1. General Information of zeolite molecular sieve
Zeolite molecular sieve is 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. M' and M are monovalent and divalent cations such as K+, Na+ and Ca2+, Ba2+, etc., respectively. It has many pores with uniform pore size and neatly arranged pores in its structure. 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 can be obtained. The general type of zeolite molecular sieves are 3A (potassium A type), 4A (sodium A type), 5A (calcium A type), 10X (calcium Z type), 13X (sodium X type), Y (sodium Y type), sodium mordenite type etc.. It has high adsorption capacity, strong selectivity and high temperature resistance. It is widely used in organic chemical industry and petrochemical industry and is also an excellent adsorbent for gas dehydration. Increasing attention has also been paid to exhaust gas purification.
The general formula of the chemical composition of molecular sieve is: (M)2/nO·Al2O3·xSiO2·pH2O, M stands for metal ion (usually Na in artificial synthesis), n stands for metal ion valence, x stands for moles of SiO2, also called Is the ratio of silicon to aluminum, and p represents the number of moles of water. The most basic structure of the molecular sieve framework is SiO4 and AlO4 tetrahedrons, which form a three-dimensional network structure of crystals through the combination of shared oxygen atoms. This combination forms cavities and pores with molecular level and uniform pore size. Due to different structures and different forms, the "cage"-shaped space pores are divided into "cage" structures such as α, β, γ, hexagonal columns, and faujasite. The crystal structures of A-type, X-type and Y-type molecular sieves are shown in Figure 1 (The main structure of type A molecular sieve), Figure 2 (The main structure of X-type and Y-type molecular sieves).
Since the AlO4 tetrahedron has a negative charge, it can combine with sodium ions to become electrically neutral. In an aqueous solution, Na can easily exchange with other cations. Most molecular sieve catalysts are exchanges of multivalent metal cations or H. Molecular sieves have acidity and selectivity to molecular size, and can be used as catalysts or carriers. High-silica zeolite exhibits a high affinity for organic groups. In contrast, low-silica zeolite exhibits hydrophilicity due to its Lewis and Bronsted acid properties. Silicon and aluminum atoms form an oxygen ring through oxygen, and the size of the oxygen ring determines the pore diameter of the zeolite. The number of oxygen atoms in each oxygen ring is 4-12. Generally, there are eight-membered ring (0.4-0.5nm), ten-membered ring (0.5-0.6nm) and twelve-membered ring (0.7-0.9nm) with molecular sieve effect.
There are Y-type molecular sieves (x=3.1～6.0) and mordenite (x=9～11) with twelve-membered oxygen ring. The former can be used as a cracking catalyst and a dual-function catalyst, and the latter can be used as a disproportionation catalyst for toluene. There are some ZSM series molecular sieves such as ZSM-5 and ZSM-11 with ten-membered oxygen ring.
The eight-membered oxygen ring includes A-type molecular sieve (x=2), T-type molecular sieve and ZSM-34. Their pores are very small, and only straight-chain hydrocarbons can enter the pores. The catalyst with molecular sieve as the catalytically active component or main active component is called molecular sieve catalyst. Molecular sieves have ion exchange performance, uniform molecular size pores, excellent acid catalytic activity, and good thermal and hydrothermal stability. It can be made into a catalyst with high activity and high selectivity for many reactions.
2. The characteristics of zeolite molecular sieve
2.1 Adsorption
The adsorption characteristic of zeolite molecular sieve is a physical change process. The main reason for adsorption is a kind of "surface force" produced by molecular gravity acting on the solid surface. When the fluid flows through, some molecules in the fluid collide with the surface of the adsorbent due to irregular motions, causing molecular concentration on the surface. Reduce the number of such molecules in the fluid to achieve the purpose of separation and removal.
Since there is no chemical change in adsorption, as long as we try to drive away the molecules concentrated on the surface, the zeolite molecular sieve will have adsorption capacity again. This process is the reverse process of adsorption, called analysis or regeneration.
Since the zeolite molecular sieve has a uniform pore size, only when the molecular dynamics diameter is smaller than the zeolite molecular sieve can it easily enter the inside of the crystal cavity and be adsorbed. Therefore, the zeolite molecular sieve is like a sieve for gas and liquid molecules.
Since the zeolite molecular sieve has a strong polarity in the crystalline cavity, it can have a strong effect on the surface of the zeolite molecular sieve with molecules containing polar groups, or by inducing the polarization of the polarizable molecules to produce strong adsorption.
This kind of polar or easily polarized molecules is easy to be adsorbed by polar zeolite molecular sieve, which reflects another adsorption selectivity of zeolite molecular sieve.
2.2 Ion exchange
Generally speaking, ion exchange refers to the exchange of compensation cations outside the framework of the zeolite molecular sieve. The compensation ions outside the framework of the zeolite molecular sieve are generally protons and alkali metals or alkaline earth metals, which are easily ion-exchanged into various valence metal ion-type zeolite molecular sieves in the aqueous solution of metal salts.
Ions are easier to migrate under certain conditions, such as aqueous solutions or higher temperatures. In aqueous solution, due to the different ion selectivity of zeolite molecular sieves, different ion exchange properties can be exhibited. The hydrothermal ion exchange reaction between metal cations and zeolite molecular sieves is a free diffusion process. The diffusion rate restricts the exchange reaction rate.
Through ion exchange, the pore size of the zeolite molecular sieve can be changed, thereby changing its performance, and achieving the purpose of shape-selective adsorption and separation of the mixture.
After ion exchange of zeolite molecular sieve, the number, size and position of cations change. For example, the exchange of high-valent cations for low-valent cations reduces the number of cations in the zeolite molecular sieve, which often results in vacancies in the zeolite molecular sieve and enlarges its pore size; and ions with a larger radius After the ions with a smaller radius are exchanged, the pores are easily blocked, and the effective pore diameter is reduced.
2.3 Catalytic performance
Zeolite molecular sieves have a unique regular crystal structure, each of which has a pore structure of a certain size and shape and has a large specific surface area. Most zeolite molecular sieves have strong acid centers on the surface, and there is a strong Coulomb field in the crystal pores for polarization. These characteristics make it an excellent catalyst.
Heterogeneous catalytic reactions are carried out on solid catalysts, and the catalytic activity is related to the size of the crystal pores of the catalyst. When a zeolite molecular sieve is used as a catalyst or a catalyst carrier, the progress of the catalytic reaction is controlled by the pore size of the zeolite molecular sieve. The size and shape of the crystal pores and pores can play a selective role in the catalytic reaction. Under general reaction conditions, zeolite molecular sieves play a leading role in the reaction direction and exhibit shape-selective catalytic performance. This performance makes zeolite molecular sieves a new catalytic material with strong vitality.
3. Application of zeolite molecular sieve
3.1 Application in dehydration and purification
a. Gas 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.
b. 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.
3.2 Application in the field of adsorption and separation
a. 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.
b. 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 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.
c. 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 alkanes 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.
3.3 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 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 acts as a liquid-phase alkylation catalyst to catalyze the reaction of benzene and ethylene to produce ethylbenzene, which not only improves the selectivity of ethylbenzene, but also MCM-22 itself 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 in two aspects: butene skeletal isomerization and n-heptane isomerization.
The wide application of zeolite molecular sieve materials (for example: adsorption separation, ion exchange, catalysis) is inseparable from its structural characteristics. For example, the performance of adsorption and separation depends on the size of the pores and pore volume of the molecular sieve; the ion exchange performance depends on the number and position of cations in the molecular sieve and the permeability of the pores; the shape selectivity shown in the catalytic process and the size of the pores of the molecular sieve , The trend is related, and the intermediate product and final product in the catalytic reaction are related to the pore dimension of the molecular sieve or its cage structure. Therefore, the structure of molecular sieves is a basic problem in the study of molecular sieve materials.
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