It Stores, It Filters, It's...

It Stores, It Filters, It's...

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Dr. Sacco on space shuttle July 10, 2000 Huntsville, AL What is as hard as a rock but works like a sponge? This isn't a difficult riddle - such substances, called "zeolites," are actually found throughout nature. Zeolites have a rigid crystalline structure with a network of interconnected tunnels and cages, similar to honeycomb. But while a sponge needs to be squeezed in order to release water, zeolites only give up their contents when they are heated. The name "zeolite" comes from the Greek words zeo (to boil) and lithos (stone), literally meaning "the rock that boils."

There are nearly 50 different types of naturally-occurring zeolites, and they vary in crystal structure, molecular pore size, and chemical composition, among other things. Variations can also occur between zeolites of the same group, due to the different environmental conditions where a zeolite may be found. Impurities in the environment, for instance, can affect how a zeolite forms.

The ability of zeolites to adsorb liquids and gases makes them useful in such products as air-fresheners, kitty litter and laundry detergents. Because zeolites have extremely small pores, they can also be used as filters, such as air and water filters that help clean up the environment. Zeolites are also used as fillers for composite paper, rubber, plastics, or ceramics, and can be enhanced to become specialty lightweight ceramic and concrete products.

zeolite crystals grown in microgravity In fact, zeolites are so useful, they form the backbone of the chemical process industry. While zeolites are currently used in everything from animal feed supplements to the decontamination of radioactive waste, there are even more uses for zeolites waiting in the wings. The Center for Advanced Microgravity Materials Processing (CAMMP), for instance, is working to improve zeolite materials for storing hydrogen fuel. CAMMP is also applying zeolites to detergents, optical cables, gas and vapor detection for environmental monitoring and control, and chemical production techniques that significantly reduce by-products hazardous to the environment.

A large part of CAMMP's research depends on figuring out the molecular structure of zeolites. Due to the extremely small size of these molecules (2 to 8 microns), it is difficult to get accurate structural information about zeolites. Such information can be readily achieved, however, if zeolite crystals 200 to 1,000 times their normal size can be grown. CAMMP has discovered that it is possible to grow such large crystals in the microgravity conditions on the Space Shuttle and the International Space Station.

Once enough is known about these materials to be able to manipulate both their nucleation and growth, CAMMP can custom design them for specific applications. By selectively processing molecules, CAMMP could make chemical processes more energy efficient, and also reduce unwanted chemical byproducts. This type of processing ultimately could reduce both production costs and pollution. CAMMP plans to target zeolite membranes towards isomerization, dehydrogenation, and desulfurization - reactions that are all critical to the worldwide processing of petroleum and petrochemical products. In addition, novel applications are being developed to use zeolite membranes to separate and purify gases and liquids for pollution control.

Zeolites could help us move from an economy that relies on petroleum to one that uses hydrogen for its fuel needs. Hydrogen would be an infinitely renewable, pollution-free fuel: Hydrogen is the most abundant element in the universe, and the main product of hydrogen combustion is H2O (water). One of the major problems remaining to be solved is the efficient storage of hydrogen, but zeolite and zeo-type materials are being tested as a possible storage medium.

Currently, zeolites are a 2 billion-dollar-a-year market, but development of zeolite technology could increase that figure dramatically. Since chemical processing is a trillion-dollar industry worldwide, any improvement in the understanding of zeolite materials could have an enormous economic impact. Perhaps more importantly, zeolites could be used in even more ways to make Industry safer and less damaging to the environment. Thanks to research in space, CAMMP may someday make zeolites more useful than ever before.