## 2 Zeolites
### Synthesis Methods
#### 2.9.6 Methods for Heteroatom Zeolite Synthesis  
These methods are perfectly described by Pang et al. [14] (p. 3). The heteroatom zeolites have received increased attention for the development of synthesis methods, mostly due to changes in such characteristics as microporous structure, particle size and surface acidity. One of the methods involved to obtain this framework is direct synthesis [181]. This process is conducted as a traditional hydrothermal synthesis where the heteroatom is inserted and directly adding a metal salt solution or a metal organic [182]. The hydrolysis of heteroatom precursors occurs simultaneously with the addition of silicon/aluminum sources, giving the formation of a homogeneous sol alkaline or neutral, crystallized into the heteroatom zeolite at a certain temperature, performing the uniform insertion of plenty of heteroatoms in the zeolite framework during the crystallization [183]. The function of heteroatoms is to substitute silicon or aluminum and to form a new framework with connection by SiO\({}_{4}\) tetrahedrons. When the execution of this method occurs under alkaline conditions, there is the possibility of hydroxides forming from the transition metals used, resulting in an inhomogeneous distribution of heteroatoms on the surface and in the channels of zeolite that may block the micropores [184]. The production of Ga-ZSM-5 zeolite by this process gives the advantage of reducing the decomposition of the structure at the temperature necessary to remove the template. The acidity required for some catalytic processes increases, such as the insertion of Ga as of Fe in this MFI structure [185]. TS-1 is another type of structure with insertion of Ti atoms forming the peroxotitanate species and replacing the aluminum atoms. The experimental procedure for the production of TS-1 zeolite has two steps: the mixing of alkoxides with solutions of TPAOH, TEOS  
Figure 12: Microwave-assisted synthesis method and differences in product morphology. Reproduced from [177]. Copyright 2020 Elsevier. and TEOT suffering a hydrolysis and the dissolution of titanium mixing TEOT in water and then hydrogen peroxide, after which the system is cooled, forming the clear orange solution of peroxotitanic acid, and then inserting aqueous solution of TPAOH, forming a yellow solution of peroxotitanate. The process is finished by adding colloidal SiO\({}_{2}\) with mild heating. The use of F- as mineralization agent helps Ti atom incorporation in the framework, producing a Ti-\(\beta\) zeolite with high hydrophobicity and selective oxidation for organic substrates with H\({}_{2}\)O\({}_{2}\). The formation of TS-1 is highly dependent on the pH of the synthesis medium, which influences the number of Ti atoms per unit cell. If the pH is near 7.0, there are near 2.3 atoms/unit cell and the anatase phase may be formed. As the pH values increase, the anatase phase extinguishes with increased unit cell volume. The direct method to heteroatom zeolite production is readily applicable in industrial scaling up, with the product showing high crystallinity, and control of heteroatom content, topology, morphology, and particle size. Nevertheless, some deficiencies related to the mismatching of transition metals in hydrolysis rate relative to Al/Si ions as well as the radius differences are present in this method. Other inconvenient features include the long reaction time for crystallization, partial incorporation of heteroatoms in alkaline conditions, facile formation of extra-framework heteroatoms, severe and narrow synthesis conditions, and in some cases metal agglomeration [186]. Another type of pathway to obtain heteroatom zeolites is the postsynthesis method, which incorporates the metal in the zeolite framework of as-synthesized zeolites in liquid-solid, solid-solid, and gas-solid systems combining the dealumination and desilication with isomorphic substitution of framework atoms under specific conditions [187].