Metal organic frameworks (MOFs)

Metal-organic framework structures are crystalline, porous coordination polymers that construct one, two, or three-dimensional networks by attaching metal ions (M+) to poly-functional organic molecules (Ligand, L). It is feasible to create well-defined structures with a wide surface area, structural diversity, adjustable pore size, and easy workability using these two structural units, which combine some qualities of organic and inorganic porous materials. This is related to organic molecules’ symmetry and is dependent on the coordination number of metal ions.

In the geometry of the Metal Organic Framework, the angle of attachment of the ligands to the metal atoms, the bond length, and the frequency of attachment vary and the surface area, pore size and pore volume vary as a result of these factors.

The basic structure of Metal organic framework

The Metal Organic Framework has a three-dimensional high crystalline structure, which is higher than zeolites, thanks to the unique feature of the organic molecule used as a binder. It has a huge pore capacity and has a lot of active surface area. These materials frequently crystallize into well-defined three-dimensional (3D) cages in the micro/meso and nanoporous range. This sort of cage is utilized for gas storage and allows for reversible hydrogen adsorption at cryogenic temperatures (CO2, CH4 etc.).

Nowadays one of the most fascinating nanotechnology research topics in recent years are about Metal Organic Frameworks. Metal Organic Frameworks can also be produced at high density and purity under the right conditions, according to the literature.

Because of its high active surface area, mesoporous structure, high adsorption capacity, and high surface reactivity, Metal Organic Frameworks are employed as an excellent adsorbent. The pore volume of Metal Organic Framework materials is frequently larger than 0.2 cm3/g, with an interior surface area of more than 400 m2 (value measured by BET). Pore diameter that can be adjusted can be 3 Å or greater. The gas to be employed in the adsorption process is directly proportional to the pore diameter. Gas adhesion cannot be achieved in pores with a diameter smaller than the gas molecule.

Different Metal organic frameworks nanoporous architectures

CO/CO2 emissions, the majority of which come from industrial areas and power plants, are one of the most serious and pressing issues today, as they contribute to global warming. As a result, since 2006, researchers have been focused on the development and implementation of highly efficient CO/CO2 capture and storage (CCS-Carbon Capture Storage) processes. Today, a lot of work is being done in order to apply in industrial areas by increasing the CO/CO2 capture and adsorption performances of metal organic framework structures with modifications due to their high porosity, defined morphology, adjustable high active surface area, and pore size features.

Among the features that distinguish metal organic framework structures from other adsorbents:

– Particle size and bonding angles that can be designed according to the application area,
– Easy synthesis and modification process,
– High adsorption capacity at moderate temperatures,
-Low cost preparation,

It has advantages such as relatively easy regeneration and reusability .

History

With the discovery of zeolites, researchers were able to meet the need for porous materials with high adsorption capacity for the first time. Until the end of the twentieth century, alumina silicate crystals, a zeolite derivative, held the record for the material with the greatest known pore volume. Hofmann et al. discovered a coordination network linked by CN groups with the formula Ni(CN)2(NH3).C6H6 in 1897, which started the history of metal-organic structures.

In the early 1990s, some MOF structures were discovered in the literature. Omar Yaghi et al.  coined the phrase “Metal Organic Framework Structure” in 1995. In 1999, the MOF published two key articles. Ian Williams et al found HKUST-1, a MOF structure consisting of copper-based aggregates and benzene tricarboxylate linkers, in February. MOF-5, a structure made up of benzene dicarboxylate binders and the metal ion zinc, was discovered by Yaghi et colleagues in November. These materials have been demonstrated to be more than three times the porosity and surface area of the zeolite derivative with the highest porosity and surface area known at the time.

Makoto Fujita and colleagues researched the chemical diversity and alterations of MOFs in 1996, and good results were obtained employing them in catalytic applications. More than thirty thousand investigations on MOF have been conducted from the beginning to the present, with over two hundred MOF structures reported in the literature. It has been offered to gain qualities for the application area and to boost its efficiency as a result of its changes with various functional groups. Metal organic framework structures have been researched in a variety of applications, including catalysis, water treatment, adsorption, gas separation, and storage, due to their features.

Usage areas of MOFs

MOFs offer a broad range of uses due to their tunable porosity and large surface area as well as their ability to be functionalized as needed. Separation, purification, storage, medication storage and controlled release, catalytic support material, sensor medium, magnetic material, and energy generation are just a few of the applications for Metal Organic Frameworks.

Seperation

Separation is one of the most critical and hardest procedures in chemical processes. Chemical separation methods in industry use a significant amount of time and money. Extraction, distillation, membrane separation, adsorption, absorption, and crystallization technologies are some of the techniques used in separation operations. Metal Organic Frameworks, which can be employed as adsorption materials in a variety of separation processes, have good selectivity, low cost, and low energy consumption.

Via coordinating components to be separated or removed from different environments, making acid-base interactions, establishing a complex, adsorption by hydrogen bonding, or electrostatic interactions from unsaturated regions, Metal Organic Frameworks can attract related compounds from their environment. Furthermore, they provide for a pure physical separation by enclosing them in the pores.

Adsorption

The adsorption phenomenon is caused by an imbalance of forces between molecules in the solid or liquid matter’s boundary regions. Adsorption occurs when gas or liquid molecules are attached to a solid surface by the application of forces. The substance to be adsorbed is referred to as the “adsorbent,” while the liquid or gas particles sticking to the surface are referred to as the “adsorbate.” Desorption refers to the return of adsorbed substances to the environment.

The active pore diameter, pore volume, surface area and functional groups are the most essential characteristics determining the adsorption process for adsorbents. The adsorption capacity increases as the active surface area and porosity increase. Adsorbents are inexpensive, readily available, and can be reused, making the adsorption process cost-effective.

The adsorbed substance is affected by concentration, pressure and chemical similarity to the adsorbent. The amount of adsorption increases as the adsorbate concentration and pressure rise. Similarly, the temperature at which the adsorption process is carried out has an impact on the amount of adsorption. The adsorption process is divided into three primary steps. The gas or liquid molecules to be adsorbed are first adsorbed on the adsorbent’s outer surface. The adsorbents are subsequently adsorbed on the walls of the interior pores as it advances towards the inner of the adsorbent pores.

Gas Storage

MOFs have become one of the most successful materials for gas adsorption due to their high surface areas and designable pores. Metal Organic Frameworks with varying selectivity for different gases can be made by mixing the organic binders and metals that make up the MOF structure in various ways. In recent years, investigations for high gas storage capacity have focused on MOFs, which can be produced at values beyond 6000 m2/g surface area.

Due to the movements of gas molecules in the structure, such as dispersion, repulsion, and attraction, gas molecules physically bind to the pores in the framework mesh and are held there throughout gas storage. The amount of adsorbed gas is directly proportional to the surface area and pore volume of the adsorbing material, according to experimental and theoretical investigations. As a result of the ease with which these parameters can be customized, Metal Organic Frameworks have an opportunity in this field.

Drug Release

MOFs have been shown to be useful as a biocompatible capsule for drugs with low biocompatibility, such as cancer drugs, and as a tool that can be controlled by external effects such as magnetism in drugs that need to be delivered in a controlled manner, and that can release the drug when it reaches the appropriate region. MOFs can be specially designed and functionalized to carry certain drugs, and they can be used in a way that will allow for slow or rapid drug release.

Sensor

MOFs can only be built for a certain host and can provide detection of the host molecule due to changes in their physical, chemical, optical, or magnetic properties while interacting with the host because their structures can be modified as needed. Selective H-binding and van der Waals contacts, open metal sites and p-p interactions may all contribute to MOF selectivity towards host molecules. Electrochemical sensing has lately become one of the areas of concentration, owing to benefits such as low cost, quick turnaround, crisp results, and ease of usage.

Catalysis

There has been a spike in interest in the development of heterogeneous catalysts in past few years. Solid catalyst systems have gotten lots of interest as a heterogeneous set of catalysts with a lot of advantages over their liquid counterparts. The overall atomic efficiency of the reactions and the turnover number of the catalysts (TON) are enhanced by using solid systems instead of liquid systems in stoichiometric proportions. Furthermore, solid systems are less irritating and can generate scar treatment issues. Metal Organic Frameworks have opened up new possibilities for the construction of solid systems.

In solid catalyst systems, MOFs can serve as either the catalyst support material or the catalyst itself. For organic catalysis and photocatalysis, Metal Organic Framework building blocks can be used as active sites. Using enantiotropic ligands, homochiral MOFs for non – symmetric catalysis may be produced comparatively easily compared to other porous materials.

Metal Organic Frameworks offer unique qualities that go beyond what one would expect from a basic blend, combining the good aspects of inorganic and organic components. Unlike standard solid systems, MOFs can provide metal knots and organic ligands a variety of properties, resulting in a diversity of useful catalysts with diverse characteristics.

Furthermore, in catalysis involving transmetallic cycles involving metals such as platinum and palladium, Metal Organic Framework structures can improve catalysis efficiency by boosting selectivity thanks to the functional groups they contain, while maintaining the catalyst in the pores and increasing its stability. Several solids systems generated from MOFs have been explained. The structure of basic MOFs can theoretically be built by selecting the correct metal centers with organic ligands, resulting in nearly limitless variants.

Synthesis of metal organic frameworks

Initially, metal organic framework structures were made by dissolving metal salts and binders in a solvent at a specific pH, pressure and temperature. In addition to this approach, new MOF methods for obtaining more stable and porous structures in less time have been developed.

The notion of dissolving metal salts and organic binders in a solvent at a specific pressure, temperature, and pH provides the basis for a general MOF synthesis technique. Different types of synthesis have also been devised in order to obtain MOFs under various conditions and in various sorts.

The main idea is relied on the realization of complexation between organic ligands and metal ions, however the conditions of synthesis processes differ. In general, metal organic framework structure synthesis processes can be divided into six categories. Solvothermal, microwave, slow evaporation, electrochemical, mechanochemical and sonochemical methods are examples of these.

he synthesis methods and application areas of MOFs

MOF synthesis by solvothermal method

Solvothermal reactions use autogenous pressure that above the solvent’s boiling point and take place in closed containers. Under solvothermal circumstances, many starting materials can experience unusual chemical modifications, which are frequently followed by the creation of nanoscale morphologies that are not feasible by traditional means. Solvothermal reactions have often been carried out with organic solvents that have high boiling point.

Diethyl formamide, acetonitrile, dimethyl formamide, acetone, methanol, ethanol and other organic solvents are often utilized. Solvent mixtures have also been utilized to avoid difficulties with different starting components having variable solubilities. Solvothermal reactions can be performed at a range of temperatures, based on the reaction’s conditions.

Glass bottles are typically used for lower temperature reactions, but Teflon-lined autoclaves are required for temperatures beyond 400 K. When the solvent is water , the technique called hydrothermal method instead of solvothermal method. Lots of materials like inorganic chemicals or inorganic organic hybrid materials have been successfully synthesized using the solvothermal technique.

MOF synthesis by solvothermal method

MOF synthesis by microwave-assisted solvothermal method

The synthesis of MOFs can be done relatively quickly using microwave-assisted synthesis. To make nanoscale metal oxides, microwave-assisted techniques have been employed extensively. Microwaves are used to heat a solution for around an hour to form nanosized crystals in these methods. The microwave-assisted approach produces crystals of the same quality as the traditional solvothermal method. However, the time that required fort he synthesis is reduced.

MOF synthesis by microwave assisted solvothermal method

MOF synthesis by slow evaporatin method

When mof synthesis is performed with approach named slow evaporation method, external energy is not needed. Even if the slow evaporation synthesis method is more prefable because it can realise at room temperature, it has a disadvantage that is it requires more time than other traditional techniques.

In a slow evaporation method, a solution of the beginning components is concentrated by slow evaporation of the solvent at a specific temperature, usually room temperature. A solvent mixture is sometimes used to enhance the reagents’ solubility and accelerate the process by allowing low boiling solvents to evaporate quickly.

MOF synthesis by electrochemical synthesis method

Although massive MOF crystals can be made in mild conditions by altering the pH/solvent at room temperature, the development and understanding of new mild and rapid synthetic methods is ongoing for definite uses, as well as for the purpose of producing huge quantities of MOF samples in a repeatable manner. One such technique is electrochemical synthesis, which does not need salts of metal and allows for the constant formation of MOF crystals. That is a significant benefit in an industrial operation. The core concept involves anodic dissolution of metal ions into synthesis mixtures containing electrolytes and organic linkers.

MOF synthesis by electrochemical synthesis method

MOF Synthesis by mechanochemical synthesis method

Synthesis of Metal Organic Framework using mechanochemical method is a solvent-free method. The use of mechanical force to a chemical reaction is known as mechanochemistry. Modern synthetic chemistry is fascinated by the construction of bonds by easy, cost-effective, and environmentally acceptable mechanochemical processes. Liquid-assisted grinding (LAG), which involves introducing a small amount of solvent to a solid reaction mixture, has recently been employed to successfully synthesize Metal Organic Frameworks.

Friscic and colleagues proved that in a LAG process, one-dimensional, two-dimensional, and three-dimensional coordination polymers may be generated from the same reaction mixture by modifying the added solvent. Given method was then utilized to create a variety of zeolitic imidazolate frameworks.

MOF synthesis by mechanochemical synthesis method

MOF synthesis by sonochemical synthesis method

The use of intense ultrasonic radiation (20 kHz–10 MHz) causes molecules to undergo chemical transformation, which is known as sonochemistry. Ultrasound causes chemical or physical changes in a liquid through a cavitation process that involves the production, development, and quick collapse of bubbles, resulting in local hot spots with high temperature and pressure with a brief lifetime.

Extreme temperatures can speed up chemical reactions by forming an excess of crystallization nuclei right away. When compared to traditional hydrothermal procedures, sonochemical approaches can produce homogeneous nucleation centers and a significant reduction in crystallization time. MOF-5 crystals of 5–25 mm may be grown in 30 minutes using sonochemical irradiation in 1-methyl-2-pyrrolidinone (NMP), which is comparable to MOF-5 grown using solvothermal or microwave techniques.

MOF synthesis by sonochemical synthesis method

Properties of metal organic framework structures

Surface area

The active surface area is the most defining and significant physical feature of Metal Organic Frameworks. The Brunauer-Emmet-Teller (BET) method is used to compute the surface area. The active surface area of nitrogen gas adsorbed to the material is measured using the physical adsorption characteristic of the gas and the monolayer principle of the gas in this approach. The amount of gas adsorbed on the material surface by the adsorption process formed at varying pressures is used to compute the surface area.

The amount of adsorbed material changes in direct proportion to the size of the MOF’s active surface area during the adsorption process. Although surface area is an important component in the adsorption process, other factors that influence the amount of adsorbed material include porosity, pore diameter, and the chemical characteristics of the surface that will be created as a result of changes.

Pore diameter and porosity

The structure and size of the pores are another essential aspect of MOFs. The metal organic framework structure has a wide pore size distribution that ranges from macro to nano. Porous material is defined by the International Union of Pure and Applied Chemistry (IUPAC) as any solid substance with pores, channels, or cavities in its structure. The pore size (r) according to the radii of the pores is checked in four stages, according to IUPAC standards.

1. Macropores (r > 25 nm)
2. Mesopores (1 nm< r < 25 nm)
3. Micropores(0.4 nm< r < 1 nm)
4. Nano/Submicropores (r < 0.4 nm)

The abundance of micro and nanopores enhances the active surface area of the metal-organic framework structure, allowing Metal Organic Framework to adsorb to solvents and gases with great efficiency. Mesopores, which are abundant in the metal organic framework structure, are a key variable in terms of adsorbing big molecules. The adsorption process is unaffected by macropores. The channels that allow gas to be adsorbed to flow into the micro and mesopores are regarded to constitute the macropores’ primary function.

Representative Metal Organic Framework pore structures

Surface features and crystalline structure

Metal Organic Framework structures can be divided into four categories. Rigid framework structures, flexible/dynamic framework structures, surface functional framework framework, and open metal areas are all examples of these.

Rigid Metal Organic Frameworks feature a porous framework structure that is permanent, stable, and non-degradable. During the adsorption and desorption processes, rigid framework structures maintain their shape and pore sizes. During the entry and exit of molecules to be adsorbed into the pores, flexible Metal Organic Frameworks, on the other hand, show shape changes according to external influences such as pressure and temperature.

This shape variation, on the other hand, has no effect on porosity rate, which is advantageous because it does not decrease due to the material’s flexibility at high pressures. This property of flexible framework structures improves the efficiency of gas molecule capture and release. Flexible Metal Organic Frameworks, on the other hand, are more difficult to compare to rigid framework structures when evaluating their efficiency.

The adsorption isotherm of hard MOFs is normal (type I). Flexible MOFs show a ladder (type VI) adsorption isotherm, and hysteretic isotherm occurs during desorption because it changes shapes at different pressures during gas adsorption.

On the other hand, open metal framework structures perform well in the separation of polar/non-polar gas couples. The adsorption capacity of CO2 is increased by the presence of water molecules in the Metal Organic Framework structure. The CO2 adsorption capability of the HKBTU-1 material, which includes 4% water by mass and has an open metal cage structure, was found to increase fourfold in the investigation.

The development of an electrostatic field by the water containing Cu+2 ions caused the adsorption mechanism to occur in this example, and this electrical attraction force boosted CO2 adsorption. The metal type and anionic/cationic characteristics of the material are crucial parameters in this scenario for the material’s adsorption efficiency. During the gas adsorption stage in the material, functionalized framework structures cause strong electrostatic interactions between molecules, as well as the grafting of functional groups with high affinity to the surface of porous materials (arylamine, alkylamine, hydroxyl groups, etc.).

The structure whose surface is changed with a functional group with a basic character is easily attracted and maintained by chemical action by CO2 gas, which has an acidic character, and the CO2 adsorption capacity is high even at low pressures and high temperatures. Although the functionalization process partially clogs the material’s surface pores and reduces pore volume, it results in a better CO2 adsorption efficiency than unfunctionalized materials.

Corrosion resistance, hardness, porosity ratio, and particle size distribution are all other essential physical features of metal-organic framework systems. The adsorption rate improves as particle size decreases, however in column applications, tiny particle size generates a pressure drop, which reduces efficiency.

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