**1. Introduction**

### **1.1 General introduction**

Metal–organic frameworks (MOFs) are crystalline three-dimensional (3D) hybrid materials composed of metal ions and metal clusters linked by polydentate organic ligands [1]. MOF metal centers act as templates, connecting to organic linkers via coordinative metal–ligand interactions and electrostatic attraction. MOFs esthetical chemistry is determined by the interaction of a specific metal secondary building unit with organic ligands [2]. The organic linker may have the same topology but a different metric, creating an isoreticular set of structures that share the same basic net. The importance of coordination bonds and other weak interactions (pi-electron, H-bond, or

#### **Figure 1.**

*Schematic representation of the synthesis of MOF-5 and HKUST-1 using different secondary building units (SBUs) and organic linkers. Free spaces in the framework are represented by yellow and blue spheres (reproduced from Ref. [8]).*

Van der Waals interaction) in MOF synthesis could be taken for granted. MOFs have high crystallinity, large surface area, high pore volume, and low framework density [3]. They are promising materials for a variety of applications, including clean energy storage (methane and hydrogen), CO2 capture, absorption, and various separation processes [4–6]. In general, MOFs are made up of two parts: cluster or metal ion nodes and organic linkers that connect the SBUs, resulting in crystalline structures with significant porous texture development. MOFs can also be used as thin-film devices, for biomedical imaging, light harvesting, optical luminescence, catalysis, and other various applications [7]. MOF-5 and HKUST-1 are two well-known MOFs used as a photocatalyst in the synthesis of synthetic organic molecules (**Figure 1**) [9]. The interactions of a specific metal secondary building unit with organic SBUs determine the chemistry of MOFs. The combination of these structures results in an enormous number of possibilities for synthesizing various MOFs with tailored functional properties [10].

The various types of MOFs are produced by using various SBU and organic linkers. As can be seen, different pore shapes of the MOFs framework can be achieved depending on the organic linker. Polytopic organic linkers include carboxylates, phosphonates, sulphides, azoles, and heterocyclic compounds [8]. Several SBUs and organic ligands used in the synthesis of MOFs are depicted in **Figure 2**. When Yaghi et al. synthesized MOF-5 in 1995, they made the first reference to the synthesis of metal–organic frameworks. Since then, a large number of these materials have been studied and classified into various categories in the literature [3, 11, 12]. UIO-66 from Universiteti I Oslo, MIL from Materials of Institute Lavoisier, and ZIF-based MOFs from Zeolite Imidazolate Framework, among many others MOFs have been used in the photocatalytic degradation [13, 14].

#### **1.2 Historical developments**

MOF-5 was first synthesized by Yaghi and his colleagues in 1999 as Zn4O (BDC)3(DMF)8(C6H5Cl) using zinc nitrate and H2BDC (1,4-benzenedicarboxylate) as a precursor. Structural transformation of MOFs occurs when exposed to variable

*Historical Developments in Synthesis Approaches and Photocatalytic Perspectives… DOI: http://dx.doi.org/10.5772/intechopen.107119*

**Figure 2.**

*Synthetic scheme for different zirconium-based metal–organic frameworks (MOFs) were synthesized using the same secondary building units (SBUs) and different organic ligands (reproduced from Ref. [8]).*

water concentration environments [15]. Despite the fact that such a structure can be reversed by thermal treatment of the frameworks. Hausdorf and his colleagues studied the photocatalytic activity of zinc carboxylate-based MOFs (MOF-5) in water. Furthermore, Laurier et al. reported in 2013 that when exposed to visible light, iron (III)-based MOFs can photodegrade Rhodamine 6G in an aqueous solution [16]. Meanwhile, Serre and Sanchez synthesized the Ti8O8(OH)4(O2C–C6H4–CO2)6 (MIL-125(Ti)) in 2009 [17]. When exposed to visible light, iron(III)-based MOFs can photodegrade Rhodamine 6G in an aqueous solution. The Fe-O cluster itself could indeed act as a semiconductor to absorb visible light and then induce electrons from organic ligands and entire photocatalysts'surface [18].
