Thermophysical properties and phase equilibria study of the binary systems {N-hexylquinolinium bis(trifluoromethylsulfonyl)imide + aromatic hydrocarbons, or an alcohol}

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Abstract

The new quinolinium ionic liquid has been synthesised as a continuation of our work with quinolinium-based ionic liquids (ILs). The work includes specific basic characterisation of synthesized compounds: N-hexylquinolinium bromide, [HQuin][Br] and N-hexylquinolinium bis{(trifluoromethyl)sulfonyl}imide [HQuin][NTf2] by NMR spectra, elementary analysis and water content. The basic thermal properties of the pure [HQuin][NTf2] i.e. melting and glass-transition temperatures, the enthalpy of fusion as well as heat capacity have been measured using a differential scanning microcalorimetry technique (DSC) and thermal analysis instrument (TA). Densities and viscosities were determined as a function of temperature. Phase equilibria for the binary systems: {[HQuin][NTf2]) + aromatic hydrocarbon (benzene, or toluene, or ethylbenzene, or n-propylbenzene), or an alcohol (1-butanol, or 1-hexanol, or 1-octanol, or 1-decanol)} have been determined at ambient pressure. A dynamic method was used over a broad range of mole fractions and temperatures from (270 to 320) K. For all the binary systems with benzene and alkylbenzenes, the eutectic diagrams were observed with immiscibility gap in the liquid phase beginning from (0.13 to 0.28) mole fraction of the IL with very high an upper critical solution temperature (UCST). For mixtures with alcohols, the complete miscibility was observed for 1-butanol and immiscibility with UCST in the liquid phase for the remaining alcohols. The typical dependence was observed, that with increasing chain length of an alcohol the solubility decreases. The well-known NRTL equation was used to correlate experimental (solid + liquid), SLE and (liquid + liquid), LLE phase equilibria data sets. For the systems containing immiscibility gaps, (IL + an alcohol) parameters of the LLE correlation were used to the prediction of SLE.

Research highlights

► We synthesized new ionic liquid, [HQuin][NTf2] with low viscosity, and low density. ► We found high heat capacity, high enthalpy of melting and low melting temperature. ► HQuin][NTf2] is proposed for possible use in the phase change materials (PCM). ► We examine phase equilibrium changes, SLE and LLE with hydrocarbons and alcohols. ► [HQuin][NTf2] may be proposed as entrainer for the separation proceses.

Introduction

Ionic liquids (ILs) are under intense investigation, especially as replacement solvents for reactions and separations as they reveal negligible vapour pressure and do not contribute to air pollution. Until recently, the most important and popular ILs considered as extractive solvents: imidazolium, pyridinium and pyrrolidinium based ionic liquids with anions as e.g. bis{(trifluoromethyl)sulfonyl}imide anion [NTf2], or trifluoromethanesulfonate anion [CF3SO3], or thiocyanate, [SCN] have attracted the attention in different laboratories [1], [2], [3], [4]. Polyaromatic quinolinium-based ionic liquids have shown great extraction potential in the desulfurization of oils (extraction of dibenzothiophene from n-dodecane) [4]. The quinolinium-based ionic liquid was used in the selection of entrainers in the 1-hexene/n-hexane separation problem with moderate result [5]. Usually, the activity coefficients at infinite dilution show the possibility of using the IL in the extraction process. The first experimental work was presented from our laboratory for isoquinolinium-based IL. The activity coefficients at infinite dilution, γ13 for 37 solutes: alkanes, alkenes, alkynes, cycloalkanes, aromatic hydrocarbons, alcohols, ethers, ketones and water in the ionic liquid N-octylisoquinolinium bis{(trifluoromethyl)sulfonyl}imide [C8iQuin][NTf2] were determined by gas–liquid chromatography at the temperatures from (328.15 to 368.15) K [6]. It was found that this IL shows definite lower selectivity at infinite dilution than other, most popular ionic liquids with the bis{(trifluoromethyl)sulfonyl}imide anion and lower than the entrainers used in industry such as N-methyl-2-pirrolidynone (NMP) or sulfolane used in the processes of separation of aliphatic hydrocarbons from aromatic hydrocarbons. For example, the selectivity of the separation of n-hexane/benzene at T = 328.15 K was 8.06 and 17.3 for [C8iQuin][NTf2] and 1-butyl-4-methylpyridynium bis{(trifluoromethyl)sulfonyl}imide, [BMPy][NTf2], respectively. A slightly better result was shown for the n-heptane/tiophene separation problem at T = 328.15 K: 10.96 and 25.5 for [C8iQuin][NTf2] and [BMPy][NTf2], respectively.

The phase equilibria (solid + liquid) phase equilibria, SLE and (liquid + liquid) phase equilibria, LLE of binary systems containing the N-butylquinolinium bis{(trifluoromethyl)sulfonyl}imide [BQuin][NTf2] IL and a different solvents have shown typical phase equilibria – eutectic systems with immiscibility in the liquid phase with aromatic hydrocarbons and longer chains alcohols [7]. Interesting was the complete miscibility in the liquid phase up to the high mole fraction of aromatic hydrocarbon [7]. Knowledge of the phase equilibria is fundamental for the ILs to be used effectively as solvents in extractive distillation or liquid–liquid extraction [8], [9], [10]. Particularly, by varying the cation, anion and/or substituent groups, the physico-chemical properties of the IL could be tailored to fit into specific requirements.

This paper is a continuation of our wide ranging investigation into the physicochemical properties and phase equilibria containing the ILs. The new quinolinium-based IL [HQuin][NTf2] was proposed to compare with [BQuin][NTf2] in thermophysical properties and phase equilibria measurements. The [BQuin][NTf2] was solid at room temperature, thus the longer alkyl chain at the quinolinium ring should decrease the melting temperature of the IL.

The quinolinium-based ILs were found to reveal high values for the enthalpy of fusion [7] and can be proposed as a heat-transfer fluids. It was shown that some ionic liquids have thermophysical and chemical properties that may be suitable for heat-transfer and short heat term storage materials in power plants using parabolic solar collectors [11], [12], [13]. Because, in practice the ILs are liquid around room temperature, they may be handled like ordinary solvents. We have to remember however, that ILs reveal strong ion–ion interactions that are not often seen in higher temperature molten salts. Heat-transfer fluids have wide applications from refrigeration systems at low temperatures to solar collection and storage at high temperatures. The specific application for ILs as a heat-transfer fluid is in the solar electric generating system (SEGS) in electric power plants [11], [14]. The heat storage could be provided by sensible heat, by reversible chemical reactions, or by phase change, especially by heat of fusion and solid–solid phase transitions at convenient temperatures. Heat storage depends on the density and heat capacity of the fluid as well as of heat of phase transitions calculated per mass. As a first proposition, the three chosen ILs were presented: 1-ethyl-3-methylimidazolium tetrafluoroborate, [EMIM][BF4], 1-butyl-3-methylimidazolium tetrafluoroborate, [BMIM][BF4] and 1,2-dimethyl-3-propyl bis{(trifluoromethyl)sulfonyl}imide [DMPIM][NTf2] [11]. Nowadays, the tetrafluoroborate-based ILs are eliminated from the possible industry use because they are not water stable. A much wider perspective of phase change materials based on imidazolium-based ILs was presented for 44 ILs using a quantitative structure–property relationship (QSPR) model for heat of fusion calculations [13]. The substance with the highest enthalpy of fusion, i.e. 1-hexadecyl-3-methylimidazolium bromide, [C16MIM][Br] presented the enthalpy 152.56 J  g−1 has quite high melting temperature (Tfus = 337.06 K) [13]. Among the phase change materials (PCM), some can be used to store thermal energy temporarily, and release it when the energy is needed. This effect can be used in fibre and textiles which have automatic acclimatising properties to the human body [15]. The technology for incorporating PCM microcapsules into textile structure to improve their thermal performance is popular to use in fabrics for astronauts, miners, military personnel to provide improved thermal protection against the extreme temperature fluctuations especially in outer space. The PCM materials absorb energy during the heating process as phase change takes place and next is transferred to the environment in the phase change range during a reverse cooling process [16]. The ILs proposed for these smart materials must show a low melting temperature, about the temperature of human body, high enthalpy of fusion per gram of substance, high heat capacity, low density and sharp freezing/melting characteristics.

The aim of the present work is to study the thermophysical and physicochemical properties of a new IL in perspective of use as PCM and/or as an extractive substance for separation of aromatic hydrocarbons from aliphatic hydrocarbons. The phase equilibria are under study, the SLE and LLE of N-hexylquinolinium bis{(trifluoromethyl)sulfonyl}imide, {[HQuin][NTf2]) + aromatic hydrocarbon (benzene, or toluene, or ethylbenzene, or n-propylbenzene)}, and with alcohols (1-butanol, or 1-hexanol, or 1-octanol, or 1-decanol) at ambient pressure. The data obtained are analysed to determine the influence of the nature of the cation in comparison with other ionic liquids on solubility; the role of the alkyl chain length at the quinolinium ring; the role of the alkyl chain length of an alcohol and benzene derivatives. Herein, the densities, viscosities and heat capacities were measured as a function of temperature. The molecular interpretation of the solubility, in order to gain important insights on how the molecules interact or how they behave in solution and thermal properties are discussed in a possible use in new technologies.

Section snippets

Materials

Synthesis of 1-hexylquinolinium bromide [HQuin][Br]. To a solution of 120.08 g quinoline (distilled, Aldrich 0.98 mass fraction purity) (0.9297 mol) in 200 cm3 acetonitrile, 179.21 g hexylbromide (as received, Aldrich 0.98 mass fraction purity), (1.0856 mol, 1.17 equivalents) were added. The stirred mixture was heated to reflux for 24 h. Solvent was removed in vacuum. The purple product was diluted with 200 cm3 of water, and extracted with diethyl ether until latter remained colourless (∼10 × 70 cm3).

Results and discussion

The basic thermal properties of the ionic liquids i.e. temperature of fusion (Tfus), enthalpy of fusion (ΔfusH), have been measured. The average value of the melting temperature was Tfus = (317.20 ± 0.05) K (average over two scans). The enthalpy of fusion was (63.54 ± 0.08) kJ  mol−1 and the Tg,1 was (219.3 ± 0.1) K with ΔCp(g),1 of (356.4 ± 3) J  K−1  mol−1. The enthalpy of fusion was found to be high as for IL and to be not very high (128.50 ± 50 J  g−1) in comparison with enthalpy of a typical PCM i.e. paraffins

(Solid + liquid) phase equilibrium

The equation frequently applied to the (solid + liquid) equilibrium data calculations is [24]:-lnx1=ΔfusH1R1T-1Tfus,1-ΔfusCp,1RlnTTfus,1+Tfus,1T-1+lnγ1,where x1, γ1, ΔfusH1, ΔfusCp,1, Tfus,1, T, are mole fraction, activity coefficient, enthalpy of fusion, difference in solute heat capacity between the liquid and solid phase at melting temperature, melting temperature, equilibrium temperature, respectively. The thermophysical data of the pure compound are presented in table 1 and the experimental

Concluding remarks

The thermal properties of [HQuin][NTf2] presented here indicate that the proposed IL is suited for use as a phase change material. In many ways it is superior to present commercial heat-transfer fluids. It is stable over a wide temperature range, has a low melting temperature, has a relatively low density, can store substantial heat, and has the advantage of low vapour pressure.

The results obtained in the phase equilibria study show better solubility with aromatic hydrocarbons and alcohols than

Acknowledgement

Funding for this research was provided by the Polish Ministry of Education and Sciences for the Grant II-24.

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