Teflon solves sticky problem for catalysts
remove and recycle the catalyst once the process is complete. A new
approach, using Teflon, could make recovery simpler and cut down on
the use of solvents, writes Phil Taylor.
In many reactions, recovery of catalysts is crucial because they often contain rare metals such as platinum, they can be difficult and expensive to produce, and they can contaminate the product of the reaction they catalyse.
This is particularly difficult when the catalyst is dissolved in the same solvent as the reagents. A relatively new but popular technique uses 'ponytails' - carbohydrate chains rich in fluorine atoms - which are attached to the catalyst molecules. The catalysts can be extracted with highly fluorinated solvents quite simply - but there is a downside. The solvents required are very expensive and pose a risk to the environment.
Now, scientists at the University of Erlangen-Nüremberg in Germany have developed a new technique that promises to be cleaner, greener and more effective.
The researchers, John Gladysz and Long Dinh investigated whether a fluorine-containing solid could work as well as a liquid solvent. They showed that a rhodium catalyst for hydrosilylation of ketones - common reaction used in the synthesis of chiral pharmaceuticals - can be recovered efficiently and easily using tape made of Teflon or PTFE (polytetrafluoroethylene).
The catalyst - incorporating a fluorous ponytail - was dissolved with the reagents in the reaction flask, along with a strip of Teflon tape. Upon cooling, the orange-coloured catalyst was adsorbed onto the tape - which turned from white to orange as a result - and could be recycled simply by heating the tape in a fresh reaction mixture.
The approach allows even tiny amounts of catalyst to be recovered, according to the researchers, whose findings have been published as an early view paper on the website of the journal Angewandte Chemie International Edition.
The scientists suggest that entire reactor linings or reactor parts made of Teflon could be used to house the catalysts, which could then be released into the reaction, or removed. Alternatively, the catalyst could be supplied already adsorbed onto Teflon tape, with the desired amount added to a reaction simply by adding the required length.
