The Progress of Metal-Organic Framework for Boosting CO2 Conversion

Abstract

With the rapid development of modern society, environmental problems, including excessive amounts of CO₂ released in the atmosphere, are becoming more and more serious. It is necessary to develop new materials and technologies to reduce pollution. Among them, metal–organic frameworks (MOFs) have shown potential for application in the area of catalysis due to their ultra-high specific surface area, structural versatility, and designability as well as ease of modification and post-synthesis. Herein, we summarize recent research advances by use of MOFs for boosting CO₂ conversion. Furthermore, challenges and possible research directions related to further exploration are also discussed.

1. Introduction

As the global economy has grown at a rapid pace since the 21st century, energy and environmental issues have become increasingly prominent. In recent years, the consumption of fossil fuels has provided 85% of the world’s total energy consumption, and this proportion is likely to remain high in the years to come [1,2,3,4,5]. However, fossil fuels, for example, are being depleted, and their combustion is causing serious environmental pollution problems such as global warming [6,7,8]. Therefore, the search for green and efficient new energy sources has become a hot topic of research for scientists worldwide. Just as plants can convert CO₂ in the atmosphere into organic compounds such as oxygen and sugar through photosynthesis [9,10,11,12,13], if humans can efficiently use CO₂ in the air to produce small organic molecules of high energy density such as carbon monoxide, formic acid, methanol, and methane, they can both effectively reduce the accumulated CO₂ in the atmosphere and the greenhouse effect, and also produce fuel and more fine chemical products, alleviating mankind’s heavy reliance on fossil fuels [14,15,16,17].

While CO₂ is a major component of greenhouse gases, it is also an important renewable carbon resource. The conversion of CO₂ into value-added chemicals and fuels is a promising pathway for reducing CO₂ emissions and will contribute to the goal of sustainable development and carbon neutrality [18,19,20]. The soaring CO₂ levels in the atmosphere due to the excessive use of fossil fuels have broken the original carbon cycle system and degraded our living environment, forcing us to urgently adjust and optimize the current energy supply structure [21]. At the same time, CO₂ shows great potential for exploitation as a cheap, abundant, non-toxic, and renewable carbon feedstock [22]. The conversion of CO₂ into value-added chemicals is therefore of great importance for the reduction of fossil fuel use and the achievement of carbon neutrality targets [23].

However, the CO₂ reduction reaction (CO₂–RR) now requires a high level of CO₂ gas purity, and although direct CO₂ reduction using air is a goal pursued by scientists, no related work has been reported [24,25,26,27]. In recent years, researchers have developed various methods for CO₂ reduction, such as electroreduction, photoreduction, thermal reduction and bioreduction. To date, a variety of catalytic materials including graphene, graphitic carbon nitride (g-C₃N₄), layered oxides and hydroxides, metal–organic framework materials (MOFs) and covalent–organic framework materials (COFs) have been extensively investigated in the field of CO₂ catalysis due to their unique physicochemical properties [28,29,30,31].

Metal–organic framework materials are a promising new generation of adsorbent materials due to their high specific surface area, high porosity, ease of functionalization, and adjustable structure [32,33,34]. They are a promising new generation of materials for catalytic CO₂ conversion. In particular, MOFs have the following advantages as catalysts for photocatalytic or electrocatalytic CO₂ reduction: (1) the structural diversity and designability of MOF components (nodes, linkers, and pores) enables catalyst design at the molecular level; (2) the high porosity of MOFs facilitates efficient transport of reactants and products across the active site and catalyst; (3) the precise structure of MOFs allows the molecular level to explore the mechanism of photocatalysis or electrocatalysis; (4) the excellent CO₂ adsorption capacity of MOFs leads to high CO₂ concentration around the active site, which is ideal for photocatalytic or electrocatalytic CO₂ reduction. With all these advantages, MOF has been successfully applied in photocatalytic and electrocatalytic CO₂ reduction. This paper provides a comprehensive review of recent advances in the research of MOF materials for CO₂–RR catalysis and a brief overview of several commonly used research areas for catalytic CO₂ conversion and introduces the research progress in CO₂ reduction in the recent five years [35,36,37,38].

2. Strategies for CO₂ Recycling

It has been found that carbon dioxide is a linear molecule with a C=O bond energy of about 750 KJ mol⁻¹. Therefore, its conversion must involve the breaking of C=O, a process that requires a large amount of energy [39,40,41,42,43]. The literature reports that the direct reduction of CO₂ involves the gain of an electron in the CO₂ molecule, which changes the geometrical configuration of the CO₂ molecule from linear to curved, forming a CO₂ anion radical intermediate (CO₂⁻). This process requires a negative potential to be achieved (−1.9 V vs NHE), making the catalytic reduction of CO₂ difficult. However, when a proton source is introduced into the reaction system, proton-assisted multi-electron transfer allows the CO₂ catalytic reduction process to bypass the CO₂ anion radical intermediate (Figure 1), allowing the catalytic reduction of CO₂ to proceed at a lower potential [44,45,46,47,48]. While the explicit impact of pH on the HER is still an ongoing area of research, it has been thought that increasing local pH helps favor CO₂–RR over HER, mainly due to the decreasing overpotential for the formation of C₂⁺ products, as discussed earlier. On the other hand, a high local pH shifts equilibria of the acid–base reactions toward (bi)carbonates, which can reduce the concentration of CO₂ near the surface, decreasing selectivity toward CO₂–RR and favoring the HER instead [36]. Depending on the number of protons and electrons involved in the catalytic reduction process (Table 1), CO₂ can be reduced to carbon monoxide [CO(2e⁻)], formic acid [HCOOH(2e⁻)], formaldehyde [HCHO(4e⁻)], methanol [CH₃OH(6e⁻)], methane [CH₄(8e⁻)], ethylene [C₂H₄(12e⁻)], ethanol [CH₃CH₂OH(12e⁻)], ethane [C₂H₆(14e⁻)], and propanol [C₃H₇OH(18e⁻)], among other products [49,50,51,52,53,54,55,56].

For the production and final combustion of CO₂ reduction/hydrogenation products to represent a closed cycle (Figure 1), the final source of electrons and protons for CO₂ reduction must be water (H₂O), since the combustion of hydrogenated carbon products releases H₂O [4]. CO₂ reduction/hydrogenation/fixation therefore follows the overall equation:

xCO₂ + yH₂O → product + zO₂ CO₂ recycling reaction(1)

Although photosynthesis by plants and other phototrophic organisms fixes a much larger amount of carbon globally than is released by human activity, using biomass as a carbon source is an easy carbon cycling strategy. However, the use of bio-based carbon is limited by the amount of land required. Markets for biofuels such as ethanol and biodiesel require energy-intensive care (e.g., fertilisers, pesticides, etc.) and in some cases have led to deforestation, which significantly reduces the capacity for CO₂ bioconversion. As mentioned earlier, biomass energy with carbon capture and storage (BECCS) has the potential to achieve net negative emissions; however, the land requirements are huge, meaning that some 14 million square kilometers of land is needed for plants to offset the current global carbon emissions of 11 GtC/year. It is clearly not feasible to reuse roughly the same area of land for global food crop production. Therefore, although important, harnessing the power of natural photosynthesis can only play a limited role in achieving net zero emissions, leaving room for other CO₂ recovery strategies [57].

3. Electrochemical CO₂ Reduction

3.1. Basic Issues of CO₂ET

CO₂ electro-chemical transformation (CO₂ET) systems consist of electrodes, electrolytes, and electrolytic cells. Metal-based materials, metal-organic frameworks, and carbon materials are widely used as electrocatalysts for CO₂ET due to their low cost, high conductivity, and tunable properties [58]. CO₂ET electrodes are usually made from powder catalysts, polymer binders, and collectors. However, the binder usually leads to pore blockage of the catalyst, hindering the performance of the powder-based electrode due to loss of mass transfer channels [59]. Fortunately, emerging self-assembly and electrochemical deposition technologies offer stand-alone and binder-free strategies for electrode preparation with enhanced catalytic activity, improved stability, and reduced cost [60].

3.2. Construction of Electrolytic Cells

The electrolytic cell is the site where the electrocatalytic reduction of carbon dioxide takes place, and its design has a great influence on the mass transfer and thus on the efficiency of the carbon dioxide reduction. Electrolytic cells for general laboratory use fall into two main categories: H-type electrolytic cells and microfluidic flow electrolytic cells. A typical H-type electrolytic cell consists of a cathode and anode chamber and three electrodes (working electrode, reference electrode, and counter electrode). The cathode and anode chambers are separated by a proton exchange membrane or ion exchange membrane (nafion 117, etc.), which enables conduction while avoiding the diffusion of cathode products to the anode for oxidation, and the two chambers can be filled with different electrolytes so that the working electrode and counter electrode can operate under optimum conditions, respectively [61].

The working electrode and counter electrode of a microfluidic flow electrolytic cell are separated by a flowing electrolyte. The working electrode is usually a gas diffusion electrode, where the gas is slowly introduced into the cell by the diffusion layer, which ensures that the carbon dioxide is better covered by the catalyst and increases the chance of the carbon dioxide participating in the reaction. This ensures that the carbon dioxide is better covered by the catalyst and increases the chance of the carbon dioxide participating in the reaction, thus increasing the conversion rate of carbon dioxide and improving the electrolysis efficiency [22].

3.3. Type of Electrolyte

The electrolyte provides the medium of electron and proton transfer for the electrocatalytic reduction of carbon dioxide. Differences in type and concentration will greatly affect the activity and selectivity of the catalyst, and it should be noted that impurities in the electrolyte (metal ions or organic matter) in the electrolyte may poison the catalyst, causing a reduction in activity or even deactivation. In general, there are three types of electrolytes used in the laboratory: aqueous electrolytes, organic phase electrolytes, and ionic solutions electrolytes [62].

3.4. Metal–Organic Frameworks in Electrocatalytic CO₂–RR

As a class of porous materials, MOFs contain both a large number of single-molecule metal junctions and structurally controlled organic groups. In contrast to conventional inorganic porous materials, MOFs are long-range ordered porous crystalline materials with a highly periodic arrangement of organic ligands and metal nodes [62,63]. In addition, the physical and chemical properties of MOFs can be adjusted by modulating the composition of the compound, metal nodes, and organic ligands at the molecular level; these excellent properties allow them to be used in a wide range of applications in electrocatalytic CO₂–RR [64].

3.4.1. Fe–MOFs with Iron as the Active Centre

Due to the tunable structure and properties of porous materials, researchers have developed a series of MOFs and their derivatives as CO₂–RR electrocatalysts, with the goal of designing and synthesizing highly active, selective, and stable electrocatalysts to achieve efficient electrocatalytic CO₂–RR. According to relevant studies, the selection of suitable metal centres is extremely important for the synthesis of high-performance catalysts. So far, transition metals such as Fe, Co, Ni, and Cu have been good catalytic active sites in MOFs catalysts because they can form reduced or oxidised states and have good chemical stability. For example, Fe-MOFs with Fe as the active centre have shown good catalytic performance in electrocatalytic CO₂–RR reactions by exploiting the Lewis acidity and redox activity of Fe [65]. Hods’ group [66] reported the electrophoretic deposition of Fe-MOF-525 films on fluorine-doped tin oxide (FTO) glass. The catalyst has a large active site, because the Lewis acid of the active center iron has good adsorption performance for CO₂ molecules. It can reach a current density of 150 mA/cm² during catalytic reduction and has a Faraday efficiency of approximately 100% for the reduction of CO₂ to CO. The same Fe-PCN-222 MOFs have also been investigated for electrocatalytic CO₂–RR, where the metal cluster zirconium oxygene as a linker of the MOFs does not have a catalytic active role and the porphyrin central iron is the catalytic centre, catalyzing CO₂ to CO with a selectivity of 99%.

3.4.2. Co-MOFs with Cobalt as the Active Centre

Cobalt is also a well performing catalytic active site in electrocatalytic CO₂–RR [67], with cobalt as the catalytic active centre mainly exploiting the diversity of cobalt valence states (Co²⁺, Co³⁺, and Co⁴⁺) [68]. To demonstrate that metallic Co also has CO₂–RR properties, Yang’s group [69] prepared a kind of dispersed nano cobalt porphyrin MOFs in situ with a Faraday efficiency of about 76% CO under potentiostatic electrolytic conditions at −0.7 V (vs. RHE) and was able to maintain stability for >7 h. To further understand the process of the reduction reaction, in situ spectroelectrochemical measurements of the prepared samples were carried out (Figure 2), and the results showed that the CO₂ adsorbed on the cobalt porphyrins may be subjected to a reduction reaction through the interaction of Co¹⁺ with single electrons [70].

3.4.3. Cu-MOFs with Copper as the Active Centre

Copper is one of the most widely studied catalytic active centres because of its large CO₂ molecular adsorption energy [71], which can trap other CO₂ molecules to participate in the reaction while the catalytic reaction is incomplete, resulting in the production of polycarbonate products. The onset potential for CO₂ reduction of Cu-MOF’s electrode occurred at potentials approximately 0.2 V more positive than those observed with a Cu metal electrode. The Hing group [72] prepared a Cu-MOF material by mixing red ammonia with aqueous copper sulphate solution and applied it to electrocatalytic CO₂–RR. The authors found that in a 0.5 M KHCO₃ solution, the MOF material with Cr–Mn as the metal node had an onset potential around 0.2 V (vs. RHE) higher than that of the Cu metal electrode when catalysed under the same conditions. To explain this phenomenon, the authors show by means of density flooding (DFT) calculations that the reason for this phenomenon is mainly due to the lower electron cloud density of copper, which leads to a stronger adsorption of CO₂ onto Cu-MOFs and a higher amount of CO₂ per active site, thus exhibiting a lower onset potential for Cu-MOFs. Since then, MOFs materials with copper as the catalytic active centre have been widely used in electrocatalytic CO₂–RR reactions. In subsequent studies, researchers have used the specific adsorption of metallic copper on CO₂ molecules to generate polycarbonate products in electrocatalytic CO₂–RR reactions. A typical MOFs catalyst is Cu₃(BTC)₂, generated using benzoic acid as the organic ligand, and experimental results show that this copper-based MOF can effectively catalyse the reduction of CO₂ to 90% pure oxalic acid with a Faraday efficiency of 51% [73]. Yang and co-workers [74] also reported an IL-templated electro-synthesis of MOF [Cu₂(L)] on a copper electrode. This integrated electrode consisting of a thin dense coating of Cu₂(L) on Cu foam incorporates the uncoupled Cu(II) active site and shows high conductivity and stability (Figure 3). This electrode shows excellent activity for the electro-reduction of CO₂ to formic acid, with a low onset potential of −1.45 V vs. Ag/Ag⁺, and the FE_HCOOH reaches 90.5% at −1.80 V vs. Ag/Ag⁺ with a current density of 65.8 mA·cm⁻². Experimental (EPR spectroscopy) and theoretical (DFT) methods confirm that the reaction is driven by defects within the structure of the decorated electrode.

3.4.4. Other Metal as the Active Centers

MOFs using metallic nickel as the catalytic active site have also been extensively investigated. For example, Jiang’s group [75] has proposed a post-ion exchange strategy to successfully obtain low-coordinated, single-atom Ni catalysts based on Zn-based MOFs (ZIF-8) using a two-step ion-exchange strategy (Figure 4). The material morphology can be well maintained from the ZIF-8 precursor to Zn-N₃-C and then to the Ni-N₃-C material obtained by ion exchange. The MOFs with metallic nickel as the catalytic active centre were experimentally verified to have a Faraday efficiency of 95% in catalyzing the formation of CO from CO₂. Cao’s group has constructed nickel-based MOFs with phthalocyanine as the organic ligand; the Faraday efficiency of the catalytic CO generation can also reach 96.4%, and the material can be electrolysed for 10 h with almost constant activity [76].

3.4.5. Application of Porphyrin-Based Metal–Organic Frameworks in Electrocatalytic CO₂–RR

MOF-based catalysts play a good catalytic role in electrocatalytic CO₂–RR as a class of porous materials, where the key influence is the choice of organic ligands and metal nodes. As an important organic group in nature, porphyrins are heterocyclic molecules consisting of four pyrroles linked by hypomethyles that can carry different metals in their centres and thus have a variety of properties. Metal porphyrins and their derivatives are commonly found in nature and in organelles associated with the human body, and play an important role (Table 2). The use of metalloporphyrins as molecular catalysts has been widely studied for small molecule activation reactions and has the following advantages in terms of coordination chemistry. First, the rigid and robust porphyrin backbone can be systematically designed and synthesized based on the porphyrin framework; second, many functional groups can be on the porphyrin backbone, and these play a key role not only in improving catalysis but also in controlling the framework structure and morphology; third, porphyrins can be used as structural units and catalytic sites in frameworks based on MOFs. Bimetallic and polymetallic porphyrin-based frameworks can be used to construct catalysts for synergistic catalytic effects. Thus, the application of porphyrins as organic ligands to MOF materials has led to important advances in electrocatalytic CO₂–RR.

PCN-222 showed excellent electrocatalytic CO₂–RR performance. For example, Professor Jiang’s group chose a MOF (PCN-222) constructed from porphyrin tetracarboxylic acid ligands and zirconium ions for electrocatalytic CO₂–RR and found that the Faraday efficiency of catalytic CO₂ to CO reached 87%, while the catalytic performance did not decrease after 10 h of continuous electrolysis [77]. Lan and co-workers [78] have proposed a new strategy to insert metal clusters into porphyrin-based backbones by chemical vapour deposition (CVD) to achieve electrocatalytic CO₂–RR selectivity. By optimizing the experimental conditions, the resulting complexes have a Faraday efficiency of 97% for CO at −0.7 V (vs. RHE) using MOF-545-Co as the backbone. Not only is the introduction of metal clusters an electron donor and carrier, but also the strong binding interactions between the metal clusters and the metal active site of the porphyrin centre reduce the CO₂ adsorption energy.

Table 2

MOFs with active metal nodes for CO₂ reduction.

MOF	Metal Node	Product	Reference
NH2-UiO-66 (Zr/Ti)	(Zr/Ti)6O4(OH)4	formate	[79]
MIL-101 (Fe)	Fe–O cluster	formate	[80]
NNU-13	Zn-ε-Keggin cluster	CH4	[81]
Co-ZIF-9	Co–O cluster	CO	[82]
MOF-Ni	Ni cluster	CO	[83]
BIF-29	Cu cluster	CO	[84]
Ni MOLs	Ni2(OH)2	CH3OH	[85]

4. Photocatalytic CO₂ Reduction

Photocatalysis is an efficient, low-energy, clean, and non-secondary pollution-free technology [86,87,88,89,90]. The three important parameters of a photocatalyst are: (1) the spectral response range; (2) the effective separation efficiency of electrons and holes; (3) the redox reaction efficiency of the charge carrier. Organic ligands provide the highest occupied molecular orbital (HOMO), and metal centres provide the lowest unoccupied molecular orbital (LUMO), so that metallic crystalline materials can exhibit similar semiconductor behavior [91,92,93]. Irradiation of MOFs by a certain light source can lead to metal charge transfer (LMCT) of the organic ligands, where electrons from the organic ligands are transferred to the metal centres and electron and hole separation occurs [94,95,96,97]. The photocatalytic activity of MOFs is further modulated by improving the photo-response range of the ligand. As a result, MOFs and MOF derivatives are rapidly being developed for photocatalytic applications, widely used in the photocatalytic reduction of CO₂ [82,98,99].

4.1. Metal Porphyrin Ligands for MOF Photocatalysts

Metalloporphyrins are widely used in photocatalytic reactions due to their broad absorption spectra [100]. Ye et al. [101] prepared highly catalytically active MOF-525-Co by post-synthetic modification using MOF-525 as the parent body and found that under visible light irradiation (Figure 5), the porphyrins were excited by light absorption and photogenerated electrons were transferred to the Co²⁺ present in the MOF in the form of single atoms, reducing it to form the true catalytic centre; Co⁺ is subsequently oxidised to Co²⁺ by CO₂ molecules, completing a catalytic cycle in which CO₂ is reduced to carbon monoxide and methane; The intermediates of the whole process can be captured by electron paramagnetic resonance [101]. The introduction of Co²⁺ was found to promote the separation of photogenerated charges and thus enhance the catalytic activity, with an increase of about six-fold in the rate of methane production and about three-fold in the rate of carbon monoxide production for MOF-52-Co compared to the parent.

Zhang et al. [63] used cobalt porphyrin derivatives containing polyphenolic groups to coordinate with zirconium oxygen clusters to generate ZrPP-1 and ZrPP-2. Unlike other carboxy porphyrin MOFs, this MOF utilises -OH for coordination and is therefore highly acid and base resistant. The carbon dioxide adsorption of ZrPP-1 was found to be as high as 90 cm³g⁻¹, with a carbon monoxide production rate of 14 μmol g⁻¹ h⁻¹ under visible light (λ > 420 nm) irradiation. Based on theoretical calculations and experimental data, The researchers concluded that ZrPP-1 is excited by the absorption of light by the porphyrin ligand under visible light irradiation, and the excitation electrons are transferred to Co²⁺, followed by a continuous quenching process that generates Co⁰, which can activate carbon dioxide to form Co²⁺-CO and eventually release carbon monoxide, completing a catalytic cycle. Jiang et al. [77] reported the use of a MOF (PCN-222) formed by zirconium oxygen clusters and tetracarboxy porphyrins for the photocatalytic reduction of carbon dioxide to formic acid. The ultrafast transient absorption spectra showed that the zirconium oxygen clusters acted as a catalytic centre to inhibit the complexation of electrons and holes, thus enhancing the catalytic activity.

4.2. Bipyridyl Metal Complex Ligands for MOF Photocatalysts

Bipyridyl metal complexes are widely used in the study of photo(electro)-catalytic CO₂ reduction, but the stability of molecular catalysts is poor, and making them into MOFs can significantly improve the stability and catalytic activity. Yang et al. used dicarboxylated bipyridyl rhenium complexes (H₂ReTC) to replace some of the biphenyldicarboxylic acid (H₂BPDC) to generate the hybrid UIO-67 MOF (Figure 6). The presence of too many H₂ReTC ligands in the MOF cell increases the collision between ligands, which is detrimental to the catalytic activity and reduces the specific surface area of the MOF, leading to the obstruction of the diffusion of reaction substrates and products; when silver nanoparticles with surface plasmon resonance effect are encapsulated in Re₃-MOF, the catalytic activity can be increased by 7 times [102].

Lin et al. [103] grafted the catalytically active M(bpy)(CO)₃X (M = Re or Mn) onto a Hf₁₂-Ru monolayer MOF with photosensitive properties by means of carboxylic acid ligand exchange to form a Hf₁₂-Ru-M hybrid MOF with both photosensitivity and catalytic activity, and since the distance between the photosensitive and catalytic centres is only 1.3 nm, photogenerated electrons can be quickly transferred to the catalytic site to participate in the reaction, so this MOF shows excellent catalytic activity: the conversion number of Hf₁₂-RuReMOF is 3500 and that of Hf₁₂-Ru-MnMOF is 1000 after 25 h of irradiation by artificial light, and more excitingly, the conversion number of Hf₁₂-RuReMOF reaches 670 after 6 h of irradiation by natural light.

4.3. MOF Photocatalysts Containing the -NH₂ Functional Group

Li et al. [104] formed NH₂-MIL-125(Ti) from amino terephthalic acid (NH₂-BDC) and titanium oxide clusters. Since amine sites are highly effective for CO₂ adsorption and can act as light-absorption chromophores, NH₂-MIL-125(Ti) produced 8 μmol of formic acid, while MIL-125(Ti) produced almost no product. The UV-vis absorption spectra indicated that the introduction of amino groups in the ligand could broaden the absorption band of MOF to 550 nm, thus increasing the energy uptake of the reaction system. Meanwhile, the introduction of amino groups on the benzene ring could significantly increase the carbon dioxide adsorption of MOF, thus leading to the high catalytic activity of NH₂-MIL-125(Ti) [105]. When the amount of amino groups introduced on the benzene ring increases, the reaction activity is further enhanced. The group partially replaced 2-amino-terephthalic acid with 2,5-diamino-terephthalic acid to form a mixed ligand NH₂-UIO-66 (Zr) (mixed NH₂-UIO-66 (Zr)), and experiments revealed that mixed NH₂-UIO-66 (Zr) catalyzed the reduction of carbon dioxide formic acid, which was found to be more active than NH₂-UIO-66 (Zr), which was analysed to indicate that this was due to the introduction of more amino groups to broaden the absorption spectrum of MOF while further increasing the amount of CO₂ adsorbed, leading to a stronger catalytic activity.

4.4. Others

Ligands with special characteristics, such as large conjugated group and high visible-light harvesting abilities, have also been applied for constructing MOFs with photocatalytic CO₂–RR properties. Recently Lan et al. [106] reported a two-in-one strategy for efficient CO₂–RR. Two eco-friendly biomimetic MOFs were prepared for their study ([Co₂(AD)₄H₂(BDA)] (ADMOF-1) and [Co₂(AD)₄H₂(IBA)₂] (AD-MOF-2)). First, the -NH₂ of HAD ligands acted as auxochromic groups to make MOFs responsive to visible light. Second, the uncoordinated aromatic nitrogen of HAD (on the o-position of -NH₂) acted as an active position for CO₂–RR. Photocatalytic CO₂–RR was carried out in pure aqueous solution without any other photosensitizer. Under the visible-light irradiation for 5 h, HCOOH production over AD-MOF-2 was detected to be as high as 443.2 mol g⁻¹ h⁻¹. These results indicated that the two-in-one strategy efficiently improved the photocatalytic CO₂-RR performance of the MOFs.

4.5. The Dynamic Role/Mechanism of MOFs Works as a Catalyst

Up to now, the examples of CO₂ reduction based on MOF-based photocatalysts are still insufficient to elucidate the structure–activity relationship and reaction mechanism in the photocatalytic process. The active centers, such as open metal sites, are required to be chosen judiciously to achieve efficient activation of CO₂ molecules. For heterogeneous reactions, CO₂ molecules generally need to diffuse into the channels of catalysts to increase the utilization of the active sites within the solid.

5. Conclusions

In summary, MOF-based materials have important applications in the conversion of CO₂, as the MOF catalysts contain transition metal sites in the porous structures, giving them a rich arrangement of electrons outside the nucleus, which can be used to change the reaction path during the reaction and thus reduce the activation energy. The porous framework of the MOF material can be used as a good support structure, allowing it to be compounded with other materials to form a wider variety of new materials. Due to the regular structure of MOF materials, calcination of MOF materials as precursors can lead to inorganic nanomaterials with special morphology and chemical composition, and the carbon elements in MOF can also form carbon coating films during calcination to obtain non-metal doped composites.

Although great progress has been made in the application of MOF-based materials in the catalytic conversion of CO₂, there are certain limitations. First, the separation, transport, and complexation of photogenerated carriers (electrons and holes) in the photocatalytic process has always been a problem across all types of materials. A good photocatalyst should have the advantage of a high number of photogenerated carriers, fast transport rates, and less complexation during the transport of carriers from the catalyst interior to the surface. Therefore, there is room for further development of MOF-based materials. The doping of MOF-based materials with photosensitive components can effectively improve the response of the material to light. For example, ligands containing strong conjugated groups have a certain absorption in the visible region, and the introduction of such ligands or molecules can lead to photocatalytic activity of the material under visible light irradiation.

Constructing heterojunctions can improve the separation and transport efficiency of photogenerated carriers. Changing the morphology of the material and shortening the transport distance of photogenerated carriers can effectively control the compounding of electrons and holes during the transport process. Changes in the chemical composition of the catalyst surface can enhance the interaction between the material and CO₂, and the fabrication of more pore structures can expand the specific surface area of the material and expose more catalytic active sites to the reaction system, thus improving the catalytic efficiency of the material. Secondly, the stability and recyclability of the catalyst material is also an important indicator of the quality of the catalyst, especially in liquid phase reaction systems where the MOF structure is somewhat unstable in the presence of other Lewis acids and bases. For photocatalytic CO₂ reduction, many current systems require the use of organic sacrificial agents (e.g., TEOA) to provide electrons, whereas a more ideal system would use water as the electron donor. Once again, MOF-based materials have a relatively low product generation rate and are not sufficiently selective to be used industrially. Finally, the economic value and environmental friendliness of the catalyst material itself are also issues that need to be considered for the industrial application of MOF-based materials.

There are many important issues to be solved in the field of MOF-based materials for catalytic CO₂ conversion, and the future development directions are mainly in the following two areas:

• (1). design and construct MOF-based materials with high activity, high selectivity, high stability, low production cost, and low toxicity to achieve efficient and green conversion of CO₂ under mild conditions;

• (2). further broaden the types of reactions for CO₂ conversion by MOF-based materials, develop new ways of CO₂ conversion, and convert CO₂ into a variety of high value-added chemicals.

Author Contributions

Writing—original draft preparation, Y.Q. and X.Y.; writing—review and editing, F.H. and Z.D.; resources, project administration, funding acquisition, Z.D. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Figures and Tables

Figure 1. Schematic illustration of CO2–RR toward artificial carbon cycle.

Figure 2. Electrocatalytic CO2–RR performances of H−POM@PCN−222 (Co), H−POM@PCN−222 (Fe), H−POM@PCN−222 (Mn), H−POM@PCN−222 (Ni), PCN−222 (Co), and POM. (a) Linear sweep voltametric scans. (b) Faradaic efficiencies for CO with different metals and at different potentials. (c) Maximum FECO for six samples at −0.8 V (vs RHE). (d) CO current density at different potentials. (e) Tafel plots. (f) The turnover frequency for different metals and PCN−222 (Co). Re−printed with permission from Ref. [70]. Copyright 2021, Wiley–VCH.

Figure 3. Photograph (a) and SEM image (b) of a fresh Cu-foam electrode (0.5 × 1.0 cm2). (c) Photograph of the as-prepared electrode Cu2(L)-e/Cu (0.5 × 1.0 cm2). (d–f) SEM images of the as-prepared electrode Cu2(L)-e/Cu. (g,h) Views of the crystal structure of [Cu2(L)]. Hydrogen atoms are omitted for clarity. The scale bars of (b,d–f) are 300 µm, 300 µm, 300 nm and 100 nm, respectively. Reprinted with permission from Ref. [74]. Copyright 2020, Springer Nature.

Figure 4. Fabrication of low-coordination single-atom Ni electrocatalysts via a PSMS strategy. Reprinted with permission from Ref. [75]. Copyright 2021, Wiley-VCH.

Figure 5. (a) View of the 3D network of MOF-525-Co featuring a highly porous framework and incorporated active sites. (b) Fourier transform magnitudes of the experimental Co K-edge EXAFS spectra of samples (not corrected for phase shift). Key: Co foil (blue), Co@C (orange), MOF-525-Co (green). (c) Wavelet transform for the k3-weighted EXAFS signal of MOF-525-Co, based on Morlet wavelets with optimum resolution at the first (lower panel) and higher (upper panel) coordination shells. Vertical dashed lines denoting the k-space positions of the Co-N and Co-Co contributions are provided to guide the eye. (d) Comparison between the Co K-edge XANES experimental spectrum (orange) of MOF-525-Co and the theoretical spectrum calculated with the depicted structure. For clarity, the non-convoluted theoretical spectrum is also shown. The energy-dependent exchange–correlation potential was calculated in the real Hedin–Lundqvist scheme, and then the spectra are convoluted using a Lorentzian function with an energy-dependent width to account for the broadening due both to the core–hole width and to the final state width. (e) UV/Vis spectra of MOF-525 (green), MOF-525-Co (purple), and MOF-525-Zn (orange). Reprinted with permission from Ref. [101]. Copyright 2016, Wiley-VCH.

Figure 6. Structures of Ren-MOF and Ag⊂Ren-MOF for plasmon-enhanced photocatalytic CO2 conversion. (a) Zr6O4(OH)4(−CO2)12 secondary building units are combined with 4,4′-biphenyldicarboxylate (BPDC) and ReTC linkers to form Ren-MOF. The structure of Re3-MOF is shown, as identified from single-crystal X-ray diffraction. The 12 coordinated Zr-based metal clusters are interconnected with 21 BPDC and 3 ReTC linkers in a face-centred cubic array. Atom labeling scheme: C, black; O, red; Zr, blue polyhedral; Re, yellow; Cl, green; H atoms are omitted for clarity. (b) Ren-MOF coated on an Ag nanocube for enhanced photocatalytic conversion of CO2. Reprinted with permission from [102]. Copyright 2017, American Chemical Society.

Table 1

Standard electrochemical potentials for CO₂ reduction.

Reaction	E0/[V vs RHE (ReversibleHydrogen Electrode)]
2H2O → O2 + 4H+ + 4e−	1.23
xCO2 + nH+ + ne− → product + yH2O	-
CO2 + 2H+ + 2e− → HCOOH(aq)	−0.12
CO2 + 2H+ + 2e− → CO(g) + H2O	−0.10
CO2 + 4H+ + 4e− → C(s) + 2H2O	0.21
CO2 + 6H+ + 6e− → CH3OH(aq) + H2O	0.03
CO2 + 8H+ + 8e− → CH4(g) + 2H2O	0.17
2CO2 + 12H+ + 12e− → C2H4(g) + 4H2O	0.08
2CO2 + 12H+ + 12e− → C2H5OH(aq) + 3H2O	0.09
2CO2 + 14H+ + 14e− → C2H6(g) + 4H2O	0.14
3CO2 + 18H+ + 18e− → C3H7OH(aq) + 5H2O	0.10
2CO2 + 2H+ + 2e− → (COOH)2(s)	−0.47
2CO2 + 8H+ + 8e− → CH3COOH(aq) + 2H2O	0.11
2CO2 + 10H+ + 10e− → CH3CHO(aq) + 3H2O	0.06
3CO2 + 16H+ + 16e− → C2H5CHO(aq) + 5H2O	0.09
xCO + nH+ + ne− → product + yH2O	-
CO + 6H+ + 6e− → CH4(g) + H2O	0.26
2CO + 8H+ + 8e− → CH3CH2OH(aq) + H2O	0.19
2CO + 8H+ + 8e− → C2H4(g) + 2H2O	0.17
2H+ + 2e− → H2	0

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