The focus of the review is on zeolite characterization, synthesis, and applications. Zeolite is a hydrated aluminosilicate with a tetrahedral structural framework that contains exchangeable active metal ions and water molecules in channels and cages. Different synthesis methods, including hydrothermal and green synthesis methods, were used to make zeolite. 

The results of the characterization demonstrate that zeolite has a number of distinct properties, including uniform pore size, acidic qualities, thermal stability, mobile extra cation, hydrophilicity, and hydrophobicity. Catalysis, water purification, adsorption, and agriculture are only a few of the applications.

Zeolites: 

Zeolites are hydrated aluminosilicates of alkaline or alkaline earth metals that are crystalline, microporous, and hydrated. The frameworks are made up of corner-shared SiO4 and AlO4 tetrahedra that form various open structures. The tetrahedra are coupled together to form cages that are connected by pore holes of varying sizes. The pore sizes vary depending on the structural type. The positive charge of cations within the material’s pores balances out the negative charge on the lattice. These are mainly univalent and bivalent metals, or a mixture, in basic zeolites.

Zeolite Synthesis: 

Because of the purity of crystalline products and particle size consistency, synthetic zeolites are employed commercially more frequently than natural zeolites. Standard chemical reagents were used to make the first zeolites. Natural zeolites were used for much of the basic zeolite science research. Synthetic zeolites have several advantages over naturally occurring zeolites, including the ability to construct a wide range of chemical characteristics and pore sizes, as well as higher heat stability.

The hydrothermal crystallization of aluminosilicate gels or solutions in a basic environment is used to make zeolite. Crystallization takes place in a closed hydrothermal system at various temperatures, autogenous pressures, and times (a few hours to several days).

Synthetic zeolites: 

Many studies have been conducted with the goal of producing zeolites from low-cost silica-alumina sources. Under hydrothermal circumstances, zeolites are frequently produced in an alkaline phase.

Fly ash and kaolinite are two sources of silica alumina. Zeolites are comparable to clay minerals in terms of composition (kaolinite). They’re both aluminosilicates. Their crystalline structure, on the other hand, is different. Kaolinite has a poor exchange capacity and a mineral layer charge. It has a small surface area and has a low absorption capability. However, careful treatment can improve kaolin’s characteristics.

Methods of synthesizing: 

1. Hydrothermal synthesis: This method is the most widely utilized. This technique closely resembles the natural conditions under which zeolite-bearing rocks evolved. The supply of components that are a source of Si and Al, followed by treatment with an alkaline solution (pH > 8.5), is required for hydrothermal (80–350°C) synthesis of zeolites. The reactions that take place in autoclaves, including dissolution, condensation, gelatinization, and crystallization, are frequently carried out at high pressure. The production of desired products is aided by proper control of process parameters.

The cost of zeolite material generated using the methods described above is anticipated to be somewhere between natural and synthetic zeolite. However, given the fact that rates for garbage storage and disposal are likely to rise, the installation of one is unlikely.

2. Seeding technique in Secondary Growth process: The deposition of zeolite crystals seed on the support is the first stage in the secondary hydrothermal growth process before hydrothermal synthesis. The seed crystals’ properties, such as crystal size, thickness, density, and seed layer consciousness, are critical parameters that influence the quality and performance of the produced zeolite membrane. To obtain compact and high coverage of the zeolite seed layer, the seed crystals should be homogeneously spread onto the support. To develop a continuous zeolite membrane layer with few defects, homogeneous seed coverage is critical. 

As mentioned in the previous section, a variety of seeding methods have been used in the manufacture of zeolite membranes. Seeding is a key step in the fabrication of zeolite membranes because it gives benefits such as crystal development on the support rather than in the solution and avoidance of nucleus transition into undesired zeolite phases.

Factors affecting crystallization product: 

The following factors influence the crystalline zeolite structure-

• Composition of the reaction mixture.
• Nature of reactants and their pretreatments.
• Initial and final pH of the system.
• The temperature of the process and its variation with time.
• Time allowed for the reaction to take place, including the calcination time. 
• The mixture, whether homogeneous or heterogeneous.
• Seeding.
• Template molecules.

Benefits of synthetic zeolite: 

• Synthetic zeolites have a substantially bigger pore size than wild zeolites. This allows for the adsorption of bigger molecules, broadening the application possibilities. Synthetic zeolites, for example, have two times the oil sorption capacity of natural clinoptilolite, making them a prospective option for natural mineral sorbents in the cleanup of land-based petroleum spills.
• Furthermore, natural zeolites with smaller pore diameters suffer from pore obstruction, poisoning, and deactivation when used as catalysts, whereas manufactured zeolites with huge interconnecting channels remain stable in reactions for a considerably longer period.

Conclusion: 

To summarise, zeolite synthesis processes are influenced by a variety of parameters, including the composition of precursors, reaction pH, temperature, pre-treatment of precursors, seeding time, reaction time, and template employed. However, using a two-step crystallization approach in hierarchical porous zeolite synthesis is promising; combining hydrothermal crystallization with microwave heating is another synthesis method that produces smaller, more uniform particles in less time.