Sieving property of molecular sieves

From the basic characteristics of molecular sieves, we can learn that molecular sieve crystals have a honeycomb-like structure. The cavities and channels within the crystal are interconnected, and the pore sizes are uniform and fixed (the diameter of the molecular sieve cavities is generally between 6 - 15 Å), which is comparable to the size of common molecules. Only those molecules with relatively small diameters can pass through the zeolite channels and be adsorbed by the molecular sieve. Large - sized molecules cannot enter the molecular sieve channels and thus are not adsorbed. Based on this, we can conclude that molecular sieves have strong sieving properties. In contrast, silica gel, activated alumina, and activated carbon do not have uniform pore sizes, and their pore size distribution ranges are very wide, so they do not have sieving performance.
According to the selective adsorption based on molecular polarity, unsaturation, and polarizability, molecular sieves have a high affinity for polar molecules and unsaturated molecules. Among non - polar molecules, they have a higher sieving adsorption advantage for molecules with higher polarizability. In addition, molecules with lower boiling points are less likely to be adsorbed by molecular sieves.
In summary, molecular sieves selectively adsorb molecules based on their size, polarity, and activity. Therefore, molecular sieves have strong sieving properties.
Remarks: Basic characteristics of molecular sieves

1. Molecular sieves can reversibly adsorb and desorb water or various gaseous and liquid compounds.

2. Metal cations in molecular sieves can be easily exchanged.

3. The internal cavities and channels of molecular sieves form a very high internal surface area. The internal surface area can be 10,000 - 100,000 times higher than the external surface area of molecular sieve particles.