**3.3 Vapor diffusion method**

The vapor diffusion method was the first synthetic route used to create MOFs structure. This method produces high-quality crystals, but it requires high ligand

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

#### **Figure 10.**

*Schematic representation of the microwave-assisted method for synthesis of metalorganic frameworks (reproduced from Ref. [40]).*

#### **Figure 11.**

*Schematic representation of the vapor diffusion method for synthesis of metal–organic frameworks (reproduced from Ref. [55]).*

solubility. For the first time, Smaldone and his colleagues characterized a series of cyclodextrin-based metal–organic frameworks (CD-MOFs) [54]. In addition, Forgan's group created cyclodextrin-based metal–organic frameworks (-CD-MOFs) by combining -CD with K+ , Rb<sup>+</sup> , and Cs+ in an aqueous medium and then vapor diffusion with MeOH [54]. They obtained single crystals of -CD-MOFs with crystal sizes ranging from 200 to 400 nm in 2–7 days (**Figure 11**). Wu et al. used this method to make [Pb(1,4-NDC)(DMF)] by dissolving Pb(NO3)2 and H2-1,4-NDC (naphthalene dicarboxylate) in DMF inside one vessel and triethylamine in the other. The continuous growth of MOF crystals was caused by the sustained diffusion of triethylamine from the outer to the inner container [56].

### **3.4 Gel crystallization**

Gel crystal growth is a useful strategy for MOF synthesis that involves adding an emulsifier to the reaction medium. Das et al. synthesized MOF [Ba2(O3P(CH2)3PO3)] 3H2O by first dissolving the metal component, barium(II) chloride in water [57]. The

**Figure 12.** *Synthesis of Fe-MOFs via PdCl2-mediated gel crystallization methods (reproduced from Ref. [58]).*

mixture was thoroughly mixed prior to forming a gel, and sheets of ethanediphosphonic aqueous acidic remedy were equipped and nurtured for three weeks. Despite the fact that this procedure is time-consuming, additional separation phases are required to detoxify the product using the gel crystallization method (**Figure 12**).
