Ind. Eng. Chem. Res. 2006, 45, 6279-6283
6279
Bulk-Fluid Solubility and Membrane Feasibility of Rmim-Based Room-Temperature Ionic Liquids Dean Camper,*,† Jason Bara,† Carl Koval,‡ and Richard Noble† Department of Chemical and Biological Engineering and Department of Chemistry and Biochemistry, UniVersity of Colorado, Boulder, Colorado 80309
This paper shows how gas solubility can be predicted in Rmim-based room-temperature ionic liquids (RTILs). We have developed a model to illustrate that ideal, single-gas solubility in Rmim-based RTILs is primarily a function of molar volume of the RTIL. This method is used to estimate bulk-fluid gas solubility, showing that for Rmim-based RTILs there is a particular molar volume where a maximum amount of gas per volume of RTIL occurs at a given pressure and temperature. This method is also used to estimate gas permeability and gas pair separation selectivity for ideal CO2/N2 and CO2/CH4 separations. A comparison to traditional polymer membranes utilized in these separations is included in the form of a “Robeson plot”. 1. Introduction Room-temperature ionic liquids (RTILs) are organic salts that are liquid at or below 298 K.1 RTILs remain fluid at low temperatures primarily because of the large size and asymmetry of the cation, coupled with resonance-stabilized anions. Applications of RTILs include, but are not limited to, “green” solvents for reactions,1-6 bulk-fluid and membrane separation mediums,1,3,5,7-14 and electrochemical applications.4,8 The properties of RTILs that make them of interest for these applications include nonflammability,1,4-6,8 negligible vapor pressure,1,3-6,8 and high thermal stability.4,5,8 The use of RTILs is becoming more common,1-27 where 1-R3-methylimidazolium (Rmim)-based RTILs are of particular interest because of their tendency to be less viscous than other types of RTILs. Solubility of gases of industrial importance such as carbon dioxide, nitrogen, and hydrocarbons are of particular interest for Rmim-based RTILs.8,9 RTILs are also being used to store and deliver reactive and hazardous gases in the semiconductor industry. A recent presentation at the 1st International Congress on Ionic Liquids (COIL)10 described the use of Rmimbased RTILs to store BF3 and PH3 at subatmospheric pressures. Understanding gas solubility is also important for using Rmimbased RTILs for reaction and separation mediums. This paper provides a method that can be used to choose a particular Rmim-based RTIL for bulk-fluid separations or storage of gases, and it shows how these RTILs can also be used as membranes for gas separations. A method to predict an optimal Rmim-based RTIL for maximum bulk-fluid absorption is described. This method then is used to compare Rmim-based RTILs to polymers for gas separations using “Robeson plots”, which illustrate a “flux-selectivity tradeoff” and an apparent upper limit of gas-separation selectivity vs permeability for polymers. 2. Experimental Methods 2.1. Materials. The solubility data used in this paper to validate the methods were found in the literature or measured in our lab. All solubility data for carbon monoxide were found in Ohlin et al.15 The CO2 solubility data along with their source * Corresponding author. E-mail:
[email protected]. † Department of Chemical and Biological Engineering. ‡ Department of Chemistry and Biochemistry.
are listed in Table 1 for 1-ethyl-3-methylimidazolium trifloromethanesulfone ([emim][CF3SO3]), 1-ethyl-3-methylimidazolium dicyanamide ([emim][dca]), 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([emim][Tf2N]), 1-butyl-3methylimidazolium tetrafluoroborate ([bmim][BF4]), and 1butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([bmim][Tf2N]). The CO2 solubility data measured in our lab for 1-decyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([decmim][Tf2N]) at 25, 40, and 50 °C and for [emim][Tf2N] and 1-hexyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide [hmim][Tf2N] at 40 °C are also shown in Table 1. The solubility data for CH4 and N2 are shown in Table 2. The solubility data recorded in our lab were measured using a dual-transducer, dual-volume pressure decay method as described by Camper et al.8 The molar volumes of the RTILs at 22 °C are shown in Table 3. The molar volumes are assumed to be constant during gas absorption, as the moles of gas absorbed were sufficiently small. The values used for the molar volumes of [emim][Tf2N] for the various temperatures were calculated from the density equation presented by Cadena et al.,16 and the values used for the molar volume of [bmim][BF4] were calculated from the density equations by Anthony et al.2 All other densities used were measured in 1 mL volumetric flasks at 22 °C. The halide contents for all RTILs, except for [decmim][Tf2N] and [hmim][Tf2N], are listed in the reference from which the data originated, along with either water content or methods to reduce water content. The [emim][Tf2N] used in the CO2 solubility measurement at 40 °C was the same as the [emim][Tf2N] used by Camper et al.24 The residual halide content for [hmim][Tf2N] and [decmim][Tf2N] was
