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DBL%20Hendrix%20small.png College chemistry, 1983

Derek Lowe The 2002 Model

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Derek Lowe, an Arkansan by birth, got his BA from Hendrix College and his PhD in organic chemistry from Duke before spending time in Germany on a Humboldt Fellowship on his post-doc. He's worked for several major pharmaceutical companies since 1989 on drug discovery projects against schizophrenia, Alzheimer's, diabetes, osteoporosis and other diseases. To contact Derek email him directly: Twitter: Dereklowe

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June 11, 2013

The Overselling of Ionic Liquids

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Posted by Derek

Ionic liquids (molten salts at relatively low temperatures) have been a big feature of the chemical literature for the last ten or fifteen years - enough of a feature to have attracted a few disparaging comments here, from me and from readers. There's a good article out now that talks about the early days of the field and how it grew, and it has some food for thought in it.

The initial reports in the field didn't get much attention (as is often the case). What seems to have made things take off was the possibility of replacing organic solvents with reusable, non-volatile, and (relatively) non-toxic alternatives. "Green chemistry" was (and to an extent still is) a magnet for funding, and it was the combination of this with ionic liquid (IL) work that made the field. But not all of this was helpful:

The link with green chemistry during the development of the IL field, propelled both fields forward, but at times the link was detrimental to both fields when overgeneralizations eroded confidence. ILs were originally considered as green since many of these liquid salts possess a negligible vapor pressure and might replace the use of volatile organic solvents known to result in airborne chemical contamination. The reported water stability and non-volatility led to the misconception that these salts were inherently safe and environmentally friendly. This was exacerbated by the many unsubstantiated claims that ILs were ‘green’ in introductions meant to provide the motivation for the study, even if the study itself had nothing to do with green chemistry. While it is true that the replacement of a volatile organic compound (VOC) might be preferred, proper knowledge of the chemistry of the ions must also be taken into account before classifying anything as green. Nonetheless, the statement “Ionic Liquids are green” was widely published (and can still be found in papers published today). Given the number and nature of the possible ions comprising ILs, these statements are similar to “Water is green, therefore all solvents are green.”

There were many misunderstandings at the chemical level as well:

However, just as the myriad of molecular solvents (or any compounds) can have dramatic differences in chemical, physical, and biological properties based on their chemical identity, so too can ILs. With the potential for 10^18 ion combinations, a single crystal structure of one compound is not a good representation of the chemistry of the entire class of salts which melt below 100 °C and would be analogous to considering carbon tetrachloride as a model system for all known molecular solvents.

The realization that hexafluorophosphate counterions can indeed generate HF under the right conditions helped bring a dose of reality back to the field, although (as the authors point out), not without a clueless backlash that decided, for a while, that all ionic liquids were therefore intrinsically toxic and corrosive. The impression one gets is that the field has settled down, and that its practitioners are more closely limited to people who know what they are talking about, rather than having quite so many who are doing it because it's hot and publishable. And that's a good thing.

Comments (12) + TrackBacks (0) | Category: Chemical News | The Scientific Literature


1. NoHFforme on June 11, 2013 11:46 AM writes...

I stayed away from ionic liquid catalyzed hydrocarbon chemistry because the research reactors kept catching fire; i.e. they're on my 'Will not work with' list. That said ionic liquids appear to be a more "pleasant" alternative to HF based alkylation process units one finds in refineries. I guess it's an improvement to go from getting a chemical burn from HCl rather than HF.

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2. BG on June 11, 2013 1:59 PM writes...

I know ionic liquids are good solvents for reactions done in a microwave, on account of their high dialectric constant. But, I've never used them or seen anyone use them.

Are they being applied as solvents in industry at all, where they are being recycled?

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3. Anonymous on June 11, 2013 3:28 PM writes...

On my plant, purchase and disposal of solvents typically contributes less than 1% to the overall cost. It would have to be an exceptional project where a costly ionic liquid would be considered; the plant time associated with recycling isn't trivial either.

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4. milkshake on June 11, 2013 4:37 PM writes...

one of early pioneers of ionic liquids was one John Clark, of the Ignition! fame, in 50s. He was making monopropelants consisting of organic amine nitrate salts dissolved in fuming nitric acid. He observed that ethylamine nitrate salt is a liquid at room temperature. His goal was to have some stable storable liquid monopropellant composition that does not crystallize in cold weather...

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5. SmallMoleculeCrystallographer on June 11, 2013 5:41 PM writes...

There has been some industrial use of ILs, and there's certainly plenty of companies making them - or there were when I was working with them five years ago! BASF has at least one synthetic process using a biphasic system with IL as one phase - the advantage of this being that you get clean phase mixing for reaction, then separation under (I think) thermal control, which enable repeated cycling of the IL solvent. BASF has also done plenty of work on other separation-type chemistry involving ILs - a variety of it in collaboration with the Rogers lab (authors of the article mentioned). Of course, if you look back through some of their earlier work, you do see a certain amount of the same "upselling" of ILs as they are being critical of in the current article!

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6. gippgig on June 12, 2013 12:28 AM writes...

In 2015 NASA plans to launch the Green Propellant Infusion Mission testing the ionic liquid monopropellant AF-M315E (AF-315E (which seems to be the same thing) is a mix of hydroxylammonium nitrate (HAN), hydroxyethylhydrazinium nitrate (HEHN), & water.) It's much safer than the currently used hydrazine and significantly more powerful with a specific impulse (the rocket version of MPG) of 263 vs. 234 for hydrazine.

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7. sepisp on June 12, 2013 1:54 AM writes...

Fundamentally it's your own fault if you take hype too seriously. You have to stuff the introduction with *something*, and particularly if you want to publish in Green Chemistry, you have to explicitly claim it's green. The potential environmental toxicity of imidazoliums has been known for a while, and the problem has been already discussed in the literature.

On recycling, there are also ionic liquids you can distil, see for instance 10.1002/ange.201100274

The field seems to be entering a phase where the hype dies down, but you start to see serious patents and process evaluations appearing. There are more than 100 of these now, and I know of three more.

The question really boils down to if you need an "ultra-polar" solvent. For instance, basic ionic liquids can dissolve cellulose and other polymers with a solvent you can dip your hand in. That's not empty hype.

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8. Paul on June 12, 2013 7:50 AM writes...

The startup Fluidic Energy is working on zinc-air batteries using ionic liquids for the electrolyte. For air batteries, the ionic liquids eliminate the need for water level control, since the electrolyte does not evaporate.

I don't know just how "real" this technology is, but it has been undergoing testing in the field at various customer sites (I think mostly as backup power at cell phone towers) around the world.

Zinc-air batteries, if they can be made effectively rechargeable, may be very important, since they have the potential to be among the least expensive ways to store electrical energy.

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9. a. nonymaus on June 12, 2013 11:36 AM writes...

Re: #7
That's some interesting solubilizing. I can see how cellulose would dissolve, especially with a hdroben-bond accepting cosolvent or basic anion. IIRC an earlier approach used something like saturated lithium salts in DMF or NMP with an amine oxide. What surprises me is that it's apparently not a good solvent for the keratin of one's epidermis. What gives?

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10. Curt F. on June 12, 2013 12:25 PM writes...

I thought one of the problems with ionic liquids was viscosity. Never having worked with any myself, I don't know for sure. Aren't they much, much more viscous than most other solvents? I can see how that would be annoying.

"Deep eutectic solvents", pseudo-ILs that can be formed by mixing two organic solids, like choline chloride and urea, were a neat idea I read up on awhile ago, but viscosities were something like 100x that of water, which seemed like it would be bad for anything at process scales.

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11. SmallMoleculeCrystallographer on June 13, 2013 12:38 PM writes...

Re: #10
Viscosity in ILs is one of those properties that can be varied by choice of cation/anion, and tuning towards desired particular properties. From memory of the systems I worked with they would have been more viscous than many common solvents, but not to the point where they were impossible to work with.

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12. Tatvachintan on April 10, 2014 12:18 AM writes...

No matter how much hype is created about Ionic Liquids, you can't ignore its importance as they are indeed very useful chemical & in India, where i live, it makes a significant part of overall chemical production!

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