**2. Design of MOFs**

## **2.1. Crystal engineering of MOF**

The term of metal organic framework was introduced by Yaghi in 1995 [29, 30], however, such structures were known until 1964 when Bailar first reported them [31]. The resurgence of the structures has been accompanied by the application of these materials in various areas, including: catalysis [2–4], guest adsorption (molecular recognition) [5], drug delivery [6, 7], gas storage [8–13], optical applications [14–16], composites [17], water treatment [18, 19] and sensor technologies [20], among others [21–26].

The structural characteristics of the MOFs are mainly determined by the nature of the metal center and the organic linker, yet, during the synthesis of these materials, solvents and/or counterions are typically used [32] and they also play an important role. The counterions change the environment of the metal ion and may generate overlaps with the structure resulting in weak interactions with the MOF. Meanwhile, solvent molecules with the MOF generally crystallize during synthesis, modifying the pore size.

Generally, the transition metal ions used can generate a wide range of structures. The prop‐ erties of these metals, including the oxidation state and coordination number (typically varies from 2 to 7), produce a linear, trigonal, square planar, tetrahedral, trigonal pyramidal, trigonalbipyramidal, octahedral, and pentagonal bipyramidal geometries as well as some other distorted forms [32]. The lanthanoidions, whose coordination number varies between 7 and 10, have polyhedral geometries and can generate MOFs with particular topologies [33].

In the formation of MOFs, the organic linkers must meet certain requirements to form coordination bonds, mainly being multidentate having at least two donor atoms (N-, O- or S-) and being neutral or anionic. The structure of MOF is also affected by the shape, length, and functional groups present in the organic linker. The linkers commonly used in the MOFs synthesis are piperazine [34], 4,4′-bipyridine [34–37] (neutral ligands), and polycarboxylates (anionic ligands). Polycarboxylates may be di- [38–43], tri- [38, 40–43], tetra- [44, 45], or hexacarboxylates [46, 47].

Some materials as metals in solution (transition metal complexes or metal salts) have been used in catalysis with excellent results. These materials are able to catalyze a variety of organic reactions, in many cases, reaching high yields and regenerating the material after the reaction. However, in many cases the metals are hardly recovered and/or decompose during the reaction due to the conditions. To achieve control these limitations, researchers have developed methods using porous materials as carriers, to achieve well-isolated, uniform single sites that don't interact between them, preventing the decomposition [2, 27]. Active sites on MOFs are located at the metal nodes on the crystalline structure; when the reaction occurs, the framework

protects their active sites and increases the efficiency and resistance of catalyst [28].

and activity, formation of active sites and limitations of these materials.

**2. Design of MOFs**

**2.1. Crystal engineering of MOF**

sensor technologies [20], among others [21–26].

96 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

generally crystallize during synthesis, modifying the pore size.

Given the variety of metallic nodes and organic linkers, it is possible to control the synthesis of MOFs to design them with modular properties, functionalized with specific sites or specific assets to catalyze organic reactions. In this chapter, we present the main results of research with MOFs in the field of catalysis, with special focus on design, relationship between structure

The term of metal organic framework was introduced by Yaghi in 1995 [29, 30], however, such structures were known until 1964 when Bailar first reported them [31]. The resurgence of the structures has been accompanied by the application of these materials in various areas, including: catalysis [2–4], guest adsorption (molecular recognition) [5], drug delivery [6, 7], gas storage [8–13], optical applications [14–16], composites [17], water treatment [18, 19] and

The structural characteristics of the MOFs are mainly determined by the nature of the metal center and the organic linker, yet, during the synthesis of these materials, solvents and/or counterions are typically used [32] and they also play an important role. The counterions change the environment of the metal ion and may generate overlaps with the structure resulting in weak interactions with the MOF. Meanwhile, solvent molecules with the MOF

Generally, the transition metal ions used can generate a wide range of structures. The prop‐ erties of these metals, including the oxidation state and coordination number (typically varies from 2 to 7), produce a linear, trigonal, square planar, tetrahedral, trigonal pyramidal, trigonalbipyramidal, octahedral, and pentagonal bipyramidal geometries as well as some other distorted forms [32]. The lanthanoidions, whose coordination number varies between 7 and 10, have polyhedral geometries and can generate MOFs with particular topologies [33]. In the formation of MOFs, the organic linkers must meet certain requirements to form coordination bonds, mainly being multidentate having at least two donor atoms (N-, O- or S-) and being neutral or anionic. The structure of MOF is also affected by the shape, length, and functional groups present in the organic linker. The linkers commonly used in the MOFs The binding of a linker to the metal center may generate a one-dimensional (1D), twodimensional (2D) or three-dimensional (3D) arrangement, which depends on the metal center (Figure 1) [48]. In a 1D network, two ligand molecules are coordinated to the metal center to generate a chain, while in a 2D network, three or four molecules of the organic linker are coordinated to generate a plane, and it grows in two dimensions. In a 3D MOF, the metal center, with high coordination number, joins three more linker molecules, along the three spatial dimensions, generating a three-dimensional structure with pores and cavities defined.

**Figure 1.** Basic building units of one-, two-, and three-dimensional MOFs [48].

Figure 2 shows examples of MOF with different dimensionalities. The helix (1D) is constituted by distorted tetrahedrons mercury (II), formed by the union of two nitrogen atoms (from two different linkers) and two terminal bromine atoms [49].

The 2D structures with grid shape are generally synthesized with a molar ratio between the ligand and the metal center of 1:2. An example of such structures is shown in Figure 3, the MOF is constituted by cobalt metal centers and ligands *N*-(3-pyridyl) nicotinamide [50]. The

metal ions are coordinated with four molecules of ligand, which result in a two-dimensional flat-shaped structure. metal ions are coordinated with four molecules of ligand, which result in a two‐dimensional flat‐shaped structure.

MOF is constituted by cobalt metal centers and ligands *N*‐(3‐pyridyl) nicotinamide [50]. The

Figure 2 shows examples of MOF with different dimensionalities. The helix (1D) is constituted by distorted tetrahedrons mercury (II), formed by the union of two nitrogen

atoms (from two different linkers) and two terminal bromine atoms [49].

**Figure 2.** Examples of MOF structures 1D, 2D, and 3D.

Figure 2. Examples of MOF structures 1D, 2D, and 3D.

The three‐dimensional MOFs are formed by the interaction of one‐dimensional chains in all three directions. Connectivity of the construction nodes depends on the metal center, and the formed structures are usually tetrahedral or octahedral. An example of such structures, wherein the metal is cadmium center and has an octahedral coordination, is given in Figure 3. The bidentate linker forms connections, where the four terminals of each linker involves oxygen atoms. The three‐dimensional growth of the framework generates cavities; generally occupied by solvent molecules [51]. The three-dimensional MOFs are formed by the interaction of one-dimensional chains in all three directions. Connectivity of the construction nodes depends on the metal center, and the formed structures are usually tetrahedral or octahedral. An example of such structures, wherein the metal is cadmium center and has an octahedral coordination, is given in Figure 3. The bidentate linker forms connections, where the four terminals of each linker involves oxygen atoms. The three-dimensional growth of the framework generates cavities; generally occupied by solvent molecules [51].

### **2.2. Synthesis of MOF**

The physicochemical characteristics of MOFs can be modulated and it is clear that all these properties can be modified in the material from the synthesis process. The solvothermal synthesis is the most common way of obtaining MOFs. However, other recently studied methods of synthesis, which may cause significant changes in the MOF's properties, include (i) mechanochemical, (ii) electrochemistry, (iii) assisted synthesis (by ultrasound or micro‐ wave), and (iv) subcritical water.
