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Zeolites

When some water-containing aluminosilicate mineral crystals are burned, because the water in the crystals is driven out, a phenomenon similar to boiling occurs, namely zeolite. In the structure of zeolite (A_(x/q) [ (AlO₂)_x (SiO₂)_y ] ·n(H₂O)), a represents metal ions, and ion A can be replaced by other ions, which will change its performance. The Al-Si (aluminum- silicon) framework in the structure is also an important basis for distinguishing different zeolites. Therefore, the performance of the zeolite can be improved by adjusting the structure of metal ions and Al-Si framework, such as improving the acid resistance and thermal stability of the zeolite. As shown in Figure 1, the common vertices of silicon-oxygen tetrahedron and aluminum-oxygen tetrahedron form various forms of skeletons, such as A-type and Y-type.

Zeolites Figure 1. Al-Si framework structural.

Applications:

Zeolite has the advantages of uniform pore size and large internal surface area, which has been widely used in catalysis, separation, adsorption and other fields. Zeolite molecular sieve has good adsorption performance, ion exchange performance and catalytic performance.

Application in the field of drying and essence: Zeolites with a low silicon-aluminum ratio have good polar hydrophilicity, and can be used as a dehydrating agent in the reaction or as an air desiccant. Zeolite can use its large surface area inner wall to absorb toxic gases in the air, such as H₂S、SO₂、NO_X, to achieve the purpose of air purification.
Application in the field of adsorption separation: When the diameter of the adsorbed molecule is smaller than the pore diameter of the zeolite, the adsorbed molecule is easy to enter the crystal cavity and get adsorbed, and the zeolite has a uniform distribution channel. Therefore, zeolite can be used as a sieve for gas and liquid molecules. It is commonly used in industry to separate mixed xylene and N₂/ O₂ to increase the octane value of gasoline.
Applications in the field of catalysis: Zeolites have a unique regular crystal structure, each of which has a certain size and shape of the channel structure, and has a large specific surface area. Most zeolites have strong acid centers on the surface and strong polarization in the pores. These characteristics make it an excellent catalyst.

Classification:

Zeolite can be simply divided into natural zeolite and artificial zeolite. There are more than 80 kinds of zeolite minerals common in nature, such as mordenite, clinoptilolite, cristobalite and so on.

Production Processes:

Hydrothermal method: Many factors need to be considered in the hydrothermal synthesis method, such as the selection and ratio of various reaction raw materials, reaction temperature and reaction time. The combined chemical hydrothermal method has high synthetic efficiency and rapid screening.
Gas-phase transfer method: This method uses a large amount of templates, and the zeolite product needs to be separated from the mother liquor. It does not need solid liquid separation,and the liquid part can be reused, reducing the amount of template and cost. It is a kind of economic, environmental protection method for the preparation of zeolite.
Dry glue method: The zeolite synthesized by this method has the advantages of high yield, small amount of waste liquid, wide range of application and short crystallization time. The dry glue method separates the solid raw material from the liquid phase in the synthesis system, and the vapor after the liquid phase vaporization at high temperature interacts with the solid raw material to promote the molecular sieve crystallization process.

References:

Pavla Eliášová, Jiří Čejka. Two-dimensional Zeolites [J]. Zeolites in Catalysis: Properties and Applications., 2017.
Millini R., Bellussi G. Zeolite Science and Perspectives [J]. Zeolites in Catalysis: Properties and Applications, 2017.
Li Jiyang, Cormab Avelino, Yu Jihong. S Synthesis of new zeolite structures [J]. Chem. Soc. Rev., 2015, 44, 7112-7127.

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