Molecular sieves are a group of adsorptive desiccants
which are crystalline aluminosilicates, chemically
similar to clays and feldspars, belonging to a class of
minerals known as zeolites. The most important
characteristic of these materials is their ability to undergo
dehydration with little or no change in crystal structure.
The dehydrated crystals are interlaced with regularly
spaced channels of molecular dimensions, comprising
almost 50% of the total volume of crystals.
The empty cavities in activated molecular sieve
crystals have a strong tendency to recapture the water
molecules that have been driven out. This tendency is so
strong that if no water is present, they will accept any
material that can enter or filter into them. However, only
those molecules that are small enough to pass through the
pores of the crystal can enter the cavities and be adsorbed
on the interior surface. This sieving or screening action,
which makes it possible to separate smaller molecules
from the larger ones, is the most unusual characteristic of
molecular sieves.
Molecular sieves are used in many fields of
technology to dry gases and liquids. They are also used for selective molecular separations based on such size
and polar properties as ion-exchangers, as catalysts and
in petroleum industry to remove normal paraffins from
distillates.
