Is catalysis in ionic liquids a potentially green technology?

The amount of interest in ionic liquids over the past decade has been simply staggering. This issue of Green Chemistry contains a review of the use of catalysts bearing a charged functional group designed to impart better retention of the catalyst in the ionic liquid in biphasic and/or continuous operation. Much, even most, of the activity in this field has centred on the claimed use of ionic liquids as so-called green solvents. This claim has been, to say the least, controversial, and I have seen debates around this get quite animated. Claims and counter claims are made, positions taken and plenty of heat generated, but seldom much light. Although I write from the position of working with ionic liquids, similar debates can be found elsewhere.

Early claims to the greenness of ionic liquids rested on their extremely low vapour pressures. Since the ionic liquids cannot evaporate under typical operating and storage conditions, they cannot leak into the atmosphere, as do most organic solvents that are VOC's. Many people have rightly pointed out that this does not on its own constitute a reason to declare all processes conducted in ionic liquids to be green. They note that ionic liquids are often made from ions that are toxic and that may accumulate in the environment and that they are usually made by multi-step syntheses, often using the very solvents that they are replacing. Others have countered that ionic liquids can be made to be less toxic, more biodegradable, from more environmentally benign sources and that they can offer process advantages that outweigh the other disadvantages.

Some would say that the thorough way to resolve this issue is to use cradle-to-grave life-cycle analysis. However, to do this requires information about how the particular ionic liquid performs in comparison to the solvent to be replaced in the process in which it is to be used, as well as detailed information about the source, number of recycles possible and disposal of both the ionic liquid and the replaced solvent. LCA is a lengthy and costly technique, and having performed the analysis the results tell you little about how this or other ionic liquids will perform in other processes. It is very good at providing the information required to make decisions about the final version of an industrial process to be implemented, but it is less able to give guidance about whether some general area of research is worth pursuing.

How does one escape this impasse? First, I would say that it important to recognise what question should be being asked at the beginning of the research process. If one had sufficient information to hand to be able to undertake a detailed LCA there would probably be no need to conduct further studies. The question that is really being asked is whether the technology to be applied offers the potential to provide greener chemicals processes. One could try testing this question against the 12 principles of green chemistry. So this is what I have tried to do for catalysis in ionic liquids below using information contained in the review and my background knowledge of the area:

1. Prevent waste. Some catalytic ionic liquid processes are more selective and give higher yields of the product than alternative routes. Some catalysts have been designed to be sufficiently retained and stabilized in the ionic liquid solution so that many recycles can be achieved. It is possible to think of ways in which these catalysts can be converted into recoverable forms, but much less is known about this.

2. Design safer chemicals and products. If we think of the ionic liquid as the product in this case, they are generally non-volatile and non-flammable liquids and work is already underway to make these from non-toxic materials.

3. Design less hazardous chemical syntheses. See 2 above.

4. Use renewable feedstocks. There are already ionic liquids made from renewable resources in use. There has been a great deal of recent activity on the use of ionic liquids as process solvents for biomass derived starting materials.

5. Use catalysts, not stoichiometric reagents. Catalysis in ionic liquids does this.

6. Avoid chemical derivatives. Catalytic routes often avoid protection/deprotection steps. It is possible that the careful selection of ionic liquid for the process may improve the selectivity of a catalyst and so aid this.

7. Maximize atom economy. Some catalytic ionic liquid processes are more selective and give higher yields of the product than alternative routes.

8. Use safer solvents and reaction conditions. Ionic liquids are generally non-volatile and non-flammable solvents, work is underway to reduce their toxicity and improve their biodegradability.

9. Increase energy efficiency. Ionic liquids have been shown to increase the rates of some reactions, and even act as co-catalysts. Increased rates allow for less energy input.

10. Design chemicals and products to degrade after use. There is work in the literature on designing biodegradable ionic liquids.

11. Analyze in real time to prevent pollution. Ionic liquids are compatible with most spectroscopic techniques.

12. Minimize the potential for accidents. Ionic liquids are generally non-volatile and non-flammable solvents.

So, the answer to the question of whether catalysis in ionic liquids has the potential to provide greener chemicals processes is overwhelmingly yes. It does not mean that it will always provide the greenest process for all products. Solving that problem is what research is for.

Tom Welton

Professor of Sustainable Chemistry, Imperial College of Science, Technology and Medicine, South Kensington, London, SW7 2AX, UK

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