A. Yokozeki

Physical and chemical absorptions of carbon dioxide in room-temperature ionic liquids.

Gaseous solubilities of carbon dioxide (CO2) in 18 room-temperature ionic liquids (RTILs) have been measured at an isothermal condition (about 298 K) using a gravimetric microbalance. The observed pressure-temperature-composition (PTx) data have been analyzed by use of an equation-of-state (EOS) model, which has been successfully applied for our previous works. Henrys law constants have been obtained from the observed (PTx) data directly and/or from the EOS correlation. Ten RTILs among the present ionic liquids results in the physical absorption, and eight RTILs show the chemical absorption. The classification of whether the absorption is the physical or chemical type is based on the excess Gibbs and enthalpy functions as well as the magnitude of the Henrys constant. In the chemical absorption cases, the ideal association model has been applied in order to interpret those excess thermodynamic functions. Then, two types of the chemical associations (AB and AB2, where A is CO2 and B is RTIL) have been observed with the heat of complex formations of about -11 (for AB) and from -27 to -37 (for AB2) kJ x mol(-1), respectively.

Separation of N2O and CO2 Using Room-Temperature Ionic Liquid [bmim][BF4]

We have developed a ternary equation of state (EOS) model for the N(2)O/CO(2)/1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF(4)]) system in order to understand separation of these gases using room-temperature ionic liquids (RTILs). The present model is based on a generic RK (Redlich-Kwong) EOS, with empirical interaction parameters for each binary system. The interaction parameters have been determined using our measured VLE (vapor-liquid equilibrium) data for N(2)O/[bmim][BF(4)] and CO(2)/[bmim][BF(4)] and literature data for N(2)O/CO(2). The binary EOS models for the N(2)O/[bmim][BF(4)] and CO(2)/[bmim][BF(4)] systems correctly predicted the liquid-liquid phase separation found in VLLE experiments. The validity of the ternary EOS model has been checked by conducting VLE experiments for the N(2)O/CO(2)/[bmim][BF(4)] system over a range in temperature from 296 to 315 K. With this EOS model, solubility (VLE) behavior has been calculated for various (T, P, and feed compositions) conditions. For both large and small N(2)O/CO(2) feed ratios, the N(2)O/CO(2) gas selectivity [α(N(2)O/CO(2)) = (y(N(2)O)/x(N(2)O))/(y(CO(2))/x(CO(2)))] is α = 1.4-1.5, compared with (α = 0.96-0.98) in the absence of ionic liquid. While the concentration of the ionic liquid does not affect the selectivity, the addition of an ionic liquid provides the only practical means of separating CO(2) and N(2)O.

Phase Behavior of CO2 in Room‐Temperature Ionic Liquid 1‐Ethyl‐3‐Ethylimidazolium Acetate

Carbon dioxide solubility (vapor-liquid equilibria: VLE) in an ionic liquid, 1-ethyl-3-ethylimidazolium acetate ([eeim][Ac]) was measured using a gravimetric microbalance at four isotherms (about 283, 298, 323, and 348 K) up to about 2 MPa. An equation-of-state (EOS) model was used to analyze the VLE data and has predicted vapor-liquid-liquid equilibria (VLLE: or liquid-liquid separations) in CO(2)-rich solutions. The VLLE prediction was confirmed experimentally using a volumetric method and likely the liquid-liquid equilibria will intersect with the solid-liquid equilibria such that no lower critical solution temperature can exist and the binary system may be classified as Type III phase behavior. Carbon dioxide solubility in the ionic-liquid-rich solution show extremely unusual behavior. CO(2) dissolves in the ionic liquid at large concentrations (up to about 20 mole % of CO(2)) with almost no vapor pressure above the mixtures. This result is similar to our previous findings with 1-butyl-3-methylimidazolium acetate ([bmim][Ac]) and 1-ethyl-3-methylimidazolium acetate ([emim][Ac]). In all three cases the CO(2) forms a molecular complex (or chemical reaction) with the ionic liquid. (13)C NMR spectroscopy has identified the structure for CO(2) absorbed in [eeim][Ac] to be [eeim]-2-carboxylate. Addition of water to the carboxylate leads to the dissolution of CO(2). The thermodynamic excess properties (enthalpy, entropy, and Gibbs energy) for all three systems have been calculated using the EOS and support the complex formation of the type AB(2) (where A is CO(2) and B is ionic liquid). Isothermal differential scanning calorimetry has verified the heat of reaction calculations and found for CO(2) absorbing in [emim][Ac], [eeim][Ac] and [bmim][Ac] to be about -38 kJ mol(-1). Additional experiments have examined the effect of water on the density, viscosity and CO(2) solubility in [eeim][Ac] and the CO(2) solubility in mixtures of [eeim][Ac] with other acetate salts.

Separation of N2O and CO2 using Room-Temperature Ionic Liquid [bmim][Ac]

We have developed a ternary equation of state (EOS) model for the N2O/CO2/1-butyl-3-methylimidazolium acetate ([bmim][Ac]) system in order to understand the separation of N2O and CO2 using room-temperature ionic liquids (RTILs). The present model is based on a generic RK (Redlich-Kwong) EOS, with empirical interaction parameters for each binary system. The interaction parameters have been determined using our measured VLE (vapor-liquid-equilibrium) data for N2O/[bmim][Ac] and literature data for CO2/[bmim][Ac] and N2O/CO2. The binary EOS model for the N2O/[bmim][Ac] system correctly predicted the liquid-liquid phase separation found in VLLE experiments. The validity of the ternary EOS model has been checked by conducting VLE experiments for the N2O/CO2/[bmim][Ac] system over a temperature range from 296 to 313 K. With this EOS model, solubility (VLE) behavior has been calculated for various (T, P, and feed compositions) conditions. Over a range of N2O/CO2 feed ratios, the N2O/CO2 gas selectivity [α N 2 O/CO 2 = (y N 2 O /x N 2 O )/(y CO 2 /x CO 2 )] increases by at least 5 orders of magnitude when adding [bmim][Ac] (α = 1 × 102 to 1 × 107), compared with the absence of the ionic liquid (α = 0.96 to 0.98). The addition of [bmim][Ac] may provide a practical means of separating CO2 and N2O. Supplemental materials are available for this article. Go to the publishers online edition of Separation Science and Technology to view the free supplemental file.

Comparison of the Sorption of Trifluoromethane (R-23) on Zeolites and in an Ionic Liquid

In this study, trifluoromethane (R-23) has been adsorbed on three zeolites, namely 5A, LSX and 13X, and their adsorption isotherms have been measured at approximately 298 and 323 K using a gravimetric microbalance. All cases belong to the adsorption Type II (one of the six IUPAC classifications), and the adsorption (and desorption) processes are reversible. Three different adsorption models [original Langmuir, multi-site Langmuir and BET equations] have been adopted to analyze the data, with a particular interest in calculating the heat of adsorption (–ΔH), which was found to be about 10, 30 and 40 kJ·mol−1 for zeolite 5A, LSX and 13X, respectively. These values are within the range of typical physical adsorptions. Solubility of R-23 in ionic liquid 1-octyl-3-methylimidazolium 1,1,2,2-tetrafluoroethanesulfonate ([omim][TFES]) has been measured at approximately 298 and 323 K using the same gravimetric microbalance and the value was compared with the adsorption results. The solubility capacity of R-23 in ionic liquid reaches about 3 mol kg−1 around 2.5 MPa at 298 K, while the adsorption capacity on zeolites becomes around 3 mol kg−1 at around 0.25 MPa (ten times smaller). The adsorption on zeolites took more time than the absorption in the ionic liquid to reach thermodynamic equilibrium.