Article 2 : Origin and Effect of Impurities in Protic Ionic Liquids Based on 2-

Applications in Electrochemistry

Ce second chapitre de la discussion est un article qui a été publié en 2009 dans le volume 54 de la revue Electrochimica Acta aux pages 7422–7428. J'ai réalisé la majeure partie de la rédaction de cet article ainsi que toutes les expériences qui y sont présentées.

Origin and Effect of Impurities in Protic Ionic Liquids

Based on 2-Methylpyridine and Trifluoroacetic Acid for

Applications in Electrochemistry

Laurence Mayrand-Provencher and Dominic Rochefort

Département de chimie, Université de Montréal

C.P.6128 Succ. Centre-ville, Montréal, Québec, Canada H3C 3J7

Abstract

Protic ionic liquids (PILs) based on 2-methylpyridine (2-MePy) and trifluoroacetic acid (TFA) have been synthesized under multiple conditions and with different proportions of their constituents. These PILs present a brown color of variable intensity and the factors responsible for the presence of these impurities have been examined. UV-vis spectroscopy analyses revealed that both TFA and 2-MePy can be thermally degraded during the synthesis process. Distillation of the IL can be used to quickly obtain mixtures without any traces of coloration, but can leads to a severe alteration of the relative proportion of the constituents. This work shows that high-yield syntheses of PILs obtained with different contents in colored impurities can be compared to evaluate the effect of these impurities not only on the electrochemical behavior (on Pt and on GC) but also on the physicochemical properties of interest for PILs as applications in electrochemistry (conductivity, density, and viscosity).

1. Introduction

One of the aspects which makes ionic liquids (ILs) particularly interesting is that their properties can be tailored by choosing the anions and cations composing them. This allows ILs to fulfill a wide variety of very specific needs, and it is one of the key reasons why they have been the object of so much research interest.

ILs have found many applications so far, which have been discussed in an abundance of recent reviews.1-8 Several of these applications necessitate optically pure ILs (e.g., solvent in spectroscopy9-11_{), but while pure ILs are generally colorless,}

most synthesis routes explored in the literature lead to the formation of colored mixtures. This is mainly because impurities that absorb strongly in the visible region of the light spectrum are formed during one or several steps of the syntheses. These colored impurities impose severe constraints on the utilization of ILs for such applications where a good optical purity is primordial.

Another important concern that arises from the impurities of an IL is that they may affect its electrochemical behavior. Indeed, electroactive impurities can narrow the electrochemical window of an IL,1 and changes in its viscosity due to these impurities would influence its ionic conductivity. Therefore, in order to characterize the electrochemical stability and the physicochemical properties of an IL correctly, these impurities must be eliminated or it must be demonstrated that they have no significant effect on their key properties.

Several methods have been proposed to remove the colored impurities from ILs. Recrystallization can be used for ILs that can crystallize near room temperature, but most of them do not fit this criteria since they are viscous liquids in these conditions.11 Purification with decolorizing charcoal is probably the most widely used method to increase the optical purity of ILs. It involves the adsorption of colored impurities on the surface of activated charcoal (typically for 24 h), followed by a filtration step. The increase in optical purity provided by this technique is not always significant12 and it was reported as inefficient for the ultraviolet region.11

Distillation is another purification process for ILs that has attracted attention lately to reach high purity levels and also for recycling purposes. Processes involved in distillation are strongly dependant on the type of IL. Hence, some distinctions

between the families of ILs will be discussed briefly. Protic ionic liquids (PILs) are a subclass of ILs that regroups those obtained by the proton transfer from a Brønsted acid (HA) to a Brønsted base (B). All other ILs will be referred to as aprotic ionic liquids (AILs). The latter present a higher ionic character than most PILs and are thus characterized by a lower vapor pressure. Despite this, the distillation of several AILs was recently shown to be possible at low pressure, and it was done without a significant thermal degradation.13 Yet, these ILs can only be distilled very slowly even in extreme distillation conditions which would be impractical to use for large scale syntheses of optically pure AILs. On the other hand, PILs are generally more volatile because they contain both ionic (A-, BH+) and neutral species (HA, B), among which an equilibrium exists (see eq. 3.1 for an example). The energy needed to reform the neutral species depends upon the difference in basicity of the constituents and it is generally low. Hence, most PILs are considered to be of “poor” quality according to the classification established by Angell et al.14 _{and they can have}

a sufficient vapor pressure to be distilled quickly in large amounts.

MacFarlane et al. mentioned that the distillation of some imidazolium and pyrrolidium PILs removes their initial coloration.15 Since it was not the objective of their article to focus on this particular aspect and on the effect of impurities, no comparison of the properties between the crude and distilled PILs was provided. Also, they did not verify the optical purity of their PILs (e.g., by UV-vis spectroscopy) so it remains unclear to what extent does the distillation can increase the spectral window of PILs.

In this work, the effect of colored impurities on the electrochemical and physicochemical properties of PILs made from 2-methylpyridine (2-MePy) and trifluoroacetic acid (TFA), which are well-known in the literature,16-19 is investigated. While different PILs do not necessarily contain similar impurities, a methodology to confirm if colored impurities affect or not the properties of any given PIL can be developed even if using exclusively 2-MePy:TFA melts. This was done here, using different sets of syntheses conditions, and the method described is a powerful syntheses validation tool. Also, this study sheds some light on the formation of impurities in analogous PILs based on N-heterocyclic bases and TFA, PILs that have

been sought-after for applications such as electrolytes in energy-storage devices like metal-oxide supercapacitors.18

This work brings insights on practical aspects in the syntheses of PILs and on the utilization of distillation. We show that while it is possible to obtain PILs without any trace of coloration by distillation in a very short amount of time, a change in the relative proportion of the constituents of the PILs is hard to avoid and the effect of this change on the physicochemical properties of the melts must be taken into account.

2. Experimental Section

2.1. Materials

2-Methylpyridine (98%), trifluoroacetic acid (99%), and the KCl conductivity standard solution (0.117 M, 15 000 μS at 25 °C) were obtained from Alfa Aesar. Acetonitrile (>99.99%) was bought from EMD Chemicals. All reagents were used as received.