## 2 Zeolites
### Synthesis Methods  
#### 2.9.1 Hydrothermal  
The hydrothermal process of preparation of zeolites involves two main processes: the dissolution of Si and Al and the nucleation and growth of crystals for the formation  
Figure 9: Bronsted acid sites (BAS) after different calcination temperatures determined by three methods. Reproduced from [132]. Copyright 2016 ACS. of zeolite [133]. Hydrothermal synthesis is a multiphase process involving a crystallization reaction, comprising a liquid phase and a solid phase, which can be amorphous and become crystalline. Some authors define the process as in situ reactions that occur at a temperature above the ambient temperature in an aqueous medium and at a pressure of about 14.5 psi [134]. The advantages of using this process include the low energy consumption involved, the high reactivity of the reagents used, the impossibility of environmental pollution, and the formation of metastable phases [135]. In the work of Hartati et al. [136], synthesis in the hydrothermal condition must occur through reagents containing Si and Al being mixed with a cation source of basic character. The reaction must occur in a closed flask in an autoclave at a temperature above 100 \({}^{\circ}\)C. During the synthesis, the structure of the Si and Al sources is amorphous and the obtaining of crystalline zeolite depends on passing through the induction period. The initial function of used alkali (NaOH) is to supply hydroxyl ions for the process of dissolution of Si and Al, thus acting as a mineralizer. This can be evidenced by the disappearance of quartz peaks in a diffractogram as the amount of sample inserted is increased, as Si becomes more available in the mother solution [137]. Some authors also note the effect on the crystallization process, supplying sodium ions that improve the crystallinity of the structure zeolite [133]. In the synthesis process, Cundy and Cox [138] showed that the induction period is between the beginning of the reaction and the observation of the first formation of crystalline material. In the synthesis process, the amorphous phase is considered constant. This phase is responsible for releasing soluble species and is called the \"primary amorphous phase,\" forming as the reagents are mixed, forming a gel, which can also be called \"clean solution\" when it has a colloidal form, being translucent. The primary amorphous phase is formed by the immediate appearance of the product, existing in an unbalanced condition, and as a heterogeneous system, which may contain aluminosilicate precipitates, coagulated silica, and precipitated alumina or untransformed reagents [139]. However, the formation of the secondary amorphous phase occurs under conditions of medium heating and formation of a pseudo-transition-state intermediate, which follows the reaction path involving silicates and aluminosilicates, monitoring the pH of the medium, and at the end of the process, the formation of crystalline zeolite product [138]. According to Abdullahi et al. [134], the complexity of the hydrothermal synthesis process exists due to the different factors that can affect it, among which the following stand out impurities from the starting materials, the composition of the synthesis gel, the pH of the medium, time and temperature of the calcination and crystallization steps, the formation of intermediate metastable phases, the Si/Al ratio, the water content used, the alkalinity, the use of organic drivers, and the nucleation and growth steps of more stable phase crystals in digestion conditions. In accordance with the studies by Hartati et al. [136] (p. 18), the presence of impurities in the starting material can influence the crystallization of the zeolite structure. One of the impurities that fulfills this role are the Fe atoms present as oxides or not. The reactivity in the presence of this element is influenced by temperature. The removal of impurities can be carried out with a dispersing agent. However, this decreases the SiO\({}_{2}\) content and increases the Al\({}_{2}\)O\({}_{3}\) content.