Fundamental Characterization of Solvent Effects on Electrochemical Properties of Silylamine-Type Reversible Ionic Liquids

Abstract

Traditional salts dissolved in aqueous solutions have been used in electrochemical systems as electrolytes. While their high conductivity allows good performance of electrochemical reactions within the aqueous system, their use is limited in organic solvents due to poor solubility. This in turn, restricts the conductivity of the solution. Additional challenges associated with traditional electrolytes are product separation from the electrolyte and downstream electrolyte recycling concerns. If the solubility and electrolyte recovery challenges could be solved, wider-scale practical use of electrochemical process as an alternative to traditional chemical processes could occur[1]. Alternative electrolytes for organic systems are ionic liquids (ILs), defined as ionic salts with low melting points (below 100oC). The properties of ILs can be widely tuned by changing the cation or anion to have favorable solubility in organic solvents while having good conductivity. To address the electrolyte recovery and products separation, switchable IL-like solvents could be used[2, 3]. These switchable ILs are able to be in an ionic state during electrochemical processing and be converted to a molecular state for separation and/or recovery thereby making the overall electrochemical process more viable. Silylamines can be switchable solvents. With externally added CO2, they form a reversible ionic liquid (RevIL) and switch back to a molecular liquid (ML) with mild heating[4]. CO2 capture applications have been the primary use of these RevILs in the past[5-7]. When the model silylamine, (3-aminopropyl)triethoxysilane (TEtoxySA), was investigated electrochemically to determine the conductivity, it was found that the conductivity was unsatisfactory for electrolyte use in the neat RevIL state[8]. However, with the addition of protic polar organic solvents, conductivity of the TEtoxySA-RevIL remarkably increased up to feasible values for electrochemical use[8]. It was also found that non-polar and/or aprotic solvents did not increase conductivity near as much. In our previous study[8], we also revealed that H-bonding ability and the dielectric constant of solvents played a significant role in the resulting conductivity of RevIL-solvent mixtures. However, specific interactions were not characterized experimentally. This work presents fundamental characterization for molecular interactions between silylamine type RevILs and different solvents. Nuclear magnetic resonance and infrared spectroscopy were utilized to characterize the molecular behaviors within mixtures of RevILs and organic solvents. These results will be compared to the hypothetical ion-pairing models presented by our group[8] and others[9].