Zeolite molecular sieve structure and characteristics of knowledge

Zeolite molecular sieves are crystalline silicates or aluminosilicates that are formed by connecting a silicon tetrahedron or an aluminoxy tetrahedron through an oxygen bridge. The pore size and cavity system of molecular size (usually 0.3~2.0 nm) has the characteristics of sieving molecules. However, with the deepening of zeolite molecular sieve synthesis and application research, researchers have discovered aluminosilicate zeolite molecular sieves, and the framework elements of the zeolite molecular sieve (silicon or aluminum or phosphorus) can also be B, Ga, Fe, Cr, Ge, Substitutions of Ti, V, Mn, Co, Zn, Be, and Cu can also achieve pore sizes and cavities greater than 2 nm. Zeolite molecular sieves can therefore be divided into aluminosilicate molecular sieves and aluminophosphate zeolites by the framework element composition. Zeolite and framework heteroatoms Zeolites are classified as micropores, mesopores, and large pore zeolites by the pore size of pores smaller than 2 nm, 2 to 50 nm, and greater than 50 nm. Because of its large pore size, it becomes a good carrier for large-scale molecular reactions, but the pore wall of the mesoporous material is amorphous, making its hydrothermal and thermal stability unable to meet the harsh conditions required for petrochemical applications. . The
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Zeolite molecular sieves contain metal ions with lower ionic valence and larger ionic radius and combined water. The water molecules lose continuously after heating, but the crystal skeleton structure does not change, forming many cavities of the same size. Many micro-holes with the same diameter are connected. These tiny holes have uniform diameters. They can adsorb molecules smaller than the diameter of the pores into the interior of the pores, and they can exclude molecules larger than the pores, so they can vary in shape and diameter. Molecules, molecules with different polarities, molecules with different boiling points, and molecules with different degrees of saturation are separated. They have the function of "sieving" molecules and are called zeolite molecular sieves. At present, zeolite molecular sieves are widely used in metallurgy, chemical industry, electronics, petrochemical, natural gas and other industries.
Zeolite molecular sieves are natural zeolites and synthetic zeolites. 1 Natural zeolites are mostly formed by the reaction of volcanic tuff and tuffaceous sedimentary rocks in the marine or lacustrine environment. At present, more than 1,000 zeolite minerals have been discovered, 35 of which are more important. Commonly known are clinoptilolite, mordenite, erionite and chabazite. Mainly distributed in the United States, Japan, France and other countries, China has also found a large number of mordenite and clinoptilolite deposits, Japan is the largest natural zeolite extraction zeolite molecular sieve countries. 2 Because of the resource constraints of natural zeolites, a large number of synthetic zeolites have been used since the 1950s (see table). The
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Commercial zeolite molecular sieves are commonly used to prefix zeolite molecular sieves with different crystal structures, such as 3A, 4A, and 5A zeolite molecular sieves. Type 4A is shown in Table A and has an aperture of 4A. The Na+-containing zeolite A molecular sieve is designated as Na-A. If Na+ is replaced by K+, the pore size is about 3A; ie, 3A zeolite molecular sieve; for example, Na+ is replaced with Ca2+ by more than 1/3 of Na+. About 5A; that is, 5A zeolite molecular sieve.