Trioctylmethylammonium thiosalicylate (TOMATS)

By: Roland St. Kalb and Michael J. Kotschan, Aldrich ChemFiles 2006, 6.9, 13.

Aldrich ChemFiles 2006, 6.9, 13.

A novel, high-performance, task-specific ionic liquid for the extraction of heavy metals from aqueous solutions


Roland St. Kalb and Michael J. Kotschan, proionic Production of Ionic Substances GmbH, Leoben, Austria; www.proionic.at; info@proionic.at

Task-specific ionic liquids for the selective liquid/liquid extraction of heavy metals from aqueous systems were first published by Robin D. Rogers and co-workers in 2001.1 Functionalized imidazolium cations with thioether-, urea- or thiourea-derivatized side chains act as metal ligating moieties, whereas the PF6 – anion provides the desired water immiscibility (Figure 1). Nernst distribution ratios were reported for Cd2+ and Hg2+ to be < 380.


Figure 1: A thiourea derivatized ionic liquid

These ionic liquids were the first to contain specific functionalities to enable well defined chemical properties and may therefore be called “designer solvents.”

However, these pioneering ionic liquids have some major drawbacks: the hexafluorophosphate anion is known to be quite unstable with respect to hydrolysis, producing toxic and corrosive HF or fluorides. The toxicity of the imidazolium cation is difficult to estimate and a toxicological study would be expensive. The disposal of these fluorous compounds is expensive and problematic. Additionally, the synthesis on a larger scale is complicated and the starting materials are expensive.

Trioctylmethylammonium thiosalicylate – a novel task-specific ionic liquid

To overcome these drawbacks – especially in consideration of possible industrial applications at larger scales – and to enhance the performance, proionic Production of Ionic Substances GmbH developed a new ionic liquid: trioctylmethylammonium thiosalicylate (TOMATS) (Figure 2).


Figure 2: TOMATS

TOMATS contains no fluorine and is absolutely stable towards hydrolysis. Therefore, it does not release HF or fluorides, is noncorrosive, and is much easier to dispose. The low toxicity of the cation is known from related compounds like trioctylammonium chloride (a phase transfer catalyst) and thiosalicylic acid or its salts are typically classified as irritants. The distribution coefficients of heavy metals typically show values in the range of 50,000 to more than 2,000,000, which may be explained by the chelating effect of the ortho-positioned carboxylate group relative to the thiol functionality (which is well-known to form metal thiolate complexes). The synthesis is simple and can be done at industrial scales.

Application of TOMATS



Figure 3: Extraction of Cu2+

Figure 3 shows the extraction of copper from a blue colored aqueous Cu2+-tetramine phase. After addition of the TOMATS ionic liquid, and before shaking the test tube, diffusion zones can be seen (second test tube) showing a copper-free, colorless region and a dark copper-containing upper region. After shaking and separation of the phases, all the copper is extracted into the upper phase, as evidenced by the completely dark-colored ionic liquid layer (third test tube).

Phase separation sometimes takes quite a long time due to the high viscosity of TOMATS at 20 °C (1500 mPa.s). This drawback can be overcome by either adding a water-immiscible organic solvent like ethyl acetate, or by gentle heating of the mixture (to decrease the viscosity). Phase separation can be optimized by using a centrifuge or adding a small amount of sodium sulfate to the aqueous phase before shaking. If the aqueous phase still looks turbid, it can be filtered through a common membrane filter.

Characterization of TOMATS3

Appearance: Olive green, viscous liquid
Relative Molecular Mass: 521.89 g/mol
Empirical Formula: C32H59NO2S
Solubility: Soluble in alcohols, ethyl acetate, THF, acetonitrile, acetone, dichloromethane, DMSO; insoluble in water, hexane
Nernst Distribution Coefficients4: Cd2+ >50,000; Cu2+ >50,000; Pb2+ and Hg2+ >100,000
Melting Point: <30 °C
Refractive index: nD 20 = 1.5185
Leaching into aqueous phase: <100 ppm

Viscosity and Density Temperature Dependence Data


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Materials

     

References

  1. Visser, A. E.; Swatloski, R. P.; Reichert, W. M.; Rogers, R. D.; Mayton, R.; Sheff, S.; Wierzbicki, A.; Davis, Jr., J. H. Chem. Commun. 2001, 135.
  2. Decontamination of Heavy Metal polluted Process Water, Waste Water and Filter Cake with High Performance, Roland St. Kalb, Regina Krachler, and Bernhard K. Keppler, EMChIE 2006 Conference Book, Vienna, May 3–5, 2006.
  3. Aqueous phase with 5 to 50 ppm metal, 1:1 extraction, detection using AAS and ICP-AES.

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