Molecular sieves are materials which can separate molecules based on their dimensions. This capacity is based on the presence of precisely and uniformly sized tiny pores in the material, with a diameter between 3 and 10 Angstrom depending on the material. The molecular sieves have a very large surface area inside the pores (600–700 m2/g). Molecules larger than the pores cannot enter the material, while those small enough to penetrate them can pass. Generally polar molecules are absorbed on the internal surface of the pores and trapped there, while non-polar molecules are not retained. The materials most used as molecular sieves are alumino-silicates and zeolites in particular, both natural and synthetic.