Imidazolium-based Room Temperature Ionic Liquids Research Articles

Double layer in room temperature ionic liquids: influence of temperature and ionic size on the differential capacitance and electrocapillary curves

Differential capacity-potential curves, C(E), were obtained from electrochemical impedance spectra (12 kHz-2 Hz) for the interfaces between Hg and a series of alkyl imidazolium-based room temperature ionic liquids having the same anion, bis(trifluoromethanesulfonyl) imide: 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl) imide [EMIM][Tf(2)N], 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl) imide [BMIM][Tf(2)N], 1-hexyl-3-methylimidazolium bis(trifluoromethanesulfonyl) imide [HMIM][Tf(2)N]. The electrocapillary curves were obtained from drop time measurements and the values of the pzc were calculated. The pzc apparently becomes more negative as the imidazolium alkyl chain length increases. A small effect of the cation is seen on the C(E) curves at negative potentials. The effect of the aromatic nature of the cation is assessed by comparing 1-butyl-1-methylimidazolium bis(trifluoromethanesulfonyl) imide, with 1-butyl-3-methylpyrrolidinium bis(trifluoromethanesulfonyl) imide [BMPyr][Tf(2)N]. The effects of temperature on the capacitance, drop time electrocapillary curve and on the pzc were also obtained. The capacity was found to increase with increasing temperature in the whole range of accessible potentials.

Room-Temperature Ionic Liquids and Composite Materials: Platform Technologies for CO2 Capture

Clean energy production has become one of the most prominent global issues of the early 21st century, prompting social, economic, and scientific debates regarding energy usage, energy sources, and sustainable energy strategies. The reduction of greenhouse gas emissions, specifically carbon dioxide (CO(2)), figures prominently in the discussions on the future of global energy policy. Billions of tons of annual CO(2) emissions are the direct result of fossil fuel combustion to generate electricity. Producing clean energy from abundant sources such as coal will require a massive infrastructure and highly efficient capture technologies to curb CO(2) emissions. Current technologies for CO(2) removal from other gases, such as those used in natural gas sweetening, are also capable of capturing CO(2) from power plant emissions. Aqueous amine processes are found in the vast majority of natural gas sweetening operations in the United States. However, conventional aqueous amine processes are highly energy intensive; their implementation for postcombustion CO(2) capture from power plant emissions would drastically cut plant output and efficiency. Membranes, another technology used in natural gas sweetening, have been proposed as an alternative mechanism for CO(2) capture from flue gas. Although membranes offer a potentially less energy-intensive approach, their development and industrial implementation lags far behind that of amine processes. Thus, to minimize the impact of postcombustion CO(2) capture on the economics of energy production, advances are needed in both of these areas. In this Account, we review our recent research devoted to absorptive processes and membranes. Specifically, we have explored the use of room-temperature ionic liquids (RTILs) in absorptive and membrane technologies for CO(2) capture. RTILs present a highly versatile and tunable platform for the development of new processes and materials aimed at the capture of CO(2) from power plant flue gas and in natural gas sweetening. The desirable properties of RTIL solvents, such as negligible vapor pressures, thermal stability, and a large liquid range, make them interesting candidates as new materials in well-known CO(2) capture processes. Here, we focus on the use of RTILs (1) as absorbents, including in combination with amines, and (2) in the design of polymer membranes. RTIL amine solvents have many potential advantages over aqueous amines, and the versatile chemistry of imidazolium-based RTILs also allows for the generation of new types of CO(2)-selective polymer membranes. RTIL and RTIL-based composites can compete with, or improve upon, current technologies. Moreover, owing to our experience in this area, we are developing new imidazolium-based polymer architectures and thermotropic and lyotropic liquid crystals as highly tailorable materials based on and capable of interacting with RTILs.

Diol-Functionalized Imidazolium-Based Room-Temperature Ionic Liquids with Bis(trifluoromethanesulfonimide) Anions that Exhibit Variable Water Miscibility

The synthesis and characterization of a new class of 1-(2,3-dihydroxypropyl)-3-alkylimidazolium bis(trifluoromethanesulfonimide) room-temperature ionic liquids (RTILs) with tunable water miscibility are reported. The presence of a vicinal diol substituent on these RTILs allows for variable water miscibility, depending on the nature of the alkyl substituent on the cation. Water-miscible, imidazolium-based RTILs with the bis(trifluoromethanesulfonimide) anion are unprecedented.

Electrochemical stability of bis(trifluoromethanesulfonyl)imide-based ionic liquids at elevated temperature as a solvent for a titanium oxide bronze electrode

Four different electrolytes are prepared by dissolving a Li salt in three different room-temperature ionic liquids (RTILs) and also in a conventional organic solvent. The cathodic (electrochemical reduction) stability of these electrolytes is compared at both ambient and elevated temperature by potential cycling on a TiO 2 -B electrode. At room temperature, the stability of pyrrolidinium- and piperidinium-based RTILs is comparable with that of the carbonate-based organic solvent, which is in contrast to the severely decomposed imidazolium-based RTIL. At elevated temperature (120 °C), the imidazolium-based RTIL undergoes even more significant cathodic decomposition that results in the deposition of a resistive surface film and leads to eventual cell degradation. By contrast, the cathodic decomposition and concomitant film deposition are not serious with pyrrolidinium- and piperidinium-based RTILs even at this high-temperature, so that the TiO 2 -B/Li cell operates with reasonably good cycle performance. The latter two RTILs appear to be promising solvents for lithium-ion batteries that are durable against occasional exposure to high-temperature.

The double role of ionic liquids in organic electrosynthesis: Precursors of N-heterocyclic carbenes and green solvents. Henry reaction

N-heterocyclic carbenes, electrogenerated by cathodic reduction of imidazolium-based room temperature ionic liquids (RTILs), are stable bases able to catalyze the Henry reaction. Accordingly, the electrosynthesis of β-nitroalcohols has been achieved, under mild conditions and in high yields, by stirring nitromethane and aldehydes in previously electrolyzed RTILs. RTILs have been used as green solvents as well as precursors of N-heterocyclic carbenes.

Comparison of the Blue-Shifted C−D Stretching Vibrations for DMSO-d6in Imidazolium-Based Room Temperature Ionic Liquids and in Water

The blue-shifted C-D stretching vibrations (nu(CD)) of deuteriated dimethyl sulfoxide (DMSO-d(6)) in mixtures of DMSO-d(6)/ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim][BF(4)]) were carefully investigated and compared with those in DMSO-d(6)/water. Results from FTIR studies indicate that the direct interaction of C-D...F between DMSO-d(6) and the anion [BF(4)(-)] plays a minor role in the blue-shift of nu(CD) in the mixtures of DMSO-d(6)/IL. They also show that the indirect influence of the hydrogen bond formed by the nearby functional group SO with C(2)-H of the cation significantly contributes to the blue-shift of nu(CD), as can be inferred from the significant differences of the blue-shift in [Bmim][BF(4)] and 1-butyl-2,3-dimethylimidazolium tetrafluoroborate ([Bm(2)im][BF(4)]). In contrast, in the case of DMSO-d(6)/water mixtures, direct hydration of C-H...O between the methyl groups of DMSO-d(6) with the oxygen atom of water is mainly responsible for the blue shift of nu(CD). Furthermore, theoretical calculations revealed that charge migrations as well as rehybridization have a dominant effect on the blue-shift in both mixtures.

Guide to CO2 Separations in Imidazolium-Based Room-Temperature Ionic Liquids

Room-temperature ionic liquids (RTILs) are nonvolatile, tunable solvents that have generated significant interest across a wide variety of engineering applications. The use of RTILs as media for CO2 separations appears especially promising, with imidazolium-based salts at the center of this research effort. The solubilities of gases, particularly CO2, N2, and CH4, have been studied in a number of RTILs. Process temperature and the chemical structures of the cation and anion have significant impacts on gas solubility and gas pair selectivity. Models based on regular solution theory and group contributions are useful to predict and explain CO2 solubility and selectivity in imidazolium-based RTILs. In addition to their role as a physical solvent, RTILs might also be used in supported ionic liquid membranes (SILMs) as a highly permeable and selective transport medium. Performance data for SILMs indicates that they exhibit large permeabilities as well as CO2/N2 selectivities that outperform many polymer membra...

Ionic liquid-facilitated synthesis and catalytic activity of highly dispersed Ag nanoclusters supported on TiO2

This study describes a one-pot sol–gel method for the synthesis of Ag/TiO2 nanocomposite powders. TEM images showed that the Ag clusters formed with this method are highly dispersed on the surface of TiO2 nanoparticles. UV/visible spectroscopy results indicated that [BMIM][PF6], an imidazolium-based room-temperature ionic liquid, strongly inhibited the formation and growth of Ag clusters in N,N-dimethylformamide (DMF) solvent. This made it possible to prepare a stable stock solution containing AgNO3–[BMIM][PF6]–DMF. Dilution of the stock solution followed by hydrolysis resulted in a gel that was processed to produce Ag/TiO2 nanocomposites containing Ag clusters with average size smaller than 3 nm. XPS results indicated that the silver species exist predominantly in the metallic form. Because of the high dispersion and controlled size of the Ag clusters, the Ag/TiO2 nanocomposites demonstrated excellent catalytic performance in the aqueous phase reduction of methylene blue and the hydrogenation of 4-nitrophenol using NaBH4 as the reducing agent.

In Situ Attenuated Total Reflection Infrared Spectroscopy of Imidazolium-Based Room-Temperature Ionic Liquids under “Supercritical” CO2

In situ high-pressure attenuated total reflection infrared (ATR-IR) spectroscopy has been applied to elucidate the molecular interactions between dissolved CO(2) and three different imidazolium-based room-temperature ionic liquids, 1-n-butyl-3-methylimidazolium tetrafluoroborate [bmim][BF(4)], 1-n-butyl-3-methylimidazolium hexafluorophosphate [bmim][PF(6)], and 1-n-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [bmim][Tf(2)N], and also for 1-n-butylpyridinium tetrafluoroborate [bpy][BF(4)] swollen under "supercritical" CO(2) conditions (50 degrees C, 12.0 MPa). The results show that cation species as well as anion species of the ionic liquids affect the molecular state of the dissolved CO(2), because the Lewis acid-base interaction between the ionic-liquid anion species and CO(2) generates new anion species [X-CO(2)](-) (X = anion species of ionic liquid) which are more basic than X(-). Good correlation was found between the extent of imidazolium-ring C-H stretching band shifts and that of the anion species B-F, P-F, C-F, and SO stretching band shifts after the dissolution of CO(2). Largest shift of the ring C-H bands as well as the B-F band was observed for [bmim][BF(4)], indicating that the newly formed anion species [BF(4)-CO(2)](-) is most basic, which seems to be related to the higher activity of [bmim][BF(4)] compared to [bmim][PF(6)] for the catalytic cycloaddition of CO(2) to propylene oxide. Thus, the spectroscopy was applied also for the [bmim][BF(4)]-propylene oxide-CO(2) system (3.0 MPa, room temperature and 80 degrees C) to evaluate the reactivity of [BF(4)-CO(2)](-) and to acquire mechanistic information on the cycloaddition. Based on the spectroscopic results, a new plausible catalytic cycle was proposed, in which [BF(4)-CO(2)](-) species first attacks the electrophilic epoxide carbon, followed by cyclization of the corresponding intermediate to give propylene carbonate product as well as [bmim][BF(4)]. On the other hand, the shift of B-F stretching band of [bpy][BF(4)] under identical supercritical-CO(2) conditions was less drastic compared to [bmim][BF(4)], indicating that the Lewis acid-base interaction between BF(4)(-) and the dissolved CO(2) can be tuned by the choice of the cation species of the ionic liquid. Finally, the studies revealed that the strong Lewis acid-base interaction between the ionic liquids and the dissolved CO(2) has no promotional effect on the solubility of CO(2), because [bmim][Tf(2)N] which showed very small shifts of CF(3) and SO(2) bands and no imidazolium ring C-H band shifts after the dissolution of CO(2) exhibited much higher solubilizing power for CO(2) than [bmim][BF(4)]. The solubility of CO(2) in the ionic liquids seems to be determined by other factors such as the affinity of fluorine atoms for CO(2) and free volumes.

Variations of Diffusion Coefficients of Redox Active Molecules in Room Temperature Ionic Liquids upon Electron Transfer

In ionic liquids, the diffusion coefficients of a redox couple vary considerably between the neutral and radical ion forms of the molecule. For a reduction, the inequality of the diffusion coefficients is characterized by the ratio gamma = D(red)/D(ox), where D(red) and D(ox) are the diffusion coefficients of the electrogenerated radical anion and of the corresponding neutral molecule, respectively. In this work, measurements of gamma have been performed by scanning electrochemical microscopy (SECM) in transient feedback mode, in three different room temperature ionic liquids (RTILs) sharing the same anion and with a series of nitro-derivative compounds taken as a test family. The smallest gamma ratios were determined in an imidazolium-based RTIL and with the charge of the radical anion localized on the nitro group. Conversely, gamma tends to unity when the radical anion is fully delocalized or when the nitro group is sterically protected by bulky substituents. The gamma ratios, standard potentials of the redox couple measured in RTILs, and those observed in a classical organic solvent were compared for the investigated family of compounds. The stabilization energies approximately follow the gamma ratios in a given RTIL but change considerably between ionic liquids with the nature of the cation.

Capacitance Measurements in a Series of Room-Temperature Ionic Liquids at Glassy Carbon and Gold Electrode Interfaces

An extensive study has been done for the first time on the structure of the electrical double layer (EDL) at polarized glassy carbon (GC) and gold (Au) electrode interfaces in a series of room-temperature ionic liquids (RTILs) via the measurement of capacitance-potential curves. The parabolic capacitance-potential curves similar to those observed in high-temperature inorganic molten salts were obtained at GC electrode in all of the RTILs studied. Potential of zero charge (PZC) at GC electrode in imidazolium-based RTILs depends significantly on the electrochemical pretreatment of the electrode surface: Electrochemical oxidation pretreatment generates the oxide surface on GC electrode, which results in a favorable adsorption of positively charged imidazolium cations on the electrode surface and in turn shifts the PZC to the positive direction of potential, whereas at the electrochemically reduced GC electrode, on which the adsorption of the imidazolium cations is less favorable, PZC shifts to the negative d...

Study on the excited state intramolecular proton transfer of 4′- N, N-diethylamino-3-hydroxyflavone in imidazolium-based room temperature ionic liquids

The excited state intramolecular proton transfer reaction of 4′- N, N-diethylamino-3-hydroxyflavone in imidazolium-based room temperature ionic liquids was investigated by time resolved fluorescent measurements. Dual emission from the normal and the tautomer forms were observed. The normal emission peak position gradually shifts to longer wavelength in the sub ns time region, and the tautomer emission was delayed relative to the normal emission. These results suggest that solvation processes in the earlier time region after excitation play an important role in the proton transfer mechanism.

Interpretation of CO2 Solubility and Selectivity in Nitrile-Functionalized Room-Temperature Ionic Liquids Using a Group Contribution Approach

In this work, tuning the solubility parameter of room-temperature ionic liquids (RTILs) with appended functional groups was explored using a combination of experiment and theory. By predictably altering the solubility parameters of several RTIL solvents, their gas solubility and separation performance were tailored. This concept was demonstrated by synthesizing and characterizing imidazolium-based RTILs that incorporate nitrile and alkyne functional substituents. The ideal solubility and selectivity values of CO2, N2, and CH4 at near ambient temperature and pressure were measured for these RTILs. These functionalized RTIL solvents exhibited lower CO2, N2, and CH4 solubility values but improved CO2/N2 and CO2/CH4 solubility selectivity when compared to analogous nonfunctionalized, n-alkyl-substituted RTILs. A group contribution method was used to predict the solubility parameters of the functionalized RTILs, and these values were used with regular solution theory to predict the solubility and selectivity of the three gases. These predicted gas solubility values were found to be in good agreement with those measured experimentally. Furthermore, the predictions from the group contribution method indicated that inclusion of the nitrile and alkyne functional groups increased the solubility parameter relative to the analogous, n-alkyl-substituted RTILs. These initial results show that the group contribution method offers a valuable guide for systematically designing functionalized RTILs with specific gas solubility and selectivity performance.

Highly efficient extraction of phenols and aromatic amines into novel ionic liquids incorporating quaternary ammonium cation

Two novel hydrophobic room temperature ionic liquids (RTIL) incorporating quaternary ammonium cations: tetrahexylammonium dihexylsulfosuccinate (THADHSS) and trioctylmethylammonium salicylate (TOMAS) have been obtained. The mutual solubility of the both RTILs with water and their Reichardt's polarity index have been measured. The extraction of aromatic amines and phenols into novel RTILs and two 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amides has been studied. The dependences of RTIL–water distribution ratio for the studied liquids on the phase volume ratio, the time of phase contact, pH value of aqueous solutions and the solute concentration have been obtained. In some cases, the solute distribution ratios for ammonium-based RTILs are as high as n × 10 4 that is much greater than for imidazolium ones. Notably, unlike the case of imidazolium-based RTILs, the quantitative extraction into ammonium RTILs is achieved even at the phase volume ratio V RTIL: V w = 1:20.

Interactions between Spiropyrans and Room-Temperature Ionic Liquids: Photochromism and Solvatochromism

A series of imidazolium-based room-temperature ionic liquids (RTILs) containing anions from organic carboxylic acids were prepared. A set of dye probes, including Reichardt's dye (30), 4-nitrioaniline, and N, N-diethyl-4-nitroaniline, were used to determine the ET(30) scales and the Kamlet-Taft parameters (pi*, alpha, and beta) of the RTILs. On the basis of the polarity properties, these RTILs were categorized into three groups: group A with beta >0.9, alpha <0.9; group B with beta <0.9, alpha <0.9; and group C with beta <0.9, alpha >0.9. Interactions of these RTILs with four photochromic spiropyran derivatives (SP-I, SP-II, SP-III, and SP-IV) were investigated. It was found that the spiropyrans could present photochromism (positive or negative) or not, depending mainly on the polarity properties of the RTILs and also on the structure itself. A new spectroscopic method based on the molecular transition energy of the spiropyran probes (ESP) was proposed to determine the polarity of those protic or fluorine-containing RTILs, which were failed with the Reichardt's dye (30) probe.

Interactions and Dynamics in Ionic Liquids

Precise dielectric spectra have been determined at 25 degrees C over the exceptionally broad frequency range of 0.1 <or= nu/GHz <or= 3000 for the imidazolium-based room-temperature ionic liquids (RTILs) [bmim][BF4], [bmim][PF6], [bmim][DCA], and [hmim][BF4]. The spectra are dominated by a low-frequency process at approximately 1 GHz with a broad relaxation time distribution of the Cole-Davidson or Cole-Cole type, which is thought to correspond to the rotational diffusion of the dipolar cations. In addition, these RTILs possess two Debye relaxations at approximately 5 GHz and approximately 0.6 THz and a damped harmonic oscillation at approximately 2.5 THz. The two higher-frequency modes are almost certainly due to cation librations, but the origin of the approximately 5 GHz mode remains obscure.

Ideal Gas Solubilities and Solubility Selectivities in a Binary Mixture of Room-Temperature Ionic Liquids

This study focuses on the solubility behaviors of CO2, CH4, and N2 gases in binary mixtures of imidazolium-based room-temperature ionic liquids (RTILs) using 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C2mim][Tf2N]) and 1-ethyl-3-methylimidazolium tetrafluoroborate ([C2mim][BF4]) at 40 degrees C and low pressures (approximately 1 atm). The mixtures tested were 0, 25, 50, 75, 90, 95, and 100 mol % [C2mim][BF4] in [C2mim][Tf2N]. Results show that regular solution theory (RST) can be used to describe the gas solubility and selectivity behaviors in RTIL mixtures using an average mixture solubility parameter or an average measured mixture molar volume. Interestingly, the solubility selectivity, defined as the ratio of gas mole fractions in the RTIL mixture, of CO2 with N2 or CH4 in pure [C2mim][BF4] can be enhanced by adding 5 mol % [C2mim][Tf2N].

A comparison of ether- and alkyl-derivatized imidazolium-based room-temperature ionic liquids: a molecular dynamics simulation study

Molecular dynamics simulations of ether-derivatized imidazolium-based room-temperature ionic liquids (EDI-RTILs), [C(5)O(2)mim][TFSI] and [C(5)O(2)mim][BF(4)], have been performed and compared with simulations of alkyl-derivatized analogues (ADI-RTILs). Simulations yield RTIL densities, self-diffusion coefficients and viscosity in excellent agreement with experimental data. Simulations reveal that structure in the EDI-RTILs, quantified by the extent of nanoscale segregation of tails as well as cation-ion and cation-cation correlations, is reduced compared to that observed in the ADI-RTILs. Significant correlation between ether tail oxygen atoms and imidazolium ring hydrogen atoms was observed in the EDI-RTILs. This correlation is primarily intramolecular in origin but has a significant intermolecular component. Competition of ether oxygen atoms with oxygen atoms of TFSI(-) or fluorine atoms of BF(4)(-) for coordination of the ring hydrogen atoms was found to reduce the extent of cation-anion correlation in the EDI-RTILs compared to the ADI-RTILs. The reduction in intermolecular correlation, particularly tail-tail segregation, as well as weakening of cation-anion specific interactions due to the ether tail, may account for the faster dynamics observed in the EDI-RTILs compared to ADI-RTILs.

Enhanced CO2 Separation Selectivity in Oligo(ethylene glycol) Functionalized Room-Temperature Ionic Liquids

Novel imidazolium-based room-temperature ionic liquids (RTILs) with one, two, or three oligo(ethylene glycol) substituents were synthesized. Solubilities and ideal solubility selectivities of CO2, N2, and CH4 at low pressure (1 atm) in these RTILs were determined using a pressure decay technique. Comparison to corresponding alkyl analogues of these RTILs reveals similar levels of CO2 solubility but lower solubilities of N2 and CH4. As a consequence, RTILs with oligo(ethylene glycol) substituents were observed to have 30−75% higher ideal solubility selectivities for CO2/N2 and CO2/CH4.

Tribological Characteristics of Imidazolium-based Room Temperature Ionic Liquids Under High Vacuum

Tribological characteristics of two imidazolium-based room temperature ionic liquids (RTILs), 1-hexyl-3-methylimidazolium tetrafluoroborate, and 1-hexyl-3-methylimidazolium hexafluorophosphate were investigated under high vacuum conditions. Viscosity–temperature characteristics and thermogravimetric characteristics of these compounds were also investigated. Imidazolium-based RTILs have relatively good viscosity–temperature characteristics that are comparable to those of multiply-alkylated cyclopentane (MAC). Thermogravimetric results showed that ionic liquids have high thermal stability and low vapor pressure. Ionic liquids showed low friction and low wear rate under high vacuum conditions, and high load-carrying capacity was observed.