Growing zeolite crystals in space may show how nature makes some of its tiniest traps and help researchers learn how to grow better crystals for a multitude of uses on Earth.

Learning more about the structure of these versatile, useful crystals is the goal of Principal Investigator Al Sacco Jr., of the Center for Advanced Microgravity Materials Processing (CAMMP) at Northeastern University in Boston, Massachusetts. By studying zeolite crystals grown in space, Sacco hopes to find out how the crystals may be processed synthetically to work more effectively in numerous current applications on Earth and in new applications, such as in storing hydrogen for use as a fuel.

Above: Principal Investigator Al Sacco flew as the payload specialist operating his own Zeolite Crystal Growth (ZCG) experiment onboard STS-73 in 1995. What he has learned from this and subsequent flights of ZCG could improve zeolite materials for a variety of purposes, from chemical processing to fuel storage. Credit: NASA. Source: OBPR Space Research Newsletter, September 2002 (Vol. 1, No. 4) (http://spaceresearch.nasa.gov/docs/spaceresearch_fall2002.pdf) .

Nature's Little Traps

Zeolites are made up of the elements silicon, aluminum, and oxygen. The crystals consist of alternating arrays of silica (beach sand, SiO2) and alumina (aluminum oxide, Al2O3) and can take on many geometric forms such as cubes and tetrahedrons. The crystals can be found in volcanic and sedi-mentary rocks in arid regions, and on the seafloor. There are nearly 50 different types of naturally occurring zeolites (hypothesized to be 20 percent of the total possible number of structures), and they vary in crystallographic structure, molecular pore size, and chemical composition, among other aspects. Variations can also occur among zeolites of the same group, due to the different environmental conditions under which a particular zeolite crystal was grown. Impurities in the environ-ment, for instance, can affect how a zeolite crystal forms, and the same structure can have varying ratios of silicon to aluminum.

Zeolites have a rigid crystalline structure with a network of interconnected tunnels and cages. The silica and alumina, two types of tetrahedral atomic groups, link to form complex, three-dimensional networks, with molecules of water (H2O) and ions of minerals such as sodium (Na) and calcium (Ca) housed in the cavities of the framework. The ions can be readily exchanged for ions of other types of minerals when in an aqueous solution, such as when sodium is exchanged for the magnesium and calcium in laundry water.

The cavities in zeolites are often a place for catalysis, in which the zeolite speeds up the conversion of one chemical into another without being affected or consumed by the reaction. "They are like molecular sponges," says Sacco. "They're very porous and they only allow molecules in that are a certain size. The active sites within convert these to a different molecule."

The special structure of zeolites also makes them quite useful as filters. Their small pores can sift out pollutants in water and air, and the crystals are commonly used to trap odor-causing molecules, to separate chemicals, and to adsorb gases. This filtering action enables chemists to manipulate molecules and process them individually. Because of their chambered structure, zeolites can act as tiny storage tanks as well, holding fluids, such as hydrogen, until the crystals are heated. As the crystals warm up, their framework expands, and molecules of fluid held within the chambers of the crystals can be driven off. In fact, the name "zeolite" comes from the Greek words zeo (to boil) and lithos (stone), meaning "the rock that boils." The fluid can then be recaptured when the zeolite cools.

Catch and Release

The latter two characteristics of zeolites, their ability to store and release fluids, especially interest Sacco. To better understand the special characteristics of zeolites, Sacco has been studying synthetic versions of them in his laboratory at CAMMP, one of the commercial space centers in the Space Product Development Division of NASA's Office of Biological and Physical Research. Crystals grown on Earth typically are only 2 to 8 microns in width. "That's about one-tenth the thickness of the human hair," says Sacco - much too small to accurately determine their structure. They're tiny because when zeolites nucleate (form) from a water solution, their density, which is twice that of water, causes them to sink to the bottom of an autoclave, the special container in which they are grown. This sedimentation causes the crystals to leave the nutrient-rich solution and to fall on top of each other, often merging to produce a large number of small, intergrown zeolite crystals instead of larger, separate crystals.

Sacco needed larger crystals to determine their structure, and he suspected that larger crystals could be grown in microgravity, where the effects of gravity, such as sedi-mentation, are minimized. This proved to be the case when he sent his zeolite crystal growth experiment to be conducted on missions STS-50, in 1992; STS-57, in 1993; and STS-73 in 1995, a mission on which he flew as the payload specialist operating the experiment. Improving hardware for each successive flight made it possible to grow crystals that were at least 200 times the size of crystals grown on Earth and of better quality. "In microgravity, materials come together more slowly, allowing zeolite crystals to form larger and with better order," says Sacco. He hopes to learn more about the physical and chemical mechanisms for zeolite nucleation to be able to better control nucleation on Earth. Understanding nucleation also may help him to determine how to control the location of aluminum atoms within the silica framework to affect the solid acid properties of zeo-type materials used in the petrochemical industries, and to better deter-mine locations for cations, the materials that can be used to block the pores of zeolites.

Above: Zeolites grown on Earth range in size from 2 to 8 microns (left). In microgravity, the crystals can grow to at least 200 times this size, allowing researchers a better view of their unique structure. Credit: CAMMP. Source: OBPR Space Research Newsletter, September 2002 (Vol. 1, No. 4) (http://spaceresearch.nasa.gov/docs/spaceresearch_fall2002.pdf) .

The zeolite crystal growth experiment will fly with advanced hardware on STS-107, scheduled for launch in late 2002/early 2003. The hardware consists of sample tubes that hold the components that form zeolites and the Zeolite Crystal Growth Furnace, a furnace unit used to process the samples. The mixing protocol will vary with each sample to optimize the formation of only the desired zeolite - not others of less industrial interest. Finding the right mix requires that these solutions, which begin to react on contact, are uniform in concentration across the sample tube volume. Once mixing is complete, the samples will then be placed in the furnace for automated processing. The samples, which Sacco hopes will include crystals up to 1,000 times their normal size on Earth, will be examined after their return from orbit.

Zeolite crystals also are being grown on the International Space Station (ISS). Conducting zeolite experiments on the ISS allows crystals to grow for longer periods, resulting in even larger and more structurally perfect crystals. This will make it easier for scientists to study the internal structure of different types of zeolites. The space station also will allow scientists to study results and repeat experiments, modifying experiment conditions to improve the quality of crystals, which is the same iterative process used when conducting investigations in ground-based laboratories.

The hardware for these studies on the ISS, the Improved Zeolite Electronic Control System (IZECS), was flown to the ISS on STS-108 in December 2001. This advanced hardware includes a set of 19 Teflon-lined aluminum or titanium autoclaves that fit inside the furnace. Inside each autoclave, a motor will mix the two solutions according to procedure and protocol commands sent by scientists working on the ground in the CAMMP remote operations center. The earthbound scientists also issue computer commands to the furnace to heat the samples.

The first set of samples for processing in the IZECS was carried to the space station on STS-110 in April 2002. Although a back-up system failed, the primary control system was able to process the samples. The first set of samples for postflight analysis was returned to Earth on STS-111 in June 2002. To date, half of the samples have been analyzed, and scientists have found crystals that are larger and of slightly better morphology than their Earth-grown counterparts. However, indications are that a number of samples were not mixed to completion. The reason for this inadequate mixing is being determined.

Applications on Earth

The work that Sacco and his team are doing could help them learn how to control zeolite structure to make them more effective miniature sieves, traps, and storage tanks. Zeolites form the backbone of industrial chemical processing. Sacco says, "Zeolites are now the most used material for catalysis in the chemical processing industry," and virtually all the world's gasoline is produced or upgraded using zeolites as catalysts in the refining process. Zeolites break up large, heavy oil molecules, making them smaller, and add hydrogen to the structure of the oil molecules so they burn more effectively. This function is related to the distribution of the aluminum atoms within the silica framework and is critical to improving the amount of these molecules that can be "upgraded." Industry wants to improve zeolite crystals so that more gasoline can be produced from a barrel of oil, making the industry more efficient and thus reducing America's dependence on foreign oil.

"Fine-tuning the structure of the zeolites to get more gasoline out of a barrel of oil during the refining process by as little as 1 percent would result in a $400 million [positive] swing in our [national] balance of payments," says Sacco. Zeolite improvements also can be applied to detergents, optical cables, gas and vapor detection, environ- mental monitoring and control, and chemical production techniques that reduce hazardous by-products.

In the future, zeolites may even be used for storing new fuels that are cheaper and cleaner. Hydrogen is one candidate fuel that might be stored and transported efficiently using zeolites. Sacco explains, "The problem with hydrogen storage is that it requires very cold temperatures, to make it liquid for storage, or very high pressures." Zeolites could be used to adsorb hydrogen at close to ambient conditions and then, after being heated ever so slightly, allow the hydrogen to escape. Cation material added to the zeolites would block the tiny holes in the porous zeolites, allowing the hydrogen to be stored. "These materials act just like caps on an ink bottle," says Sacco. "We heat them up and they move away from the porous area. We fill the area with hydrogen, drop the temperature a tenth of a degree, and they slide back in place and seal off the hydrogen." If zeolites do work for storing hydrogen, humans would be that much closer to using the most abundant element in the universe as a pollution-free fuel, lessening the dependence on petroleum.

Sacco dreams of returning to space to serve as a mission specialist on the ISS conducting continuous experiments on materials of interest to the petroleum and other industries as well as to researchers. He explains the value of this unique laboratory: "The ISS for the first time allows scientists and engineers a functioning laboratory to do investigations and exploratory science in the way it must be performed, with hypothesis and deduction, followed by observation and analysis. Researchers can perform their investigations the way they have been trained, learning from mistakes and pushing the limits of the freefall environment of low Earth orbit."