Electroanalysis of Neutral Precursors in Protic Ionic Liquids and Synthesis of High-Ionicity Ionic Liquids

Abstract

Protic ionic liquids (PILs) are ionic liquids that are formed by transferring protons from Brønsted acids to Brønsted bases. While they nominally consist entirely of ions, PILs can often behave as though they contain a significant amount of neutral species (either molecules or ion clusters), and there is currently a lot of interest in determining the degree of "ionicity" of PILs. In this contribution, we describe a simple electroanalytical method for detecting and quantifying residual excess acids in a series of ammonium-based PILs (diethylmethylammonium triflate [dema][TfO], dimethylethylammonium triflate [dmea][TfO], triethylammonium trifluoroacetate [tea][TfAc], and dimethylbutylammonium triflate [dmba][TfO]). Ultra-microelectrode voltammetry reveals that some of the accepted methods for synthesizing PILs can readily result in the formation of nonstoichiometric PILs containing up to 230 mM excess acid. In addition, vacuum purification of PILs is of limited use in cases where nonstoichiometric PILs are formed. Although excess bases can be readily removed from PILs under ambient conditions, excess acids cannot be removed, even under high vacuum. The effects of excess acid on the electrocatalytic oxygen reduction reaction (ORR) in PILs have been studied, and the onset potential of the ORR in [dema][TfO] increases by 0.8 V upon addition of acid to PIL. On the basis of the results of our analyses, we provide some recommendations for the synthesis of highly ionic PILs.

Understanding the Role of Protic Ionic Liquids (PILs) in Reactive Systems: Rational Selection of PILs for the Design of Green Synthesis Strategies for Allylic Amines and β‐Amino Esters

“…The electrolyte was saturated with inert gas (dry N 2 ) by continuous bubbling. CVs were measured between a cathodic potential that was negative enough (<−0.7 V) to detect hydrogen evolution from the free acid, and an anodic potential that was high enough (>1.2 V) to detect the electro‐oxidation of free alkyl amine or AcOH . The potential incursion over such anodic values also allowed to keep the electrode free of fouling by amine or acid adsorption .…”

“…Notwithstanding, the complex reactive behaviour of PILs is strongly affected not only by the PIL structure but also by its parent species (free acid and base) that may be accompanying the PIL in liquid phase, either remaining from the synthesis or being released from the ionic pair via autoprotolysis equilibria (HB + A − ⇆HA+B) after being melted or dissolved. Several reports have demonstrated the presence of species other than the PILs that strongly affect their properties and performance as solvents . In many cases, the type of H‐bonded network that is present in the PIL causes a mixture of ions and neutral species, together with acid‐base pairs, leading to very particular properties .…”

“…In many cases, the type of H‐bonded network that is present in the PIL causes a mixture of ions and neutral species, together with acid‐base pairs, leading to very particular properties . Besides, the molar relations between neutral and ionized species that conform the PIL may vary depending of the used synthesis procedure, as it was shown for example by D. Rogers and co‐workers for triethylamine and acetic acid (AcOH) and by D. Walsh and co‐workers for ammonium‐based triflates and trifluoracetates …”

“…More precisely, these PILs were used for the synthesis of allylic amines from non‐polar unsaturated substrates and of β‐amino esters from polar α,β unsaturated esters. Besides, the compositions of the PILs were characterized by cyclic voltammetry of microelectrodes in the melted PILs, a technique that is capable to detect the molecular species (free acid and amine) in the PIL‐based liquid medium …”

Engineering the Interface of Carbon Electrocatalysts at the Triple Point for Enhanced Oxygen Reduction Reaction

“…[23] the catalysts urface. [53] When applieda sc arbon-based electrocatalysts for ORR, the ionic liquid usually plays ar ole as ac arbon precursor,s acrificial template, or source of heteroatom-dopant. [26] In 2017, Walsh et al investigated the synthesis of protic ionic liquids with high ion conductivity.…”