Advantages of Bimetallic Organic Frameworks in the Adsorption, Catalysis and Detection for Water Contaminants

The binary metal organic framework (MOF) is composed of two heterometallic ions bonded to an organic ligand. Compared with monometallic MOFs, bimetallic MOFs have greatly improved in terms of structure, porosity, active site, adsorption, selectivity, and stability, which has attracted wide attention. At present, many effective strategies have been designed for the synthesis of bimetallic MOF-based nanomaterials with specific morphology, structure, and function. The results show that bimetallic MOF-based nanocomposites could achieve multiple synergistic effects, which will greatly improve their research in the fields of adsorption, catalysis, energy storage, sensing, and so on. In this review, the main preparation methods of bimetallic MOFs-based materials are summarized, with emphasis on their applications in adsorption, catalysis, and detection of target pollutants in water environments, and perspectives on the future development of bimetallic MOFs-based nanomaterials in the field of water are presented.


Introduction
Since the concept of metal organic framework (MOF) was formally proposed in 1995 [1], the unique material has received extensive attention. Metal-ligand compounds, such as materials of the Institute Lavoisier (MIL) [2,3] and zeolitic imidazolate frameworks (ZIFs) [4], formed by bonding of transition metals and organic ligands, possess all kinds of unique structural properties, including but not limited to large specific surface area, developed pore structure, physical and chemical relative stability, and adjustable structure and function [5,6].
These MOFs, which contain only one transition metal, are favored by researchers, who promote the in-depth development of MOFs in energy, materials, sensors, the environment [7][8][9][10], and other fields for application. Li et al. [11] used a household microwave oven to successfully synthesize Zr 6 O 4 (OH) 4 (BTC) 2 (HCOO) 6 (MOF-808) nanoparticles with a maximum adsorption capacity of 24.83 mg/g for arsenic solution. Singh et al. [12] prepared the g-C 3 N 4 /Cu-DTO MOF nanocomposite with heterojunction structure by ultrasound to combine g-C 3 N 4 and Cu-DTO MOF in methanol solution. At room temperature, the lowest detection limit of the material for endocrine disruptors could be acquired at 0.02 µM. Nguyen et al. [13] used MIL-101(Fe) and g-C 3 N 4 as precursors and adopted solvothermal synthesis, synthesizing the MOF derivative nanomaterial of a three-dimensional g-C 3 N 4 /MIL-101(Fe). The presented material showed excellent performance in the photocatalytic degradation of paracetamol. For more application cases of monometallic MOFs, Gonzalez et al. [14] carried out a detailed review.  [33,36,42]. Copyright 2022, Elsevier.

Preparation Strategies of Bimetallic MOF-Based Nanocomposites
As is well known, the preparation methods and synthesis processes of materials are of great importance to their morphology, structure, and performance characteristics. According to statistics, the types of MOFs formed by the bonding of many organic ligands with metal ions have already exceeded 2000 [43]. Mainstream methods for synthesizing these monometallic MOF materials have been summarized in detail in the report of Mukherjee and co-workers [44], including hydrothermal, microwave irradiation, the ionic liquid method, and so on. These basic strategies for the synthesis of monometallic MOF materials have also been adopted by researchers for the construction and development of bimetallic MOF-based materials. In addition, binary metal MOF-based composites are also further extended by some major post-treatment methods [30], such as heat treatment, mechanical processing, the sol-gel method, and the like. Thus, the synthesis methods mentioned above for bimetallic MOFs can be divided into two categories. One is direct synthesis, namely hydrothermal, solvothermal, mechanochemistry, electrochemistry, sonochemistry, and spray-drying. Another is indirect synthesis or post-treatment, that is, pyrolysis, ball-milling, metal-organic chemical vapor deposition, and solution impregnation.

Direct Synthesis
Here, the direct synthesis of binary metal MOFs is the self-assembly process of metal ions with organic ligands to form a coordination polymer in one pot. Bimetallic MOFs prepared by direct synthesis have retained their physical and chemical properties since ref. [33,36,42]. Copyright 2022, Elsevier.

Preparation Strategies of Bimetallic MOF-Based Nanocomposites
As is well known, the preparation methods and synthesis processes of materials are of great importance to their morphology, structure, and performance characteristics. According to statistics, the types of MOFs formed by the bonding of many organic ligands with metal ions have already exceeded 2000 [43]. Mainstream methods for synthesizing these monometallic MOF materials have been summarized in detail in the report of Mukherjee and co-workers [44], including hydrothermal, microwave irradiation, the ionic liquid method, and so on. These basic strategies for the synthesis of monometallic MOF materials have also been adopted by researchers for the construction and development of bimetallic MOF-based materials. In addition, binary metal MOF-based composites are also further extended by some major post-treatment methods [30], such as heat treatment, mechanical processing, the sol-gel method, and the like. Thus, the synthesis methods mentioned above for bimetallic MOFs can be divided into two categories. One is direct synthesis, namely hydrothermal, solvothermal, mechanochemistry, electrochemistry, sonochemistry, and spray-drying. Another is indirect synthesis or post-treatment, that is, pyrolysis, ballmilling, metal-organic chemical vapor deposition, and solution impregnation.

Direct Synthesis
Here, the direct synthesis of binary metal MOFs is the self-assembly process of metal ions with organic ligands to form a coordination polymer in one pot. Bimetallic MOFs prepared by direct synthesis have retained their physical and chemical properties since the beginning of synthesis, including a large specific surface area, a developed pore structure, high conductivity, and a great Fermi level. The synthesis and exploration of bimetallic MOFs have become important ways to develop new MOF nanomaterials. Liu et al. [45] synthesized 36 kinds of bimetallic MOFs (MnFe-MOFs, MnMg-MOFs, MgCo-MOFs, CoNi-MOFs, FeNi-MOFs, MnNi-MOFs, and so on) using dimethyl formamide (DMF) as solvent by the hydrothermal method. The structural properties of the synthesized MOFs were investigated by UV-vis diffuse reflectance spectroscopy (DRS) and X-ray photoelectron spectroscopy (XPS). The results showed that the structural properties of bimetallic MOFs were better than those of monometallic MOF materials. Chen et al. [46] successfully prepared CoNi bimetallic MOFs (CoNi-MOFs) with a rough surface and spherical shape via a typical one-step solvothermal method ( Figure S1a). Galvanostatic charge/discharge (GCD) and cyclic voltammetry (CV) were used to test the electrochemical properties of CoNi-MOFs. The results displayed that the maximum specific capacitance of the structured material could be increased to 2608 F/g at 1 A/g, and it still maintained good stability after 5000 cycles of discharge.
In order to achieve higher structure, morphology, and performance advantages, a variety of binary metal MOF materials with strictly controlled preparation processes were compounded. Kuila and co-researchers had successfully synthesized a structurally stable 3d-4f bimetallic coordination polymer, CeFe MOFs [47]. Specifically, cerium nitrate hexahydrate, iron nitrate hexahydrate, and 2-aminoterephthalic acid were used as raw materials, with DMF, methanol, and water as solvents to prepare the CeFe MOFs. The visible light absorption characterization analysis exhibited that the absorption intensity of CeFe MOFs synthesized was obviously better than that of Fe-MOF and Ce-MOF. The photocatalytic degradation tests further confirmed the excellent performance of electron-hole pair separation in the visible band, and the degradation efficiency of 10 ppm of acetaminophen reached 94.6% in 210 min. Suksatan and colleagues synthesized nanometer nickel-zinc bimetallic MOFs (NiZn-MOFs Figure S1b) with uniform morphology with the ultrasonic method [48]. Fourier transform infrared spectroscopy, specific surface area, and thermogravimetric analysis showed that, compared with monometallic materials (Ni-MOF and Zn-MOF), the samples of NiZn-MOFs had a larger Brunauer-Emmet-Teller (BET) specific surface area and porosity and higher thermal stability.

Indirect Synthesis or Post-Treatment
Indirect synthesis, or post-treatment method, is further developed on the basis of direct synthesis for the product follow-up treatment so as to form the final binary metal MOFs-based material. The end-product is a kind of derivatively bimetallic MOF materials that are markedly different from those formed by direct synthesis in terms of structure, morphology, chemical functional groups, and properties. What needs illustration is that the target product of bimetallic MOF-based materials cannot be achieved by the one-pot synthesis method.
Guo and co-team [49] first synthesized cobalt-molybdenum bimetallic MOFs (CoMo-MI) by solvothermal method and further heated treatment to effectively design the cobaltmolybdenum bimetallic MOF-based carbon and nitrogen nanomaterials (CoMo-MI-T). Figure S2a shows the preparation process for CoMo-MI-T. These results of multiple characterization techniques (XRD, SEM, TG, and TEM) demonstrated that, compared with CoMo-MI, the crystal structure of CoMo-MOFs for CoMo-MI-T nanomaterials had collapsed and the surface morphology was rough. Meanwhile, the CoMo-MI-T's thermal stability was enhanced, and the content of micropores and mesoporous developed. Electrochemical performance analysis confirmed that the CoMo-MI-T nanomaterials possessed a better synergistic effect among the multiple components, which promoted electron conduction, thus enhancing the electrocatalytic oxygen evolution reaction. Liang and his co-research group [50] explored CoNi-MOF as a precursor to the pyrolysis preparation of CoNi@C nanowires with Janus structures ( Figure S2b). The material showed extremely strong stability and sustainable recycling in the long-term heating and cooling cycle tests, which could be used as a high-performance material to prevent electromagnetic interference in low-or high-temperature environments. Analogously, Wang et al. [51] used cobalt-cerium bimetallic MOFs as intermediates to produce the CoCeO x nanocomposites (CoCe-MOF) ( Figure S2c). In the experiments of catalytic conversion of carbon dioxide to ethanol, the ethanol conversion efficiency of the CoCe-MOF experimental groups could reach 97%, and almost no acetone byproduct was produced. In the 40 h test period, the ethanol conversion by the CoCe-MOF catalyst was only reduced by 3%, while the Co 3 O 4 /CeO 2 control groups significantly reduced it by 11%.
In Figure 2a, Yan et al. [52] demonstrated the synthesis of zinc-cobalt bimetallic MOFs (BMOFs Zn x Co y ) with methanol and ethanol as solvents. Then, ferric ions were introduced into BMOF Zn x Co y and heated in the gas mixture of H 2 and N 2 to prepare an MOF-based Co-Fe bimetallic composite (ZnCo 6 Fe). Scanning electron microscope analysis made clear that BMOF Zn x Co y was smooth and regular polyhedral, while ZnCo 6 Fe was composed of ZnCo-MOF derivatives and carbon nano tubes (CNTs) networks with a rough surface. Afterward, the material was used to test the microwave absorption properties, and the minimum microwave absorption reflection loss of ZnCo 6 Fe was −66 dB. The mechanism analysis further proved that the defective structure of the materials could greatly improve the microwave absorption characteristics and provide workable ideas for the design and construction of highly efficient microwave-absorbing materials. strong stability and sustainable recycling in the long-term heating and cooling cycle tests, which could be used as a high-performance material to prevent electromagnetic interference in low-or high-temperature environments. Analogously, Wang et al. [51] used cobaltcerium bimetallic MOFs as intermediates to produce the CoCeOx nanocomposites (CoCe-MOF) ( Figure S2c). In the experiments of catalytic conversion of carbon dioxide to ethanol, the ethanol conversion efficiency of the CoCe-MOF experimental groups could reach 97%, and almost no acetone byproduct was produced. In the 40 h test period, the ethanol conversion by the CoCe-MOF catalyst was only reduced by 3%, while the Co3O4/CeO2 control groups significantly reduced it by 11%. In Figure 2a, Yan et al. [52] demonstrated the synthesis of zinc-cobalt bimetallic MOFs (BMOFs ZnxCoy) with methanol and ethanol as solvents. Then, ferric ions were introduced into BMOF ZnxCoy and heated in the gas mixture of H2 and N2 to prepare an MOF-based Co-Fe bimetallic composite (ZnCo6Fe). Scanning electron microscope analysis made clear that BMOF ZnxCoy was smooth and regular polyhedral, while ZnCo6Fe was composed of ZnCo-MOF derivatives and carbon nano tubes (CNTs) networks with a rough surface. Afterward, the material was used to test the microwave absorption properties, and the minimum microwave absorption reflection loss of ZnCo6Fe was −66 dB. The mechanism analysis further proved that the defective structure of the materials could greatly improve the microwave absorption characteristics and provide workable ideas for the design and construction of highly efficient microwave-absorbing materials.   6 CoFe. Reproduced with permission from ref. [52]. Copyright 2020, Elsevier. (b) Leaf-shape core-shell structured ZIF-L@ZIF-67 and its transformation to NC@GC/CNTs. Reproduced with permission from ref. [53]. Copyright 2022, Elsevier. (c) Schematic diagram of the synthesis process of ZnS/CoS 2 /CC. Reproduced with permission from ref. [54]. Copyright 2022, American Chemical Society.
A novel preparation method for bimetallic MOF-based nanocomposites has been successfully brought up by Zhang et al. [53], which mainly included three stages ( Figure 2b): Firstly, Zn-MOF (ZIF-L) was synthesized by the hydrothermal method with Zn 2+ and 2-methylimidazole. Secondly, on the basis of ZIF-L, ZIF-67 was further synthesized to form the product ZIF-L@ZIF-67. Finally, under the conditions of concentrated sulfuric acid and heating, the bimetallic MOFs ZIF-L@ZIF-67 were etched and cracked into bimetallic MOF-based nanocomposites (NC@GC/CNTs) with similar leaf shapes. The highlight of the method was mainly reflected in the pyrolysis stage of MOF-based material synthesis. Through reasonable control of the amount of concentrated sulfuric acid, the physical and chemical properties of the end-products NC@GC/CNTs x (x = 1, 2, 3, and 4) could be effectively regulated. In laboratory conditions, NC@GC/CNTs 2 displayed a surprising advantage in the adsorption of NaCl (1000 mg/L sodium chloride aqueous solution), where the maximal adsorption capacity attained 77.33 mg/g after 30 min.
Recently, another interesting pyrolysis method for preparing bimetallic MOF-based nanocomposites was worth noting. Fu et al. [54] cleverly used the stepwise synthesis method to synthesize ZIF-67 and ZnCo-MOFs (ZnCo-ZIF-CC). Subsequently, in the pyrolysis stage, sulfur powder was co-pyrolyzed with ZnCo-ZIF-CC to produce sulfide bimetallic MOFs-based material (ZnS/CoS 2 /CC). The schematic diagram of the preparation process is shown in Figure 2c. The ZnS/CoS 2 /CC was used as anode material for lithium batteries, demonstrating the satisfactory advantages of electric energy storage. The battery capacity of the material was still up to 1644.7 mA h/g after hundreds of cycles of testing at the high current density of 1 A/g. Furthermore, mechanical grinding or ball-milling has also proven to be an effective strategy for the synthesis of bimetallic MOF-based nanocomposites. Panda et al. [55] synthesized amorphous bimetallic MOF intermediates using ball-milling the precursors (Al-MOF (Al-ndc) and Ga-MOF (Ga-ndc)); afterward, the bimetallic MOF-based nanomaterial AlGa-MOFs with crystal structure were reconstructed via steam treatment. This study highlighted the importance of ball-milling technology in bimetallic MOF preparation and also stressed the limitations of traditional MOF preparation methods in bimetallic MOF synthesis.
The feasible method for preparing bimetallic MOF-based nanomaterials by ball-milling and heat treatment is attracting more and more attention [56,57]. Based on the monometallic MOF (Fe-Tz and Cu-Tz) of iron and copper [56], M. Lee et al. mixed the two by mechanical grinding to prepare a bimetallic MOF intermediate (FeCu-x:y) with evenly dispersed iron and copper, then pyrolyzed, which finally formed the required bimetallic MOF-based nanomaterials (FeCu-x:y-T). For comparison, a different preparation process was also performed, which exhibited the transformation from one-pot synthesis of FeCu-MOFs (FeCu-50:50DEF) by hydrothermal method to the preparation of bimetallic MOF-based nanomaterials (FeCu-50:50-TDEF) with heat. Morphology and microstructure analysis showed that the distribution of elements in the FeCu-x:y-T sample was more uniform than that in the FeCu-50:50-TDEF. The results of the redox reaction confirmed that the synergistic effect of the FeCu-x:y-T sample was greater than that of the FeCu-50:50-TDEF, which underlined the effectiveness of mechanical grinding and pyrolysis in bimetallic MOF-based nanomaterials.

Applications of Bimetallic MOFs-Based Nanocomposites for Pollutants in Water
Bimetallic MOF-based nanocomposites have been widely regarded as novel functional materials for controlling pollutants in environmental water bodies due to their advantages in structure and performance. Therefore, many new bimetallic MOFs with different forms have been developed and constructed, which have been applied in adsorption removal, catalytic degradation, and the detection of water pollutants.

Adsorption
Bimetallic MOF-based nanomaterials are characterized by a larger surface area, a richer pore structure, and more available metal sites than monometallic MOF-based materials. Therefore, it has important practical significance in developing porous bimetallic MOFbased materials to absorb ions or molecules in water to a greater extent. At the same time, reasonable and effective adjustment of the composition of bimetallic MOFs-based materials, including metal and ligand types, can further improve the adsorption strength of the materials on target ions or molecules in water. By referring to existing reports, it can be found that the adsorption of bimetallic MOF-based nanocomposites on ions or molecules is highly correlated with the structure of MOFs [58][59][60][61][62][63][64][65][66]. Consequently, part of the latest research is summarized about the adsorption-removal of pollutants in water environments by bimetallic MOF-based materials in Table 1.

Ionic Adsorption
Sun and co-researchers constructed Fe-Co-based bimetallic MOF adsorbents with different Fe/Co molar ratios (Fe x Co y MOF-74, where x:y = 0:3, 1:2, 1.5:1.5, 2:1, and 3:0) [67]. Microstructure analysis showed that the BET-specific surface area of bimetallic Fe 2 Co 1 MOF-74 increased by about 20 times (147.82 m 2 /g) and the total pore volume increased by about 4 times (0.058 cm 3 /g) compared with that of monometallic Co MOF-74. At room temperature, compared to other Fe/Co molar ratio bimetallic MOF-based materials, the maximum adsorption capacity of Fe 2 Co 1 MOF-74 for As 5+ and As 3+ increased significantly, reaching 292.29 mg/g ( Figure 3a) and 266.52 mg/g (Figure 3b), respectively. The study displayed that the absorption capacity of Fe 2 Co 1 MOF-74 to As 5+ and As 3+ was mainly attributed to the enhancement of specific surface area, total pore volume, pore structure, and nanomaterial size.
Zhang and colleagues designed an amorphous Fe/Mn bimetallic organic framework (FeMn-MOF-74), which effectively combined the low coordination active centers with the metal atoms at the high oxidation sites to achieve the overall efficient removal of As 3+ [68]. When the molar ratio of Fe to Mn was 1.96, the synergistic effect between the active sites of Fe and Mn was the best, and the maximum adsorption capacity of the material for As 3+ could reach 161.6 mg/g under laboratory conditions. The study confirmed that the synergistic effect between the components of bimetallic MOFs in amorphous form played a decisive role in promoting the adsorption of trivalent arsenic. Similarly, a fusiform FeCo bimetallic MOFs-based nanomaterial (δ-MnO 2 @Fe/Co-MOF-74) was successfully prepared by Yang et al., which exhibited high adsorption removal rates for As 3+ in the pH range from 2 to 10 [69]. Compared with Fe/Co-MOF-74, the N 2 adsorption-desorption curves ( Figure 3d) and pore size distributions ( Figure 3c) of MnO 2 @Fe/Co-MOF-74 were significantly ameliorated. At room temperature, the maximum adsorption capacity of δ-MnO 2 @Fe/Co-MOF-74 for As 3+ was 300.5 mg/g, while that of Co-MOF-74 was 159.3 mg/g. The mechanism analysis explained that the strong coordination of Fe 3+ /Co 2+ for As was the important reason for the significant improvement of adsorption performance for bimetallic MOFs-based nanocomposites, besides the functional groups of Fe/Co-O, Fe/Co-OH, and Mn-O.

Organic Compound Adsorption
The continuous development of bimetallic MOF-based adsorbents with high physicochemical properties, high adsorption capacity and selectivity of organic matter, and low production costs is worth further exploration. Bimetallic MOFs are materials with a large surface area, high porosity, and multiple active sites that have a good development prospect in the adsorption of organic pollutants in water environments [70,71].
The introduction of a second metal into the monometallic MOF can improve the stability of the MOF as a whole and also enhance the interaction between the material and the targeted contaminant. Yang and Bai [72] prepared flower-like hierarchical NiZn MOF microspheres by hydrothermal method (Figure 3e) to improve the stability of the Nanomaterials 2023, 13, 2194 8 of 27 adsorbent and the adsorption capacity of organic pollutant Congo red. The result showed the adsorption capacity of NiZn MOF to Congo red increased to 460.90 mg/g (Figure 3f), while that of Zn MOF and Ni MOF was 276.60 mg/g and 132.20 mg/g, respectively. In addition, the density functional theory (DFT) calculation of NiZn MOF confirmed that the NiO, ZnO, and NiO-ZnO nanoclusters contained in the adsorbent were favorable for the adsorption of Congo red molecules. Similarly, Zheng et al. prepared a cobalt-zinc bimetallic MOFs-based nanomaterial (ZnCo-NPC) with a special three-dimensional porous structure [73], and the maximum adsorption capacity of the material was up to 320 mg/g for ornidazole (ODZ) in water. Adsorption mechanism analysis showed that ZnCo-NPC enhanced the capture capacity for ODZ molecules by intensifying electrostatic forces, hydrogen bonds, and π-π bonds.
Liu and co-team synthesized multi-component hollow and multi-shell Ni-Co bimetallic MOFs-based nanomaterials (Fe(OH) 3 @NiCo-LDH) to strengthen the interaction between unsaturated sites and Congo red molecules in the material's skeleton structure [74]. Within 20 min, the maximum removal efficiency of Congo red by Fe(OH) 3 @NiCo-LDH reached 100%, and the saturated adsorption capacity reached 658.52 mg/g, significantly higher than that of Fe(OH) 3 @NiMg-LDH. The remarkable difference in adsorption capacity of Congo red by bimetallic MOFs-based materials with distinct structures (Fe(OH) 3 @NiCo-LDH is a double-shell structure, and Fe(OH) 3 @NiMg-LDH is a hollow tube structure) indicated that the binding strength between the adsorption sites of the material structure and goal molecules was different. The results demonstrated that the adsorption active sites formed by NiCo-MOFs-based materials were better than those formed by NiMg-MOFsbased materials.
However, another report also showed similar research content; the maximum adsorption capacity of the prepared bimetallic MOFs on Congo red was more than three times that of the above reports. The study focused on the adsorption properties of cobalt and iron bimetallic MOFs for CoFe-BDC-(1), which was synthesized in one pot, and CoFe-BDC-(2), which was prepared by the MOF-on-MOF method (that is, CoFe-MOFs were further structured on the basis of Fe-BDC) [75]. The maximum adsorption capacity of CoFe-BDC-(1) adsorbent for Congo red reached 1935.68 mg/g, while that of CoFe-BDC-(2), Fe-BDC, and Co-BDC was 1259.52, 775.19, and 628.93 mg/g, respectively. The SEM, TEM, and N 2 adsorption-desorption analysis exhibited that CoFe-BDC-(1) had more defects than CoFe-BDC- (2), which could improve the adsorption and removal efficiency of anionic pollutants in water. The above two reports indicate that the design of an adsorbent for the same target pollutant should be fully considered from the aspects of synthesis process, metal type, material structure, and function.

Catalysis
Catalysis is an important direction in the development of bimetallic MOF-based nanomaterials. In terms of the intrinsic catalytic mechanism of MOF-based nanomaterials as catalysts, inorganic nodes owned by MOFs themselves are the important basis for the catalytic properties of catalytic materials. In fact, during the catalytic process of inorganic nodes, the intensity of the electric field around metal ions would change, which could cause fluctuations in the coordination environment between metal ions and organic ligands, further inducing the collapse of the metal-organic framework. Therefore, compared with monometallic MOF-based catalysts, the design of bimetallic MOF-based catalysts is wise; that is, one highly active metal ion mainly acts as the catalytic center, and the other maintains the stability of the catalyst structure. At the same time, the construction of bimetallic MOF-based materials can effectively optimize the distribution of metal catalytic active sites, improving their stability, catalytic activity, and selectivity [76][77][78][79][80]. The recently catalyzed degradation of pollutants in the water environment by bimetallic MOF-based nanocomposites is summarized in Table 2.

Catalytic Activation Degradation
The catalytic activation degradation process is based on the fact of activating H 2 O 2 , PMS, and PDS to produce highly active hydroxyl radicals (•OH) and sulfate radicals (SO 4 •− ), which effectively act and degrade stubborn organic pollutants. Over the past decade, hundreds of studies have confirmed that the catalytic activity of SO 4 •is superior to that of •OH and that oxidants (PMS) with asymmetric structures are more conducive to activation than those (H 2 O 2 and PDS) with symmetric structures. In response to the call for green, efficient, and sustainable development goals, heterogeneous catalysts have gradually replaced homogeneous catalysts, becoming the hot spot for catalytic activation and degradation of organic pollutants in water [81,82]. Among many heterogeneous catalysts, the bimetallic MOF-based nanocomposites, especially Co-based MOFs [83], that directly rely on the advantages of attainable and highly effective metal active sites and structures are regarded as efficient catalysts for catalytic degradation of pollutants in water.
Huang and co-workers prepared hierarchical CoFe LDH/MOFs nanorods with different cobalt and iron contents for urea oxidation in solution [84]. Compared with Co-MOF, the CoFe LDH/MOF catalyst had a larger surface activity, higher conductivity, and a better electron mass transfer path, making the material favorable to promote the catalytic reaction. The catalytic degradation of urea by bimetallic CoFe-MOFs-based materials confirmed that CoFe LDH/MOFs still had good recyclability after a long time of work, which was attributed to the improvement of the local electronic structure for the catalytic active site. Meanwhile, with the introduction of iron ions, the adsorption capacity, or controllability, of the catalyst for inorganic metal nodes and ligands gradually increased. Wang and colleagues synthesized layered Pd@ZnNi-MOF/GO catalysts to activate H 2 O 2 degrading 3,7-bis(Dimethylamino)phenazathionium chloride [85]. The introduction of Pd atoms not only greatly improved the catalytic activity of the nanocomposite on H 2 O 2 , but also reinforced its own stability.
Yang and co-team synthesized porous magnetic bimetallic MOFs-based nanocrystal catalyst CoFe 2 O 4 NC using a CoFe bi-MOFs template for degradation of bisphenol A (BPA) with activating PMS ( Figure S3a) [86]. Compared with CoFe 2 O 4 NC/BPA (only 4% BPA removal efficiency), Fe-MOF/PMS/BPA (5%), and Co-MOF/PMS/BPA (63%), the catalytic removal efficiency of BPA by the CoFe 2 O 4 NC/PMS/BPA system was more than 97% in 60 min ( Figure S3b), which illustrated that introducing iron ions into Co-MOF could significantly promote the activation of PMS and the catalytic degradation of BPA. The CoFe 2 O 3 nanoparticle (CoFe 2 O 4 NP) prepared by the traditional one-step hydrothermal method was also weaker than the CoFe 2 O 4 NC in the degradation of BPA by activating PMS, and the BPA removal efficiency of the former was 69% under the same experimental conditions. These results indicate that the synergistic catalytic effects between Co 2+ and Fe 2+ are significantly different in terms of CoFe 2 O 4 NP and CoFe 2 O 4 NC with different structures.
Li and co-researchers synthesized a series of unique macaroon-like NbCo-MOF materials for heterogeneous catalyst activation of PMS [87]. The catalytic degradation performance of tetracycline (TC) in NbCo-MOF samples with Nb/Co ratios of 1:1, 1:2, 1:3, 1:4, 1:5, 2:1, and 4:1 was investigated. In the kinetic process of TC degradation by activating PMS, the catalyst samples with the Nb/Co ratio of 1:4 showed the highest catalytic activity (within 10 min, the removal efficiency of TC was close to 100%), while the catalytic activity of other samples ranged from 35% to 79% ( Figure S3c). The anion effects of HCO 3 − , NO 3 − , C 2 O 4 2− , and Cl − co-existing with TC were investigated in the NbCo-MOF/PMS system, and the results displayed that the presence of these anions hardly affected the catalytic degradation of TC ( Figure S3d-g). In addition, NbCo-MOF exhibited excellent catalytic degradation of rhodamine B and tylosin tartrate. Within 30 min, the removal efficiency of these target molecules in water was greater than 98%. The research emphasized that the combined effect of the Nb 4+ /Nb 5+ cycle and the Co 2+ /Co 3+ cycle (Equations (1)-(6)) in the NbCo-MOF catalyst accelerated the regeneration of highly active sites and improved the electron transfer efficiency between the catalyst and PMS, thus effectively promoting the catalytic activity of the catalyst for degrading dissolved organic pollutants.
The multi-component reaction in bimetallic MOF-based materials that can be realized smoothly is based on the coordination of different active sites [88][89][90][91]. By adjusting the target metal ratio, Wang and co-members proved that the materials with spinel-structured CoMn-MOFs-based nanoparticles (Co x Mn 3−x O 4 -C) possessed high catalytic activity and were constructed with a Co/Mn bimetallic organic framework as a template [88]. A series of Co x Mn 3−x O 4 -C samples showed substantial performance in activating PMS to degrade bisphenol A (BPA), and the Co 1.5 Mn 1.5 O 4 -C had the best performance. The material could completely degrade 0.1 mM BPA in a very short time (3 min) (Figure 4a). The stability experiments measured the leaching amount of cobalt ions for prepared catalysts during the catalytic process (Figure 4b), which strongly confirmed that the structure of Co 1.5 Mn 1.5 O 4 -C had high fixability on metal ions during working. Common inorganic anions (such as HCO 3 − , H 2 PO 4 − , NO 3 − , and Cl − ) also had little effect on the catalytic degradation of BPA by the Co 1.5 Mn 1.5 O 4 -C/PMS system, but humic acid had a significantly negative effect. Finally, the mechanism analysis highlighted that there are two pathways for the catalytic degradation of BPA (Figure 4c). One is that the active sites of cobalt and manganese directly catalyze PMS to form •OH and SO 4 •− radicals working on BPA molecules, while the other is that the carbonyl group (C=O) under the action of electron transport for the carbon framework in the catalyst interacts with PMS to produce singlet oxygen 1 O 2 attacking BPA. effect. Finally, the mechanism analysis highlighted that there are two pathways fo catalytic degradation of BPA (Figure 4c). One is that the active sites of cobalt and m nese directly catalyze PMS to form •OH and SO4 •− radicals working on BPA mole while the other is that the carbonyl group (C=O) under the action of electron transpo the carbon framework in the catalyst interacts with PMS to produce singlet oxyge attacking BPA.

Photocatalytic Degradation
In recent years, bimetallic MOFs-based materials have gradually developed va excellent environmental photocatalysts [92][93][94][95][96], which could be used as prominent tocatalytic reaction active agents for the degradation of organic pollutants in wate generally acknowledged that titanium-based and zinc-based metal organic framew have become hot spots in the field of photocatalysis due to their unique absorptio (c)

Photocatalytic Degradation
In recent years, bimetallic MOFs-based materials have gradually developed various excellent environmental photocatalysts [92][93][94][95][96], which could be used as prominent photocatalytic reaction active agents for the degradation of organic pollutants in water. It is generally acknowledged that titanium-based and zinc-based metal organic frameworks have become hot spots in the field of photocatalysis due to their unique absorption and conversion properties of light [97][98][99]. Another metal combined with Ti or Zn to form the bimetallic MOFs could significantly promote the photocatalytic activity of the materials.
Li et al. investigated the application of TiIn-MOF catalysts with different proportions for adsorption and photocatalytic degradation of BPA [100]. Among them, the heterogeneous metal clusters formed by TiIn-MOF(1:1) could well regulate the surrounding electronic structure and significantly improve the absorption intensity of visible light ( Figure S4a). Moreover, the lowest optical bandgaps (2.60 eV) were found among the synthesized bimetallic MOF-based catalysts ( Figure S4b). After 30 min of dark treatment, the adsorption removal efficiency of BPA by TiIn-MOF(1:1) reached 38%, and the adsorption performance was about twice that of Ti-MOF. Subsequently, under 300 W xenon lamp irradiation, the degrading efficiency of BPA was 100% by TiIn-MOF(1:1) after 20 min, and the degradation rate was twice that of Ti-MOF. In addition, the synergistic effect of TiIn-MOF(1:1) could promote the production of •O 2 − , hole (h + ) and 1 O 2 in the photocatalytic process by a large margin. However, a part of the pores and active sites for the directly synthesized Ti-based MOFs were masked by precursor molecules (organic ligands, organic solvents, and the like), which resulted in a decrease in the activity of the catalyst. Therefore, Wang et al. performed a thermal treatment of the synthesized ZnTi-MOF ( Figure S4c) to prepare a C-doped MOFs-based bimetallic photocatalyst that displayed highly efficient visible light absorption [101]. After 45 min irradiation by a 200 W xenon lamp, the cat-alytic degradation efficiencies of all C-dope ZnTi-MOF samples were better than those of untreated samples.
Amine-functionalized bimetallic MOFs have been successfully applied in the preparation of photocatalytic materials. Amination can not only expand the visible light absorption of MOFs-based materials but also well protect the oxygen-containing functional groups on the surface, which enhances the utilization rate of light by the photocatalyst, adding to its own oxidation resistance [102,103]. Wang et al. used FeTi-MOF-NH 2 as a photo-Fenton catalyst to degrade chrysoidine at 300 W xenon lamp irradiation [103]. Within 10 min of the reaction starting, the photo-catalysis ability of Fe/Ti-MOF-NH 2 (3:1) degrading the goal pollutant increased to 100%, while FeTi-MOF-NH 2 (1:1) and FeTi-MOF-NH 2 (1:3) were only about 40%. The heterojunction structure of FeTi-MOF-NH 2 (3:1) could effectively slow down the photocatalytic electron-hole pair recombination rate and improve the effective photocatalytic e --h + pairs so as to exhibit highly efficient catalytic performance (Equations (7)-(11)).
Fe/Ti-MOF-NH 2 (3:1) + hv → Fe/Ti-MOF-NH 2 (3:1) (e − + h + ) (7) Zhong and co-workers designed an AB-type heterojunction photocatalyst, ZnO/Ni 0.9 Zn 0.1 O using MOF-NiZn as the intermediate [104]. It is worth noting that during the pyrolysis process of MOF-NiZn, some Ni metal nodes were replaced by Zn to form Ni 0.9 Zn 0.1 O. Compared with NiO (3.11 eV), the hybrid state of Ni 0.9 Zn 0.1 O (2.98 eV) had a lower band gap. Therefore, ZnO/Ni 0.9 Zn 0.1 O only needed lower photon energy to excite and generate electrons and holes in visible light, displaying better photocatalytic performance. The photocatalytic degradation efficiency of ZnO/Ni 0.9 Zn 0.1 O for phenothiazine could reach more than 97% in 60 min.
Li et al. constructed CoZn-ZIF as a porous, non-bonding nano-catalyst CoZn-N-C (MCZC) using thermal etching technology (Figure 5a), which could enhance the electron transfer between the Co active site and the Zn active site and construct the unsaturated metal sites of Co and Zn atom pairs to the maximum extent [105], improving the synergistic effect between different active sites of the catalyst. When the photocatalytic time is 24 min, the maximum removal efficiency of TC in the MCZC/PMS/Visible-light system was 99.6% (the mineralization rate was 55.8%), while that in ZCCN (ZCCN was the thermal etching product of ZIF-67) was only 74.9% without visible-light irradiation. A variety of anions (including Cl − , SO 4 2− , NO 3 − , CO 3 2− and H 2 PO 4 − ) and natural organic matter (HA) did not significantly affect the maximum degradation efficiency of the MCZC/PMS/Vis/TC system, but the removal rates were significantly different. The presence of Cland H 2 PO 4 − increased the catalytic rates of MCZC, while the rest showed inhibition (Figure 5b). The internal mechanism of the catalytic performance improvement for MCZC was explained mathematically by density functional theory (Figure 5c). When PMS was adsorbed on the Co active site at the photocatalyst surface of ZCCN or MCZC, the O-O bond length of PMS would be outstandingly stretched (from 1.483 Å or 1.476 Å to 1.527 Å) at visiblelight irradiation driving. Thus, as for MCZC, the PMS was more easily activated by photogenerated electrons to produce SO 4 •− and •OH.

Detection
Bimetallic MOF-based materials possess high electron mobility, structural adjustability, and chemical stability, which have been widely used in the detection of target ions or molecules, especially as sensors [106][107][108][109][110]. Compared with the monometallic MOF-based electrode, the bimetallic MOF-based electrode sensor has the advantages of stronger hightemperature resistance, higher sensitivity, better selectivity, and a shorter response time [111][112][113]. The bimetallic MOFs are composed of two different metal cations or metal cations' aggregates bonded with organic ligands to form functional materials with high spe- (c) Geometric optimizations of PMS adsorbed onto the Co site of ZCCN and MCZC catalysts corresponding electron density differences (Yellow and lake-blue regions represent electron-rich and electron-defective areas, respectively). Reproduced with permission from ref. [105]. Copyright 2023, Elsevier.

Detection
Bimetallic MOF-based materials possess high electron mobility, structural adjustability, and chemical stability, which have been widely used in the detection of target ions or molecules, especially as sensors [106][107][108][109][110]. Compared with the monometallic MOFbased electrode, the bimetallic MOF-based electrode sensor has the advantages of stronger high-temperature resistance, higher sensitivity, better selectivity, and a shorter response time [111][112][113]. The bimetallic MOFs are composed of two different metal cations or metal cations' aggregates bonded with organic ligands to form functional materials with high specific surface area, porosity, and structural morphology stability, which is a new sustainable-development form based on single metal MOF materials. At present, a variety of sensors with special selective performance have been developed based on bimetallic MOFs-based materials and are popularly applied in the detection of targeted ions and molecules, including glucose [114,115], bisphenol A [116], Fe 3+ [117], miRNA [118], antibiotics [119], acetone [120], organophosphate [121], and so on. The bimetallic MOFbased nanomaterials' latest progress in detecting pollutants in environmental water is summarized in Table 3.
Pan and co-workers synthesized NiCu-MOF/GCE based on a glassy carbon electrode supported by 2D/3D hierarchical NiCu-MOF, which showed great electrocatalytic performance for a non-enzyme glucose sensor [114]. NiCu-MOF/GCE electrode sensors exhibited a favorable linear response to glucose in the range of 20 µM~4.93 mM, which was three times that of Ni-MOF/GCE (5 µM~1.63 mM) (Figure 6a,b). The NiCu-MOF/GCE's performance advantage was not only manifested in the width of response to glucose but was also outstanding in anti-interference ability and stability. When the distractions of uric acid, citric acid, ascorbic acid, and Cl − coexisted, the responsiveness of Ni-MOF/GCE and NiCu-MOF/GCE for glucose was deeply explored (Figure 6c). The results showed that the detection performance of NiCu-MOF/GCE for glucose was almost unaffected in the presence of high concentrations of Cl − , while Ni-MOF/GCE decreased by 22%. In addition, Figure 6d also reflected that after continuous operation for up to 22 days, the electrode performance of NiCu-MOF/GCE only went down 10%, while that of Ni-MOF/GCE under the same conditions reduced almost 52%. Another more efficient enzyme-free glucose bimetallic MOFs-based sensor (Co/MnO@HC) was successfully prepared (Figure 6e) [115]. Chronoamperometry and cyclic voltammetry evaluated that the Co/MnO@HC sensor had high electrochemical catalytic performance in low concentration glucose (50~900 µM) and high concentration glucose (1.9~6.9 mM) solutions, and the detection limit of the electrode material was 1.31 µM.
Huang and co-researchers prepared six self-assembled multilayer M-N-MOFs-based (M=Ce, Ni, Zn; N=Co, Ni) carbon nanotubes loaded on glassy carbon electrodes (M-N-MOF/MWCNTs/GCE) and applied them to the detection of BPA in drinking water [122]. Among the above six synthetic bimetallic MOFs, CeZn-MOF-based electrodes displayed the best catalytic performance and the highest oxidation peak current value (more than 40 µA) for BPA in aqueous solution. Nyquist plots further reflected the electrode impedances of CeZn-MOF/MWCNTs/GCE, CeZn-MOF/GCE, MWCNTs/GCE, and GCE, which were 34.64, 138.2, 84.26, and 103.9 Ω, respectively. The results demonstrated that CeZn-MOF/MWCNTs/GCE electrodes had the highest charge transfer efficiency in operation. Taking advantage of electrochemical impedance spectroscopy (EIS) and differential pulse voltammetry (DPV), the working range of CeZn-MOF/MWCNTs/GCE electrodes for BPA was determined from 0.1 to 100 µmol/L along with a detection limit of 7.2 nmol/L. Wu and Huang synthesized an MgZn MOFs material with fluorescent characteristics: MgZn(1, 4-NDC) 2 (DMF) 2 (1, 4-NDCH 2 = 1, 4-naphthalene dicarboxylic acid), which was constructed by the chain of [-Mg-(COO) 2 -Zn-(COO) 2 -] n , exhibiting the two-color conversion fluorescence from green to blue [117]. The phenomenon was due to the change in interaction force and relative position between metal ions and organic ligands during the synthesis of bimetallic MOF-based materials, which led to the unique fluorescence behavior of the materials.   Wang and co-workers have designed another high-efficiency fluorescent bimetallic MOFs-based sensor, Cd 6 Na 4 (L) 4 (DMF) 2 (µ-H 2 O) 3 (H 4 L = 1,1 -ethylbiphenyl-3,3 ,5,5tetracarboxylic acid) (Figure 7a), for the detection of ODZ, cysteine (Cys), TC, and metronidazole (MDZ) in water [119]. Interestingly, Cd 6 Na 4 (L) 4 (DMF) 2 (µ-H 2 O) 3 exhibited different detection characteristics when working. On the one hand, a fluorescence quenching effect was displayed on the detection of ODZ (the detection limit was 0.32 µM), TC (0.11 µM), and MDZ (0.14 µM). On the other hand, a fluorescence-enhancing effect was revealed during the process of recognizing Cys (19.7 nM). When ODZ, TC, or MDZ were presented in water, the ligands of Cd 6 Na 4 (L) 4 (DMF) 2 (µ-H 2 O) 3 preferentially transferred energy to target molecules ODZ, TC, or MDZ, while the energy transferring to other receptors was inhibited, thereby reducing fluorescence intensity, which manifested as selective recognition of ODZ, TC, or MDZ. The fluorescence enhancement effect was the result of a synergistic effect between Cys and the porous structure of CdNa-MOFs. Targeting experiments ( Figure 7b) and interference tests of 17 amino acids (Met, Thr, Lys, and the like) proved that Cd 6 Na 4 (L) 4 (DMF) 2 (µ-H 2 O) 3 sensor owed characteristic detection performance to Cys, and cyclic tests further highlighted that the sensor possessed the properties of maintaining good stability and recycling.

Summary
The adjustable advantages of MOFs in morphology, structure, and chemical properties have made them a potential force for adsorption and catalytic treatment of polluted water and the detection of trace pollutants. Bimetallic MOFs increase the complexity of MOFbased materials on the basis of monometallic MOFs, including morphology, structure, catalytic properties, electron conduction, and stability. Direct synthesis and post-treatment have been widely adopted to prepare bimetallic MOF-based nanocomposites in order to maximize the efficiency of adsorption removal, catalytic degradation, and recycling.
The formation of heterojunctions and Schottky junctions significantly and effectively promoted electron transfer between the two metal atoms, thus enhancing the charge recombination cycle and improving the stability of the materials. In order to better explore the structure, composition, and arrangement of heterometals in bimetallic MOF-based materials, a variety of characterization techniques need to be fully utilized and combined. At present, the existing research lacks detailed and effective means to determine the arrangement of metals in the framework. However, the effective identification of metal ion arrangements in bimetallic MOFs-based materials is critical to understanding performance improvement in heterogeneous metal mixing modes.
Bimetallic MOF-based nanomaterials with complex structure and composition usually exhibit better performance than monometallic MOFs. MOFs composed of different metal centers can form bimetallic MOFs with different morphologies, structures, and functions through different preparation processes. Hydrothermal, solvothermal, and one-pot synthesis have become the basic strategies for bimetallic MOF synthesis, while ball-milling, pyrolysis, and other post-treatment methods have gradually developed into mainstream methods for the synthesis of new MOF-based materials. The synthesis process of new MOFs-based materials is usually accompanied by the formation of heterometal substitution and defective crystal structure so as to adjust the electronic structure, pore structure, and stability of bimetallic MOFs-based materials, leading to new physical adsorption and chemical catalytic properties.
At present, the effective synthesis and design of bimetallic MOF-based nanocomposites with various physical structures and chemical properties have been favored by many applications, including adsorption, catalysis, and the detection of pollutants in water environments. However, the sustainable development of bimetallic MOF-based materials is still limited by the following aspects: (a) low stability, (b) easy plugging of active sites and pore structures, (c) poor electrical conductivity, (d) unclear interaction mechanisms between bimetals, and (e) inconvenient recovery.

Outlook
A large number of studies on the characterization, recycling, and recovery of bimetallic MOF-based composites have been performed in detail and confirmed that the bimetallic MOF-based composites are more stable than the original MOFs.
Bimetallic MOF-based nanomaterials have more available active sites, relatively good stability, and electrical conductivity, which makes their applications in the fields of adsorption, catalysis, and detection under complex environmental conditions have stronger core competitiveness.
Although great progress has been made in the design of bimetallic MOFs as multifunctional materials, there are still some problems, such as the uneven distribution of metal particles, the absence of available active sites, and the difficulty of accurately controlling pore structure. In order to solve these problems as much as possible, studies using MOFs as precursors or templates are gradually developing. For example, the ball-milling method is used to control the mixing of the two precursor MOFs more fully so as to reduce the aggregation of bimetals in MOFs during the pyrolysis process. On the other hand, the doping of nonmetallic compounds can also enhance the toughness of bimetallic MOF-based nanomaterials. These strategies have improved the physical and chemical properties of bimetallic MOFs to a certain extent, but they still deserve further exploration.
The leaching of metal ions is an urgent problem to be solved in the application of bimetallic MOF-based materials in water treatment. The highly electronegative elements contained in the organic ligands can not only effectively fix metal ions in the synthesis of bimetallic MOFs-based materials, reduce ion leakage in the working process of nanomaterials, but also further optimize the pore structure and electron transport properties of the materials.
On the one hand, for the reaction mechanism and degradation path of pollutants in heterogeneous catalytic oxidation processes, many researchers only evaluated the mineralization degree of organic pollutants from the perspective of total organic carbon without considering the deeper carbon balance. In future studies, more effective evaluation methods can be considered to explain the conversion of pollutants between organic carbon and inorganic carbon. On the other hand, although bimetallic MOF-based nanocomposites have outstanding performance in removing pollutants from water environments, the comprehensive safety and ecological risk assessment of the materials still needs to be further studied.
Despite the fact that bimetallic MOF-based nanocomposites still face many challenges in water treatment, their rapid development in adsorbents, catalysts, and sensors (electrodes) in recent years indicates that they have huge potential for growth. In future research, it is expected to develop bimetallic MOF-based materials that are more efficient, cheaper, and environmentally friendly so that functional materials for water treatment can be realized with both structure and performance.

Conflicts of Interest:
The authors declare no conflict of interest.