Ionic liquids pair up for catalysis

Researchers have taken a key step towards rationalising the solvent properties of ionic liquids (ILs), which are composed entirely of ions and have been hailed the ’green’ solvents of the future (Chemistry World, June 2004).

The green credentials of ILs rely in part on their virtually non-existent vapour pressure. The downside of this is that important solvent parameters, which can normally be derived from the internal energy of evaporation, are not so easily obtained for ILs. Andrew McLean of the University of Paisley, UK, told delegates at a recent meeting of the RSC applied catalysis group how his team has studied reaction rates in different ILs so researchers might now be able to design new ILs for specific purposes.

From the bimolecular rate constants of the Diels-Alder reaction between singlet oxygen and 1,4-dimethylnaphthalene the researchers derived the Hildebrand solubility parameter for a range of ILs. These were composed of the cations 1-methyl-3-butylimidazolium, [bmim]⁺, or its methyl derivative [bm₂im]⁺, paired with a range of commonly used anions including [PF₆]^- and [BF₄]^-. This parameter is related to the cohesive pressure of a liquid and can be used to predict solvent properties.

At the same meeting, Tom Welton from Imperial College, London, UK, presented results demonstrating that ILs used in catalysis can play a number of different roles, as catalyst support, co-catalysts or activators, for example. They have proven particularly useful in combination with palladium (Pd) catalysts, where they can serve as a Pd source. A surprising result from this work was the observation that ILs used with certain catalysts can act as dehydrating agents and make molecular sieves unnecessary. This can be rationalised by the fact that water associates with the IL anions, while the catalyst is in the proximity of the cations, such that water is kept away from the reaction.

Michael Gross

Further information: K Swiderski et al, Chem. Commun, 2004, 2178