By Saren Johnston
If you're a chemist and your specialty is catalysis, then you're usually looking for ways to speed up the rates of chemical reactions so you can change molecules to other molecules more efficiently.
The results are new molecules for such
things a effective medicines, useful plastics or synthetic
fibers. But chemists also want to design new molecules for a more
basic purpose -- to help them better understand chemistry, the
properties of substances and how those substances can be changed.
Catalysts are critical to change. They make chemical reactions go faster, and they do so without undergoing permanent changes themselves. This characteristic allows catalysts to act over and over again and process huge amounts of material.
A little bit of a catalyst goes a long way in bringing about a change. But as any chemist would tell you, there is often a price to pay. Many chemical reactions that convert materials to useful products also produce wastes that can be harmful to the environment and cause significant destruction, such as the hole in the ozone layer. To address the environmental problems that can be created by chemical reactions, the world's chemical and pharmaceutical industries are looking for ways to advance "green chemistry" and make manufacturing processes more environmentally friendly.
Jim Espenson, Ames Lab program director for Molecular Processes, knows that kind chemistry isn't such a simple task. Each year, thousands of new chemicals are created throughout the world as the result of innovative chemical reactions. Espenson reminds us that those reactions take a lot of different catalysts, and many of them will produce unclean side reactions that can leave behind some pretty nasty stuff. Historically, many of the leftovers -- atoms that aren't used in reaction -- have not been easy on the environment.
Today, however, chemists are investigating methods to use every atom in a chemical reaction so that only the desired product and environmentally benign byproducts result. Espenson is helping advance that forward-thinking effort by taking a hard look at some existing ideas about a compound called methylrhenium trioxide (CH3ReO3, or MTO) and using that information to devise some new chemistry.
"MTO is proving to be a very useful compound," says Espenson. "It makes chemical reactions go quickly, and it makes them go to single products so one doesn't have to waste starting material or engage in difficult separations."
After reading some German papers that described MTO and recognizing that it had certain abilities to accelerate peroxide reactions and point them in the right direction, Espenson told the postdocs working with him at that time, Patrick Houston and Shigekazu Yamazaki, "Let's see what we can do to take advantage of this idea and do some chemistry that hasn't already been recognized." Before long, they had some new reactions going and had gained an idea of how things were taking place at the molecular level -- how the atoms and molecules come together and shuffle around. After achieving that level of success, Espenson. began thinking about some other things these compounds could do.
He began investigating hydrogen peroxide (H2O2) oxidation of organic sulfides and phosphines in collaboration with postdoctoral fellow Karlene Vassel and graduate student Mahadi Abu-Omar, respectively. H2O2 is known to bring about certain desirable chemical transformations without creating unwanted byproducts, thereby eliminating many problems and costs associated with environmental cleanup.
But what H2O2 reactions have going in terms of environmental benevolence, they lack in efficiency. The nonradical reactions of H2O2 are often very slow and produce unwanted byproducts, so a catalyst is needed to drive the reaction and bypass the undesirable side reactions of peroxide.
"In many of these reactions, the idea is to add an oxygen atom to something else," Espenson says. "You think of the formula of hydrogen peroxide, H2O2 and you take one of its oxygens and transfer it to a substrate. What you're left with, then, is H2O, that is to say, water. So it's a reaction without byproducts."
Espenson is now able to make this reaction dominate using MTO as the catalyst to transfer the oxygen atom. "We're not able to do so in every case," he admits, "but in the cases where we've had success, we can make the chemistry go very cleanly this way, and the only byproduct is a molecule of water. Not every single reaction type proved cooperative, however, so we're still pushing on some. We try to change the reaction conditions -- maybe the temperature, pH ratios of the materials or the reaction time.
"The potentially most important transformation that hydrogen peroxide brings about is in the petrochemical industry for expoxidation reactions," explains Espenson. An epoxidation reaction yields an epoxy compound, which contains a molecule in which an oxygen atom is joined to each of two carbon atoms that are already bonded. An example is the conversion of propylene to propylene oxide. Ahmad Al-Ajlouni, a graduate student working with Espenson, explored the details of these epoxidation reactions.
Although the transformations brought about by hydrogen peroxide are efficient, Espenson points out that they are not particularly economical at this time. "As efficient as the H2O2/MTO combination is, the cost of peroxide will make it impractical save for a specialized, high-value product," he explains.
Espenson would be the first to tell you that you can make all the changes in reaction conditions you want in order to achieve environmentally friendly results, but you'll be wasting your time if you ignore an important aspect of the chemistry.
"There's another part of green chemistry," he says. "You've defeated the purpose if you run a clean reaction and your solvent has to be disposed of because that solvent, itself, is an environmental problem and adds a waste-disposal cost. The fact that we can use MTO in water is really nice; we don't have to deal with organic solvents."
In addition to being stable in water, Espenson says MTO is convenient to handle. "It's a white powder, and once you prepare it and put it in a vial, you can keep it in your desk drawer. It's stable indefinitely, so you can work with it under all sorts of laboratory conditions. You don't have to treat it very delicately.
"Using MTO has given us a fundamental understanding of the way in which some small molecule can cause other molecules to rearrange their atoms in a prescribed fashion," Espenson continues. "In other words, this gives us an increasing ability to control chemical reactions in the ways that one would want."
Espenson and his co-workers are expanding their scope of and control over reactions catalyzed by MTO. They are now investigating reactions that don't take place without MTO as well as those that don't take place with it, and they're getting some interesting findings. In both instances, the reactions proceed when a small amount of a second catalyst is added. "A pinch of sodium bromide is just fine," says Espenson, "then these reactions go very cleanly. And this is very interesting because now we have two catalysts working cooperatively, doing something that neither catalyst would do on its own."
Espenson is also looking at some organic rearrangements, an area he entered in collaboration with Ziolin Zhu, a former Ames Lab graduate students. "MTO catalyzes many of the rearrangements very efficiently. One molecule is converted to another in a matter of a few hours. Had you not added MTO, the material would have sat on the shelf unchanged for a long, long time," Espenson says. "The kinetic barrier to reaction is so high that unless you add a catalyst, nothing is going to happen. MTO can interact with these molecules in such a way as to circumvent that barrier."
Espenson's accomplishments with MTO and the green chemistry it allows drew praise from the anonymous reviewer of his Petroleum Research Fund grant proposal, who stated, "This is a very interesting mechanistic and catalystic study by a very capable investigator. Jim has a good command of the chemistry, and his work has resulted in new and interesting information relating the chemistry at O[xygen], N[itrogen] and C[arbon]."
Could MTO be considered a "super catalyst" for a kinder chemistry? Espenson admits they have certainly had a lot of success with MTO. "However," he reminds us, "one has to realize that one compound is not going to catalyze all of the unsatisfactory reactions in the world. We have to choose them to match the kinds of things that MTO can offer. In particular, the thing MTO can do is transfer an oxygen atom. So when that seems to be what a reaction needs, then MTO is the catalyst to try."
For more information:
Jim Espenson, 515-294-570, espenson@ameslab.gov
Current research funded by:
DOE Basic Energy Sciences Office
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Last revision: 4/17/98 sd
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