Inorganics - Perchlorates in Ionic Liquids ?

Ionic Liquids that feature perchlorate as the anionic component ? Counterintuitive ? We don't think so. Research at Edwards AFB has demonstrated the relative stability of selected substituted triazolium perchlorate compounds, including materials having melting points below 90°C.¹ GFS advocates the development of the chemistries of imidazolium (and phosphonium) perchlorates in response to demonstrated material needs; Ionic Liquid Perchlorate (ILP) technology will benefit from several critical factors:

· ClO₄^- does not hydrolyze to toxic or corrosive by-products

· ClO₄^- is known to be inert and non-coordinating in most ambient environments

· ClO₄^- exhibits a high degree of ionic character in salt and parent acid form

· ClO₄^- salts are relatively non-moisture sensitive

· ClO₄^- compounds are not susceptible to halide contamination

The characteristics that for decades have made perchlorate compounds attractive components in battery and electrolyte technologies are indicators of the potential use of perchlorates in commercially viable, innovative Ionic Liquid technologies.

The following Q&A section covers important physical and chemical properties of perchlorates, and takes a close look at new data that address safety issues pertinent to the commercialization of ILP technology.

Q. Won't the oxidizing character of perchlorate compromise its use in ILP technology?

A. Not necessarily. For example, there are detailed studies of the reactivity of perchloric acid as a function of temperature and concentration. Significant oxidizing character of perchloric acid is rarely observed at temperatures below 150°C, and then only at high degrees of concentration.²

Q. Has the energetic behavior of perchlorate salts in extreme organic environments ever been studied?

A. Yes. Adiabatic Calorimetry has been run on a saturated (>6 M) solution of lithium perchlorate (anhydrous) in diethyl ether.³ Two key parameters were derived from the results.

· Upon heating in successive ten degree increments, a self-sustaining exotherm was not observed until temperatures exceeded 160°C.

· Observation of a controlled exotherm above 160°C continued until a temperature of over 220°C was attained, at which point the exotherm became uncontrolled.

Q. Are perchlorates commonly used in organic synthesis?

A. Yes. The seminal work of Paul Grieco in the 1990's defined the role of anhydrous lithium (and magnesium) perchlorate in the dramatic promotion of a wide variety of synthetic methodologies, especially Diels Alder reactions.^4,5

Q. What is the key property of LiClO₄ that enables this unusual behavior?

A. The anhydrous salt exhibits extraordinary solubility in diethyl ether and other organic solvents. The hydrated salt is much less soluble; other perchlorate salts also have diminished solubilities but still find use in selected synthetic reactions.

Q. Are perchlorates currently used as industrial catalysts ?

A. Yes. And it is reasonable to anticipate that the use of perchlorate-containing ionic liquids would have advantages in catalysis, especially including processes involving common perchlorate salts.

Q. How dry can bulk perchlorate salts be prepared?

A. Very dry. Ton quantities of anhydrous lithium perchlorate powder can be prepared having a moisture content of under 200 parts per million.

Q. How expensive are perchlorate compounds?

A. Relatively inexpensive. For example, research quantities of anhydrous LiClO₄ are less expensive than comparable grades of LiPF₆, LiBF₄, LiAlCl₄, and lithium triflate or triflimide.

Q. Has there been any quantitative determination of the ionic character of perchlorate compounds?

A. Yes. It has been shown that perchloric acid is so highly dissociated that a concentration of around 4M is required before significant re-association is seen; its dissociation constant is about 10¹⁴ with a pK_a of -1.6. A more authentic formulation for 72% perchloric acid would be the hydronium perchlorate hydrate: H₃O⁺ClO₄^-·H₂O. Perchloric acid is not a solution of a gas in water.

Q. What about the environmental and health effects of perchlorate?

A. The history of perchlorate use is consistently positive. Occupational health studies consistently show no abnormal effects from chronic perchlorate exposure. Perchlorate has been used effectively for 50 years to treat thyroid conditions. Most environmental exposure involves levels that are 1000 times lower than therapeutic dosage. Toxicologists have gone on record to state that long-term exposures to perchlorate at 200 times an arbitrary Public Health Goal of 1 ppb (in Massachusetts, for example) are expected to be safe for the U.S. population,⁶ and the N.I.H./NRC report issued in January 2005 judged that perchlorate is safe for consumption at levels at least twenty times the 1 ppb guideline. The natural occurrence of perchlorate in the environment is also evident from geologic studies as well as recent research at Tech Tech University.

Q. Is there a significant bibliography of perchlorate characterization?

A. Yes. In-depth perchlorate studies began nearly 100 years ago. Numerous books, monographs, and literature articles, including many published by GFS Chemicals, Inc., have addressed perchlorate behavior. (see ref. 6-8 and citations therein).

1. Drake, G. et al., Propellants, Explosives, Pyrotechnics, 2003, 28, 174.

2. Smith, G. F., The Wet Chemical Oxidation of Organic Compositions Employing Perchloric Acid With or Without Added HN0₃, H₅I0₆, H₂S0₄, GFS Chemicals Publication #213, 1965.

3. Long, J.R., Chemical Health & Safety, May/June, 2002,12 (perchlorate Safety: Reconciling Inorganic and Organic Guidelines).

4. Grieco, P.A., in Organic Chemistry: Its Language and Its State of Art, V. Kisaknerek Editor, 1993, VCH, Basel, 133.

5. Grieco, P.A., Aldrichimica Acta, 1991, 24, 59.

6. Schilt, A.A., Perchloric Acid and Perchlorates, Second Ed., 2003, published by GFS Chemicals. (see Chapter VIII by Dr. Gay Goodman, Chief Toxicologist, Intertox, Inc.)

7. Schumacher, J.C., Perchlorates, 1960, Reinhold Publ. Corp, New York.

8. Espenson, J.H., Chap. 1 in: Perchlorate in the Environment, Urbansky, E. Editor, Environmental Science Research Vol. 57, Kluwer Academic Publishers, 2000.

For more information, contact inorganicdevelopment@gfschemicals.com.