Isothermal vapour–liquid equilibria in the binary and ternary systems consisting of an ionic liquid, 1-propanol and CO2
Introduction
Ionic liquids (ILs) are mostly organic salts that are liquid at ambient conditions, generally as a result of low lattice energy due to the asymmetric cation and/or anion and weak Coulombic forces [1], [2]. The ionic liquids exhibit some interesting properties, such as: negligible vapour pressure [3] or excellent solvent power [4]. The ILs can be tailored or tuned to provide a desired density, viscosity, melting point, hydrophobicity and many other properties [5], [6] to suit the requirements of a particular process. Among the possible applications of the ILs [7], [8] the most thrilling one is their use in processes with supercritical fluids [9].
An interest in supercritical fluids (scF), mainly in supercritical carbon dioxide (scCO2) [10] has developed in the past decades. Supercritical carbon dioxide is an attractive solvent because it is readily available, inexpensive, non-toxic, non-flammable, and its critical temperature and operating pressures are relatively easy to achieve. All the aforementioned properties of the ionic liquids and supercritical gases allow classifying them as generally regarded as safe.
Furthermore due to their unique properties either ILs or scF are beneficial for the processes because they modify the efficiency and selectivity of many reactions [11], [12] comparing to the reactions with conventional solvents.
Either ionic liquids or supercritical fluids were recognised by Leitner as “advanced fluids or 2nd generation green solvents” [13]. For this reason the phase behaviour of the IL + CO2 systems is favourable for a number of applications of the IL + CO2 biphasic systems for example in biphasic separations [9], [14] or reactions [15].
Recently, investigations into the CO2 solubility in bis(trifluoromethylsulfonyl)imide [Tf2N] [16], [17], [18], [19] and trifluoromethanesulfonate [TfO] [18], [20], [21], [22] ionic liquids have received a growing interest. The CO2 was found to dissolve the fluorous-rich ILs quite well. The influence of the alkyl chain length of a cation of an IL on the solubility in CO2 was reported as significant [23].
There are only few publications on the CO2 + IL phase equilibrium with classical solvents, e.g. acetone [24], benzene, toluene, or chlorobenzene [25]. Similarly, the data on phase equilibria of CO2 + IL and an alcohol system are scarce. Until now, only four ternary systems containing CO2, an IL, and an alcohol were investigated. Ternary systems containing CO2 with methanol [26], [27] or ethanol [28], and with 1-butyl-3-methylimidazolium hexafluorophosphate [26], [27], [28] or 1-butyl-3-methylimidazolium tetrafluoroborate [27] were studied. To extend this field of research the phase equilibria of an alcohol + an IL + CO2 with 1-propanol, as never investigated, were examined in this work. The current work is a continuation of broad studies of vapour–liquid equilibria for systems consisting of an alcohol + an IL + CO2 [29], [30]. This report presents the vapour–liquid equilibrium (VLE) data for ternary systems of 1-propanol, CO2 and one of three ILs: 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C2mim][Tf2N]) or 1-ethyl-3-methylimidazolium trifluoromethanesulfonate ([C2mim][TfO]) or 1-decyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C10mim][Tf2N]) at 313.15 K and at 9, 10 and 12 MPa. Moreover, to complete the phase equilibrium studies of the systems containing CO2 and the bis(trifluoromethylsulfonyl)imide and trifluoromethanesulfonate ILs, the binary (CO2 + [C10mim][Tf2N]) system has been measured as it was never investigated before. The structures of ionic liquids used in this work are presented in Fig. 1.
Section snippets
Materials
Carbon dioxide was supplied by Air Liquide, with a stated purity of 99.998 mol%. 1-Propanol (99.5 wt%) was purchased from Sigma. The alcohol was additionally dried using 3 Å molecular sieves. [C2mim][Tf2N], [C2mim][TfO] and [C10mim][Tf2N] ionic liquids were obtained from Solchemar, Portugal with the stated purity of 98 wt%. They all were further thoroughly degassed, dried, and freed from any traces of volatile compounds by applying vacuum (0.1 Pa) at moderate temperature (60 °C). All the drying
Vapour–liquid equilibrium of binary and ternary systems
This work presents the isothermal vapour–liquid equilibrium data for the systems containing (CO2 + 1-propanol + [C2mim][Tf2N] or [C2mim][TfO] or [C10mim][Tf2N]). Experiments were performed at 313.15 K for three different pressures 9, 10 and 12 MPa. Temperature of the VLE studies (i.e. 313.15 K) was chosen on the base of the available literature data for the binary systems, such as: CO2 + 1-propanol [32], CO2 + [C2mim][Tf2N] [19], CO2 + [C2mim][TfO] [22], 1-propanol + [C2mim][Tf2N] [33], 1-propanol + [C2
Conclusions
Carbon dioxide is highly soluble in the investigated ionic liquids especially in [C10mim][Tf2N] even at relatively low pressures. The ternary systems containing 1-propanol, CO2 and one of the three presented ionic liquids were investigated at pressures from 9 to 12 MPa. The increase of pressure results in a smaller miscibility gap and extraction of a volatile compound from the liquid phase. The obtained phase envelopes were successfully correlated using the Peng–Robinson and Soave–Redlich–Kwong
Acknowledgements
This work was supported by the Fundação para a Ciência e a Tecnologia (FCT, Portugal) through the grant SFRH/BPD/26356/2006 (EBL) and SFRH/BPD/34577/2007 (RBL). DM gratefully acknowledges the ERASMUS scholarship.
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