English

• 1. Introduction

  The burning of fossil fuels contributes to a major CO₂ emission to our planet. The anthropogenic CO₂ discharge outpaces the natural carbon cycle in natural environment, leading to current global climate changes and adverse impact on humankinds. To establish a sustainable society, reliable technologies for CO₂ capture and conversion (C3) are certainly important and imperative [1-6]. CO₂ can be converted into valuable chemicals through chemical fixation [7], hydrogenation [8], photocatalysis [9], electrocatalysis [10] and photoelectrocatalysis [11]. However, the efficient CO₂ conversion is hampered by the high chemical stability of the C＝O bond (bond enthalpy, 805 kJ/mol), which requires a high energy input for bond cleavage [12]. Therefore, efficient catalysts that can capture and activate the inert CO₂ molecules are highly desirable.

  Various kinds of porous materials have been investigated for the C3, including ionic liquids (ILs), zeolites, porous carbons, porous organic polymers, covalent organic frameworks (COFs) and metal–organic frameworks (MOFs). Among them, MOFs are promising candidates for C3 application due to their high porosity, surface areas, structural diversity and functional adjustability [13-16].

  The plentiful reticular chemistry of MOF structures offers opportunities for the rational design of MOF composites [17, 18].

  Firstly, the cavities of MOFs can be used for CO₂ adsorption. In addition, a range of functional materials such as homogeneous catalysts, semiconductors, metal nanoparticles (MNPs) and quantum dots (QDs), which are beneficial for CO₂ activation, can be immobilized into MOFs, giving rise to MOF composites. Moreover, the definite and uniform structures of MOFs will benefit the study of the interactions between CO₂ and frameworks at a molecular level. Last but not the least, synergistic effect between MOFs and the functional materials can effectively improve the catalytic performances of MOFs composites. With the integration of MOF and functional materials, the MOF composites combine their merits: on one hand, the MOFs possess high porosity and tailorable structure, and on the other hand, the functional materials have excellent catalytic, optical or electrical properties, and therefore they can display enhanced activity, selectivity, and stability in the catalytic applications (Fig. 1).

  Figure 1

  

  Figure 1.  The advantages of MOF composites for the CO₂ capture and conversion.

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  This review focuses on the syntheses and catalytic applications towards C3 of MOF composites, which is divided into three sections. The first section gives a brief introduction about the synthetic strategies of MOF composites. The second section discusses the recent progress of MOF composites in C3, including CO₂ chemical fixation, hydrogenation, photoreduction, electroreduction and photoelectroreduction. The third section summarizes the challenges and future prospects of MOF composites for C3.

  2. Synthetic strategies of metal–organic framework composites

  In general, the preparation of MOF composites is achieved either via the encapsulation of functional materials in the pores or layers of MOFs or coating MOFs with functional supports/layers [17-20]. According to the different types of functional materials, MOF composites that were used for C3 application can be classified into four types as following:

  (1) MNPs@MOFs. There were three methods for the preparation of MNPs@MOFs: 1) Ship-in-bottle strategy. The precursors were dispersed inside the pores of MOFs, and followed by the in situ formation of the MNPs via reduction. 2) Bottle-around-ship strategy. The MNPs were prepared firstly, and then MOFs grew in situ on the surfaces of the MNPs. 3) One-pot synthesis. The MNPs and MOFs were formed simultaneously to afford the MNPs@MOFs.

  (2) Semiconductor (or QDs)@MOFs. They could be prepared by the ship-in-bottle or bottle-around-ship strategy. The stability of semiconductor or QDs can be improved by the encapsulation into the pores of the MOFs, and meanwhile their size- dependent electronic or optical properties can be maintained.

  (3) Homogeneous molecular catalysts@MOFs. They were prepared via covalent bonding or non-covalent interactions between the molecular catalysts and MOFs. After the homogeneous molecular catalysts were immobilized into MOFs, not only their catalytic performances remained but also the side reactions such as the oxidative self-degradation could be overcome.

  (4) Ionic liquids@MOFs (ILs@MOFs). They could be prepared through tandem postsynthetic modification, capillary action or in situ polymerization of ILs inside the MOF pores. After incorporating ILs into MOFs, the resultant ILs@MOFs were formed.

  3. Metal-organic framework composites for CO₂ capture and conversion

  3.1 Cycloaddition of CO₂ and epoxides

  Cycloaddition of CO₂ and epoxides represents one of the few promising industrial processes in which CO₂ can be utilized under mild temperature and pressure. The value-added carbonate products are important chemical intermediates that can be utilized for a variety of fine chemical synthesis and fabrication of functionalized materials. Enhanced activity and selectivity for the coupling reaction of CO₂ and epoxides can be achieved through the encapsulation of functional species within the porous framework [21-25]. As summarized in Table 1, a range of MOF composites for the CO₂ chemical conversion have been reported.

  Table 1

  

  Table 1.  Application of MOF composites in the CO₂ chemical conversion.

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  Jiang et al. incorporated the imidazolium-based poly(ionic liquid)s (denoted as polyILs) into MIL-101 to prepare the polyILs@MIL-101 via the in situ polymerization of ILs (Fig. 2) [21]. The composite polyILs@MIL-101 showed a higher isosteric heats of CO₂ adsorption (Q_st) value (44 kJ/mol) at zero coverage than that of MIL-101 (26 kJ/mol), indicating that a stronger affinity existed between CO₂ molecules and polyILs@MIL-101. The synergistic effect from the exposed Lewis acid sites (Cr³⁺) in MIL-101 and the Lewis base sites (Br⁻) in polyILs contributed to the activation and subsequent reaction of CO₂ and epoxides. The composite exhibited an excellent catalytic activity toward the cycloaddition of CO₂ and epoxides under mild conditions (1 bar CO₂, ≤ 70 ℃) in the absence of any cocatalyst (Table 1, entry 1). In addition, polyILs@MIL-101 did not show any drop of the catalytic activity at a CO₂ pressure ranging from 1.5 bar to 0.75 bar, and a decent activity (~80%) can be maintained even under 0.5 bar of CO₂ pressure. PolyILs@MIL-101 can be recycled for 10 runs without a significant loss of activity.

  Figure 2

  

  Figure 2.  The preparation of polyILs@MIL-101. Reproduced with permission [21]. Copyright 2018, American Chemical Society.

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  Sun and Ma et al. incorporated a linear ionic polymer (IP) into the pores of MIL-101 to afford the IP@MIL-101 [22]. The Lewis acid site (Cr³⁺) within the MIL-101 and the nucleophilicity of the Br⁻ ions cooperatively interacted to significantly enhance the catalytic performances. IP@MIL-101 exhibited higher conversions (99%) than the individual parts or a physical mixture of both (Table 1, entry 2).

  Yang and Chen et al. developed an in situ defect-formation strategy to obtain the hierarchical mesoCu@Al-bpydc [23]. This composite was constructed by the loading of copper precursor Cu(BF₄)₂ into the as-synthesized Al-bpydc via a wet-impregnation method (Fig. 3). The defect-mesopore was formed within microporous Al-bpydc. The BF₄⁻ anion induced the formation of mesopore within Al-bpydc, and the anchored Cu²⁺ ions functioned as Lewis acid catalysts for the CO₂ cycloaddition reaction. Under low pressures (P/P₀ < 0.2), mesoCu@Al-bpydc presented a much higher CO₂ sorption capacity than Al-bpydc. Catalytic results showed that mesoCu@Al-bpydc displayed high reactivity and selectivity for the CO₂ cycloaddition reaction with a range of epoxides, which should be attributed to the active Cu²⁺ Lewis acid sites and bipyridine N basic sites as well as the strong affinity of the composite toward CO₂ (Table 1, entry 3). Moreover, the hierarchically structure could further enhance the mass transfer efficiency of the substrate. The mesoCu@Al-bpydc can be recycled for 5 runs without loss of activity.

  Figure 3

  

  Figure 3.  The synthesis of mesoCu@Al-bpydc. Reproduced with permission [23]. Copyright 2019, Wiley-VCH.

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  3.2 Reaction of CO₂ and terminal alkynes

  Carboxylation of terminal alkynes is considerably greener over other options for the preparation of propiolic acids, which are important intermediates in chemical and pharmaceutical industries [26, 27]. MNPs@MOFs can be used for the carboxylation of terminal alkynes with CO₂ [28-34].

  Ma and Cheng et al. prepared Ag@MIL-101 using a simple liquid impregnation method, and applied the composite in the reaction of terminal alkynes and CO₂ to prepare propiolic acids [28]. Due to the large pore size (2.9–3.4 nm), high specific surface area and long-term chemical stability, MIL-101 was chosen as the support. TEM images disclosed that Ag NPs with an average size of 1.4 ± 0.4 nm were uniformly dispersed in MIL-101. Catalytic studies showed that under the optimized conditions (1.5 equiv. Cs₂CO₃, 1 bar CO₂, DMF, 50 ℃), Ag@MIL-101 could convert the alkyne substrates to the corresponding propiolic acids in excellent yields (Table 1, entry 4). The excellent catalytic activities of Ag@MIL-101 could be explained in three ways: 1) MIL-101 served as CO₂ reservoirs and then increased the local CO₂ concentration around Ag NPs, which were the active sites; 2) The inner cavities of MIL-101 provided a platform for the reaction of CO₂ with terminal alkynes; 3) Ag NPs were stabilized by MIL-101. A plausible reaction mechanism was proposed: Through the windows of MIL-101, the terminal alkynes diffused into the inner cavities and reacted with Ag NPs to generate the silver acetylide intermediates in the presence of Cs₂CO₃. Afterwards, CO₂ inserted into the C-Ag bond to generate the carboxylic acid products.

  Using an electrostatic self-assembly strategy, Sheng and Zhu et al. incorporated the anionic [Au₁₂Ag₃₂(SR)₃₀]⁴⁻ into ZIF-8 to obtain the Au₁₂Ag₃₂(SR)₃₀@ZIF-8 [29]. The high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM), TEM images as well as energy dispersive X-ray (EDX) element mapping disclosed that the [Au₁₂Ag₃₂(SR)₃₀]⁴⁻ nanoclusters were uniformly dispersed in the ZIF-8 matrix. Catalytic studies showed that Au₁₂Ag₃₂(SR)₃₀@ZIF-8 gave rise to 100% conversion of phenylacetylene into phenylpropiolate under mild conditions (50 ℃ and ambient CO₂ pressure) (Table 1, entry 5). The turn-over number (TON) was as high as 18164. In addition, the high catalytic activity could be also maintained in five consecutive runs. A possible catalytic mechanism was proposed: The [Au₁₂Ag₃₂(SR)₃₀]⁴⁻ firstly activated the terminal alkyne with a base to form a Ph—C≡C⋯Au₁₂Ag₃₂(SR)₃₀ intermediate I, and then CO₂ was fixed and activated by ZIF-8 to generate an intermediate CO₂-Lewis acid complex II. Subsequently, the activated CO₂ inserted into the nearby Au–C bond to afford the carboxylate III. Finally, the intermediate I was regenerated by the metathesis with alkyne (Fig. 4).

  Figure 4

  

  Figure 4.  Proposed reaction mechanism toward CO₂ and phenylacetylene over Au₁₂Ag₃₂(SR)₃₀@ZIF-8. Reproduced with permission [29]. Copyright 2018, Royal Society of Chemistry.

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  The different locations of MNPs relative to the MOFs in the composites contributed differently to the reaction efficiency. Sun and Wei et al. prepared 0.5Ag@ZIF-8 (where 0.5 represents the Ag/Zn ratio in the preparation mixtures) by rapidly contacting AgNO₃ and ZIF-8 in methanol at 0 ℃. Two types of Ag species were installed, including the highly dispersed Ag(I) in the backbone (Ag^HD) and the aggregated Ag(0) nanoparticles on the outer surface (Ag^NP) [30]. Under the optimized condition, 0.5Ag@ZIF-8 promoted the carboxylation of terminal alkynes with CO₂ into the corresponding propiolic acids in good to excellent yields (Table 1, entry 6). The coexistence of Ag^HD and Ag^NP had a profound effect on the catalytic performances: On one hand, Ag^NP were highly effective for the activation of terminal alkynes, and on the other hand, the modification of ZIF-8 framework with Ag^HD enhanced its CO₂ affinity and thus improved the CO₂ activation. The 0.5Ag@ZIF-8 can be recycled for 4 runs with a negligible alteration on the crystallinity and morphology.

  3.3 CO₂ hydrogenation

  3.3.1   CO₂-to-CH₃OH

  CO₂ hydrogenation can not only slow down the rising CO₂ level in Earth's atmosphere, but also produce value-added chemicals such as methanol and formate. The Cu/ZnO/Al₂O₃ is used as an industrial catalyst for CO₂ hydrogenation to prepare methanol, but however CO was observed as the byproduct due to the reverse water-gas shift reaction [35].

  Yaghi and Somorjai et al. reported the synthesis of Cu@UiO-66, which was efficient for CO₂ hydrogenation at 175 ℃ and 10 bar (CO₂ and H₂ in the 1:3 molar ratio) and produced methanol with 100% selectivity [36] (Table 2, entry 1). The turn-over frequency (TOF) was calculated to be 10 h⁻¹, which was 8 times of that of the benchmark Cu/ZnO/Al₂O catalyst. XPS results disclosed that the Zr 3d binding energy in Cu@UiO-66 was shifted toward lower oxidation state compared to UiO-66, implying that the Zr(IV) was reduced after being in contact with the Cu NPs due to the metal-support interaction effect.

  Table 2

  

  Table 2.  Application of MOF composites in CO₂ hydrogenation.

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  The Cu/ZnO_x interfaces were widely used for CO₂ hydrogenation, but however, Cu NPs slowly aggregated and separated from ZnO_x under the reaction conditions, reducing the Cu/ZnO_x interfaces and thus diminishing the catalytic activity. Wang and Lin et al. prepared the Cu/ZnO_x@UiO-bpy (bpy = 2,2′-bipyridine) composite by anchoring the ultrasmall Cu/ZnO_x nanoparticles in the framework of UiO-bpy, which can prevent the agglomeration of Cu NPs and phase separation between Cu and ZnO_x [37]. Cu/ZnO_x@UiO-bpy was prepared by the coordination of the Cu²⁺ ions into the bpy sites that were located in UiO-bpy through the postsynthetic metalation, and followed by the incorporation of Zn²⁺ ions via their reactions with the μ₃-OH sites on the Zr-O SBUs of UiO-bpy. The ultrasmall Cu/ZnO_x NPs were then in situ generated within UiO-bpy at 250 ℃ in the presence of H₂ as the reductant, giving rise to Cu/ZnO_x@UiO-bpy. Catalytic studies showed that Cu/ZnO_x@UiO-bpy-catalyzed CO₂ hydrogenation produced methanol with yields up to 2.59 g_MeOH kg_Cu⁻¹ h⁻¹ and 100% selectivity at 250 ℃ and under a pressure of 40 bar with a H₂/CO₂ ratio of 3 (Fig. 5; Table 2, entry 2).

  Figure 5

  

  Figure 5.  Catalytic behavior of Cu/ZnOx@UiO-bpy composite in CO₂ hydrogenation. Reproduced with permission [37]. Copyright 2017, American Chemical Society.

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  Due to the photothermal effect, the plasmonic Ag and Au NPs can increase the reaction temperature after the irradiation of light. Zeng and co-workers prepared Au & Pt@ZIF-8 via the encapsulation of Pt nanocubes and Au nanocages into ZIF-8, in which the Au nanocages transformed light to thermal energy, the Pt nanocubes served as the catalyst toward CO₂ hydrogenation, ZIF-8 functioned as "heat insulators" to prevent heat from dispersing in solution [38]. Au & Pt@ZIF displayed a high TOF value of 1522 h⁻¹ for the synthesis of methanol under the light irradiation, which was 13 times higher than the controlled experiment in the dark (Table 2, entry 3). The high catalytic performance can be maintained for 6 runs.

  3.3.2   CO₂-to-HCOO⁻

  Byers and Tsung et al. developed an active catalyst of Ru@UiO-66 for the hydrogenation of CO₂ to formate (Fig. 6) [39]. (^tBuPNP)Ru(CO)HCl (^tBuPNP = 2,6-bis((di-tert-butyl-phosphino)methyl)pyridine) was used for the precursor. Catalytic CO₂ hydrogenation studies were studied at 27 ℃ and under 15 bar with a H₂/CO₂ ratio of 4, which showed that Ru@UiO-66 gave rise to HCOO⁻ with a TOF value of 6 × 10⁵ h⁻¹ (Table 2, entry 4). Moreover, Ru@UiO-66 can be recycled for 5 runs without loss of activity.

  Figure 6

  

  Figure 6.  Catalysis in MOFs using aperture-opening encapsulation. Reproduced with permission [39]. Copyright 2018, American Chemical Society.

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  3.3.3   CO₂-to-CH₄

  CO₂ hydrogenation can also produce CH₄, which is also called the Sabatier reaction and is an important catalytic process to eliminate the CO₂ concentration and generate the clean CH₄ fuel. Lu and co-workers prepared Ni@MOF-5, in which Ni NPs were highly dispersed (41.8%) over MOF-5 [40] (Table 2, entry 5). The Ni@MOF-5 composite showed high activity at low temperature and exhibited almost no deactivation in the long-term stability tests up to 100 h.

  Later, this group reported a 20Ni@MIL-101 (where 20 indicate that the nickel loading was 20%) composite by encapsulating Ni NPs in MIL-101 via a double solvent method (DSM) [41]. 20Ni@MIL-101 exhibited a high thermal stability and a low activation energy (88 kJ/mol) for CO₂ hydrogenation, giving rise to CH₄ with a high TOF value of 1.63 × 10⁻³ s⁻¹ at 300 ℃ (Table 2, entry 6). The density-functional theory (DFT) calculations uncovered that the potential energy barrier was about 10.0 kcal/mol for CO₂ dissociation into CO_ads and O_ads over the Ni (1 1 1) surface, which was lower than Ni (200) plane (20.3 kcal/mol).

  Cheang and Xu et al. prepared a Ni@UiO-66 composite through the adsorption of Ni²⁺ ions into the cavity of UiO-66, and then in situ reduction under a 5% H₂/Ar atmosphere at 250 ℃ (Fig. 7) [42]. The uniform distribution of Ni NPs with an average size of 2 nm throughout the UiO-66 framework was revealed by the HAADF-STEM and EDX elemental mapping. Compared with Ni/ZrO₂ and Ni/SiO₂, Ni@UiO-66 exhibit outstanding activity and high selectivity for CO₂ methanation at low temperatures (Table 2, entry 7), which is due to the good dispersion of ultrasmall Ni NPs and the low activation energy (E_a = 68.9 kJ/mol).

  Figure 7

  

  Figure 7.  Ni@UiO-66 composite give both excellent activity and selectivity for CO₂ methanation at low temperature. Reproduced with permission [42]. Copyright 2018, Royal Society of Chemistry.

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  3.4 CO₂ photoreduction

  3.4.1   MNPs@MOFs or MNPs/MOFs

  CO₂ photoreduction is a process that uses renewable solar energy to transform CO₂ into products with different oxidation state, e.g., HCOO⁻, CO, CH₃OH and CH₄. MOFs have been extensively studied for heterogeneous CO₂ reduction, but however, the catalytic efficiency is far from satisfactory. The efficiency can be improved through incorporating precious metal nanoparticles to MOFs to form MNPs@MOFs (or MNPs/MOFs. MNPs@MOFs herein refers to that the MNPs are incorporated into the inner cavities of the MOFs, while MNPs/MOFs refers to that the MNPs are loaded on the external surfaces of the MOFs): 1) MNPs can serve as electrons trapping sites, promote the charge separation, suppress the electron-hole recombination, and behave as active reaction sites for the CO₂ reduction [43, 44]; 2) Some noble MNPs such as Cu, Ag and Au NPs display localized surface plasmon resonance (LSPR) effect, which can boost the visible-light harvesting capabilities of MOFs composites [45-48].

  The doping of noble metals such as Pt and Au into the MOFs is generally a feasible strategy to inhibit the recombination of the photo-generated electrons and holes [43, 44]. Wang and Sunet al. embedded Pt NPs inside and/or outside of NH₂-UiO-68 [43]. As for 2 wt% Pt@NH₂-UiO-68, in which Pt NPs were located inside NH₂-UiO-68, showed the highest CO₂ photoreduction activity (66.7 μmol g⁻¹ h⁻¹ for CO) under visible light irradiation (Table 3, entry 1). Pt NPs with an appropriate location and content in the resulting composites could facilitate the contact between Pt NPs and NH₂-UiO-68, thereby achieving efficient charge transfer in the photocatalytic process. More importantly, an effective charge transfer from NH₂-UiO-68 to the excited Pt NPs via the Schottky junction could inhibit the recombination of photogenerated charge carriers.

  Table 3

  

  Table 3.  Application of MOF composites in the photocatalytic CO₂ reduction.

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  Zhang and Li et al. prepared M-doped NH₂-MIL-125(Ti) (M = Pt and Au) composites and studied their catalytic applications in CO₂ photoreduction under visible-light irradiations in the presence of TEOA [44]. Pt and Au NPs played different effects on the catalytic performances. Compared to NH₂-MIL-125(Ti), Pt/NH₂-MIL-125(Ti) showed an enhanced activity for formate production, whereas Au/NH₂-MIL-125(Ti) had a negative effect on this reaction (Table 3, entries 2 and 3). Based on ESR and DFT calculations, the mechanism studies about Pt/NH₂-MIL-125(Ti)-catalyzed reaction disclosed that the hydrogen could spillover from Pt to the bridging oxygen atoms that were linked to Ti atoms, resulting in the formation of Ti³⁺ and an improved performance for CO₂ reduction to generate formate. However, it is unlikely to happen for Au/NH₂-MIL-125(Ti), which might explain its lower reaction efficiency.

  Yang and Yaghi et al. reported a core-shell Ag⊂Re₃MOF composite, which was prepared by coating the Re₃-MOF layers onto the pre-synthesized Ag nanocubes (98 nm) (Fig. 8) [45]. The strong quadrupolar LSPR scattering peak (λ_max ~ 480 nm) of Ag nanocube overlaped with the absorption range of the Re₃MOF (400 nm < λ < 550 nm). And therefore, Ag⊂Re₃MOF could maintain the characteristic LSPR features of the Ag nanocubes, and meanwhile the photoactive Re centers were spatially confined to the intensified near-surface electric fields at the surface of Ag nanocubes, contributing to a 7-fold enhancement of CO₂-to-CO conversion under visible light compared to Re₃MOF (Table 3, entry 4).

  Figure 8

  

  Figure 8.  Structures of Re_n-MOF and Ag⊂Re_n-MOF for plasmon-enhanced photocatalytic CO₂ conversion. Reproduced with permission [45]. Copyright 2017, American Chemical Society.

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  Owing to the LSPR characteristic of Ag NPs, the decorating Ag NPs onto the MOFs surface can further expand the absorption capability of the resultant Ag/MOF composites to the near IR region [46-48]. Huang, Sun and coworkers prepared Ag/MIL-101(Cr) in which Ag NPs with the size range of 80−800 nm were well-dispersed on the outer surface of the MIL-101(Cr) framework [46]. Catalytic studies showed that the photocatalytic activity of Ag/MIL-101(Cr) exhibited a strong dependence on the composite size of MIL-101(Cr), and the highest CO₂ photocatalytic reduction activities up to 808.2 and 427.5 μmol g⁻¹ h⁻¹ for CO and CH₄, respectively (Table 3, entry 5), were achieved when the size of Ag/MIL-101(Cr) was reduced to 80 nm. A set of photochemical and electrochemical studies revealed that the high catalytic performance might be ascribed to the high density of unit cells on corners and edges of Ag/MIL-101(Cr), which was beneficial to the electron transfer in the photocatalytic CO₂ reduction.

  Chen, Duan and co-workers have loaded the plasmonic Au NPs on the surface of a thin porphyrin paddle-wheel framework-3 (PPF-3) nanosheets by electrostatic interaction, giving rise to the Au/PPF-3 composites [47]. The Au NPs could serve as the light-harvesting antenna due to its strong SPR absorption peaks at 525 nm. The Au/PPF-3 composite showed a high photocatalytic activity for CO₂ reduction into HCOO⁻ with an average production rate of 42.7 μmol g⁻¹ h⁻¹ under visible-light irradiation (Table 3, entry 6), which was around 4-fold higher than that of the pristine PPF-3. Further mechanism studies disclosed that the catalytic efficiency improvement of CO₂ conversion into HCOO⁻ over Au/ PPF-3was through a plasmon resonance energy transfer process from AuNPs to PPF-3 (Fig. 9).

  Figure 9

  

  Figure 9.  Proposed mechanism for plasmon-enhanced photocatalytic activity of Au/PPF-3 towards CO₂ reduction. Reproduced with permission [47]. Copyright 2019, Royal Society of Chemistry.

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  The product selectivity of CO₂ reduction in the presence of MNPs@MOFs can be summarized as follow: Au/MOFs generate HCOO⁻, Pt@MOFs or Pt/MOFs give rise to the production of CO or HCOO⁻, whereas Ag/MOFs composites produce CO or CH₄.

  3.4.2   Molecular catalysts@MOFs

  The incorporation of molecular catalysts or photosensitizers into the pores of MOFs provides an efficient way to engineer the MOF composites for enhanced photocatalytic CO₂ reduction [49, 50]. Fontecave, Mellot-Draznieks and co-workers immobilized the molecular catalyst [Cp*Rh(4,4′-bpydc)]²⁺ and the photosensitizer [Rh(bpy)₂(4,4′-bpydc)]²⁺ into the highly porous MIL-101-NH₂(Al) via an easy post-synthetic impregnation, giving rise to the Rh–Ru@MIL-101-NH₂ composites [49]. The large cavity of MIL-101-NH₂(Al) as a nanoreactor allowed the intimate confinement of the catalyst and the photosensitizer. Catalytic studies showed that Rh–Ru@MIL-101-NH₂ exhibited a remarkable selectivity towards CO₂ reduction under visible light irradiation, and produced HCOO⁻ with the rate of 13.2 μmol g⁻¹ h⁻¹ (Table 3, entry 7). The H₂ production was totally suppressed, which was in deep contrast to the molecular catalyst alone. There was a negligible leaching of the active species in Rh–Ru@MIL-101-NH₂ during photocatalysis, which was due to the strong host–guest interactions.

  Using the "ship-in-bottle" synthetic strategy, Jin and Kong et al. encapsulated a molecular catalyst [Ni^II(bpet)(H₂O)₂] (bpet = 1,2-bis((pyridin-2-ylmethyl)thio)ethane) into a highly robust and visible-light responsive Ru-UiO-67 to obtain Ni@Ru-UiO-67 (Fig. 10) [50]. Ni@Ru-UiO-67 exhibited an efficient CO₂ reduction to CO with a TON of 581 and a high selectivity of 99% under the irradiation with a LED light (82 W) (Table 3, entry 8). Photoluminescence (PL) measurements showed the facile electron transfer from the excited Ru-MOFs to the encapsulated Ni(II) complex, which was crucial for the excellent activity of Ni@Ru-UiO-67. Advanced ultrafast absorption spectroscopy and DFT calculations further disclosed that the photoelectrons that were produced by the Ru ligand could transfer to the Ni(II) complex, affording Ni⁰ species for the reduction CO₂ to CO.

  Figure 10

  

  Figure 10.  Schematic illustration for "Ship-in-a-Bottle" synthetic strategy to prepare Ni@Ru-UiO-67 composite. Reproduced with permission [50]. Copyright 2019, Elsevier.

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  3.4.3   QDs@MOFs

  QDs are fluorescent semiconductor nanoparticles with the size range of 1–10 nm, which possess high quantum yield and strong light capture ability. Engineering MOFs and QDs to form QDs@MOFs can accelerate the separation of photogenerated charges, inhibit the recombination of photogenerated electrons and holes, and thus enhance the photocatalytic performance of the resultant composites due to the close inter-face and matched band gap between QDs and MOFs [51, 52].

  Chen's group immobilized ZIF-8 and ZIF-67 on the surface of CsPbBr₃ quantum dot to afford CsPbBr₃@ZIFs with an improved moisture stability, photo-generated charge separation efficiency and CO₂ capturing ability [51]. CsPbBr₃@ZIF-67 exhibited the broadened visible light absorption that was extended to 620 nm. The inhibited photo-generated carrier radiative recombination and effective electron transfer between CsPbBr₃ and ZIFs were confirmed by the steady-state and time-resolved photoluminescence studies. The Co^II center in ZIF-67 served as an electron reservoir to accelerate the separation of photo-induced carriers. Catalytic studies showed that only CO and CH₄ were detected, and the yield of CH₄ (10.5 μmol/g) was much higher than that of CO (2.3 μmol/g). The higher selectivity of CH₄ was attributed to the solid-gas mode catalytic reaction condition with water vapor involved, which can provide sufficient protons and help to overcome the dynamic barrier to produce CH₄. Moreover, Both CsPbBr₃@ZIF-8 and CsPbBr₃@ZIF-67 exhibited enhanced CO₂ reduction activities, which were 1.39- and 2.66-fold, respectively, of the naked CsPbBr₃ (Table 3, entries 9 and 10).

  Zhang and Lu et al. prepared a series of MAPbI₃@PCN-221(Fe_x) (x = 0–1) composites by encapsulating CH₃NH₃PbI₃ (MAPbI₃) perovskite QDs in the pores of PCN-221(Fe) through a sequential deposition route (Fig. 11) [52]. As revealed by the steady-state and time-resolved photoluminescence experiments, the photo-generated electrons of the encapsulated MAPbI₃ QDs under light irradiation quickly transferred to the photocatalytic Fe-porphyrin units of PCN-221(Fe), and thereby an efficient charge separation between the perovskite QDs and the MOF was achieved. Catalytic results disclosed that a high total yield of 1559 μmol/g for photocatalytic CO₂ reduction to CO (34%) and CH₄ (66%) was achieved using water as an electron source, which was 38 times higher than that of PCN-221(Fe_0.2) in the absence of perovskite QDs (Table 3, entry 11).

  Figure 11

  

  Figure 11.  Schematic illustrations for the synthesis of a) PCN-221(Fe_x), and b) MAPbI3 QDs (large spheres) encapsulated in the pores of PCN-221(Fe_x) by a sequential deposition route. Reproduced with permission [52]. Copyright 2019, Wiley-VCH.

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  3.4.4   MOFs/semiconductors

  Coupling MOFs with semiconductors has been proved to be an effective strategy to boost the catalytic performance in the photocatalytic CO₂ reduction. The resultant MOF composites can display an improved separation efficiency of photo-generated electrons. The most common semiconductors that are used in the MOFs composites fabrication include Zn₂GeO₄, CdS, TiO₂ and carbon nitride.

  Zn₂GeO₄ is widely used in the photocatalytic reduction of CO₂, which shows high crystallinity and light/thermal stability, but the weak CO₂ adsorption and low solar energy utilization efficiency largely inhibit its practical applications [53-55]. To overcome these shortcomings, Wang et al. immobilized ZIF-8 nanoparticles on the surfaces of Zn₂GeO₄ nanorods to obtain the ZIF-8/Zn₂GeO₄ composite, which inherited the high CO₂ adsorption property of ZIF-8 and the high crystallinity of Zn₂GeO₄ (Fig. 12) [56]. ZIF-8/Zn₂GeO₄ with 25 wt% ZIF-8 displayed 3.8 times higher CO₂ uptake than Zn₂GeO₄, which could effectively absorb CO₂ that was dissolved in water. Catalytic studies showed that ZIF-8/Zn₂GeO₄ exhibited 62% enhancement in photocatalytic conversion of CO₂ into CH₃OH (Table 3, entry 12).

  Figure 12

  

  Figure 12.  Synthesis of ZIF-8/Zn₂GeO₄ composite. Reproduced with permission [56]. Copyright 2019, Royal Society of Chemistry.

  DownLoad: Full-Size Img PowerPoint

  Later, Gao and Cao et al. prepared Mg-MOF-74/Zn₂GeO₄ through a hydrothermal method, and a close interaction was formed between Zn₂GeO₄ and Mg-MOF-74 [57]. The composite displayed high efficiency in photocatalytic CO₂ to CO using only water as the sacrificial agent, with an evolution rate of 71.9 μmol g⁻¹ h⁻¹, which was 7 times higher than that of Zn₂GeO₄ (Table 3, entry 13). The enhanced photocatalytic performances were attributed to the higher CO₂ adsorption and the slower recombination of photo-generated e-h pairs of Mg-MOF-74/Zn ₂GeO₄ than Zn₂GeO₄. Mechanistic studies disclosed that the photo-generated electrons could effectively transfer from Zn₂GeO₄ to the open alkaline metal sites (Mg²⁺) of Mg-MOF-74 for the subsequent CO₂ reduction.

  As an excellent visible-light response semiconductor, CdS has been extensively used as photo-catalyst for CO₂ reduction, but unfortunately, its applications are limited due to the fast recombination of photo-generated electron-hole pairs, the serious photo-corrosion, and the low CO₂ adsorption capability [58, 59]. Wu, Han and co-workers fabricated a ternary CdS/UiO-bpy/CoCl₂ composites, in which the semiconductor CdS and the molecular redox catalyst CoCl₂ were integrated through UiO-bpy [60]. The composite was highly efficient in the photocatalytic conversion of CO₂ to CO under visible light irradiation, giving rise to an evolution rate of 235 μmol g⁻¹ h⁻¹ and 85% selectivity (Table 3, entry 14).

  TiO₂ is a benchmark inorganic semiconductor for CO₂ photoreduction [61]. Li et al. prepared a CPO-27-Mg/TiO₂ composite through an in situ growth method, which was composed of TiO₂ nanospheres that grew on the surfaces of spindle-shaped CPO-27-Mg microcrystal [62]. The coordination between the carboxylate groups of CPO-27-Mg and Ti⁴⁺ of TiO₂ resulted in the intimate contact between MOF and semiconductor (Fig. 13). Catalytic studies showed that CPO-27-Mg/TiO₂ exhibited an enhanced performance for the photoreduction of CO₂ to CO and CH₄, and meanwhile the reduction of H₂O to form H₂ was totally inhibited (Table 3, entry 15).

  Figure 13

  

  Figure 13.  Schematic illustration of the formation of CPO-27-Mg/TiO₂ nanocomposite. Reproduced with permission [62]. Copyright 2016, Elsevier.

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  Using a similar strategy, Xu and Ye et al. reported a Co-ZIF-9/TiO₂ composite, in which TiO₂ and Co-ZIF-9 were closely bound with each other, thus leading to better charge separation [63]. The composite exhibited an enhanced total utilized photoelectron number (UPN), which was about 2.1 times of the pure TiO₂ (Table 3, entry 16). Linear sweep voltammetry disclosed that a more positive onset potential for CO₂ reduction and higher photocurrent density in the simulated photoelectron reduction of CO₂ was achieved in the composite, which indicated a better CO₂ activation and superior charge separation.

  As a metal-free and visible-light active photocatalyst, graphitic carbon nitride (CN) has been applied in the photocatalytic CO₂ reduction [64, 65]. However, due to the fast recombination of the photogenerated electron–hole pairs, the efficiency of bulk carbon nitride (bulk CN) is not satisfactory. These drawbacks can be overcome by the exfoliation of bulk CN to 2D atomic carbon nitride nanosheet (CNNS), leading to the improved photocatalytic activity [66-68]. Due to the low CO₂ adsorption capability of carbon nitride, further improvement of the efficiency is still a challenge.

  Ye and co-workers prepared a UiO-66/CNNS composite through the electrostatic reaction between the negatively charged nanosized CNNS and positively charged UiO-66 [69]. As confirmed by the TEM image of the UiO-66/CNNS composite, the CNNS were coated on the surface of UiO-66. Furthermore, the nanojunctions between the UiO-66 crystal and the CNNS were revealed by a distinguished and coherent inter-face HRTEM image, which could give rise to an efficient electron transfer. In addition, the UiO-66/CNNS composite showed a superior CO₂ uptake ability than that of CNNS. Photocatalytic CO₂ reduction in the presence of UiO-66/CNNS produced CO in the yield of 9.9 μmol g_CN⁻¹ h⁻¹, which was three times than CNNS (Table 3, entry 17). A tentative mechanism for the photocatalytic CO₂ reduction over UiO-66/CNNS was proposed as follow (Fig. 14): upon visible light irradiation, electrons were promoted from the valence band of CNNS to its corresponding conduction band. Then the photogenerated electrons migrated to the surface of CNNS and transferred to UiO-66 across the interface between CNNS and UiO-66. Subsequently, the adsorbed CO₂ was reduced to CO.

  Figure 14

  

  Figure 14.  Proposed mechanism of photocatalytic reduction of CO₂ by the UiO-66/CNNS heterogeneous photocatalyst under visible light irradiation. Reproduced with permission [69]. Copyright 2015, Wiley-VCH.

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  Zhang and Zhong et al. prepared a series of Zr-porphyrin metal-organic framework (Zr-PMOF)/ultrathin g-C₃N₄ composites, in which hollow Zr-PMOF nanotubes were surrounded by 3D ultrathin g-C₃N₄, leading to an interfacial interaction [70]. The interaction provided a platform for g-C₃N₄ as a conductor to obtain electrons from ligands or as a donor to transfer electrons to Zr-O cluster of Zr-PMOF. The composite exhibited a CO evolution rate of 5.5 μmol g⁻¹ h⁻¹ in the CO₂ reduction (Table 3, entry 18), which was 2.2 and 3.2 times higher than Zr-PMOFs and g-C₃N₄, respectively.

  3.4.5   MOFs/graphene

  To improve the conductivity of MOFs, engineering MOFs/graphene composites is a feasible strategy, considering that graphene has unique two-dimensional structure, outstanding electrical and thermal properties, and efficient electron transfer ability. Therefore, MOFs/graphene composites would be promising materials for photocatalytic reactions [71, 72].

  Li's group prepared UiO-66-NH₂/graphene composite via a microwave-induced synthesis route by in situ assembly of UiO-66-NH₂ onto graphene [71]. UiO-66-NH₂ crystals were highly dispersed on the surface of graphene, and the tight junctions between UiO-66-NH₂ and the surface of graphene was formed. The UiO-66-NH₂/2.0graphene composite displayed a strong CO₂ uptake about 73 cm³/g. Catalytic studies showed that UiO-66-NH₂/2.0graphene exhibited a high activity (418.8 μmol g⁻¹ h⁻¹) and good selectivity (78.6%) in the photocatalytic CO₂ to formic acid under visible-light irradiation (λ >410 nm, 300 W Xe lamp) (Table 3, entry 19). Controlled experiments disclosed that the introduction of graphene could suppress the competitive reaction of hydrogen evolution. The high CO₂ photo-reduction activity and excellent recyclability might attribute to the highly dispersion of UiO-66 ultrafine nanocrystals on the surfaces of graphene and the strong interaction between UiO-66 and graphene (Fig. 15).

  Figure 15

  

  Figure 15.  The working mechanism of the UiO-66-NH₂/2.0graphene composite for reducing CO₂ to formic acid under visible light irradiation. Reproduced with permission [71]. Copyright 2018, Elsevier.

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  Do and co-workers prepared Al-PMOF/NH₂-rGO (GO = graphene oxide) that was composed of a porphyrin metal–organic framework Al-PMOF and amine-functionalized graphene (NH₂-rGO) [72]. Photoluminescence and Mott-Schottky measurements disclosed that the band gap was decreased and electron–hole recombination was suppressed in the resultant composite. In the presence of Al-PMOF/NH₂-rGO (5 wt%), photoreduction of CO₂ under visible light irradiation in the presence of TEOA as a sacrificial agent led to the formation of HCOO⁻ with almost 100% selectivity and an evolution rate of 685.6 μmol g⁻¹ h⁻¹ under visible light irradiation (Table 3, entry 20), which was 76% greater than that of Al-PMOF. The N species on the rGO contributed to the catalytic efficiency of Al-PMOF/NH₂-rGO (5 wt%) in two ways: firstly, the N species on rGO enabled the connection of PMOF with the graphene and electron transfer from the excited PMOF to the graphene sheet; secondly, the amine groups of rGO could also absorb light and thereby boost the light-harvesting capability of the composite. The proposed reaction mechanism was illustrated as following: After being excited under light illumination, the ligand TCPP in Al-PMOF behaved as a visible-light-harvesting unit, and the electrons transferred from TCPP to graphene while the holes remained in the TCPP. Subsequently, the electrons from graphene transferred to the adsorbed CO₂ and led to the reduction of CO₂ towards HCOO⁻ in the presence of TEOA acting as a hydrogen source.

  3.5 CO₂ electroreduction

  Electrochemical reduction of CO₂ using renewable energy is one of the most promising approaches for CO₂ conversion to valuable products. The main challenges lie in the field of CO₂ electroreduction are the low selectivity, faradaic efficiency and catalytic efficiency.

  Li and Bao et al. reported electrocatalytic applications of Ag₂O/ZIF-7, which was prepared through one-pot hydrothermal treatment of ZIF-7 in AgNO₃ aqueous solution [73]. Catalytic studies disclosed that the faradaic efficiency and current density of Ag₂O/layered ZIF composite for CO₂ electroreduction to CO reached 80.5% and 26.2 mA/cm² at -1.2 V vs. the reversible hydrogen electrode (RHE), respectively, under CO₂-saturated 0.25 mol/L K₂SO₄ solution (Table 4, entry 1).

  Table 4

  

  Table 4.  Application of MOF composites in the electrocatalytic CO₂ reduction.

  DownLoad: CSV

  Yu and Qiu et al. fabricated Cu₂O@Cu-MOF for the electrochemical reduction of CO₂ [74]. Catalytic studies disclosed that the Cu₂O@Cu-MOF exhibited an excellent performance with a 79.4% faradaic efficiency for hydrocarbon (CH₄+C₂H₄), and especially 63.2% faradaic efficiency for CH₄ (Table 4, entry 2). The high electrocatalytic activity and high selectivity to CH₄ was attributed to the multiple functionalities of Cu₂O@Cu-MOF, such as absorption, activation, and catalysis derived from synergistic effects of Cu-MOF and Cu₂O.

  Metallocene complexes (MCp₂, Cp stands for cyclopentadienyl) are electron-rich with high stability and aromaticity, which can serve as potential electron donator and carrier to enrich the electron density of MOFs [75]. Chen and Lan et al. developed a CoCp₂@MOF-545-Co composite for the highly selective CO₂ electroreduction [76]. The composite was prepared by the incorporation of CoCp₂ into the pores of MOF-545, which display high porosity, large pore size (3.6 nm) and excellent chemical/thermal stability that can serve as an ideal platform to interact with CoCp₂. In the resultant CoCp₂@MOF-545-Co composite, the π-electron system of cyclopentadienyl ring might overlap with π-electron system of porphyrin, which would endow MOF-545 with higher CO₂ adsorption capability, larger number of active sites and more favorable electron transfer property to largely enhance the electrocatalytic activity. Catalytic studies disclosed that CoCp₂@MOF-545-Co selectively converted CO₂ to CO with 97% Faradaic efficiency under CO₂-saturated 0.5 mol/L KHCO₃ (pH 7.2) solution (Table 4, entry 3). Mechanistic studies suggested that the continuous electron transfer channel in MOFs could be created through the introduction of metallocene, and meanwhile the adsorption energy of CO₂ can be largely reduced by the strong binding-interaction with metalloporphyrin, which contributed to the excellent activity of CoCp₂@MOF-545-Co in the CO₂ electroreduction (Fig. 16).

  Figure 16

  

  Figure 16.  The schematic presentation for the advantages of MCp₂@MOF in electrocatalytic CO₂RR. Reproduced with permission [76]. Copyright 2020, Elsevier.

  DownLoad: Full-Size Img PowerPoint

  3.6 CO₂ photoelectroreduction

  An alternative approach for CO₂ transformation is the CO₂ photoelectroreduction, which integrates photocatalysis and electrocatalysis [11]. Photoelectrocatalytic reduction of CO₂ would reduce electricity consumption as compared to the CO₂ electroreduction because of the introduction of solar energy. Moreover, as compared to CO₂ electroreduction, CO₂ photoelectroreduction may achieve higher efficiency because the applied external bias voltage can drive the separation of photo-generated electrons and holes, which is the most crucial step in limiting the photocatalytic efficiency.

  Cardoso and co-workers reported the synthesis of Ti/TiO₂NT-ZIF-8 composite for the photoelectrocatalytic reduction of CO₂ to produce CH₃OH and C₂H₅OH (Fig. 17) [77]. The composite was prepared by the growth of ZIF-8 nanoparticles on the Ti/TiO₂ nanotubes through a layer-by-layer process. Photoelectrocatalysis of the Ti/TiO₂NT-ZIF-8 electrodes gave rise to the formation of up to 0.7 mmol/L of CH₃OH and 10 mmol/L of ethanol in 0.1 mol/L Na₂SO₄ at room temperature. The ZIF-8 not only participated in the CO₂ adsorption/activation, but also functionalized as a co-catalyst.

  Figure 17

  

  Figure 17.  The schematic presentation of Ti/TiO₂NT-ZIF-8 composite for the photoelectrocatalytic reduction of CO₂ to produce methanol and ethanol fuels. Reproduced with permission [77]. Copyright 2018, Elsevier.

  DownLoad: Full-Size Img PowerPoint

  4. Conclusion and future outlook

  In summary, the syntheses and catalytic applications of MOF composites towards CO₂ capture and conversion (C3), including chemical fixation, hydrogenation, photoreduction, electroreduction and photoelectroreduction of CO₂ have been discussed. Although MOF composites have great potentials towards C3 application, this research field is still in its infancy: Firstly, the chemical and thermal stability of MOF composites should be further improved. Secondly, the scalable and efficient preparation methods for MOF composites are highly desired. Until now, the fabrication of MOF composites for the target CO₂ conversion is still in the laboratory scale by time-consuming procedures, which restricts their industrial-scale applications. Thirdly, a deep understanding of the interactions between MOFs and functional materials is still not sufficient. In-depth studies of the interfacial interactions can guide the design of the next generation of MOF composites for CO₂ activation.

  For the CO₂ chemical fixation, more types of transformations should be explored to convert CO₂ into abundant value-added chemicals, which will open new ways for the CO₂ utilization using MOF composites as catalysts. And especially, MOF composites that can promote the transformations of CO₂ at room temperature, atmospheric pressure and low CO₂ concentrations are highly desired, but however only a few relevant studies have been reported. As for the CO₂ hydrogenation, MOF composites that can promote the reaction under atmospheric pressure and room temperature are highly desired, considering that some MOFs are unstable at high temperatures.

  For the photocatalytic reduction of CO₂, several problems remain to be solved. Firstly, the main products of MOF composites for the photocatalytic CO₂ reduction are limited to C₁ compounds (e.g. CO, CH₄, HCOOH and CH₃OH) and C₂ compound (e.g. CH₃CH₂OH), but the reports about the C₃₊ products (hydrocarbons with more than three carbons) are rare. Secondly, the detailed mechanism of photocatalytic CO₂ reduction on the MOF composites is still unclear. The combination of advanced characterizations and density functional theory calculations should be an efficient way to for the deep study of the structure–activity relationship, which is of vital importance to the rational design of MOF composites. Finally, most of the present catalytic systems need additional sacrificial regents, which is not economically feasible. Coupling the photocatalytic CO₂ reduction with oxidative organic reaction is an efficient strategy to tackle this problem, which produces solar fuel and value-added organic chemicals simultaneously. As for the CO₂ electroreduction, MOF composites that are based on earth-abundant metal elements and possess a good compromise of high faradaic efficiency and selectivity are highly required.

  In conclusion, MOF composites as heterogeneous catalysts for the CO₂ capture and conversion are showing bright future and will no doubt attract more attention in the CO₂ chemistry. The next challenge is to tackle the drawbacks of MOFs composites and put them into practical applications.

  Declaration of competing interest

  The authors declared that they have no conflicts of interest to this work. They also declared that they do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.

  Acknowledgments

  The authors acknowledge the support from the National Natural Science Foundation of China (Nos. 21773314, 21720102007, 21821003 and 21890382), the Guangdong Natural Science Funds for Distinguished Young Scholar (No. 2019B151502017), the Tip-top Youth Talents of Guangdong special support program (No. 20173100042150021), Science and Technology Planning Project of Guangzhou (No. 201707010168), the General Financial Grant from the China Postdoctoral Science Foundation (No. 2019M662809), and Local Innovative and Research Teams Project of Guangdong Peal River Talents Program (No. 2017BT01C161).

  
  ^* Corresponding author.
  E-mail address: zhli99@mail.sysu.edu.cn (L.Zhang).
  
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  Figure 1  The advantages of MOF composites for the CO₂ capture and conversion.

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  Figure 2  The preparation of polyILs@MIL-101. Reproduced with permission [21]. Copyright 2018, American Chemical Society.

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  Figure 3  The synthesis of mesoCu@Al-bpydc. Reproduced with permission [23]. Copyright 2019, Wiley-VCH.

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  Figure 4  Proposed reaction mechanism toward CO₂ and phenylacetylene over Au₁₂Ag₃₂(SR)₃₀@ZIF-8. Reproduced with permission [29]. Copyright 2018, Royal Society of Chemistry.

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  Figure 5  Catalytic behavior of Cu/ZnOx@UiO-bpy composite in CO₂ hydrogenation. Reproduced with permission [37]. Copyright 2017, American Chemical Society.

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  Figure 6  Catalysis in MOFs using aperture-opening encapsulation. Reproduced with permission [39]. Copyright 2018, American Chemical Society.

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  Figure 7  Ni@UiO-66 composite give both excellent activity and selectivity for CO₂ methanation at low temperature. Reproduced with permission [42]. Copyright 2018, Royal Society of Chemistry.

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  Figure 8  Structures of Re_n-MOF and Ag⊂Re_n-MOF for plasmon-enhanced photocatalytic CO₂ conversion. Reproduced with permission [45]. Copyright 2017, American Chemical Society.

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  Figure 9  Proposed mechanism for plasmon-enhanced photocatalytic activity of Au/PPF-3 towards CO₂ reduction. Reproduced with permission [47]. Copyright 2019, Royal Society of Chemistry.

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  Figure 10  Schematic illustration for "Ship-in-a-Bottle" synthetic strategy to prepare Ni@Ru-UiO-67 composite. Reproduced with permission [50]. Copyright 2019, Elsevier.

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  Figure 11  Schematic illustrations for the synthesis of a) PCN-221(Fe_x), and b) MAPbI3 QDs (large spheres) encapsulated in the pores of PCN-221(Fe_x) by a sequential deposition route. Reproduced with permission [52]. Copyright 2019, Wiley-VCH.

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  Figure 12  Synthesis of ZIF-8/Zn₂GeO₄ composite. Reproduced with permission [56]. Copyright 2019, Royal Society of Chemistry.

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  Figure 13  Schematic illustration of the formation of CPO-27-Mg/TiO₂ nanocomposite. Reproduced with permission [62]. Copyright 2016, Elsevier.

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  Figure 14  Proposed mechanism of photocatalytic reduction of CO₂ by the UiO-66/CNNS heterogeneous photocatalyst under visible light irradiation. Reproduced with permission [69]. Copyright 2015, Wiley-VCH.

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  Figure 15  The working mechanism of the UiO-66-NH₂/2.0graphene composite for reducing CO₂ to formic acid under visible light irradiation. Reproduced with permission [71]. Copyright 2018, Elsevier.

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  Figure 16  The schematic presentation for the advantages of MCp₂@MOF in electrocatalytic CO₂RR. Reproduced with permission [76]. Copyright 2020, Elsevier.

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  Figure 17  The schematic presentation of Ti/TiO₂NT-ZIF-8 composite for the photoelectrocatalytic reduction of CO₂ to produce methanol and ethanol fuels. Reproduced with permission [77]. Copyright 2018, Elsevier.

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  Table 1.  Application of MOF composites in the CO₂ chemical conversion.

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  Table 2.  Application of MOF composites in CO₂ hydrogenation.

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  Table 3.  Application of MOF composites in the photocatalytic CO₂ reduction.

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  Table 4.  Application of MOF composites in the electrocatalytic CO₂ reduction.

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