Trends in the energy and environmental applications of metal–organic framework-based materials

Abstract

Over the past 20 years, metal–organic framework (MOF) nanosheets have garnered a great deal of interest in the fields of energy and environmental management because of their inherent extraordinary qualities. These qualities include the vast surface areas, nanoscale and tunable pore sizes, adaptable structures and functions, good thermal and chemical stability, high aspect ratios, more exposed accessible active sites, flexible functionality, high electrical conductivity, and optical transparency. An overview of the current advancements in the applications of MOF-based materials in environmental science and renewable energy is provided in this review. Precisely, the advancements, advantages, history and characterization of MOF-based materials are first presented and discussed. Next, we focused on the use of MOF-based materials in the fields of environmental cleaning and monitoring, particularly for the treatment of wastewater and air purification, and energy storage and conversion. We concluded by summarizing the findings on the current state-of-the-art advancements and sharing the perspectives on the prospects and problems facing future research on MOF-based materials.

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Mohammed Yusuf received both his MSc and PhD in Chemical and Environmental Engineering from Universiti Putra Malaysia (UPM) and Universiti Teknologi Malaysia (UTM) respectively. He previously worked as Research Assistant Professor at Tokyo University of Agriculture and Technology Japan. He also worked as an Environmental Consultant at Oil and Gas Company in Al-Khobar, Saudi Arabia. Furthermore, he worked as a Post-doctoral Research Fellow at Chinese Academy of Sciences Wuhan, China and Hanyang University Seoul, South Korea. Presently, he is working as an Associate Professor at Tomsk State University Russia. His current research is directed towards the development of composite materials and their utilization as an adsorbent for the abatement of organic and inorganic pollutants from both domestic and industrial wastewater.

Nurudeen Abiola Oladoja is a Professor and Chair of the Hydrochemistry Research Group, Department of Chemical Sciences, Adekunle Ajasin University, Akungba Akoko, Nigeria. He is also the Head of the Applied Research Unit at the African Centre of Excellence for Water and Environmental Research (ACEWATER), Redeemer’s University, Ede, Nigeria. His research has focused on environmental interfacial chemistry, aquatic ecological engineering, and resource recovery from wastewater.

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1. Introduction

The intriguing structures of MOFs and their notable physical attributes, such as tunable pore size, large pore volume, high specific surface area, and the potential for case-specific customization of fundamental molecular architecture, make them an ideal platform for applications in energy storage and environmental management.^1,2 MOFs are crystalline porous solids that are made up of organic ligands and metal ions connected by coordination bonds between the metal ions/clusters and the organic ligands.³ The majority of metals used are the transition metals from the d-block (Ti⁴⁺, Zr⁴⁺, V³⁺, Cr³⁺, Fe³⁺, Mn²⁺, Ni²⁺, Cu²⁺, Zn²⁺, and Cd²⁺) or other s-block (Mg²⁺)/p-block (Al³⁺ and Ga³⁺) metals. A small number of metals are also employed from the lanthanide/actinide series. The ligands are the donors that contain nitrogen or oxygen functional groups, which act as the linkers between the metal ions.⁴ Metal ions are modeled as nodes in the MOF structure, leading to the formation of intricate configurations with hydroxy or oxide anions, ranging from trimmer to octa-nuclear.⁵ Depending on whether or not guest molecules are present, MOFs are grouped according to their dimensionality, and because structures of the MOFs are distinct, they exhibit distinctive structural dimensionality on the order of 1D, 2D, or 3D.⁶

MOFs are characterized by their high porosity, which allows for up to 90% of free space, and exhibit a high specific surface area of around 6000 m² g⁻¹.⁷ MOFs have a wide range of desirable features, which include high porosity, large surface area, and chemical and thermal stability.⁸ When there are no guest molecules present in the MOFs' pores, the porosity of the material is maintained, and the pore structure maintains its porosity, even when the guest molecules are eliminated.⁹ Materials with special qualities are created by combining elements of organic and inorganic structures. MOFs are distinguished by their notable pore volume, which accounts for at least 50% of the overall volume and their exceptional strength.¹⁰ The selection of the starting building units allows for the modification of a number of parameters, which include the pore size (increasing the pore diameter to 98 Å),¹¹ density (decreasing to 0.126 g cm⁻³),¹² and specific surface area (up to 1000–10 000 m² g⁻¹).¹³

In this discourse, we showcase the trend in the development of MOF nanosheets for the purpose of energy and environmental applications. In order to achieve the aim of this review, we have provided a brief introduction to the structural advantages of MOFs as well as their physical properties (such as stability, pore characteristics, and surface area) for practical applications. An overview of noteworthy advancements in the use of MOFs for energy storage systems (i.e., supercapacitors, batteries, solar cells, and fuel cells) is presented. The important developments in MOFs in the area of environmental applications, i.e., in the adsorptive removal of dyes and heavy metal contaminants from water and hazardous gases from the environment, were covered. The challenges and future prospects for environmental cleanup were discussed, and perspectives on the future directions in this field of research were highlighted.

1.1 Emergence of MOFs

The first mention of MOFs dates back to 1897, when Ni(CN)₂(NH₃)·C₆H₆, a coordination network bonded via the CN groups, was described in ref. 14. The molecule's crystal structure was not confirmed until 1952, when it was discovered to be a layered, square network of Ni(CN)₂(NH₃) groups with benzene occupying the channels, indicating that the structure is an extended network.¹⁵ With copper(I) as the metal node and organic nitrogen ligand as the linker, Robinson's research groups worked on the creation of a coordination polymer in 1989. They reported the synthesis of {Cu¹[C(C₆H₄·CN₄)]}_n, a polymer with 3-D network architecture, which served as the initial step toward the development of the metal–organic frameworks when they brought the idea of topology into coordination polymers for the first time.¹⁶ In 1990, Hoskins and colleagues discovered porous coordination polymers that demonstrated ion exchange capabilities, marking the beginning of the development of functional microporous materials.¹⁷ The heterogeneous catalyst {Cu¹[C(C₆H₄·CN₄)]}_n for the cyanosilylation of aldehydes was effectively prepared from a 2-D square network material created by the Japanese professor Fujita in 1994 using Cd(NO₃)₂ and 4,4-bipyridine. This material can readily form clathrates with certain aromatic guests with a specific shape. As a result, Fujita postulated that the reaction's selectivity will be influenced by the cavity size.¹⁸

It was not until 1995 that Yaghi and associates began referring to these materials as metal–organic frameworks.¹⁹ A multilayer material made of pyridine, octahedral Co centers, and 1,3,5-benzenetricarboxylate was successfully designed and synthesized, and it was able to bind aromatic molecules like benzene both reversibly and selectively. Meanwhile, as an alternative to zeolites and activated carbon, Omar M. Yaghi and colleagues synthesized a groundbreaking substance known as MOF-5, which is said to be the first strong and highly porous material.²⁰

In 2002, a number of Zn dicarboxylates gained popularity, but the concept of isoreticular chemistry²¹ was also applied to other materials. In particular, it has been found that the mixed-linker compounds [M₂(dicarboxylate)₂(diamine)] (M = Zn, Cu) are a versatile class, and several experiments have been carried out by altering the two organic components.²² The imidazolate-based compounds, now referred to as zeolitic imidazole frameworks (ZIFs),²³ were added to the MOF family in 2003.

1.2 Characterization of MOF based materials

Below is a summary of the instrumentation techniques investigated for the characterization of MOFs and their derivatives.

The microstructure of MOF materials is extensively studied using scanning electron microscopy (SEM), which offers insights into morphology, topography, crystal structure, compositional changes, phase distribution, and defect location.²⁴ In order to investigate the form and porous structure of microscale adsorbents, the analysis can be carried out at low magnification (<1000×) or high magnification (>30 000×) for nanoscale adsorbents, such as aggregate formation in metal–organic frameworks (MOFs).²⁵ In order to evaluate adsorbents post-processing and following adsorption/desorption cycles, this method is helpful for determining materials' stability.²⁶ The SEM image of yttrium-based metal oxide fillers (Y-MOFs), which is depicted in Fig. 1a, uncovered well-dispersed, sphere-like structures that ranged in size from 3 to 5 μm. The spheres are filled with nanosheets (200 nm wide and 40–50 nm thick), which resemble Allium giganteum (see the inset in Fig. 1b) in the extended pictures (Fig. 1b and c).

Fig. 1 SEM and TEM images of as-prepared Y-MOF samples at 120 °C for 24 h: (a)–(c) low- and high-magnification SEM images (inset in (b) is the photo of Allium giganteum);²⁶ (d) low- and high-magnification TEM images; (e) EDX spectrum;²⁶ (f) nitrogen adsorption/desorption isotherms of Y-MOFs:10%Eu³⁺ (inset is pore size distribution).²⁷

Particle form, thickness, and size may all be determined as well as defects such as vacancies and dislocations by using transmission electron microscopy (TEM), which is also used to examine the chemical and electrical structures of materials.²⁸ TEM is perfect for analyzing very small adsorbents because, unlike SEM, it requires the electron beam to pass through the sample. In order to determine metal distribution, agglomeration, crystal size, and particle form on substrates, TEM is frequently used with MOFs.²⁹ These data facilitate the development of separation techniques and the study of adsorbent behavior in solutions. A TEM image of Y-MOFs, which were synthesized in ref. 26, is shown in Fig. 1d. The analysis reveals radially oriented nanosheets. The presence of C, Y, O, and N is confirmed by energy-dispersive X-ray spectroscopy (EDX) in Fig. 1e, which is in line with the composition of Y-MOFs.

It is possible to determine the MOFs' surface area, pore volume, and pore size distribution using adsorption isotherms of non-reactive gases at cryogenic temperatures. For pores smaller than 0.7 nm, argon at 87.3 K is preferable, but N₂ adsorption at 77 K produces an isotherm in this case.³⁰ N₂ adsorption at 77 K indicated a surface area of 2021 m² g⁻¹, pore width of 11.7 Å, and pore volume of 0.882 cm³ g⁻¹ for the Ni-MOF produced in ref. 31 utilizing ultrasound irradiation. Their investigation produced MOFs with textural properties that were different from those found in earlier studies on surface areas and pore volumes, possibly due to ultrasonication. The N₂ adsorption–desorption isotherm and pore size distribution of the Y-MOF:10%Eu³⁺ sample are shown in Fig. 1f. Based on IUPAC standards, this isotherm is categorized as type IV with a hysteresis loop.²⁷ According to Yang et al.,²⁶ Y-MOFs have an average pore diameter of 12.5 nm and a BET surface area of about 90.1 m² per g 10% Eu³⁺.

The identification of IR-active functional groups inside MOFs is a well-established use of Fourier-transform infrared spectroscopy (FT-IR), which is essential for investigating the adsorption performance of MOFs.³² For example, FT-IR peaks at 1400–1767 cm⁻¹ for cross-linked terephthalic acid indicate the presence of carboxylic groups (–CO₂), whereas peaks at 1650–1767 cm⁻¹ for non-reacted terephthalic acid are observed.³³ A new gadolinium–porphyrin MOF nanosheet was produced and FT-IR characterization was performed³⁴ (Fig. 2a). The researchers detected absorption peaks at 963 cm⁻¹ and 3315 cm⁻³, which correspond to in-plane vibrations and N–H stretching, respectively. They also noted alterations in peak intensities subsequent to coordination with gadolinium ions (Gd). While the C=O stretching vibration of carboxyl groups dramatically attenuated to 1683 cm⁻¹ upon coordination, the C=C stretching vibration of porphyrin showed a new peak at 1651 cm⁻¹ and vibration at 1683 cm⁻¹. Furthermore, a decreased peak at 1485 cm⁻¹ owing to sulfonic acid groups and a peak at 1582 cm⁻¹ ascribed to the MOF structure's skeletal vibrations were seen (Fig. 2b shows the corresponding results for Gd-TPPS).

Fig. 2 (a) FT-IR analysis of TCPP and Gd-TCPP.³⁴ (b) FT-IR analysis of TPPS and Gd-TPPS. (c) Thermogravimetric analysis (TGA) of HKUST-1.³⁵ (d) Powder X-ray diffraction (PXRD) spectra of Fe-, Zr-, and La-based metal–organic frameworks derived from recycled PET plastic bottles.³⁶

In order to determine impurities, unreacted precursors, and breakdown temperatures, thermogravimetric analysis (TGA) profiles are first acquired for freshly synthesized MOFs (pre-activation).³⁷ MOFs break down in several ways based on the carrier gas (air, O₂, or N₂). To determine which compounds are producing mass changes and temperature variations, mass spectrometry and TGA are frequently employed in tandem.¹⁰ For HKUST-1 samples evaluated within a specific temperature range, the TGA curve in Fig. 2c shows a two-stage thermal breakdown. It is possible that the removal of lattice water/ethanol occurs in the first step (50–125 °C), and the thermal degradation of the organic ligand occurs in the second stage (300–400 °C).³⁵

The main technique for accurately figuring out the atomic arrangement and structure of MOF materials in single crystals is X-ray diffraction (XRD). PXRD, or powdered X-ray diffraction, is used to describe MOFs when single-crystal XRD is not feasible because of the size of the crystal. PXRD spectra with sharp diffraction peaks reveal the bulk MOF materials' crystallinity and structural details. To verify the authenticity of a product, experimental powder patterns can be compared with patterns predicted from single crystal structures or computational models.³⁸ PXRD analysis of Fe-, Zr-, and La-based MOFs made from recycled PET plastic bottles was performed in ref. 36 (Fig. 2d). For example, the production of Fe-BDC was confirmed by sharp (001) and (101) peaks at 9.5° and 10.7°.³⁹ According to He et al.,⁴⁰ the Zr-MOF displayed (111) and (002) peaks at 7.14° and 8.55°, respectively, suggesting intercalated molecular layers. Furthermore, Zr-MOF crystal planes were correlated with strong peaks at 11.87°, 14.71°, 17.33°, 25.73°, and 30.71°.⁴¹ Similar to this, the La-MOF showed crystalline peaks that corresponded to different crystal planes at 15.19°, 23.97°, 28.95°, and 32.87°.⁴²

2. Energy applications of MOFs

2.1 MOFs for supercapacitors

Supercapacitors, also referred to as electrochemical capacitors or ultra-capacitors, are some of the most promising energy storage technologies that are gaining international interest from researchers because of their special qualities, which include high power density, long cycling life, and quick charge rates.⁴³ Since pure MOF SCs have poor conductivity, they often have low specific capacitance despite having significant porosities. A new method for enhancing SC performance involves incorporating MOFs with conductive materials, like graphene and conductive polymers, to increase MOF conductivity. Supercapacitors can be primarily divided into three groups based on the principal charge-storage mechanism, electrode/electrolyte interface, and active materials utilized. (1) Via an electrochemical double-layer capacitor (EDLC) using electrostatic forces, electrical energy is transferred to their inside surfaces. By allowing electrolyte ions to adhere to the surface of electrode materials, the carbon-based electrodes, or their derivatives, are utilized to store charge.⁴⁴ Carbon-based active materials having large surface areas, such as microporous or mesoporous carbon,⁴⁵ graphene,⁴⁶ carbon nanotubes,⁴⁷ and so on, are typically used in these kinds of SCs. (2) Fast and reversible surface redox reactions are the foundation of pseudo-capacitors. Typical examples of active materials in pseudo-capacitors are transition metal oxides and conductive polymers,⁴⁸ and (3) hybrid supercapacitors (HSCs) combine a capacitive electrode with a battery-type electrode (such Li/Na-ion type electrodes) in an electrochemical cell to provide high energy and high power densities.⁴⁹

Excellent energy storage capabilities and high power densities can be achieved by utilizing various electrode materials, such as carbon-based electrodes for electrostatic double-layer supercapacitors or transition metal oxide electrodes for pseudocapacitors. Key factors include high conductivity, large surface areas, and convenient electrolyte access to active sites.⁵⁰ Because of the coexistence of organic and metal components in MOFs, materials based on MOFs typically have high surface areas and porous architectures that provide enough electroactive sites for supercapacitors, particularly EDLCs and HSCs. While a great deal of studies have been done on using MOFs as templates to create nanostructured materials like metal oxides or nanoporous carbon, nano-MOFs in different shapes such as particles, rods, and sheets have also been studied for use as supercapacitor electrodes. When creating electrodes, the controlled porous structure of nano-MOFs offers much-desired benefits like low density and extraordinarily large exterior surface area. Furthermore, in contrast to practically inert commercial porous carbon electrodes, the presence of redox-active metal centers may enhance pseudo-capacitance.⁵⁰ Depending on the electrode materials, MOFs can be used in three different ways: (i) utilizing pristine MOFs to store charges by physisorption of electrolyte ions on their internal surfaces or by taking advantage of reversible redox reactions of the metal centers; (ii) breaking down MOFs to obtain metal-oxides and preserving electrons through the charge transfer between the electrode and electrolyte; and (iii) pyrolyzing MOFs to yield porous carbons and increasing capacitance by increasing conductivity.⁴⁴

A porous MOF Ni₃(HITP)₂ (HITP = 2,3,6,7,10,11-hexaiminotriphenylene) with electrical conductivity has been studied for its possible application in ref. 51; when compared to the bulk electrical conductivity (>5000 S m⁻¹) of porous graphite and active carbon (≈1000 S m⁻¹), the EDLC behavior (Fig. 3) indicates 1D channels (≈1.5 nm in diameter).⁵¹ This is supposedly the first known instance of a supercapacitor made completely of pure MOFs as active materials, devoid of any binders or conductive additives. At a low discharge rate of 0.05 A g⁻¹, the Ni₃(HITP)₂ electrodes had a very high surface area normalized capacitance of 18 mF cm⁻² and a satisfactory gravimetric capacitance of 111 F g⁻¹, which was among the highest values reported for most carbon material-based supercapacitors.⁵² With over 90% capacity retention after 10 000 cycles, this MOF-based device demonstrated superior cycling performance and exhibited a high areal capacitance that surpassed most carbon-based materials.

Fig. 3 MOF-based supercapacitors. (A) Schematic construction of MOF supercapacitors.⁵³ (B) Structure and nanocrystal morphology of nMOF-867 and comparison of stack capacitances among various EDLC materials.⁵¹ (C) A space-filling diagram of idealized Ni₃(HITP)₂ (Ni, green; F, lime; N, blue; C, gray; B, brown; H, white).⁵¹ (D) Comparison of BET-surface-area-normalized areal capacitances among various EDLC materials.⁵⁴

For high-performance supercapacitors, MOFs with high electrical conductivities are therefore created as appropriate electrode materials based on the aforementioned principles. For instance, Li et al.⁵⁵ used a bottom-up synthetic approach to integrate Cu–HHTP single crystal nanowires onto carbon fibers to create conductive MOF nanowire arrays (NWAs). These NWAs were then grown on carbon fiber paper to create nanowire arrays of a copper-MOF that were 200–250 nm in diameter and 3–15 mm in length. The NWAs were then used as electrodes for solid-state supercapacitors.⁵⁵ Effective charge/electron transfer at the MOF NWAs/electrolyte interface was made possible by the NWA electrode's significant reduction of the inherent resistance and charge transfer resistance at the electrode/electrolyte interface. The Cu–HHTP NWA electrode demonstrated a high specific capacitance of 202 F g⁻¹ at a current density of 0.5 A g⁻¹, which was twice as high as those of the Cu–HHTP powder electrode. This was made possible by the composite's high porosity and strong conductivity. With a strong rate performance, the symmetric solid-state supercapacitor made with Cu–HHTP NWA electrodes had a high surface area-normalized capacitance of ∼22 μF cm⁻¹. Surprisingly, this work also showed that by manipulating the morphologies from the randomly shaped crystallite to the strongly oriented NWAs, which fully utilized their high porosity and conductivity, their electrochemical performances (e.g., rate performance and capacitance) could be improved.

Another advancement in increasing MOF conductivity was made in ref. 53, where MOFs were combined with conductive materials like graphene and conductive polymers, resulting in a novel approach for enhancing SC performance. The highest SC performance was demonstrated by Zr₆O₄(OH)₄(BPYDC)₆ (BPYDC = 2,2-bipyridine-5,5-dicarboxylate, termed nMOF-867), among the 23 distinct nanocrystalline MOFs that were examined. Its performance could be maintained for at least 10 000 cycles. Its stack and areal capacitances were 0.64 and 5.09 mF cm⁻², respectively. It was shown that MOF-801 nanocrystals, measuring approximately 100 nm, performed better than those measuring 500 nm. This was likely due to the ease with which electrolyte ions could diffuse into these tiny crystals. Liu et al.⁵⁶ reported the first preparation of NPCs using MOF-5-FA composites (FA = furfuryl alcohol) as a secondary carbon source. The as-synthesised carbon electrode in 1.0 M H₂SO₄ electrolyte demonstrated the exceptional performance of the MOF-derived NPCs for the EDLCs, providing a capacitance of 204 F g⁻¹ at a sweep rate of 5 mV s⁻¹. The Ni-based MOF electrode's structural layer structure demonstrated capacitances of 1127 and 668 F g⁻¹ at rates of 0.5 and 10 A g⁻¹, respectively, as reported by Yang et al.⁵⁷ An aqueous solution containing 6 M KOH was used to study the electrochemical behavior of the electrode, which was created by pressing a paste consisting of a 70 wt% Ni-based MOF combined with 20 wt% acetylene black and 10 wt% polytetrafluorethylene (PTFE) binders onto a stainless steel plate. Over a 3000 cycle period, the PTFE binder demonstrated over a 90% cycle lifetime and a specific capacitance of 1127 F g⁻¹ at 0.5 A g⁻¹. The greatest exposed (1 0 0) facets and layered structural characteristics were found to be highly advantageous for the kinetics of surface redox reactions and the diffusion of charged species, which explains why the material showed a satisfactory high specific capacitance and outstanding rate capability.

In a related breakthrough, Wang et al.⁵⁸ reported the creation of a flexible MOF-based supercapacitor using a two-step fabrication technique that involved coating ZIF-67 nanocrystals (∼300 nm) on a carbon cloth first and then electrochemically weaving polyaniline onto the cloth.

PANI electrochemically connected ZIF-67 had been chosen, after it had been first coated on carbon cloth; the resulting electrode was designated as PANI-ZIF-67-CC. Using polyaniline, which allowed electrons to reach the MOF surface and the open MOF pores, which made electrolyte diffusion simple, a very high areal capacitance of 2146 mF cm⁻² at 10 mV s⁻¹ was achieved. Additionally, two symmetric freestanding PANI-ZIF-67-CC electrodes were used to create a flexible solid-state SC device. After 2000 cycles, the SCC still maintained more than 80% of its initial capacitance and produced an amazing areal capacitance of 35 mF cm⁻² at a current density of 0.05 mA cm⁻².

According to Feng et al.,⁵⁹ 2D black MOFs, Cu-HAB and Ni-HAB, with an ultra-small hexaaminobenzene (HAB) linker demonstrated hexagonal pore packing with d₁₀₀ = 1.5 nm, which in turn produced extremely dense skeletons with superior capacitive behaviors for electrochemical supercapacitors that are submillimeter thick. In 1 m KOH, both HAB MOF electrodes demonstrated distinct reversible redox behaviors in addition to having sizable gravimetric capacitances of 215 F g⁻¹ for Cu-HAB and 420 F g⁻¹ for Ni-HAB. It has been determined that the main mechanism of charge storage was pseudocapacitance from ligand-based reversible redox processes, with a small contribution from EDL capacitance (<10% for Ni-HAB and <20% for Cu-HAB) obtained from moderate SSA (150–200 m² g⁻¹). Additionally, after 12 000 cycles, these HAB MOF electrodes showed good cycling stability with a capacitance retention of almost 90%.

Bi et al.⁶⁰ investigated the EDL charge storage and charging dynamics of three 2D c-MOFs (Cu-THQ, Cu-HITP, and Cu-HITN; THQ = tetrahydroxy-1,4-quinone, HITN = 2,3,8,9,14,15-hexaimino-trinaphthalene) using constant-potential molecular dynamics simulations. Experiments on the electrochemical performance of macroscale EDL capacitor devices provided support for the results of the computer models. These 2D c-MOF-based devices demonstrate unprecedentedly large specific capacitances, low cell resistances, and unprecedentedly high energy and power densities. The energy density of the supercapacitors can be further enhanced by the crystalline 2D c-MOFs since they have a wider working voltage range and a greater specific surface area, which allows for a capacity that is almost equal to the theoretical value. Additionally, Li et al., 2020⁶¹ presented a thorough investigation of a carbon material with a partial inheritance of the organized linked macroporous structures. Their research showed that when utilized as the electrode in a supercapacitor, the derivative carbon material demonstrated exceptional rate performances and improved cycle stability. These could be brought about by its enhanced diffusion, stronger structure, and decreased resistance. The technique offered significant promise for creating macro–microporous superstructures for a number of exciting energy storage device applications.

Guan et al.⁶² reported the creation of multi-shelled metal sulfide particles with a distinctive hollow structure using a MOF-engaged technique as a battery-like electrode. The Co-based MOF precursor must first be directly pyrolyzed in order to produce the onion-like Co₃O₄ particles. These multi-shelled Co₃O₄ spheres can be transformed into NiCo₂S₄ and Co₄S₃ particles, which resemble onions, by a sequential ion-change process. Due to their high capacitance and exceptional cycling stability, the complex hollow-structured NiCo₂S₄ particles exhibit enhanced characteristics as a battery-type electrode. Furthermore, an HSC device based on the combination of these onion-like NiCo₂S₄ particles and the activated carbon manifests excellent cycling stability, enhanced energy density, and power capability. The enhanced performance may largely be attributed to the complex structure and composition of the onion-like NiCo₂S₄ particles. Specifically, the complex multi-shelled structure can improve the energy density of the electrode material. Moreover, the porous and hollow structure with much higher specific surface area should also be beneficial for electrochemical applications. In addition, incorporation of Ni²⁺ cations into the cobalt sulfide could result in increased conductivity and the creation of new active sites.

2.2 MOFs for solar cells

By creating electrical charges that are free to move about in semiconductors, solar cells use the photovoltaic effect to directly convert solar energy into electrical energy and produce electricity. In contrast to nuclear power plants and coal-fired power plants, solar cells produce no hazardous emissions during the electricity-producing process, which has led to a recent surge in research into solar cells as an environmentally acceptable energy source. The photovoltaic effect, which occurs when the cell's surface is exposed to light, is the basis for how solar cells function. Common solar cells are made of semiconductor materials, which can absorb light and has a bandgap less than visible light's energy. A cell's internal electric field causes the electrical charges created by this light to move, which results in the flow of electricity. By connecting p- and n-type semiconductors, the external voltage is generated. An electric field is created between the p- and n-type semiconductors at their interface, causing band bending due to the disparity in Fermi energy. Because of the band bending of the p–n junction, electrons transported upon light irradiation to the conduction band can pass from the p-type semiconductor to the n-type semiconductor, generating an electric current that flows along the circuit and is used as energy.

The kind of materials utilized and their composition can be used to classify solar cells. The most widely used type of solar cells are the silicon-based ones, which are further classified as amorphous, hybrid, polycrystalline, and monocrystalline silicon solar cells based on the crystallinity of silicon. When compared to other types of solar cells, single-crystal silicon solar cells have the best light conversion efficiency, but they also have the highest fabrication costs. First-generation solar cells are the name given to such solar cells.⁶³

2.2.1 Dye-sensitized solar cells (DSSCs). Due to their advantages of charge separation efficiency and low cost of manufacturing, dye-sensitized solar cells (DSSCs) have drawn significant attention from researchers worldwide for more than 20 years.⁶⁴ Owing to these exceptional qualities, dye-sensitized solar cells, or DSSCs, have emerged as a viable option that might both alleviate the world's energy crisis and lead to a breakthrough in power conversion efficiency. A dye-sensitizer that may be obtained from natural resources, titanium dioxide (TiO₂), makes up the photoanode of a dye-sensitized solar cell (DSSC). In addition, the photoanode is made up of a thin layer of a conducting, transparent metal oxide that is porous and semiconducting; the most popular metal oxide for this purpose is TiO₂. A photosensitizing dye is used to sensitize the metal oxide thin layer. DSSCs can be produced using a roll-to-roll technique, which is an affordable, continuous printing approach on flexible substrates. The dye molecule absorbs photons from the sun, which results in electron injection into the semiconducting metal oxide's conduction band. As seen in Fig. 4A, the electron moves toward the counter electron CE via the outer circuit. The electrocatalyst, which is typically a thin layer of platinum, is placed as a layer on another conducting substrate to form the CE. The space between the CE and the photoanode was filled with an electrolyte solution that contained a redox pair. As DSSCs can operate well with diffused light even under overcast conditions, they can be installed in a variety of spaces, including sunroofs and windows. Due to these benefits, DSSCs have been the subject of extensive research.^65,66 Transition metal materials,⁶⁷ metal alloys,⁶⁸ carbon materials,⁶⁹ and conductive polymers⁷⁰ have been used as substitutes for the Pt counter electrode in DSSCs. Although their stability is not as good as that of metal-free electrode materials, carbon materials have a high surface area and electrical conductivity to make up for their drawbacks.

Fig. 4 (A) Schematic representation of typical dye-sensitized solar cells. (B) The use of MOFs for the photoanodes in DSSCs as the blocking layer.⁷¹ (C) Schematic illustration of the preparation of 2D MOF nanosheets and their modified electron-extraction layer for photovoltaic devices.⁷²

The application of MOFs as semiconductors has been extensively established by experimental and theoretical measurements.⁷³ Subsequent studies also validate that the semiconducting characteristics of MOFs may be tuned through metal node size adjustments, organic ligand replacements, and coordination mode adjustment between the organic and inorganic constituents. The potential of MOF-based materials as photoanodes or auxiliaries for electrode sensitization in DSSCs should be explored due to their enormous surface area and capacity to harvest light.

Li et al.⁷¹ reported the investigation of a ZIF-8-coated TiO₂ hybrid with different thicknesses, which was subsequently effectively applied in a DSSC. As a blocking layer on the photoanode, a thin layer of ZIF-8, a zinc-based zeolitic imidazolate framework, was formed on a TiO₂ surface (Fig. 4B). By altering the reaction time, the thin ZIF-8 film's thickness may be precisely regulated. By submerging the built TiO₂/ZIF-8 in the dye-containing ethanol solution, electrode sensitization is achieved. The adsorption of dyes is greatly enhanced by the thicker ZIF-8 coating layer. The interfacial charge recombination on the TiO₂ surface can be inhibited by the ZIF-8 thin layer, leading to a notably higher open circuit voltage (V_oc) than the pure DSSC without ZIF-8. It was also discovered that when ZIF-8 was present on the photoanode, the dye loading was significantly higher. With the ZIF-8 layer's assistance, the cell efficiency increased from 5.11% to 5.34% during the ideal growth period; however, when the growth time increased further, the cell efficiency decreased because of a sharp decline in the short-circuit current density (J_sc).

Lopez et al.⁷⁴ reported the development of solar cells employing the MOF thin film consisting of Al₂(bdc)₃ (bdc: p-benzene dicarboxylate) as the active component in all-solid-state DSSCs. Their investigation showed that, with an optimum thickness of 2.7 mm, Al₂(bdc)₃ containing the 1,4-dimethoxybenzene guest molecule performs better than individual components (short-circuit current density = 36 mA cm⁻², open-circuit voltage = 0.36 V, and fill factor = 40%). Additionally, materials generated from MOFs may inherit interesting properties from the related MOFs and exhibit high performance as photoanodes. With a pore size of 10 nm and a large Brunauer–Emmett–Teller surface area of 147 m² g⁻¹, Dou et al., 2016⁷⁵ created a porous TiO₂ hierarchical structure using Ti-MIL-125 as a precursor. With a 7.2% increase in efficiency over commercial TiO₂ (Degussa P25), the as-constructed TiO₂ material has been effectively employed as a photoanode for the DSSC.

2.2.2 MOFs in organic solar cells. Polymer cells composed of carbon-based materials and organic electronics are known as organic solar cells, or OSCs for short. As opposed to the expanding uses in DSSCs and PVSCs, MOFs have not yet been thoroughly investigated in OSCs, and there are currently very few research studies that address this topic. As was previously indicated, using MOFs directly as photoactive materials is very difficult. Moreover, MOF sites in OSCs are limited to interlayer engineering due to the lower semiconducting qualities of organic materials compared to their inorganic counterparts. Plastic solar cells are significantly more robust and have a lot higher coverage area than traditional solar cells, but they lack high PCEs. In contrast, organic solar cells (OSCs) have highly adjustable architectures, are flexible, lightweight, and inexpensive, and are made up of thin films. In order to increase the PCE and stability of OSCs, numerous studies have been carried out.^76–78 Furthermore, compared to silicon solar cells, organic solar cells are far thinner, providing significant material savings that are beneficial to the environment. High conductivity and charge transport mobilities are necessary for the interfacial layers, which include the hole extraction layer and electron extraction layer (EEL), which are critical components of organic solar cells (OSCs). Due to their superior electrical and optical capabilities and wide surface areas, 2D materials are employed as an addition to the interfacial layers.

By creating unique 2D tellurophene-based MOFs and exfoliating them using branched poly(ethylenimine)ethoxylate (PEIE) to create single- or few-layer MOF nanosheets, Xing et al.⁷² have presented an efficient peeling technique for MOFs. Using MOF nanosheets functionalized with PEIE as an interlayer, they were also able to improve the PCE of OSCs. According to the findings, a hybrid EEL with this kind of conductivity may outperform pristine PEIE and adjust the ETL's work function to reduce charge recombination in the resulting device. This would allow for a 15% increase in PCE when compared to the control device, as seen in Fig. 4C.⁷² Another advancement was the publication of Sasitharan et al.⁷⁹ on production of ultrathin zinc porphyrin-based MOF nanosheets (MONs).

A PCE of 5.2% was demonstrated by the OSCs using MONs as the photoactive layer, which is nearly twice as much as the reference device. Because of their structural, optical, and electrical characteristics, MONs provided a surface template for poly(3-hexylthiophene-2,5-diyl) (P₃HT) crystallization. Therefore, enhanced PCEs were obtained by lowering the grain size, increasing the hole mobility and raising the absorbance twice. These findings highlight the possibility of using tunable 2D MOF nanosheets as building blocks to raise the efficiency of various OSCs.

2.2.3 Perovskite solar cells (PSCs) with MOFs. Materials with particular crystal structures that exhibit photovoltaic (electricity from light) properties are known as perovskites. Due to their exceptional efficiency and low manufacturing costs, these materials have the potential to completely transform the solar industry. With efficiency rising quickly from reports of roughly 3% in 2009 to over 25% today, perovskite solar cells have made impressive strides in recent years.⁸⁰ Perovskites have the general structural formula ABX₃, where the X site is a halide (I⁻, Br⁻, Cl⁻, or their mixtures; SCN⁻), the B site is a divalent metal (Pb², Sn²⁺, Bi²⁺, Ge²⁺), and the A site is an organic or inorganic cation (Cs⁺, Rb⁺, methylammonium Cs⁺, Rb⁺, methylammonium (MA) CH₃NH₃, formamidinium (FA); CH₂(NH₂)₂⁺, guanidinium). Perovskite materials have several notable optoelectronic characteristics, including the capacity to transport ambipolar charges, strong and broad light absorption, extended exciting diffusion length, low cost, and solution processability. Relevant research projects are emerging because PSCs show promise for challenging the current inorganic-based photovoltaic approaches.^81,82 Despite the rapid rise in efficiency of perovskite solar cells (Fig. 5A), several obstacles need to be overcome before this technology can be competitively adopted in the commercial sector. The produced perovskite film typically has many flaws and grain boundaries because of the polycrystalline nature restricted by the solution-based fabrication procedures, which will compromise the stability of the devices' performance. Therefore, when exposed to moisture, oxygen, heat, and light, PSCs are susceptible to degradation.^83,84

Fig. 5 (A) Schematic of where MOFs can be used in perovskite solar cells (PSCs), (B) schematic of perovskite solar cells with (a) n–i–p mesoscopic, (b) n–i–p planar, (c) p–i–n planar, and (d) p–i–n mesoscopic structures,⁸⁵ and (C) (a) the transmission of TiO₂ and nTi-MOF. (b) The energy level diagram.⁸⁶

Perovskites provide excellent light-absorbing properties such as minimal recombination losses, easy bandgap tunability, long charge carrier diffusion lengths, and inexpensive manufacturing. An electron transport layer (ETL), a hole transport layer (HTL), a light absorber (a perovskite layer), a conductive substrate (either indium tin oxide (ITO) or fluorine-doped tin oxide (FTO)), and a metal electrode make up a PSC. As illustrated in (Fig. 5B), PSCs are mainly classified into mesoporous or planar structures, which are further subdivided into conventional (n–i–p) and inverted (p–i–n) structures.⁸⁵ Vinogradov et al.⁸⁷ reported the insertion of a Ti-based MOF in a standard n–i–p PSC by depositing MIL-125 onto the surface of TiO₂ nanoproducts via a single-step hydrothermal synthesis. Their research verified that micro-mesoporous MOFs can strengthen the interfacial contact between the perovskite films and promote perovskite crystallization, thereby suppressing interfacial charge recombination and improving device stability and performance. In comparison to the n–i–p structure, the p–i–n structure lowers the process temperature, which is advantageous; nevertheless, the somewhat lower efficiencies are a drawback. Moreover, Lee et al.⁸⁸ added insulating porous Zr-MOFs, MOF-808 and UiO-66 at the NiO_x/perovskite interface to give the perovskite materials another scaffold to develop on. These implanted MOFs support the crystallinity of the perovskite film formed on top, much like the advantages of the microporous scaffold that were previously discussed (Fig. 5C).

Li et al.⁸⁹ have doped an indium-based MOF, in addition to employing a MOF in the ETL, [In₂(phen)₃Cl₆]·CH₃CN·2H₂O (In₂), into the HTL of spiro-OMeTAD. The authors first showed that a device can provide more photo-response by including In₂ in the HTL. Furthermore, In₂ has a strong UV absorption due to its large E_g, even though its emission was in the visible light spectrum. In a subsequent evolution, the HTL's cube-shaped In₂ crystals might serve as the light-scattering center, generating many reflections that lengthen the light path inside the apparatus. This adds to the increase in absorption in the 500–700 nm range. With the use of rapid heat transfer, Ryu et al.⁸⁶ carried out a thorough investigation to create nanocrystalline Ti-based MOF (nTi-MOF) NPs on ITO glass. This resulted in the successful production of an nTi-MOF as opposed to a Ti-MOF (MIL-125). Their overall electrical structures differed slightly, despite the fact that the chemical state of Ti in an nTi-MOF is comparable to Ti in TiO₂. Because of its larger current energy level, an nTi-MOF has a greater transmittance than TiO₂. Its ability to function as an ETL in a device is indicated by the fact that its energy level, as demonstrated, agrees well with that of the perovskite and ITO. Rather than altering the mesoporous TiO₂ scaffold, Shen et al.⁹⁰ inserted a type of insulating ZIF-8 on top of the mesoporous TiO₂ (mp-TiO₂) layer. ZIF-8 was added to the mp-TiO₂ layer, roughening its surface in a way that encourages the crystallinity of the perovskite layer formed on top, similar to the effects reported in other investigations.

2.3 MOFs for batteries

Through an electrochemical process known as oxidation–reduction (redox), a battery directly transforms the chemical energy found in its active components into electric energy. For the purpose of powering electric vehicles, portable gadgets, grid energy storage, and renewable energy devices, in particular, dependable and efficient energy storage devices are key components of the sustainable use of renewable energy sources.⁹¹ The electrode materials are a crucial determinant of the rechargeable battery's performance. Consequently, a number of investigations have been carried out to create sophisticated electrode materials that have outstanding cycling stability, a high specific capacity, and good rate performance. Due to their unique structural and electronic properties, MOFs a novel class of porous crystalline coordination polymers have garnered significant attention in the field of batteries in recent decades. These features make them useful for investigating advanced batteries, such as zinc ion batteries (ZIBs), lithium-ion batteries (LIBs), lithium–sulfur batteries (Li–S), lithium–oxygen batteries (Li–O₂), sodium-ion batteries (SIBs), and sodium iodine (Na–I₂) batteries.^92–94 Consequently, MOF-based materials show promise as electrode materials for the upcoming generation of long-cycle, high-capacity rechargeable batteries.

2.3.1 Lithium-ion batteries (LIBs). Lithium-ion batteries (LIBs) are the most significant and widely utilized energy storage technology in our daily lives; they are vital components of computers, electrical cars, and portable electronics. One of the main factors affecting LIBs' overall performance is the electrode material. Therefore, the creation and manufacturing of novel materials or materials with distinctive architectures, like LIB electrodes, have received enormous attention. Still, they are unable to fulfill today's greater energy and power demands despite their extended lifespan and simplicity of integration into portable devices.⁴⁴ Aside from graphite, which is the material used in commercial negative electrodes, a number of nanostructured materials, including carbon nanotubes, graphene composites, transition metal oxides, etc., have been thoroughly studied recently for potential use in LIBs. The low capacity and energy density of graphite is the driving force behind the search for other suitable materials. Due to their large surface area and persistent porosity for Li⁺ ion storage and migration during charge and discharge processes, pristine MOFs have been used as replacements to traditional graphite anode materials. The MOF IN LIBs comprise the following: (1) rapid charging rates are caused by the rapid electron transport to the redox-active sites, which is facilitated by the high electrical conductivity of 2D c-MOFs.⁹⁵ (2) Li ions and other anions are made easier to diffuse and inject by the regular channels of 2D c-MOFs. (3) Stiff and elongated structures ensure superior stability during Li insertion/extraction, ensuring electrodes' electrochemical longevity.⁹⁶ (4) To obtain a better specific capacity and enough energy density, both organic linkers and transition metal ions can function as the redox active sites.⁹⁷

A conductive bis(diimino)nickel framework (Ni-HITP) is used as a high-capacity LIB cathode material.⁹⁸ The distinct energy storage mechanism that both the cation (Li⁺) and anion (PF₆⁻) provided as electron carriers to charge and discharge electrochemical energy was made clear by their discovery. Furthermore, due to the charge distribution between the metal ions and non-innocent ligands, Ni-HITP possesses several redox states. Thanks to its unique redox nature and satisfactory electronic conductivity, Ni-HITP demonstrated a faradaic reaction-driven energy storage function that demonstrated stable cycling performance for up to 300 cycles, an exciting specific capacity of 155 mA h g⁻¹, and a high specific energy density of 434 W h kg⁻¹ at a current density of 10 mA g⁻¹. A redox-active 2D copper benzoquinoid MOF (Cu-THQ) was created by Jiang et al.⁹⁹ as the cathode for rechargeable lithium-ion batteries. According to their findings, plentiful porosity and intrinsic redox properties of Cu-THQ allowed for the achievement of a maximum capacity and specific energy density of 387 mA h g⁻¹ and 775 W h kg⁻¹, respectively. In a manner similar to Ni-HITP, there, Li⁺ and PF-6 were both embedded in the pores of 2D Cu-THQ frameworks as part of the energy storage process. Moreover, research on a variety of morphologies, including hollow microspherical, pillar-layer, lamellar, and shell-like morphologies, was reported in ref. 100 and 101. Their work is helpful for accommodating volume variation, electrolyte penetration, and lithium-ion transportation during the charge/discharge processes. The study conducted by Zou et al.¹⁰² on a multipodal composite of NiO/Ni/graphene derived from MOFs in the form of a hierarchical hollow ball-in-ball nanostructure showed that the material had a high reversible specific capacity of 1144 mA h g⁻¹ and good cyclability, with nearly 100% capacity retention after 1000 cycles. Specifically, a NiO/Ni/graphene electrode has been used to create a sodium-ion battery that exhibits exceptional rate capability and great cyclability at a current density of 2 A g⁻¹, with a capacity of 207 mA h g⁻¹.

Qi et al.¹⁰³ conducted a thorough investigation that demonstrates how ZrO₂ produced from UiO-66 effectively acted as a protective layer to enhance the rate capability of the LiCoO₂ cathode material. After 100 cycles, the resulting ZrO₂@LiCoO₂ hybrid cathode produced a reversible capacity of 148 mA h g⁻¹ at a high current density of 2325 mA h g⁻¹. By contrast, the capacity of pure LiCoO₂ dropped quickly to 20 mA h g⁻¹. It is noteworthy that after 100 cycles at 55 °C, the hybrid cathode also showed a reversible capacity of 132 mA h g⁻¹, indicating remarkable thermal stability. The improved structural stability resulting from the ZrO₂ coating, which successfully reduced the volume change of LiCoO₂, may be responsible for the hybrid cathode's increased performance. Recently, Ziebel et al.⁹⁵ reported the design of two iron semiquinoid frameworks based on a deprotonated 2,5-dichloro-3,6-dihydroxybenzoquinone (Cl₂dhbqn⁻) ligand, namely (H₂NMe₂)₂Fe₂(Cl₂dhbq)₃ (MOF 1) and (H₂NMe₂)₄Fe₃-(Cl₂dhbq)₃(SO₄)₂ (MOF 2). Their findings showed a notable difference in electrical conductivity of 2.6 × 10⁻³ for MOF 1 and 8.4 × 10⁻⁵ S cm⁻¹ for MOF 2, respectively. MOF 2 demonstrated a rapid capacitance degradation with larger charging rates, along with a reasonable discharge capacity of 165 mA h g⁻¹ (90% of the theoretical value) in 0.1 m LiBF₄ propylene carbonate electrolyte at a modest charging rate of 10 mA g⁻¹. On the other hand, MOF 1 retained a comparatively high capacity of 141 mA h g⁻¹ (72% retention) even at a charging rate of up to 150 mA g⁻¹, nearly 100% of its theoretical capacity of 195 mA h g⁻¹ at 20 mA g⁻¹. Meanwhile, MOF 1 achieved a coulombic efficiency higher than 100% and a benchmark-specific energy density of 533 W h kg⁻¹ at 20 mA g⁻¹.

Yang et al.¹⁰⁴ developed porous ZnCo₂O₄ by carefully calcining Zn-doped MOF-74 at 400 °C. The substantial synergistic impact between Zn and Co has resulted in improved specific capacity, cycling stability, and rate capability of the developed porous nanostructured ZnCo₂O₄ when compared to Co₃O₄. It was discovered that adding a doping step to the MOF synthesis could result in metal/metal oxide complexes with improved characteristics during the pyrolysis process.

They created Co-MOFs using various organic linkers (1,4,5,8-naphthalene tetracarboxylic dianhydride (NTCDA) and perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA)) after realizing that Co₃O₄ had limited cycling stability and rate capability. They then treated the Co-MOFs with an organic amine solution before using calcination as a novel method to overcome these limitations, as reported by Su et al.¹⁰⁵ The material that was developed demonstrated an improved capacity for lithium storage (1370 mA h g⁻¹ reversible capacity at a mass normalized current of 100 mA g⁻¹) and this demonstrated that the synthesis process was possible for the fabrication and manipulation of metal oxides/morphology. In addition, depositing metal oxides on electro-conductive substrates can enhance Co₃O₄'s electrical conductivity. Hu et al.¹⁰⁶ carried out an experiment using pyrolyzed Se-doped ZIF-67 to produce CoSe nanoparticles enclosed in a hollow carbon shell. The as-synthesised CoSe@C demonstrated outstanding cyclability by maintaining 91.6% discharge capacity, benefiting from the structural protection and charge transport channels supplied by the hollow carbon matrix. In order to increase the theoretical capacity, Zhang et al.⁹⁷ used a newly developed MOF of Cu(2,7-AQDC) (2,7-H₂AQDC = 2,7-anthraquinonedicarboxylate), which has separate redox activities at both Cu₂(Ac)₄ nodes and anthraquinone ligands. This resulted in a high initial capacity (147 mA h g⁻¹) (Fig. 6). Regrettably, there was a small capacity decrease (≈42 mA h g⁻¹) following a reversible capacity of ∼105 mA h g⁻¹ in 50 cycles. The electroactive MOF cathodes were also built using quinone-type ligands as well as a few additional ligands, such as 1,2,4,5-tetraaminobenzene,¹⁰⁷ tetrathiafulvalene tetracarboxylic acid,¹⁰⁸ and tricarboxytriphenyl amine.¹⁰⁹ Guo et al.¹¹⁰ used a Cu–Ni bimetallic MOF as the precursor to create a binary metal oxide hybrid microsphere with multiple shells (CuO@NiO). Due to its distinct structure and molecular makeup, the CuO@NiO hybrid microsphere demonstrated a reversible capacity of 1000 mA h g⁻¹ following 200 cycles, surpassing the theoretical capacities of CuO (674 mA h g⁻¹) and NiO (718 mA h g⁻¹). In order to achieve electrochemical Li storage, Ferey et al.¹¹¹ studied the usage of the MIL-53(Fe) cathode material to exploit mixed-valence states of metals during discharge and charge. Due to the low density of the material and the restricted amount of Li ions introduced, the cathode exhibits a gravimetric capacity of 75 mA h g⁻¹ and a volumetric capacity of 140 mA h L⁻¹. MOFs have been employed as precursors for the synthesis of metal oxides, transition metal oxides, metal/metal oxides, and metal oxides/carbon composites for LIB negative electrode materials in addition to carbon materials. Yang et al.¹¹² reported the production of porous carbon-coated ZnO quantum dots by the controlled pyrolysis of Zn₄O-MOF-5. With a mass-normalized current of 75 mA g⁻¹, the as-synthesised material demonstrated a high reversible capacity of 1200 mA h g⁻¹.

Fig. 6 Structure and performances of Cu(2,7-AQDC) in a battery. (A) The single Kagome layer of Cu(2,7-AQDC). (B) The indication picture of the glided adjacent layers. (C) Charge–discharge profiles of the 10 wt% MOF battery. (D) Cycle performances of the battery for 50 cycles. Reproduced with permission.⁹⁷

2.3.2 Lithium–sulfur batteries (Li–S). Lithium–sulfur (Li–S) batteries are thought to be the most significant technologies for the upcoming generation of electrochemical energy storage. Li–S batteries have the potential to produce high energy densities of up to 2600 W h kg⁻¹ and specific capacities of up to 1670 mA h g⁻¹.¹¹³ Their high energy density and lightweight design have drawn more and more attention in recent years. Li–S batteries exhibit additional benefits over lithium-ion batteries (LIBs), including reduced fabrication costs, environmental friendliness, and widespread availability of the electrode-active sulfur material. These benefits offset some of the apparent benefits of sulfur, such as low cost, large natural abundance, and non-toxicity. Additionally, Li–S batteries outperform the existing LIBs in terms of economic viability.¹¹⁴ On the other hand, before Li–S batteries are widely used in electronic products and hybrid cars, there are a few issues that need to be resolved. The well-known “shuttle effect”, which results from the dissolution of intermediate products (i.e., polysulfides) in the organic electrolyte and causes electrode deactivation and poor cycling performance, leads to the following challenges, including low electrode utilization, poor high-rate performance, and rapid capacity fade. To address these problems, a Ni-MOF, {{Ni₆(BTB)₄(BP)₃}}_n (BTB = benzene-1,3,5-tribenzoate and BP = 4,40-bipyridyl), was chosen to confine sulfur.¹¹⁵ The typical charge and discharge behaviors of the Ni-MOF/S electrode mentioned above were revealed by galvanostatic charge and discharge tests. After 100 cycles, the Ni-MOF/S composite has a high-capacity retention of 89% at a current density of 0.1C (168 mA g⁻¹). The dissolution and shuttle effects of polysulfides are inhibited by the hierarchical porous structure of the Ni-MOF and the strong contacts between Ni metals and polysulfides. The goal of this study is to improve the cyclability of Li–S batteries by designing Li–S electrodes that include sulfur within the pores of mesoporous MOFs.

However, limited sulfur consumption and a lower performance rate are caused by MOFs' insulating characteristics. An ionic sieve called the HKUST-1@GO separator functions as a buffer against shuttling in Li–S batteries by selectively allowing Li⁺ ion transport while inhibiting polysulfide migration. The HKUST-1@GO hybrid Li–S battery, used as an ionic sieve membrane for Li–S batteries, was reported by Bai et al.¹¹⁶ to have low capacity-fading rates of roughly 0.019% per cycle across 1500 cycles. Furthermore, Baumann et al.¹¹⁷ have demonstrated how to optimize the size of nanocrystals in HKUST-1. This is because the surface of the particle, which has exposed Cu sites, increases the number of sulfur binding sites on the external surface, reducing the particle size and improving the capture of escaping polysulfides from the pores, reducing their diffusion toward the anode. Higher sulfur loading with larger nanocrystals, as illustrated in (Fig. 7), is nevertheless not advantageous to battery performance in this scenario, despite the presence of capture sites on the surface. Through cooperation, these elements can get past the inherent mechanical brittleness and create a strong ionic sieve membrane that effectively suppresses the shuttle effect. With the MOF@GO hybrid-based separator, a low capacity decay rate over 1500 cycles is guaranteed in a Li–S battery with the cathode comprising a sulfur-functionalized mesoporous carbon material (around 70% sulfur content). The HKUST-1 modified separator did not degrade after cycling, according to He et al.¹¹⁸ They reasoned that since almost all of the Cu sites were occupied by oxygen groups, the quantity of Cu–S forms would be limited. Li et al.¹¹⁹ disclosed the capacity reduction resulting from irreversible Cu–S interactions in an additional HKUST-1 modified separator. Liu et al.¹²⁰ produced a 3D monolith by inserting 20–100 nm HKUST-1 agglomerates or 200–400 nm ZIF-67 nanoparticles in a conductive polymer hydrogel. The polymer hydrogel was able to permeate both kinds of nanodomains, and their research showed that the MOFs' aperture openings ultimately determined the performance. Compared to ZIF-67, which has a tiny pore opening of 0.34 nm, HKUST-1, with its large pore opening of 0.90 nm, was more suited to encapsulate S8 (0.64 nm) and contain long chain polysulfides (>0.4 nm).

Fig. 7 (A) Activation and sulfur loading of HKUST-1 with potential binding sites at the Cu paddlewheel. (B) Electrolyte samples of 0.16 mm and 5.9 mm CuBTC, and S/C electrodes during a galvanostatic discharge of C/20. Yellow discoloration indicates the amount of leached polysulfide. (C) Coin cell performance of 3 cells as a function of particle size, left: the average maximum capacity and right: the average capacity retention over 20 cycles.¹¹⁷

Xu et al.¹²¹ conducted an experiment, which demonstrated the synthesis of sulfur-encapsulated hierarchically porous carbon nanoplates (HPCN) by one-step pyrolysis of MOF-5. The nanoplates had a huge pore volume of 1.18 cm³ g⁻¹, a high specific surface area of 1645 m² g⁻¹, and an average thickness of about 50 nm. High specific capacity and superior cycle performance are exhibited by sulfur contained in HPCN. With a reversible capacity of 730 mA h g⁻¹, the hybrid cathode demonstrated exceptional cycling performance after 50 cycles at a current of 0.5C, with charging and discharging at 837.5 mA h g⁻¹. MOFs themselves, with their highly organized pores and tunable porosity, can be used as ionic sieves in membrane separators to lessen the shuttling impact of polysulfides, in addition to the MOF-derived materials that are employed as the sulfur hosts. Wu et al.¹²² reported that the sulfur concentration in the hybrid and the type of electrolyte have an impact on the performance of MOF-derived carbon–sulfur hybrid cathodes in Li–S batteries. Furthermore, Liu et al.¹²³ constructed lithium–selenium (Li–Se) batteries using porous carbon spheres generated from a MOF. It is noteworthy that at ambient temperature, Se has a substantially higher electrical conductivity than S (1 × 10⁻⁵vs. 5 × 10⁻³⁰ S cm⁻¹), which may accelerate the cathode material's rate of electron transportation. Cerium(IV) UiO-66 (Ce-MOF-1) and MOF-808 (Ce-MOF-2) nanoparticles (∼180 nm) produced on carbon nanotubes (CNT) were used in a study conducted by Hong et al.¹²⁴ to explore the possibility of open-metal-site catalysis of long-chain polysulfide conversion. A high sulfur loading of 6.0 mg cm² and an initial specific capacity of 993.5 mA h g⁻¹ at 0.1C were achieved by the MOF-808-based hybrid, which consumed Li₂S₆ considerably better than the CNT or UiO-66-based hybrid, which had no unsaturated coordination sites at the Ce(IV) nodes.

Wu et al.¹²⁵ have reported the development of MOF-derived microporous carbon polyhedron (MCP) encapsulated PAN nanofibers as an effective sulfur immobilizer for Li–S batteries. This is because sulfur can be uniformly dispersed inside the nanofibers and in the micropores of MCPs. A combination of MCPs and PAN was electrospun to achieve the encapsulation procedure, which was then achieved by loading sulfur through a two-step reaction at 155 °C and 300 °C, respectively. The enhanced S/MCPs–PAN composite, which had 52 wt% sulfur, had a large reversible capacity of about 790 mA h g⁻¹ at first, which only slightly decreased to 789.7 mA h g⁻¹ in the second cycle. After 200 cycles, the discharge capacity remained at 666.2 mA h g⁻¹, demonstrating both good capacity retention (84.4%) and high sulfur utilization (90.7%). According to Zhou et al.,¹²⁶ the particle size reduces with increasing sulfur utilization in a series of nanoscale ZIF-8 frameworks, with less than 20 nm providing a capacity of over 950 mA h g⁻¹ at 0.5C. However, the best size for cycling stability (75% over 250 cycles at 0.5C) was ∼200 nm.

Furthermore, Hong et al.¹²⁷ found that 100 nm-sized nanoparticles outperformed 200 nm-, 500 nm-, and 1 mm-sized particles for a Cu-MOF that used both ligand Lewis acid sites and node Cu sites for polysulfide interaction. Electrochemical reactions only take place on the particle surface, where electrons and Li ions are accessible due to the conductivity constraints of the MOFs in both scenarios.

2.3.3 Sodium-ion batteries (SIB). Even though Li-ion batteries are the industry leaders in the energy market for portable electronics, they still have a lot of drawbacks, including low power density, safety concerns, and a lack of lithium resources in the Earth's crust. In addition, the price of Li-ion batteries is very high due to the limited availability of lithium (20 ppm) on Earth.¹²⁸ Furthermore, as noted by Hwang et al.,¹²⁹ large-scale electrochemical energy storage systems (EESs) may also be hampered by LIBs' low power density and lack of safety. Nevertheless, because of their lower energy density, short lifespan, and higher ionic properties to many electrode materials that are appropriate for LIBs, SIBs are no longer able to meet their needs.¹³⁰ Due to these constraints, researchers looked for appropriate electrodes for sodium-ion batteries (SIBs) that exhibit electrochemical behavior similar to that of lithium-ion batteries (LIBs). These electrodes were thought to be a viable substitute for LIBs because of the low electrochemical cell potential (−2.71 V vs. SHE) and abundance of sodium.¹³¹

While a number of materials, including Prussian blue,¹³² transition metal oxides,¹²⁸ and small organic molecules,¹³³ have been extensively employed as cathode materials for SIBs, it is still difficult to find promising anode materials with a long cycle life, high capacity, and high energy density. In this regard, MOF-derived nanostructures offer a chance to develop SIB anodes with superior electrochemical performance in a sustainable manner. For instance, numerous published studies on Na_1.40MnFe(CN)₆Na_1.72 MnFe(CN)₆ and KMFe(CN)₆ (M = Mn/Fe/Co/Ni/Cu/Zn) have demonstrated that the insertion and extraction of Na⁺ would cause a reversible reduction or oxidation of the metal ions.¹³⁴ Zhang et al.¹³⁵ developed a hollow porous CuO/Cu₂O octahedron and evaluated it as an anode for SIBs using a different alternative Cu-BTC MOF as a template. The results of the studies showed that calcination temperature was critical to the CuO/Cu₂O composite's electrochemical performance, and the optimized CuO/Cu₂O-300 had good rate capability, high capacity retention, and a long lifespan. A study conducted by Yue et al.¹³⁶ used the pore size and PBA shape to determine the rate capability in SIBs. The analysis of the KNiFe(CN)₆/t series (t = 2, 18, and 72 h) revealed that the presence of big pores would make sodium ion transport significantly easier.

Park et al.¹³⁷ reported the integration of a 2D c-MOF of Co–HAB (hexaaminobenzene) into high-power SIBs (Fig. 8(a) and (b)). HAB is a good candidate for primary design concepts for high-performance electrode materials since it can theoretically undergo a redox reaction involving up to six electrons and has the largest concentrations of redox centers. Co–HAB electrodes provided a specific capacity of 291 mA h g⁻¹ at 50 mA g⁻¹ and continued to maintain a capacity of 226 mA h g⁻¹ at a higher current density of 500 mA g⁻¹ after over 50 cycles with a coulombic efficiency of nearly 100% (Fig. 8(c) and (d)). Zhang et al.¹³⁸ reported the production of a porous CoFe₂O₄ nanocube from the CoFe-PBA (Prussian blue) precursor. For a current density of 50 mA g⁻¹, the sample showed a capacity of 394 mA h g⁻¹ and a capacity retention of 91.4%. Upon 500 cycles, a maximum capacity of 152.6 mA h g⁻¹ was retained, despite the high current density of 2.5 A g⁻¹. In order to create MnFe₂O₄ hollow microboxes, Guo et al.¹³⁹ additionally utilized MgFe-PBA as a precursor. Examined as an anode for SIBs, the as-prepared MnFe₂O₄ demonstrated good rate capability and cycle stability. With a theoretical specific capacity of 312 mA h g⁻¹, the Co–HAB was able to store three electrons per HAB unit in addition to three sodium ions, indicating that the redox-active sites of the HAB had been nearly fully used. Furthermore, Kaneti et al.¹⁴⁰ reported the synthesis of a Ni-doped Co/CoO/N-doped carbon (NC) hybrid employing bimetallic Ni-Co-ZIF as the precursor. The resulting Ni-doped Co/CoO/NC hybrid is very porous and has a specific surface area of 552 m² g⁻¹. As an electrode for SIBs, this hybrid may provide a discharge capacity of 218 mA h g⁻¹ at a high current density of 500 mA g⁻¹ while maintaining strong cycle stability and great rate performance.

Fig. 8 Electrochemical performance of the Co–HAB electrode for SIBs. (a) Chemical structure of Co–HAB. (b) A space-filling diagram of Co–HAB and the size comparison of the pore with Na⁺, solvated Na⁺ and PF₆@ions. Top: proposed three-electron reversible reaction of the HAB ligand in Co–HAB. (c) Cycling stability test of the Co–HAB electrode. (d) Comparison of areal capacity of Co–HAB with representative reported anode materials for SIBs.¹³⁷

Another breakthrough by Zou et al.¹⁴¹ employed MOF-5(Zn₄O(OOCC₆H₄COO)₃) as a precursor to create porous carbon in the form of a cube. The porous carbon that was obtained demonstrated excellent overall electrochemical performance when used as a SIB anode. A high current density of 3.2 A g⁻¹ was maintained for 5000 cycles, and a capacity of 100 mA h g⁻¹ was achieved after 100 cycles at a current density of 100 mA g⁻¹. Furthermore, MOF-derived materials such as Co₃O₄@nitrogen-doped carbon, CoP@C, CoSe/C, and TiO₂@C have benefited from the integration of metal compound components with carbons and have functioned as efficient anode materials for the SIBs.^142,143 Considering the aforementioned factors, MOFs and their derivatives are suitable electrode materials for high-power SIBs that show great promise and possess exceptional cycle stability and ultrafast storage capability.

2.3.4 Lithium–oxygen batteries. As one of the most researched types of metal–air batteries, the Li–O₂ battery is made up of Li metal as the negative electrode, i.e. the anode, reduced oxygen as the cathode active mass, and an electrolyte solution containing Li⁺. The devices are made up of a cathode, which is a composite porous matrix with electronic conductivity that allows oxygen gas and Li ions in the electrolyte solution phase to interact electrochemically. As schematically shown in Fig. 9A, it primarily operates with the deposition/dissolution of lithium metal at the anode and an oxygen reduction reaction (ORR)/oxygen evolution reaction (OER) at the cathode.¹⁴⁴ As opposed to Li–ion and Li–S batteries, which store just anode active material, the porous cathode of Li–ion batteries leverages atmospheric oxygen to produce a high energy density. Anode Li yields a high specific capacity of 3842 mA h g⁻¹ and a specific energy of 40.1 MJ kg⁻¹, which is comparable to gasoline (46.8 MJ kg⁻¹). Even at a less capacity, a robust Li–O₂ battery can supply enough power for automobiles or grid backup systems based on solar energy storage. Investigations are presently being conducted on four distinct categories of Li–O₂ systems: aprotic, aqueous, hybrid, and solid-state batteries. Regarding the types of electrolytes involved, the four types differ from one another. Subsequently, the latter ascertains the particular electrochemical processes involved in energy retention and release. Tan et al.¹⁴⁵ provided a schematic example of these four battery types in Fig. 9B. Poor cathode and electrolyte performances are the primary bottlenecks preventing Li–O₂ batteries from being commercially viable.

Fig. 9 (A) Schematic representation of a Li–O₂ battery. (B) Schematic configuration of all types of Li–O₂ cells.¹⁴⁵

The first MOF-based Li–O₂ batteries were developed by Wu et al.¹⁴⁶ A number of MOFs were carefully chosen to correlate the relationship between the structure and performance, including MOF-5, HKUST-1, and M-MOF-74 (M = Mg, Mn, and Co). According to Wu et al.,¹⁴⁶ MOF-74's accessible metal sites may result in a significant oxygen enrichment of frameworks, facilitating the Li–O₂ reaction and enhancing the performance of MOF-74-based Li–O₂ batteries. Consequently, at room temperature under 1 atm (O₂), Mn-MOF-74@carbon-black exhibits the most notable discharge capacity of 9420 mA h g⁻¹ among all cells, with an applied current density of 50 mA g⁻¹ and operating voltages of 2.6–2.7 V. The enhanced stability and efficiency in the latter case should be attributed to the synergistic effects of the bimetallic sites, which employ the bimetallic MOF, MnCo-MOF-74, as the cathode. Mu et al.¹⁴⁷ designed a soluble MOF based on heme as an electrolyte additive for Li–O₂. The MOF was synthesized using a surfactant-assisted technique. Specifically, metal nodes function as structural building blocks, heme-like TCPP(Fe) ligands operate as catalytic active sites, and polyvinylpyrrolidone (PVP) is used as a surfactant to regulate the formation of MOF crystals. According to McCloskey et al.¹⁴⁸ paper on fundamental chemistry, the discharge product during the recharging of Li–O₂ electrolytes is comparatively more stable. This can be attributed to the ether solvents' increased cathodic stability and resistance to nucleophilic attack.

Another breakthrough was reported by Liu et al.¹⁴⁹ for a Li–O₂ cell based on super-concentrated salt/DMSO that showed exceptional stability and reversibility even with an exposed Li anode. Additionally, Yuan et al.¹⁵⁰ conducted an examination of the ORR and OER performances of 2D nanosheets. In comparison to the comparable 3D Mn-MOF, the 2D Mn-MOF nanosheets (5.30 nm thickness) showed greater round trip efficiency (66.7% vs. 63.2%), lower overpotential (1.34 V vs. 1.57 V at 200 mA h g⁻¹), and higher electrochemical activity (1.66 V vs. 1.57 V at 200 mA h g⁻¹). The increased catalytic performance of the Mn–O sites was facilitated by a higher percentage of unsaturated metal sites on the surface and the edges as well as better diffusion brought about by the nanoscale 2D structure. This resulted in initial discharge capacities of 9464 mA h g⁻¹ and 1000 mA h g⁻¹ after 200 cycles at 100 mA g⁻¹. Nonetheless, Liu et al.¹⁴⁹ found that even with an exposed Li anode, a super-concentrated salt/DMSO-based Li–O₂ cell demonstrated outstanding stability and reversibility. The superior performance of the super-concentrated solutions can be ascribed to the presence of solely TFSI-Li⁺(DMSO)₃ complexes and the absence of free DMSO solvent molecules, as DMSO molecules are unstable to O₂-attack and corrosive to the Li metal anode.

Yan and colleagues examined the impact of decreasing crystal size of Co-MOF-74 rods with thicknesses of 1400 and 800 nm as well as nanofibers with a thickness of 20 nm.¹⁵¹ The enhanced performance of the Co-MOF-74 nanofiber is ascribed to the shorter ion transfer diffusion lengths and easier accessibility of the active Co sites. The existence of more defect sites in the nanofiber MOF, on the other hand, may have been caused by the employment of a size modulator during synthesis, which would account for the almost 2.5-fold increase in the initial specific capacity. Read¹⁵² reported the usage of ethers as electrolyte solvents. Utilizing DME (dimethoxyethane) and DOL (1,3-dioxolane), they reported a Li–O₂ cell with good stability and exceptional rate capability. These characteristics are ascribed to the low viscosity and high oxygen solubility, which promote oxygen transport in the Li–O₂ cell.

2.3.5 Zn-ion batteries (ZIBs). In society, rechargeable ZIBs are used for a variety of purposes. Zn-based batteries come in various varieties. According to Mao et al.,¹⁵³ Zn–air batteries (ZABs) and Zn–ion batteries (ZIBs) are further subdivided into Ni–Zn batteries (NZBs) and Zn–Mn batteries (ZMBs). The cathode materials used in these various ZIB types differ, further defining their unique characteristics.

According to Yuan et al.,¹⁵⁴ zinc-based batteries are predominantly used as primary batteries. Due to issues with recycling and disposal following usage, there has been significant resource waste and degradation of the environment.¹⁵⁵ As a result, the creation of an ideal anode made of zinc that is inexpensive, nontoxic, nonflammable, and highly water compatible is crucial. In addition to using an aqueous electrolyte, Fang et al.¹⁵⁶ reported rechargeable ZIBs with improved safety, cycle life, and accessibility, high current discharge performance¹⁵⁷ and greater ionic conductivity (10@1 to 6 S cm⁻¹).¹⁵⁸

Although the overall construction of ZMBs is similar to that of NZBs, technically speaking, ZMBs are one of the ZIBs; the cathode material is typically MnO₂. However, ZMBs are frequently discussed separately due to the excellent properties of the manganese-based materials that other positive electrode materials lack.^153,159 Given their ZIB status, ZMBs inherit all of the ZIB's benefits by default, including low reduction potential, high specific capacity, stability, safety, and so forth.¹⁶⁰ Furthermore, low toxicity, low cost, and an abundance of reserves are the benefits of using manganese oxide as the positive electrode. Mao et al.,¹⁵³ explained that Zn air batteries (ZABs) and Zn–ion batteries are further, subdivided into Ni–Zn batteries (NZBs) and Zn–Mn batteries.

In contrast to standard commercial lithium-ion batteries, ZMBs do not have an exceptional specific capacity, and manganese oxide exhibits a low electrical conductivity and a slow rate of ion diffusion.¹⁶¹ Furthermore, when manganese oxide is used, it deforms in certain ways and collapses, dissolving manganese in the electrolyte. These drawbacks have an impact on ZMB usage going forward as well.¹⁶²

Additionally, all of these various ZIB variants use zinc metal anodes. Zinc anodes frequently provide a lot of benefits. (1) According to Li et al.,¹⁶³ they are reasonably priced and very simple to obtain. (2) Because of their low reduction potential, ZIBs' open-circuit voltage is raised, assuring their stability and safety. (3) Compared to other reactive metals, zinc is more stable and less reactive. (4) According to Sun et al.,¹⁶⁴ zinc anodes have a large volume capacity and a very high theoretical specific capacity. (5) ZIBs are highly environmentally friendly and typically do not release any poisonous or damaging compounds when used.

ZIBs have great qualities, but there are a lot of practical challenges with them. During use, zinc anodes can generate zinc dendrites, which can puncture the septum and cause the ZIBs to short circuit.¹⁶⁵ Additionally, the ZIBs' ion transport capacity needs to be enhanced.

Using a 3.0 M aqueous solution of Zn(CF₃SO₃)₃ as the electrolyte (Fig. 10), Nam et al.¹⁵⁷ created a 2D c-MOF, Cu₃(HHTP)₂, as a cathode material for rechargeable ZIBs. At 50 mA g⁻¹, Cu₃(HHTP)₂ had a high reversible capacity of 228 mA h g⁻¹. These maxima occurred at 0.65/1.10 V and 0.90/1.21 V (vs. Zn/Zn²⁺), which correlate to the two-electron absorption of HHTP and the Cu²⁺/Cu⁺ redox process. Additionally, after 500 cycles, it retained 75% of its capacity, or 124.4 mA h g⁻¹ at 4000 mA g⁻¹. With a Zn ion diffusion coefficient of 3.9 × 10⁻¹⁰ cm² s⁻¹, rapid redox reactions were evident. To improve the capacity and cycling performance of ZIB cathodes, Sang et al.¹⁶⁶ created a highly crystalline 1D c-MOF called Cu-BTA-H. Cu-BTA-H has several pore configurations and a π–d conjugated structure that resembles a rod that helps with ion transport. Because of its decreased band gap energy, electrochemical performance is improved by ion–electron transport that is greatly improved. The built ZIBs maintained a capacity of 106.1 mA h g⁻¹ after 500 cycles at a current density of 2.0 A g⁻¹, and they were able to attain a reversible capacity of 330 mA h g⁻¹at 0.2 A g⁻¹. Increased Zn²⁺ storage is made possible by the extra Cu²⁺/Cu² redox pair, which improves cycle stability and reversible capacity.

Fig. 10 Cu₃(HHTP)₂ acting electrochemically as a cathode for rechargeable ZIBs. (a) Zn-Cu₃(HHTP)₂ cell schematic depiction. (b) The Cu₃ (HHTP)₂ chemical structure. (c) The anticipated redox mechanism in the Cu₃ coordination unit (HHTP)₂. (d) CV profile at 0.5 mV s @1, together with the capacitive contribution (shaded region). (e) Cycling efficiency at 500 mA g @1.¹⁵⁷

Li et al.¹⁶⁷ synthesized Ni–PTA–Mn, a 2D c-MOF intended to enhance the structural stability of electrode materials and inhibit deformation, using a hydrothermal process. This arrangement of parts resembles a flower and is characterized by a special hydrogen-bonded skeleton that was brought about by Mn. Conductivity is improved by the positively charged metal–H₂BDC framework, which increases Zn²⁺ diffusion. Because of this, ZIBs built with Ni–PTA–Mn demonstrated exceptional stability, retaining good capacity after more than 300 cycles and 93% discharge capacity after 100 cycles at 1 A g⁻¹. Outstanding long-term cycling stability was demonstrated by the reversible and steady production of zinc intermediates, as validated by in situ XRD.

He et al.¹⁶⁸ used solvent heat and a self-sacrificing method to synthesize V-MOF-48@CNTF, a 3D c-MOF. Through self-assembly, this V-based MOF created a cascading nanowire bundle structure along the CNTF substrate. ZIB electrochemical performance is greatly increased by the special design, which reduces ion diffusion distances, improves ion migration efficiency, maximizes surface area, and increases active sites. Later, Bing employed V-MOF-48@CNTF in all-solid-state fiber-optic ZIBs, which maintained over 80% capacity after 300 cycles at 2.0 A cm⁻³ and produced an energy density of 17.47 mW h cm⁻³ at 1.46 W cm⁻³. This illustrates how 3D c-MOFs can improve ZIB applications and attributes.

2.3.6 Zn–air batteries (ZABs). Aqueous zinc–air batteries are the most developed class of metal–air batteries and have the most potential for use in future energy applications. The 1930s saw the emergence of commercial items.¹⁶⁹ With a theoretical energy density of 1086 W h kg⁻¹ (including oxygen), zinc–air batteries are approximately five times more energy dense than lithium-ion batteries as they stand today. They may be produced for as little as ∼10 $ per kW per h (ref. 170), which is around two orders of magnitude less expensive than lithium-ion batteries. Of all primary battery systems, zinc–air batteries provide the best energy density possible for a variety of applications.¹⁶⁹

Fig. 11 illustrates the components of zinc–air cells: an air electrode with a gas diffusion layer and a catalytic active layer as the cathode, zinc metal as the anode, and a separator. Gaseous oxygen is used instead of liquid oxygen because oxygen is not very soluble at atmospheric pressure.^171,172 In this process, pressure differentials force atmospheric oxygen to penetrate into the porous carbon electrode, where a catalyst harnesses electrons produced by zinc oxidation at the anode to promote its reduction to hydroxyl ions in the alkaline electrolyte. This process, which is depicted in Fig. 11 (red circle), is called a three-phase reaction and involves a solid catalyst, liquid electrolyte, and gaseous oxygen. In zinc–air batteries, this structural layout improves oxygen uptake. Zinc–air batteries (ZABs) are effective in aqueous electrolytes because of their high specific energy of 1218 W h kg⁻¹ and volume energy density of 6136 W h L⁻¹.¹⁷³

Fig. 11 Working principle and each electrode reaction of zinc–air batteries. Note the red circle where a three phase reaction (oxygen (gas), catalysts (solid) and electrolyte (liquid)) occurs in the air cathode.¹⁷⁴

According to Guan et al.,¹⁷⁵ ZABs generally consist of four main parts: an alkaline electrolyte, a separator, a zinc electrode, and an air electrode with a catalyst-coated gas diffusion layer. ZABs provide electricity when they discharge by connecting zinc metal to the air electrode in an alkaline setting.¹⁷⁶ As atmospheric oxygen diffuses into the porous air electrode, an oxygen reduction process converts it to hydroxide ions. As these hydroxide ions move toward the zinc electrode, they create Zn(OH)₄²⁻, which, in supersaturated circumstances, breaks down into insoluble ZnO.

Zinc–air batteries (ZABs) rely heavily on the ORR and OER, which have a major impact on their reaction rates.¹⁷⁷ On the other hand, the oxygen redox process occurs kinetically slowly during the charge–discharge cycle. Electrocatalysts are frequently used to improve this process.¹⁷⁸ Because of their high specific surface area, porosity, and variety of architectures, MOFs are widely used in ZAB catalyst synthesis.¹⁷⁹ Nevertheless, the electrical conductivity of pure MOFs is low. Consequently, to greatly increase the electrochemical performance of ZABs, modified c-MOFs, which have enhanced electrical conductivity, are favored as electrocatalysts.¹⁸⁰

Pan et al.¹⁸¹ created a Ru-doped c-MOF, Ni_5.7Ru_0.3(HHTP)₃(H₂O)_x, for use in ZABs, based on the superior charge transport capabilities of 2D-MOFs with π–π and π–d orbital overlap. When tested at 0.05 S m⁻¹, [Ni_5.7Ru_0.3(HHTP)₃(H₂O)_x] showed better electrical characteristics than other pure MOFs. Moreover, solid-state ZABs built using this c-MOF showed good stability and strong charge/discharge performance for a period of more than 200 cycles. Motivated by this possibility, Li created a 3D c-MOF called Co-CAT/NiFe-LDH/CNFs by synthesizing Co-CAT using solvent heat and integrating c-MOFs onto NiFe-LDH/CNFs.¹⁸² It was mostly composed of nanosheets organized in arrays with embedded nanorods. With solid electrolytes, assembled ZABs demonstrated superior stability and high power density, attaining 112.04 mW cm⁻² at 11 mA cm⁻² for more than 11 hours. Furthermore, after 56 hours of cycling at the same current density, the batteries showed minimal capacity loss and stability in liquid electrolytes, underscoring their great potential for real-world uses.

2.3.7 Calcium-ion batteries (CIB). Calcium-ion batteries (CIBs) have gained interest as possible lithium-ion battery (LIB) substitutes throughout the last seven years. The fifth most abundant element in the crust of Earth, calcium has low reduction potential (−2.87 V vs. SHE, 0.17 V higher than Li/Li⁺ and 0.5 V lower than Mg/Mg²⁺), theoretically competitive volumetric and gravimetric capacities, and great promise for secondary battery applications.¹⁸³ Precursors for calcium are also far less expensive than those for sodium and lithium.¹⁸⁴

Additionally, because calcium ions are less polarizing than Mg-ions, they have better reaction kinetics, which could improve performance.¹⁸⁵ Furthermore, because CIBs cannot facilitate dendritic formation, which can result in short circuits and explosion, they are safer than LIBs.¹⁸⁶ But CIB development is still in its infancy, and before going on sale, a few characteristics like superior electrolytes and high-performance cathodes need to be refined.

Using non-aqueous electrolytes in both three- and two-electrode cells, the majority of earlier research on CIB applications was conducted.¹⁸⁷ Designing electrode materials that can withstand the enormous size of Ca²⁺ ions during charging and discharging without suffering significant structural damage is still a difficult task nevertheless. There have been several reported solutions to these cathode problems, including as the utilization of framework-based materials and metal oxides.^188,189

At a rate of 0.5C, KVO exhibited a reversible 60 mA h g⁻¹ Ca–ion intercalation capacity, maintaining 92% of its capacity after 100 cycles. The capacity increased to 78–100 mA h g⁻¹ with an average voltage of ∼3.0 V versus Ca/Ca²⁺ when the current flow was reduced to 0.1C. Prussian blue nanodisks made from Prussian green and NaI reduction were described by Vo et al.¹⁸⁹ In a full-cell configuration with Ni-MOF as the anode, the optimal sample, Na_1.09FeFe(CN)_5.11(CO)_0.89-PAAPANi11(PB41_16/PAAPANi11) demonstrated promising Ca-storage behavior, achieving a specific capacity of 77.6 mA h g⁻¹ at 100 mA g⁻¹ current density. This impressive capacity retention of 91.0% was significantly higher than that of other Prussian blue analogues. For this reason, creating high-performance cathodes for CIB applications through the use of metal oxides and framework-based materials offers considerable potential.

2.4 MOFs for fuel cells

In recent times, fuel cells have garnered considerable interest due to their exceptional efficiency, high energy density, and minimal release of detrimental gases.¹⁹⁰ Fuel cells, in contrast to most batteries, which store chemical energy inside the cell, use a continuous flow of chemical energy from outside the cell in the form of fuels (such as hydrogen, natural gas, and methanol) to produce electricity for use in stationary buildings, cars, and portable appliances. Internal combustion engines are not as eco-friendly as fuel cells; thus, they could be a viable replacement. An anode, a cathode, and an electrolyte are the three main components of fuel cells. Fuel is often oxidized at the anode using a catalyst to produce electrons and ions, which are subsequently transported to the cathode by the electrolyte for the ions and an external circuit for the electrons. At the cathode, a second catalyst is employed in conjunction with the electrons and ions to lessen the oxygen that is often provided by the air.

Research has been done on MOFs and their derivatives as potential electrode catalysts, electrolyte-holding membranes, and less expensive alternatives to catalysts based on precious metals. Several important electrocatalytic applications, including the following: (a) MOFs for the hydrogen evolution reaction (HER), (b) MOFs for the oxygen reduction reaction (ORR), and (c) composites of MOFs and proton conductive polymers for membranes, are the main drivers of the use of MOFs for increasing efficiency and lowering the cost of fuel cells.¹⁹¹

Fenoy et al.¹⁹² presented a compressive example of using a ZIF-8 top layer as an O₂ adsorbent to raise the oxygen content on the conducting polymer catalytic surface. The ZIF-8 layer improved the O₂ uptake from the solution, which improved the ORR in neutral media. Bureekaew et al.¹⁹³ reported the encapsulation of imidazole (Im) molecules into aluminum MOFs, whereby Im@{Al(μ₂-OH) (1,4-NDC)}_n (1,4-NDC = 1,4-naphthalenedicarboxylate) and Im@{Al(μ₂-OH) (1,4-BDC)}_n present enhanced conductivities of 2.2 × 10⁻⁵ and 1.0 × 10⁻⁷ S cm⁻¹, respectively, when compared to those of their parent MOFs at 120 °C. In a different investigation, Gui et al.¹⁹⁴ synthesized a Zr-MOF with anionic zirconium phosphate chains supported by NH₄⁺ cations, creating a continuous N–H/O–P chain for improved anhydrous proton conductivity. Taylor et al.¹⁹⁵ reported a different study that utilized a zirconium–sulfoterephthalate MOF as an example to show how the defect affects proton-conducting behavior. The defect-containing samples were created by either immersing the prepared sample in 0.1 M H₂SO₄ or adding excess acetic or sulfoacetic acid during the preparation process.

According to Ji et al.¹⁹⁶ multi-heteroatomic doping (e.g., N, Co, P, B, S, etc.) is frequently applied to MOF-derived 1D PCNFs or HCNFs to enhance their ORR by changing their surface polarity and electrical characteristics. They also described the one-step procedure for the tunable synthesis of ZIF-67 and PAN electrospun into carbon microsphere/nanofiber hybrids (CSFHs), and they showed how this process could be used to control the morphology of a nanofiber that resembled a pearl necklace to a microsphere/nanofiber 3D structure by varying the ZIF-67/PAN ratio.¹⁹⁷

In the past, a bimetallic Zn, Co-ZIF was combined with electrospun Co²⁺/PAN fibers to create MOF-based hierarchical carbon fibers containing N, C (Fig. 12a and b). In comparison to Zn, Co-ZIF derived carbon (ZIF-C) and Zn, Co-ZIF free carbon fibers (CP-CFs) (Fig. 12c and d),¹⁹⁸ the ZCP-CFs demonstrated high activity for the ORR in 0.1 M KOH. These features included a more positive half-wave potential (0.135 V vs. Ag/AgCl), higher diffusion-limited current density (5.95 mA cm⁻²) and kinetic limiting currents, a lower Tafel slope (62 mV dec⁻¹), and higher selectivity (the number of electrons transferred, n = 3.97). The number of researchers working around the clock to create bifunctional catalysts for water splitting has grown recently.

Fig. 12 (a) Schematic illustration of the preparation of the Zn, Co-ZIF-based hierarchical carbon fibers (ZCP-CFs) and (b) the corresponding TEM image of ZCP-CFs. (c) Tafel plots of ZIF-C, CP-CFs, ZCP-CFs, and Pt/C. (d) Kinetic-limiting current densities of ZIF-C, CP-CFs, ZCP-CFs-9, and Pt/C.¹⁹⁸ (e) Schematic illustration of the fabrication of free-standing MoS₂ nanosheet grafted Co–N-doped carbon flakes grown on electrospun carbon nanofibers (CoNC@MoS₂/CNF). (f) Polarization curves of CoNC@MoS₂/CNF, MoS₂/CNF, CoNC/CNF, and RuO₂ in 1 M KOH in the OER. (g) OER Tafel plots of CoNC@MoS₂/CNF, MoS₂/CNF, CoNC/CNF, and RuO₂ in 1 M KOH. (h) OER cycling stability of CoNC@MoS₂/CNFs in 1 M KOH.¹⁹⁶

The hydrogen evolution reaction (HER) can be effectively catalyzed by molybdenum disulfide (MoS₂); however, its low electrical conductivity limits both its charge transfer rate and electrocatalytic effectiveness. The solution to this problem is shown schematically in Fig. 12e, where MoS₂ nanosheets were formed on carbon fibers generated from ZIF-67/PAN to increase their electrical conductivity. Furthermore, using the CoNC@MoS₂/CNFs for the oxygen reduction reaction (ORR) in 1 M KOH electrolyte, they showed a better catalytic activity with a low Z10 value of 350 mV compared to MoS₂/CNFs and CoNC/CNFs (570 mV and 430 mV, respectively) (Fig. 12f). Additionally, as shown in Fig. 12g, the CoNC@MoS₂/CNFs displayed a low Tafel slope of 51.9 mV⁻¹, which was significantly lower than those of CoNC/CNF (89.7 mV⁻¹), MoS₂/CNF (119.3 mV⁻¹), and RuO₂ (98.1 mV⁻¹). This suggests that the CoNC@MoS₂/CNFs have high catalytic activity and enhanced catalytic kinetics.¹⁹⁶ Further evidence of the exceptional OER stability of CoNC@MoS₂/CNFs in a basic medium comes from the observation of a 17 mV increase in the Z100 value after 1500 continuous cycles (Fig. 12h).

Proton migration networks that are independent of water can be achieved by introducing acid–base pairs through linker alteration or guest encapsulation, as demonstrated by previous research. In an experiment, Dong et al.¹⁹⁹ embedded isomorphous UiO-66 nanocrystals (40–100 nm) attached to NH₂ and SO₃H in a sugar-based chitosan polymer network. On the other hand, Guo et al.²⁰⁰ focused on creating a hybrid in which they threaded a zwitterionic polymer containing sulfonate and quaternary ammonium groups through ZIF-8, resulting in improved proton transfer. In addition to the previous quest encapsulation technique, Chen et al.²⁰¹ reported adding several ionic liquids to the ∼500 nm pores of MIL-101 nanoparticles in order to produce enhanced proton conductivity via acid–base interactions.

A thorough explanation of the low-temperature area and the first proton-conducting MOF, {(NH₄)₂(adp)[Zn₂(ox)₃]·3H₂O}_n (ox = oxalic acid, adp = adipic acid), was provided by Sadakiyo et al.²⁰² Water molecules, NH₄⁺ ions, and the carboxyl groups of adipic acid acted as conducting media, which performed similarly to Nafion. At 98% relative humidity (RH) and 25 °C, this MOF exhibits proton conductivity as high as 8 × 10⁻³ S cm⁻¹. When compared to Nafion, this MOF exhibits a greater activation energy (E_a) of 0.63 eV, suggesting that both Grotthuss and vehicle processes are involved in the proton conduction.

3. Environmental applications of MOFs

MOFs have several potential uses due to their superior qualities (large surface area, pore size controllability, and chemical and thermal endurance). Furthermore, they are also easily tunable to get desired characteristics. MOFs are ideal adsorbents for environmental applications because of their extremely high porosity (about 90% free volume) and specific surface areas (typically greater than 7000 m² g⁻¹).²⁰³ To eliminate these contaminants from either air or water, adsorbents such activated carbon, zeolite, metal oxide, resin, and biomass are typically used. Nevertheless, the majority of these materials have issues with their low adsorption capability, especially for low-level pollutants, and their high regeneration energy consumption. Furthermore, it is simple to alter their surface in order to include more functional groups that can help with the adsorption of specific contaminants.²⁰⁴ On the effectiveness of MOFs as adsorbents, numerous studies have been published. The ability to selectively adsorb gases is made possible by MOFs' enormous surface area and various pore diameters. The storage of hydrogen, methane, and carbon dioxide has been studied in relation to MOFs. MIL-101 and ZIF-8, for example, are two MOFs that have shown promise in the adsorption of carbon dioxide. These MOFs can release carbon dioxide after selectively adsorbing it under certain conditions.^205–207 The potential application of MOFs in water filtration has been studied. MOFs can be used to remove organic materials, heavy metals, and pharmaceuticals from water.²⁰⁸ The effectiveness of MOFs as adsorbents has been the subject of numerous investigations.

3.1 Adsorption

3.1.1 Adsorption of dye pollutants. Given the availability of a library of anionic and cationic dyes, electrostatic interactions are thought to be one of the most significant types of non-covalent interactions involved in the binding of dye molecules by MOFs. For coloring cotton, silk, and wood, these dyes have been widely employed in industry.²⁰⁹ Ionic dyes are hazardous and used widely; hence, many MOFs have been created for their capture. In most cases, AC, zeolites, and chitosan beads are frequently used to remove colors like methyl orange (MO), methylene blue (MB), and malachite green (MG). Comparing MIL-100(Fe), MIL-101(Cr), MIL-53(Al), and AC, it was shown that MIL-100(Fe) had a greater adsorption capacity (146 mg g⁻¹) for MG than the other three adsorbents. The electrostatic interaction between MOF and MG was suggested as the cause of MIL-100(Fe)'s excellent adsorption capacity and selectivity.²¹⁰ The robust π–π interaction between the guest and host molecules was responsible for this performance, which exceeded several other conventional adsorbents' criteria.

An experiment to synthesize hierarchically mesostructured MIL-101(Cr) by using cetyltrimethylammonium bromide (CTAB) as a surfactant to remove methyl blue MB was reported in ref. 211. For the adsorptive removal of MB in the liquid phase, the material displayed dramatically faster adsorption kinetics. In around 110 minutes, they noticed that all of the MB molecules with a 30 ppm initial concentration had been adsorbed onto the hierarchically mesostructured MIL-101(Cr). In another development, for the purpose of removing methylene blue (MB) dyes from aqueous environments, Li et al.²¹² investigated the use of MOF/graphite oxide hybrid (MOF/HKUST1) materials. Investigations were also conducted into the isothermal, kinetics, and regeneration studies. It was discovered that the adsorption of MB followed both the Freundlich and Langmuir adsorption isotherms. Finally, it was discovered that HKUST-1/GO had superior reusability and a greater capacity for MB dye adsorption.

In order to capture the Brilliant Blue R-250 (BBR) dye molecule from aqueous solution, Liu et al.²¹³ produced a number of three-dimensional 4,4-connected MOFs based on copper paddlewheel secondary building units and tetracarboxylate linkers made from tetraphenylmethane. There was a direct correlation between the pore diameters of the 4,4-connected MOFs and the dye uptake. The maximum dye absorption capability of the MOF with the biggest pore size was 73 wt%, whereas MOFs with pores smaller than the dimensions of BBR-250 showed negligible dye uptake. The batch adsorption approach was used to remove the methyl red (MR) dye using MOFs loaded onto iron oxide nanoparticles (Fe₃O₄@MIL-100(Fe)).²¹⁴ When compared with other adsorbents, the MR removal onto MIL-100(Fe) demonstrated an improved capacity.

For the adsorptive removal of methyl orange, Haque et al.²¹⁵ reported the use of two MOFs based on chromium terephthalates, namely MIL-53(Cr) and MIL-101(Cr). Both MOFs were superior to activated carbon in the adsorption of methyl orange, with MIL-101(Cr) showing a higher adsorption capacity than MIL-53(Cr). The adsorption capacities and kinetic constants were in the following order: activated carbon < MIL-101(Cr) < ethylenediamine-grafted MIL-101(Cr) (or ED-MIL-101(Cr)) < protonated ethylenediamine-grafted MIL-101(Cr), and PED-MIL-101(Cr), with a 194 mg g⁻¹ adsorption capacity. The cationic PED-MIL-101(Cr) MOF displayed the strongest interaction with the anionic methyl orange molecules because protonated PED-MIL-101(Cr) was positively-charged, and ED-MIL-101(Cr) also displayed a partial positive charge, suggesting that electrostatic interactions were involved in the adsorption mechanism.

Adsorption of methyl orange and methylene blue onto Fe₃O₄–PSS@ZIF-67 (PSS = polystyrene sulfonic acid) with a core–shell structure, named MZIF-67, was reported by Yang et al.²¹⁶ According to the tests, an amount of 5 mg and an initial concentration of 400 mg L⁻¹ resulted in an optimum adsorption capacity of 738 mg g⁻¹ within 7 hours. This outstanding performance was attributed to its high porosity, the presence of unsaturated cobalt sites, and magnetic characteristics, which further increased the removal efficiency for methyl orange and methylene blue. Further investigation revealed that MZIF-67 has the selectivity ability to gradually separate the methyl orange dye from a mixture of solutions that contained 0.04 mg L⁻¹ methyl orange and methylene blue dyes. As a result, the removal rate and separation efficiency were both increased to 92 and 96%, respectively.

According to ref. 217, defective UiO-66(Zr) was synthesized utilizing hydrochloric acid as a post-synthetic treatment and benzoic acid as a modulator. The adsorption capacity of safranin T was found to be nine times higher in the faulty MOF (366 mg g⁻¹) than in the defect-free counterpart (30 mg g⁻¹). Because of the size-exclusion effect, the defective UiO-66(Zr) was nevertheless able to selectively adsorb safranin T over crystal violet in spite of having a bigger cavity and a more uniform distribution of pore sizes.

Under different circumstances, Lin and Chang²¹⁸ reported the adsorption of malachite green onto zeolite imidazole frameworks (ZIFs) at 20 °C, with an adsorption capacity of 1500 mg g⁻¹, which quickly increased to 2500 mg g⁻¹ at 60 °C. This demonstrated that raising the temperature of the adsorption process had a beneficial effect on the system's performance. After four cycles, the ZIF-67 regeneration and reusability study revealed a 95% discharge rate for regeneration, making it an effective adsorbent for removing the malachite green dye.

Qin and Zeng²¹⁹ reported the preparation of ultra-small nanoparticles of several MOFs, such as HKUST-1, ZIF-8, and ZIF-67, supported on copper silicate nanotubes (CuSiNT), for the adsorption of methyl orange, Congo red, and thymol blue. The CuSiNT-supported HKUST-1 nanocomposites exhibited improved methyl orange, Congo red, and thymol blue adsorption when compared with their microsized counterparts. The increased adsorption capability was due to the CuSiNT support, which might offer more accessible functional groups and open metal sites and thus reduce the dye molecules' diffusion barrier in the process. Wang et al.²²⁰ reported the potential of a crystalline triazine-based organic framework (CTF) as a promising adsorbent for removing organic dyes. This material showed an uptake of 0.48 mmol g⁻¹ for RhB within 55 min (the weight ratio of RhB/CTF was 0.24), with the adsorption capability for RhB onto CTF reaching 1.01 mmol g⁻¹, showing that CTF was more effective in removing Rhodamine B (RhB).

Haque et al.²²¹ expanded their research to a MOF based on iron terephthalate, MOF-235(Fe) or [Fe₃O(terephthalate)₃(DMF)₃][FeCl₄]. Despite being non-porous to nitrogen at low temperatures, this MOF absorbed both anionic methyl orange and cationic methylene blue dyes in the liquid phase. With MOF-235, MO and MB had adsorption capacities of 477 and 187 mg g⁻¹, respectively, in comparison with just 11 and 26 mg g⁻¹ for each respective dye with activated carbon. They also confirmed that the solution pH had a significant impact on the adsorption of the MO and MB dyes. It was posited that the positive charge density on the MOF-235(Fe) decreased at higher pH levels, leading to a reduction in the adsorption of methyl orange. A low pH value enhanced the negative charge density of the MOF's, which boosted the adsorption capacity for methylene blue.

In another development, MIL-101(Al) was functionalized with amino groups to produce NH₂–MIL-101(Al).²²² NH₂–MIL-101(Al) showed a high methylene blue adsorption capability of up to 762 mg g⁻¹. It was posited that the electrostatic interactions between the MOF and methylene blue increased as a result of the presence of amino moieties on the MOF surface. However, the MOF's X-ray photoelectron spectrum showed that structural changes happened to the MOF during the adsorption procedure and that about 30% of the Al(III) ions were lost to the solution, rendering the MOF useless. A single-walled metal–organic nanotube with an armchair (3,3) structure, ([CH₃NH₃][Zn(NTB)(NMF)])·4.5NMF (H₃NTB = 4,4′,4′′-nitrilotrisbenzoic acid, NMF = N-methylformamide), has recently been discovered by Zhou et al.²²³ and reported to display an interior channel diameter of 21 Å. The metal–organic nanotube adsorbed dye molecules (i.e., basic red 9 and basic violet 14) with remarkable efficiency (>1650 mg g⁻¹), because of the vast open mesoporous channels.

Another advancement was made by Huo and Yan,²¹⁰ when they developed MIL-100(Fe) for the adsorption of the cationic triphenylmethane dye (i.e., malachite green). The study showed that MIL-100(Fe) demonstrated an adsorption capacity of 205 mg g⁻¹ at room temperature, which was much greater than those of MIL-101(Cr) and MIL-53(Al). Electrostatic interactions played a role in the adsorption mechanism, as evidenced by the dependence of the adsorption process on pH and the zeta potential measurements. Malachite green and MIL-100(Fe)'s benzene rings also interacted with one another in a π–π fashion. Malachite green was demonstrated to adsorb on MIL-100(Fe) in an endothermic manner, with an increase in adsorption capacity at higher adsorption temperatures. Thermodynamic investigations showed that a positive entropy change is beneficial for the spontaneous adsorption of malachite green on MIL-100(Fe), whereas a positive enthalpy change is undesirable. The open metal sites in MIL-100(Fe) were occupied by water molecules, which may help to explain this. On the other hand, the weaker adsorption of malachite green would be caused by the absence of open metal sites in MIL-53(Al) and the repulsion brought about by surface positive charges on MIL-101(Cr).

In a different study, Li et al.²²⁴ prepared NH₂–MIL-53(Al) for the adsorption of the cationic dyes methylene blue and malachite green using the amino functional group. The adsorption process in this instance was not fueled by electrostatic interactions, as evidenced by the comparison of the pH-dependent adsorption studies. Instead, it was found that the amino hydrogen of NH₂–MIL-53(Al) and the nitrogen atoms on methylene blue or malachite green exhibited strong hydrogen bonding interactions. The benzene rings of NH₂–MIL-53(Al) and the dye molecules also had weak π–π interactions with one another. Seth et al.²²⁵ prepared an anionic trinuclear cadmium MOF using the tetratopic carboxylate ligand 3,3′,5,5′-tetrakis(p-carboxyphenyl)-2,2′,6,6′-tetramethoxy-1,10-biphenyl. This cadmium MOF demonstrated remarkable flexibility and durability and permitted post-synthetic metal exchange with several main groups and lanthanide ions, to produce new MOFs with the same structure. Particularly, the production of cationic MOFs, which have substantially different dye adsorption characteristics, resulted from the substitution of trivalent lanthanide ions for the divalent cadmium ions in the divalent cadmium solution. The anionic dye, bromophenol blue, was selectively adsorbed by the isostructural europium MOF, from a mixture that contained neutral or cationic dyes, in contrast to the parent cadmium MOF, which preferentially adsorbed only cationic methylene blue.

Dong et al.²²⁶ used (E)-4-(2-carboxyvinyl)benzoic acid (H₂L) to prepare the cadmium MOF, [[CdL(H₂O)]·4DMF·2H₂O]. The adsorption of rhodamine B (RhB) by the MOF led to the formation of a luminous adduct. RhB@MOF experienced ligand-to-dye energy transfer, which caused the ligand and RhB emissions to be detected at 420 and 595 nm, respectively, even though the emission of RhB was suppressed in a ground mixture with the MOF. Luo and Wang²²⁷ carried out an experiment on the use of composites formed from MIL-100(Fe) and graphene oxide nanosheets, which were discovered to have a sandwich-like structure when the decomposition temperature increased from 280 to 350 °C. At a graphene oxide loading of 5%, it was discovered that this composite material had improved methyl orange and methylene blue adsorption capabilities from aqueous solution. MOF composites made from ZIF-8 with either graphene oxide or carbon nanotubes were also employed by Abdi et al.²²⁸ The composite materials in this example showed significantly improved malachite green adsorption capabilities, with maximum values of 1667, 2034, and 3300 mg g⁻¹ for ZIF-8, ZIF-8@CNT, and ZIF-8@GO, respectively, at room temperature. The potential for practical applications of these composite materials was demonstrated by the increased uptake of malachite green and preservation of their adsorption ability in real wastewater at increasing temperatures.

Li et al.²²⁹ studied the adsorption characteristics of the Fe₃O₄/HKUST-1/GO hybrid toward methylene blue. With a better dye adsorption capacity than the more straightforward Fe₃O₄/HKUST-1 composite, the three-component Fe₃O₄/HKUST-1/GO composite demonstrated a good match in channel diameter and molecular breadth with methylene blue despite having a smaller specific surface area. The adsorption capability of Fe₃O₄/HKUST-1 was observed from desorption and regeneration studies to rapidly decrease with increasing numbers of cycles, from virtually quantitative adsorption to barely 60% after recycling for five times. The adsorption capacity of methylene blue for Fe₃O₄/HKUST-1/GO in the presence of graphene oxide as a component material was only marginally reduced, and the value remained over 90% after 5 cycles. Recent research conducted by Pei et al.²³⁰ developed MOF-polymer composites from HKUST-1 and a biocompatible binder consisting of calcium alginate and gelatin using three-dimensional (3D) printing technology. Methylene blue, malachite green, methyl violet, rhodamine B, and auramine O were the organic dyes that the 3D-printed composites adsorbed, and the adsorption performance depended on the printing geometry and the size and loading of the MOFs.

For dye removal from aqueous solution many MOF-based adsorbents have been employed, and the adsorption capacities of these arrays of adsorbents for dye removal are presented in Table 1.

Table 1 Dye removal from aqueous solution and wastewater by MOF-based adsorbents

Adsorbate	Adsorbent	Adsorption capacity (mg g^−1)	Time (min)	Ref.
Congo red	Ni–Cu MOF	999.20	240	231
	TMU-7(Cd)	97	45	232
	Ni-MOF	276.7	300	233
	Zn-MOF	132.2	300	233
	Ni–Zn MOF	460.90	300	233
	TFMOF(Zr)	252.25	10	234
	Ce(III)-doped UiO-67	799.6	80	235
	ln–TATAB	299	10	234
	Co-MOFs	4885.20	720	236
Methylene blue	UiO-66	69.8	120	237
	Zn-MOF	326	60	238
	Fe3O4@-SiO2@UiO-66	116	60	239
	Fe3O4@-SiO2@UiO-66-NH2	128	60	239
	Fe3O4@-SiO2@UiO-66-urea	121	60	239
	Fe3O4@MIL-100(Fe)	221	5–1440	240
	UiO-66-P	91.1	Over 1440	241
	MIL-100(Fe)	1105	10–1440	242
	Fe3O4@MIL-100(Fe)	73.8	420	243
	H6P2W18O62@Cu3(BTC)2	298.34	60	244
	MIL-68(Al)	1666.67	8	245
	BUT-29	1119	6	246
	UiO-66	543.48	300	247
	GO–Cu-MOF	262	1440	248
	CuBDC	41.01	20	249
	Ce(III)-doped UiO-67	398.9	80	235
	USTC-1	26.6	240	250
	ABim–Zn-MOF	174.64	60	251
Methyl orange	UiO-66	83.7	120	237
	Fe3O4@-SiO2@UiO-66	219	60	239
	Fe3O4@SiO2@UiO-66-NH2	130	60	239
	Fe3O4@SiO2@UiO-66-urea	183	60	239
	Ce(III)-doped UiO-67	401.2	80	235
	USTC-1	0.8	240	250
Rhodamine B	MIL-125(Ti)	59.92	180	252
	Zn-MOF	3.75	60	238
	MgFe2O4@MOF	219.78	5	253
	POM@UiO-66	222.6	120	254
	Fe3O4/MIL-100(Fe)	28.36	90	255
	In-MOF@GO	267	60	256
	MIL-68(Al)	1111.11	10	245
	USTC-1	13.4	240	250
Methyl orange	SCNU-Zl-Cl	285	60	257
Acid orange A	SCNU-Zl-Cl	180	60	257
Congo red	SCNU-Zl-Cl	585	60	257
Methyl blue	SCNU-Zl-Cl	262	60	257
Malachite green	Cd-ZIF	395.87	70	258
Malachite green	ZIF-8	1000	180	259
Malachite green	POM@UiO-66	190.6	120	254
Orange G	POM@UiO-66	40	120	254
Chicago sky blue	ABim–Zn-MOF	144.26	60	251
Methyl red	Fe3O4@MIL-100(Fe)	625	360	214
Acid red 18	CoOF	44.26	500	260
Rose Bengal	SmBTC	380	1440	261
Crystal violet	BUT-29	832	4	246
Acid chrome blue K	ln–TATAB	343	10	262
Acid red 26	ln–TATAB	259	10	262
Direct black 38	ln–TATAB	242	10	262
Orange II	ln–TATAB	217	10	262

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In summary, the carcinogenic effects of synthetic dyes on humans and aquatic life make them a serious environmental issue when it comes to water contamination. While adsorption is a popular method for eliminating dyes from wastewater, traditional adsorbents are sometimes ineffective for usage in industrial settings. These drawbacks can be overcome by using metal–organic frameworks (MOFs), which have good dye removal and degradation properties because of their multifunctionality, water stability, wide surface area, adjustable pore size, and recyclability. Magnetic MOFs continued to function well after several cycles.

3.1.2 Adsorption of heavy metal pollutants. A lot of research has been done on MOFs and MOF-based composites as possible adsorbents for heavy metal removal because of their intriguing structure and notable physical characteristics, which include flexibility, functionality, tunable pore size, large pore volume, high specific surface area, and the potential for case-specific customization of fundamental molecular architecture.² With careful selection of a metal ion and an organic linker, the structure of a MOF can be designed. The topology, structural properties, and metal speciation of MOFs all influence the mechanism of heavy metal ion adsorption. Metal ions may transpire via diffusion into the MOF's core or via interactions at the framework's surface.²⁶³ These interactions may take the form of chemical bonds, coordination interactions, electrostatic interactions, acid–base interactions, van der Waals contacts, or chemical bonding.^264–266 Compared to other common adsorbents such as activated carbon, zeolite, and mesoporous silica, these characteristics make the MOFs more appealing.
3.1.2.1 Arsenic. Arsenic is extremely harmful and causes cancer. Both the arsenate, As(V), and arsenite, As(III), forms are lethal to all living things.²⁶⁷ When it comes to metal ion pollution, the World Health Organization (WHO) has designated arsenic pollution as the top concern.²⁶⁸ The WHO recommends a maximum tolerable content of 10 μg l⁻¹ for arsenic in drinking water.²⁶⁹ It is estimated that each year more than 60 kg of arsenic are emitted into the atmosphere.²⁷⁰ Various approaches have been implemented to eliminate As(III) and As(V) from wastewater. For the treatment of wastewater and water containing arsenic, MOFs have been designated as promising materials.

In order to remove arsenic (As) contaminants from aqueous solutions,²⁷¹ a metal–organic coordination polymer (Fe–BTC) was synthesized using iron(III) and 1,3,5-benzene tricarboxylic acid via a solvothermal method. This Fe–BTC polymer demonstrated a significantly higher adsorption capacity for As(V) compared to commercial iron oxide powders and nanoparticles (50 nm). The Fe–BTC polymer exhibited an outstanding adsorption capacity for As(V) at 12.3 mg g⁻¹, six times higher than that of Fe₂O₃ nanoparticles. Comprehensive analyses using FTIR and XPS spectroscopy confirmed significant adsorption before and after As(V) uptake. The arsenic ions were confirmed to adsorb within the Fe–BTC polymer rather than on the surface, indicated by the Fe–O–As IR peak and the presence of As(V) in the XPS spectra.

In order to address the issue caused by secondary pollution resulting from synthetic procedures for magnetic composites,²⁷² a direct epitaxial synthesis of magnetic UiO-66 was proposed. The composite material was tested for the removal of As(V) and demonstrated an arsenic adsorption capacity of 73.2 mg g⁻¹. The characterization techniques such as SEM, TEM, and N₂ adsorption–desorption isotherms revealed that the composite has a unique core–shell structure and a high surface area (124.8 m³g⁻¹), greater than that of pristine UiO-66 (27.1 m³ g⁻¹). Furthermore, due to their ease of material recovery after adsorption, magnetically modified UiO-66 composites attracted a lot of attention.²⁷³

Liu et al.²⁷⁴ described the adsorptive removal of As(V) using heated MIL-100(Fe) to create α-Fe₂O₃ nanoparticles at various temperatures. According to the findings, there was a positive correlation between increasing particle size and rising calcination temperature (i.e., α-Fe₂O₃-350 °C (50 nm), α-Fe₂O₃-550 °C (150 nm), and α-Fe₂O₃-750 °C (200 nm)). For α-Fe₂O₃-350 °C, α-Fe₂O₃-350 °C, α-Fe₂O₃-750 °C, Fe₂O₃-550 °C, and MIL-100(Fe), the corresponding adsorption capacities were 94.9, 74.1, 70.5, 80.5, and 110 mg g⁻¹ respectively. According to ref. 275, a comparable investigation revealed that γ-Fe₂O₃ nanoparticles produced through easy thermolysis of MIL-100(Fe) had a 90.6 mg g⁻¹ adsorption capability for As(V).

Wu et al.²⁷⁶ created a variety of ZIF-8 polymers with varying cetyltrimethylammonium bromide (CTAB):His (L-histidine) molar ratios (such that the polymers were ZIF-8-H₂O, ZIF-8-MeOH, and 1 : 1 H-ZIF-8-11, 1 : 2 H-ZIF-8-12, and 1 : 4 H-ZIF-8-14). ZIF-8-H₂O has the lowest capacity among all of them. ZIF-8 was hierarchically organized and synthesized using a distinct CTAB : His molar ratio with increased CTAB, indicating that this ratio had great adsorptive capabilities. Specifically, HZIF-8-14 exhibited a 30-fold greater adsorptive ability for As(V) (90.9 mg g⁻¹) in comparison to active carbon (3 mg g⁻¹). Additionally, it was reported by Liu et al.²⁷⁷ that zeolitic imidazolate frameworks with a variety of morphologies, including cubic, leaf-shaped, and dodecahedral ZIFs, had superior As(III) adsorption capabilities of more than 100 mg g⁻¹. By contrast, γ-Fe₂O₃²⁷⁴ and ZIF-8²⁷⁸ demonstrated typical As(III) adsorption capacities of 62.9 and 49.5 mg g⁻¹, respectively.

The significant presence of the Zn–OH functional group in the ZIF-8 adsorbent produced by Li et al.²⁷⁹ was responsible for its remarkable adsorption capacity of 76.5 mg g⁻¹. ZIF-8 was functionalized with ethylenediamine by Massoudine et al.²⁸⁰ in order to increase its adsorption capacity, and the result was an adsorption capacity of 83.5 mg g⁻¹. Another type of traditional MOF, MIL-53(Fe), which was produced via a solvothermal technique, was studied by Vu et al.²⁸¹ An adsorption capacity of 21 mg g⁻¹ was attained by the adsorbent. MIL-53(Al), MIL-88A, and MIL-88B are other adsorbents exhibiting adsorption capabilities of 106, 145, and 156 mg g⁻¹, respectively.^282,283 A number of other studies have demonstrated high arsenic adsorption capacities of adsorbents, including indium-based AUBM-1,²⁸⁴ Co-MOF,²⁸⁵ Ni-MOF,²⁸⁶ and MOF-808.²⁸⁷ With an adsorption capacity of 303 mg g⁻¹, UiO-66 showed the greatest adsorption capacity among all of them.

3.1.2.2 Lead. Although lead (Pb) is a naturally occurring element in soil, its presence at lower concentrations can be considered an environmental contamination. Lead (Pb(II)) is widely used in industrial processes like metal plating, painting, smelting, oil refining, and battery manufacturing. Lead poisoning is a serious concern that has been linked to harmful and cancerous effects on human health.²⁸⁸ Little children are especially vulnerable to the toxicity of lead, which can impact many body systems such as the brain and nervous system. According to Ghorbam et al., Meng et al., and Qiao et al.,^289–291 the growing fetus may be exposed to lead toxicity due to the release of lead from bone into the circulation during pregnancy. Higher amounts of lead consumption also primarily cause renal damage, slowed bone growth, mental decline, and neurological illnesses. Pb(II) content limits have been set by the WHO, European Union, and USEPA, respectively, at 0.01, 0.01, and 0.015 mg L⁻¹.²⁹²

Huang et al.²³⁹ synthesized a range of Zr-based magnetic MOFs with core–shell amino functionalizations utilizing various functionalization agents. Three MOFs were prepared for their study: NH₂-functionalized Fe₃O₄@-SiO₂@UiO-66-NH₂ (MFC-N), urea-functionalized Fe₃O₄@SiO₂@UiO-66-urea (MFC-U), and non-functionalized Fe₃O₄@SiO₂@UiO-66 (MFC-O). When compared to MFC-O, their findings showed that MFC-N and MFC-U had better Pb(II) adsorption capacities. The high adsorption is significantly influenced by the presence of amino groups. A stronger connection between the NH₂ groups anchored on MFC-N and Pb(II) ions is indicated by the fact that MFC-N, out of the two amino-functionalized composites, has a higher adsorption capacity of 102 mg g⁻¹ towards Pb(II) ions. According to Huang et al.,²⁹³ there has been further breakthrough on the adsorption of Pb(II) using artificial zeolite-imidazolate frameworks, ZIF-67 and ZIF-8. In comparison to certain other porous materials on the market, the MOFs' adsorption capabilities of 1348 and 1119 mg g⁻¹, respectively, were higher.

In a different study, Luo et al.²⁹⁴ reported a Cr-based MOF functionalized with ethylenediamine (MIL-101) to eliminate Pb(II). In their research, the adsorption isotherms for Pb(II) adsorption on the amine-functionalized MIL-101 were compared to those for the non-functionalized MIL-101. The amine groups' partial obstruction of MIL-101's pores is indicated by the decreased pore volume following functionalization. For the adsorption of lead ions, the amino groups that have been altered on the MOF's exterior surface offer chelating binding sites. Reduced functionalization extent was observed along with a decline in maximum adsorption capabilities, which decreased from 81.09 to 15.78 mg g⁻¹.

Li et al. 2019²⁹⁵ assessed the efficacy of two amide-based COFs (COF-TP and COF-TE) in the adsorptive removal of Pb(II). Based on their findings, the maximum adsorption capacities of 140 and 185.7 mg g⁻¹ were determined for COF-TP and COF-TE, respectively. By acting as an active adsorption site for the metal ions through multi-coordination, the amide group improved the adsorption of Pb(II). A study conducted by Rivera et al.²⁹⁶ described the adsorption properties of MOF-5 for the removal of Pb(II). 750 and 660 mg g⁻¹ of adsorption were observed at pH 4 and 6, respectively, indicating a rising trend in adsorption capacity with decreasing pH.

Yu et al.²⁹⁷ evaluated the effectiveness of a new metal–organic framework embellished with O⁻ and N=N groups in order to adsorb Pb(II) from an aqueous solution. Based on their investigation, the maximum Pb(II) adsorption capacity was discovered to be 463.52 mg g⁻¹, and a noteworthy affinity for the metal ion (K_d = 8.88 × 10⁶ mL g⁻¹) was observed. The characteristics of the borderline acid were exposed by the densely packed O⁻ groups and N=N units, and they could interact with the borderline acid Pb(II) extremely quickly. Hasankola et al.²⁹⁸ reported the utilization of produced Cu-BTC and Zn-BTC MOFs by solvothermal reaction with benzene-1,3,5-tricarboxylic acid as a linker as a versatile adsorbent for the removal of Pb(II). The generated composites showed maximal adsorption capacities of 333 and 312 mg L⁻¹, respectively. When it came to adsorbing Pb(II) ions, the frameworks performed similarly and could sustain the adsorption–desorption cycle three times.

Carbon paste electrodes utilizing MOF-5 for Pb(II) detection were demonstrated by Wang et al.²⁹⁹ A carbon-MOF-5 paste was created by combining carbon powder with a few drops of ethanol, allowing the fine powder to evaporate and then mixing it with mineral oil in a mortar. To create a MOF-based carbon paste electrode, the paste was placed within a glass tube and sealed with a copper wire. Using 0.1 M acetate buffer at pH 5, various Pb(II) concentrations were used for the differential pulse voltammetry (DPV) experiments. When the concentration of Pb(II) was increased from 1.0 × 10⁻⁸ to 1.0 × 10⁻⁶ M, the DPV peak was detected at −0.45 V and moved to lower potentials. This could be because of distinct interactions between the materials at the electrode surface and the thin layer of Pb that was deposited onto the electrode. A Pb(II) removal of 312 mg g⁻¹ was accomplished by Zhang et al.³⁰⁰ using the synthesized HS-mSi@MOF-5 framework, a silica coated thiolated MOF-5 derivative. The pH of the solution had a significant impact on the MOF's performance, with low pH values producing the best results.

Yu et al.³⁰¹ described the utilization of a Zn(II) based MOF decorated with O⁻ groups for Pb(II) adsorption. The material under investigation showed a remarkable adsorption capacity of 616.64 mg g⁻¹, accompanied by 99.27% selectivity towards Pb(II) ions. Their investigation verified that the negatively charged O⁻ groups interact electrostatically with the Pb(II) ions to play a significant role in this ultrahigh adsorption capacity. Furthermore, the kinetics analysis demonstrated that Pb(II) sorption happened instantly and was attributed to the existence of several porosities that were heavily populated with O⁻ groups. A developed MOF (MnO₂-MOF) was applied to adsorb Pb(II) in a study conducted by Qin et al.³⁰² The MOF was synthesized using a simple oxidation process, and an equilibrium time of 1 hour was used to determine the metal uptake efficiency. The material revealed an adsorption capacity of 917 mg g⁻¹, which was attained as a result of inner-sphere complexation of hydroxyl groups with the metal ions.

Yin et al.³⁰³ made an effort to modify melamine using a MOF that possesses a structure similar to that of UiO-66 and used it to remove Pb(II). In comparison with the pristine MOF, the modified MOF exhibited high Pb(II) adsorption capacity (205 mg g⁻¹), at pH 6 and 40 °C. When the pH dropped from 6 to 5, a decrease in adsorption capacity of 122.0 mg g⁻¹ was noticed. The adsorption capability dropped when the pH value dropped, and the low pH caused the modified MOF to regenerate by desorbing the metal ion. Abbasi et al.³⁰⁴ created three-dimensional Co-MOF composites for Pb(II) adsorption. The pH of the solution, the concentration of metal ions, and the duration of treatment impacted the adsorbent's effectiveness.

Ricco et al.³⁰⁵ produced a magnetic framework composite based on aluminum (MFCs) for the removal of Pb(II) ions. By adjusting the 2-amino-1, 4-benzene dicarboxylic acid loadings, they created a series of amino-functionalized MIL-53(Al) MOFs. The magnetic framework composites based on aluminum demonstrated a noteworthy capacity for adsorption, reaching up to 492.4 mg g⁻¹. The investigation also found that as the degree of amino functionalization of the MOF increases, so does the metal ion absorption capability. Additionally, Sun et al.³⁰⁶ reported an Fe–BTC/PDA polymer-based MOF composite for Pb(II) adsorption, achieving an adsorption capacity of 394 mg g⁻¹. Additionally, Chakraborty et al.³⁰⁷ proposed a MOF based on tetracarboxylate and zinc(II), which demonstrated a maximum metal uptake of 71 mg g⁻¹.

3.1.2.3 Chromium. The two primary forms of chromium ions found in the environment are the trivalent (Cr(III)) and the hexavalent (Cr(VI)) species. Compared to Cr(VI), Cr(III) is less hazardous even though it is a target heavy metal pollutant in water. Because of its high toxicity, Cr(VI) has been linked to a number of disorders, including cancer, bronchial asthma, skin allergies, lung and nasal ulcers, and issues with reproduction and development.³⁰⁸ As per Lv et al.,²⁸⁶ the highest allowable concentration of chromium in drinking water is 0.1 ppm.

Using a microwave-assisted technique, Sathvika et al.³⁰⁹ created a Nitrosomonas-modified Uio-66 for the adsorption of Cr(VI). Primarily, the functional groups of the modified-MOF and the chromate ions interacted electrostatically to facilitate the uptake of Cr(VI). As for the Nitrosomonas-modified Uio-66, it attained 23.69 mg g⁻¹, while the pristine Uio-66 MOF and Nitrosomonas sp. showed Langmuir adsorption capabilities of 13.33 and 8.98 mg g⁻¹, respectively. The synergistic increase in the specific surface area and functional groups of the mixed materials is responsible for the improvement in adsorption performance. Saleem et al.³¹⁰ reported the usage of modified UiO-66 for the adsorption of Cr(III). The research yielded a resultant optimal adsorption capacity of 67.3 mg g⁻¹ for UiO-66-NHC(S)NHMe. Similar to this, post-synthetic alteration improved the performance of both the modified and unmodified UiO-66-NH₂ in Cr(III) adsorption compared to the virgin UiO-66.

A cationic Zr-MOF (ZJU-101) was developed by Zhang et al.³¹¹ to remove Cr₂O₇²⁻. By post-synthetically modifying MOF-867, they were able to create ZJU-101, which is composed of 2, 20-bipyridine-5, 50-dicarboxylate and zirconium metal ions. Tahmasebi et al.³¹² reported the synthesis and manufacture of three Zn-based MOFs using a mechanosynthesis approach: TMU-4, TMU-5, and TMU-6. According to their experimental findings, these MOFs showed maximum Cr(III) adsorption capacities of 127, 123, and 118 mg g⁻¹, respectively. In order to successfully extract Cr(VI) from aqueous solutions, Noraee et al.³¹³ used pristine Uio-66 and ZIF-8. It was shown that ZIF-8 and Uio-66 had maximal adsorption capabilities of 150 and 85.6 mg g⁻¹, respectively. The greater performance of ZIF-8 was attributed to its higher surface area; the potential impact of the MOF's composition was not considered. In contrast, Yang et al.³¹⁴ found that the adsorption capability of ZIF-67 was less than that of MMCs (18.0 mg g⁻¹), a MOF composite composed of MIL-100(Fe) and magnetic iron oxide particles.

In order to remove Cr(VI) from water, Aboutorabi et al.³¹⁵ created a novel three-dimensional framework based on lead and isonicotinate N-oxide (TMU-30). At a pH of 2–9, the optimum adsorption capacity was 145 mg g⁻¹. The adsorption of metal ions is significantly aided by TMU-30's N-oxide functional groups. In order to create an electrostatic interaction with chromate species, the positive N-oxide groups can serve as appropriate sites for their adsorption. With R² = 0.999, their findings demonstrated that the experimental isotherm data followed the Langmuir isotherm model.

An enhanced powder for Cr(VI) uptake from simulated wastewater was created by Guo et al.³¹⁶ using a BUC-17 MOF [Co₃(tib)₂(H₂O)₁₂][(SO₄)₃]. Ion-exchange and electrostatic interactions between the MOF and Cr(VI) were responsible for the 121 mg g⁻¹adsorption capacity that was attained. In order to adsorb chromate (CrO₄²⁻), Fei et al.³¹⁷ created a MOF by changing the composition ratio of two transition metals, Co(II) and Zn(II). Adsorption capabilities for CrO₄²⁻ varied across MOFs with varying Zn(II)/Co(II) ratios; the highest adsorption capacity for Cr(VI) was discovered for Zn_0.5Co_0.5-SLUG-35, which was 68.5 mg g⁻¹, where Zn and Co were distributed evenly.

A solvothermal approach was used by Maleki et al.³¹⁸ to synthesize copper-benzenetricarboxylate (Cu-BTC), which was then used to remove Cr(VI) from aqueous solution, and the produced MOF showed efficacious Cr(VI) adsorption. The utilization of a rhombic dodecahedral zeolitic imidazolate framework-67 (ZIF-67) based on Co is investigated by Li et al.³¹⁹ as a potential method for eliminating Cr(VI) from water. The first step of the Cr(VI) adsorption process was found to be rapid, and the time taken to attain equilibrium increased with the initial concentrations. With a maximum adsorption capacity of 15.4 mg g⁻¹ for Cr(VI), their findings indicate that ZIF-67's adsorption tends to follow the Langmuir isotherm model. Another development was the publication of Li et al.³²⁰ on the adsorption of Cr(VI) by a silver-triazolate MOF. A maximum absorption of 38 mg g⁻¹ of the cationic MOF was observed during the adsorption of Cr(VI), which was mostly accomplished through anion-exchange.

Recently, Jamshidifard et al.³²¹ have reported the adsorption of Cr(VI) using a hybrid system consisting of UiO-66-NH₂, chitosan, and polyacrylonitrile. The hybrid Uio-66-NH₂ was synthesized with the use of microwaves, and it was subsequently added to the chitosan/polyacrylonitrile solution by electrospinning and ultrasonication. For the regeneration of the adsorbent under study, five cautious cycles were performed, and the maximum adsorption capacity was found to be 373 mg g⁻¹. The increased surface area and surface functional groups of the composite allowed the adsorbent to perform exceptionally well.

3.1.2.4 Mercury. Mercury (Hg(II)) is a heavy metal that can be harmful and deadly and can exist in three different forms: elemental, organic, and inorganic salts.³²² However, there are major health risks associated with exposure to both organic and inorganic forms of mercury.³²³ Zhao et al.³²⁴ reported that methylmercury is the deadliest form of mercury, causing Minamata disease in individuals from Iraq and Japan who consumed foods contaminated with mercury. Furthermore, because of its affinity for the thiol group of proteins, mercury (Hg) can readily accumulate in the human body and can be harmful even at low concentrations. This means that exposure to Hg(II) can have an adverse effect on the human nervous system, brain, and kidneys.³²⁵ One of the most often employed technologies is adsorption, and several studies have documented the use of MOF-based adsorbents to extract Hg(II) from water. Although pristine MOFs can be employed for Hg(II) adsorption, most of these applications include functionalizing the MOFs to improve their Hg(II) selectivity.

According to Ke et al.,³²⁶ a thiol functionalized Cu-based MOF (Cu-BTC-DTG), which was created using a coordination-based post-synthetic approach, was able to adsorb Hg(II). The maximal Hg(II) adsorption capacity from water was found to be 714.29 mg g⁻¹. Adsorption sites are provided by the thiol groups that are present on the porous MOF's inner surface. Additionally, they observed that the pristine MOF is unable to adsorb Hg(II) in comparable circumstances. Because of its special ability to bioaccumulate and biomagnify, mercury(II) is a unique metal ion that should be taken seriously at ultra-low concentrations.³²⁷ Another breakthrough involves an experiment conducted by Li et al.²⁹⁵ in which the NH₂-MiL-68(In) MOF is synthesized using a solvothermal technique, and a thiol group (–SH) is added post-synthetically to generate SH-MiL-68(In), which is then utilized to remove Hg(II). A maximum adsorption capacity of 450 mg g⁻¹ for Hg(II) was attained. Strong acid–base interactions allowed for the high adsorption to be achieved.

Liang et al.³²⁸ reported the addition of indium(III) sulphide nanoparticles (In₂S₃) to the MOF (MIL-101) for the adsorption of Hg(II). The procedure entails utilizing In(NO₃)₃ solution to transform pristine MIL-101 into In³⁺@MIL-101, which is then transformed into In₂S₃@MIL-101 via a solid–gas reaction with H₂S, and the composite attained 518 mg g⁻¹ Hg(II) adsorption capacity. Rudd et al.³²⁹ found that this method yielded a more potent adsorbent in comparison to certain other investigations that prepared a sulfur-functionalized MOF by the traditional solvothermal methodology.

Yang et al.³³⁰ suggested growing Zr-DMBD (Zr-2,5-dimercaptoterephthalic acid-based MOF) on 3D macroporous carbon using a free-standing electrode. Therefore, a vast surface area with 3D macropores is provided by the uniformly decorated MOF nanoarray, facilitating the quick measurement of Hg(II). Zr-DMBD MOFs demonstrated a dramatic response to the addition of 2 μM mercury under optimal pH settings at 0.2 M HAc-NaAc buffer (pH – 6.0), and the electrochemical response at the modified electrode is found to be six times higher than 3D kenaf stem-derived carbon.

A photo-assisted post-synthetic modification technique was examined by Yin et al.³³¹ To evaluate the adsorption of Hg(II) from aqueous solutions, a pyrimidine–thione fragment was introduced onto ZIF-90. During synthesis, pristine ZIF-90, thioureas, and either tetrahydropyran (THP) or tetrahydrofuran (THF) under UV radiation underwent a multicomponent reaction. ZIF-90-THP and ZIF-90-THF exhibited the highest levels of Hg(II) adsorption among the functionalized materials, reaching 596 and 403 mg g⁻¹, respectively. The pristine ZIF-90 demonstrated a lower adsorption capacity of only 47 mg g⁻¹.

Li et al.³³² reported using generated ZrO_x, ZrO_xyPhos, and ZrSulf from MOFs for the adsorption of Hg(II) via a ligand extraction approach. In the ligand extraction procedure, inorganic moieties were substituted for the pure MOF's organic ligand. Here, the inorganic moieties that were utilized to make ZrO_x, ZrO_xyPhos, and ZrSulf were NaOH, Na₃PO₄ and Na₂S·9H₂O, respectively. ZrSulf had the highest adsorption capability of 824 mg g⁻¹, followed by ZrO_xyPhos and ZrO_x at 663 and 485 mg g⁻¹, respectively. In comparison to the pristine UiO-66-50Benz, which has an adsorption capacity of 363 mg g⁻¹, both demonstrated a strong removal capability.

3.1.2.5 Copper. Living organisms need trace elements like copper ions (Cu(II)) at low concentrations.³³³ However, at certain environmental concentrations, it becomes a contaminant to the environment and public health.³³⁴ Copper is released into the environment by several industries, including metal cleaning and plating, paints and pigments, mining, smelting, petroleum refinery, brass rinsing, paper board, wood pulp, and printed circuit boards. The environment frequently contains significant amounts of copper because of its widespread industrial use.³³⁵

Numerous health issues, including neuro/hepatodegenerative diseases like Alzheimer's and Wilson's diseases, have been connected to the use of Cu(II).³³⁶ Furthermore, a high consumption of Cu(II) will raise blood pressure, cause damage to the kidneys and liver, and increase respiration rates.³³⁷ Strict recommendations have been made regarding the content of Cu(II) in drinking water in order to safeguard human health. For instance, the United States Environmental Protection Agency (USEPA) sets the maximum limit of Cu(II) in drinking water at 1 mg L⁻¹,³³⁸ whereas the World Health Organization (WHO) set it at 2 mg L⁻¹.³³⁹

Ghaedi et al.³⁴⁰ investigated the usage of a Cd-based MOF (Cd-MOF-74) as an efficient adsorbent for the removal of Cu(II) from aqueous solutions. The optimum adsorption capacity of 435 mg g⁻¹ for Cu(II) is exhibited by the produced MOF. While retaining its effectiveness, three conservative cycles were also noted throughout the regeneration research. To create the Cd-TPA template, terephthalate, dimethylformamide, and Cd(CH₃COO)₂·2H₂O were combined in a room temperature ultrasonic driven reaction. Wang et al.³⁴¹ reported the use of a ceramic membrane and a Zr-MOF functionalized with amino groups to remove Cu(II). Based on their research, a comparison was made between the adsorptive capacities of a ceramic membrane and a pure Zr-MOF. Cu(II) adsorption capacities of 988 and 60 mg g⁻¹ were attained for the Zr-MOF/ceramic membrane and pristine Zr-MOF, respectively. The stability of the MOF is still a problem, despite the concerning improvement in the combined Zr-MOF/ceramic membrane adsorption capability.

Bakhtiari and Azizian³⁴² examined and documented the adsorption of Cu(II) ions using a nanoporous zinc-based MOF. Results obtained from their experimental study showed a maximum adsorption capacity of 290 mg g⁻¹ and a BET surface area of 888.5 m² g⁻¹. They also clarified that non-uniform adsorption of Cu(II) ions and slow kinetics are caused by the heterogeneity of the MOF surface. Graphene oxide was integrated into a pristine 2-aminoterephthalic acid-ZnO₄ MOF (IRMOF-3) in an experimental work conducted by Rao et al.³⁴³ The objective of the work was to remove Cu(II) from an aqueous solution. The addition of graphene oxide increased the pristine IRMOF-3's adsorption capacity and selectivity; the maximum adsorption capacity reached 254 mg g⁻¹. Zheng et al.³⁴⁴ used a solvothermal technique to create a Cd-based MOF (Cd-MOF-74) and evaluated its efficacy as an adsorbent to remove Cu(II) from solutions. An adsorption capacity of 190 mg g⁻¹ was reached by the produced MOF. Mohmoodi et al.³⁴⁵ presented an example of how a magnetic bio nanocomposite consisting of an eggshell membrane-zeolitic imidazolate framework (ZIF) can be used to extract Cu(II) from an aqueous medium. The ZIF-67 MOF was effectively stabilized on the magnetic eggshell membrane surface functionalized with iron oxide, resulting in the formation of the ZIF-67@Fe₃O₄@ESM composite. ZIF-67@ Fe₃O₄@ESM has a maximum adsorption capacity of 344.82 mg g⁻¹, which means that it may be effectively employed to remove Cu(II) ions. A significant contribution to the high Cu(II) can be attributed to the novel adsorbent's high surface area of 1263.9 m² g⁻¹.

3.1.2.6 Cadmium. Even in low amounts, cadmium (Cd) is one of the most hazardous non-essential heavy metals found in the environment. The exposure to Cd at low concentration levels is extremely harmful to the kidneys and can result in renal failure.³⁴⁶ Many industries, for example, electroplating, waste discharged nickel runoff, phosphate fertilizer, cadmium-nickel batteries, mining, pigments, stabilizers, alloys, petroleum refining, welding, and the pulp and paper sectors, release cadmium into the environment.³⁴⁷ Despite being present in the environment in small amounts, Cd is known to be bioaccumulative and to negatively impact a variety of creatures.³⁴⁸ When it comes to non-essential heavy metals in the environment, cadmium is among the most dangerous even in little amounts. The USEPA has set a maximum contamination level (MCL) of 0.005 mg L⁻¹ for cadmium in drinking water.³⁴⁹

Wang et al.³⁵⁰ performed an experiment in order to determine Cd concentration at ultra-trace levels and also to improve the conductivity of a MOF. They did this by dispersing UiO-66-NH₂ in HCl and then adding aniline and APS, which self-polymerizes to produce PANI loaded on the UiO-66-NH₂ matrix. The deposition potential response indicated a linear connection between current and Cd content across the range of 0.5 to 600 mg L⁻¹, and the detection limit was determined to be 0.3 mg L⁻¹. A deposition potential of −1.2 V, an accumulation period of 120 s, and an optimum pH of 5.0 were selected. In a separate study, Roushani et al.³⁵¹ reported the adsorption of Cd(II) on a TMU-16-NH₂MOF. It was possible to get the maximal adsorption capacity of 126.6 mg g⁻¹.

Wang et al.³⁵² conducted an experiment to test the viability of removing Cd(II) from aqueous solution using the Cu₃(BTC)₂-SO₃H framework, which was created by oxidizing Cu₃(BTC)₂. At a pH of 6, the maximum adsorption capacity was attained. Because SO₃H has numerous binding sites and coordination modes, the sulfonic groups of the MOF and Cd(II) ions exhibited a chelation reaction during the adsorption process, which increased the selectivity for Cd(II).³⁵² The PCN-100 MOF was presented by Fang et al. (2010) and was synthesized from TATAB ([[Zn₄O(C₂₄H₁₅N₆O₆)₂(H₂O)₂]·6H₂O·DMF]_n (1) using Zn₄O(CO₂)₆ secondary building units and 4,4,4′′-s-triazine-1,3,5-triyltri-p aminobenzoate) linkers. It was discovered that the MOF used linkers to engage in chelating coordination mode interactions with metal ions. Rahimi and Mohaghegh³⁵³ also assessed the Cd(II) adsorption potential of a magnetic Cu-terephthalate MOF. The maximal adsorption capacity of 100 mg g⁻¹ is achieved by the chemical adsorption technique. COOH groups on the MOF significantly improved the adsorption performance. Cd(II) adsorption by AMOF-1 was reported by Chakraborty.³⁰⁷ Utilizing Zn and tetracarboxylate linkers as its foundation, the MOF attained an adsorption capacity of 41 mg g⁻¹ (Table 2).

Table 2 Summary of reported results for heavy metal removal from aqueous solution and wastewater by MOF-based adsorbents

Adsorbate	Adsorbent	Adsorption capacity (mg g^−1)	Time (min)	Ref.
As	NH2–MIL-88(Fe)	125	60	282
	UiO-66	303	2880	268
	Fe–BTC	12.3	10	271
	Fe3O4@UiO-66	73.2	100	354
	UiO-66-(SH)2	40	1440	355
	γ-Fe2O3@CTF-1	198	1440	356
	Zn-MOF-74	As(V): 325	4	357
		As(III): 211
Pb	MIL-101(Fe)/GO	128.6	15	358
	Fe3O4–NHSO3H@HKUST-1	384.6	90	359
	Cu(TCPBDA)	300	60	360
	UiO-66-NH2@CA	89.40	4080	361
	Amide-based COF	185.7	1440	362
	PAN/chitosan/UiO-66-NH2	115	60	321
	UiO-66-RSA	189.8	180	363
	Fe–BTC/PDA	394	1440	306
	MOF-2(Cd)	434.78	180	340
	Cu-MOFs/Fe3O4	219	60	364
Cr	ZIF-8 (150)	150	350	313
	Cu-BTC	48	—	318
	UiO-66	85.6	350	313
	Nitrosomonas modified-UiO-66	921	180	309
	IMF-Cr-MOF	321	50	365
	PAN/chitosan/UiO-66-NH2	99.5	60	321
	[Co3(tib)2(H2O)12](SO4)3(BUC-17)	121	480	316
	HKUST-1	24.20	60	366
Hg	γ-Fe2O3@CTF-1	165.8	1440	356
	SH-MIL-68(In)	450	120	295
	UiO-66-50Benz	824	120	367
	In2S3@MIL-101	518.2	60	328
	ZIF-90-THP	596	10	331
	Fe–BTC/PDA	1634	1440	306
Cu	MOF-2(Cd)	769.23	180	340
	[(Zn3L3(H2O)6)][(Na)(NO3)]	379.13	—	368
	Cd-MOF-74	189.5	10	344
	IRMOF-3/GO	254.14	480	343
Cd	γ-CD MOF-NPC	140.85	60	369
	PAN/chitosan/UiO-66-NH2	107.6	60	321

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In summary, metal–organic frameworks and their composites exhibit favorable adsorptive features for heavy metal removal. Functionalization of such metal–organic frameworks can increase their environmental acceptability, strengthen their bonds with heavy metals, decrease coagulation and enhance the adsorption efficiency, resulting in increased removal of heavy metals from wastewater and aqueous solutions. Moreover, most adsorption kinetics can be illustrated by the pseudo-second-order model and the adsorption equilibrium is well described by the Langmuir isotherm model. Comparative results of heavy metal removal utilizing metal–organic frameworks and other adsorbents are summarized, clearly indicating the relatively higher removal capacities of MOFs.

3.1.3 Adsorption of toxic gases. Actually, a significant portion of MOFs have demonstrated adsorption behavior for gas molecules, and a smaller subset of these MOFs has also been examined for the combination of gases through the use of techniques like gas chromatography or breakthrough tests. Additionally, it has been demonstrated that MOFs' selective adsorption performance may be adjusted by altering their molecular architectures and pore characteristics. The majority of these gases are waste products of industrial operations that are discharged into the environment; in certain instances, individuals are even exposed to them directly. Effective capture of these dangerous gases is extremely important for the protection of the environment. According to recent findings, MOFs are superior to other materials when it comes to the high adsorption of poisonous gases like CO₂, CO, NO₂, and H₂S.³⁷⁰

According to Yazaydin et al.,³⁷¹ the carbon dioxide adsorption capacities of Ni/DOBDC and Mg/DOBDC, which are also referred to as Ni-MOF-74 and Mg-MOF-74 or CPO-27-Ni and CPO-27-Mg, are 5.95 mol kg⁻¹ and 4.07 mol kg⁻¹, respectively, at 0.1 atm and 298 K. The authors observed that MOFs with a high density of open metal sites, such as Ni/DOBDC and Mg/DOBDC, are attractive candidates for CO₂ extraction from gas samples instead of MOFs with high surface area or free volume. At 0.1 atm and 25 °C, they found that Ni/DOBDC has a greater CO₂ adsorption capacity than 5 A zeolites and NaX. The possibility of using composites of MOFs (MOF-5, Cu-BTC, or MIL-100(Fe)) and a graphitic substance (graphite or graphite oxide, GO) as adsorbents for the removal of NH₃, H₂S and NO₂ in ambient settings was investigated by Petit and Bandosz.³⁷² In order to create composites with unique characteristics, a new pore space was created at the interface between the carbon layers and the MOF units as a result of the coordination between the oxygen groups of GO and the open metal sites of porous MOFs. When a GO/Cu-BTC composite was used instead of virgin Cu-BTC, there were increases in the adsorption capacity of over 12% for NH₃, 50% for H₂S, and 4% for NO₂. The creation of additional porosity in the interface, where dispersive forces were strongest, provided an explanation for the increased adsorption capabilities for the dangerous gases. Ebrahim and Bandosz³⁷³ conducted an experiment showing that after grafting with melamine (melamine@UiO-66(Zr)-COOH and melamine@UiO-67(Zr)-COOH), respectively, the adsorption of NO₂ gas onto UiO-66(Zr)-COOH and UiO-67(Zr)-COOH was dropped from 40–73 and 79–118 g kg⁻¹ to 3–10 and 41–93 g kg⁻¹.

The study conducted by Choi et al.³⁷⁴ involved the synthesis of a three-dimensional (3D) ABDC MOF (In) through the interaction of a ditopic azobenzene-4,4′-dicarboxylic acid (H₂ABDC) and salt with 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM][BF₄]) as an ionic liquid. When the 3D ABDC (In) MOF was used for CO₂ gas adsorption, the pre-adsorbed CO₂ molecules improved the subsequent adsorption of freshly introduced CO₂ molecules. The amount of CO₂ adsorbed by the ABDC (In) MOF was 81.3 cm³ g⁻¹ (3.63 mM g⁻¹). A fluorinated metal–organic framework (FMOF-2, derived from 2,2′-bis(4-carboxyphenyl)hexafluoropropane and zinc nitrate hexahydrate) was investigated by Fernandez et al.³⁷⁵ for the potential application in the elimination of harmful acidic gases. The adsorption of SO₂ and H₂S on FMOF-2 was remarkably stable. The weight capacities of SO₂ and H₂S with FMOF-2 at room temperature and 1 bar were calculated to be 14.0% and 8.3%, appropriately.

Glover et al.³⁷⁶ investigated the adsorption of various hazardous gases, such as NH₃, CNCl, SO₂, and octane vapor, using M-CPO-27 (M Zn, Co, Ni, or Mg) in both dry and humid settings. The results of this ground-breaking experiment showed that all of these MOFs with open metal sites can only effectively adsorb the hazardous gases under study when the environment is dry. In humid environments, on the other hand, the adsorption capacity is significantly decreased as water begins to adsorb competitively. With regard to NH₃ gas, adsorption of ammonia occurred under both humid and dry circumstances, with little discernible drop in adsorption capability. Overall, it can be said that water vapor hinders different gases from adhering to CPO-27 type materials. A study conducted by Petit et al.³⁷⁷ found that the adsorption capacity of the Cu-BTC composite increased from 92 to 200 g H₂S per kg in moist settings and from 110 to 135 g NO₂ per kg under dry conditions as a result of graphite oxide (GO) grafting. However, as indicated by Petit and Bandosz,³⁷² the adsorption of H₂S and NO₂ onto the GO@Cu-BTC composite under ambient circumstances resulted in a 50% and 12% increase in the adsorption capacity, respectively.

Comparative research on MOFs that exhibit stability and ease of regeneration upon H₂S sorption via pressure swing adsorption mechanisms was conducted by Hamon et al.³⁷⁸ Low pressure causes the pores of MIL-53s(Al, Cr) to close due to the strong interaction between the polar H₂S molecules and μ₂-OH of the inorganic chain. Remarkably, as pressure increased, the strong H₂S⋯HO (on the metal site of MOFs) contacts broke, reopening the pores. This led to the filling of all the pores through weak host–guest interactions, resulting in steps in the adsorption isotherm. The initial and subsequent phases commenced at 4.5, 118 kPa and 9.0, 210 kPa for MIL-53(Al) and MIL-53(Cr), correspondingly. At high pressure (1.6 MPa), the maximum H₂S sorption capacities were reached at 13.12 and 11.77 mmol g⁻¹ for MIL-53(Al) and MIL-53(Cr), respectively. The adsorption capacities of MIL-47 and MIL-53s(Al, Cr) at high pressure were comparable, indicating that the pores in MIL-53s reopened at high pressure. Additionally, the large pores of MIL-100(Cr) and MIL-101(Cr) showed type-I shaped adsorption isotherms, indicating significant uptake of H₂S by the authors nevertheless, and the adsorption seemed to be irreversible. For MIL-100(Cr) and MIL-101(Cr), the optimum adsorbed quantities at 2 MPa were 16.7 and 38.4 mmol g⁻¹, respectively. The irreversible adsorption phenomenon was explained by the possibility of either the framework being partially destroyed or experiencing significant interactions with the H₂S molecules. Furthermore, in a separate research, it was also revealed by Hamon et al.³⁷⁹ that the adsorption of H₂S primarily takes place via the hydrogen bond formation between the H₂S molecules and the μ₂-O atom of the V=O⋯V moiety in MIL-47, wherein H₂S functions as a hydrogen donor. It was also proposed that acidic H₂S interacts with additional basic centers in MIL-47, such as the π electrons of the benzene ring and the oxygens of the carboxylate group.

Additional noteworthy instances are the flexible MOFs, where the opening of the pores is contingent upon the pressure at which the adsorbed guest molecules occur. Kitaura et al.³⁸⁰ originally noticed this phenomenon in Cu(dhbc)₂(bipy) (dhbc = 2,5-dihydroxybenzoate; bipy = 4,4′-bipyridine). In a different study, Dathe et al.³⁸¹ tested the efficacy of Ba(CH₃COO)₂ (or Ba(NO₃)₂) and BaCl₂-impregnated Cu-BTC in adsorbing SO₂. Little barium salt microcrystals were produced by impregnation in the pores of Cu-BTC, although only BaCl₂ showed signs of partially destroying the host structure. Based on the concentration of Ba²⁺, the authors noted that at high temperatures, SO₂ uptake exceeded the stoichiometric capacity. Thus, chemical interaction between the metal cations (from the MOF) and SO₂ is responsible for the excess SO₂ adsorption and for the synthesis of Cu-sulfates at the end. Cu-BTC responded well as a host material to hold widely distributed barium salts at low temperatures. Conversely, Cu-BTC broke down at high temperatures, resulting in isolated Cu species that served as SO_x storage locations before Cu-sulfates were eventually formed.

Furthermore, Garcia-Ricard and Hernandez-Maidonado³⁸² assessed the dynamics of CO₂ adsorption on three distinct forms of Cu₂(pzdc)₂(bipy) (pzdc = pyrazine-2,3-dicarboxylate) pretreated at varying temperatures, while Zhao et al.³⁸³ investigated CO₂ diffusion in cubic MOF-5 crystals and found that the process is activated and that CO₂ loading has very little effect on the rate of diffusion. Gravimetric adsorption of NO on Cu-BTC's open metal sites at 196 K (1 bar) was reported by Xiao et al.³⁸⁴ The results showed that around 9 mmol NO was adsorbed over 1 g of Cu-BTC, which was much more than any other porous solid that had been reported for the adsorption of NO. MOF-74(Mg) demonstrated a much greater CO₂ adsorption of 5.3 mM g⁻¹ when exposed to 40 °C and 150 mbar.³⁸⁵ Lower adsorption capability (2.3 mM g⁻¹) at 40 C and 150 mbar was obtained by using MIL-101(Cr)-SO₃H-TAEA [tris(2-aminoethyl) amine], which was further lowered to 1.1 mM g⁻¹ by lowering the temperature and pressure to 20 C and 0.4 mbar, respectively.³⁸⁶ Following cationic exchange, the adsorption capacity of CO₂ onto the 3D anionic MOF (Ni) was greatly enhanced.³⁸⁷

A Cu-BTC framework with 4 weight percent water molecules coordinated to the open metal sites of the framework showed remarkable improvements in CO₂ adsorption and selectivity over N₂ and CH₄, as reported by Yazaydin et al.³⁸⁸ The quadrupole moment of CO₂ interacts with the sorbent's electric field gradient, which is enhanced when water fills the copper open metal site, and it was proposed that these interactions account for the majority of the boost in CO₂ adsorption. The impact of the metal center on the CO₂ adsorption selectivity and adsorption capacity was demonstrated by Dietzel et al.³⁸⁹ using a set of isostructural MOFs M-CPO-27 (M: Ni, Co, Zn, Mg, and Mn). The CO₂ uptake values for CPO-27(Ni) and CPO-27(Mg) were reported to be 51 weight percent and 63 weight percent, respectively, at 298 K and high pressure (50 bar). According to Caskey et al.,³⁹⁰ using CPO-27(Co) and CPO-27(Mg), CO₂ uptake values were 30.6 and 35.2 weight percent at 1 atm, respectively.

Furukawa et al.³⁹¹ demonstrated a remarkable CO₂ uptake using MOFs that have an ultrahigh surface area. MOF-210 [Zn₄O(BTE)_4/3(BPDC)] comprises two components: BPDC (biphenyl-4,4′-terephthalate) and BTE (benzene-1,3,5-triyl-tris(ethyne-2,1-diyl)). At 50 bar, MOF-200 [Zn₄O(BBC)₂(H₂O)₃ in which BBC: 4,4′,4′′-(benzene-1,3,5-triyl-tris(benzene-4,1-diyl))tribenzoate] with BET surface areas of 6240 and 4530 m² g⁻¹, respectively, were able to absorb 2400 mg g⁻¹ CO₂, surpassing the uptake values of other highly porous MOFs such as MOF-177, MIL-101(Cr)c, and so forth.³⁹² Another development was the discovery by Karra and Waiton³⁹³ that Cu-BTC was a highly selective CO adsorbent at 298 K, as demonstrated by a molecular simulation analysis. The partial charges of CUS of Cu-BTC (Cu²⁺ sites) and the CO dipole were said to interact electrostatically, which dominated the adsorptive performance.

Britt et al.³⁹⁴ provided evidence regarding the potential utility of six MOFs, the Zn₄O(CO₂)₆ cluster linked by terephthalate (MOF-5), 2-amino terephthalate (IRMOF-3), benzene-1,3,5-tris(4-benzoate) (MOF-177), diacetylene-1,4-bis(4-benzoic acid) (IRMOF-62), Zn₂O₂(CO₂)₂ chains linked by 2,5-dihydroxyterephthalate (CPO-27), and Cu-BTC, in the adsorption and separation of several hazardous gases or vapors, such as SO₂, chlorine, NH₃, benzene, ethylene oxide, and tetrahydrothiophene. The outcomes were compared with BPL carbon. Numerous elements have been demonstrated to be significant in influencing the dynamic adsorption performance of these MOFs, including the open metal sites of Cu-BTC or CPO-27 (also known as M₂(dobdc)(H₂O)₂; H₄dobdc = 2,5-dihydroxyterephthalic acid) and the active adsorption site with specific functional groups (like NH₂ in IRMOF-3). The capacity of CPO-27 was six times greater than that of the BPL carbon and it outperformed the other MOFs in terms of SO₂ adsorption capacity. The presence of both the potentially reactive oxo group in CPO-27 and a highly reactive 5-coordinate zinc species is the cause of this advantageous adsorption. However, because Cl₂ does not naturally function as a ligand, Cu-BTC had a high efficiency that was on par with or higher than that of BPL carbon for all of the gases that were tested. Regarding NH₃, the adsorptive performance of IRMOF-3 is significantly enhanced by the presence of NH₂ in comparison to the virgin IRMOF-1 or MOF-5. Due to the NH₃'s propensity to form hydrogen bonds, IRMOF-3 was able to adsorb about 71 times as much NH₃ prior to breakthrough than BPL carbon.

In summary metal–organic frameworks (MOFs) are a novel class of adaptable porous materials with enormous surface areas, mechanical flexibility, programmable topologies, and tunable pore size and thickness. Adsorbents based on molecularly oxidized fuel (MOFs) are very suitable for absorbing gases, facilitating the assimilation of gaseous fuels and the elimination of greenhouse gases. A rigorous assessment was conducted on the adsorption of gaseous molecules utilizing MOFs. MOFs have potential as economical, moisture-stable adsorbents for natural gas.

3.1.4 Adsorption of antibiotics. In order to both prevent and treat major infections in living things, antibiotics are necessary. It is estimated that urine and feces excrete between 5 and 90 percent of the active antibiotic components that are eaten. According to Monahan et al.,³⁹⁵ these substances pollute groundwater and surface water, encouraging the microbial resistance of natural water bodies. They are categorized as J01 medications for systemic antibacterial usage in accordance with the WHO's therapeutic chemical categorization.³⁹⁶ Antibiotic excretion, however, can contaminate the environment and water sources, posing a threat to the world's ecology. Antibiotic overuse and drug resistance are the main causes of this pollution, which is becoming a global problem.³⁹⁷

Various methods are used to eliminate antibiotics, including membrane filtration, biological treatment, adsorption,³⁹⁸ electrochemical treatment,³⁹⁹ advanced oxidation technology,⁴⁰⁰ and photodegradation.⁴⁰¹ Adsorption and photodegradation are particularly favored for their high efficacy, simplicity, low cost, sustainability, and versatility.⁴⁰² Nevertheless, there are inherent restrictions on these techniques that make them less effective and difficult to use widely. These restrictions include things like the efficiency of removal, the complexity of function, and the production of byproducts that may cause cancer and mutations. MOFs' huge specific surface area, well-organized pores, and structural plasticity allow them to tackle these issues.

For the purpose of adsorbing nitrofuran antibiotics from wastewater, Lei et al.⁴⁰³ created a stable Cd-MOF (MOF-1) and created a composite with macroporous, elastic, and inexpensive melamine foam (MOF-1@MF) via a one-pot solvothermal synthesis. Strong hydrogen bonding between MOF-1's urea groups and the antibiotics' –NO₂ groups was made possible. A chemically resistant, hydrophobic Zn-based MOF-1 ([{Zn(hfdba)(L1)}·DMF]_n) and its membrane have been recently created by Mukherjee et al.⁴⁰⁴ for the detection of dangerous nitro-explosives and nitrofuran antibiotics, as well as for the adsorption of TNP from aqueous solutions. This MOF-1 exhibits outstanding ultralow sensitivity for precisely detecting TNP and NZF in environmental samples, thanks to its evenly functionalized lozenge-shaped pores.

Tetracycline adsorption from water was studied by Xia et al.⁴⁰⁵ utilizing three Zr-MOFs: MOF-525, NU-1000, and UiO-66. While NU-1000 had a BET surface area of 1487 m² g⁻¹ with a type-IV adsorption isotherm, UiO-66 and MOF-525 had BET surface values of 1249 and 2224 m² g⁻¹ with type-I adsorption isotherms. They were all capable of showing maximum adsorption capacities of 145, 356, and 807 mg g⁻¹. According to Yang et al.,⁴⁰⁶ doping Mn onto UiO-66 increased its surface area and active centers, which improved the material's capacity to remove tetracycline by 4.9 times when compared to unadulterated UiO-66. Because of mesopores, Zhang et al.⁴⁰⁷ produced porous UiO-66 under reflux circumstances, which resulted in a 430% increase in tetracycline adsorption efficacy.

CuCo/MIL-101, a bimetallic doped material, for tetracycline adsorption was described by Jin et al.⁴⁰⁸ They proposed that metal doping modifies surface electrical characteristics and strengthens chemical interactions, which impact antibiotics' electrostatic interactions. Zn-MIL-53(Fe) was used by Xiong et al.⁴⁰⁹ to create magnetic carbon-aFe/Fe₃C under N₂ for tetracycline adsorption. According to Sun et al.,⁴¹⁰ incorporation of additional functional groups made possible by graphene oxide (GO) allowed UiO-66-(OH)₂/GO to greatly boost adsorption efficiency when compared to UiO-66-(OH)₂ alone. Tetracycline adsorption was further improved by Wang et al.⁴¹¹ by in situ growth of UiO-66-(COOH)₂ on GO. Hydrogel–GO was used as a substrate for ZIF-67 growth by Kong et al.,⁴¹² which reduced particle aggregation and increased adsorption efficiency.

In order to activate peroxymonosulfate for the oxidative degradation of oxytetracycline (OTC), Mao et al.⁴¹³ developed Co@C-600, a highly efficient magnetic carbon generated from a Co-based MOF. Electron paramagnetic resonance experiments verified that they used both radical (OH and SO₄⁻) and nonradical pathways to achieve over 89% degradation of OTC in under 15 minutes. Tetracycline and norfloxacin were removed from an Fe MOF by both single and competitive adsorption processes, according to Zhou et al.⁴¹⁴ When comparing the competing system to the single adsorption, they discovered lower tetracycline and norfloxacin adsorption efficiency. The substance showed signs of durability and reusability.

Wu et al.⁴¹⁵ examined the use of a composite of Fe₃O₄ with HKUST-1, which is well-known for having a high surface area and accessible open metal sites (OMSs), to produce a water-stable magnetic composite (Fe₃O₄@HKUST-1). They investigated how well it eliminated medications called fluoroquinolones. Fe₃O₄ and other magnetic materials are highly valued for their ease of regeneration and separation in the presence of an external magnetic field. The protective effect of Fe₃O₄ nanoparticles on the Cu(II) metal sites, which thwarts breakdown by water molecules, is responsible for the water stability of these composites. Furthermore, Fe₃O₄@Cys@MIL125-NH₂ was created by Lian et al.⁴¹⁶ for the elimination of fluoroquinolones and could be retrieved using a magnet. MIL-101(Cr)–HSO₃ was created by Guo et al.⁴¹⁷ by functionalizing terephthalic acid with –HSO₃ in order to improve electrostatic interactions with fluoroquinolones. In order to remove levofloxacin from water, Chaturvedi et al.⁴¹⁸ produced Fe-based MOFs (MIL-100(Fe)) using a solid method. They achieved great adsorption efficiency (87.34 mg g⁻¹), principally through electrostatic interactions and hydrogen bonding mechanisms.

The best removal efficiency for ciprofloxacin and tetracycline was determined by Kim et al.⁴¹⁹ utilizing a batch model to manufacture a rGO/alginate coated Al-based MOF adsorbent. At an operating temperature of 40 °C, the adsorbent exhibited the highest removal capacity of around 40.76 mg g⁻¹for ciprofloxacin and 43.76 mg g⁻¹for tetracycline at pH 7. The contact period was approximately 12 hours. In order to study ciprofloxacin adsorption behavior with various structural features, Li et al.⁴²⁰ synthesized nanoporous carbon (NPC) using ZIF-8 derived carbonization. Yuan et al.⁴²¹ looked into the use of Konjac glucomannan (KGM) as a ZIF-8 carrier. KGM was chosen because it is inexpensive and biodegradable, which enables the formation of floating aerogels following lyophilization. According to the experimental findings, the aerogel could be effectively reused and demonstrated good adsorption performance for ciprofloxacin.

According to Moradi et al.,⁴²² the magnetic composite Fe₃O₄ MIL-100(Fe) and MOF-235(Fe) was utilized to adsorb ciprofloxacin. Additionally, Wu et al.⁴¹⁵ added Fe₃O₄ to HKUST-1 preparation to create a magnetic material (Fe₃O₄/HKUST-1) that can be recycled ten times for ciprofloxacin adsorption. Bayazit et al.⁴²³ used a sedimentation process in another advancement. In order to create composite materials with excellent ciprofloxacin adsorption capabilities, he added MIL-101(Cr) during the precipitation method of creating Fe₃O₄ by allowing Fe₃O₄ to be deposited on the surface of MIL-101(Cr).

To effectively remove cephalexin (CFX) from wastewater, Zhao et al.⁴²⁴ also synthesized another very stable Zr MOF PCN-777. The MOF in question demonstrated improved porosity, larger pore size, high specific surface area (2004 m² g⁻¹), and remarkable water stability. A mesoporous cage measuring 32 Å (a pore volume of 2.7 cm³ g⁻¹), greater than cephalexin (12 Å × 7 Å), is present in PCN-777, which exhibits exceptional water stability and a high BET surface area of 2004 m² g⁻¹. PCN-777 demonstrated a maximum adsorption efficiency of 442.48 mg g⁻¹ CFX at 303 K, higher than those of other types of adsorbents, such as activated carbon (AC), natural zeolites, SBA-15, etc. These results demonstrate PCN 777's appropriateness and potential use as an effective adsorbent for removing antibiotics from wastewater, as well as its ability to serve as a model for the synthesis and design of other adsorbents.

The effectiveness of the adsorptive removal of antibiotics from water was investigated by Gao et al.⁴²⁵ in relation to the breathing effect or framework flexibility. For the adsorptive removal of sulfamethoxazole (SMZ, a sulphonamide antibiotic often used to treat urinary tract infections) from aqueous solution, three isomorphous flexible MIL-53 adsorbents, namely MIL-53(Cr), MIL-53(Al), and MIL-53(Fe), were selected. Due to its flexibility, MIL-53 can take on several shapes, such as the narrow pore (NP), large pore (LP), and intermediate pore (INP) forms, depending on which metal ions are chosen. In order to achieve quick and selective adsorption of sulfamethoxazole from H₂O solutions, Cheng et al.⁴²⁶ created a surface molecularly imprinted polymer on a MOF (MIP-IL@UIO-66). The MOF discussed above demonstrated a maximum adsorption efficiency of 284.66 mg g⁻¹and reached adsorption equilibrium in 10 minutes. With an adsorption effectiveness of over 92% even after five cycles of reusability, the adsorbent showed good stability and reusability. Sulfamethoxazole and other antibiotics may be more effectively removed from water, thanks to the synthesized MOF's superior adsorption efficiency, selectivity, quick adsorption rate, and reusability, as demonstrated in the study.

Azhar et al.⁴²⁷ employed HKUST-1 in conjunction with open metal sites (OMS) to remove sulfachloropyridazine (SCP) from wastewater by adsorptive means. The pore volume (0.78 cm³ g⁻¹) and pore size (1.7 nm) of HKUST-1 were both greater than those of SCP molecules (0.81 × 0.87 × 0.65 nm), suggesting that SCP would preferentially adsorb into the pores of HKUST-1. The BET surface area of HKUST-1 was estimated to be 1700 m² g⁻¹. At 298 K, HKUST-1's adsorption capacity of 384 mg g⁻¹ toward SCP was observed, accompanied by a high rate of adsorption that was nearly finished in 15 minutes. An experiment was conducted by Zhao et al.⁴²⁸ to remove the antibiotic chloramphenicol from wastewater using a new MOF-based adsorbent known as PCN-222. This MOF-based material quickly reached adsorption equilibrium and showed improved adsorption efficiency. The remarkable ability of PCN-222 to eliminate substances was attributed to its unique pore structure, H-bonding, and electrostatic interactions. The study's conclusions suggest that these materials might be used to successfully remove antibiotics like chloramphenicol from aquatic ecosystems. Yang et al.⁴²⁹ created a porous magnetic derivative of MIL-101(Fe–Co) and used it in a related investigation to activate peroxymonosulfate, which broke down chloramphenicol in solution. After 120 minutes, the antibiotic had completely degraded at a catalyst concentration of 0.1 g L⁻¹ (pH = 8.2). The principal oxidative species responsible for the breakdown of chloramphenicol have been identified as hydroxyl and sulfate radicals.

In order to recover 5-nitroimidazole antibiotics using LC-MS analysis, Lu et al.⁴³⁰ employed highly porous MIL-101(Cr) as an adsorbent for the first time. They functionalized the framework with –NH₂ moieties and grafted urea or melamine into the OMS to create urea–MIL-101(Cr) and melamine–MIL-101(Cr) with BET surface areas of 1970 m² g⁻¹, 77 K from N₂ adsorption and 1350 m² g⁻¹, 77 K from N₂ adsorption, respectively, and investigated their potential for the adsorptive removal of NIABs (like metronidazole (MNZ), dimetridazole (DMZ), and menidazole (MZ)) from wastewater.

With a record-breaking adsorption capacity of 467.3 mg g⁻¹, Peng et al.⁴³¹ suggested using a highly flexible MOF MIL-53(Al) as a good adsorbent for the effective capture of dimetridazole (DMZ) from wastewater, outperforming all prior studies. Since the size of the pore channel changes with the concentration of DMZ, the results show that it may be widely employed for the adsorption of various DMZ concentrations. MOFs' great adsorption effectiveness is also a result of their enormous pore size. Additionally, for highly efficient adsorption of diclofenac sodium and chlorpromazine hydrochloride, Luo et al.⁴³² decorated a MOF material ([(CH₃)₂NH₂]{[Cu₂(L)·(H₂O)₂]·xsolvent}_n) to produce 3-D carboxylate groups because of improved π–π interactions. The adsorptive removal of antibiotics, namely the nitrofuran class (NFs) of antibiotics, using ultrarobust, hydrophobic, very porous Zr-MOFs (BUT-12 and BUT-13) was initially described by Wang et al.⁴³³ Based on 77 K N₂ adsorption, the MOFs BUT-12 and BUT-13 showed ultrahigh BET surface areas of 3387 and 3948 m² g⁻¹, respectively, with pore widths of 24.7 and 30.2 Å.

3.2 Water desalination

To create high-purity or drinkable water for industrial usage, salt ions are extracted from saline water through the process of desalination. This section addresses several methods and how MOFs fit into the process.

3.2.1 Capacitive deionization. Over the last ten years, capacitive deionization, or CDI, has drawn interest as a substitute water treatment technique. Using electrostatic forces between electrodes that are oppositely charged, CDI extracts ions from saline water. Regenerating saturated electrodes can also be accomplished by releasing the adsorbed ions into wash water through the application of a reverse potential gradient.⁴³⁴ New materials are being developed and tested for membrane capacitive deionization. While a range of porous carbons, including activated carbon, reduced graphene oxide, carbon nanospheres, mesoporous carbon, carbon nanofibers, and nanotubes, have been employed as electrodes, their propensity to oxidize restricts their long-term and large-scale applications.⁴³⁵

In order to tackle this problem, MOFs were first used as active materials for capacitive deionization without carbonization by Wang et al.⁴³⁶ They created a three-dimensional hybrid material with interconnected ZIF-67(Co) particles that promoted electron transmission by integrating ZIF-67(Co) into polypyrrole nanotubes in situ. As a result, an outstanding desalination capacity of 11.34 mg g⁻¹ and good recyclable quality were obtained. The application of bimetal MOFs (BMOFs) with different Co/Zn ratios for improved membrane-based capacitive deionization was shown by Ding et al.⁴³⁷ The specific surface area and graphitization were highly impacted by the Zn/Co molar ratio. With outstanding performance retention, the ideal porous carbon from BMOF (Zn = 3 : 1) attained a salt removal capacity of 45.62 mg g⁻¹ at 1.4 V.

3.2.2 Forward osmosis (FO). Although the concept of the forward osmosis (FO) process has been known for a long time, the process has received a lot of attention in the last ten years.⁴³⁸ FO consists of two primary processes: pure water penetration from the diluted draw solution and osmotic dilution of the draw solution.⁴³⁹ In order to remove new water from the feed solution, osmotic pressure must be created by the draw agents.⁴⁴⁰

A suitable thin-film nanocomposite (TFN) membrane was manufactured by Dai et al.⁴⁴¹ by introducing copper 1,4-benzene-dicarboxylate nanosheets (CuBDC-NS) into the PA active layer. The findings demonstrated that when 1.0 M NaCl was used in the AL-FS mode, the TFN membrane produced a 50% increase in water flux and a 50% decrease in reverse solute flux (RSF). Comparing conventional thin-film composite (TFC) membranes to a TFN membrane containing UiO-66 nanoparticles, Ma et al.⁴⁴² found that the TFN membrane doubled water flux and decreased RSF. In order to facilitate the FO process, Zirehpour et al.⁴⁴³ added rod-shaped MOFs made of silver and 1,3,5-benzene tricarboxylic acid (3HBTC) to the polyamide (PA) layer of a TFN membrane. These MOFs' dangling carboxylic acid groups increased membrane hydrophilicity when they were distributed throughout the organic phase. The TFN membrane outperformed the TFC membrane (27 L m⁻² h) in the Caspian Sea water test, with a water flux of 34 L m⁻² h.

3.2.3 Reverse osmosis. Using pressure to overcome osmotic pressure, reverse osmosis (RO) is a water purification technique that separates water molecules from impurities using a semi-permeable membrane. Considerable efforts have been undertaken to enhance the kinetics–energetics balance of the process because of its extensive global use.⁴⁴⁴

Hu et al.¹⁵⁸ proposed using MOF-embedded membranes for reverse osmosis desalination. A theoretical simulation was presented by them to illustrate the potential of ZIF-8(Zn) membranes at seawater concentrations in NaCl solutions. Hydrophobic ZIF-8(Zn) nanocrystals were experimentally added to the selective polyamide layer by Duan et al.,⁴⁴⁵ who demonstrated an 88% increase in water permeability at even a modest MOF loading (0.05 wt%). Water permeability was improved by 162% in comparison to pure PA with additional increases in ZIF-8(Zn) loading.

The impact of ZIF-8(Zn) nanoparticles of different sizes (60, 150, and 250 nm) on the reverse osmosis capabilities of thin-film nanocomposite membranes was examined by Lee et al.⁴⁴⁶ The interfacial area between ZIF-8(Zn) and the polyamide matrix was affected by the size-dependent deposition of nanoparticles on the support layer, which in turn affected the membrane's performance. Therefore, the performance of thin film nanocomposite membranes in reverse osmosis and interfacial polymerization is critically dependent on the filler size. Gupta et al.⁴⁴⁷ compared five varieties of ZIF membranes—ZIF-25, ZIF-71, ZIF-93, ZIF-96, and ZIF-97—for water desalination in a different study using simulations. ZIF-96 showed the greatest affinity for water among the studied membranes, while ZIF-71, ZIF-25, and ZIF-96 showed increased water flux in comparison to ZIF-97 and ZIF-93.

Xu and Hu⁵ developed a novel thin film nanocomposite membrane for water desalination by incorporating MIL-101(Cr) into a dense polyamide layer on a polysulfone substrate. MIL-101(Cr)'s porosity structure made it easier for direct water channels to form in the polyamide layer, which increased water permeance. Although the MOF loading was minimal (0.05%), the membrane showed a significant rejection of NaCl (>99%). Park et al.⁴⁴⁸ created a thin film composite membrane for reverse osmosis by using HKUST-1(Cu) in the support layer. Sulfuric acid treatment increased HKUST-1(Cu)'s hydrophilicity, dispersion, and hydrolytic stability inside the polysulfone membrane. When compared to pure reverse osmosis membranes, this alteration maintained salt rejection efficacy while increasing water flux by 33%.

In order to functionalize the mesoporous structure of Zr-based MOF PCN-222 for reverse osmosis membranes, Bonett et al.⁴⁴⁹ used post-synthetic modification (PSM). PCN-222 had poor salt rejection despite having a high water flux, which was improved by adding myristic acid via PSM to change the channel's dimensions and pore size distribution. The effect of adding ZIF-8(Zn) to membranes on fouling resistance was examined by Aljundi.⁴⁵⁰ The inclusion of ZIF-8(Zn) improved the anti-fouling capabilities of the reverse osmosis membrane by reducing fouling, according to an evaluation using a bovine serum albumin fouling model. Other chemically stable MOFs, such as UiO-66(Zr), have also been used in reverse osmosis procedures. Lin et al.⁴⁵¹ developed hollow fiber-based thin film nanocomposite membranes with polydopamine-modified HKUST-1(Cu) for low-pressure reverse osmosis of brackish water. MOF particles were incorporated into the polyamide support matrix via interfacial polymerization, enhancing compatibility between the organic matrix and inorganic nanofillers. The MOF-composite membrane exhibited high pure water permeability (66.94 L m⁻² h⁻¹ bar⁻¹) with NaCl rejection rates of 98.2% and 97.4% at 2 and 4 bars, respectively.

3.2.4 Membrane distillation. Membrane distillation is a thermal desalination technique that involves the use of a hydrophobic membrane to separate hot saline solution from cold water streams. A partial pressure differential caused by the temperature gradient enables vapor from the saline side to cross the membrane and reach the cooler side. Because it transports vapor selectively and blocks solid and non-volatile species, the hydrophobic character of the membrane is essential for guaranteeing the accumulation of pure water.⁴³⁶

Using a novel two-step synthesis approach, MOF crystals were produced on alumina tube supports by Zuo and Chung.⁴⁵² To increase hydrophobicity and decrease surface energy, perfluoro molecules were then added. Using this method, a membrane with good desalination performance and a strong vacuum distillation flow of 32.3 L m⁻² h⁻¹ at 60 °C was produced.

A ZIF-8(Zn)/chitosan layer was added to the PVDF membrane surface in an ultrathin thin film composite membrane by Kebria et al.⁴⁵³ in order to improve membrane distillation performance for water desalination. When exposed to seawater, the membrane's antifouling capabilities increased, and it demonstrated a 350% increase in permeate water flux with the ZIF-8/chitosan layer. It also showed very high NaCl rejection (>99.5%). In order to create a unique super-hydrophobic nanofiber membrane for direct contact membrane distillation, Yang et al.⁴⁵⁴ electrospun an Fe–BTC MOF and PVDF onto a nonwoven support material. The membrane demonstrated increased hydrophobicity (water contact angle: 138.06° ± 2.18°) and 99.9% NaCl rejection (35 g L⁻¹) in the direct contact membrane distillation process, achieving substrate attachment and a loading of up to 5 weight percent of the MOF.

3.2.5 Nanofiltration. Similar to reverse osmosis (RO), nanofiltration (NF) is a membrane liquid-separation technique.

Zhao et al.⁴⁵⁵ used blending IP and preloading IP techniques to assess the interaction between the PA layer and three water-stable MOFs (MIL-53(Al), ZIF-8, and UiO-66-NH₂). In comparison to the control TFC membrane, their investigation showed that TFN membranes integrating these MOFs exhibited rougher surfaces, bigger PA layers, and higher surface negativity. Every TFN membrane showed improved performance; the TFN membrane containing UiO-66-NH₂ had the maximum permeability, around 1.3 times higher than those of the TFN membranes containing the other two MOFs.·CuBTC was added to the substrate layer of a polysulfone substrate by Misdan et al.⁴⁵⁶ for NF procedures. With 0.25 weight percent CuBTC, they achieved a 25% improvement in pure water flux while keeping MgSO₄ rejection at 97.3%. When CuBTC was added, the membrane's hydrophilicity was enhanced, surface negativity was raised, and the surface became smoother, which decreased BSA adhesion.

Navarro et al.⁴⁵⁷ used a Langmuir–Schaefer approach to creatively insert MOFs into the PA layer instead of blending them conventionally in the organic phase. After being placed on a cross-linked polyimide (P84) support, the MOF thin film underwent the standard IP procedure. Compared to traditional techniques that distribute MOFs throughout the organic phase, our strategy reduced the likelihood of MOF aggregation and thus membrane flaws. By adding palmitoyl chloride, Liu et al.⁴⁵⁸ improved the dispersity of UiO-66-NH₂ within the PA layer. Because of this change, UiO-66-NH₂ is more polar in organic solvents, which lessens its tendency to aggregate. With 95% Na₂SO₄ rejection, the improved UiO-66-NH₂ demonstrated an increase in pure water flux from 8.1 to 12.4 L m⁻² h bar. Over 80 hours of continuous filtering, the modified UiO-66-NH₂ demonstrated strong durability and consistent membrane performance, even though the non-modified nanofillers initially had higher permeability.

Zhu et al.⁴⁵⁹ used the standard IP procedure with an organic solution after positioning UiO-66-NH₂ dispersed in aqueous solution using a vacuum filtering method. This method reduced MOF loss and allowed for regulated loading of UiO-66-NH₂, in contrast to conventional methods that cause MOF dispersion that is random and unpredictable. Better water movement access was made possible by the decreased interlocking effect without sacrificing membrane stability. ZIF-8 was used by Wang et al.⁴⁶⁰ as a sacrificial template to create a crumpled PA layer. Through the IP method, ZIF-8 and single-walled CNTs were integrated into the PA layer. Water broke the coordination bonds in MOFs due to their limited hydro-stability, which resulted in the collapse of MOF structures. According to their research, ZIF-67's geometric structure allowed for a higher effective surface area, which allowed for the accommodation of more water molecules. This led to an improved water flux of 1831 L m⁻² h and a high Na₂SO₄ rejection rate of 97.2%.

3.3 Sensing

Because of its remarkable structural diversity and qualities, as well as its capacity to detect luminescence or electrochemical changes, MOFs have demonstrated an amazing aptitude in the field of sensing applications. According to Lustig et al.,⁴⁶¹ MOFs have also demonstrated potential uses in the detection of a variety of analytes, including biomolecules, ionic species, and environmental pollutants. More precisely, detailed information will be given on DNA detection, ethylamine sensing based on MOF nanosheets, and luminous Fe³⁺ sensing.

Wang et al.⁴⁶² reported the application of surfactant as a surfactant-assisted approach in the ultrathin M-TCPP(Fe) nanosheets (M = Zn, Cu, and Co) for the quantitative detection of H₂O₂. After being constructed onto electrodes, GC/M-TCPP(Fe) nanosheets showed a standard amperometric response to H₂O₂, but with a rather low detection limit. With a detection limit in the range of 0.4 × 10⁻⁶–50 × 1 × 10⁶ M, good catalytic selectivity toward H₂O₂, high repeatability, and excellent long-term storage stability (at least five weeks), the GC/Co-TCPP(Fe) electrode demonstrated heme protein-like activity.

By taking advantage of their fluorescence quenching effect, Xu et al.⁴⁶³ investigated the possibility of extremely sensitive and fast-response luminescence sensing of Fe³⁺ using luminescent MOF Ti₂(HDOBDC)₂(H₂-DOBDC) (NTU-9-NS) nanosheets. The NTU-9-NS nanosheets' fluorescence quenching only takes 10 s after the addition of Fe³⁺, in contrast to the bulky MOFs' slow response (typically several hours to a day), and their unique pore structure allows them to recognize compounds quickly. This is mainly because of the nanosheets' high dispersion and easy access to the active center on the surface. One possible explanation for NTU-9-NS's superior luminous sensing capability is the presence of easily accessible active sites on the nanosheet surface.

For the purpose of detecting cocaine, Su et al.⁴⁶⁴ developed Au nanocluster/521-MOF nanosheet-embedded zirconium-based MOF nanosheet composites, such as ZnO@ZIF-8 nanorods, Ti@TiO₂/CdS/ZIF-67, and AuNCs@521-MOF. Typically, the immobilized AuNCs@521-MOF nanosheet with cocaine aptamer strands exhibited high electrochemical activity and bioaffinity, potentially creating a biosensitive platform for cocaine detection. This aptasensor demonstrated specific selectivity, a low detection limit of 0.44 ng⁻¹ mL⁻¹, a wide detection range of 0.001–1.0 ng⁻¹ mL⁻¹, and the ability to detect cocaine sensitivity.

The creation of a Cd-BTC MOF nanotube for the trace-level detection of nitroaromatic explosives was reported by Li et al.⁴⁶⁵ 72.5% of the MOF nanotube's fluorescence was quenched after it was immersed in saturated vapor of 2,4-dinitrotoluene for ten seconds. Because of the special composition and structural benefits of LD MOFs, the Cd-BTC nanotube's response rate for 2,4-dinitrotoluene vapor was also among the highest values for reported fluorescence-based chemical sensing materials.

In order to accomplish two-color living cell imaging of intracellular adenosine, Wang et al.⁴⁶⁶ employed lanthanide-based MOF nanosheets as the bioanalytical platforms. It's noteworthy that it is possible to modify the molecular ratio of the TAMRA-aptamer to the FAM aptamer loaded on dye-aptamer/MOF-Ln nanosheets in order to further detect tiny molecules and DNA inside neurons. Also effectively used in this system was the target intracellular ATP. Additionally, this technique successfully administered the required intracellular ATP. Exfoliated ZSB-1 nanosheets have recently been employed as a fluorescence sensor to identify Fe³⁺ ions.⁴⁶⁷ The detection limit for Fe³⁺ ions using the ultrathin ZSB-1 nanosheets was found to be 0.054 × 10⁻⁶ m, significantly lower than that of the bulk equivalent (0.110 × 10⁻⁶ m). Apart from quenching fluorescence, the LD MOF materials' fluorescence turn-on response was also examined for the purpose of detecting particular analytes, like volatile organic chemicals and uric acid.⁴⁶⁸ Song⁴⁶⁹ reported another MOF nanosheet with fluorescence sensing capabilities. Mn(C₆H₈O₄)(H₂O) nanosheets demonstrated a lower detection limit (0.2 pM) and a comparatively wide linear detection range (1–200 pM) when combined with the hybrid chain reaction techniques. Additionally, this fluorescence sensing technology based on MOF nanosheets offered a reliable way to detect live cells.

By varying the fluorescence emission intensity and elevating the ethylamine content, Li et al.⁴⁷⁰ were able to produce [Zn(BDC)(H₂O)]_n sheets with fluorescence properties that could detect ethylamine both quantitatively and sensitively. A bio-friendly Pb(II)-based MOF nanotube was created for the purpose of detecting uric acid, as an example.⁴⁶⁸ With a low detection limit of 4.3 × 10⁻³ m, the MOF nanotube exhibits strong selectivity for uric acid due to its host–guest interactions with it, resulting in responsive turn-on fluorescence.

A tunable optochemical platform based on MOF {(HNEt₃)}₂[Zn₃BDC₄] solvent {solvent = DMF or DMA} nanosheets for the sensing of different small molecules was introduced by Chaudhari.⁴⁷¹ Normally, the tiny light-emitting molecules may be absorbed by these nanosheets. These functionalized guest@MOF porous nanosheets might incorporate the peculiar optical features, given the high host–guest interaction effect. Consequently, a “guest@host” composite system with adjustable luminescence characteristics can function as a platform for chemical sensing via photonics. The outstanding capacitance and humidity-sensing characteristics of Cu₃TCPP nanosheets were reported by Tian et al.⁴⁷² Despite the hydrophobic nature of the MOF's pores, the massive –COOH functional groups that are suspended around the edges of the nanosheets give the MOF nanosheets superior proton conductivity. The Cu₃TCPP nanosheets, which had been compacted into a pellet, demonstrated a high capacitance responsiveness and sensitivity at 1 kHz, along with a specific capacitance frequency characteristic at various relative humidities (RH). It's astounding that they had also shown a clear response peak from 60% to 98% RH, as well as good reversibility and repeatability over cycles of exposure and recovery from low to high RH.

In order to detect DNA, Zhao et al.⁴⁷³ employed M-TCPP nanosheets (where M = Co, Cu, Zn, and Cd) as novel sensing platforms. More specifically, the MOF nanosheets may adsorb the ssDNA tagged with a dye, dimming the dye's fluorescence. Additionally, quantitative experiments were conducted using target DNA at different concentrations, showing very low detection limits (20 × 10⁻¹² M) and a broad detection range (0–5 × 10⁻¹² M). All of these findings demonstrated the benefits of MOF nanosheets, which go beyond their larger surface area for DNA adsorption.

3.4 Photocatalytic degradation of pollutants

Photocatalysis (PC) utilizes semiconductor materials to initiate oxidation and reduction reactions through photon absorption, generating charge carriers (h⁺/e⁻).^8,474 Extending the lifetime of these photogenerated h⁺/e⁻ pairs is crucial for effective PC, facilitating the production of reactive oxygen species (ROS) like hydroxyl radicals (˙OH), superoxide radicals (by conversion of O₂ to ˙O²⁻), and positive holes (h⁺). This approach harnesses solar radiation to enhance photoexcitation efficiency. Metal–organic frameworks (MOFs) operate similarly to semiconductors, where organic linkers act as valence bands and metallic clusters as conduction bands. In MOFs, photogenerated electrons in the lowest unoccupied molecular orbital (LUMO) convert O₂ to ˙O²⁻, while holes in the highest occupied molecular orbital (HOMO) generate ˙OH.⁴⁷⁵

3.4.1 Degradation of organic dyes. Significant amounts of wastewater are contaminated with dyes as a result of the dye industry's introduction of various synthetic dyes in recent years for use in printing, dyeing, medicine, and cosmetics. Every year, the textile sector alone generates about 100 tons of effluent from dyeing.⁴⁷⁶ This effluent presents ongoing environmental concerns because it contains complex, refractory organic components with strong colors. Consequently, because MOF-based photocatalysts can drastically change dye hues, research has concentrated on employing them for dye degradation. Cu₈Cl₅(CPT)₈(H₂O)₄(H₂O)₂₀(CH₃CN)₄, a new multi-metal MOF, was synthesized by Chen et al.⁴⁷⁷ by integrating the HCPT ligand with triazole and carboxylic acid functionalities. Under visible light with 30% H₂O₂, this MOF with a 3D interpenetrating network based on [[Cu₂(CO₂)₄]] and (Cu₄Cl)⁷⁺ clusters achieved 97% RhB degradation in 70 minutes. Mahmoodi et al. employed a green synthesis approach to produce Cu-based nanoporous MOF-199, which efficiently degraded Basic Blue 41 under UV light without oxidizing agents, achieving a 99% degradation rate at 0.04 g L⁻¹ concentration. The catalyst primarily utilized generated ˙OH radicals for degradation.⁴⁷⁸

Enhancing photocatalytic conditions of MOFs often involves incorporating external electron acceptors. Gao et al. studied Fe-based MIL-53 for acid orange 7 degradation and found initial inefficiencies due to electron–hole recombination under visible light. Addition of persulfate significantly improved degradation, nearing 100% efficiency within 90 minutes, by facilitating effective electron–hole separation and radical generation.⁴⁷⁹ Despite advancements, standalone MOFs face challenges in meeting current photocatalytic demands for organic pollutant degradation. Composite catalysts combining MOFs with functionalized counterparts or other materials have emerged as viable solutions, leveraging heterojunctions to complement MOF deficiencies.⁴⁸⁰

TiO₂, as a well-established photocatalytic semiconductor, is commonly integrated into MOF-based catalysts. Traditional MOFs like Cu-based HKUST-1, Zn-based ZIF-8, and Ti-based MIL-125 have been combined with TiO₂ to enhance their catalytic activity for degrading dyes such as RhB or methyl orange.^481,482 However, these composite catalysts often exhibit limited effectiveness under visible light. In subsequent experiments, the composite catalyst with 5% PCN-222 content demonstrated superior photocatalytic performance, achieving a 98.5% degradation rate of RhB under visible light. This rate was ten times faster compared to either pure TiO₂ or PCN-222 alone. Analysis indicated that the composite material enhances MOFs' response to visible light and effectively suppresses electron–hole recombination by matching bandgap relationships. Key active species involved in dye degradation include h⁺, O₂˙⁻, and ˙OH radicals, suggesting potential catalytic applications of MOFs under visible light.⁴⁸³

3.4.2 Degradation of pharmaceuticals and personal care products. Pharmaceuticals and personal care products (PPCPs) represent newly recognized water pollutants that have garnered significant attention. Since Daughton and Ternes first highlighted their environmental impact in 1999, countries worldwide have become increasingly concerned about PPCP pollution.⁴⁸⁴ Due to their persistence in toxicity and tendency for bioaccumulation, PPCPs enter water environments through various pathways after use, leading to severe environmental consequences. These pollutants can adversely affect human health, induce genetic mutations in bacteria, and contribute to the development of drug-resistant pathogens.⁴⁸⁵ Consequently, there is an urgent need to identify safe and effective methods for degrading PPCPs. MOF-based photocatalysts, as emerging materials, hold significant research promise for addressing this issue.
3.4.2.1 Degradation of antibiotics. Antibiotics are a prevalent category of substances within pharmaceuticals and personal care products (PPCPs). Current global annual antibiotic consumption is estimated to be between 100 000 and 200 000 tons.⁴⁸⁶ To address the environmental impact of antibiotics, numerous studies have explored the degradation of antibiotics using MOF-based photocatalysts, targeting compounds such as β-lactams, tetracyclines, and quinolones. β-Lactam antibiotics, including penicillin and cephalosporins, are the most used antibacterial drugs.⁴⁸⁷ Cephalosporins are widely utilized due to their broad-spectrum antibacterial properties. However, their extensive use necessitates the development of efficient degradation methods to mitigate environmental impact.

Askari et al.⁴⁸⁸ developed a CuWO₄/Bi₂S₃/ZIF-67 ternary MOF-based heterojunction catalyst through a simple hydrothermal synthesis method. They investigated the photocatalytic degradation of cephalexin and metronidazole in a continuous flow mode, optimizing operating parameters using central composite design. Under optimal conditions and visible light irradiation, degradation efficiencies of cephalexin and metronidazole reached 90.1% and 95.6%, respectively, with maximum total organic carbon removal rates of 74% and 83.2%. The performance of the ternary photocatalyst significantly surpassed that of Bi₂S₃ and the binary CuWO₄/Bi₂S₃ catalyst, exhibiting reaction rates nine and four times higher, respectively.

Wang et al.⁴⁶⁰ examined the photocatalytic degradation of tetracycline using various Fe-based MOFs (Fe–MIL-101, Fe–MIL-100, and Fe–MIL-53). They found that MIL-101 exhibited the highest degradation efficiency, removing 96.6% of tetracycline under visible light irradiation for 3 hours at an initial concentration of 50 mg L⁻¹, outperforming MIL-100 and MIL-53 by factors of 1.7 and 2.4, respectively. Capture and ESR experiments identified O₂˙⁻, ˙OH radicals, and h⁺ as the primary active species in the degradation process, demonstrating MIL-101's efficacy and providing insights into designing MOF-based catalysts for recalcitrant antibiotics.

Lei et al.⁴⁸⁹ further optimized the photocatalytic degradation of tetracycline by constructing a composite catalyst using Fe-based MOF MIL-101 and non-metallic red phosphorus (RP). They synthesized the RP/MIL-101 heterojunction composite via a low-temperature solvothermal method. Under full-spectrum irradiation, the RP/MIL-101 catalyst with different red phosphorus mass fractions achieved tetracycline degradation efficiencies exceeding 85% within 80 minutes, significantly higher than the approximately 50% efficiency of MIL-101 alone. The RP/MIL-101 composite with 15% red phosphorus exhibited the best performance, with a degradation efficiency of 90.1% in 80 minutes.

3.4.2.2 Degradation of non-steroidal anti-inflammatory drugs. Non-steroidal anti-inflammatory drugs (NSAIDs), commonly used for fever and pain relief, are prevalent in daily life alongside antibiotics in PPCPs. Examples include acetaminophen (paracetamol), ibuprofen, and ketoprofen.⁴⁹⁰ Despite their low biological toxicity, these drugs are stable and can accumulate in aquatic environments and organisms, leading to significant ecological impacts.⁴⁹¹

For the photocatalytic degradation of paracetamol, Gomez-Aviles et al.⁴⁹² developed a hybrid Ti–Zr MOF photocatalyst by replacing some Ti atoms in the NH₂–MIL-125 MOF with Zr atoms using a solvothermal method. The catalyst with 15% Zr doping exhibited the highest activity, completely degrading paracetamol under simulated sunlight in 90 minutes. Total organic carbon (TOC) analysis indicated a 65.3% reduction, demonstrating substantial mineralization.

Cao et al.⁴⁹³ synthesized a heterojunction composite catalyst of g-C₃N₄ and amino-functionalized In-based MOF MIL-68 using a solvothermal method with ultrasound assistance for the photocatalytic degradation of ibuprofen. The 10 wt% g-C₃N₄/MIL-68-NH₂ composite showed the highest activity, achieving a 93% degradation rate of ibuprofen under 180 minutes of visible light irradiation without additional oxidants. TOC removal efficiency reached 70%, and the reaction rate was 19.28 times that of g-C₃N₄ alone, highlighting its superior photocatalytic activity.

For ketoprofen degradation, Miao et al.⁴⁹⁴ constructed a Pt/Ag-modified Ti-based MOF (MIL-125) photocatalyst using solvothermal and light deposition methods. The Pt/MIL-125/Ag composite exhibited a significant enhancement in visible light response and photogenerated electron–hole separation due to the dual interface effect between MOF and precious metals. Using an XPA-7 photocatalytic device, the ketoprofen degradation rate reached 95.5% under 120 minutes of visible light irradiation, representing a 20-fold increase over MIL-125 alone. This demonstrates the effectiveness of dual interface design between semiconductors and precious metals for degrading recalcitrant organic pollutants.

3.4.3 Degradation of other organic pollutants. In recent years, MOF-based catalysts have also been studied for the photocatalytic degradation of residual organic pesticides in water and phenolic pollutants in chemical wastewater, such as those from petrochemicals. While these studies are less extensive than those on organic dyes and PPCPs, they primarily focus on common substances in pesticides like atrazine and malathion, as well as phenolic pollutants such as phenol and nitrophenol.
3.4.3.1 Degradation of organic pesticides. Atrazine is a widely used herbicide in agricultural crop production, but its low biodegradability results in environmental risks as it can leach into water systems through precipitation and runoff.⁴⁹⁵ Photocatalytic degradation presents a viable solution. Xue et al. developed a MOF-based composite photocatalyst using BiOBr and UiO-66 via an in situ growth method to degrade atrazine under visible light. By forming a heterojunction with BiOBr, the recombination of UiO-66 photogenerated electron holes was reduced, enhancing visible light response. In a 240-minute reaction, 88% of atrazine was degraded, compared to 50% with BiOBr alone. The study also examined the impact of environmental factors, finding that low pH (3.1) conditions facilitated degradation, while other water qualities inhibited it. Capture experiments identified h⁺ and O₂˙⁻ as the active species.⁴⁹⁶

Similarly, malathion, an organophosphorus pesticide, poses environmental challenges due to its high toxicity, persistence, and residues.⁴⁹⁷ Fakhri et al. constructed a ternary MOF-based heterojunction catalyst using graphene oxide and ZnO to degrade malathion, based on UiO-66. The Z-type heterojunction and graphene oxide's electron mobility enhanced charge carrier separation and visible light response of UiO-66. The UiO-66@45 ZG composite achieved 100% malathion degradation within 90 minutes. This study offers an effective strategy for designing MOF-based photocatalysts.⁴⁹⁸

3.4.3.2 Degradation of phenolic pollutants. Phenolic pollutants in water primarily originate from human activities, with industries such as petrochemical and pharmaceuticals often discharging these toxic, stable, and slow-degrading substances into aquatic systems.⁴⁹⁹ Common phenolic pollutants include phenol, naphthol, chlorophenol, and nitrophenol.

Chen et al. developed a composite MOF-based photocatalyst, Pt@UiO-66-NH₂, by dispersing Pt on UiO-66-NH₂. To study phenol degradation, they created a photocatalytic membrane reactor (PMR) using α-Al₂O₃ as the base carrier. This catalyst, with a Pt/Zr ratio of 0.5, achieved 70% mineralization of phenol within 300 minutes under simulated sunlight. The PMR showed good reusability and maintained high catalytic activity after multiple cycles, providing a scalable design strategy for degrading phenolic pollutants in water.⁵⁰⁰

For α-naphthol, a toxic phenolic pollutant, Abdelhameed et al. designed a multi-component nanocatalyst by combining MOFs ZIF-67, ZIF-8, and MIL-125-NH₂. The resulting ZIF-67@ZIF-8@MIL-125-NH₂ catalyst degraded 98.9% of α-naphthol under visible light within 120 minutes, significantly outperforming ZIF-8@MIL-125-NH₂ and ZIF-67@MIL-125-NH₂. This ternary MOF material retained a 94% degradation rate after five uses, showing promise for industrial wastewater treatment.⁵⁰¹

For chlorophenols, Zhang et al. synthesized a mesoporous core–shell heterojunction catalyst using NH₂–MIL-125 and Bi₂MoO₆. This catalyst improved charge separation and provided ample active surface sites, achieving 93.28% and 92.19% degradation efficiency for dichlorophenol and trichlorophenol, respectively, under visible light in 180 minutes.

In a subsequent study, Zhang et al.⁵⁰² developed a hierarchical tandem core–shell heterojunction catalyst, NH₂–MIL-125(Ti)@ZnIn₂S₄/CdS, achieving 98.6% and 97.5% degradation efficiencies for dichlorophenol and trichlorophenol, respectively. For nitrophenol, Li et al. constructed a heterostructure using aminated Fe-based MOF NH₂–MIL-53 grown on g-C₃N₄ doped with pyromellitimide (g-C₃N₄/PDI). The resulting g-C₃N₄/PDI@MOF catalyst achieved 100% degradation of p-nitrophenol within 30 minutes under visible light and in the presence of H₂O₂, demonstrating the effectiveness of energy-level matching heterojunctions in enhancing photogenerated electron transfer for MOFs.⁵⁰³

3.4.3.3 Degradation of other PPCPs. In addition to pharmaceuticals, various chemicals in personal care products can enter the environment post-use, causing ecological issues. Notable examples include the fungicide triclosan and the industrial chemical bisphenol A (BPA). Triclosan, a broad-spectrum bactericide, is present in nearly all daily toiletries.⁵⁰⁴

To investigate triclosan degradation under visible light, Bariki et al. constructed a coupled semiconductor heterojunction using heat-resistant and acid-resistant Zr-based MOF UiO-66 and CdIn₂S₄via a solvothermal method. The triclosan degradation rate reached 92% within 180 minutes, with a degradation rate constant approximately 12 times that of pure CdIn₂S₄, indicating the enhanced photocatalytic activity of the MOF-based photocatalyst. O₂˙⁻ and ˙OH radicals were identified as the main active species in the degradation reaction.⁵⁰⁵ Tang et al. developed a composite material by combining Cr-based MOF MIL-101 and the classic photocatalyst TiO₂ to degrade BPA. The TiO₂@MIL-101(Cr) composite catalyst, synthesized via a solvothermal method, demonstrated enhanced photocatalytic activity due to improved photogenerated electron–hole separation and reduced band gap width. In experiments, the composite catalyst achieved a 99.4% BPA degradation rate within 240 minutes under UV irradiation, significantly outperforming the individual catalysts. Mechanistic studies confirmed that O₂˙⁻ was the primary active species in the degradation process.⁵⁰⁶

4. Major challenges with the use of MOFs

In the last few years, great strides have been made in the energy and environmental applications of MOFs. In order to ensure robust field applications of MOF-based materials, a number of challenges as well as the potential research avenues for further investigation have been outlined here.

[1] The vast majority of documented research studies use popular MOF types like MIL, ZIF, PBAs, etc. as precursors or sacrificial templates for their investigations. In order to achieve optimal performance, it is crucial to investigate different MOFs, particularly those with characteristics that are specifically created. Furthermore, because of our limited knowledge of their reaction mechanisms, papers on metal chalcogenides, metal phosphides, and metal carbides are rather rare. Instead, the majority of documented investigations have concentrated on converting MOFs into carbonaceous materials, metal oxides, and their composites.

[2] When creating novel MOFs for the adsorption of contaminants, appropriate ligand and metal salt choices should be taken into account. It is important to monitor the reaction's operational parameters, such as temperature, pressure, solvent selection and quantity, to achieve tunable porosity forms and sizes as well as enhanced surface morphologies.

[3] A great deal of innovative MOF adsorbents still need to be created with intense and tenacious research interest for ground-breaking advancements for present and future applications, to highlight their potential for further laboratory, medical, industrial, and environmental usage.

[4] Although many MOF-derived materials have a high porosity and surface area, precise control over the size and shape of the pores is frequently lacking in the preparation processes of some of these materials. In order to effectively decontaminate water or wastewater systems containing an array of pollutants (e.g., synthetic dyes, heavy metals, endocrine-disrupting chemicals, pesticides, food additives, veterinary, pharmaceutical, personal care products, antibiotics, biological and chemical weapons, and other industrial chemicals), future research into finding more practical MOF synthetic techniques should focus on achieving very low-cost MOFs that are more useful especially as adsorbents. Such methods should yield adsorbents that are capable of taking the place of the pricy commercial activated carbons.

[5] The incomplete knowledge of self-assembly in a confined reaction environment makes it challenging to determine the morphology and structure of MOF derivatives target structures for the new form MOFs. Given our poor understanding of the evolution mechanism, the conversion process from MOFs to their derivatives remains unclear. Understanding these mechanisms is aided by emerging technologies such as aberration-corrected high-angle annular dark-field imaging scanning transmission electron microscopy, X-ray absorption near edge structure, and X-ray absorption fine structure. Our ability to design and build appropriate MOFs with regular or hierarchical porosity, customizable composition, and high surface area will depend on our profound understanding of the self-assembly mechanism.

[6] The usage of MOFs is hindered by their small pore sizes, which are within the micropore ranges in diameter. This places a substantial restriction on the amount of contaminants that the frameworks can absorb. This means that MOFs with pores in the mesoporous range must now be synthesized in order to improve their use over other adsorbent materials. However, the MOFs' regeneration mechanism and reusability continue to be crucial. Therefore, more thorough research is needed in this field to discover a better or alternate physical technique for recovering the used MOF adsorbents in order to create a more feasible and cost-effective option.

[7] Hybrid materials produced from MOFs have demonstrated promising uses in energy conversion and storage, but their volumetric energy densities are very modest. This restriction is mostly caused by their low-density porous architectures. To better understand the impact of various electronic and chemical environments as well as structural influences on materials' function, additional MOFs should be added to derivatives and composites for energy applications.

[8] Ultimately, it is necessary to conduct parallel analyses of the side effects of various MOF adsorbents and applications in order to confidently reap the benefits of employing qualified adsorbents in pollutant removal. Regretfully, most researchers consistently overlook this crucial component. As a result, it is anticipated that risk assessments will evaluate how consumers use the products, how to safely dispose the synthesized materials and apply the same for the adsorption of waste products, and how they might affect the receiving bodies.

Notwithstanding these enormous obstacles, the progress made thus far is genuinely encouraging, and studies on the materials discussed in this review are only the beginning of the research on MOF-based materials. There are excellent prospects to realize the practical applications of MOF-based materials in the realms of environmental science and renewable energy with continued research contributions in these areas.

5. Conclusion and future research perspectives

The overuse of fossil fuels, environmental pollution resulting from the indiscriminate usage, and the release or leakage of hazardous gases from industries have all contributed to a growing concern regarding the effective storage of renewable energy and the management of environmental pollution. Ensuring the efficient storage and utilization of eco-friendly energy sources and the removal of hazardous substances are critical for safeguarding the ecosystem and maintaining public health. Finding innovative materials with appropriate qualities has emerged as a powerful strategy to lessen the growing worries about environmental contamination. Current advancements in the study of metal–organic frameworks (MOFs), MOF composites, and MOF derivatives (such as metals, oxides, sulfides, and their composites) have shown the effectiveness of using MOF-based materials as precursors or sacrificial templates to prepare a variety of novel materials with unusual structures. Thus far, a multitude of MOF-derived hybrid materials with diverse compositions, morphologies, structures, characteristics, and functions have been successfully obtained and applied to a broad range of applications. Owing to their encouraging characteristics, MOF-derived hybrid micro-/nanostructures have found extensive application in energy storage and conversion devices, such as fuel cells, solar cells, batteries, and supercapacitors. Apart from their varied applications in the adsorption of dyes and heavy metals and the elimination of hazardous gases from environmental pollution control systems, they have better reusability characteristics than many traditional adsorbents, which is helpful for lowering processing expenses and related wastes using the pollutant removal procedures.

In order to gain a basic understanding of the adsorption, catalytic, and sensing mechanisms as well as the relationship between the structure and performance of MOF derivatives, numerous issues still need to be investigated at the lab scale in the future. This knowledge could offer guidance for the redesign and development of MOF-derived nanomaterials. Undoubtedly, in order to truly achieve environmental protection, more work needs to be done on MOF derivatives in order to realize their commercial applications. Scientists need to concentrate on simplified synthesis processes and optimizing costs while exploring ways to enhance their stability, selectivity, and reusability. Hence, to optimize their efficiency and guarantee their industrial applications under a variety of challenging circumstances, emphasis must be placed on the manufacture of MOF derivatives with distinctive features. Finally, future research should focus heavily on how to integrate MOF films with other parts of the devices for real-world use.

Data availability

No primary research results, software or codes have been included and no new data have been generated or analysed as part of this review.

Conflicts of interest

There are no conflicts to declare.

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