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Introduction to the development history of molecular sieves

Introduction to the development history of molecular sieves



There are two types of molecular sieves: natural zeolite and synthetic zeolite. ① Most natural zeolites are formed by the reaction of volcanic tuff and tuffaceous sedimentary rocks in marine or lacustrine environments. At present, more than 1,000 kinds of zeolite minerals have been discovered, of which 35 are more important. The common ones 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 a country where natural zeolite is mined. ②Since natural zeolite is restricted by resources, synthetic zeolite has been widely used since the 1950s. As early as more than 200 years ago, B. Kronstedt named aluminosilicate as zeolite. The general formula of chemical composition is the formula where M and n are metal ions and their valences; x is the number of silicon dioxide molecules. ; Y is the number of water molecules; p is the number of aluminum atoms; q is the number of silicon atoms. Molecular sieves are used as solid adsorbents in the chemical industry. The adsorbed substances can be desorbed, and molecular sieves can be regenerated after use. It is also used for the drying, purification, separation and recovery of gases and liquids. Since the 1960s, it has been used as a cracking catalyst in the petroleum refining industry. Now a variety of molecular sieve catalysts suitable for different catalytic processes have been developed.




What are the concept of molecular sieve and its basic information



Molecular sieves are crystalline silicates or aluminosilicates, which are formed by connecting silicon-oxygen tetrahedra or aluminum-oxygen tetrahedra through oxygen bridges.

   A system of pores and cavities with molecular size (usually 0.3~2.0 nm), which has the characteristic of sieving molecules. However, with the in-depth research on the synthesis and application of molecular sieves, researchers have discovered phosphoaluminate molecular sieves, and the framework elements (silicon or aluminum or phosphorus) of molecular sieves can also be composed of B, Ga, Fe, Cr, Ge, Ti, V , Mn, Co, Zn, Be, and Cu, etc., and the size of the pores and cavities can reach 2 nm or more. Therefore, molecular sieves can be divided into silicon-aluminum molecular sieves, phosphorus-aluminum molecular sieves and framework heteroatom molecular sieves according to the composition of the framework elements. ; According to the size of the pores, molecular sieves with a pore size of less than 2 nm, 2~50 nm and greater than 50 nm are called microporous, mesoporous and macroporous molecular sieves, respectively. Due to its large pore size, it becomes a good carrier for large-size molecular reactions, but the pore wall of the mesoporous material is amorphous, so that its hydrothermal and thermal stability cannot meet the harsh conditions required by petrochemical applications. .

Because it contains metal ions with a low electricity price and a large ion radius and water in a combined state, water molecules are continuously lost after heating, but the crystal skeleton structure remains unchanged, forming many cavities of the same size, and the cavities have many diameters The same micropores are connected. These tiny pores have uniform diameters, which can adsorb molecules smaller than the pore diameter into the inside of the pores, while repelling molecules larger than the pores, so that molecules with different shapes and diameters can be excluded. Molecules with different degrees of polarity, molecules with different boiling points, and molecules with different degrees of saturation are separated, that is, they have the function of "sieving" molecules, so they are called molecular sieves. At present, molecular sieves are widely used in metallurgy, chemical, electronics, petrochemical, natural gas and other industries.