One day industry might churn out water instead of waste. This vision - one of the main goals of a relatively new field called "green chemistry"- has recently been brought much closer to reality for two key industrial processes.
More than three billion tons of waste are released into the environment each year by the chemical industry in the United States alone. Billions of dollars are then spent every year to comply with laws for the treatment, control, and disposal of waste. Green chemistry, focusing on the prevention of pollutants from the outset, holds that the proverbial ounce of prevention is worth more than a pound of cure. Its aim is to develop new industrial processes that do not generate hazardous substances.
Targeting a major environmental challenge, Profs. David Milstein and Ronny Neumann of the Weizmann Institute's Organic Chemistry Department have cleaned up two processes that work behind the scenes to generate innumerable products ranging from compact discs to pharmaceuticals. The scientists, each working on a different process, designed them so that the only "waste"product is water. Now they are working on adapting the lab-tested methods for industrial use.
All around us, as well as within us, molecules interact with one another - joining, sharing atoms, running off with each other's atoms, or breaking into smaller molecules. For instance, sugar is created when molecules of water and carbon dioxide join. But not all molecules are happy to react with each other - some are fussy about whom they react with, and some just prefer to be left alone.
The trick behind the design of many products, including plastics and pharmaceuticals, is in causing molecules that normally would take no notice of one another to interact. Many industrial processes employ "mediators"- mainly chlorine compounds - that, after making the desired connections, end up as hazardous waste.
Milstein and Neumann, by designing two unique catalysts, have succeeded in causing such molecules to react directly with one another, without mediators.
One process, researched by Milstein's team, is the preparation of substances called aromatic alkenes, which are attractive candidates for the production of key non-steroid anti-inflammatory drugs such as Ibuprofen and Naproxen. Currently they are used for a variety of industrial materials, including plastics. To prepare them, two "unsociable"organic molecules must be merged. The metal catalyst tailored by the team causes the two organic molecules to react directly, without "middlemen,"producing the desired substance with water as the only byproduct (see "The psychol- ogy of atoms"below).
Neumann's team studied the preparation of a substance called propylene oxide (used for a wide range of plastic products), which has an annual market of $5 billion. It is generated by reacting oxygen with propylene, a compound derived from oil or natural gas. Since oxygen rarely reacts with organic materials under normal conditions (and at high temperatures can react with them explosively), Neumann designed a unique metal catalyst to couple oxygen with organic substances directly, cleaning up the process.
The scientists also hope to capitalize on another unique quality of catalysts - the ability to selectively manipulate, or "reroute,"chemical reactions to produce a desired product. "People often view catalysts as compounds that scurry around in the cell, or in flasks along factory production lines, speeding up a range of chemical reactions. But they're also masters of diversion,"says Neumann. In another of his team's green projects, catalysts are used to stop certain reactions at critical spots, yielding the desired products before waste is generated.
In addition to being environment- friendly, the reactions designed by Milstein and Neumann are much simpler than those currently in use, since they omit many stages of the process. For industry, this could translate into lower production costs. For us, it could translate into safer surroundings and a welcome breath of fresh air.
The psychology of atoms
A stable metal is a happy metal. Certain metal atoms, like those of ruthenium, will go to great lengths to achieve happiness. This property is utilized by scientists in making efficient catalysts.
In ruthenium's world, achieving balance means attaining 18 electrons in its "outer shell."Thus it will attract unguarded molecules and bind to them, draining them of electrons. Some caught molecules form partnerships among themselves and take back their electrons, leaving ruthenium to seek out other molecules.
"The 'unhappier'a metal, the more reactive it becomes,"says Milstein. "Ruthenium belongs to a group called 'transition metals,'whose atoms lack electrons and will eagerly react with a variety of molecules to attain them. The metal's 'happiness'depends, however, not only on the number of electrons it gains, but also on whom it takes them from - in other words, whom it binds with. By understanding what the metal 'wants,'we can design organic groups that induce it to seek out only specific molecules."
Milstein's team attached ruthenium to organic groups of atoms that make it "want"to bind to two specific organic molecules. It seeks each out and binds to it. The organic molecules are thus brought close to one another, in ruthenium's clutches. Displeased with their situation, they strike up a partnership and consequently unite, enabling them to escape ruthenium. The newly formed alliance between the two organic molecules is actually the desired industrial product, "catalyzed"by the tailored ruthenium complex. Later, ruthenium binds to an oxygen molecule that makes off with two hydrogen atoms, forming water.
From plastics to fuels and automobile exhaust systems, from blood clotting to food digestion - catalysts are crucial to life as we know it. These "hardworking"substances speed up the rate of diverse industrial and biochemical reactions, providing a low-energy shortcut between the reactant and product stages. Essential for nearly all cellular processes, specialized biological catalysts known as enzymes fast-forward reactions that in their absence would occur far too slowly to sustain life. Other catalysts are playing a growing role in worldwide attempts to design greener transportation and streamline factory assembly lines to cut production costs and reduce pollution. Success stories include catalytic converters (based on a metal catalyst), now required in all new cars, which reduce gasoline emissions.
Prof. Milstein's research is supported by the Levine Institute of Applied Science and the Helen and Martin Kimmel Center for Molecular Design. He holds the Israel Matz Chair of Organic Chemistry.
Prof. Neumann's research is supported by Yad Hanadiv, Israel; the Helen and Martin Kimmel Center for Molecular Design; the Fritz Haber Center for Physical Chemistry; and Minerva Stiftung Gesellschaft fur die Forschung m.b.H. He holds the Rebecca and Israel Sieff Chair of Organic Chemistry.