Metal-Organic Frameworks (MOFs) Overview

Up till now, industrial processes relied on conventional microporous inorganic and organic materials like zeolites [1] and carbonaceous materials [2] as catalyst, however, some limitations are present. Hence the need to create a 3D hybrid micro-porous material with enhanced physical, chemical and structural properties using the molecular building block approach [3].

Metal-organic frameworks (MOFs) is a new class of hybrid porous materials characterized by a well-defined crystalline structure made of metal ions (or metal clusters) and organic linker. An unlimited number of MOFs can be achieved by simply combining the metal node with an organic ligand. The latter with its length and functionality will define the void, pore size and the chemical property of the material towards the desired application. As a result, in the past years, the number of publications increase rapidly due to their vast array of applications on different fields and ease of synthesis.

The story of metal-organic frameworks begins in 1965, when, for the first time, an article on porous coordination polymer was published and during the next years more new structures were developed [4]. MOFs are a sub-class of a more vast family called metal-organic materials (MOMs) and are composed by interconnected medal nodes with organic ligands. MOMs class is comprised of hybrid organic-inorganic materials that can produce 0D, 1D, 2D, and 3D structures [5], as shown in Fig. 1.1. This class of compounds have attracted an ever increasing attention due to their tunable metal ions and organic likers, finding application on a vast number of different processes (i.e. catalysis, gas storage/separation, carbon capture) through their rational design. Professor Yaghi and co-workers in 1995 were the first to use the term metal-organic framework in relation to the hydrothermal synthesis of Cu(4,4’-bpy)_1.5·NO₃(H₂O)_1.25 framework [6]. Few years later, Williams and co-workers announced a new copper based MOF with an unprecedented high surface area (1000 m² g^-1) and named it HKUST-1, after the Hong Kong University of Science and Technology [7]. In 1999, Professor Yaghi team unraveled the MOF-5, one of the most famous and best investigated 3D-MOFs [8]. With a surface area higher than 4000 m² g^-1 it has found useful application in hydrogen nitrogen gas adsorption as well as catalyst. In the year 2002, at the Materials of Institute Lavoisier (MIL) a step further was taken by Férey and co-workers with the synthesis of MIL-53(Cr) and MIL-53(Al) [9, 10].

Furthermore, a study was conducted on MIL-101 and shown how, through both targeted chemistry and global optimization algorithm, it has been possible to predict its crystal structure [11]. Unfortunately, the metal-organic frameworks synthesized so far never displayed a remarkable chemical stability. Once again, Professor Yaghi group was up for the challenge and in 2006 presented to the scientific community the zinc-based zeolitic imidazolate frameworks (ZIFs). The ZIF-1, 4, 6, 8, 10 and -11 utilized zinc, ZIF-9 and -12 cobalt, while ZIF-5 as metal nodes had a mixture of zinc and indium [12]. Among these newly discovered frameworks, ZIF-8 quickly gained popularity due to an exceptional stability in boiling methanol, water and benzene up to 7 days and resisted in strong alkaline conditions for 24h, spawning its application in hundreds of different reactions. In 2008, Lillerud and co-workers from the University of Oslo synthesized a new zirconium-based metal-organic framework denoted as UiO-66 [13]. The Zr₆O₄(OH)₄ cluster serves as secondary building unit (SBUs) for a whole class of frameworks with a widespread range of topologies, where with the simple tuning of the organic ligand specific the desired structure can be obtained. Another noteworthy MOF have been synthesized by Schröder team at the Nottingham University in 2009 [14]. The NOTT series (100-109) utilizes copper as metal node and, through a variety of tetracarboxylate ligands, was possible to finely tune the pore cavity such that the local structure could enhance the framework H₂ adsorption capability. A year later, Rosi and collaborators developed the Bio-MOF-11 [15]. Due to the abundant presence of Lewis basic amino and pyrimidine groups on the ligand, the framework shown impressive selectivity for CO₂ over N₂ and high CO₂ capacity, outperforming other amine-functionalized MOFs-ZIFs. In 2011, Stocks and co-workers from Christian Albrechts University solvothermaly synthesized the CAU-5 [16]. This noteworthy porous air-stable framework possess a photosensitive azo-group that, upon UV irradiation, can change from trans to cis isomer.

The MOFs investigated thus far were mostly constituted by transition metal-based materials, leaving the main metal group largely unexplored. Only in recent time the synthesis of lithium and magnesium-based frameworks has been attempted, mainly because of the improvement that they can have on gas adsorption coordination polymers. In 2008, Caskey and Matzger achieved encouraging results on this field with the newly created Mg-MOF-74 [17, 18]. However, the synthesis process of these MOFs is in need of further study and optimization.

In the past fifth teen years, more than 20 000 MOFs structures have been rationally design and studied, significantly contributing to rapidly expand this new class of hybrid organic-inorganic materials [19]. The staggering interest of the scientific community on these materials is well-placed, outstanding physico-chemical properties and countless possible applications are vastly expanding new horizons for nanoscience.

Metal-organic frameworks (MOFs) are part of a vaster field that combines inorganic with coordination chemistry. MOFs can be classified ad porous coordination polymers, characterized by a crystalline structure composed of metal ions or metal clusters, while the organic linkers are usually carboxylate, imidazole or heterocyclic compounds. Usually, the MOFs structures and functions are dictated by the rationalization of organic ligands with precise lengths, geometries and functional groups [20]. Similarly, the metal nodes can also bring some additional properties. For instance, MOFs chemical stability is given by the metal-ligand bonds, which are strongly influenced through the inorganic nodes. Most of the metal cations (e.g. monovalent, divalent, trivalent and tetravalent) in the Periodic Table have been employed for the synthesis of MOFs (Fig. 1.2), or metal clusters constituted by the metal center and anions (e.g. Cl^–, F^–, O^2- or OH^–), thus obtaining the polyhedral configuration. while the one not reported are in yellow.

The MOFs self-assembly process take place when the lone pair of electrons present in the ligand’s Lewis basic site migrates to the vacant orbital of the metal ion (Lewis acid) coordinating, leading to a 1D, 2D or 3D framework [21]. As aforementioned, a well-engineered metal-organic framework is composed of a ligand, which can possess multiple binding sites, and a metal node that can assume more than one structure. Through the cautious combination of these two key elements is it possible to control the whole structure and properties of the framework. In general, the organic linkers can be divided in two major classes: N-donors (pyridil or imidazolate-based) and O-donors (carboxylate-based) [22]. In Fig. 1.3 are reported a few examples of used linkers, yet the use of long chained ligands is discouraged since they can severely compromise the structural integrity of the MOF. In order strengthen then overall MOFs structure, the use of ligands with carboxylate moieties is best.

Due to their chelating agent properties can have positive effects on the framework, enhancing its stability upon solvent and guest molecules removal or when in contact with moisture present in the air. The negative charge present in the linker counterbalance some or all the positive charges of the metal clusters in the structure, thus reducing the amount of host/guest molecules present within the framework and leading to a higher available internal surface area [23].  Additionally, O-donors carboxylate-based ligand are proven to be more versatile than N-donors because they can assume three most probable binding modes, monodentate, chelate and bridging.

To better understand what has been discussed so far, we can briefly analyze how the MOF-5 is organized (Fig. 1.4). Within the framework is it possible to identify the ZnO₄ tetrahedral structure which will extend and bond with multiple benezene-dicarboxylate linkers, mainly bi, tri or tetra-dentate anions. Then, upon the creation of a discrete structure with formula Zn₄O(CO₂)₆, six metal-ligand-metal connections are extending, thus creating specific arrays of 2D or 3D geometry. The overall topology of the newly synthesized MOF is dictated by the kinetic stability, coordination and geometry preferences of the metal ions, and additionally by flexibility and chirality of the organic linker. MOFs intrinsic structure has a higher level of structural complexity when compared to common inorganic and organic materials. Their complexity can be compared with the one that proteins possess (Fig.1.5), categorized in four level:

1. Primary, metal and organic ligand bonds together. The metal ions are enclosed by heteroatoms leading to the formation of a cluster known as secondary building units (SBUs) [24].
2. Secondary, the SBUs start to assemble into a 3D net-like structure (e.g. rht, sod, soc).
3. Tertiary, the net interact with each other forming supermolecular building blocks.
4. Quaternary, the structure is composed by multiple supermolecular units, creating crystals, nanoparticles or thin films.

In general, metal-organic frameworks have been synthesized by solvothermal methods with the aid of electrical heating [25]. A key factor in the MOFs synthesis is the temperature at which is conducted, providing the necessary activation energy for the reaction to occur. On this regard, is it possible to divide the temperature range in solvo- and non-solvothermal, whereas the nucleation take place at a temperature higher or below the boiling point of the solvent, respectively. The equipment used varies with the temperature, in a solvothermal synthesis a Teflon linen autoclave is employed allowing for the generation of autogenous pressure, therefore increasing the solubility of the reagents, while in a non-solvothermal synthesis an open flask can be used. Moreover, the solvothermal method leads to the creation of the most thermodynamically stable product, however, in the past few years, the energy-saving room temperature synthesis is gaining traction. In the solvothermal approach, a further distinction can be made based on the solvent employed during the reaction. If the solvent used is N,N-dimethylformamide (DMF) or N,N-diethylformamide (DEF) the process is defined as solvothermal, while becomes hydrothermal if deionized water is used. However, this synthetic method has some disadvantages, such as long reaction time (between 12 and 72 hours) and the solvents used are not suitable for a green-chemistry attitude. As a consequence, a variety of different methods have been developed in order to cope with the drawbacks highlighted so far and to advance the evermore important green-chemistry philosophy (Fig. 1.6).               The microwave-assisted synthesis has been used to provide energy for the MOFs crystal growth, where the electromagnetic wave irradiates and interacts with the electric charge present in the solvent (polar) and the ions in the solution. As a results, the reaction time is significantly reduced to few minutes rather than hours, a considerable amount of energy is saved, better control on the phase of the material, narrower particle size distribution and improved control of the morphology. In 2005, this technique was successfully employed for the first time on the Cr-MILL-100 synthesis [11]. The MOF was obtained in 4 hours at 220 ˚C, while under conventional hydrothermal synthesis required 4 days reaction time. The sonochemical method utilizes a funnel-shaped type of reactor equipped with a sonicator device. The ultrasonic waves lead to the formation and subsequent collapse of the in-situ generated bubbles, creating the peculiar acoustic cavitation, capable of reaching localized temperatures of 5000 ˚C and 1000 bar in pressure. In 2008, MOF-5 was synthesized sonochemically in a 30 minutes time span, obtaining a narrow crystal size distribution of 5-25 m [26]. The framework exhibited comparable properties with the conventionally synthesized sample. In the electrochemical method the origin of the metal ions is not provided by the solubilization of a desired metal salt, but rather from the continuous and gradual anodic dissolution of a metal electrode, which will react with a previous solubilized ligand. Furthermore, this technique allows a continuous production, yielding a higher amount of product compared to traditional batch methods. The first electrochemically synthesized MOFs was reported in 2005 by Mueller et al. [27]. On this instance, at a given voltage of 12-19 V and currency of 1.3 A the copper plates provided the metal necessary for the framework, while trimesic acid acted as organic ligand, leading to the formation of HKUST-1. In 2006, the same method has been applied to the synthesis of ZIFs [12]. The mechanochemical approach is a fairly straightforward process. Through the application of an external mechanical force, the metal source and linker breakage of intramolecular bonds is followed by their chemical transformation into the product. This solvent-free process has been used for long time in synthetic chemistry [28] and in 2006 was employed for the synthesis of [Cu(INA)₂] (INA = isonicotinic acid) [29] .The spray dry technique has been extensively used to dry suspensions, encapsulate, micronize and crystallize active compounds. Its main application is in the food, drug and construction sectors [30, 31] and expands to the creation of other materials, including organic crystals, silica and carbon nanotubes [32-35]. The main advantages are the low cost, fast and scalable methodology for the synthesis of a variety of materials. For the first time, in 2013, Sánchez et al. have synthesized fourteen different nano-MOFs in a cost-effective way, opening new possibilities for MOFs industrial production [36]. As mentioned before, MOFs synthesis usually requires the use of an organic solvent or deionized water, however, a new type of solvents with unique properties have attracted much interest, ionic liquids (ILs) [37]. Thanks to their particular set of properties, such as zero vapor pressure, exceptional solvatating capabilities, unreactive to moisture, thermally stable, recyclable and low price are suitable candidates for MOFs synthesis. In 2011, ILs in combination with a sonochemical device were utilized for the synthesis of HKUST-1 and its properties compared to conventional methods [38].

Metal-organic frameworks are crystalline materials with a highly ordered structure, as a result they can display a surface area far greater than commercially used zeolites. Such remarkable feature makes them ideal as gas-adsorbant storage-materials, such as carbon dioxide, hydrogen and methane [39-41]. Additionally, due to their tunable pore size, ligand functionality and structural integrity make them incredible versatile materials, finding applications as heterogeneous catalyst, gas and liquid phase separation, photocatalyst, drug delivery, sensing and more [42-47] (Fig.1.7). MOFs main application reported in literature is gas storage, given their remarkable surface areas. It has been found that the catalytic activity is associated to a high surface area and increased capabilities of substrate adsorption. In order to evaluate the permanent porosity after the removal of adsorbed guest molecules, adsorption-desorption techniques at low temperature are crucial. Usually, MOFs exhibits a Type-I isotherm with little or no hysteresis loop, indicating that the micro-porous structure of the material remained unaltered after the reversible physiosorption of the guest molecules [48]. As an example we can take the well-known MOF-5, the very first material able to maintain a permanent porosity. Its estimated Langmuir surface area reached 2900 m² g^-1, far surpassing the 500 m² g^-1 of crystalline zeolites. The advantage of this particular feature, combined with a rational chosen metal/ligand duo, can find practical application in the industry. For example, a container loaded with the designated MOFs facilitates the adsorption of a gas and its storage at a significant lower pressure than an equal container without the porous material. As a consequence, the employment of expensive high pressure tanks and multi-stage compressors is not necessary, fashioning a safer and economical gas storage option. Furthermore, MOFs can be successfully used as molecular sieves thanks to their focused-exclusion ability towards gas molecules [49]. Usually, the organic linker itself has a small influence on the gas uptake capability of the framework, while a higher surface area greatly enhances the gas sorption. This can be achieved by increasing the space that the adsorbed molecules require per weight of material. The simplest way is by modify the length of the ligand, a longer ligand can provide more space for the guest molecule. However, the trick can become counterproductive leading to cage interpenetration and only with careful design the MOF will display a single topology. The use of MOFs as heterogeneous catalyst is a well-studied branch, finding application in a vast number of reactions reaching the thousands [50-52]. When a MOF is employed as catalyst there are three structural features that are responsible for the exhibited catalytic activity, such as the metal nodes, the organic linker and the pore system. Usually, the MOF is able to perform its function utilizing one or a combination of these functionalities, even though the pore system does not participate directly in the catalytic cycle but rather acts as a molecular sieve for the substrates. Additionally, the void within the pore cage can be used to effectively encapsulating noble metals nanoparticles, or unsaturated metal nodes in combination with a specific moiety present in the organic ligand can initiate the catalytic process [53]. When the MOF is synthesized, solvent molecules coordinates with the unsaturated metal sites and upon activation by removing said molecules, the metal node is left uncoordinated and prone to react with the substrate molecule [54, 55]. The functionalization of the ligand is a more complex situation. In order to be able to express its catalytic activity, the organic linker need to possess an active moiety towards the substrate molecule. During the MOF synthesis, the ligand must have two different moieties, one for the construction of the framework and the other responsible for the catalytic properties. However, the two groups enter in competition with each other to react with the metal node leading to two possible scenarios: (i) the formation of the frameworks is compromised due to random attachment of the ligand with the metal, or (ii) the metal node interact with both the moieties and being deprived of its unsaturation, thus losing the catalytic activity [56]. To overcome these disadvantages, researchers turned to the so called post-synthetic modification technique [57, 58]. With the aid of this powerful method is it possible to introduce targeted functionalities which can have specifically chosen catalytic applications. For example, Wang et al. [59] successfully post-modified the amino groups to alkyl anhydrades or isocyanates present in the IRMOF-3 [Zn₄O(ata)₃ ; ata = 2-aminotherephtalate]. Zeolitic imidazolate frameworks (ZIFs) are a sub-class of the vast MOFs family. Reported for the first time by Yaghi and his team, the structure is composed of a metal (Zn²⁺ or Co²⁺) tetrahedrally coordinated with an imidazolate organic linker [12, 60] (Fig.1.8). As the name infers, these peculiar frameworks display a similar structure to zeolites due to their highly ordered structure and porosity. Usually, in the zeolite system, the silicon and aluminum atoms are tetrahedrally arranged as SiO₄^4- and AlO₄^4- with the oxygen at the vertices [61]. In 1756, the first mineral zeolite was discovered by Cronsted [62], a well-ordered material characterized by a robust crystalline structure. During the industrial revolution they quickly found application in the petrochemical, detergent, agricultural and construction sector [63-66].  However, their use was limited by the lack of organic moieties within the porous frameworks, only with the advent of ZIFs it was possible to tackle these limitations. The tetrahedral metal nodes (M) in combination with the imidazolate (Im) ligands form an M-Im-M bond that resembles to the Si-O-Si angle (145˚) present in the zeolites (Fig. 1.9), which lead to the rational synthesis of a vast variety of structures with different functionalities and topologies. The key advantage of these novel frameworks resides in the high tunability of the imidazolate linker, which upon reaction with the metal ions allows to formation of very stable structures [67]. Analyzing the ZIFs structure is it possible to notice that the inorganic part is composed by Zn, Co or their combination, and the organic part is constituted by the imidazolate ligand, as reported in Table 1.

Table 1. List of repoted ZIFs with their relative composition and topology.

Framework	Topology	Metal ion	Organic linker
ZIF-3	DFT	Zn	imidazole
ZIF-6	GIS	Zn	imidazole
ZIF-8	SOD	Zn	2-methylimidazole
ZIF-11	RHO	Zn	Benzilimidazole
ZIF-12	RHO	Co	Benzilimidazole
ZIF-14	ANA	Zn	Ethylimidazole
ZIF-20	LTA	Zn	Purine
ZIF-21	LTA	Co	Purine
ZIF-60	MER	Zn	imidazole/1-methylimidazole
ZIF-67	SOD	Co	2-methylimidazole
ZIF-68	GME	Zn	benzilimidazole/2-nitroimidazole
ZIF-69	GME	Zn	2-nitroimidazole/5-chloroimidazole
ZIF-70	GME	Zn	imidazole
ZIF-71	RHO	Zn	4,5-dichloroimidazole
ZIF-74	GIS	Zn	nitroimidazole/methylbenzimidazole
ZIF-75	GIS	Co	nitroimidazole/methylbenzimidazole
ZIF-76	LTA	Zn	imidazole/5-chlorobenzimidazole
ZIF-78	GME	Zn	2-nitroimidazole/6-nitrobenzimidazole
ZIF-80	GME	Zn	4,5-dichlorobenzimidazole/2-nitroimidazole
ZIF-82	GME	Zn	chloronitroimidazole/nitroimidazole
ZIF-90	SOD	Zn	imidazolate-2-carboxylaldehyde
ZIF-95	POZ	Zn	5-chlorobenzimidazole
ZIF-100	MOZ	Zn	5-chlorobenzimidazole