Introduction to molecular sieve

Molecular sieves are a variety of aluminium-silicate. Silicon and aluminum are bonded through a "bridge" of oxygen to form a porous structure, which possesses uniform porous channels and cavities which provide a large inner surface area. Besides Si, Al and O, there are metallic cations with low positive charge and relative large diameter, and chemically combined water molecules. Although these water molecules are released by heating, the crystal structure does not change, which results in many uniform cavities which are formed when abundant micropores of uniform size are linked together. The molecules smaller than the channel size can be adsorbed in the pores. On the contrary, those larger than the channel size will be excluded. Thus molecules of different size can be separated by means of sieving. Therefore, this aluminium-silicate is called a molecular sieve. Mixtures of gases or compounds that flow through the molecular sieves can be separated due to the sieving effect.
Manmade molecular sieves are a white powder, which is not soluable in water and organic solvent, but can be dissolved in strong acid. An adhesive is added to the white powder in order to extrude the material to form pellets or elongates in order to be easily handled for various applications.
Molecular sieves form highly thermal stability, a large internal surface area (600-1000 m²/g) even though its external surface area is small (1% of total surface area). The molecular sieves possess a three-dimensional frame network with a pore volume of 28% to 35% of the total volume.  Therefore the molecular sieves are able to adsorb large amounts of molecules. The inner crystal surface is highly polarized, resulting in formation of highly charged electric field. Due to its homogeneous pore size distribution and highly adsorptive selectivity, it is a selective and powerful adsorbent, catalyst, and catalytic carrier. It is widely used in industries, such as oil refining, petrochemical, chemical, metallic, electronic and those associated with military needs. In addition, they are used increasingly in medicine, light industry, agriculture and environmental protection, etc.
Characteristics of molecular sieves
1. Selective adsorption according to the the following properties of the adsorbate
(1) Molecular size
(2) Molecular polarity
(3) Degree ofsaturation of the molecule
(4) Boiling point
2. High adsorptive sensitivity: it has a higher adsorptive capacity with thin particles and a high temperature.
3. Cation exchange characteristics
4. Catalysis characteristics
(1) Catalysis effect to the molecules that are adsorbed in micropores
(2) Only Catalyzes for the molecules whose size are less than that of micropore
(3) Large surface area and high activity
(4) High resistance to poisoning and high thermal stability
(5) Ideal carrier for catalyst