Zeolitic molecular sieves are a group of natural or synthetic aluminum silicates that form a regular crystal lattice. This lattice resembles an assemblage of cages interconnected by a network of tunnels, or channels, and varies in structure depending upon the type of sieve. The alumina units of the channels and cages are negatively charged and thus require cations to balance the system. Molecular sieves generally behave excellently as physical adsorbents, holding onto molecules via Van der Waals type attractions. In addition, Molecular sieves possess a natural affinity for polar species and are ideally suited to separate component mixtures based on dipole interactions. Another important feature of molecular sieves is that the cations in the lattice structure are exchangeable. It is therefore possible to control the size of the tunnel diameters via ion-exchange. For example, a Type A sieve in the K+-form has a channel diameter of 0.29nm, while the same sieve in the Ca2+-form has a channel diameter of 0.44nm. The significance of this is that molecular sieves can also be used to separate chemical species based upon their critical molecular diameter (size selectivity) and be "tailor-made", within limits, for specific applications.
Links: Activated Alumina Claus Catalyst Adsorbents Inert Bed Support Monomer Puriifcation Desiccant Molecular Sieves