Surprise discovery that ionic liquids can be distilled
16 February 2006
Green solvents are now easier to recycle and purify, following the discovery that ionic liquids are volatile and can be distilled.
Luis Rebelo from the New University of Lisbon, Portugal, led an international team to test the hypothesis that ionic liquids (ILs) - long thought of as completely involatile - could be distilled.
'We warmed up one of these ionic liquids in vacuo and observed what the scientific community was unfoundedly denying - the evaporation of a liquid salt,' Rebelo told Chemistry World.
Rebelo expects the news to be treated with some scepticism by the IL community. 'This was precisely the original reaction of some of us,' he admitted.
ILs get their 'green' reputation from their involatility. They are green because they do not give off vapours. The latest findings do not undermine the environmentally friendly status of ILs, said Rebelo. His experiments were done under extreme conditions of high temperature and in a vacuum. He claims to have the best of both worlds - a 'green' solvent which, at high temperature, can be distilled in order to purify and recycle it after use.
Tom Welton, an IL expert from Imperial College London, UK, said the results were interesting. 'There is a race between decomposition and evaporation as temperature is increased,' he said. Ionic liquids with more reactive anions and cations decompose, but others with less reactive anions and cations can evaporate at low pressure. 'The only way to confirm that the ionic liquid is distilled as the ionic liquid species themselves is to identify the gas phase species,' he said. 'It would be very interesting to see, for example, IR spectra of the vapour.'
Rebelo sees immediate applications in distillation, leading to very pure materials, and in high-temperature crystallisation. He plans to investigate what he calls a 'still to be exploited new science': gas-phase ionic liquids - the applications of which have yet to be considered. 'I am certain that we will start looking to ionic liquids with a different perspective,' he said.
M J Earle, et al, Nature, 2006, 439, 831