Featuring "shards of soccer-ball shaped molecules jumbled in space and linked together," a new material shows promise for more efficiently producing nitrogen and oxygen--a multibillion industry, DuPont Co. and University of Delaware scientists report Sept. 17 in Science.
The carbon material, resembling bits of buckminsterfullerene or "buckyball" molecules, suggests a cheaper, lower-energy method for generating enriched oxygen and nitrogen, crucial ingredients for the production of steel and chemicals, and for the preservation of foods and medicines.
Already approved for patent protection, the material may also prove useful to the biotechnology industry for concentrating proteins in watery broths, says Henry C. Foley, a professor in UD's Department of Chemical Engineering.
"Nitrogen is the most widely used gas in the world," says Foley, coauthor of the Science article. "The world's top four industrial gas producers sell more than $22 billion worth of product annually, including over 68 billion pounds of nitrogen per year. Nitrogen and oxygen are fundamental to our global economy, and this work is a first step toward the next revolution in separation technologies."
Though not yet available as a commercial membrane, the DuPont/UD material works "remarkably well" in laboratory tests, reports lead Science article author Mark B. Shiflett, P.E., a research engineer at the DuPont Co. who is pursuing his doctoral degree at UD.
Oxygen moves through tiny pores in the material 30 times faster than slightly larger nitrogen molecules, suggesting that it would effectively separate most of the nitrogen from a sample of air, Shiflett says.
Smaller gaseous molecules travel even more rapidly through the material's pores: Hydrogen was transported 330 times faster, and helium moved 178 times faster, compared with the transport rate for nitrogen, according to Shiflett.
"That tells us that these nanopores are extremely size-selective and, therefore, very effective," Shiflett explains. "Our material offers selectivity comparable to, or better than, the conventional polymer membranes used for oxygen-nitrogen separations from air."
Once developed, Foley says, the technology may serve as an alternative to two popular, but energy-intensive separation methods: Cryogenic separation, which requires cooling oxygen and nitrogen to 77 degrees Kelvin (-321 degrees Fahrenheit), so that oxygen distills, leaving nitrogen behind; and Pressure-Swing Adsorption or PSA, in which adsorbent "beds" are alternately pressurized to release the desired gas.
Shiflett says the DuPont/UD material may initially be most useful as a preliminary step in the separation process. "You could separate out most of the nitrogen or the oxygen, significantly reducing energy costs," he says.
Harnessing fullerene fragments
What do you get when you cross soccer-ball shaped, buckminsterfullerene molecules (so named for the late inventor Buckminster Fuller) with a flat-board type of structure resembling graphite?
Foley describes the result as "chaotic arrays of fullerene-like fragments," featuring extremely tiny, uniformly spaced pores just 4.5 to 5 angstroms wide-nearly invisible to all but the most powerful of microscopes.
A key to the new material, Foley says, was ultrasonic deposition. Vibrating at a certain frequency, he explains, an ultrasonic horn atomizes a liquid polymer into many identical droplets, which then float gently onto a stainless steel tube. Because the droplets descend at "zero momentum," he says, they aren't disturbed by slamming into the substrate. And, any defects are subsequently "healed" by sequential deposition and annealing steps, brought on by a quick blast of heat in helium gas, which strengthens the film and forms carbon-carbon bonds.
The temperature in the annealing chamber, and the thickness of the material, proved critical. Heating the material to 450 degrees Celsius (842 degrees Fahrenheit) generated perfectly formed layers between 15 and 20 microns thick. (A micron is one-millionth of a meter). Thicker samples were riddled with cracks, Foley says.
The goal, Shiflett says, is to eventually produce bundles of polymer-coated tubes roughly one half-inch wide and 10 to 20 feet long. Secured inside a shell, the bundle could continuously separate nitrogen from oxygen, from a stream of air.
"You would feed the air through the inside of these tubes, for example, and allow the oxygen to preferentially diffuse through it, faster than nitrogen, so that for every 30 oxygen molecules that came through the tube, you would get one nitrogen," Shiflett explains.
In this way, he says, nitrogen could be produced more cheaply to prevent spoilage and contamination, which happens when perishable foods and drugs are exposed to oxygen. Protein-water separations may represent another application for the new technology. Combined with metals, the DuPont/UD material may even suggest new catalytic membrane reactors, or a way to enhance hydrogenation/dehydrogenation reactions.
But first, Foley and Shiflett must increase the material's throughput capacity or "flux," which corresponds with its thickness. "We still need to raise the flux by a factor of 10 without losing selectivity," Foley says. "I'm optimistic that we can jump that hurdle in the relatively near future."
Also serving on Foley's research team were former Ph.D. students Madahan Acharya and Brenda Raich, currently working for Mobil and Exxon, respectively. His research is supported by a joint DuPont Co./Delaware Research Partnership and by the U.S. Department of Energy.
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