Metal organic frameworks for wastewater treatment, renewable energy and circular economy contributions

Metal-organic frameworks (MOFs) represent a subclass of coordination polymers. They are highly effective next-generation adsorbents, an emerging class of porous crystalline materials^1,2. They are materials composed of organic linkers and metal nodes, interconnected by strong coordination bonds³. MOFs are fabricated by linking inorganic metal ions and organic ligands to form one-dimensional (1D), two-dimensional (2D), and three-dimensional (3D) structures⁴. They are an extensive crystalline material class with extremely high porosity and internal surface areas⁵. MOFs were first identified in the 1990s, and since then, more than 90,000 MOFs have been reported⁶. MOFs have grown rapidly due to their structural and functional tunability⁷. Inherently MOFs are advantageous due to their inorganic metal ions, range of organic linkers, tunable porosity, and diverse functionality has directed growing interest toward MOFs.

MOFs are one of the most investigated classes of materials of the 21st century. They bind with other materials to form functional MOF compounds and have exceptionally high molecular functionality, electron mobility, mechanical strength, chemical stability, thermal stability, and thermal conductivity⁸. Materials with metal-organic structures were first introduced by Tomic, and investigations were conducted to examine the thermal stability of coordinated polymers synthesized from ligands and selected metal ions⁹. Before the 1990s, activated carbon and zeolites were the most common porous materials used¹⁰. However, their poor structure limited their application in the sequestration of pollutants from aqueous media¹¹. MOFs consist of secondary building units connected by organic linkers and have a large surface area, making them superior to zeolites¹². However, MOFs have been developed to overcome traditional adsorbents existing constraints, such as limited surface area and reuse/regeneration difficulties. Furthermore, there is a need for highly porous, efficient, eco-friendly, cost-effective, structurally stable, and long-lasting MOF hybrids for the treatment of wastewater. MOFs are considered superior to conventional adsorbents for water treatment due to their ability to withstand high temperatures, offer large surface areas, and possess increased porosity and nanopore diameters. Novel hybrid MOFs, with their highly porous structures, can be developed and utilized for large-scale water treatment, particularly given the diverse range of organic and inorganic contaminants present in wastewater¹³. MOFs are superior to traditional adsorbents/catalysts by the following characteristics¹⁴; (i) MOFs can easily modify pore sizes and surfaces allows for excellent specificity¹⁵ (ii) isoreticular MOFs may be made from the same metal species simply by varying the length of the ligands¹⁶ (iii) most metal cations are used to make MOFs, but only a few cations (Si, Al, and P) may produce other adsorbents¹⁵ (iv) zeolite-type materials require inorganic or organic templates to develop¹⁷, whereas MOFs may be formed using the solvent itself as a template¹⁸ (v) various metallic components can be used to develop MOFs with same ligands and analogues¹⁹. The key difference is that MOFs have wide variability and diversity in their structure. Rapid growth in the properties, characterization, synthesis, design, and applications of MOFs was experienced in the past decades (Fig. 1). Unlike traditional materials, MOFs enable precise control over their composition, morphology, pore properties, and functionalities by carefully selecting linkers and metal nodes²⁰.

Schematic showing general structure of MOFs.

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Purification and activation of MOFs after synthesis, which involves the removal of pore-blocking agents, exposes their high internal surface area, thereby maximizing their porosity. However, this process may cause the framework to collapse, compromising the structural integrity of the MOF. As a result, the material may lose its porosity, rendering it ineffectiveness²¹. To develop effective activation procedures, such as solvent exchange, freeze-drying, and supercritical carbon dioxide processing, it is essential to prevent pore collapse and maintain the structural integrity of the MOF²². Progress in this regard has widened the scope for applying MOFs and improving their efficiency. Several polyatomic organic bridging ligands have been used to connect metal ions acting as coordination centers to form tailored nonporous host materials with high thermal and mechanical stability²³. This led to the development of a class of novel hybrid materials with regeneration capabilities and unique structures²⁴. The combination of organic linkers and metal ions provides endless possibilities to develop an expansive number of new MOF structures. MOFs have intriguing properties due to the sum of the physical properties of the organic and inorganic components and the synergistic interaction between them²⁵. Presently, research is focused on developing MOFs with tunable hierarchy and improved diversity, while strategies have been developed to functionalize MOFs to improve their modularity and versatility²⁶. The spatial configuration of MOFs has also extended from 2D to 3D with permanently porous structures²⁷.

Stable MOFs can be formed by many elements of the Periodic Table. The rapid growth of MOF research has been driven by advancements in coordination chemistry, the development of specialized hardware and software for evaluating sorption properties and structure determination, and progress in organic synthesis techniques related to ligand preparation and post-synthetic modifications. This interdisciplinary expansion has significantly accelerated MOF research and its potential applications, resulting in an unprecedented surge in the field²⁸. Altering the combination of organic ligands and metal nodes can produce MOFs with different properties²⁹. These properties of MOFs and the extraordinary degree of variability in their structures for organic and inorganic components make them applicable in various sectors³⁰. They are used in several applications, such as gas storage, gas separation, photocatalysis, biosensors, luminescence, catalysis, cell therapy, biomedical imaging, drug delivery, and sensing (Fig. 2). They are also being applied in diverse fields to resolve major environmental issues. Due to their vast array of applicability, they have received extensive attention among researchers in nanotechnology, environmental and material chemistry, and material science³¹. MOFs have remarkable properties and show great potential for addressing various challenges and advancing in fields ranging from energy and environmental sustainability to catalysis and healthcare as researchers continue to explore new synthetic strategies, functionalization methods, and applications. Currently, MOFs are being used in carbon dioxide capture technology to reduce carbon dioxide emissions and promote clean energy and environmental protection. MOFs are also being developed and used for hydrogen storage.

Chronology depicting the development of MOFs over time.

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The synthesis of MOFs has garnered significant attention owing to their diverse intrinsic properties and the vast potential for designing various structures, which can be leveraged for a wide range of applications across different scientific and industrial fields. The method of synthesis governs the structure, characteristics, effectiveness, and application of MOFs³². The synthesis of Metal-Organic Frameworks relies heavily on the choice of metal ions and organic linkers, which significantly influences the stability, porosity, and functionality of the MOF structure. There are numerous possibilities for synthesizing MOFs and their structural evolution through the arrangement of organic linkers and metal ions³³. There are several methods for synthesizing MOFs: sonochemical, electrochemical, hydrothermal, solvothermal, microwave, mechanochemical, carbonization, and electrospinning (Fig. 3). Each synthesis process is unique with its characteristics that play a role in forming MOFs with various properties. Synthesis processes have been established and improvements have been made to produce MOF structures with desirable characteristics suitable for specific applications³⁴ (Table 1).

Different protocols for the synthesis of MOFs and its derivatives.

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Metal ions act as coordination centre in MOFs, and their properties such as charge, size, coordination number, and electronic configuration affect the overall structure and stability^35,36,37. Highly charged metal ions form stronger bonds with organic linkers, enhancing thermal and chemical stability, whereas metals in lower oxidation states yield more flexible frameworks, improving responsiveness to temperature changes or guest molecules^38,39. Metal ions also determine the dimensionality and topology of MOFs. For example, solvents play a significant role in coordinating with metal ions, impacting the crystal structure and influencing the resulting MOFs morphology and its porosity and functionality as in the case of magnesium-based MOFs.

Organic linkers, typically carboxylates, imidazolates, or phosphonates, bind to metal ions to form the framework^40,41. Their structure, functional groups, and degree of conjugation can influence the pore size, shape, and surface area of the MOF. For example, linkers with longer chains or functional groups such as -OH or -NH₂ may produce larger pore sizes, enhancing the accommodation of guest molecule and adsorption capacity⁴⁰. Linkers also affect stability through their coordination flexibility; at different pH levels, organic linkers may adopt different coordination modes, leading to changes in network connectivity and dimensionality. The pH during reaction significantly impacts the coordination mode of organic linkers with metal ions, influences whether the network is interpenetrated (higher pH) or non-interpenetrated (lower pH) which directly affects the stability, mechanical strength, and flexibility of the MOF^42,43,44,45.

A key objective in the development of MOFs is to establish synthesis conditions that yield well-defined inorganic building blocks while preserving the integrity of the organic linker⁴⁶. Concurrently, optimizing crystallization kinetics is essential to facilitate proper nucleation and the formation of the desired phase⁴⁷. Typically, during the synthesis of MOFs, the organic linkers and the metal nodes are dissolved in a solvent under a closed reaction vessel, and the synthesis of MOFs takes place in a solvo(hydro)thermal environment at high temperature and pressure⁴⁸. Solvo(hydro)thermal reactions are carried out under autogenous pressure in closed vessels above the solvent’s boiling point between 50–260 °C⁸. Most of the MOFs synthesized and reported so far have been produced by the solvo(hydro)thermal method⁴⁹. There are several classifications in the synthesis of MOFs such as diffusion, ultrasonic, volatilization, template synthesis, solvothermal, conventional electric heating, sonochemical, hydrothermal, mechanochemical, electrochemical, and microwave-assisted⁵⁰. Other specific methods for synthesis are also established to produce structures made of MOFs. Hundreds of thousands of MOF and MOF-based materials have been developed through numerous synthesis techniques. The advancement of sophisticated methods has rendered it effective to govern and adapt the morphology and size of MOFs and their respective properties, functions, and applications⁵¹. Synthesis techniques have also improved due to advancements in scientific research; hence efficiently producing tunable MOFs through the choice of metal ions, organic linkers, and their derivatives⁵². Some of the synthesis methods are as following:

Conventional synthesis

MOFs are conventionally synthesized through conventional electric heating with no parallelization of reactions. Temperature is one of the main parameters for synthesis, and it influences the resulting structure and properties of the MOF. Reactions occur between room temperature and 250 °C in a solvent. The reaction setup depends on the two distinguished temperature ranges⁵³. The solvothermal reaction takes place at autogenous pressure above the boiling point of the solvent in a closed vessel and the non-solvothermal reaction takes place under ambient pressure below the boiling point or at boiling point⁵⁴. Non-solvothermal reactions therefore do not require elevated pressure and reactions are classified based on the temperature at which the reaction occur i.e. at room temperature or elevated temperature⁵⁵. Reaction temperatures can influence the product quality, as denser structures are formed at higher temperatures⁵⁶. Reaction temperatures can influence crystal morphology as elevated temperatures are necessary for certain MOFs to attain suitable crystallinity and reaction rates. However, prolonged reaction times can lead to the degradation of the MOFs³⁸. Synthesis methods such as direct precipitation that occur at room temperature have been successful in developing well-known MOFs, including MOF-5, MOF-74, MOF-177, HKUST-1, and ZIF-8^39,57. Some MOFs crystallize relatively quickly, and structures can be synthesized rapidly under mild conditions, with MOFs exhibiting good thermal and chemical stability which are highly relevant for practical applications⁵⁶. Early work by Hoskins and Robson relied on low temperature and emphasized the use of precipitation, recrystallization, and solvent evaporation that were effective in growing simple molecular/ionic crystals, where reaction conditions could be fine-tuned⁵⁸. Coordination chemistry plays a vital role in the process of MOF self-assembly as metal ions behaving like coordination centres interact with organic linkers to form the characteristic porous frameworks. Controlling reaction conditions such as temperature, pH, and solvent composition can aid with the crystallization process involves nucleation and crystal growth⁵⁶. MOFs are prepared on a smaller scale by solvothermal hydrothermal synthesis through electrical heating⁵⁹. Conventional synthesis of MOFs typically involves electric heating through solvothermal methods, conducted in vials or sealed NMR tubes, often on a small scale¹⁴. High-throughput solvothermal synthesis is an effective method to accelerate the identification of novel MOF structures and streamline synthesis procedures⁴⁰.

Sonochemical synthesis

The process of chemically transforming molecules using high-energy ultrasonic radiation is known as sonochemistry. Sonochemical methods facilitate homogeneous and accelerated nucleation, leading to the formation of smaller particle sizes and a reduction in crystallization time⁶⁰. These are much more efficient when compared with conventional solvothermal synthesis as they have faster reaction times while yielding high-quality products at ambient pressure and under moderate conditions. Irradiation of a reaction solution with ultrasound radiation forms bubbles that create a local hot spot with high temperature of up to 5000 K and pressure of up to 1000 bar for a short time that accelerate coordination reactions between metal ions and organic linkers, which promotes chemical reactions that crystallize the nuclei immediately⁶¹. Immediate nucleation occurs when energy is released rapidly due to localized high temperatures which promoted the growth of MOF crystals⁵⁶. The sonochemical synthesis of [Zn₃(BTC)₂] demonstrates the impact of reaction time on particle morphology. Spherical particles are formed during shorter reaction times, while needle-like particles are generated when the reaction is extended³⁷. In the synthesis of MOF-5 and HKUST-1, the choice of solvent plays a critical role in crystal growth and phase purity. For HKUST-1, dimethylformamide (DMF) was essential for producing phase-pure nanocrystals. In the case of FeMIL-88A, pH also influences the synthesis, with low temperatures and the addition of acetic acid leading to the formation of small, monodisperse nanoparticles⁵⁶.

Microwave synthesis

Microwave-assisted synthesis is often used to synthesize organic and nonporous inorganic materials and is advantageous due to rapid reaction, low cost, and high yield⁶². The motion of the molecules from the microwaves facilitates nucleation, and the formation of crystals with controlled shape and size by appropriately adjusting the reaction’s temperature and concentration⁶³. Commercial microwave equipment has fiber optic temperature and pressure controllers with changeable power outputs⁵³. Metal ions act as connectors and organic ligands act as linkers, forming extended frameworks that assist in the self-assembly of MOFs⁵⁶. Synthesis parameters such as temperature, pH, and solvent influence the structure and properties of the resulting product by altering the coordination environment around the metal centers. For example, nucleation is accelerated under higher temperature while a lower pH might affect ligand deprotonation and coordination preferences, leading to structural variations⁵⁶. Cr MIL-101 was used to generate nanocrystals under microwave conditions within minutes while avoiding the use of hazardous solvents like HF³⁶. Control over time, temperature, and concentration influences the coordination chemistry, with higher nucleation rates, improved product purity, and selective polymorph formation.

Electrochemical synthesis

MOFs use an anodic dissolution as a constant source of metal ions that reacts with the dissolved linker molecules and a conducting salt in the reaction liquid⁶⁴. In contrast to conventional batch processes, this synthesis can operate continuously to provide greater content of solids⁶⁵. This method synthesizes MOF in a powder form on an industrial scale. Electrochemical synthesis occurs extremely quickly at slighter temperatures under milder conditions⁶⁶. For example, the applications in gas purification, hydrogen storage, and the separation of Kr-Xe mixtures were used to synthesize HKUST-1³⁵. Similarly, highly porous Cu- and Zn- compounds were synthesized from the combination of different anode materials (Zn, Cu, Mg, Co) and linkers such as 1,3,5-H₃BTC, 1,2,3-H₃BTC, H₂BDC, and H₂BDC(OH)₂³⁵. Although the electrochemical method is advantageous, it produces MOFs of inferior quality due to the incorporation of linker molecules/conducting salts in the pores during crystallization, as seen in the case of HKUST-1, where electrochemically synthesized product had lower purity and performance in sorption experiments⁶⁷. Adjusting reaction parameters achieved uniform film growth and extending the reaction time has enabled the self-completing growth of uniform HKUST-1 films⁶⁸.

Mechanochemical synthesis

This involves a chemical transformation after the intramolecular bonds are mechanically broken down⁶⁹. Instead of using a solvent, mechanical force is used to grind the reagents to form coordination bonds⁷⁰. In cases where organic solvents can be bypassed, room-temperature mechano-chemical reactions conducted under solvent-free conditions are typically favored²⁰. Mechanochemical synthesis is however limited in its production capacity and is limited to specific MOFs types⁷¹. To obtain 1D, 2D, and 3D coordination polymers, solvents in small amounts may be added to the solid reaction mixture. It is an environmentally sound method that produces materials of high quality in a short duration⁷².

Mechanical forces accelerate the reaction time by facilitating the mass transfer, heating and melting the reagent, and reducing particle size. Small amounts of solvents can be added in cases where solvent-free reactions are not feasible, which enhances the mobility of the reactants and can also exert a structure-directing effect⁸. For example, grinding of Cu(OAc)₂·3H₂O and isonicotinic acid without a solvent for a few minutes produces a highly crystalline and porous [Cu(INA)₂] framework⁴⁵. Since mechanochemical synthesis takes place under room-temperature without the need for large amounts of organic solvents, it has environmental benefits⁷³. Short reaction times also lead to quantitative yields. Varying synthesis parameters such as solvent type, ammonium salts, and grinding time can influence the resulting structure of the MOF. For example, if HKUST-1 is synthesized using copper acetate as the starting material and employing LAG produces better crystalline products with improved sorption; however, mechanochemically synthesized HKUST-1 retains residual acetic acid and water in its pores, which impacts its surface area⁴⁴. Mechanochemical synthesis is limited to specific MOFs and its application for industrial applications requires further research.

Ionothermal synthesis

Ionothermal synthesis is a method utilizing ionic liquids as a solvent and a structure-directing agent/template to produce a unique ionic environment for the synthesis of MOFs^43,74. The unique characteristics such as zero vapor pressure, solvating properties, recyclability, and high thermal stability enable synthesis⁷⁵. During this process, the precursors of MOFs dissolve in the ionic liquid solution and the anions or cations serve as charge compensators in the ionic liquids and are embedded into the MOF framework, resulting in an electrically neutral composite⁷⁶. Primarily, the cationic parts of ionic liquids integrate into the framework compared to the anionic parts. Although, ionothermal synthesis is simple and environmentally friendly, has its limitations due to the strong interaction between ionic liquid cations and the MOF framework, which can alter the properties of the ionic liquid when embedded⁷⁶. Furthermore, all MOFs are not charged, with only a limited number of ionic liquids that can replace water or organic solvents which significantly limits the broader application of this method⁸. MOFs have mostly been synthesized from 1-alkyl-3-methylimidazolium ionic liquids but mixtures of two or more compounds called deep eutectic solvents have lower melting points compared to any of their constituents and have been employed for the synthesis of MOFs. They are easy to prepare, cheap, and relatively unreactive to atmospheric moisture⁷⁷.

Microfluidic synthesis

Microfluidic synthesis is a promising approach to address commercial and industrial requirements for the continuous, rapid, and scalable production of MOFs⁸. This method allows for more efficient synthesis with reduced reaction times, energy consumption, and environmental impact. Large-scale production can be accomplished by quick reactions that could be used for continuous synthesis, leading to energy-efficient processes⁷⁸. A dry amorphous aluminosilicate gel is converted to crystalline zeolite via the dry gel conversion (DGC) method when it comes into contact with water vapor and volatile amine⁴⁹. Zeolites and membranes made of zeolites are produced using this method⁷⁹. A dry gel is crystallized using the vapor-phase transport (VPT) method, which involves vaporizing water and a volatile amine, and the steam-assisted crystallization (SAC) approach, which uses steam to help a dry gel containing a non-volatile amine crystallize. The dry gel conversion method minimizes waste generation and disposal, reduces reaction volume, and produces high yield⁸⁰. Microfluidic synthesis offers a more efficient alternative compared to traditional synthesis techniques like hydrothermal and solvothermal methods that require longer durations for crystallization and encounter challenges with controlling crystal size⁴². For examples, Co₃BTC₂ was successfully synthesized using segmented flow in a PFA tube with a reaction temperature of 140 °C, and crystals were identified after just 1 minute when compared to the 24-hour crystallization period required for conventional hydrothermal synthesis. Other MOFs synthesized by the same microfluidic approach include HKUST-1, UiO-66, MOF-5, and IRMOF-3, etc⁸¹. Coordination chemistry plays a key role in the self-assembly of MOFs during microfluidic synthesis, as the mixing of metal and ligand precursors in T-junction devices allows for droplet formation and rapid crystallization⁴². The size, shape, and particle size distribution of the crystals can be fine-tuned by adjusting synthesis conditions such as temperature, pH, solvent choice, and residence time. By varying key synthesis parameters, microfluidic synthesis also leads to the production of hierarchical structures, offering a solution to diffusion limitations in catalytic and adsorption processes which is advantageous over traditional batch processes, making it a viable application for large-scale production of MOFs. Microfluidic systems produced MOFs of higher quality and offers opportunities for advanced control over the synthesis process⁸¹. Technologies like microfluidic pen lithography and digital microfluidics enable precise patterning and the generation of uniform MOF crystals and innovations like this could enhance application in sensing systems and other advanced materials.

Carbonization

The functionalization of MOF-derived carbonaceous materials with metal/metal oxide nanoparticles is a process that involves heating the MOF under specific conditions such as inert atmospheres^82,83. When MOFs are carbonized, metal and metal-oxide particles are concurrently developed, and in the carbonized structure, metal nanoparticles of various sizes, shapes, oxidation levels, and crystal phases are formed⁸⁴. The process varies depending on the precursor used and desired properties in the resulting MOF. During the carbonization process parameters such as temperature, heating rate, and duration can be adjusted to produce the desired material for specific applications. The organic ligands break down due to thermal decomposition resulting in the release of volatile compounds such as water, carbon dioxide, and other gases, leaving behind a carbon-rich structure within the MOF framework⁸⁵.

The role of coordination chemistry is critical in the self-assembly process of MOFs as it governs the interaction between metal ions and organic ligands. For example, during the synthesis of cobalt organic framework (CoOF), coordination chemistry facilitates the formation of CoOF through the reaction of cobalt (II) chloride (CoCl₂) and terephthalic acid in DMF solution. During the process, coordination between cobalt ions and the terephthalic acid leads to the development of a nanocomposite structure, which can be carbonized to form hexagonal structures of CoOF, as demonstrated by the SEM images of pristine and carbonized CoOF⁸⁴. The formation mechanism and resulting structure of MOFs are highly sensitive to the synthesis conditions. Refluxing the mixture at 125 °C for 24 h produced a homogeneous nanocomposite, and subsequent carbonization at 600 °C under an inert atmosphere altered the structure by introducing cobalt oxide nanoparticles. Adjusting synthesis conditions can lead to different structural outcomes which can be attributed to the influence of coordination chemistry during synthesis and the subsequent transformation during carbonization. The synthesis conditions not only affect the structural properties but also the functional characteristics of MOFs. For example, the capacity of CoOF to adsorb dyes is influences by porosity and surface area, which are determined by the synthesis and carbonization process⁸⁴. Other reported MOFs synthesized by carbonization includes ZIF-8, MIL-53, EDTA/C-MIL-101, nickel-incorporated Ce-based MOFs etc^86,87,88.

Electrospinning

Electrospinning is employed to produce composite nanofibers made from MOFs and polymer. The mechanism is based on the ejection and elongation of a solution under a high-voltage electric field or a viscous polymer melt, which is solidified in the electrified fluid jet on a collector⁸⁹. The integration of MOF particles with polymer matrices to form nanofibers allows for various applications, such as gas adsorption, filtration, or catalysis. Viscosity, flow rate, voltage, humidity, slurry concentration, and working distance are some factors that influence the electrospinning process and the structure of the resulting nanofibers⁹⁰. Nanofibers are desirable for various applications due to their large specific surface area to volume, high aspect ratio, high flexibility, and multiscale porosity⁹¹. Direct electrospinning and surface decoration are the two primary routes for structuring MOF-polymer nanofibers^92,93. Direct electrospinning makes use of a polymer solution mixed with a slurry of MOF particles, and the resulting mixture is electrospun into a composite nanofiber⁹³. The quality of the resulting nanofibers is dependent on parameters such as concentration of MOF particles, viscosity of the slurry, voltage, and distance between the needle and the collector. Direct electrospinning has limited applications in gas adsorption/catalytic applications since the polymer covers the MOF particles thereby limiting their accessibility and performance. Surface decoration addresses some limitations of direct electrospinning by growing MOF particles directly on the surface of preformed polymer nanofibers. A polymer nanofiber layer is developed through electrospinning without the MOF particles and the polymer nanofibers are immersed in a solution containing MOF precursors which allows the MOF crystals to grow on the nanofiber surface. Surface decoration is advantageous as it preserves the pore structure and surface properties of the MOF and ensures that the MOF particles are exposed so that their active sites fully accessible⁹². However, surface decoration requires careful selection of stable polymer fibres that can withstand the aggressive solvents or high temperatures used during synthesis^92,93. For example, combining MOFs like ZIF-8, UiO-66-NH₂, and MOF-199 with polymers such as Polyacrylonitrile (PAN) and Polystyrene (PS) develop composite nanofibers for air pollutant filters while in situ growth of MOFs on nanofiber surfaces, treating polymer surfaces improved the adhesion and distribution of MOF particles⁴¹.

Postsynthetic modifications

Postsynthetic modification is a method for incorporating functional groups into MOFs, including covalent and dative modifications, as well as post-synthetic deprotection³. Adding functional groups to the MOFs after their synthesis, they undergo a chemical transition after being isolated. Building block replacement, a type of post-synthesis modification, involves the heterogeneous exchange of metal ions or ligands by breaking and establishing chemical bonds inside the original MOF. The fundamental structural units of the MOFs are replaced during these reactions⁸. Mixed-metal MOFs are prepared by post-synthetic methods and the resulting materials possess new properties due to the presence of the second metal ion. This technique produces isostructural MOFs with various physical and chemical characteristics⁹⁴.

Environmental applications

The main aims of functionalization of MOFs in water treatment and gas purification are to enhance the selectivity and adsorption efficiency of the MOFs towards specific adsorbates. For example, the amine-functionalization of MOF-Fe (amine-MOF-Fe) is evaluated for increasing the selectivity towards the removal of methylene blue dye from aqueous solution. The electrostatic interaction between the amino group of amine-MOF-Fe and MB had contributed to the high adsorption capacity 312.5 mg/g compared to 149.25 mg/g for MOF-Fe⁹⁵. Also, to improve the selectivity of MOF adsorbents towards Au(III), the researchers functionalized MOFs with thioctic acid⁹⁶. Additionally, functionalized MOFs were used to improve the overall efficiency of the membrane in the catalytic membrane reactors depended on sulfate-based advanced oxidation processes. For example, Cu-MOF-74 was used to modify the polyvinylidene fluoride layer. The functionalized Cu-MOF-74 greatly increased the anti-fouling capacity of the membrane. Simultaneously, the modified membrane catalyzed peroxymonosulfate (PMS) to achieve 97.1% Rhodamine B breakdown efficiency within 60 min and 90.2% dye penetration recovery after 30 min⁹⁷. Functionalization of MOFs with hydrogen bonding was also investigated in adsorptive purification of liquid and vapor/gas phases from various gases, particularly polar or polarizable gases such as NH₃, CO₂, and aniline, organic compounds with -N, -NH, -O, or -OH groups and volatile organic compounds such as formaldehyde. Furthermore, adsorbates containing -O- or -OH groups (which may be deprotonated), such as oxyanions (vanadates, chromates, molybdenites, sulfates, phosphates, etc.), can be readily removed via the development of N-O-N′ bonds where N′ and N are the metallic components of the pollutant and MOF, respectively. This process utilizes the -OH groups present within the MOF. Given the established exchange of -OH sites with fluoride ions, further investigation into the removal of additional anions, such as halides, may be investigated further⁹⁸.

CO₂ capture and separation

The amine function group showed promising results in the field of post-combustion carbon dioxide capture and separation. Among these, the -mmen-functionalized structure exhibits superior carbon dioxide uptake and enhanced regenerability under the flue gas mixtures and conditions investigated⁹⁹. Also, ethyleneamines such as ethylenediamine (ED), diethylenetriamine (DETA), and tetraethylenepentamine (TEPA), were used to functionalize MOF-808, to enhance its selectivity for CO₂ capture¹⁰⁰.

Drug delivery

MOF nanoparticles have been used in numerous antimicrobial actions owning to their permeability, constant drug delivery capability, and structural flexibility in grouping with several materials such as polymers, nanoparticles, antibiotics and drugs. There are numerous routes to functionalize MOFs with antibacterial agents for drug delivery including covalent binding, surface adsorption, functional molecules and substrate encapsulation. Although several functionalization approaches showed good results; however, it has some limitations. For example, molecules encapsulated by substrate encapsulation and adsorption may progressively leak because of the weak interaction forces. The covalent bonds are strong, but it requires compound synthetic arrangement and might impact the molecules capabilities. In addition, the functional ligands suitable for MOF synthesis are typically rigid and extremely symmetrical, which makes it problematic to directly use biomolecules as the building block. Further research is required to functionalize MOF with better biostability, antibacterial efficiency and biocompatibility¹⁰¹. In the field of co-drug delivery and bioimaging, with special focus in cancer treatment, functionalization of MOF recorded promising results. For, example, the situ development of Fe-MOF in the presence of layered double hydroxides intercalated with 2-aminoterephthalic acid (NH₂-BDC) showed good entrapment efficiency of methotrexate (28 wt%) and doxorubicin (21%) indicating a promising potential for anticancer drug delivery¹⁰².

Renewable energy

The increasingly demand for renewable energy sources has prompted the functionalization of MOFs in order to improve the efficiency of the materials and enhance the performance of systems/devices. For example, a new wood-based solar evaporator with a porous wood substrate, zeolitic imidazolate framework-8 (ZIF-8), and polydopamine (PDA) as a light absorption layer is developed. The in-situ loading of ZIF-8 in the microchannels gives the wood evaporator a crucial function: it reduces the equivalent evaporation enthalpy owing to the decreased hydrogen bonding density of water molecules as they pass through the wood channels, significantly increasing solar evaporation efficiency. Under 1.0 solar irradiation, the evaporation rate reaches 2.70 kg m⁻² h⁻¹, compared to the standard 2D photothermal evaporator ( ~ 1.46 kg m⁻² h⁻¹). The proposed wood/ZIF-8@PDA additionally has high removal efficiency for hazardous ions and organic contaminants¹⁰³. Additionally, a redox-active 1,2,4,5-tetrazine moiety is investigated for the functionalization of MOF to enhance the performance of the cathode in lithium-oxygen batteries. Specifically, the material improves sustainable capacity while having a smaller overpotential opening up a new route for the development of useful materials for energy storage and conversion¹⁰⁴. Moreover, different elements, such as metal sulphides, are frequently coupled with MOFs to improve the performance of H₂ production photocatalysts. For example, Zr-MOF-S@CdS recorded excellent photocatalytic activity for H₂ evolution rate of up to 1861.7 μmol g⁻¹ h⁻¹, which is 4.5 times greater than pure CdS and 2.3 times higher than Zr-MOF/CdS¹⁰⁵.

MOFs display significant topological diversity and structurally appealing architectures, coupled with a broad spectrum of desirable properties and a wide range of potential applications³². Numerous MOF structures with projected topology and dimensions have been synthesized through the arrangements of organic linkers and metal ions in numerous combinations¹⁰⁶. MOFs are fourth-generation polymers, and the MOF superstructure is classified into four dimensionalities¹⁰⁷. The functional groups introduced by the linker dictate the chemical properties and functionalities of MOFs¹⁰⁸. MOFs are characterized by tunable pore structure, functional group diversity, high thermal stability, large surface area, abundant active sites, and mechanical stability¹⁰⁹. Properties also include low density, well-defined pore size, optical and magnetic responses to the inclusion of guest molecules, and the selective uptake of guest molecules⁵². MOFs also have low carbon content, crystalline integrity, and large chemical energies¹¹⁰. They have excellent structure, topology, flexibility, robustness, stability and structural integrity, post-synthetic modifications, and non-permanent porosity associated with inseparable host-guest dependence¹¹¹. Even without guest molecules in their pores, MOFs maintain their porosity and the pore structure retains its porous nature and does not collapse²⁰. The pore size depends on the length of the carbon chains in the organic linker⁵⁹. The pores that are responsible for the functionality of MOFs are developed by the organic linkers¹¹².

Structure of MOFs

The structure of MOFs is determined by the geometry of secondary building units and the shape and size of the organic ligands. Hence, MOFs can be tuned by selecting secondary building units and linkers to have the required pore size, structure, and functionality¹¹³. Porosity is a prominent feature of MOFs required for various functions. MOFs have a range of pore sizes capable of accommodating a wide variety of species; including nanoparticles, single metal atoms, organic dyes, metal complexes, polyoxometalates, polymers, and small enzymes. The applications of MOFs can be extended due to the presence of guest species that cannot be achieved with single MOFs⁵⁴. MOFs are categorized depending on the presence or absence of guest molecules and every single MOF structure has a characteristic structural dimension in the order of 1D, 2D or 3D²⁷.

1D MOFs have coordination bonds spread unidirectionally over the polymer and their structure is represented in a chain or rod form¹¹⁴. They are porous and hollow nanofibrillar structures that are advantageous for energy storage and conversion devices due to their increased active sites and reduced mass and charge transport paths¹¹⁵. Electrospinning with carbonization is a simple and scalable technique for the synthesis of 1D MOFs. 1D MOFs are applicable in drug delivery for controlled release systems and in catalysis for specific adsorption processes; however their dimensionality restricts surface area and adsorption capacity compared to MOFs of higher dimensions³³.

2D MOFs have single layers superimposed through edge-to-edge stacking or staggered stacking, developing a weak interaction within the layers¹¹⁶. The nature of the stacking can be altered by changing the organic linker which can accommodate guest molecules in the voids¹¹⁷. Highly exposed catalytic sites make 2D MOFs superior compared to 1D. They also possess better electron transferability, better charge and mass transfer characteristics, and nanoscale thickness¹¹⁸. Advances in MOFs are focused on 2D nanosheets due to their unique properties such as atomic-level thinness, large lateral size, mechanical flexibility, high surface area, optical transparency, and a high surface-to-volume ratio¹¹⁹. 2D MOFs provide higher stability and accessibility to reactive sites, making them efficient in catalysis, gas storage and gas separation, energy storage and conversion, electronics, and medical fields³³. However, limited pore connectivity across the third dimension can reduce their efficiency for storing larger gas volumes.

3D MOFs have coordination bonds that have a multidirectional spread resulting in a more complex, cage-like structure that is highly porous and stable. The most common are the 3D grid MOFs and 3D pillar layered MOFs synthesized from the strong chemical coupling of organic ligands and metal ions⁹. They have a high surface area, large pore volume, and uniform pore size distribution¹²⁰. MOFs have diverse topology, structure, and pore characteristics and by manipulating the choice of synthesis method and conditions and precursors their properties can be tuned. Various crystalline structures serve as the foundation for the wide range of functionalities of MOFs and the metals or metal clusters are joined with organic linkers like carboxylic acid or ligands containing nitrogen to develop MOFs¹²¹. 3D MOFs are highly ordered structurally and their crystalline nature along with their porosity and these properties make them excellent candidates for an array of diverse applications such as sensing, hydrogen storage, CO₂ storage, gas separation, catalysis, magnetism, and drug delivery³³. Despite numerous synthesis methods, the demand for application-based MOF superstructures capable of performing core functions efficiently is rising. Several notable 3D MOFs have been synthesized using innovative strategies¹²². Advancements in 3D nanoporous MOF superstructures involve using pretreated substrates such as semiconductors, metals, and polymers to deposit MOFs as thin films, known as surface-mounted 1D MOFs. Despite the advantages, 3D MOFs can sometimes suffer from reduced structural stability and interpenetration, which can decrease porosity and negatively impact performance in some applications.

Stability of MOFs

MOFs have poor water, mechanical, thermal, and acid/base stability. The stability of MOFs can be improved by increasing the strength of the coordination bonds between secondary building units and organic linkers¹²³. Water-stable MOFs such as zeolitic imidazolate frameworks, aluminum-based carboxylates, zirconium-based carboxylates, and pyrazole-based MOFs have been studied recently. At the same time, new strategies for creating hydrophobic surfaces are being developed to improve the water stability of MOFs¹²⁴. By heating organic ligands on the surface of MOFs under nitrogen while preserving the pore size distribution and structure, the water stability of IRMOF-1 was increased¹²⁵. By adding hydrophobic and permeable polydimethylsiloxane layers to the surface of MOFs using vapor deposition, which preserves the MOF’s original properties without changing the surface area, the water stability of MOFs was increased. This would generate high water resistance and maintain the stability of the MOF under aqueous conditions without blocking the inherent porosity material. These materials could separate the organic solvent chloroform and water¹²⁶. The internal channels of MOF were modified through in situ polymerization of aromatic acetylenes which produced a MOF that captures CO₂, retards the diffusion, and repels water molecules¹²⁷. The thermal stability of MOFs is determined by the number of linkers connected to each metal node and the bond strength between them¹²⁸. The strength of the coordinate bonds formed between metal ions and organic ligands determines the framework’s stability with stronger bonds resulting in more stable structures^1,39. Stable MOFs are typically constructed using hard bases with high-valent metal ions or soft bases with soft divalent metal ions. The operating environment also affects stability, particularly pH levels, as acidic or basic conditions can lead to degradation of the framework through competing reactions³³. For example, in basic conditions, hydroxide ions may replace organic ligands, whereas in acidic media, competition from protons can cause decomposition. To enhance stability, strategies such as functionalizing MOFs with hydrophobic groups to resist water interactions can help prevent hydrolysis reactions that compromise structural integrity^43,82,83. The instability of MOFs can cause partial collapse of pores, amorphization, and phase changes⁵⁵. Resistance to other stresses can be predicted by thermal stability¹²⁹.

Post-synthetic modifications can further enhance the properties and introduce new functionalities in MOFs. These modifications can improve stability, selectivity, and catalytic activity. For example, incorporating hydrophobic groups, like CF₃ or long alkyl chains, onto the surface of MOFs can provide resistance against water and enable solvent separation applications whereas, altering the metal ion and ligand combinations can lead to frameworks with tailored properties suited for specific applications, such as catalysis or gas storage^40,130. The robustness of MOFs can also be significantly affected by internal factors like the rigidity of ligands and the strength of metal-ligand bonds as stronger metal-ligand interactions allow for structural repair, leading to enhanced stability. Rigidity in ligands contributes to a framework’s resilience, as shorter and more rigid ligands can increase activation energy for decomposition¹³¹. The mechanical and thermal stability of MOFs are critical for their performance. Mechanical forces, such as those from high pressures or milling, can induce irreversible phase changes and loss of porosity while thermal stability is dependent on the strength of the metal-ligand bonds and the number of coordinating ligands⁴⁰. Therefore, MOFs constructed with hard Lewis’s acids and bases tend to exhibit greater stability under extreme conditions. New MOFs must be evaluated since they are crucial for applications requiring elevated temperatures. The MOFs structures and their properties make them prime materials for numerous applications such as gas storage, separation, and purification, energy storage, biomedical purposes, sensing, catalysis, and environmental applications. Therefore, careful selection of metal ions and organic linkers, combined with strategic post-synthetic modifications, allows for the development of MOFs with tailored properties, ultimately expanding their applications¹³². New MOFs must be evaluated since they are crucial for applications requiring elevated temperatures. The mechanical stability of MOFs is also an essential factor in enabling their practical applications.

MOFs in real-world applications

The rationalization of the synthesis of MOFs is yet only done in a very incomplete way and for a few chemicals. As a result, this opens up a wide range of study opportunities. The transfer of MOFs to real-world applications may promote such attempts to establish greater control over synthesis by understanding. Commercial manufacture of MOFs has just commenced at BASF in Germany, highlighting their market potential³⁵. The diverse chemical makeup of MOFs allows for toxicologically safe formulations, and their high degree of functionality enables usage as imaging agents and medicinal agent delivery vehicles. The difficulties in the domain include not only the invention of novel solids, but also advancements in material formulation and processing, such as matching the frameworks shape and surface chemistry to the intended applications¹³³. MOFs have been effectively used in a wide range of applications, including catalysis, energy storage, drug delivery, nonlinear optics, and gas storage. The sorptive capacity of MOFs is unquestionably their most extensively studied property. This research area has matured significantly and is now widely utilized for the separation and storage of various gaseous components (e.g., hydrogen, CO₂, CH₄, O₂, etc.). Luminous and conductive CP/MOFs have facilitated the use of MOFs in highly sensitive and specialized sensing devices for detecting small molecules, solvents, contaminants, and explosives. Given the unique advantages of MOFs, such as their high surface area, porosity, chemical and thermal stability, luminescence, high sorptive capacity, molecular sensing abilities, receptor molecule incorporation, and regeneration potential, MOF research is expected to expand in the coming years. However, several challenges, including complex synthesis and characterization, aqueous insolubility, insulating properties, and high production costs, currently limit their widespread application. Addressing these obstacles, various strategies are being explored, such as combining MOFs with biomolecules, graphene, CNTs, metal nodes, guest molecules, and redox or catalytic sites, which not only aim to overcome these limitations but also enhance MOF properties, such as conductivity. MOFs and their composites are increasingly being investigated for innovative industrial applications, including gas separation/storage, small molecule sensing, and clean energy generation through batteries, fuel cells, and capacitors. Consequently, as significant advancements are made to improve structural stability, compositional flexibility, and cost-efficiency, the practical applications of MOFs and their composites are expected to broaden substantially in the coming years¹³⁴.

Despite the numerous biological uses of MOFs, commercial applicability has yet to gain significant attention. The main limiting issue in the use of MOFs is their toxicity and lack of biodegradability. Thus, the market potential and economic worth of MOFs are still in their infancy. However, the rapid increase in patents and publications on biological uses of MOFs demonstrates their widespread acceptance in the scientific community, which will undoubtedly expand their relevance in the healthcare system¹³⁵.

Different studies have shown that certain MOFs are safe at the preclinical level, with suitable degradability under in vitro physiological conditions. A key advantage of MOFs is their ability to incorporate bioactive molecules as linkers or bioactive metals as the inorganic component, enabling potential use in theranostics. However, several challenges remain for bioapplications, including controlling particle size, preventing aggregation, and engineering particle surfaces with reactive groups to enhance stability and introduce properties like stealth, active site targeting, and bioadhesion. Understanding nanoparticle transport across biological barriers, investigating different MOF compositions, and developing stable formulations are crucial for practical applications¹³³.

The unique properties of MOFs, such as their structure, large surface area, and excellent pore distribution, make them favorable for a wide range of applications (Fig. 4). They have wider applications in air quality monitoring, drug delivery systems, gas purification, solar devices, wastewater treatment etc.

Applications of MOFs in various fields.

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MOFs in air quality monitoring

MOFs have the potential for application in air quality monitoring at an urban and industrial scale due to their advantages over other materials. Monitoring pollutants and indoor air quality has been made possible by changes in the physical and chemical properties of MOFs following the adsorption of gas molecules¹³⁶. MOFs exhibit luminescent sensory characteristics that are useful to detect the presence of highly selective and sensitive determinations of environmental contaminants in the presence of other coexisting species⁵². Such luminescent MOFs are used in gas sensing due to their optical response toward guest molecules inside their cavities. At low operating temperatures, MOF sensors exhibit high sensitivity and selectivity that make them suitable for gas sensing²³. However, MOF used for gas sensors undergo partial degradation when exposed to moisture, therefore, long-term stability and reusability are essential properties for sensing materials to possess and developing such material is necessary¹³⁷. The low electrical conductivity of most MOFs limits the application of MOFs in gas and liquid sensors. The practical success of modern sensors is limited and challenging; however, as chemo-resistive sensors, MOFs are being researched and explored because of their designed surface reactivity, which provides capability and high selectivity⁵². MOFs with accessible open metal sites are suitable to chemisorb odour-generating molecules that are rich in electrons, such as amines, oxygenates, phosphines, water, and alcohols, and could be beneficial in the removal of sulfur traces from various gases¹³⁸.

Metal-organic frameworks enhance sensitivity and selectivity in sensing technologies. The 3D Ag@MOF platform optimizes analyte capture and signal amplification through Surface-Enhanced Raman Scattering (SERS), enabling real-time detection of airborne chemicals like volatile organic compounds and greenhouse gases. Stand-off Raman spectroscopy leverages molecular vibrational fingerprints for remote air quality monitoring but faces sensitivity challenges with low concentrations¹³⁹. Integrating MOFs improves detection efficacy by amplifying Raman signals via plasmonic nanoparticles, increasing analyte sorption, and enabling rapid monitoring of air quality changes, which is crucial for indoor and industrial applications. Integration of MOF-based sensors within HVAC systems can enhance indoor air quality monitoring by detecting harmful gases and VOCs, thereby facilitating timely interventions¹⁴⁰. The sensors could also help monitor emissions and ensure compliance with regulatory standards, enhancing safety and environmental protection in manufacturing and chemical processing. Moreover, a quartz crystal microbalance-based sensor array incorporating a photoresponsive nanoporous MOF as the active sensing layer has been reported. This 4-channel electronic nose (e-nose) was evaluated for its ability to detect and identify volatile organic compound (VOC) vapors. Although the discrimination accuracy of the e-nose with sensors in the same state was initially poor, selectively switching the sensors significantly improved the accuracy, resulting in an array with high specificity and optimal sensor performance. The use of light to remotely modify the characteristics of individual sensors enables the reversible programming of the sensor array’s performance¹⁴¹.

MOFs in drug delivery and telemedicine

The performance of MOFs can be enhanced through surface modifications that render a platform suitable for biomedical applications such as drug delivery and magnetic resonance imaging (Table 2). Conventional drug delivery systems such as capsules, tablets, and granules have certain limitations. MOF membranes have a large surface area, high porosity, and special chemical and thermal stability that make them suitable for therapeutic and medicinal targets. This makes them versatile and popular for drug delivery systems¹⁴². MOF membranes synthesized from solvothermal, sonochemical, mechanochemical, and electrochemical processes have low side effects, multiple drug-loaded properties, and stimulus-based delivery systems¹⁴³. MOFs control drug release due to their high specific surface area, adjustable structure, biodegradability, and modifications¹⁰¹. Biological camouflage can be achieved by using MOFs to modify platelets, red blood cells, neutrophils, macrophages, dendritic cells, and cancer cell membranes^{144,145,146,147}. By using one-pot synthesis, bioactive compounds can be added to MOFs, and the loading approach makes use of the coordinative capabilities by using a saturated solution of the bioactive molecules to immerse the MOF system¹⁴⁸. Bioactive substrates can be designed by immobilizing MOFs onto flexible porous membranes¹⁴⁹. Small molecular weight nitric oxide donors and anti-thrombotic medicines are thestandard treatments for coronary artery disorders^149,150. In addition, drug-eluting stents are inserted into the artery to ensure long-term prevention of occlusion¹⁵¹.

Both copper & non-copper-based MOFs have been evaluated for cardiovascular implants, reported as a metal-free stent material serving as a sustained drug release vehicle, mechanical reinforcement, and a contrast agent for MRI¹⁴⁹. Studies indicated that MOFs prevented arrhythmia and reduced sympathetic excitability in rat model; however, the field is poorly studied, and further research is needed to compare it to clinically used material to determine the safety and effectivity for widespread medical application¹⁵². MOFs are suitable for combining short peptides, nucleic acids, and antibodies, and cellular behavior is supported by MOF layers^153,154. MOFs are ideal for bone tissue engineering due to their chemical stability, thermalstability, negligible cytotoxicity, and pH sensitivity²⁸. MOF materials are suitable for drug delivery systems, cancer, and clinical tumor therapies because they retain their physicochemical characteristics after modification without altering their controlled shape, size, and uniformity¹⁵⁵. MOFs are advantageous in monitoring, diagnosis, and treatment as they can perform both diagnosis and treatment¹⁵⁶.

The integration of 2D nanomaterials with artificial intelligence (AI) enhances telemedicine by optimizing drug delivery, improving diagnostic accuracy, and enabling personalized treatment through real-time data analysis. This synergy holds promise for better patient outcomes, fewer side effects, and faster development of innovative healthcare solutions, paving the way for a promising future in telemedicine¹⁵⁷. Furthermore, MOFs may respond to external stimuli such as pH, temperature, light, and electrical fields, allowing drugs to be released in regulated quantities at particular target sites in the body. This response reaction increases the accuracy and selectivity of personalized medicine, reducing off-target effects and systemic toxicity. Overall, MOFs have demonstrated great promise as smart drug delivery systems for the treatment of a variety of disorders. More research is needed to solve issues, including scaling up production, long-term stability, and clinical translation¹⁵⁸.

MOFs and their derivatives in 3rd generation solar cells

Metal-organic framework (MOF) is gaining immense interest in photovoltaics. Different types of 3rd generation solar cells including perovskite solar cells (PSCs), dye-sensitized solar cells, organic solar cells and quantum-dot solar cells etc. are reported to involve MOF, having better performance and stability (Fig. 5). MOFs are usually crystalline materials with periodic networks formed by organic ligands and inorganic metal ions or metal clusters. They are well known for their porous structure, high specific surface area, and structural stability.

Application of MOFs within the third-generation solar cells.

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MOFs in dye-sensitized solar cells

The implementation of MOFs in dye-sensitized solar cells (DSSC) has become very researchable. For improving the performance of these solar cells, a lot of optimizations were made in the photosensitizer, counter electrode, and photoanode. The utilization of MOFs in dye-sensitized solar cells shows promise in improving dye loading, charge transport, stability, and electrolyte immobilization. MOFs are incorporated in DSSC for modification of photoanode to improve the dye loading capacity. MOFs can serve as efficient host materials for dyes due to their high surface area and porosity. By incorporating the sensitizing dye within the MOF structure, it is possible to increase the dye loading capacity, thereby enhancing light absorption, and improving the overall efficiency of the DSSC (Table 3). The surface area of the MOFs modified photoanode as obtained from the BET analysis was 3.06 times and 3.37 times higher than that of the reference RGO/TiO₂-based photoanode. This improved the dye loading and enhanced the photo generation of carriers¹⁵⁹. MOFs can act as electron transport materials in DSSCs, facilitating efficient charge transfer from the sensitizing dye to the electrode. The tunable electronic properties of MOFs allow for the optimization of charge transport and reduction of charge recombination, leading to improved device performance. Uio-66-RGO/TiO₂ and ZIF-8-RGO/TiO₂ were used as photoanode exhibiting an efficiency of 7.33 And 7.67%, respectively. MOFs can serve as matrices for immobilizing the liquid electrolyte used in DSSCs. Incorporating the electrolyte within the porous structure of MOFs mitigates issues related to electrolyte leakage and volatility, leading to improved stability and longevity of the solar cell¹⁶⁰.

While the integration of MOFs into DSSCs is a relatively new research area, several studies have demonstrated the potential benefits of using MOFs in DSSC architectures. MOFs such as ZIF-67, MIL-101, and UiO-66 have been investigated for their application in DSSCs, showing improved device performance and stability. However, challenges such as optimizing the MOF properties for DSSC applications, achieving good compatibility between the MOF and other components of the device, and scalability of fabrication techniques still need to be addressed.

MOFs in perovskite solar cells

MOFs and their composites are widely applied in perovskite solar cells (PSCs) because of their low and flat charge/discharge potential plateau, stability, and repeatability. One of the most common applications of MOFs in perovskite solar cells is as electron or hole transport materials. MOFs possess high surface area and tunable electronic properties, which can facilitate charge transfer and extraction in the device. By incorporating MOFs as selective transport layers, it is possible to enhance the charge collection efficiency and reduce charge recombination, thereby improving the overall power conversion efficiency of the solar cell. Another potential use of MOFs in perovskite solar cells is as protective coatings. Perovskite materials are known to be sensitive to moisture, oxygen, and other environmental factors, which can degrade their performance over time. MOFs can act as encapsulating layers that provide a barrier against moisture and other detrimental species, thereby enhancing the stability and lifetime of perovskite solar cells. Furthermore, MOFs can serve as host materials for incorporating perovskite nanoparticles. By dispersing perovskite nanocrystals within the porous structure of MOFs, it is possible to improve the charge transport properties and stability of perovskite solar cells. The MOF matrix can provide a stable environment for the embedded perovskite nanoparticles and protect them from degradation. With the addition of MOFs, according to the composition materials of the active layer and transport layers, high-performance perovskite solar cells were fabricated¹⁶¹. The interface modification of these devices using MOFs was reported as an efficient way to enhance device performance. Different MOFs and their derivatives reported in perovskite solar cells include MOF-Cu, ZIF-8, UiO-66-NDC/GO, MOF-ZnO, MOF-In, MOF-525, MIL-101, etc. One more important usage of MOF is the reduction of Pb leakage or recycling Pb from halide perovskites¹⁶².

MOFs and their derivatives in ETLs

TiO₂, ZnO and SnO₂ are the most used oxide-based ETL for perovskite solar cells. Cobalt-doped TiO₂ based on MOFs were used as an efficient electron transport layer in perovskite solar cells with an architecture of FTO/TiO₂/MOF doped TiO₂/perovskite/Spiro-OMe-TAD/Au. A PCE improvement from 14.42 to 15.6% was reported by Nguyen et al. ¹⁶³. MOF-derived ZnO with a dodecahedron porous architecture was used as an efficient ETL, reporting a PCE of 18.1%¹⁶⁴. MIL-125(Ti) was used to make Na-TiO₂ as an ETL layer for perovskite solar cells, which have exhibited an improvement of PCE from 17.21% to 20.49% with excellent repeatability and lesser defect density¹⁶⁵.

MOFs and their derivatives in HTLs

MOFS were also reported in the applications of hole transport layers in solar cells¹⁶⁶. [In_0.5K(3-qlc)Cl_1.5(H₂O)_0.5]_2n,(In10) was incorporated in the Spiro-OMe-TAD in an n-i-p structured device with an efficiency improvement from 14.1% to 17.% because of the improved charge transport¹⁶⁷. Li-TFSI@NH₂-MIL-101 doped HTL was reported by Wang et al. in the perovskite solar cells with a planar device structure FTO/C-TiO₂/PC₆₁BM/perovskite/HTL/Au. Champion device showed an efficiency of 19.01%. After the modification, these devices showed very high stability under the ambient condition after 3600 h by retaining more than 85% of the optimal efficiency¹⁶⁸. NiOx is one of the most efficient HTLs for perovskite solar cells. MOF-derived NiO_x nanoparticles for the HTL were synthesized by using four different ligands like DHPTA, BTC, ATPA, and TPA. Islam et al. ¹⁶⁶. fabricated PSCs involving the structure FTO/TiO₂/CsPbBr₃/NiO_x/C, and incorporated MOF-derived NiOx, which showed a PCE of 13.9%.

MOFs and their derivatives in active layer

A light-absorbing perovskite− MOF hybrid layer for PSCs was reported by Lee et al. involving triple cation perovskite, with a power conversion efficiency of 20.5%. Cu-BTC MOF with an average size of 20 nm was incorporated in the perovskite layer. It was reported that the intermediate monohydrate phase survives longer in the perovskite−MOF hybrid absorber layer, slowing structural degradation into PbI₂¹⁶⁹. Recently, a Zn-based MOF [((Me₂NH₂)₃(SO₄))₂(Zn₂(ox)₃₎]n (ZnL) was incorporated into the perovskite layer to extend the role of the MOF in the passivation of perovskite layers. Several oxygen sites from (Zn₂(ox)₃)₂− formed solid electrostatic interactions with Pb²⁺, thus anchoring the atoms of the perovskite and improving the stability. The efficiency improved from 19.75% to 21.15% with high stability over 3000 h¹⁷⁰. 2D polyoxometalate (POM)-based MOF (C₅NH₅)₄(C₃N₂H₅)₂Zn₃(H₈P₄Mo₆O₃₁)₂·2H₂O (POMOF) was used in the perovskite active layer which resulted with very high efficiency (23.3%) and excellent stability ( ≈ 90% of the original PCE after 1600 h. Highly ordered POMOF helped grow highly crystalline perovskite films with lower defect density. This also contributes to the reduction of environmental risks by effectively inhibiting lead leakage through chemical anchoring and adsorption involving the phosphate groups¹⁷¹. Different MOFs and their derivatives used as active layer transport for energy applications are illustrated in Table 4.

MOFs and their derivatives as interfacial modifier

ZIF-8 was introduced at the interface between the perovskite and TiO₂. This has caused a scaffolding effect for the perovskite layer. Yazdi et al. used ZIF-8 in place of mesoporous TiO₂ and observed an enhancement of PCE from 14.5 to 16.8% which was attributed to the defect reduction at the interface¹⁷². Co₃O₄@NC on ZIF-67 as a sacrificial template was reported as an interfacial modifier in carbon-based perovskite solar cells. By interface engineering, Geng et al. found this hole buffer layer in an n-i-p device architecture effectively stimulates charge separation and extraction and reduces charge carrier recombination at the interface between MAPbI₃ and the carbon electrode. This has improved power conversion efficiency to 14.63% while the reference showed an efficiency of 11.63%¹⁷³. UiO-66-NDC/GO was used for interface engineering between the perovskite and SnO₂ (ETL) in SnO₂/perovskite/Spiro-OMe-TAD/Au device architecture. This has resulted in improved charge transport because of interfacial dipoles. The PCE improved to 18.2% from 16.3% which is a 12% improvement¹⁷⁴.

MOFs for preventing lead leakage

POMOF reported by Dong et al. were used to prevent lead leakage as the PO₄³⁻ groups efficiently capture heavy metal ions by forming strong chemical bonds¹⁷¹. ZrL3:bis-C60 EEL was incorporated in the p-i-n-device architecture, showing 90% of its initial PCE for over 1100 h¹⁷⁵. While incorporating MOFs into perovskite solar cells is currently at a nascent investigation stage, promising outcomes have emerged from various studies. These research efforts have demonstrated enhancements in device performance, bolstered stability, and minimized hysteresis. Nonetheless, practical implementation faces obstacles like ensuring compatibility between MOFs and perovskite materials, developing scalable fabrication methods, and addressing long-term stability concerns. These challenges must be effectively tackled to fully realize the potential benefits of MOFs in perovskite solar cell technology.

MOFs in organic solar cells (OSC)

Two-dimensional MOF nanosheets show great promise as nanomaterials for organic solar cells (OSCs) due to their adjustable electronic and optical properties. These nanosheets can potentially be blended with polymers, although significant optimization of processes and mechanisms are still required. A novel approach utilizes a tellurophene-based 2D MOF combined with a branched polymer surfactant, polyethylenimine ethoxylate (PEIE), to develop single- and few-layer nanosheets. This hybrid ink is directly applied as an electron extraction layer in high-performance OSCs, resulting in a power conversion efficiency of 10.39%, surpassing PEIE-based devices. The improvement is attributed to reduced charge recombination, adjustable work function, and enhanced conductivity with the ZnO/MOF-PEIE layer¹⁷⁶. Sasitharan et al. reported the development of ultrathin zinc-porphyrin-based metal-organic nanosheets (MONs) with electronic and optical properties well-suited for integration into polythiophene–fullerene (P3HT–PCBM) organic solar cells. Notably, incorporating these MONs into the photoactive layer of the photovoltaic device achieved a power conversion efficiency of 5.2%¹⁷⁷. Ongoing research in the use of MOFs for organic solar cells continues to focus on refining their synthesis and functionalization to tailor their properties for specific applications.

MOFs in quantum dot solar cells (QDSCs)

Metal-organic frameworks (MOFs) have attracted attention in quantum-dot solar cells due to their porous structure and customizable chemistry, allowing for the integration of diverse quantum dots. This capability optimizes light absorption and enhances charge transport. A composite of CdTe quantum dots with a europium-MOF was utilized as a novel photoanode, resulting in an increase in short-circuit current density from 19.8 mA/cm² (CdTe QD) to 28.45 mA/cm² (CdTe QD/EuMOF). Correspondingly, the power conversion efficiency (PCE) improved from 1.67% to 3.02%. This 1.35% absolute enhancement in PCE can be attributed to the augmented surface area and enhanced photon absorption capacity facilitated by the MOF structure¹⁷⁸. The use of MOFs in quantum-dot solar cells is a developing area with substantial growth potential. Future research should focus on optimizing the synthesis and integration of these materials to further enhance performance metrics such as efficiency, stability, and scalability. With ongoing advancements, MOFs may pave the way for next-generation solar technologies that capitalize on the unique attributes of quantum dots.

MOFs in water treatment

Metal-Organic Frameworks have emerged as promising materials for water treatment especially in the removal of heavy metal ions, degradation of organic pollutants, and desalination^14,179,180. For example, A TiO₂/NH₂-MIL-125(Ti) nanorod array on carbon fiber cloth (CFc/TiO₂/Ti-MOF) was designed for effective self-cleaning solar desalination. CFc/TiO₂/Ti-MOF demonstrates super-hydrophilicity (contact angle of 0° at 5 s) and increased photoabsorption (89.5%) due to surface modification and light-trapping effect from TiO₂/Ti-MOF nanorod array, when compared to CFc substrate. The membrane removed 58.5% ofloxacin (OFLX) and 62.8% tetracycline (TC), 94.1% methylene blue (MB), and 76.4% Rhodamine B (RhB) in 120 min under one sun exposure. As a result, this work sheds light on how to build innovative photothermal/photocatalytic membranes for effective auto-cleaning solar desalination¹⁸¹. The unique properties of MOFs make them efficient for adsorption and separation capabilities in complex water matrices. Recent advances in MOF composites and hybrid materials have enhanced their efficiency and reusability, making them ideal for challenging environments that contain mixed contaminants. In wastewater treatment, MOFs serve as both adsorbents and catalysts, effectively addressing the pollution caused by harmful chemicals from industrial, agricultural, and domestic sources. MOFs are used to remove heavy metal ions, which are non-biodegradable and highly toxic, and agricultural pollutants, even at low concentrations, through various mechanisms such as adsorption, catalysis, and photocatalysis^14,179,182. The effectiveness of MOFs in these applications is influenced by their pore size, functional groups, and structural design, allowing them to be tailored for specific contaminants. The mechanisms through which MOFs remove toxic elements primarily involve π − π interactions, ion exchange, and electrostatic interactions^14,179. The electrostatic attraction between surface charges on pollutants and oppositely charged MOFs plays a crucial role in adsorption processes, with protonation and deprotonation facilitating these interactions. Hydrogen bonding between the hydrogen atoms in N − H, F − H, and O − H bonds and the lone pairs of electronegative atoms enhances the adsorption of pollutants¹⁸⁰. MOFs are capable of degrading various contaminants through catalytic processes, often combined with reactive species like sulphate radicals or hydrogen peroxide, to oxidize pollutants into less toxic products. For example, NbCo-MOF demonstrates an impressive removal efficiency of 99.7% for tetracycline in wastewater within just 30 min¹⁸².

Membrane separation technology has gained attention for its potential in energy conservation and environmental protection. MOFs stand out due to their tuneable crystalline microporous structure, which allows for the adjustment of pore sizes and chemical properties. Adsorption technology is gaining popularity in water treatment due to its low cost, ease of operation, and broad applicability. MOFs have adjustable porosity and functionalization capabilities which enable effective removal of toxic contaminants from wastewater¹⁸³. The diverse range of central metals and coordinated unsaturated sites in MOF frameworks make them powerful adsorbents for wastewater purification¹⁷⁹. Effective removal of these metals is crucial for protecting human health and maintaining a healthy ecosystem. MOFs have been effective in adsorbing a wide range of heavy metals from wastewater, such as mercury, lead, and cadmium¹⁴. By incorporating sulphur-containing functional groups into MOFs, high adsorption capacities and rapid kinetics can be achieved. For example, sulphur-functionalized MOFs have demonstrated exceptional capabilities in removing mercury ions (Hg²⁺). Studies have also explored ammonia-functionalized MOFs, which utilize free amine groups for Hg²⁺ adsorption and effectively reduces mercury levels to acceptable limits for drinking water¹⁸⁴. The increasing concern over agricultural pollutants, such as pesticides and fertilizers, has also led to the development of MOF-based materials capable of effectively degrading these harmful substances, thereby mitigating their adverse effects on ecosystems and human health. Advancements in the development of MOF composites have led to the development of materials that maintain high stability and selectivity in aqueous environments¹⁸³. For example, the NENU-401 MOF, which incorporates thioether groups, demonstrates excellent mercury removal capabilities and high reusability over multiple cycles¹⁸⁵. The introduction of thioether side chains has not only improved adsorption performance but has also enhanced the stability of the MOF frameworks in challenging conditions¹⁸³. Overall, the advancements in MOF technology represent a significant step forward in wastewater treatment, offering sustainable solutions to the growing problem of water contamination. Their versatility, high efficiency, and potential for functionalization make MOFs a valuable material to address challenges with water treatment.

MOFs in dye removal

High porosity, large surface area, adsorption capacity, and surface functionalization propertied make MOFs ideal for removing toxic dyes in water treatment plants³¹. MOFs have several advantages over porous adsorbents due to their large surface area, tunable porous structure, thermal stability, suitable shape and size, removal efficiency, ease of synthesis, and adsorption capacity. These properties make MOFs a prime candidate for removing toxic dyes from water¹⁸⁶. There are numerous pollutants in water bodies and toxic organic dyes that pose a risk to the environment, humans, and animals^187,188. MOF materials show excellent properties for the removal of dyes in industrial wastewater. Chemical modification of MOFs with functional groups or other materials and surface modification helps remove dyes¹⁸⁹. Adsorption on MOF fibers is more effective and desirable compared to powdered MOF adsorption due to sludge formation by powdered MOFs¹⁹⁰. Sponge and monolith MOFs are highly reusable compared to powdered MOFs¹⁹¹. Nanofiber MOFs are highly efficient in the selective removal of methylene blue¹⁹². Positively charged tin MOF was synthesized and showed promising results for the removal of anionic dyes¹⁹³. Adsorption on the surface of MOFs could be slow or rapid in cationic dyes, depending on the surface functional groups and the molecular structure or size of the dye¹⁹⁴. Moreover, cationic dyes were adsorbed onto MOFs in a solution of pH 6 or above due to deprotonation of the MOF surface¹⁹⁵.

MOFs in soil remediation

MOFs are extremely porous materials having a high adsorption capability for environmental contaminants. The integration of stimuli-responsive moieties in their structures provides them with additional features such as easy recovery and recyclability. As a result, stimuli-responsive MOFs are increasingly being used for environmental remediation. Some researchers have successfully integrated photoactive species into MOF structures, resulting in the development of light-responsive composites. An MOF’s structure contains thermo-responsive polymeric units, which give it physicochemical features that are sensitive to temperature changes. Incorporating magnetic nanoparticles into MOF composite structures can help in their recovery¹⁹⁶. Also, MOFs and their composite were used for remediation of soil from toxic heavy metals. For example, nano zero-valent iron (NZVI) supported by MOF (MOF-NZVI) utilizing NaBH₄ and FeCl₃ was developed to tackle the soil Cd remediation problem. The MOF-NZVI was capable of changing Cd in soil from weak acid extractable and reducible fractions to oxidizable and residual forms, thereby lowering the toxicity of Cd in soil¹⁹⁷. Additionally, to remove Cr(VI) from agricultural soil, a new combined remediation technique was used: multiple-pulse water soil flushing (MF) with innovative aminated cellulose nanofibers (A-NFCs)/AEM/AM@MIL-100(Fe) nanocomposite hydrogel adsorbent. In addition, when MF@ANCMH was applied to growing wheat plants, Cr bioaccumulation was reduced to less than 0.1 ppm, physiological phytotoxicity symptoms were avoided, and photosynthesis, antioxidant enzyme activity, yielding and growth indicators were increased as compared to untreated soil. Furthermore, it enhanced microecological activities, microbial community, soil organic matter, and diversity¹⁹⁸. On the other hands, MOFS composites was developed to determine/remove fungicides from soil samples. For example, a sensitive magnetic solid-phase extraction method using mag-ZIF-7@GO and LC-MS is developed to determine seven fungicides (bitertanol, diniconazole, pyrimethanil, hexaconazole, triadimenol, tebuconazole, and flutriafol) as a function of extraction time, optimized salt concentration, desorption time and pH. Under ideal conditions, the approach has excellent repeatability, reproducibility and linearity. The quantification and detection limits for the seven fungicides are 1.95–7.94 ng/L and 0.58–2.38 ng/L respectively. Furthermore, the suggested approach is used to identify trace fungicides in soil samples, with recoveries of 82.4–93.4%¹⁹⁹.

MOFs in CO₂ capture and sequestration

MOFs, MOF membranes such as ZIF-90, ZIF-69, ZIF-8, ZIF-62, Cu₃(BTC)₂ and their different composites such as IL/MOF composite have been reported for CO₂/CH₄, CO₂/H₂ and CO₂/N₂ separations^{200,201,202,203,204}. They possess excellent mechanical properties, and their production costs can be considerably lower than those of inorganic membranes. The separation performance of MOFs membranes is typically evaluated through single gas permeation tests, which yield ideal selectivity’s and gas permeabilities²⁰⁵. However, as gases in industrial applications commonly exist in mixtures, conducting tests under relevant gas mixture conditions would provide significantly more accurate and practical results than those obtained from single gas assessments. Recent advancement in synthesis procedure not only cover stability issues but also improves separation performance. The diverse structural and chemical properties of MOFs present considerable opportunities for the synthesis and development of more efficient MOF membranes for CO₂ capture.

MOFs in smart wearable sensors

Wearable electronics, the next generation of flexible electronic devices, require stable and continuous monitoring of physiological data without the need for an external power supply. A self-powered smart system comprising flexible solid-state Zn-Co MOFs@MXene supercapacitors and polyacrylamide-BaTiO3/NaCl (PAM-BTO/NaCl) organic ionic hydrogel sensors has been constructed. The supercapacitor has a high energy density of 51 Wh/kg and a power density of 1.59 kW/kg. In particular, it retains remarkable mechanical flexibility in a variety of bending situations, as well as outstanding electromechanical stability, with nearly unaltered performance under numerous similar stresses. Furthermore, the self-powered integrated system is suitable for real-time sensing of human movements and tiny stresses²⁰⁶.

MOFs have been also incorporated in smart sensors. For example, a highly stretchy wearable electrochemical sensor made of Ni-Co MOF/Ag/reduced graphene oxide/polyurethane (Ni-Co MOF/Ag/rGO/PU) fibers is designed to constantly and correctly quantify the quantity of glucose in sweat. To construct the Ni-Co MOF/Ag/rGO/PU (NCGP) fiber bioelectrode, the rGO/PU fiber was easily spun using an improved wet spinning approach, and the Ni-Co MOF nanosheet was coated on its surface for continuous sweat-based glucose sensing²⁰⁷.

MOFs have become increasingly popular, and they have been investigated and applied to be used in different fields; however, there are several challenges to be overcome. MOFs are commonly used for biomedical applications in drug delivery systems. Despite advances in laboratory research MOF-based composites face challenges. MOFs are toxic and poisonous due to their morphological properties, composition, size, and stability, and these toxicity issues need to be addressed prior to or during clinical trials. A comprehensive assessment of MOFs toxicity is therefore required. The toxicity of MOFs scaffolds has been researched extensively; however, long-term toxicity is a critical aspect that has not been addressed and must be considered for biomedical applications of MOF scaffolds. Long-term accumulation monitoring in tissues and extensive in vivo studies is required to evaluate the toxicity of MOFs. Functional MOFs can be developed from highly biocompatible metal ion nodes and ligands to avoid toxicity³⁴. Despite the exceptional performance of MOFs as adsorbents for removing various pollutants from water, their limited stability in aqueous environments hampers their practical applications²⁰⁸. To overcome this issue, MOFs can therefore be synthesized with appropriate structure and molecular design. Although MOFs have been used effectively for the remediation of potentially toxic trace elements (PTE) in wastewater, they remain a challenge to be applied in industrial and large-scale applications. Limitations with stability, production cost, toxicity, mass production, specific and multi-metal PTE adsorption, and regeneration potential prevented several MOFs from industrial and large-scale applications²⁰⁹. The effectiveness of MOFs in industrial applications is largely contingent upon their stability in water. For MOF adsorbents to efficiently remove potentially toxic elements (PTEs) and other pollutants from aqueous environments, they must exhibit high water stability. However, their susceptibility to degradation upon prolonged exposure to humidity limits their practical utility and commercialization prospects²⁰⁸. The coordination bonds between metal ions and linkers are vulnerable to hydrolysis, leading to displacement of the linkers and structural alterations that compromise their stability in water²⁰⁹.

For industrial applications, the large-scale production of MOFs is essential, yet their synthesis is constrained by low mass production rates and impractical high-temperature requirements. Methods such as vapor/base diffusion and solvent layering are more suited to small reaction volumes, limiting their scalability. Additionally, reproducibility poses significant challenges, as synthesis outcomes are influenced by variables such as reaction conditions, solvent selection, and ligand-metal ratios¹⁷⁹. Although MOFs are characterized by high surface areas and tunable pore structures, their performance varies with specific applications and target molecules, necessitating extensive characterization and optimization for desired selectivity and efficacy¹⁸⁰.

Moreover, the potential for regeneration is critical for the sustainable application of MOFs, as it aids in resource conservation, energy efficiency, waste reduction, and environmental safety. The effective regeneration of MOFs also plays a role in pollution control and economic viability. However, the environmental toxicity of MOFs remains a significant concern. Organic linkers like phosphonates, imidazolates, amines, and carboxylates can induce mild toxicity, while solvents used during synthesis may pose risks to organisms. A comprehensive understanding of MOF toxicity is still lacking, highlighting the need for further research in this area.

A circular economy aims to remove waste and develop a sustainable, efficient production and consumption system. It focuses on minimizing waste and maximizing resource use through improved recycling and the reuse of materials, thereby reducing the depletion of finite natural resources. This model seeks to balance economic growth with environmental stewardship. In the chemical industry, it encourages efficient resources and energy use, promoting sustainable practices that benefit both the economy and the environment. Applying circular economy principles can enhance the sustainability and scalability of MOFs used in activation and purification processes^210,211. Recycling, reusing, and regenerating MOFs represent the fundamental hierarchies essential for advancing sustainable and circular opportunities within this field. When MOFs are no longer suitable for reuse, recycling them reduces the amount of waste disposed. However, recycling MOFs is less sustainable in terms of efficient resource use, profitability, and material limitations. Recycling MOFs involves the recovery and purification of unreacted materials, solvents, and spent MOFs for further application. MOFs act as adsorbents in several industrial processes and after fulfilling their role they may become saturated. This renders the saturated material less efficient and recycling it aims to regenerate the spent MOFs to their original capacity. This reduced the need for constant production and contributes to a circular economy²¹².

Maximum resource conversion and the efficient use of energy without releasing toxic pollutants into the environment are key to reduction processes. The reduction can be implemented by minimizing raw materials used, energy consumed, and the waste and emissions generated by applying MOFs. The reduction principle can be implemented through efficient synthesis processes by reducing the duration of these processes that ultimately have a higher yield and conversion efficiency, purification, and activation routes. Minimizing toxic reagents used throughout MOF production can reduce waste generation while the by-products generated during production can be reused for production or alternative processes. Reusing MOFs requires fewer resources and less energy when compared to manufacturing a new product. It exerts less demand on the environment due to less waste disposal. Unreacted material and solvent from synthesis, purification, and activation are collected and reused for multiple cycles of adsorption and desorption. To be able to reuse MOFs, their structure needs to be stable to be able to regenerate them multiple times whilst they retain their functional properties²².

MOFs can be regenerated to restore their adsorption capacity after they have been saturated and deactivated. Regeneration involves desorption using solvents, thermal treatment, and pressure swing adsorption and these processes restore the MOFs to their original state allowing them to be reused. Regeneration and reuse optimize the lifespan of MOFs and reduce waste. MOFs can be regenerated by removing the adsorbed species using solvents and the choice of solvent depends on the specific MOF and the adsorbate. The MOF is immersed in a solvent that selectively desorbs the target molecule. This process may involve heating or pressure changes to enhance desorption, and the recovered MOF is reused. Thermal treatment is also used for regeneration by heating the MOFs to a high temperature which causes the adsorbed species to desorb²¹⁰. This renders the MOF material ready to be reused. The method used for regeneration should be based on the suitability of the specific MOF, the nature of the adsorbed species, and the intended application since some MOFs may decompose or lose their structural integrity at high temperatures. When desorption by solvents or thermal methods is not effective, mechanical or ultrasonic treatment can help remove adsorbed species from MOFs. The ultrasonic energy disrupts the adsorption forces and facilitates the release of the adsorbed molecules. When MOFs cannot be regenerated or reused, they may be recovered and used for the synthesis of new MOFs. The organic ligands and metal ions can be combined to form new MOFs through synthetic methods after they are separated and purified.

Recycling, reusing, and regenerating MOFs pose several challenges due to their complex structures, interaction with the adsorbate, and sensitivity to various conditions. The process of recycling, reusing, and regenerating MOFs is an active area of research, and new techniques and strategies continue to emerge to improve their sustainability and commercial viability. New and innovative approaches and techniques are being studied to improve the efficiency of these processes. Adopting sustainable practices for MOFs can reduce their environmental implications and reduce synthesis costs while contributing to a greener and more sustainable future. The circular economy of MOFs will make them more viable for a wide range of applications as research progresses and new methods are developed.

Given the current environmental concerns worldwide by rapid industrialization, developing novel functional materials is gaining growing importance. As MOFs and MOF-derived materials emerge as superior candidates for application in several sectors, promoting their practical utilization for green chemical engineering with a sustainable approach by taking advantage of their superior functions is crucial. MOFs demonstrate superior properties; these adsorbents are promising and efficient alternatives to traditional materials. MOFs have been successful in several applications due to their structural tunability, functionality, design, stability, and high surface area. The synthesis of MOFs using non-toxic, environmentally benign reactants and solvents under mild conditions, with minimal byproduct formation, is essential for sustainable green synthesis. Promoting sustainable consumption and production enhances resource and energy efficiency, facilitates the transition to a greener economy, and significantly reduces pollutant emissions. The United Nations’ Sustainable Development Goals (SDGs) offer a comprehensive framework for addressing global challenges and advancing sustainable development on a global scale.

The Sustainable Development Goals (SDGs) relevant to MOFs emphasize several factors including affordable and clean energy, innovation, infrastructure, responsible consumption, climate action, and environmental impact. MOFs play a crucial role in the development of advanced energy storage and conversion technologies which help increase access to affordable, sustainable, reliable, modern, and clean energy sources and fulfill SDG 7. The goal of SDG 7 is to ensure sustainable energy for all which promotes economic growth, improves health and education, and ensures the eradication of poverty as well as addresses climate change. To achieve sustainable development across various sectors, it is crucial to provide affordable clean energy. MOFs have the potential to revolutionize various industrial processes by improving the efficiency and sustainability of these processes. MOFs can contribute to SDG 9 by promoting sustainable industrialization. SDG 9 focuses on innovation, building resilient infrastructure, and promoting sustainable industrialization. To enhance economic development and considerably increase employment and gross domestic product, specific objectives include constructing high-quality, trustworthy, sustainable, and resilient infrastructure. Efficient use of resources increases by upgrading and improving processes to make them sustainable, clean, and environmentally friendly technologies.

MOFs can be used in infrastructure and construction as coatings and filters to reduce energy consumption. MOFs thereby contribute toward SDG 11 which aims to make cities and human settlements resilient, sustainable, and safe. Specific targets of SDG 11 ensure enhancing sustainability and livability of urban environments through access to safe affordable housing and access to sustainable transport systems for all. MOFs have the potential to improve resource efficiency by promoting sustainable consumption and production which contribute to SDG 12. To drastically reduce negative effects on human health and the environment, SDG 12 includes the efficient use of natural resources and the sustainable and safe management of chemicals and wastes throughout their life cycles according to international guidelines. SDG 12 also emphasizes the reduction, recycling, reuse, and regeneration of waste. MOFs can be used in carbon capture and storage technologies to mitigate greenhouse gas emissions and thus contribute to SDG 13 by supporting climate action and its impacts.

SDG 13 targets the inclusion of climate change measures into national strategies, policies, planning, and adaptive capacity. It includes the implementation of the Paris Agreement on Climate Change which involves preventing the increase in global average temperatures while continuing efforts to limit the rise in temperature. Promoting mitigation methods to reduce greenhouse gas emissions and adaptation strategies to deal with the repercussions of climate change is the ultimate objective. MOFs are used in water treatment systems to remove contaminants and pollutants and thus contribute toward the preservation and restoration of marine and terrestrial ecosystems. MOFs contribute to SDG 14 and SDG 15 by promoting sustainable use of our water resources and terrestrial ecosystems respectively. Both emphasize the importance of protecting and conserving our ecosystem and biodiversity. The goal of both SDG 14 and SDG 15 is to prevent pollution, habitat loss and promote sustainable practices, and ensure that resources are being used sustainably to preserve the health and resilience of our ecosystem. MOFs hold great potential for sustainable development, however further research and development is required before implementation.

MOF and MOF-based materials are of great interest and applicability in various industries. MOFs were originally synthesized to make new structures and compounds with interesting properties. However, the field has broadened in scope over the years. MOFs have promising applications due to their unique properties and will likely give way to new research to benefit new and existing processes. New materials are being discovered and applied in different fields due to the possible combinations of linker molecules and metal ions. The outstanding quality and potential of MOFs for various post-synthesis organic modifications is attracting a lot of attention. MOFs can be synthesized with different structures and chemical compositions. MOFs’ covalent or noncovalent functionalization, such as composite creation, can be used to customize their properties and applications. Because of their porous structures, MOFs can hold gases, metallic species, and organic molecules. This makes them suitable for biological, environmental, industrial, and energy applications. For industrial implementation, synthesis with high hydrothermal stability should be preferable. It is possible to combine the synthesis of MOFs with the synthesis of other organic-inorganic compounds. MOFs are typically synthesized on a small scale, and the processes are often not optimized; therefore, reliable methods and protocols are needed to enable large-scale production. The properties, including the textural characteristics, of MOFs can vary depending on the specific synthesis conditions. Before industrial-scale implementation, further research is necessary to scale up the synthesis of MOFs in an optimal and cost-effective manner. While MOFs show promise in air quality management applications due to their beneficial properties, they face significant challenges, including the lack of standardized methods for assessing key performance metrics such as low production costs, hydrothermal stability, chemical stability at varying temperatures and humidity, and recycling and disposal of spent materials. Due to research gaps in the emerging field, a systematic evolution and investigation of MOFs for air purification must be evaluated. The chemistry of MOFs also needs to be specified for practical application. The tunable properties of MOFs enable enhanced performance in gas capture technologies and pollutant storage, offering significant advantages over conventional materials. Low-cost hierarchical and functional pore structures are desired for developing novel approaches for the post-synthetic modification of MOFs which expand the practical applicability and implementation at an industrial scale. These applications can be practically implemented on an industrial scale. MOFs are extremely desirable materials for air purification technologies. MOFs are advantageous for the removal of anionic and cationic dyes from water. Hybrid MOFs have higher adsorption capacity and removal efficiency compared to bare MOFs. Stability, synthesis techniques, cost of production, regeneration, and real-time application are some challenges that need to be overcome for further deployment of MOFs. Although MOFs have been studied extensively and show promising results, the use of MOFs as efficient adsorbents in the future requires research for the production of MOFs at a large scale with a green approach with improvised properties such as high stability, surface area, selectivity, reusability, and adsorption capacity. Environmental concerns must be accounted for and researched to enhance the understanding and interactions of MOFs in the environment. Systematic studies need to be performed under well-defined scenarios for different synthetic procedures. The expansion of MOF varieties requires more research and knowledge about the formation of compounds. In in-situ and ex-situ procedures, theoretical methods and analytical equipment are crucial to examine the stages of crystallization, while systematic research can also be helpful in discovering certain parameters. Not only do they help explain crystallization, but also help in the synthesis of MOFs with new or improved properties. Future investigations should be extended toward the wide array of MOF compounds and their diverse application in other fields also could be explored.