Liquid−Vapor Equilibrium of the Systems Butylmethylimidazolium Nitrate−CO₂ and Hydroxypropylmethylimidazolium Nitrate−CO₂ at High Pressure: Influence of Water on the Phase Behavior

Abstract

Ionic liquids (IL) are receiving increasing attention due to their potential as "green" solvents, especially when used in combination with SC-CO2. In this work liquid-vapor equilibria of binary mixtures of CO2 with two imidazolium-based ionic liquids (IL) with a nitrate anion have been experimentally determined: butylmethylimidazolium nitrate (BMImNO3) and hydroxypropylmethylimidazolium nitrate (HOPMImNO3), using a Cailletet apparatus that operates according to the synthetic method. CO2 concentrations from 5 up to 30 mol % were investigated. It was found that CO2 is substantially less soluble in HOPMImNO3 than in BMImNO3. Since these ILs are very hygroscopic, water easily can be a major contaminant, causing changes in the phase behavior. In case these Ils are to be used in practical applications, for instance, together with CO2 as a medium in supercritical enzymatic reactions, it is very important to have quantitative information on how the water content will affect the phase behavior. This work presents the first systematic study on the influence of water on the solubility of carbon dioxide in hygroscopic ILs. It was observed that the presence of water reduces the absolute solubility of CO2. However, at fixed ratios of CO2/IL, the bubble point pressure remains almost unchanged with increasing water content. In order to explain the experimental results, the densities of aqueous mixtures of both ILs were determined experimentally and the excess molar volumes calculated.

Phase Behavior at High Pressure of the Ternary System: CO2, Ionic Liquid and Disperse Dye

“…It is possible to notice that CO 2 solubility in the IL increases continually with pressure, reaching 0.50 at 333 K and 9.83 MPa. Such behavior is also reported by Bermejo et al [27], Kamps et al [14], and Shariati and Peters [12]. Figure 2 and Table 2 show the experimental phase behavior for the ternary system CO 2 (1) + [bmim] [PF 6 ] (2) + disperse dye Red 60 (3).…”

Influence of water on the carbon dioxide absorption by 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide

“…Fu et al [23] have not observed a regular variation of the carbon dioxide solubility with the water mole fraction in [C 4 mim][PF 6 ]. An average variation of 6.5% on the solubility with a maximum variation of 15% was measured while in our case, a 47% decrease of the Henry's law constant for a water mole fraction of 0.27 is observed at 303 K. On the contrary, Bermejo et al [24] have observed a decrease of the carbon dioxide solubility in the presence of water. As it has been demonstrated by our group, using molecular simulation calculations, both carbon dioxide [44] and water [45] are preferentially solvated in the charged domains of the ionic liquids, the decrease of the carbon dioxide solubility in the presence of water being attributed to the competition of the two molecular components by the same ionic liquid solvation sites.…”

“…Such a result was already observed for carbon dioxide and others gases in ionic liquids and in mixtures of ionic liquid and a molecular solvent [41,42,26] but is not in line with the results determined by Scovazzo et al [43] that show no difference on carbon dioxide solubility between the dried ionic liquid and the water saturated solvent. The present results can be compared to literature studies where mixtures of an ionic liquid, a molecular solvent and a high pressure gas are studied [23,24]. Fu et al [23] have not observed a regular variation of the carbon dioxide solubility with the water mole fraction in [C 4 mim][PF 6 ].…”

“…These authors have observed an average variation of the solubility of 6.7% with the addition of water (water mass fraction varying from 0.0067 to 1.6%). Similarly, Bermejo et al [24] studied the influence of water on the high pressure solubility of carbon dioxide in two hygroscopic imidazolium based ionic liquids containing a nitrate anion. It was observed that the presence of water reduces the absolute solubility of CO 2 .…”

Understanding CO2 capture kinetics and energetics by ionic liquids with molecular dynamics simulation

“…In contrast, high concentration of water reversely lowered the CO 2 solubility. 16 Unfortunately, the molecular mechanism of the inuence of water on the performance of ILs is still unknown. This situation becomes to be the main obstacle to the design and development of highperformance ILs for the aim of effective CO 2 capture and storage.…”