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What is molecular sieve?

What is molecular sieve?
A synthetic hydrated aluminosilicate (zeolite) or natural zeolite with the function of screening molecules. Its general chemical formula is (M'2M)O·Al2O3·xSiO2·yH2O, and M' and M are monovalent and divalent cations such as K+, Na+, Ca2+, Ba2+, etc. respectively. It has many channels with uniform pore size and neatly arranged pores in its structure, and molecular sieves with different pore sizes separate molecules of different sizes and shapes. According to the different molecular ratios of SiO2 and Al2O3, molecular sieves with different pore sizes are obtained. Its models are: 3A (potassium A type), 4A (sodium A type), 5A (calcium A type), 10Z (calcium Z type), 13Z (sodium Z type), Y (sodium Y type), sodium mordenite type Wait. It has high adsorption capacity, strong selectivity and high temperature resistance. It is widely used in organic chemical industry and petrochemical industry, and it is also an excellent adsorbent for gas dehydration. It has also received increasing attention on exhaust gas purification.
What are molecular sieves used for?
Applications in the field of drying and purification
(1) Dehydration. Using the polar hydrophilicity of zeolite molecular sieves with low silicon-alumina ratio (such as A-type, X-type, etc.), air drying can be carried out. In addition, in recent years, the incorporation of ethanol into gasoline to replace part of gasoline has received extensive attention. As a fuel, ethanol requires a water content of less than 0.8%. However, due to the azeotropy of ethanol and water, only 95% of ethanol can be obtained through rectification. , for ethanol dehydration with low water content, adsorption dehydration by zeolite molecular sieve is the best choice.
(2) Purify pollutants in the air. With the rapid development of industry, the emissions of H2S, SO2, NOX and formaldehyde are increasing day by day, and the resulting pollution has brought serious harm to people's lives and the environment.
Application in the field of adsorption and separation
(1) Separation of mixed xylenes. Mixed xylene is generally used as a solvent and gasoline admixture to sell cheaply, and the waste of resources is very serious. But the four isomers of mixed xylene: ethylbenzene, para-xylene, meta-xylene and ortho-xylene are important chemical raw materials, so it is necessary to separate them one by one.
(2) Separation of N2/O2. In the pressure swing adsorption (PSA) method, the zeolite molecular sieve utilizes the difference in equilibrium adsorption of N2/O2 on its surface to selectively adsorb N2.
(3) Increase the octane number of gasoline. Since the octane number of isoparaffins is much higher than that of normal paraffins, normal paraffins can be removed by adsorption separation. In practical applications, adsorption separation is generally combined with C5/C6 alkane isomerization, and the normal alkanes separated by adsorption are isomerized, thereby increasing the octane number of gasoline to a greater extent.
Applications in the field of catalysis
Zeolite molecular sieve has a complex and changeable structure and a unique pore system, and is a catalyst with excellent performance. ZSM-5 is used in FCC reaction with Y-type zeolite molecular sieve to obtain higher yields of gasoline, propylene and butene.
How do molecular sieves remove water?
Molecular sieves are crystalline metal aluminosilicates having a threedimensional interconnecting network of silica and alumina tetrahedra. Natural water of hydration is removed from this network by heating to produce uniform cavities which selectively adsorb molecules of a specific size.
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