Characterization of Imidazolium Chloride Ionic Liquids Plus Trivalent Chromium Chloride for Chromium Electroplating

A series of mixtures consisting of the ionic liquids (ILs) 1-ethyl-3-methylimidazolium chloride, 1-butyl-3-methylimidazolium chloride, and 1-hexyl-3-methylimidazolium chloride ([emim]­[Cl], [bmim]­[Cl], and [hmim]­[Cl], respectively) and trivalent chromium chloride have been prepared. Physicochemical and electrochemical properties of these mixtures have been studied and the potential applications of these mixtures for chromium electroplating, as an alternative to the conventional hard chromium electroplating processes using hexavalent chromium baths, have been examined. To optimize the transport properties of the mixtures, different amounts of ultrapure water were added to the Cr­(III) salt–IL mixtures, although the ultimate goal is to reduce or eliminate water. As shown previously for choline chloride/Cr­(III) salt mixtures, we found that the physicochemical and electrochemical properties of the mixtures are affected by the relative water content. Our preliminary electroplating results show that these types of Cr­(III) salt–IL mixtures could be promising alternatives to Cr­(VI) containing baths for chromium electroplating applications with the advantage of avoiding the use of highly toxic hexavalent chromium

Physical Properties and CO₂ Reaction Pathway of 1‑Ethyl-3-Methylimidazolium Ionic Liquids with Aprotic Heterocyclic Anions

Ionic liquids (ILs) with aprotic heterocyclic anions (AHA) are attractive candidates for CO₂ capture technologies. In this study, a series of AHA ILs with 1-ethyl-3-methylimidazolium ([emim]⁺) cations were synthesized, and their physical properties (density, viscosity, and ionic conductivity) were measured. In addition, CO₂ solubility in each IL was determined at room temperature using a volumetric method at pressures between 0 and 1 bar. The AHAs are basic anions that are capable of reacting stoichiometrically with CO₂ to form carbamate species. An interesting CO₂ uptake isotherm behavior was observed, and this may be attributed to a parallel, equilibrium proton exchange process between the imidazolium cation and the basic AHA in the presence of CO₂, followed by the formation of “transient” carbene species that react rapidly with CO₂. The presence of the imidazolium-carboxylate species and carbamate anion species was verified using ¹H and ¹³C NMR spectroscopy. While the reaction between CO₂ and the proposed transient carbene resulted in cation-CO₂ binding that is stronger than the anion-CO₂ reaction, the reactions of the imidazolium AHA ILs were fully reversible upon regeneration at 80 °C with nitrogen purging. The presence of water decreased the CO₂ uptake due to the inhibiting effect of the neutral species (protonated form of AHA) that is formed

Origin of Catalytic Effect in the Reduction of CO₂ at Nanostructured TiO₂ Films

Electrocatalytic activity of nanostructured TiO₂ films toward the reduction of CO₂ is probed by depositing a nanostructured film on a glassy carbon electrode. The one-electron reduction of CO₂ in acetonitrile seen at an onset potential of −0.95 V (vs NHE) is significantly lower than the one observed with a glassy carbon electrode. The electrocatalytic role of TiO₂ is elucidated through spectroelectrochemistry and product analysis. Ti³⁺ species formed when the TiO₂ film is subjected to negative potentials have been identified as active reduction sites. Binding of CO₂ to catalytically active Ti³⁺ followed by the electron transfer facilitates the initial one-electron reduction process. Methanol was the primary product when the reduction was carried out in wet acetonitrile

Solid–Liquid Equilibria Measurements of Mixtures of Lithium Bis(trifluoromethanesulfonyl)imide with Varying Alkyl Chain Length Ammonium Bis(trifluoromethanesulfonyl)imide Ionic Liquids

Ionic liquid (IL)–metal mixtures are potential solvents for a variety of applications, including metal electrodeposition, electrolytes for batteries, catalysis, and for separations processes involving liquid–liquid extraction. The solubility of a metal in an IL is fundamentally important to selecting an appropriate IL for a potential process; however, relatively few measurements have been reported in the literature. The solid–liquid equilibria of binary mixtures of lithium bis­(trifluoromethanesulfonyl)­imide and three ammonium bis­(trifluoromethanesulfonyl)­imide ILs are investigated. Measurements are made through two different methods. A visual method allows direct observation of the phase behavior between room temperature and 373 K and a differential calorimetry method provides solid–liquid equilibria information up to 623 K. The activity coefficients of the solid in the liquid are calculated from the measured phase equilibria and the pure component physical properties. The mutual solubilities of lithium bis­(trifluoromethanesulfonyl)­imide and hexadecyl-trimethylammonium bis­(trifluoromethanesulfonyl)­imide are found to be higher than expected given the long alkyl chain length of the IL cation

The Viscosity and Density of Ionic Liquid + Tetraglyme Mixtures and the Effect of Tetraglyme on CO₂ Solubility

We show that the addition of tetraglyme (TG), which is a low viscosity liquid with relatively low vapor pressure, is effective in reducing the viscosity of ionic liquids (ILs). In particular, we measure the viscosities of mixtures of 18 ionic liquids with tetraglyme at temperatures between 278.15 and 323.15 K, with a focus on mixtures that are primarily ionic liquid. Thirteen of the ionic liquids contain aprotic heterocyclic anions (AHA ILs) paired with tetra-alkylphosphonium and imidazolium cations, which we have developed for cofluid vapor compression refrigeration and postcombustion CO₂ capture applications. Three bis­(trifluoromethylsulfonyl)­amide ([Tf₂N]⁻) ionic liquids are included for comparison, as well as trihexyl­tetradecyl­phosphonium acetate and trihexyl­tetradecyl­phosphonium dicyanamide ([P₆₆₆₁₄]­[acetate] and [P₆₆₆₁₄]­[DCA]). In addition, we present the densities of trihexyltetradecylphosphonium 1,2,3-triazolide ([P₆₆₆₁₄]­[3-Triz]) + tetraglyme mixtures at temperatures between 283.15 and 353.15 K. Finally, we show that the solubility of CO₂ in mixtures of [P₆₆₆₁₄]­[3-Triz] + 30 mol % tetraglyme and trihexyltetradecylphosphonium 1,2,4-triazolide ([P₆₆₆₁₄]­[4-Triz]) + 30 mol % tetraglyme at 313.15, 333.5, and 353.6 K and pressures to 34 bar can be represented reasonably well by a mole fraction weighted sum of the solubilities (on a mole ratio basis) in the two pure components

Effect of Cation on Physical Properties and CO₂ Solubility for Phosphonium-Based Ionic Liquids with 2‑Cyanopyrrolide Anions

A series of tetraalkylphosphonium 2-cyanopyrrolide ([P_<i>nnnn</i>]­[2-CNPyr]) ionic liquids (ILs) were prepared to investigate the effect of cation size on physical properties and CO₂ solubility. Each IL was synthesized in our laboratory and characterized by NMR spectroscopy. Their physical properties, including density, viscosity, and ionic conductivity, were determined as a function of temperature and fit to empirical equations. The density gradually increased with decreasing cation size, while the viscosity decreased noticeably. In addition, the [P_<i>nnnn</i>]­[2-CNPyr] ILs with large cations exhibited relatively low degrees of ionicity based on analysis of the Walden plots. This implies the presence of extensive ion pairing or formation of aggregates resulting from van der Waals interactions between the long hydrocarbon substituents. The CO₂ solubility in each IL was measured at 22 °C using a volumetric method. While the anion is typically known to be predominantly responsible for the CO₂ capture reaction, the [P_<i>nnnn</i>]­[2-CNPyr] ILs with shorter alkyl chains on the cations exhibited slightly stronger CO₂ binding ability than the ILs with longer alkyl chains. We attribute this to the difference in entropy of reaction, as well as the variation in the relative degree of ionicity

Effect of Structure on Transport Properties (Viscosity, Ionic Conductivity, and Self-Diffusion Coefficient) of Aprotic Heterocyclic Anion (AHA) Room-Temperature Ionic Liquids. 1. Variation of Anionic Species

A series of room temperature ionic liquids (RTILs) based on 1-ethyl-3-methylimidazolium ([emim]⁺) with different aprotic heterocyclic anions (AHAs) were synthesized and characterized as potential electrolyte candidates for lithium ion batteries. The density and transport properties of these ILs were measured over the temperature range between 283.15 and 343.15 K at ambient pressure. The temperature dependence of the transport properties (viscosity, ionic conductivity, self-diffusion coefficient, and molar conductivity) is fit well by the Vogel–Fulcher–Tamman (VFT) equation. The best-fit VFT parameters, as well as linear fits to the density, are reported. The ionicity of these ILs was quantified by the ratio of the molar conductivity obtained from the ionic conductivity and molar concentration to that calculated from the self-diffusion coefficients using the Nernst–Einstein equation. The results of this study, which is based on ILs composed of both a planar cation and planar anions, show that many of the [emim]­[AHA] ILs exhibit very good conductivity for their viscosities and provide insight into the design of ILs with enhanced dynamics that may be suitable for electrolyte applications

Effect of Structure on Transport Properties (Viscosity, Ionic Conductivity, and Self-Diffusion Coefficient) of Aprotic Heterocyclic Anion (AHA) Room Temperature Ionic Liquids. 2. Variation of Alkyl Chain Length in the Phosphonium Cation

A series of room-temperature ionic liquids (ILs) composed of triethyl­(alkyl)­phosphonium cations paired with three different aprotic heterocyclic anions (AHAs) (alkyl = butyl ([P₂₂₂₄]⁺) and octyl ([P₂₂₂₈]⁺)) were prepared to investigate the effect of cationic alkyl chain length on transport properties. The transport properties and density of these ILs were measured from 283.15 to 343.15 K at ambient pressure. The dependence of the transport properties (viscosity, ionic conductivity, diffusivity, and molar conductivity) on temperature can be described by the Vogel–Fulcher–Tamman (VFT) equation. The ratio of the molar conductivity obtained from the molar concentration and ionic conductivity measurements to that calculated from self-diffusion coefficients (measured by pulsed gradient spin–echo nuclear magnetic resonance spectroscopy) using the Nernst–Einstein equation was used to quantify the ionicity of these ILs. The molar conductivity ratio decreases with increasing number of carbon atoms in the alkyl chain, indicating that the reduced Coulombic interactions resulting from lower density are more than balanced by the increased van der Waals interactions between the alkyl chains. The results of this study may provide insight into the design of ILs with enhanced dynamics that may be suitable as electrolytes in lithium ion batteries and other electrochemical applications

Switching the Reaction Course of Electrochemical CO₂ Reduction with Ionic Liquids

The ionic liquid 1-ethyl-3-methylimidazolium bis­(trifluoromethylsulfonyl)­imide ([emim]­[Tf₂N]) offers new ways to modulate the electrochemical reduction of carbon dioxide. [emim]­[Tf₂N], when present as the supporting electrolyte in acetonitrile, decreases the reduction overpotential at a Pb electrode by 0.18 V as compared to tetraethylammonium perchlorate as the supporting electrolyte. More interestingly, the ionic liquid shifts the reaction course during the electrochemical reduction of carbon dioxide by promoting the formation of carbon monoxide instead of oxalate anion. With increasing concentration of [emim]­[Tf₂N], a carboxylate species with reduced CO₂ covalently bonded to the imidazolium ring is formed along with carbon monoxide. The results highlight the catalytic effects of the medium in modulating the CO₂ reduction products

Predicting the Solubility of CO₂ in Toluene + Ionic Liquid Mixtures with PC-SAFT

Perturbed-chain statistical associating fluid theory (PC-SAFT) was applied for modeling the vapor–liquid equilibrium of CO₂ + toluene + ionic liquid (IL) mixtures and the molar volume of their liquid phases at temperatures between 298.15 K and 333.15 K and at pressures up to 80 bar. ILs used for this study contain the bis­(trifluoromethylsulfonylimide) anion ([Tf₂N]⁻) and imidazolium, pyridinium, thiolanium, and phosphonium cations. The pure-IL PC-SAFT parameters were fit to pure-IL liquid density data. Temperature-dependent binary interaction parameters were fit to binary liquid–liquid equilibrium data (i.e., toluene + IL) obtained from the literature and some points measured for this work. Temperature independent binary interaction parameters were fit to vapor–liquid equilibrium data (CO₂ + IL, CO₂ + toluene) from the literature. The availability of the pure-IL parameters and binary interaction parameters allowed prediction of CO₂ solubility in toluene + IL mixtures with an absolute average relative deviation (AARD) of 6.8%, as well as molar volumes of CO₂ + toluene + IL mixtures with an AARD of 5.0%, for the four ternary systems under investigation