Ionic liquids in Green Chemistry

The launch of Green Chemistry in 1999 coincided with the explosion of interest in ionic liquids that was associated with the arrival of good quality, accessibly priced, commercial ionic liquids. This led to large numbers of publications on the potential of ionic liquids to act as solvents for chemicals synthesis, often justified by the ionic liquids being a green alternative to conventional molecular solvents. Also, a number of papers reporting the synthesis and properties of ionic liquids were published in the journal. After all, if ionic liquids are green solvents then where else could be more appropriate to report their various properties and uses?

In those earlier days the claim that ionic liquids were green solvents largely rested on the (then believed) non-volatility of ionic liquids and its associated properties (low flammability, ease of containment etc.). This claim has since been challenged on several occasions, particularly with the toxicity and environmental persistence of the most widely used ionic liquids being noted as important negative green factors. In response to these challenges ionic liquids have been being designed for low toxicity and biodegradability.

Alongside this debate, the subject of green chemistry has been evolving rapidly. Today we would consider the notion of any solvent being intrinsically green to be based upon too narrow a view of what it means to be green. As a discipline, we would now judge a process solvent in terms of the overall environmental impact of the process that it is a part of. For instance, we would now recognise that if the use of a more hazardous solvent gave significant improvements to the total environmental impact of the overall process than a less hazardous alternative, then the greener choice is the former. Similarly, one could imagine a situation wherein a commercial product may have its overall environmental impact reduced by using a more hazardous liquid than a less hazardous one. So, if there is no such thing as an intrinsically green solvent, should Green Chemistry now stop publishing papers on ionic liquids?

The simple answer to this question is, of course, NO. However, we do now need to be a little more sophisticated in our selection of which papers are appropriate for Green Chemistry and which are not. As demonstrated by the contributions to the recent Green Solvents for Synthesis conference in Berchtesgaden, a solvent only becomes green in its use. It is clear that ionic liquids are capable of delivering chemical synthesis processes that are greener than the ones they replaced, partly because they are already doing so in a number of commercial applications. It is also clear, again because such things exist, that they can provide components in devices whose use can lead to environmental benefits. Articles that are about subjects such as these are entirely appropriate for Green Chemistry. That is not to say that these articles must always be at the near application stage of the research, with full life-cycle analysis completed, but an environmental benefit must be the purpose of the work. For example, I have myself published on the subject of physical measurements to describe the polarity of ionic liquids. In spite of the fact that this is important information for green chemists who are interested in using ionic liquids as solvents, it is not itself green chemistry. Hence, it should be and was published elsewhere in a more appropriate journal. On the other hand a paper that describes how using an ionic liquid with a particular polarity has led to a greener chemical process would be appropriate for Green Chemistry.

One subject that has been discussed in-depth by the Editorial Board in this respect has been ionic liquid toxicity. The outcome of these discussions has been a decision that ionic liquid toxicity should be treated in the same way as physical properties. Papers that are simply on the subject of ionic liquid toxicity will no longer be accepted by Green Chemistry. For papers including toxicity studies to be accepted, they must demonstrate how these lead to the selection or design of ionic liquids for some green process or product.

Journals must evolve and develop with the subjects that they cover and, unless a journal is going to become one that is ready to accept papers on all subjects, it is necessary to set boundaries. This inevitably means the some submissions will be considered to be outside the scope of the journal. This is in no way a criticism of the quality of the work being reported, just a judgement on its fit. We do accept that there are often fine judgements where this is concerned, and differences of opinion can occur. However, we assure all potential authors that these decisions are always taken after thorough consultation with experts in the field.

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