What Is a Molecular Sieve?

A synthetic hydrated aluminosilicate (zeolite) or natural zeolite with the function of screening molecules. Its chemical formula is (M2M) O · Al2O3 · xSiO2 · yH2O, and M and M are monovalent and divalent cations such as K +, Na + and Ca22 +, Ba22 +, etc. It has many uniform pores and neatly arranged holes in the structure. Molecular sieves with different pore sizes separate molecules of different sizes and shapes. According to 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. Widely used in organic chemical industry and petrochemical industry, it is also an excellent adsorbent for gas dehydration. Attention has also been paid to exhaust gas purification. ^[1]

There is a natural aluminosilicate in nature, which has sieving molecules, adsorption,

According to catalytic properties, molecular sieve catalysts can be divided into the following points:

(1) An acid catalyst, which uses the surface acidity of a molecular sieve to catalyze a reaction.

(2) Dual function catalyst, molecular sieve can be supported

The wide application of zeolite molecular sieve materials (such as adsorption separation, ion exchange, catalysis) is inseparable from its structural characteristics. For example, the adsorption and separation performance 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 the cations in the molecular sieve and the passageability of the pores; the shape selectivity shown in the catalytic process and the size of the pores And orientation are related, while the intermediate products and final products in the catalytic reaction are related to the channel dimension of the molecular sieve or its cage structure. Therefore, the structure of molecular sieves is the basic problem for studying molecular sieve materials. ^[5]

In-depth research on the formation and growth mechanism of zeolite molecular sieves will help people to better design and synthesize new zeolite molecular sieve topologies, expand new routes for the synthesis of zeolite molecular sieve materials, and develop new properties and applications of zeolite molecular sieve materials. Although zeolite molecular sieves have been developed for many years, there has been no real conclusion on its synthetic mechanism. It is of great theoretical significance to study the crystallization mechanism of molecular sieves, as well as practical guidance for the synthesis of new zeolite molecular sieves. At present, the three most representative mechanisms are the solid hydrogel transformation mechanism, the solution-mediated transport mechanism, and the two-phase transformation mechanism. ^[5]

Hydrothermal synthesis is the most common and effective way in the synthesis of zeolite molecular sieves. The main difficulty in studying the hydrothermal synthesis of molecular sieves is that the mechanism of molecular sieve formation is not well understood. However, no matter which generation mechanism is used for the synthesis of zeolite molecular sieves, the crystallization process must go through the same basic steps: repolymerization of polysilicate and aluminate, nucleation of molecular sieve, nuclear growth, and growth of molecular sieve crystals. And the resulting secondary nucleation. In order to control and tune the synthesis reaction of zeolite molecular sieve well, the most important thing is to study the influence of reaction conditions on the synthesis reaction. According to many years of practical experience, the following influencing factors play an important role in the synthesis of zeolite molecular sieves, mainly including: the composition of the reactants, the ratio of silicon to aluminum,

In recent years, zeolite molecular sieves have been widely used in adsorption separation, catalysis and other fields due to their unique properties. However, the performance advantages and disadvantages of some zeolite molecular sieves are not sufficiently understood, and some are even superficial. In order to more effectively utilize the advantages of zeolite molecular sieves in the field of adsorption separation and catalysis, we must pay attention to the following aspects: 1) Develop low-priced zeolite molecular sieves with the goal of reducing production costs; 2) study the effects of synthesis and modification of zeolite molecular sieves on structure, composition, and performance, and find ways to increase their adsorption capacity and selectivity; 3) establish and improve evaluation Quantitative indicators of adsorbent performance can better guide actual production. ^[4]