Molecular Sieve Basics: Crystals Help Determine the Pore Size of Molecular Sieve

This article is a kind of a continuation on an article we wrote in 2011 that discusses the pores sizes of molecular sieve.

Molecular sieve are crystalline metal aluminosilicates that belong to the zeolite family.  That means that the molecules and atoms that make up a molecular sieve are made out of alumina, silicon, and oxygen and because they are crystalline they have a strong degree of order in the way they are laid out.

Molecular sieves specialize in separating very small molecules and atoms apart from one another.  Being part of the zeolite family, molecular sieve has a three dimensional network of pores which can adsorb molecules of a specific size.  The pores on a molecular sieve is what makes sieve special, this is because they can separate any substance down to the 1/10,000,000,000th of a meter, or an Angstrom.  There are four standard pore sizes that a molecular sieve can have:

• 3A, 3 Angstrom pore size
• 4A, 4 Angstrom pore size
• 5A, 5 Angstrom pore size
• 13X, 10 Angstrom pore size (depending on the manufacturer the pore size may be either 8 or 9 Angstrom)

The pores on molecular sieve could have one of two structure types: A structure or X structure.  3A, 4A, and 5A are made from an A structure while 13X is made from an X structure.  The A structure is smaller and more square-shaped than the X structure which is larger and circle shaped.

Aluminum Hydroxide, Sodium Hydroxide, Sodium Bicarbonate, and clay are used in the sieve manufacturing process, when the process is created this combination of material will make 4A molecular sieve when created with a type A structure or 13X molecular sieve when created with a type X structure.

3A and 5A molecular sieve are made once they are ion exchanged with the originally cre

ated 4A sieve.  4A molecular sieve is ion exchanged with potassium to create 3A sieve, the potassium molecules are larger than the sodium molecules they were exchanged with shrink the pore size.  5A sieve is created when 4A sieve is ion exchanged with calcium, calcium molecules are exchanged in a 1:2 ratio.  Every calcium molecule removes two sodium molecules thus increasing  the size of the pore.

The various pore sizes of molecular sieve offer a great variety of services to anyone looking to separate different combinations of molecules from one another.