## 2 Zeolites
### Crystallinity of Zeolites  
The crystalline structure of zeolites is a remarkable characteristic of their properties, depending on the mechanisms involved in the crystallization process. The process of crystallization occurs in two steps: nucleation of discrete particles of the new phase and growth of zeolite crystals. In the nucleation stage, small particles called crystal nuclei represent a new crystalline phase formed. Crystal growth is depicted by the crystallization curve, which shows the process from gel diffusion to equilibrate composition, followed by nucleation and polymerization, and finally crystal growth [90]. However, the synthesis temperature and the time of reaction have great influence in zeolite crystallization, due to the metastable nature of material [91]. Other important factors are the purity and reactivity of raw materials (precursors), agitation of the system, order of mixing, and aging [92]. The initial mixtures have gel-like viscosity because of the amorphous aluminosilicate gel particles suspended in the basic medium [93]. The temperature is directly related to the contact time in synthesis and therefore with degree of crystallization for interconversion between zeolitic phases, e.g., zeolite A to S. The formation of a given zeolite depends on solubility at a certain temperature and equilibrium between the liquid phase and gel [94]. Changes in zeolite structure occur depending on reaction time, since, e.g., the sodalite may be formed in 2-3 h at boiling temperature, while being transformed into zeolite A after 9 h [95]. Figure 7 shows the influence of temperature in a constant time of 2 h in zeolite crystallinity to obtain NaA zeolite, based on [91]. The maximum degree of crystallinity may be controlled with H\({}_{2}\)O/Na\({}_{2}\)O ratio, which is very dependent on the solution's pH and strongly influences the kinetics of structure formation [96]. Studies show that the precipitation of zeolite from the mother solution is increased with high alkalinity and is the first step of crystallization; therefore, pH is an important parameter for this goal [97]. The Si/Al ratio in some cases may influence induction time less [98], defined as the time that the gel precursors are forming and rearranging to make viable the crystal growth from the nuclei, i.e., this is a step before the crystallization [42] (p. 8). The aging time has a great influence on the growth process and alters the crystal size, affecting also the appearance of another phase, such as in synthesis of Y-type zeolite, where the aging time of 72 h is enough to generate a zeolite P phase. The average crystal diameter may be changed by prolonged aging and crystallization times. The agglomeration of small crystals formed in a synthesis is dependent on the Al\({}_{2}\)O\({}_{3}\) content and the alkalinity, that is, the small crystals formed in low alumina content may agglomerate significantly, as the alkalinity of the mixture is high. As the Al\({}_{2}\)O\({}_{3}\) increases, the crystal size and reaction time decreases; therefore, the Al provides a site of nucleation and so the nucleation  
Figure 7: XRD patterns of products synthesized at different crystallization temperatures. Reproduced from [91]. Copyright 2013 Elsevier. accelerates with increase in alumina content. The elevated alkalinity values make difficult the incorporation of tetrahedrally coordinated Si in the framework because of the high solubility in this condition [99]. This can change the Si/Al ratio in the crystallized material and even affect performance during its application. The Si/Al ratio is a fundamental parameter to determine thermal and acid stability, and affects both the crystallization behavior and application performance. The alkalinity may generate a structure of low density, with value above 0.82, and the aluminosilicate layers tend to aggregate, forming another phase. The reduction in OH\({}^{-}\) concentration in the synthesis medium leads to lower monotonic zeolite growth and crystallinity [100].