Basic Characteristics Of Molecular Sieves

- Apr 02, 2019-

What are the basic characteristics of molecular sieves? The following molecular sieve manufacturers will introduce them in detail!


a) Molecular sieves can reversibly adsorb and desorb water or various gases and liquid compounds.


b) Metal cations are easy to replace.


c) The internal cavities and channels of the molecular sieve form a very high internal surface area. The inner surface may be 10,000 to 100,000 times higher than the outer surface area of the molecular sieve particles.


1. Select adsorption according to the size and shape of the molecule. ——Molecular sieve effect


The molecular sieve crystal has a honeycomb structure, and the crystal pores and pores in the crystal are connected to each other, and the pore diameter is uniformly fixed (the molecular sieve chamber diameter is generally between 6 and 15 angstroms), which is equivalent to the size of a usual molecule, and only those diameters are compared. Molecular sieves can adsorb small molecules through the zeolite pores, while molecular sieves do not adsorb macromolecules because they cannot enter the zeolite pores. However, silica gel, activated alumina and activated carbon do not have a uniform pore size, and the pore size distribution range is very wide, so there is no screening performance.


2. Selective adsorption based on molecular polarity, unsaturation and polarizability


Molecular sieves have high affinity for polar and unsaturated molecules; in non-polar molecules, they have a higher selective adsorption advantage for molecules with polarization. In addition, the lower the boiling point of the molecule, the less likely the molecular sieve will adsorb.


High-efficiency adsorption characteristics of molecular sieves:


Molecular sieves have high affinity for H2O, NH3, H2S, CO2 and other polymer polarities, especially for water, low partial pressure (even below 133 Pa) or low concentrations, and high temperatures (even above 100 °C) still exist. High adsorption capacity under harsh conditions.


1. Low partial pressure or low concentration adsorption


At a relative humidity of 30%, the water absorption of the molecular sieve is higher than that of the silica gel and the activated alumina. As the relative humidity decreases, the superiority of molecular sieve becomes more and more obvious. For silica gel, the adsorption amount of activated alumina increases with the increase of humidity, and when the relative humidity is very low, their adsorption amount is small. .


2. High temperature adsorption


Molecular sieves are the only available high temperature adsorbents. At 100 ° C and 1.3% relative humidity, the molecular sieve can absorb 15% by weight of water, 10 times larger than the activated alumina under the same conditions; 20 times larger than the silica gel. Therefore, at higher temperatures, molecular sieves can still absorb large amounts of water, and activated alumina, especially silica gel, greatly loses its adsorption capacity.


3. High speed adsorption molecular sieves have a much higher adsorption rate for polar molecules such as water at partial or very low concentrations than silica gel and activated alumina. Although the equilibrium water absorption of silica gel is higher than that of molecular sieve when the relative humidity is high, the water absorption rate of silica gel is not as high as that of molecular sieve with the increase of adsorption linear velocity.


Molecular sieve ion exchange capacity


An important property of molecular sieves is the ability to perform reversible ion exchange. Through this exchange, the adsorption and catalytic properties of the molecular sieve are improved, thereby obtaining a wide range of applications (for example, for demineralized water and wastewater treatment).


Catalytic properties of molecular sieves


Molecular sieve crystals have a uniform pore structure with pore sizes comparable to normal molecules; they have a large surface area. Moreover, the surface polarity is very high; the cations that balance the negative charge of the skeleton can be ion-exchanged; some catalytically active metals can also be exchanged and introduced into the crystal, and then reduced to an elemental state with extremely high dispersion; and the stability of the molecular sieve skeleton structure very high. These structural properties make molecular sieves not only excellent adsorbents, but also effective catalysts and catalyst supports.