An Overview of Enabling Catalysts for Carbon Dioxide Conversion Aiming at the Two-carbon Target

Received: May 30, 2022
Revised: July 14, 2022
Accepted: July 17, 2022

 Copyright The Author(s). This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are cited.

Cite this article:

Lv, T., Wang, B., Wang, R. (2022). An Overview of Enabling Catalysts for Carbon Dioxide Conversion Aiming at the Two-carbon Target. Aerosol Air Qual. Res. 22, 220227. https://doi.org/10.4209/aaqr.220227

HIGHLIGHTS

• Major CO₂ conversion catalysts were included in this review.
• The status of CO₂ conversion catalysts covers articles since 2019.
• The problems unresolved together with future research trend were proposed.

Keywords: Carbon dioxide, Catalytic conversion, Catalyst, Research progress

1 INTRODUCTION

Since the industrial revolution, humans have burned large amounts of fossil fuels, such as coal and oil, leading to a dramatic increase in the amount of carbon dioxide in the atmosphere and an increasingly severe greenhouse effect. Various extreme weather, such as the once-in-a-century rain in Henan, China, in July 2021 and the sudden snowfall in Brazil in July 2021, is gradually increasing. The greenhouse effect is having a huge impact on the global climate. To mitigate global warming, the UN climate conferences have been held several times, and in 2015 the 21st UN Climate Change Conference adopted the Paris Agreement, which sets out arrangements for global action to address climate change after 2020. The Chinese government has announced that it aims to achieve peak CO₂ emissions by 2030 and carbon neutrality by 2060. Therefore, the development of efficient carbon emission reduction technologies is the general trend, the key to combating global warming, and the need of mankind.

Among the many carbon reduction measures, converting CO₂ into fuels and chemicals helps create a carbon-neutral cycle, thus mitigating the rapid depletion of fossil resources and the increase in CO₂ emissions. However, the presence of covalent double bonds makes carbon dioxide highly thermodynamically stable and kinetically inert. Therefore, it is difficult to open the covalent double bonds to achieve the conversion of carbon dioxide. The development of suitable catalytic systems to facilitate the breaking of covalent double bonds and the synthesis of new chemical bonds for the efficient conversion of carbon dioxide is an effective means to solve this problem. For many years, researchers around the world have been investigating the catalytic conversion of CO₂. Innovations and breakthroughs have been made in the conversion of CO₂ to methanol, cyclization of CO₂ with epoxides, co-conversion of CO₂ and methane to synthesis gas, and hydrogenation of CO₂ to ethylene. However, there are still some problems to be solved in this field, and the key to solving them lies in further optimization of catalysts used for catalytic reactions.

The conditions required for the catalyst, the catalytic efficiency of the catalyst, the selectivity of the product, and the recoverability of the catalyst are critical to the catalytic reaction. These properties depend to a large extent on the materials chosen for the catalysts, and it is, therefore, necessary to discuss the catalysts in terms of the materials used. Many scholars have published several reviews on CO₂ conversion catalysts, ranging from a discussion of the catalysts involved in the occurrence of a certain reaction of CO₂ to a discussion from the perspective of a certain material used in the catalyst, and this review summarizes the various CO₂ catalytic materials studied by researchers since 2019 from the perspective of the material chosen for the catalyst. The catalytic materials covered include ionic liquids (ILs), metal-organic frameworks (MOFs), covalent organic frameworks (COFs), metal-based materials, porous organic polymers, nanocatalysts, metal-free green catalysts, and catalysts from other materials. The carbon dioxide conversion catalysts involved in the article are shown in Fig. 1. In addition to the increasing impact of carbon dioxide in recent years, the impact of other types of polluting gases still requires attention, such as VOCs. Most VOCs are prone to complex photochemical reactions that ultimately produce a variety of toxic and carcinogenic secondary pollutants that pose a threat to human and plant life (Bae et al., 2022). Related studies have also been published in recent years (Wang et al., 2020b; Li et al., 2021b). Also as acid gases, NO_x and SO₂ research seem to be more mature. Zhu et al. (2021b), Yu et al. (2021a) and Zhu et al. (2021a) have been working on NO_x removal and have recently published innovative papers. Some of the catalysts they have studied provide some worthwhile insights into the catalytic conversion of CO₂. This review aims to summarize the types of existing carbon dioxide conversion catalysts and the latest research advances in these catalysts for researchers who want to learn about carbon dioxide conversion catalyst research. Finally, this article also outlines some of the issues facing catalysts and the future direction of catalyst development.

 
Fig. 1. Types of catalytic materials covered in this review.

2 RECENT RESEARCH RESULTS OF VARIOUS CO₂ CONVERSION CATALYTIC MATERIALS

 
2.1 Ionic Liquids

Ionic liquids remain liquid below 100°C and are usually composed of organic/inorganic cations and anions with the advantages of high tunability, high stability, low pressure, high polarity, and low melting point. The introduction of specific functional groups into ionic liquids can change the chemical structure and physical properties of ionic liquids, and their applications are changed accordingly. For this reason, ionic liquids have been called "solvents of the future", "design solvents" or "green solvents and catalysts" (Chen and Mu, 2019). Currently, ionic liquids have attracted a lot of attention and are used in the fields of CO₂ capture/conversion, biomass dissolution/conversion, SO₂ absorption, materials synthesis, metal passivation, and battery electrolyte production. Researchers have innovated and developed a variety of ionic liquid catalytic systems, often including ionic liquid coordination with co-catalysts, immobilized ionic liquid catalysts, functionalized ionic liquid catalysts, and polyionic liquid catalysts. In addition, other ionic liquid catalytic systems, including ionic liquid catalysts with multiple active sites, the combination of ionic liquids with metal-organic frameworks, dual-centered cationic liquids catalysts, and ionic liquid nanocatalysts have also been initially explored and developed.

 
2.1.1 Coordination of ionic liquids with co-catalysts

The coordination and recycling of ionic liquid catalysts with co-catalysts is a key research element in CO₂ cyclization reactions. Zhou et al. (2019b) developed a catalytic system combining a cobalt source with an ionic liquid as a catalyst for the cyclization of propargylamine with CO₂ carboxyl groups to prepare 2-oxazolidinones. Under the synergistic effect of cobalt and IL, the catalytic efficiency of the catalytic system is high, and the catalytic activity can still be maintained after ten cycles. Xu et al. (2019) prepared an efficient binary homogeneous catalyst for catalyzing the cycloaddition reaction of CO₂ with propylene oxide (PO) by physically mixing both the catalyst functionalized aminoimidazole ionic liquid (MIM-NH₂) as well as the co-catalyst tetrabutylammonium bromide (TBAB). It was found that when the mass ratio of MIM-NH₂/TBAB was 1:1, the reaction was carried out at 75°C and CO₂ pressure of 0.35 MPa. After 48 h, the yield of propylene carbonate (PC) was ≥ 98%.

 
2.1.2 Immobilized ionic liquids

The preparation of immobilized ionic liquid catalysts using carriers and ionic liquids allows not only for the phase change of the catalyst but also for higher CO₂ conversion and catalyst recovery. Yan et al. (2019) prepared immobilized ionic liquid catalysts ZSM-5-[BMIM]Zn₂Br₅ (BMIM = 1-N-butyl-3 methylimidazole) using a double salt ionic liquid [BMIM]Zn₂Br₅ reacted with the carrier ZSM-5. The immobilized bed was used to produce propylene carbonate by catalytic oxidation of propylene and carbon dioxide using ZSM-5-[BMIM]Zn₂Br₅ under the reaction conditions of a pressure of 3.0 MPa, temperature of 130°C, and space velocity of 0.25 h^–1. The conversion of the feedstock was as high as 88.3% and the product selectivity reached 97.1%. And the catalyst maintained good catalytic performance after 10 h of reaction. Cui et al. (2019) prepared immobilized ionic liquid catalyst Al-SBA-15-[BMIM][Zn₂Br₅] using the carrier metal aluminum modified molecular sieve Al-SBA-15 with the composite ionic liquid [BMIM][Zn₂Br₅]. The performance of its catalytic synthesis of propylene carbonate from propylene oxide and carbon dioxide was investigated experimentally. It was found that the conversion of propylene oxide and product selectivity reached 93.9% and 97.4%, respectively, when the reaction temperature was 90°C, the pressure was 2.0 Mpa, and the catalyst mass fraction was 2.5%. Liu et al. (2021b) prepared polystyrene-supported ionic liquid (PS-IMPCOOHTMGBr) catalysts catalyzing the cycloaddition reaction of carbon dioxide and epichlorohydrin (ECH) using the new material polystyrene as a carrier. The synthesis steps of PS-IMPCOOHTMGBr are shown in Fig. 2.

Fig. 2. Synthesis of PS-IMPCOOHTMGBr (Liu et al., 2021b).

The product yield reached 97.2% at 80°C as well as at atmospheric pressure and could be recycled more than 9 times. And the catalytic activity of this catalyst increased with the amount of immobilized ionic liquid. Later, Shi et al. (2020) combined the advantages of multiple sites and mesoporous channels to prepare immobilized ionic liquid catalysts catalyzing the conversion of CO₂ to cyclic carbonates (Fig. 3) on modified molecular sieve SBA-15 using ionic liquids with multiple active sites. The catalytic schematic is shown in Fig. 4.

Fig. 3. Typical synthetic procedure of cyclic carbonates (Shi et al., 2020).
 

 
Fig. 4. Catalytic schematic (Shi et al., 2020).

The utilization of ionic liquids was improved and the catalysts were easily separated after the reaction. Among all the immobilized ionic liquids, [IMCA]₂Br₂@SBA-15 showed the best catalytic activity. The yield of propylene carbonate was as high as 97.4% with a selectivity close to 100%. The catalyst was still well recovered after five cycles. Another team of Liu et al. (2022) prepared a mesoporous SBA-15-supported imidazole-functionalized ionic liquid (SBA-15@DMIL-HOCH₂COO) by combining an imidazole-based ionic liquid and an aluminum-modified molecular sieve SBA-15, 2-oxazolidine. SBA-15@DMIL-HOCH₂COO possesses an abundance of active centers, which allows which was used as a catalyst to catalyze the cycloaddition reaction of CO₂ and aziridine to produce it to exhibit excellent activity in terms of yield and selectivity. In addition, the catalyst was easily recyclable and maintained good catalytic activity after five repeated use cycles. Peng et al. (2021) prepared a clay-supported CO₂ conversion catalyst by encapsulating ZnBr₂-based Lewis acidic ionic liquid (ZnBr₂/IL) in kaolin (Hal) in a one-step method. The carrier material used for this catalyst is greener.

2.1.3 Functionalized ionic liquid

In the absence of co-catalysts and organic solvents, a large number of ionic liquid catalysts were developed to catalyze the coupling reaction of carbon dioxide with epoxides, and although the catalytic activity was satisfactory with > 90% yield of cyclic carbonates, the reaction conditions, especially the reaction temperature and the initial pressure of carbon dioxide, were still not mild enough (Yue et al., 2014). The introduction of functional groups such as amino, carboxyl, and amine groups in ionic liquids can reduce the reaction temperature and pressure of the catalyst-catalyzed carbon dioxide conversion, resulting in milder reaction conditions and improved catalyst recovery. Yue et al. (2014) synthesized amino-functionalized imidazole-based ionic liquid catalysts to catalyze the carbon dioxide and epoxide under mild conditions as well as without any co-solvent cyclization reaction to synthesize cyclic carbonates. The catalytic system could be reused at least nine times without any significant loss of activity and selectivity. Moreover, the activation and subsequent conversion of carbon dioxide can be accomplished in one step at room temperature and atmospheric pressure. And then, Zhang et al. (2019a) synthesized four new amino-functionalized pyrazole ionic liquids, 2-(2-aminoethyl)-1-methyl-pyrazolium bromide (AEMPzBr), 2-(3-aminopropyl)-1-methyl-pyrazoliumbromide (APMPzBr), 2-(2-aminoethyl)-1-ethyl-pyrazolium bromide (AEEPzBr), and 2-(3-aminopropyl)-1-ethyl-pyrazolium bromide (APEPzBr), for the first time using inexpensive starting materials. The structural formulae of the four catalysts are shown in Fig. 5.

Fig. 5. Four amino-functionalized pyrazolium ionic liquids (Zhang et al., 2019a).

Compared with other single-component ionic liquids, the catalyst requires a 10–20°C lower reaction temperature and can catalyze the coupling reaction of carbon dioxide and propylene oxide to produce propylene carbonate with a yield of over 94%. Also, the initial pressure of carbon dioxide was maintained at 1.5 MPa, which was superior to many other ionic liquids. Wang et al. (2019) synthesized for the first time five hydroxyl-functionalized pyrazole ionic liquids including 1-methyl-2-hydroxyethyl pyrazolium bromide (HEMPzBr), 1-ethyl-2-hydroxyethyl pyrazolium bromide (HEEPzBr), 1-ethyl-2-hydroxypropyl pyrazolium bromide (HPEPzBr), 1-ethyl-2-hydroxyethyl-3-methylpyrazolium bromide (HEEMPzBr), and 1-ethyl-2-hydroxyethyl-3,5-dimethyl pyrazolium bromide (HEEDMPzBr) with different chain lengths or substituents (Fig. 6).

Fig. 6. Synthetic route and structures of the hydroxyl functionalized pyrazolium ILs (Wang et al., 2019).

The yield of HEEMPzBr reached 92.9% within 4 h at 110°C, 1.0 MPa initial CO₂ pressure, using 1 mol% catalyst, and this hydroxyl-functionalized pyrazole ionic liquid catalyst synthesized further reduced the CO₂ catalytic pressure. Zhang et al. (2019b) synthesized and characterized 10 carboxyl-functionalized pyrazole ionic liquids. The carboxymethyl pyrazole ionic liquid (CMEPzBr) performed best under the optimal reaction conditions (reaction temperature of 110°C, initial CO₂ pressure of 2.0 MPa, catalyst dosage of 1.0 mol%, and reaction time of 4.0 h), and the yield of the product could reach 99.4%. More importantly, however, the yield of polycarbonate is still considerable even when the reaction temperature is further reduced. When the reaction temperature and initial pressure of CO₂ were lowered to 70°C and 1.0 MPa, respectively, and the reaction time was 4.0 h with 1.0 mol% of catalyst, the coupling reaction of CO₂ and PO catalyzed by CMEPzBr for 24.0 h, the yield of polycarbonate (PC) could still reach a high yield of 90.4%. Chen et al. (2019), on the other hand, synthesized amine-based functionalized ionic liquids (AFILs), which have the dual function of ionic liquid and organic base, and which can effectively catalyze the conversion of carbon dioxide and epichlorohydrin to 3-chloro-1,2-propene. Mujmule et al. (2019) used N-vinyl imidazole and 2-chloroethane to synthesize imidazole-based ionic liquids 3-(2-hydroxyethyl)-1-vinyl-1H-imidazol-3-ium chloride [EvimOH][Cl]/DBU, which is a novel dual catalytic system (Fig. 7). It was used to catalyze the reaction of epoxide with carbon dioxide and could convert propylene oxide to polycarbonate in 60 min with more than 99% conversion and selectivity. In addition, the synthesized 3-(2-hydroxyethyl)-1-vinyl-1H-imidazol-3-ium sulphate [EvimOH][HSO₄]/DBU catalyst system can also outperform good catalytic performance.

Fig. 7. Synthesis of n-vinyl imidazolium hydroxyl functionalized ionic liquids (a) [EvimOH][Cl] and (b) [EvimOH][HSO₄] (Mujmule et al., 2019).

By a simple post-synthetic approach, Dai et al. (2021) obtained polar group-functionalized ionic liquid porous organic polymers (POP-PA-COOH, POP-PA-OH, POP-PA-NH₂, and POP-PA-Et). The structural formulae of the four ionic liquids are shown in Fig. 8.

Fig. 8. Structure illustration of the various polar group-functionalized porous organic polymers (Dai et al., 2021).

This material acts as an efficient heterogeneous catalyst to catalyze the cycloaddition reaction of carbon dioxide with epoxide under mild, co-catalyst-free conditions. Notably, POP-PA-NH₂ still exhibits good catalytic activity at low CO₂ concentration and is an ideal material for CO₂ removal under mild conditions. Liu et al. (2020) first proposed a novel square amine-based ionic liquid. It was found that the square amine group easily forms strong hydrogen bonds with the oxygen atoms of the epoxide, leading to strong activation of the epoxide, resulting in a good selectivity of the reaction under optimal reaction conditions. In addition, the catalyst is easily recoverable and has good reusability. This study provides insight into the synergistic catalytic mechanism of multiple hydrogen bond donors and bromide anions. In relevant CO₂ conversion applications, multiple hydrogen bonding catalyzes show a green and promising alternative to Lewis acid catalysis.

 
2.1.4 Polyionic liquid

Polyionic liquids (PILs) are characterized by a large number of active sites and large porosity, which facilitate CO₂ uptake and catalytic conversion. Sang and Huang (2020) prepared a new type of PIL by a one-pot FeCl₃-catalyzed Friedel- Crafts reaction and prepared a novel benzimidazole-based supercrosslinked polyionic liquids (HPILs), whose nitrogen content and porosity can be easily adjusted by BMZ ratios, and HPILs have strong CO₂ absorption and catalytic capacity. Taking HPILs-Cl₂ as an example, the conversion of propylene oxide to cyclic carbonate can reach 99% under mild conditions (0.1 MPa, 70°C) catalyzing the reaction of CO₂ with propylene oxide for 9 h. Xie et al. (2019) prepared a series of super-crosslinked polyionic liquids with high nucleophilic/ electrophilic density and abundant mesopores using the free-radical polymerization of imidazolium-based monomers with six ionic pairs (Im-6) and bipyridine (Py)-based monomers, followed by the efficient post-synthetic metallization and supercritical drying (SCD) (Fig. 9).

Fig. 9. Preparation of Py-Im-6 via solvothermal synthesis and Py-Im-6-Zn-x via post-synthetic modification (Xie et al., 2019).

The high density of bromide anion/zinc sites (six bromide anions in one monomer, 5.15 wt% zinc loading), abundant mesopores (pore size of 13.1 nm, pore volume of 1.17 cm³ g^–1), large specific surface area (483.8 m² g^–1) and high CO₂ adsorption capacity (1.74 mmol g^–1, 273.15 K, 1 atm) allow MPILs to act as efficient catalysts for the cycloaddition reaction of CO₂ with epoxides. Py-Im-6-Zn-5-SCD catalyzed the cycloaddition reaction of CO₂ for 18–48 h at solvent-free, additive-free, 1 atm, and 30°C, obtaining higher than 92% on a wide range of epoxide substrates of cyclic carbonate yield on a wide range of epoxide substrates. Furthermore, the catalyst could be recycled at least six times without significant loss of reactivity. Cui and Guo (2020) synthesized three complex polyether imidazole ionic liquids (PIILs) with terminal hydroxyl, carboxyl, and amino groups supported on molecular sieves (MCM-22) using the silane coupling agent 3-chloropropyltriethoxysilane (CPTES). This catalytic system was used to catalyze the cycloaddition reaction of carbon dioxide and propylene oxide (PO) in a fixed-bed reactor.

MCM-22-CPTES-HOOC-[PECH-Mim]Cl/[ZnBr₂] (PECH = polyepichlorohydrin) was found to have the best catalytic performance in this reaction. Its conversion and selectivity reached 92.7% and 82.7%, respectively. Yang et al. (2022) synthesized novel friendly nonmetallic catalysts using polymeric ionic liquids (PIL) with mesoporous silica (mSiO₂). Under solvent-free and co-catalytic-free conditions, mSiO₂-PIL-n exhibited good catalytic activity for the CO₂ cycloaddition of epoxides. Under the conditions of 120°C, 2 Mpa, and 6 h, mSiO₂-PIL-2 could effectively catalyze the cycloaddition reaction with 96% yield and 99% selectivity, and its catalytic activity did not decrease after 10 cycle runs. In addition, the catalytic activity of mSiO₂-PIL-2 was extended for other substituted epoxides. Thus, PIL immobilized on mesoporous SiO₂ by copolymerization is a promising synthetic strategy to combine multiple active components in a single catalyst.

 
2.1.5 Other ionic liquid

In addition to the above-mentioned typical ionic liquid catalysts, iron-containing ionic liquids, ionic liquids with multiple active centers, the combination of ionic liquids with MOFs and nanocatalysts, and double-centered cationic liquids catalysts have also shown good catalytic effects in the catalytic conversion of CO₂. Leu et al. (2019) synthesized an ionic liquid containing imidazoline in an iron-containing ionic liquid [BMIM][Fe(NO)₂Cl₂]. The Fe-containing ionic liquid was found to be an active catalyst for CO₂ cyclization with high conversion of various substrates under mild conditions. Wang et al. (2021) synthesized an ionic liquid C[CMIMBrTMG]₂ (TMG = 1,1,3,3-Tetramethylguanidine) with multiple active centers, which achieved multiple goals of CO₂ adsorption, activation, and conversion as a single-component catalyst and provided strong electrophilic and nucleophilic moieties. The reaction at 50°C and 1 bar CO₂ pressure for 6 h resulted in a 98.5% yield of chloropropane carbonate (CPC). Furthermore, washing with ethyl acetate, C[CMIMBrTMG]₂ could be easily separated from the catalytic system, thus enabling the recovery of the catalyst. Bahadori et al. (2020) synthesized imidazolium-based ionic liquids combined with MIL-101 (Cr) via coordination and covalent bonding as a heterogeneous catalyst for the efficient capture of CO₂ at low pressure and the carbon dioxide with epoxide immobilization. In MIL-IL (A), the ionic liquid is covalently linked to the Cr center, while in MIL-IL (B), the ionic liquid is covalently linked to MIL-101 (Cr), and the adsorption capacities of MIL-IL (A) and MIL-IL (B) for CO₂ are 5.46 and 7.84 times higher than those of the parent MOF, respectively. The tight bonding between the ionic liquid and MIL-IL(B) frameworks makes them recoverable heterogeneous catalysts for catalyzing the reaction of CO₂ with epoxide. Analytical techniques confirmed the grafting effect of ionic liquids on the MOF structure and that the backbone structure of the catalysts remained unchanged after five CO₂ cyclizations of epoxystyrene. Zhang and Zhiani (2020) synthesized imidazole-based ionic liquid nanocatalysts to achieve the combination of nanocatalysts and ionic liquids. On this basis, porous dendritic fiber nanosilica (DFNS) catalysts containing ionic liquids (IL/DFNS) were designed and synthesized. This porous IL/DFNS catalyst showed good catalytic performance for the cyclization reaction of carbon dioxide and epoxide under mild conditions. In addition, Guglielmero et al. (2019) synthesized a double-centered cationic liquid catalyst, investigated the catalytic effect of this catalyst for the reaction of epoxide and carbon dioxide, and proposed a simple and easy method for the recovery and reuse of brominated DILs. Yue et al. (2022) synthesized novel one-component metal-free functionalized ionic liquids ([N_2,2,2,2OH][BA], [N_2,2,2,2OH][SA], [N_2,2,2,2OH][OAc], [N_2,2,2,2OH][GA], [N_2,2,2,2][BA]) for the catalytic conversion of carbon dioxide to cyclic carbonates in the absence of solvents and co-catalysts. These ionic liquids showed outstanding catalytic activity for the conversion of carbon dioxide and various epoxides to cyclic carbonates. The catalytic activity of [N_2,2,2,2][BA] without the OH functional group was significantly lower than that of [N_2,2,2,2OH][BA]. A relationship between the higher catalytic reactivity and the presence of OH functional groups was shown.

Some of the typical ionic liquid catalysts mentioned above are briefly compared in Table 1.

2.2 Metal-Organic Frameworks (MOFs)

The large pore size and specific surface area of MOFs play an important role in the cyclization reaction of CO₂ under mild conditions. The development of more MOF catalytic systems for CO₂ conversion under mild conditions is also one of the important research directions for CO₂ conversion catalysts (Huh, 2019). Since 2019, many papers on MOFs have also been published. The MOFs involved can be broadly classified into types of copper-based MOFs, zinc-based MOFs, zirconium-based MOFs, bimetallic hybrid MOFs, and two- and three-dimensional MOFs.

 
2.2.1 Copper-based metal-organic frameworks

The introduction of metal Cu into MOFs can effectively catalyze the binding of carbon dioxide to epoxides. Kurisingal et al. (2020a) synthesized two new Cu(II)-based metal-organic frameworks (MOFs), [Cu(BDC)(BPDB)_0.5]_n (PNU-25) and [Cu(NH₂-BDC)(BPDB)_0.5]_n (PNU-25-NH₂) (where BPDB = 1,4-bis(4-pyridyl)-2,3-diaza-1,3-butadiene. BDC = benzene-1,4-dicarboxylic acid). Both catalysts catalyze the fixation of CO₂ to cyclic carbonates at atmospheric pressure, low reaction temperature, and neutral conditions. The structural integrity, ECH conversion, and activity of the PNU-25-NH₂ catalysts remained essentially unchanged after four cycles of repeated use. On this basis, Dhankhar et al. (2021) further prepared porous metal-organic backbone {[Cu₆ (TATAB)₄(DABCO)₃(H₂O)₃]·24DMF}_n(Cu(II)-MOF) (where H₃TATAB = 4,4′,4′′-triazine-1,3,5-trial-triaminobenzoic acid. DABCO = 1,4 diazabicyclo[2.2.2] octane DMF = N, N-dimethylformamide), this catalyst provides a high density of Lewis acid (LA), Cu(II) ions and basic-NH sites and could be used as an efficient heterogeneous catalyst for the solvent-free chemical immobilization of CO₂ in cyclic carbonates with little change in activity after multiple cycles of reuse. Gu et al. (2020) investigated the performance of isomeric polyhedral copper-based MOFs (Cu-PMOFs) in catalytic reactions for the conversion of carbon dioxide and epoxides to cyclic carbonates. The structures of these heterogeneous polyhedral Cu-based MOFs are shown in Fig. 10.

 Fig. 10. Ball-and-stick view of (a) JLU-Liu20, (b) JLU-Liu21, (c) JLU-Liu22 and (d) JLU-Liu46, removing the guest molecule. Color code: Cu = green, C = gray, N = blue and O = red. All hydrogen atoms are omitted for clarity (Gu et al., 2020).

It was demonstrated that the presence of Lewis acid site and Lewis base site can simultaneously facilitate the chemical conversion of CO₂. In a series of controlled experiments, [Cu₆(TADIPA)₃(H₂O)₆]·16H₂O·8DMF (JLU-Liu21) (where H₄TADIPA = 5-5'-(1H-1,2,4-triazole-3,5-diyl) diisophthalic acid) exhibited the most efficient catalytic performance among the four copper PMOFs as well as good cycling stability. Due to the restriction effect of pore size, JLU-Liu21 has good catalytic selectivity for smaller epoxides and low catalytic yield for larger epoxides.

 
2.2.2 Zinc-based metal-organic frameworks

The introduction of zinc metal into MOFs can also enhance the catalytic activity of MOFs. Mohammadian et al. (2020) introduced zinc metal into MOFs and obtained a novel homochiral zinc-containing metal-organic framework for the catalytic conversion of carbon dioxide by defect engineering. The defects introduced in the structure of the zinc-containing MOF not only greatly affected the basic properties of the parent MOF (MOF-5), including stability, morphology, and crystallinity, but also introduced new features in its backbone. properties, including stability, morphology, and crystallinity, but also introduced new features such as chirality in its backbone. Kurisingal et al. (2020b) synthesized Zn metal-based multivariate MOFs by integrating different functionalized ligands into a single framework. Chemically functionalized 1,4-phthalic acid in the form of 2-amino-terephthalic acid (NH₂BDC) and 2-hydroxy-terephthalic acid (OH-BDC) were combined into different groups of multicomponent MOFs. In a solvent-free gas-liquid-solid reactor under mild reaction conditions, this catalytic system was used to catalyze the reaction of epoxides with carbon dioxide to form cyclic carbonates with the selectivity of the reverse use greater than 99%. Compared with other catalysts (MOF-5, MOF-5-NH₂, MOF-5-OH), MOF-5-MIX exhibited higher catalytic activity (98% ECH conversion in 6 h at 50°C) and was a promising catalyst system to catalyze the synthesis of cyclic carbonates at high temperature (120°C) in the absence of TBAB and the presence of TBAB, then catalyze the synthesis of carbonates at low temperatures (50°C). The adaptability of the catalyst to the reaction conditions of fixed CO₂ was further improved. Lan et al. (2021) introduced carboxyl groups in the Zn-MOF catalytic system, and they synthesized Zn-MOF [Zn₂L₂MA·2DMF] (MA = melamine, H₂L = 2,5-thiophenedicarboxylate), a carboxyl-rich zinc group, by a simple room temperature stirring method (Fig. 11).

Fig. 11. Feasible synthesis procedure of Hie-Zn-MOF (Lan et al., 2021).

This metal-organic framework (labeled Hie-Zn-MOF) with layered pores can effectively catalyze the fixation of CO₂ by epoxide. The propylene carbonate yield of Hie-Zn-MOF-TEA (where TEA = triethylamine) with TEA as a protonation agent was 97% at room temperature. In particular, the catalytic activity for the bulky epoxide was significantly higher than that of the corresponding microporous-only Zn-MOF (labeled Mic-Zn-MOF), and the product yield was significantly increased from 64% to 88% when butyl glycidyl ether was used as the substrate. In addition, Lan et al. (2021) proposed a preliminary synthesis of Hie-Zn-MOF catalysts and a feasible catalytic mechanism to immobilize CO₂ on epoxide. The time-saving and labor-saving synthesis strategy and the efficient Zn-MOF catalyst are attractive for CO₂ capture and CO₂ chemical conversion.

 
2.2.3 Zirconium-based metal-organic frameworks

In addition to introducing metals such as Cu and Zn into MOFs, researchers have also tried to introduce Zr into MOFs to prepare various Zr-based MOF catalysts for catalytic conversion of CO₂ with good results. Gao et al. (2022) mixed 4-guanidinobenzoic acid (Gua) with BDC in different molar ratios and injected the guanidinium-based compounds into zirconium MOFs, and designed and synthesized zirconium metal-organic skeletons. Small Zr-MOFs (350 nm), denoted as UiO-66-Gua_0.2(s) (where = (Zr₆O₄(OH)₄(CO₂)₁₂), Gua = 4-guanidinobenzoic acid), could be prepared by implanting Gua containing 20 mol% ligands. Compared to the large size and different guanidinium-based Zr-MOFs, UiO-66-Gua_0.2(s) showed the best activity in catalyzing the cyclization reaction of epoxide with CO₂ in the presence of nitrile rubber catalyst. The yield of the chloropropane carbonate product reached 97% at 90°C and 1 atm of CO₂. In addition, the heterogeneous catalyst of UiO-66-Gua_0.2(s) showed great versatility and better recovery in the conversion of other epoxides. Mu et al. (2020) prepared MOF-808(Zr) catalysts with ligand-unsaturated Zr⁴⁺ centers by a thermal reflux method and a series of catalysts with different Au/MOF808(Zr) catalysts with different Au/MOF808 (Zr) loading rates were prepared by impregnation reduction. These catalysts can effectively catalyze the conversion of CO₂ and aniline/H₂ to N-methylation and N-carbonylation products. The conversion of aniline was 18.4%, the selectivity of N-methyl aniline was 80.9%, and the selectivity of N, N-dimethylaniline was 19.1% over 3.0 wt% Au/MOF-808 (Zr) catalysts.

 
2.2.4 Bimetallic mixed metal-organic frameworks

The introduction of suitable metals such as Zn, Cu, and Zr in MOFs can effectively improve the adsorption as well as the catalysis of carbon dioxide by MOFs. On this basis, the catalytic selectivity of MOFs was improved by introducing two metals into the MOFs to make the reaction conditions milder and the catalytic efficiency higher by using the synergistic catalytic effect of the two metals.

Kang et al. (2019) prepared two stable two-dimensional metal MOFs, {Na[LnCo(DATP)₂(Ac)(H₂O)](NO₃)·DMA·11H₂O}_n (Ln = Er(1) and Yb(2), H₂DATP = 4′-(3,5-dicarboxyphenyl)-2,2′:6′,2′′′-terpyridine), by solvothermal synthesis technique. They have high thermal stability and are highly tolerant to various organic solvents and acid-base solutions over a wide pH range (1–13). Heterogeneous catalysts 1 and 2 showed good catalytic activity and cyclic stability in catalyzing the cycloaddition reaction of 10-substituted azopyridines with CO₂ to produce high-value products. Catalyst 1 was the first heterogeneous metal-MOFs-based catalyst for the cyclization reaction of azopyridine and CO₂. Wu et al. (2020) prepared [CoZn][(BDC)(DABCO)_0.5] (CZ-BDO), [CoNi][(BDC)(DABCO)_0.5] (CN-BDO) and [NiZn][(BDC)(DABCO)_0.5] (NZ-BDO) three bimetallic mixed MOFs to catalyze the cyclization reaction of epoxides with CO₂. The presence of solid solutions in the Co and Zn bimetallic samples provided synergistic catalysis for the cyclization reaction of CO₂. The conversion of epichlorohydrin and the yield of CPC was 99.31% and 97.05%, respectively, within 5 h at 100°C and 3.0 MPa over the best CZ-BDO catalyst with 0.5 wt% ECH as the sample. Kurisingal et al. (2019) synthesized by solvothermal method containing Cu-benzene-1,3,5-tricarboxylate (Cu-BTC) and UiO-66 MOFs binary catalysts as novel catalysts for cycloaddition reactions under solvent-free conditions. They found that UiO-66/Cu-BTC and Cu-BTC/UiO-66 were strong catalysts for the cycloaddition reaction of epichlorohydrin and carbon dioxide under the condition of tetrabutylammonium bromide as the active catalyst, and the selectivity of epichlorohydrin carbon could reach 99%. The binary catalyst UiO-66/Cu-BTC was used repeatedly more than 6 times and the catalytic activity remained essentially unchanged. Ji et al. (2021) synthesized Zr-porphyrin MOFs (PCN-224) MOF using porphyrin ligands as building blocks of the metal-organic backbone. The metalization reaction was carried out by adding different ratios of Zn and Co metal ion solutions, which were pyrolyzed to stable CO₂ conversion catalysts under an argon atmosphere. The concentration of N-containing radicals in the pyrolysis products gradually increased with the increase of the Zn/Co ratio in the precursors. Among them, the PCN-224-ZnCo1-950 catalyst with Zn/Co = 1:1 is a catalyst with higher basicity than the precursor. It has a suitable acid/base center concentration ((acid center concentration)/(base center concentration) = 37.4), which can effectively catalyze the cyclization reaction of carbon dioxide with epoxide. Due to its macroporous structure and laminar pore structure, this catalyst has a good catalytic effect on long carbon chain epoxides.

 
2.2.5 Two-dimensional and three-dimensional MOFs

Yuan et al. (2021) successfully prepared two-dimensional porous metal-organic frameworks [Cd₂(DCBA)·(H₂O)₃]·2DEF, (H₄DCBA = 4″,6′-diamino-5′,5″-bis(4-carboxyphenyl)-[1,1': 3′,1'':3″,1‴-quaterpHenyl ]-4,4‴-dicarboxylic acid, DEF = N,N-dimethylformamide). This catalyst showed better selectivity for CO₂ than CH₄ at 273 K and 298 K and 1 atm. The pore structure, better CO₂ adsorption, open Cd(II) sites, and good recoverability favor the catalytic CO₂ cyclization reaction. Sharma et al. (2019) constructed a novel visible-light-responsive, microporous, three-dimensional supramolecular Mn(II)-porphyrin metal-organic backbone. It efficiently catalyzed the visible-light-assisted cyclization reaction of CO₂ with epoxides to generate the corresponding cyclic carbonates at room temperature and under mild conditions of 1 bar CO₂, with high yields and selectivity of the reaction, and the catalyst could be recycled several times. Das et al. (2020) synthesized a novel three-dimensional microporous bifunctional MOF, {[Zn₂(TDC)₂(DATRZ)]·3H₂O·(DMF)}_n(Zn·DAT) (where TDC = 2,5-thiophene dicarboxylate ion and DATRZ = 3,5-diamino-1,2,4-triazole). The presence of the basic-NH₂ functional pore allows the Zn-DAT MOF to show selective adsorption of CO₂ as a bifunctional heterogeneous catalyst that can catalyze the conversion of CO₂ to cyclic carbonates under mild conditions at room temperature without solvent. This work demonstrated the effect of basic-NH₂ groups on selective CO₂ capture and conversion of CO₂ to cyclic carbonate under mild conditions. Zhang et al. (2021c) prepared bifunctional co-catalysts without co-catalysts under mild conditions to achieve efficient CO₂ fixation. They reported a bifunctional core-shell catalyst MOF@POP (NH₂-UiO-66(Hf)@CoTPy-CAP) (where Co-TPy 5,101,520-Tetrakis(4-pyridyl)porphyrin cobalt(II)) constructed from amino-functionalized NH₂-UiO-66(Hf) and ionic porphyrin-based CoTPy-CAP to catalyze the cyclization reaction of CO₂. It effectively catalyzed the fixation of CO₂ and epoxide under mild conditions (80°C, 0.5 MPa), and the conversion of propylene oxide (PO) could reach 98%. In addition, this bifunctional co-catalyst was easily recovered with no significant decrease in activity after 5 cycles.

The cycloaddition reaction of carbon dioxide with aziridine is an important reaction for obtaining high-value products. Porous MOFs can catalyze this reaction but require the addition of a co-catalyst to improve its catalytic performance. Therefore, Shi et al. (2021) synthesized a porous and stable three-dimensional (3D) framework {[Ni(DCTP)]·6.5DMF}_n (H₂DCTP = 4′-(3,5-dicarboxyphenyl)-4,2′:6′,4′′-terpyridine), which can adsorb up to 104.0 cm³ g^–1 of CO₂ at 273 K. It can effectively catalyze the addition reaction of CO₂ and aziridine without the addition of a co-catalyst and retains its catalytic activity after five reactions. This catalyst is the first MOF material that can be used as a heterogeneous catalyst for the conversion of CO₂ and aziridine without the use of a co-catalyst.

 
2.2.6 Other metal-organic frameworks

Rani et al. (2019) synthesized a series of metal-organic frameworks [Zn-BTC(1), Co-BTC(2), Ni-BTC(3), and Cu-BTC(4)] based on benzotricarboxylic acids [BTCs] with different structures and catalytic properties by solvothermal method. All catalysts followed first-order kinetics and under mild conditions, Zn-BTC(1) showed the strongest catalytic activity with a yield of about 100% for cyclic carbonate and an initial turnover of 946 h^–1 for a total conversion of 18845 mol^–1 cyclic carbonate. The yields of products catalyzed by other metals (Co, Ni, Cu)-BTC were also above 94%.

The reactions of alkylamines with CO₂ can provide high-value-added chemical products. However, most catalysts in such reactions use noble metals to obtain high yields, so it is important to find environmentally friendly noble metal-free MOFs catalysts. Cao et al. (2020) investigated the first environmentally friendly metal-free MOFs catalyst for the cyclization reaction of propargylamine with carbon dioxide. They synthesized a large, lantern-like [Zn₁₁₆] nanocage consisting of six [Zn₁₄O₂₁] clusters and eight [Zn₄O₄] clusters in the zinc tetrazolium 3D framework [Zn₂₂(Trz)₈(OH)₁₂(H₂O)₉·8H₂O]n·Trz = (C₄N₁₂O)₄. It can effectively catalyze the cycloaddition reaction of propargylamine with carbon dioxide, producing only various 2-oxazolidinones under mild conditions. Another Zhou group used a flexible post-synthetic approach for the first time to prepare triazole-like ionic metal-organic frameworks (IMOFs) using a simple click reaction. IMOFs are efficient and recyclable heterogeneous catalysts for the conversion of CO₂ under mild catalyst-free conditions. Hydroxy-functionalized 1,2,3-triazolium (MIL-101-tzmOH-Br) exhibited excellent catalytic in synergy with different active centers’ performance (Zhou et al., 2019a).

Wang and Lv (2021) prepared ultrathin polydopamine microcapsules with layered structures and porosity using MOFs as templates. Three enzymes, carbonic anhydrase (CA), formate dehydrogenase (FateDH), and glutamate dehydrogenase (GDH) were encapsulated in polydopamine (PDA) microcapsules, and this PDA microcapsule containing three enzymes was used to catalyze the reaction of carbon dioxide to formic acid. The batch reaction performed by immobilizing multiple enzymes and NADH with a reaction time of 0.5 h was observed and the highest HCOOH yield of (4.4 ± 1.4) mmol L^–1 was found for the multi-enzyme system based on PDA microcapsules. The conversion of CO₂ was 13.3%. Several types of metal-organic skeletal core-shell microspheres were successfully synthesized by Tsai et al. (2022). They were modified and the coating method was optimized to obtain high stability and good catalytic properties. The modified SiO₂@MOF core-shell microspheres were used as catalysts for the cycloaddition reaction of carbon dioxide and propylene oxide with tetrabutylammonium bromide as a co-catalyst, and the maximum conversion of SiO₂@ZIF-67 could reach 97%, and SiO₂@ZIF-67 could be used for five reaction cycles while maintaining good catalytic performance. Ye and Chen (2021) synthesized organically linked metal-organic frameworks (PS/MOFs) with 3-(pyridinium)-1-propane sulfonic acid (PS) inner salt for catalytic carbon dioxide (CO₂) reductive functionalization to benzimidazole and N-carboxamide. PS/MOFs can efficiently promote a wide range of benzimidazoles by reductive cyclization in 88–99% yields. At low conversion levels of 1,2-phenylenediamine, a linear relationship between catalytic performance (in terms of turnover frequency) and the specific surface area of PS/MOFs was observed. In addition, PS/MOFs can easily catalyze the N-carboxylation of various monoamines to quantitatively synthesize N-carboxylation using CO₂ as the carbonyl source.

Some of the typical MOFs catalysts mentioned above are briefly compared in Table 2.

2.3 Covalent Organic Frameworks (COFs)

As an emerging crystalline porous material, covalent organic frameworks (COFs) have been shown to adsorb gases and act as catalysts for chemical transformations, and many researchers have attempted to use them for the catalytic conversion of CO₂. A multiphase catalyst with a covalent organic skeleton (POM@ImTD-COF) modified by polymetallic oxalate imidazolium ionic liquid was constructed by Zhang et al. (2021c) through the covalent modification of COFs with ionic liquid and electrostatic interaction between pom and ionic liquid. It exhibited high catalytic activity for CO₂ cycloaddition reaction in the presence of additives and at 1 atm, 80°C. The catalytic activity of POM@ImTD-COF did not decrease significantly after 5 times of repeated use. The multiple activations of ionic liquids and paraformaldehyde, the adsorption of CO₂, and the well-dispersed active sites in the COFs were responsible for the remarkable catalytic performance. Li et al. (2021a) reported a COF (Co-BD-COF) (BD = 1,8-dimethylbiphenylene) with an imine-linked cobalt sandwich composite with high crystallinity and a large specific surface area. Co-BD-COF has abundant metal sites and high porosity, which can effectively catalyze the conversion of carbon dioxide to cyclic carbonates. Moreover, due to the strong steric hindrance of phenyl molecules around the cobalt complex, Co-BD-COF exhibits high catalytic selectivity for small ethylene oxide derivatives in the cycloaddition reaction. This novel metal intercalation structure provides a new strategy to improve the catalytic selectivity of COF. With high specific surface area, high nitrogen content, and high physicochemical stability, covalent triazine frameworks (CTFs) have promising applications in gas adsorption and separation, pollutant adsorption, and heterogeneous catalysis. The introduction of both Lewis acid active centers (cations) and Lewis base active centers (halogen anions) into the structures of CTFs can not only improve the adsorption capacity of CO₂ but also greatly improve the catalytic performance of CO₂ cyclization reactions. The carboxylation reaction of terminal alkynes with carbon dioxide to produce propionic acid is an atomically economical and high-value route for carbon dioxide fixation and utilization, but the conversion under mild conditions requires transition metal catalysts. The first transition metal-free organocatalysts (1,3,5-trimethylphenol (Tp)-derived Schiff bases) for this reaction, either homogeneous (discrete molecules) or heterogeneous (COFs), were demonstrated by Zhang et al. (2021a). Their key catalytic sites are phenoxy and imine groups, which activate CO₂ via phenoxy-CO₂ complexation and also C(sp)-H bonds via bifurcated C-H⋯imine and C-H⋯phenoxy hydrogen bonds. COF catalysts are less active than molecular catalysts but have the advantage of heterogeneous catalysis, combining silver nanoparticles (AgNPs) with intrinsically catalyzed COFs for higher catalytic performance. This work opens up the potential for the development of transition metal-free catalysts for CO₂ conversion reactions. Zhao et al. (2022a) successfully synthesized two types of CTFs based on 1,3,5-tris(4-cyanopyridine-1-ylmethyl)-benzene tribromide (TPM) based novel cationic CTFs (CCTFs) for the trimerization reaction of cyanide moieties. The synthesis reaction process is shown in Fig. 12.

Fig. 12. Synthetic route of CTF-TPMs (Zhao et al., 2022a).

The obtained CTF TPMs have abundant pyridine cation sites, high specific surface area (1206 m² g^–1), high CO₂ adsorption capacity (up to 61.4 CCG^–1, 1 bar, 273 K), and good chemical and thermal stability even in strong acid and base solutions. Moreover, in the absence of co-catalysts and noble metals, CTF TPMs can synergistically catalyze the cycloaddition reaction of carbon dioxide with epoxides in the presence of pyridine and halide ions to form the corresponding cyclic carbonates. More importantly, the catalytic performance and structural ramework of CTF TPMs remained essentially unchanged after five cycles, showing good prospects for practical applications.

A hollow porous organic framework (HPOFs) with unique large hollow cavities, graded porous shells, and well-dispersed catalytic active sites was proposed by Xu et al. (2022). The shell thickness of HPOFs can be controlled by adjusting the number of building blocks. Compared with solids, hollow HPOFs accelerate the mass transfer of reactants to catalytic sites and the mass transfer of products outside the catalyst skeleton. At the same time, the layered porous thin shells expose more active catalytic species to interact with the substrate molecules. The catalytic activity gradually increases with decreasing shell thickness using the cycloaddition reaction of CO₂ with 2-chloromethoxyhexane in the absence of solvents, auxiliaries, and additives. This work provides a new idea for the study of multiphase of hollow porous organic skeleton homogeneous catalysts.

 
2.4 Metal-based Materials

Metal-based materials are commonly used as catalysts for CO₂ reduction. Grasso et al. (2019) proposed an efficient process for the in situ recovery of CO₂ and its conversion to CH₄, avoiding the storage and transport of CH₄. The process selectively converts CO₂ to CH₄ under controlled experimental conditions using Mg₂FeH₆ as a hydrogen source. The molar ratio of H₂:CO₂ was found to be 4:1 after heating at 400°C for 5 h. Partial oxidation of magnesium and deposition of carbon were observed after 10 h of reaction. The method involves a dual utilization of low-cost and portable metal hydride phases. On the one hand, the complex hydrides act as a source of hydrogen, and on the other hand, these complex hydrides provide highly selective catalytic species that facilitate the complete conversion of CO₂ to CH₄. A series of Co-based catalysts modified by transition metals (Y, Gd, and Mn) were synthesized by applying the impregnation method by Zhang et al. (2022b). Except for the catalysts modified with Mn promoter, the catalysts modified with other promoters showed better catalytic performance than the 20Co/WC-AC catalyst. Among them, the 20Co-10Y/WC-AC catalyst showed the best catalytic performance in the DMR reaction. the addition of Y enhanced the interaction between the active metal and the carrier and effectively inhibited the sintering of metallic Co at high temperatures, and the Y promoter facilitated the formation of oxygen vacancies and promoted the activation of CO₂. Al-Fatesh et al. (2019) synthesized a nickel-based catalyst using mesoporous silica (MCM-41) as a carrier and metallic gadolinium (Gd) as a promoter and tested its catalytic activity for the dry methane reforming (DRM) reaction when the Gd content was 0.1 wt%, the catalytic performance of the catalyst was significantly improved increasing the conversion of CH₄ and CO₂ to > 87% and > 91%, respectively, and making the ratio of CH₄ and CO₂ to H₂/CO₂ almost equal to 1. Therefore, Gd can be used as a promoter for nickel-based catalysts in the DRM reaction. Kim et al. (2020) synthesized a mesoporous bimetallic aluminum spinel oxide (MAl₂O₄, where M is Mg, Co, Cu, or Zn) and used it as a heterogeneous-based catalyst to catalyze the hydrogenation of CO₂ without additional catalytic metal components. The main product produced by most catalysts is CO (> 80%), with relatively small amounts of CH₄ and CH₃OH. When CoAl₂O₄ is used as a catalyst, CH₄ becomes the major product (> 80%). A systematic relationship was found between the number of strongly basic sites and the CO₂ conversion rate. As the number of strongly basic sites increased, the CO₂ conversion increased linearly at all tested reaction temperatures (300–400°C). Furthermore, due to the high specific surface area of the mesopores, molecules could easily diffuse through their mesoporous channels, leading to an increase in the catalytic activity of the spinel oxides. It is demonstrated that the strongly basic sites contribute to the effective activation of carbon dioxide with small energy input. Therefore, these spinel oxides can be further used to design versatile catalysts. Velpuri and Muralidharan (2021) reported a catalytic system based on copper-zinc mixed metal oxide (CuO-ZnO) nanosheets/macro sheets (Cozi-nmf) catalyzing the addition reactions of CO₂ with various epoxides at room temperature and solvent-free conditions, with yields above 95% for most reactions synthesized with cyclic carbonates, catalysts, and cyclic carbonates are synthesized more environmentally friendly, starting materials are cheaper, and the preparation of efficient iron-based catalysts is a reliable and achievable goal for catalytic CO₂ hydrogenation. Kosol et al. (2021) found that ethylenediamine as a benign modifier can well modulate the surface properties of carbon carriers and achieve good dispersion of active small-sized iron carbide sites. The selectivity of heavy hydrocarbons (C₅₊) increased from 14.8% to 39.8% with the further addition of alkaline K additives. The simultaneous incorporation of nitrogen atoms and alkali metals provides a good approach for the rational design of carbon catalysts with CO₂ immobilization and functionalized metal carriers. Wang et al. (2020a) developed a multifunctional catalyst consisting of sodium-modified iron-based catalyst and hollow acidic zeolite H-ZSM-5 to catalyze the single-pass CO₂ hydrogenation to aromatics reaction and obtained high aromatic yields (203.8 g (CH₂) kg(cat)^–1 h^–1). A mesoporous Ta-W composite oxide prepared by a modified hydrolysis method catalyzed the cycloaddition reaction of epoxide and carbon dioxide by Yin et al. (2022). The best yield of the resulting styrene carbonate was obtained when the Ta/W molar ratio was 2:1 (Ta_0.67W_0.33Os). Under the optimal reaction conditions, the styrene oxide conversion and styrene carbonate selectivity reached 95 and 97%, respectively. With the increase of tungsten content, the particle size distribution of tungsten became more and more uniform. Among them, the Ta_0.67W_0.33Os catalyst has the strongest acid-base strength. It is shown that the acidity of the catalyst surface and the synergistic effect of adjacent bases are responsible for its excellent catalytic activity.

 
2.5 Porous Organic Polymers (POPs)

POPs with an intact metalloporphyrin structure is typically efficient catalysts for CO₂ cycloaddition reactions.

By polymerizing 1,10-phenanthroline and triphenylene followed by Co coordination, Zhang et al. (2021b) synthesized a new Co-Phen-POP material that heterogeneously catalyzes the generation of PC from atmospheric CO₂ and PO at room temperature very efficiently and economically. the TOF is 263 h^–1, more than 6 times that of the homogeneous molecular analogue. this high performance of Co-Phen-POP originates from the combination of cobalt reactive material and a polymer with permanent porous organic polymers with permanent porosity, high BET surface area, and high CO₂ capture capacity. Yuan et al. (2022) synthesized a series of metal-functionalized porous organic polymers (M-salen-POP, M = Mn²⁺, Co²⁺, Ni²⁺) by a simple metal-assisted method Co-salen-POP catalyzed the cycloaddition reaction of CO₂ and styrene oxide, and the yield, as well as the selectivity of the products, could reach 96% and 99%, respectively, within 30 min at 100°C and 1 MPa CO₂ atmosphere. After five cycles, the catalytic yield of styrene carbonate was still 94% under the same conditions. In addition, other epoxides, such as epichlorohydrin and phenyl glycidyl ether, could also be completely converted within 5 min at 100°C and 1.0 MPa. This study provides a simple and effective method for constructing efficient CO₂ immobilization heterogeneous catalysts with abundant active centers. The porous organic polymers CorPOP-1(Mn) and CorPOP-1(FeCl) constructed from custom-designed Mn and Fe-corrole composite building blocks were reported by Zhao et al. (2022b). Under mild reaction conditions, CorPOP-1(Mn) containing Mn-corrole active center exhibits superior multiphase catalytic activity for the solvent-free cycloaddition of carbon dioxide with epoxides to form cyclic carbons compared to homogeneous catalysts. CorPOP-1(Mn) can be readily recovered and does not exhibit a significant loss of reactivity after seven consecutive cycles. This work highlights the potential of metallocorrole-based porous solid catalysts for targeting CO₂ transformations and would provide a guide for the task-specific development of more corrole-based multifunctional materials for extended applications. Zhang et al. (2022a) prepared porphyrin-based conjugated microporous polymer (Co-Po-POPs) microspheres using a simple self-polymerization strategy, which exhibited good catalytic performance for the cycloaddition reaction of free solvents with CO₂ at ambient temperature and pressure. Even with dilute CO₂ (15% CO₂ in N₂), Co-Po-POP₂ can provide up to 8.1 h^–1 TOF at ambient conditions (25°C, 1 bar). this study highlights the potential for the sustainable industrial synthesis of cyclic carbonatites. A series of two-dimensional (2D) bimetallic salt-based porous organic polymers (BSPOPs) (BSPOP-Al, BSPOP-Co, and BSPOP-Ni) were synthesized by the reaction of 2,3,6,7,14,15-hexaammonium tributylenes with 2,6-diformyl-4-methylphenol in the presence of metal salts (i.e., aluminum chloride, cobalt acetate, and nickel acetate) by Zheng et al. (2021). They have a high affinity for carbon dioxide. Their porosity and the synergistic effect arising from the high affinity between carbon dioxide and Lewis acidic metal ions, as well as the many active catalytic sites in the channel, lead to an efficient coupling reaction of epoxides with carbon dioxide on BSPOP-Co.

 
2.6 Nanocatalysts

Nanocatalysts for the catalytic conversion of CO₂ are a class of catalysts that have been studied by researchers. For nanocatalysts, the catalyst particle size and particle size distribution, the ratio of contained particle sizes, and the carbon deposition on the catalyst after multiple cycles have a great impact on the catalyst performance. Catalysts with smaller particle sizes, more uniform particle size distribution, and less carbon deposition tend to be more beneficial for CO₂ resource recovery.

The nanocatalysts studied so far have been mainly used for addition reactions of carbon dioxide and epoxides and the co-conversion of carbon dioxide and methane to synthesis gas. For the reaction of CO₂ with epoxides, Yao et al. (2020) prepared and characterized periodic mesoporous organosilica-supported cobalt nanoparticles Co-PMO-IL(x) (where POM = periodic mesoporous organosilica) based on benzotriazole ionic liquid as efficient and practical heterogeneous nanocatalysts to catalyze the cyclization reaction of carbon dioxide and epoxide under solvent-free and additive-free conditions (Fig. 13).

Fig. 13. Catalytic cycloaddition of CO₂ to epoxides with Co-PMO-IL (Yao et al., 2020).

The catalysts exhibited strong catalytic activity in the reaction, especially the Co-PMO-IL(1.0) catalyst. Moreover, these nanocatalysts can be easily recycled and reused at least five times with essentially unchanged catalytic activity. Xiao and Hu (2021) synthesized periodic mesoporous organosilica PMO@ILCu₂(OH)₃Cl_x modified copper oxychloride anionic benzotriazole ionic liquids as efficient, green, recyclable heterogeneous nanocatalysts to catalyze the cyclization of carbon dioxide with cyclization reaction of epoxides for the synthesis of cyclic carbonates. Compared with other nanocatalysts, PMO@ILCu₂(OH)₃Cl₂(1.0) nanocatalysts exhibited higher catalytic activity with good yield and selectivity under solvent-free and co-catalyst-free conditions. The catalytic process showed good stability and recoverability with no significant loss of catalytic activity for at least five runs.

For the co-conversion reaction of methane and carbon dioxide, Ali et al. (2020) reported an efficient coke-resistant Ni-based nanocatalyst. Nickel-based catalysts supported by alumina with a metal loading of 5 wt% were prepared by solution combustion synthesis (SCS) and conventional wet impregnation methods. The Ni-SCS nanocatalysts exhibited higher activity and stability in the dry conversion of methane compared to the conventional nickel-impregnated (Ni-I) catalysts. In addition, the conversion of methane and carbon dioxide, as well as the percentage yields of hydrogen and carbon monoxide were significantly higher. After 50 h of catalytic reaction, the Ni-I catalyst was severely deactivated due to severe carbon deposition, in contrast, the Ni-SCS catalyst exhibited slight carbon deposition and maintained high activity. Chen et al. (2020) prepared aluminum-doped MgH₂ nanocatalysts by reactive ball milling in an H₂ atmosphere using magnesium as raw material. The aluminum-doped MgH₂ nanocatalyst can achieve 88.4% CH₄ selectivity and 27.1% CO₂ conversion at 320°C when the H₂/CO₂ ratio is 5/1. Shafiee et al. (2021) prepared Nie-Co-Al₂O₃ catalysts with different Co loadings using a simple solid-phase synthesis method. It was found that the addition of cobalt to the Ni-based catalysts improved the catalyst activity. The 15 wt% Ni-12.5 wt% Co-Al₂O₃ sample with a specific surface area of 129.96 m² g^–1 showed the highest catalytic performance in the CO₂ methanation reaction (76.2% CO₂ conversion and 96.39% CH₄ selectivity at 400°C), and the catalyst exhibited a high degree of stability within 10 h of operation.

In addition to cycloaddition reactions of carbon dioxide and dry reforming reactions of methane, some researchers have combined nanocatalysts with semiconductor materials, plasmas, and other materials for the conversion of carbon dioxide into other high-value products such as formic acid, ethylene, 2-oxazolidinone, and benzimidazole.

Metal nanoparticles supported on semiconductor surfaces are considered potential nanocatalysts for converting carbon dioxide into energy-carrying molecules, such as formic acid or carbon monoxide, which can be used as feedstock for fuel synthesis. Dziadyk et al. (2020) studied bimetallic Cu/Ni nanoparticles supported on zinc oxide. They found that the hydrogenation of CO₂ to formate over CuNi/ZnO catalysts was more favorable than the carboxylate route. A schematic representation of the reaction of the conversion process on the CuNi/ZnO nanocatalyst model is shown in Fig. 14.

Fig. 14. Schematic representation of the reaction of the conversion process on the CuNi/ZnO nanocatalyst model (Dziadyk et al., 2020).

Sivachandiran et al. (2020) used a Ni/γ-Al₂O₃ nanocatalyst coupled non-thermal plasma dielectric barrier discharge reactor (NTP-DBD) to hydrogenate CO₂ to CH₄. Compared to conventional thermal catalysis (300°C), plasma catalysis shows a temperature shift (T shift) of 50°C. The plasma discharge slightly increases the particle size of a nickel, but it does not affect the conversion of CO₂ and the selectivity of methane. At 250°C and specific input energy (SIE) of 340 J L^–1, the 10 wt% Ni/γ-Al₂O₃ nanocatalyst showed about 40% CO₂ conversion and 70% CH₄ selectivity. At lower operating temperatures, increasing SIE resulted in about a 10% improvement in CO₂ conversion and methane selectivity. Yu et al. (2021b) used a direct hydrothermal synthesis method to obtain the Al/FPS catalytic system by the controlled assembly of catalytic active sites on the fibrous backbone of silicate phosphate (FPS), and aluminum (Al) was used to modify its surface properties by doping as heteroatoms with different silica-aluminum ratios. Compared with other reported catalysts, the Al/FPS catalytic system can catalyze the carboxylation reaction of styrene oxidation by carbon dioxide in higher yields and under milder conditions. He et al. (2021) synthesized PrVO₄ nanoparticles supported by Sn-doped dendritic nanosilica (SnD) as catalysts (PrVO₄/SnD) by an in situ procedure. It is a potentially highly active catalyst to stabilize the CO₂ production of 2-oxazolidinone and benzimidazole and is easily recyclable and reusable. Rajesh et al. (2020) developed a unique one-pot hydrothermal preparation of polyols for the lattice growth of AgO in Fe₃O₄ octahedra (Fe₃O₄@Ag-40, 10, 6, 2 at%). The AgO nanoparticles on the material surface and within the Fe₃O₄ body were 40 at%, and the AgO lattice was extensively doped with Oh Fe²⁺ substitution vacancies even at concentrations as low as 2 at%. These hybrid interfacial catalysts effectively activated alkynes and CO₂ substrates to form new lactones in yields as high as 85%, and the materials could be magnetized for recovery for further use. A novel nanocatalyst Ru-Co-Pt/C was prepared by Yu et al. (2022). It was found that Ru-Co-Pt/C exhibited excellent catalytic performance in the conversion of CO₂ and H₂ to C²⁺ compounds (C₂–C₂₂) with an exceptional selectivity of 90.1% at 130°C. DFT calculations showed that compared to Ru or Pt nanoclusters, small Ru clusters anchored on Pt nanoclusters exhibit lower energy barriers in CH₂ + CH₂ coupling and those for CH₂ or CH₃ hydrogenation, which is excellent selectivity for C²⁺ products. This work not only provides insight into the origin of the high C²⁺ selectivity of the prepared novel catalysts but also provides an ideal target structure for the development of efficient and highly selective catalysts for the conversion of CO₂ to multi-carbon compounds, providing a promising pathway for understanding the extraordinary catalytic properties of metal nanocluster catalysts based on theoretical studies.

 
2.7 Metal-free Green Catalysts

The development of environmentally friendly metal-free green catalysts is an important and challenging research direction for the future. Research articles have been gradually published on metal-free catalysts, such as biomass catalytic materials and nitrogen-doped carbon catalytic materials. These catalytic materials are less harmful to the environment, very economical, do not require the use of expensive metals, and are very promising.

 
2.7.1 Nitrogen doped carbon catalyst

The development of environmentally friendly metal-free green catalysts is an important and challenging research direction for the future. Research articles have been gradually published on metal-free catalysts, such as biomass catalytic materials and nitrogen-doped carbon catalytic materials. These catalytic materials are less harmful to the environment, very economical, do not require the use of expensive metals, and are very promising. For the reaction of CO₂ with epoxide, Mujmule et al. (2020) synthesized heterogeneous carbonaceous materials containing different carboxyl, hydroxyl, and amine groups by a simple and straightforward hydrothermal method using carbonaceous materials of glucose, oxalic acid, and urea under solvent-free conditions, and added a certain amount of KI as a co-catalyst in a catalytic system, which can efficiently and stably catalyze the epoxide reaction with good recyclability of the catalyst. Liu et al. (2021a), on the other hand, synthesized N-doped mesoporous carbon spheres (N-MCSs) catalysts by the dissolution-recombination method using ammonium hydroxide as the nitrogen source and a base catalyst. The BET-specific surface area and porosity of N-MCSs were 1341 m² g^–1 and 1.47 cm³ g^–1, respectively. The catalysts were a one-component halogen-free and metal-free catalyst that catalyzes the cycloaddition reaction of carbon dioxide and epichlorohydrin with a TOF of up to 236 h^–1. This N-MCS material has good stability and reusability. Zhang et al. (2020) synthesized and characterized a novel 1-bromomethyl-4-hydroxycalix[4]ane and tris(4-imidazolyl)amine group based cationic polymers as metal-free bifunctional heterogeneous carbon dioxide catalysts. The polymer has an excellent activity to catalyze the formation of cyclic carbonates from epoxides. More importantly, this bifunctional catalyst is easily recyclable and maintains its catalytic properties well, providing a new approach for the synthesis of competitive green heterogeneous catalysts. Tang et al. (2019) prepared N-doped porous carbon (NPCs) materials by pyrolysis of a mixture of tannic acid and urea for the methylation conversion of CO₂. The synthesis process and analysis are shown in Fig. 15. Under mild conditions (1 bar CO₂, 75°C), the catalyst showed good catalytic activity for the methylation reaction of CO₂. The feasibility of using NPCs as a C₁ resource to catalyze the methylation of amino compounds for the production of N, N-dimethylamine was demonstrated for the first time.

Fig. 15. (a) Synthesis of NPC. (b) XRD patterns of NPC samples. TEM and HRTEM images of (c and d) NPC(1/2); (e and f) NPC(1/3); (g and h) NPC(1/5); and (i and k) NPC(1/7) (Tang et al., 2019).

 
2.7.2 Biomass catalyst

Carbonic anhydrase (CA) has shown great potential in mitigating CO₂ emissions through enzymatic conversion of CO₂, while its commercial implementation is limited by the drawbacks of thermal and chemical instability, high environmental sensitivity, and high-cost. To overcome these drawbacks and develop advanced technologies for mitigating CO₂ emissions, Zhang et al. (2021d) designed and synthesized a mimetic CA (ZnHisGly), a zinc-based deep eutectic solvent (DES) with the characteristics of natural CA, to facilitate the hydration and conversion of CO₂. This bionic CA is characterized by easy synthesis, high stability, and low cost. Its catalytic performance increases greatly with the increase of pH and temperature, which provides a broad prospect for the industrial application of DES-based CA mimics. Two novel Zn-capped polyoxometalate-based organic-inorganic hybrids, {[α-PMo^V₂Mo^VI₁₀O₃₉(OH)Zn₂][bbbm]₃}·0.5C₂H₅OH (1) and TBA₂{[ε-PMo^V₈Mo^VI₄O₃₇(OH)₃Zn₄][phim]₃} (2) ((where bbbm = 1-(4-imidazol-1-ylbutyl) imidazole) and Phim = 2-phenylimidazole) were successfully synthesized by hydrothermal method by Zhu et al. (2022) (where bbbm = 1-(4-imidazole-1-butyl)imidazole) and phim = 2-phenylimidazole). Compounds 1 and 2 acted as efficient multiphase catalysts and showed good catalytic activity in the desulfurization process. In particular, Compound 2 not only enabled satisfactory catalytic oxidation of dibenzothiophene (DBT) at 55°C to give the more valuable dibenzothiophene sulfone with almost 99% conversion but also showed satisfactory catalytic effects for the oxidation of various epoxides in CO₂ cycloaddition reactions, which demonstrated the potential of Compound 2 as sulfur oxidation and CO₂ cycloaddition dual This indicates the potential of Compound 2 as a dual functional material for sulfur oxidation and CO₂ cycloaddition. A novel arginine-glucose-derived carbonaceous material-loaded silica composite catalyst (Ar-CM/SiO₂) was synthesized by a simple hydrothermal process using non-toxic and non-hazardous reagents (arginine, glucose, and tetrachlorosilicate) by Ye et al. (2022). The silica served as a support for the carbonaceous material and provided an additional hydrogen bond donor (HBD) group. The carbon dioxide cycloaddition reaction with propylene oxide under metal-free and solvent-free conditions at 40 degrees C and 2 MPa resulted in a 62% yield and 99% selectivity for the production of propylene carbonate. Repeated use for six cycles showed no significant decrease in catalytic activity or structural deterioration.

 
2.8 Catalysts of Other Materials

In addition to the above types of catalysts, many other types of catalysts have been creatively developed to expand the research on CO₂ conversion catalysts. Ye et al. (2020) obtained a bifunctional Ir1-In₂O₃ single-atom catalyst to catalyze CO₂ hydrogenation reaction in liquids by immobilizing single atoms of Ir on an In₂O₃ carrier and integrating two active catalytic centers. The catalyst showed high selectivity for ethanol (> 99%) and a high initial turnover rate (481 h^–1). Characterization revealed that the isolated Ir atom couples to an adjacent oxygen vacancy to form a Lewis acid-base pair, which activates CO₂ and forms a carbonyl group (CO*) adsorbed on the Ir atom. This CO* couples with methanol adsorbed on In₂O₃ to form a C-C bond. This bifunctional monoatomic catalyst achieved the catalytic conversion of CO₂ by synergistically exploiting the different catalytic effects of monoatomic sites and substrates, providing a new idea for catalyst design for complex catalysis. Xin et al. (2020) synthesized and characterized a series of rare-earth metal complexes (including Y, Sm, Nd, and La) stabilized by polydentate N-methylethylenediamine linked tris(phenolate) ligands. Under ambient conditions (i.e., room temperature, 1 bar CO₂), the lanthanide complexes showed good activity in catalyzing the cyclization reaction of terminal epoxides with CO₂ with a 49–99% yield of cyclic carbonates. In addition, the lanthanide complexes are well reusable and their activity remains essentially unchanged after six cycles. This is the first rare-earth-based catalyst to achieve efficient cyclization of carbon dioxide and epoxides under ambient conditions. Song et al. (2020) prepared a lignin-based porous organic polymer (P-(L-FeTPP)) (TPP = tetraphenylporphyrin) with a specific surface area of up to 1153 m² g^–1 by Friedel-Crafts reaction of lignin and metalloporphyrin. The P-(L-FeTPP) with iron porphyrin as the active center can effectively catalyze the cyclization of epoxide with CO₂ under solvent-free conditions. The product yield was up to 99.6%. Wang et al. (2022) synthesized new Ni-LDO catalysts derived from water talc by sequential precipitation for CO₂-catalyzed reduction synthesis of natural gas (SNG) and obtained significant performance improvement by tuning the precipitation process, with a 7-fold higher CO₂ conversion per gram of nickel than conventional comparisons at 573 K and atmospheric pressure. In addition, the desired product selectivity was up to 99%. When some Mg²⁺ and Al³⁺ are precipitated in the first step, the optimized catalyst sample has an increased Ni particle size and increased coverage of medium to strong basic sites. The enrichment and reduction of Ni on the catalyst surface can also be achieved by using the sequential precipitation method. The results of this work provide a new route for the preparation of more active hydrotalcite-derived Ni-LDO layered catalysts. Su et al. (2020) prepared novel inexpensive CO₂ imprinted adsorbents on the surface of sunflower seed activated carbon using glycolic acid as a template molecule and acrylamide as a functional monomer by surface imprinting technique. The effects of the ratio of KOH to sunflower-based activated carbon, carbon dosage, and adsorption temperature were investigated, and it was found that the adsorption performance was optimal at the alkali-carbon ratio of 0.75:1, carbon dosage of 0.75 g, and adsorption temperature of 20°C. Under these conditions, the maximum adsorption capacity of the adsorbent for CO₂ was up to 1.71 mmol g^–1. The adsorbent after adsorption was effectively regenerated by N₂ purging at 120°C. After five adsorption/desorption experiments, the CO₂ adsorption capacity decreased by only 11%. The results indicate that sustainable and inexpensive CO₂ imprinted adsorbents have good adsorption, selectivity, and regeneration properties, and the idea of combining such imprinted adsorbents with catalytic materials with active sites for proper activation is worthy of further study and application in the field of CO₂ conversion.

 
3 SUMMARY AND OUTLOOK 

To achieve large-scale processing of CO₂ resources, CO₂ conversion catalysts must, on the one hand, be able to achieve high CO₂ conversion efficiency and high selectivity of the target product at ambient temperature and pressure conditions. On the other hand, the catalytic materials needed to make the catalysts should be cheap enough to be produced in large quantities at a low cost. In addition, the cycle stability of the catalyst is also very important, as good cycle stability can reduce the cost of CO₂ resource utilization. Ionic liquids seem to be the most promising catalytic materials among these materials. Ionic liquid-based catalysts have higher catalytic activity and are more environmentally friendly. In addition, ionic liquid catalysts are also more modifiable, thus making ionic liquid-based catalysts more versatile. As shown in Table 1, Proper immobilization and functionalization of ionic liquid catalysts can achieve phase change of ionic liquid catalysts, improve the stability and catalytic activity of ionic liquid catalysts, and reduce the temperature and pressure required for catalyst operation. As seen above and in Table 2. The introduction of Cu, Zn, and Zr metals into MOF can significantly improve the catalytic conversion of CO₂. The adsorption of CO₂ by MOF catalytic materials has an impact on the efficiency of CO₂ catalytic conversion by MOF, and the two-dimensional and three-dimensional MOF further improves the adsorption capacity of CO₂. Therefore, it is believed that the efficiency of CO₂ catalytic conversion can be further improved if the two-dimensional three-dimensional metal-organic frameworks and ionic liquid catalytic materials are further combined. Compared with MOFs, COF catalytic materials have been less studied and applied. To improve the catalytic activity of COFs, we can refer to the improved method of MOFs and combine COFs with metal or ionic liquid materials to improve the catalytic performance by using the synergistic effect. However, the disadvantages are that the raw materials are expensive and the metal-based materials are harmful to the environment. Secondly, the recycling techniques and means need to be further optimized to improve the stability of the metal materials and achieve hundreds or even thousands of cycles. Multi-vacancy organic polymers have high CO₂ adsorption properties, and it is believed that better catalytic performance can be achieved by introducing other catalytic materials with higher active sites such as transition metal ionic liquids with appropriate functionalization. The catalytic activity of nanocatalysts is mainly influenced by the particle size of nanoparticles, the ratio of active sites, and the carbon deposition after recycling. The catalytic activity of these catalytic materials as well as metal-free green catalytic materials such as nitrogen-doped carbon is not as good as the more mature materials in previous studies, but these metal-free green materials are very promising and need further optimization to achieve breakthroughs in high catalytic activity. Therefore, there may be more room for the development of these materials. Biomass materials are usually rich in functional groups, such as hydroxyl, alcohol hydroxyl, and carboxyl groups that can be used as hydrogen bond donors (Guo et al., 2020). Appropriate adjustment of biomass materials can make them non-toxic, efficient, green, cheap, and renewable catalytic materials for CO₂ absorption and conversion, further improving the greenness of catalysts, and using materials from nature to control pollution and protect nature, such materials are highly desirable, but these materials are still relatively backward in all aspects of catalytic performance, and more basic research is needed before they can reach the level of practical application.

In addition to some problems faced by each material, in general, catalysts can still be broken through from the following points. (1) Most of the end products of CO₂ thermal conversion reactions reported in current research are compounds containing CO bonds, which limits the innovation of CO₂ conversion reactions, so we can further investigate new products of CO₂ resourcefulness, for example, whether we can consider using CO₂ to react with sulfur-containing waste gas to construct CS compounds, using CO₂ and nitrogen oxides to react to construct CN compounds, etc. (2) The simultaneous removal or resource recovery of multiple waste types in one reaction is highly desirable both from the perspective of resource conservation and pollutant management efficiency. Therefore, the synergistic catalytic conversion of CO₂ and other pollutants such as H₂S, SO₂, NO_x, and VOCs can also be an innovative direction for CO₂ resource recovery. (3) Catalysts that meet the final industrial applications need to have low cost, high stability, low viscosity, high efficiency, high selectivity, and easy separation. Therefore, catalyst materials and structures need to be adapted and refined to meet these performance requirements (Chen and Mu, 2019). (4) Improving the ability of catalysts to handle low CO₂ gas concentrations. (5) Develop more efficient and low-cost catalyst synthesis methods to achieve a large-scale synthesis of catalysts in industrial production. (6) Further explore environmentally friendly, green, and low-cost catalytic materials, such as biomass catalysts and nitrogen-doped carbon catalysts.

 
ACKNOWLEDGMENTS

This work was supported by the Key Research and Development Program of Shandong Province, China [2017GSF217006] and the Scientific Innovation Program of Shenzhen City, China, under basic research program [JCYJ20170818102915033].

 
DISCLAIMER

The authors declare no conflict of interest with this study.