Ionic liquid ion exchange: exclusion from strong interactions condemns cations to the most weakly interacting anions and dictates reaction equilibrium

Abstract

It is well-known that for a fixed ionic liquid cation, variation of the anion determines the physicochemical properties and the activity as solvent or catalyst. Although ion exchange is an acknowledged phenomenon, it is often overlooked in ionic liquid applications due to a lacking framework. We propose here a framework to describe and predict ionic liquid ion exchange with a gain in interaction energy as driving force. The low charge density ionic liquid cations display low interaction energies and are condemned to pair with the most weakly interacting anions in the system, after all stronger interactions have formed. This is demonstrated both for phosphonium and imidazolium ionic liquids, with and without hydration effects. The importance on processing is exemplified by esterification, in which the degree of exchange of carboxylic acid for carboxylate and the availability of protons determines the conversion.