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4A molecular sieve has a wide range of applications. It is not only a new type of high-performance microporous adsorbent, but also a catalyst carrier with excellent performance. As a desiccant, molecular sieve has strong drying efficiency and can be used to dry a variety of gases and liquids. Molecular sieves cannot be used to dry unsaturated compounds because they have a strong adsorption effect on unsaturated molecules, polar molecules and easily polarized molecules. Molecular sieve has a small water absorption capacity. If the material to be dried contains too much water, it should be dehydrated with other desiccants first, and then dried with molecular sieve.
4A and 5A molecular sieves are currently widely used molecular sieves in various major industries. 4A molecular sieve is a kind of sodium aluminosilicate. The apparent diameter of its micropores is 420pm, which can adsorb molecules with a diameter of less than 400pm. The 5A molecular sieve is sodium calcium aluminosilicate with a micropore diameter of 500 pm, which can absorb molecules below 500 pm, and the diameter of water molecules is about 300 pm. So what is the activation method of 4A molecular sieve?
If you want to completely remove the water absorbed by the molecular sieve, do not exceed 550°C under normal pressure. This is the general condition for determining the adsorption capacity of the molecular sieve. If the adsorbed water is desorbed industrially, such a high temperature is unnecessary or inappropriate, generally between 200°C and 300°C. Insufficient water absorption increases the number of adsorption-desorption operations, but reduces the activation temperature of the 4A molecular sieve. If it is "vacuum desorption", the required activation temperature can be appropriately lowered.
(1) Drying in a laboratory muffle furnace can activate dehydration, and dry at 350°C and atmospheric pressure for 8 hours;
(2) If there is a vacuum pump, it can be dried at 150°C for 5 hours.
(3) The activated molecular sieve should be cooled to about 200°C in the air, and should be stored in a desiccator immediately. If conditions permit, N2 protection should be applied during cooling and storage to prevent moisture in the air from being absorbed again.
3. Storage method after activation of 4A molecular sieve
The activated molecular sieve was cooled in the air to about 200°C (about 2 minutes) and immediately stored in a desiccator. If conditions permit, dry nitrogen should be used for protection during cooling and storage to prevent water vapor from being absorbed again. The used molecular sieve has contaminants. When activated, the temperature must not only be as high as 450°C, but also steam or inert gas. It should be introduced to replace other substances in the molecular sieve, and avoid contact with oil and liquid water when molecular sieve is used.
Molecular sieves are crystals of hydrated aluminosilicates. After high temperature activation and loss of water, many micropores with uniform pore size are formed inside the crystal. It has a strong adsorption capacity and can suck molecules with a diameter smaller than its pore size into the pores, but cannot absorb molecules larger than its pore size, so it can be used as a sieve molecule. Molecular sieve is non-toxic, non-corrosive, insoluble in water and organic solvents, and can be used in the pH range of 4-13.
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