True Molecular Sieving for Small Molecules

Whereas the separation of nitrogen and oxygen from air over cationic zeolites results from preferential adsorption, rather than molecular sieving, the well-defined pore struetures of microporous framework solids do permit the 

The ability of microporous solids to act as high-capacity molecular sieves has long been exploited in a wide range of applications in adsorption and separation. The electrostatic interactions of the traditional cationic forms of aluminosilicates are well suited for the uptake of polar molecules (such as H2O) and are also able to separate oxygen from air. The development of microporous solids with varied chemistry has enabled adsorption and diffusion properties to be finely tuned for particular technologies. Pure silica zeolite polymorphs such as silicalite have particular importance, because they enable separation on the basis of a different range of polarity and on molecular size the absence of aluminium in the framework also prevents the presence of unwanted acidity, so adsorbed hydrocarbons do not undergo any catalytic transformation. 

Most of the above uses rely on physisorption rather than strong chemical bond formation. Chemisorption, however, is of particular importance in heterogeneous catalysis, where it is a necessary precursor to reaction. Chemisorption sites include Bronsted and Lewis acid sites as well as coordinatively unsaturated transition metal cations, either within the framework or as charge-balancing cations outside the frameworks. Their catalytic activity is discussed further in the following Chapters 8 and 9. 

Baerlocher, W. M. Meier and D. H. Olson, Atlas of Zeolite Framework Types , Elsevier, Amsterdam, 2001. 

Diaz-Cabanas, J. Martinez-Triguero, F. Rey and J. Rius, Nature, 2002, 418, 514. 

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