Promising a greener future

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Ionic liquids have long been hailed as the future of green chemistry but can they live up to their promise? Andrew West investigates

’At the present rate of progress, unless there are significant changes in the state of the art, I think that it is unlikely that we will see widespread use of ionic liquids within the next 10 years.’

These harsh words from Alan Curzons (Chemistry World, June 2004, p11), from the corporate environment, health and safety department of GlaxoSmithKline (GSK), UK, seemingly condemn ionic liquids (ILs) to a long-term future with only academic interest rather than the industrial applications that have long been heralded by research leaders.

Curzons continues that, without further study on the safety, disposal and environmental impacts of ILs, the statement by scientists that they are green chemicals is highly debateable. While ILs have no measurable vapour pressure and so are non-volatile, Curzons argues ’this is only a relatively small component’ of a solvent’s green qualities.

It may seem surprising therefore that, one month after Curzons made his comments, Scionix, a joint venture between the University of Leicester, UK, and Genacys, a subsidiary of Whyte Chemicals, UK, won the prestigious Crystal Faraday Green chemical technology award for developing two processes using ILs. According to Crystal Faraday, these processes ’offer significant improvements in chemical processes, products and services so to achieve a more sustainable, cleaner and healthier environment as well as creating competitive advantage’.

The award clearly flies in the face of Curzons’ caution, but with all the hype surrounding ILs, it remains difficult to tell whether they will be the next big thing or consigned to the academic backburner as a curiosity.

So just how green are ionic liquids? Was Curzons right to question the term’s validity when applied to ILs, or did Crystal Faraday, the body hoping to improve green chemistry interactions between the UK science and technology base and industry, make the right choice when it awarded Scionix with its Green chemical technology award? Will it really be 10 years before ILs see widespread application?

ILs are mixtures of anions and cations with a melting point around 100?C and there is little doubt that they are useful and effective in chemical syntheses. ILs as alternative solvents for organic transformations have been extensively investigated by academics around the world, with particular attention being paid to Heck reactions using palladium catalysts. However, for researchers to hail ILs as green simply on the basis of their lack of vapour pressure and reusability seems highly questionable. There are far more factors to consider.

To begin with, it is worth clarifying what the term ’green’ actually means. Anastas and Warner defined and set out the principles of green chemistry in 1998 (see box). Their 12 principles include statements such as waste prevention is better than clean-up or disposal, catalytic reactions are better than stoichiometric ones and atom economy in reactions is desirable, with each principle being general in its definition. Unfortunately, this allows researchers to apply the term ’green’ to a chemical process to make it fit in with in vogue chemistry, often with only the curtest of nods given towards these principles.

This confusion surrounding the definition of a green process only clouds the issue and makes chemists and the general public sceptical of technologies hailed as environmentally friendly. ’Clearly, when a subject becomes fashionable, people will look at their own work to see how it fits in with the new area for work being created. This isn’t a bad thing; it’s one of the ways in which new ideas are generated,’ says Tom Welton from Imperial College London, UK. ’However, one of the things that we all do is to focus on our own, inevitably small, part of the picture and although our bit may fit a green idea, one often only needs to go back a step to find dirty chemistry.’

When ILs are considered generally in terms of the 12 principles of green chemistry, they do fit in with the ideas of safer solvents compared to standard organic solvents because they are non-flammable, have no vapour pressure and they can be reused. However, the major concerns, as highlighted by Curzons, are the unknown toxicity of ILs to humans or the environment and the unexplored disposal methods for most ILs. Those ILs based on imidazole and fluorinated anions are likely to be toxic and, while they cannot enter the environment by evaporation, most ILs are water soluble and could easily enter the biosphere this way.

To assess fully a compound’s green qualities, we need to consider how the material is made. Imidazolium ions are generally made by reacting an imidazole-based starting material with a linear halogenoalkane, such as 1-bromohexane. ILs with alternative anions can then be prepared either by reaction with salts, such as silver tetrafluoroborate for anion exchange, or metal halides such as aluminium trichloride.

Imidazole and halogenoalkanes come from petroleum feedstocks that are neither green nor sustainable; hexafluorophosphate anions are often generated using toxic silver or arsenic salts and the phosphorus obtained electrochemically, requiring vast amounts of energy and generating much pollution; chloroaluminate anions are generated using aluminium trichloride, which comes from the aluminium industry. In short, at the moment ILs are not made using green chemicals, so it should be unsurprising that ILs themselves can be classed as ’light green’ at best.

But ILs cannot be written off so easily as dubiously green. To get a better picture, we need to look at specific examples of IL use rather than generic applications.

The Crystal Faraday award was given to Scionix in recognition of the advances researchers had made in two specific chemical technologies using ILs. The first was using an ionic liquid to replace chromic acid in the chromium plating industry. Chromic acid is a highly toxic, carcinogenic compound that is so hazardous it is banned from most processes. However, a lack of alternatives means it is still used to chrome plate more reactive metals like aluminium.

Using an ionic liquid formed by reacting chromium(iii) chloride with choline chloride, the team at Scionix significantly reduced the hazards associated with the process while increasing overall current efficiency from around 15 per cent to over 90 per cent. The change from using the electrochemical chromic acid chromium plating process using Cr(vi) to the electrochemical ionic liquid process using Cr(iii) fulfils at least eight of the 12 green chemistry principles, making it hard to argue that the process is not green.

’ILs are particularly beneficial to this project because not only do they replace the hazardous plating solution, they also lead to a considerable increase in the electrochemical efficiency of the process,’ says Khalid Shukri from Whyte Chemicals. He adds ’The working conditions for operators would also be significantly improved.’

The second process that Scionix has pioneered is environmentally friendly electropolishing. Normally, concentrated sulfuric or phosphoric acids are used to electropolish stainless steel to increase corrosion resistance and smoothness. Replacing the acids with an ionic liquid again increases current efficiency to over 90 per cent, while improving corrosion resistance and smoothness further. The improved process using an ionic liquid fulfils more than six of the green-process criteria.

’Scionix has spent a significant amount of effort in developing the technology,’ Shukri explains. ’Winning the Crystal Faraday Green chemical technology award was a very important stage for us as it acknowledged the major breakthrough in green technology that was developed’.

These examples clearly show how ILs can be used in green chemical technology without themselves being classed as green; the IL used in chromium plating for example is based on chromium chloride, which is still produced by heavy industry. This explains GSK and Crystal Faraday’s differing opinions.

Curzons quite rightly claims that it is inappropriate to class all ILs as green without further testing and research, but Crystal Faraday’s decision to give its Green chemical technology award to Scionix for specific green chemical processes rather than general, so-called green chemicals was wholly appropriate. ’In process chemistry no one component is green, only the overall process,’ comments Welton. ’It is only in the eventual application that the "greenness", or otherwise, will be apparent.’

If a truly green ionic liquid were to be produced, the components would need to be sustainable, easy and clean to prepare, non-toxic and biodegradable. This can’t be achieved when standard halogenated anions and imidazolium-based cations are used.

However, ILs can be made using simple organic halide salts and complexing agents that form a hydrogen bond. This complexing agent decreases the interaction between the anion and the cation and therefore lowers the mixture’s freezing point.

Work at the University of Leicester on these so-called eutectic solvents has shown that a wide variety can be produced using cheap, non-toxic and biodegradable starting materials. One good example is Reline 203, a mixture of choline chloride and urea. Choline chloride is vitamin B4 and is produced on a mega tonne scale as a chicken feed additive. Like urea, it is readily available and very cheap.

Andrew Abbott, researcher at Leicester, explains, ’for specific applications, particularly large volume ones, imidazolium-based ILs are not suitable because of their cost and necessity for registration from health and safety perspectives. Our eutectic solvents are analogous to ILs in that they have similar properties but, because they are mixtures of bulk commodity chemicals, they do not require registration and they are trivial to make.’

These ILs have other benefits. The toxicity and disposal methods for the parent compounds are already well known. Maybe of more significance to industry however, such ILs are much cheaper than current examples; Reline 203 can be bought for around ?200 per kilogram as opposed to over ?1000 for some of the ILs that have become standards. These factors should encourage their use in industrial applications, which will bring costs down as demand increases.

However, fewer reactions have been carried out in this type of media than in imidazolium-based ILs and their effectiveness as solvents compared to other ILs is less well understood. ’They have totally different chemistry to the imidazolium-based ILs,’ says Abbott. ’The applications we are looking at primarily centre on electrochemical techniques such as metal plating, batteries and metal extraction. Since most electroplating processes require about a tonne of electrolyte and work upward, there is no way that imidazolium-based ILs are applicable,’ he adds

Looking to the future, Welton remains pragmatic about ILs. ’The one thing I have learned in working with ILs is that any predictions that you make will be proved wrong.’ On the future of ILs, he continues, ’I believe that what is required now is a more selective and rational approach. What we want to avoid is an unrealistic battle between "ionic liquids are going to save the world" and "if this can’t be achieved in three months for the price of a postage stamp it should all be abandoned". I’m exaggerating to make the point, but you see elements of both of these stances at pretty much every ionic liquids meeting.’

What of the statement that it will be 10 years before ILs see widespread use? Abbott comments: ’I would agree that using imidazolium salts as simple solvents for organic reactions is probably not going to find widespread application for some time. I would strongly disagree, however, that ILs in general will not be used in this time scale. I think that they will find successful commercialisation relatively soon, probably using their electrochemical properties or phase separation capabilities.’ It seems that, as far as ILs are concerned, green chemistry’s green qualities are in the eye of the beholder.

Andrew West is a science writer and post-doctoral researcher at Queen’s University in Belfast, UK