|Introduction to Molecular Sieve
Molecular sieve is one kind of aluminosilicate. Silicon and aluminum are bonded though the "bridge" of oxygen to form porous structure, which possesses uniform porous channel and cavity of large inner surface area. Besides Si, Al and O, there are metallic cations of low positive charge and relative large diameter, and chemically combined water molecules. Although these water molecules could be released upon heating, the crystal structure does not changed, resulting in formation of many uniform cavities, where there are abundant micropores of uniform size linked together. The molecules less than channel size could be adsorbed in the pores, on the contrary, these larger than the channel size will be excluded. Thus molecules of different size can be separated by means of sieving. Therefore this aluminosilicate is named as molecular sieve. Mixture of gases or compounds that flow through molecular sieve can be separated due to sieving effect.
Man-made molecular sieves are white powder, that is undissolved in water and organic solvent, but usually dissolves in strong acid. Adhesive is added to the white powder to make extruded sphere and pellet in order to easily handle for the application.
Molecular sieve has highly thermal stability, large inner surface area (600-1000 m2/g), although its external surface area is small (1% of total surface area). It possesses three-dimensional frame network, the pore volume is of 28%-35% of total volume, so that it is able to adsorb large amount of molecules. Its inner crystal surface is highly polarized resulting in forming highly electric field. Due to its homogeneous pore size distribution and highly adsorptive selectivity, it is powerful adsorbent, catalyst and catalyst support. Therefore, it is widely used in industries, such as oil refining, petrochemical, chemical, metallic, electronic and military filed. Besides, it is used more and more in medicine, light industry, agriculture and environmental protection etc.
Characteristics of molecular sieves
1. Selective adsorption according to adsorbate property
(1) Molecular size
(2) Molecule polarity
(3) Saturability of unsaturated hydrocarbons
(4) Boiling point
2. High adsorptive sensitivity: it is still effective at low adsorbate concentration and high temperature
3. Cation exchange property
4. Catalytic propertiy
(1) Catalytic effect to the molecules that are adsorbed in micropores
(2) Only Catalysis for the molecules which size are less than that of micropore
(3) Large surface area and high activity
(4) High resistance to catalytic poison and high thermal stability
(5) Ideal support material for catalyst