## 1 Introduction
The use of slag from glass production, generated from the pyrometallurgical process, stands out notably due to its proportion of Si and Al as well as Ca and Mg. On the other hand, electric arc furnace slag has a more variable composition of silica and alumina e.g. the by-product from Ti extraction that generates the Ti-bearing electric arc furnace slag [19]. The formation of mesoporous zeolite structures may be accompanied by the development of microporous ones that make the diffusion of molecules into the structure difficult. At this point, new strategies for synthesis are proposed, such as leaching treatments by acid or alkali, dry-gel conversion for the transformation of amorphous mesoporous silica into zeolite and hard-template-directed synthesis. Some template methods offer crystalline and ordered zeolite structure with narrow pore sizes. Alternatively, the use of a mixture of templates can be applied, where two or more macromolecules are used to direct the structure's formation. In this process, the structure may be bulky with amorphous mesoporous material formed. An improvement is the use of organosilane and functionalized polymers that influence the crystal growth. Besides these experimental remarks, a computational screening is an approach that Blatov et al. [20] suggest to help the new zeolite developments. The ordered mesoporous structure is a type of hierarchical zeolite structure that may be synthesized by a bottom-up method. This method occurs with the crystallization of aluminosilicate gel with the aid of mesoporous templates generating mesoporous channels. The mesoporous templates might be then removed by calcination. While the use of these substances is important to define the size and shape of mesoporous channels, their decomposition generates greenhouse gases at high temperatures. The green synthesis process may be classified into methods without template molecules or recyclable and renewable templates; methods using sustainable silicon or aluminum sources; solvent-free methods; and facile methods, such as the use of microwave to reduce the time of crystallization [21]. Figure 2 shows a template synthesis mechanism according with Li et al. [22]. The introduction of mesopores in zeolites makes the diffusion of bulky molecules inside their channels easier and decreases the intracrystalline diffusion length. The synthesis of mesoporous zeolites is adequate to obtain ultrafine structure not  
Figure 2: Template synthesis mechanism of zeolite Ni-MFI. Reproduced from [22]. Copyright Elsevier 2009. 100 nm of size. Postsynthesis treatments such as steaming, acid/base leaching and chemical treatment are eco-unfriendly and produce mesopores without good definition. Some other improved methods for mesoporous zeolite synthesis were proposed, including the aggregation of a metal in the structure, e.g., nimpregnation of Nb, Ta and Pd form a zeolite encapsulation system [23] and in incorporation of iron forming Fe-ZSM-5 zeolite [24]. Other methods are microwave-assisted single-surfactant templating [25] and pseudomorphic transformation [26]. A synthesis strategy using silica-carbon composites obtained by pyrolysis of carbonaceous gases in the presence of silica-gel was also described as useful for mesoporous zeolite production [27]. There are many possibilities of synthesis methods to obtain different zeolites structures with applications focusing on environmental protection, such as the removal of dyes, heavy metals, and other recalcitrant pollutants. The removal of greenhouse gases and catalytic reactions are other applications found in zeolite research. Figure 3 shows relevant publications involving zeolite synthesis and applications. This review aimed to explore the main aspects for the synthesis of zeolites, exploring factors such as the chemical nature of precursor, the chemical and physical properties of zeolites, the main precursor used for synthesis and the different applications in chemical processes, biochemical processes and the relevance of the material for environmental protection.