The working principle and types of molecular sieves
Adsorption function: The adsorption of molecular sieves to substances comes from physical adsorption (van der Waals force). The crystal pores have strong polarity and Coulomb field, and show strong adsorption capacity for polar molecules (such as water) and unsaturated molecules.
Sieving function: The pore size distribution of the molecular sieve is very uniform, and only substances with a molecular diameter smaller than the diameter of the pores can enter the crystal cavities of the molecular sieve.
The molecules of different substances are distinguished by the order of priority and size of adsorption, so they are vividly called "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. 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 discovered a large number of mordenite and clinoptilolite deposits, and Japan is the country with the largest amount of natural zeolite mining. ②Since natural zeolite is restricted by resources, synthetic zeolite has been widely used since the 1950s.
Commercial molecular sieves often use prefix numbers to classify molecular sieves with different crystal structures, such as 3A, 4A, and 5A molecular sieves. Type 4A is type A in the table, with a pore size of 4Å;. A molecular sieve containing Na+ is denoted as Na-A. If Na+ is replaced by K+, the pore size is about 3Å; that is 3A molecular sieve; if more than 1/3 of Na+ in Na-A is replaced by Ca2+, the pore size is about 5Å;, that is, 5A molecular sieve.
Molecular sieves are powdered crystals with metallic luster, hardness of 3～5, relative density of 2～2.8, natural zeolite has color, synthetic zeolite is white, insoluble in water, thermal stability and acid resistance increase with the composition ratio of SiO2/Al2O3 Increase and improve. Molecular sieve has a large specific surface area of 300～1000m2/g. The inner crystal surface is highly polarized. It is a kind of high-efficiency adsorbent and a kind of solid acid. The surface has a high acid concentration and acid strength, which can cause normal carbon. Ionic catalytic reaction. When the metal ions in the composition are exchanged with other ions in the solution, the pore size can be adjusted to change its adsorption properties and catalytic properties, thereby preparing molecular sieve catalysts with different properties.
The resulting proteoglycan polymer twists and turns to form a microporous sieve-like structure, which is called a molecular sieve. Molecular sieves only allow substances smaller than the pores of their micromolecular sieve to pass through, and have a barrier effect on macromolecular substances and bacteria that are larger than their pores. Make the matrix a defensive barrier to limit the spread of harmful substances such as bacteria. Streptococcus hemolyticus and cancer cells can produce hyaluronidase, break down proteoglycans, destroy the matrix structure, and spread. The proteoglycan polymer is also bound with many hydrophilic groups, which can bind a large number of water molecules to form an extracellular "reservoir".