Physicochemical properties and activity coefficients at infinite dilution for organic solutes and water in a novel bicyclic guanidinium superbase-derived protic ionic liquid

Abstract

The activity coefficient at infinite dilution, ${\gamma }_{13}^{\infty }$, was determined by gas–liquid chromatography at temperatures from T = 308.15 to T = 348.15 K for 54 assorted solutes—among them, alkanes (linear, branched, or cyclic), alkenes, alkynes, aromatic hydrocarbons, alcohols, water, thiophene, tetrahydrofuran, 1,4-dioxane, ethers, acetone, ketones, acetonitrile, pyridine, and 1-nitropropane—in the novel bicyclic guanidine superbase-derived protic ionic liquid 1,3,4,6,7,8-hexahydro-1-methyl-2H-pyrimido[1,2-a]pyrimidine bis(pentafluoroethyl)sulfonylimide, [MTBDH][BETI]. The partial molar excess Gibbs free energy $\mathrm{\Delta }{G}_1^{\text{E,}\infty }$, the partial molar excess enthalpy $\mathrm{\Delta }{H}_1^{\text{E,}\infty }$, and entropy at reference temperature $T_{\text{ref}}\mathrm{\Delta }{S}_1^{\text{E,}\infty }$ at infinite dilution were calculated from the experimental ${\gamma }_{13}^{\infty }$ values obtained over this temperature range. The gas–liquid partition coefficient, $K_{\text{L}}$, was calculated for each solute and discussed in light of the Abraham solvation parameter model. The density of the IL as a function of temperature was also measured. The selectivities for heptane/benzene, heptane/thiophene, heptane/pyridine, and heptane/nitropropane separation problems were calculated from ${\gamma }_{13}^{\infty }$ and compared to literature values for select ILs, as these separation problems are central to the petroleum industry.

Introduction

Ionic liquids (ILs) are attracting momentous attention as innovative alternatives to conventional organic solvents as N-methyl-2-pyrrolidinone (NMP) or sulfolane for countless solvent extraction and separation processes [1], [2], [3], [4]. Not limited to academic interest, the industrial importance of ILs is amply shown by the number of newly-emerged technologies currently entering or already on the market [5]. The prevalence of ILs stems from their unique and tailored properties, such as strong solvation and (in general) non-flammability and negligible volatility [1]. The sensible design of IL-based extraction processes requires a knowledge of the activity coefficients at infinite dilution, ${\gamma }_{13}^{\infty }$, making possible the calculation of entrainer selectivity, $S_{12}^{\infty }$, and capacity, $k_2^{\infty }$ [6]. The rational selection of ILs for practical purposes in aliphatic/aromatic hydrocarbon separations was recently discussed in a fascinating review on the subject [7]. To aid technologists and engineers in selecting the appropriate IL for solving a range of separation problems, we propose that measurement of ${\gamma }_{13}^{\infty }$ for a great many solutes in numerous and diverse ILs is a necessary step, particularly as studies of these materials shift from understanding the parameters that govern their fundamental properties to pursuing useful applications. The solute–solvent interactions in solution are controlled by ${\gamma }_{13}^{\infty }$ and, thus, deeper knowledge of ${\gamma }_{13}^{\infty }$ will make ILs more widely applicable. Furthermore, the range of extraction-robust cations and anions has been extended in recent years, allowing for the preparation of next-generation ILs with enhanced utility. In this regard, perfluorinated anions such as trifluorotris(perfluoroethyl)phosphate (FAP) [2], [8], [9], [10], [11] or nitrile-functionalized 1-alkylpyridinium and N-alkyl-N-methylpiperidinium cations [12], as prominent examples, have generated new opportunities in advanced extractions. Sustained effort on the part of synthetic chemists among the community of IL researchers is required to prepare additional and functional cations and anions, pushing the experimental space that can be explored in the generation of reliable data to advance this new separation technology.

Recently, a family of ILs based on N-alkylisoquinolinium or N-alkylquinolinium cations was fully described by our laboratory [13], [14], [15], [16], [17]. It was found, for example, that N-octylisoquinolinium bis(trifluoromethylsulfonyl)imide, [OiQuin][NTf₂], was an excellent entrainer for the extraction of 2-phenylethanol from aqueous bio-media [17]. These sorts of studies remain limited, however. For instance, there exists in the literature but a single study of ${\gamma }_{13}^{\infty }$ for different organic solutes in an IL based on the bis(pentafluoroethyl)sulfonylimide anion [BETI]^–, in this case paired to the 1-butyl-3-methylimidazolium cation, [BMIM]⁺ [11]. In this particular study, it was revealed that [BMIM][BETI] exhibits both high selectivity (33.3) and capacity (2.51) in the heptane/pyridine separation process [11]. The prospect for tailoring the properties of the IL to meet the requirements of a specific application is particularly attractive. Recent results have suggested that double aromatic rings such as that found in benzimidazolium deserve further contemplation in separations. We feel that, likewise, bicylic cations warrant additional scrutiny.

Here, we report the activity coefficients at infinite dilution, ${\gamma }_{13}^{\infty }$, for 54 solutes, including saturated and unsaturated alkanes, cycloalkanes, aromatic hydrocarbons, alcohols, water, thiophene, tetrahydrofuran, ethers, acetone, ketones, pyridine, and 1-nitropropane in the novel bicyclic guanidine superbase-derived protic IL 1,3,4,6,7,8-hexahydro-1-methyl-2H-pyrimido[1,2-a]pyrimidine bis(pentafluoroethyl)sulfonylimide, [MTBDH][BETI]. Values of ${\gamma }_{13}^{\infty }$ were determined by gas–liquid chromatography at 10 K intervals from T = 308.15 K to T = 348.15 K. This work also provides a unique opportunity to make direct comparisons with previous results with [BMIM][BETI]. By way of additional characterization, we also determined the density of [MTBDH][BETI] as a function of temperature.