Martyn J. Earle

Ionic liquids. Green solvents for the future

Ionic liquids, being composed entirely of ions, were once mainly of interest to electrochemists. Recently, however, it has become apparent that, inter alia, their lack of measurable vapor pressure characterizes them as green solvents, and that a wide range of chemical reactions (reviewed here) can be performed in them.

The distillation and volatility of ionic liquids

It is widely believed that a defining characteristic of ionic liquids (or low-temperature molten salts) is that they exert no measurable vapour pressure, and hence cannot be distilled. Here we demonstrate that this is unfounded, and that many ionic liquids can be distilled at low pressure without decomposition. Ionic liquids represent matter solely composed of ions, and so are perceived as non-volatile substances. During the last decade, interest in the field of ionic liquids has burgeoned, producing a wealth of intellectual and technological challenges and opportunities for the production of new chemical and extractive processes, fuel cells and batteries, and new composite materials. Much of this potential is underpinned by their presumed involatility. This characteristic, however, can severely restrict the attainability of high purity levels for ionic liquids (when they contain poorly volatile components) in recycling schemes, as well as excluding their use in gas-phase processes. We anticipate that our demonstration that some selected families of commonly used aprotic ionic liquids can be distilled at 200–300 °C and low pressure, with concomitant recovery of significant amounts of pure substance, will permit these currently excluded applications to be realized.

Diels–Alder reactions in ionic liquids. A safe recyclable alternative to lithium perchlorate–diethyl ether mixtures

Diels–Alder reactions in neutral ionic liquids (such as 1-butyl-3-methylimidazolium trifluoromethanesulfonate, 1-butyl-3-methylimidazolium hexafluorophosphate, 1-butyl-3-methylimidazolium tetrafluoroborate, and 1-butyl-3-methyl-imidazolium lactate) are reported. Rate enhancements and selectivities similar to those of reactions performed in lithium perchlorate–diethyl ether mixtures have been observed. As the ionic liquids used have no measurable vapour pressure, are thermally robust, will tolerate impurities such as water, and are recyclable, it is envisaged that these systems could be used on an industrial scale.

Separation of aromatic hydrocarbons from alkanes using the ionic liquid 1-ethyl-3-methylimidazoliumbis{(trifluoromethyl) sulfonyl}amide

The liquid–liquid equilibrium for the ternary system formed by hexane, benzene and the ionic liquid 1-ethyl-3-methylimidazoliumbis{(trifluoromethyl)sulfonyl}amide, [C2mim][NTf2], has been experimentally determined at 25 °C and 40 °C. The results show that the [C2mim][NTf2] can selectively remove benzene from its mixtures with hexane, suggesting that this ionic liquid can be used as an alternative solvent in liquid extraction processes for the removal of aromatic compounds from their mixtures with alkanes.

Antibiofilm activities of 1-alkyl-3-methylimidazolium chloride ionic liquids

Microbial biofilms are ubiquitous in nature and represent the predominant mode of growth of microorganisms. A general characteristic of biofilm communities is that they tend to exhibit significant tolerance to antimicrobial challenge compared with planktonic bacteria of the same species The antibiofilm activity of a series of 1-alkyl-3-methylimidazolium chloride ionic liquids has been evaluated against a panel of clinically significant microbial pathogens, including MRSA. A comparison of antimicrobial activity against planktonic bacteria and established biofilms is presented. In general, these ionic liquids possess potent, broad spectrum antibiofilm activity.

The first high yield green route to a pharmaceutical in a room temperature ionic liquid

The synthesis of the pharmaceutical Pravadoline is achieved in high yield without the production of noxious waste products; the ionic liquid can be recycled and reused in this methodology.

Bis{(trifluoromethyl)sulfonyl}amide ionic liquids as solvents for the extraction of aromatic hydrocarbons from their mixtures with alkanes: effect of the nature of the cation

The liquid–liquid equilibria of two ternary systems comprising hexane, benzene and a bis{(trifluoromethyl)sulfonyl}amide ionic liquid, [A][NTf2] (where A = cation), have been determined at 40 °C. Specifically, the cations of the ionic liquids investigated are 1-ethylpyridinium, [C2py]+, and (2-hydroxyethyl)trimethylammonium, [N1 1 1(C2OH)]+. The effectiveness of each ionic liquid as solvent for the separation of benzene from its mixtures with hexane was evaluated, and compared to that of 1-ethyl-3-methylimidazolium bis{(trifluoromethyl)sulfonyl}amide, [C2mim][NTf2], as well as of trihexyl(tetradecyl)phosphonium bis{(trifluoromethyl)sulfonyl}amide, [P6 6 6 14][NTf2]. It was observed that the ammonium ionic liquid leads to higher selectivities in a certain range of global compositions, but it presents lower distribution ratios. No significant difference was observed between the pyridinium and the imidazolium ionic liquids. It was concluded that these ionic liquids can be envisioned as potential alternatives for the replacement of conventional organic solvents currently used for the separation of aromatic and aliphatic hydrocarbons. The liquid–liquid equilibrium data were acceptably correlated by means of the ‘Non-Random Two-Liquid’ (NRTL) equation.

FRIEDEL-CRAFTS REACTIONS IN ROOM TEMPERATURE IONIC LIQUIDS

Friedel–Crafts reactions in the ionic liquid system 1-methyl-3-ethylimidazolium chloride–aluminium(III) chloride can be performed with excellent yields and selectivities, and in the case of anthracene, have been found to be reversible.

Mutually immiscible ionic liquids

This work presents the novel discovery of room-temperature ionic liquids that are mutually immiscible, some of which are also immiscible with solvents as diverse as water and alkanes; an archetypal biphasic system is trihexyltetradecylphosphonium chloride with 1-alkyl-3-methylimidazolium chloride (where the alkyl group is shorter than hexyl).

Regioselective alkylation in ionic liquids

The room temperature ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate, [bmim][PF6], is used as a ‘green’ recyclable alternative to dipolar aprotic solvents for the regioselective alkylation at the heteroatom of indole and 2-naphthol.