Application of Zeolite Molecular Sieve
Application in the field of drying and purification
(1) Dehydration. Using the polar hydrophilicity of zeolite molecular sieves with low silicon-to-aluminum ratio (such as type A, type X, etc.), air drying can be carried out. In addition, in recent years, the blending of ethanol into gasoline to replace part of gasoline has received widespread attention. The water content of ethanol as a fuel requires less than 0.8%. However, due to the azeotrope of ethanol and water, only 95% of ethanol can be obtained through rectification. For the dehydration of ethanol with lower water content, zeolite molecular sieve adsorption dehydration is the best choice.
The zeolite molecular sieve used in this method is of type A or X, and type KA is the best. On the one hand, the polarity of type A zeolite molecular sieve is used. On the other hand, since the pore diameter of KA zeolite molecular sieve is about 0.3nm, water molecules can be free. Enter, and ethanol molecules with a diameter greater than 0.3nm cannot enter the pores of the zeolite molecular sieve. This zeolite molecular sieve dehydration process is the preferred process for industrial production of fuel ethanol.
(2) Purify pollutants in the air. With the rapid development of industry, the emissions of H2S, SO2, NOX and formaldehyde are increasing, and the pollution caused 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 blending agent and sold at low prices, which is a serious waste of resources. But the four isomers of mixed xylene: ethylbenzene, p-xylene, meta-xylene and o-xylene are all important chemical raw materials, so it is necessary to separate them one by one.
There are many separation methods for mixed xylenes, such as rectification, precision rectification, pressure crystallization, cryogenic crystallization, etc., which are traditional separation methods, but their common shortcomings are high energy consumption, huge equipment, and high operating requirements. .
The adsorption separation method is an efficient separation method, the key to which is the preparation of the adsorbent. Due to the special structure of zeolite molecular sieve and the diversification of its types, using zeolite molecular sieve as adsorbent to separate mixed xylene has a good application prospect
(2) Separation of N2/O2. In the pressure swing adsorption (PSA) method, zeolite molecular sieves use the difference in equilibrium adsorption of N2/O2 on their surface to selectively adsorb N2. Because of the higher polarizability of N2, the interaction between N2 and the cations in the zeolite molecular sieve and its polar surface is stronger than that of O2. The LiA zeolite molecular sieve has a higher N2/O2 selective ratio and N2 adsorption capacity, but its thermal stability is poor. Therefore, the A-type zeolite molecular sieve after Li+ and alkaline earth metal mixed cation exchange has a higher N2/O2 selective separation coefficient, N2 adsorption capacity and higher thermal stability. In addition, the X-type zeolite molecular sieve with a low silicon-to-aluminum ratio has attracted people's attention. People have carried out various ion exchanges, and its N2/O2 separation selectivity is high and its thermal stability is good.
(3) Increase gasoline octane number. Since the octane number of isoparaffins is much higher than that of normal paraffins, the adsorption separation method can be used to remove normal paraffins. In practical applications, adsorption separation is generally combined with C5/C6 alkane isomerization to isomerize the normal paraffins separated by adsorption, thereby increasing the octane number of gasoline to a greater extent. When the sodium ions in the A-type zeolite molecular sieve are exchanged by calcium ions for more than 40%, its effective pore size can be increased to 0.5nm, which can meet the requirements of this separation. During the separation, the hydrocarbon mixture passes through the adsorption bed and the normal alkane is The molecular size is smaller than the pore size of the zeolite molecular sieve and can freely enter the pores and be adsorbed. If the molecular size of isoparaffin is larger, it cannot enter, and the outflow of the adsorption bed is a material rich in isoparaffin and high octane. After the adsorption bed is saturated with adsorption, the normal paraffin is desorbed by a desorbent and sent to the isomerization reaction. [4]
Application in the field of catalysis
Zeolite molecular sieve has a complex and changeable structure and unique pore system, which is a kind of catalyst with excellent performance. ZSM-5 and Y-type zeolite molecular sieve are used together in FCC reaction to obtain higher yield of gasoline, propylene and butene. MCM-22 zeolite molecular sieve has significant advantages in the alkylation reaction. For example, MCM-22 is used as a liquid-phase alkylation catalyst to catalyze the reaction of benzene and ethylene to produce ethylbenzene. With high stability and low dosage, it can be regenerated in-situ in the reactor, while other types of catalysts must be taken out of the reactor to be regenerated. In the synthesis reaction of short-chain alkyl substituted aromatics, MCM-56 has better activity and is not easy to deactivate. ZSM-22 is used as a catalyst in many processes, but it is mainly used for two aspects: butene skeletal isomerization and n-heptane isomerization.
The main applications of molecular sieves as catalysts are shown in the following table.
|
Catalytic process |
Type of catalyst |
Example of catalyst |
|
Petroleum catalytic cracking |
solid acid |
rare earth-Y molecular sieve, ultra-stable-Y molecular sieve |
|
Hydrocracking |
dual-function |
palladium-ultra-stable Y-type molecular sieve |
|
|
|
molybdenum-nickel ultra-stable Y-type molecular sieve |
|
C5 and C6 alkane isomerization |
dual function |
platinum-mordenite |
|
Diesel or lubricating oil with hydrogen and condensation reduction |
dual function,shape selective |
Palladium-mordenite, palladium-zinc-ZSM-5 molecular sieve |
|
Selective reforming |
dual function,shape selective |
Palladium-erionite, chabazite |
|
Xylene Isomerization |
dual function,shape selective |
platinum-mordenite |
|
Toluene disproportionation |
solid acid, shape selective |
Mordenite, ZSM-5 molecular sieve |
|
Alkylation (synthesis of ethylbenzene) |
solid acid, shape selective |
ZSM-5 molecular sieve |
|
Methanol synthesis gasoline |
solid acid, shape selective |
ZSM-5 molecular sieve |
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