Molecular Sieve

Introduction

Fig.1. Molecular sieve

Molecular sieve is a material with small pores of uniform size. The pore diameters are similar in size to small molecules which can enter the pores, and thus large molecules cannot enter or be adsorbed. When a mixture of molecules migrate through the stationary bed of porous, the semi-solid substance referred to as a sieve, the components of highest molecular weight leave the bed first, followed by the smaller molecules. The diameter of a molecular sieve is measured in ångströms (Å) or nanometres (nm).

Typical molecule sieves are synthetic silicates (zeolite) or natural zeolites, the chemical formula of them is Mx/m [(AlO₂)x·(SiO₂)y]·zH₂O, M represents cation, such as K⁺, Na⁺ and Ca²⁺, Ba²⁺ etc., m represents valence electrons number, z means the amount of crystal water, x and y are integer. By adjusting the molecular ratios of SiO₂ and Al₂O₃, the molecular sieves with different pore sizes can be obtained. The models of molecule sieves include: 3Å (potassium), 4Å (sodium), 5Å (calcium), 10Z (calcium), 13Z (sodium), Y (sodium) etc.

Fig.2. Adsorption of an acetone molecule into a zeolite Y

Applications

• 3 Å molecular sieves do not adsorb molecules whose diameters are larger than 3 Å. The characteristics of these molecular sieves include fast adsorption speed, frequent regeneration ability, good crushing resistance and pollution resistance. 3 Å molecular sieves are the necessary desiccant in petroleum and chemical industries for refining oil, polymerization, and chemical gas-liquid depth drying.

• 4 Å molecular sieves are widely used to dry laboratory solvents. They can absorb water and other molecules with a critical diameter less than 4Å such as NH₃, H₂S, SO₂, CO₂, C₂H₅OH, C₂H₆, and C₂H₄. It is widely used in the drying, refining and purification of liquids and gases (such as the preparation of argon).

• 5 Å molecular sieves are often utilized in the petroleum industry, especially for the purification of gas streams and in the chemistry laboratory for separating compounds and drying reaction starting materials. They contain tiny pores of a precise and uniform size, and are mainly used as an adsorbent for gases and liquids.