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Scientists develop recyclable catalyst for solvent-free reactions

DOE/Brookhaven National Laboratory : 19 May, 2007  (Technical Article)
Chemists at the U.S. Department of Energy
Such a complete transformation of reactants to products without the need for additional separation steps is particularly attractive in the manufacture of pharmaceuticals, which must be pure and free from residual metal catalysts. But it should be of interest to all in industry who seek to reduce waste and the cost of manufacturing processes.

“Avoiding the use of solvents is an important way to prevent waste in chemical manufacturing processes,” said Brookhaven chemist Morris Bullock, who led the research. This new catalyst, developed by Vladimir Dioumaev, a postdoc in Bullock’s lab, achieves that goal in two ways:

First, as a homogeneous catalyst, it dissolves in the reactants. That eliminates the need for a solvent to mix the reagents to get the reaction going. Second, it precipitates as a solid at the end of the reaction, so no solvents or additional steps are required to separate the catalyst from the products. “It separates itself. You can simply pour the products into another container and use the catalyst again,” Dioumaev said.

“The concept is simple, but striking the right balance between maintaining catalyst solubility throughout the reaction and precipitation at the end is a diabolical problem,” Bullock said. Homogeneous catalysts usually remain dissolved at the end of the reaction, presenting a problem in recovery of the catalyst. But a catalyst that precipitates before the reaction is over would lead to incomplete conversion of the reactants. The Brookhaven team was trying to develop a catalyst that would retain some solubility in the liquid phase until all the reactants were used up, but which would then precipitate to facilitate recovery and recycling of the catalyst.

The catalyst is soluble in one of the reagents and remains soluble when the other reagent is added. As the reaction goes on, and the product builds up, the catalyst precipitates from the mixture as oil. This oil - liquid clathrate - remains to be an active catalyst, as the reagents are able to penetrate into it. When all the reagents are converted into products, the oily catalyst turns into a sticky solid, which can be easily separated and recycled.

To explore the concept, the scientists investigated a variety of catalysts designed to react ketones, a class of organic molecules, with organic silicon compounds. These reactions, called hydrosilylation reactions, can be used in the manufacture of drugs, pesticides, and other organic compounds. The products of such reactions are also used in the preparation of ceramic materials.

The catalysts they experimented with had been found in their earlier work to dissolve in ketones but form oily precipitates, known as liquid clathrates, from many other solvents. Such an oily phase, they thought, might be ideal for maintaining the ability to catalyze the reaction to completion, because the oil would allow access to the liquid reactants rather than settling out as a solid precipitate would. By experimentally altering the structure of the catalyst, the scientists arrived at a formulation containing the metal tungsten that accomplished their goal.

At first, the catalyst mixes readily with the reactants. Then, as the reaction progresses to near completion, the catalyst begins to precipitate, but remains suspended as an oily liquid clathrate, clearly visible in the tube where the reaction is taking place. “The reagents can penetrate the oil to keep reacting until all the reactants are used up,” Dioumaev said. When that happens, the no-longer-soluble catalyst precipitates out as a solid, which settles to the bottom of the test tube. Analysis revealed that there was essentially no catalyst remaining in the liquid products.

The Brookhaven scientists plan to continue their research to see if this general method of catalyst self-separation using liquid clathrate catalysts can be applied to other reactions. This research often requires knowledge of the molecular structure of the catalyst, and the tungsten catalyst used in this research was deciphered at the National Synchrotron Light Source at Brookhaven Lab. A paper on the structural determination was recently published in Chemical Communications.
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