Solubility of hydrofluorocarbons in phosphonium-based ionic liquids: Experimental and modelling study

doi:10.1016/j.jct.2014.08.001

The Journal of Chemical Thermodynamics

Volume 79, December 2014, Pages 184-191

https://doi.org/10.1016/j.jct.2014.08.001 Get rights and content

Highlights

•
Gas solubilities of CHF₃, CH₂F₂ and CH₃F in phosphonium-based ILs measured at 1 atm.
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CHF₃, CH₂F₂ more soluble in [P_4,4,4,2][(C₂)₂PO₄] while CH₃F is in [P_6,6,6,14][Cl].
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Solubility of HFC/RTIL systems were modelled using CPA-EoS and RST.
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CPA-EoS describes well the experimental data with ARDs of 3.5% using a 2B scheme.
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Negative enthalpy and entropy of solvation is observed for all HFC/RTIL systems.

Abstract

In this work, experimental values of gas solubility of hydrofluorocarbons (CHF₃, CH₂F₂ and CH₃F) in three room-temperature ionic liquids (RTILs) were determined within the temperature range 288 K to 308 K and at atmospheric pressure. The RTILs used were trihexyltetradecylphosphonium chloride ([P_6,6,6,14][Cl]), tributyl(methyl)phosphonium methylsulfate ([P_4,4,4,1][C₁SO₄]), and tributyl(ethyl)phosphonium diethylphosphate ([P_4,4,4,2][(C₂)₂PO₄]). The data gathered have been modelled using two approaches: the Cubic plus Association equation of state (CPA EoS) and the regular-solution theory (RST). The experimental measurements were then discussed critically and the modelling results compared.

Introduction

This study aims to measure the temperature dependence at atmospheric pressure of hydrofluorocarbons (HFCs) solubility in three room-temperature ionic liquids (RTILs), trihexyltetradecylphosphonium chloride ([P_6,6,6,14][Cl]), tributyl(methyl)phosphonium methylsulfate ([P_4,4,4,1][C₁SO₄]), and tributyl(ethyl)phosphonium diethylphosphate ([P_4,4,4,2][(C₂)₂PO₄]). It extends the experimental work carried out previously [1] by measuring the solubility of trifluoromethane (CHF₃), difluoromethane (CH₂F₂), and fluoromethane (CH₃F) to a new set of unstudied RTILs.

Earlier, some authors have determined the solubility of alcohols [2], alkanes and alkenes [3], [4], [5], [6], [7], [8], [9], oxygen [8], carbon monoxide [9] or carbon dioxide [2], [4], [5], [6], [7], [10], [11] in phosphonium-based RTILs. Ferguson and Scovazzo [3] have shown that imidazolium and phosphonium-based RTILs have similar values of solubility for several gases, while Anthony and co-workers [4] suggested that the nature of the anion has the most significant influence on the gas solubility. Yokozeki, Shiflett and co-workers have determined the solubility of hydrofluorocarbons and hydrofluoroethers in some non phosphonium-based RTILs [12], [13], [14], [15]. Despite the significant amount of work contained in the open literature, experimental measurements of hydrofluorocarbon solubility in phosphonium-based RTILs are still unavailable. These solvents have several advantages that could make them an obvious choice for industrial purposes. Contrary to imidazolium and pyridinium, phosphonium-based RTILs are usually less dense than water and exhibit an extremely low melting temperature, providing potential benefits for some industrial applications, e.g. separation purposes [16]. Furthermore, they are more stable in basic and nucleophilic conditions due to the absence of acidic protons in their moieties [17].

In the present work, the volumetric method with an automated apparatus [18] is used to determine the solubility of CHF₃, CH₂F₂ and CH₃F in [P_6,6,6,14][Cl], [P_4,4,4,1][C₁SO₄], and [P_4,4,4,2][(C₂)₂PO₄], as a function of temperature at atmospheric pressure. This procedure has shown to be highly accurate and precise in the past for various gases and solvents, making it a preferable choice.

The experimental results were then modelled to provide information about the HFC-RTIL systems and grasp their behaviour. Yokozeki, Shiflett and co-workers modelled the solubility of gases in RTILs with modified cubic equations of state (EoS). They concluded that all the EoS tested work equally well for the modelling of PTx phase equilibria (solubility) [19]. Another approach based on the regular solution theory (RST) was employed by Camper and collaborators to model the solubility of simple gases in RTILs [20], [21], [22], [23], [24]. Although RST was originally conceived for non-electrolyte solutions, they successfully showed that simple thermodynamic relationships could be drawn between gas solubility and the cohesive energies of gases and RTILs. As pointed out by Scovazzo et al. [21], the electrolyte character of the solutions containing ionic liquids (ILs) is dependent whether the Coulombic or the van der Waals forces are the dominant interactions. Because in ILs the ion charge is usually disperse, Coulombic forces are commonly weak compared with the van der Waals forces.

Two modelling approaches were used. Values of the observed solubility are first analysed by a model based on a Cubic plus Association equation of state (CPA EoS) to verify the accuracy of the CPA EoS for the description of the physical interactions and the intermolecular specific association interactions in HFC-RTIL systems. Secondly, the RST is used to verify the relationship between Henry’s law constant values at a reference temperature and the molar volume of RTILs as well as to estimate the partial molar enthalpy and entropy of solution of the HFCs.

Section snippets

Experimental

The experimental technique used in this work is based on a volumetric method. The apparatus used for the determination of solubility was described previously in detail [18]. The RTILs and HFCs studied in this work are presented in table 1, along with their full name, abbreviations, chemical structures, origin and purities in mass fraction for RTILs and mole fraction for HFCs. The water and volatile compounds contents in ILs were not measured, however all ILs were degasified by applying vacuum

Gas/liquid equilibria modelling

The contact of a gas with a liquid produces changes of enthalpy and entropy along with a decrease in gas volume as dissolution takes place until an equilibrium state is reached. Mathematically, the (gas + liquid) equilibrium condition for the solute can be expressed as a stationary point of Gibbs function, i.e. iso-fugacity condition: $f_{2}^{G} = f_{2}^{L}$ . To model gas and liquid fugacities of HFCs ( $f_{2}^{G}$ and $f_{2}^{L}$ , respectively) and calculate their solubility in RTILs, two strategies are employed. In the former,

Experimental results

The solubility data and the Henry’s law constants for HFCs in RTILs solvents, for several temperatures at about 101.3 kPa are presented in TABLE 3, TABLE 4, TABLE 5. The complete raw experimental values can be found in the supporting information. The results show that the solubility increases in the following order: [P_4,4,4,1][C₁SO₄] < [P_6,6,6,14][Cl] < [P_4,4,4,2][(C₂)₂PO₄] for CHF₃ and CH₂F₂, and [P_4,4,4,1][C₁SO₄] < [P_4,4,4,2][(C₂)₂PO₄] < [P_6,6,6,14][Cl] for CH₃F. The behaviour of the HFCs solubility

Conclusions

The solubility of HFCs (CHF₃, CH₂F₂ and CH₃F) in three phosphonium-based RTIL solvents ([P_6,6,6,14][Cl], [P_4,4,4,1][C₁SO₄], and [P_4,4,4,2][(C₂)₂PO₄]) determined within the temperature range between 288 K and 308 K at 101.3 kPa has been reported in this paper. It was found that among the solvents, values of the solubility for CHF₃ and CH₂F₂ were greater in [P_4,4,4,2][(C₂)₂PO₄] while CH₃F presented higher solubility values in [P_6,6,6,14][Cl]. Moreover, for the solvents studied, the solubility values

Acknowledgements

The authors acknowledge financial support provided by Fundação para a Ciência e Tecnologia (Portugal) through the Ph.D. Grant SFRH/BD/64338/2009 of J.F. Granjo and the Ph.D. Grant SFRH/PROTEC/67482/2010 of J.M.M.V. Sousa.

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Solubility of hydrofluorocarbons in phosphonium-based ionic liquids: Experimental and modelling study

Highlights

Abstract

Introduction

Section snippets

Experimental

Gas/liquid equilibria modelling

Experimental results

Conclusions

Acknowledgements

J. Chem. Thermodyn.

Fluid Phase Equilib.

Chem. Eng. J.

J. Chem. Thermodyn.

Ecotoxicol. Environ. Saf.

J. Chem. Thermodyn.

J. Supercritical Fluids

J. Chem. Thermodyn.

Fluid Phase Equilib.

J. Supercritical Fluids

J. Phys. Chem. B

Ind. Eng. Chem. Res.

J. Phys. Chem. B

J. Chem. Eng. Data

Eng. Chem. Res.

J. Phys. Chem. B