Measurement of infinite dilution activity coefficients of n-alkanes in 4-methyl-n-butylpyridinium tetrafluoroborate using gas–liquid chromatography

doi:10.1016/j.fluid.2006.10.016

Fluid Phase Equilibria

Volume 251, Issue 1, 15 January 2007, Pages 17-23

https://doi.org/10.1016/j.fluid.2006.10.016 Get rights and content

Abstract

The infinite dilution activity coefficients of octane, nonane and decane in 4-methyl-n-butylpyridinium tetrafluoroborate (C₁₀H₁₆BF₄N, MW = 237.05) were measured by gas–liquid chromatography (GC) using the ionic liquid as stationary phase. The measurements were carried out at several temperatures between 297 and 344 K under atmospheric pressure. The infinite dilution activity coefficients were affected by the flow rate of carrier gas. The infinite dilution activity coefficients were determined by extrapolating the flow rate of carrier gas to zero. The infinite dilution activity coefficients determined in this work somewhat are larger than those of the literature values because the effect of flow rate of carrier gas was not considered in the literature. The partial molar excess enthalpy at infinite dilution and the heat of solution for the solutes in the ionic liquid were calculated from the infinite dilution activity coefficients.

The experimental results were correlated by using the ASOG, which is one of the successful group contribution activity coefficient models. The group interaction parameters were determined by using the Complex method to minimize the deviation between the experimental and calculated values. The correlated results are in good agreement with the experimental results.

Introduction

Ionic liquids have received increased attention in recent years. They are particularly suitable as new solvents or electrolytes because of their low vapor pressures, high boiling points, nonflammability and existence as liquids over wide temperature range. Most works have been invested in the development of synthetic methods and applications of ionic liquids for reaction processes [1], [2]. Recently, the applications of ionic liquids as heat transfer media [3], separation media [4], [5] or lubricant [6] are also discussed. However, the thermodynamic properties of the mixtures containing ionic liquids have not yet been studied systematically. Relatively few papers have been published on the liquid–liquid equilibria [7], [8], [9], the vapor–liquid equilibria [10], [11], [12], [13], [14], [15] and the extraction [16].

The infinite dilution activity coefficients of solutes are important for design of separation processes when the trace contents or impurities have to be removed. In particular, reliable data of equilibria at infinite dilution are required for purifying ionic liquids by stripping process or for separation of products from ionic liquids used as reaction solvents.

The infinite dilution activity coefficients of octane, nonane and decane in 4-methyl-n-butylpyridinium tetrafluoroborate (C₁₀H₁₆BF₄N, MW = 237.05) shown in Fig. 1 were measured by gas–liquid chromatography (GC) using the ionic liquid as stationary phase. GC method is suitable for measurement of the infinite dilution activity coefficients because of a negligible vapor pressure of ionic liquids. GC method is well known as the useful one, by which the data can be measured quickly. However, the infinite dilution activity coefficients are not measured directly by the GC method. Careful measurement is necessary for accuracy. The procedure of the measurement has been investigated in this study. Especially, the effect of flow rate of carrier gas and the effect of the sample size, which have not been discussed in previous study [13], have been considered. The partial molar excess enthalpy at infinite dilution and the heat of solution for the solutes in the ionic liquid have been calculated from the infinite dilution activity coefficients. The experimental results were correlated by using the ASOG [17], which is one of the successful group contribution activity coefficient models.

Section snippets

Materials

Reagent-grade octane, nonane and decane with a purity of 98 wt% were supplied by Wako Pure Chemical Industries Co. Ltd. The ionic liquid 4-methyl-n-butylpyridinium tetrafluoroborate was supplied by Merck. Chromosorb W AW DMCS (mesh 100/120) was supplied by GL Science Inc. Japan, and was used as the support material for the ionic liquid in the GC column.

Apparatus and procedure

The schematic diagram of the experimental apparatus is shown in Fig. 2. As a solvent, dichloromethane was used to coat the ionic liquid onto the

Correlation

The experimental results were correlated by using the ASOG [17], which is one of the successful group contribution activity coefficient models. The activity coefficient of component i is given by Eqs. (10), (11), (12), (13), (14), (15). $\ln γ_{i} = \ln γ_{i}^{FH} + \ln γ_{i}^{G}$ $\ln γ_{i}^{FH} = \ln \frac{ν_{i}^{FH}}{\sum_{j} ν_{j}^{FH} x_{j}} + 1 - \frac{ν_{i}^{FH}}{\sum_{j} ν_{j}^{FH} x_{j}}$ $\ln γ_{i}^{G} = \sum_{k} ν_{k, i} (\ln Γ_{k} - \ln Γ_{k}^{(i)})$ $\ln Γ_{k} = - \ln \sum_{p} X_{p} a_{k, p} + 1 - \sum_{p} \frac{X_{p} a_{p, k}}{\sum_{m} X_{m} a_{p, m}}$ $X_{k} = \frac{\sum_{i} x_{i} ν_{k, i}}{\sum_{i} x_{i} \sum_{l} ν_{l, i}}$ $\ln a_{k, p} = m_{k, p} + \frac{n_{k, p}}{T}$ where γ_i, $γ_{i}^{FH}$ and $γ_{i}^{G}$ are the activity coefficient of component i, the size contribution term and the

Conclusions

The infinite dilution activity coefficients of octane, nonane and decane in 4-methyl-n-butylpyridinium tetrafluoroborate were measured by GC at 297–344 K under atmospheric pressure. The infinite dilution activity coefficients were affected by the sample size of solutes. Therefore, they were determined to satisfy the infinite dilution condition. The values of the infinite dilution activity coefficients were also affected by the flow rate of carrier gas. Therefore, they were determined by

Acknowledgement

The present study was supported in part by a Grant-in-Aid for the 21st century center of excellence (COE) program, “Functional Innovation of Molecular Informatics”, from the Ministry of Education, Science, Sports and Culture of Japan.

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trends in solvent impact on infinite dilution activity coefficients of solutes reviewed and visualized using an algorithm to support selection of solvents for greener fluid separations
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The infinite dilution activity coefficient, ${γ_{i}}_{}^{\infty}$ , is a frequently used molecular descriptor to pre-select a solvent for various kinds of fluid separations. Unfortunately, information related to these coefficients is scattered through-out the open literature. Therefore, an open-source ${γ_{i}}_{}^{\infty}$ -database containing 77.173 $γ_{i}^{\infty}$ data points over the temperature interval 243 K < T < 555.6 K for 268 solutes and 692 solvents is provided in the electronic supporting information of this work. Additionally, we performed an inter- and extrapolation data analysis algorithm using the Van ‘t Hoff equation to extend the $γ_{i}^{\infty}$ data points at 298.15 K. The ${γ_{i}}_{}^{\infty}$ for five solutes (n-hexane, benzene, chloroform, acetone and ethanol) in a wide range of molecular solvents and ionic liquids (ILs) were compared. Various trends between the molecular solvent structure and the ${γ_{i}}_{}^{\infty}$ are visualized, which allows for not only a pre-selection of solvents for targeted ${γ_{i}}_{}^{\infty}$ but also visualizes the effect molecular modification of the solvent will have on the ${γ_{i}}_{}^{\infty}$ . Overall, the presented methodology is complementary to approaches using simulation software, and helps acquiring a detailed understanding of the effect of the solvent structure on the ${γ_{i}}_{}^{\infty}$ , which will facilitate solvent pre-selection in an early process design stage towards greener fluid separations.
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There are also a substantial amount of literature data according physicochemical properties of binary [26,34–37] and ternary [38] systems of ([b4mpy][BF4]). It is known, that ([b4mpy][BF4]) improves enantioselectivity of lipases [39], can be used as a solvent for separation of n-alkanes [40,41] and has an inhibition effect of the copper corrosion [42]. There is also information on ([b4mpy][BF4]) toxicity [2,43,44].
Ionic liquids (ILs) have attracted much attention for their unique physicochemical properties, which can be designed as needed by altering the ion combinations. Besides experimental work, numerous computational studies have been concerned with prediction of physical properties of ILs. The results of molecular dynamics simulations of ILs depend strongly on the proper force field parameterization. Classical force fields and the force fields designed specifically for ILs have been found to yield diffusion coefficients, that are too low for the liquid state. This has been attributed to the lack of electronic polarizability in the most force fields. One simple solution to this problem has been uniform by scaling down of the partial charges of all ions. In this work we investigated the influence of the charge scaling, the size of the simulated system and the temperature factor on calculated density, radial distribution function and the diffusion coefficients of the cation and anion of two methylpyridinium based ionic liquids: 1-butyl-4-methylpyridinium tetrafluoroborate ([b4mpy][BF₄]) and 1-butyl-4-methylpyridinium chloride ([b4mpy][Cl]). We show that the parameterization is the key for a proper reproduction of the diffusion coefficient and as a consequence the melting temperature and ionic conductivity. We were also able to get some structural informations on the cation-anion relationships in the investigated ILs.
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Interestingly, the pyridinium–based ILs were fund to be a good solvents to extract sulfur compounds from alkanes [10,24–27] and to separate alkanes from aromatic hydrocarbons [21,28–33]. The pyridinium-based ILs, which have been shown to have potentially excellent entrainer properties for the separation of aliphatic compounds from aromatic hydrocarbons by extractive distillation or extraction are pyridinium ethoxyethylsulfate, [Py][C2H5OC2H4SO4] [28], N-ethyl-pyridinium bis{(trifluomethyl)sulfonyl}imide, [EPy][NTf2] [28], N-butyl-4-methylpyridinium tetrafluoroborate [BM4Py][BF4] [29–32], and N-butyl-4-methylpyridinium bis{(trifluomethyl)sulfonyl}imide, [MB4Py][NTf2] [33]. Generally, the selectivity for the separation decreases with increasing length of the alkyl chain length on the cation, or anion of the IL.
The activity coefficients at infinite dilution, $γ_{13}^{\infty}$ for 65 solutes, including wide spectra of alkanes, alkenes and alkynes as well as aromatic hydrocarbons, alcohols, water, ethers, ketones, acetonitrile, pyridine, 1-nitropropane, thiophene, and esters in the 1-butyl-4-cyanopyridinium bis{(trifluoromethyl)sulfonyl}imide [BCN⁴Py][NTf₂] were determined by gas–liquid chromatography for all solutes at seven temperatures in range of (308.15 to 368.15) K. The gas–liquid partition coefficients, $K_{L}$ at infinite dilution and the fundamental thermodynamic functions, the partial molar excess Gibbs energy, $Δ G_{1}^{E, \infty}$ , enthalpy $Δ H_{1}^{E, \infty}$ , and entropy term $T_{ref} Δ S_{1}^{E, \infty}$ were calculated. The values of selectivity and capacity for four separation problems as hexane/hex-1-ene, hexane/benzene, cyclohexane/cyclohexene and ethylobenzene-styrene were calculated from $γ_{13}^{\infty}$ and compared to literature values for bis{(trifluoromethyl)sulfonyl}imide-based ionic liquids (ILs) for the same separation problems. In comparison with the former measured ILs, the [BCN⁴Py][NTf₂] present quite high selectivity for the chosen separation problems, and an average capacity for hex-1-ene, benzene, cyclohexene and styrene. The data presented here shows that [BCN⁴Py][NTf₂] may be proposed as an alternative solvent for the chosen separation problems in new technological projects.
Group contribution lattice fluid equation of state (GCLF EOS) for ionic liquids
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This work focuses on the extension of group parameters and application of the group-contribution lattice-fluid equation of state (GCLF EOS) for the systems with ionic liquids (ILs). The new group parameters e_kk and R_kk for ILs in GCLF EOS were obtained by means of correlating the liquid density of pure ILs at different temperatures, while the group binary interaction parameters α_mn were obtained by means of correlating the activity coefficients of solutes at infinite dilution in ILs at different temperatures, which were exhaustively collected from references. New IL groups were added into the current parameter matrix. It was verified that GCLF EOS for ILs can be used for predicting liquid densities of pure ILs or the mixture of IL and solute, vapor–liquid equilibria (VLE) and liquid–liquid equilibria (LLE) at finite concentration, as well as identifying the structure–property relation in separation science. This is the first work for us to extend the predictive GCLF EOS to the systems with ILs.
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A comparison of the most usual gas chromatographic methods for the calculation of partial molar enthalpies of solvation (Δ_solH^o) has been carried out. Those methods based on the fitting of lnV_g or ln(k/T) vs. 1/T and ln(k/T) vs. (1/T and the temperature arrangement, T_a) are the most adequate ones for obtaining Δ_solH^o values. However, the latter is the only reliable option for Δ_solH^o estimation when commercial WCOT capillary columns are used, since in this case the estimation of some variables involved in the V_g determination is less accurate or even impossible. Consequently, in this paper, Δ_solH^o obtained from ln(k/T) vs. (1/T + T_a) fitting at 373.15 and 298.15 K for n-alkanes and n-alkylbenzenes on 12 commercial capillary columns coated with stationary phases covering the 203–3608 McReynolds polarity range are reported. Moreover, molar heat capacities of solvation at constant pressure ( $Δ_{sol} C_{p}^{o}$ ) have also been calculated using this method. A clear influence on Δ_solH^o of the type and content of the substitution group in the stationary phase was observed. In addition, a linear relationship of $Δ_{sol} C_{p}^{o}$ with the van der Waals volume of the n-alkanes and the temperature gradient of density of the stationary phase was found. The effect of the size of the hydrocarbon on both thermodynamic variables was also investigated.
Phase behavior of mixtures of ionic liquids and organic solvents
2010, Journal of Supercritical Fluids
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For the subcritical components, Campanella et al. [9] relied on available methods for the excess Gibbs free energy to obtain solute-free activity coefficients, γ, a function only of temperature and the relative composition of the solvent mixture, x. Such methods are less matured for ionic liquid systems, although developments are in-progress [53–58]. Thus, the perspectives for methods of the present kind may depend to some extent on these or related developments.
A corresponding-states form of the generalized van der Waals equation, previously developed for mixtures of an ionic liquid and a supercritical solute, is here extended to mixtures including an ionic liquid and a solvent (water or organic). Group contributions to characteristic parameters are implemented, leading to an entirely predictive method for densities of mixed compressed ionic liquids. Quantitative agreement with experimental data is obtained over wide ranges of conditions. Previously, the method has been applied to solubilities of sparingly soluble gases in ionic liquids and in organic solvents. Here we show results for heavier and more-than-sparingly solutes such as carbon dioxide and propane in ionic liquids.

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Measurement of infinite dilution activity coefficients of n-alkanes in 4-methyl-n-butylpyridinium tetrafluoroborate using gas–liquid chromatography

Abstract

Introduction

Section snippets

Materials

Apparatus and procedure

Correlation

Conclusions

Acknowledgement

Tetrahedron Lett.

J. Mol. Catal. A Chem.

Thermochim. Acta

J. Membr. Sci.

Tsinghua Sci. Tech.

Tribol. Int.

Fluid Phase Equilib.

Fluid Phase Equilib.

Fluid Phase Equilib.

Chem. Eng. Sci.

Fluid Phase Equilib.

J. Phys. Chem. B

J. Phys. Chem. B