The University of Alabama, Tuscaloosa; Queen’s University Belfast, Northern Ireland
A chemist believes that an ionic liquid is the place for a noxious gas.
As a ‘green chemist’, I worry about the potential dangers of moving toxic and flammable gases around. Most nasty gases are transported in pressurized canisters to save space, posing the risk of hazardous compounds being expressed over people and pleasant greenery on the rare occasions that a container breaks.
Recently, some scientists at Air Products and Chemicals, a chemicals supplier in Allentown, Pennsylvania, found a way to store phosphine (PH3) and boron trifluoride (BF3) — both toxic gases — in ionic liquids, and then recover the gases without introducing impurities (D. J. Tempel et al. J. Am. Chem. Soc. 130, 400–401; 2008).
The advantage of transporting gases in ionic liquids is that many such liquids have no measurable vapour pressure. So were a container to burst, the gases inside it would remain as chemical complexes in a liquid state, making them much easier to mop up. Furthermore, ionic liquids can be recycled in subsequent shipments.
Dan Tempel and his team used a computer model to consider two ionic liquids — the cation 1-butyl-3-methylimidazolium paired with either Al2Cl-7 and Cu2Cl-3 — for phosphine transport. They then tested the latter in the lab; the positively charged copper atoms bound the lone electron pair on phosphine. Similarly, the electron-deficient boron atom in boron trifluoride facilitated the formation of a covalent bond with a fluorine atom in another ionic liquid, in which the same cation is paired with BF-4.
In both cases, more than 90% of the ionic liquid’s reactive sites formed complexes at room temperatures. This means that relatively small volumes of ionic liquids could move a lot of toxic gas around. I think this could revolutionize the industry.