Structure of molecular sieve membrane

Zeolite molecular sieves are widely used because they have excellent properties. In order to understand these properties deeply, it is necessary to clarify the structure of molecular sieves, and to study the structure and properties of molecular sieves in depth will in turn further promote its application and development. Molecular sieves are crystalline aluminosilicates, so x-ray diffraction can be used for structural analysis. Generally, synthetic molecular sieves are powders below 104. When testing with powder diffraction, it is very difficult to index and collect strength for zeolite types with poor symmetry. So far, only more than forty have been determined. About half of the zeolite structure has not been determined.

If larger single crystals of the Buddhist stone can be obtained, the single crystal rotation method using x-ray diffraction is more effective. Larger A-type molecular sieve single crystals can be obtained by recrystallization of seed crystals. Using the single crystal rotation method of x-ray diffraction, not only the unit cell parameters can be derived to determine the skeleton structure before indexing, but also non-framework atoms (or ions) and molecules and positions can be estimated. Each molecular sieve has a characteristic x-ray powder diffraction pattern, so by comparing the diffraction patterns, the type of zeolite can be determined. Amorphous gels do not generate diffraction, so x-ray analysis can also be used to observe the crystallization of molecular sieves. The diffraction pattern of the mixture is formed by superposing the powder lines of each component, and the diffraction intensity varies with the content. . So x-ray radiation is also used to determine the purity of molecular sieves.

There is now a new infrared spectroscopy method to determine the structure of molecular sieves. By measuring the infrared spectrum of a molecular sieve of known structure, the relationship between the characteristic frequency of the general band and the skeleton structure groups is found, and then the spectrum of the unknown structure is measured to derive the skeleton structure. Generally use infrared frequency of 1300-200 cm. Because this range contains (Si, Al) 0, the basic vibration of the tetrahedron, reflecting the characteristics of the skeleton structure. At present, infrared spectroscopy has been used to determine the structure type of zeolite framework, the structure groups, the silicon-aluminum composition of the framework, the structural changes during thermal decomposition, and the migration of cations during dehydration and dehydroxylation.

In the application of molecular sieves, surface properties are very important. With the help of infrared spectroscopy, we can more thoroughly understand the surface properties of zeolites and the changes in various treatments. For example, based on the spectral changes caused by the adsorption molecules, we can know the interaction between the zeolite surface and the adsorption molecules, the position of the adsorption molecules, and the catalytic activity And surface properties. Due to the high sensitivity of the infrared spectrum, small changes in the structure of the zeolite are reflected in the spectrum. Other physical testing techniques such as ultraviolet spectroscopy can also help determine the structure of the molecular sieve, but x-ray diffraction and infrared spectroscopy are mainly used at present. .

Zeolite A, zeolite x, zeolite Y, and mordenite are the most widely used, and their structure and performance are also the most deeply studied.

The first section is an overview of the molecular sieve structure. Molecular sieves are a type of aluminosilicate crystals with a skeletal structure. The cations and water molecules in the crystals have a large degree of freedom of movement in the skeletal structure, which can undergo cation exchange and reversibly dehydrate. The chemical composition of the molecular sieve can be expressed by the following experimental formula: M2 / n0.Al203.xSiO2 yH2O M is a metal ion n is the valence of M is the number of molecules of Si02, also SiO2 / Al203; the molar ratio. Y is the number of water molecules. The above formula can be rewritten as Mpinl (AIO2), p () ql yH2O P is the number of AI0 molecules and q is the number of SiO2 molecules. Mny can be seen from the above formula: each aluminum atom and silicon atom have two oxygen on average Atoms: If the valence of metal atom M is n = 1, the number of atoms of M is equal to the number of aluminum atoms. If n = 2, then the number of atoms of M is equal to half the number of aluminum atoms. each

The difference between these molecular sieves is the first difference in chemical composition. For example, M in the empirical formula can be metal ions such as Na, K, Li, Ca, Mg, or organic amines or composite ions. An important difference in chemical composition is the different molar ratio of silicon to aluminum. For example, the silica to alumina ratios of zeolite A, zeolite x, zeolite Y, and mordenite are 1.5-2, 2.1-3.0, 3.1-6.0, and 9-11, respectively. When the x value in the formula is different, the acid resistance, thermal stability and catalytic activity of the molecular sieve are different. The larger the general value, the higher the acidity and thermal stability. The most fundamental difference between various molecular sieves is the difference in crystal structure, so different molecular sieves have different properties.