*2.2.1. Solvothermal synthesis*

metal ions are coordinated with four molecules of ligand, which result in a two-dimensional

1D 2D 3D

Co(SCN)2(3‐pna)2]}<sup>∞</sup> (SCN = tiocianate), (3‐pna = N‐(3‐pyridil) nicotinamide) [50]

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

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

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‐

[Cd(3,3´‐AZDB)2]H2NMe)2 (NH4)(3,3´‐AZDB = 3,3´‐ azodibenzoate) [51]

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

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].

98 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

flat-shaped structure.

flat‐shaped structure.

HgBr2(2,5‐bis(3‐piridil)‐ 1,3,4‐oxadiazole) [49]

occupied by solvent molecules [51].

wave), and (iv) subcritical water.

**2.2. Synthesis of MOF**

occupied by solvent molecules [51].

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

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

The solvothermal synthesis comprises the reaction of the metal salt and the organic ligand in the presence of organic solvents or solvent mixtures typically involving formamide [52–55], alcohols [56, 57], or pyrrolidones [58]. The reaction temperature is usually considerably less than 523 K, but depends on the boiling point of the solvent used, for example, in the case of methanol, the synthesis temperature is 338 K. Upon heating, the blends in sealed containers such as teflon or glass pressure tubes produced compounds incorporating solvent molecules in the pores of the material. These organic solvents are often toxic, carcinogenic, and/or dangerous to the environment. For example, dimethylformamide (DMF), one of the most commonly used solvents in the synthesis of MOFs is contaminant, mutagenic, and toxic [59]. Additionally, DMF decomposes when heated at high temperatures for long periods and therefore cannot be reused [60].

The concern about the use of these organic solvents has increased due to their negative impact on the environment [61]. Therefore, the replacement of polluting solvents in the synthesis of MOFs with solvents that do not pose a risk to the environment has been established as a primary objective, committing to the principles of Green Chemistry [62]. When the solvent is water, the main problem encountered during synthesis is the low or nonsolubility of the reactants and organic ligands, which generally prevents the formation of coordination polymer.

Other important parameters in the solvothermal synthesis are: temperature, concentration of reactants (which can be varied over a wide range) and the pH of the solution.

One of the advantages of this synthesis are the yields ranging from 60% to 95%; however, removing solvent molecules occluded in the pores of the MOF is not a simple process, the solids must be washed several times, which can take several days and considerably reduce performance.
