Purification of undiluted ionic liquids from trace-colored impurities for spectroscopy by octadecylsilyl solid-phase extraction

https://doi.org/10.1016/j.seppur.2007.08.018Get rights and content

Abstract

Ionic liquids are often contaminated by trace-colored impurities. In this paper, a solid-phase extraction (SPE) method for purification of undiluted imidazolium ionic liquids was described. A SPE apparatus with octadecylsilyl (ODS) solid phase was proposed to separate these impurities from ionic liquids via hydrophobic interaction. Solid-phase extraction with ODS can improve the spectroscopic quality obviously for the tested ionic liquids. Without introducing any solvent to ionic liquid, the purified ionic liquid can be used directly in spectroscopic measurements. It was found that short alkyl chain ionic liquids were purified more efficiently than the long alkyl chain ones. This method can be used to purify the hydrogen-bonding anion containing ionic liquids, which cannot be decolorized efficiently by conventional active charcoal method. In comparison with other existing purification methods, ODS is advantageous to solve the purification problem of ionic liquids.

Introduction

Ionic liquids have been studied extensively as potentially benign solvents for a wide range of chemical processes [1], [2], [3]. Various cation–anion combinations of ionic liquid were proposed, but most investigations have been devoted to the 1-alkyl-3-methylimidazolium salt. However, many synthetic methodologies of ionic liquids lead to the final product with yellowish or brown color. Thus, most commercially ionic liquids are still visibly colored.

Pure ionic liquids have a low molar extinction coefficients in the near-ultraviolet and visible spectral regions. Thus, they were used as an interesting medium for photophysical and photochemical researches [4], [5], [6], [7], [8] and photochemical reaction studies [9]. The optic emission of lanthanide salts and complexes in ionic liquids has been reported [10]. Another spectroscopic application of ionic liquids is the monitoring of organic reactions by absorption spectroscopy [11], [12]. The state of water in ionic liquid was investigated by IR spectroscopy [13].

With regard to the spectroscopic quality of ionic liquids, there have been many attempts to remove these chromophoric impurities when they were used for spectroscopic study. However, these impurities are usually below the detection limit of NMR spectroscopy and mass spectroscopy. Thus, it is still unknown what they are. Welton and co-worker reported that these impurities may be the products of side reactions occurring at high temperature in the alkylation reaction when ionic liquids halides are synthesized [14]. To obtain “spectrograde” ionic liquids, some strategies have been proposed, which can be represented in four ways: purification of starting materials, control of conditions for quaternization reaction, ion exchange and cleaning of the ionic liquids [15]. Nockemann et al. reported that yellowish ionic liquid can partially be decolorized by treating with active charcoal column or recrystallization. Alumina or silica columns are unsuitable for the ionic liquids purification [16]. Decolorizing with active charcoal, Burrell et al. developed a method for producing high quality ionic liquids at kilogram scale [17]. Recently, Mantyn et al. reported the best purification strategy is to clean up the final ionic liquids. An active charcoal column was designed for the decolorization. They also reported that active charcoal cannot decolorize efficiently for the ionic liquids which contain strong hydrogen-bonding anion [18].

In this work, the problem of how to remove the colored impurities from the ionic liquids, especially from the ionic liquids, which contain strong hydrogen-bonding anion, was focused on. An ODS-based solid-phase extraction method is developed and found to be useful in solving the above-mentioned problem. The change of the colored impurities during the purification process was monitored by reversed-phase chromatography and UV–vis spectrometry. A comparison between the proposed solid-phase extraction method with ODS and existing purification methods, including active charcoal decolorization and recrystallization, was also made.

Section snippets

Experimental

The ionic liquids used in this work were purchased from Chemer Chemical (Hangzhou, China), including 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM][BF4]), 1-decyl-3-methylimidzolium tetrafluoroborate ([DMIM][BF4]). Their marked purities were more than 97%. 1-Butyl-3-methylimidazolium chloride ([BMIM]Cl) and 1-butyl-3-methylimidazolium trifluoroacetate ([BMIM][TFA]) were synthesized according to the literature procedures [13], [19]. These synthesized ionic liquids were verified by MS and

Results and discussion

In initial chromatographic investigation, among the different tested stationary phases, we found that ODS column was the best one to efficiently separate the colored impurities from ionic liquid. Using ODS as the sorbent, a solid-phase extraction method for purification of ionic liquids was developed.

Conclusion

When suitable stationary phase was selected, solid-phase extraction technology can be used to purify imidazolium ionic liquids. ODS can adsorb the colored impurities from ionic liquid via hydrophobic interaction to improve the spectroscopic quality for the tested ionic liquids. We found that the short alkyl chain ionic liquids were purified more efficiently than the long alkyl chain ones. Comparing with activated charcoal, ODS can be used as an alternative solid phase material, especially

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 20575019 and 20335020).

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