Shining a light on ionic liquid photodegradation

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  • Published: Nov 15, 2017
  • Author: Ryan De Vooght-Johnson
  • Channels: HPLC
thumbnail image: Shining a light on ionic liquid photodegradation

Detailed knowledge of degradation products lacking for many ionic liquids

For many years ionic liquids have been assumed to be ‘green’ solvents in the academic literature, despite only patchy information being available on their environmental effects and that of their degradation products. Where studies have been carried out, potential problems have often been flagged up. Large-scale use of ionic liquids will inevitably lead to some losses to the environment, so it is important that the environmental hazards of degradation products are properly investigated. The introduction of the REACH (regulation, evaluation, authorisation and restriction of chemicals) regulations into the EU means that more information is required on the environmental characteristics of industrial solvents than was the case in the past.

The Turin researchers investigated the effect of UV irradiation on solutions of BMPA in water, using titanium dioxide as the photocatalyst. The aim was to mimic the photodegradation that occurs when the ionic liquid is dissolved in the upper layers of bodies of water. Toxicity studies were therefore carried out on aquatic bacteria.

BMPA degradation products identified by HPLC and LTQ-Orbitrap MS

A sample of BMPA in ultrapure water was irradiated in the presence of titanium dioxide (P25 grade) with a Philips TL-K 40W/05 UV lamp (maximum emission 360 nm). The resulting mixtures were filtered through a PTFE membrane to remove solids prior to HPLC.

HPLC was carried out using a Thermo Scientific UltiMate 3000 instrument with a Phenomenex Gemini NX-C18 column. Gradient elution was employed with a flow rate of 200 μL/min using acetonitrile and 5 mM aqueous heptafluorobutanoic acid, the proportion of the former being increased from 5 to 95% during the run. High-resolution mass spectrometry (HRMS) used a Thermo Scientific LTQ-Orbitrap instrument, which combined a linear ion trap with an Orbitrap, with electrospray ionisation (ESI) in positive mode. Molecular ions were detected, along with (in most cases) daughter MS2 ions.

After 45 minutes of photodegradation, 32 transformation products were formed, but BMPA was no longer detected. As might be expected, many of these products were formed by oxidation, with –OH or carbonyl groups being present. Products involving loss of butyl and some with ring opening were also noted. Careful examination of the daughter ions enabled nearly all the compounds to be identified. It was found that prolonged irradiation (2 hours) led to the disappearance of nearly all the transformation products, with compete mineralisation occurring by 4 hours. The nitrogen atoms present in ring of the ionic liquid were mainly transformed to ammonium ions, which were detected by ion chromatography (70% of the initial nitrogen in BMPA gave ammonium).

Toxicity studies on the degradation mixture were carried out using the luminescent bacterium Aliivibrio fischeri (previously known as Vibrio fischeri), which is found in most oceans. The inhibition of luminescence by the degradation products was noted. BMPA was non-toxic itself, but inhibition started to occur on irradiation. After 5 minutes, a maximum of 35% inhibition was reached, which decreased to 5% after 15 minutes. The inhibition then increased again, levelling off at 30% between 30 and 120 minutes.

Degradation of ionic liquid gives mixture moderately toxic to bacteria

The HPLC and the LTQ-Orbitrap mass spectrometer formed a powerful combination that successfully identified nearly all the degradation products from the ionic liquid. The moderate inhibition of bacteria is interesting, but the effects on higher organisms also require study. Although ionic liquids are more widely used in industry than in the past, many still require much toxicological and degradation work before they can be used on a large scale within the EU.

Related Links

Rapid Communications in Mass Spectrometry, 2017, Early View Paper. Fabbri et al. HPLC-HRMS for the characterization of transformation products of ionic liquids.

Journal of Chemical Technology and Biotechnology, 2016, 91, 2882-2887. Munoz et al. Degradation of imidazolium-based ionic liquids by catalytic wet peroxide oxidation with carbon and magnetic iron catalysts.

Wikipedia, Orbitrap

Article by Ryan De Vooght-Johnson

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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