Molecular sieves are materials with tiny pores of a precise and uniform size. These pore diameters are similar in size to small molecules, and thus large molecules cannot enter or be adsorbed, while smaller molecules can. And they are primarily used for the separation of gases and low-molecular-weight hydrocarbons.
The pore diameter of a molecular sieve is measured in ångströms (Å) or nanometers (nm). For example, 3A molecular sieve means the molecules with the pore size of 3Å or 3 angstroms and it simply signifies that molecule with the pore size larger than 3Å cannot be used for adsorption.
Molecular sieves are composed of sodium and aluminosilicate, which are grown to form a three-dimensional crystal structure. According to their crystal structures, the common molecular sieves can be divided into A-type, X-type, Y-type, ZSM-type, and carbon molecular sieve.
Fig. 1 Crystal structures of molecular sieves
When A-type molecular sieve is grown, it initially is type 4A, meaning the pore openings are 4 Ångstroms in diameter. If the sodium ion is exchanged with the larger potassium ion, the pore opening is reduced to approximately 3 Ångstroms, called 3A molecular sieve. On ion exchange with calcium, one calcium ion replaces two sodium ions. Thus, the pore opening increases to approximately 5 Ångstroms, called 5A molecular sieve[1].
X-type crystal is shaped differently from A-type crystal, and it is to offer much larger pore sizes, about 10 Ångstroms in diameter, which is called 13X molecular sieve. It is commonly used to separate nitrogen from the air for high purity oxygen and is also used to hairdryer and remove H2S and CO2 in natural gas and liquefied petroleum gas.
While some crystals can be produced to have pore openings about 8 Ångstroms in diameter, which are referred to as type 10X molecular sieves.
Y-type molecular sieve, together with X-type molecular sieve, belongs to the family of aluminosilicate molecular sieves with a faujasite-type structure. The difference between Y-type and X-type molecular sieve is the Si/Al atomic ratio. Y-type molecular sieve has a higher Si/Al atomic ratio than X-type (typically in the range 1-1.5). Therefore, the higher content of silica confers Y-type molecular sieve with higher thermal stability.
The ZSM-type molecular sieve is a highly siliceous aluminosilicate zeolite with an intersecting and three-dimensional channel system. And the ratio of the amount of silicon molecules and aluminum molecules can be changed within a pretty large range. The basic structure is composed of eight 5-membered rings, which has channels only and is cage-free cavities.
Fig. 2 Common ZSM-type molecular sieves and their pore dimensions
The carbon molecular sieve (CMS) is a slit-like micro-pore structure of carbon-based adsorbents (pore size: 0.4 nm~0.9nm), capable of separating molecules with different molecular sizes. It is commonly used for the separation of air into nitrogen and oxygen.
Fig. 3 Preparation process and pore size of CMS[2]
Alfa Chemistry is a leading manufacturer of molecular sieves. We offer molecular sieves in various of specification which can be used in various applications. To place an order or request a quote, please don't hesitate to contact us.
