(92c) Frontiers of Shape Selective Separation

Separations based on differences in molecular shapes, instead of molecular sizes or boiling points, have been successfully used in industry with small molecules. The zeolites used have 8 to 12 oxygen atoms forming almost circular windows, with diameters from 3.0 to 7.4 Å. The search for a suitable zeolite to separation two molecules begins with comparing their diameters, as a window should admit a smaller molecule but not a larger molecule. This "hard sphere" method of search does not always work well, since molecules and windows are not circular, and a larger molecule can nevertheless penetrate a smaller window, especially at higher temperatures. The "hard ellipse" method of search is an improvement for elliptical molecules, and the "strain energy" method of search is much more versatile and accurate.

Biological molecules have a large variety of conformations and shapes, and are targets for shape selective separations. Molecules such as fibrinogen with a large aspect ratio of L/D can enter pores that are far too small for globular proteins of the same molecular weight. Many new zeolites have been synthesized with a wide variety of window shapes, such as ellipses, squares, and even clover-leave. However the traditional zeolites cannot accommodate molecules larger than molecular weight of 300. Currently the largest zeolite VPI-5 has a window diameter of 12.7 Å, which extends the range to MW=800. The carbon nano-tubes have diameters from 5 to 24 Å, and the mesoporous MCM-41 has diameters from 25 to 100 Å. These developments open the opportunity to search for mesoporous material that would separate biological molecules as large as MW= 400,000.