## 2 Zeolites  
### Remarks about Chemical Composition  
Zeolites are crystalline solids with the general formula M\({}_{2}\)/nO, Al\({}_{2}\)O\({}_{3}\), zSiO\({}_{2}\), where \"n\" is the valence of the metal \"M.\" In these crystalline polymers composed of TO\({}_{4}\) tetrahedra (T = Al or Si), the stoichiometric ratio \"z\" can vary from 2 to infinity. According to Loewenstein's rule, Al-O-Al bonds are not allowed in aluminosilicates, and thus an important parameter for obtaining a zeolitic structure is the Si/Al ratio [28]. The modification of chemical composition in zeolites lead to different properties, such as pore volume, channel dimensionality and framework density (FD). In framework composition, the Si/Al ratio is the main parameter that brings an identity for a particular structure: there are those  
Figure 3: Evolution of the number of publications on zeolites obtained from the Web of Science database from 2010 to 2022.  
with high silica content (Si/Al > 12), e.g., the MFI structure ZSM-5 [29], intermediate Si/Al ratio between 1.2 and 3.0 [30], and same proportion of alumina and silica (Si/Al = 1.0), e.g., zeolite A [31]. The framework composition has an indirect effect on framework structure; therefore, zeolites with high silica content are rich in five-ring types [32]. Besides the framework chemical species, the extra-framework species are important for physicochemical properties such as ion-exchange capacity, catalytic selectivity, and adsorption. The extra-framework species formation studied in the postsynthesis process called dea-lumination is generally used to obtain high-silica structures such as USY. The consequences of this process include changes in Si/Al ratio, structure, acidity, selectivity, reactivity and cooking behavior for a certain catalytic reaction. Citric acid is one of the chemicals used to remove Al from zeolite [33]. The process of deaulumination may increase the acidity by many factors, among which are revealing catalytic Lewis acid activity, stabilization of negative charge on the lattice after the removal of protons, and the synergism of Lewis and Bronsted acid sites due their proximity [34]. Some studies proved that the impact of EFAL (extra-framework aluminum) species on the effective size of supercage voids resulted in the confinement and stabilization of transition states [35; 36]. The extra-framework cations (EFC) are Lewis acids, influencing the chemical and optical properties of adsorbing molecules as well as generating a strong electric field in comparison to the presence of protons (Bronsted acids). It is important to know the location and acidity of these extra-framework cations for a rational modification of acidity from zeolites. The acidity of these species can be evaluated through their interaction with a weak basic molecule such as CO. Effectively, its adsorption depends on the size of zeolite pore and channel, the zeolite topology, the cation size, and the Si/Al ratio [37]. The EFC influences the activity of aluminum extra-framework, and the Na\({}^{+}\) may influence the activation barrier of conversion reaction between the AlOH\({}^{+2}\) and Al(OH)\({}_{2}\)\({}^{+}\) that are formed in the process of deaulumination and are extra-framework aluminum species, together with Al\({}^{+3}\), AlCOOH, Al(OH)\({}_{3}\) and AlO\({}^{+}\)[35; 36; 37; 38]. The increase and decrease in activation are a function of Na\({}^{+}\) coordination and the presence of other ions such as K\({}^{+}\) and Ca\({}^{+2}\), which are important for energy configuration of zeolite structure [34; 35; 36; 37; 38; 39]. Another extra-framework cation is La\({}^{+3}\), present in structures such as zeolite L and may be hidden, altering ion exchange and thermodynamic properties [40]. Figure 4 shows the topology and location of extra-framework sites. In the crystal structure of zeolite, Al\({}^{3+}\) ions together with Si\({}^{4+}\) arranged in tetrahedrons may generate an excess of negative unit charge. This charge can be compensated by Na\({}^{+}\) ions.