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۱٫ Introduction

Various applications of a new class of compounds, room-temperature ionic liquids (RTILs), have been widely proposed in the last several years. RTILs are molten salts at room temperature without practically any vapor pressure and thus regarded as ‘‘green solvents’’, due to the lack of volatile organic compounds (VOCs). Among many possible RTIL applications, a promising application may be the capture (or sequestration) of sour gases like CO2, H2S, and SO2 in refinery, coal combustion, and synthesis of gas streams, using the unique absorption characteristics of RTILs. Researchers at the University of Notre Dame, E.J. Maginn, J.F. Brennecke and their coworkers, have initiated such studies a few years ago, and are making steady progress in this area, as described in their Technical Reports to DOE (Department of Energy, USA) [1]. One of their RTILs for the CO2 solubility studies is 1-butyl-3-methylimidazolium acetate, [bmim][Ac], which shows very high solubility of CO2. Their results imply possible chemical reactions [1], although their experimental solubility data at only one temperature have been quite limited. In this respect, Chinn et al. have proposed the {CO2 + [bmim][Ac] + water} system as the CO2 absorbent (or sequestration) in their US patent [2]. The purpose of the present study is to re-examine in detail their findings of the very high solubility of CO2 in [bmim][Ac] and to shed some light on this interesting binary system. To do so, we measured more detailed solubility data {(vapor + liquid) equilibria: VLE} than those in the literature. Then, we made a thermodynamic model for this system using an equation of state, which has predicted a very rare phase behavior including liquid–liquid separations {(vapor + liquid + liquid) equilibria: VLLE}. VLLE are confirmed experimentally for the first time in this system. Furthermore, in order to understand possible chemical reactions or complex formation in the present system, we have conducted several other experiments: making CO2 absorbed [bmim][Ac] solutions, and analyzing them by 1 H NMR, TGA-IR, FT-IR and ATR-IR for various sample conditions. Some reasonable models are discussed for the chemical and thermodynamic behaviors of this binary system.

۲٫ Experimental

۲٫۱٫ Materials

Carbon dioxide (purity >0.9999 mass fraction, CAS no. 124-38-9) was purchased from MG Industries (Philadelphia, PA). The [bmim][Ac] (assay P0.95 mass fraction, C10H18N2O2, CAS no. 284049-75-8, Lot and Filling code S25803 444041302) was obtained from Fluka (Buchs, Switzerland). The as-received mass fraction of water was measured by Karl–Fischer titration (Aqua-Star C3000, solutions AquaStar Coulomat C and A) and the undried sample contained 4699 ppm H2O. The [bmim][Ac] was dried and degassed by first filling a borosilicate glass tube with about 10 g of ionic liquid and pulling a coarse vacuum with a diaphragm pump (Pfeiffer, model MVP055-3, Nashua, NH) for about 3 h. Next, the [bmim][Ac] was completely evacuated using a turbopump (Pfeiffer, model TSH-071) to a pressure of about 4 Æ ۱۰۷ kPa while simultaneously heating and stirring the ionic liquid at a temperature of about 348 K for 5 days. The final mass fraction of water was agained measured by Karl–Fischer titration and the dried sample contained 703 ppm H2O. Detailed descriptions of experimental equipment and procedures for the VLE and VLLE are given in our previous reports [3–۶]; therefore, only the basic experimental techniques and measurement uncertainties are presented here.

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