## 2 Zeolites
### Isomorphic Substitution  
According to Pang et al. [14] (p. 3), the presence of heteroatoms in an aluminosilicate framework increases material performance in catalysis, ionic exchange, and adsorption. The grafting of material with cation and anion modification has shown good catalytic activity due to better accessibility of its active sites. Xia and Mokaya [41] performed anion modification using NH\({}_{3}\), bringing a partial substitution of oxygen atoms by nitrogen, at high temperatures, forming a web of oxynitrates and transforming the material in a basic catalyst. In general, the Al and Si atoms present in the zeolite structure can undergo an isomorphic substitution, i.e., replace another cation of similar size, by other tri- or tetravalent elements. As examples, the incorporation of Ge in thomsonite and the addition of Ga in zeolite A and faujasite can be mentioned. The incorporation of B makes possible the generation of a material with weak acid strength. Ti can be incorporated into an MFI structure, generating TS-1 zeolite. Isomorphic substitution has the likely function of stabilizing the secondary building units (SBUs), favoring the crystallization of zeolites, but making it difficult to obtain an aluminosilicate system. With this, there is the possibility of obtaining zeolites with low framework density, that is, the number of atoms in tetrahedral geometry per 1000 A. Thus, the structures can have a high volume of micropores if the framework has a large number of three- and four-ring members, resulting in a structure with less stability than those with five- and six-membered rings. Moreover, the presence of specific heteroatoms can stabilize the three- and four-membered rings [42]. The formation of an SBU structure with four-membered double rings (D4R) can occur with the presence of Ge during the formation of zeolite. In this case, the insertion of more than three metal atoms can stabilize the D4R units, as the formation of a small mean angle of the Ge-O-Ge bond occurs. making the structure energetically stable [43]. The concentration of heteroatoms may be a parameter to measure the selectivity of a zeolitic material for a proposed catalytic conversion, such as in methanol to olefin reaction [10] (p. 3). In some studies, heteroatoms changing with Si and Al in the structure may cause the production of a large-pore framework, conferring dynamic flexibility in framework and enhancing the molecular diffusion in zeolite. It should be noted that the substitution of heteroatoms requires a charge compensation with extra-framework cations [44]. The amount and location of heteroatoms incorporated impact the chemical and physical properties [45]. The heteroatoms may be distributed in a zeolite framework to different degrees, as a full order or as a random order. Some computational studies have been done on heteroatom distribution and limitations in zeolites with high Si/Al ratio or small differences in energy for configurations [46]. While with high differences in energy configurations, the thermo dynamic factors will determine the distribution of heteroatoms and zeolite frameworks, a small difference leads to heteroatom distribution determined by synthetic conditions [47]. The isomorphic substitution of Si\({}^{4+}\) atoms for trivalent cations such as Ga\({}^{3+}\), Fe\({}^{3+}\) and B\({}^{3+}\) changes the acid strength of a structure. The deprotonation energy (DPE) is the parameter that estimates the material strength [48]. As heteroatoms in structure may alter one of the main properties from zeolite (acid strength), the insertion of heteroatoms may increase the functionality of zeolites in a catalytic approach. Bian et al. [49] presented a new zeolite synthesis methodology called interlayer expansion reaction that may be made with insertion of heteroatoms such as Fe as Sn, generating materials with catalysis activity. Besides these metal species, Zn and Co may be used for this purpose.