Significance of Metal-Organic Frameworks Consisting of Porous Materials

Introduction

Materials of porous nature are abundantly available in nature in a variety of forms. A few porous materials are mentioned in Figure 1.1. Metal-organic frameworks (MOFs) are a new class of hybrid porous solids, w'hich are potentially a type of prominent porous adsorbent; and they can also exist in an empty guest-free state [1].

FIGURE 1.1 Materials with a porous nature [4].

MOFs are defined by Yaghi et al. as porous structures made with coordinative bonding between metal ions and organic linkers [2]. MOFs have grown to become the leading domain of solid-state chemistry [3,4]. This special case of crystalline materials presents a high degree of functional and structural tenability [5,6] which is not possible with other traditional porous materials like zeolites and activated carbons [3].

Even though the general porous materials have many valuable attributes, [7] techniques for controlling the individual crystal locations, and coatings with particularly designed pore sizes, their arrangement/distribution is not yet optimized [8]. Among all kinds of porous materials, MOFs are a special kind of ultra-porous material with an extraordinary accessible surface area because of the framework generated by the inorganic nodes and organic compounds [2,9]. These surface areas range between 1000 and 10,000 m/g, which exceed the values of other porous materials such as carbons, zeolites, and mesoporous-based oxides [10]. A few artificially made commonly used products with a porous nature are illustrated in Figure 1.2.

It is significant to note that MOFs are called by many names, such as porous coordination networks, porous coordination polymers, etc. The fast rate of growth in the synthesis, characterization, and analysis of MOFs could be noted in recent years. These kinds of materials are produced in such a way that they have permanent porosity [11]. The flexibility with MOFs is that their secondary building units (SBUs) and organic linkers can be varied, which has led to the formation of thousands of MOF compounds. Specifically, they have been extensively used in the energy domain, including fuel cell technology, super capacitors, and catalytic converters [12,13]. In order to utilize the positive features of both inorganic and organic porous compounds, porous hybrids (MOFs) are being generated which are stable, ordered, and have high surface areas.

FIGURE 1.2 Artificially developed common porous materials [7].

1.1.1 Definition of Porosity

Porosity of any solid material can be realized with the presence of cavities, void space, and/or inter-channels. Materials consisting of a regular organic-inorganic hybrid framework and acting as a regular porous structure with pores of the size range 0.2 x 10⁹ to 100 x 10⁹ m are called nanoporous materials [13].

Inferences Obtained from the Wide Range of Relevant Research Articles

Various published research articles related to porosity for MOF materials have been referred to and the important elements are presented in this section.

1.2.1 Introduction to Porous MOFs

At present, MOF chemistry has grown well enough to the point where the chemical composition, structure of the compounds, specific functionality, and the nature of porosity of a metal-organic structure can be made for the desired application. This exclusive control over the assembly of compounds propels this area further into a new domain area for synthetic chemistry, in which further, more sophisticated materials may be approached. For example, materials can be visualized which have: ^{[1] [2] [3] [4]}

In recent years, researchers have carried out extensive works on crystalline extended structures [15,16]. Even though these structures are extended crystal structures and do not have large detached molecules like polymers, they are dubbed coordination “polymers” -MOFs [17],because these structures are constructed from long organic linkers which are surrounded by void space. MOFs are known to have the potential to be permanently porous like in the case of zeolites. The porosity of MOFs was investigated in the 1990s by forcibly sending gas molecules into the narrow openings at high pressure [18].

1.2.2 Zeolites—An Amorphous and Inorganic Porous Material

Zeolites are an ideal type of structure which belong to the group of purely inorganic materials, and which are a benchmark in the field of solid-state porous materials. Zeolites are readily rehydrated and dehydrated which makes them useful in various commercial areas [7]. Porous materials include a wide range of applications in industry, such as catalysis and absorption. Zeolites are the most perfect examples among the group of crystalline alumino silicate materials with interlinked pores of size 4 to 13 A [19,20]. In comparison with zeolites, activated carbons have high degrees of porosity and specific surface area. Activated carbon also belongs to amorphous porous materials, which rule a major area of the market of solid-state porous materials [21].

Inorganic porous frameworks exhibit a highly ordered structure (e.g. zeolites). Synthesis processes often require an organic or inorganic template with strong interactions between the template formed during the process and inorganic framework. As the outcome, elimination of the template can result in the collapse of the framework. Inorganic frameworks are also influenced by many factors such as lack of diversity. On the other hand, inorganic frameworks are being used in applications like catalysis and separation of gases [3].

1.2.3 Activated Carbon—An Organic Porous Material

Porous materials are utilized widely in gas storage, adsorption-based gas and vapor separation, selective catalysis, storage and delivery of drugs, and as templates in the synthesis of low geometric materials [22]. Conventionally, porous materials are of either inorganic or organic type. Possibly the most general organic type of porous material is activated carbon which is normally produced by decomposing carbon- rich materials at high temperature. Activated carbon has a high surface area and a good degree of adsorption capability, but it does not have an ordered structure. Though activated carbon has a lack of order, porous carbon materials are being used in many application areas including the separating and storing of gases, purifying water, and removing and recovering solvents [23].

1.2.4 Formation of Pores in MOFs

Pores are known to be the voids present within the porous materials while removing the guest molecules [24]. Even though MOFs are constructed by combining inorganic and organic compounds to have a large number of pores, frameworks will often merge with one another to improve the packing efficiency [25]. In such cases, the sizes of the pores are considerably reduced, but this would also be useful for a few applications. In fact, merged frameworks are being deliberately produced and this phenomenon has been found to be useful in improving the performance. Example: in the storage of H₂ [26]. Assessment of porous materials is currently focused on the adsorption of pure methane. Even though methane is the major constituent (95%), commercial natural gas also contains other impurities,including 3.2% of ethane, 0.2% of propane, and 0.5% of carbon dioxide [27]. Porous carbon materials, especially carbon nanotubes and activated carbons, are the most focused kinds of porous materials for storing methane [28].

1.2.5 Types of Pores

The adhesion of the guest molecules and the surface of the adsorbent, as well as the relationship between the adsorbent’s surface and guest molecules, plays needful roles in predicting the characteristics of the porous structures, which are strongly ruled by the shape and size of the pores. In the physical system, pores are categorized based on their sizes as listed and shown in Figure 1.3.

Liu et al. stated that microporous systems can be used in upgrading the energy density. Also, they have reported that mesoporous systems are used to improve the power density [29]. It is possible to have a flexible structure just by altering the inorganic or organic linkers [30].

Microporous materials are used in valuable applications like redox catalysts, in the petroleum industry, and in the synthesis of chemical items for different kinds of shape-selective transformation and detachment processes. They create the fundamentals of new environment-friendly technologies which involve cheaper and more efficient conditions for performing chemical reactions. Transition metals modified

FIGURE 1.3 Types of pores based on pore size [29].

microporous molecular filters with alumino silicate and aluminophosphate frameworks accelerate a wide range of artificially effective oxidizing transformations with impurity-free oxidants like hydrogen peroxide under comparably light conditions, providing the advantage of recovering and recycling complex structures. MOFs are utilized in a significant number of applications in waste treatment activities including detachment of heavy metals and radio active species, ammonia, various kinds of phosphates, and harmful gases from soil, water, and air because they have unique structural and physicochemical characteristics. In earlier times, silica-aluminum- based zeolite microporous materials were mainly used. In recent times, several types of microporous materials are being produced with the aid of metal oxides, metal phosphates, and inorganic-organic hydride materials [31].

1.2.6 Characterization of Porous MOFs

As clearly explained, pore engineering is a powerful path to direct the structure and functionality of pores, which drastically promotes the development of MOFs for recognizing differential molecules. It is possible to engineer the pores of MOFs by tuning their sizes or channels, functional sites, and surface areas to achieve unique MOF materials to be utilized for specific gas separations [32].

This special characteristic makes MOFs unique regarding their structural properties, and which offers the desirable potential to use in different sectors. The comparably easiest production method of MOFs is one of the main reasons that they are a better choice for different applications. Their chemical properties, customized pore structure, and thermal stability make them preferable for sound application domains such as separation of specific gases [33], catalysis [34], and conduction of protons [35]. MOFs are usually characterized with the help of the X-ray diffraction (XRD) method, surface area analysis, electron microscopy (EM), thermogravimetric analysis for predicting thermal stability of various constituents, and Fourier transform infrared (FTIR) technique to characterize the molecules and atomic structures. Since MOFs hold the nature of both crystalline materials as well as highly porous materials, powder X-ray diffraction (PXRD) is usually used to characterize the adsorption measurements, phase purity, and crystalline nature to check for the porosity [36].

1.2.7 Checking for Permanent Porosity

In order to ensure permanent porosity, it needs determination of reversible gas sorption isotherms at low temperatures and low-pressure conditions. Nevertheless, as we stated at that time [37], it has become commonplace to refer to the materials as “open framework” and “porous” even though such validation was missing. The ultimate proof for permanent porosity of MOFs can be obtained by estimating the carbon dioxide and nitrogen isotherms on layered zinc terephthalate MOF [37]. The overall order of porosity in a microporous molecular sieve depends on the template molecule, composition of inorganic materials, the condition of reaction, and the formation mechanism.

A significant advancement in the chemistry of MOFs happened in 1999 when the synthesis and determination of a single crystal structure using XRD, and low temperature-low pressure gas sorption characteristics were described for the first powerful porous MOF [38]. In order to prepare MOFs with a further higher surface area, which require an increased storage space per weight of the component, that phenomenon is termed as ultrahigh porosity. Organic linkers with large lengths offer a larger storage space and higher number of adsorption sites within a given component. Nevertheless, the more space within the framework makes it liable to make impregnating structures, in which two or more crystal frameworks get increased in size and mutually twirled together. By making MOFs with topology that obstructs impregnation, it can be effectively prevented from impregnation, which will require the further framework to be with a different topology [10,39].

1.2.8 Advantages of MOF Porous Materials

The common advantages of MOF porous materials are presented in Figure 1.4. The foremost advantage of MOF porous materials is that the quantity of possible combinations of organic and inorganic parts to make the resulting expected structures is excellent [40]. Moreover, MOFs hold some unique properties like luminescence [41] and magnetism [42] in comparison with other porous materials.

MOFs have a few other advantages such as low-cost design, light weight, etc., which can probably be used to decrease the growing carbon dioxide level in the atmosphere due to the burning of fossil fuels [5].

A tank charged with a porous adsorbent permits storage of a gas at a very low pressure compared to a similar tank without adsorbent. Thus, tanks with high pressure and multi-stage compressors can be modified by providing a safe and economical gas storage method. Many gas storage analyses have been performed on porous adsorbents such as carbon nanotubes, activated carbon, and zeolites [31].

FIGURE 1.4 Common advantages of MOF porous materials [40].

1.2.9 Porous MOFs in Separation of Gases

As already discussed, MOF materials play vital roles in many application areas. Major application areas of MOF materials are tabulated in Table 1.1.

The insight of the first few porous MOFs with permanent porosity implemented by gas adsorption analyses significantly facilitated the development of these novel adsorbents for storing and separating gases [37,43]. For separating specific gases, the early researchers conducted tests based on single component adsorption and/or desorption isotherm prediction of pure gases. The accumulation of sorption information (from a number of MOFs) exhibited a positive potential for purification and separation of vapor or gas mixtures. However, normal separation of mixtures of gases with the help of MOFs was hardly understood until gas chromatography (a new evaluating method) was introduced into this field in 2005 [44]. In 2007, an experimental fixed bed breakthrough was also applied to the prediction of separation of MOFs [45].

Based on these technologies, MOFs have revealed real separation of gas mixtures from their intrinsic porous properties. In turn, the breakthrough experiment became a powerful tool for evaluating the separation of MOFs that can imitate the industrial process which cannot be implemented by a simple static single-component gas sorption analysis. Then, a number of significant and challenging separations such as capturing C0₂, separating CO, from nitrogen or methane, separating light hydrocarbons, separating isomers, separating noble gases, etc., have been accomplished by utilizing the unique MOFs as adsorbent materials [46,47].

1.2.10 Nano Porous MOFs

Currently, nano porous materials are being focused with the view of nano science and nanotechnology that is an evergreen multi-disciplinary domain of analysis, which attracted lots of effort in R&D around the world. Nano porous materials (as a subset of nano structured materials) possess unique surface, structural, and chemical

TABLE 1.1

General Application Areas of MOF Porous Materials [37]

properties which show their importance in various sectors such as ion exchange [48], separation [49], catalysis [50], gas storage [51-53], lithium ion batteries [54], biological molecular isolation [55], and purification fields because of their flexible frameworks, uniform pore size, controlled chemistry, and high internal surface area. In recent years, porous materials have also been used extensively in optical transparency, [56] photovoltaic solar cells [57], nano generators [58], nanotechnology [59], sensors [60], optoelectronic devices [61], biomedical imaging [62], and biomedical sciences [63]. However, they are capable of interacting with the atoms, ions, and molecules at their surfaces as well as throughout the portion of the materials [7].

Hence, nano porous materials also have scientific technological importance due to their flexibility to interact with atoms, ions, and molecules on their large interior surfaces and in their nanometer-size pore spaces. They provide new opportunities in many areas including inclusion chemistry and guest-host synthesis [64].

• [1] combined compartments which operate separately, but their function is
• [2] integrated;
• [3] ii. ability to perform simultaneous operations; and
• [4] iii. dexterity to count, classify, and code data [14].