Physical Properties of 1-Butyl-3-methylimidazolium Tetrafluoroborate/N-Methyl-2-pyrrolidone Mixtures and the Solubility of CO 2 in the System at Elevated Pressures Shidong Tian, † Yucui Hou,* ,‡ Weize Wu,* ,† Shuhang Ren, † and Kun Pang † † State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China ‡ Department of Chemistry, Taiyuan Normal University, Taiyuan 030012, China ABSTRACT: Densities and viscosities of 1-butyl-3-methylimidazolium tetrafluor- oborate ([bmim][BF 4 ]) (1) + N-methyl-2-pyrrolidone (NMP) (2) with w 1 = 0.0000, 0.5000, 0.6999, 0.8981, and 1.0000 at T = (298.15 to 318.15) K were measured and fitted to standard equations. The results show that densities and viscosities of the hybrid solvents decrease with the increase of temperature and mass fraction of NMP. A high-pressure variable-volume view cell technique was used to determine the solubility of CO 2 in pure solvents and mixtures under elevated pressures up to 6 MPa at temperatures from (298.15 to 318.15) K. The results indicate that the solubility of CO 2 in the pure solvents and mixtures increases with the increase of pressure and with the decrease of temperature. The solubility of CO 2 in the mixtures increases with the increase of the mass fraction of NMP, but it is very close to that in pure [bmim][BF 4 ] as the mass fraction of NMP is around 0.1019. ■ INTRODUCTION Carbon dioxide (CO 2 ) is one of the greenhouse gases which may lead to the global-warming, and the emissions of CO 2 from burning of fossil fuels have caused great concern all over the world. Currently, there are many technologies developed for the removal of CO 2 . 1-3 One of the technologies is to employ aqueous amines, such as methyldiethanolamine, monoethanol- amine, and diethanolamine, as chemical solvents to absorb CO 2 . They are used commercially for a high CO 2 absorption capacity. However, there are several drawbacks of this technology, such as corrosion problems, high volatility of amines, and high energy consumption for desorption. 4 Using physical solvents such as methanol, N-methyl-2-pyrrolidone (NMP), propylene carbonate (PC), and dimethylether of poly(ethylene glycol) is also an efficient way to capture CO 2 . Methanol is one of the most studied and used solvents, and it shows great advantages when used at higher CO 2 pressures. However, methanol needs extremely low operating temper- atures for its high vapor pressure. 2 Therefore, nonvolatile liquid solvents with a high capacity may be more attractive for the removal of CO 2 . Currently, ionic liquids (ILs) have drawn much attention for their negligible vapor pressure, low toxicity, high thermal stability, and tunable structure. Many groups have studied the solubility of CO 2 in different kinds of ILs. Blanchard et al. 5 found that ILs can dissolve massive amounts of CO 2 , while the solubility of ILs in supercritical CO 2 was extremely low. Since then, the absorption of CO 2 with ILs has drawn much attention all over the world. The ILs used to absorb CO 2 can be classified into normal ILs and task-specific ILs. CO 2 can be only physically soluble in the normal ILs, and its solubility increases with increasing pressure and decreasing temperature. 6-13 Bates et al. 14 reported the first task-specific IL used to absorb CO 2 at ambient pressure and room temperature; the uptake of CO 2 per mole IL could reach to 0.5 which was very close to the theoretical maximum capacity of the IL. After that, many kinds of task-specific ILs were reported. 11,15-21 The studies of the absorption of CO 2 by ILs have made great progress, and ILs have a potential use in real industries. However, the viscosities of pure ILs are often much higher than those of traditional organic solvents. These disadvantages of ILs severely limit their applications. Many researchers have reported the viscosities of ILs with water or organic solvents and found that the viscosities of pure ILs decrease sharply by adding small amounts of molecular solvents. 22-26 As a result, the blends of ILs with other solvents may show great advantages when used for the removal of CO 2 . 27,28 Actually, the absorption of CO 2 by the hybrid solvents has been widely studied. 4,29-35 As is known, 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF 4 ]) is one of the most efficient ILs for its high absorption capacity of CO 2 , 36 and NMP is the solvent used in the Purisol process 37 for its wonderful properties, such as low vapor pressure, low viscosity, and high CO 2 absorption capacity. In this work, we selected NMP as a cosolvent to improve the high viscosity of [bmim][BF 4 ], and we studied the physical properties such as densities and viscosities of [bmim][BF 4 ] (1) + NMP (2) with w 1 = 0.0000, 0.5000, 0.6999, 0.8981, and 1.0000 at T = (298.15 to 318.15) K. Besides, the solubilities of CO 2 in pure solvents and the hybrid solvents were determined at 298.15 K under elevated pressures, Received: August 7, 2011 Accepted: January 30, 2012 Published: February 29, 2012 Article pubs.acs.org/jced © 2012 American Chemical Society 756 dx.doi.org/10.1021/je200886j | J. Chem. Eng. Data 2012, 57, 756-763