Characteristics and types of molecular sieves.
Molecular sieves have extremely strong moisture absorption capacity (so they are widely used as desiccants). For gas purification treatment, they should be avoided from being directly exposed to the air during storage. Molecular sieves that have been stored for a long time and have absorbed moisture should be regenerated before use. Molecular sieves are incompatible with oil and liquid water. When in use, contact with oil and liquid water should be avoided as much as possible. In industrial production, the gases that are dried include air, hydrogen, oxygen, nitrogen, argon, etc. Two adsorption dryers are connected in parallel. One is working while the other can be regenerated. They work and regenerate alternately to ensure continuous operation of the equipment. The dryer works at 8 - 12 °C and is regenerated by purging gas at 350 °C. The regeneration temperatures of different specifications of molecular sieves are slightly different. Molecular sieves have good catalytic effects on some organic gas-phase reactions.
Also known as zeolite or zeolite, it is a crystalline aluminosilicate. In its crystal structure, there are regular and uniform channels with pore diameters on the order of molecular size. It only allows molecules smaller than the pore diameter to enter. Therefore, it can sieve the molecules in a mixture according to their sizes. Hence it is called a molecular sieve. As early as more than 200 years ago, B. Cronstedt first named aluminosilicate as zeolite. The general chemical composition formula is: where M and n are metal ions and their valences; x is the number of molecules of silicon dioxide; y is the number of molecules of water; p is the number of atoms of aluminum; q is the number of atoms of silicon. In the chemical industry, molecular sieves are used as solid adsorbents. The substances adsorbed by them can be desorbed. Molecular sieves can be regenerated after use. They are also used for drying, purification, separation and recovery of gases and liquids. Since the 1960s, they have been used as cracking catalysts in the petroleum refining industry. Now, a variety of molecular sieve catalysts suitable for different catalytic processes have been developed.
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 a marine or lacustrine environment. At present, more than 1000 kinds of zeolite ores have been found. There are 35 relatively important ones. Commonly seen ones are clinoptilolite, mordenite, erionite and chabazite. They are mainly distributed in countries such as the United States, Japan and France. China has also found a large number of mordenite and clinoptilolite deposits. Japan is the country with the largest exploitation of natural zeolites.
2. Due to the resource limitations of natural zeolites, since the 1950s, a large number of synthetic zeolites have been widely used (see the table).
Commercial molecular sieves are often classified by prefix numbers according to molecular sieves with different crystal structures. For example, 3A-type, 4A-type, and 5A-type molecular sieves. 4A-type is category A in the table with a pore diameter of 4 Å. A-type molecular sieve containing Na+ is denoted as Na-A. If the Na+ is replaced by K+, the pore diameter is about 3 Å, which is a 3A-type molecular sieve. If more than 1/3 of the Na+ in Na-A is replaced by Ca2+, the pore diameter is about 5 Å, which is a 5A-type molecular sieve.
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