Preparation of MOFs and MOFs derived materials and their catalytic application in air pollution: A review

Journal Pre-proof Preparation of MOFs and MOFs derived materials and their catalytic application in air pollution: A review Hanbing He, Ren Li, Zhihui Yang, Liyuan Chai, Linfeng Jin, Sikpaam Issaka Alhassan, Lili Ren, Haiying Wang, Lei Huang

PII:

S0920-5861(20)30092-4

DOI:

https://doi.org/10.1016/j.cattod.2020.02.033

Reference:

CATTOD 12701

To appear in:

Catalysis Today

Received Date:

10 September 2019

Revised Date:

14 January 2020

Accepted Date:

20 February 2020

Please cite this article as: He H, Li R, Yang Z, Chai L, Jin L, Alhassan SI, Ren L, Wang H, Huang L, Preparation of MOFs and MOFs derived materials and their catalytic application in air pollution: A review, Catalysis Today (2020), doi: https://doi.org/10.1016/j.cattod.2020.02.033

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Preparation of MOFs and MOFs derived materials and their catalytic application in air pollution: A review

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Hanbing He a, Ren Li a, Zhihui Yang a, Liyuan Chai a, b, Linfeng Jin a, Sikpaam Issaka Alhassan a , Lili Ren a, Haiying Wang a, b*, Lei Huang a, b* a School of Metallurgy and Environment, Central South University, Changsha 410083, China b Chinese National Engineering Research Center for Control & Treatment of Heavy Metal Pollution, Changsha 410083, China E-mail address: [email protected], [email protected]

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Graphical abstract

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Highlights:

 MOFs were used as catalysts in different applications of air pollution.

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MOFs were modified to enhance catalytic performance in different ways.  Different kinds of materials were prepared by using MOFs as precursors or composites.

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 MOFs are promising materials in the field of gas-phase catalysis.

Abstract

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Metal-organic framework (MOFs) composites are new kind of porous materials, with great specific surface area and extremely high porosity. MOFs have been in the catalytic application due to excellent catalytic properties such as, their unique structure and dispersed active centers. Hence, the chemical, environment and energy-related fields disciplines have made substantial applications of MOFs and its composites. Moreover, several researchers have made conducted extensive studies regarding the application of MOFs materials for air pollution control such as, including functionalization, loading, carbonization, oxidation of MOFs materials, and application of MOFs as sensors for air pollution monitoring. This review article is aimed at providing precise information about the efforts made by some researchers in the field of MOFs and MOFs derived materials and their application in other disciplines. The following areas have been succinctly reviewed: (1) general introduction of MOFs materials, (2) synthesis of various MOFs materials, composites and their derivatives, (3) catalysis of MOFs materials, composites and their derivatives on different air pollution, (4) summary and future prospects. We hope this review will help researchers understand more about the research progress of MOFs materials in the catalysis of air pollution.

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Keywords: Metal-organic framework; MOFs derivatives; Catalysis; air pollution

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1. Introduction In recent years, metal-organic frameworks (MOFs) have been developed as a functional material in the past few years, and various new structures have been developed and widely used in optics [1], sensors [2], Gas adsorption and separation [3], gas storage [4], catalysis [5, 6], drug delivery [7] and other fields. The MOFs materials are not only structurally adjustable (chemical tunability, surface topography), but also have high porosity and surface area. MOFs are crystalline, porous solids, whose structure are defined by nodes of metal ions or clusters of metal ions held in the lattice by organic linkers [143]. The directionality of the metalligand coordination bond is responsible for creating empty spaces and voids in the lattice, and the stability of the structure depends on the strength of these coordinated forces, while these coordinated forces have medium energy between covalent bonds. Possible active sites in MOFs include metal nodes with freely coordinated sites, functional linkers, or guests accommodated in voids. Structural defects are also considered as potential active sites in catalysis. All these factors, including the possibility of synthesizing MOFs with any metal, large pore size and surface area, stable lattice, certain unsaturated metal sites, ease of design and synthesis have made MOFs good materials in solid catalysts. Understanding the limitations of MOF is critical for catalytic applications. Compared with inorganic porous solids, MOF has relatively low thermal and chemical stability, and some MOFs are highly sensitive to moisture [144]. Moreover, increasing atmospheric pollution has become a hot issue of concern. Among them, heavy metal air pollution, volatile organic compounds (VOCs) (such as benzene, toluene, and acetone) and other toxic gases (such as H2S, NO2, and SO2) and other pollutants discharged from industrial and agricultural production processes seriously endanger public health and safety. So far, several researchers have conducted extensive studies regarding the application of MOFs materials for air pollution control such as; functionalization [8-10], carbonization [14-15], oxidation of MOFs materials [11-13] compound [16-17] of MOFs materials and the application of MOFs as sensors for air pollution monitoring. MOFs are compound with 3D framework structure with porosity and/or guest exchange properties, which have a very strict definition. [139] In this paper, we organize and classify these materials based on MOFs, and systematically review the application of MOFs materials in atmospheric catalytic pollution. The schematic diagram of different applications of MOFs materials is shown in Figure 1. Additionally, synthetic methods for the application of catalytic reactions in various reactions have also been discussed. This review is therefore expected to offer basic understanding of MOFs materials to researchers and further explore their use in atmospheric pollution catalysis. 2. Modificatory classification of MOFs catalysts By modifying the MOFs catalyst, its morphology and structure were improved, strengthening the interaction between the active site of the catalyst and the carrier. These methods improved the redox ability of the catalyst, the thermal stability, and mechanical properties of the materials in Figure 1. 2.1 Alternative Routes and improvement of synthetic methods Chemical reactions require some form of energy input, and the chemical reaction stops only at temperatures near 0 K. The synthesis of MOF is usually carried out in a solvent at a temperature ranging of <250℃ [18]. Conventional electric heating is typically used to introduce energy from the heat source (oven). Alternatively, energy can also be introduced by other

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means, such as by electric potential, interfacial diffusion [19], electromagnetic radiation (microwave synthesis [20]), mechanical waves (sonochemical synthesis [21]) or mechanical means [22]. It is important to use different synthesis methods. Obviously, different methods produce new compounds that cannot be obtained in other ways. In addition, alternative pathways may result in compounds having different particle sizes and morphology that may affect material properties. For example, different particle sizes in porous materials can affect the diffusion of guest molecules, which directly affects the adsorption or separation of catalytic reactions or molecules. [139] At present, the preparation of catalytic materials with uniformity, better crystal form and high crystallinity by various synthetic methods have very good research significance. The solvothermal method is generally used to synthesize MOFs at low temperatures (<250℃) [18]. There are other common methods for preparing MOFs such as, interfacial diffusion [19], microwave synthesis [20], ultrasonic synthesis [21], and mechanochemical methods [22]. In addition to metal ions and ligands, factors such as solvent, solution concentration, temperature, and reaction time play an important role in the crystal structure. The physicochemical and structural properties of microwave products are very similar to products of hydrothermal synthesis [23]. The advantages of the microwave method [24] are that it facilitates large-scale synthesis [1], rapid reaction (5-10 min), phase selectivity (selective synthesis of specific and unique crystal structures from the same reaction components) and reduction of crystallites size [25] to improve catalytic performance. Cao et al. [1] used the microwave-assisted large-scale synthesis of a series of microporous lanthanide metal-organic frameworks (Ln-MOFs). In addition, sonochemical synthesis as an effective method for synthesizing MOFs has many advantages, such as a short crystallization time, a low reaction temperature, and a significant reduction of the particle size. The smaller the size, the more catalytic sites are exposed with better catalytic performance. Sung et al. [21] synthesized CuBTC at normal pressure and room temperature by ultrasonic wave, and Cu-BTC was obtained in 1 min in the presence of DMF (N, N-dimethylformamide). At present, the hydrothermal method/solvothermal method still remains the common preparation method of MOFs with good effect. It has the potential to effectively synthesize a uniform catalytic material, and it is easy to further improve the catalytic site. According to the literature [26, 27], Kun et al. [28], used Cr(NO3)3, TPA, HF, H2O as raw materials, carried out the hydrothermal synthesis of MIL-101 (Cr) at 220℃. In order to reduce the preparation cost and experimental risk, Masood et al. [29] used HNO3 instead of HF in hydrothermal synthesis to synthesize MIL-101-HNO3 with larger specific surface area and used for H2S adsorption. Zhao et al. [98] performed the synthesis of HF-free MIL-101 (Cr) and used it for the removal of Hg0. These amended methods did not use hydrofluoric acid, leading to a less dangerous synthesis protocol and the addition of other ions improved the catalytic performances. The most important point for mechanically activated MOF synthesis is an environmental issue. The reaction can be carried out at room temperature in the absence of a solvent without a long reaction time [141] (usually in the range of 10-60 minutes). Moreover, in some cases, the metal salt can be replaced by a metal oxide as a starting material. For example, Zn-MOF74 was synthesized directly from ZnO without requiring amount of solvent. And ZIF-90 nanopolyhedron (Zn(ICA)-1) and ZIF-90 amorphous network (Zn(ICA)-2) were both successfully prepared at room temperature. These catalytic materials are possible candidates for massive

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applications in catalytic action for air pollution control. 2.2 Adjustment of the Physic-chemical property The duration of energy and introduction, the pressure and the energy of each molecule are closely related. [140] In the system, each of these parameters can have a significant impact on the product being formed and its morphology. 2.2.1 Adjustment of morphology The morphology and structure of the materials have a significant impact on the catalytic performance of MOFs, and many researchers have made improvements in this area. A modified MIL-101(Cr), usually denoted as M-MIL-101(Cr), was synthesized with a new linker to cluster ratio. A considerable proportion of crystals in M-MIL-101(Cr) samples were coupled with each other (compared to the MIL-101(Cr)) resulting in further porosity and surface area. Using modulators (i.e. HNO3 and HF) increased the average particle size, uniformity, and octahedral shape of the M-MIL-101(Cr). Based on other researchers [33,34], the effects of solvothermal temperature and co-solvents on the properties of MOF-74 materials have been explored [122]. When using isopropanol as a co-solvent, the Cu-MOF-74 sample synthesized at 80 ℃ showed a rod-like morphology, while the sample synthesized with ethanol as a co-solvent at 65 ℃ and 80 ℃ showed a one-way and two-way bouquet structure. Co-MOF-74 synthesized at 100 ℃ had a flower-like morphology with a larger specific surface area and pore volume; a hexagonal prism structure was obtained at other temperatures. At the same time, Mn-MOF-74 with a hollow spherical structure was successfully prepared by the hydrothermal method. By controlling time and solvent, researchers [22] [35] reported that it was possible to control the crystallinity and adsorption properties of the graded microporous-mesoporous Zn-MOF-74 while the reaction happens at room temperature. Some studies [36-39] prepared Cu-BTC and used it for NH3-SCR. Due to the difference between the reaction temperature and the reaction pressure [40], there were some differences in the characteristics of the reported Cu-BTC. A recent study [39] showed that acetic acid was an effective modulator for these crystal structure and morphology during its synthesis, and also help achieve a layered porous structure with lattice defects. 2.2.2 Changing of metal ions and ligands Many non-porous MOFs have other important applications, such as magnetism, luminescence, and sensors, depending on the type of metal and ligand. The treatment of metal centers can promote the adsorption and catalysis of the reactant molecules. Specifically, it could be divided into metal center substitution and coordination unsaturated metal sites. For example, Ni was used to replace the active sites of Fe in MIL-100(Fe) and to prepare a crystalline porous Ni-MOF catalyst by hydrothermal method [41]. In NH3-SCR system, its stability was better than other reported Cu-BTC or MIL-100(Fe). The geometry and connectivity of the ligand determine the structure of the final MOF. The geometry, length, and functional group of the ligand significantly influences the size, shape, and chemistry of the MOF [47]. For example, a new Cu-MOF [48, 49] was constructed in DMF by solvothermal method using the novel ligands AIPA. [Cu(AIPA)·DMF]n had a twodimensional crystal structure and generated a three-dimensional skeleton through non-classical hydrogen bond interaction. A range of different ligands was investigated for MOFs with the same activity center but different topologies, pore topography and catalytic center size and distribution. This study was for a better understanding of the relationship between structure

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and catalytic performance [50]. Zr-MOFs [3], synthesized by a series of different ligands (such as H2-2, 6-NDC, H2Fum and so on), showed different pore sizes and separation performance for C3H8/CH4. Spencer et al. [51] synthesized two types of Zn-MOFs by solvothermal method, while inorganic structural units were bridged to form a three-dimensional network. In a recent study, a new style Ni+-1,3,5-tribenzyl-1,3,5-triazine-2,4,6-(1H,3H,5H)-trione coordinated polymeric MOF was synthesized with microwave-assisted hydrothermal technique [18]. 2.2.3 Defect engineering Catalysis at the nodes by creating defects has become a reasonable way to design MOF materials. Generating defects through direct methods or post-synthesis processing is a common strategy for very active catalytic systems. Thermal pre-treatment of catalysts used to release catalyst sites is a very common process. Lewis acidity in MOFs corresponded to an accessible metal site with a low coordination number, also known as coordinatively unsaturated sites (CUSs). The activation of MOFs has raised the number of CUSs and promoted adsorption of reactant molecules and catalysis [42] [141]. Based on porous MIL-100(Fe) [43], a mesoporous iron triacetate MIL-100(Fe) with Fe(Ⅲ)/Fe(II) mixed state [44] [45] was prepared by vacuum treatment at about 150℃. A mixedvalence state of MIL-100(Fe) was used for H2S selective catalysis and NH3-SCR. The mixedvalence Ce3+ /Ce4+-BTC (MVCM) was prepared by in-situ partial oxidation in a mixed solution of NaOH-H2O2, as shown in Figure 2 [46]. Furthermore, coordination of metal ions can be changed and new active sites can be formed during the reaction by destroying weakness of materials in a synthetic method [147]. The weak combination was identified as the coordination bond that held the linker to the metal. This weakness can lead to selective substitution of ligands or metals in MOF materials and may generate catalytic sites based on defects. For example, two adjacent Cu2+ CUS sites (only 8.1Å apart) can synergistically interact with the corresponding reaction intermediate [145]. Similarly, through hydrolysis, the number of joint vacancies in UiO-66 increased with the number of washes [146]. Metal cation isomorphous substitution on MOF was a common method of defect engineering. A series of [130,131] A-Cu-BTC (A=Fe, Ni, Co, Mn, Sr, La, Ce, Al) were prepared by direct ion exchange pre-assembly method. When the outer surface of the crystallite got close to the active site, the catalytic activity was improved by reducing the crystallite size or forming voids inside the crystallite. By adjusting the concentration of the additives, the nucleation rate of [Cu3(BTC)2] skeleton was significantly adjusted to control the crystal size. Finally, earlier studies have proven that it is possible to synthesize nanocrystals with size from a few tenths of nanometer to a few microns in a controlled manner. [142] 2.2.4 Functional modification In addition to the advantages of high porosity, large specific surface area and structural diversity, the most attractive feature of MOFs is the modulation of their chemical composition. The functional group can be stably incorporated into the MOF and reacted with the incoming molecule. For instance, comparing the classic amino-functionalized modified MOF NH2-MIL-53 (Fe) with its parent skeleton MIL-53 (Fe), the introduction of an amino group reduced the activation energy of the reaction and impacted on a moderately basic site to the surface of the catalyst [8]. For a series of Fe-based MOF materials (MIL-101 (Fe), MIL-53 (Fe) and MIL-88 (Fe)), adding amine functional groups significantly improved their photocatalytic reduction of carbon

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dioxide activity [9]. Similarly, the introduction of the thiol (-SH) functional group in UiO-66 also significantly reduced the adsorption energy [10] [52]. The results showed that the thiol skeleton had good adsorption performance for Hg(II) in both the aqueous solution and gas phase. Zhang et al. [53] synthesized a metal skeleton containing bromine group (Br-MOF), which had a high Hg0 capture efficiency. Among unmodified UiO-66 (Zr) and its functionalized derivatives, UiO-66-(COOH)2 and UiO-66-COOH showed a better adsorption performance in sulfur-containing binary mixed gases [54]. 2.3 Other method of enhanced catalysis For the first time, the MOFs-NTP synergy method was studied for the denitrification performance [28]. The Cu-BTC was synthesized by microwave method and combined with non-thermal plasma (NTP), the removal rate of NO reached 97.87%. It was 76.77% higher than the Cu-BTC-separate method and 64.43% higher than NTP-separate denitrification method. The authors reported that NTP can activate Cu-BTC to form coordinated unsaturated Cu(I)/Cu(II) sites that had the potential to significantly improved the surface catalytic activity of Cu-BTC. Among all the factors that affect the catalytic reaction, the active site plays a key role in catalysis. Considering NH3-SCR on MIL-100-Fe as an example [117], the Lewis acid sites played a crucial role in activating the reactants, while the NO molecule was easily oxidized to the NO2 species at the Fe Lewis acid site. 2.4 Derived materials from MOFs 2.4.1 Preparation of carbonized materials by MOFs In addition to being a crystalline porous material, MOFs are good precursors for metal/metal oxide nanoparticles within the carbon framework, or as structural templates. Due to the high porosity and uniformity of carbon skeleton, they have a high activity as single-atom catalysis and received increasing attention. 2.4.1.1 Self-template method Since the template method uses MOFs as a template, the synthetic route does not contain a surfactant, a carbon source or other complicated additives, and thus has advantages. Using Cu-BTC as the precursor. These carbon-based catalysts all have an octahedral structure of Cu-BTC [14]. A new method has been reported that through selective diffusion of H3PO4 molecules and etching between MOFs crystal layers, the exfoliation treatment of MOFs nanosheets is possible could succeed. By carbonization of Cu-MOF/Zn-MOF in Figure 3, Cu@C or Zn@C catalysts (Cu or Zn nanoparticles covered by porous carbon) were finally synthesized [15]. The influence of temperature during pyrolysis was studied, and ZIF-67 was oxidized to porous carbon (CoOx@PC-T) [35] [55] [57]. The pyrolysis temperature had a significant effect on the carbonization degree of carbon in ZIF-67, and affected the exposure and oxidation of cobalt nanoparticles on surface of the catalyst. Carbonized materials using MIL-101(Cr) as a template had also been reported, and the average particle sizes of chromium oxide nanoparticles (CrOx/C) successfully synthesized had reached 3 nm [56]. Based on UiO-66-NH2 (Zr-MOF) as a sacrificial template, a Pt/ZrO2@C catalyst was treated by a double solvent method (DSA) and pyrolyzed at 800 ℃ [61]. The double solvent method was used in order to prevent agglomeration of the nano Pt particles, as clearly shown in Figure 4. By simply roasting from a mixed metal Co/Ni-MOF-74, researchers obtained the CoxNi1x@CoyNi1-yO@C, while small-sized nanoparticles were highly dispersed embedded in a carbon matrix [64]. According to this research, the densely coated carbon shell prevented particle

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agglomeration and further improved the stability of the nanostructure. 2.4.1.2 Sacrificial template method One of the challenges of metal/metal oxide@carbon (M/MO@C) composites is the adjustment of the micropores and mesopores of the carbon skeleton as well as keeping the M/MO particles sizes under control during performing while maintaining their original morphology [58]. The use of an additional carbon source [59], the preservation of crystal morphology by a two-step calcination process [58], the use of aliphatic ligands to control porosity can solve some problems [60]. Using MOF-5 as a template and furfuryl alcohol (FA) as a carbon precursor, the composite material was carbonized at 1000℃ for 8h to prepare porous carbon [59]. Metal/metal oxide@carbon (M/MO@C) composites were converted from MOFs by the same method [58]. The porosity of carbon and the particle size of M/MO were controlled by a simple two-step process: phenolic resin was formed in the MOF pore by vapor phase polymerization (Vpp) and then thermally decomposed in Figure 5. Such as the Cu/Cu2O/C, it used a metal-organic framework Cu3(BTC)2 as a template and a phenolic resin as a carbon precursor. The Cu/Cu2O nanoparticles (Cu/Cu2O NPs) had a diameter of about 40 nm and were evenly distributed on the inner and outer surfaces of the porous carbon sheets [62]. Luo et al. [63] used MIL-101 (Fe)/C as precursor pyrolysis at 650℃ to produce the FeNC-650. FeNC-650 had a core-shell structure, and Fe and Fe3O4 nanoparticles were intercalated in the pore carbon layer. In the carbonization process of MOFs, the carbon layer will act as a carrier for metal NPs/metal oxides, which is beneficial for enhancing the electron transfer and stability of nanostructures because of its high electronic conductivity and inherent chemical inertness. The nanoparticles are randomly dispersed and vary in size from ten nanometers to a hundred nanometers. The carbonized materials are prepared with MOFs, which have stable physical and chemical properties and inherit the high specific surface area of MOFs. They have good prospects in the field of atmospheric pollution catalysis. 2.4.2 Preparation of metal oxides by MOFs MOFs precursors have special structural features such as ordered alignment of metal ions and nano-size effect. The metal oxide prepared by the MOFs precursor method generally has a more dispersed active center and larger specific surface area and exhibits stronger physical and chemical properties. 2.4.2.1 Self-pyrolysis Self-pyrolysis uses MOFs as a template without additional additives; the prepared metal oxide morphology inherits the morphology and specific surface area of MOFs. Through thermal decomposition, spherical mesoporous MnOx nanoparticles were prepared using MnMOF-74 as a precursor [11]. The structure of the MnOx particles were adjusted by changing the thermal decomposition conditions. By the thermal decomposition of three different coordinations: Cu-MOFs, porous CuO/Cu2O heterostructures, pure phase CuO and Cu2O were synthesized respectively as shown in Figure 6 [12]. According to the study, CuO/Cu2O heterostructure had a larger pore volume, a higher BET specific surface area, a stronger acidity, and more Lewis acid sites than pure CuO or Cu2O. Peng et al. [65] compared the catalytic effect of CuO prepared by thermal decomposition method and co-precipitation method in the NH3-SCR system. The specific surface area of CuO-p (15.4m2 / g) obtained by pyrolysis of HKUST-1 was much larger than that of CuO-c (3.4m2 / g) produced by coprecipitation. Ce-

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BTC was used as a precursor to synthesize three-dimensional penetrating mesoporous CeO2 for toluene combustion [46] [66]. Similarly, Li et al. [67] prepared a porous hollow structure Co3O4 polyhedron by direct pyrolysis of ZIF-67 crystal in air. 2.4.2.2 Loading self-pyrolysis Our group [130,131] prepared A-Cu-BTC (A=Fe, Ni, Co, Mn, Sr, La, Ce, Al) precursors by direct ion exchange pre-assembly method, then it burnt at 600℃ in the nitrogen atmosphere and AOx/CuOy was obtained for CO-SCR. Other researchers developed a new CuO @ Cu-MOF core-shell material, and Cu3 (BTC)2 was synthesized by in-situ growth method with CuO as the core [70]. The prepared CuO @ Cu-MOF material combined CuO with high NO catalytic activity, large specific surface area, and rich acid sites, which was highly resistant to H2O and SO2. Liu et al. [13] synthesized a set of highly dispersed CuO/CeO2 catalysts using Ce-MOF nitrogen-rich precursors. Compared with the Ce-MOF precursor without nitrogen atom, the MOFs with the nitrogen-containing ligand could further promote the dispersion of the active center on the surface of the CuO/CeO2 catalyst. Using Ni/UiO-66 precursor as template and filtering, drying and calcining at 450℃, a non-precious metal-based Ni/ZrO2 catalyst was successfully prepared [68]. Toluene oxidation catalyst, MnOx-CeO2 and MnOx, were prepared by pyrolysis of MOF-74 precursor [5]. The introduction of Ce in MnOx-MOF inhibited excessive shrinkage of precursors during pyrolysis [69]. Mn-doped magnetite (γ-Fe2O3) particles were formed by thermal decomposition of binary metal Fe-Mn-MOF [6]. The presence of Mn ions prompted the magnetite particles thermally stable in air, rather than normal Fe2O3. MOFs are promising catalyst precursors, and their highly ordered metal ions are well separated by organic linkers, which play an important role in preventing metal and metal oxide aggregation. Therefore, when the high-temperature heat treatment is performed as a precursor, the MOFs can suppress sintering or fusion of the bulk phase to a significant level. 2.4.3 Preparation of composite materials by MOFs Comparatively, heterogeneous catalysts have higher stability than homogeneous catalysts and can make significant progress in catalytic activity, cycle performance, and economic benefits. Commonly used load methods are in situ growth [71], epitaxial growth [72], one-step synthesis [73], post-treatment [74] and so on. 2.4.3.1 Composite with metal oxides Wang et al. [74] synthesized a high-performance material coated with nano-cerium oxide in MIL-100(Fe) pores by the impregnation method for NH3-SCR. Li et al. [76] generated a series of uniform nano-MOFs in situ into the flower-like MgAl-LDH with a hierarchical structure, including single metal MOFs (ZIF-8, ZIF-67, Cu-BTC) and bimetallic MOF(Co/Zn-ZIF). The synthesized MgAl-LDH/MOFs was used as a general platform for preparing integrated nano catalysts by controlling thermal decomposition as shown in Figure 7. Zhang et al. [71] used a mild in-situ deposition method to load Mn on a UiO-66 [77] support. Characterization of the prepared MnOx/UiO-66 catalyst showed that the presence of a crystal structure, porous UiO66 carrier and uniform distribution of the manganese particles on the catalyst surface. The epitaxial growth method has also been reported as a common method for preparing metal oxides@MOFs materials such as MoO3@ZIF-8 [72] and Fe3O4@Cu3(BTC)2 [78]. The hydrothermal synthesis for MO@MOF preparation has been investigated. Zhao et al. [75] loaded microporous MOF-74(Ni) to mesoporous γ-Al2O3 microbeads to prepare a layered

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porous composite material MOF-74(Ni)@γ-Al2O3. The introduction of γ-Al2O3 significantly increased the surface area and specific surface utilization, and the desulfurization adsorption efficiency was improved. Mesoporous Cu2O@MIL-100(Fe) was synthesized by hydrothermal method, and the loading effect of Cu2O loading on the catalytic efficiency of NH3-SCR was also investigated [16]. 2.4.2 Composite with metal and metal salt loading Zhang et al. [74] successfully synthesized Fe-Mn-MIL-100 bimetallic MOFs by hydrothermal method and prepared Fe-Mn-MIL-100 by mixing trimethyl sulfonic acid with Fe0, Mn(NO3)2, HF and deionized water. Kim et al. [79] prepared a Cu-loaded MIL-100 (Fe) material by CuCl2 wet immersion, washed with dichloromethane (DCM) and filtered. Xiao Zhang et al. [80] prepared metal-organic framework MnCe @ MOF catalyst by impregnating UiO-67 with an ethanol solution of manganese nitrate and lanthanum nitrate. Zhao et al. [81] prepared UiO-66 [82] by hydrothermal method and then doped with nano silver, which was recorded as UiO-66-Ag. Gisela et al. [4] impregnated three different pore size metal organic frameworks (MOFs) with lithium crown ether complex solution, and the samples were named LiCrw@Cr-MIL-101, LiCrw@Fe-MIL-100 and LiCrw@Ni-MOF-74. Three MOFs were successfully combined with organometallic units. Qi et al. [89] impregnated MOFs with FeCl3 solution to prepare the adsorbent FeCl3@MIL-101(Cr). Morever, when metal-organic frameworks were impregnated metal salt, it resulted in the reduction of the pore volume and small metal salt crystals were formed in the pores [86] [87]. Comparing immersion with in-situ doping on MOFs, the results showed that different doping methods lead to different forms of Mn-Ce in MOFs, showing different redox characteristics and resulting in different catalytic properties [88]. Different load capacity, load ratio, and metal type also have been studied. Different amounts of Pt particles were directly deposited on the surface of MIL-96 (Al) by the hydrothermal method to prepare Ptx-MIL-96 (X represents Pt content). The electronic structure of MIL-96 remained intact during Pt deposition [83]. Our group [84] prepared a bimetallic organic framework Agx-Cu-BTC by pre-assembly method, which showed a regular octahedral shape and a high specific surface area. The content of load metal incorporation in MOFs significantly affected the morphology of MOFs, which ultimately affects its SSA, pore volume and CUS [85]. Ali Mohammad Pourreza [90] studied the silver-ion functionalized metal-organic framework and developed an adsorbent (MIL-101(Cr)-SO3Ag) with a variety of interaction centers for adsorbing sulfur components. Yang et al. [91] synthesized selenium functionalized metal organic framework Se/MIL-101 with in-situ growth and prepared sulfur functionalized MIL101 (Se/MIL-101). Han et al. [17] assembled Zn-MOF-74 by ultrasonic-assisted hydrothermal method and prepared Au@Pd@MOF-74, Pt/MOF-74 and Pt/Au@Pd@MOF-74 based on literature [92]. Among them, gold nanoparticles (NPs), as the core of Pd’s shell epitaxial growth, were encapsulated in MOF-74 nano shuttle to control the morphology of MOF-74, and Pt NPs, which were loaded onto the surface of MOF-74. 2.4.3 Composite with carbon Composites of metal-organic frameworks (MOFs) and nano-carbon materials have better performance than single MOFs. By combining MOFs with suitable materials such as graphene derivatives [93] or carbon nanotubes, their thermal stability, chemical stability, and adsorption capacity were significantly [148].

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Barbara et al. [94] reported that the [Al8(OH)8(BTB)4(HCOO)4] in three-dimensional graphene (MG) with a larger pore volume, resulting in a mass ratio of MG/MOFs=1: 2 graphene-MOFs composites. Ram et al. [95] synthesized graphene-MOFs composites with graphene oxide (GO), benzoic acid functionalized graphene (BFG) and a two-dimensional layer film MOFs (CDPBM). Li et al. [96] proposed a solvent-free mechanochemical method for the rapid synthesis of Cu-BTC and graphite oxide composites (Cu-BTC@GO). The synthesized Cu-BTC@GO had a higher specific surface area and pore volume, with better stability in water than Cu-BTC. Additionally, there are some other MOFs composites. For example, Gamze et al. [97] constructed a layered metal sulfide (NiCo-LDH) by in-situ transformations of ZIF-67-derived hollow diamond dodecahedron and vulcanization of nickel-cobalt layered double hydroxide (NiCo-LDH/Co9S8) system. The synthesis of MOFs composites is often complex and timeconsuming. In addition, the accumulation of chemical species often occurs on the outer surface, resulting in non-optimal chemical interactions [76]. Moreover, in recent years, MOFs composites have been widely used in various catalytic reactions, and therefore has been established that MOFs composites have excellent activity in various catalysis. 3. Catalytic applications to fight air pollution The application of MOFs materials and their derivatives in catalytic applications for air pollution abatement is shown in Figure 8, such as mercury removal, VOCs catalytic oxidation, desulfurization, denitrification, CO catalytic oxidation, and so on. 3.1 The adsorption of Hg0 The MOFs carrier plays an important role in promoting the dispersion of active components, increasing Hg0 adsorption and catalytic performance, and improving oxygen utilization, thereby producing an ideal catalytic activity. Compared with traditional support materials, MOF carriers show superiority in utilizing O2. The adsorption of Hg0 on the MOF support was stronger than traditional material because of the special structure and Lewis acidity. Flue gas composition showed a significant impact on mercury removal on MOFs [36]. For example, Cu-BTC had limited Hg0 removal ability without HCl; HCl played an important role in promoting removal of Hg0, while SO2 had an inhibitory effect. It was also interesting to find NO promoted the removal of Hg0, and H2O had little effect on the removal of mercury. When the concentration of HCl and O2 was high, the effects of SO2, NO and H2O were not significant. And for MIL-101 (Cr), higher oxygen content improved Hg0 removal efficiency [89][98]. The introduction of functional groups and supported metals affected the mercury adsorption stability of MOFs. Se/MIL-101 maintained high mercury adsorption stability in an atmosphere containing SO2, NO and H2O [91]. After the introduction of SO2 into the flue gas, the removal rate of Hg0 by Br-MOF was improved [53]. However, the Hg0 removal rate was lower when H2O was present. As shown in Figure 9, the adsorption of elemental mercury (Hg0) and mercury species (HgCl2, HgO and HgS) on Mg/DOBDC (2,5- dihydroxy terephthalic acid) had been studied through theoretical calculations [99]. The results showed that Hg0 was stably adsorbed on magnesium ions, and the binding energy (BE) was -27.5 kJ/mol. HgCl2 was combined with two magnesium atoms by Cl- to obtain a strong adsorption strength (89.0 kJ/mol). The adsorption energies of HgO and HgS on Mg/DOBDC reached -117.0 kJ/mol and -169.7 kJ/mol, respectively.

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The Hg0 removal rate’s influence of MOFs types, temperature and oxygen content in sintering flue gas [98]. Compared with Cu-BTC, UiO-66, and activated carbon, MIL-101 (Cr) had the best Hg0 removal performance which can reach about 88% at 250℃. The removal mechanism of Hg0 was analyzed by XPS and Hg-TPD methods. The Hg0 was first adsorbed on the surface of MIL-101(Cr) and then oxidized by the open metal center Cr3+. The generated Hg2+ was adsorbed on the surface of the adsorbent to form HgO, and then the surface of active oxygen was used to oxidize the metal center Cr2+ to Cr3+. Reaction temperature was also an important condition for affecting adsorption. For adsorption of Hg0 on MOFs, the removal efficiency of Hg0 almost increased with increasing temperature, and the Hg0 was adsorbed on the surface of adsorbent to form HgO [98]. In contrast, Hg0 was converted into stable, water-insoluble HgSe on Se/MIL-101, and the adsorption capacity of mercury decreased with the increase of adsorption temperature. TPD experiments showed that the mercury on Se/MIL-101 could be released above 150℃ [91]. For halogen functional groups, the Gibbs free energy required for oxidizing Hg0 was very low, and too high a temperature may inhibit the activity of the active components. [89] Furthermore, results from the simultaneous removal of mercury (Hg0) and NO from flue gas by MnCe @ UiO-67 have been studied and the result was compared with Mn-Ce-ZrO2 [80]. The influence of Hg0 removal on NH3-SCR also was studied. The addition of Hg0 in the flue gas had little impact on the removal of NO while long-term removal of Hg0 may negatively affected the activity of MnCe @ UiO-67 in SCR. The results showed that MnCe @ UiO-67 had better activity than the conventional catalyst Mn-Ce loaded ZrO2 at low temperature (<250℃). The study also observed that Mn3+/Mn4+ remained constant during the reaction at different temperatures, while the proportion of Ce3+/Ce4+gradually decreased with the reaction time. This probably led to a decrease in chemisorbed oxygen on the catalyst. The introduction of different groups improved the stability and catalytic activity of MOF materials in different atmospheres, and forms new reaction sites based on these groups. For example, although the specific surface area and pore volume decreased after impregnation, more active sites were obtained [89]. Under anaerobic conditions, Cl- ions acted as oxides in metal chlorides; while under aerobic conditions, metal oxides provided lattice oxygen to the reaction of oxidizing Hg0. By simulating the flue gas fixed-bed adsorption test, the adsorption amount reached 14.27 mg/g. And the introduction of selenium on MIL-101 improved its mercury adsorption capacity (148.19 mg·g−1), initial adsorption rate (44.8μg·g−1·min−1, which was 89~1659 times higher than the adsorption rate of mercury adsorbent reported in other literature) and its ability to resist SO2, NO, and H2O [91]. But its thermal stability was decreased, the TPD experiments showed that the mercury on Se/MIL-101 could be released above 150℃. As for Br-MOF [53], bromine functional group on MOF was the main active site for capturing Hg0. After treatment with Br-MOF for 48h at 200℃, the optimum removal rate of Hg0 was still over 99%, while the efficiency of non-functionalized MOF and conventional bromine impregnated activated carbon decreased to 59.8% and 91.2%, respectively. The removal rate of Hg0 by Br-MOF was improved in the presence of SO2. However, the Hg0 removal rate was lower when H2O was present. The addition of Ag did not change the morphology of UiO-66 while increasing redox activity, while Hg0 adsorption capacity reached 3.7 mg/g at 50℃ [81]. The study of reaction mechanism showed that Hg0 was removed by the formation of amalgam at low temperature and channel adsorption at high temperature, it was

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oxidized by reactive oxygen and adsorbed on the surface of Ag. Yee et al. compounded mercaptan (−SH) in MOFs for the removal of Hg0 in the gas phase and Hg(II) in water [10]. These frameworks were assembled by reacting ZrCl4 or AlCl3 with 2,5-dimercapto-1,4-phthalic acid (H2DMBD). The synthesized Zr-DMBD and Al-DMBD frameworks were represented as UiO-66 and CAU-1, and the carboxyl group was bonded to the hard Zr(IV) or Al(III) center, and the thiol group was used to modify the pores. Sun et al. [100] prepared COF-V (COF-S-SH) by treating COF-V with 1,2-ethanedithiol. The adsorption capacity of Hg2+ and Hg0 was 1350 mg/g from aqueous solution and 863 mg/g from the air, respectively, exceeding all the thiol and thioether functionalized materials reported. More importantly, COF-S-SH had an ultra-high partition coefficient (Kd) of 2.3×109 mL•g−1. This enabled it to rapidly reduce Hg2+ concentration from 5 ppm to less than 0.1 ppb, far below the limit (2 ppb). The study attributed the impressive performance to the synergistic effect of dense chelating groups with strong binding capacity in ordered mesopores, allowing mercury species to diffuse rapidly throughout the material in ordered mesopores. In COF-S-SH, each Hg combined with two S by intramolecular synergy. The possible adsorption mechanism of Hg0 on MOFs as reported was as follows： 1. The adsorption of Hg0 by benzene-bromo-doped MOFs was similar to the Grignard reaction. Hg0 was first adsorbed on the surface of the MOFs and then reacted with additional phenyl bromide. Due to the transfer of electrons, benzene radicals, mercury radicals, and Brwere formed. The Hg radical combined with Br- and then coupled with benzene radical to form a benzene-HgBr complex on the surface of the MOFs [101]. 2. Halogen catalytic adsorption [101]: The X functional group (halogen ion represented by X ) in the catalyst had a partial X-characteristic [89, 102], and HCl in the atmosphere also promoted the adsorption reaction [36]. Initially, Hg0 was adsorbed on the surface of the adsorbent, and reacted with the halogen on the surface of the adsorbent through a chemical adsorption process. Finally, mercury complexes such as [HgX] and [HgX2] were produced. For example, a functional group containing Cl- formed chemisorption of Hg0 by the following reaction [103]. Hg0+[Cl]-→[HgCl]++2e Hg0+2[Cl]-→[HgCl2]++2e In the presence of additional Cl species, mercury even turned to use four coordination numbers: [HgCl2] +2[Cl]-→[HgCl2]++2e 3. Mars-Van-Krevelen redox mechanism: was the reaction of the reactants with the catalyst lattice oxygen ions. The first step is the reduction of oxygen vacancies between the reactants and the catalyst. The second step is that the catalyst is deoxidized by dissociation of the adsorbed oxygen to supplement the oxygen vacancies and is regenerated. Since the first step is the reduction of the oxide catalyst and the second step of the catalyst is oxidized, this mechanism is also called the redox mechanism. The removal of Hg0 on MnCe @ MOF sample was an example [80]. The first step in catalytic oxidation was likely the adsorption of Hg0 on MnCe @ MOF (Eq) (1). The adsorbed Hg0 was catalytically oxidized by oxygen to form HgO (Eq) (2). When the active metal component was present in a low-valent state, it exhibited a tendency to accept oxygen, forming oxygen vacancies, while the O adsorbed on the oxygen vacancies was called chemisorbed oxygen.

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Chemically adsorbed oxygen was the most active form of oxygen in the catalytic reaction. Lattice oxygen (Olat) was present in the lattice structure of metal oxide. Olat participated in Hg0 oxidation. The lattice oxygen consumed was supplemented by Oads (chemical adsorbed oxygen) formed by O2 during the reaction. 4. Synergistic catalytic mechanism: The adsorption process of the system was completed by the synergistic coordination of multiple catalytic sites. The metal Ag, Se [91] and thiol(-SH) [104] supported on the MOFs material were active sites for adsorption. Through higher charge transfer, a stronger binding force was produced, which stabilized the existence form of Hg. 3.2 The removal of VOCs 3.2.1 VOCs adsorption and sensing Volatile organic compounds (VOCs), including benzene, toluene, ethylbenzene, xylene, aldehydes, ketones, and chlorinated hydrocarbons, are among the most common contaminants in indoor/outdoor air, that have attracted worldwide attention. They are mainly produced by chemical processing industries (such as thinners, detergents, lubricants and liquid fuels) and pose a significant threat to humans [105]. VOCs in the environment also lead to other serious environmental and health risks, such as photochemical smog [106]. At present, research on the removal of VOCs by MOFs focuses on the removal and recovery of VOCs from the air as adsorbents [107]. The higher interaction affinities of MOFs with VOCs are mainly attributed to physical adsorption [110] (hydrogen bonding and л-complexation) and pore-filling mechanisms [28]. The adsorption of VOCs on MIL-101 was mainly a pore-filling mechanism that was selective for the size and shape of VOC molecules as shown in Table 1[28]. A new ligand-tocluster ratio was used to synthesize modified MIL-101(Cr), expressed as M-MIL-101 (Cr) [32]. The adsorption behavior of different volatile organic compounds (VOCs) on the M-MIL-101 (Cr) has been studied. The specific surface area and pore volume of M-MIL-101@Free at 77K were 4293 m2/g and 2.43 cm3/g, respectively. This promoted the adsorption of gasoline molecular and its organic components with various molecular sizes, shapes and polarities. Benzene adsorption under the practical conditions (in the range of 0.1-50 ppm) on MOFs (such as MOF-199 and UiO-66) was studied, compared with the activated carbon [108]. In terms of maximum adsorption capacity (mg g-1), MOF-199 (94.8) was slightly better than AC (93.5) at the highest tested concentration of 50 ppm benzene. However, in terms of partition coefficient (PC) at BTV10 values (at 0.01 Pa), the performance of AC was about 14 times that of MOF199 and 46 times that of UiO-66. Barbara et al. [94] crystallized Al-MOF in the pores of threedimensional graphene (MG) to synthesize high-porosity composite MG-MOF. The material improved the poor benzene adsorption when MOF was at low pressure. This clearly showed that benzene has better adsorption over a wider pressure range with better thermal stability. Kowsalya et al. [109] studied the adsorption of toluene (MOFs: UiO-66, UiO-66 (NH2), ZIF67, MOF-199, MOF-5 and MIL-101 (Fe)) at atmospheric pressure, and its equilibrium adsorption capacity ranged from 159 mg/g (MOF-199) to 252 mg/g (UiO-66-NH2). Due to the loss of structure and porosity of the water-soaked Cu-BTC, mechanochemical synthesis of CuBTC@GOs composites was treated as an effective strategy to improve Cu-BTC adsorption performance and its water stability [96]. The toluene absorption also reached 9.1mmol/g at 298 K, much higher than traditional activated carbon and zeolite. New Cu-MOF synthesized by 5aminoisophthalic acid (AIPA) was capable of formaldehyde capture [49].

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At the same time, some researchers have made some progress in applying MOFs nanoparticles to VOCs sensing devices. Zeinali [110-111] applied Cu-BTC nanoparticles to capacitive sensing devices and interdigital electrodes (IDE) of VOCs (methanol, ethanol, isopropanol, and acetone) for higher high VOCs abatement efficiency. 3.2.2 VOCs catalytic oxidation Sun et al. [5] prepared MnOx-CeO2 and MnOx catalysts by using MOF-74 precursor pyrolysis. The catalytic activity of toluene oxidation was better than that of MnOx-CeO2-CP and MnOx-D catalysts prepared by both coprecipitation method and MnOOH thermal decomposition method. The introduction of Ce inhibited excessive shrink on the precursor during pyrolysis [69], increasing the specific surface area and external content of Mn4+. Hence, surfacial Mn4+ played a crucial role in increasing toluene oxidation activity. The T50 (Transformation rate) and T90 values of the MnOx-CeO2 catalysts were 210℃ and 220℃, respectively. The apparent activation energy (EA) was 82.9 kJ/mol, which was lower than other catalysts. Chen et al. [66] used Ce-MOF as raw material to synthesize the mesoporous CeO2 catalyst (CeO2-MOF). The results showed that the CeO2-MOF/350 catalyst (350℃ pyrolysis) had higher catalytic activity for toluene oxidation. The conversion rates at 180℃, 211℃ and 223℃ were T10%, T50%, and T90%, respectively. Especially in the high temperature zone, the CeO2MOF/350 catalyst had 100% conversion rate. The toluene decomposition mechanism over CeO2-MOF/350 was depicted in Figure 10. The catalytic performance of CeO2-MOF/350 could be reasonably attributed to a series of better properties, such as three-dimensional penetrating mesoporous channels, large specific surface area, small average grain size, high relative percentage of Ce3+/Ce4+ and oxygen storage, higher oxygen vacancy concentration, higher active oxygen content and more acid sites. Chen et al. [112] prepared Pd@Cu(II)-MOF(2) hybrid materials by solution impregnation method based on the novel Cu(II)-MOF(1) material. The fluorine atom-containing Cu(II)-MOF effectively stabilized the Pd nanoparticles in the crystal. The obtained Pd NPS-embedded Pd@Cu(II)-MOF was used as a highly active heterogeneous catalyst. It sped up the oxidation of the aromatic alcohols in air to form corresponding benzaldehydes with high conversion and the selectivity almost reached 100% in Figure 11. 3.3 The removal of sulfide Hydrogen sulfide (H2S) and sulfur dioxide (SO2) are invisible gases with an unpleasant odor. They are converted into more stable products (such as sulfuric acid) in the atmosphere, posing threat to vegetation and aquatic life in ecologically sensitive areas and causing damage to human organs. Effective treatment of H2S and SO2 in sulfur-containing gases is essential for pollution control and industrial operation safety. Song et al. [113] simulated the adsorption of sulfur dioxide by metal organic frameworks (MOFs) and studied the effects of temperature, free volume and surface area on the adsorption of sulfur dioxide. MIL-101(Cr)-SO3Ag was prepared by Alimohammad et al. [90] and interacted with H2S through various routes. Its adsorption capacity (96.75 mg/g) was four times higher than that of MIL-101 (Cr) with good cyclic stability. The electrostatic interaction between S and Ag was the main reason for H2S adsorption. When Cu-BTC was impregnated with different Ba salt, the Ba/Cu-BTC (Cl-) samples had the highest ability to absorb SO2 at temperatures below 673 K, even more than BaCO3/Al2O3/Pt based materials [86]. In containing

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trace sulfur-containing gas mixtures, the H2S and SO2 capture properties of different functionalized UiO-66(Zr)-XN materials were studied by molecular simulation [54]. Among them, UiO-66-(COOH)2 and UiO-66-COOH were more promising porous adsorbents than other functionalized UiO-66(Zr) derivatives. The use of MOFs (UiO-66(Zr)-(COOH)2) as fillers also significantly improved the permeability of the polymer and the separation of H2S/CH4 mixed gas. [114] 3.3.1 Selective oxidation of sulfide At present, there are few studies on the catalytic oxidation of SO2 in the oxidation of sulfides by MOFs. Selective oxidation of H2S is an attractive treatment. This is because H2S can be completely oxidized to sulfur under the action of a catalyst (H2S+1/2O2→(1/n) Sn+H2O), which is not limited by thermodynamic equilibrium.[115] Zheng et al. [44] prepared porous MIL-100(Fe) with a coordination unsaturated (CUS) Fe2+/Fe3+ site as a selective catalytic oxidant for H2S. At 100~190℃, CUS-MIL-100 (Fe) had H2S conversion close to 100%, S selectivity close to 100%, and exhibited good thermal stability and S selectivity. The process of selective H2S oxidation to form elemental sulfur follows a redox mechanism on CUS-MIL-100(Fe) [115]. Then, they prepared a classic aminofunctionalized iron organic framework NH2-MIL-53(Fe) by a simple hydrothermal method [8]. The NH2-MIL-53(Fe) catalyst had a higher H2S conversion and S selectivity close to 100% over a temperature range of 130-160℃, superior to Fe2O3 and activated carbon. The introduction of an amino group reduced the activation energy of H2S oxidation and imparted a certain basic site to the surface of the catalyst. The catalytic oxidation reaction [115] of H2S was (H2S+1/2O2→(1/n) Sn+H2O), and the Mars-Van-Krevelen redox mechanism on MOFs was as follows: 1. Taking NH2-MIL-53 (Fe) catalyst as an example in Figure 12 [8]: for this Fe-based catalyst, the formation of strong Fe-S bonds was not obvious, so the H2S molecules were mainly adsorbed at the amide groups. The oxygen molecules on the surface were converted to O2- and O- active species. H2S in the surface reacted with O- to form elements S and H2O. 2. Taking CUS-MIL-100(Fe) catalyst as an example [44,116]: The H2S molecules were diffused into CUS-MIL-100 (Fe) and adsorbed on the Lewis acid sites (Fe2+/Fe3+ CUS) on the outer and inner pores in Figure 13. On this basis, H2S was directly oxidized by Fe3+ to form S and Fe2+. Finally, Fe2+ reacted with reactive oxygen species to regenerate the Fe3+ active site and further participated in the oxidation reaction. At the same time, H2S was adsorbed and dissociated at the Lewis acid site to form HS-, and then interacted with the adsorbed oxygen to form elemental sulfur. 3.4 The removal of nitrogen oxides NOx emissions have a major direct impact on human health and serious consequences for the global environment. State of the art approaches employ, selective catalytic reduction (SCR) is an effective control technique for removing NO. In this process, the development of efficient SCR catalysts is the key to achieving efficient NOx conversion.[45] At present, researchers have applied MOFs materials to three effective NOx selective catalytic reduction routes, namely NH3-SCR, H2-SCR, and CO-SCR. 3.4.1 NH3-SCR The SCR mechanism on MIL-100(Fe) was studied by density functional theory calculations considering van der Waals interactions [117]. Both the Lewis acid site and the Bronsted acid

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site were present on MIL-100-Fe and active in the SCR reaction. The NH3-SCR mechanism was subdivided into two parts: (1) NO oxidation and (2) rapid SCR reaction. The results indicated that the Lewis acid sites played a significant role in activating the reactants. The NO molecule was easily oxidized to the NO2 species at the Fe Lewis acid site, and the adsorbed NH3 species reacted with the gas NO2. Hence, the formation of the intermediate NH2NO was a determining step of the reaction rate. Wang et al. [118] used MIL-100 (Fe) as a catalyst for the catalytic reduction of nitrogen oxides by NH3, and the conversion of nitrogen oxides at 245-300℃ reached 97%. In addition, the effect of H2O and SO2 on the catalytic activity was reversible, and the NOx conversion rate was restored after the removal of H2O and SO2. NH3 existed in the form of ion NH4+. NH4+ reacted with the adsorbed NO2 at the Fe(II) CUS central site and followed the L-H reaction mechanism. Since the vacuum drying can make the iron sites in MIL-100 (Fe) form many coordination unsaturated metal centers (CUS), the MIL-100 (Fe) with Fe2+/Fe3+ mixed state was recently researched [45]. And the importance of activation temperature on catalytic reduction was also proved. The results showed that the sample was dried in vacuum at 473 K for 12 h, and the sample had a NO conversion rate of 100% at 200℃. NO was adsorbed significantly on Fe2+/Fe3+ CUS of MIL-100(Fe), thereby promoting the NH3-SCR reaction of the (E-R) mechanism. Some researchers also performed load experiments. Other elements doped in MIL-100(Fe) were also investigated. The MIL-100(Fe-Mn) catalyst [73] had higher NOx conversion than MIL-100(Fe) and MIL-100(Mn). The NOx conversion rate reached 96% at 260℃. Moreover, MIL-100(Fe-Mn) had good stability and resistance to SO2 and H2O. In the NH3-SCR process, the nitrogen oxide conversion was slightly increased (about 7%) in the presence of H2O and SO2, which was attributed to the formation of sulfate on the catalyst surface as a new acid center. Combing with in situ FT-IR results, the SCR reaction of MIL-100 (Fe-Mn) belonged to the Ely-Rideal (E-R) mechanism. Copper was also a popular element of NH3-SCR research. By the hydrothermal method, the maximum catalytic efficiency of Cu/MIL-100 (Fe) at 300℃ was 76%, increased 16% compared to MIL-100 (Fe) [16]. In addition, the type of copper significantly promoted the NO reduction activity of Cu/MIL-100 (Fe), and the conversion of NO improved with the increased amount of copper. Cerium oxides were also often used for flue gas catalysis. Wang et al. synthesized MIL-100(Fe) coated with nano-CeO2 by impregnation method (IM) as shown in Figure 14 [74]. The prepared CeO2/MIL-100(Fe) catalyst had good catalytic activity in a wide temperature window range and achieved 90% NOx conversion at 300℃. The nano-ruthenium coated in MIL-100 (Fe) promoted the generation of chemisorbed oxygen on the surface of the catalyst, greatly promoting the formation of NO2 species. A rapid SCR reaction was performed. The study observed that sulfur dioxide had a certain influence on the adsorption of nitrogen oxides on the catalyst surface. In addition, the formed sulfate covered the active center of the catalyst surface and inhibited the conversion of NOx [119]. A series of MOF-74 have been studied, especially in the field of denitration. The effects of metal species, metal ratio, reaction temperature, and solvent species on the MOF-74 were observed. [11] [85] [120] [121] [122] The M-MOF-74(M=Mn, Co and Cu) showed different topography while prepared Mn-MOF-74, having a hollow spherical structure, Co-MOF-74 with petal structure and Cu-MOF-74 exhibited a rod-like morphology. [120] [122] Co-MOF-

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74 and Mn-MOF-74 had high adsorption and activation ability for NO and NH3. The denitrification rates in the presence of NH3 were 99% at 220℃(Mn-MOF-74) and 70% at 210℃(Co-MOF-74), then the NO conversion rate of Cu-MOF-74 was 97.8% at 230℃. In addition, these materials had certain heat resistance, excellent resistance to SO2 and H2O poisoning in the temperature range of low-temperature SCR. Among them, Co-MOF-74 had the best sulfur resistance that SO2 had little effect on the catalytic activity. During the reaction, the conversion of M-MOF-74 (M = Mn and Cu) decreased with the introduction of H2O and SO2. Subsequently, it recovered substantially when the gas was shut off. The effect of Co/Mn atomic ratio of Co/Mn-MOF-74 on a solid structure also has been studied [85]. Very high NO conversion (>96.0%) at 180-240℃ was obtained using the chemical composition of Mn0.66Co0.34-MOF-74 (optimal content of Co). In addition, Mn0.66Co0.34-MOF74 showed excellent resistance to SO2 poisoning. In the case of H2O and SO2, the NO conversion rate was reduced from 99% to 70% at 200℃. After the introduction was terminated, the NO conversion rate gradually recovered to 97-98%. The addition of Co weakened the SO2 adsorption strength on the surface of the catalyst, thereby improving the anti-SO2 poisoning ability of the catalyst. The solvothermal synthesis temperature affected a lot on the structure of MOF-74 [121]. The Co-MOF-74 synthesized at 100℃ had a flower-like morphology, while a hexagonal prism structure was obtained at a temperature range of 120℃ to 150℃. Compared with hexagonal prisms, the flower-like morphology had a larger specific surface area and pore volume. The CUSs (coordinatively unsaturated sites) were exposed after solvent removal at the same time. Solvothermal temperature and cosolvent were the key factors affecting the physicochemical properties of MOF-74 [122]. The Cu-MOF-74 sample synthesized at 80℃ with isopropanol as a cosolvent exhibited a rod-like morphology, while the product was synthesized with ethanol as a cosolvent at 65℃ and 80℃, showing a unidirectional and bidirectional bouquet-like structure. Then the Cu-MOF-74-iso-80 catalyst had the highest NH3-SCR activity with the conversion rate of 97.8%，where “iso” and “80” mean the cosolvent (isopropyl alcohol) and the preparation temperature of 80 °C, respectively. The BET test results showed that the specific surface area of Cu-MOF-74 was larger, and the adsorption capacity of NH3 was stronger, which was beneficial to the adsorption of SCR at low temperatures. The catalyst had good resistance to water. Through thermal decomposition by the Mn-MOF-74 template, spherical mesoporous MnOx nanoparticles were also excellent catalysts [11]. The morphology and crystals of MnOx nanoparticles were inherited from Mn-MOF-74 templates. By changing the thermal decomposition conditions, the structure of the MnOx particles were adjusted. The sample prepared at 700℃ under N2 atmosphere had the largest specific surface area (89.1m2/g) and the highest denitrification activity (98% NO conversion at 260℃). A hollow porous MnxCo3-xO4 nanocage with a spinel structure was also prepared by pyrolysis, which was derived from Mn3[Co(CN)6]2·nH2O precursor [127]. Compared with the MnxCo3-xO4 nanoparticles, MnxCo3-xO4 nano nanocages had better low-temperature catalytic activity, higher stability and sulfur dioxide tolerance. The hollow structure and the porous structure provided a larger specific surface area and more active centers for adsorption and activation of the reaction gas, leading to a higher catalytic activity. In addition, the uniform distribution and strong interaction of manganese and cobalt oxide species not only enhanced

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the catalytic cycle but also inhibited the formation of manganese sulfate, resulting in higher catalytic cycle stability and better resistance to SO2. Zhang et al. [123] studied the adsorption characteristics of six small gases associated with NH3-SCR, such as NH3, NO2, NO, O2, H2O and SO2 in Mn-MOF-74 during NH3-SCR process. According to the study, the Mn-MOF-74 structure bound the molecules of the mall gases in the following order of increasing bond: NH3 > NO2 > NO > O2. In addition, a possible pathway for the conversion of NO to NO2 was also calculated. The competitive adsorption on NH3 and H2O, NH3 and SO2 were shown under certain conditions. Both H2O and SO2 were replaced by NH3, indicating that those two impure gases affected the activity of NH3-SCR reaction. Furthermore, the characteristics of carboxy-oxygen vacancies and hydroxyl oxygen vacancies and the influence of H2O on the stability and catalytic performance of Mn-MOF-74 were investigated by using density functional theory (DFT) method [129]. The study pointed out that oxygen vacancies did not only activated the reactive species but also promoted the desorption of NO2 molecules from the metal center. Additionally, the authors further noted that H2O attenuated the Mn-O bond interaction of Mn-MOF-74. Cu-BTC, named HKUST-1, was the focus of research about NH3-SCR. The influence of modulation and thermal treatment had been studied [39]. They introduced acetic acid as a modifier and prepared a layered porous structure with beneficial lattice defects. The NO conversion rate of modified Cu-MOF was the highest at 280℃ (95.5%), compared with a NO conversion of 83.9% at 280℃. The pyrolysis temperature had a significant influence on the activation process because the thermosensitive organic ligands primarily affected the active sites on the surface. Li et al. [37] found that the activation temperature had a significant effect on Cu-BTC. The Cu-BTC catalyst, activation at 230℃ in N2, had complete NO conversion efficiency within the range of 220℃ to 280℃. This excellent catalytic activity was mainly attributed to the unsaturated Cu+ position in Cu-BTC. Wang et al. [12] successfully synthesized porous CuO/Cu2O heterostructures using metalorganic framework (MOFs) assisted template method. The tunable production of pure phase CuO and Cu2O was achieved by changing the coordination environment of copper. Compared with pure CuO and Cu2O, the heterostructure CuO/Cu2O had obvious NH3-SCR deNOx activity and N2 selectivity in the low-temperature range of 170~220℃. In addition, the CuO/Cu2O heterostructure exhibited excellent durability of H2O, SO2, and alkali metals. The NH3-SCR process of NOx on CuO/Cu2O heterostructures conformed to the Eley-Ride (E-R) mechanism. The rich Lewis acid sites in the CuO/Cu2O heterostructure promoted the E-R reaction pathway. At the same time, the synergistic effect of Cu2+ and Cu+ at the CuO/Cu2O heterostructure interface initiated a cyclic reaction to remove NO in Figure 15 [124] [125]： Cu2++NH3+NO→Cu++N2+H2O+NH4+ Cu++1/2O2+NH3+NO→Cu2++N2+H2O+OHComparing CuO prepared by different methods, the CuO-P, prepared by pyrolysis of HKUST-1 template, had a better catalytic performance than CuO-C (co-precipitation methods) [65]. CuO-P (15.4m2/g) had a larger surface area than CuO-C (3.4m2/g) and showed the superior activity of NO conversion. Further calcination at 600℃ in a nitrogen atmosphere was beneficial for increasing the specific surface area and obtained the desired SCR performance. Cu-MOF was also synthesized as core-shell material to improve the stability at the reaction conditions. A new CuO@Cu-MOF core-shell material was synthesized. CuO was used as the core, and Cu3(BTC)2 as shell by in-situ growth method [70]. The prepared CuO@Cu-MOF

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material combined CuO (high NO catalytic activity) with Cu3(BTC)2 (large specific surface area and rich acid point). More importantly, CuO@Cu3(BTC)2 showed satisfactory tolerance to H2O and SO2. When H2O and SO2 were added, the activity decreased, which was related to the rapid formation of ammonium sulfate and metal sulfate [126]. Nevertheless, the reactivity was greatly increased after the removal of H2O and SO2. The effects of temperature during pyrolysis on the microstructure, texture and chemical properties of CoOx@PC-T (PC represented porous carbon, and T represented pyrolysis temperature) were investigated from ZIF-67[55]. The results showed that temperature had a significant effect on the degree of carbonization of ZIF-6 and affected the exposure and oxidation of cobalt nanoparticles on the catalyst surface. The experimental results as reported also indicated that CoOX@PC-800 obtained the highest NOx conversion due to its higher Co3+/Co2+ ratio and larger specific surface area. CoOx@PC-800 had good catalytic stability and excellent H2O resistance, but less general sulfur resistance. In the presence of SO2, surfacedeposited sulfite/sulfate species blocked the active center of the catalyst. Lee et al. [6] prepared an Mn-doped magnetite (γ-Fe2O3) particle by thermally decomposing a binary metal Fe-Mn-MOF. Although γ-Fe2O3 was a thermodynamically unstable structure, Mn ions inhibited the conversion of γ-Fe2O3 to Mn-doped α-Fe2O3, because the formation of Mn-doped α-Fe2O3 required higher activation energy. The sample annealed at 600℃ showed a mixed hysteresis loop, indicating that an intermediate structural phase existed between γ-Fe2O3 and α-Fe2O3 during the FeMn-MOF phase transition. The Mn-doped γ-Fe2O3 calcined at 320℃ had a NO conversion of 97% at 250℃, which obtained good catalytic activity. Converting the crystal structure was converted to α-Fe2O3, causing a reduction in its catalytic activity. Yu et al. [56] successfully synthesized composite materials (CrOx/C) using MIL-101 (Cr) as raw material. The chromium oxide nanoparticles had an average particle diameter of 3 nm, covering with amorphous carbon. The synthesized CrOx/C material exhibited excellent NH3SCR activity and had a good ability of regeneration in the presence of SO2. In the CrOx/C sample, a certain amount of unstable lattice oxygen was exposed to the surface of the CrO x nanoparticles. It obtained an average size of 3 nm, which resulted in a good performance of oxidizing NO to NO2. The formed NO2 reacted with the catalyst through the "rapid NH3-SCR" route, thereby improving the NH3-SCR performance of the CrOx/C catalyst. In addition, the stable lattice of the CrOx species retarded the sulfation process of CrOx/C catalyst, which resulted in a better ability of regeneration. Effects of metal impregnation and in-situ deposition on the metal organic framework (MOF) were studied [88]. The existing forms of Mn-Ce in MOFs were different by using impregnation and in-situ. The Mn-Ce introduced by the impregnation method was deposited on the surface of MOFs, and the catalytic efficiency was higher above 98% between 200-300℃. And under the in-situ doping method, Mn-Ce was inserted into the lattice structure of MOFs. This increased the specific surface area of Mn-Ce. It resulted in lower Mn concentration and the poorer redox performance, leading to lower activity of MnCe. Howerer, the in-situ doped MnCe was released from the MnCe-MOFs lattice by thermal decomposition, and then the redox performance and catalytic activity increased. The study concluded that different doping methods lead to different forms of Mn-Ce in MOFs, showing different redox characteristics, which directly led to different catalytic properties. Zhang et al. [71] prepared an Mn-based catalyst on UiO-66 support by a mild in-situ

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deposition method. In the range of a wide temperature window (100 to 290℃), when the MnOx content was 8.5%, the NO conversion of the catalyst was the highest (90%). The characterization of MnOx/UiO-66 catalyst showed that the catalyst had the crystal structure and porosity of the UiO-66 carrier, and the manganese particles were uniformly distributed. The interaction between the surface MnOx and Zr oxide of the catalyst had a positive effect on the catalytic activity. The 8.5 wt% MnOx catalyst maintained a good activity after 24 h work with good resistance to SO2 poisoning. However, the porosity decreased about 9%, possibly due to the folding of some structures in the catalyst [128]. The crystalline porous Ni-MOF catalyst prepared by Sun et al. [41] had good thermal stability. After preheating in an N2 atmosphere, the catalytic activity of Ni-MOF was significantly improved. The activated Ni-MOF at 220℃ had a conversion of NO of 92% and a reaction temperature of 275 to 440℃. The main catalytic mechanism of NH3-SCR was as follows： 1. Eley-Ride (E-R) mechanism: the reaction is between the adsorbed molecule and liquid or gas phase molecules. In this mechanism, proposed in 1938 by D. D. Eley and E. K. Rideal, only one of the molecules adsorbs and the other one reacts with it directly from the gas phase, without adsorbing: A(g) + S(s) ==AS(s) AS(s) + B(g) → Products Taking the NH3-SCR reaction of NOx on CuO/Cu2O heterostructure as an example [12], the rich Lewis acid sites in CuO/Cu2O heterostructure promoted the E-R reaction pathway. NH3(g)→NH3(a) O2→O2(a) NH3(a)+O(a)→NH2(a)+OH(a) NO(a)+NH2(a)→NH2NO(a) NH2NO(a)→N2(g)+H2O(g) 2. Langmuir-Hinshelwood mechanism: the reaction occurs between the adsorbed molecule, also known as the L-H mechanism. A heterogeneous catalytic mechanism in which a surface reaction is used as a control step for surface reaction with two adsorbed molecules. It is that the two reactants are first adsorbed on the solid catalyst, and then reacted on the surface, and the product is desorbed again. The surface reaction is a control step, and the adsorption and desorption rates are much greater than the surface reaction rate. The rate of reaction is proportional to the coverage of the two reactants on the surface of the catalyst. NH3-SCR of the CrOx/C sample was an example [56]. NH3(g)→NH3(a)→NH4+(a)→NH4+(a)+NO2(a)→N2(g)+H2O(g) 3. The Langmuir-Hinshelwood (L-H) mechanism and the Eley-Ride (E-R) mechanism existed simultaneously: take the MIL-100 (Fe) catalyst as an example [118]. (L-H) mechanism NH3(g)→NH3(a)→NH4+(a)→NH4+(a)+NO2(a)→N2(g)+H2O(g) (E-R) mechanism NH3(g)→NH3(a)→NH2(a)→NH2(a)+NO(a)→N2(g)+H2O(g) 4. Mars-Van-Krevelen redox mechanism: the MIL-100 (Fe) catalyst was still regarded as an example [118]. During the reaction, Fe(III)CUS reduced to Fe(II)CUS in the oxidation stage while NO formed NO2, and then Fe(II) was oxidized to Fe(III) by O2, thereby completing the redox cycle.

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Fe3+-NO→Fe2+-NO2+NH4+→N2(g)+H2O+Fe2+→Fe3+ 3.4.2 H2-SCR Wang et al. [61] prepared a 0.5% Pt/ZrO2@C catalyst using UiO-66-NH2 (Zr-MOF) as a template for H2-SCR. This was a carbon-coated octahedral ZrO2 on which Pt particles were highly dispersed with an average particle size of about 5 nm. The conversion rate of NOx of Pt/ZrO2@C was close to 100% at 90℃. Under optimal conditions of activity, the catalyst had an N2 selectivity of 70%. The residual carbon formed by the pyrolysis treatment was coated on the octahedral ZrO2, effectively preventing the agglomeration of the platinum particles on the surface. It was demonstrated that MIL-96(Al) effectively and uniformly disperse Pt particles [83]. Different amounts of Pt particles were deposited directly on the surface of MIL-96 by the hydrothermal method to prepare Ptx-MIL-96 (X represented the platinum content of the catalyst). The Pt contents were 0.03%, 0.11%, 0.27%, and 0.41%, respectively. For Pt5-MIL96/CP, the NO removal rate reached 100% at 60℃. Electron transfer between Pt and MIL-96 was observed by X-ray photoelectron spectroscopy (XPS). The results showed that the deposited Pt was in a metallic state. At the same time, the electronic structure of MIL-96 remained intact during the Pt deposition process. H2-SCR mechanism: Take Ptx-MIL-96 as an example [83], the core part of the NO reduction process was the adsorption and dissociation of NO on Pt. During the adsorption process, no molecules were first adsorbed at the Pt position to form a Pt-NO bond. The NO molecules were then dissociated into N(ad) and O(ad). Pt was essentially in a metallic state, mainly presented on the surface of MIL-96. At the same time, MIL-96 adsorbed and transferred H2 to Pt with a high specific surface area. 3.4.3 CO-SCR Our group's research on MOFs was concentrated in the CO-SCR system. The CO-SCR material was prepared as carbon-based catalyst by using Cu-BTC as the precursor. All carbonbased catalysts had an octahedral structure of Cu-BTC and consisted of face-centered cubic copper. The introduction of Ag further increased the catalytic activity of CuOx/C. The catalyst with Cu: Ag molar ratio of 6:1 and activation temperature of 500℃ had the best catalytic performance while the NO conversion rate of the catalyst reached 100% at 235℃. During the catalytic reaction, Cu+ mainly played a catalytic role. What was more, Agx-Cu-BTC was prepared by pre-assembly method, which had a regular octahedral shape and a high specific surface area [84]. At the same temperature (<240℃), the addition of Ag ions increased the activity of CO-SCR, and the mixed node Agx-Cu-BTC showed better performance for COSCR. The addition of Ag ions to the metal skeleton increased the activity of the catalyst and increase the reactive center. Then a series of A-Cu-BTC (A=Fe, Ni, Co, Mn, Sr, La, Ce, Al) [130,131] were prepared by direct ion exchange pre-assembly method, which also used as a precursor to obtain AOX/CuOy/C under a nitrogen atmosphere at 600℃. A-Cu-BTC have been successfully applied to CO-SCR and obtained similar performance in catalytic denitration, while La-Cu-BTC and Fe-Cu-BTC had poor denitrification performance. The results showed that AOX/CuOy/C with A atom had excellent catalytic effect on NO-CO reaction, which was better than CuOy/C. Among them, the NO conversion of SrOx/CuOy/C reached 100% at 160℃. Recently, we [132] synthesized Me-ZIF-67@CuOx (Me=Mn, Fe, Al or Zn) by colloidal chemical synthesis and

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encapsulation at room temperature. The Co ion was replaced by another metal ion to form a different Me-ZIF-67, and then Me-ZIF-67 was loaded on CuOx to form Me-ZIF-67@CuOx. The ability of the catalyst of NO conversion at low temperatures was over 90%, and the value of Co2+/Co3+ remained high in Me-ZIF-67@CuOx. Mn-ZIF-67@CuOx had the best sulfur resistance, and were attributed to the active center of Me-ZIF-67@CuOx. The mechanism of CO-SCR reaction was organized as follows, and the reaction followed the (L-H) mechanism. Take Agx-Cu-BTC as an example:[131] CO reduced Cu2+, which led to the formation of Cu+ active species. CO was easily absorbed by Cu+ and formed Cu+(CO)n (n=1-3) with CO molecules. NO was also easily absorbed by Cu+ to obtain Cu(NO)n (n = 1-2). [133] 3.5 CO catalytic oxidation In recent years, studies on the catalytic oxidation of CO to CO2 have been extensively studied for its potential applications in automotive exhaust gas treatment, air purification, and CO selective oxidation. Nowadays, many researchers have been used MOFs as precursors to prepare metal oxide catalysts for catalytic oxidation of CO. Huo et al. [135] compared the effects of Pt loading on Co3O4. Pt@Co3O4 composite still maintained the shape of ZIF-67 with a hollow structure. The metal ions effectively promoted CO oxidation reaction at lower temperatures (100% conversion at 110℃). Compared with Co3O4, Pt@Co3O4 composite catalyst exhibited excellent performance in CO oxidation reaction due to Pt coated nanoparticle. The influence of calcination temperature and Ag loading on ZIF-67 was investigated [67]. The authors pointed out that Co3O4 nanoparticles and pore size enlargened with increasing calcination temperature. Compared with a single Co3O4-350 catalyst, the CO oxidation activity of Ag/Co3O4-350 interfacial catalyst was significantly improved from 100 ℃ to 120 ℃. The study concluded by attributing such a significant improvement in CO oxidation to the improvement of surface-active oxygen at the interface between Ag and Co3O4. The interface between Ag and Co3O4 promoted the activation of oxygen and the adsorption of CO, thereby reducing the potential barrier of the reaction and improving the oxidation performance of CO. Its catalytic mechanism was as follows [67]: CO species were easily adsorbed on the surface exposed Co3+ sites and reacted with surface-active oxygen (such as O2-) to form Co2+. The role of Ag in CO oxidation was advantageous for adsorption and activation of oxygen. The adsorption of oxygen was improved after the introduction of the Ag species. Previous studies [136] again indicated that the electron transfer occurs between Ag and O2 molecules, weakening the O-O bond and forming O2- species to increase oxygen activation. Ionic Ag facilitated the adsorption of CO, while CO adsorption did not occur on bare Ag metal. The formed Ag-O compound facilitated the adsorption of CO and the activation of oxygen. Li et al. [57] prepared a nano-Co-based carbon material by a simple thermal decomposition method using a cobalt-based metal organic framework ZIF-67 and studied its catalytic properties at low temperature. Co/C-600 obtained by pyrolysis from ZIF-67 at 600℃ was completely converted CO at 0℃ even in the presence of moisture. The apparent activation energy of CO oxidation on Co/C-600 catalyst was about 22 kJ•mol-1. At the same time, the CO conversion rate remained unchanged at 100% after 24 hours of operation at room temperature with good long-term stability. Nitrogen in MOF precursors had a certain effect on catalytic performance. Chen et al. [13] prepared a CuO-CeO2 catalyst with a nitrogen-containing ligand MOF as a precursor for

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preferential oxidation of carbon monoxide in a hydrogen-rich stream. Due to the coordination interaction between the metal ion and the nitrogen atom, the copper ion was adsorbed into the pore of the Ce-MOF and stabilized in the pore wall by the ordered nitrogen atom. Cu0.04Ce0.96A-700 not only completely oxidized CO at 80℃ but also had a wide active window (20℃) with 100% conversion and selectivity. The different catalytic activities of the CeO2-CuO catalysts, and the Ce-BTC precursors were prepared by one-step and post-treatment methods as comparison [134]. The Ce-Cu-Ox with one-step preparation had a stronger interaction between CuO and CeO2, making the reduction of CuO species more difficult. At the same time, the strong interaction also led to the weak adsorption of copper-CO. Under the post-treatment method, highly dispersed CuOx in the CeO2-CuO sample was beneficial to the formation of Cu(I). In the redox process of Ce(IV)/Ce(III), the adsorption of Cu(I) carbonyl species by CO was enhanced, so that the CeO2-CuO catalyst exhibited higher activity for CO oxidation in Figure 16. Another researcher compared the effects of in situ synthesis, mechanical mixing and impregnation methods on Ce-BTC precursors for the catalysis of CuO/CeO2 [138]. Among them, the CuCeO-ETH catalyst prepared by the impregnation method had good catalytic activity for CO oxidation at 100℃-180℃. The addition of H2O and CO2 slightly reduced the CO conversion rate, and the CuCeO-ETH catalyst remained 100% CO conversion rate at 130℃. The increase in catalytic performance of CO oxidation and preferential CO oxidation were attributed to more Cu+, oxygen vacancies, and surface lattice oxygen. Zhang et al. [137] successfully synthesized a highly uniform and well-dispersed octahedral CuO/Cu2O composite with Cu-BTC as a raw material, which had high CO catalytic activity at 100-135℃. In addition, CuO/Cu2O composites had good catalytic properties, which were related to high Cu2O/CuO ratio and high surface-active oxygen content. 3.6 Others 3.6.1 Catalytic oxidation of other gases Ping et al. [68] successfully prepared a high-efficiency, non-precious metal-based Ni/ZrO2 catalyst using Ni/UiO-66 precursor as a template, and applied it to CO selective methanation in hydrogen-rich gas. The catalyst had excellent activity, selectivity, and high stability at an extremely wide temperature window of 215- 350℃. Its excellent performance was attributed the following factors: increased specific surface area of the nano-Ni nanoparticles, smaller grain size (3.5 nm), higher dispersion (15.3%) and the enhanced chemisorption capacity of CO. Han et al. [17] prepared Au@Pd@MOFs-74, Pt/MOFs-74, and Pt/Au@Pd@MOFs-74. On the core-shell Au@Pd, gold NPs were used as the core of Pd shell epitaxial growth, and the morphology of MOF-74 was controlled and its NP function was given. Pt NPs were loaded onto the MOF-74. The catalyst Pt/Au@Pd@MOF-74 not only catalyzed the conversion of CO2 to CO but also catalyzed CO2 to CH4 at low concentrations. Luo et al. [63] used MIL-101 (Fe) as the precursor and synthesized iron-based catalyst by pyrolysis at 650℃. FeNC-650 had good ORR (Oxygen reduction reaction) catalytic performance, stability, and methanol resistance. The effect of temperature on during pyrolysis the morphology of the material was studied. The study reported that the synthesized FeNC-650 hybrid had a core-shell structure in which Fe and Fe3O4 nanoparticles were embedded in the mesoporous carbon layer, while FeNC-600 and FeNC-700 contained Fe3O4 and Fe3C, respectively.

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3.6.2 The stability of MOFs at the reaction conditions and the price of MOFs MOF stability was important in catalysis, such as catalytic stability, sulfur resistance and water resistance. Some MOF materials were inherently stable, such as MIL-100 (Fe) and MOF74 [118] [120] [122]. These materials had a certain heat resistance, resistance to SO2 and H2O poisoning in the temperature range of catalytic reaction. Metal doping or replacement was suggested to improve catalyst stability. For example, the Co-MOF-74 and the Mn0.66Co0.34MOF-74 showed excellent resistance to SO2 poisoning compared with the Mn-MOF-74 [85] [120] [122].The MIL-100(Fe-Mn) catalyst even had a higher nitrogen oxide conversion (about 7%) in the presence of H2O and SO2, attributed to the formation of sulfate on the catalyst surface as a new acid center. Recent studies on the preparation of carbonized materials or oxides using MOF as precursor have been found to improve the stability of catalyst. The CuO/Cu2O heterostructure exhibited excellent durability of H2O, SO2, and alkali metals [12]. And the CoOx@C synthesized from ZIF-67 showed good catalytic stability and excellent H2O resistance [55]. The preparation of composite materials has also been reported as a good method for improving the stability of catalyst. For instance, the CuO@Cu3(BTC)2 core-shell material showed satisfactory tolerance to H2O and SO2 [70]. Similar studies further revealed that removal of H2O and SO2 significantly improves the reactivity of CuO@Cu3(BTC)2. Comparatively, the Cu-BTC@GO, composites of Cu-BTC and graphite oxide (Cu-BTC@GO), had a higher specific surface area and pore volume, water stability than the Cu-BTC [96]. The primary issue faced with the industrial scale application of MOFs in atmospheric pollution was the limited production volume. Producing MOFs fit for industrial scale is a major problem in the future application. As shown in Table 2, the price of MOFs is high and the use of MOF is almost economically unsustainable at this stage. 4. Conclusions A comprehensive review on the preparation of MOFs and MOFs derived materials for their catalytic application in air pollution control has been presented. This review has attempted to cover a wide range of MOFs and their derived composites available in literature. Serious atmospheric pollution in recent times calls for the need for effective pollution control alternatives. The high surface area, clearly defined and designable structure, adjustable and uniform pore structure, closed space nanopore microenvironment and the presence of active sites make MOFs suitable for atmospheric catalytic applications. So far, MOFs have made extensive attempts and breakthroughs in the application of air pollution. In order to improve the catalytic ability and the thermal and chemical stability of MOFs, researchers have investigated different types of modification methods, including functional modification, carbon, metal, loading metal oxide, carbonization, and composites of MOFs materials. These MOFsderived materials can provide highly adsorbed, stable porous structures with many active sites for catalytic reactions. This paper elaborated MOFs materials had been applied in different contaminants of atmospheric catalysis. These materials and atmospherical contaminants were listed in Table 3. Furthermore, different modified methods for MOFs and their derived materials in different atmospheric contaminants such as; removal of mercury, catalytic oxidation of VOCs, desulfurization, selective catalytic reduction of NOx and catalytic oxidation of CO have been well-reviewed and further elaborated. These investigations have brought some development about the scope of atmospheric pollution catalysis to MOFs materials. Moreover,

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some key reaction mechanisms of atmospheric pollution control using MOFs have been discussed including; Mars-Van-Krevelen redox mechanism, Langmuir-Hinshelwood mechanism, and the Eley-Ride mechanism. Based on literature, the following concluding remarks can be made: (1) Combining MOFs with appropriate materials such as graphene derivatives and Mxene, composite materials can be prepared to improve their catalytic performance, thermal stability, chemical stability, and adsorption capacity. (2) Monoatomic catalysis using MOFs as a precursor is one of the current research hotspots and remains an important direction for atmospheric catalysis. (3) Theoretical and computational methodologies can become a way to comprehend and improve the catalytic action of materials. Predicting the performance of specific MOF-based catalysts through theoretical research is complex, and a solid basic theory and optimized standard experimental system is still absent (4) The current production volume of MOFs materials is limited to small batches, and the production of MOFs is a major obstacle to their future applications. Based on the above viewpoints, the development of MOFs catalysts with the advantages of cheaper, more stable, and more efficient is the focus of the next research. Hanbing He, Ren Li: Writing. Haiying Wang, Lei Huang: Design, Revision, Drawing

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pictures. Zhihui Yang, Liyuan Chai, Linfeng Jin and Lili Ren: Discussion. Sikpaam Issaka

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Alhassan: Revised English grammar.

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Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Acknowledgements This research is financially supported by the National Key R&D Program of China (2017YFC0210405), Key R&D Program of Hunan Province (2018SK2026), and National Natural Science Foundation of China (51634010).

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Figures

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Figure 1. The schematic diagram for modification-based Classification of MOFs Materials. Figure 2. The scheme for rapid synthesis of the MVCM. Ce3+ white, Ce4+ yellow (shown as

lP

polyhedra), O red and C black. [46]

Figure 3. The synthetic strategy of the Cu@C and Zn@C nanocatalysts derived from Cu-MOF and Zn-MOF. [15]

octahedra. [61]

na

Figure 4. Schematic illustration of the fabrication of MOFs-derived porous Pt/ZrO2@C nano-

ur

Figure 5. Shape-persistent transformation of MOF into M/MO@C. MOF undergoes (a) vapor phase polymerization and (b) thermolysis to form the M@C composite. (c) Post-thermolytic

Jo

treatment may turn the metal nanoparticles into oxide species. [58] Figure 6. Building unit of Cu-MOF 1-3 with atomic labeling scheme. All H atoms are omitted for clarity. [12] Figure

7.

Synthesis

Procedure

for

Hierarchically

Structured

MgAl-LDH/MOFs

Nanocomposites. [76] Figure 8. The schematic diagram for different applications of MOFs Materials in Atmospheric Catalysis.

Figure 9. Adsorption of elemental mercury (Hg0) and mercury species (HgCl2, HgO, and HgS) in novel metal-organic frameworks (MOFs) (gray: C; red: O; white: H; green: Mg; pink: Hg; cyan: Cl; yellow: S). [99] Figure 10. Mechanism of toluene degradation over CeO2-MOF/350. [66] Figure 11. Pd@Cu(II)-MOF-Catalyzed Aerobic Oxidation of Benzylic Alcohols. [112] Figure 12. Schematic diagram of oxidation of H2S on NH2-MIL-53(Fe). [8] Figure 13. Schematic diagram of H2S selective oxidation on CUS-MIL-100. [44] Figure 14. The reaction mechanism on CeO2/MIL-100 (Fe) catalyst. [74]

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Figure 15. Proposed reaction mechanism of NH3-SCR-NO over CuO/Cu2O. [12]

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lP

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Figure 16. Reaction mechanism on CeO2-CuO and Ce-Cu-Ox, respectively. [133]

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lP

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Figure 1. The schematic diagram for modification-based Classification of MOFs Materials.

Figure 2. The scheme for rapid synthesis of the MVCM. Ce3+ white, Ce4+ yellow (shown as

Jo

ur

na

lP

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polyhedra), O red and C black. [46]

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Figure 3. The synthetic strategy of the Cu@C and Zn@C nanocatalysts derived from Cu-MOF

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ur

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lP

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and Zn-MOF. [15]

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Figure 4. Schematic illustration of the fabrication of MOFs-derived porous Pt/ZrO2@C nano-

Jo

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na

lP

re

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octahedra. [61]

Figure 5. Shape-persistent transformation of MOF into M/MO@C. MOF undergoes (a) vapor phase polymerization and (b) thermolysis to form the M@C composite. (c) Post-thermolytic

Jo

ur

na

lP

re

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treatment may turn the metal nanoparticles into oxide species. [58]

Figure 6. Building unit of Cu-MOF 1-3 with atomic labeling scheme. All H atoms are omitted

Jo

ur

na

lP

re

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for clarity. [12]

7.

Synthesis

Procedure

for

Hierarchically

Structured

Jo

ur

na

lP

re

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Nanocomposites. [76]

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Figure

MgAl-LDH/MOFs

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Figure 8. The schematic diagram for different applications of MOFs Materials in Atmospheric

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lP

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Catalysis.

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Figure 9. Adsorption of elemental mercury (Hg0) and mercury species (HgCl2, HgO, and HgS) in novel metal-organic frameworks (MOFs) (gray: C; red: O; white: H; green: Mg; pink: Hg;

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lP

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cyan: Cl; yellow: S). [99]

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Figure 10. Mechanism of toluene degradation over CeO2-MOF/350. [66]

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Figure 11. Pd@Cu(II)-MOF-Catalyzed Aerobic Oxidation of Benzylic Alcohols. [112]

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Figure 12. Schematic diagram of oxidation of H2S on NH2-MIL-53(Fe). [8]

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na

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Figure 13. Schematic diagram of H2S selective oxidation on CUS-MIL-100. [44]

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Figure 14. The reaction mechanism on CeO2/MIL-100 (Fe) catalyst. [74]

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Figure 15. Proposed reaction mechanism of NH3-SCR-NO over CuO/Cu2O. [12]

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Figure 16. Reaction mechanism on CeO2-CuO and Ce-Cu-Ox, respectively. [134]

Tables: Table 1 [28,108] Selected properties of VOC molecules. Table 2 The price of MOFs and COFs from some companies. air

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Table 3 Summary of investigation on MOFs and MOFs derived materials for catalyzing pollution.

Table 1 [28,108] Selected properties of VOC molecules. SPd Xe Ye Ze (kPa,25℃) Acetone 0.786 58 0.270 30.414 6.600 4.129 5.233 Benzene 0.876 78 0.305 12.573 6.628 3.277 7.337 Toluene 0.865 92 0.344 3.776 6.625 4.012 8.252 Ethylbenzene 0.901 106 0.368 1.320 6.625 5.285 9.361 m-Xylene 0.877 106 0.379 1.117 8.994 3.949 7.315 o-Xylene 0.858 106 0.375 0.876 7.269 3.834 7.826 p-Xylene 0.861 106 0.380 0.725 6.618 3.810 9.146 a Density, b Molecule weight, c Molecule cross-sectional area, d Saturation pressure, e X, Y and Z are the molecular width, thickness and length, respectively ρa(g/mL, 25℃) MWb(g/mol) σc(nm2)

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VOCs

56

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Table 2 The price of MOFs and COFs from some companies. MOF Particle size US$/ga ZIF-8 0.6~1μm,100~400 nm 64.98~115.52 ZIF-67 100 nm~2 μm 64.98~115.52 HKUST-1 (MOF-199) 100 nm~3.5 μm 72.20~115.52 MOF-74 / 64.98~158.54 MOF-808 1~1.5um 122.74 MOF-UiO-66(Zr) ~180 nm 144.4 MOF-UiO-66(Zr)-NH ~600 nm 173.28 ACS Material DAAQ-TFP-COF / 15884 ACS Material COF-TpPa-1 / 15884 ACS Material COF-LZU1 / 14440 a. the price of MOFs on market came from several companies, like Nanjing XFNANO Materials Tech Co. Ltd. (https://www.xfnano.com/) and Hangzhou Nano-Mall Technology Co. Ltd. (https://shiyanjia.com/) in China.

57

Table 3 Summary of investigation on MOFs and MOFs derived materials for catalyzing air pollution. Refe S. Catalytic S. Catalytic Refere Material rence Material N type N type nces s NO(NH3 1 Hg0 Cu-BTC [36] 28 Cu-BTC [37] -SCR) NO(NH3 CuO/Cu2O from 2 Hg0 MIL-101 (Cr) [98] 29 [12] -SCR) Cu-MOF Hg0

UiO-66

[81]

30

NO(NH3 -SCR)

CuO from Cu-BTC

[65]

4

Hg0

MnCe@MOF

[80]

31

NO(NH3 -SCR)

CuO@Cu3(BTC)2

[70]

5

Hg0

FeCl3@MIL101(Cr)

[89]

32

NO(NH3 -SCR)

6

Hg0

Br-UiO-66

[53]

33

NO(NH3 -SCR)

7

Hg0

Mg/DOBDC

[99]

34

NO(NH3 -SCR)

8

Hg0

Se/MIL-101

[91]

9

Hg0

UiO-66 and CAU-1

10

Hg0

11

VOCs(to luene )

COF-V (COF-SSH) MnOx-CeO2 from MOF-74

12

VOCs(to luene)

13

36

[100] 37

[55]

γ-Fe2O3 from FeMn-MOF

[6]

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NO(NH3 -SCR)

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lP

na

[10]

CoOx@PC from ZIF-67

CrOx/C from MIL101 (Cr) MnxCo3-xO4 from Mn3[Co(CN)6]2·nH 2O

[56]

[127]

NO(NH3 -SCR) NO(NH3 -SCR) NO(NH3 -SCR)

MnCe-MOF

[88]

MnOx/UiO-66

[71]

Ni-MOF

[41]

38

CeO2 from CeMOF

[66]

39

NO(NH3 -SCR)

Mn-MOF-74

[129]

VOCs(B enzylic Alcohol)

Pd@Cu(II)-MOF

[112]

40

NO(H2SCR)

Pt/ZrO2@C from UiO-66-NH2

[61]

H2S

CUS-MIL-100 (Fe)

[44]

41

NO(H2SCR)

Pt-MIL-96/CP

[83]

Cu-BTC and Agx/CuOx/C from Cu-BTC

[14]

Agx-Cu-BTC

[84]

ur

[5]

Jo 14

35

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3

15

H2S

NH2-MIL-53(Fe)

[8]

42

NO(COSCR)

16

NO(NH3 -SCR)

MIL-100 (Fe)

[118]

43

NO(COSCR)

58

17

NO(NH3 -SCR)

MIL-100 (Fe)

[45]

44

NO(COSCR)

A-Cu-BTC (A=Fe, Ni, Co, Mn, Sr, La, Ce, Al) and AOX/CuOy/C from A-Cu-BTC

18

NO(NH3 -SCR)

Cu+/MIL-100 (Fe)

[16]

45

CO

CeO2-CuO from Ce-MOF

MIL-100(Fe-Mn)

[73]

46

CO

CeO2/MIL-100(Fe)

[74]

47

CO

[120] 48

CO

[85]

49

CO

[121] 50

CO

[11]

CO

22 23 24

Mn-MOF-74 and Co-MOF-74 Mn0.66Co0.34-MOF74

NO(NH3 -SCR)

NO(NH3 25 -SCR)

Co-MOF-74 MnOx from MnMOF-74 Cu-MOF-74

NO(NH3 -SCR)

Cu-MOF

27

NO(NH3 -SCR)

Mn-MOF-74

[122] 52

[39]

53

lP

26

51

CO methanat ion CO2 methanat ion

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[123] 54

59

ORR

[134] [135] [67] [13]

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21

Pt@Co3O4 from ZIF-67 Ag/Co3O4 from ZIF-67 CuO-CeO2 from Ce-MOF

Co/C from ZIF-67

[57]

CuO/Cu2O from Cu-BTC

[137]

CuO/CeO2 from Ce-BTC

[138]

Ni/ZrO2 from Ni/UiO-66

[68]

Pt/Au@Pd@MOF74

[17]

FeNC-650 from MIL-101 (Fe)

[63]

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20

NO(NH3 -SCR) NO(NH3 -SCR) NO(NH3 -SCR) NO(NH3 -SCR) NO(NH3 -SCR)

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19

[130,1 31]