True molecular sieving, small

The most straightforward cause of shape selectivity is the discrimination between molecules on the basis of their diffusion rates through the channels or cage windows. Microporous solids act as true molecular sieves, because the well-defined pores are able to select molecules on the basis of differences in dimensions of 0.1 A or less. Examples of strong molecular sieving effects include the selection of normal alkanes over branched ones by small-pore solids and the selection of para-substituted over ortho- and meta-substituted aromatics over medium-pore zeolites. This type of selectivity according to molecular diffusion rate may act on both reactant and product molecules. The much faster dehydration of n-butanol compared to isobutanol over Ca-A demonstrated by Frilette and Weisz is the classic example of reactant diffusion... 

The last contribution (Chapter 7) deaHng with the role of Mossbauer spectroscopy in the science of molecular sieves was provided by Lovat V.C. Rees, one of the pioneers in this field. Although Mossbauer spectroscopy is appHcable in zeolite research only to a small extent because of the limited number of suitable Mossbauer nuclei, we are indebted to this technique for valuable knowledge of and a deeper insight into some special groups of zeoHtes and zeolite/guest systems. This is particularly true of molecular sieves, which contain the most important Mossbauer nucleus Fe in their framework and/or extra-framework guests (cations, adsorbates, encapsulated complexes, and so on). 

---