Better Living Through Catalysts?
For the past 10 years, between teaching budding chemical-engineering students at California Institute of Technology, Professor Mark E. Davis has repaired to his campus lab to play with piles of sand. Davis' specialty is zeolites--grains made of silicon, aluminum, and oxygen that are used as catalysts in oil refining. Molecules of crude enter thousands of tiny holes in zeolites, where they react with an acid and break down into gasoline, heating oil, and various byproducts. Davis' goal is to create synthency of catalysts would save 22 million barrels of oil a year and cut the U.S. trade deficit by more than $400 million.
The prospect of this and other payoffs has lit a burner under chemistry labs around the country. For decades, new catalysts have been developed mainly by trial and error. Scientists knew that they worked without understanding how. But recent advances in lab equipment, such as the scanning-tunneling microscope, have provided the first glimpse of how individual atoms are arranged on the surfaces of catalytic materials.
This knowledge is turning what has long been an art into a science aimed at designing catalysts for specific jobs. Among the many advances this could eventually lead to are miraculous new medicines, "nanoscale" computer circuits, and more efficient solar-power devices. And instead of just cleaning up pollution, "designer catalysts" might avoid it in the first place. "We can now design catalysts to make only a product--without polluting byproducts," says James A. Cusumano, president of Catalytica Inc., a catalyst maker in Mountain View, Calif.
GOLDEN AGE? Catalysts are the marriage brokers of chemistry: They help form new chemical bonds in other materials to make a desired product, then emerge unchanged and ready to start over again. Products made this way already represent about a quarter of America's gross national product. Industrial catalysts turn chemicals into everything from plastics to paint to drugs. And bio-logical catalysts, called enzymes, help make food, drink, and detergents. Now, the latest advances indicate that the heyday of catalysts may lie just ahead.
Zeolites are a case in point. Nature's brand is dug from rock quarries. But chemists such as Davis are crystallizing new varieties from different materials. For instance, one way to get the bigger holes for processing heavier oil is to substitute phosphorous for silicon in a zeolite's structure. For the moment, such molecular sieves, as they're called, can't withstand the pressure and heat of refining. So Davis is trying to make larger-pored sieves from sturdier silicon. He and others are also designing zeolite membranes that would streamline the refining process by separating oil products in one step. Currently, energy-intensive distillation columns vaporize oil and cool the resulting gases to separate various products.
Zeolites can do many other jobs, too. They're replacing polluting phosphates as water softeners in laundry detergents, where they act as minuscule sponges to soak up minerals that make water hard. Meanwhile, some scientists hope to turn sieves into tiny electronic and optical devices. One goal is making chemical sensors that would trigger an alarm when molecules of a particular gas or pollutant entered their pores. Another use, says Geoffrey Ozin, a chemist at the University of Toronto, might be to house tiny semiconductor transistors, or switches, that have superior electronic and optical properties compared with today's semiconductors.
HIT LIST. But for all their diversity, zeolites can't match the exquisite precision of enzymes. These biological catalysts influence all chemical reactions in the body--and some outside it. The ability of enzymes to break down carbohydrates and process sugars, for example, helps in making food, beverages, and stain-eating detergents. And now, enzymes are finding some more exotic uses.
Celgene Inc., a biotech company in Warren, N.J., has built a library of 7,000 microbes, or microorganisms that contain enzymes. Among these is a bacteria that attacks methylene chloride, a carcinogen on the Environmental Protection Agency's most-wanted list. Placed in a bioreactor in a factory waste stream--at one General Electric Co. plant so far--the microbes convert the toxin into carbon dioxide, water, and salts.
Celgene is also using enzymes to purify drugs. The key molecules in many drugs occur in two slightly varying forms, each of which may have hugely different effects. The birth defects caused by thalidomide, normally a sedative, may result from one of these variants. Eliminating the undesirable form is usually difficult and costly. But Celgene has found enzymes that can do the job quickly and cheaply. Celgene says the market for purifying drugs this way may exceed $3 billion within a decade.
An even more dramatic development is the engineered abzyme, or catalytic antibody. This technology gives antibodies--disease-fighting proteins--the ability to enhance chemical reactions the way enzymes do. Since antibodies can be tailored to latch on to almost any molecule, abzymes could enable scientists to change almost any chemical reaction in the body. "Once you can custom-tailor enzymes," says Peter G. Schultz, a chemist at the University of California at Berkeley, "you have many more opportunities to cure illness." Some 50 abzymes have been developed so far, including one that may fight skin cancer by repairing DNA damage caused by ultraviolet light from the sun.
Meanwhile, scientists are using light and sound as catalysts. In photosynthesis, sunlight acts as a catalyst to spur chlorophyll-containing cells to make food for plants. Now, researchers at Arizona State University and Argonne National Labs, to name two, are close to capturing the energy generated by this molecular activity. Says Devens Gust, a professor at Arizona State: "What we have is a solar battery that, in principle, you could use to drive anything you want."
In another form of photocatalysis, scientists from the National Renewable Energy Lab in Golden, Colo., and from Sandia National Laboratories in Albuquerque, are purifying groundwater contaminated by the solvent trichloroethylene, which is used in cleaning machinery. The water is run through tubes containing titanium dioxide crystals. Sunlight reacts with the crystals to degrade the contaminants into harmless products. Project Leader Hal Link at the National Renewable Energy Lab points out that existing methods of cleaning groundwater merely transfer toxins onto filters or into the air.
Scientists are even looking to high-frequency sound waves, or ultrasound, to spur catalytic reactions. When such sound is radiated through liquids, it causes gas pockets to expand and collapse within fractions of a second. This creates tiny spots as hot as the surface of the sun--9,600F--plus pressures equal to those on the sea floor. These conditions might make catalysts work better in many production processes. Indeed, chemist Kenneth S. Suslick at the University of Illinois at Champaign-Urbana has used ultrasound to make a purer form of "amorphous" iron. Amorphous metals are used in magnetic recording heads and as a corrosion-resistant coating.
From oil to ultrasound, catalysts have come a long way. And as chemists learn more about how catalysts work and edge closer to designing them from scratch, they'll be building a lot more than castles in the sand.