3 June 2005: Catalyst recycling on tape
Chemists in Germany have demonstrated that certain classes of catalyst can be efficiently and simply recovered from and released into reaction mixtures by using cheap, commercially available Teflon tape.
Recycling catalysts from reaction mixtures makes sound economic sense - catalysts can be expensive or difficult to manufacture and they can contaminate the final product. But many homogeneous catalysts can be tricky to remove from the mixture once the reaction is complete.
Work by John Gladysz and Long Dinh at the University of Erlangen-Nüremberg opens the intriguing possibility of lining an entire reactor with Teflon as a way of controlling the release and recovery of catalysts in reactions. It might also be feasible to load Teflon strips with catalysts as a way to deliver the catalyst to a reaction mixture, making it unnecessary to weigh the reagent; small amounts of catalyst could be measured out by length of tape.
The system relies on two phenomena: the fact that some catalysts exhibit thermomorphic behaviour - their solubility is temperature-dependent; and the ability to attach a flourine-containing 'ponytail' - an aliphatic hydrocarbon chain containing many fluorine atoms - to catalytic molecules to render them soluble in fluorous solvents or amenable to adsorption on solid fluorous supports.
Gladysz and Dinh tested their system by carrying out a ketone hydrosilation reaction using rhodium complexes as catalysts to which fluorine-containing ponytails had been incorporated. These red-orange catalysts are insoluble in organic solvents at room temperature, but dissolve at higher temperatures.
A strip of Teflon tape was introduced into the reaction vessel. When the temperature of the reaction mixture was elevated the catalyst dissolved and participated in the reaction. When the temperature was reduced the catalyst precipitated from the mixture and was adsorbed onto the Teflon tape highly efficiently.
Such 'ponytailed' molecules can be recovered by dissolution into fluorous solvents, but these solvents are expensive and have potential environmental drawbacks. Similarly, Teflon shavings have been used in the past. The advantage of the tape is that it is cheap and easily engineered.
Gladysz speculates that it might be possible to line entire reactors with Teflon as a simple way to recover 'ponytailed' catalysts. The catalyst could be released into or removed from the reaction mixture simply by controlling the temperature. Alternatively, a prefabricated catalyst-coated Teflon strip could be used to deliver the catalyst to the reaction mixture. 'Even small amounts of catalyst could then be measured out by units of length,' said Gladysz. 'No weighing necessary.' Simon Hadlington
L V Dinh and J A Gladysz, Angew. Chem. Int. Ed., (DOI: 10.1002/anie.200500237)