## 2 Zeolites
### Synthesis Methods
#### 2.9.4 Alkaline Activation  
The synthesis method with alkaline activation is mainly applied to obtain geopolymers, which can be derived from the same precursor as zeolites, a source of Si and Al. These are polycondensed aluminosilicate amorphous structures. The alkaline activator is a concentrated alkali that can be a hydroxide, silicate, carbonate or sulfate [159]. However, it has been used to obtain zeolitic structures [160; 161; 162; 163; 164]. Alkaline activation is a hydration reaction of aluminosilicates with alkaline and alkaline earth substances. The mechanism of the process is complex, involving reactions of dissolution, coagulation, condensation and crystallization, the first step being the breaking of Si-O-Si and Al-O-Si covalent bonds, occurring when the pH of the solution increases, transforming itself into a colloidal solution. After this process, the cleaved structures unite with time, forming a coagulated structure and thus generating a condensed structure in which--as time progresses and temperature varies--a crystallization process can develop [165]. The initial phase consists of silica dissolution, followed by the transport and polycondensation phase. Studies show that the dissolution of aluminosilicates present in the alkaline medium is the primordial and fundamental step for its realization; thus, hydrolysis reactions are those that occur more frequently and according to the following reactions [166]. \[\text{Al}_{2}\text{O}_{3}+3\text{H}_{2}\text{O}+2\text{OH}^{-}\!\to 2\;[ \text{Al}(\text{OH})_{4}]^{-}\]  
\[\text{SiO}_{2}+\text{H}_{2}\text{O}+\text{OH}^{-}\!\to[\text{SiO}(\text{OH} )_{3}]^{-}\]  
\[\text{SiO}_{2}+2\text{OH}^{-}\!\to[\text{SiO}_{2}(\text{OH})_{2}]^{2-}\]  
The literature shows that the presence of hydroxyl ions reveals that alkalinity plays an important role in the reaction. In addition, temperature and reactivity of the raw material play a fundamental role. The solubility of SiO\({}_{2}\) grows as the pH of the medium increases, while Al\({}_{2}\)O\({}_{3}\) has high solubility in extreme pH conditions (acid and basic). Alkaline attack releases silicate and aluminate ions in the medium, with the Al atoms changing from coordination 5 or 6 to coordination 4. Because they have faster release, the Al atoms react with any silicate present in the activating solution, forming oligomers. Thus, the use of sodium silicate (Na\({}_{2}\)SiO\({}_{3}\)) as an activator allows the generation of a product with greater mechanical resistance than those formed only by the use of NaOH [167]. Several works report that in the condensation step, there is a nucleophilic substitution reaction between the species [Al(OH)\({}_{4}\)]\({}^{-}\) and [SiO(OH)\({}_{3}\)]\({}^{-}\), forming an intermediate complex from the attraction of OH\({}^{-}\) groups and Al\({}^{*3}\) ions. This step is accompanied by dehydration to form the aluminosilicate network. In the process, cations with a smaller radius form a less ordered system accompanied by greater condensation than those with a larger radius [167]. In general, many works report that in the synthesis process, alkali  
Figure 11: Sub-molten salt method for synthesis of hierarchical sodalite. Reproduced from [158]. Copyright 2020 Elsevier. metal hydroxides or a mixture of these with an alkali metal silicate are used. Alkali metal cations play a fundamental role in controlling synthesis steps, such as hardening and crystal formation [168]. Silicate allows the generation of a product with higher Si content and greater mechanical strength. According to Severo et al. [167], mixtures activated from the combined use of alkali silicate with alkali metal hydroxide allow the reaction to progress to a greater degree, as silicate silica reacts before precursor silica. Temperature and time of alkaline activation can determine proportions of zeolitic products, and the formation of polymorphic structures of NaAlSiO\({}_{2}\) can be observed. In a study by Monzon et al.