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Brookhaven-developed Recyclable Catalyst May Help to Reduce Hazardous Industrial Waste

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.

Soluble in a reagent (A), the new catalyst remains soluble when another reagent is added (B). As the reaction goes on and the product builds up (C), the catalyst precipitates from the mixture as an oil (D), which continues as an active catalyst because the reagents can penetrate it. When the reagents are converted into product, the oily catalyst turns into a sticky solid (E), which can be easily separated and recycled.

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 its ideal form, ‘green’ chemistry eliminates the production of hazardous waste from the start of a process.”
- Morris Bullock

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).


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