Measurement and modeling of infinite dilution activity coefficients of organic compounds in an equimolar ionic liquid mixture of [Bmim]Cl and [Bmim][Tf2N]
Graphical abstract
Introduction
Ionic liquids are being proposed as solvents for environmental and chemical applications due to their low volatility and their ability to dissolve many types of compounds. Some ionic liquids such as 1-butyl-3-methylimidazolium chloride, [Bmim]Cl, are able to dissolve lignocellulosic biomass [1,2], whereas water or most organic solvents are unable to dissolve biomass even under harsh conditions. Challenges in biomass processing with ionic liquids are related to the separation of products that tends to require thermal energy and the mixing or processing of biomass-ionic liquid solutions that have high viscosities [3]. CO2 is often selected as a separation solvent in ionic liquid solutions [[4], [5], [6]], because it does not require thermal energy and it has the advantage of reducing solution viscosity according to its solubility in the ionic liquid phase [7]. While some ionic liquids readily dissolve cellulose [2], other ionic liquids (e.g. [Bmim][Tf2N], 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide) are unable to dissolve biomass [1] but have high CO2 solubility [8]. Although it is possible to synthesize ionic liquids that have the desired characteristics, a simple approach is to use ionic liquid mixtures that have different functionalities (e.g. [Bmim]Clx[Tf2N]1-x) to design a solvent that would dissolve biomass and would have high CO2 solubility and low viscosity. However, physical properties are needed to assess ionic liquid mixtures for processing biomass. In previous research, high-pressure densities [9] and CO2 solubilities [10] in ionic liquid mixtures were reported. Partition coefficients for solutes in CO2-biphasic systems have been measured for many ionic liquids as these are important for assessing extraction processes. Solute partition coefficients in CO2-biphasic ionic liquids systems are scarce for systems that use ionic liquid mixtures.
Solute partition coefficients in CO2-ionic liquid mixture systems at high-pressure can be determined experimentally [[11], [12], [13], [14]], however, development of a scheme for their prediction in which solute-ionic liquid interactions are determined at atmospheric pressure would save considerable experimental effort and provide a better understanding of specific interactions in the system. For example, the selectivity and resolution factors of a solute in an ionic liquid mixture in gas chromatography can be considered as properties that describe solute interactions with ionic liquid mixtures [15,16]. Since the retention factor in gas-liquid chromatography is related to the solute activity coefficient, correlation of the activity coefficients measured at low-pressure with an equation of state provides a method to estimate or predict the solute partition coefficient at high-pressures for similar solvent systems.
The first objective of this work was to measure infinite dilution activity coefficients (IDACs) of benzene derivatives, furan derivatives and primary alcohols in ionic liquids, [Bmim]Cl, [Bmim][Tf2N] and their equimolar mixture denoted as [Bmim]Cl0.50[Tf2N]0.50, at atmospheric pressure and at (313 to 353) K with a chromatographic technique. IDACs in the pure ionic liquids, [Bmim]Cl [17] or [Bmim][Tf2N] [[18], [19], [20]], have been reported, whereas values in ionic liquid mixtures have not been reported. The second objective of this work was to correlate IDAC data with an equation of state (EoS) and to determine the EoS parameters. The EoS chosen was the ε*-modified Sanchez-Lacombe EoS due to its reliability for ionic liquid systems. The third objective was to apply interaction parameters determined from low-pressure data to predict high-pressure partition coefficients in CO2 - ionic liquid systems and to analyze their trends.
Section snippets
Materials and pretreatment
The source and purity of solutes, solvents and the mobile phase are given in Table 1 and those specifications for the ionic liquids are given in Table 2. Water and volatile impurities in the ionic liquids were removed by subjecting samples to vacuum (ca. 3 Pa) and temperature (353 K) for at least 24 h. After the first degassing procedure, the two ionic liquids were mixed and then subjected to treatment again. The ionic liquid mixture [Bmim]Cl0.50[Tf2N]0.50 was prepared by mass to within 0.1 mg
Correlation model for partition coefficient
The ε∗-mod SL EoS [26] was chosen for application to the measured IDAC since it provides reliable correlation of ionic liquid systems. The ε∗-mod SL EoS has the same functional form as the original Sanchez-Lacombe equation [27,28]:where T∗, P∗, ρ∗ are characteristic pure substance parameters. However, while ε∗ is a characteristic constant in the original SL-EoS, the ε∗-mod SL EoS treats the interaction energy ε∗
Experimental infinite dilution activity coefficients (IDACs)
Before measuring IDACs in ionic liquid mixtures, the validity of IDAC values were confirmed by comparing IDACs of benzene, toluene, methanol and ethanol in [Bmim][Tf2N] with literature values [20] (Fig. S1). It was also confirmed that the IDAC values did not vary with the flow rate of helium (Figs. S2 and S3), so that the flow rate was set constant for all solutes at 100 mL/min.
Fig. 2 shows experimentally-determined IDACs for benzene derivatives (benzene and toluene), primary alcohols (methanol
Conclusions
In this work, activity coefficients at infinite dilution (IDACs) for benzene derivatives, furan derivatives and primary alcohols in three ionic liquids ([Bmim][Cl], [Bmim][Tf2N] and their equimolar mixture were determined with a chromatographic technique. The IDAC values for the respective benzene- and furan-derivative solutes were higher for [Bmim]Cl than for [Bmim][Tf2N], whereas those for primary alcohols with [Bmim]Cl were lower than those for [Bmim][Tf2N]. The lower IDAC values for
Acknowledgements
This research was supported by JSPS KAKENHI (R.S.), Grant-in-Aid Scientific Research (B) Number 16H04549.
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