For instance, a water molecule may be small enough to pass through
the pores while larger molecules are not, so water is forced into the
pores which act as a trap for the penetrating water molecules, which
are retained within the pores. Because of this, they often function
as a desiccant. Molecular
sieves are desiccants with varying
properties to those of silica gel. molecular sieves which appear as
small silicon pinkish beads, are synthetically produced, highly
porous crystalline metal-alumina silicates. They have many internal
cavities that are linked by window openings of precise diameters,
(measured in Angstroms) that classify molecular sieves - 3A,
4A, 5A, and 10A (also known as 13X). That is, in detail these are
the 3A
Molecular Sieve, 4A Molecular Sieve,
5A
Molecular Sieve...and so on.But the 10A
series molecular sieve is also know as 13X Molecular Sieve.

A molecular sieve can adsorb water up to 22% of its own
weight. The principle of absorption to molecular sieve particles is
nearly similar to that of size exclusion chromatography. Often they
consist of alumina silicate minerals, clays, porous glasses, micro
porous charcoals, zeolites, active carbons, or synthetic compounds
that have open structures through which small molecules, such as
nitrogen and water can diffuse. Molecular sieves are often utilized
in the petroleum industry, for purifying gas streams and are also
used in the chemistry laboratory for separating compounds and their
drying reaction. Methods for regeneration of molecular sieves undergo
the process of pressure change (as in oxygen concentrators), heating
and purging with a carrier gas (as when used in ethanol dehydration),
or heating under high vacuum. Temperatures typically used to
regenerate water-adsorbed molecular sieves range from 130°C to
250°C.

The United States FDA has not yet approved molecular sieve for
direct contact with consumable items. Lack of government approval for
the use of molecular sieves in food and drug packaging has limited
its widespread use. Independent testing suggests that molecular
sieves meets all government requirements but the expensive funding
required for the testing for government approval is being denied.
Europe is the only country where molecular sieves have limited its
usage in pharmaceuticals too.