## 2 Zeolites
### Synthesis Methods
#### 2.9.5 Microwave-Assisted Synthesis  
The process of obtaining a porous material from the use of microwaves brings numerous unique synthesis characteristics. Among them is the reduction in process time to develop and the production of products with a distribution of sizes and pores more uniform. These characteristics can extend both to the nucleation step and to the growth of formed crystals. In obtaining zeolites, the process occurs for a great range of Si/Al ratios, highlighting the possibility of incorporating metals in zeolite structures [172; 173]. Studies always emphasize that improvement in the synthesis of nanoporous materials by microwaves may be due to the following factors: heating by radiation increases the rate of temperature rise of the medium; radiation leads to greater uniformity of heating in the synthesis medium; there is a change in association between species in the medium; hot spots are created in the reaction mixture; and radiation enables the ideal dissolution of the precursor gel [173; 174]. Empirically, there have been some findings in obtaining zeolites. The beginning of the application of this method is in its synthesis, and a patent from the company Mobil verified that the crystallization processes can be similar to the use of the traditional method of synthesis. Furthermore, the particle size distribution of the product is greater than that for the use of a hydrothermal method [175].  
Other developments in the method lie in obtaining zeolite A after 12 min of synthesis, studying the effect of digestion on the formation of this structure [176]. Synthesis of fauja-site zeolites is capable of being completed in a short period of time at temperatures close to 150 \({}^{\circ}\)C. Faujasites with high Si content can be obtained by assisting the use of microwaves with optimal characteristics for use in catalysis, such as high selectivity, activity, and stability, at a reduced reaction time from 48 h to 30 min compared to the conventional process [175]. In obtaining the Y type, the advantages of the method lie in maintaining the characteristics of those structures obtained industrially. The evolution of the products obtained as temperature advances can be amorphous phase, zeolite Y, and zeolite P2. Moreover, among the phenomena that can explain the synthesis process in this method, the interaction that water molecules promote with radiation and dispersion of the aluminosilicate gel in the medium should be noted [177; 178].  
Studies show that microwave-assisted synthesis can be used in the so-called dry synthesis and in the method of growth on surfaces (membranes), both efficiently [175]. In obtaining zeolitic membranes, three strategies can be combined with the use of microwaves: in situ hydrothermal synthesis, secondary growth synthesis, and vapor transport synthesis. Among the zeolite structures obtained in the form of membranes by these methods assisted with microwaves are LTA, MFI, AFI, FAU, SOD and ETS-4 [174]. In addition to these, MOF zeolitic structures can be obtained by the microwave method and solvothermal synthesis [179]. Some authors demonstrated that the synthesis of NaP zeolite from NaY gel can occur with the use of microwaves, generating pure products that can go through a recrystallization process, and the synthesis can be composed of crystallization and rapid cooling in the microwave chamber, with wave digestion at room temperature and crystallization at a temperature higher than the first step, since the temperature reached in the system allowed the network density to vary [180]. Figure 12 shows differences in zeolite morphology obtained by a microwave-assisted method.