Molecular Sieve 4A is used as a desiccant in array of industrial applications. 4A molecular sieve means the molecules with the pore size of 4A0 or 4 angstrom are used for the adsorbing process.It simply signifies that molecules with the pores size larger than 4a cannot be used for adsorption.Has a pore size of 4A or 4 angstrom. Any molecule larger than 4A will not be able to be adsorbed. 4 Angstroms are the sodium forms of the Type A crystal structure. 4A Molecular Sieve is primarily for removing moisture from liquefied and gaseous materials.