Angewandte Chemie International Edition
© WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
For full article and contact information, see Angew. Chem. Int. Ed. 1999, 38 (18), 2730 - 2732
Organic solids with molecule-sized
pores can do more than their
inorganic counterparts, "zeolites"
"Boiling stones" - behind this mysterious term lie minerals that contain molecule-sized pores. These solids - their technical name is "zeolites" - got their name from a remarkable effect: the pores of the first zeolites discovered in nature contained water molecules, which exited on heating and made the mineral appear to boil. Chemists have been using the pores of these minerals for decades. Scientists like Taiwanese chemist Kuan-Jiuh Lin are now trying to synthesize similar substances out of organic molecules. These variants should be able to do more than their inorganic counterparts.
Thanks to their pores, zeolites have many applications. Some zeolites allow small molecules to pass through, whereas larger ones - such as pharmaceuticals - get stuck in these "molecular sieves". In other examples, ions - such as those that render water "hard" - stick to the walls of the pores. Some of the small channels serve chemists as tiny reactors, and therefore play an important role in the chemical industry. For example, in detergents, zeolites may be used to soften water.
The organic zeolites now synthesized by Lin, however, have little in common with minerals. Lin linked cobalt and manganese ions with slightly modified "porphyrins" - these are disc-shaped molecules that are also found in hemoglobin in the blood and in chlorophyll in leaves. In Lin's flasks, these fragments bound together in an extended, three-dimensional network. The most striking characteristics of this network are its extended channels and pores, which are equivalent to those found in the inorganic minerals. The new zeolites are robust as well; they even withstand heating to 200 degree centigrade in boiling crude oil.
In contrast to the inorganic representatives of "boiling stones", the organic zeolites can be quickly and easily changed through modification of the porphyrin components. In this way, the zeolite pores can, for example, be equipped with special coatings through the simple reactions learnt by every chemistry student. This may make it possible to lure molecules into the pores more selectively than before - for example those of an alcohol, while smaller water molecules remain outside. Industrial chemists are already excited about their new playground.