**2. Advance features of MOFs**

### **2.1 Ultrahigh porosity of MOFs**

Metal–organic frameworks (MOFs) with ultrahigh porosity are useful in a variety of applications, such as gas storage, separation, and catalysis. It is usually vulnerable to conscience because of the large void space inside the crystal framework. Expanding the organic linker chains should lead to increased porosity of MOFs in general [19]. The porous nature of high porosity MOFs was first demonstrated in the 1990s. The reported metal–organic framework (MOF-2010) had a large Langmuir surface area (6240 m<sup>2</sup> g<sup>1</sup> ) and pore volume (3.60 cm<sup>3</sup> g<sup>1</sup> and 0.89 cm3 cm<sup>3</sup> ) (**Figure 3**) [21]. The porous nature of high porosity MOFs was first demonstrated in the 1990s, with no encapsulation of guest molecules in their pores. MOFs frameworks exhibit ultrahigh porous behavior with reversible gas storage properties [22]. They are excellent candidates for use in the creation of novel and valuable MOF materials.

#### **Figure 3.**

*(a) The connectivity of pyr and qom nets along with [3, 6] coordinates; (b) pairs of pyr nets; (c–e) qom is not selfdual; (d) qom connectivity with dual tiling net; (e) different net from the original net of the (c); (f) porous net of MOF-177; (g) porous net of MOF-180; and (h) porous net of MOF-200. Where yellow ball indicates the porous cages, Zn is blue; O is red; C is black, and hydrogen atoms are omitted for clarity (reproduced from Ref. [20]).*

## **2.2 Ultrahigh surface area of MOFs**

The isoreticular expansion premise has made significant progress in the generation of ultrahigh surface area MOFs. This method has been used to summarize some of the appreciable surface areas of MOFs, such as MOF-2105 and NU-1004 [23, 24]. When the solvent is removed, an increase in linker causes MOFs to collapse. Supercritical carbon dioxide activation has proven useful in addressing this issue. Although a long chain of linkers can result in the formation of interpenetrated structures, synthesizing MOFs in topographic networks can mitigate this tendency. Ultrahigh surface area of MOFs has been developed to overcome the problem of water shortage (or high humidity) consistency [22, 25]. This has led to the development of NU-1106 and DUT-327, both of which are based on the *rht* and *umt* topologies. Large surface area MOFs (NU-1103) have been reported with a larger surface area of 5646 m<sup>2</sup> g<sup>1</sup> (BET area of 6550 m<sup>2</sup> g<sup>1</sup> ) (**Figure 4**) [23, 26, 27].

### **2.3 MOFs with Lewis acid frameworks**

Multicomponent reactions (MCRs) combine three or more reaction partners in one skillet to produce organic products. MCRs have played an important role in drug discovery and pharmaceutical applications. Brønsted and Lewis acids have been used to accelerate multiple MCR reactions at the same time [28]. Metal–organic frameworks (MOFs) have emerged as an important class of crystalline porous materials for

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

#### **Figure 4.**

*Representation of large pores (indicated by blue spheres) and small pores (indicated by purple spheres) in the* **ftw** *topological networks of NU-1103 (reproduced from Ref. [23]).*

#### **Figure 5.**

*The generation of strongly Lewis acidic Zr-OTf sites in Zr6OTf-BPDC, Zr6OTf-BTC, and Zr6OTf-BTB is illustrated and compared using (a) MOF nodes and ligands, (b) structures and pore distributions, and (c) coordination defects or capping residues (yellow color) (purple: Zr, red: O, gray: C, yellow: Lewis acidic site). Where H atoms are omitted for clarity (reproduced from Ref. [28]).*

the development of high-efficiency single-site solid photocatalysts. MOFs are composed of inorganic metal ions or clusters and organic linkers with organic atoms and molecules (**Figure 5a**) [20, 29]. A set of strict MOFs with acidic sites based on electron-deficient high-valent metallic sources (ZrIV, HfIV, etc.) have been developed and used to catalyze biologically important transformations. The acidity of Brønsted and Lewis acids was increased by converting immaculate doped or other Zr-capping substituents (**Figure 5b**) [28, 30]. A 2D MOF with self-supporting nanostructure morphological characteristics and freely available Lewis acidic Zr-OTf sites has outperformed two three-dimensional (3D) MOFs for the fabrication of a wide range of synthesized tetrahydroquinoline and aziridine carboxyl group derivative products

#### **Figure 6.**

*The reversible phase transformation for stabilization of soft nature MOFs (reproduced from Ref. [37]).*

(**Figure 5c**). Zr6OTf-BTB outperformed the relatively homogeneous standard Sc (OTf)3 in terms of significantly higher turnover numbers and 9–14 times longer catalyst lifetime [31]. It was eventually used to effectively create a few biologically active drug targets via MCRs.

#### **2.4 Flexible and porous MOFs**

Porous coordination polymers (CPs) or metal–organic frameworks (MOFs) have received a great deal of attention as smart materials. MOF-based materials, such as MOF composite materials, have piqued the interest of electrochemical energy storage and conversion researchers [32]. In addition to MOFs, there are also soft porous crystals (SPCs), which appear to be reversible or multistable crystalline solids with long-range structural ordering and repairable state transformation [33]. Flexibility frequently comes at the expense of decreased stability, and porosity loss is common. Polymeric guests prevent the framework from collapsing spontaneously, resulting in novel and stable porous phases [34]. This strategy was also used to stabilize highly porous MOFs after activation, preserving porosity. Polymerization of monomer units within MOF pore spaces is widely acknowledged as a simple and convenient method for polymer-porous material interbreeding. Kitagawa and colleagues demonstrated the incorporation of common vinyl polymers, polypyrrole (PPy), and polythiophene (PTh) into appropriate nanochannels of different MOFs [35]. Polypyrrole (PPy) is a polymer that has been shown to be a good electrical conductor for superconductors and has been used to improve MOF electrical properties and encourage their use in energy storage applications and supercapacitors [36] (**Figure 6**). Researchers believe that inserting a conducting polymer guest into such materials could change the porous behavior of the host frameworks.
