Page 27 - Adsorption Technology & Design, Elsevier (1998)

24 Adsorbents

relatively inexpensive and can be used for re-refining edible and mineral
oils, adsorbing toxic chemicals, removing pigments, etc. The cationic forms
are capable of adsorbing a range of polar molecules and non-polar molecules
if some water is present.
The spaces between the natural layers can be enlarged to form pillared
interlayered clays. This is carried out by ion exchanging the charge
compensation cations with polynuclear metal ion hydro-complexes which
are formed in hydrolysed solutions of polyvalent metal ions such as Al(III)
or Zr(IV). The polynuclear cations dehydrate on calcination to create metal
oxide clusters which act as pillars between the clay layers and create spaces
of molecular dimensions. Example separations with pillared clays include
the separation of oxygen and nitrogen, and the separation of isomers.

2.9 ZEOLITES

Zeolites are porous crystalline aluminosilicates which comprise assemblies
of SiO4 and AIO4 tetrahedra joined together through the sharing of oxygen
atoms. More than 150 synthetic zeolite types are known, the most important
commercially being the synthetic types A and X, synthetic mordenite and
their ion-exchanged varieties. Of the 40 or so mineral zeolites the most
important commercially are chabazite, faujasite and mordenite. Cavities (or
cages) are contained within the framework of a zeolite and are connected by
regular channels (pores) which are of molecular dimensions and into which
adsorbate molecules can penetrate. In crystal form, zeolites are distinct
from other adsorbents in that, for each type, there is no distribution of pore
size because the crystal lattice into which the adsorbate molecules can or
cannot enter is precisely uniform. The internal porosity is high and thus the
majority of adsorption takes place internally. For this reason zeolites are
capable of separating effectively on the basis of size and they have been
assigned the popular description of molecular sieves. The processes of
adsorption and desorption of molecules in zeolites are based on differences
in molecular size, shape and other properties such as polarity. For physical
adsorption the cavities fill and empty reversibly and the mechanism is
generally considered to be one of pore filling. Hence the surface area
concepts presented for other types of adsorbent strictly do not apply.
The channel size is determined by the number of atoms which form the
apertures (or windows) leading to the cages. For example, apertures may be
constructed from rings of 6, 8, 10 or 12 oxygen atoms together with the same
number of aluminium and/or silicon atoms. Cages formed with 6 oxygen
atom apertures can admit only the smallest molecules such as water and
ammonia. Zeolites containing 8, 10 and 12 oxygen atom rings have limiting