Synthesis and Applications of Metal Organic Frameworks

Metal Organic Frameworks (MOFs) are the important class of porous materials that comprise of organic-inorganic building blocks, in which organic components are called linkers and inorganic parts are called nodes or Secondary Building Units (SBUs). Linkers are usually bi- or multi-dentate organic ligands and SBUs are inorganic metal ions or metal-oxo clusters [1]. MOFs are also known as three-dimensional porous coordination polymers (PCPs). MOFs have attained considerable research interest due to their tunable pore size, enhanced surface area, low framework density, high thermal and chemical stability and diverse functionality, which enable them to stand with higher impacts over other porous materials [2].

From last few years, MOFs have been considered as the structurally sound organic-inorganic hybrid materials due to their vast structural diversity and ease of synthesis. Therefore, researchers are frequently developing different types of MOFs and till date more than 20,000 MOFs have been rationally designed with advanced topology, which are sincerely taking part in various fields of applications such as catalysis, sensing, adsorption and separation, gas storage etc [2].

MOFs consist of organic linkers and inorganic metal/metal-oxo clusters (SBUs). Self-assembly of linkers and SBUs gives rise to three dimensional arrays, in which linkers and SBUs are joined together by covalent bonds. The pore dimension and surface area of MOFs can be frequently tailored by altering the length of organic linkers or coordination geometry of SBUs. Thus, designing of great variety of MOFs can be possible with different void structures and diverse structural geometries. The common rigid organic linkers include di-, tri-, tetra-topic carboxylate groups, neutral nitrogen heterocycles etc. Terephthalate (benzene-1,4-dicarboxylate), one of the most common di-topic organic linkers is linear in shape. It has fascination to interact with paddle-wheel, octahedral or trigonal-prismatic metal/meta-oxo clusters. Similarly, trigonal planer tri-topic organic linker, for example trimesate (benzene-1,3,5-tricarboxylate) or tetrahederal/tetra-topic organic linker, for example methanetetra (4-benzoate) are also able to form multi-dimensional MOFs structures with paddle-wheel, octahedral, trigonal-prismatic, cubical, square planar, hexagonal bipyramidal SBUs [3]; (Figure 1).

Figure 1:Represents the structures of a few commonly used rigid organic linkers in MOFs synthesis

The variation of functional groups in linkers or change in linkers’ length lead to the formation of MOFs with tunable structure, geometry, pore dimension and surface area. For example, terephthalate, the well-known di-topic organic linker, when connected to Zn₄O clusters, leads to the formation of IRMOF-1 (MOF-5). But, the replacement of one or two hydrogen atoms with different functional groups such as -Br, -NH2, -OC₃H₇, -OC₅H₁₁, -C₂H₄ (cyclobutyl) and -C₄H₄ (fused benzene) in phenyl ring of terephthalate results in the formation of IRMOF-2, IRMOF-3, IRMOF-4, IRMOF-5, IRMOF-6, IRMOF-7 respectively. Notably, the variation of functional groups in linkers leads to the disparity of surface area and pore size of IRMOFs. Additionally, increase in linker’s length leads to increase in pore dimension [4], as represented in Figure 2.

Figure 2:Crystal structures of various MOFs synthesized using different carboxylate ligands (the yellow spheres represent the voids).

It is worth mentioning that fabrication of MOFs not only depends on linkers but the structure of secondary building units strictly regulate the specific design of MOFs. In the context of permanent porosity, O. M. Yaghi and his co-workers first developed three dimensional, permanently porous, rigid MOF structure namely MOF-5, which contains octahedral Zn-based metal clusters Zn₄O and BDC ligands [5]. Thereafter, a number of porous, rigid MOFs structures were developed with various SBUs. For example, MOF-199 (HKUST-1) can be developed by using tetrahedral Cu(II) metal sites [Cu₂(-COO)₄] and trimesic acid. Cu(BDC) MOFs are fabricated by connecting the Cu(II) dimers with BDC linkers in bidentate bridging fashion [6]. MIL-n series (where, MIL=Matériaux de I′Institut Lavoisier) were first discovered by Ferey and coworkers in 2002. MIL-53-M (M = trivalent metal ions such as Al3+ / Fe³⁺ / Cr³⁺ etc.) were fabricated by corner sharing the octahedral MO₄(OH)₂ clusters with terephthalate linkers [7]. MIL-88B (B implies BDC ligands) and MIL-101 can be synthesized using trigonal prism [M₃O(OOCR)₆L₃]n⁺ (L implies terminal ligand viz. H₂O or Cl−) SBUs, coordinated with six terephthalate ligands [8]. Zeolite type MOFs such as ZIFs can be built up by connecting the tetrahedrally coordinated MN4 (where, M=Zn/Co) clusters with ditopic imidazolate linkers [9]. Geometries of a few secondary building units and the corresponding crystal structures of MOFs are shown in Figure 3.

Figure 3:A few representative SBUs and corresponding crystal structures of different MOFs.

Synthesis of metal organic frameworks

Metal organic frameworks are synthesized by various methods as shown in Figure 4. The most common methods are conventional method, microwave assisted synthesis, mechanochemical synthesis, sonochemical synthesis and electrochemical synthesis.

Figure 4:Various methods of MOFs synthesis.

Conventional method of MOFs synthesis: MOF synthesis in conventional method usually refers to the method that involves electric heating during synthesis. Conventional method includes solvothermal and non-solvothermal processes. In solvothermal process, the reaction is carried out in sealed container and the temperature of the reaction mixture is raised above the boiling point of the solvent. In this process, high pressure is generated in the closed container that predominantly regulates the reaction. In the closed reaction chamber, the linkers interact with metal ions, subsequently nucleation occurs followed by growth that lead to the formation of highly crystalline MOFs structures. There are numerous metal organic frameworks synthesized using this approach. Highly crystalline Zn-based MOFs were synthesized via solvothermal method [10]. Two dimensional fluorinated metal organic frameworks such as F-MOF-4, Cu-F-MOF-4B, Zn-F-MOF- 4B were synthesized solvothermally [11]. Highly crystalline MIL- 101(Cr) was synthesized via hydrothermal approach [12].

Whereas, in non-solvothermal process, the reaction is carried out at room temperature or the temperature of the reaction can be fixed at boiling point of the solvent. Room temperature nonsolvothermal approach was adopted for the synthesis of various MOFs such as Cu-BTC MOF, UiO-66, Zn-based metal organic frameworks etc [13,14]. In non-solvothermal approach, the rate of nucleation and growth of metal organic frameworks can be tuned by changing the reaction temperature or evaporation of the solvent at slightly higher temperature.

Microwave assisted MOFs synthesis: In microwave (MW) assisted MOF synthesis, electromagnetic waves interact with solid or liquid materials, which results in high molecular orientation in solvent or reactant materials. These lead to the elevation of temperature in the reaction medium. In MW assisted synthesis, the high and homogeneous temperature can be maintained throughout the reaction medium. The solvents used in MW assisted synthesis can be selective, so that the electromagnetic waves can interact strongly with the reaction medium. Cr-based MIL-100 metal organic framework was first synthesized by this approach [15]. Later on, IRMOF-1, 2, 3 were synthesized using this process [16].

Mechanochemical synthesis of MOFs: In mechanochemical synthesis, the mechanical force breaks the inter-molecular bonds and simultaneously chemical transformation occurs in the reactant molecules to yield the product. The entire process occurs in solventfree condition, therefore, the process is very much eco-friendly. The first MOF synthesized in this process was three dimensional Cubased MOF [Cu(INA)2], where isonicotinic acid was used as linker. After the first report, HKUST-1, MOF-14, [Zn(EIm)2] MOFs were synthesized by mechanochemical process [17-19].

Sonochemical synthesis of MOFs: In sonochemical process, high energy ultrasonic vibrations are applied to the reaction mixture. This energy-efficient process is used to synthesize a number of metal organic frameworks. The first MOF, synthesized using this approach was [Zn3(BTC)2. 12H2O] [20]. Sonochemical approach was also employed to synthesize MOF-5, HKUST-1, ZIF-8 etc [21-23].

Electrochemical synthesis of MOFs: In electrochemical synthesis, metal ions are allowed to pass through the anodic dissolution to the reaction medium, where they interact with dissolved linkers to form metal organic frameworks. The first electrochemically synthesized MOF was Cu-BTC MOF [24]. Thereafter, Ni-BTC, HKUST-1, ZIF-8, MIL-100(Al), MIL-53(Al), and NH2-MIL-53(Al), MOF-5 were also framed with electrochemical approach [25-27]..

Applications of metal organic frameworks

Metal organic frameworks have attained remarkable research interest due to their wide-spread applications in the field of catalysis, sensing, adsorption and separation, biological applications etc., as shown in Figure 5.

Figure 5:Various applications of metal organic frameworks.

Gas storage, adsorption and separation: MOFs are crystalline solids having permanent porosity and large surface area, for which they are capable of storing various gas molecules, solids and liquids within their structures. MOF with divalent metal ions such as HKUST-1 exhibits excellent porosity, therefore, they can uptake methane (CH₄) gas molecules [28]. PCN-250(Fe2M) MOFs, where, M=Fe, Co, Ni, Mn and Zn are used in CH4 storage applications [29]. The flexible MOFs are advantageous over rigid MOFs in gas storage performance. For example, MIL-53(Al) can uptake huge amount of CH₄ gas at room temperature [30]. MOF-177 and was reported for CO₂ storage application [31]. MOFs are also proficient to adsorb toxic chemicals from water. For example, UiO-66, PCN-222 were used as toxic dye adsorbents [32,33]

Catalysis: MOFs act as very good catalyst in many heterogeneous chemical reactions. The catalytic activity of MOFs is directly correlated to metal centers, as the coordinatively unsaturated secondary building units act as Lewis acids for many chemical reactions. For example, MIL-100(Fe) was used as Lewis acid catalyst in Friedel-Crafts reactions [34], regioselective ring-opening reactions of epoxides [35], Claisen-Schmidt condensation reactions [36], Knoevenagel condensation reaction [37], cyanosilylation reaction [38], etc. UiO-66 can be used in aldol condensation reaction [39]. Besides these, MOFs can act as promising light harvesting materials in photocatalysis. For example, PCN-22 was used as photo catalyst in light driven alcohol oxidation reaction [40]. MIL- 101(Fe) also acts as excellent photocatalyst in photocatalytic water oxidation reaction [41].

Sensing: MOFs are used as excellent chemical sensors. For instance, water soluble Cd-based MOF [Cd₂(TIB)₂(BDA)₂] was used as chemical sensor for the detection of ketones in aqueous medium [42]. Fluorescent metal organic framework, MIL-53(Al) was used for the detection of Fe3+ ions in aqueous solution [43]. Antibiotics and explosives in water could be detected by Zr-based MOFs such as Zr₆O₄(OH)₈(H₂O)₄(CTTA)_8/3 and Zr₆O₄(OH)₈(H₂O)₄(CTTA)_8/3 [44]. Toxic heavy metals in water can be detected by UiO-66 [45]. Fumarate based RE-fcu-MOF thin film has the ability for selective detection of H2S gas [46].

Biomedical applications: In the field of biomedicine, MOFs play a vital role owing to their well-defined structures, tunable pore size and large specific surface area. MOFs with non-toxic metal-sites were used as host matrices to incorporate biologically active compounds such as drugs, enzymes etc [28]. The pioneering work on MOF based drug delivery was carried out by Férey et al. [47] in 2005. Based on Férey’s work, lots of research works are being continued to further explore the MOF’s proficiency in the field of targeted drug delivery systems. For example, flexible metal organic frameworks MIL-53(Cr, Fe) were used for in vitro release of ibuprofen [48]. ZIF-8 was also used as host matrix for the release of anticancer drug doxorubicin [49]. Non-conventional anti-cancer drug such as [Ru(p-cymene)Cl₂(pta)] (RAPTA-C) was successfully released into SBF solution by Ni-based MOF (CPO-27-Ni) [50].

This review endeavored to the concepts, various synthetic strategies and applications of MOFs. The MOFs can be prepared by Conventional methods, microwave assisted, sonochemical, mechanochemical and electrochemical approaches. The structural analysis established the MOF crystal structure. These MOFs have been successfully used for various applications such as catalysis, adsorption, gas storage, sensing, and biomedical applications.

There are no conflicts to declare.

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