Phase Behavior of Binary Ionic Liquid Systems: Ionic Liquids with Ammonia, Carbon dioxide, and Dihydroxy Alcohols
University of Kansas
Chemical & Petroleum Engineering
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Solvents play a crucial role in industrial processes, which might directly or indirectly have a bearing on the environment. As engineers and scientists, our goal is to advance or develop more sustainable chemicals to overcome the environmental challenges of the 21st century. Therefore, ionic liquids (ILs) might offer a unique solution. Ionic liquids are low melting point salts composed entirely of ions. The characteristics of ILs can be designed by varying both the cation, anion, and substituents. Therefore, ILs can be designed to be non-volatile, non-toxic, and environmentally benign. ILs are soluble with a wide range of compounds, allowing the use in various applications such as catalysis, separation, and solvents, to name a few. In order to develop these processes, fundamental phase behavior knowledge is required. The main objective of this thesis is to investigate the phase behavior of ionic liquids with gases (ammonia and carbon dioxide) and organic solvents (diols) over a wide temperature and pressure range. ILs are relatively viscous compared to traditional solvents like water; therefore, in addition to the thermodynamic measurements and modeling, the kinetics of gas dissolution in the ILs were also explored. Nuclear Magnetic Resonance (NMR) spectroscopy was utilized to further advance the understanding of the interaction in binary ionic liquid mixtures (i.e., NH3+ILs). In the investigation of the ionic liquid and ammonia system, vapor-liquid equilibrium (VLE) measurements for the binary systems of ammonia (NH3) with aprotic and protic ionic liquids have been successfully measured using a new Hiden XEMIS gravimetric microbalance. This study reports the first gravimetric measurements conducted for the solubility of NH3 in ionic liquids and provides the most accurate data to date. The NH3 sorption measurements were conducted at temperatures of 283.15, 298.15, 323.15, and 348.15 K and at pressures up to 0.7 MPa. The VLE data were correlated using the Peng-Robinson equation of state, the Non-Random Two Liquid (NRTL), and the Flory-Huggins model. All models are in excellent agreement with the experimental data. The Flory-Huggins model demonstrated that the non-idealities in NH3 solubility in the imidazolium-based ILs are due to both entropic and enthalpic impacts. The Fickian diffusivities of NH3 in imidazolium-based ILs were obtained by fitting experimental concentration to the one-dimensional (1D) mass diffusion equation and found to be about 3 to 5 times slower than the diffusion of NH3 in water. The semi-theoretical Stokes-Einstein equation was used to model diffusivities and to obtain the diffusing radius of NH3 in imidazolium-based ILs. NMR spectroscopy is utilized to investigate the interaction between NH3 and imidazolium-based ILs. NMR spectra of the NH3 systems revealed that the NH3 interacts with all protons in the cation while interacting with the most acidic hydrogen more profoundly. One exception is the system of ammonia and 1-ethyl-3-methylimidazolium 1,1,2,2-tetrafluoroethanesulfonate where NH3 interacts with all hydrogens in a similar manner. In the investigation of IL and carbon dioxide system, the high-pressure vapor-liquid equilibrium for the binary systems of carbon dioxide and a series of 1-alkyl-1-methyl pyrrolidinium bis(trifluoromethylsulfonyl)imide ionic liquids ([CnC1pyr][NTf2] (n = 3,4,6)) are measured at 298.15, 318.15, and 338.15 K and at pressures up to 20 MPa. Experiments were conducted using gravimetric (IGA and XEMIS microbalances) and volumetric (high-pressure view cell) methods. In this study, the solubility of CO2 in pyrrolidinium ionic liquids increases with increasing pressure and decreasing temperature. However, the high-pressure behavior (above 10 MPa) approaches almost a vertical slope, which indicates the CO2 solubility only slightly increases despite large increases in pressure. The CO2 solubility is found to be slightly dependent on the alkyl chain length on the pyrrolidinium cation, which is potentially due to the steric impacts. Molar volume and volume expansion of CO2 + IL mixtures at high pressures were also measured and reported. The Fickian diffusion of CO2 in pyrrolidinium-based ionic liquids (~10-10 m2·s) was calculated at pressures up to 2 MPa and found to be slightly lower than the diffusivity of CO2 in an imidazolium-based ionic liquid with the [NTf2] anion. In the investigation of ionic liquids and dihydroxy alcohols system, liquid-liquid equilibria (LLE) for the mixtures with three imidazolium-based ionic liquids were measured. The dihydroxy alcohols were 1,3-propanediol, 1,4-butanediol, and 1,5-pentanediol and the ionic liquids were 1-ethyl-3-methylimidazolium tetrafluoroborate ([C2C1im][BF4]), 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C2C1im][NTf2]), and 1-ethyl-3-methylimidazolium 1,1,2,2-tetrafluoroethanesulfonate ([C2C1im][TFES]). The experimental LLE data were well correlated using the NRTL activity coefficient model, which allows quantification of the the miscibility gaps. All binary diol systems with [C2C1im][BF4] or [C2C1im][NTf2] demonstrated an upper critical solution temperature (UCST) between 310 to 360 K. An equimolar mixture of diols and [C2C1im][TFES] showed complete miscibility between 293.15 to 373.15 K. An increase in alkyl chain length of the dihydroxy alcohols and/or changing the anion from [BF4] to [NTf2] for a given [C2C1im] cation results in an increase in the UCST. The excess molar volume of diols with ILs was, in most cases, larger than those of ordinary solutions.
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