Brookhaven-developed recyclable catalyst may help to reduce hazardous industrial waste
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At industrial plants, where the household products in our kitchens, garages, backyards, and bathrooms are made, generating waste continues to be an inevitable part of the creation process. Even the production of organic compounds, such as pharmaceuticals, results in unusable by-products that can be hazardous to the environment and costly to eliminate.
These waste chemicals are generated from reactions that produce plastics, herbicides, pesticides, paper, cleansers, rubber, and lubricants, to name a few. According to the U.S. Environmental Protection Agency, industrial manufacturing processes produce tens of millions of pounds of organic waste chemicals each year. Many of these substances have already been documented as contaminants to wildlife and natural resources.
At Brookhaven Lab, chemists Morris Bullock and Vladimir Dioumaev have joined in the movement of scientists looking for ways to address this problem. Using funding from the U.S. Department of Energy's Office of Science, they have developed a catalyst that converts chemical reactants into usable products without producing waste, allowing it to be used over and over again. The discovery may help reduce the amount of hazardous waste entering the environment, as well as the amount of money that businesses spend to treat and dispose of waste.
The new "green" catalyst accomplishes two significant goals. First, it works by removing one stage of the reaction: the new catalyst eliminates the need to use solvents in the process by which many organic compounds are synthesized. The catalyst does this by dissolving into the reactants, thus eliminating the need for a separate solvent in which the reagents are mixed to set the reaction in motion.
"Avoiding the use of solvents is an important way to prevent waste in chemical processes," says Bullock.
Second, the catalyst has the unique ability of being easily removed and recycled. This is so because, at the end of the reaction, the catalyst precipitates out of products as a solid material, allowing it to be separated from the products without using additional chemical solvents. Instead, "It separates itself," comments Dioumaev. "You can simply pour the products into another container and use the catalyst again."
Trial and Error
The procedure may sound straightforward, but finding a catalyst that works correctly was a complicated task. "The concept is simple, but striking a balance between maintaining catalyst solubility throughout the reaction and precipitation at the end was a diabolical problem," explains Bullock.
To work, the catalyst must stay dissolved long enough for the reactants to interact with it, and then must precipitate when the reaction is complete. If these two events do not happen, then the reactants will not generate the products properly, and the catalyst will not be easy to recover and recycle after the reaction. As a result, more steps would be needed to use up the reactants and collect the catalyst, and so the reaction would produce unwanted waste.
To solve these problems, Bullock and Dioumaev employed a tungsten catalyst that they had prepared and have shown to be effective for hydrogenation of ketones, a class of organic compounds. They found that this tungsten complex also catalyzed the reaction of ketones with organic silicon compounds to produce alkoxysilanes, which are used in the manufacture of drugs, pesticides, and other familiar organic compounds, as well as in the preparation of ceramic materials.
In earlier research, the two chemists had discovered that, while this class of catalysts could dissolve in ketones, it does not dissolve in certain other solvents. Instead, the catalysts forms oily precipitates called liquid clathrates.
Observing this, Bullock and Dioumaev wondered if using the oily substance in the reaction might be an ideal way to keep the catalyst suspended in the reactants until the reaction concluded. They postulated that the oil, when compared to a solid precipitate, might offer some advantages in the reaction because it would not obstruct the interaction between the liquid reactants in the way that a solid would.
The researchers worked with this idea experimentally, developing a suitable catalyst, a formulation containing the metal tungsten. Because this research required knowledge of the catalyst's molecular structure, the structure of the tungsten catalyst was determined using x-rays at the National Synchrotron Light Source (NSLS; see related story).
Catalysts at Work
The new catalyst begins working as it mixes readily with the reactants. When the reaction is nearly complete, the catalyst begins to precipitate, yet it remains suspended within the reactants as an oily liquid clathrate clearly visible in the reaction tube. "The reagents penetrate the oil to keep reacting until the reactants are used up," Dioumaev explains. When that happens, the catalyst is no longer soluble, so it precipitates out of the reaction as a solid, settling to the bottom of the test tube.
Further analysis revealed that virtually no trace of the catalyst is left in the liquid reaction products. This property is particularly desirable, for instance, in the creation of pharmaceutical compounds, which must be free of metal-based catalyst residue to be safe for use.
Now, the Brookhaven scientists will see if catalyst self-separation using liquid clathrate catalysts can employed in other reactions. "At present, we are trying to determine the scope of reactions that can be carried out using this type of catalyst," says Bullock. "In the future, we will seek to understand the criteria that influence the behavior of catalysts and their ability to mix with different solvents, which may allow us to design more catalysts that are readily recyclable."
Meet Morris Bullock and Vladimir Dioumaev
Though the liquid in their test tubes has a purplish-red hue, Morris Bullock and Vladimir Dioumaev see only green -- a fine example of "green chemistry," that is.
"Our interest in developing readily recyclable catalysts and the use of solvent-free conditions arose out of the 'green chemistry' movement -- the growing recognition of the importance of using chemical reactions that can contribute to a sustainable future by minimizing the generation of hazardous waste," says Bullock, who has been a member of Brookhaven's Chemistry Department since 1985.
"Green" chemistry is defined as the design of chemical processes and products that minimize or, ideally, eliminate the production of hazardous substances. That approach has multiple benefits, since avoiding both the production of waste and waste treatment can save the environment, lives, and money.
It was, in fact, the economical, practical idea of developing a self-separating catalyst that first drew the interest of Dioumaev, Bullock's postdoctoral fellow. "Morris was the first to realize the environmental 'green' implications when I showed him how the catalyst self-separates," Dioumaev says.
The team's approach to designing catalysts is to use fundamental mechanistic information about the individual steps of the reactions and then to design catalysts that use those individual reactions.
"Our long-term goal is rationally to design catalysts that have high reactivity, but that, at the same time, are used in processes that are environmentally benign," Bullock explains. Morris Bullock grew up in North Carolina and earned a B.S. in chemistry from the University of North Carolina at Chapel Hill in 1979, and a Ph.D. in chemistry from the University of Wisconsin–Madison in 1983. He did postdoctoral research at Colorado State University before coming to the Lab.
Vladimir Dioumaev did his undergraduate work at Moscow State University in his native Russia, before earning an M.S. in organic and organometallic chemistry through a joint program of Yale University and Moscow State University in 1988.
Dioumaev earned his Ph.D. in inorganic and polymer chemistry from McGill University in 1997 before coming to the United States. He joined the Lab's Chemistry Department in 2001 and became a naturalized U.S. citizen in 2003.
More Information
Funding: Division of Chemical Sciences, Office of Science, U.S. Department of Energy
Paper: "A Recyclable Catalyst That Precipitates at the End of the Reaction," Nature, 2003, 424, pp. 530-532.
Contact: Morris Bullock, bullock@bnl.gov or (631) 344-4315
Morris Bullock's web site , Vladimir Dioumaev's web site