algorithms based on several key chemical and physical characteristics. These characteristics break down the overall family of MOFs into several subgroups for the purpose of classification. This classification is based on characteristics such as surface chemistry, metal cluster families, and crystallographic properties. Thus, researchers can determine how MOFs interact with specific molecules or assess their physicochemical stability. Researchers can understand and query various aspects, such as metal clusters, network and pore dimensions, surface chemistry (i.e., functional groups), and chirality [16].

2.1.3. Classification Based on Substructure Units (SBUs)

MOFs can be categorized based on the secondary structural units (SBUs), with this categorization method emphasizing the nature of the nodes and connectors, thereby enabling MOFs to develop unique applications in numerous fields. Given that the structure of metal-organic frameworks consists of metal ion clusters and organic connectors, the SBUs are related to metal-containing nodes, and the nature of the nodes and connectors can determine the network structure, topology, physical, mechanical, and morphological characteristics of MOFs and many other properties. Following the classification of MOFs into designated systems, the manipulation of nodes and organic connectors can yield a substantial number of MOFs with distinctive properties, which possess unique applications in numerous fields [17].

2.1.4. Classification by Function

Metal-organic frameworks (MOFs) can be divided into five categories: adsorption and separation, sensing, catalysis, supercapacitors, and luminescence. The applications of MOFs are very broad, including catalysis, gas adsorption and separation, membrane separation, energy storage devices, and sensor applications.

The classification of metal-organic framework materials is a complex and extensive task that requires various classification methods. Researchers and practitioners are advised to adopt classifications that meet their specific needs to ensure a comprehensive understanding and effective use of MOFs.

2.2. Synthesis of Metal-Organic Skeleton Material MOFs

There are several general methods for synthesizing MOFs:

(1)

Solvothermal/Hydrothermal Synthesis

Hydrothermal synthesis is the most traditional method of MOFs preparation. It generally involves the use of organic solvents to dissolve the metal precursor and the organic linker. The mixture is then placed into the lining of polytetrafluoroethylene (PTFE). The autoclave is subsequently used to heat the mixture, thereby obtaining MOF crystals. However, the use of organic heating requires the employment of toxic solvents, such as methanol, ethanol, and acetonitrile, among others. These solvents are typically used above their respective boiling points, a practice that can cause environmental concerns due to the release of volatiles. Furthermore, the extended reaction times inherent to this process can lead to significant energy consumption and reduced yields [18].

(2)

Microwave-Assisted Synthesis

The employment of microwave-assisted synthesis, which involves the utilization of microwave heating within an oscillatory field, the amalgamation of substrates (metal salts and organic ligands), and suitable solvents after permanent dipole moment coupling, results in molecular rotation. This, in turn, leads to rapid heating, which accelerates the kinetics of the chemical reaction, reduces the nucleation time, and decreases the crystal growth time. The crystals obtained by this method are small in size, making them well-suited for the synthesis of nanoscale MOFs [18].

(3)

Mechanochemical Synthesis

The direct reaction of the metal salt with the ligand is driven by mechanical forces, such as ball milling, and no (or only trace) solvent is required. The products prepared by this method are suitable for reactions involving insoluble ligands. Furthermore, the products may be amorphous or of low crystallinity, requiring further processing.

(4)

Diffusion Method

The rate of crystal growth is governed by the slow diffusion of reactants (e.g., in layered solvent systems). Typically, this process requires prolonged standing of the reaction solution, often taking several days.

(5)

Ultrasound-Assisted Synthesis

Ultrasonication is a synthesis method in which the energy source is ultrasound. This method utilizes the cavitation effect of ultrasound to promote the mixing of reactants and the nucleation of crystals, resulting in a significant increase in the nucleation rate [18].

(6)

Electrochemical Synthesis

Electrochemical methods utilize a metal electrode as a metal source, which dissolves and combines with a ligand under the influence of an electric field to produce MOFs. In general, electrochemical synthesis enables the continuous dissolution of the anode, thereby driving the reaction of metal ions with a polyacid, which can result in the formation of highly crystalline powdered MOFs. The electrochemical method also has clear advantages, such as high efficiency, low-temperature requirements, and the absence of the need for metal salts. It is imperative to acknowledge that the addition of organic solvents is frequently employed to facilitate the dissolution of organic ligands [18].

(7)

Specialized Methods

The advent of research has given rise to a plethora of novel methodologies for the synthesis of MOFs, resulting in a wide array of outcomes.

A statistical summary of the advantages and disadvantages of several major methods for preparing MOFs is provided in

Table 2. Comparison of advantages and disadvantages of several major synthesis methods for MOFs.

Synthesis Method	Vantage	Drawbacks
Hydrothermal method	1. Simple equipment and operation, low production costs, suitable for large-scale preparation, and better control of the frame structure 2. Preparation of MOF materials with good stability, small particle size, and light particle agglomeration 3. Mature system, easy to modify	1. Reaction systems are difficult to observe in closed containers 2. Longer reaction times (usually hours to days), higher energy consumption, and lower yields. 3. The use of organic solvents (e.g., DMF, DEF, etc.) may cause environmental problems and require high-temperature and high-pressure reactors, which is a big investment.
Microwave method	1. Improve the selectivity of the reaction, which can control the product particle size and physical phase, which can help to quickly screen the optimal synthesis conditions and improve the yield 2. Fast reaction rates and crystallization efficiencies, high process efficiencies, significant reduction in reaction times, often from hours to minutes to hours	1. The equipment is more expensive and complex, requiring specialized design 2. Heating too fast will lead to “hot spot” and “thermal runaway”; the sample temperature is difficult to measure accurately. 3. For some precursors that are not sensitive to microwaves, the effect is not obvious, and there may be an impurity reaction. 4. Safety hazards, violent reaction to phase contamination of the insulation layer.
Diffusion method	1. Mild reaction conditions, slow crystal growth, easy to obtain high-quality, large-size single crystals 2. Convenient for research, suitable for fine characterization of crystal structures, e.g., single crystal X-ray diffraction.	1. High requirements on equipment sealing and temperature gradient make it difficult for large-scale production. There are certain requirements for the reactants, such as a certain solubility, and at the same time, it is convenient to purify. Time-consuming 2. Time-consuming (may take days to weeks), low birth rate.
Mechanosynthesis	1. Simple preparation process, low preparation cost, simple, better adaptability to thermally unstable ligands or metal salts 2. Green environmental protection, the amount of solvent is very small or even no solvent, which can reduce the effect of solvent evaporation on environmental pollution. 3. Short reaction times, high yields in minutes to tens of minutes, suitable for large-scale preparations	1. Single crystals are not easily obtained, and the MOF materials that can be synthesized are relatively homogeneous 2. Difficulty in accurately controlling grain size, morphology, and defect concentration 3. Product purity is not ideal, needs to be ground and use other mechanical force equipment, equipment is more expensive
Ultrasonic method	1. Fast ultrasonic cavitation effect promotes rapid nucleation and crystal growth with short reaction times (often minutes to hours) 2. MOF particles with small particle size or special morphology can be obtained. 3. Uniform nucleation is possible	1. It is not easy to get single crystals, ultrasonic conditions (frequency, power) on the product have a greater impact and need to be optimized 2. Reactors are vulnerable 3. High equipment requirements are not suitable for mass production, ultrasonic energy distribution is not balanced, amplification is more difficult
Electrochemical method	1. Can be synthesized at room temperature and pressure with low energy consumption 2. Thin films or coatings can be directionally grown on electrode surfaces for sensing, electrocatalysis, and other applications. 3. The reaction conditions are mild and high temperature decomposition can be avoided.	1. Applies only to metal ions capable of participating in reactions in electrochemical systems. 2. There are certain requirements for electrode materials and technical challenges for large-scale production.

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2.3. General Modification Strategies for Metal-Organic Skeleton Materials MOFs

The three existing modification strategies are as follows: the introduction of metal sites, the introduction of functional groups, and the preparation of composites with carbon nanomaterials. These three modification methods overlap with the aforementioned strategies. In addition, the modification of MOFs can be achieved by altering the reaction conditions during hydrothermal synthesis.

The incorporation of metal sites into metal-organic skeleton materials has been demonstrated to engender structural and property diversity, thereby enhancing the selective absorption of CO₂. A notable example is the work of Bai Juan, who modified Zr-MOF through the introduction of metal doping and alterations to organic ligands. Subsequent to Ce doping, significant alterations were observed in the crystallinity, crystal morphology, particle size, pore volume, oxygen vacancy content, and acid-base properties of the material. The DMC yield of the resulting Zr-Ce-MOF was determined to be 5.02 mmol at 433 K and 4.6 MPa, indicating its optimal weak basicity and the presence of medium-strength acid sites. Furthermore, experiments involving the alteration of the structure of organic ligands revealed that Zr-1C=C-2C yielded 2.78 mmol DMC at 423 K and 2.6 MPa, while Zr-BDC-Br yielded 5.82 mmol DMC at 423 K and 4.6 MPa, thereby enhancing the material’s properties [19]. Lillerud’s team successfully synthesized a Zr-MOF (metal-organic framework) with the acronym UiO-66, which possesses an ultra-high specific surface area and excellent thermal stability properties. They achieved this feat by linking hexanuclear Zr clusters with a simple ligand, 1,4-benzene dicarboxylate (BDC), to obtain a robust three-dimensional porous structure [20]. Mutyala et al. synthesized MOFs (UiO-66) and TETA/UiO-66 with Zr as the metal center by adding triethylenetetramine (TETA) to the synthesis process. The results of comparative tests revealed that the modified materials produced much higher CO₂ uptake and reduced uptake of other substances, thereby improving the separation parameter [21]. Pazoki revealed that the incorporation of Li+ into Cu-BDC diminishes CH₄ adsorption at lower pressures, while the converse is observed at higher pressures, a finding that holds considerable promise for industrial CO₂/CH₄ adsorption separations [22]. In his study, Wu Dong examined the interaction of metal Li-modified chem-4Li MOF (doped with metal Li in IRMOF-1) with CO₂. His research encompassed a systematic investigation into the CO₂ trapping and separation efficacy of this modified material for various industrial systems, including CO₂/H₂, CO₂/N₂, CO₂/O₂, CO₂/CO, and CO₂/C₂H₄. Additionally, he conducted a detailed analysis of the alterations in the pore structure and properties induced by the introduction of the metal Li modification. Furthermore, he analyzed the reasons and mechanisms for the strong interaction between the material and CO₂, as well as the changes in the adsorption sites of adsorbed guest molecules in different pore structures within the material and the resulting changes in the pore structure of the adsorbed guest molecules. The study also examined the reasons and mechanisms behind the strong interaction between the material and CO₂, as well as the changes in adsorption sites of adsorbed guest molecules in different pore structures within the material and the resulting microphase separation effect. The findings of this study demonstrate that metal Li modification is an effective method to enhance the separation performance of the materials [23]. Asghar et al. [24] synthesized MOFs (Cu-BDC) with Cu as the metal-centered site and prepared hexamethylenetetramine (HMTA)-loaded Cu-BDC ⊃ HMTA through ligation. The results demonstrated that the adsorption capacity of the modified MOFs was significantly higher than that of the pre-modified MOFs [24]. Wu Dong studied Cu-BTC with a pore-cage-pore structure (also known as [Cu₃(TMA)₂-(H₂O)₃]). The material is prepared by the reaction of copper nitrate solution with mesitylene triformate under certain conditions and has a three-dimensional intersecting central pore of 9.5 Å. Typically, the ligand-unsaturated sites on the metal are occupied by water molecules, which are delocalized upon heating, leaving the ligand-unsaturated sites (bare). It has been shown that high porosity and good chemical stability are also reflected in Cu-BTC, making it potentially useful in gas adsorption and separation [23].

The incorporation of functional groups has been demonstrated to enhance the CO₂ uptake capacity of MOFs. For instance, Yurduen et al. synthesized a MOF (MIL-88B) with Fe(III) as the metal center by following the method of functional groups [25]. Similarly, Mahdipoor et al. synthesized MOFs (MIL-101(Fe)-NH₂) using amino substituent-modified terephthalic acid as the ligand [26]. The resulting materials exhibited superior CO₂ adsorption and separation properties. In a similar study, Mahdipoor et al. prepared MOF-5, ZIF-8, and other materials, which exhibited superior performance compared to conventional adsorbent materials due to the precise control of the Fe/BDC ratio during the reaction [26].

A comparison of the aforementioned approaches reveals the distinct advantages of composites prepared with carbon nanomaterials. These composites offer a superior degree of selection and manipulation while preserving the structural and morphological integrity of the raw materials. Extant research has demonstrated that the integration of nanofiber membranes and the synthesis of MOF nanoparticles can yield adsorptive high-performance materials with the capacity to separate CO₂ from mixed gas streams [27].

Modifying the reaction conditions in hydrothermal synthesis is a strategy to alter the structure and properties of MOFs. This modification is achieved by adjusting reaction conditions, including the type and ratio of solvents, reaction temperature and time, concentration of metal ions, and the amount of ligands. These conditions are based on the preparation of MOFs through hydrothermal synthesis. For instance, the choice of solvent affects solubility and polarity, which in turn influence the solubility and reactivity of metal ions and organic ligands. This, in turn, impacts the crystallization process and the structure of MOFs.

3. CO₂ Capture and Catalytic Conversion by MOFs

Considering the aforementioned strategies for structural adjustments of MOFs, metal doping, and functionalization improvements, these methods have considerable potential for optimization in the adsorption performance and the mechanism of CO₂ capture.

3.1. CO₂ Capture by MOFs

Substantial research has demonstrated that MOFs exhibit remarkable properties, including a low regeneration temperature, high porosity and adjustability, good chemical stability, and selective adsorption. This suggests that MOFs possess considerable potential in the field of CO₂ capture. This potential extends beyond the scope of single adsorption and encompasses its potential for development, manufacturing, and industrialization. For instance, Yaghi’s team developed MOF-303 (chemical formula Al(OH)(PZDC)), which can adsorb water efficiently at a relative humidity as low as 16% and release the water by mild heating at only 287 K, with a regeneration enthalpy as low as 5 kJ·mol⁻¹. The synthesis on a kilogram scale is enabled by multi-metal coordination and green water-based synthesis methods. Notably, the regeneration energy consumption of this material is 60% lower than that of traditional silica gel [28]. As an illustration, Fe₃O₄@UiO-66-SO₄ is a material that, through sulfate interfacial modification, reduces CO₂ desorption energy consumption by 44.7% and the regeneration temperature to less than 353 K (whereas conventional materials usually require 393 K). The core–shell structure of Fe₃O₄ (the core) and UiO-66-SO₄ (the shell) combines the advantages of magnetic separation with low regeneration energy consumption, making it suitable for industrial flue gas treatment [29]. Lian Shen’s research utilized the facile modification properties of metal nodes in high porosity MOFs to prepare enhanced catalytic materials using the post-modification method and investigate their applications in oxygen and hydrogen peroxide reduction [30]. Wu Dong’s research demonstrated that the UiO-66 (Zr) series materials exhibit elevated levels of thermal and chemical stability. This enhanced stability allows the materials to maintain their structural integrity during processes such as adsorption, separation, and other applications. Consequently, this ensures the maintenance of their properties and provides a reliable basis for their utilization in practical industrial settings [23]. In the study of noble gas adsorption, Chen Hao used a Monte Carlo method based on high-throughput screening to investigate the effects of various factors in MOFs on the adsorption and separation performance of Kr/Xe. Four types of functional groups were computationally screened along with three types of backbone structures. The findings of the study indicated that the functional groups had a more significant influence on the adsorption and separation properties than the skeletons and metal centers. The interpenetrating structures demonstrated superior performance in single-component adsorption, while the non-interpenetrating structures exhibited enhanced selectivity for Xe in the binary hybrid system [31]. Three primary factors have been identified as influential in the process of CO₂ capture by MOFs: the presence of organic functional groups, the composition of the metal, and the conditions under which adsorption occurs [19,26]. The present study demonstrates that enhancing the adsorption capacity of MOFs for CO₂ can be achieved by altering the organic ligands, incorporating metals as dopants, reducing the regeneration temperature, and modifying the external conditions.

Alteration of organic ligands: In general, the solvothermal method is employed to enhance the material’s capacity for CO₂ adsorption by modifying the carbon chain structure, the number of coordination groups, and the functional groups of the organic ligands. These modifications result in substantial structural alterations in the material’s properties, including crystallinity, crystal morphology, particle size, pore structure, acidity, and alkalinity. Consequently, these alterations lead to an improvement in the material’s adsorption capacity for CO₂. For instance, alterations in the carbon chain structure of organic ligands have been observed to result in changes in the crystallinity of the material, the evolution of the crystal morphology from tetrahedral to rectangular, a shift in pore size from microporous to mesoporous, a decrease in thermal stability, and an increase in Lewis acid sites. These changes in properties have been shown to lead to an increase in the adsorption capacity of the material for CO₂. Microporous MOFs (pore size < 2 nm) possess a high specific surface area and structural selectivity for small molecules; however, they are often poorly suited due to difficulties in mass transfer and the limited encapsulation of large functional guest molecules [32]. To overcome these limitations, mesoporous MOFs (2–50 nm) have emerged as a subject of intense research interest. Synthesized MOFs frequently contain guest molecules, such as solvents, unreacted linkers, clusters, and modifiers. These guest molecules generally require removal through an activation process. However, mesoporous MOFs are prone to collapse during this process. Consequently, research into the design and control of porosity, including methods to better maintain porosity for the intended application, is a perpetual central theme [33]. In a seminal study, Wu et al. pioneered a novel method for synthesizing a highly ordered macroporous MIL-125 material via a solvent evaporation-induced self-assembly route [34]. This innovative approach yielded a macroporous MIL-125 material with a remarkable specific surface area of up to 1083 m²·g⁻¹. Subsequent investigations revealed that this material exhibited remarkable stability in immobilizing CO₂, demonstrating good substrate tolerance and excellent yields in UV-irradiated CO₂ carbonylation coupling reactions [35].

Doped metals: Metal elements, as the earliest studied electrocatalysts for CO₂ reduction, hold an important position in the field of electrocatalysis. The CO₂ adsorption properties of MOFs centered on different metals containing modified metals are shown in Table 3. During the 1980s and 1990s, Hori et al. published a series of research results on the use of various metals for electrocatalytic CO₂ reduction [36,37,38], which still have a profound impact on today’s scientific research. A taxonomy of metals based on their reduction products has been proposed, classifying metals into four categories. The first category includes Pt, Ni, Fe, Co, and other metals with low hydrogen overpotential and a strong carbon monoxide adsorption capacity. These metals can poison the CO₂ reduction site, thereby converting hydrogen into formate. The second category includes Sn, Pb, Bi, In, etc., due to the difficulty in adsorbing CO intermediates. These metals can be easily protonated after detachment, thus generating formic acid or formate. The third group includes Au, Ag, Zn, and others. These metals can adsorb CO₂ intermediates and break the C-O bond, with the generated carbon monoxide easily detaching from the surface of the electrode to become the main product. Copper is the only metal element capable of generating C₁–C₃ hydrocarbon products due to its ability to react with them, making hydrogen the main product. The selective adsorption capacity of copper for carbon monoxide, along with its ability to hydrogenate adsorbed carbon monoxide, leads to the generation of C₁–C₃ hydrocarbon products. The subsequent formation of C-C bonds enables the production of C₂ and higher hydrocarbon products. The performance of the catalytic conversion of CO₂ is significantly influenced by doping with different metal elements during the preparation of MOFs. It has been demonstrated that Zr/Ce-based MOFs with the UiO-66 structure can be synthesized by doping Ce metal into the MOFs. This material has the potential to catalyze the conversion of CO₂ to DMC. It has been observed that doping with Ce metal leads to substantial alterations in the structural properties of the MOFs, including an enhancement of the crystallinity, evolution of the crystal morphology from tetrahedral to spherical, reduction in particle size, increase in pore volume, increase in oxygen vacancy content, and changes in acid-base properties, among others. These property changes lead to enhanced adsorption of CO₂ by the materials [19]. The enhancement of defects and acidic sites in bimetallic MOFs contributes to their superior performance. This is attributable to the elevated number of metal defect sites and Lewis acid content present in bimetallic MOFs compared to monometallic ones. These properties are advantageous for CO₂ adsorption and separation, resulting in enhanced CO₂ adsorption efficiency [27].

A reduction in the regeneration temperature has been demonstrated as a viable strategy for achieving simultaneous energy savings and enhanced service life of MOFs. This approach is supported by existing studies, which have shown that modifying existing MOFs can result in a decrease in the regeneration temperature. For instance, a modified MOF developed at the University of California, Berkeley, exhibits a regeneration temperature of 323 K, while the regeneration temperature of an amine solution (currently employed for CO₂ absorption) ranges from 353 to 383 K. The regeneration temperature of the modified MOF is considerably lower than that of the amine solution. The lower regeneration temperature, combined with the advantage that the modified MOFs do not require heating of the aqueous solution used by conventional amine absorbents, results in reduced energy consumption for the new material when utilized as a CO₂ trap. Yaghi’s team developed MOF-303 (chemical formula: Al(OH)(PZDC)), which exhibits remarkable efficacy in adsorbing water at relative humidity as low as 16%. This material requires only mild heating at 287 K to release the water, and its regeneration enthalpy is as low as 5 kJ·mol⁻¹. The material’s synthesis is scalable to a kilogram through multi-metal coordination and green water-based synthesis, and its regeneration energy consumption is 60% lower than that of traditional silica gel [28].

Alterations in external conditions: Modifications to the external conditions during the reaction have been demonstrated to enhance the efficacy of adsorption. For instance, researchers at Oregon State University observed that copper-based MOFs exhibited more than a twofold increase in CO₂ adsorption efficiency when initially exposed to ammonia, requiring less energy for regeneration and demonstrating enhanced stability compared to amine-based adsorbents [27]. Beyond this approach to enhancing performance, it has been determined that MOFs have the capacity to further improve their adsorption performance through activation. For instance, Yang et al. [39] augmented the CO₂ adsorption of prepared MIL-100 (Cr) from 3.52 to 7.18 mmol·g⁻¹ by calibrating the vacuum activation temperature. In the case of UiO-66, Ahmadijokani et al. demonstrated that varying activation solvents and vacuums yielded different product yields and CO₂ adsorption, underscoring the pivotal role of the vacuum activation temperature in product formation [40]. Furthermore, the layering of this substance with acid treatment, followed by the synthesis of plagioclase zeolite with the aid of PEG or PVP and a layered composite, has been shown to result in the construction of a disordered layered UiO-66 heterostructure on plagioclase zeolite. This has been demonstrated to exhibit excellent adsorption selectivity for CO₂ and CH₄ [41]. It is important to note that trapping efficiency can be influenced by several external factors. For instance, Yan et al. were able to enhance CO₂ adsorption from 7.23 mmol·g⁻¹ to 10.67 mmol·g⁻¹ at 273 K and 10000 Pa by soaking and allowing HKUST-1 in ethanol containing a small amount of NaCl and NH₄Cl at room temperature several times [42]. In addition to the aforementioned factors, it has been demonstrated that the incorporation of nanofibrous membranes and the synthesis of MOF nanoparticles yields materials with high performance in adsorption, capable of separating CO₂ from mixed gas streams [43].

Table 3. CO₂ Adsorption Properties of MOFs Centered on Different Metals Containing Modified Metals Table.

MOF	Metal Center	Specific Surface Area/m2·g−1	CO2 Adsorption Capacity/mmol·g−1	Pressure/MPa	Selective CO2/N2	Reference
MIL-53	Al	634	10.4	30	50	[44,45]
MIL-53	Mn0.27Al0.73	1748	11.8	30	83	[46]
MIL-53	Mn0.5Al0.5	1817	13.3	30	95	[46]
MIL-53	Mn0.6Al0.4	1576	12.8	30	45	[46]
MIL-100	Fe	1811	2.6	1	49.6	[47]
MIL-100	Al, Fe	1993	3.3	1	76.5	[46]
MIL-120	Al	3084	4.8	10	-	[46]
MOF-74	Mg	1640	8.0	1	233	[48,49,50]
MOF-74	Zn0.14Mg0.86	1277	0.5	1	-	[51]
MOF-74	Zn0.48Mg0.52	794	2.1	1	-	[51]
MOF-74	Zn0.75Mg0.25	668	3.2	1	-	[51]
MOF-74	Ni	1274	5.2	1.1	11.3	[41,47]
MOF-74	Pd, Ni	1115	12.2	32	14.6	[52]
MOF-74	Co	1404	3.7	1.1	9.7	[52]
MOF-74	Pd, Co	1088	11.4	32	12.4	[52]
MOF-5	Zn	1858	0.5	1	-	[53]
MOF-11	Zn	2096	14.7	35	-	[54]
MOF-210	Zn	6240	-	50	-	[55]
UiO-66	Zr	1455	4.3	20	-	[56]
HKUST-1	Cu	1440	3.4	1	-	[57]
HKUST-1	Li, Cu	1000	2.6	2	-	[58]
Cu3(BTC)2	Cu	1781	10.7	35	-	[54]
[Ni6(OH)4(COO)8(H2O)6]	Ni	468	1.4	1	-	[58]
[Ni4.1Co1.9(OH)4(BTB)8/3(H2O)6]	Ni4.1Co1.9	491	1.7	1	-	[58]
[Ni3.1Co2.9(OH)4(BTB)8/3(H2O)6]	Ni3.1Co2.9	526	1.9	1	-	[58]
[Ni2.8Co3.2(OH)4(BTB)8/3(H2O)6]	Ni2.8Co3.2	819	2.3	1	-	[58]

Table 3. CO₂ Adsorption Properties of MOFs Centered on Different Metals Containing Modified Metals Table.

MOF	Metal Center	Specific Surface Area/m2·g−1	CO2 Adsorption Capacity/mmol·g−1	Pressure/MPa	Selective CO2/N2	Reference
MIL-53	Al	634	10.4	30	50	[44,45]
MIL-53	Mn0.27Al0.73	1748	11.8	30	83	[46]
MIL-53	Mn0.5Al0.5	1817	13.3	30	95	[46]
MIL-53	Mn0.6Al0.4	1576	12.8	30	45	[46]
MIL-100	Fe	1811	2.6	1	49.6	[47]
MIL-100	Al, Fe	1993	3.3	1	76.5	[46]
MIL-120	Al	3084	4.8	10	-	[46]
MOF-74	Mg	1640	8.0	1	233	[48,49,50]
MOF-74	Zn0.14Mg0.86	1277	0.5	1	-	[51]
MOF-74	Zn0.48Mg0.52	794	2.1	1	-	[51]
MOF-74	Zn0.75Mg0.25	668	3.2	1	-	[51]
MOF-74	Ni	1274	5.2	1.1	11.3	[41,47]
MOF-74	Pd, Ni	1115	12.2	32	14.6	[52]
MOF-74	Co	1404	3.7	1.1	9.7	[52]
MOF-74	Pd, Co	1088	11.4	32	12.4	[52]
MOF-5	Zn	1858	0.5	1	-	[53]
MOF-11	Zn	2096	14.7	35	-	[54]
MOF-210	Zn	6240	-	50	-	[55]
UiO-66	Zr	1455	4.3	20	-	[56]
HKUST-1	Cu	1440	3.4	1	-	[57]
HKUST-1	Li, Cu	1000	2.6	2	-	[58]
Cu3(BTC)2	Cu	1781	10.7	35	-	[54]
[Ni6(OH)4(COO)8(H2O)6]	Ni	468	1.4	1	-	[58]
[Ni4.1Co1.9(OH)4(BTB)8/3(H2O)6]	Ni4.1Co1.9	491	1.7	1	-	[58]
[Ni3.1Co2.9(OH)4(BTB)8/3(H2O)6]	Ni3.1Co2.9	526	1.9	1	-	[58]
[Ni2.8Co3.2(OH)4(BTB)8/3(H2O)6]	Ni2.8Co3.2	819	2.3	1	-	[58]

3.2. Catalytic Conversion of CO₂ by MOFs

The catalytic conversion of CO₂ by MOFs will vary depending on the material itself, and in the reduction process, MOFs contain the following main catalysts.

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Photocatalysis

MOFs have the capacity to function as a photocatalyst in the conversion of CO₂. MOFs are hybrid materials formed by the coordination of metal ions or clusters of ions with organic molecules. To a certain extent, MOFs can be regarded as both organic molecules linked by inorganic nanoparticles and nanoparticles isolated by organic molecules. There are several reasons for using MOFs in the photocatalytic reduction of CO₂ [59,60,61]: from a structural point of view, MOFs are characterized by their porous structure and large specific surface area. This feature not only indicates that they have more active sites, but also that they can adsorb more CO₂. Meanwhile, the special structure can increase the local CO₂ concentration, which is conducive to the reaction. Furthermore, the integration of isomeric organic ligands with chromophoric groups or metal ions that possess an isomeric structure during the synthesis of MOFs has been demonstrated to enhance the light absorption efficiency of MOFs, broaden the absorption spectra, or improve catalytic performance. With respect to the ligand type, the organic ligands of MOFs typically contain functional groups such as N-H₂, -OH, -CH=CH₂, and others. These functional groups not only provide a substantial number of bonding sites but also introduce new ligands when they are unsaturated or coordinated with solvent molecules. The introduction of molecules with -OH, -NH₂, -COOH, etc., into MOFs through coordination enables dye functionalization, and their combination with other inorganic materials to form new hybrid materials can enhance catalytic performance. In terms of plasticity, the morphology and size of MOFs can be controlled by manipulating them during synthesis, for instance, by adding surfactants, using solvents, and generating ultrathin nanosheets, thereby exposing more catalytic sites and enhancing catalytic activity.

According to existing literature, MOFs play at least one role in catalyzing and absorbing light in the photocatalytic CO₂ reduction reaction. Accordingly, MOFs involved in catalytic reactions can be categorized into three types: MOF photocatalysts, MOF catalysts, and MOF photosensitizers. For instance, Wang et al. [62] synthesized ZIF-9 through a series of controls involving the use of Co²⁺, benzimidazole, tris-bipyridine ruthenium as a photosensitizer, and triethanolamine as a sacrificial agent. The photocatalytic reduction of CO₂ was subsequently carried out in a mixture of acetonitrile and water, and a total of about 350 μmol of hydrogen and carbon monoxide was produced from 0.8 μmol of ZIF-9 after a 2.5-h reaction (hydrogen: carbon monoxide = 3:4), with a conversion number of 450. Subsequent X-ray diffraction analysis revealed that the crystalline structure of ZIF-9 remained unaltered, indicating that the MOFs did not function as reactants and were capable of maintaining their stability during the catalytic process. It is noteworthy that the authors’ experiments demonstrated that the MOFs functioned as a catalyst in the catalytic reaction exclusively as the catalytic component, with the light-absorbing component typically comprising a metal complex (e.g., tris(pyridine)ruthenium/iridium) or a semiconductor (e.g., metal-semiconductor, organic-semiconductor) or semiconductors (e.g., metal semiconductors, organic semiconductors). In this system, metal complexes or semiconductors may exhibit minimal catalytic activity, and the activity of the reaction system is significantly enhanced by the incorporation of the MOFs. Consequently, this type of MOF is frequently referred to as a cocatalyst in the literature [63].

Two broad strategies have been developed to enhance photocatalytic efficiency. The first strategy involves the use of a functionalized ligand, such as a porphyrin ligand. PCN-222 has been shown to achieve visible-light-driven CO₂ reduction (CO generation at rates of up to 120 μmol/g·h) via a porphyrin ligand [64]. Metal doping has also been demonstrated to enhance photocatalytic efficiency to 0.8% by loading Co²⁺ onto Ti-MOF-NH₂ [65]. These enhancement strategies generally begin with charge transfer and photocatalytic processes. There are three charge transfer pathways: LMCT (ligand-metal charge transfer), MLCT (metal–ligand charge transfer), and LLCT (ligand-ligand charge transfer). The electron transfer process is closely associated with chemical bonding, as illustrated in Figure 4. A variety of materials and systems exhibit electron transfer and redox processes under photoexcitation, including TTPA materials, MIL-125-NH₂ materials, the TCH-related photocatalytic reaction process, the BDC-NH₂ removal process, porphyrin-[Ni] cluster-related photocatalytic reactions, and the photocatalytic degradation of CBZ/IBP under visible light. Refer to Figure 5 for a visual representation of each of these elements.

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Electrocatalytic

Electrocatalysis is defined as the catalytic action that accelerates the charge transfer reaction between the electrode and electrolyte interface. MOFs have emerged as a subject of intense research interest in the field of CO₂ trapping and electrocatalytic reduction due to their advantageous properties, including tunable pore structures, high specific surface areas, and well-defined active sites. However, the inherent limitations of pristine MOFs, including conductivity, stability, and selectivity, impose significant constraints on their practical applications. To address these challenges, modification strategies such as metal doping, ligand functionalization, and composite hybridization have been employed to enhance the performance of MOFs. The selection of suitable electrode materials is paramount for optimizing battery performance, thereby accelerating the reaction rate. During the discharge process, the selected electrode materials function as catalysts, modulating the reaction rate or electrode orientation without undergoing substantial changes themselves. In the domain of electrocatalysis, MOFs have demonstrated efficacy in the production of hydrogen and oxygen, the reduction of oxygen, and the reduction of CO₂, among other applications. Xu et al. modified hybrid nanostructures with CeO_x nanoparticles on ZIF-67-derived hollow CoS surfaces through in situ generation [67]. It was found that proper control of the amount of CeO_x on the CoS surface could achieve precise tuning of the Co²⁺/Co³⁺ ratio, which could induce defects and thus enhance the oxygen release reaction (OER) activity. Wang et al. synthesized a series of FeNi bimetallic MIL-101 materials by the hydrothermal method and used them for OER directly [68]. These metal-organic frameworks (MOFs) contain a substantial number of catalytically active MO₆ octahedra and bare hydrophilic carboxyl groups, which enhance OER activity. MOFs possess a greater number of active sites and a larger surface area, making them highly useful in electrocatalysis. The combination of MOFs with other materials can improve their stability and thus enhance their electrocatalytic activity, providing a direction for the development of electrocatalysis. A plethora of strategies have been proposed for enhancing electrocatalytic modification, including metal-centered modulation, mono- and bimetallic doping, and ligand-functionalized amine and polar group modification. A domestic study has been undertaken, focusing on the following areas: Xiao’s team enhanced CO₂ adsorption up to 3.8 mmol·g⁻¹ (1 bar) by amine-functionalizing MOF-177 (TEPA-modified) [69], and the data significantly showed that it was superior to conventional physical adsorption materials. Hydrogen bonding modulation: The team from Sun Yat-sen University introduced NH₂ groups into Co porphyrin MOFs and stabilized the Co-CO₂ intermediates through weak hydrogen bonding, with 99.4% FE and 2.7 s⁻¹ TOF, which is leading performance in the international arena. A comprehensive analysis reveals that domestic and international studies have distinct foci within the broader context of MOF-based electrocatalysis. Foreign countries are at the forefront of developing mechanisms and designing highly selective catalysts, while domestic efforts prioritize cost-effective modifications and industrial adaptation.

Figure 5. Selectivity of different metal catalysts for electrochemical CO₂ reduction products Ref: [70].

Figure 5. Selectivity of different metal catalysts for electrochemical CO₂ reduction products Ref: [70].

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Thermal Catalysis

The thermal catalytic reduction of CO₂ is the process of converting CO₂ into high-value-added chemicals, such as methane, methanol, and low-carbon olefins, through the action of catalysts under high-temperature conditions (as shown in Figure 6). MOFs with tunable metal nodes and organic ligands are promising catalysts for a variety of applications, including CO₂ cycloaddition reactions, hydrogenation of unsaturated hydrocarbons, and epoxidation of olefins. The modification of MOFs has been shown to significantly enhance catalytic activity and selectivity. This enhancement results from the unique pore structure, tunable active sites, and the metal–ligand synergistic effect of the modified materials. The modification strategies for thermal catalysis can be broadly categorized into three distinct classes: metal doping and alloying, structural modulation and functionalization, and composite hybridization and pyrolytic derivatization. The core mechanisms include active site modulation, where metal nodes (e.g., Zr, Fe, Cu, etc.) in MOFs can act as Lewis acid sites to promote CO₂ adsorption and activation. Electronic structure optimization: Metal doping or alloying (e.g., bimetallic Ag/Cu-MOF) has been demonstrated to regulate the electron density and accelerate the formation and conversion of intermediates (e.g., -COOH, -HCOO). Pore domain-limiting effect: The microporous structure of MOFs has been shown to stabilize reaction intermediates through the spatial domain-limiting effect and prolong their residence time, thus improving product selectivity. The use of organic ligands containing sulfoxide groups, amide, amino, or pyridine groups is a viable approach for synthesizing functionalized MOFs. The advantage of these groups is that they can undergo further modification to enhance efficiency and achieve targeted modifications. For instance, Zou et al. successfully assembled Zn²⁺ with tricarboxylic acid ligands to form new MOFs [71]. Subsequent to this, single-crystal to single-crystal metal cation exchange was performed with Cu²⁺ and Co^2+. Of particular note, a catalytic study of the cycloaddition reaction of CO₂ with epoxides to generate related carbonates showed that the MOFs of Zn²⁺ had the highest catalytic activity. Aniruddha et al. prepared a highly crystalline Hf (IV) MOF, and the resulting Hf-UiO-66-N₂H₃ was found to be useful for the benzaldehyde and malononitrile Knoevenagel condensation reaction, showing effective catalytic activity [32].

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Multiphase Catalysis

MOFs have been identified as optimal carriers for multiphase catalytic CO₂ conversion due to their high specific surface area, tunable pore structure, and well-defined active sites. Figure 7 shows the microstructure of Bi₂O₂S/PCN-20 obtained by using different detection methods. Through modification strategies such as metal doping, ligand functionalization, and complex hybridization, MOFs can achieve efficient catalytic activity and selectivity. These modified frameworks can be applied in classical multiphase catalytic reactions, including the CO₂ cycloaddition reaction, CO₂-assisted alkane dehydrogenation, and electrocatalytic CO₂ reduction. Enhanced strategies for multiphase catalysis encompass metal node modulation and bimetallic synergistic effects. Bimetallic MOFs (e.g., BiZnMOF) markedly reduce the binding energy of reaction intermediates by optimizing the electronic structure of BiZn bimetallic centers and exhibit a 92% Faraday efficiency in the electrocatalytic reduction of CO₂ to formate (HCOO-) (−0.9 V vs. RHE) [73]. Metal cluster active site: trinuclear metal clusters in Fe₂M-TATAB (M=Cu, Co, Zn, etc.). MOFs provide open Lewis acid sites to catalyze the cycloaddition reaction of CO₂ with epoxides >95% conversion [74]. Ligand functionalization, porphyrin ligand with a photosensitive moiety: PCN-222 (Zr-based porphyrin MOF) has been shown to drive the reduction of CO₂ to CO in the visible spectrum with a quantum efficiency of 0.8%, a process that occurs through the light-trapping ability of the porphyrin ligand [75].

The comparison of several CO₂ catalytic conversion technologies are shown in

Table 4. Comparison of several CO₂ catalytic conversion technologies.

Types	Reaction Temperature	Relative Energy Consumption	Dominance	Limitation
Photocatalysis	room temperatures	Low	Reaction conditions are mild and environmentally friendly; catalytic materials are diverse and functionality is easy to regulate; catalysts are highly stable and reusable	Narrow light absorption range, low solar energy utilization; high photogenerated carrier complex rate, limited catalytic efficiency; poor selectivity, easy to inactivate in complex systems; high cost for large-scale application, difficult engineering
Electrocatalysis	Room temperatures	high	Green efficiency and controllability; high selectivity and multifunctionality; energy efficiency and device compactness; emerging materials driving performance breakthroughs	High cost and precious metal dependence; low current efficiency and energy challenges; mass transfer limitations and unclear mechanisms; side reactions and stability issues
Thermocatalysis	473–623 K	Middle	Easy separation and recycling of catalysts, high industrialization maturity; adaptable to harsh reaction conditions	Higher energy consumption and carbon emissions; catalyst deactivation issues; mass transfer limitations; precious metal dependence and cost
Polyphase catalysis	573–873 K	Middle	Easy separation and recycling of catalysts; high stability and tolerance; industrial maturity	Mass transfer limitations; catalyst active sites are susceptible to carbon, sulfide, or metal toxicity; high energy and cost; deactivated catalysts require high temperature calcination or chemical treatment, and regeneration is energy intensive

. After comparative analysis, it is believed that electrocatalysis may be more suitable in distributed environments (such as exhaust gas treatment for electric vehicles). It can achieve low-carbon conversion by using off-peak electricity from the grid, but the issue of high energy consumption still needs to be addressed. Photocatalysis is particularly suitable for remote areas with abundant sunlight, such as desert photovoltaic power stations equipped with carbon capture devices, which aligns with the ‘solar energy + CO₂ conversion’ green energy strategy. However, its large-scale application still requires further research and breakthroughs. In large-scale industrial applications, such as CO₂ recovery at coal-to-gas plants, thermocatalysis appears more suitable and relies on advanced high-temperature treatment technologies for efficient conversion. Although heterogeneous catalysis holds an irreplaceable position as a core industrial technology, achieving its sustainability still requires further breakthroughs in material innovation and process optimization. The development of non-precious metal catalysts (such as Fe/Co bimetallic MOFs) and enhancing their activity through improved synergistic effects could become a new trend in future research.

4. Challenges and Future Perspectives

In recent years, MOFs have demonstrated significant advancements in the field of CO₂ capture and catalytic conversion. However, to further enhance the efficacy of these materials, it is imperative to overcome the current technical limitations through material innovation and exploration of mechanisms.

4.1. Technological Problems

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Stability

The harsh preservation conditions and stability of some MOF materials require enhancement because their structures are vulnerable to damage in harsh environments, including humidity, elevated temperatures, and strong acids and bases. This can compromise their performance and service life. For instance, some MOF materials based on carboxylic acid ligands are susceptible to hydrolysis in water, resulting in structural collapse. Chae et al. reported that MOF-5 was rapidly deactivated in the presence of water. Although its high specific surface area was favorable for gas adsorption, the intrusion of water led to the fracture of the framework structure, which seriously limited its reliability in practical applications [77].

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Cost

The industrialization of MOFs applications is hindered by the necessity of precious metal modification or complex synthesis steps in some MOFs. The use of precious metals in the modification process or the employment of complex synthesis steps in some MOFs results in high preparation costs and complex production processes, thereby hindering industrialization. Consequently, recent studies have focused on the development of cost-effective, readily scalable modification strategies for synthesis. Wang et al. demonstrated that MOFs derived from noble metal modification exhibited substantial activity enhancement in CO₂ reforming catalysis. However, due to the cost of the noble metal itself and the complexity of the modification process, the overall preparation cost remained high, impeding the industrialization and promotion of the scheme [78].

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Adaptation to Working Conditions

The CO₂ capture and catalytic conversion performance of MOFs in an atmosphere containing impurities (e.g., SO_x, NO_x, H₂O) is affected. Kim et al. demonstrated that the CO₂ adsorption capacity of certain MOFs was considerably diminished, and the catalytic activity was significantly suppressed in an environment containing water vapor and other impurities. This resulted in considerable uncertainty regarding their application under practical working conditions [79].

4.2. Frontier Research Directions

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Machine Learning Assisted Design

The current state of research on MOF catalytic mechanisms is characterized by two significant limitations. Firstly, the experimental dataset is relatively small, with approximately 200 data points, which restricts the scope of analysis and hinders the development of more sophisticated machine learning methods. Secondly, the predictive accuracy of traditional machine learning methods, such as random forest and decision tree algorithms, ranges from 0.65 to 0.72, which falls short of the precision required for advanced design [80]. To address these challenges, an active learning framework with a migration learning strategy has been proposed. The utilization of an existing inorganic porous material database (e.g., CSD) for pre-training has been demonstrated to effectively mitigate the learning errors of small samples. Concurrently, the active learning framework is constructed, where the key synthesis parameters (metal electronegativity, ligand rigidity index) are screened using Bayesian optimization, thereby enhancing experimental efficiency. The algorithm in question has the capacity to predict the structure of MOFs with a high degree of stability and activity, while concurrently constructing efficient catalytic sites.

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Replacement of Precious Metals with Cheaper Metals and Green Synthesis

Ru-based metal-organic frameworks (MOFs) (e.g., Ru-UiO-67) exhibit remarkable catalytic activity (TOF: 250 h⁻¹), yet the metal cost share is substantial at 78% (320/g). The employment of a Fe/Co bimetallic system (FeCo-MOF-74) enables the maintenance of high activity while concomitantly reducing the cost [81]. The development of continuous production technology is imperative. Conventional solvothermal synthesis of ZIF-8 yields 65% (batch mode), whereas the microreactor continuous flow process can enhance the yield to 92%, achieving a 5.6-fold increase in the spatiotemporal yield. This technology has been successfully implemented in a kiloton MOF production line, resulting in a 42% reduction in unit energy consumption [82].

The development of high-performance modified MOF materials is expected to provide new technical means and solutions to solve the global CO₂ emission problem, promote the sustainable development of the energy sector, and have broad application prospects in the fields of industrial exhaust gas treatment, energy storage and conversion, and environmental protection. A comprehensive investigation into the structure-property relationship of modified MOFs is essential to elucidate the impact of these materials on the CO₂ capture and conversion mechanism. Such an investigation will contribute to the enrichment and development of theoretical knowledge in the fields of materials science and chemical engineering, thereby providing theoretical guidance for the design and development of novel and efficient CO₂ capture and conversion materials. In the context of big data, there is a need to prioritize system integration and innovation in constructing a MOF/membrane composite system and an artificial intelligence-assisted platform. Future research will focus on multifunctional integrated design, real environment adaptation, and cross-disciplinary technology fusion to promote the practical application of MOFs in the field of carbon neutralization.